GB2480682A

GB2480682A - Temporomandibular joint prosthesis

Info

Publication number: GB2480682A
Application number: GB1008989A
Authority: GB
Inventors: Eric Abel; Philip Martin Mcloughlin
Original assignee: Individual
Current assignee: Individual
Priority date: 2010-05-28
Filing date: 2010-05-28
Publication date: 2011-11-30
Also published as: GB201008989D0

Abstract

A temporomandibular joint replacement prosthesis with a condylar component 40 that abuts a resected condyle 43 and provides the principal means of load bearing for forces transmitted across the replacement joint. A fixation region 41 may serve to hold the condylar component 40 in place when acted upon by forces parallel to the planar surface contact region 44. The fixation means may be screw, clip or clamping means. A modified condylar component may serve as a cutting guide for resecting the condyle 43 before the condylar component 40 is attached.

Description

HELD OF THE D4VENTTON The present invention relates to the design of a temporomandibular joint prosthesis

BACKGROUND TO THE ThWENTION

A temporomandibular joint (TMJ) prosthesis is a structure that replaces all or part of the temporomandibular joint, commonly known as the jaw joint. A schematic representation of the temporomandibular joint is shown in Figure 1. The temporomandibular joint 1 is the articulation between the condyle 2 of the mandible 3 (also known as the jaw bone), and the fossa 4 of the temporal bone 5. Then temporomandibular joint is a complex joint, both anatomically and biomechanically. Its wide range of motion enables speaking, chewing, grinding and other actions. Damage to or disease of the joint can cause severe pain and degeneration of the joint, which may require the replacement of the articulating regions of the joint. Tf both the articulating region of the condyle 2 and the articulating region of the fossa 4 are replaced with implanted parts then the prosthesis is known as a total TMJ prosthesis. Tf one of these regions only is replaced then the prosthesis is known as a partial TMJ prosthesis. The present invention relates to both total and partial TMJ prostheses.

A substantial proportion of temporomandibular prostheses are used for women of child bearing age, the reasons for which are not fully understood. Their life expectancy may be forty to fifty years from the time a first prosthesis is implanted. Even with the best materials currently available for use in the articulating surfaces of joint prostheses in the human body, such as cobalt-chrome alloys and high density polyethylene, prosthetic joints have a typical normal lifespan of fifteen to twenty years. Most patients who have a temporomandibular joint prosthesis implanted will therefore be expected to require at least one additional TMJ prosthesis, known as a revision prosthesis, during their lives.

During the surgical implantation of a total or partial TMJ prosthesis, some bone in the region of the joint is removed to accommodate the prosthesis. II is highly desirable for a TMJ prosthesis to require the removal of as little bone as possible during implantation so that in the event of the prosthesis having to be removed and replaced with a revision prosthesis there is as much of the patient's original bone as possible remaining for attachment to and fixation of the revision prosthesis.

Previous and currently manufactured versions of the condylar component require removal of all the condyle 2 and part of the ramus 6 (in Figure 1), much of this removed bone being healthy. The need for its removal is not necessary and is carried out because of limitations associated in the prior of condylar components of TMJ prostheses. Examples of such condylar are disclosed in US Patent 5445650 (Nealis), US Patent 5549680 (Gordon), US Patent 6132466 (Hoffmann), US Patent 5405393 (Falkenstrom) and US Patent 4917701 (Morgan). A schematic illustration of a condylar component representing the prior art is shown schematically in Figure 2, in which the extent of the bone removed is represented by the dashed line. The condylar component 20 comprises a condyle 21, a neck 22 and screw fixation area 23 containing holes 27 for screws to fix the condylar component 20 to the ramus 24. The condyle 25 is removed and part of the ramus 26 is also removed during implantation, even if the lower part of the condyle and the ramus are not diseased or damaged. II would be desirable if a condylar component could be configured such that it required only removal of the diseased bone of the mandible, which is normally only in the region of articulating surface of the condyle.

In the natural temporomandibular joint muscular actions generate mechanical forces, which are borne by the area of contact at the joint at the condyle and the fossa. A large proportion of the transmifted forces are compressive forces. It would be desirable for this natural means of force transfer across the joint to be replicated in a TMJ prosthesis. This is not possible in the prior art because the condyle is removed, so the natural route of force transmission across the joint being bypassed and the forces are instead transferred to the mandible at the ramus 24 via the screws and at the areas of contact of the prosthesis with the ramus 24. This means of transmission of forces to the mandible causes shear forces and stresses at the contact areas between the condylar component and the bone, which are known to be undesirable at interfaces between a prosthesis and a bone, partly because bone is stronger when compressed than when sheared.

