MX2008005811A - A method of reducing loading failure for a prosthetic component - Google Patents

Abstract

A method for reducing prosthetic loading failure including the steps of providing a prosthesis for a vertebral column comprising at least an upper part for attachment to an upper vertebrae and a lower part for attachment to a lower vertebrae, the upper part having a lower curved surface and the lower part having an upper curved surface, wherein the upper and lower curved surfaces have a centre of radius of curvature offset rearwardly with respect to a central vertical axis through the upper and lower vertebrae, and positioning the centroid of at least one of the upper and lower parts substantially on the same vertical axis of the centre of radius of curvature.

Description

METHOD TO REDUCE THE FAILURE BY LOAD PAPA A PROSTATE COMPONENT FIELD OF THE INVENTION The present invention relates to a prosthesis for use in a skeletal structure. In one application, the invention relates to a prosthesis for use as an artificial intervertebral disc predominantly, but not exclusively for use in the spine of humans.

BACKGROUND OF THE INVENTION Intervertebral discs in humans maintain a junction between adjacent vertebrae of the spine. They must satisfy numerous important functions that include supporting the load and cushioning the impact forces. In addition, it must allow a complex pattern of movement and resist various stresses, pure or combined, in the sagittal, coronary and axial planes. Assisted by the mucoligamentous structures that surround the spine, the intervertebral disc should also help maintain normal alignment of the vertebrae of the spinal column. An ideal replacement for an artificial disk will accurately reproduce all disk functions - - intervertebral. However, although there have been many different artificial discs which have been described and tested, until now all have failed to reproduce the capabilities of an intervertebral disc. Typical failures of previous artificial discs have included loss or dislocation of vertebral fixation, premature wear of materials or structural failure, poor duplication of spinal or physiological segmental movement, and predisposition to loss of normal neutral vertebral alignment. An important aspect of the normal movement of the spinal column and the kinematics of the various methods of intervertebral movement is the behavior of the segments of movement during the movements of flexion and extension in the sagittal plane. Fundamental to kinematics is the location of the instantaneous rotation axis (IAR). The IAR varies from level to level within the spinal column and through the movements of flexion and extension for any given segment (level) of movement. A type of spinal disc prosthesis is described in the U.S. patent. 5674296. The disclosed stent consists of a resilient body having a generally elliptical shape. The endoprosthesis is fixed between the adjacent upper and lower vertebrae through the - - L-shaped support, each one confronted with concave-convex legs for coupling the thicknesses of adjacent bone section on a surface and which retain the resilient endoprothesis between them. The endoprosthesis is located centrally between the superior and inferior vertebrae to allow the central pivoting of the superior vertebrae in relation to the inferior vertebrae. In addition, from the above, a joint and a seal are located in the anterior and posterior regions between the vertebrae to seal the endoprosthesis in its position between the superior and inferior vertebrae. The Patent of E.U.A. No. 5556431 describes another type of intervertebral disc endoprosthesis in which the upper and lower plates are used instead of the L-shaped supports of the U.S. patent. identified in the above. The disclosed endoprostheis includes a core which has spherical upper and lower surfaces which, from the drawings, appear to be aligned with a central vertical axis through the upper and lower vertebrae. In contrast to the document of E.U.A. 5674296, the prosthesis core of this patent has a border edge which limits the range of movement of the core and ensures even under extreme conditions, cohesion of the prosthesis This patent also discloses the displacement of the articulation center and the prosthesis towards the rear in relation to the center of the vertebral end plates so as to provide sufficient space in the ventral edge area of the upper and lower plates of the prosthesis. way that allows the reception of screws attached to the bone. Other artificial prostheses seek to reproduce the normal variation in the location of the IAR using various mechanisms that include the use of deformable viscoelastic cores. An example of this is shown in the patent of E.U.A. No. 5824094. Unfortunately, this type of artificial discs is subjected to wear and premature materials and failure by tension. In addition, artificial discs with metal springs have not yet found their use in the clinical setting. All of the artificial discs described in the above have inherent problems which eventually generate unnatural tensions and resultant pain for a receiver of the artificial disc implant. The present invention provides an alternative prosthesis which aims to mitigate at least some of the problems associated with prior art prostheses.

