In this retrospective cohort study, acetabular erosion occurred at a relatively high frequency when BHA was performed in young patients, increasing the conversion rate to THA. This supported our hypothesis and demonstrated that acetabular erosion after BHA was highly related to age at the time of surgery and BMI, which was closely related to the patients’ activity but had little relationship to other factors including poor bone quality. The strengths of this study were the analysis of relatively large follow-up data from young patients and the study design in which prostheses from a single manufacture were implanted in consecutive patients by one surgeon.
Many reports have presented a comparison of the results of BHA and THA [3, 11, 12]. BHA has an advantage in terms of preventing postoperative dislocation and is considered a good option when the acetabular cartilage is preserved in old-aged patients [1–3, 6, 7]. Moreover, BHA is still used as a salvage procedure for patients with poor general condition or tumors [13, 14]. In the case of THA, despite its excellent postoperative functional outcomes, its use is limited in patients with old age or musculoskeletal disorders due to dislocation-related problems. In recent years, the frequency of use of BHA had gradually decreased because of the wear resistance of a polyethylene liners, increased use of ceramic bearing, and dual mobility in THA. However, BHA is still required in some cases, determination of long-term survival after BHA requires reports of the long-term results. However, long-term follow-up is very difficult owing to low life expectancy and low compliance in old-aged patients. For these reasons, we targeted young patients who are relatively easy to follow-up. Further, the association between acetabular erosion and the patients’ activity could be analyzed by targeting younger patients with higher activity than older patients.
Several reports have been published on the risk factors associated with acetabular erosion after BHA. Hsu et al. concluded that increased leg length was significantly associated with early acetabular failure after BHA for femoral neck fracture among geriatric patients [15]. For a > 6 mm increasing in leg length, the OR of early acetabular failure was 25-fold greater than that in the control group. Kwok et al. surmised that leaving a longer neck may cause overtightening of the periprosthetic soft tissues leading to increased stress across the hip joint, and resultant increased wear [16]. However, their data were limited owing to the small number of patients, and, in reality, the goal of hip arthroplasty is to prevent leg length discrepancy. In elderly patients, the soft tissue tension during surgery is weak and surgery is often performed for a longer leg length. At present, rather than increasing the leg length, it is necessary to re-check the offset or evaluate and correct impingement or the soft tissue condition. Therefore, we did not analyze postoperative leg length discrepancy as a risk factor for acetabular erosion after BHA.
Other studies suggested that a smaller femoral head size is another risk factor associated with acetabular erosion after BHA. Schiavi et al. reported that a smaller head size of the BHA prosthesis leads to polar wear, implying a higher risk of acetabular erosion and migration. In their population, this risk was consistent with the use of an implant head < 48 mm in diameter [17]. A small head distributes all forces to a rather small area of articular cartilage within the acetabulum, whereas a larger head transmits all forces initially at the entrance to the acetabulum [18]. However, the femoral head size when performing BHA is actually the patient’s own measurement. In other words, a small or large femoral head size is only the operator’s relative choice for the patient’s native size. Therefore, the result according to the absolute size of the femoral head is important and no difference was noted in this study. That is, if the femoral head is accurately measured using a template before surgery and a calibre during surgery, it is reasonable to presume that it has no effect, as the present results showed.
In a study of 69 patients who underwent Thompson hemiarthroplasty, Phillips reported that the physical activity level and duration of follow-up had the highest correlation with the severity of acetabular erosion [19]. Moreover, obesity was also reported as a risk factor for acetabular erosion [20]. Presumably, increased body weight leads to increased wear of the acetabulum, causing more acetabular erosion. The present results showed that acetabular erosion after BHA was highly related to age at the time of surgery and BMI, which is closely related to the patients’ physical activity. However, sex, preoperative bone quality, and underlying disease were not associated with the occurrence of acetabular erosion after BHA. Studies on avascular necrosis of the femoral head demonstrated that degenerative changes of the acetabular cartilage are common in patients with osteonecrosis of the femoral head, even when radiographs of the acetabulum appear normal [21, 22]. In such cases, evidence showed that BHA resulted in unacceptably high failure rates, mainly owing to central migration of the prosthetic femoral head. In the multiple logistic regression analysis for the risk factors of acetabular erosion after BHA in this study, avascular necrosis of femoral head, which was statistically associated with acetabular erosion in the univariate model, was not an independent risk factor in the final multivariate logistic regression.
Among 44 reoperations after BHA, reoperation was performed in five hips for a cause other than acetabular problems, including stem loosening, periprosthetic femoral fracture, and recurrent instability. The incidence of postoperative periprosthetic fractures was reported to be higher among patients who underwent cementless BHA than that among patients who underwent cemented BHA (at 5 years, 5.7–15.2% in cementless BHA and 0.9–5.9% in cemented BHA) [23–25]. However, this issue cannot be discussed here because the high periprosthetic fracture rates in previous studies may be attributed to the technical challenges of surgery and/or the implant design during those earlier study periods.
In this study, the incidence of acetabular erosion after BHA continued to increase during the follow-up, and reoperation was performed at a mean of 10.2 years after surgery. In other words, it is difficult to expect long-term safety with BHA when considering reoperation for acetabular erosion in patients with an expected survival of > 10 years, or in those with good physical activity levels. Furthermore, when a considering a hip arthroplasty in young patients with a high risk of dislocation, the recently introduced dual-mobility-articulation THA may be a good option [26].
This study had some limitations. First, this was a single-center retrospective cohort study. However, we accounted for all postoperative radiologic outcomes in our consecutive patients. Second, although most of the patients had a femoral neck fracture, surgery was needed for various diseases. The possibility that the disease type requiring surgery affects acetabular erosion after BHA cannot be excluded. However, because the number of disease groups was not large, analysis by disease was not performed in this study. Third, a comparative analysis according to the femoral stem fixation method (i.e. cemented vs. cementless) was not performed. Cemented stems have the potential advantage of a reduced risk of periprosthetic fracture in the elderly population with poor bone integrity. However, cemented stems also may carry the risk of increased operative time and perioperative mortality secondary to fat and bone marrow emboli when compared to cementless stems. In this study, we focused on the long-term results related to acetabular erosion after BHA. Finally, 25% of our patients who underwent initial BHA were incompletely followed-up. Although patient compliance to clinical follow-up after BHA remains challenging, the 114 patients followed up for > 10 years do not represent a small number. These limitations are obvious obstacles to the generalization of our results, and further multicenter prospective studies are needed to verify their authenticity. We are also continuing further follow-up in these patients.