Following hip arthroplasty carried out using the Slooff-Ling impaction grafting technique micro-motion of the acetabular cup is frequently seen within the bone graft bed. In some cases this can lead to gross migration and rotation of the acetabular cup, resulting in failure of the arthroplasty. The movement of the cup is thought to be due to the irrecoverable deformation of bone graft under shear and compressive forces. Previous experimental studies have addressed ways in which the behaviour of the bone graft material may be improved, for example through washing and the use of improved particle size distribution. However there has been a limited amount of research carried out into assessing the behaviour of the acetabular construct in-vivo.

This study presents a 3D finite element model of the acetabular construct and hemi-pelvis following impaction grafting of a cavitory defect. A sophisticated elasto-plastic material model was developed based on research carried out by the group to describe the bone graft bed. The material model includes the non-linear stiffness response, as well as the shear and consolidation yield response of the graft. Loading associated with walking, sitting down, and standing up is applied to the model. Distinct patterns of migration and rotation are observed for the different activities. When compared in a pseudo-quantitative manner with clinical observations results were found to be similar. Walking is found to account for superior migration, and rotation in abduction of the acetabular cup, while sitting down and standing up are found to account for posterior migration, and lateral rotation. The developed 3D model can be used in the assessment of cup designs and fixation devices to reduce the rate of aseptic failure in the acetabular region.

Previous experimental studies of the pelvis have been carried out on cadaveric samples stripped of soft tissue. Investigations of the stress concentrations present in the pelvis due to the application of force through the hip joint have been conducted with the superior iliac crests cast in resin or cement. Thus stress concentrations are observed towards the superior iliac crests, and to some extent the pubic symphysis (these being the areas in which force transfer can occur). Due to the rigid fixing of the pelvis in these experiments, the pelvic bone has become viewed as a ‘sandwich beam’ acting between the sacro-iliac and the pubic joints. Numerical models employing similar fixed conditions have shown good agreement with the experimental studies.

However it is clear that these experiments, and the accompanying computational models are not representative of the in-vivo situation, in which the muscles and ligaments of the pelvis and hip joint provide resistance to movement, and in the case of muscles place additional forces on the pelvis, not addressed in the experimental studies. This study presents a finite element model of the pelvis in which novel techniques have been used to include the pelvic ligaments, and hip joint muscles using realistic attachment areas on the cortex, providing a more realistic comparison to the in-vivo environment. Joint interactions at the pubic symphysis and sacro-iliac joints are also simulated. A fixed boundary condition model is also presented for comparison.

The resulting stress concentrations in the pelvis for single leg stance observed in the in-vivo boundary condition model are dramatically different to those presented in studies in which the pelvis is rigidly fixed in place. The abductor muscles are seen to play a significant role in reducing stress concentrations towards the sacro-iliac joints and superior to the acetabulum, in comparison to fixed boundary condition analyses. Stress reductions away from the acetabulum are also observed in the underlying trabecular bone for the in-vivo boundary condition model. Similar stresses are observed within the acetabular region for the fixed, and in-vivo boundary condition models.

Morsellised bone graft is used extensively in revision arthroplasty surgery. The impaction technique at the time of surgery has a significant effect on the subsequent elastic and inelastic properties of the bone graft bed. Differences in values reported in the literature for the mechanical properties of morsellised cortico-cancellous bone (MCB) can be attributed to the different loading histories used during testing. We performed serial confined compaction tests to assess the optimum compaction strategy. Compaction of the samples was carried out using repeated standardised loading cycles. Optimal preparation of MCB is dependant on the force and frequency of compaction. The maximum compactive pressure the samples were subjected to was 3 N/mm² based on the clinical experience of Ullmark & Nilsson¹ in MCB preparation at the time of surgery. This paper presents the Young’s Modulus, E, vs. number of compaction cycles and inelastic strain, ^ie, vs. number of compaction cycles curves for MCB. Qualitative and quantitative descriptions of the material behaviour of MCB are developed. The importance of frequent percussive episodes prior to implant insertion is illustrated.

MCB was also found to exhibit significant visco-elastic response, with stress relaxation under displacement controlled loading continuing for several hours following initial load application. Bone graft substitutes do not at present exhibit a similar beneficial shock absorbing visco-elastic response.

Our experiments indicate that the material properties of MCB are dependent on the force of impaction and the number of impactions applied with a hammer at the time of surgery. A minimum of 10 to 20 compaction episodes, or hammer blows are required for MCB to achieve 60 to 70% of its long term predicted stiffness.