The co-pending application of the applicant identified as application number 2005901682 and entitled "A Prosthesis", is incorporated herein by reference. A prosthesis is described in this patent application comprising an upper portion for attachment to an upper vertebra, a lower portion for attachment to a lower vertebra and a middle portion that is located between the upper and lower portions. The radius of curvature center of the coacting surfaces of all the parts is deviated backwards with respect to the central vertical axis through the upper and lower vertebrae. The upper part which is in the form of an upper end plate typically has an upper surface which is connected to the lower surface of a superior vertebra. Research has shown that the upper end plate can suffer from descent. Typically, plate failure occurs by tilting with the posterior face of the prosthesis descending. It has not been shown that the anterior face descends. In addition, said descent occurs almost exclusively in the upper rear corner of the upper plate.

BRIEF DESCRIPTION OF THE INVENTION In accordance with the present invention, there is provided a method for reducing load failure for a prosthetic component such as an upper end plate. The method is also applicable to a lower end plate. According to one aspect of the present invention there is provided a method for reducing failure by prosthetic loading that includes the steps of providing a prosthesis for the spine comprising at least one upper portion for attachment to an upper vertebra and a lower portion for attachment to a lower vertebra, the upper part has a lower curved surface and the lower part has a superior curved surface, providing the upper and lower curved surfaces with a center of radius of curvature deviated backward with respect to a central vertical axis to through the upper and lower vertebrae, and placing the centroid of at least one of the upper and lower portions substantially on the same vertical axis of the center of the radius of curvature. Preferably, the method includes the step of placing the centroid of a contact surface of at least one of the upper and lower portions substantially on the same vertical axis as the center of the radius of curvature, with the contact surface contacting an adjacent vertebra. Preferably, the prosthesis includes a middle part which is capable of rotating and moving with respect to the upper and lower parts. According to another aspect of the present invention, there is provided a prosthesis for a skeletal body comprising an upper surface and a lower surface, wherein one of the surfaces is a contact surface configured for contact with an adjacent surface of a body part. Skeletal, the contact surface has a centroid located substantially vertically aligned with the center of rotation of the prosthesis. Preferably, the prosthesis is offset with respect to a central vertical axis through the upper and lower skeletal body parts. The prosthesis may comprise an end plate for attachment to the skeletal body part. The prosthesis preferably comprises a top or bottom end plate for attachment to the skeletal body part. The prosthesis may comprise an insert configured for location between an end plate and a part of the skeletal body.

The skeletal body part can be a vertebra. It is preferred that the skeletal body include any skeletal structure for a biological or mechanical structure. It is preferred that a prosthesis refers to any component which is designed to replace parts of a skeletal structure, which simulates or improves the movement of a skeletal structure. According to another aspect of the present invention, there is provided a method for reducing the load failure for a prosthetic component, comprising: identifying the center of the radius of curvature for a prosthesis, wherein the center of the radius of curvature is deviated from a central vertical axis through a skeletal structure, identify a vertical axis through the center of radius of curvature and configure the upper prosthetic part or a lower prosthetic part with a centroid for its contact surface, which is located substantially on the central vertical axis when in situ. According to a further aspect of the present invention, there is provided a method for reducing load failure for a prosthetic component, comprising identifying the center of rotation of curvature for a prosthesis, identifying an equivalent location for the center of rotation of curvature on a contact surface of an upper and lower prosthetic part, contact surface which is configured to be fixed to a skeletal part and to configure the contact surfaces with a centroid substantially in an equivalent location for the center of rotation of curvature. Preferably, the configuration step includes configuring a surface of the skeletal part which is the surface to which the contact surface is attached. The configuration step preferably includes designing, manufacturing, producing, manipulating and any equivalent action resulting in a contact surface with a centroid that is substantially located in the equivalent location. The method may include the step of supplying a prosthesis comprising at least two parts including an upper part and a lower part which are capable of pivoting with respect to each other when they are used. The method also includes supplying a prosthesis with a core which is capable of pivoting or moving with respect to the upper and lower parts.

