Posteromedial Bow of Tibia

Congenital posteromedial bowing of the tibia (CPMBT) presents at birth with posterior and medial tibial angulation, tibiofibular shortening and usually a calcaneovalgus foot. The angular components usually remodel substantially and percentage shortening may partly improve, but an absolute limb-length discrepancy can persist or increase as the child grows. The long-term task is serial measurement of length, alignment, ankle motion and function, not automatic reassurance or automatic reconstruction.

Who. CPMBT is rare, reported in older series as roughly 1 per 1,000 to 1 per several thousand live births; the exact incidence is uncertain because most of the evidence comes from small single-centre cohorts. It is usually unilateral, and the left side is more frequently affected in collated case-series data. Pooled literature reports a slight male predominance, though individual series vary.

Inheritance. The condition is sporadic, with no Mendelian inheritance pattern and no reproducible familial clustering. An association with firstborn status has been noted, which supports an intrauterine-packaging mechanism.

Why it matters. Small historical cohorts found substantial angular remodelling but persistent shortening, and worse early deformity often accompanied greater later discrepancy. The widely quoted maturity ranges come from selected, incompletely mature cohorts, so they should frame the uncertainty in counselling rather than predict an individual child.

The normal tibia. 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 (Hueter-Volkmann) forces, which is why a flexible neonatal angular deformity can correct.

The deformity. The diaphysis is angulated posteriorly and medially, apex posteromedial and concavity anteromedial, and the initial bow at birth ranges from roughly 25 to 70 degrees. The bone architecture is normal: there is no narrowed or sclerotic canal, no cystic lesion and no hamartomatous fibrous tissue, which are features of the CPT spectrum rather than of posteromedial bow. The tibia and fibula are short from birth, and the shortening is progressive in absolute terms through growth.

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

Aetiology. The cause is still uncertain. The leading hypothesis is intrauterine moulding or packaging, fetal constraint of a dorsiflexed, externally rotated foot pressing the leg posteromedially, supported by the firstborn association, the almost 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.

What comes with it. The calcaneovalgus foot is almost universal at birth and usually resolves. Reduced ankle dorsiflexion, sometimes with calf or peroneal weakness, may persist into later childhood and has its own section below. The progressive limb-length discrepancy is the dominant long-term issue.

History. The bowing is noted at birth or in early infancy, sometimes with a calcaneovalgus foot. There is usually no pain or functional limitation and no history of trauma, and a family history is rare but may be present.

Look and feel. The tibia is bowed posteriorly and medially, and the bow is palpable. Look for the calcaneovalgus foot and for other deformities. Unlike CPT, there is no pseudarthrosis site and the bone structure is normal.

Movement. At birth the ankle sits in the characteristic calcaneovalgus posture, the foot dorsiflexed against the shin; later, reduced ankle dorsiflexion may persist. Test active plantarflexion and toe-walking, because calf and peroneal weakness can be present. The knee and hip are usually normal.

Measurements. The limb-length discrepancy is the key longitudinal measurement.

• Angular bow, clinically and by radiographic interphyseal angles on AP and lateral views
• Limb-length discrepancy, measured clinically (block test, Galeazzi) and confirmed radiographically
• Foot position (calcaneovalgus assessment)

Tibial radiographs. AP and lateral films of the tibia confirm the posteromedial bow, measure it, and show whether the bone is normal, as it should be: normal cortical thickness and medullary canal, with no canal narrowing or sclerosis, no cyst and no pseudarthrosis. The tibia and fibula are short, often with distal fibular hypoplasia, and the distal tibial changes described under Classification may appear over time.

Measuring the bow. The angle is monitored on AP and lateral films as interphyseal or diaphyseal angles, and the length is tracked as percentage tibial shortening (affected/normal length).

View larger

View larger

Full-length standing radiographs. These measure the bowing angle accurately and show overall alignment, the leg-length discrepancy and the position of the foot.

Other tests. CT and MRI are not routinely indicated and are reserved for surgical planning or atypical features. Genetic or NF1 testing is needed only if NF1 stigmata raise the possibility of an anterolateral (CPT) lesion instead.

An isolated tibial bow may be detected antenatally, but direction, mineralisation, the other bones, fractures and associated anomalies determine the differential and the counselling.

The ultrasound appearance. The features that point to CPMBT:

• An isolated, usually unilateral long-bone bow angulated posteriorly and medially, with the concavity anteromedial
• Normal bone echogenicity and mineralisation: the shaft is not thin, wide, fractured, sclerotic or cystic
• Normal length and morphology of the other long bones, a normal chest circumference and no other skeletal anomaly
• A commonly associated dorsiflexed (calcaneovalgus) foot pressed against the shin, a supportive clue consistent with the intrauterine-moulding hypothesis
• Mild-to-moderate tibial shortening, which may already be measurable relative to the contralateral side

The fetal differential. The mimics of a bent or short fetal long bone are the reason the read matters:

Counselling. Antenatal counselling mirrors the postnatal message and avoids two opposite errors: needless alarm, mistaking a benign bow for a dysplasia, and false reassurance that "it will simply straighten". Parents should be told that the angular deformity is expected to remodel after birth, that the leg is nonetheless likely to remain somewhat short with a progressive discrepancy requiring surveillance and possibly later equalisation, and that the condition does not fracture or form a pseudarthrosis. Postnatal radiographs and examination, including a review of the newborn for the calcaneovalgus foot and reduced ankle dorsiflexion, complete the 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.

