Reliability of Metacarpal Subsidence Measurements after Thumb Carpometacarpal Joint Arthroplasty.

Background: Thumb metacarpal subsidence after trapeziectomy can affect clinical function over time. Methods for measuring subsidence after trapeziectomy have been described, and they rely on an intact thumb metacarpal or proximal phalanx for measurement. The authors evaluated the reliability and reproducibility of measuring the trapezial space ratio, using previously described methods. In addition, the authors evaluated a new method that measures trapezial space on a posteroanterior (PA) hand/wrist radiograph that does not rely on an intact thumb metacarpal or proximal phalanx for measurement, which can often be altered by degenerative changes or in cases in which metacarpophalangeal arthrodesis is performed during carpometacarpal (CMC) joint arthroplasty to correct excessive hyperextension. The authors hypothesized that a new method of calculating trapezial space would have comparable reliability and reproducibility to previously proposed methods.

Methods: Thirty-seven PA hand/wrist radiographs from patients who had trapeziectomy with ligament reconstruction and tendon interposition were evaluated. Trapezial space was measured using PACS (Picture Archiving and Communication System) digital tools as the distance perpendicular to the tangents of the scaphoid and first metacarpal joint surfaces. All X-rays were evaluated individually by five fellowship-trained hand surgeons, twice, 4 weeks apart. The reviewers calculated trapezial space ratios, using three different methods, two previously described and a novel one: (1) trapezial space relative to first metacarpal length (classic 1); (2) trapezial space relative to proximal phalanx length (classic 2); and (3) trapezial space relative to capitate height (novel). Inter- and intraobserver reliabilities were measured using intraclass correlation coefficients (ICC) and limits of agreement for each method.

Results: The authors identified excellent agreement between the classic 1, classic 2, and novel methods with an ICC greater than 0.8, indicating excellent agreement. The average trapezial space ratios for the thumb proximal phalanx, thumb metacarpal, and capitate methods were measured as 0.19, 0.12, and 0.24, respectively. The upper and lower limits of the 95% confidence intervals for both the inter- and intraobserver agreements of the aforementioned trapezial space ratios were (0.17-0.26), (0.11-0.17), and (0.21-0.34) for the interobserver rates and (0.11-0.25), (0.06-0.16), and (0.12-0.33) for the intraobserver rates, respectively.

Conclusion: Measuring trapezial space is an important diagnostic tool to assess postoperative changes in thumb length. The trapezial space indexed to the capitate height method (novel) provides a simple and similarly reliable method for calculating the trapezial space ratio on a PA radiograph of the hand/wrist when other measurement techniques are unavailable and when the thumb metacarpal or proximal phalanx is not intact. The authors found a high degree of reproducibility and inter- and intraobserver reliability as measured by the ICC and the 95% limits of agreement that compare with previous agreements in the literature.