WO2021151151A1

WO2021151151A1 - Variable length bone screw

Info

Publication number: WO2021151151A1
Application number: PCT/AU2021/050052
Authority: WO
Inventors: David Graham Little; Tegan Laura CHENG; Andrew Lillia; Sarah Frances Williamson
Original assignee: The Sydney Children's Hospitals Network (Randwick And Westmead)
Priority date: 2020-01-31
Filing date: 2021-01-28
Publication date: 2021-08-05

Abstract

An orthopaedic fixation device and method of insertion. The fixation device includes a first elongate member which comprises a first shaft which has a first threaded portion, a first drive feature for connecting to a drive device and a longitudinal channel. A second elongate member comprises a second shaft slidably receivable in the longitudinal channel of the first elongate member, the second shaft having a second threaded portion and a second drive feature for connecting to the drive device. When the first and second drive features are connected to the drive device, the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device.

Description

Variable Length Bone Screw

Technical Field

[0001] The present disclosure relates generally to orthopaedic fixation devices, and more generally to variable length bone screws.

Background

[0002] Certain types of bone fractures, injuries or conditions may require internal fixation to restore proper anatomical structure and promote healing. Fixation is commonly achieved through the use of orthopaedic screws, nails, pins, and the like.

[0003] Slipped capital femoral epiphysis (SCFE) is a hip condition in which the head of the femur (the capital epiphysis) loses the appropriate strength of the connection of the femoral head to the femoral neck at the growth plate, resulting in increasing deformity and hip dysfunction. This condition occurs primarily in children prior to skeletal maturity. Treatment for most forms of SCFE may involve fixation into the femoral head via a single screw through the femoral neck. Traditionally, the aim in screw fixation of SCFE is to stop the growth of the bone in the region of the slip, in the hope of preventing the deformity from recurring or progressing. However, such fixation usually results in cessation of growth and may also lead to premature fusion of the growth plate. Both growth inhibition and premature fusion of the growth plate lead to subsequent imbalanced growth of the femur. This may adversely impact hip biomechanics and lead to long-term complications.

[0004] Any discussion of documents, acts, materials, devices, articles or the like which has been included in the present specification is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the present disclosure as it existed before the priority date of each of the appended claims. Summary

[0005] According to one aspect of the present disclosure there is provided a bone screw comprising: a first elongate member, and a second elongate member longitudinally slidably connectable to the first elongate member, wherein the first and second elongate members are each connectable to a drive device and wherein, when the first and second elongate members are connected to the drive device, the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device.

[0006] According to one aspect of the present disclosure, there is provided a bone screw including: a first elongate member comprising: a first shaft extending from a proximal end to a distal end, the first shaft having a first threaded portion; a first drive feature at a proximal end of the first shaft for connecting to a drive device; and a longitudinal channel; and a second elongate member comprising: a second shaft extending from a proximal end to a distal end and slidably receivable in the longitudinal channel of the first elongate member, the second shaft having a second threaded portion; a second drive feature at the proximal end of the second shaft for connecting to the drive device, wherein, when the first and second drive features are connected to the drive device, the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device.

[0007] In some embodiments, the screw may be configured for implantation in a human femur. The screw may be configured for implantation in the a femur of a child or adolescent, for example, for treatment of SCFE. In some embodiments, the screw may be configured for removal after a predetermined period of time, or once predetermined clinical indications have been met. In other embodiments, the screw may be configured to be left in situ indefinitely.

[0008] The first and second elongate members may be assembled to form the bone screw by inserting the second shaft into the longitudinal channel. When assembled, the elongate members may be longitudinally slidable relative to one another. In some embodiments, when assembled, the elongate members may be rotationally movable relative to each other. The elongate members are typically not translationally movable relative to each other. The screw may be axially extendable while implanted, allowing for growth of the bone in the longitudinal direction of the screw while maintaining proper axial alignment of bone components. The axial extension may occur without the application of compressive forces.

[0009] The screw may be configured to have an initial assembled length of between about 40 mm and about 110 mm. For example, the screw may have a length of up to about 40 mm, about 55 mm, about 60 mm, about 65 mm, about 70 mm, about 75 mm, about 80 mm, about 85 mm, about 90 mm, about 95 mm, about 100 mm, about 105 mm, about 110 mm or more.

[0010] In some embodiments, the screw may be telescopically extendible. The longitudinal channel may be substantially centrally located such that, when assembled, the first and second elongate members share a common longitudinal axis. In some embodiments, the longitudinal channel may extend the entire length of the first elongate member.

[0011] In some embodiments, the longitudinal channel of the first elongate member and the second shaft of the second elongate member may each have a substantially cylindrical shape such that the second shaft is rotatably and axially slidably receivable in the longitudinal channel. In such embodiments, the interconnection of the first and second elongate members may allow for longitudinal sliding and rotational movement of the first and second elongate members relative to each other, along or about the common axis. For example, the first shaft may comprise a hollow tube, wherein the longitudinal channel is a central cannulated portion. In other embodiments, the longitudinal channel may have a non-circular cross-section configured to receive a correspondingly shaped second shaft such that the first and second elongate members are axially slidable but not rotatable relative to one another.

[0012] The second shaft may be partially receivable in the longitudinal channel. As such, the second shaft may comprise a receivable portion and a non-receivable portion. A cross-section of the non-receivable portion may differ from a cross-section of the receivable portion, such that the non-receivable portion cannot be inserted into the longitudinal channel. In embodiments having a cylindrical channel, for example, an outer diameter of at least a proximal end of the non-receivable portion may be larger than the diameter of the longitudinal channel. By contrast, the outer diameter of the second shaft at the receivable portion may substantially equal to (or, in some embodiments, smaller than) the inner diameter of the longitudinal channel.

[0013] The non-receivable portion may comprise the second threaded portion, such that the second threaded portion is exposed when the receivable portion is received in the longitudinal channel.

[0014] The receivable portion may be substantially non-threaded. In other embodiments, the receivable portion may be partially or wholly threaded. The receivable portion may have a length greater than that of the first elongate member such that, when assembled, the receivable portion of the second shaft extends all the way through the longitudinal channel and is sleeved by the first elongate member. In such embodiments, when fully inserted, the proximal end of the second elongate member may protrude from a proximal opening of the longitudinal channel.

[0015] In some embodiments, the screw may further comprise a cap for sealing the proximal end of the longitudinal channel after insertion of the screw, for example to prevent tissue ingrowth. [0016] The first and second elongate members (and, optionally, the cap) of the present disclosure may be made from a number of suitable biocompatible materials. For example, the elongate members may be made from a metal or a metal alloy. Alternatively (or additionally), the elongate members may be made from a polymeric material. Examples of suitable materials include, but are not limited to, stainless steel and its alloys, titanium and its alloys, cobalt chrome and its alloys, tantalum and its alloys, polyether ether ketone (PEEK), MP35N and its alloys, and graphite/pyrocarbon.

