Gastrocnemius Anatomy

Origins. The medial head arises from the posterior aspect of the medial femoral condyle, immediately proximal and posterior to the adductor tubercle, and from the adjacent posterior capsule of the knee; it is the larger and longer of the two. The lateral head arises from the lateral aspect of the lateral femoral condyle, immediately proximal to the lateral epicondyle, and from the adjacent posterior capsule and the supracondylar line. Both heads receive a contribution from the oblique popliteal ligament and the posterior capsule.

The fabella. A sesamoid bone within the tendon of the lateral head, present in a variable minority of knees. It articulates with the lateral femoral condyle and gives attachment to the fabellofibular ligament. It is a recognised radiographic mimic of a loose body and a rare cause of posterolateral knee pain (fabella syndrome) and of common peroneal nerve irritation.

Length asymmetry, the flap number. The medial head extends 3-5 cm further distally than the lateral head, the single most practically useful dimension on this page. The medial head is therefore longer, has a larger surface area and reaches further, which, combined with the absence of the common peroneal nerve on the medial side, is why it is the flap of choice and the lateral head a second choice used mainly for lateral defects.

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Insertion. The two heads converge and their aponeuroses fuse at about the midpoint of the calf, forming a broad aponeurosis on the deep surface. This fuses with the aponeurosis of soleus to form the Achilles tendon, typically at the junction of the middle and distal thirds of the leg, and the tendon inserts into the middle third of the posterior surface of the calcaneal tuberosity, with the retrocalcaneal bursa deep to it and the subcutaneous (superficial) calcaneal bursa between the tendon and the skin.

The spiral. The Achilles tendon spirals through roughly 90 degrees as it descends, so that fibres originating medially insert posterolaterally on the calcaneus. The spiral concentrates stress at the hypovascular watershed zone 2-6 cm above the insertion, the site of both midportion tendinopathy and the majority of ruptures.

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The musculotendinous junction. The medial gastrocnemius musculotendinous junction lies roughly at the junction of the middle and distal thirds of the calf. It is the site of the tennis leg tear and the level at which a Strayer recession divides the aponeurosis.

Variants. Accessory heads occur. A five-headed gastrocnemius is a popliteal fossa variant whose accessory heads can lie beside the neurovascular bundle and alter dissection planes or compress the popliteal vessels and tibial nerve. Gastrocnemius tertius is an accessory head that may arise from either condyle and pass close to the popliteal neurovascular bundle; recognise it on imaging and during posterior approaches.

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Motor supply. The tibial nerve, S1 and S2. Separate, short branches to each head arise in the popliteal fossa and enter each head on its deep (anterior) surface in its proximal third, generally within about 5 cm of the origin. Because the motor branches enter proximally, close to the pedicle, the muscle can be raised as a flap without denervation provided the dissection stays distal to the pedicle. Conversely, if a denervated, non-contractile flap is preferred (to avoid a distracting muscle twitch under a graft), the branch can be deliberately divided at the time of transfer.

The sural nerve, the key relation. The medial sural cutaneous nerve arises from the tibial nerve in the popliteal fossa and descends in the groove between the two heads of gastrocnemius, lying on the deep fascia. It pierces the deep fascia at about the middle of the calf, and is there joined by the sural communicating branch of the lateral sural cutaneous nerve (a branch of the common peroneal nerve) to become the sural nerve proper. The sural nerve then runs subcutaneously with the small saphenous vein, crossing the lateral border of the Achilles tendon roughly 9-12 cm proximal to the calcaneal insertion and passing behind the lateral malleolus, to supply the posterolateral leg, the lateral heel and the lateral border of the foot to the fifth toe.

Why this matters. The nerve is at risk in every operation on the calf, and it is closest to the midline in the mid-calf, precisely where the Strayer incision is made. Injury produces numbness over the lateral heel and lateral border of the foot to the fifth toe, and can produce a painful neuroma. It is a common consent point and a common medico-legal complaint.

The pedicles. Each head has its own dominant pedicle, the medial sural artery to the medial head and the lateral sural artery to the lateral head, both arising directly from the popliteal artery at or just above the level of the knee joint. Each artery enters the deep (anterior) surface of its head, typically within 3-5 cm of the muscle's origin, in the proximal quarter of the muscle. Pedicle length is approximately 4-6 cm when dissected back toward the popliteal artery, with an arterial calibre around 2-3 mm and paired venae comitantes.

Mathes-Nahai type I. A single vascular pedicle supplies the entire muscle. This is the classification that matters: a type I muscle can be raised entirely on one pedicle with complete confidence, which is why the gastrocnemius flap is so reliable. The two heads have independent supplies, so one head can be harvested with no compromise to the other.

