The Trethowan sign is used to diagnose slipped capital femoral epiphysis (also called slipped upper femoral epiphysis). In this sign, the line of Klein passes above the femoral head 1.
It is best seen on the frontal projection 2. On the AP view of a normal hip, the Klein line intersects a small portion of the lateral capital epiphysis.
In typical posterior displacement of the capital epiphysis, the line intersects a smaller portion of the epiphysis or does not intersect it at all - the positive Trethowan sign.
It is one of the signs that may help in the initial phase (mild slip) with typical posterior displacement, where AP radiographic findings are known to be subtle.
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