- - The upper part may be able to slide (move with respect to the upper part). The upper or lower prosthetic part may comprise an insert. The upper or lower prosthetic portion preferably comprises an end plate or an insert. The insert can be configured to accumulate a relevant portion of a top surface of an upper part or a lower part of the lower part. The contact surface may comprise an upper surface of the upper part or a lower surface of the lower part. The method may include trimming a part of the contact surface. The method may include separating a portion of the contact surface. The configuration step may include supplying the contact surface with a recessed region. Preferably, the recessed region is located between the opposite sides of the contact surface. The method may include trimming a portion against the centroid. The method may include creating a recess between - - opposite sides and an anterior portion of the upper surface. Preferably, the method includes forming substantially identical side portions which are separated by a recess. The recess can be rectangular. The side portions may have substantially parallel edges. The side portions may have a generally rectangular cross section. The anterior portion may comprise a plurality of recesses or holes. It is preferred that according to at least one of the above aspects of the invention that if descent occurs for an upper end plate in a prosthesis, this happens without inclination and that it is a parallel descent. According to one aspect of the present invention, there is provided a method for reducing load failure for a prosthetic component, comprising identifying the center of rotation of curvature for a prosthesis, identifying an equivalent location for the center of rotation of curvature on a contact surface of a prosthetic upper or lower part, contact surface to which it is configured for fix to a skeletal part and configure the contact surface with a centroid which moves towards the equivalent location for the center of rotation of curvature. Although it is desirable to configure the contact surface with a centroid substantially in an equivalent location for the center of rotation of curvature, a reduction in load failure can be obtained by any movement of the centroid toward the vertical axis will be aligned with the center of rotation of curvature of the skeletal part. Preferably, the method includes separating part of the prosthetic component so that it moves the centroid to a position which is closer to the center of rotation of curvature. It is preferred that the method includes creating a recess between opposing sides of an anterior person of a top surface of at least one upper and lower prosthetic portion.

BRIEF DESCRIPTION OF THE DRAWINGS A preferred embodiment of the present invention is now described by way of example only, with reference to the attached Figures 1 to 9D, in which: Figure 1 shows a schematic side view of a prosthesis; Figure 2 shows the prosthesis shown in Figure 1 with a non-uniform print distribution; Figure 3 shows a top view and a top plate of a prosthesis according to a preferred embodiment of the present invention; Figure 4 shows a graphic representation of the parameter b in figure 3 versus in surface area; Figure 5 shows a top view of a proposed cut to prove that it has a net moment of zero; Figure 6 shows a comparison of load displacement curves for an unmodified prosthesis and a prosthesis as shown in Figure 5; Figure 7 shows a perspective view of an upper end plate of a prosthesis according to an embodiment of the present invention; Figure 8A shows a perspective view of an end plate insert for use with a lower plate prosthesis according to an embodiment of the present invention; Figure 8B shows a top view of an endplate insert shown in Figure 8A; Figure 8C shows a front view of the endplate insert shown in Figure 8A; Figure 8D shows an end view of the endplate insert shown in Figure 8A; Figure 9A shows a perspective view of an end plate insert according to a further embodiment of the present invention; Figure 9B shows a top view of the end plate insert shown in Figure 9A; Figure 9C shows a front view of the endplate insert shown in Figure 9A; and Figure 9D shows an end view of the end plate insert shown in Figure 9A.

