Finite element analysis: a comparison of an all-polyethylene tibial implant and its metal-backed equivalent.

PURPOSE: The hypothesis of this study is that all-polyethylene (APE) tibial implants offer a biomechanical profile similar to metal-backed tray (MBT). There are significant financial implications, in selected patient groups, if APE can be deemed to perform as well as MBT. METHODS: Using a finite element analysis of CAD models provided by DePuy (Leeds), stress distributions were investigated for both an APE and MBT tibial implant. The performance was assessed for cancellous bone at 700 MPa (normal) and at 350 MPa (less stiff). Plots were recorded along the length of the tibia, showing the loads carried by the bone (cortical and cancellous), the implant interface, cement interface and the stem. von Mises stress distributions and percentage volumes were used to assess bone resorption and hence potential for failure (fracture). RESULTS: Higher stress shielding (resorption) occurred around the keel and stem of the MBT revealing greater potential for bone loss in these areas. APE had no areas of bone resorption (being more flexible resulting in less stress shielding). The stiffer MBT carries a higher proportion of the load down the stem. MBT stress in cancellous bone is lower than APE, as load is distributed to the cortical rim. APE has a marginally favourable strain state in cancellous bone and spreads loads more at the cement interface than MBT. CONCLUSION: Modern-day APE bearings may be superior to previously designed implants due to improvements in manufacturing. In the correct patient group, this could offer substantial cost savings.

Finite element assessment of block-augmented total knee arthroplasty.

Loosening and migration of tibial prostheses have been identified as causes of early total knee replacement (TKR) failure. The problem is made more complex when defects occur in the proximal tibia compromising fixation and alignment. Clinical studies using metal augments have shown these to be an alternative to other means of defect treatment. Finite element (FE) analysis can be used to identify regions that may be prone to loosening and migration. In the current work, 3D FE models of TKR uncontained type-2 defects treated with block augments have been constructed and analysed. It has been shown that a metal augment is the most suitable. The use of bone cement (PMMA) to fill proximal defects is not considered suitable as stresses carried by the cement block exceed those of the fatigue limit of bone cement. It has been shown that the stresses in the proximal cancellous bone of block-augmented models are significantly below levels likely to cause damage due to overloading. Furthermore, the use of stem extensions has been shown to reduce the cancellous bone stresses in the proximal region thus increasing the likelihood of bone resorption. Given this, it is recommended that stem extensions are not required unless necessary to mitigate some other problem.

Maximum surgical blood ordering schedule in a district general hospital saves money and resources.

A 6-month prospective audit was carried out in three surgical departments of a district general hospital. Over that period, 2720 units of red cells were electively cross-matched, 957 being transfused. The overall cross-match-to-transfusion ratio (CTR) was 2.8, but this varied from over 40 for some gynaecological procedures to 1.5 for major surgical procedures. The average CTR for general surgery was 2.2, orthopaedics 2.3, and obstetrics and gynaecology 5.7. A maximum surgical blood ordering system (MSBOS) was introduced and a second 6-month audit carried out. The number of units cross-matched had fallen by 36% to 1746, with a CTR of 1.8. The change in activity had led to a saving conservatively estimated at 11,616.00 Pounds per annum. Local audit and the introduction of a MSBOS in a district general hospital is an exercise which can demonstrate inefficiencies in blood ordering practices and can lead to large financial savings without detracting from standards of patient care.