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.

Micro level finite element models of bone have been extensively used in the literature to examine its mechanical behaviour and response to loads. Techniques used previously to create these models involved CT attenuations or images (e.g. micro-CT, MRI) of real bone samples. The computational models created using these methods could only represent the samples used in their construction and any possible variations due to factors such as anatomical site, sex, age or degree of osteopo-rosity cannot be included without additional sample collection and processing. This study considers the creation of virtual finite element models of trabecular bone, i.e. models that look like and mechanically behave like real trabecular bone, but are generated computationally.

The trabecular bone is anisotropic both in terms of its micro-architecture and its mechanical properties. Considerable research shows that the key determinants of the mechanical properties of bone are related to its micro-architecture. Previous studies have correlated the apparent level mechanical properties with bone mineral density (BMD), which has also been the principal means of diagnosis of osteoporosis. However, BMD alone is not sufficient to describe bone micro-architecture or its mechanical behaviour. This study uses a novel approach that employs BMD in conjunction with micro-architectural indices such as trabecular thickness, trabecular spacing and degree of anisotropy, to generate virtual micro-architectural finite element models. The approach permits generation of several models, with suitable porous structure, for the same or different levels of osteoporosity. A series of compression and shear tests are conducted, numerically, to evaluate the apparent level orthotropic elastic properties. These tests show that models generated using identical micro-architectural parameters have similar apparent level properties, thus validating this initial bone modelling algorithm. Numerical tests also clearly illustrate that poor trabecular connectivity leads to inferior mechanical behaviour even in cases where the BMD values are relatively high. The generated virtual models have a range of applications such as understanding the fracture behaviour of osteoporotic bone and examining the interaction between bone and implants.

Morsellised cortico-cancellous bone (MCB) is used extensively in impaction grafting procedures, such as the filling of cavitory defects on the femoral and acetabular sides during hip arthroplasty. Several experimental studies have attempted to describe the mechanical behaviour of MCB in compression and shear, and it has been found that it’s properties can be improved by washing and rigorous impaction at the time of surgery. However their focus has not been on the development of constitutive models that can be used in computational simulation.

The results of serial confined compaction tests are presented and used to develop constitutive models describing the non-linear elasto-plastic behaviour of MCB, as well as its time dependent visco-elastic behaviour. It is found that the elastic modulus, E of MCB increases linearly with applied pressure, p, with E achieving a value of around 30 MPa at a pressure of around 1 MPa. The plastic behaviour of MCB can be described using a Drucker Prager Cap yield criterion, capable of describing yielding of the graft in shear and compression. The time dependent visco-elastic behaviour of MCB can be accurately modelled using a spring and dashpot model that can be numerically expressed using a fourth order Prony series. The role of impaction in reducing subsequent plastic deformation was also investigated. The developed relationships allow the constitutive modelling of MCB in finite element simulations, for example of the acetabular construct following impaction grafting. The relationships also act as a gold standard against which to compare synthetic graft and graft extender materials.

Introduction: Several studies have shown that malalignment of primary knee replacement of more that three degrees result in rapid failure and less than optimal functional outcome. We investigated whether inaccuracies in registration resulted in malalignment and also what degree of registration error would result in an unacceptable result. The Stryker image free navigation system was used.

Method: Using a dry bone leg with restrained joints, we varied the points where registration was set and measured the resulting errors in alignment. Registration points used were 1: centre of the knee, 2: AP axis, 3: centre of the ankle and 4: medial malleolus. The true registration point was translated medially or laterally up to 10 mm or inclined up to ten degrees to simulate inaccurate placement.

Results: 1: less than 1° varus for 10mm of medial translation, 1.5° valgus for 10mm lateral translation on distal femoral cut. 2a: 0.5° valgus for 10mm medial translation and 0.5° valgus for 10° medial angulation on distal femoral cut. 2b: 0.5° varus for 10mm medial translation and no effect for 10° angulation on tibial cut. 3a: Software error with 10mm translation laterally and 1.5° varus for 10mm of lateral translation. 3b: 1.5° posterior slope and valgus angle for 30 degrees of lateral angulation. 4: 1° valgus for 10mm lateral translation and no effect on tibial slope.

Conclusion: There is not only no significant intra-observer error, but also that even combining registration errors may result in a cut error of only 1.5 degrees. The software itself also protected against any significant errors.

Introduction: We present our early results using the Stryker navigated knee system, since March 2003. There have been several papers showing an improvement in alignment of prostheses using navigation but few series have mentioned the problems of introducing this new technology.

Method: 214 consecutive operations were audited retrospectively from operation notes, discharge summaries and clinic notes.

Results: 11 surgeons performed 214 operations on 196 patients. 205 operations were primary knee joint replacements and 9 revisions. Average operation time was 149 minutes. 96% had an excellent outcome (pain free with a good range of motion), 2.6% had a moderate outcome and 1.4% had a poor outcome. 17 patients had superficial wound infections; 4 patients required an MUA for stiffness (with a good outcome); 3 DVTs (all below knee); 1 acute and 3 delayed haemarthroses; 1 temporarily unstable knee; 5 suffered prolonged pain, 1 peri-prosthetic fracture due to anterior notching of the femur requiring revision and there was 1 quads tendon rupture. There were 4 procedures abandoned, 2 because the femoral pin was unstable in osteoporotic bone and because of 2 software errors. Average range of motion was 0–110°. There was one deep infection following pyelonephritis. Average follow up has so far been 20.6 (2–104) weeks.

Conclusion: We have found that our results compare favourably with conventional techniques. We found it particularly useful for revision surgery and those patients who had intramedullary devices for previous fractures of the femur where conventional jigs could not be used.

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.

Non-union of femoral and tibial shaft fractures is a serious complication, prolonging patient morbidity and ultimately influencing functional recovery. The aim of the study was to assess the effectiveness of different surgical options in the treatment of non-union of femoral shaft fractures after initial intramedullary nailing.

Between January 1995 and November 2003, 320 patients with femoral or tibial shaft fractures were treated with closed intramedullary nailing. The mechanism of injury, fracture pattern, concomitant injuries, subsequent surgical treatment and complications were prospectively recorded and retrospective analysis was performed.

16 of the 157 patients (10%) with femoral fractures and 31 of the 161 patients (19%) with tibial fractures developed non-union after initial primary intramedullary nailing. This group of patients had 2–3 further operations before union was established. 26 patients had initial dynamisation and 11 had exchange nailing alone. The remaining patients had autologous bone grafting and/or internal fixation with a plate. Subsequently a further 3 patients required dynamisation, 2 required exchange nailing and another 3 bone grafting. Finally 2 patients required a fourth procedure to reach solid union.

Our experience showed that exchange nailing and dynamisation are the most effective method of treatment of non-union of femoral and tibial shaft fractures after intramedullary nailing.