Semitendinosus Anatomy

Proximal Origin

• Site: the posteromedial aspect of the ischial tuberosity, as part of the conjoint tendon shared with the long head of biceps femoris.
• Relationship: the conjoint footprint lies posteromedial; the semimembranosus footprint lies anterolateral to it. The two are contiguous but distinct on the tuberosity.
• Surgical consequence: semitendinosus and biceps long head avulse together because they share a tendon. A "two-tendon avulsion" therefore usually means the conjoint tendon plus a partial semimembranosus, and a "three-tendon avulsion" means the conjoint tendon has separated from semimembranosus as well.

The Tendinous Raphe

• An oblique fibrous intersection crosses the muscle belly at about the level of the mid-thigh, running from proximal-medial to distal-lateral.
• It divides semitendinosus into two functional compartments, each with an independent motor nerve.
• Identification value: at open dissection or in an MRI cross-section, the raphe is the reliable feature distinguishing semitendinosus from gracilis and semimembranosus.

Distal Insertion

• The tendon becomes free at roughly the junction of the middle and distal thirds of the thigh, runs distally and anteriorly around the medial femoral condyle, and inserts into the superomedial surface of the tibia as the deepest layer of the pes anserinus.
• Precise footprint: on the anteromedial tibia, roughly 2-3 cm distal to the medial joint line and just medial to the tibial tuberosity, beneath the sartorius fascia.
• The pes anserinus is the conjoined insertion of sartorius (most superficial and anterior), gracilis (middle) and semitendinosus (deepest and most posterior), resembling a goose's foot.
• Layer note: the pes tendons are superficial to the superficial MCL, with the pes anserine bursa interposed between them. The bursa is the anatomical basis for pes anserine bursitis.

Harvestable Tendon

• Length: typically 24-30 cm of usable tendon from a standard harvest — longer than gracilis, which yields about 20-26 cm.
• Diameter: a doubled semitendinosus alone gives roughly 6-8 mm; a quadrupled semitendinosus alone or a doubled semitendinosus-gracilis construct gives 7-10 mm.
• Fascial expansions: the tendon gives an expansion into the crural fascia distally and, critically, one or more fascial bands to the medial head of gastrocnemius proximally.

Nerve Supply

• Nerve: the tibial division of the sciatic nerve.
• Root values: L5, S1, S2.
• Dual motor supply: semitendinosus is unusual in receiving two separate motor branches — one entering the muscle proximal to the tendinous raphe and one entering distal to it.
• Entry: both branches enter on the anterior (deep) surface of the muscle, in the proximal and middle thirds of the thigh, arising from the medial side of the tibial division.

Clinical Application

• Localising a sciatic lesion: all hamstrings except the short head of biceps femoris take the tibial division. Preserved short-head function with denervated semitendinosus, semimembranosus and biceps long head places the lesion in the tibial division.
• Foot drop with normal hamstrings points instead to a common peroneal lesion at the fibular neck, not a sciatic lesion.
• Harvest and innervation: harvesting the tendon does not denervate the muscle. The motor branches enter well proximal to the harvest field, which is why the belly survives, retracts proximally and often regenerates a new distal tendon.

Testing

• Prone knee flexion with the tibia internally rotated against resistance preferentially loads semitendinosus and semimembranosus.
• Tibia externally rotated biases biceps femoris.
• Post-harvest, the most reproducible deficit is at deep flexion (greater than 70-90 degrees), not at mid-range.

Arterial Supply

• Proximal: the inferior gluteal artery and the medial circumflex femoral artery to the conjoint tendon origin.
• Belly: the first and second perforating branches of the profunda femoris artery — the dominant supply, entering the deep surface in the proximal and middle thirds.
• Distal tendon: small branches from the superior medial genicular artery and the descending genicular artery, plus the periosteal supply at the pes insertion.
• Mathes-Nahai pattern: type IV (segmental), which is why semitendinosus is a poor pedicled flap donor. Its value is as a free tendon graft, not a muscle flap.

Vascular Consequences of Harvest

• Harvest divides the distal tendon and its small distal vessels; the belly retains its perforator supply and remains viable.
• The retracted belly re-scars to the posteromedial capsule and adjacent fascia, and a regenerated tendon commonly forms along the scar, typically inserting more proximally and posteriorly than the original pes attachment.
• Bleeding at harvest is usually from the saphenous branches and the crural fascial vessels, not from a named artery; a subcutaneous haematoma in the medial calf is a recognised nuisance complication.