[0017] Fixation of the first and second elongate members to surrounding bone may be achieved by the first and second threaded portions, respectively. Typically, the first and second threaded portions are fixed to respective, opposed bone regions, such that when the bone grows, the elongate members are pulled apart and the length of the screw increases. The screw may increase in length until the bone ceases its growth. Natural growth in the femoral neck in children is typically in the order of 3 mm per year.

[0018] The screw may be configured such that, when the second shaft is received in the longitudinal channel to assemble the screw, the second threaded portion is distal to the first threaded portion.

[0019] For example, in embodiments configured for treatment of SCFE, the second threaded portion may be distal to the first threaded portion and adapted for fixation to the femoral head (epiphysis), while the first threaded portion may be adapted for fixation to at least a metaphyseal portion of the femur, such that the screw extends through the neck of the femur and across the growth plate. In some embodiments, the first threaded portion is adapted for fixation to a metaphyseal and a diaphyseal portion of the femur. The unthreaded region is positioned across the growth plate. As the femoral neck lengthens during growth, the length of the screw may increase in length accordingly while maintaining axial alignment of the femoral head relative to the other portions of the femur. [0020] The screw may be configured to avoid the application of compressive forces across the growth plate. In some embodiments, when assembled, the screw has an unthreaded region between the first and second threaded portions. For example, the unthreaded region may be formed by one or a combination of an unthreaded region on the first elongate member, an unthreaded region on the second elongate member or unthreaded mating features forming an unthreaded mating region. The absence of screw threads at or adjacent the growth plate may substantially inhibit bone attachment to the screw in this region.

[0021] The first threaded portion may extend from the proximal end of the first shaft towards the distal end of the first shaft. The first threaded portion may extend fully along a nominal length of the first shaft, that is, from the proximal end of the first shaft (exclusive of any head) to the distal end of the first shaft. In other embodiments, the first threaded portion may extend partially along the nominal length of the first shaft. For example, the first threaded portion may extend along at least 10%, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90% or more, including up to 100% of the length of the first shaft.

[0022] The second threaded portion may be located at or adjacent to the distal end of the second shaft. The thread of the second threaded portion may be substantially identical in major diameter, root diameter (minor diameter) and/or pitch to the first threaded portion.

[0023] In some embodiments, the major diameter of the threads of the first and/or second threaded portions may be between about 6.5 mm to about 7.3 mm. For example, the major diameter may be up to about 6.5 mm, about 6.6 mm, about 6.7 mm, about 6.8 mm, about 6.9 mm, about 7.0 mm, about 7.1 mm, about 7.2 mm, about 7.3 mm, about 7.4 mm, about 7.5 mm, or more. The root diameter may be between about 4.8 mm to about 5.6 mm. For example, the root diameter may be about 4.8 mm, about 4.9 mm, about 5.0 mm, about 5.1 mm, about 5.2 mm, about 5.3 mm, about 5.4 mm, about 5.5 mm, about 5.6 rumor more. [0024] In some embodiments, the thread of the second threaded portion may have a smaller root diameter than the thread of the first threaded portion. That is, the depth of the thread may be greater in the second threaded portion than in the first threaded portion. A deeper thread may provide for more secure fixation of the second threaded portion, which may have a shorter length than the first threaded portion. Further, a reduced root diameter may facilitate easier insertion of the second threaded portion into a non-drilled portion of the bone. In some embodiments, the root diameter of the second threaded portion may be substantially equal to the diameter of the receivable portion of the second shaft. In other embodiments, the root diameter of the second threaded portion may be larger than the diameter of the receivable portion of the second shaft.

[0025] In some embodiments, the second threaded portion may extend fully along the non-receivable portion. In other embodiments, the second threaded portion may extend partially along the non-receivable portion. The pitch and length of the thread of the second threaded portion may be configured to provide a predetermined number of turns of the thread on the second threaded portion. In some embodiments, the second threaded portion may be configured to have between about 3 to about 5 turns. For example the second threaded portion may be configured to have at least 2 turns, at least 3 turns, at least 4 turns or at least 5 turns of the thread.

[0026] Further, the length of the second threaded portion may be configured such that, when the screw is fully implanted, the distal end of the second shaft is positioned in trabecular bone short of the subchondral bone plate. For example, the distal end of the second shaft may be positioned about 2mm to 3mm away from the cartilage of the femoral head, so as not to risk penetration into the hip joint.

[0027] In some embodiments, the major diameter (i.e. the outer diameter) of the thread of the first threaded portion may be greater than an external diameter of an unthreaded portion of the first shaft. The root diameter (minor diameter) of the thread of the first threaded portion may be substantially equal to the external diameter of the first shaft at the unthreaded portion. [0028] The assembled screw may be adapted for insertion through a canal drilled into the bone. The canal may be configured to have a diameter substantially equal to the minor diameter of the first threaded portion. In some embodiments, the screw is adapted for insertion through a canal drilled into a femur to the level of the growth plate. In such embodiments, the second threaded portion may be adapted for insertion into an undrilled portion of bone. In some embodiments, the second elongate member comprises a tapping feature at its distal end for tapping through the undrilled bone. In such embodiments, the second threaded portion may be fixed in the femoral head via an interference fit.

[0029] The screw may be cannulated along its length to allow for insertion of the screw over a guidewire or a guide pin. For example, the second elongate member may comprise a longitudinal bore adapted for sliding over a guidewire or guide pin.

[0030] The first and second elongate members may further comprise respective first and second mating features. In some embodiments, the mating features are configured for rotational coupling of the first and second elongate members. The mating surfaces may be configured to abut, interlock, key, mesh or otherwise engage with one another to rotationally couple the first and second elongate components.

[0031] In some embodiments, the mating features transmit torque between the first and second elongate members when the elongate members are locked longitudinally relative to one another, such as when the drive features are connected to the drive device.

[0032] In some embodiments, the first mating feature may be located at the distal end of the first shaft (i.e., at the distal end of the longitudinal channel). The second mating feature may be located part way along the second shaft. For example, the second mating features may be located at the distal end of the receivable portion of the second shaft, such that, when the receivable portion is fully inserted into the longitudinal channel, the first and second mating features are brought into mating engagement. [0033] The first and second mating features may contact each other via respective contact surfaces. In some embodiments, the first and second mating features may be freely separable (distractable) from each other when the first and second drive features are not connected to the drive device (or when the first and second elongate members are not otherwise longitudinally locked relative to each other). For example, in some embodiments, when the elongate members are not locked longitudinally relative to each other contact surfaces of the mating features may slide relative to one another upon relative rotation of the elongate members.

[0034] In some embodiments, the contact surfaces of the first and second elongate members may be substantially flat, each lying in a single plane. In some embodiments, the contact surfaces may each lie in a respective plane perpendicular to the longitudinal axes of the first and second elongate members. In such embodiments, the rotational coupling may be achieved primarily by friction between the contact surfaces when the elongate elements are locked longitudinally relative to one another.

