Changes in proximal femoral strain after insertion of uncemented standard and customised femoral stems

Abstract

We have compared the changes in the pattern of the principal strains in the proximal femur after insertion of eight uncemented anatomical stems and eight customised stems in human cadaver femora. During testing we aimed to reproduce the physiological loads on the proximal femur and to simulate single-leg stance and stair-climbing. The strains in the intact femora were measured and there were no significant differences in principal tensile and compressive strains in the left and right femora of each pair. The two types of femoral stem were then inserted randomly into the left or right femora and the cortical strains were again measured. Both induced significant stress shielding in the proximal part of the metaphysis, but the deviation from the physiological strains was most pronounced after insertion of the anatomical stems. The principal compressive strain at the calcar was reduced by 90% for the anatomical stems and 67% for the customised stems. Medially, at the level of the lesser trochanter, the corresponding figures were 59% and 21%. The anatomical stems induced more stress concentration on the anterior aspect of the femur than did the customised stems. They also increased the hoop strains in the proximomedial femur. Our study shows a consistently more physiological pattern of strain in the proximal femur after insertion of customised stems compared with standard, anatomical stems.

Differences in External and Internal Cortical Strain with Prosthesis in the Femur

“…Our findings correspond to the results reported by other authors [5, 18, 19]. However, strain studies on the external cortex have been used uncritically to predict the in vivo performance, as the stress at the internal interface between the implant and endosteal bone never has been studied.…”

“…Much work has been done to obtain a design of the stem that maintains physiological strain after insertion of the implant [5, 18-20]. Strain gauge measurements usually have been employed to study the pattern of load transfer from the stem to the femur after insertion of a prosthesis.…”

“…Thereafter 1000 N axial load was applied and strain gauge measurements on the intact femur were recorded with a sampling rate of 50 Hz during the middle 3 seconds of the loading cycle while the load was maintained at 1000 N. The minimum and maximum principal surface strain and corresponding principal direction were calculated through strain transformation [5]. Determination of principal direction in degrees from the strain gauges was done by defining the horizontal gauge of each rosette as the zero degree axis.…”

“…This theory has been validated by in vitro [4] and in vivo strain gauge measurements [5]. Insertion of a prosthesis has been shown to alter the strain patterns of the proximal femur which may have adverse effects on the dynamic bone remodeling and the ultimate fate of the prosthesis [6, 7].…”

Changes in strain patterns after implantation of a short stem with metaphyseal anchorage compared to a standard stem: an experimental study in synthetic bone

“…6,8,10,23,24 These studies uniformly revealed a dramatic reduction of the principal strains in the proximal part of the femur for both stem types with the greatest decreases at the calcar especially for the standard stems. Customised proximal fit stems and traditional stems all display non-physiological strain patterns within the proximal femur, with slightly more physiological patterns for the customised stems.…”