Plantar Interossei Anatomy

The plantar interossei are three small unipennate muscles of the fourth (deepest) plantar layer, one for each of the third, fourth and fifth metatarsals. Each arises from the base and medial side of the shaft of its own metatarsal and inserts on the medial side of the base of the proximal phalanx of the same toe, so it draws that toe medially, toward the axis of the second ray.

They are individually trivial and collectively indispensable. Together with the dorsal interossei and the lumbricals they form the intrinsic sling that flexes and stabilises the metatarsophalangeal joints, and their loss is the mechanical event that permits a normal forefoot to become a clawed, ulcerating, intrinsic-minus foot. Say permits rather than causes, and know why: the Bus MRI study cited below found profound atrophy — muscle fell from 30.8% to 8.3% of forefoot cross-sectional area — in neuropathic feet whose toe joint angles were no different from controls, with clawing in only two of eight. Intrinsic loss is necessary for the deformity and is not sufficient for it. Almost every important question about these muscles is nonetheless a question about neuropathy.

Actions

• Adduction of the third, fourth and fifth toes toward the axis of the second ray - their eponymous action, and clinically almost irrelevant in isolation.
• Flexion of the metatarsophalangeal joints of those toes, because the tendons pass plantar to the metatarsophalangeal joint axis. This is their functionally important action.
• Stabilisation of the metatarsophalangeal joints in the transverse and sagittal planes during the stance phase, holding the toes down against the ground.
• Transverse stabilisation of the metatarsal parabola, acting with the deep transverse metatarsal ligaments and the transverse head of adductor hallucis to resist forefoot splay.
• Contribution to the intrinsic muscle sling supporting the longitudinal arch, working in parallel with the plantar aponeurosis.
• No action at the interphalangeal joints - they have no extensor expansion.

The Four-Way Balance of the Lesser Toe

The Cascade of the Intrinsic-Minus Foot

1. Interossei and lumbricals are denervated. The only intrinsic flexor moment at the metatarsophalangeal joint is lost.
2. Extensor digitorum longus hyperextends the metatarsophalangeal joint through the extensor sling; the long and short flexors flex both interphalangeal joints. Claw toe.
3. The plantar fat pad is dragged distally by the hyperextended proximal phalanx, unroofing the metatarsal head.
4. Plantar pressure under the metatarsal head rises sharply, and pressure also rises at the toe tip and over the dorsal proximal interphalangeal joint (against the shoe).
5. Callus forms at these three points. In a foot with intact sensation this hurts and the patient adapts. In an insensate foot the callus becomes an ulcer, then infected, then osteomyelitic.
6. In parallel, the peroneus longus overpowers a weakened tibialis anterior, plantarflexing the first ray; the forefoot pronates on the hindfoot; and the hindfoot compensates into varus - the cavovarus foot of Charcot-Marie-Tooth disease.
7. The deformity becomes fixed as the plantar fascia and the intrinsic muscles contract, and the toes become rigid.

Length-Tension and What Happens When It Fails

Palpation and Inspection

• The interossei are not individually palpable. What is visible and clinically decisive is their bulk.
• Inspect the dorsum of the foot with the patient seated and the foot relaxed: in a normal foot the intermetatarsal spaces are gently filled. In intrinsic wasting they become hollowed and gutterred, and the extensor tendons stand out - exactly analogous to guttering of the dorsal interosseous spaces of the hand in ulnar palsy.
• Inspect the plantar forefoot for callus under the metatarsal heads and at the toe tips, and for distal migration of the fat pad (the metatarsal heads become easily palpable through thin plantar tissue).
• Inspect the dorsum of the proximal interphalangeal joints for corns from shoe contact.
• Compare the two feet, and look at the hands. Bilateral symmetrical wasting with hand involvement points to a hereditary neuropathy; unilateral wasting demands a search for a nerve lesion, an old compartment syndrome, or a spinal cause.

