Malalignment of knee arthroplasty components can lead to abnormal wear, premature loosening and patellofemoral problems. Computer assisted surgery has been developed to improve surgeons ability to achieve correct alignment and thereby improve outcomes. This project compares the accuracy of computer assisted total knee arthroplasty with a conventional jig-assisted technique.

A total of 150 patients were recruited. Selection criteria included patients presenting with degenerative or inflammatory joint disease who were candidates for total knee arthroplasty. Patients having revision procedures were excluded as were patients who previously had a corrective osteotomy. Ethics approval was obtained and patients consented for the study. Patients were randomly allocated to either the computer navigated or jig alignment groups via the sealed envelope system. Demographic patient data and intraoperative data were collected. Quality of life and function assessments made using the WOMAC and Knee Society Scores. The component position was assessed using the Perth CT protocol.

One hundred and fifty patients were recruited from Dec 2005 to July 2007. Five patients were excluded due to machine malfunction and two others were excluded due to insufficient data collected leaving 143 patients for the study. There was no difference in blood loss, post operative hemoglobin or patient length of hospital stay. There was no difference in the Knee Society knee or function scores at 12 weeks or the knee flexion range either at discharge or at 3 months follow up. There was a significant correlation in duration of surgery (p< 0.05) with the navigated cases taking an average 23 minutes longer. Both the conventional jig alignment and computer navigated techniques produced accurate results in all CT measurements except for the tibial slope where the navigated group (4.8+/−1.6) was closer to the elected posterior slope of 3.5+/− 1.5 than the jig system (6.4+/−2). Statistically significant differences in favour of the navigated group were also found for both femoral component rotation and tibial coronal alignment but the outliers beyond the accepted ideal alignment of 0+/−3 degrees for each parameter were minimal and equivalent for the two groups.

Computer navigation in knee replacement surgery is gaining popularity to improve component alignment and consequently the outcome of total knee arthroplasty. This study has shown only marginal benefits in alignment of the navigated group but this needs to be considered against the increased surgical time despite familiarity with the hardware.

Introduction: Prevalence of femoral neck fracture in resurfacing hip arthroplasty continues to question if failure is technique-related or due to the inherent bone quality. This study aimed to correlate cement penetration profile during resurfacing hip with inherent bone density. The hypothesis is that osteoporotic bone is unable to support the prosthesis leading to fatigue failure.

Methods: Fifteen patients scheduled for total hip replacement (THR) were recruited to undergo resurfacing arthroplasty prior to THR. Each patient was implanted with a resurfacing femoral component (BHR, Smith & Nephew, Memphis, TN). Antibiotic simplex cement was inserted one minute after mixing at 18°C to fill 10% of the femoral component volume. The femoral head-implant section was removed and kept in buffered formalin. The patients then proceeded with standard THR. The femoral head-cement-prosthesis section was separated using electrical discharge (ED) machining technique and CT-scanned. The depth and volume of cement penetration were measured from the CT scans and correlated with femoral neck bone densities.

Results: Cement penetration was compared for three groups of bone density: normal, osteopenic, osteoporotic. Average cement thickness were found to be 0.36 ± 0.16mm (proximal), 0.28 ± 0.11 mm (centre) and 0.12 ± 0.05 mm. During hip resurfacing, cement is forced into the porous structure, e.g. the trabeculae and airspaces when the femoral component is fixed onto the head of the femur. In normal bone, the trabeculae is dense and air spaces occupy a small volume of the bone. Greater cement penetration was expected in osteopenic and osteoporotic bones. However, no significant difference was found between cement thickness and volume against inherent patient bone density (p> 0.05). High viscosity of the cement may have prevented more cement to penetrate the bone. While the exterior cortex of the femoral head is strengthened by a cement layer, the interior structure of the femoral neck is still susceptible to fracture at high loads. In addition, increased bone necrosis due to the exothermic reaction during cement fixation may predispose patients to fracture.

Discussion: Resurfacing hip replacement is a viable technique if the fracture risk can be reduced by gaining the best possible cement penetration. This would provide continuous cement stiffness with the bone.