WO2016102026A1

WO2016102026A1 - Method for manufacturing a device to connect to a bone

Info

Publication number: WO2016102026A1
Application number: PCT/EP2014/079307
Authority: WO
Inventors: Frederik Gelaude; Ward An Johan BARTELS; Wilhelmus Antonius Johannes WOUTERS
Original assignee: Mobelife N.V.
Priority date: 2014-12-24
Filing date: 2014-12-24
Publication date: 2016-06-30

Abstract

Method for manufacturing a device to connect to a bone, such as an implant, wherein the method comprises the steps of: - determining by pre-operative planning, on the basis of an image of the bone to which the device is to be connected, a plurality of screw trajectories for connecting said device to the bone, wherein each screw trajectory having a position, length and orientation is represented by a screw line segment connecting the head and the tip of a screw; - designing the device provided with a plurality of screw receiving elements, wherein the screw receiving elements of said device are positioned and oriented corresponding to the pre-operatively planned screw trajectories; and - manufacturing said device, wherein the step of determining the screw trajectories using a planning comprises defining at least one pair of screw trajectories, wherein the screw line segments of the screw trajectories in said pair are planned to extend non-parallel with a mutually crossing orientation.

Description

Method for manufacturing a device to connect to a bone

The present invention relates to a method for manufacturing a device to connect to a bone. In 2007, 14.469 patients were treated with a primary total hip prosthesis and more than 1800 patients underwent a revision of their implant. The last 10 years, the amount of planned surgeries (non-traumatic) shows a mean growth of 3.5% per year. The largest growth is found in the population younger than 60 years old. By consequence, hip revision surgeries are becoming more prevalent and the younger patients are more demanding towards the quality/life span of the implant and the surgery.

Standard implants for primary hip arthroplasty generally do not contain holes for screws in the acetabular components. In standard implants for revision however, extra fixation is necessary and can be achieved using bone screws. These bone screw typically have an outwardly extending head, with regard to the remainder of the screw, arranged to contact the implant in a hole in said implant. The part distally of this implant connecting part is usually arranged to engage the bone for connecting and fixating the implant to the bone. This distal part is typically provided with a sharp tip or point. The success rate of for instance such a bone reconstructing operation using an implant is largely dependent on the manner of fixing the implant to the bone and whether the implant in implanted situation is capable of withstanding the loading conditions. For instance, loosening of the implant due to incorrect fixation of the implant by a bone screw is detrimental to the success rate of such a reconstructing operation. More specifically, screw fixation serves at short term fixation of the implant, whereas biologic fixation of the implant serves as mid to long term fixation. If an implant only relies on traditional screws for fixation, these screws will typically fail in mid term, and with this the entire reconstruction.

In standard implants, bone screws will be inserted in the predefined holes of the implant. The location, direction and the amount of screws used is decided upon during the operation. In custom implants however, the screw configuration can be predetermined during the pre -operative planning. For example, a bone quality map gives an indication of the local bone quality, on the basis of which good fixation of screws can be obtained. It is a goal of the present invention, next to other goals, to provide an improved method for designing and manufacturing a device to be connected to the bone and/or which method results in a device wherein the risk of loosening of the device is decreased. This goal, amongst other goals, is met by a method according to appended claim 1. More specifically, this goal, amongst other goals, is met by a method for manufacturing a device to connect to a bone, such as an implant, wherein the method comprises the steps of:

determining by pre-operative planning, on the basis of an image of the bone to which the device is to be connected, a plurality of screw trajectories for connecting said device to the bone, wherein each screw trajectory having a position, length and orientation is represented by a screw line segment connecting the head and the tip of a screw;

designing the device provided with a plurality of screw receiving elements, wherein the screw receiving elements of said device are positioned and oriented corresponding to the pre- operatively planned screw trajectories; and

