Entrapment of First Branch of Lateral Plantar Nerve (FB-LPN)/Baxter’s neuritis
Commonly overlooked – has been suggested a being responsible for up to 20% chronic heel pain. FB-LPN bifurcates from lateral plantar nerve, having three divisions. One division innervates periosteum over medial plantar tuberosity of calcaneus; second supplies flexor digitorum brevis; third supplies abductor digiti quinti brevis .
Usually occurs where nerve changes from vertical to horizontal around medial plantar aspect of heel – compression is between deep fascia of adductor hallucis muscle and medial margin of quadratus plantae muscle or by compression from a calcaneal spur. Also can get entrapment where nerve passes just distal to medial tuberosity of calcaneus (especially if there is a heel spur present).
Unclear – could be trauma to abductor hallucis muscle from overuse type problem; osseous heel spur or inflammation from insertional plantar fasciitis may impinge on or irritate the nerve; pronated foot may increase tension on nerve and/or forces medial tuberosity of calcaneus to compress the nerve. May coexist with insertional plantar fasciitis.
Maximum pain is usually where nerve is compressed between abductor hallucis and the quadratus plantae muscle – over plantar medial aspect of heel. Usually worse later in day after activity. Difficult to differentiate from insertional plantar fasciitis/heel spur syndrome – first step pain after rest (post-static dyskinesia) not as severe or may be absent. Pain on medial compression of calcaneus as well as plantar. Pain can occasionally radiate from medial calcaneal area to lateral foot. May be able to elicit neuritic pain by palpating the nerve in area of medial plantar heel along abductor muscle or plantar fascia withdrawal response. Sensory changes are uncommon. Tinnels sign may be elicited in some. (Up to 20% will have radiographic evidence of osseous heel spur, of no pathognomonic significance). May not be able to abduct lessor digits – compare to asymptomatic side, as branch to abductor digiti quinti brevis is commonly affected (normally this is difficult for many people to perform). May have symptoms in third interspace (may be confused with Morton’s neuroma).
Phalen’s manoeuvre may produce pain – plantarflexion and inversion of foot reduces width of the porta pedis abductor hallucis compresses nerve symptoms. May be able to differentiate from insertional plantar fasciitis/heel spur syndrome by changing neural tension – with finger pressure on the painful area, ankle dorsiflexed, subtalar joint everted and the forefoot perform a single leg raise or slump test. If the nerve is involved, the pain will alter.
Differential diagnosis – insertional plantar fasciitis/heel spur syndrome (should produce pain on forced hallux and ankle dorsiflexion with palpation of plantar fascia); plantar fad pad disorders; rheumatological conditions; stress fracture
Shock absorbing heel cups; foot orthoses; avoidance of barefoot walking; activity modification; NSAID’s; electrotherapeutic modalities; stretching of achilles tendon and plantar fascia; cortisone injections; surgical resection of nerve .