A rapid prototyping model for biomechanical evaluation of pelvic osteotomies

The biomechanical consequences of Salter pelvic osteotomy are difficult to assess due to the complex three-dimensional anatomy of the pelvis. Therefore, models of the dysplastic pelvis are required to allow realistic biomechanical simulation of possible outcomes. A polyamide reversed-engineering model of the left hemipelvis and proximal femur was produced from a computed tomography dataset of an 8-year-old child with severe dysplasia of both hips using selective laser sintering. Hip joint forces before and after Salter osteotomy of the hip were measured using an experimental setup in which an industrial robot was exerting hip joint forces and moments representing one-legged stance. Hip extensor and abductor actuator forces were measured which counterbalanced the joint moments. The preoperative hip joint resultant force was 583 N (270% body weight), while after the operation a mean force of 266 N (120% body weight) was measured. The resulting bony model was geometrically accurate, while apparent joint incongruencies were due to the neglected cartilaginous structures in the model. The preoperative joint resultant force was within the limits reported in the literature. The results suggest that Salter innominate osteotomy not only increases joint contact area but also reduces the hip joint force.