Examination of ten fractured Oxford unicompartmental knee bearings

Abstract

Since the Oxford knee was first used unicompartmentally in 1982, a small number of bearings have fractured. Of 14 retrieved bearings, we examined ten samples with known durations in situ (four Phase 1, four Phase 2 and two Phase 3). Evidence of impingement and associated abnormally high wear (> 0.05 mm per year) as well as oxidation was observed in all bearings. In four samples the fracture was associated with the posterior radio-opaque wire. Fracture surfaces indicated fatigue failure, and scanning electron microscopy suggested that the crack initiated in the thinnest region. The estimated incidence of fracture was 3.20% for Phase 1, 0.74% for Phase 2, 0.35% for Phase 3, and 0% for Phase 3 without the posterior marker wire. The important aetiological factors for bearing fracture are impingement leading to high wear, oxidation, and the posterior marker wire. With improved surgical technique, impingement and high wear should be prevented and modern polyethylene may reduce the oxidation risk. A posterior marker wire is no longer used in the polyethylene meniscus. Therefore, the rate of fracture, which is now very low, should be reduced to a negligible level.

Fracture of mobile unicompartmental knee bearings: A parametric finite element study

“…The radio-opaque posterior marker wire was replaced with marker balls in 1999; , the results from the present study indicate that this change has reduced the residual tensile stress within the marker hole by 53% and also caused a small reduction in tensile stress at the side of the bearing. I; it is therefore likely that this has reduced the fracture risk, which is supported by the clinical evidence [3].…”

“…In four of the ten fractured bearings reported previously, the marker wire was implicated as a possible cause [3]. In these cases, the components were small bearings, which are known to have the marker wire closer to the centre; and the fracture was in the region of the posterior marker wire rather than the anterior wire.…”

“…General wear of the bearing is known to be an important factor; , all of the bearings which fractured clinically had worn excessively and demonstrated evidence of impingement and oxidation [3]. Impingement Wear has been related to wear impingement [4,5], and oxidation of UHMWPE has also been shown to increase wear and decrease fracture resistance [31].…”

“…With increasing usage of these components [1] and longer life expectancy [2], it is essential to reduce the risk of fractures occurring. In a recent study, ten retrieved Oxford Unicompartmental Knee Replacement (UKR) (Biomet UK Healthcare Ltd., Swindon, UK) ultra-high molecular weight polyethylene (UHMWPE) bearings were examined that had fractured during use [3]. The average time in situ prior to fracture was 16.8 years and the incidence of fracture was 3.2% for the Phase 1 (used prior to 1987), 0.74% for Phase 2 (used prior to 1998) and 0.35% for Phase 3 bearings manufactured prior to 1999.…”

“…Increased in vivo wear rates have been linked with impingement of the UHMWPE bearing on the surrounding bone or cement [4,5],5]; this wear can lead to weakening and an increased risk of fracture [6]. Impingement and high wear was evident in all the fractured bearings and is thought to play a major role in the mechanism of fracture [3]. The quality of the UHMWPE used to manufacture a component is also thought to have been a factor [3,7].…”