[In vivo determination of patello-femoral separation and linear impulse forces]

Problems associated with the patellofemoral joint account for nearly half of all total knee arthroplasty (TKA) revisions. Under in vivo conditions, we previously determined that TKA subjects experience patellofemoral separation while performing dynamic, weight-bearing activities. This study investigates the impulse loading conditions that may exist at the time the patella impacts the femur during knee flexion. Fifty-seven subjects (68 knees) performed three successive deep knee bends under fluoroscopic surveillance. Eleven subjects (14 knees) had a posterior cruciate retaining (PCR) TKA, 19 subjects (25 knees) had a posterior cruciate substituting (PS) TKA, 15 subjects (17 knees) had a normal knee, and 12 subjects (12 knees) had an anterior cruciate ligament deficient (ACLD) knee. Velocities of each subjects' patella relative to a fixed point on the tibia were used as input to a mathematical model incorporating the impulse-momentum equation. At full extension, 12 of 14 PCR knees, 11 of 25 PS knees, 1 of 12 ACLD knees, and none of the 17 normal knees exhibited patellofemoral joint separation. The maximum separation, detected in a PCR knee, was 12 mm. The relative force determined upon patellofemoral impact was minimal (1.0 N). Simulated walking conditions for each subject were then entered into the mathematical model at a rate of 100 Hz and the calculated patellofemoral impact forces ranged from 78 N to 213 N. Since impulse loading conditions occur over a very small period of time, it was concluded that capturing fluoroscopy images at a rate of 30 Hz was too slow. Under simulated walking conditions, the impact forces due to impulse loading could contribute to polyethylene failure if these conditions induce fatigue of the polyethylene.