*3.2. Alumina-on-Alumina Total Hip Replacement in Developmental Dysplasia of the Hip*

THA in sequelae of developmental dysplasia of the hip (DDH) is a challenging and difficult procedure in terms of restoring the normal biomechanics. In case of dysplastic patients, the pre- and intraoperative process of decision making is also complex due to the underdeveloped acetabulum, the contracture of the surrounding soft tissues and often the high riding head; on the pelvic side, the choice is frequently linked to the need of using small-diameter acetabular cups because DDH causes a lack of bone under the prosthesis, and sometimes, it may not be possible to place a large acetabular cup in the anatomical position with a medialization close to the radiological tear drop because of the hypoplasic acetabulum and thus the risk of miscoverage of the shell (Figure 2). We are convinced that obtaining the correct location of the acetabular cup, and thus providing the best function of the artificial hip, is dependent on the outer diameter of the shell because, typically, a cup with a small diameter is used, and as a result, the thickness of the acetabular insert decreases, the two components being inextricably linked. We are aware of manufacturer information stating that a 32 mm HXLPE inlay may only be utilized with a cup diameter of 52 mm or more [30]. The presence of small-diameter cups affects the size of the insert which will be proportionally smaller. The use of the AMC in these patients is essential because it allows the combination of very thin inserts even in acetabular cups with a reduced diameter, combining them with large diameter heads [31]. Inadequate polyethylene thickness is identified as the source of plastic particle-mediated osteolysis, not only in conventional polyethylene [32] but also in vitamin E-diffused HXLPE, and hence significant liner thinning is not advised [33]: Higher peak contact loads and smaller contact areas result from a decrease in polyethylene liner thickness or headliner conformance, resulting in reduced biomechanical wear factors. On the other hand, since larger femoral heads have been widely advocated to improve implant stability and range of motion, especially in patients with spinopelvic alignment, such as patients with hip osteoarthritis secondary to DDH, prosthetic heads must be enlarged, implying a reduction in liner thickness [34–40].

**Figure 1.** Female, 14 years old, osteonecrosis of the femoral head in her left hip: preoperative radiological features in AP (**a**), axial view (**b**) and magnetic resonance imaging (**c**). Postoperative AP X-ray (**d**) of THA with a 32 mm AMC-on-AMC coupling.

**Figure 2.** Male, 55 years old, right hip osteoarthritis secondary to development dysplasia: preoperative AP and axial radiographs (**a**,**b**), and postoperative X-ray (**c**) of the AMC-on-AMC THA using screws for implementation of cup fixation and a conical stem below it.