- manufacturing said device,

wherein the step of determining the screw trajectories using a planning comprises defining at least one pair of screw trajectories, wherein the screw line segments of the screw trajectories in said pair are planned to extend non-parallel with a mutually crossing orientation. The device resulting from the method of the invention is designed to be connected to bone with a plurality of screws, of which two extend non-parallel and with a mutually crossing orientation. This results in an improved connection of the device to the bone, such that the risk of loosening of the device after connection is decreased. It is noted that the position, orientation and length of a screw can be mathematically represented by the screw line segment which connects the head of the screw to the tip of the screw. The length of this segment therefore corresponds to the distance between the head and the tip. The terms screw trajectory and screw line segment as used herein can therefore be exchanged. Both the screw trajectory and screw line segment lie on an infinite line which is referred to as the screw line. In the step of pre-operative planning the screw trajectories, at least two screw trajectories are planned to extend non-parallel with a mutually crossing orientation in connected, for instance implanted, situation. The device is hereby preferably designed such that the orientation of a screw with respect to the device is completely determined by the respective screw receiving element of the device. The screw receiving element may for instance comprise a hole in said device having a shape in accordance with the shape of the screw, particularly the head thereof. The planning of the screw trajectories and/or the design of the device is preferably based on a three-dimensional bone model of at least the part of the bone to which the device is to be connected. Providing the three-dimensional bone model may comprise the step of obtaining an image of the bone to which the device is to be connected. Digital patient-specific image information can be provided by any suitable means known in the art, such as for example a computer tomography (CT) scanner, a magnetic resonance imaging (MRI) scanner, an ultrasound scanner, or a combination of Roentgenograms. A summary of medical imaging has been described in "Fundamentals of Medical imaging", by P. Suetens, Cambridge University Press, 2002. For example, the step of obtaining an image of the bone and the defect therein may comprise the steps of obtaining 2D datasets of the bone and reconstructing a 3D virtual bone model from said 2D datasets. Indeed, the first step in a planning is the construction of a 3D virtual model of the bone. This reconstruction starts with sending a patient to a radiologist for scanning, e.g. for a scan that generates medical volumetric data, such as a CT, MRI scan or the like. The output of the scan can be a stack of two-dimensional (2D) slices forming a 3D data set. The output of the scan can be digitally imported into a computer program and may be converted using algorithms known in the field of image processing technology to produce a 3D computer model of a relevant bone.

Preferably, a virtual 3D model is constructed from the dataset using a computer program such as Mimics(TM) as supplied by Materialise N.V., Leuven, Belgium. Computer algorithm parameters are based on accuracy studies, as for instance described by Gelaude at al. (2008; Accuracy assessment of CT-based outer surface femur meshes Comput. Aided Surg. 13(4): 188- 199). A more detailed description for making a perfected model is disclosed in U.S. Patent No. 5,768, 134 entitled 'Method for making a perfected medical model on the basis of digital image information of a part of the body'. The three-dimensional model of the bone is reconstructed for instance as disclosed in Gelaude et al. (2007; Computer-aided planning of reconstructive surgery of the innominate bone: automated correction proposals Comput. Aided Surg. 12(5): 286-94).

An improved fixation of the device is obtained if the screw line segments in a pair are planned to be non-parallel and non-intersecting, i.e. as skew lines. More specifically, the trajectories in a pair are preferably planned such that the orthogonal projections of the two screw line segments in said pair onto a projection plane intersect, wherein the projection plane is oriented parallel to both screw line segments.

A further improved retention is obtained if the trajectories in a pair are planned such that an angle enclosed between the heads of the two screw trajectories in said pair is between 20 and 120 degrees, preferably between 60 and 90 degrees for an even further improved retention of the device to the bone in connected situation. The screw trajectories in a pair hereby preferably extend approximately orthogonally. The angle is defined by the angle between the sections of screws provided with the heads. In terms of the projection plane as defined above, it is preferred to obtain the improved retention if the trajectories in a pair are planned such that an angle between the projections of the screw line segments enclosed between the heads of the screws is between 20 and 120 degrees, preferably between 60 and 90 degrees.

According to a further preferred embodiment, the trajectories in a pair are planned such that an intersection line segment which is defined as the shortest possible line segment connecting two points on the two screw lines, preferably screw line segments, in said pair has a length smaller than a predetermined maximum distance. Assuring that the screws in a pair do not extend too far apart improves the fixation capability of said pair, and thereby improves the retention of the device on the bone. It has been found that a maximum distance of 20 mm results in said improved retention. The maximum distance may further be defined on the basis of the maximum screw shaft diameter of any of the screws in the pair. This has shown to be a reliable parameter for determining said distance. The maximum screw shaft diameter is measured at the widest region of the threaded portion of the screw. Preferably, the maximum distance is defined as three times the maximum screw shaft diameter.

However, according to a further preferred embodiment, the maximum distance is defined as the largest of three times the maximum screw shaft diameter or 20 mm. It was found that screws having larger diameters are allowed to have a larger mutual distance, while still ensuring the improved fixation of the device.