The skin territory. Musculocutaneous perforators from the gastrocnemius supply the overlying skin, and a myocutaneous flap can be raised. The perforators are concentrated over the proximal and middle portions of the muscle, so the skin paddle should not extend far beyond the distal muscle belly. The medial sural artery perforator (MSAP) flap is a fasciocutaneous free flap based on a perforator of the medial sural artery, allowing the skin to be taken without sacrificing the muscle, a modern refinement worth knowing about.

The Achilles watershed. The Achilles tendon receives blood from the posterior tibial artery proximally and distally and from the peroneal artery in its middle portion, leaving the relatively hypovascular zone above the calcaneal insertion described under Attachments. That watershed is the site of most ruptures and of midportion tendinopathy, and it is the reason percutaneous and minimally invasive Achilles techniques emphasise preserving the paratenon.

Popliteal fossa. The two heads form the inferior (distal) boundaries of the diamond-shaped popliteal fossa, with biceps femoris superolaterally and semimembranosus and semitendinosus superomedially. Deep (anterior) to the muscle, the popliteal artery, vein and tibial nerve lie on its deep surface and on the popliteus and the posterior capsule: the artery is the deepest of the three (closest to bone), the vein is intermediate and the tibial nerve is the most superficial. Between the heads lie the medial sural cutaneous nerve and, in the distal part of the groove, the small saphenous vein; superficial are the deep fascia, the subcutaneous tissue and the skin.

The medial head as a retractor. It is the retractor of choice for protecting the popliteal vessels during a posteromedial approach: retract it posterolaterally and the vessels come with it, away from the tibia.

Calf. Deep to gastrocnemius lies soleus, and between them the plantaris tendon, running obliquely from lateral proximally to medial distally; deeper still lies the deep posterior compartment with the posterior tibial neurovascular bundle. Superficial is the crural fascia, with the sural nerve and small saphenous vein in the subcutaneous plane from mid-calf downward. The plane between the medial gastrocnemius and soleus is the interval used for a Strayer recession and is where a tennis leg haematoma collects.

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At the knee. The medial head lies immediately posterolateral to the semimembranosus tendon; the interval between them is the posteromedial approach to the knee and the site of a Baker cyst, a distended gastrocnemio-semimembranosus bursa communicating with the joint through a valvular slit. The lateral head lies adjacent to the common peroneal nerve, which runs along the posteromedial border of the biceps femoris tendon toward the fibular neck; this proximity is a principal reason the lateral head is the second-choice flap.

The workhorse. The medial gastrocnemius flap is the workhorse for soft tissue cover over the knee and the proximal third of the tibia; for a fellowship examination, this is the single most important thing to know about gastrocnemius. The tibia is reconstructed by thirds, and the table gives the first choice for each.

For the distal third, a local fasciocutaneous or perforator flap is the alternative to free tissue transfer.

Why the medial head rather than the lateral. Its extra length and size give greater reach, and its arc of rotation covers the medial and anterior knee and proximal tibia more effectively. The common peroneal nerve lies adjacent to the lateral head at the fibular neck and is at risk during a lateral harvest, and a lateral harvest may require fibular head osteotomy to gain reach, further endangering the nerve.

Arc of rotation and reach. The standard reach is the superior pole of the patella anteriorly, the medial and anterior knee, and the proximal third of the tibia. To gain further reach, in escalating order:

1. Complete release of the muscle's origin from the femoral condyle, which adds several centimetres
2. Scoring the deep aponeurosis transversely in multiple places (like scoring a mesh graft), which lengthens the flap without dividing muscle fibres; the most useful and the safest manoeuvre
3. Dividing the Achilles insertion of the medial head aponeurosis distally, extending the flap distally
4. Tunnelling the flap beneath the deep fascia to the defect, with a tunnel generous enough not to strangle the pedicle
5. Freeing the pedicle back toward the popliteal artery, the last resort, and the one that risks the flap

Both heads can be raised together for a very large defect, at the cost of significant plantar flexion power, which is generally acceptable in a limb-salvage setting but should be a conscious decision.

Harvest.

1. Position supine with a tourniquet and the knee flexed, or lateral. Mark the posterior midline of the calf and the medial border of the tibia
2. A longitudinal posteromedial incision approximately 2 cm posterior to the posteromedial border of the tibia, from just below the knee to the mid-calf; keeping 2 cm posterior to the tibial border protects the subcutaneous perforators of the skin over the tibia
3. Incise the deep fascia and identify the medial head
4. Develop the plane between gastrocnemius and soleus, the key interval: a clean, avascular plane, but the plantaris tendon runs within it and can be divided
5. Identify the midline raphe between the two heads, where the sural nerve lies; protect it, and divide the raphe to separate the medial head from the lateral
6. Divide the distal aponeurosis of the medial head, taking as much length as the defect requires
7. Identify and protect the medial sural artery pedicle entering the deep surface
8. Release the origin and score the aponeurosis if more reach is needed
9. Tunnel or rotate the flap to the defect and inset without tension
10. Cover the muscle with a split-thickness skin graft, immediately or as a delayed procedure
11. Close the donor site over a drain; the defect is closed directly in almost all cases

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Donor morbidity. Plantar flexion power is reduced but generally well compensated by the soleus and the remaining head; most patients regain the ability to perform a single-leg heel raise, though with reduced endurance. There is a contour deformity, a visible flattening of the medial calf which some patients find cosmetically troubling, and the risks of sural nerve injury if the intermuscular raphe is divided without identifying the nerve, and of donor site seroma and haematoma. The overall morbidity is low enough that the flap remains the first choice for the proximal third, even in comparison with free tissue transfer.

