Load transfer across the pelvic bone.

Earlier experimental and finite element studies notwithstanding, the load transfer and stress distribution in the pelvic bone and the acetabulum in normal conditions are not well understood. This hampers the development of orthopaedic reconstruction methods. The present study deals with more precise finite element analyses of the pelvic bone, which are used to investigate its basic load transfer and stress distributions under physiological loading conditions. The analyses show that the major part of the load is transferred through the cortical shell. Although the magnitude of the hip joint force varies considerably, its direction during normal walking remains pointed into the anterior/superior quadrant of the acetabulum. Combined with the fact that the principal areas of support for the pelvic bone are the sacro-iliac joint and the pubic symphysis, this caused the primary areas of load transfer to be found in the superior acetabular rim, the incisura ischiadaca region and, to a lesser extent, the pubic bone. Due to the 'sandwich' behavior of the pelvic bone, stresses in the cortical shell are about 50 times higher than in the underlying trabecular bone (15 to 20 MPa vs 0.3-0.4 MPa at one-legged stance). Highest intraarticular pressures are found to occur during one-legged stance and measured about 9 MPa. During the swing phase, these pressures decrease less than linearly with the magnitude of the hip joint force. Muscle forces have a stabilizing effect on the pelvic load transfer. Analysis without muscle forces show that at some locations stresses are actually higher than when muscle forces are included.