WO2019081909A1

WO2019081909A1 - Device

Info

Publication number: WO2019081909A1
Application number: PCT/GB2018/053055
Authority: WO
Inventors: David Dunaway; Will Rodgers; Alessandro BORGHI; Silvia Schievano; Owase JEELANI
Original assignee: Ucl Business Plc
Priority date: 2017-10-25
Filing date: 2018-10-23
Publication date: 2019-05-02
Also published as: GB201717548D0; EP3700448A1

Abstract

A fully implantable distractor (10) comprises a fixable portion (12), a movable portion (14) and a sing le distractor arm (16) which defines a movement path of the movable portion (14) relative to the fixable portion (12). The distractor (10) further comprises a resilient member provided between the fixable portion (12) and the movable portion (14) to, in use, move the movable portion (14) relative to the fixable portion (12) along the movement path defined by the distractor arm (16). The resilient member is formed from a shape memory material.

Description

Device

The present invention relates to a device. In particular, though not exclusively, it concerns a medical device comprising a shape memory material for the modulation of biological tissue and/or bone conformation.

The need to lengthen and remodel the shapes of tissues and/or bones poses many difficult challenges particularly, though not exclusively, in the areas of orthopaedic and cranio-maxillofacial reconstruction.

Starting in the 1970s, Ilizarov (The principles of the Ilizarov method . Bull. Hosp. Jt. Dis. Orthop. Inst. 48 : 1, 1988) described a process of distraction osteogenesis (DO) to lengthen the long bones in limbs. The process involved weakening the bone to be lengthened by performing corticotomies (cuts in the outer layer of the bone) . An external adjustable framework was then used to distract the bone across the corticotomy. Bone lengthening occurs as callus is formed in response to the microfractures that occur in response to the distracting force.

Subsequently, McCarthy (Lengthening the human mandible by gradual distraction. Plast. Reconstr. Surg . 103 : 1592, 1999) introduced more sophisticated devices that could lengthen bones in the craniofacial skeleton and since then ever more complex distractors have been developed . An instance where mandibular distraction is required is in the treatment of Craniofacial M icrosomia (CFM). CFM is a rare disease affecting ~1 in 5,600 live births and is the second most common congenital disorder of the face after cleft lip and palate. The aetiology remains debated though is likely related to a disruption of 1st and 2nd branchial arch structures during the first 6 weeks of gestation, as demonstrated by artificially induced CFM phenotypes produced in the laboratory by reducing flow in the stapedial artery. The structures affected include the ears; facial soft tissue; facial nerve and mandible, though increasing ly extracraniofacial anomalies are described, diagnosis is based on clinical findings. Deformity of the mand ible in CFM is complex and typically includes asymmetry of ramus height and length of hemimandible with a degree of rotational deformity, often involving the temporomand ibular joint (TMJ), glenoid fossa and cranial base. It will be understood that there exist instances other than CFM where mandibular distraction, or indeed other forms of osseous distraction, may be deemed necessary for a patient.

Current concepts in mandibular DO leave scope for considerable further improvement. Today a typical protocol for DO in the mandible includes a latent period of 5 days; distraction at lmm/day - with 0.5mm in the morning and 0.5mm in the evening - typically performed by the patient or parent; a 3 month period of consolidation followed by removal of the distraction device. The distractor device consists of proximal and distal attachments with a variable length span connected to an actuator which protrudes through the tissue to allow for daily winding .

Current mandibular distractors are bulky, and there may exist wound problems relating to the external siting of portions of the distractor. Current distractors have a limited geometry of possible distraction and the final result achieved can be unpredictable. Such distractors typically function in a d iscontinuous rather than a continuous manner and require daily intervention, such as winding, by the patient or a medical practitioner. The aforementioned issues with known mandibular distractors are discussed in further detail below.

Bulky design : Although there has been incremental improvement to device desig n since conception with an evolution from external devices, as used in limb DO, to internal semi-buried devices they remain bulky. This is due to the need for multiple moving parts and an actuator which must penetrate the skin to be externally manipulated . There is therefore a risk of infection, which may jeopardise the outcome of distraction. Limited geometry: Current devices are restricted by a limited geometry of possible d istraction, those devices most commonly used distract only in a single plane and cannot accommodate rotational deformity or complex multiplanar 3 dimensional distraction. Although work on 3 dimensional distractors is in development, devices which permit movement in multiple planes remain in the prototype stage.

