Although recent protocols for standardized knee radiography afford highly reproducible radioanatomic alignment of the joint and measurement of joint space width (JSW) in repeat radiographs acquired on the same day, the sensitivity of these techniques to joint space narrowing (JSN) over time in subjects with knee osteoarthritis (OA) is unknown. The present study was undertaken to compare the metatarsophalangeal (MTP) view and the semiflexed anteroposterior (AP) view with respect to sensitivity to JSN in knee OA.
In 49 subjects with definite knee OA, 2 MTP radiographs and 1 semiflexed AP radiograph were obtained at baseline. Each examination was repeated 14 months later. In MTP views, minimum JSW and the distance between the anterior and posterior margins of the medial tibial plateau (intermargin distance [IMD], an indicator of parallel alignment of the tibial plateau and the x-ray beam) were measured with a pair of calipers and a magnifying lens fitted with a graticule. JSW in semiflexed AP views was measured by digital image analysis.
The mean of within-knee standard deviations of JSW in the baseline MTP examinations (n = 52 OA knees) was 0.24 mm (coefficient of variation 5.8%). Although IMDs in the 2 baseline MTP views were very highly correlated (+0.88), IMDs in the serial examinations were only moderately correlated (+0.45). Serial MTP views showed a small increase in mean JSW over 14 months that was not significantly greater than zero (mean +/- SD +0.09 +/- 0.66 mm; P not significant). In contrast, concurrent semiflexed AP examinations showed a marginally significant decrease in mean JSW (-0.09 +/- 0.31 mm; P = 0.10).
These results demonstrate that evidence of the short-term reproducibility of a radiographic protocol is an insufficient basis on which to predict the quality of its longitudinal performance.