Elsevier

The Knee

Volume 19, Issue 6, December 2012, Pages 918-922
The Knee

Histology of the bone-cement interface in retrieved Oxford unicompartmental knee replacements

https://doi.org/10.1016/j.knee.2012.03.010Get rights and content

Abstract

Introduction

Radiolucent lines (RLL) are commonly seen at the cement–bone interface of knee replacements, yet are poorly understood. Although thin RLL are not associated with implant loosening or poor patient outcome there is still concern that they indicate sub-optimal fixation. The primary study aim is to characterise the histology at the cement–tibia interface in Oxford unicompartmental knee replacement (UKR). The second aim is to assess whether a correlation exists between the presence of a RLL and the type of tissue that predominates at the interface.

Methods

The radiology and histology of retrieved specimens of the interface from around firmly fixed tibial trays in ten patients undergoing revision between 1 and 19 years after Oxford UKR were studied.

Results

Pre-revision radiographs showed the presence of both full and partial RLL. On contact radiographs of 5 mm thick sections of the interface the total percentage of radiolucency ranged from 0 to 90% between patients. There was no consistent pattern for the distribution of radiolucency. Histological assessment demonstrated that under every tibial component there were areas where there was direct contact and interdigitation between bone and cement. The amount of direct bone–cement contact was between 19% and 95% of the tibial tray surface area. The remaining tissue was mainly fibrocartilage but there was also fibrous tissue. The presence of radiolucency was strongly inversely correlated with the percentage of cement–bone contact.

Conclusion

This study demonstrates that even with partial or complete RLL seen on radiographs there is still cement–bone contact, thus indicating that there is stable fixation.

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)

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