Posterior Thigh

• Superficial: the deep fascia of the thigh, gluteus maximus proximally, and the posterior femoral cutaneous nerve in the subcutaneous plane.
• Deep: semimembranosus for most of the thigh — semitendinosus lies posterior and superficial to it.
• Lateral: the long head of biceps femoris (shared origin) and, deep to it, the sciatic nerve.
• Medial: gracilis and the adductor magnus.

At the Knee

• The crossing: proximally semitendinosus is superficial to semimembranosus; distally it swings anteriorly around the medial femoral condyle to reach the pes, ending up anterior to semimembranosus at the tibia.
• Order at the pes (superficial to deep): sartorius fascia, gracilis, semitendinosus, then the pes anserine bursa, then the superficial MCL, then bone.
• Immediately deep and posterior to the harvest field: the medial head of gastrocnemius, connected to the semitendinosus tendon by fascial bands.
• Anterior to the field: the infrapatellar branch of the saphenous nerve, running transversely across the sartorial fascia.

Structures at Risk in the Harvest Field

• Infrapatellar branch of the saphenous nerve — the commonest injury.
• Sartorial branch of the saphenous nerve and the main saphenous trunk — deep to sartorius, running with the great saphenous vein.
• Great saphenous vein — subcutaneous, medial, at the level of the harvest incision.
• Medial head of gastrocnemius — the destination of the errant stripper.
• Popliteal neurovascular bundle — only at risk if a stripper is driven blindly posteriorly and laterally, which is the mechanism of the rare catastrophic harvest complication.

Indications for Harvest

• ACL reconstruction — the commonest indication worldwide; doubled semitendinosus-gracilis or quadrupled/quintupled semitendinosus alone.
• MCL and posteromedial corner reconstruction — semitendinosus, sometimes left attached distally at the pes as a natural tibial fixation.
• MPFL reconstruction — usually gracilis, but semitendinosus is used when a larger graft is required or the gracilis is inadequate.
• PCL and posterolateral corner reconstruction, multiligament knee reconstruction, and ankle lateral ligament reconstruction.
• Paediatric and adolescent ACL reconstruction — a physeal-sparing or transphyseal soft-tissue graft avoids the bone plug of a patellar tendon graft crossing the physis.

Step-by-Step Technique

1. Position: supine, knee flexed to about 90 degrees, hip flexed and externally rotated (figure-of-four) to bring the tendons subcutaneous and away from the popliteal vessels.
2. Incision: a 2-3 cm oblique or vertical incision over the pes, roughly 2-3 fingerbreadths distal to the medial joint line and 1-2 cm medial to the tibial tuberosity. An oblique incision parallel to the infrapatellar branch reduces the risk of nerve division.
3. Expose the sartorial fascia; identify the gracilis as the more proximal, thinner cord and the semitendinosus as the distal, thicker cord palpable through the fascia.
4. Incise the sartorial fascia along the superior border of the gracilis (an "L" or transverse incision) and deliver both tendons with a right-angled clamp. Preserve the fascial flap for closure.
5. Whip-stitch the free end of each tendon after detaching it from the tibia, or detach it later — practice varies.
6. Release the fascial bands. With firm distal traction on the tendon, sweep a finger along its deep and posterior surface toward the popliteal fossa and sharply divide the accessory insertion and the dense investing fascia a few centimetres proximal to the tibial insertion.
7. Pass the closed-loop stripper over the tendon, keep it coaxial with the tendon under constant tension, and advance with a smooth push toward the ischium. Never force it.
8. Detach the tibial insertion subperiosteally if this has not already been done, preserving a periosteal cuff for repair.
9. Prepare the graft on the back table: strip muscle, whip-stitch both ends, measure length and diameter, pre-tension.
10. Close the sartorial fascia to reduce the risk of a symptomatic donor-site defect and haematoma.

Yields

Premature Graft Amputation

• The classic pitfall. The stripper follows an undivided fascial band to the medial gastrocnemius and transects the tendon early, leaving an unusably short graft.
• Reported incidence varies widely between series but is consistently higher for surgeons early in their learning curve, and it is more common when the harvest is performed through too small an incision or with the knee inadequately flexed.
• What to do when it happens:
  • Measure what you have. A short graft can often still be used by increasing the number of strands or by using adjustable-loop suspensory fixation at both ends, which needs less graft in the tunnels.
  • Harvest the gracilis as well if it has not already been taken.
  • Convert to a quadriceps tendon or bone-patellar tendon-bone autograft, or use an allograft — this is why consent for an ACL reconstruction should always mention alternative graft sources.
  • Recover the retracted proximal stump only if it is safely accessible; do not chase it blindly into the posterior thigh.
• Prevention is the answer the examiner wants: adequate incision, adequate knee flexion, firm distal traction, digital and sharp release of every band under vision, and a coaxial stripper that is never forced.

