Unfavorable Hip Stress Distribution in a Patient with Cerebral Palsy as a Cause for Hip Dislocation

Abstract

Hip dislocation is a relatively common complication in cerebral palsy [1]. Surgical intervention in the early phase is the preferred choice of treatment which however requires following the migration of femoral head with respect to acetabulum in hips at risk [1]. Standard anteroposterior radiograms are used to assess the Reimers index (RI) which expresses the percent of epiphyseal ossific nucleus located outside the acetabular roof ( Figure 1a); the hip is considered displaced if RI is greater than 0.3 [1]. While RI reflects the consequences, a biomechanical parameter -contact hip stress -may also elucidate the mechanism of hip dislocation. According to the mathematical model within the HIPSTRESS method, unfavorable stress distribution in the one-legged stance is characterized by a monotonous decrease of contact hip stress in the medial direction and by small functional angle of the weight bearing area. This is expressed by a positive hip stress gradient index (G p ) [2]. HIPSTRESS method requires as an input the geometrical parameters of pelvis and hips which can likewise be obtained from standard anteroposterior rentgenograms (Figure 1).