PATELLAR TRACKING MEASUREMENT IN THE NORMAL KNEE

Eleven fresh frozen cadaveric knee specimens were mounted in a knee ki nematics test device, and normal patellar movements were evaluated wit h use of an external device for direct measurement of patellar movemen ts. The effects of four different measurement conditions were assessed through alteration of one condition and determination of its effect o n patellar kinematics with the use of six specimens. The four conditio ns included (a) change of the measuring axis from an axis parallel to the central axis of the femur (femoral axis) to one parallel to the ce ntral axis of the tibia (tibial axis), (b) rotation of the femoral axi s internally 6 degrees, (c) change of the direction of the quadriceps force from parallel to the mechanical line of the lower extremity to a direction parallel to the femoral shaft, and (d) increase of the magn itude of the quadriceps force from 111 to 500 N. During knee flexion, the patella shifted laterally after a slight initial medial shift, til ted laterally from midflexion to 90 degrees, and gradually rotated med ially. The patellar shift relative to the tibial axis appeared to be m ore medial than the shift measured relative to the femoral axis; the d iscrepancy was caused by the valgus position of the tibia relative to the femur. Changing the rotational angle of the femoral axis artificia lly changed the patellar position. Varying the direction of the quadri ceps within the narrow range and increasing the quadriceps force did n ot affect patellar movements. It is likely that the anatomic configura tion of the patella allows the patella to seat in a stable configurati on so that it resists moderate changes in the load and direction of th e quadriceps. Tibial rotation exerted a major influence on patellar sh ift and tilt in the early phase of knee flexion: the patella rotated m edially when the tibia was externally rotated and rotated laterally wh en the tibia was internally rotated. These results indicate that the p atella