Fibular Hemimelia

Fibular hemimelia is the most common congenital long bone deficiency: partial or complete absence of the fibula, on a spectrum that runs from mild hypoplasia to complete absence. A limb length discrepancy and a deformed foot come with it.

Who. About 1 in 40,000 live births, with a male to female ratio of 1.5:1. The right side is slightly more often affected, and bilateral involvement is reported in anywhere from 9 to 52% of cases.

Cause. The fibula fails to develop normally during embryogenesis, and the exact cause is unknown. It may involve:

• Vascular insult during development
• Genetic factors, in rare familial cases
• Teratogenic exposure
• Failure of mesenchymal condensation

What the fibula normally does. The fibula is the lateral bone of the lower leg. It carries minimal weight, 10-15%, but it contributes to the ankle mortise, gives the ankle its lateral stability, and anchors the peroneals and flexor hallucis longus.

What its absence does. Without the fibula the ankle loses its lateral support and lateral tether, and it drifts into valgus. The tibia bows anteromedially, a compensatory deformity and the characteristic one, and the lateral column of the foot is affected. Because the fibula contributes to length, the limb is short, and that limb length discrepancy is the main problem.

View larger

The whole limb is involved. Associated anomalies are common and run the length of the limb, so assess all of it:

• Foot: equinovalgus, the most common deformity; absent lateral rays and tarsal coalition, both common; syndactyly
• Knee: cruciate ligament deficiency, genu valgum, lateral femoral condyle hypoplasia
• Hip and femur: PFFD in 50%, coxa vara, acetabular dysplasia
• Upper limb: ulnar hemimelia may be associated

Three classifications are in use. Achterman-Kalamchi describes severity by whether the fibula is present and the state of the foot. Birch sorts by whether the foot can be preserved, which is what matters most for treatment. Paley combines foot rays with the tibia-femur ratio to guide lengthening.

History. A short lower limb is noted at birth, and a foot deformity may be noted with it. Ask about family history, which is rare but may be present, difficulty with walking or activities, and previous treatment in an established case.

Inspection and palpation. The limb is short, with anteromedial tibial bowing that can be felt as well as seen and a foot deformity, equinovalgus most commonly. The fibula may be absent or hypoplastic. Look at both legs for bilateral involvement and at the hip, knee and upper limb for associated deformity.

The foot and ankle. Count the functional rays (the toes) and record the foot position, equinovalgus or equinovarus, any tarsal coalition and the ankle valgus angle. Assess ankle stability; the ankle may have limited motion and a valgus deformity.

Above the ankle. At the knee, look for valgus and assess cruciate function. At the hip, assess for PFFD and coxa vara. Look for contractures throughout, examine the upper limbs for ulnar hemimelia, and check the cardiac and renal systems for their rare associations.

Measurements. True leg length is measured from the ASIS to the medial malleolus, apparent leg length from the umbilicus to the medial malleolus, and tibial length from the knee joint line to the ankle. The final discrepancy is then predicted (see Investigations).

Radiographs. AP and lateral films of the lower limb show whether the fibula is present, the anteromedial tibial bowing, the limb length discrepancy, the ankle valgus angle and the foot (ray count and tarsal coalition). Dedicated foot radiographs count the functional rays and assess the tarsal coalition, the foot structure and the ankle valgus.

Full-length standing radiographs (scanogram). These give an accurate LLD measurement, show alignment and the tibia-femur ratio, and are the basis for planning treatment.

View larger

CT and other imaging. CT is used if reconstruction is being considered, for detailed foot and ankle anatomy, the tarsal coalition and planning the foot centralisation. MRI may assess the soft tissue structures; ultrasound is usually not needed.

Predicting the discrepancy. The LLD percentage is constant throughout growth, so the final discrepancy can be predicted. The multiplier method multiplies the current LLD by the multiplier to give the predicted final LLD. The growth remaining method accounts for growth and is more complex.

The combination of a short lower limb, anteromedial bowing and a valgus foot has a focused differential. The discriminator is usually the radiographic status of the fibula, the apex/direction of any bowing, and the pattern of associated anomalies.

