This study revealed that the postoperative JOA score of THA after RAO was comparable to that of the control group. In addition, radiologically, the RAO group showed increased lateral coverage of the acetabulum and characteristic bone defect in the posterior wall of the acetabulum due to the excessive anterior rotation of the osteotomy fragment. Hence, the cup was required to be positioned on the upper and lateral sides where there was more bone mass, which may cause limited range of motion and bony impingement.
RAO is a joint-preserving surgery in which the acetabulum is osteotomized into a spherical shape and rotated laterally to increase the coverage of the femoral head by the acetabulum to improve joint congruity. It is commonly performed in young and adolescent patients with DDH [15, 16]. While positive postoperative outcomes of RAO have been reported , there are cases of advanced osteoarthritis leading to THA [17, 18]. It has been reported that THA after RAO is more challenging to perform due to bone deformity and the operative time is significantly longer than that of primary THA . In addition, bone defects in the posterior acetabular wall with large osteophytes are factors that complicate THA after RAO when compared with after Chiari osteotomy and shelf acetabuloplasty . As shown in these reports, bone defects in the acetabular wall are characteristic of RAO. However, there are no reports that have examined bone coverage in the anterior-posterior direction in CT after THA, as ACE and PCE angles. In this study, the osteotomy fragments were rotated anteriorly to increase the anterior bony coverage, resulting in a significantly lower PCE angle than that in the control group; thus, posterior acetabular bone defect was more likely to occur. It has been reported that in THA after RAO, the cup was often placed more laterally compared to that in primary THA . When comparing the RAO group to the control group, the hip center was laterally and superiorly positioned in the RAO group. This was thought to be the result of placing the cup in the superolateral region, where there was more bone volume, according to the shape of the RAO acetabulum. These results suggest that the posterior acetabular bone defect and cup position should be carefully considered in THA after RAO.
Bulk bone grafting is useful for acetabular defects [19, 20]; however, dislocation of grafted bone can occur in bulk bone grafting for bone defects after RAO . We performed bulk bone grafting for a posterior acetabular bone defect, and the grafted bone survived without any postoperative dislocation. While it is necessary for the cup to be placed in an area of high bone mass to obtain good initial fixation, care should be taken in high placement. Elevating the center of the hip increases the bone coverage of the cup, but it also decreases the range of motion and is a risk factor for dislocation and THA failure [21–23]. Although the cup requires placing as close to the original acetabulum as possible, there is no difference in clinical outcomes or implant survival if the center of the head is no higher than 35 mm above the inferior edge of the teardrop in primary THA . However, in THA after RAO, the osteotomy fragment is rotated more anteriorly, so the possibility of impingement of the anterior acetabular osteophyte, cup, or anterior inferior iliac spine with the femur is even higher with a high hip center. Therefore, careful preoperative planning and intraoperative confirmation of impingement and resection of the impinging bone are necessary. These were thought to be the reasons why the postoperative range of motion of THA after RAO was worse than that of the control group.
Although it has been known that bone defects in the anterior and posterior acetabular walls occur after RAO, this study is the first to evaluate bone defects in the anterior and posterior acetabular walls in THA after RAO using sagittal sections of CT. There were various limitations in this study. First, this was a retrospective study and patients were not randomized. There was no significant difference in patient background in terms of sex, age, follow-up period after THA, or surgical approach, although there could be bias due to unmeasured factors. Second, some patients are only a few years post-operative, and long-term follow-up data for such patients is not yet available. Because of the strong deformation of the acetabulum in these patients compared to those with primary THA, we will continue to follow-up the survival rate of THA, especially the cup survival rate, dislocation rate, and clinical evaluation over time.