Medial and lateral hamstring lengthening for flexed-knee crouch gait in cerebral palsy · advanced
- Indication requires a documented dynamic or fixed knee-flexion contracture with a positive popliteal angle contributing to crouch gait; instrumented gait analysis is the gold standard for confirming the contribution of hamstring spasticity versus other drivers such as hip-flexor tightness or ankle plantarflexor weakness before single-event multilevel surgery.
- Over-lengthening is the most common cause of post-operative genu recurvatum; this converts crouch into a stiff hyperextended knee that is functionally worse because hip-flexor and plantarflexor-knee-extension coupling is ignored — always assess and address these coupled deformities simultaneously.
- The posteromedial approach protects the sciatic and tibial nerves and the popliteal vessels by staying medial to the midline and using careful blunt dissection; the semitendinosus and gracilis are lengthened first, followed by fractional lengthening of semimembranosus, with biceps femoris addressed laterally only if residual contracture persists after medial release.
- Complete fractional (intramuscular aponeurotic) lengthening of semimembranosus is performed at the musculotendinous junction while preserving the muscle belly; Z-lengthening of semitendinosus and gracilis allows controlled lengthening and repair under tension to avoid over-correction.
When & Why
Indication. Flexed-knee crouch gait in cerebral palsy in which instrumented gait analysis confirms that hamstring spasticity or contracture is a primary contributor to excessive stance-phase knee flexion — a positive popliteal angle (greater than 50 degrees under anaesthesia with the hip flexed to 90 degrees) or a fixed knee-flexion contracture (greater than 15–20 degrees) that limits community ambulation or causes knee pain, and that has failed conservative management (physiotherapy, serial casting and botulinum toxin). Crouch driven primarily by hip-flexor tightness or ankle plantarflexor weakness, without a real hamstring contribution, is managed without isolated hamstring surgery. Absolute indications.
- Instrumented gait analysis confirming hamstring spasticity or contracture is a primary contributor to flexed-knee crouch gait in stance phase.
- Fixed knee-flexion contracture greater than 15–20 degrees that limits community ambulation or causes knee pain.
- Positive popliteal angle greater than 50 degrees under anaesthesia with the hip flexed to 90 degrees, not correctable by ankle dorsiflexion alone.
- Failed conservative management including physiotherapy, serial casting and botulinum toxin injections with documented deterioration in gait kinematics. Relative indications.
- Dynamic knee-flexion contracture contributing to an increased energy cost of walking in a child planned for single-event multilevel surgery (SEMLS).
- Crouch gait with compensatory lumbar lordosis and anterior pelvic tilt that improves with hamstring lengthening in the gait laboratory.
- A patient and family motivated for comprehensive SEMLS including rectus femoris transfer and foot or ankle correction in the same anaesthetic session. Contraindications. Absolute: primary crouch driven by hip-flexor tightness or ankle plantarflexor weakness without significant hamstring contribution on gait analysis; severe quadriceps weakness that would leave the patient unable to extend the knee after hamstring lengthening; active infection or uncontrolled seizures precluding elective surgery. Relative: very young children (under 5–6 years) with high growth velocity, in whom recurrence is almost certain and conservative management is preferred; non-ambulatory patients whose functional goal is seating rather than gait; previous selective dorsal rhizotomy with already reduced spasticity, in whom more conservative lengthening is required. The one decision that matters. Once gait analysis confirms the hamstring contribution, the only real choice is which tendons to lengthen and whether to combine the release with a rectus femoris transfer:
Z-lengthen semitendinosus and gracilis and fractionally lengthen semimembranosus through a posteromedial approach. Addresses most crouch gait and is almost always sufficient.
Add lateral fractional lengthening only if the popliteal angle is still greater than 20 degrees after complete medial release. Increases recurvatum risk, so use sparingly.
Convert the rectus from a knee extensor to a knee flexor when swing-phase knee flexion is limited. Isolated hamstring lengthening otherwise worsens stiff-knee gait.