[0035] In other embodiments, one or more contact surfaces may lie in one or more planes oblique to the longitudinal axes of the first and second elongate members. The mating of contact surfaces on an oblique plane may provide rotational stability of the first and second elongate components relative to each other when the components are mated and locked longitudinally relative to each other. Additionally, mating of contact surfaces on an oblique plane may allow for free separation of the first and second elongate members (and inhibit seizing). Each oblique contact surface may act as a ramp surface upon relative rotation of the elongate members, such that the contact surfaces slide relative to one another, pushing the elongate members apart.

[0036] The first and second mating features may be mateable to define a mating region. The screw may be adapted for implantation such that the mating region is positioned substantially at or adjacent the growth plate. In some embodiments, the mating region may be substantially non-threaded. When mated, the mating features may define a substantially smooth outer surface. The outer surface may be substantially cylindrical. The absence of screw threads at the mating region may further avoid application of compression forces across the growth plate and thus avoid growth inhibition or premature fusion of the growth plate.

[0037] The outer diameter of the cylindrical mating region may be smaller than the major diameter of the threads of the first and second threaded portions. In some embodiments, the outer diameter of the cylindrical surface may be substantially equal to the minor (root) diameter of the threads of the first and second threaded portions.

[0038] In some embodiments, the first and second mating features comprise respective complementary teeth. Each of the mating features may comprise one or more teeth. For example, each of the mating features may comprise at least 1, at least 2, at least 3, at least 4, at least 5, at least 6 , at least 7, at least 8, at least 9 or more teeth. The teeth may extend parallel to the longitudinal axis of the screw. In other embodiments, the mating features may comprise a serrated surface.

[0039] In some embodiments, the teeth may have a trapezoidal profile. In some embodiments, the teeth may have an isosceles trapezoidal profile. In other embodiments, the teeth may have alternatively shaped profiles, such as rectangular, square, triangular or curved profiles, for example. A trapezoidal profile may provide increased shear strength at the base region of the tooth and allow for uninhibited distraction.

[0040] In alternative embodiments, the first mating feature may be located on the interior of the longitudinal channel while the second mating feature may be located on the exterior of the receivable portion of the second shaft. For example, the first and second mating features may comprise complementary non-circular cross sections of the second shaft and longitudinal channel.

[0041] The first and second elongate members may be connected to the drive device by the first and second drive features, respectively. Connection of both the first and second elongate members to the same drive device may allow both components of the screw to be inserted simultaneously with correct alignment and engagement. Further, locking of the components provides a relatively easy method of implantation, as the screw may be inserted in a manner similar to that for a single cannulated screw. Further, locking of the elongate members relative to each other and the drive device allows the screw assembly to be manipulated by means of the drive device without risking either elongate member separating from the other, or from the drive device. Once connected to the drive device, the screw may be manipulated without the need to touch the screw.

[0042] In some embodiments, one of the drive features may be configured to be axially slidably but not rotationally connectable to the drive device. The other of the drive features may be configured to be rotationally but not axially slidably connectable to the drive device.

[0043] In some embodiments, one of the drive features may comprise a drive head having a non-circular cross-section suitable for transmission of torque. The head may be configured to be received in a correspondingly shaped recess on the drive device. For example, the head may have a substantially hexagonally shaped outer profile adapted for insertion in a correspondingly shaped hexagonal socket on the drive device.

[0044] Alternatively (or additionally) the drive head may comprise a drive recess having a non-circular cross section suitable for transmission of rotational force. The recess may be configured as a socket for receiving a correspondingly shaped head on the drive device. For example, the recess may have a substantially hexagonally shaped inner profile.

[0045] In some embodiments, the other of the drive features may comprise a third threaded portion. The third threaded portion may be an external thread configured to engage an internally threaded feature of the drive device. Alternatively, the third threaded portion may be an internal thread configured to engage an externally threaded portion on the drive device. The thread may be of opposite hand to the threads of the first and second threaded portions (i.e., reverse threaded). In such embodiments, the second drive feature may be connectable to the drive device (or another retrieval device) for removal of the second elongate member from the bone.

In other embodiments, the third threaded portion may be threaded in the same direction as the first and second threaded portions.

[0046] In some embodiments, the first drive feature may comprise a hexagonal drive head at the proximal end of the first elongate member while the second drive feature may comprise a third threaded portion in the proximal end of the second shaft. In such embodiments, a major diameter of the third threaded portion may be substantially equal to (or, alternatively, smaller than) the diameter of the second shaft at the receivable portion such that the second drive feature is slidably receivable in the longitudinal channel. However, the reverse arrangement is also contemplated. That is, the first drive feature may be a third threaded portion in or on the head of the first elongate member, while the second drive feature may be a drive head or drive recess at the proximal end of the second elongate member.

[0047] Related to this, there is provided a drive device configured to engage the first and second drive features for inserting the bone screw into a bone. The drive device may comprise a handle. A first connecting feature may be attached to the handle and adapted for connecting to the first drive feature of a bone screw according to embodiments of the present disclosure. A second connecting feature may be attached to the handle and adapted for connecting to the second drive feature of a bone screw according to embodiments of the present disclosure.

[0048] The first and second connecting features may be directly or indirectly attached to the handle. The first connecting feature may be fixedly attached to the handle. The second connecting feature may be axially slidably and rotatably attached to the handle. The first connector may be configured to be slidably but not rotationally attachable to one drive feature of the screw while the second connector may be configured to be rotationally but not axially slidably connectable to the other drive feature of the screw. [0049] The drive device may comprise a female portion and a male portion. In some embodiments, the female portion may comprise the handle and the first connecting feature. The second connecting feature may be comprised in the male portion. The female portion may be configured to receive the male portion.

[0050] The male and female portions of the drive device may be axially slidable and/or rotatable relative to one another to permit sequential attachment of the drive device to the first and second elongate members. In some embodiments, the degree of relative axial movement possible may correspond to a length of the thread of the third threaded portion of the second elongate member. In other embodiments, the drive device may further comprise a retaining member (for example, a removable clip) for inhibiting relative longitudinal movement between the male and female components of the drive device during insertion of the screw.

[0051] In some embodiments, the first connecting feature is a hexagonal socket, configured to receive a correspondingly shaped first drive member, while the second connecting feature is a threaded portion configured to engage a correspondingly threaded second drive member.

[0052] In some embodiments the drive device is cannulated (for example, to the same bore as the second elongate member) to allow for insertion of the screw using the drive device over a guidewire or a guide pin.

[0053] According to another aspect of the present disclosure, there is provided a method for inserting a bone screw according to embodiments of the present disclosure into a femur, the femur having a femoral head, a growth plate and a metaphyseal portion, the method comprising: fully inserting the second shaft into the longitudinal channel such that the first and second mating features are brought into mating engagement; connecting the first and second drive features to the drive device, such that the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device; driving the screw into the femur through the metaphyseal portion and the growth plate such that the second threaded portion is engaged in the femoral head and the first threaded portion is engaged with the metaphyseal portion of the femur.

