The aetiology of clinodactyly is unknown. The common mechanism of deviation involves an abnormality in the growth of the middle phalanx that tilts the alignment of an interphalangeal joint surface - commonly the distal interphalangeal joint - relative to the axis of the digit.
One common aetiology is the formation of a 'delta phalanx' within the middle phalanx. This is also known as a longitudinal bracketed epiphysis or longitudinal epiphyseal bracket. Rather than the normal transverse physis, this longitudinally orientated physis extends around the phalanx in a 'C' shape with the open part facing the ulna. The longitudinal restriction on the non-open side results in unconstrained growth of the opposite side. A wedge-shaped middle phalanx results with deviation of the distal interphalangeal joint articular surface.
Occasionally a similar deformity is due to a fibrous band on the ulna side of the digit acting as an external constricting band.
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