Consent specifically for sciatic or tibial nerve injury (less than 1 percent), popliteal vessel injury (rare but serious), over-lengthening leading to genu recurvatum (8–15 percent), recurrence requiring repeat surgery (15–25 percent), wound infection, and the frequent need for simultaneous rectus femoris transfer and other SEMLS procedures. Setup. Prone with the knees at the end of the table or in slight flexion over a padded bolster; both lower limbs are prepared and draped free to allow bilateral comparison and measurement of the popliteal angle under anaesthesia. A tourniquet is applied to the thigh but inflated only if needed for haemostasis. General anaesthesia with muscle relaxants avoided until after the intraoperative assessment; an epidural or caudal block gives excellent post-operative analgesia for bilateral procedures. Loupe magnification (2.5–3.5×) and a headlight are mandatory, and a sterile goniometer measures the popliteal angle before and after every lengthening step.
With the contralateral hip flexed to flatten the lumbar lordosis, flex the ipsilateral hip to 90 degrees and extend the knee — the angle between the tibia and the vertical is the popliteal angle. Greater than 40–50 degrees in a child older than 5 years indicates clinically significant hamstring tightness contributing to crouch.
Genu recurvatum risk after hamstring lengthening is highest when hip-flexor or plantarflexor weakness is unaddressed. Always evaluate psoas and gastroc-soleus length and strength before deciding on isolated hamstring surgery — single-event multilevel surgery (SEMLS), combining hamstring lengthening with rectus femoris transfer, adductor release and foot or ankle procedures in one anaesthetic, is the modern standard and minimises repeated hospitalisations.
The Operation
The goal is to expose the medial hamstrings through the posteromedial approach, protect the tibial nerve and popliteal vessels, lengthen the medial hamstrings in a controlled, tension-matched fashion, re-balance the knee to a 10–15 degree residual popliteal angle, and combine the release with a rectus femoris transfer when swing-phase knee flexion is limited. The exposure is laid out in full as the opening steps below (and in depth on the posteromedial approach to the knee page).
Operative sequence
- Prone, knees at the end of the table or in slight flexion over a padded bolster; both lower limbs prepared and draped free for bilateral comparison and popliteal-angle measurement under anaesthesia.
- Tourniquet on the thigh, inflated only if needed for haemostasis; muscle relaxants withheld until after the intraoperative assessment; an epidural or caudal block for bilateral post-operative analgesia.
- Loupe magnification (2.5–3.5×) and a headlight are mandatory; a sterile goniometer measures the popliteal angle before and after every step.
- Palpate the landmarks: the ischial tuberosity proximally, the popliteal fossa, the posteromedial border of the distal thigh over the semitendinosus, and the fibular head laterally for the biceps if needed.
- A longitudinal incision runs along the posteromedial distal thigh centred on the semitendinosus tendon, from about 8–10 cm proximal to the joint line to 4–5 cm distal to it if the gracilis is to be released; it is placed medial to the midline to keep the neurovascular bundle lateral and safe.
- Divide the subcutaneous tissue and identify the semitendinosus by its long, cord-like tendon lying superficially; the gracilis lies anterior and slightly medial to it and the semimembranosus is deep and slightly lateral.
- Pass a vessel loop around each of the three medial hamstring tendons before going deeper — this maintains orientation and guards the tibial nerve, which lies just lateral to the semimembranosus.
- Start with the semitendinosus because it is the most superficial and easiest landmark, then locate the gracilis anteriorly and the semimembranosus deep and lateral; control superficial subcutaneous veins to avoid troublesome bleeding.
- Gently retract the semimembranosus medially and identify the tibial nerve lying just lateral and slightly posterior to the semimembranosus tendon; pass a vessel loop around it for gentle retraction and constant visualisation.
- Palpate the popliteal artery and vein in the midline deep to the semimembranosus and confirm their position before any deeper dissection.
- Never divide a tendon until both the tibial nerve and the popliteal vessels are under direct vision or at least palpated and protected; use a finger to sweep the neurovascular bundle laterally and confirm it is clear before touching the semimembranosus.
- Remember that previous botulinum toxin injections can scar and distort the normal planes, so dissect bluntly and expect altered anatomy.
- Divide the semitendinosus tendon in a Z-fashion at the musculotendinous junction, about 5–6 cm proximal to its insertion, with proximal and distal limbs each at least 3 cm long to allow overlap and repair.