Variability of distraction : In spite of pre-surgical planning the outcome of the distraction is heavily reliant on the surgeon's eye, a judgement which is frequently hampered by soft tissue swelling in addition to the introduction of human bias.

Discontinuous distraction : Currently used devices distract the callus discontinuously, usually twice per day, in spite of the fact that the advantages of continuous distraction are well described including : lower distraction forces; potential for faster rates of distraction; improved quality of bone regeneration.

Patient compliance: Devices used today require the patient or parent to actuate the distractor by means of daily winding . This can be distressing and painful for both and introduces the possibility of erroneous distraction.

CN 201094659 discloses a transport distractor device where two fixed guides are provided . The device requires an exogenous heat source in order to effect distraction movement. WO 98/25538 discloses an intra-oral bone distraction operable to distract in a straight line due to the provision of parallel guide rails. The present invention seeks to provide a device and method that can achieve multi-planar continuous distraction osteogenesis without the need for patient involvement. The present invention further seeks to provide a device that can be internally sited and thus reduce the potential for wound problems.

According to a first aspect of the present invention there is provided a fully implantable distractor comprising a fixable portion, a movable portion, a sing le distractor arm which defines a movement path of the movable portion relative to the fixable portion, and a resilient member is provided between the fixable portion and the movable portion to, in use, move the movable portion relative to the fixable portion along the movement path defined by the distractor arm, wherein the resilient member is formed from a shape memory material. The present invention provides a distractor that is fully implantable within a human or animal body and thus is, in use, internally sited . In use, the necessary force for osseous distraction is provided by the shape memory resilient member. The present invention thus eliminates the need for patient involvement.

The shape memory material forming the resilient member is provided initially in its deformed or temporary shape and reverts to or towards its original or permanent shape in order to move the movable portion along the movement path. The shape memory material may expand from its deformed or temporary shape to or towards its original or permanent shape in order to move the movable portion along the movement path. The shape memory material may revert to or towards its original or permanent shape due to an increase in temperature of the material. More specifically, the shape memory material may revert to or towards its original or permanent shape when heated to the temperature of the body within which it has been implanted . The shape memory material thus remains in its deformed or temporary shape at temperatures below normal body temperature i.e. approximately 36.5 degrees C to 37.5 degrees C.

The distractor arm may define a movement path in at least 2 dimensions. More specifically, the distractor arm may define a 3 dimensional movement path.

The required movement path, and hence the shape, curvature, deflection and such like of the d istractor arm from straight may be determined virtually, for example with surgical modelling software. The shape memory material may be a shape memory alloy and/or a shape memory polymer. The shape memory material may comprise a shape memory alloy. The shape memory alloy may be an alloy of nickel and titanium. More specifically, the shape memory alloy may be nitinol.

The resilient member may be configured to move the movable portion along the movement path defined by the distractor arm from an initial position proximal to the fixable portion to a final position distal to the fixable portion.

The resilient member may be a coil spring. The coil spring may be provided around the distractor arm. The fixable portion may be provided with one or more formations configured to allow the fixable portion to be connected to bone. The formations may include an aperture. More specifically, the formations may include a plurality of apertures. The or each aperture may be provided in an extension of a body of the fixable portion. The extension may be a lateral extension of the body of the fixable portion. The body of the fixable portion may be provided with lateral extensions on opposing sides of the body.

The movable portion may be provided with one or more formations configured to allow the movable portion to be connected to bone. The formations may include an aperture. More specifically, the formations may include a plurality of apertures. The or each aperture may be provided in an extension of a body of the movable portion. The extension may be a lateral extension of the body of the movable portion. The body of the movable portion may be provided with lateral extensions on opposing sides of the body. The distractor arm may extend from the fixable portion through an aperture of the movable portion. The distractor arm and movable portion may be provided with complementarily shaped formations configured to guide the movable portion along the path defined by the distractor arm. The complementarily shaped formations may comprise projections of one of the distractor arm and movable portion and recesses of the other of the distractor arm and movable portion. More specifically, the complementarily shaped formations may comprise projections of one of the movable portion and recesses of the distractor arm.

