A radiographic method for evaluation of the index-hypoplastic thumb angle

Abstract

Purpose: Thumb metacarpophalangeal joint radial instability occurs during the pinch motion in patients with hypoplastic thumb because of thumb-index web narrowing. We devised a radiographic technique to measure the thumb-index angle, applied while the patient holds a styrene foam cone, to evaluate the relationship between the thumb and the index finger. We used this technique to compare different types of thumb hypoplasia and the groups before and after surgery.

Methods: Twenty patients with hypoplastic thumbs held the styrene foam cone. The average age of the patients was 6.5 years (range, 1.6 to 12.0 y). We obtained an overhead radiograph while the patient held the cone and evaluated the apparent thumb-index web angle, the thumb to index finger metacarpal angle (1-2MCA), and the first metacarpophalangeal angle (1MPA).

Results: In the 9 unilateral cases, no significant difference was detected between the unaffected side and the affected side in terms of thumb-index web angle, but the data showed meaningful differences in terms of 1-2MCA and 1MPA. In addition, the data showed meaningful differences between the groups before surgery and 2 years after surgery in terms of 1-2MCA and 1MPA.

Conclusions: Measuring both 1-2MCA and 1MPA enables evaluation of the severity of the deformity, and these parameters allowed for comparative evaluation of the severity of preoperative and postoperative narrowing of thumb-index web space and the radial instability of the thumb metacarpophalangeal joint. By focusing on these characteristics, we devised a novel approach for imaging of the thumb-index web space.