OTA 1997 Posters - Pelvic & Acetabular Fractures

Location Effects Peak Articular Cartilage Stress in Displaced Acetabular Fractures through the Weight Bearing Dome

Michael J. Voor, PhD, Arthur L. Malkani, MD

University of Louisville, Louisville, Kentucky, USA

Purpose: Recent biomechanical and finite element investigations have examined the effect of displacement or "stepoff' on the generation of contact stress in the articular cartilage and its implications for later development of arthritis. The purpose of this study was to determine the effect of fracture location on the peak contact stress in the articular cartilage at the weight bearing dome.

Methods: Two-dimensional plane strain finite element analyses (Modeling: I-DEAS, SDRC, Milford, OH; Analysis: ABAQUS, HKS, Pawtucket, RI) were performed to simulate an idealized hip joint with a transverse acetabular fracture through the weight bearing dome. Loading was applied to simulate single leg stance at an angle of 15 degrees medial to a vertical line through the center of the femoral head. Models were created simulating fractures with 2 mm of "stepoff" at 10 different locations. Describing the fracture line as an angle with respect to a vertical line through the center of the femoral head, the fracture locations ranged from -18.65 degrees (lateral) to +18.96 degrees (medial). In the intact model, the weight bearing dome extended to 22.4 degrees laterally. The articular cartilage thickness was 2.0 mm. In all cases the displacement of the lateral fragment was in the superior direction. The models contained regions representing cancellous bone (modulus: 1000 MPa), cortical bone/subchondral plate (modulus: 3000 MPa), calcified cartilage (modulus: 1400 MPa), and articular cartilage (modulus: 10 MPa). A model of an intact joint was also analyzed to provide a baseline for comparison.

Results: The peak articular cartilage contact stress of the intact hip joint model was 1.71 MPa. There was an increase in peak contact stress in every fracture model. As a percentage of baseline, the following peak contact stresses were produced for each model described by its location with respect to vertical (Table 1).

Table 1. Peak Articular Cartilage Contact Stress in Ten Models with Transverse Fractures at Various Locations Through the Weight Bearing Dome of the Acetabulum

	Lateral					Medial
Location (deg)	-18.65	-14.92	-11.19	-7.46	-3.73	0.0	+4.74	+9.48	+14.22	+18.96
% Intact Stress	110	122	136	150	177	226	342	342	287	267

Discussion: Peak contact stress due to fracture displacement increased as the fracture location approached the line of action of the hip joint load vector. The greatest peak occurred in the model where the fracture was 9.48 degrees medial to the vertical line and approximately 5.5 degrees lateral to the line of action of the hip joint load. When the fracture was located more medially, the peak contact stress was not as great because the femoral head had slipped off of the "step" and was partially supported by the superiorly displaced lateral fragment. In combination with the previous studies looking at "stepoff" height and its effect on contact stress, this study demonstrates the importance of fracture location as well. For the same amount of displacement in a fracture through the weight bearing dome, location alone can be responsible for an almost 3.5 fold difference in peak contact stress.

Conclusion: This study demonstrates the sensitivity of articular cartilage contact stress to the location of a fracture through the weight bearing dome. Fractures in locations well removed from the high load bearing region may lead to small increases in stress while others closer to the hip load vector lead to large increases. Along with the many factors which contribute to outcome of acetabular fractures, we feel that fracture location also plays a significant role with respect to peak articular cartilage contact stress at the weight bearing dome.