The distractor arm may be flexible so as to allow the movement path of the movable member to be defined. The distractor arm may be provided with a stop so as to limit the movement of the movable member on the distractor arm in a direction distal to the fixable portion. The distractor arm may be provided with a plurality stops.

The distractor may be a mandibular distractor.

According to another aspect of the present invention there is provided a kit comprising a distractor as defined herein and a plurality of pins and/or screws for attaching the device to a section of bone.

According to another aspect of the present invention there is provided a process for distraction osteogenesis, the process comprising :

(i) Surgically separating a bone structure requiring distraction osteogenesis, and (ii) Attaching a fully implantable distractor has hereinbefore described on opposing sides of the separated bone structure and allowing it to warm to body temperature. An embodiment of the present invention will now be described with reference to the accompanying figures in which :

Figure 1 shows a top plan view of a mandibular distractor according to the present invention;

Figure 2 shows a perspective view of the mandibular distractor of figure 1 ;

Figure 3 shows another perspective view of the mandibular distractor of figure 1 ;

Figure 4 shows a further perspective view of the mandibular distractor of figure 1 ; Figure 5 shows a top plan view of a fixed portion of the mandibular distractor having a distracting arm;

Figure 6 is a side view of the portion of the mandibu lar distractor of figure 5;

Figure 7 is an end view of the portion of the mandibular distractor of figure 5; ure 8 is a cross-sectional view of the distracting arm Figure 9 is a perspective view of the portion of the mandibular distractor of figure 5;

Figure 10 shows a side view of a movable portion of the mandibular distractor;

Figure 11 shows an end view of the movable portion of the mandibular distractor; Figure 12 shows a top plan view of the movable portion of the mandibular distractor;

Figure 13 shows a perspective view of the movable portion of the mandibular distractor;

Figures 14a and 14b shows a top plan views of the mandibular distractor of the present invention in, respectively, initial and final extension conditions; Figures 15a and 15b show side and front views of a human mandible and mandibular distractor, where the mandibular distractor is in an initial extension cond ition; and

Figures 16a and 16b show side and front views of a human mandible and mandibular distractor, where the mandibular distractor is in a final extension condition.

Referring to the accompanying figures, there is shown a mandibular distractor generally designated 10. The distractor 10 comprises a fixable portion 12 and a movable portion 14. The fixable portion 12 includes a single distractor arm 16 which, in use, guides the movement of the movable portion 14 of the distractor 10 relative to the fixable portion 12 of the distractor 10. Disposed between the fixable portion 12 and movable portion 14 of the d istractor 10 there is provided a spring 18. The fixable and movable portions 12,14 and distractor arm 16 may be formed from metal, for example titanium. The fixable and movable portions 12,14 and distractor arm 16 may be made from other biocompatible materials. The spring 18 is formed from a shape memory material as will be described in greater detail below.

The fixable portion 12 comprises a body 20 having a pair of lateral extensions 22, each of which extends from an opposite side of the body 20. Each extension 22 is provided with a plurality apertures 24. In the embodiment shown, each extension 22 is provided with three circular apertures 24 which are aligned in a direction which is substantially parallel to the axis of the distractor arm 16. It will be appreciated that this arrangement for the apertures 24 is provided by way of example only, and that other arrangements with a greater or lesser number of apertures 24 may be provided . It will further be appreciated the arrangement of apertures 24 on each extension 22 may be d ifferent, and thus not the same for each side of the fixable portion 12 as shown in the figures.

The apertures 24 are sized and shaped to receive a fastener, such as threaded fastener or bone screw, therethrough so as to enable the fixable portion 12 of the distractor 10 to be fixed to a location on the mandible of a patient.

The distractor arm 16 comprises a rod having a generally circular cross-section. The distractor arm 16 extends initially from the body 22 of the fixable portion 12 in the same direction as the extensions 22. Figures 1 to 9 show the distractor arm 16 in the initial position where upon it is straight and extends along a sing le axis A-A. As will be described below, before attachment to the mandible of a patient the distractor arm 16 requires realignment so as to define a travel path for the movable portion 14.