II would be preferable for the condylar component of a TMJ prosthesis to be of a configuration whereby as much of the condyle as possible could be retained, with only the diseased bone removed and for the condylar component of the prosthesis to rest on the remaining part of the condyle so as to enable natural, physiological loading as closely as possible rather than to transmit the joint forces to the ramus. This could have the benefit of a smaller prosthesis as there would not be the need to provide such a sizeable structure as that disclosed in the prior art and illustrated in Figure 2 to support the high shear stresses that occur at the interface between the condylar component and the ramus.

There are several benefits of using a smaller size of condylar component than that which has been disclosed in the prior art. In the currently performed surgical implantation procedure, two incisions on the face are required for implanting a total TMJ prosthesis, the locations of which are shown in Figure 3. There is a pre-auricular incision 31 is commonly about 40 mm long for accessing the fossa and the condyle and a submandibular incision 32 is commonly about 60 mm long for accessing the ramus of the mandible. If the condylar component could be made smaller yet still provide an adequate structural support, then the submandibular incision could be shortened in length or even become unnecessary if the condylar component were small enough to be inserted through a pre-auricular incision 31. A smaller surgical incision reduces trauma to the muscles and other tissues and the risk of paralysis of the facial nerve, which commonly occurs during temporomandibular joint replacement surgery. Although the paralysis is usually temporary it may last several weeks and is distressing for the patient. The extent of facial scarring is reduced and subdermal tissue scarring is also reduced, the former being important for the patient's appearance and the lafter being important for retaining as much as possible the integrity of the tissues around the joint.

A ftirther benefit of a smaller condylar component is that less muscle needs to be removed from the ramus of the condyle to make space for the condylar component to abut the bone with the consequence that muscle function is less adversely affected and there is less post-surgical morbidity, which helps the prospect of improved restoration of the range of motion of the joint after implantation of the prosthesis and quicker post-operative recovery for the patient.

A temporomandibular joint prosthesis should not result in facial deformity due to its bulk or shape. The components of the prosthesis must therefore conform well to the contours of the patient's face. Manufacturers of temporomandibular joint prostheses therefore tend either to have a very large inventory of sizes and shapes to enable a reasonably good prosthesis fit, or preferably to produce customized prostheses for the individual patient based on data from computerized tomographic images. Both methods are used in current practice. The most common method for customization of prosthetic joint components, entails the initial fabrication of a plastic skull or part-skull containing the temporomandibular joint using three-dimensional rapid prototyping printing methods using data from computerized tomographic x-ray images. The parts of the fossa component and the condylar component are fabricated to fit the model and with minor modification at the time of surgeiy can be an accurate fit to the patient. The machining and fabrication processes for these customized prostheses are time consuming to perform but latest methods of computer aided design and computer aided manufacturing are not always possible because of the high level of complexity of the component parts. It would be desirable for TMJ prosthesis components to be easier to manufacture using modern computer based methods.

It is among the objects of the present invention and its embodiments to seek to address

these and other limitations in the prior art.

SUMMARY OF THE INVENTION

According to one aspect of the present invention there is provided a condylar component for a total or partial TMJ prosthesis part of which is in contact with the residual bone of a partially resected condyle of in which only the diseased part or a minimal additional amount of bone of the condyle is removed and such that forces transmifted across the temporomandibular joint pass through the area of contact between the residual condylar bone and the condyle of the condylar component.

The condylar component is manufactured as a customized fit to the mandible of an individual patient using anatomical reference data from computerized tomographic x-ray or other three-dimensional tomography or a combination of tomographic methods. The method of fabrication of the condylar component may be by rapid prototyping making use of a plastic or similar model for creating the desired shape or preferably using computer aided design methods to generate data for automated manufacture of the component. The fossa component is preferably a surface replacement of the fossa surface as described generally in US Patent 3178728, now expired, but advantageously produced to a high level of congruency with the fossa of the patient by customization methods, to provide a close fit to the fossa. Currently produced fossa components rarely achieve a wide area of contact between the fossa and the fossa component and may achieve no more than three points of contact. The manufacturing methods used in the present invention provide a good fit with many points of contact, thereby conferring improved stability of the condylar component in the fossa and enabling a reduction of area needed for screw fixation into the fossa eminence.

