Histology of the bone-cement interface in retrieved Oxford unicompartmental knee replacements
Introduction
Radiolucencies are commonly seen at the interface of both total and unicompartmental knee replacement (TKR and UKR). They tend to be more common on the tibial rather than the femoral side. For a radiolucency to be well seen there has to be a large, flat interface which is not obscured and the x-ray beam has to be parallel to the interface. With many TKRs the wide keel obscures the interface and radiolucencies are not seen. The best way to reliably align the X-ray beam parallel to the interface is to use an image intensifier. If standard radiographs are taken, which are not usually parallel to the interface, then radiolucencies are rarely seen. The tibial component of the Oxford UKR is an ideal implant with which to study radiolucencies as there is a large, flat surface, not obscured by a keel and the loading is well defined. The incidence of radiolucencies on fluoroscopically screened radiographs, where the X-ray beam is parallel to the tibial tray, under the Oxford tibial component varies from about 40%–100% [1], [2], [3], in contrast with standard radiographs where it is 0%–20% [4], [5], [6].
Goodfellow et al. [7] described two different types of radiolucency. The commonest type is the radiolucent line (RLL), which is usually 1 mm or less thick and has a sclerotic margin. It tends to develop and consolidate during the first and second postoperative years and thereafter does not progress. It is common and not associated with significant problems and is, therefore, termed a physiological radiolucency [7]. In contrast, when there is loosening or infection the radiolucencies tend to be more than 2 mm thick, progressive and without a sclerotic margin. These have been called pathological radiolucencies.
Tibrewal et al. [1] have previously described the histology obtained from biopsies of the cement–bone interface of securely fixed tibial components revised for recurrent bearing dislocation. The radiolucency was composed of fibrocartilaginous connective tissue with a sclerotic margin beneath made of a thick lamella of bone. It has been hypothesised that the development of the sclerotic line is due to an increase in stress in the bone beneath the soft tissue present in a RLL [8]. Ryd and Linder reported the histology of the interface in three patients after revision of the Marmor unicompartmental knee replacement [9]. This was found to be either fibrous tissue or fibrocartilage which they felt was consistent with their findings of initial migration but stable fixation, as measured by radiostereometric analysis. Even though the presence of a physiological radiolucent line is not associated with an inferior clinical outcome [10], the appearance often causes concern, particularly if the patient has continuing pain. It is, therefore, important to identify the constitution of the tissues in the radiolucent line in order to understand why they occur and their significance.
The aim of this study is to identify both the type and distribution of tissue at the cement–bone interface of a well fixed UKR. The secondary aim is to assess whether there exists a correlation between the presence of soft tissue at the cement–bone interface and the presence of radiolucent lines beneath the tibial tray.
Section snippets
Patients and methods
Ten patients who were undergoing revision of a cemented Oxford UKR (Biomet, Swindon, UK) were recruited to this study. The patient demographics and reason for revision were collected (Table 1). Seven patients were revised in the Nuffield Orthopaedic Centre, Oxford and three in Skovde, Sweden. The mean age at index surgery was 62.5 years (range 43 to 70 years, SD 9.1 years). There were nine females and a single male patient. All original operations were primary Oxford UKR and the mean time in situ
Statistical analysis
The distribution of tissue percentages was assessed using frequency histograms and was non-normally distributed. In addition, tissue thickness was categorised and was treated as an ordinal variable. Therefore non-parametric statistical tests were used with correlation coefficients being calculated using Spearman's rank correlation test. Reliability between two independent observers (BJLK & ARJ) for assessment of radiolucency was tested using the intraclass correlation coefficient (ICC).
Results
The percentage of tibial tray surface area with cement–bone contact had a median of 52.5% (range 19% to 95%). Fibrocartilage was more common than fibrous tissue with a median of 27% (0%–49%) compared with 14% (0%–48%). The individual distribution of different tissues beneath each tibial tray showed marked variability (Fig. 3). Eight of the specimens had both fibrocartilage and fibrous tissue present, in differing proportions as shown, with one specimen each having only one tissue type in
Discussion
The histological constitution of RLLs beneath well fixed tibial components in Oxford UKR has previously been unknown. While the presence of thin, non-progressive RLLs beneath the tibial component are well recognised their significance with regard fixation is poorly understood.
The main weakness of this study is the specimen group is not standardised, in that both lateral and medial compartment UKRs were retrieved. The kinematics of each compartment is different, resulting in different mechanical
Conflict of interest
One or more of the authors (DWM) has received funding from Biomet, Swindon, UK. In addition, the institution of the authors (NDORMS, University of Oxford) has received funding from Biomet, Swindon UK.
Acknowledgements
Financial support has been received from the NIHR Biomedical Research Unit in Musculoskeletal disease, Nuffield Orthopaedic Centre and University of Oxford. We would like to thank Dr Ulf Svard for his help with this study.
References (15)
- et al.
Mobile vs. fixed bearing unicondylar knee arthroplasty: a randomized study on short term clinical outcomes and knee kinematics
Knee
(2006) - et al.
The sclerotic line: why it appears under knee replacements (a study based on the Oxford knee)
Clin Biomech (Bristol, Avon)
(2010) - et al.
On the correlation between micromotion and histology of the bone–cement interface. Report of three cases of knee arthroplasty followed by roentgen stereophotogrammetric analysis
J Arthroplasty
(1989) - et al.
A new approach to quantify trabecular resorption adjacent to cemented knee arthroplasty
J Biomech
(2012) - et al.
Mechanical interface conditions affect morphology and cellular activity of sclerotic bone rims forming around experimental loaded implants
J Orthop Res
(2004) - et al.
The radiolucent line beneath the tibial components of the Oxford meniscal knee
J Bone Joint Surg Br
(1984) - et al.
The Oxford medial unicompartmental knee replacement using a minimally-invasive approach
J Bone Joint Surg Br
(2006)
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