Posteromedial Bow of Tibia

Congenital posteromedial bowing of the tibia (CPMBT) is a sporadic deformity present at birth in which the tibia (and usually the fibula) is angulated posteriorly and medially, accompanied by tibiofibular shortening and a calcaneovalgus foot. It is almost always unilateral. The deformity is benign in the sense that it does not fracture or form a pseudarthrosis, but it is not simply self-resolving: while the angular bow remodels, the limb-length discrepancy is progressive and frequently requires treatment.

Epidemiology:

• Rare; reported as roughly 1 per 1,000 to 1 per several thousand live births in older series (exact incidence uncertain — most evidence is small single-centre cohorts)
• Usually unilateral; the left side is more frequently affected in collated case-series data
• A slight male predominance is reported in pooled literature, though individual series vary
• Sporadic — no Mendelian inheritance pattern and no reproducible familial clustering
• An association with firstborn status has been noted, supporting an intrauterine-packaging mechanism

Why it matters clinically: the two components behave oppositely over time. According to PubMed, Pappas (1984) followed 33 patients and showed that the posterior and medial angular components reduce markedly with growth, but tibial shortening persists and is proportional to the initial bow, with absolute discrepancies of 3.3-6.9 cm at maturity (DOI/PMID 6490868).

Normal tibial anatomy: the tibia is normally straight in both the sagittal and coronal planes, with a triangular cross-section. Bone remodels throughout childhood in response to physeal growth and mechanical (Heuter-Volkmann) forces, which is why a flexible neonatal angular deformity can correct.

The deformity in CPMBT:

• Posterior + medial diaphyseal angulation — apex posteromedial, concavity anteromedial; initial bow at birth ranges from roughly 25 to 70 degrees
• Normal bone architecture — no narrowed/sclerotic canal, no cystic lesion and no hamartomatous fibrous tissue (these are features of the CPT spectrum, not of posteromedial bow)
• Tibial and fibular shortening present from birth and progressive in absolute terms through growth
• Distal tibial changes can develop — wedging of the distal tibial epiphysis, fibular hypoplasia and valgus inclination of the distal articular surface in some children

Two mechanisms of angular correction were described by Shah, Doddabasappa and Joseph (2009): rapid early physeal realignment and slower diaphyseal remodelling. Most correction occurs in the first year, slows markedly thereafter, and may leave residual deformity beyond 4 years of age — so it is incorrect to promise complete resolution by 2-3 years (PMID 19339901).

Aetiology (still uncertain): the leading hypothesis is intrauterine moulding / packaging (fetal constraint of a dorsiflexed, externally rotated foot pressing the leg posteromedially), supported by the firstborn association, the universal calcaneovalgus foot and prenatal ultrasound detection. It is not a positional deformity in the trivial sense, because the shortening is a true growth disturbance, not just a postural curve.

Associated findings:

• Calcaneovalgus foot — almost universal at birth, usually resolves
• Reduced ankle dorsiflexion and sometimes calf/peroneal muscle weakness — may persist into later childhood (Pappas described early posterior soft-tissue fullness evolving into relative muscle atrophy)
• Progressive limb-length discrepancy — the dominant long-term issue

History:

• Bowing noted at birth or early infancy
• May have calcaneovalgus foot noted
• Usually no pain or functional limitation
• Family history (rare but may be present)
• No history of trauma

Physical Examination:

Inspection:

• Posterior and medial bowing of tibia
• May be bilateral (50-60%)
• Calcaneovalgus foot (common association)
• Assess for other deformities

Palpation:

• Bowing is palpable
• No pseudarthrosis site (unlike CPT)
• Normal bone structure

Range of Motion:

• Ankle: characteristic calcaneovalgus posture at birth (foot dorsiflexed against the shin); later, reduced ankle dorsiflexion may persist
• Test active plantarflexion / toe-walking — calf and peroneal weakness can be present
• Knee and hip: usually normal

Measurements:

• Angular bow (clinical + radiographic interphyseal angles, AP and lateral)
• Limb-length discrepancy — measure clinically (block test, galeazzi) and confirm radiographically; this is the key longitudinal measurement
• Foot position (calcaneovalgus assessment)

The diagnosis is usually clinical, but the direction of the bow and the surrounding features separate it from dangerous mimics. The single most important distinction is from anterolateral bowing (the congenital pseudarthrosis spectrum), which is managed completely differently.

Radiographs:

AP and Lateral Tibia:

• Assess posteromedial bowing
• Measure bowing angle
• Evaluate bone structure (should be normal)
• Assess for any cystic changes (should be absent)
• Evaluate medullary canal (should be normal)

Full-Length Standing Radiographs:

• Measure bowing angle accurately
• Assess alignment
• Evaluate for leg length discrepancy
• Assess foot position

Key Radiographic Features:

• Posteromedial diaphyseal angulation with normal bone architecture — normal cortical thickness, no canal narrowing/sclerosis, no cyst, no pseudarthrosis (these features exclude the CPT spectrum)
• Tibial and fibular shortening, often with fibular hypoplasia distally
• Possible distal tibial epiphyseal wedging and increased lateral distal tibial angle deviation (ankle valgus) over time

Serial assessment — this is mandatory:

• Track the angular bow (AP and lateral interphyseal angles) — expect rapid early correction
• Track the percentage tibial shortening and predict LLD at maturity using the multiplier method — the proportional discrepancy is stable after about 12 months, making prediction reliable
• Follow at 6-12 month intervals in early childhood, then periodically; continue surveillance until skeletal maturity because LLD progresses and ankle deformity can evolve — do NOT discharge once the angle looks better

Prenatal: posteromedial bowing can be detected on antenatal ultrasound (reported from the second trimester); recognising it prenatally allows appropriate parental counselling and avoids misdiagnosis as a skeletal dysplasia or fracture.

