Congenital lower limb conditions can occur due to a variety of reasons: maternal factors, such as diabetes and teratogens, as part of a genetic syndrome, or due to an amniotic constriction band.
Depending on the type of deformity and its severity the condition may be just a cosmetic 'problem' to the patient or it may cause psychological and/or functional difficulties.
The most common congenital lower limb conditions are:
As with all congenital conditions, good knowledge of the normal embryological process is needed to be able to understand why, and how, a condition occurs.
The limbs start to develop from the 4th week post-fertilization, with the lower limb buds present 2 days after the upper limb buds. Each limb bud consists of a mass of mesenchyme covered by a layer of ectoderm. At the tip of each limb bud, the ectodermal cells form an apical ectodermal ridge (AER) which promotes the development and growth of that limb in the proximo-distal axis, i.e. from hip to foot.
Fibroblast growth factors and Tbx-4 and Tbx-5 genes are released from the AER and activate the mesenchymal cells in the zone of polarizing activity (ZPA) on the posterior margin of the bud. The ZPA expresses sonic hedgehog (Shh), which helps with the development and growth of the limb in the anterior-posterior axis, i.e. from big toe to little toe, by signalling to activate the expression of genes that determine the number and identity of the digits.
By the 6th week the distal ends of the limb buds flatten and become hand/footplates. Ridges, called digital rays, form with loose mesenchyme inbetween them. The interdigital tissue is then separated by apoptosis to leave individual digits, [Image 1].
The most common congenital lower limb defect is polydactyly of the toes, which means there are additional toes present from the normal 5 digits with or without corresponding metatarsal duplication, [Image 2].
Polydactyly affects on average 1.7 per 1000 live births, but people of Afro-Caribbean ethnicity are more commonly affected (3-14 per 1000 live births) and there is a slight male predominance. It usually occurs as an isolated trait but can be of autosomal dominant transmission and variable penetrance.
It can also be associated with genetic syndromes such as Ellis-van Creveld syndrome, Patau's syndrome (Trisomy 13) and Down's Syndrome (Trisomy 21). Therefore, when examining a patient, one must also look for other signs that may be present in a syndrome e.g. a cleft lip and undescended testicles, as seen in Ellis-van Creveld syndrome.
Polydactyly can be classified by clinical features into 3 types:
The most common type is post-axial with the least common being central polydactyly.
Another classification, the Venn-Watson classification, can also be used. It is based on the radiological findings, such as the metatarsal abnormalities found on radiology [Image 3]. However, as such this method is not commonly used, so as to allow the child's bones to ossify.
Whichever type of polydactyly, they all occur when either an environmental factor, or, more commonly, a genetic abnormality, lead to an extra separation in the footplate during the first trimester of pregnancy.
It has been shown that in pre-axial polydactyly a defect in Shh expression is present.
Unlike with polydactyly of the hand, polydactyly of the foot does not normally cause any functional problems and treatment is therefore usually for cosmetic reasons or to allow easier fitting of shoes.
Postaxial or central polydactyly may require no surgery if the extra digit can be correctly aligned. However, if the extra digit is malaligned, such as in preaxial polydactyly, excision/resection can be performed. Surgery is usually performed when the child is approximately 12 months old, so that the child is young enough for their development not be greatly impaired (by post-op immobility), but old enough for the surgeon to be certain that ossification has occurred within the affected digit to ensure correct anatomical assessment.
Generally the most medial toe/s of preaxial, and the most lateral toe/s of postaxial polydactyly are removed to allow the foot to be narrower with a straight border, to make shoe-fitting easier.
Postaxial polydactyly surgery usually has an excellent result but patients may have angulation at the MTP joint and bowing of the metatarsal.
Preaxial polydactyly surgery can lead to hallux varus formation, which can cause pain and difficulty with shoe-fitting.
Central polydactyly surgery needs care to be taken to ensure there is a good cosmetic affect as widening of the foot is a common complication.
Syndactyly occurs when adjacent digits are not completely separate form each other, the word coming from the Greek 'syn' (together) and 'dactyly' (fingers/digits). It occurs in approximately 1 in every 2000 live births, and is the most common congenital limb deformity. It usually occurs between the second and third toe, with boys twice as likely to have syndactyly than girls.
CLASSIFICATION (Diagram 1):
Syndactyly can be either simple (involving just the soft tissue), complex (involving bone and/or nail), fenestrated (joined by tissue but with gaps) or polysyndactyly (an extra digit present which is 'webbed').
Syndactyly occurs due to failure or incomplete apoptosis during the first trimester of pregnancy. The exact cause for this is unknown, however there are over 100 genetical conditions that syndactyly is associated with, e.g. Down's syndrome, Apert's syndrome and Poland syndrome.
Unlike with syndactyly of the fingers, syndactyly of the toes rarely requires surgery for a functional reason, unless the syndactyly is complex. If surgery is performed, either for a functional or cosmetic reason, only one side of any digit can be released at any one time to prevent ischaemic compromise and all but the minority of incomplete syndactyly cases will require a full-thickness skin graft.
Once the digits are divided the surgical wound is resurfaced with a dorsal trapezoidal-shaped flap, interdigiting fasciocutaneous flaps and skin grafts. There are therefore possible complications including digit ischaemia, graft failure, hair growth in grafted skin (skin-graft is usually taken from the groin) and recurrence, also known as web creep.
Amniotic constriction syndrome (ACS), also known as ADAM complex, occurs when the fetus is entangled in fibrous bands of the surrounding amniotic sac.
It affects 1 in every 10000-15000 live births, and whilst it can occur around the babies' head or umbilical cord, it more commonly forms around limbs and digits and is strongly associated with clubfoot. ACS does not necesserily indicate a problem, and it can be just an aesthetic concern.
However if the constricting bands are deep and/or tight it can cause restricted venous flow and lymphatic flow (congenital lymphodema) as seen in Image 4. The bands can also interfere in the development of the limb or digit and may even cause congenital amputation occuring in utero. ACS can also cause syndactyly.
The cause of ACS is unknown, however there are two theories:
ACS is not normally diagnosed until after birth, as the fibrotic bands are small and not easily seen on ultrasound. Swelling of a limb may be seen on ultrasound however it is uncommon to perform fetoscopic surgery unless the band is around a vital organ or the umbilical cord.
An X-ray is usually conducted to assess the depth and tightness of the band. If the band is restricting the blood circulation the baby is usually treated immediately, otherwise most surgery is postponed until the infant is approximately 12 months old.
Surgical treatment is to excise the amniotic band with 1-2mm of normal skin to minimise the risk of recurrence. The excision is performed by Z-plasty for lesser constrictions and by W-plasty if the band is tight/large as a Z-plasty would cause a longer scar and scars contract as they heal (Diagram 2). Care must be taken to ensure that the neurovascular bundles close to the skin are not damaged.
Smaller and relatively shallow constrictions have a good outcome both in terms of aesthetics and low recurrence. Deeper and tighter bands may require further treatment for lymphatic or neurovascular problems even after surgery to remove the bands.
An infant with a congenital amputation of the lower limb usually adapts well to this, and a prosthesis can be worn if the amputation occured proximal to the ankle joint to enable full function.
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