What it is. Instead of the normal hinged mortise, the talar dome becomes rounded and the distal tibial articular surface correspondingly concave, producing a ball-and-socket articulation on the AP radiograph.

Why it forms. It is an acquired, adaptive change. The absent or deficient lateral malleolus removes the normal lateral bony buttress, and an underlying tarsal coalition, usually talocalcaneal, abolishes subtalar inversion and eversion. The ankle remodels into a ball-and-socket to substitute a multiaxial range of motion, including the coronal-plane motion the fused subtalar joint can no longer provide.

Why it matters. On a paediatric film it is a red flag, not an isolated normal variant. It also signals that the ankle's stability and coronal alignment are abnormal, which is relevant before any lengthening.

View larger

The principle. Treatment is based on whether the foot can be preserved, not on the length of the fibula. Four factors decide it:

1. Foot rays: under 3 rays (Birch Type 2) usually means amputation; 4-5 rays (Type 1) make reconstruction possible
2. Predicted LLD: over 30-40cm favours amputation; under 30cm, reconstruction is possible
3. Ankle valgus: over 30 degrees favours amputation; under 30 degrees may be correctable
4. Family preference, after counselling about both options

The old teaching and the evidence. Syme amputation has been taught to give better function than reconstruction in severe cases. The only study to compare the two paths directly found no clinically significant functional or psychosocial difference in mid-childhood (see Outcomes), so there is no "wrong" choice between them and the decision is shared with the family.

Algorithm

View larger

What it is. SUPERankle, Paley's Systematic Utilitarian Procedure for Extremity Reconstruction of the ankle, is a single-stage foot-and-ankle realignment. It converts a fixed equinovalgus, ball-and-socket, subluxated ankle into a stable, plantigrade foot positioned under the tibia, so that the limb can then be lengthened rather than amputated.

What it corrects. In one sitting it combines:

• Release of the tight lateral/posterolateral structures and the fibular anlage, the fibrocartilaginous band that tethers the ankle into valgus and equinus
• A supramalleolar (distal tibial) osteotomy to correct the distal tibial valgus and procurvatum
• Resection of the tarsal coalition/subtalar realignment to reposition the hindfoot
• Realignment of the ankle to make it congruent and plantigrade, held with wires/an external fixator

Where it sits. It is the enabling first step of the reconstruction (preservation) route for a preservable foot, performed before or with the first lengthening. It is the advance that has shifted some historically "amputation-only" few-ray feet into the reconstructable group.

The trade-off. It is a demanding, specialist procedure with a real risk of stiffness, recurrence of deformity and neurovascular compromise, and it commits the family to the long multi-lengthening course. It does not abolish the shared decision with a well-executed Syme amputation.

View larger

View larger

After reconstruction. Early problems are infection, in 20-30% with external fixators, wound healing problems, neurovascular injury and inadequate correction. Late ones are persistent LLD that may need additional lengthening, ankle instability, stiffness or arthritis, knee valgus and cruciate deficiency, recurrence of foot deformity or tarsal coalition, ankle and knee contractures, and hardware problems.

From lengthening.

• Pin site infection (common)
• Ankle and knee stiffness
• Contractures
• Delayed union
• Premature consolidation
• Nerve injury, peroneal most commonly
• Refracture

After Syme amputation. Wound healing problems occur in 5-10%; infection is rare. The heel pad may migrate if it is not properly fixed, and migration or bony overgrowth may need revision later. Problems with prosthetic fitting are rare.

Prevention. Prevention rests on careful patient selection, meticulous surgical technique and realistic expectations, with aggressive physical therapy and long-term bracing after reconstruction.

After Syme amputation. Pain management, wound care and a cast for 2-3 weeks. Once healed, the prosthesis is fitted at 6-8 weeks, followed by gait training and return to activities. The prosthesis is adjusted as the child grows, and the stump is watched for heel pad migration and bony overgrowth.

After reconstruction. Immediate care is pain management, wound care and care of the cast or external fixator. The lengthening phase needs daily pin site cleaning, the distraction protocol, regular radiographs and physical therapy, which is critical.