DETAILED DESCRIPTION OF THE DRAWINGS In accordance with a preferred embodiment of the present invention, the center of instantaneous rotation (ICR) needs to be in the posterior portion of the upper end plate of the lower vertebral body. The inability to obtain this position will impede the normal movement of the prosthesis and the movement of the facet will be abnormal. So far little attention has been paid to statically loaded disc prostheses - in other words, when it is not in motion. This is the position in which the implant finds itself the most part of the time In this position, the neuromuscular control system recruits any muscle that is necessary to maintain a static posture. In a situation where a person is standing in a static position, the upper and lower end plates are parallel (figure 1). Assuming that the applied load is distributed evenly across the surface of the implant will have a uniform pressure distribution. If this is the case with the fingerprint adrift there would be an unbalanced net moment that causes the prosthesis to tilt before flexion and descend to the posterior end plate. This can be perceived conceptually by considering a series of small areas of the prosthesis end plate. If the pressure distribution is uniform, the strength of these small areas is equal. However, the moment around the pivot point of the prosthesis depends on the distance of the small area from the pivot. If all the moments are added together, there will be a moment of net bending. Another way to consider this is that the center of mass of the drifting prosthesis faces the pivot point and thus causes a net moment. This will make the pressure distribution uneven. A non-uniform pressure distribution can cause the prosthesis to fail when descending on the plate. rear end (figure 2). To solve the previous problem, the upper end plate has been designed with a sufficient amount of the surface area removed from the front of the prosthesis so that the net momentum is zero. This results in a substantially uniform pressure distribution. Figure 3 shows an embodiment of the invention in which the upper surface of the upper end plate has a rectangular "cutout" from the front of the drift end plate. As shown, the area which has been generally separated is rectangular in shape and, as more clearly shown in Figure 7, comprises lobes 11, 12 above with a space 13 therebetween. The posterior region of the prosthesis remains substantially the same as the existing prosthesis upper end plate. Similarly, the lower surface has a lower center of radius of curvature according to the requirements of a previously designed prosthesis. For a pure mathematical analysis of the effects of the surface area that is removed from the anterior region of the prosthesis, it is necessary to consider a uniform pressure distribution that acts on the entire surface area. In each small area - - a uniform force will act. With reference to the first principles, a mathematical analysis for the prosthesis can be provided as shown in Figure 3 based on those moments that are balanced around the pivot line. Therefore, the moments that act behind the pivot equals the moments that act on the frof the pivot. The resulting mathematical equation is as follows: Fa 2b Fe2. (f - 2b) = Fe2 2 + 2 2 2a2b + c2 (f - 2b) = e2f c2 (f - 2b) = e2f - 2a2b 2 e2f - 2a2b C "(f - 2b) Since the parameters a, e and f are fixed by the dimensions of the drifting prosthesis, equation 1 generates the relation between c and b that will result in a number - - of solutions that will still have balanced moments. The total cct area will vary according to the equation. area = ef + 2ab + cd Using a reasonable calculation for parameter "b" of 7 mm, the shape in figure 4 is subjected to engineering drop tests against saw bone foam. The center of mass in the y-axis for the above diagram is calculated as 10.6 mm from the posterior edge of the prosthesis using the formula: Mo? « where MO = moment and a = area. This also suggests that the prosthesis can not be tilted when loaded. The hypothesis is further established that although the deformation load should decrease due to the reduction in surface area to some extent, this would be compensated by avoiding a high load on the posterior edge of the prosthesis due to a non-uniform pressure distribution. . This can be confirmed by a finding of a lower percentage reduction in load - - of failure than would be expected by the reduction only in the surface area. The preliminary test when compressing the disc prosthesis to drift in soft foam suggests that the inclination occurs and that it is associated with a translatory movement. The load displacement characteristics of the drift disk is tested against the saw bone material using a roller bearing to allow lateral displacement and to compare the results with at least one machined surface of each shape shown in the figure 5. The experimental results show the following: 1. Under load conditions that do not allow the upper end plate to flex relative to the lower end plate, the drifting disc prosthesis collapses in a flexion position when descending on the back end plate. 2. The anterior edge of the prosthesis does not make cct with the saw bone and grooves appear to stop 10 mm in frof the sphere (equal to the distance of the sphere from the posterior edge). 3. The modified prosthesis cut area is 69% of the area of the original drift prosthesis. Although the deformation load is 89% of the original. 