Plantaris Anatomy

Origin. Plantaris arises from the inferior part of the lateral supracondylar line of the femur, immediately proximal and medial to the origin of the lateral head of gastrocnemius. It has an additional origin from the oblique popliteal ligament, the broad superolateral expansion of the semimembranosus tendon across the posterior capsule. The belly is small, thin and fusiform, typically 7-10 cm long overall with only 2-4 cm of true muscle fibres, and it runs obliquely.

Course. The belly runs inferomedially between the lateral head of gastrocnemius behind it and the popliteus and knee capsule in front. The tendon becomes free at about the level of the gastrocnemius musculotendinous junction and then runs obliquely and medially in the plane between soleus (deep) and the medial head of gastrocnemius (superficial). That oblique course, lateral proximally to medial distally, is the defining anatomical feature, and it is why the tendon crosses the very plane in which a tennis leg haematoma collects. In the distal third of the leg it runs along the medial border of the Achilles tendon.

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Insertion. Most commonly the tendon inserts on the posterior surface of the calcaneus, medial to the Achilles insertion, either separately or blending with the medial edge of the Achilles. The insertion pattern determines whether plantaris can be harvested as a free tendon and how easily it separates from the Achilles, and the variants change what you find at surgery:

• Blending entirely with the medial Achilles several centimetres above the calcaneus
• Into the flexor retinaculum or the deep crural fascia
• Into the medial calcaneal periosteum, separate from the Achilles
• Rarely, into the plantar aponeurosis, the ancestral insertion that gives the muscle its name

Dimensions for graft planning. The tendon is occasionally longer than 40 cm. It is approximately 3-5 mm wide, flattening distally, and approximately 1-2 mm thick. Its tensile strength is substantial for its size: the collagen density is high, and the strength comfortably exceeds the requirements of most reconstructions for which it is used.

Variants. In Freeman's series of 46 cadaveric knees the muscle varied from the standard description in 30 per cent, including interdigitation with the lateral head of gastrocnemius (about 20 per cent) and a strong fibrous extension to the patella (about 11 per cent), the latter implicated in patellofemoral pain; how often it was absent is discussed in the alert below. Aberrant origins are also described from the posterior capsule, the lateral meniscus, or the medial rather than the lateral supracondylar line.

A hypertrophied or aberrant plantaris has been implicated as a rare cause of popliteal artery entrapment syndrome and of posterior knee impingement, though the far commoner cause of the former is an aberrant medial head of gastrocnemius.

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Nerve supply. The tibial nerve, root values S1, S2 (some descriptions include L4 and L5). A small branch arises in the popliteal fossa, usually from the same region of the tibial nerve that gives the branches to the lateral head of gastrocnemius, and enters the belly on its deep (anterior) surface in the proximal popliteal fossa. The branch is short and easily divided during a posterior approach to the knee or exposure of the popliteal vessels, which is of no functional consequence.

Clinical application. Isolated plantaris denervation is silent: there is no clinical test, no measurable deficit and no imaging appearance that matters. Electrodiagnostic studies do not sample it separately, because it is too small and too variably present.

The examination grouping. The nerve is worth knowing for completeness and for the standard question that groups plantaris with gastrocnemius and soleus as the superficial posterior compartment. All three are supplied by the tibial nerve, S1-S2, and all three plantar flex the ankle. Gastrocnemius, with its two heads, and plantaris cross the knee; soleus, the uniarticular workhorse of the calf, does not.

Arteries and veins. The belly is supplied by the sural arteries, the same popliteal branches that supply the gastrocnemius heads, typically through a small branch from the lateral sural artery or directly from the popliteal artery. In the distal leg, small branches of the posterior tibial artery reach the tendon through the paratenon, and the peroneal (fibular) artery contributes to the distal tendon via the posterior intermuscular septum. Small venae comitantes drain to the popliteal and posterior tibial veins.

Consequences for surgery. The muscle is too small and too poorly vascularised to be of any use as a flap. Tendon harvest is essentially bloodless: the small vessels in the paratenon are the only source of bleeding, and a tendon stripper produces almost none.

The watershed. The plantaris tendon shares the distal leg with the Achilles watershed zone 2-6 cm above the calcaneal insertion, the region of relative hypovascularity where midportion Achilles tendinopathy and rupture occur. That anatomical coincidence is part of why a plantaris adherent to the medial Achilles at that level is a plausible contributor to tendinopathy.

