Capsular distance in the hip of the healthy child - normal values with sonography and MR imaging.

Abstract

To determine the normal values of the anterior and posterior capsular distances of the hip joint in healthy children by means of US, using MR imaging as reference, and to evaluate any possible correlation between age, length, weight and anterior capsular distance (ACD). In our first study both hips in 14 healthy children (5-18 years old) were examined with US and MR to obtain measurements of the ACD and the posterior capsular distance (PCD). The distance from the anterior or posterior aspect of the femoral neck to the anterior or posterior aspect, respectively, of the outer limit of the capsule was determined. The distances were measured both with the hips in spontaneous external rotation of 10-15 degrees and in internal rotation of 45 degrees. In our second study, both hips in 28 healthy children (3-16 years old) were examined with US to determine the ACD. Age, length and weight were recorded. Study I: There was good correlation between the US and MR measurements in all positions. The ACD measured by US was significantly increased in inward rotation of the hip. Study II: There was no correlation between ACD and age, length or weight. The PCD of the hip joint can be accurately measured by US with the hip in internal rotation of 45 degrees. When compared with MR values, the ACD measured by US was dependent on the degree of rotation of the leg and increased significantly in internal rotation. Because the outer limit of the external layer of the joint capsule is sonographically more distinct, we suggest that the capsular distance should be measured from the outer limit of the joint capsule to the anterior or posterior aspect of the femoral neck. The measurement should be made perpendicular to the femoral neck, at the position where the greatest numerical value is obtained.