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Triple Deformity of the Knee

Definition

1. Flexion deformity of the knee joint
2. Posterior subluxation of the tibia on the femur
3. External (lateral) rotation of the tibia

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Schematic Diagram of Triple Deformity

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Etiology (Causes)

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Pathology

Stages of Tuberculous Knee (the model disease for triple deformity)

• Mycobacterium tuberculosis seeds the synovial membrane via hematogenous spread
• Granuloma formation with epithelioid cells, Langhans giant cells, lymphocytes, and central caseation
• Synovial hypertrophy and effusion develop (boggy, doughy swelling - "white swelling" or "cold swelling")
• Pannus (vascular granulation tissue) forms and begins to creep over the articular cartilage
• Joint space is preserved or slightly widened (effusion)
• No articular cartilage destruction yet

• Pannus invades and erodes articular cartilage from the periphery inward
• "Kissing lesions" - cartilage destroyed from both surfaces simultaneously
• Marginal erosions appear; osteolytic foci develop in periarticular bone
• Joint space narrows progressively
• Night pain is characteristic at this stage

• Complete destruction of articular cartilage; bone exposed
• Severe periarticular muscle wasting (especially quadriceps)
• Ligamentous laxity and capsular fibrosis develop simultaneously
• Massive joint destruction with virtually no joint space
• Triple deformity becomes clinically evident:
  • Flexion: posterior capsule and hamstring contracture
  • Posterior subluxation: gravity + capsular destruction allows tibia to slip backward
  • External rotation: contracture of the iliotibial band, which acts as a bowstring along the lateral aspect, pulling the tibia into external rotation
• Cold abscess and sinuses may form; melon seed bodies (rice bodies) may be found in joint fluid

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Mechanism of Formation

How Each Component Develops:

• Joint effusion causes reflex spasm of the hamstrings (flexors), holding the knee slightly flexed (the position of maximum joint volume - ~25° flexion)
• Prolonged flexion leads to shortening of the posterior capsule and hamstring tendons
• Quadriceps wasting removes the antagonist pull
• Result: fixed flexion contracture

• Articular destruction and ligamentous laxity (especially posterior cruciate ligament involvement)
• In the flexed position, gravity and body weight act on the tibia, pulling it posteriorly on the femoral condyles
• The flexed posture with weight bearing causes the tibia to progressively sublux backward
• On lateral X-ray: the tibia sits posterior to the normal alignment with the femoral condyles

• The iliotibial band (ITB) runs obliquely from the iliac crest to the lateral tibial condyle (Gerdy's tubercle)
• As the knee flexes and the posterior structures contract, the ITB - which cannot elongate - acts as a taut lateral bowstring
• It mechanically rotates the tibia externally relative to the femur
• This is further assisted by contracture of the biceps femoris (external rotator of the flexed knee)

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Signs and Symptoms

Early Stage

• Limp - the first symptom noticed in children
• Aching pain around the knee, worse on activity
• Joint swelling - diffuse, boggy, doughy consistency ("white swelling" - no warmth or redness, unlike pyogenic)
• Slight flexion attitude - knee held in mild flexion
• Enlarged inguinal lymph nodes (popliteal nodes may also be enlarged but harder to palpate)
• Wasting of quadriceps - visible even early

Intermediate Stage

• Night pain - characteristic of tuberculosis (released muscle spasm during sleep causes sudden pain)
• Increasing stiffness of the joint
• Marked quadriceps wasting
• Effusion detectable on clinical examination (bulge sign, patellar tap)
• Restricted range of movements with pain on movement

Late Stage - Triple Deformity

• Visible deformity:
  • Knee held in flexion
  • Tibia pushed posteriorly
  • Foot externally rotated
• Fixed flexion deformity - knee cannot be fully extended even passively
• Posterior fullness in the popliteal fossa (subluxed tibia)
• Severe quadriceps wasting - thigh appears grossly wasted
• Cold abscess - fluctuant, non-tender, non-warm swelling, may track along fascial planes
• Sinuses - discharging sinuses may develop in untreated cases
• Shortening of the limb - due to flexion deformity
• Positive four-finger test - examiner can place four fingers in the popliteal fossa due to posterior tibial shift
• General signs of tuberculosis: evening rise of temperature, night sweats, weight loss, anorexia, pallor

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X-Ray Findings (Investigations)

Radiological Progression

• Generalized or localized decalcification (osteoporosis) around the joint
• Increased joint space (effusion)
• Periarticular soft tissue swelling

• Diminished joint space (cartilage destruction)
• Irregular joint line ("moth-eaten" appearance)
• Marginal erosions and periarticular bony foci

• Practically no joint space remaining
• Posterior subluxation of tibia visible on lateral view (posterior tibial shift - confirmed by drawing dotted line along posterior surface of femoral condyles)
• External rotation of tibia visible on AP view
• Flexion deformity visible clinically and on lateral view
• Generalized severe osteoporosis
• Possible collapse/destruction of femoral condyles or tibial plateau

X-Ray - Triple Deformity (S Das, 13th Ed.)