Examination of the Intrinsic-Minus Foot

Imaging

• Weight-bearing foot radiographs - assess metatarsophalangeal subluxation or dislocation, metatarsal length pattern, hindfoot alignment, Meary's angle and calcaneal pitch for cavus, and the lateral talo-first metatarsal angle.
• MRI - the modality that shows the muscles. Look for fatty atrophy of the interossei and the other intrinsics (the earliest structural marker in diabetic and hereditary neuropathy), denervation oedema in acute lesions, and selective abductor digiti minimi atrophy if the lateral plantar nerve territory is under suspicion. MRI of the whole spine is mandatory in a unilateral or asymmetric paediatric cavovarus foot.
• Nerve conduction studies and electromyography - to characterise the neuropathy (demyelinating versus axonal), which is the key discriminator in the Charcot-Marie-Tooth subtypes.
• Genetic testing - PMP22 duplication for CMT1A, the commonest subtype, and panel testing where the phenotype is atypical.
• Plantar pressure mapping where available - quantifies the metatarsal head overload that drives ulceration, and guides insole design.

Related pages. The rest of the intrinsic sling: Dorsal Interossei — DAB about the same second-ray axis — and Lumbrical Muscles, whose small cross-sectional area Kura quantifies above. The deformity their loss permits: Claw Toes, Hammer Toes and Lesser Toe Deformities, plus Metatarsalgia for the metatarsal head overload that follows fat pad migration. Why they waste: Diabetic Foot, Diabetic Foot Ulcers and Diabetic Foot Infections for the neuropathic cascade, Charcot-Marie-Tooth Disease and Cavovarus Foot for the inherited one, and Charcot Neuroarthropathy. The acute cause of irreversible clawing: Foot Compartment Syndrome — Manoli's nine compartments include four separate interosseous ones — with Compartment Syndrome and Calcaneal Fractures, the injury that hides it. Their nerve supply: Tibial Nerve Anatomy. The hand comparison, and its limits: Intrinsic-Minus and Intrinsic-Plus Hand.

1. Charcot-Marie-Tooth Disease (Hereditary Motor and Sensory Neuropathy)

• The archetypal cause of a bilateral, progressive, intrinsic-minus cavovarus foot in a young patient. CMT1A, caused by a duplication of the PMP22 gene, is the commonest subtype, inherited in an autosomal dominant pattern.
• Foot findings: intrinsic wasting, claw toes, a plantarflexed first ray, forefoot pronation on the hindfoot, hindfoot varus, a high arch with a positive Meary angle and increased calcaneal pitch, peroneal weakness with recurrent lateral ankle instability, and later a fixed cavovarus with plantar and lateral border callus.
• The muscle imbalance to be able to recite: tibialis anterior and the peroneus brevis weaken while peroneus longus and tibialis posterior are relatively preserved. The strong peroneus longus plantarflexes the first ray; the strong tibialis posterior inverts the hindfoot; the weak tibialis anterior cannot lift the first ray; and the denervated intrinsics allow the toes to claw. Every component of the deformity follows from that imbalance.
• Assessment: neurological examination including the hands, family history and examination of relatives, nerve conduction studies, genetic testing, and a Coleman block test to determine whether the hindfoot varus is flexible and forefoot-driven.
• Management principles:
  • Non-operative: orthoses with a lateral forefoot post and a first ray recess to accommodate the plantarflexed first ray, ankle-foot orthoses for foot drop, physiotherapy, and footwear.
  • Operative, tailored to the deformity: plantar fascia release; first metatarsal dorsiflexion (dorsal closing wedge) osteotomy for the plantarflexed first ray; peroneus longus to brevis transfer to remove the plantarflexing force on the first ray and augment eversion; tibialis posterior transfer for foot drop; lateralising (Dwyer or lateral closing wedge, or lateral translational) calcaneal osteotomy if the hindfoot varus is rigid on the Coleman block test; claw toe correction by flexor-to-extensor transfer or proximal interphalangeal fusion; and arthrodesis (triple fusion) reserved for the severe rigid deformity or salvage.
  • Sequence: correct the plantar and forefoot drivers before the hindfoot, and reassess the hindfoot after the forefoot has been corrected.
• Progression is expected, so counsel accordingly and plan for staged intervention over years.

