Hip Center

In principle, the goal of the cup placement height is the true anatomical position. Previous biomechanical studies have shown that the true anatomical position is the most efficient position for the abductor muscles to exert an abduction force with the least stress on the cup [1]. This is also understandable from the viewpoint of the postoperative range of motion and impingement. Regardless of whether a cemented or cementless cup is used, the concept of aiming for the true anatomical position to obtain sufficient bony coverage and strong initial fixation is commonly accepted. However, in Japan, most patients with osteoarthritis of the hip joint who are candidates for total hip arthroplasty (THA) have secondary osteoarthritis of the hip joint due to acetabular dysplasia. In dysplastic hips (especially Crowe grade 2 or higher), acetabular deformity and hypoplasia may result in inadequate bony coverage of the acetabular cup in the true anatomical position. Insufficient bony coverage of the cup is an important problem that affects initial fixation and cup laxity [2, 3]. In highly dysplastic hips, placing a cup in the true anatomical position results in a large residual bone defect above the cup, necessitating augmentation or the use of a massive bone graft to augment the cup fixation. When a cementless cup is used, care must be taken because the risks of cup laxity, migration, and collapse of the grafted bone are high if a large, massive bone graft is placed with insufficient bony coverage of the cup by the matrix bone [4]. Elevation of the cup position is a technique that can be used to increase contact with the host bone and obtain sufficient bony coverage by raising the cup height in severe dysplastic hips (Fig. 43.1). In the case of cemented cups, it has been reported that high cup placement results in a high rate of implant loosening [5–7], In principle, the primary cup is placed in the primary position using bone grafts, such as massive bone grafts, impaction bone grafts, and KT plates [8–10]. However, in the case of cementless cups, the concept of high cup placement has been widely accepted in recent years because of its good long-term results and the evolution of cup surface treatment [11]. However, there are concerns that elevated cups may increase stress on the cups and decrease abductor muscle strength; moreover, there is still room for debate as to what level of elevation is acceptable from the perspective of joint biomechanics and what level of bone covering is necessary. Previous biomechanical tests using cadaveric bones reported that the stress on the cup did not increase significantly even when the cup was placed 3.7 cm above the teardrop line [12, 13]. In both studies, lateral placement of the cup was cited as a defective factor. The results of other basic and clinical studies have also reported many problems with lateral placement of the cup, and caution should be exercised because it may have a negative effect on treatment outcomes [1, 2, 14]. When considering the appropriate cup height, it is important to fully understand the advantages and disadvantages of elevation (Table 43.1). The installation should be made close to the original mortar site to avoid unnecessary cup elevation.