X-Ray - TB Knee Before and After Arthrodesis (Campbell's Operative Orthopaedics, 15th Ed.)

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Other Investigations

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Treatment

I. Medical (Antitubercular Therapy - ATT)

II. Conservative / Non-Surgical

• Rest and immobilization - posterior splint or POP cast to rest the joint in functional position
• A knee with a normal-appearing radiograph or mildly confined osteomyelitic changes frequently responds to multidrug chemotherapy; these patients may tolerate early range-of-motion and mobilization procedures - Campbell's, p.8029
• Physiotherapy - quadriceps strengthening, gentle ROM exercises once inflammation subsides
• Traction - skin or skeletal traction to correct early flexion deformity and relieve muscle spasm
• Serial plastering - gradual correction of deformity by progressive plaster casts

III. Surgical Treatment

• Repeated aspiration of joint fluid relieves tension pain
• Abscess: drained; localized bone lesion: curetted - Campbell's, p.8026

• Indicated in persistent synovitis not responding to chemotherapy
• Can be done open or arthroscopically (less morbidity)
• Removes the infected pannus and granulation tissue
• Best results in stage 1 (pre-arthritic stage)
• Partial synovectomy described by Wilkinson (1962) for TB knee - Campbell's, p.9277

• For localized bone lesions with sequestrum
• Curettage of adjacent bone lesion if distal femur or proximal tibia involved

• Gradual correction: Serial plasters / traction + ATT (for mild-moderate fixed flexion)
• Posterior capsulotomy: Release of tight posterior capsule for flexion contracture
• Hamstring lengthening: Surgical lengthening (recession or tenotomy) to correct flexion
• Iliotibial band release: Division/lengthening of ITB to correct external rotation component
• Pre-arthroplasty: reduction of subluxation is always attempted before any arthroplasty

• Indicated for severe joint destruction with significant degenerative changes when a more aggressive open approach is needed - Campbell's, p.8051
• Provides a pain-free, stable, weight-bearing limb (though with no movement)
• Preferred in young patients with single-joint disease where arthroplasty is inappropriate
• Immobilization in a long-leg cast post-operatively
• Results in solid fusion typically within months (see X-ray above: Figure 25.11)

• Considered in older patients with end-stage disease
• Technically challenging due to posterior subluxation in triple deformity; subluxation must be reduced before arthroplasty can be performed
• Limited success described; risk of reactivation of quiescent TB process is a concern
• Must be preceded by at least 6 months of ATT; some recommend 1-2 years of disease quiescence
• Gaining knee flexion is considered the main goal of rehabilitation following surgery - Relainstitute

Treatment Summary Flowchart:

Triple Deformity of Knee
        |
        v
Start ATT (all stages)
        |
        +---> Early disease (Stage 1-2, no/mild deformity)
        |         -> Rest + Immobilization + Physiotherapy
        |         -> Arthroscopic/open synovectomy if no ATT response
        |
        +---> Moderate disease (Stage 2, early deformity)
        |         -> Serial plaster + traction to correct deformity
        |         -> Posterior capsulotomy + hamstring release
        |         -> Synovectomy + sequestrectomy
        |
        +---> Advanced disease (Stage 3, full triple deformity)
                  -> Correction of deformity (ITB release, capsulotomy)
                  -> Young patient: Arthrodesis (fusion)
                  -> Older patient: TKR (after subluxation reduction)

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Differential Diagnosis

• Acute pyogenic arthritis - warm, red, tender, rapid onset; high fever; synovial fluid shows neutrophilia
• Osteoarthritis - disease of old age; bony swelling; osteophytes on X-ray; no lymphadenopathy
• Rheumatoid arthritis - bilateral, morning stiffness, RF positive; pannus formation; can also cause triple deformity in late stage
• Charcot (neuropathic) joint - associated with diabetes, tabes dorsalis; marked hypermobility; bizarre X-ray destruction
• Haemophilic arthropathy - young males; history of bleeding disorder; episodic hemarthrosis
• Gout/pseudogout - acute attacks; crystal analysis of synovial fluid diagnostic

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Key Clinical Pearls

• Triple deformity = flexion + posterior subluxation + external rotation - all three must be present
• It is a late/end-stage sign, classically of tuberculosis of the knee
• The iliotibial band is the key structure responsible for the external rotation component (acts as a lateral bowstring)
• The "four-finger test" (palpation of posteriorly subluxed tibia) is a bedside sign
• "Night pain" is a hallmark of stage 2 (arthritic) TB knee - pain on movement diminishes at rest, but released muscle spasm during sleep wakes the patient
• X-ray: Always draw a line along the posterior femoral condyles on the lateral view to demonstrate posterior tibial subluxation
• ATT is the backbone of treatment - surgery is adjunctive
• TKR is technically difficult because of the subluxation and must be planned carefully

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• S Das, A Manual on Clinical Surgery, 13th Edition, pp. 257-258
• Campbell's Operative Orthopaedics, 15th Ed. 2026, pp. 8026-8058
• Rela Institute, Triple Deformity of the Knee: Treatment and Prevention