1. Field of the Invention
The invention relates to prosthetic joints, such as prosthetic knee joints.
2. Description of the Related Art
A typical prosthetic knee includes a tibial component for mounting to the resected proximal end of the tibia, a femoral component for mounting to the resected distal end of the femur, a bearing between the tibial and femoral components and a patellar component mounted to the posterior face of the patella. The tibial and femoral components typically are made of metal and the bearing typically is made of plastic, such as UHMWPe. The proximal or superior surface of the bearing is formed to define medial and lateral concave regions. The distal or inferior surface of the femoral component is formed to define medial and lateral convex condyles that articulate in bearing engagement with the concave regions of the bearing. Some prosthetic knees include a mobile bearing that is permitted to undergo controlled rotational and translational movement relative to the tibial component. Other prosthetic knees include a bearing that is fixed relative to the tibial component.
Knee motion is highly complex and includes flexion-extension, axial rotation, anterior-posterior translation, and adduction-abduction. Incongruency between the femoral component and the bearing enables these complex motions to be carried out with enhanced mobility for the patient who has a prosthetic knee joint Accordingly, many prosthetic knee joints provide highly incongruent contact between the femoral component and the bearing. Incongruent contact causes a specified load to be applied to a small area, and hence causes the contact stress (load per unit area) to be higher than in a knee joint with more congruent contact. The metallic and plastic materials currently used in joint replacement permit normal knee motion with contact stresses that can accommodate normal physiological loads over an extended period of time in mobile bearing prosthetic knees. For example, U.S. Pat. Nos. 4,309,778 and 4,340,978 disclose mobile bearing prosthetic knee joints with tibiofemoral articulation surfaces that have demonstrated an ability to last for an extended time.
Incongruent contact is particularly important in fixed bearing designs in view of the complex combinations of flexion-extension, axial rotation, anterior-posterior translation, and adduction-abduction associated with knee motion. However, fixed bearing prosthetic knee joints can produce contact stresses greatly in excess of acceptable limits associated with the strength of UHMWPe normally used for the tibial articulation surface. The dilemma for designers of fixed bearing knees is to effect a compromise between the conflicting requirements for joint motion mobility (which is accomplished by increasing contact surface incongruity and thus contact stress) and low contact stress (which requires high congruity and thus low joint mobility) to prevent rapid failure of the plastic used in current prosthetic joint articulations. Unfortunately a satisfactory compromise has yet to be found where fixed bearing knee components can be considered safe for extended use under normal physiological loads. A similar situation is true for other load bearing condylar joints such as the tibiotalar ankle joint.
The United States Food and Drug Administration (USFDA) requires extensive and rigorous clinical testing before approval of most mobile bearing joint replacements, and hence inhibits the use of such devices. The USFDA does not require similar testing for fixed bearing devices. Thus, most knee devices and all ankle devices that are generally available in the United States are the lower performing fixed bearing devices.
Improved fixed bearing articulating surfaces are possible by limiting the degree of incongruity in such devices. This may be accomplished by using a congruent, spherical surface on the medial condyle of the knee or ankle and mildly incongruent line contact on the more lightly loaded lateral condyle rather than the typical point contact on both sides used for fixed bearing designs. This design recognizes the fact that the medial condyles of both the femur and the patella of the knee joint and the medial condyle of the ankle joint are subject to greater loads than the lateral condyles thereof. The congruent contact at the more highly loaded medial condyle results in lower stress (i.e. force per unit area) due to the higher surface contact area achieved with congruency. On the other hand, the line contact at the less highly loaded lateral condyle results in acceptably low stress despite the smaller surface area due to the lower load on the lateral condyle. However, the line contact at the lateral condyles can achieve greater joint mobility without using a mobile bearing joint design.
Such a surface can be designed to accept normal walking loads within the allowable stress limits of the materials used in such joint replacement while still providing needed joint mobility. Expected stresses on the lateral condyle will, however, be substantially greater than that of a comparable mobile bearing with congruity on both sides. The combined congruent-incongruent articulating surface is thus an acceptable, although less desirable, design compromise to accommodate the regulatory requirements of the USFDA and the many surgeons who have become accustomed to fixed bearings.
