Retrieval studies based on revision operations at King Edwards VII Hospital reveal that, although micro-cracks develop in the cement mantle, it is the debonding between cement and bone that often defines the final failure of cemented acetabular replacements. This was illustrated at the revision surgeries by the easy removal of the acetabular cups with cement mostly attached to the cup. It is felt that a fundamental understanding of the mechanisms that initiate and propagate the interfacial failure at the bone-cement interface is the key towards solving the problem. In this work, in-vitro fatigue tests were carried out on cemented acetabular replacements using third-generation of composite pelvic bones. Standard Charnley cups were implanted using common bone cement, CMW, following the standard surgical procedures. The implanted hemi-pelvic bone model was then constrained at the sacro-iliac and pubic joints to represent the anatomic constraint conditions. Cyclic loads representing the maximum range of the hip contact force during normal walking were used and the direction of the maximum hip contact force was achieved by using angled plates. In addition to standard cup position, open cup and retroverted cup positions were also examined to assess the significance of cup orientation under fatigue loading conditions. Damage development in the reconstruction was monitored using CT scanning at regular intervals. Permanent records were collected and the sample was eventually sectioned and polished for microscopic studies. Results show excellent correlations between the results from the CT images and the microscopic studies, indicating progressive bone-cement interfacial failure in the posterior-superior quadrant. The significance of the work in the studies of ‘aseptic loosening’ will be discussed.
A new hip simulator has been developed at the University of Portsmouth and manufactured at Simulation Solutions, Ltd. (UK) for the purpose of fatigue testing of implanted acetabula. Although hip simulators for in vitro wear testing of prosthetic materials in total hip arthroplasty (THA) have been available for many years, similar equipment has yet to appear for endurance testing of fixations in cemented THA, despite of considerable evidence of late aseptic loosening as one of the most singnificant failure mechanisms in acetabular replacements [1]. In this study, a new four-station hip simulator designed for in vitro fatigue testing of implanted acetabula is described. The four-station machine has spacious test cells that can accommodate full hemi-pelvic bones with implants. The machine was designed to simulate the direction and the magnitude of the hip contact force relative to the acetabular cup coordinate system, as reported by Bergmann et al. [2], under typical physiological loading conditions, including stair climbing as well as walking. The controls were designed as such that each station may operate independently with a loading waveform that is fully programmable. The motions were achieved through two encoded servomotors suitably connected to gearboxes; while the loading was realised through a close-looped pneumatic system. The motions and the resultant hip contact force of the new hip simulator were evaluated, and found to be satisfactory in reproducing the typical physiological loading waveforms including normal walking, ascending and descending stairs. Experiments have been carried out using third generation composite bones (Pacific Research Laboratories, Inc.) and bovine bones. Both hip simulator and conventional fatigue testing were carried out. The implanted acetabula were CT scanned periodically to monitor the damage development in the fixation. Preliminary results seem to suggest that both magnitude and direction of the hip contact force influence the integrity of the fixa-tion, and failures appear to occur earlier in samples tested using the hip simulator. The predominant failure mechanism appears to be interfacial fracture, consistent with clinical observation of radiolucent lines and bone-cement interfacial failure.
At the position of maximum load, the maximum principal strain in the un-cemented specimens was 14.4 times higher than that for the cemented specimens (T-value = −96.40, P-value = 0.007). The highest recorded tensile strains in these specimens were localised to the acetabular rim of the posterior-superior quadrant. For the cemented specimens, the maximum principal strains are highest in the dorsal acetabulum, at a location that approximates to the centre of rotation of the replaced hip joint. Shear strains in the posterior-superior quadrant of both cementless and cemented acetabuli surpass the maximum principal strains.
Rib asymmetry is confirmed as a negative prognostic sign. Early treatment gives better results.
Introduction: Complications of homologous blood transfusion include transmission of infection and development of antibodies. Autologous pre-donation, acute normo-volaemic haemodilution and cell salvage have been used to reduce the use of homologous transfusions. Surgery for spinal deformities often requires blood transfusion. In February 1999, we started an autologous pre-donation programme for children undergoing spinal deformity surgery. Methods and results: The case records of the first 15 patients who took part in the programme have been scrutinised and data about pre-donation, haemoglobin, pre- and post-operative hameoglobin, blood loss, blood transfusions, use of blood products, and complications related to pre-donation of blood were obtained and analysed. Similar data from case records of 15 patients, who had surgery for spinal deformities before start of the programme, were used as control. In the autologous pre-donation group, four received homologous transfusion and 11 escaped exposure to homologous blood or blood products. In comparison in control group 14 out of 15 received homologous transfusion. There was no significant difference between the two groups in terms of diagnosis, operating time, postoperative haemoglobin, body weight and age. Mean operative blood loss in autologous group was less (1190 mls) than in that of the control group (1529 mls). Of the four patients who received homologous transfusion, two were transfused outside the hospital protocol. Complications from pre-donation of blood occurred in three patients and were minor. They included minor bruising in two and difficult and painful venous cannulation in one. Conclusion: In our practice autologous pre-donation resulted in avoidance of homologous blood transfusion in three quarters of patients undergoing spinal deformity surgery. By adopting strategies such as acute normo-volaemic haemodilution, cell salvage and strictly adhering to protocols for prescribing transfusion, we believe that the need for homologous transfusion could be obviated except in extreme cases.