2. Diabetic Peripheral Neuropathy

• The commonest cause of intrinsic-minus forefoot changes overall. Distal symmetrical polyneuropathy denervates the interossei and the other intrinsics, producing claw toes, forefoot splay, metatarsal head plantar prominence and distal fat pad migration.
• Intrinsic muscle fatty atrophy on MRI is one of the earliest structural manifestations of diabetic neuropathy - detectable before deformity is clinically obvious. It should therefore be read as a call to action, not an incidental finding.
• The pathway to amputation runs through this anatomy: intrinsic denervation gives claw toe, claw toe gives metatarsal head and toe tip overload, overload gives callus, callus in an insensate foot gives ulceration, ulceration gives osteomyelitis, and osteomyelitis gives amputation. Every step is preventable.
• Prevention and management:
  • Annual neuropathy screening with monofilament and vibration testing, and pulse assessment.
  • Protective footwear and total-contact or pressure-relieving insoles; a recess under the metatarsal heads and adequate toe box depth for clawed toes.
  • Regular callus debridement - callus is the pre-ulcer.
  • Percutaneous flexor tenotomy for a claw toe with an apical (tip) ulcer - a simple, local-anaesthetic, minimal-equipment procedure with a high healing rate and low morbidity.
  • Offloading of an established ulcer, most reliably with a total contact cast or removable device.
  • Surgical correction of fixed deformity where it is the mechanical driver of recurrent ulceration - metatarsal head resection, joint resection or fusion, or metatarsal osteotomy.
  • Vascular assessment before any forefoot surgery - remember the dorsal-plantar perforators in each interspace are the collateral pathway.
• Charcot neuroarthropathy is a separate but overlapping problem in the same population, driven by neuropathy plus repetitive trauma, and produces midfoot collapse with a rocker-bottom deformity.

3. Other Causes of the Intrinsic-Minus Foot

4. Interosseous Compartment Syndrome

• Each interosseous space is a separate compartment. In a forefoot crush or Lisfranc injury they can be involved with or without the medial, central and calcaneal compartments.
• They are decompressed from the dorsum, through longitudinal incisions over the second and fourth intermetatarsal spaces, spreading down onto the metatarsal shafts on either side to open all four spaces. A medial (Henry) approach does not decompress them.
• Missed, the interossei fibrose and the forefoot becomes rigidly clawed, with no good reconstruction.

5. Interdigital (Morton) Neuroma - The Anatomy That Explains the Approach

• A perineural fibrosis of a common plantar digital nerve, most often in the third interspace (where the communicating branch between the medial and lateral plantar nerves makes the nerve relatively tethered) and next most often the second.
• Presentation: forefoot pain radiating into the adjacent toes, worse in tight footwear, often described as walking on a pebble or with a sock rolled up, sometimes with a Mulder click on transverse compression of the forefoot.
• Anatomical point that determines the operation: the common plantar digital nerve runs PLANTAR to the deep transverse metatarsal ligament, whereas the interossei lie DORSAL to it. A dorsal approach therefore requires division of the deep transverse metatarsal ligament to reach the nerve, working between the interossei.
• Non-operative treatment first - metatarsal dome or bar, footwear with a wide toe box and a low heel, and an injection. Excision or decompression for refractory cases, with counselling about permanent numbness in the adjacent toe web.

6. Falls, Balance and the Ageing Foot

• Age-related loss of intrinsic muscle bulk reduces toe purchase and postural control and is a recognised contributor to falls risk in older people.
• Toe flexor and intrinsic strengthening (including short-foot exercises) has a legitimate role in balance rehabilitation, alongside footwear assessment.