The distractor arm 16 is provided along its length with a pair of recesses or channels 26. The channels 26 are provided on opposing lateral sides of the distractor arm 16. In use, the channels 16 receive guide projections (described below) of the movable portion 14 of the distractor 10. The interaction of the guide projections with the channels 26 ensures that the movable portion 14 of the distractor 10 follows an intended path. The interaction of the guide projections with the channels 26 further counteracts unwanted torqueing and/or rotational forces that may be experienced by the distractor after attachment to the mandible of a patient.

The distractor arm 16 is further provided with a pair of through apertures 28,30. A first distal aperture 28 is located at the end 31 of the distractor arm 16 which is d istal to body 20. A second proximal aperture 30 is provided substantially at the midway point of the distractor arm 16 between the body and the end 32 of the distractor arm 16. In use, the apertures 28,30 may be utilised to define stopping points on the distractor arm for the movable portion 14 of the distractor 10.

Stopping of the movable portion 14 on the distractor arm 16 may be effected by, for example, a formation of the movable portion 14 which is received in one of the apertures 28,30. In such an embodiment, the movable portion 14 may require manipulation to release the formation from an aperture 28,30 and thus allow the movable portion 14 to move along the distractor arm 16.

In an alternative embodiment, stopping of the movable portion 14 on the d istractor arm 16 may be effected by, for example, an insert or pin received in each aperture 28,30. In such an embodiment, a pin may require removal from an aperture 28,30 so as to allow the movable portion 14 to move along the distractor arm 16. The position of the distal and proximal apertures 28,30 of the distractor arm 16 define, respectively, the final and positions of the movable portion 14 of the distractor 10 on the distractor arm 16. It will be appreciated that the positions and spacing of the apertures 28,30 in the accompanying figures are illustrative, and that other positions and spacings may be used depending upon the surgical intervention required for the mandible of a given patient.

The movable portion 14 comprises a body 32 having a pair of lateral extensions 34, each of which extends from an opposite side of the body 32. Each extension 34 is provided with a plurality apertures 36. In the embodiment shown, each extension 34 is provided with three circular apertures 36. It will be appreciated that this arrangement for the apertures 36 is provided by way of example only, and that other arrangements with a greater or lesser number of apertures 36 may be provided . It will further be appreciated the arrangement of apertures 36 on each extension 34 may be different, and thus not the same for each side of the movable portion 14 as shown in the figures. The apertures 36 are sized and shaped to receive a fastener, such as threaded fastener or bone screw, therethrough so as to enable the movable portion 14 of the distractor 10 to be fixed to a location on the mandible of a patient.

The body 32 of the movable portion 14 is provided with a through bore 38 through which the distractor arm 16 passes, in use. The wall 40 of the through bore 38 is provided with two pairs of opposed projections 42,44 which define the aforementioned guide projections of the movable portion 14. As described above, the interaction of the guide projections 42,44 with the channels 26 ensures that the movable portion 14 of the distractor 10 follows an intended path as defined by the d istractor arm 16. The interaction of the guide projections 42,44 with the channels 26 further counteracts unwanted torqueing and/or rotational forces that may be experienced by the distractor 10 after attachment to the mandible of a patient.

It will be appreciated that the use of two pairs of opposed projections 42,44 is illustrative of one of many projection configurations that may be used .

The spring 18 is a coil spring that is located around the distractor arm 16 and is constrained axially between the fixed and movable portions 12,14 of the distractor 10. The spring 18 is formed from a shape-memory material such as a shape-memory alloy or polymer.

A shape-memory alloy (often alternatively referred to in the art as a smart metal, memory metal, memory alloy, or smart alloy) and a shape memory polymer are smart materials that have the ability to return from a deformed state (temporary shape) to their original (permanent) shape when induced by an external stimulus (trigger), such as a temperature change. Suitable memory shape alloys include alloys comprising at least two metals selected from titanium, aluminium, zinc, nickel, copper, gold and iron.

