Radiographic Features of the Metacarpal in Blauth Type III Thumb Hypoplasia

Abstract

The purpose of this study was to investigate whether radiographs can be used to aid in the determination of Blauth IIIA and IIIB thumbs. Six pediatric hand surgeons were asked to evaluate the radiographs of 77 thumbs and classify the thumb as IIIA or IIIB and indicate which morphologic features influenced their decision. Quantitative measurements and ratios of radiographs were obtained and compared between IIIA and IIIB thumbs. The radiographic features selected for type IIIA thumbs include near-normal length and near-normal width and for type IIIB thumbs, abnormally short, tapered proximal end, and round proximal end. The six surveyed surgeons reached consensus in 82% (63/77) of thumbs, and this matched the enrolling surgeon's classification in 77% (59/77) cases. The ratio of the length of the thumb metacarpal compared with the length of the index metacarpal was different between IIIA and IIIB thumbs (66% ± 0.08% and 46% ± 0.18%, respectively). The ratio of the width of the thumb metacarpal shaft at its narrowest aspect to the width of the thumb metacarpal base was notably different between IIIA and IIIB (68% ± 0.13% and 95% ± 0.28%, respectively). Near-normal length and near-normal width of the metacarpal were used to predict IIIA and abnormally short, abnormally narrow, and a round or tapered base of the metacarpal were used to predict IIIB classification. The length of the thumb metacarpal relative to the index metacarpal is on average 66% of the length of the index metacarpal in IIIA thumbs compared with 46% in IIIB thumbs. The width of the shaft of the thumb metacarpal at its narrowest is 68% of the width of the thumb metacarpal base in IIIA thumbs, indicating a flared base. In IIIB thumbs, the shaft width was on average 95% of the base width, indicating a tapered base. Diagnostic level III.