Selective Distribution of Flexor Hallucis Longus-Derived Slips at the Knot of Henry and Fifth Toe Functional Independence: A Narrative Review.

The tendon interconnection between the flexor hallucis longus (FHL) and flexor digitorum longus (FDL) at the knot of Henry has important anatomical and clinical implications, particularly in tendon transfer surgery. Although previous anatomical studies have consistently shown that FHL-derived slips are distributed mainly to the second through fourth toes, the reason for their minimal contribution to the fifth toe remains unclear. This narrative review examined anatomical and functional studies identified through PubMed and Google Scholar, focusing on the FHL-FDL interconnection, toe-specific tendon distribution, and the role of the quadratus plantae (QP). Six major anatomical studies were identified. Across studies, FHL-derived slips consistently supplied the second and third toes and occasionally the fourth toe, whereas direct contribution to the fifth toe was absent or extremely rare (<1%). Anatomical evidence also demonstrated close integration of the QP with the lateral digital flexor system. Biomechanical findings suggest that the FHL primarily supports medial forefoot propulsion, whereas the lateral column, including the fifth toe, is more closely associated with balance and stabilization through the FDL-QP complex. Together, these findings support the concept of functional differentiation within the long digital flexor system. The selective distribution of FHL-derived slips appears to represent a consistent anatomical pattern rather than random variation, with the fifth toe functioning as a relatively independent unit associated with the FDL-QP complex. This concept may provide a useful framework for interpreting checkrein deformity and for planning tendon-level surgical procedures.