Tibial Hemimelia

Tibial hemimelia (longitudinal tibial deficiency) is a rare congenital absence or hypoplasia of the tibia, with a relatively intact fibula that migrates proximally, and a rigid, varus, deformed foot. Its incidence is about 1 in 1 000 000 live births.

It is usually sporadic and unilateral, but about 30% are bilateral, and it is often part of a syndrome. An autosomal dominant pattern with variable penetrance is described in familial cases, with a link to the sonic hedgehog signalling enhancer (a ZRS or other variant); associated conditions include polydactyly, split hand and foot, femoral deficiency, and cardiac, renal and gastrointestinal anomalies. About 75% of cases have another skeletal anomaly.

The Jones classification describes the extent of tibial absence: types 1a and 1b (complete absence, with 1a also having a hypoplastic distal femoral epiphysis), type 2 (distal tibia absent with the proximal tibia present), and types 3 and 4 (milder distal deficiency and tibiofibular diastasis). Complications are severe limb length discrepancy, knee instability and flexion contracture, foot deformity, and multiple surgeries or amputation.

Pathophysiology.

The tibia fails to form in early limb development (week 4 to 8), perhaps through altered sonic hedgehog signalling. In the absence of the tibia, the fibula is unopposed and migrates proximally, the foot falls into varus and equinus, and the quadriceps mechanism (the extensor mechanism) is deficient in complete absence, which decides if the knee can be saved.

A short, bowed leg with a rigid varus foot, with the sole facing the opposite leg and the fibular head prominent at the knee. Knee flexion contracture and instability, absent or weak quadriceps (in type 1), and extra or missing toes. Polydactyly or split hand features are common.

Examine the limb length, the knee (active extension, flexion contracture, stability), the foot position, quadriceps function, the hands, hips, spine and other organs.

• Prenatal ultrasound may show the condition at the second trimester scan.
• Radiographs: absence or hypoplasia of the tibia (complete in type 1, with the fibula alone distal to the knee), a proximally migrated fibula, and a hypoplastic distal femur (type 1a). In infants, the cartilaginous tibial anlage is not visible, and MRI or ultrasound identifies it and the femoral condyle.
• MRI or ultrasound of the knee and tibia to show the cartilage anlage and the quadriceps mechanism, which affects the decision for knee salvage.
• Echocardiography, renal ultrasound, hand and foot radiographs, spine films, and genetic counselling with testing (microarray, targeted gene analysis).

Differential diagnosis.

• Fibular hemimelia and proximal femoral focal deficiency.
• Congenital pseudarthrosis of the tibia and clubfoot.
• Tibial dysplasia within syndromes (Werner syndrome, Langer-Giedion syndrome, CHARGE syndrome).
• Amniotic band syndrome and other longitudinal limb deficiencies.

Management depends on the type, and aims at a functional, stable limb with a prosthesis in most children. The choice is made early with a limb reconstruction team.

Type 1a (hypoplastic distal femoral epiphysis, no extensor mechanism). Knee disarticulation is the usual treatment at 9 to 18 months of age (or through knee amputation), with a prosthesis; reconstruction is not reliable.

Type 1b (normal distal femur, with an extensor mechanism). Fibular centralisation (Brown procedure: the fibula is placed under the femur) with later conversion to a Syme amputation, if a quadriceps mechanism exists; otherwise, knee disarticulation. Results are variable, with the risk of flexion contracture and instability, and some surgeons prefer knee disarticulation.

Type 2 (proximal tibia present). Tibiofibular synostosis (fusion of the fibula to the proximal tibia), Syme or Boyd amputation of the foot, and prosthetic fitting, with an expected knee function and a durable result. Some mobile feet are reconstructed with limb lengthening and ankle stabilisation.

Types 3 and 4 (distal deficiency, diastasis). Reduction of the ankle diastasis or fusion, fibular talar stabilisation, and lengthening, or amputation for severe deformity.

Orthoses and prostheses. Prosthetic fitting for amputees from about 12 months, and regular replacement during growth. A knee-ankle-foot orthosis for reconstructed limbs.

Operative antibiotic prophylaxis. Cefazolin 30 mg/kg to a maximum of 2 g intravenously (2 g in adolescents; 3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose; with severe penicillin allergy, clindamycin 10 mg/kg (maximum 900 mg) intravenously or vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose.

Analgesia.

• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily) and ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily), for up to 5 days; avoid ibuprofen with kidney impairment, dehydration or active bleeding.
• After amputation or major surgery, oral morphine 0.1 to 0.2 mg/kg every 4 hours as needed for severe pain for up to 3 days (maximum 10 mg a dose); monitor sedation and respiration, and prescribe a laxative. Phantom and stump pain: gabapentin 5 mg/kg orally at night (maximum 300 mg), increased every 1 to 3 days to 5 to 10 mg/kg three times daily (maximum 900 mg three times daily in older children; reduce at creatinine clearance under 60 mL/min); monitor sedation and dizziness.

Monitoring. Wound and stump review after surgery, and growth of the stump (bony overgrowth in children, which needs revision in about 10 to 20% of transosseous amputations). Prosthetic fit every 3 to 6 months in growth, and limb length and alignment yearly. Psychological and school support, and long term review of the other joints and the spine.

Referral and escalation. Refer every infant with suspected tibial hemimelia to a paediatric limb reconstruction centre in the first months of life, with genetics, cardiology and nephrology review. Urgent referral for neurovascular compromise, stump infection or overgrowth with skin breakdown.

Special groups.

• Infants: early surgery before walking age improves prosthetic adaptation.
• Familial cases: genetic counselling (autosomal dominant with variable penetrance).
• Bilateral disease: staged treatment, and early independence with prostheses.
• Families: early meetings with the team, prosthetic users and counsellors.