Salvage. Monitor the flap clinically, by colour, turgor and bleeding on pinprick, and with a monitoring skin island where one is available. Venous congestion is the commoner failure mode, a dusky, swollen flap with brisk dark bleeding; check the tunnel first, because a tight tunnel strangling the pedicle is the commonest correctable cause. Arterial insufficiency gives a pale, cool flap with no bleeding: check for a kinked or twisted pedicle, excessive tension, or a haematoma compressing the pedicle.

Take the patient back early. A flap that is failing at 6 hours is salvageable; one that is failing at 48 hours is not. Prevention is a generous tunnel or an open transposition, tension-free inset, meticulous haemostasis, and a drain that does not lie on the pedicle.

The procedures. The lengthenings are distinguished by level, and the level decides how much of the complex is lengthened and how forgiving the operation is.

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The central rule. Match the procedure to the Silfverskiold test. A positive test, an isolated gastrocnemius contracture, gets a gastrocnemius recession, Strayer or Baumann, and not an Achilles lengthening. A negative test, a combined contracture, will not be corrected by a gastrocnemius recession alone and needs an Achilles lengthening or a Vulpius-type combined procedure, once a bony block has been considered: anterior ankle osteophytes, an os trigonum, or post-traumatic joint change.

Why it matters so much. Achilles lengthening is far less forgiving. An over-lengthened Achilles produces a calcaneus gait with loss of push-off, and it is essentially irreversible.

Indications in the adult foot. Gastrocnemius recession is offered for:

• Recalcitrant plantar fasciitis with a demonstrable isolated gastrocnemius contracture
• Metatarsalgia and forefoot overload
• Insertional and non-insertional Achilles tendinopathy, as an adjunct
• Adult acquired flatfoot deformity, an essential adjunct in most reconstructions, because leaving the equinus uncorrected loads the reconstruction
• Neuropathic diabetic forefoot ulceration, reducing forefoot pressure to allow healing and to prevent recurrence
• Charcot midfoot reconstruction
• Ankle and hindfoot arthrodesis and arthroplasty, where equinus would compromise the result

A Strayer recession. Prone, or supine with the hip externally rotated, through a longitudinal or transverse posteromedial incision at the mid-calf, at the level of the gastrocnemius-soleus junction. Identify and protect the sural nerve first: it pierces the deep fascia at this level and runs across the operative field, which is why the operation is done open. Develop the plane between the gastrocnemius aponeurosis and the soleus, sweeping the soleus muscle fibres away, and divide the aponeurosis transversely under direct vision.

Setting the length. Dorsiflex the ankle with the knee extended and the gastrocnemius recedes proximally. Aim for approximately 10 degrees of dorsiflexion with the knee extended and the hindfoot in neutral, not more. The recessed aponeurosis is repaired or left according to preference; some surgeons suture the recessed edge to the soleus fascia to control the amount of lengthening. Rehabilitation is early weight-bearing in a boot, dorsiflexion stretching, and strengthening from about 2-6 weeks.

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Equinus in cerebral palsy. The commonest deformity in ambulant cerebral palsy, the one most often operated on, and the one most often over-treated. Spasticity of the gastrocnemius-soleus complex progresses from a dynamic contracture, correctable under anaesthesia or with the knee flexed, to a fixed contracture with muscle shortening and, eventually, fixed bony change. The gastrocnemius is usually affected before and more than the soleus in ambulant diplegia, which is the anatomical justification for gastrocnemius-specific surgery.

The plantar flexion-knee extension couple. In mid and terminal stance the plantar flexors control the forward progression of the tibia over the fixed foot. By restraining tibial advancement they generate an external knee extension moment: the ankle helps extend the knee. Weaken the plantar flexors by over-lengthening and the tibia advances uncontrolled, the ground reaction force falls behind the knee, and the knee flexes throughout stance. That is crouch gait, and this single mechanism explains why an apparently minor over-lengthening at the ankle destroys walking.

