Normal radiographic values for cartilage thickness and physeal angle in the pediatric hip

Ninety-three standing anteroposterior (AP) pelvis roentgenograms in 87 patients were measured for a total of 186 normal hips in children aged 1-17 years. For each hip, the physeal angle relative to the floor, the physeal angle relative to the pelvis, the cartilage thickness perpendicular to the floor, and the cartilage thickness perpendicular to the physis were measured and recorded. The physeal angle varied from ages 1-7 years, stabilizing at age 8 at a mean of 23 degrees . Physeal angle is best measured relative to the floor because pelvic obliquity introduces significant variability to the measurements. Cartilage thickness ("joint space") declined after age 7 years, with measurements in three statistically distinct groups. There was a statistically significant difference between cartilage-thickness measurements of boys versus girls, with girls showing a slightly smaller cartilage thickness than boys. Cartilage thickness measured perpendicular to the floor was not statistically significantly different from that measured perpendicular to the physis. We describe and recommend standard measurement techniques for physeal angle and cartilage thickness. These established normal values may be helpful in the diagnosis and evaluation of coxa vara and chondrolysis, and in identifying the head at risk for slipped capital femoral epiphysis or Legg-Calvé-Perthes disease.