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Posterior Capsule Tightness, FFD (Fixed Flexion Deformity), and Extension Lag in Post-op Knee / OA — Causes

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Definitions First

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Causes of Posterior Capsule Tightness / FFD

1. Prolonged Positioning in Flexion (Most Common)

• In OA and post-op patients, pain forces the patient to rest with the knee slightly bent (flexion is the position of comfort as it reduces joint compression and pain).
• Over time, the posterior capsule, posterior cruciate ligament, and posterior soft tissues adaptively shorten and fibrose.
• Common in patients who use pillows under the knee post-operatively or sit in chairs for prolonged periods.

2. Posterior Osteophytes (OA-specific)

• In osteoarthritis, bony spurs develop along the posterior femoral condyles and tibial plateau.
• These osteophytes act as a mechanical block to extension — the joint literally cannot close fully.
• In TKA, if posterior osteophytes are not adequately resected, FFD persists post-operatively.

3. Posterior Capsular Contracture / Fibrosis

• Long-standing OA (often >5–10 years) leads to progressive capsular contracture.
• Post-operatively, scar tissue formation (fibrosis) — especially if physiotherapy is delayed or inadequate — causes stiffening of the posterior capsule.
• Arthrofibrosis is a significant post-TKA complication where excessive fibroblast proliferation and collagen deposition restrict motion.

4. Hamstring Tightness/Spasm

• Tight or spastic hamstrings actively pull the tibia posteriorly and maintain the knee in flexion.
• In OA patients, hamstring co-contraction is a pain-protective mechanism.
• Post-operatively, pain and swelling trigger reflex hamstring guarding.

5. Incorrect Component Sizing or Positioning (Post-TKA specific)

• Flexion gap > extension gap: If the flexion gap is too large (over-resected distal femur or under-sized femoral component), the knee sits in slight flexion.
• Tibial component slope: Excessive posterior tibial slope can cause recurvatum or, if the slope is insufficient, limit extension.
• PCL over-tightening (in CR-TKA): A retained PCL that is too tight pulls the femur posteriorly, creating an FFD.
• Component flexion: If the femoral component is placed in too much flexion, extension is mechanically limited.
• Inadequate posterior osteophyte removal during TKA.

6. Polyethylene Insert Too Thick (Post-TKA)

• Upsizing the polyethylene insert to balance flexion instability can tighten the extension gap and create an FFD.

7. Patella Baja (Infrapatellar Contracture Syndrome)

• Scarring and inferior migration of the patella post-op tethers the anterior soft tissues and indirectly limits full extension.

8. Heterotopic Ossification

• Ectopic bone formation in the posterior capsule or soft tissues (rare but possible) can mechanically block extension.

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Causes of Extension Lag

1. Quadriceps Weakness / Inhibition (Most Common)

• The vastus medialis oblique (VMO) and quadriceps as a whole are significantly weakened by:
  • Pre-operative disuse atrophy (chronic OA)
  • Post-operative pain inhibition (arthrogenic muscle inhibition)
  • Swelling/effusion → reflexly inhibits quadriceps contraction via Ib afferents
• Even a small effusion can reduce quadriceps force by 20–30%.
• According to Bailey & Love's Surgery (p. 512): inability to re-straighten the knee against gravity (lag test) indicates significant weakness of the quadriceps mechanism.

2. Arthrogenic Muscle Inhibition (AMI)

• Pain and joint effusion generate afferent signals that centrally inhibit the alpha motor neuron pool supplying the quadriceps.
• This is a neurophysiological phenomenon — the muscle is structurally intact but cannot fire fully.
• Very common in the early post-operative period.

3. Extensor Mechanism Disruption (Post-TKA)

• Quadriceps tendon rupture — rare but catastrophic cause of complete extension lag.
• Patellar tendon rupture — similarly catastrophic.
• Patellar component maltracking or subluxation — reduces mechanical efficiency of the extensor mechanism.
• Patellar fracture post-TKA (especially if patella resurfaced) can disrupt the mechanism.

4. Patella Baja

• Low-riding patella increases the moment arm inefficiency of the extensor mechanism.
• The quadriceps must generate disproportionately more force to extend the knee, and if weak, extension lag appears.

5. Nerve Injury

• Femoral nerve palsy (rare) from retractor pressure during TKA causes complete quadriceps paralysis and severe extension lag.
• L3–L4 radiculopathy in elderly OA patients may contribute to pre-existing quadriceps weakness.

6. Post-surgical Pain and Swelling (Early Post-op)

• Swelling increases intra-articular pressure → inhibits quadriceps.
• Pain causes active avoidance of terminal extension effort.

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Why OA Patients Are Particularly Vulnerable

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Relationship Between the Three

Posterior Capsule Tightness
         ↓
    → Leads to FFD (if passive extension also lost)
    
Quadriceps Weakness / Extensor Disruption
         ↓
    → Leads to Extension Lag (passive extension preserved)

Both can coexist → Extension lag + FFD simultaneously

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Clinical Differentiation

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Management Principles

• Posterior capsule tightness/FFD: Aggressive prone-lying, prone hang stretching, serial casting, physiotherapy; intra-op — adequate osteophyte removal, correct component positioning, posterior capsular release if needed.
• Extension lag: Quadriceps strengthening (SLR, terminal knee extension), effusion management (ice, elevation, compression), neuromuscular electrical stimulation (NMES), treating AMI.
• Post-TKA persistent FFD: Manipulation under anesthesia, revision surgery if component malposition is the cause.

