Assessment of hip dysplasia in adults

Degeneration off the articular cartilage and osteoarthritis of the hip joint are most often idiopathic (primary osteoarthritis). In patients with known pathogenesis (secondary osteoarthrtis), the disease is most commonly due to mechanical bone deformation, termed hip dysplasia. The treatment is planned on the basis of clinical, anatomic and biomechanical characteristics of the hip joint. The biomechanical pattern is most frequently determined by the Wiberg centre-edge angle of lateral covering of the femoral head. Dysplastic hips seem to be characterized by an unfavourable stress distribution to the weightbearing area. A mathematical model is used to find out which geometric parameters in addition to the Wilberg centre-edge may influence the distribution of stress on the weight-bearing surface. The author describes the method for assessing these parameters from a standard antero-posterior rentgenogram. The procedure was tested on healthy subjects with small centre-edge angles, reporting no clinical problems. We analyzed the mathematical model for determining stresses exerted on the weight-bearing articular surface of the hip during the one-legged stance, and determined the parameters affecting stress distribution in the hip joint for various hip and pelvis configurations. Standard anteroposterior rentgenograms of 411 subjects were examined and the subjects with centre-edge angles smaller than 30° were selected. The mathematical model was used to determine the values of radial stress for the selected subjects. The following parameters are relevant to the assessment of hip dysplasia: the radius of the sphere that corresponds to the hip weight bearing surface r, the magnitude of the force R, and the parameter termed functional angle of the femoral head covering, which is used here for the first time and equals the sum of the centreedge angle and the angle of the vertical inclination of the resultant hip force R. Further, it was found that subjects with small centre-edge angles are relatively rare in the healthy population. In only 9 of the 411 subjects examined was this angle smaller than 30°, and in 4 of them it was less than 20°. In 5 of these 9 subjects the peak stress values were found to be comparable with the reference values of in vivo mesurements. This is attributable to the compensating effects of the favorable hip and pelvis geometry and low body weight. The functional angle of the femoral head covering, the femoral head radius and the size of the force R are the parameters that can be determined from a standard AP roentgenogram using a relatively simple method and therefore constitute valuable elements for assessing hip dysplasia. They are particularly important in patients with borderline values of the centre-edge angle, where greater inclination of the force R, large femoral head radius and small body weight compensate for unfavourable lateral covering.