[0054] According to another aspect of the present disclosure, there is provided a method for inserting a bone screw into a femur, the femur having a femoral head, a growth plate and a metaphyseal portion, the method comprising: providing a first elongate member comprising: a first shaft extending from a proximal end to a distal end, the first shaft having a first threaded portion; a first drive feature at the proximal end of the first shaft for connecting to a drive device; and a longitudinal channel; providing a second elongate member comprising: a second shaft extending from a proximal end to a distal end and slidably receivable in the longitudinal channel of the first elongate member, the second shaft having a second threaded portion; a second drive feature at the proximal end of the second shaft for connecting to the drive device, and inserting the second shaft into the longitudinal channel; connecting the first and second drive features to a drive device, such that the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device; and driving the screw into the femur through the metaphyseal portion and the growth plate such that the second threaded portion is engaged with the femoral head and the first threaded portion is engaged with the metaphyseal portion of the femur.

[0055] In some embodiments, the screw may be driven through the metaphyseal portion and a diaphyseal portion of the femur such that the first threaded portion is engaged with the metaphyseal portion and the diaphyseal portion of the femur. [0056] The method may further comprise drilling or reaming a canal in the femur. In some embodiments, a first guidewire may be inserted in the femur prior to drilling the canal. The canal may be drilled by over-drilling the first guidewire with a cannulated drill bit. The first guidewire may be a fluted tip guidewire. A tip of the first guidewire may be positioned at or adjacent the growth plate. The tip of the first guidewire may be positioned substantially central to the growth plate. The first guidewire may be positioned under fluoroscopy guidance. In some embodiments, the canal may be drilled without the first guidewire.

[0057] The method may further comprise removing the first guidewire. The method may further comprise inserting a second guidewire through the cannulated drill bit. The second guidewire may be inserted through the growth plate and into the trabecular bone of the femoral head. The second guidewire may be a threaded tip guidewire. The second guidewire may have a diameter smaller than that of the first guidewire. The method may further comprise removing the cannulated drill bit, leaving the second guidewire in situ. Swapping the first guidewire for a second guidewire of smaller diameter may allow the second elongate member to have a smaller longitudinal bore, which may increase the strength of the second elongate member.

[0058] In some embodiments, the method may comprise measuring the protruding length of the second guidewire to determine the inserted length of the second guidewire. For example, a depth gauge may be used to ensure proper selection of the device length. Proper positioning may comprise positioning in the trabecular bone of the femoral head but 2 to 3 mm away from the articular cartilage.

[0059] Connecting the first and second drive features to the drive device may comprise connecting a first connection feature of the drive device to the first drive feature of the first elongate member, and subsequently connecting a second connection feature of the drive device to the second drive feature of the second elongate member. For example, in embodiments where the first drive feature is a hexagonal head and the second drive feature is a third threaded portion, the drive device may be connected by sliding a hexagonal socket of a female component of the drive device onto the hexagonal head of the first elongate member, and subsequently rotating a male component of the drive device to bring a threaded second connection feature into threaded engagement with the third threaded portion of the second elongate member.

[0060] In some embodiments, driving the screw into the femur comprises driving the screw over the second guidewire. Driving the screw may comprise turning the drive device by hand while applying longitudinal force to advance the screw into the bone. In some embodiments, the screw may be configured to be driven in a clockwise direction. In other embodiments, the screw may be configured to be driven in a counter-clockwise direction.

[0061] The screw may be driven into the bone until the distal end of the second elongate member is engaged with the trabecular bone of the femoral head, 2 to 3 mm away from the articular cartilage. An unthreaded portion of the screw may be positioned at or adjacent the growth plate.

[0062] According to another aspect of the present invention, there is provided a surgical kit comprising a bone screw according to embodiments of the present disclosure and a drive device. The kit may be configured for use in a method according to embodiments of the present disclosure.

[0063] According to another aspect of the present disclosure, there is provided a surgical kit comprising at least one bone screw and a drive device, the at least one bone screw including a first elongate member comprising: a first drive feature for connecting to the drive device; a first shaft extending from a proximal end to a distal end, the first shaft having a first threaded portion; and a longitudinal channel; and a second elongate member comprising: a second shaft extending from a proximal end to a distal end and slidably receivable in the longitudinal channel of the first elongate member, the second shaft having a second threaded portion; a second drive feature at the proximal end of the second shaft for connecting to the drive device, and a drive device comprising: a handle; a female member connected to the handle, the female member comprising a first connection feature for connecting to the first drive feature; and a male member connected to the handle, the male member comprising a second connection feature for connecting to the second drive feature, wherein, when the first and second drive features are connected to the first and second connection features, respectively, the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device.

[0064] In some embodiments, the kit may further comprise one or more guidewires.

In some embodiments, the kit comprises at least a first guidewire and a second guidewire. In some embodiments, the first and second guidewires have different diameters.

[0065] The first guidewire may have a fluted tip. The first guidewire may be adapted for insertion into a femur to the level of the growth plate. The first guidewire may be adapted for over drilling with a cannulated drill bit. The first guidewire may have a length longer than that of the cannulated drill bit. In some embodiments, the first guidewire may have a diameter of between about 2.5 mm to 3 mm, for example, about 2.5 mm, about 2.6 mm, about 2.7 mm, about 2.8 mm, about 2.9 mm or about 3.0 mm.

[0066] The second guidewire may have a threaded tip. The second guidewire may be adapted for insertion into the trabecular bone of the femoral epiphysis. The second guidewire may have a diameter smaller than that of the first guidewire. The second guidewire may have a diameter between about 1.4 mm to about 2 mm, for example, about 1.4 mm, about 1.5 mm, about 1.6 mm, about 1.7 mm, about 1.8 mm, about 1.9 mm or about 2.0 mm.

[0067] The kit may further comprise a drill. The kit may further comprise one or more cannulated drill bits. The drill bit diameter may be configured to drill a canal of preselected diameter in the bone. The diameter of the canal may be selected for suitability for the size of the bone screw. For example, the diameter the canal may be selected to be substantially equal to the diameter of an unthreaded portion and/or unthreaded mating region. The canal diameter may be between about 4.8 mm to about 5.6 mm. For example, the canal diameter may be about 4.8 mm, about 4.9 mm, about 5.0 mm, about 5.1 mm, about 5.2 mm, about 5.3 mm, about 5.4 mm, about 5.5 mm, about 5.6 mm or more. The drill bit may be shorter in length than the first and second guidewires.

[0068] In some embodiments, the kit may include a measurement tool for determining the inserted length of one or more of the guidewires. For example, the kit may include a depth gauge.

[0069] Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.