For example, the two main preferred types of shape memory alloys are copper-aluminium-nickel, and nickel-titanium (N iTi) alloys, although shape memory alloys can also be created by alloying zinc, copper, gold and/or iron. Although iron-based and copper-based shape memory alloys, such as Fe-Mn-Si, Cu-Zn-AI and Cu-AI-N i, are commercially produced and potentially cheaper than nickel-titanium alloys, nickel-titanium-based shape memory alloys (particularly nitinol) are more preferable for most applications due to their stability, practicability and superior thermo-mechanical performance.

Nickel-titanium, also known as nitinol, is a metal alloy of nickel and titanium, where the two elements are present in roughly equal atomic percentages, e.g . N itinol 55, N itinol 60. Nitinol is preferred in the context of the present invention as it is highly biocompatible and has properties suitable for use in orthopaedic implants.

Furthermore, nitinol alloys exhibit two closely related and unique properties: shape memory and superelasticity (also called pseudoelasticity). Shape memory is the ability of nitinol to undergo deformation at one temperature, then recover its original, undeformed shape upon heating above its "transformation temperature". Superelasticity occurs at a narrow temperature range just above its transformation temperature; in this case, no heating is necessary to cause the undeformed shape to recover, and the material exhibits enormous elasticity, some 10-30 times that of ordinary metal.

In addition, the phase transformation exhibited by nitinol is "reversible", meaning that heating above the transformation temperature will revert the crystal structure to the simpler austenite phase. Another key feature is that the transformation in both directions is instantaneous. At high temperatures (e.g . at and above body temperature of approximately 37 °C), the nitinol for use in the invention assumes an interpenetrating primitive cubic crystal structure referred to as austenite (also known as the parent phase) . At low temperatures (e.g . near to and below body temperature of approximately 37 °C), the nitinol spontaneously transforms to a more complicated monoclinic crystal structure known as martensite (daughter phase). The temperature at which austenite transforms to martensite is generally referred to as the transformation or transition temperature. When the alloy is fully austenite, martensite begins to form as the alloy cools at the so-called martensite start, or Ms temperature, and the temperature at which the transformation is complete is called the martensite finish, or Mf temperature. When the alloy is fully martensite and is subjected to heating, austenite starts to form at the As temperature, and finishes at the Af temperature.

In the present invention, the transition from martensite to austenite preferably occurs at approximately body temperature (i.e. at about 37 °C). This means that the return of the device from its deformed shape back to its original, pre-programmed shape may be triggered simply by use of the device with a human subject. In this case, the working temperature is body temperature.

Martensite's crystal structure has the unique ability to undergo limited deformation in some ways without breaking atomic bonds. This type of deformation is known as twinning, which consists of the rearrangement of atomic planes without causing slip, or permanent deformation. It is able to undergo about 6-8% strain in this manner. When martensite is reverted to austenite by heating, the original austenitic structure is restored, regardless of whether the martensite phase was deformed . Thus, the name "shape memory" refers to the fact that the shape of the high temperature austenite phase is "remembered", even if the alloy is severely deformed at a lower temperature.

One of the possible reasons that nitinol works so hard to return its original shape is that it is not just an ordinary metal alloy, but is what is known as an intermetallic compound . In an ord inary alloy, the constituents are randomly positioned in the crystal lattice, whereas in an ordered intermetallic compound, the atoms (in this case, nickel and titanium) have very specific locations in the lattice. This means that a large degree of force can be produced by preventing the reversion of deformed martensite to austenite, such as from 35,000 psi to, in many cases, more than 100,000 psi (689 MPa) .

Nitinol is typically composed of approximately 40 to 60% nickel by atomic percent, preferably 50 to 60% nickel by atomic percent, more preferably 52 to 58% nickel by atomic percent. The Af temperature can be controlled in nitinol to some extent depending on the content of nickel and titanium. The invention is effective when the Af temperature is below the working temperature. Convenient working temperature ranges are from about -20 °C to 60 °C, preferably 0 °C to 50 °C, more preferably 5 °C to 40 °C or 30 °C to 40 °C (e.g . body temperature of approximately 37 °C). At such temperatures, nitinol d isplays hyperelastic properties.

Polymers may also be employed as shape memory materials in the present invention. Polymers exhibiting a shape memory effect have both a visible, current (temporary) form and a stored (permanent) form. Once the latter has been manufactured, the material is changed into another, temporary form by processing through heating, deformation, and finally, cooling . The polymer maintains this temporary shape until the shape change into the permanent form is activated by a predetermined external stimulus.

