Teaching Case

A 9-year-old boy with a history of prostatic embryonal rhabdomyosarcoma, treated with chemoradiotherapy and transurethral resection, presents with limping and hip pain on the left side. Lab results were normal. The boy was of normal weight and height. There was no recent trauma.

On the suspicion of transient synovitis of the hip, an ultrasound was performed. A unilateral joint effusion in the left hip was confirmed. However, when compared with the contralateral side, there was not only a joint effusion, but also an asymmetrical shape of the femoral head with a malalignment of 8 mm between the epiphysis and metaphysis. A pelvic MRI 9 months prior, showed no abnormalities in both hip joints.

After consultation with the paediatrics and orthopaedics department radiographic anteroposterior (AP) and frog-leg views of the pelvis were taken. Both views showed a clear slipped upper femoral epiphysis (SUFE). There also was a clear antalgic pelvic tilt to the left while performing the frog-leg position.

Slipped upper femoral epiphysis (SUFE) is a type I Salter-Harris growth plate injury and is the most common hip disease of adolescence, with an incidence of 4.8/100000 in 0–16-year-olds [1]. SUFE occurs when repeated shearing forces applied on the femoral head exceed the strength of the capital femoral physis. Risk factors include: normal periosteal thinning and widening of the physis during rapid growth acceleration, trauma, obesity, inflammation, genetic predisposition, endocrine and metabolic disorders (hypothyroidism, hypopituitarism, hyperparathyroidism, renal osteodystrophy) and (as is the case in our patient) radiation and/or chemotherapy [2,3].

Plain radiographs are the most common imaging tool to evaluate SUFE. The minimal examination consists of an AP view and lateral projections of both hips. The lateral projections are most often obtained with a frog-leg view. When acute instability is suspected, alternatives like the cross-table lateral or true lateral pelvic view might be preferred to reduce the risk of further displacement. In the case of extreme obesity, other views like the unilateral Lauenstein view might be considered [4,5]. As the slip of the epiphysis is directed posterior and only to a lesser extent medial, lateral views are essential [6].

In the pre-slip stage, there is widening of the physis, with irregular and blurred edges of the metaphysis. Once the epiphysis has slipped posteriorly, it might appear smaller on AP views. The line drawn along the superior edge of the femoral neck (Klein’s line) on AP views fails to intersect the lateral portion of the femoral head as it passes outside of the epiphysis, known as the Trethowan sign. However, as this sign has a very low sensitivity, an overlap of 2 mm might be suggested as a threshold to increase sensitivity for SUFE [7]. Another sign of SUFE on AP views is the metaphyseal blanch sign, where superposition of the femoral neck and the posteriorly displaced capital epiphysis results in increased density [8]. On lateral views Southwick’s head-shaft angle is used to grade the severity of SUFE. A slip-angle of <30° is considered mild, 30-30° moderate and >60° is considered severe [8,9].

Ultrasound has little role in the routine evaluation of SUFE. Its use has been described in the diagnosis and follow-up of SUFE, and might even confirm the diagnosis when plain radiographs are normal. However, MRI is more commonly used as it can easily detect widening of the physis and surrounding oedema. Furthermore, MRI can assess the extent and distribution of osteonecrosis, if present [8].

All patient data have been completely anonymised throughout the entire manuscript and related files.