Medial and lateral hamstring lengthening for flexed-knee crouch gait in cerebral palsy | advanced
Surgical Imaging
Location: The sciatic nerve divides into tibial and common peroneal branches in the upper popliteal fossa; the tibial nerve lies medial and slightly posterior to the semimembranosus tendon at the level of hamstring release.
Risk: Aggressive medial retraction or blind deep dissection can stretch or lacerate the tibial nerve; the nerve is most vulnerable when the knee is flexed and the hamstrings are tight.
The fix: Stay strictly medial to the midline, use blunt spreading dissection only, and identify the nerve under direct vision before any tendon division; never use sharp retractors deep to the semimembranosus.
Location: The popliteal artery and vein lie in the midline of the popliteal fossa, deep to the semimembranosus and just lateral to the tibial nerve.
Risk: Overzealous lateral retraction or inadvertent deep incision can injure the vessels; bleeding in this confined space is difficult to control and can lead to compartment syndrome.
The fix: Maintain a medial trajectory throughout the approach; when dissecting the semimembranosus insertion, keep the blade or scissors directed medially and use finger palpation to confirm the vascular bundle before any deep cut.
Mechanism: Excessive hamstring lengthening removes the knee-flexion moment without addressing hip-flexor or ankle plantarflexor weakness; the quadriceps then hyperextends the knee during stance.
Consequence: The patient converts from a crouched gait to a stiff, painful recurvatum gait with increased energy cost and risk of knee ligament attenuation.
The fix: Preoperative gait analysis must quantify the contribution of each deforming force; combine hamstring lengthening with rectus femoris transfer and address hip flexion and equinus simultaneously in SEMLS.
Why it happens: In growing children, spasticity returns with skeletal growth; incomplete fractional lengthening of semimembranosus leaves residual contracture that recurs within 2-3 years.
Prevention: Perform complete intramuscular aponeurotic lengthening of semimembranosus at the musculotendinous junction; Z-lengthen semitendinosus and gracilis under controlled tension; plan for possible repeat lengthening at skeletal maturity if growth velocity is high.
Clinical picture: After over-lengthening, the patient loses active knee flexion in swing phase and develops compensatory quadriceps overuse; this leads to fatigue, patellofemoral pain, and eventual recurvatum.
Examination clue: Popliteal angle less than 10 degrees post-operatively with inability to flex the knee beyond 30 degrees in swing.
The fix: Intraoperative assessment with the patient under anaesthesia should confirm that knee extension is achieved with no more than 10-15 degrees of residual popliteal angle; do not chase a zero popliteal angle.
The trap: Treating only the hamstrings when hip-flexor tightness or ankle equinus is the primary driver of crouch; isolated hamstring surgery then fails or produces recurvatum.
Gait analysis requirement: Instrumented analysis must demonstrate that hamstring lengthening will improve knee extension in stance without creating compensatory hyperextension; otherwise address the primary driver first.
The fix: Always plan SEMLS with simultaneous correction of hip flexion (psoas lengthening), knee flexion (hamstrings), and ankle equinus (gastroc-soleus lengthening or transfer) when indicated by the kinematic data.
H.A.M.S.T.R.I.N.GHAMSTRING — Surgical Anatomy and Lengthening Principles
C.R.O.U.C.HCROUCH — Preoperative Decision Framework
L.E.N.G.T.H.E.NLENGTHEN — Intraoperative Safety Checklist
Surgical Indications
Absolute Indications
- Instrumented gait analysis confirming that 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 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 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
- Patient and family motivated for comprehensive SEMLS including rectus femoris transfer and foot/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 result in inability 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 — recurrence is almost certain and conservative management is preferred
- Non-ambulatory patients where the functional goal is seating rather than gait improvement
- Previous selective dorsal rhizotomy with already reduced spasticity — more conservative lengthening is required
Evidence for Hamstring Lengthening
Natural History and Timing
- Untreated crouch gait in cerebral palsy progresses with growth; knee-flexion contracture increases on average 1-2 degrees per year during the rapid growth phase
- Early intervention with botulinum toxin and physiotherapy can delay surgery but does 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 of Hamstring Lengthening
- Modern series using instrumented gait analysis for patient selection report 70-85% improvement in knee extension at stance phase at 2-year follow-up
- Recurrence requiring repeat lengthening occurs in 15-25% of patients by skeletal maturity when fractional lengthening is incomplete
- Genu recurvatum develops in 8-15% of cases when hamstring lengthening is performed without simultaneous correction of hip-flexor or ankle equinus deformities
Combination with Rectus Femoris Transfer
- Isolated hamstring lengthening often worsens stiff-knee gait in swing phase; simultaneous rectus femoris transfer improves peak knee flexion in swing by 10-15 degrees in most patients
- The combination is now standard in SEMLS protocols and reduces the need for subsequent isolated knee surgery
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 increased energy cost of walking. What has happened and how do you manage it?”