Biomechanical evaluation of a low anterior wall fracture: correlation with the CT subchondral arc.

OBJECTIVE

To measure the effect of a simulated low anterior wall fracture of the acetabulum on load transmission in the hip joint.

DESIGN

We measured the contact areas and pressure between the acetabulum and the femoral head of cadaveric pelves in three different conditions: intact, with an operatively created fracture of the anterior wall, and after anatomic reduction and internal fixation of the fracture.

SETTING

Hips were loaded in simulated single-limb stance. Pressure and area measurements were made with Fuji pressure-sensitive film.

SPECIMENS

Seven hip joints in seven whole pelves were tested.

INTERVENTION

Anterior wall fractures were anatomically reduced and fixed.

MAIN OUTCOME MEASUREMENTS

Contact area, load, and mean and maximum pressures were measured.

RESULTS

Anterior wall fractures in our specimens entered the hip joint an average of 9.7 millimeters from the vertex of the acetabulum, corresponding to a 45-degree roof arc measurement. Peripheral loading seen in the intact acetabulum was disrupted after fracture. The loading pattern was not restored to preinjury levels with anatomic reduction and fixation. There was no significant change in the contact area (p = 0.43), force (p = 0.06), or mean (p = 0.57) or maximum (p = 0.20) pressures in the superior aspect of the acetabulum after creation of the anterior wall fracture.

CONCLUSIONS

These results differ from those of previous studies with posterior wall acetabulum fractures, where significant increases in force and mean and maximum pressures were noted in the superior acetabulum after fracture. The lack of significant increases in superior acetabular pressures is discussed in relation to the mean computed tomographic subchondral arc of approximately ten millimeters in our specimens.