Neurovascular Injury

• Infrapatellar branch of the saphenous nerve — by far the commonest. It crosses the field transversely, running from posteromedial to anterolateral, deep to or through the sartorial fascia. A vertical incision cuts it; an oblique incision parallel to its course substantially reduces the risk. The result is numbness or a painful neuroma over the anterolateral proximal leg and the lateral aspect of the incision.
• Saphenous nerve trunk — deeper, running with the great saphenous vein under sartorius. Injury numbs the medial leg to the medial malleolus and is a much more disabling deficit.
• Popliteal vessels — the rare catastrophic complication, caused by a stripper driven posterolaterally with the knee extended. Knee flexion of 90 degrees moves the neurovascular bundle away from the harvest axis and is the primary protection.
• Great saphenous vein — bleeding, and rarely thrombophlebitis.

Other Donor-Site Problems

• Haematoma in the posteromedial calf and thigh, tracking distally.
• Painful scar and hypersensitivity at the pes.
• Persistent deep-flexion weakness, the reproducible functional deficit.
• Loss of a future graft source — relevant in the young multiligament or revision patient.

Does the Tendon Grow Back?

• Often, but neither reliably nor completely. MRI and ultrasound studies demonstrate a regenerated tendon-like structure in many harvested tendons, but rates differ markedly between series — a two-year MRI study found regeneration of both the semitendinosus and gracilis in only 35 per cent of patients, while a review of donor-site morbidity described regrowth of the hamstring tendons within two years.
• The catch: the regenerated tendon usually inserts more proximally and more posteriorly than the original pes attachment, often onto the posteromedial capsule or the medial gastrocnemius fascia, and the muscle-tendon junction migrates proximally.
• Functional consequence: because the insertion is more proximal, the moment arm is shorter, which is the anatomical explanation for the persistent deep-flexion deficit despite apparently successful regeneration.
• Where the tendon fails to regenerate, the loss of donor muscle size is greater and correlates with the knee flexion strength deficit.

Can It Be Re-Harvested?

• A regenerated tendon can be harvested at revision, but the tissue is smaller, more scarred, more variable in quality, and the harvest is technically harder because the normal fascial planes are obliterated.
• Practical advice: do not plan a revision reconstruction on the assumption that a regenerated hamstring will yield an adequate graft. Have a quadriceps tendon, contralateral hamstring or allograft plan.

Compensatory Changes

• Biceps femoris and semimembranosus hypertrophy partially compensate for the lost cross-sectional area.
• Semitendinosus belly volume decreases and its architecture changes, with a shorter, more proximally located musculotendinous junction.
• These morphological changes have been shown to persist at two years after harvest.

Pes Anserine Bursitis and Tendinopathy

• Bursa location: deep to the three pes tendons and superficial to the superficial MCL, roughly 4-5 cm distal to the medial joint line, anteromedially.
• Population: middle-aged and older patients, often with medial compartment osteoarthritis, obesity, diabetes or a valgus thrust; also runners and swimmers with a whip kick.
• Discriminators from medial joint pathology: tenderness is below and in front of the joint line, McMurray is negative, and passive rotation of the knee does not reproduce the pain.
• Management: load modification, correction of biomechanics (foot posture, hip abductor strength), eccentric loading, and image-guided corticosteroid injection into the bursa for refractory cases. Surgery is very rarely required.

Proximal Hamstring Injury

• Semitendinosus is injured as part of the conjoint tendon with the biceps long head, either as a strain at the musculotendinous junction or as a proximal avulsion.
• Free tendon involvement on MRI is a poor prognostic sign and predicts a longer return to play.

Snapping Pes Syndrome

• A rare cause of audible or palpable snapping at the posteromedial knee, produced by the semitendinosus or gracilis tendon flicking over the medial tibial condyle or over a prominent semimembranosus insertion during flexion and extension.
• Distinguish from: a snapping medial meniscus, a snapping medial plica, and a snapping biceps femoris tendon laterally.
• Treat non-operatively first; refractory cases may need release or Z-lengthening of the offending tendon.

Semitendinosus Rupture

• Isolated distal semitendinosus rupture is rare and usually well tolerated, requiring no repair — the same reasoning that makes it acceptable to harvest.