THE EFFECT OF POST IMPINGEMENT IN POSTERIOR STABILIZED TOTAL KNEE REPLACEMENTS ON FEMORAL ROTATION AND DAMAGED AREA AS DETERMINED FROM ANALYSIS OF RETRIEVED TIBIAL INSERTS

Abstract

This anterior post impingement would also potentially affect the kinematics of the knee by altering or limiting internal/external rotation of the femur. The consequences of anterior post impingement on the movement of the femur have not been previously reported. We present here the first study of the effect of anterior post impingement on femoral rotation and damaged area as determined by analysis of retrieved PS knees. Materials and Methods 38 retrieved tibial inserts from 3 different designs were examined, 12 PFC (Johnson & Johnson), 11 Insall Burstein (Johnson & Johnson) and 15 Genesis (Smith & Nephew Richards). The post wear was assessed as well as the apparent rotation of the femur relative to the tibia and overall damage of the articulating surface of the insert. The presence and location of significant wear to the post was determined as previously described. Post wear was classified as significant when at least .3 mm of wear was observed. Determination of the rotation of the femur relative to the tibia as well as the overall damaged area was achieved through assessment of all 7 major damage modes on the surface of the inserts. Digital images of a superior view of the inserts were obtained, and then the damaged areas manually selected. All damage modes were considered equally, such that an overall pattern of damage was obtained for each insert. The damage area was then quantified using Optimas software, and the area normalized to the intended articulating area of the implant. In general, this was a region that excluded the central portion of the inserts. All damage areas are therefore expressed as a percentage of the possible intended wear areas. The average angle of rotation of the femoral component relative to the tibia was determined from the center points of the wear areas in reference to a defined line. The center of each of the damaged areas was determined based on the location of the midpoint between the extreme edges of damage in both the anterior-posterior and medial-lateral directions. The line that the angle is in reference to was a horizontal line connecting the posterior edges of each condyle for all designs except the Genesis. Due to asymmetry, the Genesis design was oriented such that the anterior edge was level, as per surgical placement, and all angles were defined with respect to this. All comparisons with post wear data were made independent of rotation direction (internal or external).