Patent Application
20110178606
This disclosure relates generally to surgical devices and procedures, and more particularly, to implantable, total knee replacement prostheses.
The most widely-used type of knee prosthesis for implantation into a patient during a total knee replacement (TKR) procedure includes three components: a metallic, femoral component that attaches to the distal femur; a metallic, tibial component (or tray) that attaches to the proximal tibia; and a polymeric (UHMWPE), insert (also called a bearing or an inlay) that fits between the femoral and tibial components. Various types of patella replacements are also available for use in combination with some of these knee prostheses. Two types of knee prostheses are a posterior-stabilized (PS) prosthesis, for when the posterior cruciate ligament is no longer viable, and a (posterior) cruciate-retaining (CR) knee prosthesis. Each of these two types of knee prostheses may be provided as a fixed bearing knee prosthesis, in which the insert does not move relative to the tibial component, or a mobile bearing knee prosthesis, in which the insert rotates upon a smooth platform of the tibial component. Whether to use a mobile insert or a fixed insert depends largely on the condition of the patient's knee ligaments and other soft tissues.
A knee prosthesis system may include numerous sizes of femoral, tibial and insert components to accommodate the variation of patient anatomies in the worldwide TKR patient population. The design of a knee prosthesis system requires trade-offs among many important factors related to kinematic performance, clinical outcomes, implant longevity, cost, and ease of use, to name just a few. An important consideration relative to both the kinematic performance and the life of the knee prosthesis is the degree of conformity between the femoral component bearing surfaces and the insert bearing surfaces.
Investigators typically characterize conformity in either the coronal plane or sagittal plane as the ratio of the convex radius of a femoral condyle of the femoral component to the concave radius of the interfacing insert surface. A conformity ratio of zero represents a flat insert surface, corresponding to very high contact stress at high loads. A conformity ratio of 0.99 represents high conformity, corresponding, in general, to high contact area, relatively low contact stress and, subsequently, reduced wear rate of the polyethylene surface of the insert.
Investigators have found that conformity in the coronal plane may affect prosthesis life more than conformity in the sagittal plane. For example, in an article by Kuster, et al, “The effects of conformity and load in total knee replacement” (Clinical Orthopaedics and Related Research, Number 375, pp. 302-12, June 2000), the authors found that the compressive surface stress, the shear stress and the von Mises stress were affected by changes to the conformity ratio and to a lesser extent by load changes. In a more recent article by Berend, et al, “Effects of coronal plane conformity on tibial loading in TKA: a comparison of AGC flat versus conforming articulations” (Surgical Technology Int., Number 18, pp. 207-212, 2009), the authors studied the effect of conformity on loading of the proximal tibia of the patient. Improper loading of the proximal tibia may lead to aseptic loosening of the tibial component in the tibia and eventually prosthesis failure requiring revision surgery. The authors found that coronally dished components created a strain increase in the anterior medial tibia while creating a significant strain decrease in the posterior tibia. They also found that proximal tibial strains were decreased and centralized in conforming versus flat articulations.
It is known in the art, however, that very high conformity may also lead, for example, to undesirable loading conditions on the insert surface or to excessive constraint of the femoral component, thereby inhibiting joint motions important to joint performance and patient comfort. Therefore, designs with intermediate values of contact area may be optimal as long as the stresses are below the yield strength of the insert material, in order to provide the optimal combination of joint laxity and conformity.
Complicating the challenge faced by knee prosthesis designers is the variability of patient anatomies in the worldwide, TKR patient population. Smaller patients with smaller femurs require, obviously, smaller knee prostheses. Each of the medial and lateral condyles of a femoral component of a small femoral component has a smaller coronal radius than a large femoral component for a large patient. To maintain the appropriate comformity ratio, as well as other geometrical relationships including condylar spacing, the small femoral component must be matched to a properly sized insert. In addition to the wide range of patient sizes, however, the dimensional proportionality between the femur and tibia bones also varies widely. For example, some patients, have a larger distal femur than other patients for a given size of the proximal tibia. In such cases when using currently available knee prosthesis systems, the surgeon may need to choose to implant a femoral component that is slightly mismatched with the femur and matched with the insert, or a femoral component that is matched with the femur and slightly mismatched with the insert.
Therefore, in view of the foregoing considerations, there is a need for a knee prosthesis system that allows the surgeon to select a femoral component that is sized to fit the femur of a particular patient, a tibial component that is sized to fit the tibia, and an insert that optimally matches the femoral component and is compatible with the tibial component. Such a knee prosthesis system should include both fixed and mobile types of prostheses and provide for both CR and PS procedures. Furthermore, the system should accommodate the wide variety of patient anatomies in the worldwide population.
