IHDI method is a new radiographic classification of the severity of hip dislocation in DDH. It is based on the location of the midpoint of superior part of ossified metaphysis (H-point) relative to acetabulum. In contrast to Tonnis classification method, IHDI method:
- Has superior reliability
- Can be applied reliably even when the ossification center is absent
- Uses Hilgenreiner’s line (H-line) instead of Superolateral margin of acetabulum line (SMA-line)
Tonnis classification:
It is assessed according to the relative position of the femoral proximal ossific nucleus to Perkin’s line (P-line) and the superolateral margin of the acetabulum line (SMA-line). The P-line is a perpendicular line from the superolateral margin of the acetabulum SMA, and the SMA-line is a single line drawn through the superolateral margin of the acetabulum bilaterally.
- Grade I: Ossific nucleus is medial to P-line (Infero-medial quadrant)
- Grade II: Ossific nucleus is lateral to P-line but inferior to SMA-line (Infero-lateral quadrant)
- Grade III: Ossific nucleus is lateral to P-line leveled to SMA-line
- Grade IV: Ossific nucleus in superior to SMA-line (Supero-lateral quadrant)
IHDI classification
The position of the proximal femoral metaphysis (instead of the ossific nucleus) is used as the important reference landmark to determine the position of the femoral head. This classification requires supine AP X-ray of pelvis with the hips at rest while the lower limbs are gently held in neutral position without traction and patellae forward.
- H-point is the midpoint of the superior margin of the ossified metaphysis
- H-line is drawn through the top of the tri-radiate cartilages bilaterally
- P-line is then drawn perpendicular to the H-line at the superolateral margin of the acetabulum
- An additional diagonal line (D-line) is then drawn 45 degrees from the junction of H-line and P-line
The position of the H-point then determines the IHDI grade:
- IHDI grade I: H-point is at or medial to the P-line
- IHDI grade II: H-point is lateral to the P-line and at or medial to the D-Line
- IHDI grade III: H-point is lateral to the D-line and at or inferior to the H-line
- IHDI grade IV: H-point is superior to the H-line
References:
1. IHDI Standards – International Hip Dysplasia Institute
2. Pasupathy B, Sathish M. Reliability of a New Radiographic Classification System for Developmental Dysplasia of the Hip. International Journal of Paediatric Orthopaedics Jan-April 2020; 6(1): 16-19.
He is the section editor of Orthopedics in Epomedicine. He searches for and share simpler ways to make complicated medical topics simple. He also loves writing poetry, listening and playing music. He is currently pursuing Fellowship in Hip, Pelvi-acetabulum and Arthroplasty at B&B Hospital.