- Z-lengthen the gracilis at the same level and repair both with absorbable suture (2-0 or 3-0 Vicryl) under slight tension that allows the knee to extend to the planned popliteal angle (typically 10–15 degrees residual).
- Lengthen the semitendinosus and gracilis first because their long tendons give immediate length; repair under slight tension so that on knee extension the repair is under physiologic load, which prevents over-lengthening and recurvatum.
- The semimembranosus is the most important medial hamstring for knee-flexion contracture; its short tendon demands fractional (intramuscular aponeurotic) lengthening rather than Z-lengthening.
- At the musculotendinous junction divide the aponeurotic fibres on the anterior surface of the muscle transversely while preserving the muscle belly posteriorly; make three to five transverse cuts until the muscle lengthens under gentle traction, leaving the posterior muscle fibres intact.
- Extend the knee and re-measure the popliteal angle; make further cuts only if residual contracture greater than 15–20 degrees persists, stopping at 10–15 degrees residual flexion — never zero.
- Protect the tibial nerve, which lies immediately lateral, during stretching, and avoid over-aggressive cuts that can rupture the muscle or compress the popliteal vessels.
- After complete medial release, extend the knee and re-assess the popliteal angle; if residual contracture greater than 20 degrees persists, expose the biceps femoris long head through a separate posterolateral incision or by extending the medial incision across the midline.
- Fractionally lengthen the biceps at its musculotendinous junction exactly as for the semimembranosus; avoid complete tenotomy, which increases recurvatum risk and weakens swing-phase knee flexion.
- This is rarely needed — complete medial release usually achieves adequate extension; when it is done, lengthen fractionally and conservatively because the lateral hamstrings contribute less to crouch but more to recurvatum risk if over-lengthened.
- With the hamstrings lengthened, document the popliteal angle and assess the rectus femoris for tightness; if swing-phase knee flexion was limited on preoperative gait analysis, transfer the rectus to the semitendinosus or gracilis stump through the same or an extended incision.
- Detach the rectus tendon from the patella and transfer it to the hamstring stump under appropriate tension to convert it from a hip flexor and knee extensor into a knee flexor, improving swing-phase foot clearance.
- Perform the transfer after hamstring lengthening so the new knee-flexion moment can be balanced against the lengthened hamstrings.
- Achieve haemostasis; cover the repaired tendons with local muscle or fat to prevent adherence; close the subcutaneous tissue with absorbable suture and the skin with absorbable subcuticular suture (or non-absorbable interrupted sutures in older children).
- Apply a well-padded long-leg cast or removable knee immobiliser with the knee in slight flexion (10–20 degrees) to protect the repair.
The tibial nerve lies just lateral and slightly posterior to the semimembranosus at the level of release and is most vulnerable when the knee is flexed and the hamstrings are tight; stay strictly medial to the midline, use blunt spreading dissection only, identify the nerve under direct vision before any tendon division, and never place sharp retractors deep to the semimembranosus. The popliteal artery and vein lie in the midline deep to the semimembranosus and just lateral to the nerve; bleeding in this confined space is hard to control and can cause compartment syndrome, so maintain a medial trajectory, keep instruments directed medially, and palpate the vascular bundle before any deep cut. If a vessel is injured, obtain proximal and distal control, call for vascular help, repair primarily or with an interposition graft, and perform a fasciotomy if compartment pressures rise.
Re-measure the popliteal angle under anaesthesia (hip at 90 degrees) after every lengthening step and aim for 10–15 degrees of residual flexion. Chasing a zero popliteal angle is the most common cause of post-operative genu recurvatum — the knee then hyperextends in stance and is functionally worse than the original crouch.
Aftercare & Complications
Rehabilitation | Phase | Timing | Immobilisation | Therapy | |-------|--------|----------------|---------| | 1 | 0–2 weeks | Long-leg cast or knee immobiliser in 10–20 degrees of flexion | Touch weight-bearing with crutches for bilateral procedures; full weight-bearing as tolerated for unilateral; epidural or caudal analgesia | | 2 | 2–6 weeks | Removable knee immobiliser; night knee-extension splint | Gentle active-assisted knee flexion and extension; quadriceps activation and hamstring stretching within comfort; gait re-education | | 3 | 6 weeks–3 months | Splint for heavy tasks; night splinting | Progressive range of motion and isometric quadriceps and gluteal strengthening from 4 weeks; custom or ground-reaction AFO | | 4 | 3–6 months | Night splint to skeletal maturity | Intensive gait training; return to school and light activity at 6–8 weeks; sports at 4–6 months with protective bracing | Most patients return to school and light activity at 6–8 weeks and to sport at 4–6 months. Repeat instrumented gait analysis at 6–12 months quantifies improvement and plans any further SEMLS stages. Night splinting continues until skeletal maturity in growing children, with annual clinical review of the popliteal angle.