A further improvement of the fixation of the device is obtained if according to a further preferred embodiment, the trajectories in a pair are planned such that the intersection line segment completely extends through bone. In particularly for this embodiment, it is preferred if the planning is based on the three-dimensional bone model of at least the part of the bone to which the device is to be connected as mentioned above.

A further preferred embodiment of the invention comprises the step of providing bone quality data which is representative of the quality of bone to which the device is to be connected. This data may for instance be obtained by medical imaging, such as a CT- or MRI-scan or similar, and provides detailed information about the bone to which the device is to be connected. It is in particular preferred if the bone information comprises bone (mineral) density data, for instance obtained by a DEXA-scan. It is preferred that the screws in a pair extend through bone having a high quality to improve the retention. To however further improve the retention, it is preferred if the trajectories in a pair are planned such that the intersection line segment and the two screw line segments extend through high quality bone. It was found that also high quality bone between the two screws, in particularly along the intersection line segment, improves the retention.

To further improve the retention of the device, it is preferred if said step of determining one or more screw trajectories using a planning further comprises taking into account one or more of the following criteria:

- obtaining an optimal number of non-intersecting drill directions for screw trajectories; ensuring that said screw trajectories run through bone volume with the optimal available quality;

ensuring optimal screw trajectory length; and

ensuring that the surrounding healthy soft tissue is optimally preserved.

The method according to the invention is preferably an automated method, for instance in the form of a computer implemented method. A computing device provided with a processor, input means, output means and storage means may be for instance be arranged to execute the method. In order to ensure that the defined screw trajectories in a pair have a mutual orientation which improves the fixation of the device, it is preferred if the method comprises a checking step or confirmation step to check or confirm whether the screw trajectories in a pair are in accordance with at least one, preferably all, of the requirements as described above. More specifically, it is preferred to check whether the pair, that is the screw trajectories of said pair, meets at least one, preferably all, of predefined screw crossing criteria, wherein the screw crossing criteria are preferably defined as whether:

the screw line segments in the pair are non-parallel and non-intersecting, preferably such that the orthogonal projections of the two screw line segments in said pair onto a projection plane intersect, wherein the projection plane is oriented parallel to both screw line segments;

- an angle enclosed between the heads of the two screw trajectories in the pair is between 20 and 120 degrees; and/or

an intersection line segment which is defined as the shortest possible line segment connecting two points on the two screw line segments in a pair has a length smaller than a predetermined maximum distance.

The method hereto preferably comprises at least one of the steps of: calculating and checking whether the screw line segments of a pair extend non-parallel and non-intersecting, preferably determining the orthogonal projections of the two screw line segments in said pair onto a projection plane, wherein the projection plane is oriented parallel to both screw line segments, and checking whether said orthogonal projections intersect; and/or

- calculating an angle enclosed between the heads of the two screw trajectories in the pair and checking whether this angle is between 20 and 120 degrees; and/or

calculating the length of an intersection line segment which is defined as the shortest possible line segment connecting two points on the two screw lines, preferably screw line segments, in a pair and checking whether the intersection line segment has a length smaller than a predetermined maximum distance.

If it is determined that a pair does not meet the criteria, the pair can be redefined, for instance in an iterative process by incrementally adjusting the position, length and/or orientation of at least one of the screw trajectories. This can be done automatically and/or by input of a user.

More specifically, according to a preferred embodiment, the step of determining the screw trajectories comprises defining a pair of screw trajectories and subsequently checking whether the pair meets at least one, preferably all, of predefined screw crossing criteria and repeating the steps of defining the pair and checking until the defined pair meets at least one, preferably all, of the predefined screw crossing criteria.

According to a further preferred embodiment of the method according to the invention, the step of determining the screw trajectories comprises determining a first screw trajectory and preferably outputting the first screw trajectory to a user. The outputting may for instance include outputting the trajectory to a screen, wherein the trajectory is outputted in combination and in relation to the bone image on which the planning is determined. This provides a visual representation of the screw trajectory.

