[Labrum lesions in residual dysplasia of the hip joint. Biomechanical considerations on pathogenesis and treatment].

The capsular-labrum-complex consists of the triangular fibrocartilaginous labrum, which is fixed at the bony acetabular ring and supported by the ligamentum transversum actetabuli in the caudal part. In a normally developed and correctly orientated acetabulum, the femoral head is symmetrically covered by the contact area of the lunate surface. The vertical component of the resultant hip joint force concentrically meets the horizontal weight bearing surface: Evenly distributed compressive forces are transmitted; the capsular-labrum-complex does not have to compensate excessive shear or tension. In contrast, in a "dysplastic" acetabulum the femoral head is poorly covered by the pathologically orientated lunate surface: The lunate surface is sloping in an anterolateral direction, leading to tension and shear on the superolateral capsular-labrum-complex, which becomes a secondary stabilizer ("guide rail") against the decentering femoral head. The vertical component of the resultant hip force eccentrically meets the oblique weight bearing surface, causing extra stress in the labrum: The capsular-labrum-complex is trying to compensate this biomechanical stress with hypertrophy (Type IB). Progressive mechanical decompensation of the capsular-labral-complex might lead to tears (Type IIB) or complete avulsions (Type IIIB) of the labrum. In conclusion labral lesions result from pathobiomechanics caused by residual hip dysplasia (RHD). Logically, the basic therapeutic principle can only be corrective osteotomy.