The soleal veins. The soleal veins in this compartment are the classical site of calf deep vein thrombosis. That is why the differential diagnosis of a painful swollen calf after a snap always includes DVT.

In the popliteal fossa. The lateral head of gastrocnemius lies superficial (posterior) to the plantaris belly and overlies it; the posterior knee capsule, the oblique popliteal ligament and popliteus lie deep (anterior) to it. The popliteal artery, vein and tibial nerve lie medial and deep to the belly. Laterally lies the common peroneal nerve, running along the posteromedial border of the biceps femoris tendon toward the fibular neck.

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In the calf. The tendon runs in the interval between soleus (deep) and the medial head of gastrocnemius (superficial). The sonographic sign of a medial gastrocnemius tear is fluid between the medial gastrocnemius and soleus, exactly where the plantaris tendon lies, and that coincidence explains a century of misattribution. The sural nerve runs more superficially in the subcutaneous plane, having pierced the deep fascia at about the mid-calf.

In the distal leg. The tendon lies medial to the Achilles, usually within or immediately adjacent to its paratenon, with the crural fascia and subcutaneous tissue behind it. Deep to it are the deep flexor compartment structures: flexor hallucis longus, tibialis posterior, flexor digitorum longus and the posterior tibial neurovascular bundle. The structures at risk in a harvest are tabulated under Surgical Relevance.

The syndrome. Tennis leg describes acute midcalf pain with a sudden snap or the sensation of being struck on the calf. It typically affects a middle-aged recreational athlete pushing off with the knee extended and the ankle dorsiflexed, the lunge in racquet sports.

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The misattribution. The historical teaching attributed tennis leg to rupture of the plantaris tendon, an inference from anatomy rather than evidence. Ultrasound and MRI studies of large consecutive series overturned it: the great majority of patients have a tear at the medial gastrocnemius musculotendinous junction, with fluid collecting between the medial gastrocnemius and soleus, and plantaris rupture accounts for only a small minority. In Delgado's 141 patients referred with the diagnosis, two thirds had a medial gastrocnemius rupture, a further fifth had fluid between the two muscles without a rupture, and 1.4 per cent had a plantaris rupture.

The examination answer. Tennis leg is a medial gastrocnemius musculotendinous tear. Plantaris rupture exists, but it is uncommon and is not the explanation for the syndrome.

Clinical features. The patient feels sudden sharp pain in the medial calf with an audible or perceived snap, often described as being hit with a ball or a racquet, and has immediate difficulty weight-bearing, with pain on push-off and on passive dorsiflexion. Tenderness is focal at the medial musculotendinous junction. Ecchymosis develops over 24-72 hours and tracks distally to the ankle and foot under the influence of gravity.

Plantar flexion. Power is reduced but present; completely absent plantar flexion or a positive Thompson test means an Achilles rupture, not a tennis leg.

Isolated plantaris rupture. It gives an identical history, a snap and midcalf pain, but a much less impressive clinical picture, with less swelling, less ecchymosis and a faster recovery. Ultrasound or MRI shows a retracted plantaris tendon with the medial gastrocnemius intact. Management is entirely non-operative: it heals or it does not, and either way the patient is unaffected.

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Management. Tennis leg is managed non-operatively in essentially all cases, and the muscle heals reliably.

• Acute phase (0-72 hours): relative rest, ice, compression, elevation and analgesia, with a heel raise to shorten the muscle-tendon unit and ease pain
• Subacute phase: progressive weight-bearing and gentle range of motion, then eccentric and concentric calf loading with the knee both extended and flexed, to load gastrocnemius and soleus separately
• Return to sport typically at 4-8 weeks for a partial tear, longer for a large tear with a substantial haematoma
• Surgery only for the rare large symptomatic haematoma requiring drainage and the very rare complete rupture with a functional deficit

Thrombosis risk. A significant calf haematoma with reduced mobility raises the risk of DVT, and a substantial proportion of tennis leg patients have been shown to develop one. Assess the risk formally, and consider prophylaxis and a duplex scan.

The subgroup. Plantaris-associated midportion Achilles tendinopathy is a specific and treatable subgroup in which the plantaris tendon is the driver. Plantaris involvement is identified in a meaningful minority of patients with recalcitrant midportion tendinopathy, and it is disproportionately represented among those who have failed a well-conducted eccentric loading programme. Alfredson's surgical series found an invaginated or closely located enlarged plantaris far more often, in 58 of 73 chronic tendons, but that was the operating surgeon's unblinded judgement, with no comparison against asymptomatic tendons.

