The aim of the study was to examine the stress and strain relationships in proximal femurs, using finite element analysis techniques. We looked at normal, osteoporotic and osteoarthritic models, to detect any differences, and specifically, in relation to neutral or valgus alignments of the femoral components in a cemented prosthetic femoral head resurfacing situation. A CAD model of a third-generation composite femur was virtually operated upon to implant the femoral component. The femoral component, geometry was of a 54 mm Birmingham hip resurfacing. A 1 mm cement mantle was allowed for. Finite element model is were generated with 10 node tetrahedral elements. The material properties of both cortical and cancellous bone were assigned according to standard parameters.

Our analysis of the stress and strain in the resurfaced femoral head under the implant showed significant reductions in the stress and strain compared to the intact femur and this was the case for all stem-bone interface conditions. This region of high stress and strain was not seen in the model with the stem was overreamed and there was no bone contact with the stem. The stress and strain levels were generally higher when osteoporotic bone was modelled. The peak maximum tensile stress and strain in the cortical bone at the superolateral femoral neck was 4% to 24% greater in the resurfaced femur for all by the conditions with valgus implant positioning experiencing high at peak stresses and strain then neutral alignment. Maximum tensile stress in the cement at the had- implant rim junction was not greatly different for the different bone conditions except for osteoporosis where the stress was almost 50% greater than the other bone conditions. Generally the highest tensile stresses occurred anteroinferiorly and were greater in the neutral alignment than in the valgus alignment. The superolateral offset associated with a valgus orientation, rather than the valgus orientation itself maybe what reduces the stress and strain in the neck leading to a lower incidence of fracture. Stresses were lower than 8 MPa, the fatigue strength of cement, for all the valgus models except osteoporosis. All neutral models contained some locations where the tensile stress exceeded 8 MPa.

The postoperative stress and strain in the femoral head and neck maybe increased in comparison to the intact femur. Under the component there may be significant reduction in stress and strain, causing resorbtion. The biomechanical reason why a more valgus orientation protects against femoral neck fracture is more complex, sends in some critical locations stress and strain has reduced but in others it is increased. Further study is being planned.

Introduction This prospective outcome study presents the results and complications of 41 lateral unicompartmental knee joint replacements.

Methods One surgeon operated on all the patients in this series. The surgery was performed through minimally invasive techniques wtih the patients being day-stay or overnight stay patients. Assessment was made using SF-36 and WOMAC questionnaires, physical examination, x-ray pre-operatively and at six monthly intervals. Kaplan-Meier survival analysis was carried out.

Results Forty-one knees (39 patients) underwent surgery. There were 15 males (average age 64 years) and 26 females (average age 68 years). Mean follow-up time was 3.2 years (max 4.6 years). Of these four were Repicci inlay components, 10 were Repicci onlay components and 28 were Oxford mobile bearing implants. Three patients required reoperation, all having mobile bearings in place. One required revision to total knee replacement for progression of arthritis in the medial compartment, and two for bearing dislocation. The operation consisted of a change of bearing to a thicker one. One of these patients had a further complication, a deep infection which was treated successfully with arthroscopic debridement and antibiotics. Another of the mobile bearing patients had a DVT. WOMAC and SF36 show a reduction in pain and stiffness, and an increase in physical function and quality of life. Kaplan-Meier analysis shows 100% survival of the fixed implants but 96% in the mobile bearing group. However 11% of the mobile bearing group required reoperation.

Conclusions Lateral compartment replacement is a technically demanding procedure. This study looks at the early results, and does not examine long-term wear. A significant difference in the complication rates for the different type of implant is noted, with the mobile bearing having a higher reoperation rate.

In relation to the conduct of this study, one or more of the authors is in receipt of a research grant from a non-commercial source.