4. The descent by inclination of the unmodified prosthesis is associated with 2 mm of posterior displacement of the lower vertebrae in relation to the fixed upper vertebrae. This can cause additional facet loading. 5. Modified trimming prosthesis does not tilt down and collapses on the end plate with its end plate parallel to the end plate of the saw bones. This is carried out with very little lateral displacement (0.15 mm). Figure 6 shows a comparison of the load displacement graphs of an unmodified prosthesis compared to a modified prosthesis as shown in Figure 5. From this comparison the following observations can be made. 1. The peripheral force and relative weakness of the central portion of the endplate means that if an anterocentral trimming is performed it should cause less reduction in the performance of the total endplate than if the material were uniform. This can increase the discrepancy between the deformation load that is obtained and that expected by reduction only in the surface area. 2. The rear end plate is more - 1 - resistant than the previous end plate. The center of the mass of the prosthesis therefore does not need to be exactly in the axis of rotation of the prosthesis and therefore the trimming that may be needed is less. 3. When adding more lateral area near the pivot point the contact surface area will increase with little effect in the net moments. A more rounded prosthesis will also have a center of mass closer to the pivot point. It is known that the vertebral end plate is in the form of a cardioid2. 4. The effect of the cyclic loading is not known. It is possible that it is significant with high cyclic loads at the posterior edge of the prosthesis and that the effect indicated above can be exaggerated. Based on the foregoing, it is considered that, in addition to redesigning the upper surface of the upper end plate, an alternative strategy for providing an insert plate configured to move the centroid of the combined end plate and insert should be provided. According to another embodiment of the invention, a lower end plate with an insert plate can be provided to avoid redesigning the lower surface. Figures 8A to 8D show one embodiment of the lower end plate which has a region 20 - central spherical which is coupled with an upper end plate having a similar shaped recess (receptacle) formed on its lower surface. This insert plate can be attached to the adjacent vertebrae. The back region 16 of the plate 15 can effectively cover most of the rear section of the lower end plate and the anterior region 17 can be provided with side lobes 18 and 19 to effectively accumulate the sides of the lower end plate. The result will be an effective recess that is formed between the front sides of the lower end plate. This can result in effective movement of the centroid for the lower end plate towards the center of rotation of the prosthesis. Figures 9A to 9D show an alternative embodiment of a lower end plate 30 which generally has a more rectangular shape than the previous embodiment. Beneath the insert plate is a festooned external region 31, provided above with its center aligned with the center of the portion 32 of the hemispherical sphere on its upper surface. As shown in Figures 9A and 9D, the scalloped outer region 31 begins at a forwardmost end of the insert and curves concavely towards back to the lower surface more than half the way along the length of the end plate (measured from the front to the rear) to a point which is rearward of the center of the portion 32 of the hemispherical sphere. This effectively moves the centroid backwards. Figure 9A shows that the scalloped region is partly circular in shape. Therefore, it can be seen that the modality shown in Figures 9A to 9D uses a scalloped outer region to provide a change in centroid position while the modality shown in Figures 8A to 8D achieves the same or similar purpose in shaping the insert with lateral loads 18, 19. The inserts shown in Figures 8A to 9D can be configured to be coupled with an upper end plate in a manner similar to that shown in Figures 1 and 2. Since the design of both embodiments shown in Figures 8A to 9D result in a change in the position of the centrolide of the lower end plate, this may have advantages as indicated previously when considering the previous experimental results. According to an alternative embodiment of the invention, the lower end of the vertebrae adjacent to the upper end plate can be physically altered so that the recess is provided in an anterior center section to provide an effect similar to that described above in providing a recess in the anterior center region of the upper end plate. In accordance with alternative aspects of the present invention, the basis of the same theory can be applied to the lower end plate of a prosthesis. It is understood that, if any prior art publication referred to herein, such a reference does not constitute an admission that the publication forms part of the general common knowledge of the art, in Australia or in some other country. In the claims that follow and in the foregoing description of the invention, unless the context otherwise requires due to the expression of the language or the necessary implication, the word "comprises" or variations such as "comprising" or "understood" "are used in an inclusive sense, that is, to specify the presence of established features but not to prevent the presence or addition of additional features in the various embodiments of the invention.

