Hip stress distribution may be a risk factor for avascular necrosis of femoral head

Avascular necrosis of femoral head (AN) is a hip disorder with various risk factors, however, the underlying mechanisms are not yet understood. In order to elucidate the effect of the mechanical factors on AN we have compared a group of hips at risk for AN and a group of healthy hips with respect to biomechanical parameters: functional angle of the weight bearing area (theta(F)), position of the stress pole (Theta), index of the gradient of the contact stress at the lateral border of the load bearing area (G(p)) and peak contact hip stress (p(max)). The test group representing hips at risk for AN consisted of 32 male hips contralateral to the necrotic hips while the control group consisted of 46 healthy male hips. The biomechanical parameters we computed with the HIPSTRESS method (based on measurements of geometrical parameters from standard anterior-posterior pelvic radiographs). The average values of parameters pertaining to both groups were compared by the impaired two-sided Student t-test. The functional angle of the weight bearing area was on the average larger (more favorable) in the control group (112.9 degrees +/- 13.5 degrees) than in the test group (105.0 degrees +/- 12.4 degrees), the difference (7%) being statistically significant (p < 0.01). The position of the stress pole was more lateral (less favorable) in the test group (15.44 degrees +/- 7.23 degrees) than in the control group (11.80 degrees +/- 7.58 degrees), the difference (27%) being statistically significant (p = 0.037). The index of the hip stress gradient was higher (less favorable) in the test group (-17.23 degrees +/- 17.16 degrees x 10(3)m(-3)) than in the control group (26.05 +/- 16.85 x 10(3)m(-3)), the difference (40%) being statistically significant (p = 0.028) while we found no statistically significant difference in the peak contact stress between the two groups. Our results indicate that a less favorable steep stress distribution over a smaller load-bearing area is a risk factor in AN.