The mechanical properties of recovered PMMA bone cement: a preliminary study.

Samples of polymethylmethacrylate (PMMA) bone cement, used in the fixation of hip prostheses, have been recovered from 11 patients after in service life spans of between 15 and 24 years. Eighteen samples in total have been recovered from the acetabular and/or femoral cement. Samples were subjected to three point bending tests, their density, porosity and microhardness determined and all specimens were examined using EDX and X-ray techniques. Since the porosity of many of the samples is very high, the continuous matrix properties are inferred from the performance of individual specimens. No evidence has been found to suggest that the PMMA has deteriorated whilst in-vivo and the mechanical properties of the cement matrices appear to be comparable to freshly made PMMA.

Assessment of wear on the cones of modular stainless steel Exeter hip stems.

The wear on the stem cones of retrieved Exeter Universal hip stems has been assessed using scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX) and surface profilometry. The in-service life of these prosthetic stems varied, up to a maximum of 7 years. A combination of SEM, EDX and visual assessment indicates that the stem cones have not suffered from any corrosion. SEM scans indicate that damage to stem cones (excluding extraction and post-removal damage) can be categorised into insertion marks and fretting marks. In some cases there are signs of material being deposited on the cone surface. Surface profilometry suggests that the levels of debris generation at the cone/internal head interface are very low relative to those that are likely to be associated with head articulation against the acetabular cup. A total of 20 stem cones underwent SEM scans. From these, 10 subsequently have undergone surface profilometry along with the corresponding internal head surfaces. There is a good correlation between surface roughness measured by surface profilometry and the topography observed in the SEM images. The surface roughness of each stem cone is similar to that of the corresponding internal head surface.