In joint reconstructive surgery, it is important that the implant components be installed in correct alignment and positioned so as to produce proper motion with respect to the surrounding joint structures. For total knee arthroplasty, for example, the femoral, tibial and patella implants must all be properly installed and positioned in the knee joint and be in balance with the existing soft tissue joint support structures.
Proper alignment and placement of the implant components, as well as the proper balancing and tensioning of surrounding joint structures is imperative to the long term success of the arthroplasty. Improper alignment, placement and balancing may result in joint instability and unsatisfactory function, as well as possible increased wear rates between components.
Several methods have been used in the past for determining the proper alignment and placement of joint implant components. For example, one such method involves the visualization of alignment checks by the surgeon. As the surgeon moves the joint through its range of motion, the relative motion of the components is visualized and the implant components are placed accordingly. This method has weaknesses, however, as it is difficult to view the forced motion between the knee components with the patella reduced into its natural position. Moving the patella does not help since true motion of the components cannot be obtained unless the patella is in its natural position.
Another known method involves viewing the rollback of the femur on the tibia with the posterior cruciate ligament ("PCL") spared and in place. Again, difficulties arise in attempting to view the rollback with the patella reduced into its natural position. Several current implant designs feature dished condyles and a femoral component with a decreasing radius of curvature which make viewing of the joint contact point difficult during rollback. Yet in order to obtain satisfactory results from a PCL sparing arthroplasty it is helpful to determine the effectiveness of the PCL during rollback.
Still another known method involves determining the joint tension between the femoral and tibial components. The separation of the knee joint is viewed and felt as the knee is forced by the surgeon into varus or valgus at several degrees of flexion. A calibrated distractor, such as that shown in U.S. Pat. No. 4,501,266, is then used to apply a known force across the femoral-tibial joint and the resulting alignment is observed. The joint tension is further altered by inserting various heights of tibial provisional components and observing the resulting stability. One of the difficulties with this procedure is that it relies on the surgeon's "feel" and thus is subjective. The results of the implant procedure might vary from operation to operation and might be affected by the weight or size of the limb. The procedure also provides limited quantitative data on the joint force between the femoral and tibial components and provides limited quantitative estimates of the differences in joint contact forces between the medial and lateral condyles of the femoral component. It is also difficult with this procedure to monitor or determine the forces between the patella and femur.
The alignment and placement of the patella is determined primarily by visualization while moving the joint through a range of motion to see if it operates properly. This is affected by the choice of joint line reconstructed by the implant components and visualization does not detect the magnitude or direction of the forces in the joint. The proper joint line is often difficult to determine, however, and an improper joint line can cause excessive or insufficient joint forces between the patella and femoral components.
It is an object of the present invention to provide improved methods and devices of determining proper alignment, functioning, tensioning and positioning of joint reconstruction components.
It is another object of the present invention to provide improved methods and devices for determining the forces in the joint and thus determining the proper placement, alignment and positioning of implant components, particularly for knee joint reconstructions.
It is a further object of the present invention to provide improved methods and devices for determining femoral-tibial joint forces and relative movement by means of a provisional implant and use of force transducer mechanisms.
It is a still further object of the invention to provide a method and device for determining patella-femoral joint forces.
It is another object to provide a method and device for determining proper alignment and placement of a patella implant component through use of a patella provisional component and force transducer mechanisms.
A more specific object of the invention is a provisional component for use in knee joint reconstruction procedures with a force transducer which comprises a base member, at least one bearing element, a rocker means, and means for connecting said bearing element to said base member with said rocker means in sufficient contact with said base element that a force transducer positioned between the base member and the rocker means will be exposed to forces placed upon the bearing element during flexion of the joint.
Another more specific object of the invention is a method for assessing and determining proper alignment and placement of permanent implant components during knee joint reconstruction which comprises the steps of affixing a first permanent implant prototype on one side of a joint; preparing a plateau on another side of the joint for receiving a second permanent implant; provisionally affixing a base to the plateau and interposing a bearing element between the first permanent implant prototype and the base rockable relative to the base in response to manipulation of the joint; provisionally interposing a force sensor between the base and the bearing element adapted to generate force signals when the bearing element rocks relative to the base; manipulating said knee joint through a desired range of flexion to cause the bearing element to rock relative to the base and generate a pattern of force magnitude and location signals indicative of knee joint alignment and contact areas; making adjustments to the joint as necessary to balance the forces and assure proper alignment and positioning of the permanent implants in view of the pattern of force signals, removing the base, bearing element and force sensor from the plateau; substituting the first permanent implant for its prototype; and placing a second permanent implant on the plateau.
Still other objects, features, benefits and advantages of the present invention will become apparent from the following statement of the invention, description of the preferred embodiments, and appended claims and drawings.