Distances and Safe Zones

• Deep plantar arch and the deep branch of the lateral plantar nerve: run transversely across the sole at the level of the metatarsal bases, immediately plantar to the interossei and deep to the oblique head of adductor hallucis. Deep plantar dissection at this level should be transverse in orientation.
• Perforating (deep plantar) artery: in the proximal first intermetatarsal space at the level of the metatarsal bases - the structure that limits proximal extension of a dorsal first web space dissection, and an important forefoot collateral.
• Dorsal digital branches of the deep peroneal nerve and superficial peroneal cutaneous branches: subcutaneous on the dorsum of the interosseous spaces, within about 3 to 5 mm of the skin. Incise skin only, then spread.
• Common plantar digital nerve: lies PLANTAR to the deep transverse metatarsal ligament between the metatarsal heads; the interossei lie DORSAL to it. Dividing the ligament is what gives dorsal access to the nerve.
• Interosseous fasciotomy incisions: dorsal, over the second and fourth intermetatarsal spaces, releasing fascia on both sides of each incision to open all four compartments.
• Nerve branch entry to the interossei: from the plantar aspect at the level of the proximal metatarsal shafts, from the deep branch of the lateral plantar nerve running with the arch.

Anatomical Variation Across Populations

• The three plantar and four dorsal interossei pattern is highly constant. Accessory slips, fusion of adjacent interossei and duplicated tendon slips are described but rare and clinically silent.
• Variation in the innervation of the fourth interspace interossei (superficial versus deep branch of the lateral plantar nerve) is described across dissection series, and the communicating branch between the medial and lateral plantar nerves in the third interspace is variably present - the anatomical explanation for the third interspace predominance of interdigital neuroma.
• Variation in the deep plantar arch and in the dominance of the dorsalis pedis versus posterior tibial contribution is clinically the most important vascular variation, because it determines the collateral reserve of the forefoot.
• Prevalence of the relevant diseases is what varies between populations, not the anatomy. Diabetes prevalence is rising fastest in low- and middle-income countries, so intrinsic-minus forefoot pathology and diabetic foot ulceration are a growing global burden. Charcot-Marie-Tooth disease prevalence is broadly similar across populations, in the region of 1 in 2,500. Leprosy remains a significant cause of intrinsic-minus foot deformity and plantar ulceration in endemic regions and must not be forgotten as a differential.

Side-by-Side Guidance and Practice Differences

Areas of Genuine Uncertainty

• Optimal timing of cavovarus reconstruction in a progressive hereditary neuropathy - operating early risks recurrence as the neuropathy advances, operating late means correcting a rigid deformity.
• Joint-preserving reconstruction versus triple arthrodesis in the moderately rigid Charcot-Marie-Tooth foot; long-term comparative data are limited.
• Whether prophylactic surgical correction of claw toes reduces ulceration in the neuropathic foot, as opposed to offloading and footwear alone.
• Whether intrinsic muscle strengthening can slow the progression of deformity in early neuropathy - biologically plausible but not established.
• Interosseous compartment pressure thresholds in the foot are less well defined than in the leg, and the number and boundaries of foot compartments still differ between anatomical models.
• Excision versus decompression for interdigital neuroma - excision reliably relieves pain but leaves permanent numbness; decompression by dividing the deep transverse metatarsal ligament preserves sensation but with less certain relief.

High- versus Limited-Resource Practice

• Well-resourced settings: MRI to quantify intrinsic atrophy, nerve conduction studies and genetic testing, plantar pressure mapping, custom total-contact insoles and orthoses, total contact casting, and access to staged reconstructive surgery and multidisciplinary diabetic foot services.
• Limited-resource settings: the highest-value interventions cost very little. Monofilament and tuning fork screening, daily foot inspection education, callus debridement, a well-fitting shoe with an adequate toe box, and a simple pressure-relieving insole prevent the majority of ulcers. A percutaneous flexor tenotomy for an apical toe ulcer needs a blade, local anaesthetic and knowledge of the anatomy. Where leprosy is endemic, active case finding and protective footwear programmes prevent an enormous burden of plantar ulceration in exactly this anatomical pattern.
• Everywhere: in an insensate foot, the absence of pain removes the warning signal. Structured examination and offloading must substitute for symptoms, and callus must be treated as a pre-ulcer rather than as protective skin.