Brief Description of Drawings

[0070] By way of example only, embodiments are now described with reference to the accompanying drawings, in which:

[0071] Figure 1 is a front view of a bone screw according to one embodiment of the present disclosure;

[0072] Figure IB is a cross section of the screw of Figure 1, taken along axis A; [0073] Figure 2 is a front view of the first elongate member of the bone screw of Figure 1;

[0074] Figure 3 is a front view of the second elongate member of the bone screw of Figure 1;

[0075] Figure 4 is a perspective view of a bone screw according to another embodiment of the present disclosure;

[0076] Figure 5 is a front view of a bone screw according to another embodiment of the present disclosure;

[0077] Figure 6 is a perspective view of a bone screw according to another embodiment of the present disclosure;

[0078] Figure 7 is a partial perspective view of a bone screw according to another embodiment of the present disclosure;

[0079] Figure 8 is a perspective view of a bone screw according to another embodiment of the present disclosure;

[0080] Figure 9 is a perspective view of the bone screw of Figure 8 shown in a partially extended state;

[0081] Figure 10 is a perspective view of a bone screw according to another embodiment of the present disclosure;

[0082] Figure 11 is a perspective view of a bone screw according to another embodiment of the present disclosure;

[0083] Figure 12 is a perspective view of a drive device according to one embodiment of the present disclosure; [0084] Figure 13 is a perspective view of the female component of the drive device of Figure 12;

[0085] Figure 14 is a perspective view of the male component of the drive device of Figure 12; and

[0086] Figure 15 is a partial perspective view of the drive device of Figure 12 aligned for connection with the bone screw of Figure 1 ;

[0087] Figure 16 is a front view of a drive device according to another embodiment of the present disclosure connected to the bone screw of Figure 10;

[0088] Figures 17A to 171 show steps in a method of implanting a bone screw according to embodiments of the present disclosure in a femur.

[0089] Figure 18 shows components of a surgical kit for use in the method of Figure 17A to 171; and

[0090] Figure 19 shows extension of the bone screw of Figure 1 during growth of a femur.

Description of Embodiments

[0091] A bone screw of the present disclosure is generally shown as 100 in drawings. The bone screw 100 may be adapted for securing portions of a femur for treatment of SCFE, as discussed further below.

[0092] Figure 1A and IB show one embodiment of a bone screw 100 adapted to be implanted into a femur to span the growth plate. The bone screw 100 includes a first elongate member 200 configured to receive a second elongate member 300. The first and second elongate members 100, 300 are freely slidable relative to each other along longitudinal axis A, allowing the screw to increase in length to accommodate natural growth of the bone at the growth plate. The screw may ensure proper alignment of bone components as the SCFE heals without applying compressive forces which might inhibit normal growth.

[0093] Figures 2 and 3 show the first elongate member 200 and the second elongate member 300 of the bone screw 100 in a disassembled state.

[0094] Referring to Figure 2, The first elongate member 200 includes a drive head 210 having a hexagonal outer profile. A first shaft 220, extends from a proximal end 221 connected to the drive head 210 to a distal end 222, and includes a first threaded portion 223. A cylindrical longitudinal channel 230 extends as a central cannula through the length of the first elongate member 200. A mating feature 240 is provided at the distal end 222.

[0095] The second elongate member 300 comprises a cylindrical second shaft 320, extending from a proximal end 321 to a distal end 322, and having a second threaded portion 323. The second elongate member 300 further includes a second drive feature in the form of a third threaded portion 310 at the proximal end 321 of the second shaft 320 for connection to a drive device. A second mating feature 340, configured for mating engagement with the first mating feature 240 is provided at the proximal end of the threaded portion 323. The second elongate member 300 further comprises a longitudinal bore 300 (visible in Figure IB) for sliding over a guidewire.

[0096] The first and second elongate members 200, 300 may be assembled by sliding the second shaft 320 into the longitudinal channel 230 such that the first shaft 220 sheaths the second shaft 320. The longitudinal channel 230 is centrally located in the first elongate member 200 such that, when assembled, the first and second elongate members 200, 300 share a common longitudinal axis A as shown in Figure 1. The first and second elongate members 200, 300 may slide relative to each other, along or about the axis A, allowing for telescopic extension of the screw 100. Further, in this embodiment, the second shaft 320 and longitudinal channel 230 are each cylindrical, allowing the first and second elongate members 200, 300 to rotate relative to one another about axis A when assembled. [0097] As seen in Figure 3, the second shaft 300 is only partially receivable in the longitudinal channel, the second shaft 300 comprising a receivable portion 324 and a non-receivable portion 325 accordingly. The non-receivable portion 325 has an outer diameter larger than that of the longitudinal channel 230 and therefore cannot enter the longitudinal channel 230. The second threaded portion 323 is positioned on the non-receivable portion 325. As such, when the elongate members 200, 300 are assembled, the second threaded portion 323 is exposed. The receivable portion 324 has a length longer than that of the first elongate member 200 such that, when fully assembled, the receivable portion 320 extends all the way through the longitudinal channel 230. In this state, the proximal end 321 protrudes beyond a proximal opening of the longitudinal channel 230 in the head 210, as can be seen in Figure 1A and IB. In other embodiments, the proximal end 321 may protrude out of the head 210 to a lesser extent than illustrated in Figure 1, or may remain sheathed by the head 210.

[0098] In the embodiment of Figures 1 to 3, the first mating feature 240 is located at the distal end of the first shaft 220 and the second mating feature 340 is located at a distal end of the receivable portion 324 of the second shaft 320. As such, when the receivable portion 324 of the second shaft 320 is fully inserted into the longitudinal channel 230, the first and second mating features 240 and 340 are brought into mating engagement, as shown in Figures 1A and IB. While in Figure 1A a small separation is depicted between the mating features 240 and 340, this is simply to illustrate the individual features more clearly. Typically, when fully assembled, the two mating features 240, 340 are flush with one another at respective contact surfaces, for example, surfaces 241 and 341.

[0099] When the first and second elongate members 200, 300 are longitudinally locked (such as when connected to a drive device) the mating features rotationally couple the first and second elongate members 200, 300 to transmit drive force there between. However, when the first and second elongate members 200, 300 are not longitudinally locked, the mating features 240, 340 are freely separable from each other and do not inhibit longitudinal movement of the first and second elongate members 200, 300 relative to each other. As such, the first and second elongate members 200, 300 may be rotationally locked relative to each other when attached to a drive device for insertion, but not rotationally locked once implanted.

[0100] In the embodiment of Figures 1 to 3, the mating features 240, 340 comprise respective complementary interlocking teeth which mate to define a mating region 150. The teeth are free of threads, and mate to create a substantially smooth, cylindrical outer surface at the mating region.

[0101] The teeth in Figures 1 to 3 have an isosceles trapezoidal profile. A trapezoidal shape may substantially inhibit seizing of the mating features 240, 340 as relative rotation of the first and second elongate members 200, 300 causes oblique contact surfaces (for example 241, 341) of the trapezoidal teeth to slide relative to one another, pushing the elongate members 200, 300 axially apart. Further, increased width at the base of each tooth may strengthen the mating features 240, 340 enabling them to withstand greater shear stress when used to transmit driving forces. However, in other embodiments, the mating features may comprise teeth having alternatively shaped profiles, such as rectangular, square, triangular or curved profiles. An embodiment showing mating features 240a, 340a having substantially square teeth is shown in Figure 4, for example.

