Plantaris Anatomy

Origin

• Site: the inferior part of the lateral supracondylar line of the femur, immediately proximal and medial to the origin of the lateral head of gastrocnemius.
• Additional origin: the oblique popliteal ligament of the knee — the broad superolateral expansion of the semimembranosus tendon across the posterior capsule.
• Character: a small, thin, fusiform belly typically 7-10 cm long overall with only 2-4 cm of true muscle fibres, running obliquely.

Course

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

Insertion

• Most commonly: the posterior surface of the calcaneus, medial to the Achilles tendon insertion, either separately or blending with the medial edge of the Achilles.
• Variants (relevant because they change what you find at surgery):
  • Blending entirely with the medial Achilles tendon several centimetres above the calcaneus.
  • Inserting into the flexor retinaculum or the deep crural fascia.
  • Inserting into the medial calcaneal periosteum separate from the Achilles.
  • Rarely, into the plantar aponeurosis — the ancestral insertion that gives the muscle its name.
• Practical consequence: the insertion pattern determines whether the plantaris can be harvested as a free tendon and how easily it separates from the Achilles.

Dimensions for Graft Planning

• Tendon length: commonly 30-40 cm, occasionally more.
• Tendon width: approximately 3-5 mm, flattening distally.
• Tendon thickness: approximately 1-2 mm.
• Tensile strength: substantial for its size — the plantaris tendon has a high collagen density and a tensile strength that comfortably exceeds the requirements of most reconstructions for which it is used.

Nerve Supply

• Nerve: the tibial nerve.
• Root values: S1, S2 (some descriptions include L4 and L5).
• Entry: 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 muscle 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 during exposure of the popliteal vessels. This 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.
• In electrodiagnostic terms plantaris is not separately sampled; it is too small and too variably present.
• The nerve is worth knowing only for completeness and for the standard examination question grouping plantaris with gastrocnemius and soleus as tibial nerve, S1-S2, superficial posterior compartment.

Root Value in Context

Arterial Supply

• Sural arteries — the same popliteal artery branches that supply the gastrocnemius heads. The plantaris belly is typically supplied by a small branch from the lateral sural artery or directly from the popliteal artery.
• Posterior tibial artery — small branches supply the tendon through the paratenon in the distal leg.
• Peroneal (fibular) artery — contributes to the distal tendon via the posterior intermuscular septum.

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.
• Watershed relevance: 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.

Venous

• Small venae comitantes drain to the popliteal and posterior tibial veins.
• The soleal veins in this compartment are the classical site of calf deep vein thrombosis — relevant because the differential diagnosis of a painful swollen calf after a snap always includes DVT.

Popliteal Fossa

• Superficial (posterior): the lateral head of gastrocnemius, which overlies the plantaris belly.
• Deep (anterior): the posterior knee capsule, the oblique popliteal ligament and the popliteus muscle.
• Medial: the popliteal artery, vein and tibial nerve — the neurovascular bundle lies medial and deep to the plantaris belly.
• Lateral: the common peroneal nerve, running along the posteromedial border of the biceps femoris tendon toward the fibular neck.

Calf

• The tendon runs in the interval between soleus (deep) and the medial head of gastrocnemius (superficial).
• This is the same plane in which a tennis leg haematoma collects — the sonographic sign of a medial gastrocnemius tear is fluid between the medial gastrocnemius and soleus, exactly where the plantaris tendon lies. This anatomical coincidence explains a century of misattribution.
• The sural nerve runs in the subcutaneous plane, more superficially, having pierced the deep fascia at about the mid-calf.

Distal Leg

• Medial to the Achilles tendon, usually within or immediately adjacent to its paratenon.
• Anterior (deep): the deep flexor compartment structures — flexor hallucis longus, tibialis posterior, flexor digitorum longus, and the posterior tibial neurovascular bundle.
• Posterior (superficial): the crural fascia and subcutaneous tissue.

Structures at Risk in a Plantaris Harvest

• Sural nerve — subcutaneous, running with the small saphenous vein, crossing the lateral border of the Achilles in the distal third of the leg. It is the structure at risk if the distal incision is placed laterally rather than medially.
• Achilles tendon — the plantaris tendon must be dissected free from the medial Achilles without incising the Achilles itself.
• Posterior tibial neurovascular bundle — deep to soleus, at risk only if the dissection is taken too deep.
• Small saphenous vein — accompanies the sural nerve.

The Classical Misattribution

• Tennis leg describes acute midcalf pain with a sudden snap or the sensation of being struck on the calf, typically in a middle-aged recreational athlete pushing off with the knee extended and the ankle dorsiflexed — the lunge in racquet sports.
• The historical teaching attributed it to rupture of the plantaris tendon. This was inference from anatomy, not from evidence.
• The imaging evidence overturned it. Ultrasound and MRI studies of large consecutive series of patients presenting with tennis leg found that the great majority have a tear at the medial gastrocnemius musculotendinous junction, with fluid collecting between the medial gastrocnemius and soleus, and that plantaris rupture accounts for only a small minority of cases.
• The correct answer in an examination: tennis leg is a medial gastrocnemius musculotendinous tear. Plantaris rupture exists but is uncommon and is not the explanation for the syndrome.

Clinical Features

• Sudden sharp pain in the medial calf with an audible or perceived snap, often described as being hit with a ball or a racquet.
• Immediate difficulty weight-bearing, with pain on push-off and on passive dorsiflexion.
• Focal tenderness at the medial musculotendinous junction, roughly at the junction of the middle and distal thirds of the calf.
• Ecchymosis developing over 24-72 hours and tracking distally to the ankle and foot under the influence of gravity.
• Plantar flexion power is reduced but present — a completely absent plantar flexion or a positive Thompson test means an Achilles rupture, not a tennis leg.

