Purpose: Coxa magna is well known in Perthes’ disease but a quantitative evaluation of the early, in particular cartilaginous, enlargement of the femoral head and the necessary adaptive changes of the acetabulum (widening and/or growth) does not exist. We would like to present MR-based volumetric data.

Methods: We measured the volume of the femoral head and the acetabulum in MRI by means of a software established for the sizing of tumours before therapy and the determination of liver lobe volume prior to transplantation.

We evaluated MRI exams in 47 children with Perthes’ disease and 72 normal children from 4 to 9 years and present data of the affected hip in comparison to the unaffected hip and to normal hips.

Results:

Femoral head:

On the average the affected head had a volume that was 47% (range 42 – 57%) larger than on the unaffected side and 44 % (range 13 – 59%) larger than in hips of healthy children.

Cases with serial exams showed that the volume of the affected head increased in the course of time.

Acetabulum:

On the average the acetabular volume was 21% (range 13 to 30%) larger on the affected side than on the unaffected side and 20% (range 10 to 29%) larger than in healthy children.

In patients who underwent surgery (pelvic osteotomy, alone or together with intertrochanteric varus osteotomy) the acetabular volume was 24% larger (range 9 – 33%) on the affected side than on the unaffected side. In patients without surgery the acetabular volume was 16% larger (range 10 to 33%) on the affected side.

Conclusions:

We found that Perthes’ disease is associated with an average increase of femoral head volume of 47% in comparison to the unaffected side and of 44% in comparison to healthy children.

There was an average increase of the acetabular volume of 21% in comparison to the unaffected side and of 20% in comparison to healthy children.

These data may allow a better understanding of the disease and a reappraisal of current forms of treatment.

Significance: Given a chronic disproportion between the size of the femoral head and the acetabulum therapy should aim at:

Retardation of the (cartilaginous) enlargement of the femoral head

Promotion of widening or growth of the acetabulum.

We believe that current conservative modes of treatment are effective through rationale A and B.

Operative modalities, in particular pelvic osteotomies and/or intertrochanteric varus osteotomy, seem to be mainly effective through rationale B. By reorientation of the acetabulum and/or the proximal femur they should favour a better distribution of forces through the hip joint allowing for a gradual widening of the acetabulum. In addition, the operative trauma in the vicinity of the triradiate cartilage may have a stimulating effect on acetabular growth.