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 effect of activity levels of total hip arthroplasty patients on socket penetration.

Survivorship of total hip arthroplasties (THAs) has been linked to penetration of the femoral head into the polyethylene acetabular cup and to polyethylene wear. The activity level of patients with THAs is considered to be an important factor affecting wear, and the purpose of this study was to explore the relationship between activity as recorded by pedometers and cup penetration. The measurement of daily activity levels of normal subjects and THA patients of various ages are discussed in another article ([1]). Subjects were monitored continuously during 2- to 4-week periods using simple pedometer devices. Patients (n = 54; mean age, 58 years) from the Centre for Hip Surgery at Wrightington Hospital for whom the penetration data also were available were included in the study. The average activity level for the patient group was 1.426 million loading cycles on each hip joint per year. Radiographic penetration measurements were compared for 81 hips in 54 patients with a mean follow-up of 13.1 years (range, 1.3-26.4 years). The overall correlation of penetration with implantation period is known to be poor, however, and did not improve significantly when the penetration was plotted against a new parameter that took account of not only implant life, but also the level of activity and patient weight. The considerable scatter of penetration levels was noted to increase with increasing implantation period, which indicates that in the multifactorial problem of polyethylene wear, other factors, particularly femoral head surface finish or polyethylene deterioration, may predominate.

Comparative study of the activity of total hip arthroplasty patients and normal subjects.

The walking activity of normal subjects and total hip arthroplasty (THA) patients from the Wrightington Hospital for Joint Disease and The General Infirmary at Leeds was assessed by means of electronic pedometers. The principal objectives were to establish the extent to which joint arthroplasty patients recover their activity relative to normal subjects and to establish the number of loading cycles to which prostheses should be subjected in joint simulator studies of implant performance. A further objective was to establish an experimental procedure for the assessment of the role of activity in contributing to the well-known scatter in the measurements of femoral head penetration into acetabular cups in in vivo studies of implant performance. The last-mentioned issue is addressed in another article ([1]). The walking activity of 2 normal subjects of disparate ages was assessed during 1 full year. It was concluded that fair estimates of activity could be achieved by recording pedometer readings during successive 2- to 4-week periods. This approach was adopted in the full assessment of the walking activity of cohorts of normal subjects and THA patients. Linear regression expressions relating the number of steps taken daily and the annual number of loading cycles on each leg to age are presented for normal subjects and THA patients. In all cases, activity declines with age, but it is shown that total joint arthroplasty is not at all restrictive on walking activity-a remarkable testimony to the efficacy of total joint arthroplasty. Attention is drawn, however, to different levels of activity of THA patients recorded in the present United Kingdom study and a similar survey conducted in California.

The effect of femoral head diameter upon lubrication and wear of metal-on-metal total hip replacements.

It has been found that a remarkable reduction in the wear of metal-on-metal hip joints can be achieved by simply increasing the diameter of the joint. A tribological evaluation of metal-on-metal joints of 16, 22.225, 28 and 36 mm diameter was conducted in 25 per cent bovine serum using a hip joint simulator. The joints were subject to dynamic motion and loading cycles simulating walking for both lubrication and wear studies. For each size of joint in the lubrication study, an electrical resistivity technique was used to detect the extent of surface separation through a complete walking cycle. Wear of each size of joint was measured gravimetrically in wear tests of at least 2 x 10(6) cycles duration. Joints of 16 and 22.225 mm diameter showed no surface separation in the lubrication study. This suggested that wear would be proportional to the sliding distance and hence joint size in this boundary lubrication regime. A 28 mm diameter joint showed only limited evidence of surface separation suggesting that these joints were operating in a mixed lubrication regime. A 36 mm diameter joint showed surface separation for considerable parts of each walking cycle and hence evidence of the formation of a protective lubricating film. Wear testing of 16 and 22.225 mm diameter metal-on-metal joints gave mean wear rates of 4.85 and 6.30 mm3/10(6) cycles respectively. The ratio of these wear rates, 0.77, is approximately the same as the joint diameters ratio, 16/22.225 or 0.72, as expected from simple wear theory for dry or boundary lubrication conditions. No bedding-in was observed with these smaller diameter joints. For the 28 mm diameter joint, from 0 to 2 x 10(6) cycles, the mean wear rate was 1.62 mm3/10(6) cycles as the joints bedded-in. Following bedding-in, from 2.0 x 10(6) to 4.7 x 10(6) cycles, the wear rate was 0.54 mm3/10(6) cycles. As reported previously by Goldsmith et al. in 2000 [1], the mean steady state wear rate of the 36 mm diameter joints was lower than those of all the other diameters at 0.07 mm3/10(6) cycles. For a range of joints of various diameters, subjected to identical test conditions, mean wear rates differed by almost two orders of magnitude. This study has demonstrated that the application of sound tribological principles to prosthetic design can reduce the wear of metal-on-metal joints, using currently available materials, to a negligible level.

Wear of fixed bearing and rotating platform mobile bearing knees subjected to high levels of internal and external tibial rotation.