Differential Diagnosis

Management of Tennis Leg

• Non-operative in essentially all cases. The muscle heals reliably.
• Acute phase (0-72 hours): relative rest, ice, compression, elevation, and analgesia. A heel raise shortens the muscle-tendon unit and eases pain.
• Thromboprophylaxis consideration: a significant calf haematoma with reduced mobility raises DVT risk, and a substantial proportion of tennis leg patients have been shown to develop a DVT. Assess the risk formally and consider prophylaxis and a duplex scan.
• Subacute: progressive weight-bearing, 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 is reserved for the rare large symptomatic haematoma requiring drainage, and for the very rare complete rupture with a functional deficit.

Plantaris-Associated Midportion Achilles Tendinopathy

• A specific and treatable subgroup of midportion Achilles tendinopathy in which the plantaris tendon is the driver.
• Prevalence within the tendinopathy population: plantaris involvement is identified in a meaningful minority of patients with recalcitrant midportion Achilles tendinopathy, and it is disproportionately represented among patients who have failed a well-conducted eccentric loading programme.

Clinical Signature

• Pain and tenderness localised to the MEDIAL side of the Achilles at the midportion, typically 2-7 cm above the calcaneal insertion.
• A discrete medial tender point rather than the diffuse fusiform tenderness of classical midportion tendinopathy.
• Failure of a well-conducted eccentric loading programme — the single most useful pointer, because standard tendinopathy responds well to eccentric loading and this subgroup does not.
• The Achilles may be only minimally thickened, so a relatively normal-looking tendon with severe medial pain should raise the suspicion.

Investigation

• Ultrasound with Doppler: shows the plantaris tendon closely applied to or invaginated into the medial Achilles, with neovascularisation at the interface. Dynamic examination can demonstrate the tendons failing to slide independently.
• MRI: axial sequences at the level of maximal tenderness demonstrate the plantaris tendon medial to or within the Achilles, sometimes with a shared paratenon.
• 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: high-volume injection, sclerosing injection of the neovessels, and shockwave therapy.
• Surgery when loading fails: excision of the plantaris tendon together with release of the medial Achilles paratenon and any adhesions. This can be performed open through a small medial incision or ultrasound-guided and minimally invasive in experienced hands.
• Outcomes: published series report high rates of return to previous activity in appropriately selected patients — that is, those with medial-sided pain, imaging evidence of plantaris involvement, and failure of a proper loading programme.
• The counselling point: excising the plantaris costs the patient nothing functionally. This makes it a low-risk operation, which in turn makes patient selection the only variable that matters.

Why It Is a Good Graft

• Long: 30-40 cm, enough for most reconstructions requiring a long, thin graft.
• Thin and of uniform calibre: 3-5 mm wide and 1-2 mm thick, ideal where a bulky graft would be a problem.
• Strong: high collagen density gives a tensile strength that exceeds the requirements of the reconstructions for which it is used.
• Expendable: no functional deficit whatsoever.
• Low donor-site morbidity: two small incisions, minimal bleeding, and no immobilisation required for the donor site.

Applications

• 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.
• Achilles surgery: as a local augmentation graft, sometimes taken as a distally based turn-down.
• Knee: occasionally used 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. Distal incision: a short longitudinal or transverse incision over the medial border of the Achilles tendon, roughly 2-4 cm above the calcaneal insertion. Keep medial — the sural nerve crosses the lateral border.
4. Identify the tendon as a distinct thin cord medial to the Achilles, and confirm by tracing it distally to its calcaneal insertion. It is easily confused with a slip of the Achilles paratenon or with 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, prepare and measure it.
8. Close in layers with attention to the paratenon.

Alternatives if Absent

• 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.

Isolated Plantaris Rupture

• It exists but is uncommon, accounting for only a small minority of tennis-leg presentations.
• Presentation: identical history — a snap and midcalf pain — but a much less impressive clinical picture, with less swelling, less ecchymosis and a faster recovery.
• Diagnosis: ultrasound or MRI showing a retracted plantaris tendon with the medial gastrocnemius intact.
• Management: entirely non-operative. It heals or it does not, and either way the patient is unaffected.

Plantaris and the Thompson Test

• A commonly repeated teaching point is that an intact plantaris can produce a false negative Thompson test in a complete Achilles rupture.
• The more important source of a false negative is the intact deep posterior compartment — tibialis posterior, flexor hallucis longus and flexor digitorum longus can plantar flex the foot.
• The practical lesson stands regardless: squeeze the entire calf mass firmly, compare with the contralateral side, and combine the Thompson test with the Matles test and with palpation for a gap. Do not exclude a rupture on a single test.

Plantaris in Compartment Anatomy

• Plantaris lies in the superficial posterior compartment with gastrocnemius and soleus.
• In a fasciotomy for compartment syndrome, the superficial posterior compartment is released through the medial incision, longitudinally over the calf, and plantaris is encountered incidentally within it.
• The deep posterior compartment, containing tibialis posterior, flexor digitorum longus, flexor hallucis longus and the posterior tibial neurovascular bundle, is released by detaching soleus from the tibia through the same medial incision — a step frequently done inadequately, leaving the deep compartment unreleased.

Anatomical Variants

• Absence in about 13 per cent of knees in a cadaveric series of 46 specimens.
• Variation from the standard description in a further 30 per cent of that series, 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.
• Aberrant origins from the posterior capsule, the lateral meniscus, or the medial rather than the lateral supracondylar line.
• Variant insertions into the flexor retinaculum, the medial calcaneal periosteum, the deep crural fascia or, rarely, the plantar aponeurosis.
• 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.