Post-operative bracing in knee extension or slight flexion for 4–6 weeks followed by night splinting prevents rapid recurrence; aggressive physiotherapy begins at 2 weeks to regain strength without stretching the repair too early.
Complications
- Incidence
- less than 1 percent
- Recognition
- Immediate sensory loss or motor deficit in the tibial distribution; foot drop if the common peroneal branch is involved; neuropathic pain
- Prevention & management
- Prevention: identify and protect the tibial nerve under direct vision before any tendon division; blunt dissection only; avoid aggressive retraction. Management: immediate exploration and repair if recognised intraoperatively; late recognition needs nerve conduction studies and possible exploration with grafting
- Incidence
- rare (less than 0.5 percent)
- Recognition
- Intraoperative bleeding; expanding haematoma; absent distal pulses; compartment-syndrome signs
- Prevention & management
- Prevention: palpate and protect the popliteal vessels before deep dissection; maintain a medial trajectory; use finger dissection rather than sharp retractors. Management: immediate vascular repair; fasciotomy if compartment pressures are raised; anticoagulation and monitoring
- Incidence
- 8–15 percent
- Recognition
- Knee hyperextension in stance on gait analysis; loss of active knee flexion in swing; patellofemoral pain; quadriceps fatigue
- Prevention & management
- Prevention: preoperative gait analysis to identify coupled deformities; address hip-flexor and ankle equinus simultaneously; aim for 10–15 degrees residual popliteal angle, not zero. Management: knee-ankle-foot orthosis locked in slight flexion for 3–6 months; quadriceps and gluteal strengthening; revision rarely required
- Incidence
- 15–25 percent by skeletal maturity
- Recognition
- Return of a positive popliteal angle and crouch gait within 2–4 years; increased energy cost of walking
- Prevention & management
- Prevention: complete fractional lengthening of semimembranosus; controlled Z-lengthening of semitendinosus and gracilis; night splinting for 6–12 months. Management: serial casting, botulinum toxin and physiotherapy for mild recurrence; repeat lengthening at skeletal maturity; consider selective dorsal rhizotomy in refractory spasticity
- Incidence
- 10–20 percent when rectus transfer omitted
- Recognition
- Loss of active knee flexion in swing; compensatory circumduction; increased energy cost
- Prevention & management
- Prevention: combine hamstring lengthening with rectus femoris transfer when indicated by gait analysis; preserve muscle fibres during fractional lengthening. Management: formal physiotherapy for swing-phase training; consider rectus transfer revision if it was not performed initially
- Incidence
- 2–4 percent
- Recognition
- Erythema, warmth, discharge; wound breakdown over the repair site
- Prevention & management
- Prevention: meticulous haemostasis; layered closure; perioperative antibiotics in high-risk patients; immobilisation to protect the repair. Management: oral or IV antibiotics; wound washout if deep infection; delayed primary closure or skin graft if dehiscence occurs
- Incidence
- rare — commoner after aggressive quadriceps lengthening
- Recognition
- Anterior knee pain; radiographic patellar fracture; quadriceps lag
- Prevention & management
- Prevention: avoid simultaneous quadriceps lengthening with hamstring release; protect the patellar tendon during rectus transfer. Management: activity modification; patellar tendon strap; surgical fixation if a displaced fracture occurs
Viva & Exam Focus
HAMSTRINGHAMSTRING — anatomy and lengthening principles
CROUCHCROUCH — preoperative decision framework
Clinical Decision Scenarios
Practise clinical reasoning and management decisions out loud
“A 10-year-old boy with spastic diplegic cerebral palsy presents with progressive crouch gait. Instrumented gait analysis shows a popliteal angle of 65 degrees, increased knee flexion in stance, and limited swing-phase knee flexion. Hip-flexor tightness and mild ankle equinus are also present. How do you plan his surgical correction?”
