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Table 1 The Graf classification system of developmental dysplasia of the hip, based on sonographic angles of the hip

From: Tübingen hip flexion splints for developmental dysplasia of the hip in infants aged 0–6 months

TypeDescriptionBony RoofBony RimCartilage Roofα-angleβ-angle
IMature hipGoodAngular/bluntCovers the femoral head≥60< 77
IIaPhysiological (< 3 month)DeficientRoundedCovers the femoral head50–59> 55
IIbDelay of ossification (> 3 month)DeficientRoundedCovers the femoral head50–59< 55
IIcCritical hipSeverely deficientRounded to flattenedStill covers the femoral head43–49< 77
DDecentering hipSeverely deficientRounded to flattenedDisplaced43–49> 77
IIIDislocated hipPoorFlattenedPressed upward, perichondrium slopes cranially< 43> 77
IVDislocated hipPoorFlattenedPressed downward, perichondrium is horizontal or dips caudally< 43