1. Proximal focal femoral deficiency (PFFD):

PFFD is a rare abnormality of the proximal femur with an incidence of 1 in 52,000 live births ^[4]. It can range from mild shortening to severe deficiency of the femoral head, acetabulum, and femoral shaft.  Most cases are sporadic.  Autosomal Dominant inheritance, presence of bilateral PFFD with abnormal facies. Common associations are fibular hemimelia and absent patella.  

Figure 1 shows the Aitken classification of PFFD ^[5,6]. 

Fig. 1: Aitken Classification of PFFD. Reference: M. a Bedoya, N. a Chauvin, D. Jaramillo, R. Davidson, B. D. Horn, and V. Ho-Fung, “Common Patterns of Congenital Lower Extremity Shortening: Diagnosis, Classification, and Follow-up.,” Radiographics, vol. 35, no. 4, pp. 1191–207, 2015 [6].

Ultrasound findings of PFFD include normal anatomic landmarks such as the acetabular labrum, acetabular roof, lower edge of the ilium, cartilage-bone interface, and normal round configuration of the femoral head in the coronal view not being identifiable ^[6].  

Magnetic Resonance imaging of PFFD allows for complete anatomic characterization in skeletally immature. It helps to define the cartilaginous proximal femur and the presence or absence of a cartilaginous connection to the femoral head. Gradient images ­­­help to depict cartilage. MRI also helps to assess the soft tissue structures, guiding the surgical approach and preparation of limb prosthesis.

Fig. 2: Proximal focal femoral deficiency

2. Fibular hemimelia (FH):

The most frequent congenital anomaly of the fibula and the most common long bone agenesis ^[6]. The absent bony part is commonly replaced with a thick fibrous or fibrocartilaginous palpable band. The clinical findings can vary from mild to severe lower limb deformity, and includes equinovalgus foot, shortening of the leg, tibial anterior bowing, and variable knee valgus ^[7]. Common associations are PFFD, absence of lateral rays and phalanges of lateral toes, syndactyly and polydactyly. 

Imaging is done to assess the degree of fibular hypoplasia or agenesis; evaluation of tibial shortening, femoral shortening, tibial bowing, and tibial epiphyseal involvement; assess valgus deformity and stress changes such as cortical sclerosis and fractures. MRI is helpful in the detection of the soft-tissue deformities associated with FH and for analysis of joint stability. MRI can characterize the size and course of the fibrous or fibrocartilaginous band.

Fig. 3: Proximal focal femoral deficiency and fibular hemimelia

Fig. 4: Fibular Hemimelia with syndactyly

3. Tibial hemimelia (TH):

TH is usually sporadic, with an autosomal dominant inheritance pattern in bilateral cases. TH can range from isolated mild shortening to complete tibial absence. The deficient bone part is usually replaced with a strong fibrous or fibrocartilaginous band. 

Fig. 5: Tibial hemimelia

4. Sirenomelia:

Presentation is of a single fused lower limb, with multiple urogenital, anorectal and vascular malformations. There is association with maternal diabetes.

5. Syndactyly:

Syndactyly is congenital fusion of the finger or toes. Several different types are defined, and the majority follow an autosomal dominant inheritance pattern. Simple syndactyly involves only fusion of the soft tissue, whereas complex syndactyly also involves fusion of the bones. Smith-Lemli-Opitz syndrome manifests as syndactyly of the 2nd and 3rd toes, with associated other congenital anomalies.

Fig. 6: Syndactyly

6. Oligodactyly:

Oligodactyly refers to the congenital absence of one or more toes. It occurs due to improper differentiation of the apical ectodermal ridge during development, likely caused by teratogenic insults, impaired blood flow and amniotic bands. A positive family history in some cases. Some of the associated conditions are FH, tarsal coalition and syndromes including VACTERL and Fanconi.

7. Congenital amputation:

Congenital limb amputation is the absence of a fetal limb or part of a limb that usually occurs due to disruption of vascular supply. Common causes are amniotic band syndrome (ABS), amniotic adhesion, resorptive necrosis and vasoactive teratogens.

ABS arise due to entrapment of various body parts in a disrupted amnion. Spectrum of abnormalities depends on which part becomes entrapped and stage of gestation. Features are often asymmetrical. Limb defects tend to be the commonest ^[8], presenting as variable levels of amputation and/or constriction with possible distal atrophy, lymphedema, pseudosyndactyly, club foot and phocomelia.

Fig. 7: Amniotic band syndrome

8. Polydactyly:

Polydactyly refers to presence of more than five digits in a hand or foot. It is broadly classified as pre-axial (extra digits towards hallux), post-axial polydactyly (extra digits towards little toe) and central (involvement of middle three digits).

Fig. 8: Polydactyly

Fig. 9: Multiple congenital lower limb anomalies

9. Congenital tibial bowing:

This is a rare condition resulting from an abnormal intrauterine position. It is usually convex posteriorly and medially, and less commonly, it is convex laterally. The fibula is also bowed. The foot shows marked dorsiflexion at birth.  At radiography, there is cortical thickening along the concavity of the curvature, with diaphyseal broadening.  

Fig. 10: Congenital knee dislocation

10. Congenital vertical talus (CVT):

CVT, also known as congenital convex pes valgus, is a severe form of congenital rigid flatfoot. Fifty percent of CVT cases present as an isolated (idiopathic) deformity, whereas the other 50% occur in association with neuromuscular or genetic disorders. The cause of this deformity is unknown; however, Autosomal Dominant inheritance is suggested.

Radiographically, the talus is in a vertical position and the navicular is dorsally dislocated when the foot is placed in maximum dorsiflexion and plantar flexion.  

Fig. 11: Congenital vertical talus

11. Club foot:

It includes the following spectrum:

1. Talipes equinovarus (adduction of the forefoot, inversion of the heel and plantar flexion of the forefoot and ankle)

2. Talipes calcaneovalgus (dorsal flexion of the forefoot with the plantar surface facing laterally)

3. Metatarsus varus (inversion and adduction of the forefoot alone)

It has multiple associations such as chromosomal anomalies, syndromes like Meckel Grubers and Roberts, renal, connective tissue, spinal and skeletal anomalies. 

Fig. 12: Club foot

Fig. 13: Bilateral talipes equinovarus

12. Tarsal Coalition:

Although congenital, patients typically present in adolescence with foot pain and flat foot. It can be of bony, cartilaginous or fibrous types. The most common bones involved are talocalceanal and calcaneonavicular. Radiographic features of the former include the C-sign and talar beak sign; and latter include the anteater nose sign.