Clinical signature. Pain and tenderness localise to the medial side of the Achilles at the midportion, typically 2-7 cm above the calcaneal insertion, at a discrete medial tender point rather than the diffuse fusiform tenderness of classical midportion tendinopathy. The Achilles may be only minimally thickened, so a relatively normal-looking tendon with severe medial pain should raise the suspicion.

The most useful pointer. Failure of a well-conducted eccentric loading programme is the single most useful pointer, because standard tendinopathy responds well to eccentric loading and this subgroup does not.

Investigation. Ultrasound with Doppler shows the plantaris tendon closely applied to, or invaginated into, the medial Achilles, with neovascularisation at the interface in symptomatic patients, a useful confirmatory finding, and dynamic examination can show the two tendons failing to slide independently. Axial MRI at the level of maximal tenderness shows the plantaris tendon medial to or within the Achilles, sometimes with a shared paratenon. An ultrasound-guided injection of local anaesthetic into the interface can be used diagnostically.

Management. First line remains a properly conducted loading programme, heavy slow resistance or eccentric loading over at least 12 weeks; do not diagnose a plantaris problem in someone who has not done the rehabilitation properly. Adjuncts are high-volume injection, sclerosing injection of the neovessels and shockwave therapy.

Surgery when loading fails. The operation is excision of the plantaris tendon together with release of the medial Achilles paratenon and any adhesions. It can be performed open through a small medial incision or, in experienced hands, ultrasound-guided and minimally invasive; the open steps are under Surgical Relevance.

Outcomes and counselling. Published series report high rates of return to previous activity in appropriately selected patients: those with medial-sided pain, imaging evidence of plantaris involvement and failure of a proper loading programme. Excising the plantaris costs the patient nothing functionally, which makes it a low-risk operation, which in turn makes patient selection the only variable that matters.

Why it is a good graft. Its length is enough for most reconstructions requiring a long, thin graft, and its thin, uniform calibre is ideal where a bulky graft would be a problem. It is strong for its size and expendable, with no functional deficit whatsoever. Donor-site morbidity is low: two small incisions, minimal bleeding, and no immobilisation required for the donor site.

Where it is used. The applications group by region:

• Hand and wrist - flexor and extensor tendon grafting, ligament reconstruction including scapholunate and thumb ulnar collateral ligament reconstruction, and pulley reconstruction in the flexor sheath
• Foot and ankle - lateral ankle ligament reconstruction, spring ligament and deltoid reconstruction, peroneal and tibialis posterior tendon reconstruction, and augmentation of an Achilles repair
• Knee - occasionally, for small-graft applications, though the hamstrings are preferred where a larger graft is needed
• Elsewhere - as a suture-like biological material in reconstructive and plastic surgery

Harvest technique.

1. Confirm presence pre-operatively by ultrasound or MRI, or by exploring the distal medial Achilles border first.
2. Position prone, or supine with the hip externally rotated.
3. Make the distal incision, short and longitudinal or transverse, over the medial border of the Achilles tendon, roughly 2-4 cm above the calcaneal insertion. Keep medial, because the sural nerve crosses the lateral border.
4. Identify the tendon as a distinct thin cord medial to the Achilles, and trace it distally to its calcaneal insertion. It is easily confused with a slip of the Achilles paratenon or a medial Achilles fibre bundle; traction on it should produce movement in the proximal calf, which is the confirmatory test.
5. Divide the tendon distally, whip-stitch the free end, and apply distal traction.
6. Pass a closed-loop tendon stripper proximally along the tendon, keeping it coaxial with the tendon and advancing with steady traction toward the popliteal fossa. Never force it: resistance means an adhesion or an aberrant band, which should be released through a small counter-incision.
7. Deliver the graft, then prepare and measure it.
8. Close in layers with attention to the paratenon.

Alternatives if it is absent. Have one planned:

• Palmaris longus, the direct upper limb equivalent, itself absent in a comparable proportion
• Extensor digitorum longus to the lesser toes, or an extensor hallucis brevis slip
• Peroneus longus or peroneus brevis split graft
• Semitendinosus or gracilis, where a larger graft is acceptable
• Allograft, reliable but with cost, availability and, in some reconstructions, incorporation considerations