In addition to providing optimally matched knee prosthesis components, there is an ongoing need to maintain or lower the costs and complexity of knee prosthesis systems. A knee prosthesis system may include femoral, tibial and insert components in a number of sizes, for each of the right and left knees, to accommodate variations in patient anatomies and conditions. In addition, each of inserts may be provided in a number of thicknesses so that the surgeon may select the one that results in the appropriate joint tension. Consequently, knee prosthesis manufacturers must provide a very large inventory of components representing a large number of different size combinations to accommodate the worldwide patient population. What is needed, therefore, is an improved, knee prosthesis system that allows component interchangeability to provide the necessary size combinations with a minimal number of components.
Another consideration during the design of knee prosthesis systems is bone preparation for implantation of the PS femoral component. Both the PS and the CR femoral components have a pair of spaced-apart condyles that are somewhat similar to the natural condyles of the distal femur. For the PS femoral component, a box (or intracondylar notch) positioned between the condyles includes features for interaction with a spine on the PS insert. Implantation of the PS femoral component requires cutting a recess into the distal femur to receive the box. In some current, knee prosthesis systems, the size of the box is the same for all of the PS femoral component sizes, thereby requiring cutting the same size recess into the distal femur, even for smaller femurs. It is desirable, however, to conserve natural bone, if possible, during preparation of the femur for attachment of the femoral component. There is a further need, therefore, for a knee prosthesis system in which each of the PS femoral components has a box that is sized proportionately to the femur size, while also addressing the previously described needs.
Yet another consideration during the design of knee prosthesis systems is bone preparation for implantation of the tibial component. Currently available, mobile and fixed TKR prosthesis systems include tibial components for a range of anatomical sizes. For some of these systems, the tibial component for a mobile TKR prosthesis of a particular size has a different configuration than that of a fixed TKR prosthesis of the same size. Specifically, the platform that supports the fixed bearing insert may have a different shape than the platform that supports the mobile bearing insert. This may result in a small, but possibly significant, difference in coverage of the resected, tibial plateau surface. Although less than ideal, one way surgeons may obtain the desired, tibial bone coverage is to select a larger size tibial component. What is more desirable is a TKR system that has mobile and fixed tibial components with a common platform profile shape that is optimized for interaction with surrounding tissues, kinematic performance, etc.
Also, currently available TKR systems have tibial components with stems of variable lengths to accommodate different tibial bone conditions. Furthermore, the stems for mobile tibial components may have a different configuration than the stems for fixed tibial components. Subsequently, such systems require that a number of different reaming instruments be available for each surgical procedure. A preferable TKR system would have mobile and fixed tibial components with stems of different lengths, but not requiring several different reaming instruments for preparing the tibia. This would also provide the surgeon with the intraoperative flexibility to select the appropriate type of tibial component, while reducing the number of instruments that would need to be available during the surgical procedure.
While this specification concludes with claims that particularly point out and distinctly claim the invention, the following description and the accompanying figures further illustrate some non-limiting examples of the claimed invention. Unless otherwise indicated, like reference numerals identify the same elements.
In this disclosure, the terms “anterior, posterior, lateral, medial” generally refer to the front, back, outside and midline of the surgical patient, respectively, although we also use these terms in reference to the devices.
The following are incorporated herein by reference in their entirety:
Hazebrouck discloses a fixed bearing, knee prosthesis system in which each of differently sized inserts are compatible with each size of tibial component, so that it is possible for a surgeon to select a tibial component that is properly sized for a patient's tibia, and an insert that is matched with the femoral component.
Wyss discloses a knee prosthesis system having a plurality of distinctly-sized PS inserts (fixed or mobile) having a spine extending superiorly from an inferior surface. The spine has a posterior side that has a concave cam surface and a convex cam surface. Each of the PS femoral components has a pair of spaced-apart condyles defining an intracondylar notch that has a posterior cam. The posterior cam includes a concave cam surface and a convex cam surface. The concave cam surface of the posterior cam contacts the convex cam surface of the spine during a first range of flexion and the convex cam surface of the posterior cam contacts the concave cam surface of the spine during a second range of flexion.
One characteristic of the distinctly-sized femoral and tibial components of knee prosthesis system 100 is proportionality of each component to the particular size of bone to which the component is to be attached. In general, the dimensional scale of the component varies, but not the shape. For example, the femoral component may have a proportionally-sized, intercondylar distance, such that a large femoral component has a proportionally longer intercondylar distance than that of a small femoral component. Similarly, a large tibial component may have a proportionally wider and deeper, posterior notch than that of a small tibial component.
In addition, each size of each of inserts 30, 40, 50 and 60 may be provided in a plurality of thicknesses.
