Contact pressures in the patellofemoral joint during impact loading on the human flexed knee

Currently, a bone fracture criterion is used by the automotive industry to assess the potential for a lower extremity injury from impact directed on the flexed knee. However, recent studies with animal models indicate irreversible damage to articular cartilage due to overpressures generated within the patellofemoral (P-F) joint without bone fracture, and suggest this injury may lead to a progressive, degenerative disease of the joint. The purpose of this investigation was to measure contact pressure in the human P-F joint during impact loading on the isolated, flexed knee. Loads were delivered on the patella with a free-flight inertial mass that had a rigid or foam padded interface. The experiments were conducted by serially increasing the impactor velocity in repeated tests until bone fracture was observed. The distribution of maximum pressures generated within the joint was recorded with a pressure-sensitive film. Fracture of the patella or the femur occurred at impact loads of approximately 8.5 kN. The average P-F pressure was approximately 25 MPa for 8 kN of impact load on the 90 degrees flexed joint. The P-F contact area varied with the level of contact load and degree of joint flexion. The distribution of P-F pressures was nonuniform. At approximately 70% of the fracture load for the 90 degrees flexed knee, nearly 35% of the contact area was exposed to pressures greater than 25 MPa. In an earlier study by others using subchondral bone-cartilage preparations, this level of pressure resulted in fissures and lacerations of the cartilage.