Usually not required:

• CT/MRI — not routinely indicated; reserve for surgical planning or atypical features
• Genetic / NF1 testing — only if NF1 stigmata raise the possibility of an anterolateral (CPT) lesion instead

Natural-history sequelae (the things that actually cause morbidity):

• Progressive limb-length discrepancy — the dominant problem; mean 3-4 cm, frequently over 2.5 cm, occasionally over 5 cm. NOT a trivial sub-centimetre difference
• Residual angular deformity — uncommon but may persist beyond age 4 if the initial bow was severe
• Persistent reduced ankle dorsiflexion and calf/peroneal weakness
• Ankle valgus from distal tibial wedging / fibular hypoplasia

Surgical complications (with lengthening or osteotomy):

During treatment / early:

• Pin-tract infection
• Premature/delayed consolidation, regenerate problems
• Neurovascular irritation, joint subluxation during distraction

Late:

• Regenerate fracture and tibial valgus malalignment
• Ankle stiffness/equinus
• Recurrence of discrepancy after early lengthening (higher when lengthening is done young — recurrence rates are significantly higher in children lengthened under age 10)
• Need for repeat lengthening if performed far from maturity

Prevention:

• Accurate prediction (multiplier method) and correct timing of equalisation
• Circular/hexapod fixation or internal lengthening with adequate stability
• Proactive ankle protection and physiotherapy during distraction

Key point: the untreated complication that matters is the progressive discrepancy, not fracture (this condition does not pseudarthrose); the treatment-related complications are those of limb reconstruction.

Surveillance protocol (non-operative period):

• At diagnosis — baseline clinical exam, radiographs, document angle, percentage shortening, foot/ankle posture
• 6-12 monthly through early childhood, then periodic review until skeletal maturity
• Each visit: re-measure interphyseal angles and LLD, re-run the multiplier prediction, reassess ankle dorsiflexion
• Do not discharge once the angle improves — the discrepancy and ankle deformity evolve over years

After limb lengthening:

• External fixator until regenerate consolidation (healing index roughly 30-50 days/cm); protect against valgus drift
• Aggressive ankle and knee physiotherapy throughout distraction and consolidation to prevent stiffness/equinus
• Staged weight-bearing as regenerate matures; remove fixator after radiographic union of all cortices
• Continue follow-up for recurrence of discrepancy (especially after early lengthening) and for regenerate fracture after frame removal

After epiphysiodesis:

• Brief recovery, early mobilisation
• Monitor to confirm the predicted equalisation is achieved at maturity

Angular deformity: prognosis is excellent — the posteromedial bow remodels spontaneously and substantially, with most correction in the first year and the majority of children needing no angular surgery. A mild residual medial component may remain.

Limb-length discrepancy: prognosis is guarded without planning — the discrepancy is progressive and proportional to the initial bow:

• Hofmann & Wenger (1981): mean LLD 3.1 cm at follow-up, with 9 of 13 children over 2.5 cm and usually progressing
• Pappas (1984): absolute tibial discrepancy 3.3-6.9 cm at maturity, greater with greater initial bow
• Modern series confirm progressive shortening and frequent need for equalisation (epiphysiodesis or lengthening)

Ankle/foot: the calcaneovalgus foot resolves, but reduced ankle dorsiflexion may persist and ankle valgus can develop from distal tibial changes.

Predictors of a worse length outcome:

• Greater initial angular bow (the single best predictor)
• Greater early percentage shortening
• Distal tibial epiphyseal wedging / fibular hypoplasia (predict ankle valgus)

Overall quality of life: generally good. With appropriate prediction and timely, well-executed equalisation, most patients reach maturity with near-equal leg lengths, neutral alignment and good function; outcomes after lengthening are good in modern series, though at the cost of the morbidity of reconstruction. The key message is that good outcomes depend on surveillance and planning, not on assuming the condition disappears on its own.

According to PubMed, the evidence below has been verified against the primary sources.

Global epidemiology: congenital posteromedial bowing is rare and sporadic worldwide, reported across all populations without a clear geographic or ethnic predilection. Evidence is dominated by small single-centre cohorts (typically 13-51 patients) from Europe, North America, the Middle East and South Asia — there is no large population registry specific to this deformity, and no dedicated society guideline. International multicentre work is now emerging through limb-difference outcome programmes (e.g. the LIMB-Q Kids patient-reported outcome measure, validated across 16 sites in 7 countries; Chhina et al 2025).

Where guidance converges (society/textbook level):

All converge on the same core message: do not osteotomy the remodelling bow, and do plan for the progressive length discrepancy.

High- vs limited-resource practice variation:

• Well-resourced settings: magnetic internal lengthening nails, hexapod fixators and guided-growth implants reduce external-fixator burden; serial scanograms / EOS imaging and routine multiplier prediction.
• Limited-resource settings: reliance on clinical block-test measurement and plain radiographs; Ilizarov / circular external fixators remain the workhorse for lengthening (large Indian and Middle-Eastern series), with epiphysiodesis as a low-cost option for moderate discrepancies; longer follow-up logistics and counselling are the main constraints.