Injuries produced by blunt trauma to the human patellofemoral joint vary with flexion angle of the knee.

Patellofemoral joint impact trauma during car accidents, sporting activities, and falls can produce acute gross fracture of bone, microfracture of bone, and soft tissue injury. Field studies of car accidents, however, show that most patellofemoral traumas are classified as 'subfracture' level injuries. While experimental studies have shown that the influence of flexion angle at impact is not well understood, flexion angle may influence injury location and severity. In the current study, 18 pairs of isolated human cadaver knees were subjected to blunt impact at flexion angles of 60 degrees, 90 degrees, or 120 degrees. One knee from each cadaver was sequentially impacted until gross fracture of bone was produced. The contralateral knee was subjected to a single, subfracture impact at 45% of the impact energy producing fracture in the first knee. The fracture experiments produced gross fracture of the patella and femoral condyles with the fracture plane positioned largely within the region of patellofemoral contact. The fracture location and character changed with flexion angle: at higher flexion angles the proximal pole of the patella and the femoral condyles were more susceptible to injury. For the 90 degrees flexion angle, the patella was fractured centrally, while at 60 degrees the distal pole fractured transversely at the insertion of the patellar tendon. In addition, the load magnitude required to produce fracture increased with flexion angle. In the 'subfracture' knees, injuries were documented for all flexion angles; occult microfractures of the subchondral and trabecular bone and fissures of the articular surface. Similar to the fracture-level experiments, the injuries coincided with the patellofemoral contact region. These data show that knee flexion angle plays an important role in impact related knee trauma. Such data may be useful in the clinical setting, as well as in the design of injury prevention strategies.