Suitable shape memory polymers include physically crosslinked polymers, chemically crosslinked polymers, light-activated polymers, and electro-activated polymers. Representative physically crosslinked polymers include polyurethanes, e.g . polyurethanes with ionic or mesogenic components made by a prepolymer method, other block copolymers, such as block copolymers of polyethylene terephthalate (PET) and polyethyleneoxide (PEO), block copolymers containing polystyrene and poly(l,4-butadiene), and an ABA triblock copolymer of poly(2-methyl-2-oxazoline) and polytetrahydrofuran. In addition, linear, amorphous polynorbornene or organic-inorganic hybrid polymers consisting of polynorbornene units that are partially substituted by polyhedral oligosilsesquioxane (POSS) may also be used . Suitable chemically crosslinked polymers include crosslinked polyurethane, produced by using an excess of diisocyanate or by using a crosslinker such as glycerin or trimethylol propane, PEO-PET block copolymers, such as those produced by using maleic anhydride, glycerin or dimethyl 5-isopthalates as crosslinking agents, and thermoplastic polymers, most notably polyether ether ketone (PEEK). The introduction of covalent crosslinking improves creep, and increases the recovery temperature and recovery window.

An embodiment of the present invention utilises a nitinol coil or helical spring 18 which reverts from a compressed state to an extended state when heated . The nitinol spring 18 is martensitic below human body temperature to allow for easy manipulation and application during the surgical procedure, but becomes austenitic when warmed to human body temperature. The hysteresis of the nitinol forming the spring 18 is required to be low (ideally within 10- 15°C) for ease of working . The nitinol spring 18 delivers sufficient force which is quasi constant over a displacement that is compatible with mandibu lar DO.

The force of the nitinol helical spring is proportional to the wire diameter, spring diameter, pitch, transition temperature and number of active turns. An embodiment of the present invention has a wire diameter of 1.25mm and a pitch of 5mm, and exhibits a quasi-constant force of roughly 30N over a 15mm range. It will be appreciated that, depending upon the surgical requirements of a particu lar patient, a differing quasi-constant force over a different range may be required . A spring can be thus be produced by changing such features as the wire diameter, spring diameter, pitch, transition temperature and number of active turns to achieve the desired spring characteristics.

Referring now to figures 14a and 14b there is shown a mandibular distractor 10 according to the present invention in an initial extension cond ition (figure 14a) and a final extension condition (figure 14b). For the sake of describing the operation of the distractor 10, the distractor arm 16 is shown in its straight configuration and not in the curved condition necessary for a successful surgical outcome.

The initial extension condition (figure 14a) of the d istractor 10 corresponds to the condition of the distractor 10 prior to and during implantation. The spring 18 is it its deformed state, i.e. where its length is shortest. The spring 18 is positioned between the fixed and movable portions 12,14 of the distractor 10 and may exert minimal or no force on the portions 12,14. Additionally, the movable portion 14 may be stopped or retained in position on the distractor arm 16 by interaction with the proximal aperture 30 of the arm 16.

The final extension condition (figure 14b) of the distractor 10 corresponds to condition of the distractor 10 prior to removal of the device from a patient. The spring 18 is it its original state, i.e. where its length is greatest. The spring 18 is positioned between the fixed and movable portions 12,14 of the d istractor 10 and may exert quasi-constant force on the portions 12,14. Additionally, the movable portion 14 may be stopped or retained in position on the distractor arm 16 by interaction with the distal aperture 28 of the arm 16. The movement of the spring 18 from its deformed state to its original state occurs after it has been heated to human body temperature. Figures 15a to 16b again show the distractor 10 in its initial and final extension conditions while located on a mandible 46. The distractor 10 is located to the mandible 46 on opposing sides of a separation 48 of the mandible 46. The separation is produced by surgical osteotomy. The distractor arm 16 is shaped so as to define a desired movement path for the portion of the mandible 46 connected to the movable portion 14 of the distractor 10. The distance over which the portion of the mandible 46 connected to the movable portion 14 is movable by the spring 18 is defines by the position of the stops on the distractor arm 16.