“During a posteromedial hamstring lengthening on a 9-year-old girl, you identify the tibial nerve but cannot clearly visualise the popliteal artery. The semimembranosus is very tight. How do you proceed safely?”
“A 12-year-old boy with crouch gait undergoes bilateral hamstring lengthening. At the 6-month review he has excellent knee extension in stance but complains of difficulty clearing his feet in swing phase and an increased energy cost of walking. What has happened and how do you manage it?”
Key indications
- Instrumented gait analysis is mandatory — confirms hamstring contribution to stance-phase knee flexion
- Popliteal angle greater than 50 degrees under anaesthesia with the hip at 90 degrees
- Fixed knee-flexion contracture greater than 15–20 degrees limiting ambulation
- Failed conservative management (physiotherapy, casting, botulinum toxin)
- SEMLS is the modern standard — combine with rectus transfer, psoas lengthening and foot correction
Critical anatomy
- Medial hamstrings: semitendinosus and gracilis (Z-lengthening), semimembranosus (fractional intramuscular aponeurotic lengthening)
- Tibial nerve: lies immediately lateral and slightly posterior to semimembranosus — identify and protect first
- Popliteal artery and vein: midline deep to semimembranosus — palpate before any deep cut
- Biceps femoris: lengthen only if residual contracture after complete medial release — increases recurvatum risk
- Rectus femoris: often transferred simultaneously to improve swing-phase knee flexion
Danger zones
- Tibial nerve stretch or laceration: less than 1 percent with direct visualisation but catastrophic if missed
- Popliteal vessel injury: rare but requires immediate vascular repair and possible fasciotomy
- Genu recurvatum: 8–15 percent when hip-flexor or ankle equinus is not addressed simultaneously
- Over-lengthening: aim for 10–15 degrees residual popliteal angle, never zero
- Recurrence: 15–25 percent by skeletal maturity — night splinting and complete fractional lengthening reduce the risk
Operative technique — key steps
- Prone position, loupe magnification, headlight, sterile goniometer
- Posteromedial incision; identify semitendinosus, gracilis and semimembranosus with vessel loops
- Identify and protect the tibial nerve and popliteal vessels before any tendon division
- Z-lengthen semitendinosus and gracilis at the musculotendinous junction; repair under slight tension
- Fractionally lengthen semimembranosus — multiple transverse aponeurotic cuts preserving the muscle belly
- Re-measure the popliteal angle after each step; aim for 10–15 degrees residual
- Consider biceps femoris lengthening only if residual contracture greater than 20 degrees
- Combine with rectus femoris transfer when swing-phase knee flexion is limited
- Long-leg cast or immobiliser in slight flexion for 2 weeks; night splinting for 6–12 months
Complications
- Sciatic or tibial nerve injury: immediate exploration and repair if recognised
- Genu recurvatum: prevention is key — address coupled deformities and avoid over-lengthening
- Recurrence: repeat lengthening at skeletal maturity; night splinting reduces the risk
- Stiff-knee gait: rectus femoris transfer is both prevention and treatment
- Wound infection: 2–4 percent; layered closure and immobilisation are protective
Post-op protocol
- Week 0–2: immobilisation in slight flexion, touch weight-bearing, epidural analgesia
- Week 2–6: gentle mobilisation, night splint, progressive physiotherapy
- Month 2–6: gait training, custom orthoses, return to school at 6–8 weeks
- Long-term: night splinting until skeletal maturity; annual popliteal-angle review
- Repeat gait analysis at 12 months to quantify improvement and plan further SEMLS stages
Special situations
- Recurvatum risk is highest with previous rhizotomy or adductor release — more conservative lengthening
- Very young children (under 6 years): high recurrence; delay surgery if possible
- Non-ambulatory patients: consider seating goals rather than gait improvement
- Combined rectus transfer: improves swing-phase knee flexion by 10–15 degrees in most patients
- Recurrence management: serial casting first; repeat lengthening at skeletal maturity
Background & Evidence
Pathomechanics of crouch gait. Crouch results from a balance of spasticity, contracture and weakness: hamstring spasticity or contracture produces an excessive stance-phase knee-flexion moment; hip-flexor tightness (psoas) and rectus femoris spasticity drive anterior pelvic tilt and knee flexion; ankle plantarflexor weakness or equinus prevents adequate knee extension through the plantarflexion–knee-extension couple; and quadriceps weakness fails to counter the flexion moment, perpetuating the crouch. Understanding which force dominates on instrumented gait analysis is essential before deciding which structures to lengthen. Surgical anatomy. The medial hamstrings are the primary targets; the lateral hamstring is addressed only secondarily.
