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Stem
A 4-week-old female infant is brought to the well-baby clinic. She was born at term by vaginal breech delivery (frank breech) to a primigravida mother. On examination of the hips, the right hip abducts fully but the left hip abducts only to 50 degrees. With the hips flexed to 90 degrees, abduction of the left hip produces a palpable "clunk" as the femoral head reduces into the acetabulum. There is an extra thigh skin fold on the left. The baby is otherwise well, afebrile, feeding normally, with no other findings.
Questions
a) What is the most likely diagnosis, and which physical sign is pathognomonic in this age group? (2 marks)
Developmental dysplasia of the hip (DDH), left side. The pathognomonic sign is a positive Ortolani test — the palpable clunk of reduction as the dislocated femoral head returns to the acetabulum on abduction. The risk factors (breech, female, firstborn, left side) make this the leading diagnosis.
b) List four risk factors for DDH evident or implied in this case, and explain why breech presentation increases the risk. (3 marks)
Risk factors present: (1) female sex (4 to 8x risk; 80% of cases), (2) breech presentation (single biggest modifiable risk; up to 12 to 25% of breech babies affected), (3) firstborn (tight primigravid uterus), (4) left side affected (3x commoner, due to left occiput anterior intra-uterine position holding the left hip adducted). Mechanism of breech risk: in the frank breech position the foetal hips are flexed and knees extended, so the femoral head rests against a shallow, posteriorly inclined acetabulum; after delivery the sudden extension of the knees levers the head out posteriorly and superiorly.
c) Outline the appropriate imaging strategy and definitive management, with the key parameters of treatment. (3 marks)
Imaging: because the infant is under 4 to 6 months and the femoral head is cartilaginous, the modality of choice is hip ultrasound (Graf technique with dynamic stress). Plain X-ray is unhelpful at this age as the head is not yet ossified. Management: a Pavlik harness fitted within the next 1 to 2 weeks, holding the hip flexed to about 100 degrees and abducted 30 to 50 degrees (the human position), worn 23 hours per day. Ultrasound confirmation of reduction within 1 to 3 weeks is mandatory; the harness is continued for 6 to 12 weeks until the hip is stable and the acetabulum has matured (Graf type I). The harness should be abandoned if reduction is not achieved by 3 to 4 weeks, to avoid posterior acetabular erosion.
d) What is the most feared complication of treatment, and how is it minimised? (2 marks)
Avascular necrosis (AVN) of the femoral head — caused by compromise of the medial circumflex femoral artery from forced or excessive abduction, or extreme flexion compressing the vessels. It is minimised by using the moderate-abduction human position (never forced wide abduction), confirming reduction early by ultrasound, and performing adductor longus tenotomy when tension is high. AVN leads to coxa magna, coxa breva, premature physeal closure, leg-length discrepancy, and early osteoarthritis.