The crouch gait warning. This is the most important clinical point on this page for a paediatric examination. Over-lengthening the Achilles tendon in an ambulant child with spastic diplegia produces an irreversible calcaneus gait and crouch, and there is no reliable operation to reverse it. Consequently:

• In ambulant spastic diplegia a gastrocnemius recession (Strayer or Baumann) is preferred over an Achilles lengthening, because it is a graduated, gastrocnemius-specific procedure that does not risk over-lengthening the whole complex
• Achilles lengthening is reserved for hemiplegia with a genuinely combined contracture, for non-ambulant children where the goal is bracing and hygiene rather than gait, and for severe fixed deformity
• Never lengthen the Achilles in a child with a positive Silfverskiold test, existing crouch, weak plantar flexors, or a knee flexion contracture; these are the risk factors for iatrogenic crouch

The multilevel problem. Concurrent knee and hip pathology must be addressed: correcting an equinus in a child with hamstring and psoas contractures without addressing them unmasks the crouch. This is the rationale for single-event multilevel surgery, planned on the basis of instrumented gait analysis.

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Timing and adjuncts. Botulinum toxin A to the gastrocnemius, with serial casting, is the standard first-line management of dynamic equinus in a young child, deferring surgery and allowing growth, and ankle-foot orthoses control the deformity and improve gait efficiency. Surgery is generally deferred until at least 6-8 years of age where possible, because recurrence rates after early surgery are high and because gait analysis is more informative in an older child. Recurrence is common in a growing child regardless of technique, and this should be part of the counselling.

Tennis leg. A tear at the medial gastrocnemius musculotendinous junction, caused by forced push-off with the knee extending and the ankle dorsiflexing: the lunge in racquet sports, a sudden acceleration, or a step off a kerb. The patient describes a sudden snap or a sensation of being struck on the calf, immediate pain and difficulty weight-bearing, with ecchymosis tracking distally to the ankle over 24-72 hours.

Not a plantaris rupture. Imaging series of consecutive patients presenting with tennis leg have consistently found a medial gastrocnemius musculotendinous tear with fluid between the medial gastrocnemius and soleus in the great majority, with plantaris rupture in only a small minority.

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Always exclude two things. An Achilles rupture, by the Thompson and Matles tests and palpation for a gap, and a deep vein thrombosis, by duplex ultrasound: a significant proportion of tennis leg patients have a concurrent DVT, and the two are clinically indistinguishable.

Management. Non-operative in essentially all cases: rest, ice, compression, elevation, a heel raise, then progressive loading with the knee both extended and flexed. Return to sport is at 4-8 weeks for most tears, longer with free tendon involvement. Formally assess thromboprophylaxis risk in a patient with a large haematoma and reduced mobility.

Popliteal artery entrapment syndrome. The commonest anatomical cause is an aberrant medial head of gastrocnemius, arising too laterally so that the popliteal artery is displaced medially and compressed. The patient is young and athletic, with exercise-induced calf claudication relieved by rest and normal resting pulses that disappear on active plantar flexion or passive dorsiflexion. The Whelan / Insua classification, simplified:

• Type I: the artery deviates medially around a normally positioned medial head
• Type II: the medial head arises too laterally, displacing the artery medially
• Type III: an accessory slip of the medial head entraps the artery
• Type IV: the artery lies deep to popliteus or a fibrous band
• Type V: any of the above with venous involvement
• Type VI (functional): hypertrophied but anatomically normal muscle compressing a normally positioned artery, the type seen in bodybuilders and elite athletes

Investigation and treatment of entrapment. Duplex ultrasound and MR or CT angiography, both performed with provocative manoeuvres (active plantar flexion and passive dorsiflexion); a study performed at rest will be normal and will miss the diagnosis. Treatment is surgical release of the aberrant muscle or band through a posterior approach, with arterial reconstruction if the vessel is damaged. Delay risks arterial thrombosis, aneurysm and distal embolisation in a young patient.

Baker cyst. In adults a symptom rather than a diagnosis: look for the intra-articular driver, usually a medial meniscal tear or osteoarthritis. Rupture mimics a deep vein thrombosis and produces ecchymosis around the malleoli. Duplex scan first, always.

Denervation and weakness. An S1 radiculopathy is the commonest cause of plantar flexion weakness with a reduced or absent ankle jerk and sensory loss over the lateral foot, the same territory as the sural nerve; distinguish by the presence of other S1 findings and by the ankle reflex. A tibial nerve injury in the popliteal fossa produces gastrocnemius, soleus and deep compartment weakness with sensory loss on the sole. Isolated denervation of one head is rare and functionally minor.

Compartment syndrome. Gastrocnemius occupies the superficial posterior compartment, released through a medial calf incision. The deep posterior compartment, tibialis posterior, flexor digitorum longus, flexor hallucis longus and the posterior tibial bundle, is released through the same medial incision by detaching soleus from the posteromedial tibia. This step is frequently done inadequately, leaving the deep compartment unreleased, and is a recognised cause of a missed deep compartment syndrome.