Quantitative assessment of joint space width with an electronic caliper

Abstract

The progression of joint space narrowing (JSN) is considered to be the best available marker of osteoarthritis (OA) progression. Several techniques have been proposed for the measurement of joint space at its narrowest point in OA of the hips and knees.

Objective: To evaluate the properties of the technique using an electronic caliper for the measurement of JSN in OA patients.

Design: We used an electronic caliper to measure joint space width (JSW) for hips on 100 plain radiographs. JSW was measured in the vertical position at the center of the femoral head. Femoral head diameter was also determined to correct for variations due to differences in magnification of digitized X-rays. All films were read twice by each of two rheumatologists (one junior, one senior) and two radiologists (one junior, one senior). Intraclass correlation coefficients and their 95% confidence intervals were calculated.

Results: Detailed results are given for right hips (38 with OA, 18 inflammatory, 44 normal); very similar results were obtained for left hips. For JSW, the intraclass correlation coefficient was between 0.96 and 0.99 for intraobserver reliability. The level of reliability was similar for analysis of the diameter of the femoral head (R:0.84 to 0.98) and for the ratio of these two measurements (0.96 to 0.99). The most reliable measurements were those made by the senior radiologist, followed by those made by the two rheumatologists. In assessments of interobserver reliability for the measurement of JSW, R varied from 0.91 to 0.96 for the first reading and from 0.88 to 0.96 for the second reading. For the measurement of femoral head diameter, R varied from 0.86 to 0.96 for the first reading and from 0.74 to 0.96 for the second reading.

Conclusion: The electronic caliper technique is an accurate method for measuring JSW in the hip. This technique seems to be reproducible, is simple, and could be used for routine evaluation. Further validation is required, with the measurement of serial X-rays from the same patients.