The effect of gap and step-off displacement of infratectal T-type acetabular fractures on contact pressures of the acetabulum: a biomechanical analysis.

PURPOSE: To determine the effect of gap and step-off displacement of infratectal T-type acetabular fractures in a biomechanical model and understand the effects of displacement on acetabular joint pressures when the fracture lies outside of the weight-bearing dome. METHODS: Infratectal T-type acetabular fractures were created with osteotomies in cadaveric acetabuli and the hip joints were assembled in a servohydraulic test frame. Intra-articular pressure sensors were used to measure contact area, contact pressure, and peak forces through increasing gap and step-off displacement during simulated positions of standing, walking, and stair climbing. RESULTS: With 0 mm articular step-off and increasing gap from 0 mm to 5 mm, there was no significant change in contact area, contact pressure, or peak force throughout most testing conditions. With a 2 mm step-off, contact area (-5.9 ± 1.8cm2, -23.9% ± 7.3%, p = 0.002) and contact pressures (10.9 ± 4.2 N/cm2, 43.5% ± 15.8%, p = 0.004) changed significantly once a 5 mm gap was reached. CONCLUSION: This biomechanical study suggests that hip conditions are not significantly altered in T-type acetabular fractures outside the weight-bearing dome when there is an articular gap but no step-off and when there is an articular step-off and a gap of 2 mm or less. Infratectal T-type acetabular fractures with a combination of 2 mm articular step-off and gap greater than 5 mm may benefit from surgical reduction as malunion may lead to increased contact pressures within the joint.