Many patients who receive knee and ankle implants are quite elderly and inactive and thus produce loads that are substantially less than normal. This lower loading level (producing lower contact stresses for a given articulation geometry), coupled with the reduced time and frequency of use (which reduce the accumulated damage for given contact stresses) can allow articulating surfaces with a greater degree of incongruity and thus allow the use of fixed bearing components. Since fixed bearings do not require a supporting prosthetic platform, they can be fixtured directly to bone, saving the cost of the platform. The US medical care system is under considerable pressure to lower costs, and hence many hospitals would prefer to use a low cost device. A low cost, fixed bearing, device can be used as tibial or patellar components of a total knee in an elderly, inactive, patient. Therefore, the added cost of multi-part tibial or patellar replacements are not justified economically if a lower cost set of components are adequate.
An articulation surface with partially incongruent contact surfaces can produce substantially lower contact stresses than existing incongruent, fixed bearing devices. Lowering contact stresses in incongruent fixed bearing devices reduces wear and fatigue damage of the prosthetic articulating surfaces, thereby increasing their service life and increasing the population group to which such components can safely be used. The articulating surfaces of the subject invention can have similarities to the articulating surfaces shown in U.S. Pat. No. 5,871,539 and U.S. Pat. No. 6,074,425, the disclosures of which are incorporated herein by reference. However, the articulating surfaces of the subject invention are formed by means that are different from the means used to generate the articulating surfaces in these earlier patents. Additionally, the articulating surfaces of the subject invention are configured to achieve line contact in only one of the condyles of the subject invention as compared to both condyles of the earlier patents. Thus, this invention improves the fixed bearing articulating surfaces.
The geometry of the femoral articulating surface 12 of the femoral component 10, as shown in
The tibial component 20 has a tibial articulating surface 22 that is generated using the same generating curve 13, except for different connecting tangents. However, only the medial articulation 28 of the tibial articulating surface 22 is a surface of revolution, and the lateral surface 29 is not a surface of revolution. Rather, the lateral tibial articulating surface 29 is generated by simultaneously rotating a surface of revolution about different axes. This generation method is unique and useful. The surface of revolution for the medial articulation 28 of the tibial articulating surface 22 preferably is configured relative to the compound surface of revolution of the femoral articulating surface 12 to achieve congruency to at least about 40-50 degrees of flexion. Line contact may exist between the femoral component 10 and the tibial component 20 at greater flexion.
The tibial articulating surface 22 may be formed on a tibial component blank 23, as shown in
This resulting surface will be referred to here as a “medial-pivot” surface since motion on the medial articulation of the tibia 21 relative to the femur will take place about the origin of the X, Y and Z axes, fixed to the tibia with the X, Y, Z coordinate system origin at the center of the spherical medial articulating surfaces.
Loads that press the patellar component 30 to the femoral component articulating surface 12 are low at full extension. However, at about 35-45 degrees flexion, the substantial load caused by the quadriceps pulls the patellar component 30 medially into the sulcus. Thus, the medial patellar articulation surface 32 carries most of the load, Often the lateral patellar articulating surface 33 lifts off the femoral component articulating surface 12, as shown in
Where the medial component of the patellofemoral compressive load is sufficient so as not to produce lift off of lateral patellar articulation surface 33, as shown in
A replacement knee in accordance with a third embodiment is identified by the numeral 400 in
Referring to
The medial-pivot surface need not be formed by use of a cutter such cutter 34, which is used primarily for purposes of illustration. A medial-pivot surface can be machined by a variety of cutters including form cutters, point cutters, and ball mills using two and three dimensional computer driven machines.
A medial-pivot surface is unique within and without the field of orthopedic surgical appliances. In human replacement joints its primary application is in condylar joints such as the knee, ankle great toe, pip joint of the finger, and the thumb and in the elbow.
This application claims priority on U.S. Provisional Patent Appl. No. 61/098,824 filed on Sep. 22, 2008, the entire contents of which are incorporated herein by reference.
Number | Date | Country | |
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61098824 | Sep 2008 | US |