MRI evaluation of interconnections between flexor hallucis longus and flexor digitorum longus around the Master knot of Henry

To demonstrate the magnetic resonance imaging (MRI) findings of interconnections between flexor hallucis longus (FHL) and flexor digitorum longus (FDL) around the Master knot of Henry (MKH). Fifty-two MRI scans of adult patients were retrospectively analyzed. The types and subtypes of interconnections between the FHL and FDL were evaluated using the classification suggested by Beger et al based on the direction and number of the tendon slips and contributions to the lesser toes. The layering organization formed by the FDL, quadratus plantae, and tendon slip from the FHL was evaluated. The distance between bony landmarks and the branching site of tendon slips and the cross-sectional area (CSA) of the tendon slips were measured. Descriptive statistics were reported. MRI scans revealed that type 1 interconnection was the most common (81%), followed by type 5 (10%) and types 2 and 4 (4% each). All tendon slips from the FHL contributed to the second toe, and 51% of the tendon slips contributed to the second and third toes. For the layering organization, the two-layered type was the most common (59%), followed by the three-layered (35%) and single-layered (6%) types. The mean distance between the branching site and bony landmarks was longer in the FDL to FHL cases than that in the FHL to FDL cases. The mean CSA of the tendon slips from the FHL to FDL was larger than that of the FDL to FHL. MRI could provide detailed information about the anatomical variations around the MKH. In lower extremity reconstruction surgery, the flexor hallucis longus and flexor digitorum longus tendons serve as donor tendons. A preoperative MRI scan could provide information on anatomical variations around the Master knot of Henry, which can help predict postoperative functional outcomes.