After union. Protected weight-bearing, then a long-term AFO, physical therapy and a gradual return to activities; foot centralisation is followed by the same protected weight-bearing and long-term AFO. Assess annually until skeletal maturity, monitoring the progression of the LLD and function and addressing complications.

Amputation versus reconstruction. Amputation has been taught as functionally superior for forty years, and the one study that compared the two paths directly found no such difference. Birch 2019 compared 20 primary amputations with 22 staged reconstructions, at a mean age of 9 and a minimum of 2 years after treatment:

• No significant functional or psychological differences between the groups
• Psychosocial and quality-of-life scores in both groups were comparable to healthy populations, and to each other
• Gait-analysis and timed-dash differences were statistically but not clinically significant
• The majority of patients and parents in both groups were satisfied and would choose the same again

The caveat that travels with it. Families choosing amputation had lower economic and educational levels and were more ethnically diverse, so the groups differed in who they were as well as in what was done. The authors attribute the single difference they found, lower perceived school-related quality of life among boys treated by amputation, to exactly that. Socioeconomic circumstance influences these scores independently of the operation: a reason to support families, not a reason to prefer an operation. Psychosocial support matters in both pathways.

What still differs. These differences are not in dispute:

How to counsel. The honest position is that the functional evidence does not favour either path in mid-childhood, that the burden of treatment differs enormously and predictably, and that the decision therefore belongs to an informed family. Say plainly that the long-term adult comparison has not been made: Birch's cohort was assessed at a mean age of nine, which the authors themselves call an interim stage.

What predicts success. Appropriate patient selection on either path. For Syme, proper technique and preservation of the heel pad; for reconstruction, a motivated family, adequate rays and a manageable LLD.

The long term. After Syme the child needs prosthetic adjustments while growing and is otherwise stable. After reconstruction additional procedures may be needed and ankle problems may worsen. On both paths most patients function independently.

Global epidemiology:

• Most common congenital long-bone deficiency; reported birth prevalence approximately 1 in 40,000 (longitudinal limb deficiencies overall ~1 per 20,000)
• Male predominance (~1.5:1); right side slightly more common; bilateral in roughly 10-25% of series
• Predominantly sporadic - the large majority are isolated with no Mendelian inheritance; rare familial/syndromic cases exist
• Strongly co-segregates with ipsilateral femoral deficiency (PFFD) and cruciate/lateral femoral condyle dysplasia

Side-by-side practice positions (no single global guideline exists):

Amputation versus reconstruction in severe disease. The comparative cohort data under Outcomes (Birch/Paley/Herzenberg 2019) challenge the historic teaching that amputation gives superior function in Type II, and the "right" answer is increasingly shared decision-making rather than a fixed algorithm.

How many foot rays are "reconstructable"? The under-3-rays threshold for amputation is a guide, not a hard rule. With modern foot and ankle reconstruction (SUPERankle) some experienced centres reconstruct feet that older series would have amputated, and definitions of a "functional plantigrade foot" are not standardised.

Lengthening nails versus external fixation. Magnetic intramedullary nails (PRECICE) reduce pin-site morbidity and improve comfort, but they require adequate bone stock, add cost, and may not suit the smallest or most deformed segments, where Ilizarov frames remain the workhorse. Long-term implant data are still maturing.

Timing and total burden of lengthening. The optimal age to start, the number of lengthenings, and the trade-off between repeated distraction (stiffness, contracture, neurovascular risk) and contralateral epiphysiodesis remain individualised. The cumulative anaesthetic and psychosocial burden over 10-15 years is hard to quantify.

Unresolved questions. Three remain open:

• No validated, disease-specific patient-reported outcome measure for fibular hemimelia exists; cross-study comparison is limited
• The genetic/embryological basis is still unclear (vascular insult vs mesenchymal condensation failure); most cases remain sporadic and unexplained
• Whether proactive cruciate reconstruction at the time of lengthening should be routine (versus reactive) is not yet supported by high-level evidence