Shepherd’s crook deformity | Radiology Signs

Answer:

A shepherd’s crook deformity is severe coxa vara, with the femoral neck-shaft angle reduced well below normal, combined with lateral bowing of the proximal femoral shaft, so that the proximal femur takes the shape of a shepherd’s crook. It is the most characteristic deformity of polyostotic fibrous dysplasia and of McCune-Albright syndrome, and is also described in Paget disease and osteogenesis imperfecta. Radiographs show the deformity together with the underlying lesion: expansile, ground-glass medullary lucency with endosteal scalloping, cortical thinning and often a well-defined sclerotic rim. It carries a high risk of pathological fracture, progressive limb shortening and painful gait, and correction is generally considered when the neck-shaft angle falls below about 120 degrees. Surgery is difficult because the dysplastic bone holds fixation poorly and the deformity recurs; valgus osteotomy with intramedullary rather than plate fixation is favoured, sometimes combined with curettage and grafting, and no single technique is agreed.

Why is it called so?

The varus neck with the bowed shaft below reproduces the curved hook of a shepherd’s crook or staff.

Pathophysiology

In fibrous dysplasia a post-zygotic activating GNAS mutation causes the marrow space to be replaced by structurally weak fibro-osseous tissue containing immature woven bone that never matures into lamellar bone. This tissue cannot bear load, so the proximal femur, which carries the highest bending moments in the skeleton, deforms progressively under body weight: the neck angulates into varus and the subtrochanteric shaft bows laterally. Repeated microfractures through the weak bone accelerate the process, and because the abnormal tissue continues to be produced, correction achieved surgically tends to be lost over time.

Alternative names: Shepherd crook deformity

Other associated named signs: Ground-glass matrix and the rind sign of fibrous dysplasia, café-au-lait lesions with a coast-of-Maine border, and the leontiasis ossea of craniofacial involvement

References

1. Ippolito E, Farsetti P, Boyce AM, Corsi A, De Maio F, Collins MT. Radiographic classification of coronal plane femoral deformities in polyostotic fibrous dysplasia. Clin Orthop Relat Res. 2014;472(5):1558-67. PMID: 24249535.