In order to extend the lifetime of total knee replacements (TKR) in vivo, reduction of the volumetric wear rate of ultra high molecular weight polyethylene (UHMWPE) bearings remains an important goal. The volume of wear debris generated in fixed bearing total knee devices increases significantly when subjected to higher levels of internal-external rotation and anterior-posterior displacement. Six PFC Sigma fixed bearing TKR were compared with six LCS rotating platform mobile bearing knees using a physiological knee simulator with high rotation kinematic inputs. The rotating platform polyethylene inserts exhibited a mean wear rate which was one-third of that of the fixed bearing inserts despite having increased femoral contact areas and additional tibial wear surfaces. The rotating platform design decouples knee motions, by allowing unidirectional motion at the tray-insert articulation, which reduces rotation at the femoral-insert counterface. This translation of complex knee motions into more unidirectional motions results in molecular orientation of the UHMWPE and reduced volumetric wear.

A frictional study of total hip joint replacements.

Polymeric wear debris produced by articulation of the femoral head against the ultra-high-molecular-weight polyethylene socket of a total hip replacement has been implicated as the main cause of osteolysis and subsequent failure of these implants. Potential solutions to this problem are to employ hard bearing surface combinations such as metal-on-metal or ceramic-on-ceramic prostheses. The aim of this study was to investigate the difference in lubrication modes and friction of a range of material combinations using synthetic and biological fluids as the lubricants. The experimental results were compared with theoretical predictions of film thicknesses and lubrication modes. A strong correlation was observed between experiment and theory when employing carboxy methyl cellulose (CMC) fluids as the lubricant. Under these conditions the ceramic-on-ceramic joints showed full fluid film lubrication while the metal-on-metal, metal-on-plastic, diamond-like carbon-coated stainless steel (DLC)-on-plastic and ceramic-on-plastic prostheses operated under a mixed lubrication regime. With bovine serum as the lubricant in the all ceramic joints, however, the full fluid film lubrication was inhibited due to adsorbed proteins. In the metal-on-metal joints this adsorbed protein layer acted to reduce the friction while in the ceramic coupling the friction was increased. The use of bovine serum as the lubricant also significantly increased the friction in both the metal-on-plastic and ceramic-on-plastic joints. The friction produced by the DLC-on-plastic joints depended on the quality of the coating. Those joints with a less consistent coating and therefore a higher surface roughness gave significantly higher friction than the smoother, more consistently coated heads.

A comparative joint simulator study of the wear of metal-on-metal and alternative material combinations in hip replacements.

While total hip replacement represents the major success story in orthopaedic surgery in the twentieth century, there is much interest in extending even further, early in the twenty first century, the life of implants. Osteolysis has been identified as a major factor limiting the life of prostheses, with indications that fine polyethylene wear debris, generated primarily at the interface between the femoral head and the acetabular cup, promotes the process. There is therefore considerable interest in the introduction of alternative wear resistant systems to limit the deleterious effects of wear. These alternatives include ceramic-on-ceramic and metal-on-metal configurations and the present paper is primarily concerned with the latter. Some six pairs of new metal-on-metal implants of 36 mm diameter and four pairs of existing metal-on-metal implants of 28 mm diameter were tested in a ten-station hip joint simulator in the presence of a 25 per cent bovine serum solution. The implants were tested in the anatomical position to 5 x 10(6) cycles. The new heads and cups were manufactured from CoCrMo alloy with careful attention being paid to sphericity and surface finish of both components. The wear performance of the new and existing metal-on-metal total hip replacements have been evaluated and compared. The overall wear rates have then been compared with previously reported wear rates for a zirconia-on-polyethylene prosthesis of 22 mm diameter tested on the same simulator. The comparison is taken further by recalling published penetration data for metal-on-polyethylene implants of 22 and 28 mm diameter and converting these to volumetric wear rates. It was found that the heads and cups in metal-on-metal joints wore by almost equal amounts and that the opposing surfaces converged to similar surface roughness as the testing time increased. Steady state wear rates were generally achieved after 1-2 x 10(6) cycles. The mean long-term wear rates for the metal-on-metal prostheses were very low, being 0.36 mm3/10(6) cycles and 0.45 mm3/10(6) cycles for the new implants of 36 mm diameter and established implants of 28 mm diameter respectively. These wear rates compare with 6.3 mm3/10(6) cycles for zirconia-on-ultra-high molecular weight polyethylene tested on the same simulator and representative clinical values for metal-on-polyethylene of 36 mm3/year for heads of 22 mm diameter and a reported range of 60-180 mm3/year for 28 mm heads. These values do not translate directly into numbers of particles, since the metallic debris from metal-on-metal joints is very fine. The number of metallic particles may exceed the number of polyethylene wear particles from an otherwise similar metal-on-polyethylene joint by a factor of 10(3). A detailed discussion of the size and morphology of wear debris and tissue reaction to various forms of debris is beyond the scope of this paper, but the biological response to polymeric, metallic and ceramic wear debris forms a major subject for further study. The present investigation nevertheless confirms the potential of carefully designed and manufactured metal-on-metal total replacement joints for the treatment of diseased and damaged hips.