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Genu Valgum | Genu Varum | Genu Recurvatum

Comprehensive Guide: Biomechanics · Types · Anatomy · Diagnosis · Ortho & Physio Management

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SECTION 1 — ANATOMY & NORMAL ALIGNMENT

Key Anatomical Axes

Relevant Anatomical Structures at the Knee

• Medial compartment: Medial femoral condyle, medial tibial plateau, medial meniscus, MCL, pes anserinus
• Lateral compartment: Lateral femoral condyle, lateral tibial plateau, lateral meniscus, LCL, IT band, biceps femoris, popliteus
• Sagittal stabilizers: ACL (prevents anterior tibial translation), PCL (prevents posterior translation), posterior capsule, popliteus
• Extensor mechanism: Quadriceps, patellar tendon, patella (transmits force across the joint)
• Neurovascular: Popliteal artery & vein, tibial nerve, common peroneal nerve — all at risk in severe deformities

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SECTION 2 — GENU VARUM (Bow Legs)

Definition

Types / Classification

• Radiographic sign: Drennan metaphyseal-diaphyseal angle >16° = abnormal
• Langenskiöld classification (I–VI): progressive metaphyseal-epiphyseal changes
• Associated with internal tibial torsion and obesity

Biomechanics of Genu Varum

"In genu varum, the center of the knee joint is lateral to the mechanical axis... the medial joint complex is subjected to abnormal pressure, while the lateral joint structures (LCL, IT tract, biceps femoris) are subjected to abnormal tension." — THIEME Atlas of General Anatomy, p. 431

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SECTION 3 — GENU VALGUM (Knock Knees)

Definition

Types / Classification

Biomechanics of Genu Valgum

• Intermalleolar distance >5 cm with knees touching = abnormal
• Normal: when feet together, both medial malleoli and both knees should touch simultaneously (THIEME Atlas of General Anatomy, p. 431)

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SECTION 4 — GENU RECURVATUM (Hyperextension Deformity)

Definition

Normal knee extension = 0°. Genu recurvatum = hyperextension typically >5°, up to 15–20° in severe cases.

Types / Classification

Biomechanics of Genu Recurvatum

Normal knee stance:
Knee in ~5° flexion → quadriceps active → stable

Recurvatum:
Knee hyperextends → weight-bearing line passes ANTERIOR to knee axis
→ Posterior capsule and PCL must resist hyperextension passively
→ Quadriceps mechanism becomes inefficient (shortened moment arm)
→ Over time: posterior capsule stretches further → deformity worsens

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SECTION 5 — DEVELOPMENTAL (PHYSIOLOGICAL) PROGRESSION

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SECTION 6 — DIAGNOSTIC CRITERIA

Clinical Assessment

• Age of onset, rate of progression
• Family history, nutritional history (rickets)
• Pain location, gait complaints, functional limitations
• Prior trauma, infections, surgeries

Radiological Assessment

• Mild: 5–10°
• Moderate: 10–20°
• Severe: >20°

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SECTION 7 — ROLE OF ORTHOPAEDICS

Genu Varum

Genu Valgum

Genu Recurvatum

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SECTION 8 — ROLE OF PHYSIOTHERAPY

General Physiotherapy Goals Across All Three Deformities

1. Correct/compensate alignment through muscle rebalancing
2. Reduce pain (manual therapy, electrotherapy)
3. Improve function and gait pattern
4. Post-operative rehabilitation (peri- and post-surgical)
5. Prevent progression in mild/physiological cases

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Physiotherapy for Genu Varum

Physiotherapy for Genu Valgum

Physiotherapy for Genu Recurvatum

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SECTION 9 — QUICK COMPARISON TABLE

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Sources

• Miller's Review of Orthopaedics, 9th Ed, pp. 278–279 (Genu varum, Blount disease, Genu valgum)
• THIEME Atlas of General Anatomy and Musculoskeletal System, pp. 46, 431 (Axis deviation, Mikulicz line, Physiologic alignment)
• Physical Therapist Management of Total Knee Arthroplasty, p. 17
• Bailey & Love's Short Practice of Surgery, 28th Ed, p. 512
• Grainger & Allison's Diagnostic Radiology (Radiological assessment of limb alignment)This comprehensive guide covers all three knee deformities. Here's a quick orientation to the content:

1. Anatomy — Mechanical axis (Mikulicz line), femorotibial angle (normal 174°/6° valgus), key structures of each compartment
2. Types — Physiologic vs pathologic, Blount disease (infantile + adolescent), metabolic, traumatic, neuromuscular, degenerative, post-surgical
3. Biomechanics — For each deformity: which compartment is overloaded, which structures are stretched, gait deviations
4. Developmental timeline — When varum/valgum is physiologic vs pathologic by age
5. Diagnostic criteria — Clinical tests (intercondylar/intermalleolar distance, recurvatum test, thrust test) + radiological (MAD, MPTA, LDFA, Drennan angle, tibial slope)
6. Orthopaedic role — Observation → bracing → hemiepiphysiodesis → osteotomy (HTO/DFO) → reconstruction → Ilizarov, stratified by severity and skeletal maturity
7. Physiotherapy role — Muscle rebalancing targets, specific exercises, orthoses, gait retraining, and post-op rehab for each condition