The lack of reproducibility of radiographic joint space measurements in longitudinal assessment is one of the main shortcomings of radiography, especially for the extended-knee radiograph (that is, a bilateral weightbearing AP view of both knees in full extension) [5
]. Changes in knee pain between examinations can introduce systematic measurement error for radiographs in full extension [2
]. Mazzuca and colleagues [9
] detected significant increases in tibiofemoral JSW in extended knee radiographs taken 7 to 14 days apart of OA subjects who had experienced relief of an induced flare of knee OA pain. Notable differences in JSW may be demonstrated depending on the angulation of the knee (Figure ). Thus, the lack of reproducibility of positioning in longitudinal studies and large multicenter trials remains a major drawback to the use of radiography.
Figure 1 Radiographs at baseline and 2-year follow-up of a 61-year-old woman with osteoarthritis. At two time points, radiographs were taken with 5°, 10°, and 15° angulation of the knee. (a) Anteroposterior (AP) radiograph taken at 5° (more ...)
To overcome this problem, various protocols have been proposed to enable standardized radiographic assessment of the knee [2
]. Of those, the fixed flexion view with the use of a positioning frame (for example, SynaFlexer™; CCBR-SYNARC, Newark, CA, USA) [10
] is the most widely used protocol in ongoing large-scale multicenter OA studies, such as the Osteoarthritis Initiative (OAI) and the Multicenter Osteoarthritis Study (MOST). Other protocols use fluoroscopy to confirm satisfactory anatomical positioning of the medial tibial plateau prior to acquisition of the radiograph [11
Such measures may alleviate the problems associated with positioning, but even if the image acquisition method is optimized, interpreting JSN may still be problematic. Based on data from the OAI, a recent study by Guermazi and colleagues [13
] showed marked differences in thresholds for scoring of JSN between expert readers. In light of these findings, research studies using radiographic semiquantitative grading of OA should rely on centralized adjudicated reading for all grading in order to minimize variability.
The inability of radiography to directly visualize cartilage and many other OA features is the insurmountable shortcoming of this imaging modality (Figure ). Recently, Amin and colleagues [14
] examined the relationship between progression of JSN on radiographic images and cartilage loss on MRI. While their results provided longitudinal evidence that radiographic progression of JSN is correlated with cartilage loss assessed on MRI, the authors concluded that if radiography were used alone, a substantial proportion of knees with cartilage loss would be missed [14
]. In longitudinal studies, radiographic progression of JSN is neither a sensitive [14
] (Figures and ) nor a specific [1
] (Figure ) measure of OA disease progression when compared with MRI findings.
Figure 2 Comparison of an anteroposterior radiograph and a coronal magnetic resonance image of the knee. (a) Anteroposterior radiograph of the left knee demonstrates marginal osteophytes of the medial and lateral femur and tibia (arrows). Joint space width appears (more ...)
Figure 3 Example of non-sensitivity of radiography. (a) Baseline coronal intermediate-weighted magnetic resonance imaging shows hyperintensity in the weight-bearing portion of the lateral tibial plateau but no definite cartilage defect. (b) At 24-month follow-up, (more ...)
Figure 4 Example of non-sensitivity of radiography. (a) Baseline sagittal intermediate-weighted fat-suppressed image shows normal articular cartilage coverage in the medial femur and tibia. (b) At 24-month follow-up, there is circumscribed thinning of cartilage (more ...)
Figure 5 Example of non-specificity of radiography. (a) Baseline sagittal intermediate-weighted fat-suppressed image shows discrete superficial cartilage loss at the central part of the medial femur (arrows). (b) No progression is seen at 24-month follow-up (arrowheads). (more ...)
It was previously believed that JSN and its changes reflect only articular cartilage thinning, but several studies have shown that alterations in the meniscus, such as meniscal extrusion or subluxation, also contribute to JSN [1
]. A more recent study, by Hunter and colleagues [1
], compared MRI and weight-bearing posteroanterior radiographs to explore the relative contribution of several morphologic features, including cartilage, meniscal damage, and position, to the radiographically detected JSN. The authors found that a substantial proportion of the explained variance in JSN was due to meniscal position and degeneration, and a substantial proportion of change in JSN resulted from change in meniscal position.
Lastly, one should note that patients usually have medial knee OA and the regular use of JSW measurements is performed in the medial compartment. However, the lateral joint space may become widened as a result of severe medial JSN. In such secondary widening of joint space, articular cartilage in the lateral compartment may be normal or abnormal, but even if there is cartilage thinning, it is very difficult to assess by radiography alone [17