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Biomedical subjects

R M Hooper

Publications and source records attributed to R M Hooper.

7 recordsLinked to original sources
PubMed2006

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.

Arthroplasty, Replacement, Hip↗
PubMed2004

In vivo surface wear mechanisms of femoral components of cemented total hip arthroplasties: the influence of wear mechanism on clinical outcome.

The appearance and mechanism of femoral stem wear was studied in 172 retrieved femoral components, of which 74 stems had been stable in vivo. Macroscopic, microscopic, and nano-level scales of examination were used. Loss of stem surface in response to micromotion (wear) was found to affect 93% of stems. However, changes were frequently difficult to see with the naked eye, and in 19% of cases they would have been missed completely without the use of light microscopy. The surface finish of the prosthesis determined the mechanism of stem wear. Matte surfaces showed typical abrasive processes that also damage the cement, releasing particulate debris from the cement and metal surfaces. This may destabilize the stem within the cement. Polished stems showed a typical fretting appearance with retention of debris on the stem surface and without significant damage to the cement. These differences in wear mechanism between matte and polished stems have significant effects on stem function.

Arthroplasty, Replacement, Hip↗
PubMed2004

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.

Arthroplasty, Replacement, Hip↗
PubMed2003

Investigation of the mechanical performance of young glass-ionomer cement using dynamic mechanical analysis.

Glass-ionomer (or more correctly, glass polyalkenoate) cements have wide applications in dentistry. This paper reports an investigation using dynamic mechanical analysis (DMA) on the setting of typical conventional glass-ionomer cements of varying age. Rectangular section cement samples were stored for four weeks in distilled water at 37 degrees C before being tested. The experimental procedure involved the clamping of the sample in tensile mode and heating through a 37-95 degrees C temperature range in water. A general behavioral trend was followed where all the samples showed increased flexibility with rise in temperature until a "threshold" temperature was reached, whereupon sudden tensile stiffening was observed. The temperature at which the stiffening took place was dependent on the age of the cement, and was interpreted in terms of the secondary cement forming reactions of silica and phosphate. The younger samples stiffened at significantly lower temperatures than the older ones. The activated nature of glass-ionomer setting chemistry meant that younger cements could be prematurely aged through heating. Differential scanning calorimetry was used to study the effect of heating on the distribution of loosely bound water in the cements that had, and had not been exposed to a DMA cycle. Most notably, it was seen that the DMA process did not affect the water in the matrix. This leant further credence to the hypothesis that the stiffening observed during the DMA heating process was caused by accelerated network formation.

Journal Article↗
PubMed2002

Stress relaxation modelling of polymethylmethacrylate bone cement.

This paper describes tests that were carried out to model the stress relaxation behaviour of polymethylmethacrylate (PMMA) bone cement. Stress relaxation of bone cement is believed to be a significant factor in the mechanism of load transfer in the femoral stem of a polished, collarless taper-fit replacement hip joints. It is therefore important that this condition and its implications are understood. Stress relaxation was carried out on PMMA samples of varying age in four-point bending configuration. It was shown that the samples stiffened with age and that the amount of stress relaxation reduced as the samples aged. The experimental results of the stress relaxation were accurately modelled on the double exponential of the Maxwell model so that long-term predictions of the stress condition could be made from short-term mechanical tests.

Bone Cements↗