While the embodiments described above relate to mandibular distraction, the use in this instance is not intended to be limiting and it will be appreciated that the method and device of the present invention may be used at other locations on the human or animal body where osseous distraction is required .

Claims

1. A fully implantable distractor comprising a fixable portion, a movable portion, a sing le distractor arm which defines a movement path of the movable portion relative to the fixable portion, and a resilient member is provided between the fixable portion and the movable portion to, in use, move the movable portion relative to the fixable portion along the movement path defined by the distractor arm, wherein the resilient member is formed from a shape memory material.

2. A distractor as claimed in claim 1 wherein the distractor arm defines a movement path in at least 2 dimensions.

3. A distractor as claimed in claim 2 wherein the distractor arm defines a 3 dimensional movement path.

4. A distractor as claimed in any preceding claim wherein the shape memory material is a shape memory alloy and/or a shape memory polymer.

5. A distractor as claimed in claim 4 wherein the shape memory material comprises a shape memory alloy.

6. A distractor as claimed in 5 wherein the shape memory alloy is an alloy of nickel and titanium.

7. A distractor as claimed in claim 6 wherein the shape memory alloy is nitinol.

8. A distractor as claimed in any preceding claim wherein the resilient member is configured to move the movable portion along the movement path defined by the distractor arm from an initial position proximal to the fixable portion to a final position distal to the fixable portion.

9. A distractor as claimed in any preceding claim wherein the resilient member is coil spring .

10. A distractor as claimed in claim 8 wherein the coil spring provided around the distractor arm.

11. A distractor as claimed in any preceding claim wherein the fixable portion is provided with one or more formations configured to allow the fixable portion to be connected to bone.

12. A distractor as claimed in claim 11 wherein the formations include an aperture.

13. A distractor as claimed in claim 12 wherein the formations include a plurality of apertures.

14. A distractor as claimed in 12 or claim 13 wherein the or each aperture is provided in an extension of a body of the fixable portion.

15. A distractor as claimed in claim 14 wherein the extension is a lateral extension of the body of the fixable portion.

16. A distractor as claimed in claim 15 wherein the body of the fixable portion is provided with lateral extensions on opposing sides of the body.

17. A distractor as claimed in any preceding claim wherein the movable portion is provided with one or more formations configured to allow the movable portion to be connected to bone.

18. A distractor as claimed in claim 17 wherein the formations include an aperture.

19. A distractor as claimed in claim 18 wherein the formations include a plurality of apertures.

20. A distractor as claimed in 18 or claim 19 wherein the or each aperture is provided in an extension of a body of the movable portion.

21. A distractor as claimed in claim 20 wherein the extension is a lateral extension of the body of the movable portion.

22. A distractor as claimed in claim 21 wherein the body of the movable portion is provided with lateral extensions on opposing sides of the body.

23. A distractor as claimed in any preceding claim wherein the distractor arm extends from the fixable portion through an aperture of the movable portion.

24. A distractor as claimed in claim 23 wherein the distractor arm and movable portion are provided with complementarily shaped formations configured to guide the movable portion along the path defined by the d istractor arm.

25. A distractor as claimed in claim 24 wherein the complementarily shaped formations comprise projections of one of the distractor arm and movable portion and recesses of the other of the distractor arm and movable portion.

26. A distractor as claimed in claim 25 wherein the complementarily shaped formations comprise projections of one of the movable portion and recesses of the distractor arm.

27. A distractor as claimed in any preceding clam wherein the distractor arm is flexible so as to allow the movement path of the movable member to be defined .

28. A distractor as claimed in any preceding claim wherein the distractor arm is provided with a stop so as to limit the movement of the movable member on the distractor arm in a direction d istal to the fixable portion.

29. A distractor as claimed in claim 28 wherein the d istractor arm is provided with a plurality stops.

30. A kit comprising a distractor as claimed in any preceding claim and a plurality of pins and/or screws for attaching the device to a section of bone.

31. A process for distraction osteogenesis, the process comprising :

(i) Surgically separating a bone structure requiring d istraction osteogenesis, and

(ii) Attaching the fully implantable distractor of any of claims 1 to 29 opposing sides of the separated bone structure and allowing it to warm to body temperature.