Radiography is the least expensive method of imaging joints and is more readily available than MRI. In man, radiographic OA is defined on the evaluation of joint space width (JSW) and osteophytes [1–8, 21, 22]. Severity of radiographic OA can be estimated using semi-quantitative scoring systems. For nearly half-century the Kellgren and Lawrence system has extensively been used in large clinical and epidemiological studies. However, such a global assessment is invalid as it assumes that changes in radiographic features are linear over the course of the disease with constant relationship. Clinical studies and research on OA pathogenesis require separate assessment of the different OA features [9, 10, 16, 19, 59–69]. To overcome this problem, Altman and others developed a radiographic grading scale of individual OA features. This scale is illustrated in a radiographic atlas which has been updated in 2007 and published on line by the Osteoarthritis Research Society International (OARSI) .
To the authors' best knowledge, prior to this report, while being well characterized by histology and by gross examination in the literature no radiographic atlas of the CCLT rabbit model of OA has been described. Equally neither recommendations nor detailed description of the radiographic procedure could be found. In man JSN and osteophyte scoring provide better reproducibility for grading ROA than cyst, sclerosis or bone contour. We based the CCLT rabbit model composite ROA atlas and grading scale on osteophyte and JSN grading. ICCs of each individual ROA features demonstrated excellent agreement inter- and intra-readings. Global ROA score gave the highest ICCs value for both between and within observer agreements. In all individual ROA features, JS narrowing gave the highest overall reliability and reproducibility.
The simple "rabbit bed" permitted easy standardization of the fully extended caudocranial view of the rabbit femorotibial joint, with both rabbit metatarsi parallel to the sagittal plane of the animal, but precluding weight-bearing. We are currently working on a dedicated system to perform in vivo weight-bearing radiographs of the rabbit femorotibial joints.
Poor radio-anatomic positioning is known to be an important source of error of JSW (and JSN) evaluation. Manually applied stress on non weight-bearing caudocranial radiographs of the canine femorotibial joint, has the most marked effect on the JSW . In contrast, mild to moderate decentering of the Xray beam along the long or transversal axes of the hind leg does not alter JSW as much as rotation or manually applied mediolateral stress. To limit positioning artifact we fixed the rabbit metatarsi to the bed extensions ensuring simple and reproducible positioning. As radiographic procedure was performed by the same person (CB) this also helped to reduce positioning errors. Further work is needed to evaluate the consequences of the OA changes on the positioning of the femorotibial joint for the ROA grading as OA impairs the joint range of motion.
In CCLT rabbit model of OA, the severity of the chondropathy is reported to vary widely between subjects and to be associated with meniscal lesions and it is not clear whether structural cartilage or meniscal lesions appear first in the CCLT rabbit. Worthy of notice, at five months after surgery ROA severity was positively correlated both with meniscal and cartilage macroscopic lesions. Further work is needed to distinguish the sequential development of these lesions [36–42].
Similarly to the CCLT canine experimental model of OA, the radiographic details of the operated rabbits' femorotibial joint allowed observation of central osteophyte and subchondral cystic lesions. Gross examination does not allow quantification of subchondral lesions. We are currently investigating histological analyses to integrate these features in a ROA grading scale.
Poor correlation has often been reported between macroscopic and histologic tibial cartilage thickness of the rabbit femorotibial joint [50, 54]. This is due to the inherent problems of using a unidimensional measure (histology) to indirectly evaluate changes in 3D structures within a joint compartment.
Non-invasive surrogates such as tomographic imaging procedure (μCT, μMRI) still need to be validated in CCLT rabbit model of OA.