CIM Case: Congenital Pseudoarthrosis of Tibia

Clinical Scenario

Patient: 6-year-old boy Presentation: Fell at school, unable to weight-bear on left leg, closed injury to left tibia Relevant history: Parents noted progressive anterolateral bowing of the left leg since infancy, previous X-rays at age 2 showed cystic changes in the tibia, multiple café-au-lait spots noted since birth Examination findings:

• Left leg shorter than right (2cm discrepancy)
• Anterolateral bowing of left tibia
• Mobile pseudoarthrosis at junction of middle and distal thirds
• Multiple café-au-lait spots (>6, largest 4cm, coast of California borders)
• Axillary freckling present
• No skin nodules palpated
• Intact sensation and pulses distally
• Unable to weight-bear

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Investigations Provided

Laboratory Results

Imaging

Image 1: AP and Lateral X-rays Left Tibia

Radiological features:

• Anterolateral bowing at junction of middle and distal third
• Complete fracture through area of cystic change
• Atrophic bone ends with sclerotic margins
• Tapered ("pencil-point") proximal fragment
• Widened medullary canal at pseudoarthrosis site
• Fibula also shows dysplastic changes
• Cortical thickening proximally
• No periosteal new bone

Image 2: Contralateral Tibia X-ray

Findings:

• Normal tibial alignment
• No dysplastic changes
• Used for comparison and LLD assessment

Image 3: Previous X-rays (Age 2 years)

Historical findings:

• Anterolateral bowing present
• Cystic lesion in distal third of tibia
• Thinned cortex overlying cyst
• Findings consistent with pre-pseudoarthrosis

Questions & Model Answers

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Key Teaching Points

Pattern Recognition

This pattern suggests Congenital Pseudoarthrosis of Tibia:

• Anterolateral bowing of tibia from infancy
• Fracture at junction of middle and distal third
• Café-au-lait spots (NF1 association)
• Atrophic bone ends on X-ray
• Often fibula also involved
• Progressive deformity before fracture

NF1 Features to Look For:

Critical Management Points

1. CPT is strongly associated with NF1 - screen all patients
2. Crawford Type IV has worst prognosis - counsel family
3. Pseudoarthrosis tissue must be resected - abnormal biology
4. IM rod should cross the ankle - protect distal tibia
5. Vascularised fibula graft has best union rates - consider if conventional fails
6. Amputation is a valid option - discuss early, decide together

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Common Examiner Follow-ups

Q: "What is the natural history if untreated?"

Without treatment:

• Progressive deformity and shortening
• Recurrent fractures
• Severe leg length discrepancy
• Non-functional limb
• Eventually requires amputation

The condition does NOT heal spontaneously - the abnormal periosteum prevents union.

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Q: "What is the Farmer procedure?"

The Farmer procedure (or cross-union technique):

• Vascularised contralateral fibula transfer
• Fibula is transposed with its blood supply intact (rotation plasty concept)
• Used when ipsilateral fibula is also dysplastic
• Provides structural support and blood supply
• Now largely replaced by free vascularised fibula graft

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Q: "What is the role of BMP in CPT?"

BMP-2 (bone morphogenetic protein-2):

• Off-label use in CPT
• May improve union rates when added to bone graft
• Studies show benefit in some series
• Concerns about cost, off-label status, cancer risk in NF1
• Used as adjunct, not primary treatment
• Dose: Variable, typically 1.5-12mg

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Q: "Why does the rod need to cross the ankle?"

The distal tibial segment in CPT is:

• Small and fragile
• At high risk of refracture
• Protected by rod extending into talus/calcaneus
• Crossing ankle provides mechanical protection
• Accept some ankle stiffness for limb salvage
• May be removed at skeletal maturity if stable

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Related Topics

• Neurofibromatosis Type 1
• Tibial Bowing in Infancy
• Paediatric Non-union
• Vascularised Bone Grafting
• Limb Lengthening in Children
• Paediatric Amputation