The method preferably further comprises determining the second trajectory by adjusting the position, length and/or orientation of the second screw trajectory to meet at least one, or more preferably all of a screw crossing criteria which will result in the improved retention according to the invention. It is for instance possible to generate a plurality of second screw trajectories by incrementally adjusting the position, length and/or orientation of the second screw trajectory and to subsequently evaluate said trajectories to determine whether the trajectories meet at least one, preferably meet all, of a screw crossing criteria. These trajectories may for instance be outputted for selection to the user. It is also possible that the method comprises receiving input from the user regarding the position, length and/or orientation of the second screw trajectory and outputting feedback to the user whether the inputted second screw trajectory of the pair meets at least one, or more preferably all of the screw crossing criteria. The input may further include adjustments to the position, length and/or orientation of the second screw trajectory. Based on the input for the second screw trajectory, it is determined whether this pair of screw trajectories will result in an improved retention according to the invention. Based on the input of the user, which may be adjusted due to the feedback, the pair of screw trajectories on the basis of the input of the user is determined.

According to a further preferred embodiment, the step of determining the second screw trajectory comprises determining a position of the second screw trajectory, for instance the position of the head of the screw or a point based on the maximum distance from the first trajectory, and adjusting, preferably by input of the user, the length and/or orientation of the second screw trajectory. It is for instance possible that the position of the second screw trajectory is based on a preferred location of a screw receiving element on the device to improve fixation at that location. The orientation and length can then be adjusted such that the screw trajectory meets one or more, or more preferably all of the screw crossing criteria. A further preferred embodiment further comprises the step of identifying a first surface in the device for designing therein a first screw receiving element positioned and oriented in accordance with one of the trajectories of a pair, the one trajectory being substantially perpendicular to the first surface, and identifying a second surface in the device, the surface being substantially

perpendicular to the other trajectory of the pair, for designing therein a screw receiving element positioned and oriented in accordance with said other trajectory. This results in an improved fixation of these surfaces due to the substantially perpendicular orientation of the screw with respect to this surface. It is also allowable that the screw line segment is under an angle of 70 to 110 degrees with respect to the surface. The method may further comprise the first step of selecting the first surface in the device, wherein the step of planning the screw trajectories in a pair is based on the selected first surface such that one of the trajectories is substantially perpendicular, or under an angle of 70 to 110 degrees as mentioned above, to the selected first surface. To improve the retention of the device, it is preferred if the step of determining the screw trajectories comprises determining a plurality of said pairs of screw trajectories. It may for instance also be possible that screws from different pairs are designed in a crossing manner according to the invention. According to a further preferred embodiment, the step of determining the screw trajectories comprises defining a first set of screw trajectories extending substantially parallel to each other and defining a second screw trajectory, or a second set of screw trajectories extending substantially parallel to each other, which extends non-parallel with a mutually crossing orientation, both with respect to the first set of screw trajectories and which preferably meets at least one of, preferably all of the screw crossing criteria. This improves the retention of in particular larger devices. Evaluating the screw crossing criteria for a plurality of screws may include evaluating the screw crossing criteria for any combination of screws from the first set with the screws from the second set of screws. In case the criteria are met, or an optimum number of criteria are met, the planning can be considered as optimal.

It is further possible that planning a pair comprises taking as a basis an already planned screw trajectory, for instance from an earlier planned pair, and planning an additional screw trajectory to form an additional pair in a non-parallel and mutually crossing manner according to the invention. This further improves the retention, in particular for larger devices.

The invention further relates to a method for manufacturing a device according to the invention arranged for joint replacement, wherein the step of designing comprises designing a cup region arranged to receive another part of said joint and designing at least one flange region, wherein one screw trajectory of a pair is planned to originate in said cup region and wherein the other of the screw trajectories of the pair is planned to originate in said flange region. This results in an improved fixation of the implant. To be able to reliably and accurately manufacture the device in accordance with the design thereof, the step of manufacturing preferably comprises using a three-dimensional printing technique, also referred to as rapid manufacturing technique, layered manufacturing technique, additive manufacturing technique or material deposition manufacturing technique. Rapid manufacturing includes all techniques whereby an object is built layer by layer or point per point by adding or hardening material (also called free-form manufacturing). The best known techniques of this type are stereo lithography and related techniques, whereby for example a basin with liquid synthetic material is selectively cured layer by layer by means of a computer-controlled electromagnetic beam; selective laser sintering, whereby powder particles are sintered by means of an electromagnetic beam or are welded together according to a specific pattern; fused deposition modelling, whereby a synthetic material is fused and is stacked according to a line pattern; laminated object manufacturing, whereby layers of adhesive -coated paper, plastic, or metal laminates are successively glued together and cut to shape with a knife or laser cutter; or electron beam melting, whereby metal powder is melted layer per layer with an electron beam in a high vacuum.