Claims (26)

1. Method for reducing the failure by prosthetic load that includes the steps of providing a spinal prosthesis comprising at least one upper part for attachment to an upper vertebra and a lower part for attachment to a lower vertebra, the upper part has a surface curved lower and the lower part has a curved upper surface, wherein the upper and lower curved surfaces have a center of radius of curvature deviated backwards with respect to a central vertical axis through the upper and lower vertebrae, and centroid placement of at least one of the upper and lower portions substantially on the same vertical axis of the center of radius of curvature. Method as described in claim 1, characterized in that it includes the step of placing the centroid of a contact surface of at least one of the upper and lower portions substantially of the same vertical axis as the center of radius of curvature, with the contact surfaces making contact with the adjacent vertebrae. 3. Method as described in claim 1 or 2, wherein the prosthesis includes a middle part which is able to pivot and move with respect to the upper and lower parts. 4. Prosthesis for skeletal body comprising an upper surface and a lower surface, with one of the surfaces being a contact surface configured for contact with an adjacent surface of a skeletal body part, the contact surface having a localized centroid aligned substantially vertically with the center of rotation of curvature of the prosthesis. 5. Prosthesis as described in claim 4, including an end plate for attachment to a skeletal body part. Prosthesis as described in claim 4, comprising a top or bottom end plate for attachment to a skeletal body part. 7. Prosthesis as described in claim 4, comprising an insert configured for location between an end plate and a skeletal body part. 8. Prosthesis as described in claim 4, wherein the upper and lower surfaces are configured so that the prosthesis is deviated from the central vertical axis through portions of the prosthesis. Upper and lower skeletal body. 9. Method for reducing load failure for a prosthetic component, comprising: identifying the radius of curvature center for a prosthesis, wherein the center of radius of curvature is deviated from a central vertical axis through a skeletal structure, identify a vertical axis through a center of radius of curvature and configure an upper prosthetic part or a lower prosthetic part with a centroid for its contact surface, which is located substantially on the vertical axis when in situ. Method as described in claim 9, wherein the configuration step includes configuring a surface of the skeletal part which is the surface to which the contact surface is attached. The method as described in claim 9, which includes the step of providing a prosthesis comprising at least two parts including an upper part and a lower part which are capable of pivoting with respect to each other when used . 1

2. Method as described in claim 11, which includes providing a prosthesis with a core which is capable of pivoting and / or moving with respect to the upper and lower parts. The method as described in claim 11, wherein at least one of the upper and lower prosthetic portions comprises an insert. Method as described in claim 11, wherein the contact surface comprises at least one of an upper surface of the upper part and a lower surface of the lower part. 15. Method as described in claim 11, which includes trimming a part of the contact surface. 16. Method as described in claim 11, wherein the configuration step includes supplying the contact surface with a recessed region. 17. Method as described in claim 16, wherein the recessed region is located between opposite sides of the contact surface. 18. Method as described in claim 9, which includes the step of trimming a portion against the centroid. 19. Method to reduce load failure for a prosthetic component, which comprises identifying the center of rotation of curvature for a prosthesis, identifying an equivalent location for the center of rotation of curvature in a contact surface of a superior or inferior prosthetic part, contact surface which is configured to be fixed to a skeletal part and to configure the contact surface with a centroid which is more near the equivalent location for the center of rotation of curvature. The method as described in claim 19, which includes the step of creating a recess between opposite sides and an anterior portion of a top surface of at least one of the upper and lower prosthetic portion. 21. Prosthesis as described in claim 4, comprising a recessed region that is located between opposite sides of the contact surface. 22. Apparatus as described in claim 21, wherein the recess is located between opposite sides of an anterior portion of the prosthesis. 2

3. Prosthesis as described in claim 22, comprising substantially identical side portions separated by the recess. 2

4. Prosthesis as described in claim 22, wherein the anterior portion comprises a plurality of holes. 2

5. Prostheses as described in claim 4, wherein a surface opposite the contact surface has a radius of curvature configured to coincide with the radius of curvature of a prosthetic core part, the opposite surface is capable of articulation. 2

6. Method as described in claim 9 or 19, wherein the prosthetic component comprises side portions separated by a recess in an anterior portion of at least the upper surface thereof.