- Origin
- Ischial tuberosity
- Insertion
- Pes anserinus (anteromedial tibia)
- Lengthening method
- Z-lengthening — long cord-like tendon, superficial, easy to repair under tension
- Origin
- Inferior pubic ramus
- Insertion
- Pes anserinus, anterior to semitendinosus
- Lengthening method
- Z-lengthening — long tendon, lies anteromedial to the other medial hamstrings
- Origin
- Ischial tuberosity (deep to semitendinosus)
- Insertion
- Posteromedial tibia
- Lengthening method
- Fractional intramuscular aponeurotic lengthening at the musculotendinous junction — short tendon
- Origin
- Ischial tuberosity (short head from linea aspera)
- Insertion
- Fibular head
- Lengthening method
- Fractional lengthening only if residual contracture persists after complete medial release
Neurovascular structures at risk. The sciatic nerve divides into the tibial and common peroneal branches in the upper popliteal fossa; the tibial nerve lies immediately lateral and slightly posterior to the semimembranosus tendon at the level of release, and the popliteal artery and vein lie in the midline deep to the semimembranosus and just lateral to the nerve. The sural nerve runs in the midline of the calf and is at risk only with a very distal extension. The posteromedial approach exploits the safe interval between the medial hamstrings and the midline neurovascular bundle — staying strictly medial to the tibial nerve and palpating the popliteal vessels before any deep cut minimises injury, and the semimembranosus musculotendinous junction is a safe zone for fractional lengthening because the nerve has already branched more proximally. Natural history and timing. Untreated crouch progresses with growth: knee-flexion contracture increases on average 1–2 degrees per year during the rapid growth phase. Botulinum toxin and physiotherapy can delay surgery but do not prevent eventual contracture in moderate-to-severe cases. Single-event multilevel surgery performed between 8 and 12 years of age optimises the balance between recurrence risk and growth remaining. Outcomes. Modern series using instrumented gait analysis for patient selection report a 70–85 percent improvement in stance-phase knee extension at two years. Recurrence requiring repeat lengthening occurs in 15–25 percent of patients by skeletal maturity when fractional lengthening is incomplete, and genu recurvatum develops in 8–15 percent when hamstring lengthening is performed without simultaneous correction of hip-flexor or ankle equinus deformities. Isolated hamstring lengthening often worsens stiff-knee gait in swing, whereas simultaneous rectus femoris transfer improves peak swing-phase knee flexion by 10–15 degrees and reduces the energy cost of walking by 15–25 percent in appropriately selected patients.
References
Distal hamstring lengthening in ambulatory children with cerebral palsy: primary versus revision procedures
- Primary and revision distal hamstring lengthening improve knee extension in crouch gait; primary procedures show greater kinematic gains
- Careful protection of the tibial nerve and popliteal vessels is essential during the posteromedial approach
Monitoring of the sciatic nerve during hamstring lengthening by evoked EMG
- Intraoperative evoked EMG reliably identifies the sciatic nerve during hamstring lengthening and reduces iatrogenic injury risk
- Nerve monitoring confirms the safe plane of dissection medial to the midline in the popliteal fossa
Influence of hamstring lengthening on muscle activation timing
- Hamstring lengthening normalises muscle activation timing during stance phase in children with cerebral palsy crouch gait
- Improved activation patterns correlate with better knee extension and reduced energy cost of walking
Management of severe crouch gait in children and adolescents with cerebral palsy
- Multilevel surgery incorporating hamstring lengthening corrects severe crouch gait and improves ambulatory function at long-term follow-up
- Instrumented gait analysis is critical for planning combined procedures and avoiding over-lengthening leading to recurvatum