Aim

The incidence of prosthetic joint infections can be severe to monitor, as they are rare events. Recent publications from National registries points toward a significant underestimation of reported infections. The aim of this project was to develop a complication register that could report the “true” and momentaneous incidence of prosthetic infections after total knee and hip arthroplasty.

Objective

In ex vivo hip fracture studies femoral pairs are split to create two comparable test groups. When more than two groups are required, or if paired femurs cannot be obtained, group allocation according to bone mineral density (BMD) is sometimes performed. In this statistical experiment we explore how this affects experimental results and sample size considerations.

Background: Resurfacing hip arthroplasty has re-emerged as an option in total hip arthroplasty and by 2008 these prostheses constituted 7.8% of the total number of primary hip replacements in Australia. In the Scandinavian countries the use of resurfacing prostheses is substantially less, reported from 0.6–2.8% in the different national arthroplasty registries. The resurfacing implant preserves proximal bone stock and is expected to retain a physiological load transfer in the proximal femur. Mid-term results for the resurfacing implants are promising, but periprosthetic neck fractures remains the most frequent complication. Finite element analyses have suggested increased strains in the femoral neck area after resurfacing arthroplasty. This has not yet been proved in a cadaver model.

Purpose: This study compared the strain pattern of the femoral neck and the proximal femur in cadaver femurs before and after insertion of a resurfacing femoral component.

Material and method: When load transfers trough the hip joint to the femur, the bone undergoes a deformation, which can be measured by strain gauges. In this study, ten strain gauge rosettes were distributed on the femoral neck and proximal femur of thirteen human cadaver femurs. The femurs were loaded in a hip simulator for single leg stance and stair climbing. Cortical strains were measured on the femoral neck and proximal femur before and after implantation of a resurfacing femoral component (DePuy ASRTM).

Results: After resurfacing the mean tensile strain increased by 15 % (CI: 6 – 24%, p=0.003) on the lateral femoral neck, and mean compressive strain increased by 11 % (CI: 5 – 17%, p=0.002) on the medial femoral neck during single leg stance simulation. On the anterior side of the femoral neck the strain increased up to 16%, however this difference was not found statistically significant. On the proximal femur the deformation pattern remained similar to the strains measured on the unoperated femurs.

Discussion: Both patient related factors such as female gender, obesity and high age, and surgical factors such as notching, lack of seating and varus-orientation of the implant have been associated with increased risk of neck fracture after resurfacing arthroplasty. We asked ourselves if there could be a biomechanical factor contributing to the risk of periprosthetic fracture. The small increase of strains in the neck area would probably not alone be sufficient to cause a neck fracture. Acting together with patient-specific and surgical factors it may however contribute to the risk of early periprosthetic fracture.

The cortical strains on the femoral neck and proximal femur were measured before and after implantation of a resurfacing femoral component in 13 femurs from human cadavers. These were loaded into a hip simulator for single-leg stance and stair-climbing. After resurfacing, the mean tensile strain increased by 15% (95% confidence interval (CI) 6 to 24, p = 0.003) on the lateral femoral neck and the mean compressive strain increased by 11% (95% CI 5 to 17, p = 0.002) on the medial femoral neck during stimulation of single-leg stance. On the proximal femur the deformation pattern remained similar to that of the unoperated femurs.

The small increase of strains in the neck area alone would probably not be sufficient to cause fracture of the neck However, with patient-related and surgical factors these strain changes may contribute to the risk of early periprosthetic fracture.

Hydroxyapatite-coated standard anatomical and customised femoral stems are designed to transmit load to the metaphyseal part of the proximal femur in order to avoid stress shielding and to reduce resorption of bone. In a randomised in vitro study, we compared the changes in the pattern of cortical strain after the insertion of hydroxyapatite-coated standard anatomical and customised stems in 12 pairs of human cadaver femora. A hip simulator reproduced the physiological loads on the proximal femur in single-leg stance and stair-climbing. The cortical strains were measured before and after the insertion of the stems.

Significantly higher strain shielding was seen in Gruen zones 7, 6, 5, 3 and 2 after the insertion of the anatomical stem compared with the customised stem. For the anatomical stem, the hoop strains on the femur also indicated that the load was transferred to the cortical bone at the lower metaphyseal or upper diaphyseal part of the proximal femur.

The customised stem induced a strain pattern more similar to that of the intact femur than the standard, anatomical stem.

In the past it has been widely accepted that bone remodelling of the proximal femur after cementless total hip replacement is a result of the altered mechanical environment. Usually, there is are distribution of the stresses in the bone, and subsequently bone mass, from the metaphysis to the proximal part of the diaphysis. The design rationale for some cementless stems is to transmit load to the proximal femur and thus to preserve the bone mineral content in this area. The aim of the present study was to investigate the relationship between postoperative strain shielding of the proximal femur and the bone remodelling after insertion of two different cementless femoral stems.

Experimental study: Twelve pairs of human cadaveric femurs were instrumented with strain gauge rosettes in Gruen zones2 to 7 and the cortical strains were measured during simulation of one leg stance before and after insertion of a custom stem (Unique, SCP) or an anatomic stem (ABG, Stryker-Howmedica).

Clinical study: In a prospective, randomized study including 80 patients, the same types of stems were inserted and the bone mineral density (BMD) was measured during the first two years postoperatively using DEXA. Then, the pattern of remodelling was compared with the gradient of strain shielding in each of the Gruen zones in the frontal plane.

In Gruen zone 7 the relative cortical strain shielding was45% in the femurs with a custom stem and 87% in the femurs with an anatomic stem. In zone 6 the corresponding figures were 2% and 38%, in zone 5 0% and15% and in zone 3 0% and 20%. The DEXA measurements showed a decrease in BMD in zone 7 of 22% and 23% for the two stems, respectively. In the other zones the bone loss was smaller and there was no difference between the stems.

In the proximal zones there was a highly significant difference in strain shielding between femurs receiving a customor an anatomic stem. However, there was no difference in the pattern of bone remodelling. The bone remodelling around these two stems does not seem to mirror the gradient of strain shielding.