In all embodiments of the present invention the condylar component makes contact with the resected condyle and fixation means of holding it in this place are provided. The fixation means may be to the ramus, it may be a clip or clamp or other fixation means to the underlying face of the resected condyle in the region where it joins to the ramus or it may be other fixation means. In the case of a clamp or a clip fixation means the clamp or clip may be in an open position initially and then closed once the condylar component is in place on the resected condyle. A clamp of clip arrangement may be made from any suitable material but a preferred material is a superelastic alloy such as nitinol or other superelastic material, which has the advantage of being highly deformable.

The condylar component may be configured to include a shaped peg or other projection for fitting into the residual condylar bone to provide a means for holding the condylar component in place. The peg or other means may be combined with other fixation means to hold the condylar component in place.

The condylar component may be fabricated from a single material such as a cobalt chrome alloy or high density polyethylene, materials which have been shown to be effective for articulating surfaces of TMJ replacements. The condylar con1ponent may comprise more than one material, for example high density polyethylene in combination with a supporting structure of titanium or titanium alloy both of which are known for their compatibility with bone. The material of the condylar component may be coated with a material such as titanium or titanium alloy or hydroxyapatite or other material with desirable biomechanical properties.

According to a second aspect of the present invention a modified condylar component also having a customized shape is provided to be used as a template to enable an accurate resection of the diseased condylar bone to be carried out, after which the modified condylar component is removed and replaced by the condylar component. The modified condylar component has part or all of its superior portion replaced with a cutting guide for accurately positioning and guiding the trajectory of a cutting device such as a saw or other cutting tool to enable the cutting tool to be positioned, aligned, supported and guided while it traverses the cutting guide, to enable the operator to cut the bone of the diseased condyle to produce an accurate straight or profiled cut in the condlyar bone at the desired place and plane of orientation on the bone. The modified condylar component is held in place on the mandible using fixation means that may be the same as or similar to the fixation means used in the corresponding condylar component or it may incorporate alternative fixation means, provided it locates accurately to cut the diseased condyle in the place required for the customized condylar component such that the resected condyle makes contact with the condylar component.

a preferred embodiment of the second aspect of the invention, the modified condylar component has the same customized shape as that of a condylar component represented by the first aspect of the present invention and contains a corresponding similarly located screw attachment means for locating it to the ramus of the mandible. Undulations in the shape of the surface of the ramus allow the modified condylar component to interlock with the ramus, after which the modified condylar component is screwed into position into the ramus in its interlocked position. A cutting tool, such as a power saw, is used to resect the diseased condyle, after which the modified condylar component is removed and the replaced by the condylar component, fixing it in place using screws in at least the same screw holes are used to fix it to the mandible.

The modified condylar component is disposed of after the condylar component is fitted so may be made of a different, preferably cheaper material than that of the condylar component, such as medical grade stainless steel or a polymer or a combination of two materials.

Figure 1 illustrates the location and anatomical features of the temporomandibular joint Figure 2 illustrates a representative example of prior art of the condylar component Figure 3 illustrates the two incisions used in the implantation of prior art TMJ prostheses.

Figure 4 illustrates a first preferred embodiment of the present invention Figure 5 illustrates a second preferred embodiment of the present invention Figure 6 illustrates a third preferred embodiment of the present invention Figure 4 shows a condylar component 40 held in place by a screw fixation region 41 smaller than that used in the prior art and represented in Figure 2. The screw fixation region is attached to the ramus 42. The underside surface of the condular component 40 is abutted to the resected surface of the resected condyle 43 such that a planar surface contact region 44 between the two parts acts as the primary load bearing surface for the condylar component 40. The screw fixation region 41 acts to hold the condylar component 40 in place when acted upon by forces parallel to the planar surface contact region 44.

Other embodiments of condylar component with different shapes of contact region may be used to replace the planar surface contact region 44 in Figure 4, with shapes that are for example corrugated, domed or recessed, to provide better interlocking at the contact region and to reduce further the reliance on screws or other means of fixation to hold the condylar component 40 in place. Appropriate cutting instruments can readily be envisaged without further explanation that can cut these surface profiles and others like them.

Figure 5 shows another embodiment of condylar component 50, being smaller in size and with screw fixation means 51 for holding it to the region below the resected condyle 52.

Figure 6 shows an embodiment of a condylar component that employs a spring arrangement 61 containing one or more springs to grip the surface of the resected condyle 62. The spring arrangement 61 is held in an open position while the condylar component is placed on the resected condyle, after which the spring arrangement 61 is allowed to close on to the resected condyle 62. In a preferred embodiment illustrated in the figure, there are three springs (two of which are shown as 63, the third being out of view) made from a superelastic material such as nitinol, a nickel-titanium alloy commonly use in spring mechanisms in the body because of its proven biocompatibility. The springs shown are cantilevered plates and are customized to fit that part of the resected condyle 62 that they close on to. An alternative preferred embodiment is for the nitinol material to operate in a shape memory mode, whereby the springs can be made to close in a one way transformation by increasing their temperature to a few degrees above human body temperature.