[0102] Further, while the embodiments of Figure 1 shows mating features 240, 340 each having three teeth other suitable numbers of teeth may be used. For example, Figure 5 shows an alternative embodiment in which mating features 240b, 340b each comprise only two trapezoidal teeth. In other embodiments, the mating features may each comprise only one tooth, or four or more teeth. Further, although the illustrated embodiments show teeth which are substantially uniform, other embodiments may have teeth that differ from each other in size and/or shape. In other embodiments, the mating features may comprise respective serrated contact surfaces.

[0103] Figure 6 shows an alternative embodiment, in which the first mating feature includes a projection (not visible) on the internal surface of the longitudinal channel 230c adapted to engage a second mating feature in the form of a groove 340c on the second shaft 320c. Other embodiments, may similarly have complementary non circular cross sections for rotationally coupling the first and second elongate members 200c, 300c. In some such embodiments, when assembled, the first and second elongate members 200c, 300c may be rotationally coupled regardless of whether they are also longitudinally locked.

[0104] In still further embodiments, the shaft and longitudinal channel may be substantially cylindrical and the mating features comprise opposing flat contact surfaces, such as surfaces 241c and 341c in Figure 6. These surfaces may be perpendicular to the longitudinal axis of the screw. In such embodiments, rotational coupling of the elongate members 200c, 300c may be achieved by friction between the contact surfaces 241c and 341c when the elongate elements 200c, 300c are locked longitudinally relative to one another. By contrast, in Figure 7, the opposing flat contact surfaces 24 Id, 34 Id each lie in a plane oblique to the longitudinal axes of the screw lOOd. Contact surfaces in an oblique plane may provide for increased rotational stability compared to a perpendicular plane.

[0105] Returning to Figures 1 to 3, the elongate members 200, 300 are each provided with respective threaded portions 223 and 323 for attaching to surrounding bone.

[0106] The first threaded portion 223 extends from the proximal end 221 of the first shaft 220, adjacent the head 210, towards the distal end 222. In the illustrated embodiments, the first threaded portion 223 extends along a significant length of the first shaft, for example, approximately 70-90% of the nominal length of the first shaft 220 (that is, exclusive of the head), leaving an unthreaded portion 224 at the distal end 222. However, in other embodiments, the first shaft 220 may be threaded along a smaller, or greater, percentage of its length.

[0107] Referring to Figure 3, the second threaded portion 323 extends from the distal end 322 of the second shaft 300, substantially along the length of the non-receivable portion 325. The second threaded portion may extend fully along the length of the non-receivable portion 325, (with the exception of the mating features which are non- threaded). However, in other embodiments, the non-receivable portion 325 may also include a non- threaded portion (for example, as shown in Figure 6). The second threaded portion 323 may have a self-tapping feature 326 at its distal end, such that the second threaded portion 323 is adapted for insertion into an undrilled portion of the bone. When assembled, the second threaded portion 323 is distal to the first threaded portion 223, As can be seen in Figure 1 A.

[0108] In the embodiment of Figures 1 to 3, the first and second threaded portions 223, 323 have substantially identical major diameter, root diameter and pitch.

Further, the root diameter of the threaded portions is substantially equal to the outer diameters of the unthreaded portion 224 of the first elongate member 200 and the smooth cylindrical outer surface of the mating features 240, 340 in the mating region 150. The smaller diameter of the unthreaded portions 224 and mating region 150 relative to the major diameter of the screw threads may allow for drilling of a correspondingly sized canal in the bone, allowing the threads to more securely ‘bite’ into the surrounding bone when inserted.

[0109] In other embodiments, the second threaded portion 323 may have a reduced root (minor) diameter relative to the root diameter of the first threaded portion 223, such that the thread of the second threaded portion 323 is deeper than that of the first threaded portion 223. Examples of this are shown Figure 8, Figure 9 and Figure 11.

[0110] The pitch of the threads of the first and second threaded portions 223, 323 may be configured to ensure a minimum number of turns on the second threaded portion 323, which is typically shorter in length than the first threaded portion 223. For example, comparing Figure 10 and Figure 11, it can be seen that the threads of the first and second threaded portions 223, 323 of the screw 100 of Figure 11 have a reduced pitch compared to those of Figure 10. As a result, the second threaded portion 323 in Figure 11 has approximately 4.5 turns of the thread, whereas the second threaded portion 323 in Figure 10 has only about 3.5 turns of the thread. [0111] Each of the elongate members is provided with a respective drive feature for connecting to a drive device. In the embodiment of Figure 1, the first drive feature is in the form of the hexagonal head 210 of the first elongate member 200 which is axially slidably but not rotationally connectable to the drive device. The second drive feature is in the form of the third threaded portion 310 on the proximal end 321 of the second elongate member 300. The third threaded portion can be rotationally (but not axially slidably) connected to the drive device by threading into a complementary threaded recess on the drive device.

[0112] In some embodiments, the third threaded portion 310 may be reverse-threaded (that is, of opposite hand) relative to the first and second threaded portions 223, 323 to facilitate retrieval of the second elongate member 300. This can be seen, for example, in the embodiments of Figures 1, 3 and 8. However, in other embodiments, for example, as shown in Figures 5 and 10, the thread of the third threaded portion 310 is of the same direction as the threads of the first and second threaded portions 223, 323.

[0113] Figures 8 and 9 show a perspective view of a screw 100 with a hexagonal head 210 and third threaded portion 310 similar to those of Figure 1. In this embodiment, the head 210 has a hexagonal outer profile and a hexagonal inner profile, which may be engaged on the outer surface, inner surface, or both inner and outer surfaces by a correspondingly shaped connecting feature of a suitable drive device.

[0114] In other embodiments, for example as shown in Figures 10 and 11, the outer surface of the head 210 may be rounded, such that only the inner surface has a hexagonal profile. Although in Figure 10, the third threaded portion 310 is threaded in the same direction as the first and second threaded portions 223, 323, other embodiments may employ a rounded head 210 in combination with a reverse third threaded portion 310.

[0115] The drive features 210, 310 are adapted for connection to a corresponding drive device 500. One embodiment of a drive device 500 is shown in Figure 12. The drive device comprises a female component 510 and a male component 520, which are shown separately in Figures 13 and 14, respectively.

[0116] As shown in Figure 13, the female component 510 includes a handle 511 and first connecting feature 515 fixedly connected to the handle and adapted for connection to the drive head 210. The female component 510 may include a central bore 516 for receiving the male component 520.

[0117] The first connecting feature 515 may be a hexagonally shaped recess in a distal end of the female component 510 configured to slidingly receive the hexagonal drive head 210 to rotationally couple the first elongate member 200 to the drive device 500.