In particular embodiments, Rapid Prototyping and Manufacturing (RP&M) techniques are used for manufacturing the device of the invention. Rapid Prototyping and Manufacturing (RP&M) can be defined as a group of techniques used to quickly fabricate a physical model of an object typically using three-dimensional (3-D) computer aided design (CAD) data of the object. Currently, a multitude of Rapid Prototyping techniques is available, including stereolithography (SLA),

Selective Laser Sintering (SLS), Fused Deposition Modeling (FDM), foil-based techniques, etc. A common feature of these techniques is that objects are typically built layer by layer.

Stereolithography (SLA), a common RP&M technique, utilizes a vat of liquid photopolymer "resin" to build an object a layer at a time. On each layer, an electromagnetic ray, e.g. one or several laser beams which are computer-controlled, traces a specific pattern on the surface of the liquid resin that is defined by the two-dimensional cross-sections of the object to be formed.

Exposure to the electromagnetic ray cures, or solidifies, the pattern traced on the resin and adheres it to the layer below. After a coat has been polymerized, the platform descends by a single layer thickness and a subsequent layer pattern is traced, adhering to the previous layer. A complete 3-D object is formed by this process.

Selective laser sintering (SLS) uses a high power laser or another focused heat source to sinter or weld small particles of plastic, metal, or ceramic powders into a mass representing the 3- dimensional object to be formed.

Fused deposition modeling (FDM) and related techniques make use of a temporary transition from a solid material to a liquid state, usually due to heating. The material is driven through an extrusion nozzle in a controlled way and deposited in the required place as described among others in U.S. Pat. No. 5.141.680.

Foil-based techniques fix coats to one another by means of gluing or photo-polymerization or other techniques and cut the object from these coats or polymerize the object. Such a technique is described in U.S. Pat. No. 5.192.539. Typically RP&M techniques start from a digital representation of the 3-D object to be formed, in this case the design of the device. Generally, the digital representation is sliced into a series of cross-sectional layers which can be overlaid to form the object as a whole. The RP&M apparatus uses this data for building the object on a layer-by-layer basis. The cross-sectional data representing the layer data of the 3-D object may be generated using a computer system and computer aided design and manufacturing (CAD/CAM) software.

The device of the invention may be manufactured in different materials. Typically, only materials that are biocompatible (e.g. USP class VI compatible) with the human body are taken into account. Preferably the device is formed from a heat-tolerable material allowing it to tolerate high- temperature sterilization. In the case SLS is used as a RP&M technique, the device may be fabricated from a polyamide such as PA 2200 as supplied by EOS, Munich, Germany or any other material known by those skilled in the art may also be used. The present invention is further illustrated by the following Figures, which show a preferred embodiment of the method according to the invention, and are not intended to limit the scope of the invention in any way, wherein:

Figure 1 is a perspective view schematically showing two screws orientated in accordance with the invention;

- Figure 2 corresponds to the orientation of figure 1 and is a view parallel to the projection plane;

Figure 3 corresponds to the orientation of figure 1 and is a view perpendicular to the projection plane;

Figure 4 schematically shows the steps of a method for determining a screw pair in accordance with the invention;

Figure 5 schematically shows the output on a screen during the design of a device according to the invention;

Figures 6 and 7 show alternative steps of a method for determining a screw pair in accordance with the invention. Figures 1 - 3 explain the orientation of two screws 1 , 2 in a pair of screws which are intended to fix an implant to a bone. The concept according to the invention is to deliberately use cross fixation of at least two screws. That is, at least two screws are oriented such that they extend non-parallel and non-intersecting. A device is thereto designed which accordingly orients the screws upon insertion in the device, for instance by designing specially oriented screw holes. The position, orientation and length of a screw 1 , 2 can be mathematically represented by a line segment 11, 21 which connects the head H_{1 2} of the screw 1, 2 to the tip T_{1 2} of the screw 1,2. This segment will be referred to as screw line segment 11, 21. The screw line segments 11, 21 lie on infinite lines called the screw line 12, 22. Furthermore, only non-parallel and non-intersecting pairs of screws will be considered.

For any given non-parallel pair of screw line segments 11, 21 in three-dimensional space, a projection plane P can be determined that is parallel to both lines 12, 22. This projection plane P can be positioned anywhere, but must be oriented parallel to both screw line segments 11, 21. For the purpose of this description, the projection plane P will be positioned to coincide with one 22 of the screw lines 12, 22.

