Characterization of ultra-high molecular weight polyethylene.

The density, ash content and hardness were determined for ultra high molecular weight polyethylene from a single block of medical grade RCH 1000. The effect on these parameters of position within the block and of 2.5 Mrad irradiation was determined. The density was 942 +/- 1 kgm-3. It varied slightly with position and increased slightly after irradiation. The ash content was less than 200 ppm, and varied slightly with position. The ash contained Ca, Al and Si. The hardness (DIN 53456) was 36 MNm-2 and no variation due to position or irradiation could be detected. It was concluded that the observed variations would be unlikely to have a significant effect on the wear behaviour.

Metal ion release after total hip replacement.

The concentration of cobalt (Co), chromium (Cr), molybdenum (Mo), nickel (Ni), iron (Fe) and zinc (Zn) was measured using neutron activation analysis in tissue taken from an 81 year old female at necropsy. The patient had bilateral cobalt chromium molybdenum (Co-Cr-Mo) total hip replacements: one, a metal-on-metal had been in place for 14 years, the other a metal-on-plastic for 5.5 years. Although the metal-on-metal side had become painful, the patient remained active until she died. The measurements indicated that the concentrations of Co and Cr in the lung, kidney, liver and spleen were up to fifty times "Standard Man" values. High values occurred also in the urine and in the hair. The tissue adjacent to the metal-on-metal joint was heavily laden with metal wear debris, whereas that adjacent to the metal-on-plastic joint was relatively uncontaminated. The concentration varied with distance from the implant. Co predominated in the urine, whereas Cr predominated near the implants. The existence of such high levels, especially in the organs, is a possible cause for concern.

Wear studies on prosthetic materials using the pin-on-disc machine.

The wear of ultra high molecular weight polyethylene in combination with cobalt-chromium-molybdenum alloy was investigated by pin-on-disc experiments in which the alloy pin was loaded against the rotating polyethylene disc. In some experiments the pin was stationary, but in others it rotated about its axis. The effect of lubricant type, pin rotation, magnitude of the applied load, magnitude of the relative surface velocity, and disc storage in various environments was investigated. Wear of the polyethylene was assessed at 100 h intervals by both volumetric and gravimetric methods. Each experiment lasted 1000 h. It was concluded that of the lubricants tested bovine serum was the most suitable and that pin rotation produced more suitable polyethylene surfaces than did the stationary pin. The wear rate increased slightly with load (possibly owing to creep), was insensitive to speed, and was not affected by disc storage. The dynamic coefficient of friction increased with running tme. Results obtained by the two wear assessment methods were comparable and the reproducibility of the results was good.

Survivorship of total hip replacements.

The survivorship of total hip replacements in patients operated on at the Royal National Orthopaedic Hospital has been analysed using standard actuarial methods. Between 1963 and 1972 173 metal-on-metal prostheses of various developmental designs were inserted. Between 1969 and 1972 248 prostheses of one design using a metal femoral component and plastic cup were used. The patients were surveyed retrospectively and for each the actual follow-up period was determined as well as the eventual outcome. The criterion of "survival" was that the prosthesis was still in situ and all such prostheses were regarded as "survivors", regardless of whether the patients experienced pain or loss of function. The survivorship was then determined by constructing life tables. The results indicate that for metal-on-metal prostheses the overall probability of survival was only 53 per cent after 11 years and the average annual probability of removal, irrespective of cause, was 5.5 per cent. The results were better for metal-on-plastic prostheses with figures of 88 per cent after eight years and 1.5 per cent respectively. For both prostheses the predominant reason for failure was loosening and the annual rate of removal for loosening increased as the follow-up time increased, suggesting that loosening was a wearing out process. The advantages of the survivorship method of analysis as compared with conventional methods are discussed.

Endoprosthetic replacement of the proximal femur and acetabulum. A survival analysis.

A series of 81 patients has been reviewed to determine the value of endoprosthetic replacement of the proximal femur and hip in the treatment of bone tumours. Standard statistical methods were used to evaluate the survival of the replacements and the patients. Taking removal of the prosthesis, irrespective of the cause, as the criterion for failure the survival of the replacements was found to be 63 per cent after 10 years. If deaths are regarded as failures, then the survival value falls to 48 per cent. The survival of patients with chondrosarcoma and osteoclastoma treated by endoprosthetic replacement compares favourably with survival after amputation or excision of the tumour.

A model to predict the incidence of fracture in femoral components.

The paper presents a probability model for the fracture of femoral components based on the assumption that the components are defect-free, and that they fracture by a fatigue process. The model predicts the probability of fracture for patients of different body weight, and the incidence of fractures versus body weight, and versus time. The results are compared with recently published survey data. In addition the model is used to demonstrate the effect of improvements in the mechanical properties of components. Finally the paper discusses the modifications needed to demonstrate the effect of variations in component design, prosthesis orientation, and level of patient activity, and the effect of time-dependent processes such as bone resorption, acrylic disintegration and shock loads.

A model femur for in vitro testing of femoral components.

A total hip replacement femoral component was embedded using acrylic bone cement into a full length model femur consisting of a thin-walled cylindrical fixture and was loaded anatomically. The strains in the prosthesis, the fixture and the cement were determined as was the effect of prosthesis misorientation. This method of fixation, unlike others tested, gave prosthesis stresses of magnitude and distribution similar to those which occurred in similar prostheses implanted in human femora. A metal fixture was found to be suitable for in vitro testing of femoral components, since the fixture stresses were adequately low. High stresses occurred in the cement, and in spite of acrylic precompression due to polymerization in situ, could result in acrylic fatigue fracture and interface failure, both possible causes of the loosening which occurs clinically. The load transmission from prosthesis to model femur was discussed.

The incidence and significance of rub marks on the stem of removed total hip replacements.

This article presents data on the incidence of rub marks (burnishing) on the stems of 180 removed total hip replacement femoral components most of which were McKees and Stanmores. The incidence varied with the reason for removal. It was low for assorted removals excluding fractured stem removals, higher for "loose stem" removals, and highest for fractured stem removals. For "loose stem" removals, it appeared to increase with implantation time. For fractured stem removals it appeared to increase with the time elapsed between fracture and removal. On not-fractured removals, rub marks appeared to indicate that the component had been loose, but on fractured stem removals they appeared to indicate only that the component had been loose after fracture.