Peri-prosthetic osteolytic lesions around orthopaedic implants are a recognised cause of bony matrix instability leading to mechanical failure [1–5]. While several postulates have been suggested to explain this frequently observed phenomenon, the exact mechanism remains controversial [1, 3, 6–8] and is the subject of current international scrutiny . What does appear to be universally accepted is the need to recognise the onset and progression of osteolytic lesions. This is aimed to be ascertained at the earliest possible point so that appropriate management can provide the best possible clinical and patient outcome [8, 10]. To facilitate such practice, there is a need for an accurate and reliable non-invasive technique to allow both lesion identification and morphologic (volumetric) description.
In many countries total knee replacement (TKR) is the most common form of joint replacement . Extensive epidemiological data indicate that the trend towards an increasing incidence of TKR is likely to continue . In a population with an increasing life expectancy , there are ever-greater expectations for the preservation of mobility and physical activity . While the vast majority of cases show good clinical outcome and improvement in post-procedural standard-of-life , implant failure (through a variety of mechanisms) remains a problematic clinical issue . Particle-induced wear-related bone loss (osteolysis) is a recognised precursor to implant loosening and mechanical instability [8, 12]. Osteolysis is often insidious and asymptomatic [10, 13, 14] until it reaches critical levels, with subsequent implant failure. For this reason, peri-prosthetic osteolysis following TKR has become a significant clinical problem [8, 15]. Periodic radiographic surveillance post-joint replacement is often prospectively recommended [8, 16, 17], especially for young and active recipients . This allows early detection and thus instigation of management pathways [8, 18, 19], aiming to achieve better long-term patient outcomes.
In the majority of cases, post-surgical or follow up plain film X-rays form the routine basis for assessment of implant positioning, stability and integrity, as well as evaluation of the condition of adjacent bony domains [20–22]. A small number of institutions employ conventional CT-based follow up either as an adjunct to, or in lieu of, plain film examinations . However, in most cases, such practice is likely to involve isolated patients on a purely case-by-case basis, commonly with a more pressing secondary indication.
Historically, the use of plain film X-ray examinations as a screening tool for osteolysis, despite multi-angle and multi-projection approaches, has proved unreliable [5, 21, 23]. Concerns have been raised regarding the inability to accurately delineate the peripheral margins of osteolytic lesions, often resulting in under-estimation of lesion size [10, 18, 21, 23–25] (especially in close proximity to the bone/implant interface). Additionally, they often lack consistency and repeatability in sequential (follow-up) examinations, limiting direct comparability and hence clinical benefit in the accurate monitoring of progressive change . The latter is heavily influenced by subtle variations in patient presentation and radiographic technique (i.e. patient positioning, central beam orientation, exposure parameters, projection series performed and structural superimposition) [18, 21, 22, 27, 28]. Although often advocated [10, 28], the application of conventional CT for non-invasive osteolytic lesion description, has been limited by poor scan alignment on longitudinal assessment. This has subsequently resulted in inaccurate extrapolation of volume estimates when viewing sectional images. Also, the presence of metal (i.e. implant) in the scan field causes significant image distortion due to beam hardening artefact [5, 28–30] and inherently limits the clinical value of obtained images [28, 31].
The description of osteolytic lesions and their size around total hip replacements (THR) has been reported previously [5, 32, 33] and appears to be relatively common . However, there is little evidence in the contemporary literature to suggest that substantial application of such approaches have been extrapolated to other body regions, including the human knee.
There is increasing suggestion that CT-based assessment of peri-prosthetic bone around TKRs may provide a quick, technically simple, highly accurate and reliable form for volume measurement of both discrete pathology and normal anatomy [5, 28, 30]. Ongoing advancements in CT scanner-based algorithms for the reduction (or amelioration) of metal (i.e. implant) induced beam hardening artefact [5, 26, 31, 34], combined with next generation software-based correction techniques , have largely overcome many of the pitfalls previously associated with orthopaedic imaging. These technologies provide a non-invasive imaging modality, which may be inherently suited to analysis of osteolysis in the peri-prosthetic region [5, 14, 26, 30, 34].
Given the clinical relevance of accurate description of TKR-associated peri-prosthetic osteolysis, and the lack of evidence indicating previous similar work, the aim of this study was to assess lesion recognition and description using a rapid-acquisition CT-based imaging technique, and to contrast this to standard X-ray examination approaches.