Furthermore, an intersection line segment I is defined as the shortest possible line segment connecting two points on the two different screw lines 12, 22. An intersection line segment I can always be defined for any given non-parallel and non-intersecting pair of screw lines 12, 22. It is perpendicular to the projection plane P.

In short, the screw line segments 11, 21 are defined as Hi-Ti and H₂-T₂ and have lengths corresponding to the respective distances between the heads H_{1 2} and the tips T_{1 2}. The projection plane P is parallel to both screws 1, 2 and has been positioned to coincide with screw 2.

Also shown in the figures is the projection 11 ' onto the projection plane P of the line segment 11 , which extends between projected points Hi'-Ti' . As the screw line 22 is in the plane of P, the screw line segment 21, between H₂-T₂, is equal to its own projection H₂'-T₂' . The projected screw line segments 11 ' and 21, or Hi' -Ti' and H₂'-T₂', intersect in the intersection point X₂. The smallest distance d between both line segments is Xi-X₂ (X₂ is the projection of Xi) and is also indicated with I.

With reference to figure 3, the angle between the screws can be quantified as a which corresponds to the angle between the projections of the screw heads.

According to the invention, it is found that optimal device fixation is obtained if crossing screws meet the following criteria:

The projected screw line segments 11, 21 (Ηι'-Τι' and H₂'-T₂') must intersect in a point (X₂) inside both line segments; The distance d between X_x and X₂ must be smaller than 3 times the largest screw shaft diameter (measured at widest region of the threaded portion) or 20 mm, whichever of the two values is largest;

Angle a must be between 20 and 120°, and is ideally between 60 and 90°.

The principle of deliberate cross fixation can be implemented in procedures that are embedded in design workflows or software to (semi-automatically) generate designs that meet the criteria of cross fixation as previously described. Planning screws in cross fixation can be a part of a design step that is started after planning the implant/device (surface), or beforehand. In the latter case the implant is designed to be compatible with previously planned screws. Possible embodiments include but are not limited to applications of the following procedures.

Figure 4 shows a first embodiment of a method for planning a pair of screw trajectories in accordance with the invention. In the first step 1001, one screw line segment is planned starting from an implant/device or bone surface into an underlying bone region. This planning can for instance be based on a part of the device which needs fixation. The segment is oriented such that a portion of the screw is positioned into a bone region and the head position is determined by a location that is indicated on the implant/device or bone surface. Figure 5 shows an example of a computer screen which allows the user to select the position of a screw. In an output region 101, a representation of the implant 3 on the bone 4 is shown. This representation is based on a medical image of the bone 4 and the implant 3 in this example is shaped to be congruent to the surface 42 of the bone 4. The lower surface 32 of the implant 3 is thus designed to accurately fit on the surface 42 of the bone 4.

The position, orientation (indicated with the dashed line 12) and the length of the first screw 1 is already determined by selection by the user. A screw hole 31 shaped to receive and orientate the screw 1 is also visible. In a next step 1002, see figure 4, the head position of the second screw is determined. This can be done by selecting a location on the surface of the bone 42 and/or the implant 3. This is schematically indicated with the arrow 23. An initial orientation, indicated with the screw line 22, is then determined. In a next step 1003, the length and orientation of the screw line 22 of the second screw is adjusted. In this example, the screen 100 is provided with an input region 102 provided with one slider 103 for adjusting the length, indicated with 103a in the output region 101, and two sliders 104 and 107 for adjusting the orientation or angles of the screw line segment of the second screw, indicated with 104a and 107a in the output region 101. While sliders 104 and 107 change the orientation (104a and 107a) of the screw line 22, the screw line 22 is constrained so it always passes through the manually selected head position 23 of the second screw. Movement of slider 104 causes the screw line 22 to rotate (104a) around an axis 104b perpendicular to both screw lines 12 and 22, which is parallel to the intersection line segment I in figures 1 and 2.

Movement of slider 107 causes the screw line 22 to rotate (107a) around an axis 107b parallel to screw line 12.

On the basis of the position and adjusted length and orientation of the two screw line segments of the two screws, it is determined whether the crossing criteria as defined above are met (step 1004). This is outputted in the feedback region 105 of the screen 100. It is indicated whether each of the criteria is met. In case the criteria are not met, the user may adjust the length and orientation of the screw line segment, indicated with the arrow 1004a in figure 4. When the criteria are met and the screw configuration is satisfactory to the user, the process may be finished (step 1005), for instance by pressing the OK button 106. The process may then be repeated to generate other screw pairs in cross fixation. Although it is possible to generate a new pair with two new, different screws, it is also possible that one of the screws 1 , 2 of a previous pair is taken as a first screw of a new pair to be formed, wherein a second, new screw is planned in accordance with the invention with respect to the first screw.