[0118] Figure 14 shows the male component 520 of the drive device 500. The male component 520 may include an elongate rod 521, slidably receivable in the central bore 516 of the female component 510 to assemble the drive device 500, and a second connecting feature 525 adapted for connection to the third threaded portion 310. The second connecting feature 525 may be an internally threaded portion of the male component 520 configured for threaded engagement with the third threaded portion 310 to longitudinally secure the second elongate member 300 to the drive device 500. The male component may further include a knob 527 for rotating the male component to thread the second connecting feature onto the third threaded portion 310.

[0119] When assembled, the male and female components 510, 520 of the drive device may be axially slidable and rotationally movable relative to one another to permit sequential attachment of the first and second connection features 515, 525 to the head 210 and third threaded portion 310, respectively.

[0120] The drive device 500 may include a removable clip 528 to fill an axial gap between the knob 527 of the male component and a proximal end of the female component 520 when the second connecting feature 525 is fully threaded onto the third threaded portion 310 of the second elongate member 300. The clip 528 may be configured to prevent relative movement between the male and female components 510, 520.

[0121] Figure 15 shows a distal portion of the drive device 500 of Figure 12, with the handle not visible to more clearly show the connection of the female and male components 510, 520. To connect the assembled screw 100 and drive device 500 of Figure 15, the hexagonal head 210 of the first elongate member 200 is slidingly inserted into the first connecting feature 515 of the drive device 500. The male component 520 of the drive device 500 is then axially rotated (in this case in a clockwise direction) to thread the second connecting feature 525 onto the third threaded portion 310 of the second elongate member 300. When connected, the female component 510 is rotationally locked to the first elongate member. Accordingly, when the head 210 and third threaded portion 310 are connected to the drive device 500, the first and second elongate members 200, 300 are longitudinally and rotationally locked relative to each other and the drive device 500. This allows both components of the screw 100 to be inserted simultaneously and with correct alignment and engagement.

[0122] An alternative embodiment of a drive device 500 is shown in Figure 16. In this embodiment, the female member 510 comprises a first connecting feature 515 in the form of a hexagonal head, adapted to be slidingly received by a hexagonal recess in the drive head 210 of the first elongate member 200. Further, in this embodiment, the length of the male component is configured such that the knob 527 abuts the proximal end of the female member 510 when the connecting feature 525 is threaded onto the third threaded portion 310, negating the need for a clip.

[0123] During insertion, the drive device 500 rotates the screw 100 to drive it into bone. Rotation of the drive device 500 transmits torque to the first elongate member 200 via the drive head 210. The torque is transmitted from the first elongate member 200 to the second elongate member 300 via the mated mating features 240, 340. [0124] Figures 17A to 171 illustrate steps in one embodiment of a method for implanting a bone screw according to embodiments of the present disclosure in a femur (for example, for treatment of SCFE).

[0125] Referring first to Figure 17A, a 2.8 mm stiff fluted tip guide-wire 610 is inserted into the femur 400. The fluted tip guide-wire 610 is inserted under fluoroscopy guidance from near the anterior intertrochanteric line of the femur 400, through the centre of the femoral neck to the level of the growth plate 420 such that a distal tip of the fluted tip guide-wire 610 is positioned substantially centrally to the growth plate.

[0126] The fluted tip guidewire 610 is then over-drilled with a 5.5 mm cannulated drill bit 620, as shown in Figure 17B. The drill diameter of 5.5 mm is selected in this particular embodiment to suit a screw having a root diameter of 5.5 mm at the first threaded portion 223 and an equivalent diameter at the mating region 150. In other embodiments, other drill diameters may be selected in accordance with the screw to be inserted.

[0127] The fluted tip guidewire 610 is then withdrawn, leaving the cannulated drill bit 620 in situ , as shown in Fig 17C. A 1.6 mm threaded-tip guidewire 630 is then inserted through the cannulated drill bit 620 and into the femoral head 430 as shown in Figure 17D. Swapping of the guidewires in this manner allows for a thicker, stronger guidewire to be used for the drilling step and a thinner guidewire to be used for the step of inserting the screw. This in turn allows the longitudinal bore 330 of the second elongate member 300 to have a smaller diameter.

[0128] The cannulated drill bit 620 is then withdrawn, leaving the threaded-tip guidewire 630 in situ as shown in Figure 17E. A depth gauge 640 may be used to determine the inserted length of the threaded tip guidewire 630, as shown in Figure 17F. This determines the correct selection of screw length. [0129] The second elongate member 300 is inserted into the first elongate member 200 to assemble the bone screw 100. The screw 100 is connected to the drive device 500 to longitudinally and rotationally lock the elongate members 200, 300 relative to each other and the drive device 500. Figure 17G shows the screw 100 partially attached to the drive device and not yet longitudinally locked. Once fully attached and longitudinally locked to the drive device 500, the screw 100 can be manipulated without touching the screw. Further, the screw can be positioned at any angle without the elongate elements 200, 300 separating from each other or from the drive device 500.

[0130] The bone screw 100 is then driven into the femur 400, through the drilled canal, over the threaded-tip guidewire 630. Insertion of the screw is via a continuous rotational driving force applied by hand to the drive device 500, in a similar manner to insertion of a conventional single cannulated screw. The torque applied to the hexagonal head 210 drives the first elongate member 200, which in turn drives the second elongate member 300. Longitudinal and rotational stability is achieved by the connection of the first and second elongate members to the drive device 500 and the mating of the mating features 240, 340.

[0131] The screw follows the path of the pre-drilled canal until it reaches the growth plate 420. After this point, the self-tapping feature 326 taps a hole for the second threaded portion 323. As such, the second threaded portion 323 engages the bone by an interference fit.

[0132] The screw reaches its final position once the first threaded portion 223 is secured in the femur, the second threaded portion 323 is secured in the trabecular bone of the femoral head short of the subchondral bone plate and approximately 2 to 3mm away from the femoral head articular cartilage. The smooth, unthreaded mating region 150 is positioned at the growth plate 420, as shown in Figure 17H. [0133] The drive device 500 may then be disconnected from the screw 100, leaving the screw 100 in situ as shown in Figure 171. The hexagonal head 210 and third threaded portion 310 remain proud of the bone.

[0134] A kit for performing the embodiment of the method described above is shown in Figure 18. The kit includes the fluted-tip guidewire 610, the cannulated drill bit 620, the threaded tip guidewire 630, the depth gauge 640, the drive device 500 and the screw 100.

[0135] Figure 19 shows the extension of an implanted bone screw 100 as the femur 400 grows. The first threaded portion 223 is attached to at least a metaphyseal portion of the femur 410, while the second threaded portion is inserted through the growth plate 420 and attached to the femoral head 430. The unthreaded mating region 150 is positioned in the region of the growth plate 420 such that growth in this region is substantially not inhibited. As shown in panels B and C of Figure 19, as the femoral neck grows at the growth plate 420, the elongate members 200, 300 are pulled apart, increasing the length of the screw 100 in accordance with the growth of the bone.

