Session VII - Pelvic Trauma

The Effects of Transverse, Anterior Column and Posterior Column Acetabular Fractures on the Stability of the Hip Joint

Mark S. Vrahas, MD, Kirstin K. Widding, MS, Kevin A Thomas, PhD

Louisiana State University Medical Center, New Orleans, Louisiana, USA

Purpose: Posttraumatic arthrosis may result from unreduced acetabular fractures. This is especially true if the fracture crosses a portion of the acetabulum necessary for weightbearing. However, the exact portion of the acetabulum necessary for normal weightbearing is not known. At the very least, normal weightbearing requires a stable hip joint. Thus, an acetabular fracture that affects hip joint stability can be assumed to involve an important weightbearing area. Such a fracture, if left unreduced, most likely would lead to arthrosis. The purpose of this study was to assess how various transverse, anterior column and posterior column acetabular fractures affect hip joint stability.

Methods: A total of 24 fresh-frozen cadaver hip joints were used in this investigation. Each specimen was mounted in a mechanical testing machine so that compressive forces of various magnitudes could be applied. The femur was mounted vertically on a freely moving instrumented x-y displacement table. Stability of the hip joint depended upon only the intrinsic stability of the articulation between the femur and the acetabulum. The acetabulum was mounted in a fixture that allowed simulation of the various fractures and allowed positioning of the joint so that the magnitude and direction of the peak forces seen during normal gait could be applied. For each of the fracture types, the specimens were loaded in compression four times at each of three loads (800 N, 1200 N, and 1600 N). Each specimen was first tested intact, and again following each of the various simulated fractures. Simultaneous recordings were made of the load, actuator displacement, and translation of the femur. A hip was considered stable if no dislocations occurred over the four loading cycles.

Transverse acetabular fractures (OTA classification 62-B1) were evaluated in 12 specimens. Each specimen was tested with successive transverse fractures having roof-arc angles of 60°, 50°, 40°, 30° and 20°. The roof-arc angles were determined radiographically. Anterior column fractures (OTA 62-A3) were evaluated in 6 specimens. For each specimen, very low, low, intermediate, and high fractures were tested successively. Posterior column fractures (OTA 62-A2) also were evaluated in 6 specimens; only very low posterior column fractures were tested.

Results: All 24 hips were stable when loaded intact to 800 N, 1200 N and 1600 N. For the transverse fractures, for each of the applied loads, the number of stable specimens decreased with each successive fracture. For these fractures, stability was significantly affected by both the location of the fracture (roof-arc angle) and the magnitude of the applied load. For the anterior column fractures, all specimens with very low, low, and intermediate fractures remained stable throughout all tests. With high anterior column fractures, 33% of specimens were unstable when loaded to 800 N, 67% were unstable when loaded to 1200 N, and all were unstable when loaded to 1600 N. Overall, the posterior column fractures were the least stable. All six specimens were unstable with the simulated very low fracture (exiting the posterior column just superior to the ischial spine).

Discussion: The simulated fractures produced in this investigation do not exactly imitate fractures that occur in vivo. However, this study does provide new information concerning what portions of the acetabulum may be critical for weightbearing. This study suggests that the radiographic roof-arc angle is a useful technique for evaluating transverse acetabular fractures. It is generally accepted clinically that transverse fractures with roof-arc angles of 40° or less affect an important weightbearing portion of the acetabulum. The present results support this belief. Nearly 100% of the specimens (11 of 12) were unstable with transverse fractures at the 40° roof-arc angle. No specimens were stable with roof-arc angles of 30° or 20°. However, a substantial number of specimens were unstable with transverse fractures at the 50° and 60° roof-arc angles, with instability directly proportional to the magnitude of the applied load.

Anterior column fractures were more stable than expected. No specimens were unstable unless the fracture was of the very high type. Fractures at the base of the pubic rami did not affect stability. Only fractures that exited through the iliac crest caused instability. This suggests that fractures at the base of the pubic ramus may be less important then previously thought. Posterior column fractures were much less stable than expected. Even low fractures, that is, fractures at the level of the ischial spine, rendered the hip unstable in all six specimens, even at the lowest applied load.

Conclusions: The results of this study have direct implications in the treatment of anterior column and T-type fractures. Based on these results, currently only anterior column fractures that exit through the iliac crest are treated surgically. For T-type fractures, the posterior column is the greater concern, and requires reduction and fixation even if the fracture is very low. The anterior column portion of the fracture is of concern only if it very high.