As noted earlier, the anatomies of patients vary not only in size, but also in femur/tibia, size proportionality. Using historical data for TKR procedures, it is possible to determine the size combinations that would be needed for the majority of patients in the worldwide population. For example, each of practically all patients may be accommodated with a knee prosthesis distinctly-sized to fit both the femur and the tibia by pairing a femoral component that is sized either up two sizes or down two sizes from a tibial component. A “size 3” CR femoral component may be used with any one of a “size 1, 2, 3, 4 or 5” tibial components (fixed or mobile), whereas a “size 1” CR femoral component may be used with any one of a “size 1, 2 or 3” tibial components (fixed or mobile). Similarly, a “size 5” fixed tibial component may be used with any one of a “size 3, 4, 5, 6 or 7” fixed insert (CR or PS). Using knee prosthesis system 100, each of these pairings allows optimally matching the femoral component to the insert to maintain desirable geometrical relationships.
Tables 1 lists the components of an exemplary embodiment of knee prosthesis system 100. Table 2 lists the femoral component sizes provided for each femoral component listed in Table 1. Table 2 also shows for each femoral component size the compatible insert size for each insert listed in Table 1 and the compatible tibial component sizes for each tibial component listed in Table 1.
3N
4N
5N
6N
The embodiment of knee prosthesis system 100 shown in Table 1 and Table 2 provides 2176 unique combinations of prosthesis components. In each of these combinations, the femoral component is distinctly-sized to fit the patient's femur while optimally matched to the insert, and the tibial component is distinctly-sized to fit the patient's tibia while compatible with the insert. As a result, knee prosthesis system 100 may allow surgeons to avoid compromising kinematic performance and life of the implanted joint for each patient of the worldwide patient population.
As previously noted, patella components may also be provided for implantation in combination with the knee prosthesis. The patella components may be provided in a plurality of sizes. Examples of patella implants that may be adapted for use in knee prosthesis system 100 are the “P.F.C. Sigma Patellar Implants” available from DePuy Orthopaedics, Inc., Warsaw, Ind. Another embodiment of knee prosthesis system 100 may also include two unique types of patella components, each type having five sizes, thereby allowing the surgeon to select from 21,760 unique combinations of components. In each of these combinations, the femoral component is distinctly-sized to match the patient's femur while optimally matched to the insert, and the tibial component is distinctly-sized to fit the patient's tibia while compatible with the insert.
Knee prosthesis system 100 allows the surgeon to select a combination of knee prosthesis components for implantation into the patient, wherein the components are distinctly-sized to fit the femur and tibia of the patient, while also optimally matched to avoid compromising performance of the reconstructed joint. Knee prosthesis system 100 further provides PS femoral components that are proportionally sized to the femur since the PS insert (fixed or mobile) is matched to each PS femoral component. Knee prosthesis system 100 also may lower the cost and complexity of the necessary inventory of implant components to accommodate the worldwide patient population, due primarily to the interchangeability of the components.
As previously explained, there is a need for a knee prosthesis system that has mobile and fixed tibial components with stems of different lengths, but that does not require several different reaming instruments for preparing the tibia. As shown in
As shown in
Distal portions 156, 166, 176 and 186 may also have the same, generally conical shape. Proximal portions 158, 168, 178 and 188 may have approximately the same frustoconical shape and vary primarily in length. Keels 151, 153, 161, 163,171, 173, 181, 183 may have approximately similar configurations and orientations. As would be apparent to those skilled in the art, a surgeon may use the same reaming instrument to form a cavity to the desired depth in the proximal tibia to receive any one of the various sizes of stems 154, 164, 174 and 184. Because the external sizes and configurations of each of the plurality of distinctly-sized fixed tibial components may be approximately similar or identical to the corresponding one of the plurality of distinctly-sized mobile tibial components, the surgical preparation of the proximal tibia may be the same for a given size of either the fixed or mobile tibial components, and the required instrumentation may be the same for all sizes of both the fixed and mobile tibial components.
As previously explained, it is also desirable that the total knee replacement system have mobile and fixed tibial components with a common, platform profile or “footprint” that is optimized for coverage of the tibial plateau, interaction with surrounding tissues, kinematic performance and other factors.
For each size of tibial component, the platform profile (as viewed from the top, in the direction of the stem axis) is the same for both mobile and fixed tibial components for a particular anatomical size. There is no need to change tibial component size to get the same, tibial plateau coverage when choosing between a mobile and a fixed prosthesis. Another benefit of the common platform shape is that the same casting tool or a portion of the tool may be used in the manufacture of both tibial components, enabling reduced component cost.
We have shown and described various embodiments and examples. However, a person having ordinary skill in the art may modify the methods and devices described herein without departing from the overall concept. For instance, the specific materials, dimensions and the scale of drawings should be understood to be non-limiting examples. Accordingly, we do not intend the scope of the following claims to be understood as limited to the details of structure, materials or acts shown and described in the specification and drawings.
