Biomechanical evaluation of impaction fractures of the femoral head

Objectives: To measure the effect of an impaction fracture of the femoral h ead on load transmission in the hip joint. Design: We measured the contact areas and pressure between the acetabulum a nd femoral head of cadaveric pelves in four different conditions: intact, w ith an operatively created one-square-centimeter defect in the superior fem oral head, with a two-square-centimeter defect, and with a four-square-cent imeter defect. All defects were uniformly three millimeters deep. Setting: Hips were loaded in a simulated single-limb stance. Pressure and a rea measurements were made with Fuji pressure-sensitive film. Specimens: Seven hip joints in seven whole pelves were tested. Main Outcome Measurements: Contact area, load, and mean and maximum pressur es were measured. Results: Peripheral loading was seen in the intact acetabulum. This was not disrupted after impaction fractures of any size. A significant increase in mean maximum pressures in the superior acetabulum was seen with two-square -centimeter and foursquare-centimeter defects. Conclusions: In contrast to prior biomechanical studies of acetabular fract ures, our investigation revealed that disruption of the peripheral distribu tion of load does not occur with impaction fractures of the femoral head. C linical series indicate that impaction injuries to the femoral head are ass ociated with a poor prognosis. Previous biomechanical data on acetabular fr acture patterns associated with a poor prognosis have shown increases in me an and peak pressures in the superior acetabulum. This was seen with two-sq uare-centimeter and four-square-centimeter impaction injuries. Other factor s, such as wear of the articular cartilage during joint motion or associate d microscopic damage to the remainder of the joint surface at the time of i njury, may also contribute to the rapid joint deterioration seen in these i njuries. Further study is indicated.