Making the distinction. Direction is essential but not sufficient. Confirm the posterior and medial components separately on orthogonal films, then look at the architecture: cortical or canal dysplasia, or a fracture, redirects the child to a CPT pathway. When CPT is suspected, the skin, the family history and other NF1 features matter.

View larger

The other causes of a bowed infant or toddler tibia are separated by direction, symmetry and the rest of the skeleton:

Natural-history sequelae. These are the things that actually cause morbidity:

• Residual limb-length discrepancy, which may remain clinically important even as percentage shortening partly improves
• Residual angular or rotational deformity, judged by function and mechanical axis rather than age alone
• Ankle motion limitation and calf asymmetry
• Ankle valgus or other distal alignment change from distal tibial and fibular morphology

Complications of lengthening or osteotomy. During treatment and early:

• Pin-tract infection and other pin or wire problems
• Premature or delayed consolidation and regenerate problems
• Neurovascular irritation or injury, and joint subluxation during distraction

Late:

• Regenerate fracture, axis drift and tibial valgus malalignment
• Ankle stiffness and equinus
• Repeat discrepancy after early lengthening as growth continues
• Need for staged or repeat equalisation when the original target or growth trajectory changes

Prevention. Accurate prediction (multiplier method) and correct timing of equalisation, circular or hexapod fixation or internal lengthening with adequate stability, and proactive ankle protection and physiotherapy during distraction.

The distinction to draw. Untreated, the complication that matters is the progressive discrepancy, not fracture, because this condition does not pseudarthrose. The treatment-related complications are those of limb reconstruction.

After reconstruction. Progress loading and frame or device removal from the evidence of regenerate and fixation, not a universal healing index. Protect knee and ankle motion throughout lengthening and consolidation, and monitor for axis drift, joint contracture, regenerate fracture and recurrent discrepancy.

After epiphysiodesis. Recovery is brief and mobilisation early. Follow the child to confirm that the predicted equalisation is achieved at maturity.

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

View larger

The length. The prognosis is guarded without planning, because 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 a frequent need for equalisation by epiphysiodesis or lengthening

View larger

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

Predictors. A greater initial angular bow is the single best predictor of a worse length outcome, and greater early percentage shortening also predicts it. Distal tibial epiphyseal wedging and fibular hypoplasia predict ankle valgus.

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

Natural history. Pappas (1984) described enlargement of the posterior soft tissues of the affected leg in early life that evolves into a relative muscle atrophy in later years, together with an initial calcaneovalgus foot and a decrease in ankle motion that does not improve with age (PMID 6490868). Shah, Doddabasappa and Joseph (2009) likewise recommended following all children to maturity specifically to detect residual muscle weakness alongside limb-length inequality and ankle deformity (DOI).

The deficit. The neonatal calcaneovalgus posture resolves, but the underlying posterior-compartment weakness and a fixed reduction in ankle dorsiflexion frequently persist and do not fully normalise. The functional consequence is weak plantarflexion and push-off: the child may have difficulty with toe-walking and heel-rise, and a subtly weaker calf on the affected side. The calcaneovalgus foot itself is a flexible neonatal posture covered as its own entity; the concern here is the leg's posterior musculature and the ankle's motion arc, which are CPMBT-specific and long-lasting.

Assessment. At every review, compare calf girth side to side and document active and passive ankle dorsiflexion and plantarflexion, adding single-leg heel-rise as the child matures. Reduced dorsiflexion here is a motion and strength deficit, separate from the later ankle valgus produced by distal tibial wedging and fibular hypoplasia; the two can coexist and should be recorded separately.

Management. Physiotherapy and stretching for the ankle motion arc and calf strength are the mainstay, and surgery is rarely needed. A fixed equinus or limited-dorsiflexion contribution should be recognised before any lengthening, because distraction tends to aggravate equinus and valgus, so the ankle must be protected during reconstruction with physiotherapy and sometimes a temporary foot-frame extension. Operative soft-tissue correction (tendo-Achilles or posterior release) is reserved for the rare rigid or functionally limiting deformity.

There is no dedicated AAOS, BOA, EFORT or AO guideline and no population registry for CPMBT. Evidence consists mainly of small single-centre cohorts and referral reconstruction series.

High-resource systems may offer low-dose biplanar imaging, hexapod planning and internal lengthening devices. Other settings may rely on block testing, conventional standing films, epiphysiodesis and circular fixation. The invariant is not a device: it is accurate phenotype, serial measurement, honest prediction uncertainty and timely referral.

Related pages: Congenital Pseudarthrosis of the Tibia, Neurofibromatosis, Blount Disease, Fibular Hemimelia, Causes of Limb Length Discrepancy, and Limb Length Discrepancy and Epiphysiodesis.