The screw may continue to extend in length until the femur 400 ceases its growth.

[0136] The screw 100 may remain in situ indefinitely. Alternatively, the screw may be removed after a predetermined period of time, or as required. Should the screw require removal, a reverse thread on third threaded portion 310 may facilitate easier retrieval of the second elongate member 300. That is, once the first elongate member 200 has been removed, the male member 520 of the drive device (or, alternatively, a dedicated removal device) may be rotated to thread the second connecting feature 525 onto the third threaded portion 310. Once the limit of travel on the thread is reached, the male member 520 and second elongate member 300 are unidirectionally rotationally coupled such that further rotation in the same direction causes the second threaded portion 323 to unscrew from the bone.

[0137] It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the above-described embodiments, without departing from the broad general scope of the present disclosure. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS:

1. A bone screw including: a first elongate member comprising: a first shaft extending from a proximal end to a distal end, the first shaft having a first threaded portion; a first drive feature at a proximal end of the first shaft for connecting to a drive device; and a longitudinal channel; and a second elongate member comprising: a second shaft extending from a proximal end to a distal end and slidably receivable in the longitudinal channel of the first elongate member, the second shaft having a second threaded portion; a second drive feature at the proximal end of the second shaft for connecting to the drive device, wherein, when the first and second drive features are connected to the drive device, the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device.

2. The bone screw of claim 1, wherein, when the second shaft is received in the longitudinal channel to assemble the screw, the second threaded portion is distal to the first threaded portion.

3. The bone screw of claim 1 or claim 2, wherein one of the drive features is configured to be axially slidably but not rotatably connectable to the drive device and wherein the other drive feature is configured to be rotatably but not axially slidably connectable to the drive device.

4. The bone screw of any one of the preceding claims, wherein the first drive feature comprises a drive head at the proximal end of the first elongate member, the drive head having a non-circular cross section.

5. The bone screw of claim 4, wherein the drive head has a substantially hexagonally shaped outer profile.

6. The bone screw of claim 4 or claim 5, wherein the drive head comprises a drive recess having a non-circular cross section.

7. The bone screw of claim 6, wherein the drive recess has a substantially hexagonally shaped inner profile.

8. The bone screw of any one of the preceding claims, wherein the second drive feature comprises a third threaded portion.

9. The bone screw of claim 8, wherein the third threaded portion is of opposite hand to the first and second threaded portions.

10. The bone screw of any one of the preceding claims, wherein the first elongate member comprises a first mating feature and the second elongate element comprises a second mating feature configured for mating engagement with the first mating feature.

11. The bone screw of claim 10, wherein the mating features are configured for rotational coupling of the first and second elongate members.

12. The bone screw of claim 10 or claim 11, wherein the mating features only provide rotational coupling of the first and second elongate members when the first and second elongate members are longitudinally locked relative to each other.

13. The bone screw of any one of claims 10 to 12, wherein the first and second mating features are freely separable from each other when the first and second elongate members are not locked longitudinally relative to each other.

14. The bone screw of any one of claims 10 to 13, wherein the mating features are configured to transmit torque from the drive device between the first and second elongate members when the first and second elongate members are locked longitudinally relative to each other.

15. The bone screw of any one of claims 10 to 14, wherein the first and second mating features comprise respective complementary interlocking teeth.

16. The bone screw of claim 15, wherein the teeth each have a substantially trapezoidal profile.

17. The bone screw of any one of claims 10 to 16, wherein the mating features mate to define a mating region which is substantially non-threaded.

18. The bone screw of any one of the preceding claims, wherein the mating features mate to define a substantially cylindrical mating region.

19. The bone screw of claim 18, wherein an outer diameter of the mating region is substantially equal to a root diameter of the thread of the first threaded portion.

20. The bone screw of any one of the preceding claims wherein threads of the first and second threaded portions are substantially identical in major diameter and pitch.

21. The bone screw of any one of the preceding claims, wherein a root diameter of the second threaded portion is smaller than a root diameter of the first threaded portion.

22. The bone screw of any one of the preceding claims, wherein the second elongate member comprises a longitudinal bore adapted for insertion over a guidewire.

23. The bone screw of any one of the preceding claims, wherein the longitudinal channel and the second shaft each have a substantially cylindrical shape such that the second shaft is rotatably and axially slidably receivable in the longitudinal channel.

24. A method for inserting the bone screw of any one of the preceding claims into a femur, the femur having a metaphyseal portion, a femoral head and a growth plate, the method comprising: inserting the second shaft into the longitudinal channel to assemble the screw; connecting the first and second drive features to the drive device, such that the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device; driving the screw into the femur through the metaphyseal portion until the second threaded portion is engaged with the femoral head and the first threaded portion is engaged with the metaphyseal portion of the femur.

25. A method for inserting a bone screw into a femur, the femur having a femoral head, a growth plate and metaphyseal portion, the method comprising: providing a first elongate member comprising: a first shaft extending from a proximal end to a distal end, the first shaft having a first threaded portion; a first drive feature at the proximal end of the first shaft for connecting to a drive device; and a longitudinal channel; providing a second elongate member comprising: a second shaft extending from a proximal end to a distal end and slidably receivable in the longitudinal channel of the first elongate member, the second shaft having a second threaded portion; a second drive feature at the proximal end of the second shaft for connecting to the drive device, and inserting the second shaft into the longitudinal channel; connecting the first and second drive features to a drive device, such that the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device; and driving the screw into the femur through the metaphyseal portion and the growth plate such that the second threaded portion is engaged with the femoral head and the first threaded portion is engaged with the metaphyseal portion of the femur.

26. The method of claim 24 or claim 25 wherein the screw is driven through the metaphyseal portion and a diaphyseal of the femur until the first threaded portion is engaged with the metaphyseal portion and the diaphyseal portion of the femur.

27. The method of any one of claims 24 to 26, further comprising drilling a canal in the femur prior to inserting the screw.

28. The method of claim 27 wherein the canal is drilled to the level of the growth plate.

29. The method of claim 28, wherein the canal is drilled using a cannulated drill bit over a first guidewire inserted to the level of the growth plate.

30. The method of claim 29, comprising removing the first guidewire after drilling the canal and inserting a second guidewire through the cannulated drill bit and into the femoral head.

31. The method of claim 30, wherein the second guidewire has a diameter smaller than the first guidewire and wherein the screw is driven into the femur over the second guidewire.

32. A surgical kit, comprising the bone screw of any one of claims 1 to 23 and a drive device.

33. A bone screw comprising: a first elongate member, and a second elongate member longitudinally slidably connectable to the first elongate member, wherein the first and second elongate members are each connectable to a drive device and wherein, when the first and second elongate members are connected to the drive device, the first and second elongate members are longitudinally and rotationally locked relative to each other and the drive device.