Symphalangism

A congenitally straight, stiff finger with an absent flexion crease over the PIP joint and no interphalangeal motion is most likely symphalangism - specifically proximal, or true, symphalangism, which is the most common form. This is a congenital failure of differentiation, in which the interphalangeal joint never forms and the phalanges are ankylosed, so it belongs to the failure-of-differentiation group of congenital hand anomalies and is fundamentally different from a contracture. I would confirm it clinically by the absent flexion crease and the complete lack of active and passive IP motion, and radiographically by little or no joint space with bony fusion across the joint, which may progress as the child grows. I would distinguish it from camptodactyly, which is a flexion contracture with a preserved joint and a flexion crease that can be stretched or released. Because symphalangism can be isolated - often autosomal dominant, frequently bilateral and affecting several digits and the feet - or syndromic, I would look for other anomalies and take a family history, considering associations such as Apert and Poland syndromes. Management is largely supportive, because there is no true joint to mobilise or reconstruct, and function is usually good when the digit is fused in a near-straight, useful position. I would reserve surgery for a digit fused in a poor position - a repositioning osteotomy or a stable arthrodesis in a better position - rather than attempting to create motion across a joint that never formed.