Development of a ten-station, multi-axis hip joint simulator.

Joint simulators are now used extensively to evaluate the performance of materials and designs of total replacement hip and knee joints. In this Technical Note a new ten-station hip joint simulator with biaxial rotational articulation synchronized to a physiological loading cycle is described. The current simulator manufactured by ProSim Limited (Manchester, UK) is a development of a first generation machine designed and built in-house at DePuy International Limited (Leeds, UK). The use of this new form of ten-station hip simulator to evaluate the performance of 22 mm zirconia femoral heads and ultra-high molecular weight polyethylene acetabular cups over some 7 million loading cycles is described elsewhere [1].

A multi-station hip joint simulator study of the performance of 22 mm diameter zirconia-ultra-high molecular weight polyethylene total replacement hip joints.

The commissioning of a new form of 10-station hip joint simulator is described and the results of a study of the performance of zirconia-ultra-high molecular weight polyethylene (UHMWPE) total replacement hip joints in the familiar Charnley head size of 7/8 inch (22.225 mm) diameter are presented. The head size is referred to as 22 mm for brevity and consistency throughout the paper. The simulator provided very consistent and repeatable results and the new machine, together with the methods of investigation adopted, offer an excellent facility for the further evaluation of existing and new prostheses. The findings are compared with the outcome of previous laboratory simulator and clinical studies of ceramic-polyethylene implants of similar diameter. It was found that a relatively rapid penetration of the head into the cup was followed by a very low, steady, long-term penetration rate after about two million loading cycles. The mean long-term volumetric penetration rate was 6.28 mm3/10(6) loading cycles. When the linear penetration rates were assessed by direct measurement on a coordinate measuring machine, or deduced from the tunnelling expression, the resulting values were very similar and small at 0.019 and 0.016 mm/10(6) loading cycles respectively. It is generally assumed that one million loading cycles is equivalent to about one year of service in the body and if this equivalence is accepted, these penetration rates compare very favourably with a clinical evaluation of alumina heads of the same diameter, which yielded a mean long-term penetration rate of 0.022 mm/year.

Investigation into the biphasic properties of a hydrogel for use in a cushion form replacement joint.

A hydrogel with potential applications in the role of a cushion form replacement joint bearing surface material has been investigated. The material properties are required for further development and design studies and have not previously been quantified. Creep indentation experiments were therefore performed on samples of the hydrogel. The biphasic model developed by Mow and co-workers (Mak et al., 1987; Mow et al., 1989a) was used to curve-fit the experimental data to theoretical solutions in order to extract the three intrinsic biphasic material properties of the hydrogel (aggregate modulus, HA, Poisson's ratio, Vs, and permeability, k). Ranges of material properties were determined: aggregate modulus was calculated to be between 18.4 and 27.5 MPa, Poisson's ratio 0.0-0.307, and permeability 0.012-7.27 x 10(-17) m4/Ns. The hydrogel thus had a higher aggregate modulus than values published for natural normal articular cartilage, the Poisson's ratios were similar to articular cartilage, and finally the hydrogel was found to be less permeable than articular cartilage. The determination of these values will facilitate further numerical analysis of the stress distribution in a cushion form replacement joint.

Application of finite elements to the stress analysis of articular cartilage.

A common effect of arthritic disease processes in synovial joints is deterioration of the articular cartilage. Therefore, an improved understanding of the relationships between the composition and structure of articular cartilage and the mechanical behaviour is a subject of considerable interest. The numerical modelling tool of finite element (FE) analysis has been widely applied to analyse the behaviour of articular cartilage under compressive stress. FE analysis enables parameters and boundary conditions to be investigated which are not accessible experimentally or analytically. The biphasic theory describes the constitutive behaviour of soft hydrated biological tissues, such as articular cartilage, and has been successfully implemented using FE analysis. The development of successively more comprehensive biphasic models is described here detailing the use of FE analysis in modelling experimental configurations such as indentation. Key work in the area is reviewed in this paper.

Effects of acute alcohol exposure during selected days of gestation in C3H mice.

This investigation was designed to evaluate the teratogenic potential of a single alcohol exposure on one of nine specific days of gestation using a mouse model. C3H mice were intubated with alcohol on either Day 7, 8, 9, 10, 11, 12, 14, 16, or 18 of pregnancy in a dose of 0.0, 2.5, or 5.0 g/kg. On day 19 of gestation, the fetuses were removed by caesarian section, weighted, and fixed in Bouin's solution for subsequent free-hand sectioning. The results demonstrated that (1) acute prenatal alcohol exposure tended to decrease fetal body weight as the dose of alcohol increased, an effect that was most pronounced following exposure on Day 18 of gestation, and (2) neither the dose of alcohol nor the day of acute exposure significantly influenced implantations, resorptions, dead fetuses, or the incidence of fetal malformations across groups.