Session VII Pelvis and Acetabulum

*Increased Peak Contact Pressure Following Incongruent Reduction of Transverse Acetabular Fractures. A Cadaveric Model

Arthur L. Malkani, MD, Michael J. Voor, PhD, Greg Rennirt, MD, David Helfet, MD, Douglas Pedersen, MS, Thomas Brown, PhD

University of Louisville School of Medicine, Department of Orthopaedic Surgery, Louisville, KY

Hypothesis: Incongruent reduction of the acetabular articular surface following acetabular fractures will lead to excessive peak articular cartilage contact pressure.

Conclusion: Articular cartilage step-off of greater than 1mm following transverse acetabular fractures leads to significant increase in the stress distribution at the remaining contact areas.

Clinical Significance: A non-anatomic or an incongruent reduction of the acetabular articular surface following transverse acetabular fractures may play a role in the onset of post-traumatic arthritis.

Materials and Methods: Five complete fresh frozen cadaveric hip joints with an average age of 70 years were harvested and stored at -20°C. The specimens were thawed to room temperature and potted in a custom loading fixture using an epoxy-resin compound. All acetabular specimens were oriented to reproduce the normal weight bearing axis representing single-legged stance. The femur was rigidly fixed to the actuator of the MTS servohydraulic test frame (MTS Systems, Minneapolis, MN) oriented such that it could be compressed down into the open acetabulum resting on the load table. The acetabular fixture was mounted horizontally on the MTS load table supported by a ball bearing plate fixture to allow self-centering of the joint upon loading. Using pressure sensitive Fuji film, a base line contact pressure measurement was performed prior to creating a fracture line. Following base line testing, a transverse acetabular fracture was created in all five specimens through the weight bearing dome and epoxy material using a 0.5 mm thick hacksaw blade. The five specimens were then tested at loads of 445 N (100 lb) and 1335 N (300 lb) at step-off levels from 0mm to 5mm in 1mm increments. The step -offs were controlled using thin (0.5 mm) metal shims placed under the lateral side of the specimen.

Results: The mean peak pressure measured at 1335 N of loading using the Fuji film in all specimens at base line prior to transverse fracture was approximately 10 MPa. The peak pressures following transverse acetabular fracture did not change when the fracture was perfectly reduced. The peak pressures increased by approximately 20% following an articular cartilage step-off of 1mm. The increase in peak pressure was linear from 0mm to 2mm articular cartilage step-off. The peak pressure from 2mm to 5mm step-off was approximately 15 MPa but did not vary significantly at these levels.

Discussion: The results of our study demonstrated that an incongruent reduction of a transverse acetabular fracture will lead to increased peak pressures along the articular cartilage of the weight bearing dome. Peak pressures were increased by approximately 20% with an articular cartilage step-off of 1mm. The etiology of post-traumatic arthritis following acetabular fractures is multifactorial and includes the severity of the initial impact and the residual articular cartilage displacement. Based on our study, we feel that an absolute anatomic reduction should be obtained at the weight bearing dome of the acetabulum to minimize the incidence of posttraumatic arthritis.