The embodiments of the condylar component described are exemplary and that various modifications may be made without departing from the scope of the present invention.

Figure 7 shows an embodiment of a condylar component 71 and an en1bodinlent of a fossa component 72 of the present invention.

Figure 8 shows an example of an embodiment of a modified condylar component relating to the present invention The modified condylar component 80 comprises a screw fixation means 81 that has the same screw fixation means and location of the fixation screws as the condylar component that will replace it after the modified condylar component has provided its function as a guide for a cutting instrument, in the manner described earlier.

The guide 82 in this embodiment is to enable a saw to pass over its upper surface and remove the diseased bone of the condyle, which in the figure is shown after resection.

The extent of the bone removed is shown by the dotted lines and a planar surface of the top of the resected condyle remains. Other shapes of guide in combination with other cuffing instruments may be used to create different shapes of the resected condyle.

Claims

CLMMS1. A temporomandibular joint (TMJ) prosthesis with a condylar component that abuts or interlocks with the resected surface of a resected condyle to provide the principal means of load transfer between the condylar component and the mandible.
2. A TMJ prosthesis according to Claim 1 that includes a modified condylar component that may be attached to the ramus of the mandible to enable an accurately positioned and orientated cut to be made through the diseased condyle such that after removal of the modified condylar component a condylar component can be attached to the mandible and abut or interlock with the resected surface of the condyle
3. A condylar component according to Claim 1 that is customized to the shape of the patients ramus
4. A modified condylar component according to Claim 2 in which the shape of the contact region with the ramus is the same shape as that of a corresponding condylar component so that fixation means for the modified condylar component can be used for the condylar component to ensure that it is positioned accurately with respect to the resected condylar component for abutting to it or interlocking to it and to provide a close fit between the condylar component and the ramus of the mandible
5. Screw fixation means for securing the condylar component according to Claim 1 to the ramus of the mandible
6. Clip or clamping means for holding the condylar component according to Claim 1 to the bone of the resected condyle below the level at which it is cut
7. Clips made from superelastic material for holding the condylar component according to Claim 1 to the bone of the resected condyle below the level at which it is cut
8. Clips made from a shape memory material for holding the condylar component according to Claim 1 to the bone of the resected condyle below the level at which it is cut Amendments to the claims have been filed as foflows.CLAIMS1. A temporomandibular joint (TMJ) prosthesis in which a condylar component abuts with the resected surface of a resected condyle to provide principal means of load transfer between said condylar component and the mandible; 2. The TMJ prosthesis of Claim 1 wherein the form of abutment is interlocking abutment; 3. A TMJ prosthesis according to Claims 1 and 2 wherein is provided modified condylar component means that may be attached to the ramus of the mandible to enable an accurately positioned and orientated cut to be made through the diseased condyle such that after removal of said modified condylar component means said condylar component may be attached to the mandible and abut or interlock with said resected surface of said condyle; 4. A TMJ prosthesis according to Claim 1 wherein said condylar component is customized to the shape of a patient's ramus; 5. A modified condylar component according to Claim 3 wherein the shape of the contact region with the ramus and positions of location means therein for fixation means therethrough provide accurate template positioning means for said IC) condylar component corresponding to said modified condylar component said corresponding condylar component being provided with substantially identical shape for the ramus contact region and substantially identical positions of location IC) means for fixation means such that said condylar component may be positioned (\J accurately in a close abutting or interlocking fit between said condylar component and the ramus of the mandible; 6. A TMJ prosthesis according to Claims 1 to 4 wherein fixation means for securing said condylar component to the ramus of the mandible comprise screw means; 7. A TMJ prosthesis according to Claims 1 to 5 wherein attachment means for the attachment of said condylar component to the bone of said resected condyle below the level at which it is cut comprise clip means; 8. A TMJ prosthesis according to Claims 1 to 5 wherein attachment means for the attachment of said condylar component to the bone of said resected condyle below the level at which it is cut comprise clamp means;
9. A TMJ prosthesis according to Claims 1 to 7 wherein said attachment means are made from superelastic material;
10. A TMJ prosthesis according to Claims 1 to 7 wherein said attachment means are made from a shape memory material.