Three Months of Knee Pain in an Obese Adolescent Boy
An 11-year-old boy is referred from an emergency department with a 3-month history of right knee pain that began after a football game. He has been treated twice for a knee sprain, has stopped all sport, and for the last week has used a stick to walk. He is obese (78 kg, BMI 31) and has no other medical history; his mother reports he has grown 8 cm in the past year. On examination he stands with the right leg externally rotated, has a fixed external rotation deformity, and internal rotation of the right hip is 5° compared with 45° on the left. Hip flexion produces pain at the end range with obligatory external rotation. Blood results are normal, the physes are open, and the radiograph report states no fracture; only an anteroposterior pelvis view was taken.
1. What is the diagnosis, what imaging do you need, and why was the diagnosis missed?
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This is a slipped capital femoral epiphysis until proven otherwise. The history is textbook: an adolescent with a high BMI and a recent growth spurt, knee pain after sport, a progressively worsening limp, and a hip that lies in external rotation with markedly restricted internal rotation and pain at the end of flexion. The knee is an innocent bystander because the obturator nerve and the articular branches of the femoral nerve refer hip pathology to the knee; this is why the knee was examined twice and the hip was not.
The AP pelvis report of no fracture is not reassuring. Slippage is best seen on a frog-leg lateral view, and I would also obtain a cross-table lateral of the hip. The key measurement is Klein's line: a line drawn along the superior border of the femoral neck should intersect the lateral part of the epiphysis; if it does not, the epiphysis has slipped. I would assess the slip angle (Southwick) and, more importantly, whether the slip is stable or unstable using Loder's classification: stable if he can bear weight with or without crutches, unstable if he cannot. He is currently using a stick and can partly weight-bear, but that should be recorded precisely.
The system fix matters as much as the diagnosis. Any child aged 8–14 with knee pain and a limp needs a hip examination and a frog-leg lateral before discharge. I would raise this with the emergency department and add it to the referral pathway, because a delay of months is the main risk factor for a worse slip and for later avascular necrosis and femoroacetabular impingement.
2. How would you treat him, and how would you counsel the family about the contralateral hip and the risks?
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For a stable slip, the treatment is in-situ fixation with a single cannulated screw placed perpendicular to the physis in both planes, under image intensifier control. I would not attempt a reduction in a stable slip: the deformity is chronic and reduction adds risk of avascular necrosis without clear benefit. The screw should be central in the epiphysis and must not penetrate the joint; penetration causes chondrolysis and joint damage, so I would check the position on both views before leaving theatre. The screw head should sit close to the cortex to allow later growth.
The unstable slip is a different problem: the blood supply is at risk, avascular necrosis rates of 20–40% are quoted, and there is debate about gentle reduction, capsulotomy and decompression of the intracapsular haematoma. I would treat an unstable slip as an emergency and undertake gentle reduction on a fracture table only if the position is unacceptable, with a capsulotomy.
Contralateral prophylactic pinning is the other decision and must be discussed explicitly. The risk of a subsequent slip on the other side is roughly 20–40%, and with an 11-year-old boy with a very high BMI and open physes that risk is high. My inclination would be to pin the contralateral hip prophylactically after explaining the roughly 1% risk of avascular necrosis and the small risk of later screw problems, and that it commits him to follow-up. If the family declined, I would follow him closely until physeal closure.
Consent must cover avascular necrosis, chondrolysis, wound infection, screw migration, and the possibility of later surgery for impingement or deformity.
3. What follow-up would you arrange, when would you investigate for an underlying endocrine cause, and how do you counsel about the long term?
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Follow-up should continue until both physes close, with clinical review and radiographs at intervals of about 3–6 months in the first year to check screw position and growth, and then annually. I would look specifically for screw migration, physeal closure and the development of a cam-type deformity at the head–neck junction.
An underlying endocrine cause should be suspected when the child is younger than about 8–10 years, is unusually short or has atypical growth, or has a recognised syndrome, hypothyroidism, growth hormone excess or renal osteodystrophy. This boy is 11, has just had a growth spurt and has a very high BMI, which is the common idiopathic phenotype. I would check thyroid function and, if there were any atypical feature, proceed to endocrine referral; routine screening is not indicated in a typical adolescent.
The long-term conversation should cover three things. First, the residual deformity alters hip mechanics and is a recognised cause of femoroacetabular impingement and early osteoarthritis, so he will need surveillance into adulthood; some patients later need osteochondroplasty or a realignment osteotomy. Second, weight management is the single most useful long-term intervention and should be addressed now rather than after the arthritis appears. Third, he should be advised that the other hip may slip, and to return urgently with any new groin or knee pain rather than wait for another 'sprain' diagnosis.