Session VIII - Pelvis

Biomechanical Consequences of Excision of Displaced Pipkin Femoral Head Fractures

William J. Holmes, MD; Brian Solberg, MD; Brian K. Bay, PhD; Justin E. Laubach, BS; Steven A. Olson, MD, University of California, Davis, Sacramento, CA

Introduction: Femoral head fractures are an unusual injury with a range of treatment options. Current treatment of the fracture fragment includes observation, fragment excision, and open reduction and internal fixation. The purpose of this study was to measure the effect of femoral head fracture fragment excision on load transmission in the hip joint.

Methods:The authors measured the contact areas and pressure between the acetabulum and femoral head of cadaveric pelves in four different conditions: intact, removal of the femoral head fragment from the inferior margin of the articular surface to the inferior fovea (Pipkin I), fragment excision from the articular margin to superior fovea (Pipkin II), and removal of additional 1 cm of articular cartilage (Pipkin II+ 1 cm). Five whole pelves and articulated proximal femurs were harvested from fresh human cadavers. The specimens were cleaned of soft tissue leaving the pelvic ligaments and capsule of the hip joints intact. One hip joint in each pelvis was used. Pelves were prepared and mounted onto a load cell of an Instron Materials Testing Machine. Hips were loaded using a custom apparatus to approximate single-leg stance using a simulated abductor mechanism to generate the load. Pressure and area measurements were made with Fuji pressure-sensitive film. Contact area, load, and mean and peak pressures were measured. The images were digitized and analyzed using a polar coordinate system to determine change within conditions. ANOVA with Bonferroni follow up was used for statistical analysis.

Results: Normal peripheral loading patterns were seen in intact specimens. Pipkin I, II, and II+1cm fractures caused significant (p=.02, .006, &.001) changes in normal peripheral loading area to a more central position. Pipkin II and II + 1cm fractures produced an increase in central acetabular mean (.22 MPa & .65 MPa) and maximum (.90 MPa & 2.35 MPa) pressures compared to intact. Pipkin I fracture caused minimal increase in central acetabular mean (.05 MPa) and maximum (.05 MPa) pressures. Outer acetabular contact load decreased in all fractured states (p=. 02, .01 &.005). An increase in inner contact load was seen with Pipkin II + 1 cm fractures (p=. 005).

Discussion and Conclusion: Pipkin femoral head fractures are a rare injury with an array of current treatment options. No previous studies have defined the biomechanical consequences of fracture fragment excision. This study showed no change in load, and mean and peak pressures between Pipkin type I fractures and intact specimens. However, with larger fragment excision, Pipkin type II and II + 1 cm, a change in the normal peripheral loading pattern was observed with greater contact area and mean pressures located centrally within the acetabulum. This change in loading pattern and pressure distribution within the acetabulum could be responsible for poorer clinical outcomes seen with excision of larger femoral head fragments. This study Biomechanically supports the excision of displaced Pipkin type I femoral head fractures, however, larger fractures of the femoral head may cause less mechanical alteration if internally fixed. Further studies evaluating the biomechanical effect of fracture fixation need to be performed.