Demographic and occupational effects on the activity levels of normal subjects in the United Kingdom.

A study of activity levels, measured in steps per day, was made of 293 subjects from several parts of the United Kingdom. Each subject wore a pedometer adjacent to the hip for two weeks. The average number of steps taken each day over a two-week period was recorded together with additional details such as age and occupation. A general decline in activity with age was observed, which varied slightly for men and women. At twenty years of age the sample of men typically walked about 9,000 steps per day and women 9,200 steps per day, declining to 6,100 and 5,750 steps per day for men and women respectively at sixty years of age. Activity was observed to vary considerably from day to day and was more pronounced for men. The mean number of steps per day for a wide range of occupational groups varied from around 4,500 steps per day for retired persons to 12,700 steps per day for postmen. The activity data was re-analysed to remove the influence of age and gender. The activity of most occupational groups then fell within a relatively narrow range of approximately 7,700 to 8,850 steps per day. The exceptions to this were postmen, nurses and technicians, with averages of 12,750, 9,950 and 9,900 steps per day respectively, and software programmers averaging only 5,250 steps per day. Activity levels for groups of subjects from different parts of the United Kingdom were compared, including Leeds and Bradford, Belfast, Teesside, County Durham and Edinburgh. When the number of subjects in each group was sufficiently large with a wide spread of ages and occupational types, it was found that demography had little effect upon the level of activity. This study provides a substantial new database, based on the UK population. In addition, the assumption that one million cycles in vitro is equivalent to one year in vivo is well supported and relevant to simulator studies. (Hip International 2002; 1: 28-36).

The prediction of polyethylene wear rate and debris morphology produced by microscopic asperities on femoral heads.

Counterface damage in the form of scratches, caused by bone cement, bone or metallic particles, has been cited as a cause of increased wear of ultra-high molecular weight polyethylene (UHMWPE) acetabular cups. It is known that high levels of particulate wear debris lead to osteolysis. Surface damage was characterized in a series of explanted Charnley femoral heads. The heads had a mean scratch height of 1 microm with a mean aspect ratio (defined as height divided by half width) of 0.1. Wear discs were artificially scratched using these scratch geometries as a guide. In addition, the scratch geometries were incorporated into a finite element model of a stainless steel asperity repeatedly sliding over UHMWPE under conditions similar to those in an artificial hip joint. Wear tests showed a strong correlation between the average cross-sectional area of the scratch lip above the mean zero line and the measured wear factor. The finite element model predicted increases in the area of UHMWPE suffering plastic strain with increases in the cross-sectional area of the asperity above the mean line. Analysis of the wear debris showed the mode of the particle size was 0.01-0.5 microm for all cases. The morphology of the particles varied with aspect ratio of the asperity, with an increased percentage mass of submicrometer-sized debris with increased scratch lip aspect ratio. The finite element results predicted that the maximum surface strains would increase with increasing asperity aspect ratio. Examination of the worn UHMWPE pin surfaces showed an association between increased surface damage, probably due to high surface strains, and increased aspect ratio. The large areas of surface plastic strain predicted for asperities with high cross-sectional areas above the mean line offer an explanation for the positive correlation between wear rate and the average cross-sectional area of the scratch lip material. The higher surface strains predicted for the higher aspect ratios may explain the increased percentage mass of biologically active submicrometer-sized wear particles found for scratch lips with higher aspect ratios.

Modeling of damage to articulating surfaces by third body particles in total joint replacements.

Numerous small scratches and some larger scratches have been observed on metallic femoral heads of explanted hip prostheses, with the larger scratches believed to be a major contributor to increased wear of the polyethylene acetabular cups. Previous work in our group has shown that smaller scratches, with a mean lip height up to 0.35 microm, can be caused by bone cement and bone particles up to 500 microm in size. However, the larger scratches were not readily replicated with these particles. Therefore in this study experimental and theoretical models have been developed to investigate the damage caused by harder metallic and ceramic particles. Small 10 microm diameter spherical metallic particles were also found to produce small fine scratches on the metallic counterface. However larger diameter spherical metal particles greater than 100 microm in diameter, which were embedded in polyethylene pins, caused severe sharp scratching of the metallic counterface with scratch lips greater than 0.5 microm. This level of damage, which was comparable to the severe damage found in vivo, was also simulated by a three body finite element model. Thus the larger metal particles led to the type of damage which was predicted to increase wear dramatically. This technique for simulating severe in vivo third body damage using spherical metal particles was found to be reproducible and reliable and will be used in the future in hip simulator testing to replicate third body damage and wear.