Symphalangism results from the failure of an interphalangeal joint to form. There may be complete or partial lack of development of the joint. When the interphalangeal joint is entirely absent, there is no dividing line between the two phalanges. Rather, they appear as a single long phalanx with a common bone marrow cavity. The trabecular structure of the bone is changed to conform to the new stresses of pull and pressure. When the interphalangeal joint has developed slightly, there are two distinct bones and two bone marrow cavities. The ligaments are shorter than normal, and there is a solid fibrous connection between the two bones. Motion is impossible. When the development of the interphalangeal joint has progressed a little further, some bending and stretching become possible.
Usually one person will exhibit the various degrees of symphalangism in different fingers. Any or all of the four extremities may be involved. The
FREUD P, SLOBODY LB. SYMPHALANGISMA FAMILIAL MALFORMATION. Am J Dis Child. 1943;65(4):550–557. doi:10.1001/archpedi.1943.02010160034004