Introduction 

Metatarsus adductus (MA) is the most common congenital foot deformity in newborns. It is a uniplanar transverse plane deformity in which the metatarsal bones of the forefoot are angled toward the midline (adducted) at the Lisfranc joint.

Clinically, there is a concavity of the medial border of the foot, a convexity of the lateral border, and a prominence of the head of the fifth metatarsal bone.¹

The deformity is different from others such as clubfoot, where the entire foot and ankle are involved and tend to point downward and inward, and skewfoot, where there is forefoot adduction and heel valgus.

It occurs in 1 to 3 per 1,000 live births¹and is more likely to happen in girls than boys.⁴

While metatarsus adductus is mainly diagnosed in infancy, it can persist into adolescence and adulthood.

If left untreated, it can lead to walking problems, pain, shoe-fitting problems, shoe wear unevenness, and a greater risk of other foot deformities.¹

Anatomy and Pathophysiology of Metatarsus Adductus

Normal Foetal Foot Development

In utero, the foetal foot forms in the first trimester. Its final form is influenced by muscle balance, ligamentous tension, and uterine space later in pregnancy. Movement and fetal position can also influence bone alignment and soft tissue flexibility by the third trimester.

Pathoanatomy in Metatarsus Adductus

The deformity happens at the Lisfranc joint, which is the location where the metatarsals articulate with the bones of the midfoot. These metatarsals are angled inward in a pure transverse plane, while the heel and rearfoot are in normal alignment beneath the ankle joint.⁵ 

The most widely accepted theory of the cause of metatarsus adductus is that of abnormal intrauterine position.⁵ 

It occurs more commonly in first pregnancies (primigravida), likely because the mother's abdominal and uterine walls are tighter, with less space for the foetus.⁵ 

This would mean that mechanical forces in the womb can shape the foot before birth.

Several other factors can also contribute to this, such as:, 

• Soft tissue tightness of structures like the abductor hallucis muscle and medial foot capsule
• Abnormal tendon insertions of muscles like tibialis anterior, tibialis posterior, and abductor hallucis⁵
• Intrauterine compression and osseous abnormalities
• Family history, with a probable genetic influence⁵

Clinical Features

 The signs of metatarsus adductus include: 

• A "C"- shaped foot with a curved inside border and convex outside border
• Prominent base of the fifth metatarsal bone
• Increased gap between the first and second toes
• Forefoot adduction and slight turning upward (supination)
• A high medial arch
• Resistance to abduction of the forefoot
• A crease on the inner side of the arch
• No deformity of the ankle pointing downward (equinus), unlike clubfoot
• Can be associated with internal tibial torsion⁷

Classification

Metatarsus adductus is graded based on flexibility.⁴

• Flexible- the forefoot can be corrected beyond neutral
• Semi-flexible- the forefoot can be corrected to neutral but not beyond
• Rigid- the forefoot cannot be corrected to neutral

Intrauterine Positioning: Mechanisms and Variations

The position of a fetus in the womb is not constant during pregnancy. As it progresses, the space gets limited, and certain positions can result in continuous pressure over parts of the body.

Some of the significant influences include:

• Breech position- buttocks or feet first, altering the alignment of the legs and feet.

Studies have found higher rates of metatarsus adductus in breech-born children or in children with low amniotic fluid.⁶

• Oligohydramnios- reduced amniotic fluid reduces cushioning and mobility
• Multiple pregnancies- twins or triplets may have reduced space, with compression of limbs by other fetuses
• First pregnancy- stiffer abdominal wall can restrict foetal movement
• Uterine anomalies- abnormal uterine shapes like a bicornuate uterus can infringe on space
• Large infant size may lead to compression against the uterine walls

These mechanical pressures can, over time, deform the forefoot inward, forming the fixed angulation of metatarsus adductus.

Although intrauterine position is a factor, there are other multifactorial causes like: 

• Genetic predisposition- family history can be pertinent⁵
• Positional moulding- uterine mechanical forces
• Associated conditions- higher incidence in children with developmental dysplasia of the hip (DDH) or congenital muscular torticollis

Clinical Evaluation and Diagnosis

Evaluation and diagnosis involves

• History taking- obstetric history (presentation, amniotic fluid volume, multiple birth)
• Physical examination-  The Heel bisector line is a visual grading system for severity⁷

Testing for flexibility by forefoot manipulation manually.

• Imaging- not usually necessary unless operating for rigid or severe deformities is anticipated⁴

Management and Prognosis

Most flexible cases resolve spontaneously within the first year without the need for medical intervention. However, if the condition persists beyond this period, several conservative management strategies can be effective, including:

• Gradual stretching exercises performed regularly by parents
• Use of splints or specialised corrective footwear to support proper alignment
• Serial casting, particularly for more resistant or rigid cases
• Modifying sitting and sleeping positions to encourage correct posture and alignment²

These approaches aim to improve flexibility and alignment over time, often avoiding the need for surgical intervention.

Surgical Treatment

It is reserved for severe rigid deformities that do not improve with conservative management. The outcome is better with early detection, and treatment is most effective when started before 9 months of age.³

Preventive and Antenatal Considerations

Ultrasound can sometimes pick up abnormal foot positions antenatally. Parental counselling is also important, in particular to inform that most cases spontaneously resolve.

Conclusion

Intrauterine position plays a significant role in the aetiology of metatarsus adductus, particularly in crowding, restricted motion, or abnormal presentation. Despite the fact that most cases correct themselves, an understanding of risk factors will facilitate early diagnosis and management. Additional prospective research is needed to establish how much position and genetics truly affect his condition.