Study of the three-dimensional orientation of the labrum: its relations with the osseous acetabular rim

Abstract

Understanding the three-dimensional orientation of the coxo-femoral joint remains a challenge as an accurate three-dimensional orientation ensure an efficient bipedal gait and posture. The quantification of the orientation of the acetabulum can be performed using the three-dimensional axis perpendicular to the plane that passes along the edge of the acetabular rim. However, the acetabular rim is not regular as an important indentation in the anterior rim was observed. An innovative cadaver study of the labrum was developed to shed light on the proper quantification of the three-dimensional orientation of the acetabulum. Dissections on 17 non-embalmed corpses were performed. Our results suggest that the acetabular rim is better represented by an anterior plane and a posterior plane rather than a single plane along the entire rim as it is currently assumed. The development of the socket from the Y-shaped cartilage was suggested to explain the different orientations in these anterior and posterior planes. The labrum forms a plane that takes an orientation in between the anterior and posterior parts of the acetabular rim, filling up inequalities of the bony rim. The vectors V(L) , V(A2) and V(P) , representing the three-dimensional orientation of the labrum, the anterior rim and the posterior rim, are situated in a unique plane that appears biomechanically dependent. The three-dimensional orientation of the acetabulum is a fundamental parameter to understand the hip joint mechanism. Important applications for hip surgery and rehabilitation, as well as for physical anthropology, were discussed.

Fig. 1

Lateral view of a left acetabulum. The anterior and posterior horn tips are indicated by the letters A and D. Along the rim two inflexions can be systematically identified: B represents the most cranial inflexion where the indentation of the anterior part of the rim appears; C, the most caudal inflexion, was placed at the beginning of the curvature of the posterior horn. B and C allow the division of the rim in two parts named the anterior and posterior acetabular rims. The landmarks 1 and 2 (indicated by crosses) correspond to the insertion of the ilium and the ischium on the acetabular rim.

Fig. 2

Successive points 1 mm apart were digitised along (A) the intact labrum, (B) the totality of the acetabular rim, (C) the posterior part of the acetabular rim and (D) the anterior part of the acetabular rim. With these data four planes named, respectively, P_L, P_T, P_A and P_P combined with their respective direction vector V_L, V_T, V_A and V_P were obtained. Due to a methodological problem (see text), the anterior rim was modelled using a regression plane P_A2 based on points A, B and C (large points in D) resulting in a vector V_A2.

Fig. 3

The planes P_A2 and P_P, computed using the anterior and posterior acetabular rims, are not in the same plane but they formed a mean angle of 158.6 °. (a) A general view illustrating the plane P_A2 of the anterior rim (in red) and the plane P_P of the posterior rim (in blue). A transversal slice was observed along the dotted line and present in (b). (b) The vector V_A2 (red dotted line) and V_P (blue dotted line) form a mean angle of 21.4 °. The plane of the labrum is represented by the grey dotted line.

Fig. 4

(a) There is a significant positive correlation between the angle V_AV_P and the angle V_LV_A (r_Pearson = 0.83; P < 0.0001; N = 31), which is represented in the graph by a regression line (grey solid line). The light grey dotted line represents shapes for which V_L would be exactly at the middle of V_A2 and V_P. A comparison between the two lines shows that V_L is generally closer to V_A2 than V_P, and that V_L is near to an intermediate position between V_A2 and V_P when the angle V_A2V_P decreases. (b) Two cases were represented to illustrate the changes of the relationships between V_A (in red), V_P (in blue) and V_L (in green) when the angle V_A2V_P takes its minimal or maximal value (individuals 1 and 2, respectively, marked by arrows in a). These two cases are viewed in the plane of the three vectors V_L, V_A2 and V_P, i.e. at the perpendicular to P_C. The hatched areas illustrate the height of the labrum in its anterior and posterior parts.

Fig. 5

The irregularity in the morphology of the acetabular rim can be explained by the Y-shape of the growth cartilage (solid lines). The posterior part grows from the ilio-ischiatic branch, while the anterior part is formed by the ilio-pubic and ischio-pubic branches. Each branch of the Y cartilage is not submitted to the same constraints, resulting in growth in a different direction. The acetabulum is integrated in the complex biomechanical system of the hip bone. The acetabulum is surrounded by a network of different trabecular systems (thick dotted lines), which is load history dependent. A link between the orientation of the plane P_C and the biomechanical loading context was identified. The points A–C, which correspond more generally to the intersection of the pubis, ilium and ischium with the acetabular rim, are located in the area of the transition between the different trabeculae, i.e. in the least constrained area. Our results showed that the plane P_C is precisely aligned with the axis formed by B and C (thin dotted line) and perpendicular to the axis formed by O and A (thin dotted line), suggesting an effect of biomechanical loading on the orientation of P_C. The scan of this left hip bone was performed using a Breuckmann® surface scanner and the view was chosen to display concurrently the anterior and posterior networks of the trabecular system, the sacroiliac joint from which they emerge, and to illustrate that A–C correspond to areas of transition between the different trabeculae.