The Effect of the Epiphyseal Growth Plate on the Length of the First Metacarpal in Triphalangeal Thumb

Purpose: Triphalangeal thumb is characterized as a congenital difference of the thumb with an extra phalanx. Additional thumb length is attributed mostly to the extra phalanx. The influence of the epiphyseal plate positions on growth in congenital hand anomalies is unclear. The purpose of this article was to compare the length of the first metacarpal in triphalangeal thumbs with the length of the same bone in normal thumbs and to investigate the influence of the often aberrant metacarpal epiphyseal plates on the length of the triphalangeal thumb metacarpal during growth. Methods: The positions of the epiphyseal plate and the relative length of the first metacarpal were examined retrospectively in 37 patients with triphalangeal thumb. Ratios of the measurements were calculated (metacarpal II/metacarpal I) and compared with a normal population. Subjects were divided into 3 groups based on the position of the epiphyseal plate (double, proximal, distal). Results: The distally placed growth plate was the most common variety, present in 22 of patients, followed by the proximal position in 14, and double epiphyseal plates in 6. All ratios of patients with triphalangeal thumb were significantly smaller than those of a normal population, indicating a longer first metacarpal in this condition. First metacarpals in triphalangeal thumb with double epiphyses grew disproportionately more than those of a normal population, whereas those with distal epiphyses grew disproportionately less than normal. First metacarpals with proximal epiphyses grew at the same rate as normal first metacarpals. Conclusions: In this study population the most common growth plate location was distal whereas the most common location in the normal population is proximal. The different positions of the epiphyseal plates correlate with growth differences in patients with triphalangeal thumb. Type of study/level of evidence: Prognostic, Level III.

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