In figure 6 an alternative process is shown, wherein instead of selecting the head position of the second screw, in step 1002a a point is selected in the bone 4 which meets the minimal distance criterion with respect to the first screw line segment. This can be done by manually indicating or automatically determining an intersection line segment I, see figure 1 and 2, by determining a point (in drawings referred to as X₂) meeting the minimal distance criterion with respect to the first screw line segment. This distance d is to be smaller than three 3 times the largest screw shaft diameter (measured at widest region of the threaded portion) or 20 mm, whichever of the two values is largest. The manual selection of the point is indicated with 24 in figure 5. In the third step 1003a, the crossing angle of the second screw line and screw length are set such that the head and tip locations are determined and that the crossing criteria are met. The calculated second screw line segment may be outputted to the user, after which the user may adjust the angle and the length (step 1004b), similar to the process of steps 1003 and 1004 in figure 4. After confirmation, the process is finished (step 1005). A further alternative is shown in figure 7. Instead of adjusting the orientation and length of the second screw (step 1003) as in the example of figure 4, the exit point of the second screw is determined (step 1002b). This is for instance preferred if the second screw is to extend through a particular section of the bone 4. This selection can take place in a way similar to the selection of the point indicated with 24 in figure 5. By the selection of the exit point, an initial screw length and orientation are determined. In a next step, the screw length may be adjusted (step 1003b), for instance with the slider 103. After, or during, adjustment, the crossing criteria are outputted, i.e. whether the requirements are met, for instance in the region 105 as mentioned earlier. On the basis of this feedback, the user may select a new exit point (following arrow 1002c) or adjust the screw length (following arrow 1003c). As an alternative, the user may adjust the angle and orientation, similar to the steps 1003 and 1004 of figure 4. After confirmation, the process is finished (step 1005).

When each of the screw trajectories of the implant 4 is determined, the design of the implant 4 can be finished. For each of the screw trajectories, a suitable screw hole 31 or similar screw receiving element is designed. The design may then be manufactured, in this example by a three-dimensional printing technique.

The present invention is not limited to the embodiment shown, but extends also to other embodiments falling within the scope of the appended claims.

Claims

Method for manufacturing a device to connect to a bone, such as an implant, wherein the method comprises the steps of:

- determining by pre-operative planning, on the basis of an image of the bone to which the device is to be connected, a plurality of screw trajectories for connecting said device to the bone, wherein each screw trajectory having a position, length and orientation is represented by a screw line segment connecting the head and the tip of a screw;

- designing the device provided with a plurality of screw receiving elements, wherein the screw receiving elements of said device are positioned and oriented corresponding to the pre-operatively planned screw trajectories; and

- manufacturing said device,

wherein the step of determining the screw trajectories using a planning comprises defining at least one pair of screw trajectories, wherein the screw line segments of the screw trajectories in said pair are planned to extend non-parallel with a mutually crossing orientation.

Method according to claim 1 , wherein the screw line segments in a pair are planned to be non-parallel and non-intersecting.

Method according to claim 2, wherein the trajectories in a pair are planned such that the orthogonal projections of the two screw line segments in said pair onto a projection plane intersect, wherein the projection plane is oriented parallel to both screw line segments.

Method according to any of the preceding claims, wherein the trajectories in a pair are planned such that an angle enclosed between the heads of the two screw trajectories in said pair is between 20 and 120 degrees, preferably between 60 and 90 degrees.

Method according to claim 3 and 4, wherein the trajectories in a pair are planned such that an angle between the projections of the screw line segments enclosed between the heads of the screws is between 20 and 120 degrees, preferably between 60 and 90 degrees.

6. Method according to any of the preceding claims 2 - 5, wherein the trajectories in a pair are planned such that an intersection line segment which is defined as the shortest possible line segment connecting two points on two screw lines, preferably the two screw line segments, in said pair has a length smaller than a predetermined maximum distance, wherein a screw line is defined as the infinite line on which a screw line segment lies.

Method according to claim 6, wherein the maximum distance is 20 mm.

