Anatomy of the Flexor Hallucis Longus

Courtesy : Prof Nabile Ebraheim, University of Toledo, Ohio, USA

FLEXOR HALLUCIS LONGUS
• One Of The Deep Muscles Of Posterior Compartment Of The Leg
• FHL Muscle Spans Down The Calf To The Side Of Ankle And Into Foot
• Muscle Fibers – Lowest Origin In Leg And Some Fibers Almost At The Ankle

ORIGIN AND INSERTION
• FHL Muscle Arises From Posterior Surface On Inferior 2/3rds Of Fibula
• As Muscle Extends Down Into Plantar Aspect Of The Foot , Tendon Inserts Into Distal Phalanx Of Large Toe.
• FHL Muscle Is Largest And Strongest Deep Muscle Of Leg’s Posterior Section.
• There Are Two Sesamoid Bones At Level Of The MCP Joint , One Tibial And One Fibular- That Act Like Pulleys For The Flexor Tendons and are embedded in The Flexor Hallucis Brevis Muscle
• Sesamoids Are Important To The Big Toe Region By Absorbing Weight Bearing Pressure And Reducing Friction On Metatarsal Head.
• Also They Protect The Flexor Hallucis Longus Tendon As It Glides Between Two Sesamoid Bones

FUNCTION & INNERVATION
• FHL Flexes Joints Of Big Toe, And Also Role In Plantar Flexion Of Ankle Joint
• Tibial Nerve Provides Innervation To The Flexor Hallucis Longus Muscle
• Blood Supply From Peroneal Artery(Muscular Branch) And Posterior Tibial Artery (Peroneal Branch)

ARRANGEMENT OF TENDONS
• The FHL tendon has long course from its muscle origin to its insertion
• The tendon is vulnerable to wear and tear , friction and inflammation at certain anatomic locations
• At the ankle FHL located posterior and lateral to FDL and in between posterior talar processes
• The tendon lies most lateral in medial ankle compartment
• The tendon passes in its own tunnel beneath the sustentaculum tali

• Tendinopathy is common at the tibiotalar joint as the tendon curves over the posterior talus
• Flexor hallucis tendonitis mostly happens in ballet dancers and those require excessive plantar flexion at ankle
• Distal disease of hallux sesamoids is affected next, followed by area of knot of henry
• As tendons of FHL and FDL curve under medial malleolus and talus , they begin to converge and eventually cross
• At this point KNOT OF HENRY fibrous slip connects the FDL with FHL and tendon sheath communicates allowing spreading of inflammation
• Because of the intersection of two tendons , transection of digitorum proximal to the KNOT OF HENRY to correct the tibialis posterior dysfunction could result in retention of function of hallux and lesser digits