Method according to claim 6 or 7, wherein the maximum distance is defined on the basis of the maximum screw shaft diameter of any of the screws in the pair as three times the maximum screw shaft diameter. 9. Method according to claim 7 and 8, wherein the maximum distance is defined as the largest of three times the maximum screw shaft diameter or 20 mm.

10. Method according to any of the claims 6 - 9, wherein the trajectories in a pair are planned such that the intersection line segment completely extends through bone.

11. Method according to claim 10, further comprising the step of providing bone quality data which is representative of the quality of bone to which the device is to be connected, wherein the trajectories in said pair are planned such that the intersection line segment and the two screw line segments extend through high quality bone.

12. Method according to any of the preceding claims, wherein the step of determining the screw trajectories comprises defining a pair of screw trajectories and subsequently checking whether the pair meets all of predefined screw crossing criteria and repeating the steps of defining the pair and checking until the defined pair meets all of the predefined screw crossing criteria, wherein the screw crossing criteria are defined as whether:

- the screw line segments in the pair are non-parallel and non-intersecting, such that the orthogonal projections of the two screw line segments in said pair onto a projection plane intersect, wherein the projection plane is oriented parallel to both screw line segments;

- the angle enclosed between the heads of the two screw trajectories in the pair is

between 20 and 120 degrees according to claim 4 or 5; and/or

- an intersection line segment which is defined as the shortest possible line segment connecting two points on two screw lines, preferably the two screw line segments, in a pair has a length smaller than a predetermined maximum distance, wherein a screw line is defined as the infinite line on which a screw line segment lies, according to any of the claims 6 - 11. Method according to any of the preceding claims, wherein the step of determining the screw trajectories comprises determining a first screw trajectory and outputting the first screw trajectory to a user, wherein the method further comprises receiving input from the user regarding the position, length and/or orientation of the second screw trajectory and outputting feedback to the user indicating whether the inputted second screw trajectory of the pair meets all of the screw crossing criteria, and determining the pair of screw trajectories on the basis of the input of the user, wherein the screw crossing criteria are defined as whether:

between 20 and 120 degrees according to claim 4 or 5; and/or

- an intersection line segment which is defined as the shortest possible line segment connecting two points on two screw lines, preferably the two screw line segments, in a pair has a length smaller than a predetermined maximum distance, wherein a screw line is defined as the infinite line on which a screw line segment lies, according to any of the claims 6 - 11.

Method according to claim 13, wherein the step of determining the second screw trajectory comprises determining a position of the second screw trajectory, for instance the position of the head of the screw or a point based on the maximum distance from the first trajectory, and adjusting, by input of the user, the length and orientation of the second screw trajectory.

Method according to any of the preceding claims, wherein the step of determining the screw trajectories comprises determining a plurality of said pairs of screw trajectories.

Method according to any of the preceding claims, wherein the step of determining the screw trajectories comprises defining a first set of screw trajectories extending substantially parallel to each other and defining a second screw trajectory, or set of second screw trajectories extending substantially parallel to each other, which extends non-parallel with mutually crossing orientation, both with respect to the first set of screw trajectories.

17. Method according to any of the preceding claims, comprising the step of identifying a first surface in the device for designing therein a first screw receiving element positioned and oriented in accordance with one of the trajectories of a pair, the one trajectory being substantially perpendicular to the first surface, and identifying a second surface in the device, the surface being substantially perpendicular to the other trajectory of the pair, for designing therein a screw receiving element positioned and oriented in accordance with said other trajectory.

18. Method according to claim 17, comprising first the step selecting the first surface in the device, wherein the step of planning the screw trajectories in a pair is based on the selected first surface such that one of the trajectories is substantially perpendicular to the selected first surface.

19. Method for manufacturing a device arranged for joint replacement according to any of the preceding claims, wherein the step of designing comprises designing a cup region arranged to receive another part of said joint and designing at least one flange region, wherein one screw trajectory of a pair is planned to originate in said cup region and wherein the other of the screw trajectories of the pair is planned to originate in said flange region.

20. Method according to any of the preceding claims, wherein the step of manufacturing

comprises using a three-dimensional printing technique.

21. Method according to any of the preceding claims, wherein said step of determining one or more screw trajectories using a planning further comprises taking into account one or more of the following criteria:

- obtaining an optimal number of non-intersecting drill directions for screw trajectories;

- ensuring that said screw trajectories run through bone volume with the optimal

available quality;

- ensuring optimal screw trajectory length; and

- ensuring that the surrounding healthy soft tissue is optimally preserved.