Dislocation of the THA is defined as a loss of contact between the femoral head and the acetabular component, which requires intervention to relocate the joint. Many factors contribute to stability and dislocation in patients with THA, including surgical factors such as soft tissue tension, component positioning, and femoral head size [19].
The surgical approach has been recognized as a potential factor influencing THA stability and abductor function [20]. Exposure of the hip and proximal femur requires the division of the posterior hip capsule and external rotators. The main disadvantage is a higher posterior dislocation rate [21, 22]. Despite the improved clinical results observed with posterior soft tissue repair, skepticism remains regarding the long-term integrity of the results [23]. Deficiencies in native tendon-to-bone attachment have been reported in 43% of piriformis repairs and 57% of conjoined tendon repairs [24]. After tendon-to-bone repair, deficient tendon–bone interfaces are initially filled with fibrovascular tissue, following which tissue remodeling and scar formation between the tendon and bone occur [25]. After 3 months, muscle atrophy may lead to a loss of function in those undergoing repair of the short external rotator tendons [24]. However, enhanced posterior capsule and anatomic short external rotator repair have been associated with improved postoperative stability at the 4-year follow-up when the short external rotator and posterior capsule are repaired in separate layers using a transosseous technique and number 2 Ethibond sutures. Notably, patients were mobilized without weight-bearing restrictions on the first postoperative day and were instructed to avoid hip flexion greater than 90° and any internal rotation for 6 weeks after surgery [26]. Another study reported that dislocation rates for the posterior approach can be significantly reduced to as low as 0.7% when anatomical repair of the posterior capsule and external rotators is combined with increased anteversion of the cup component [27]. Thus, we suspect that lack of repair for the posterior capsule, rough short external rotator repair, and limited post-operative rehabilitation may have impaired structural and functional healing of the posterior hip envelope.
Impingement is considered the final common pathway for instability and dislocation [28]. Component orientation and implant choice both directly affect the safe range of motion in THA, with impingement potentially occurring as a result of component malpositioning; suboptimal head diameter, head-neck ratio, or geometry; or socket depth. Impingement has been defined as a mechanical abutment between the metal femoral neck and cup liner, or as bone-to-bone contact, such as that between the greater trochanter and pelvis [29]. Osseous impingement and soft tissue tension can only decrease this range of motion. Therefore, optimum positioning of the components is necessary to avoid a decrease in the stable range of motion owing to prosthetic impingement.
Although component positioning has been shown to play an important role in hip stability and risk of postoperative dislocation, there is currently no consensus regarding the safe zone for acetabular component positioning. These differences among studies may be attributable to differences in the surgical approach, methods of measuring component positioning, and limitations in statistical power [13, 30–32]. Computer modeling studies have indicated that the abduction angle for optimal cup position ranges from 45–55°. Angles < 55° require an anteversion of 10–20° for both the stem and cup to minimize the risks of impingement and dislocation [33]. Studies on the 3-D orientation of the acetabular cup have shown that the majority of dislocations have an acetabular cup position that resides within the “safe zone” [34]. Recent studies have questioned the validity of this so-called “safe zone” in explaining dislocations and the variety of definitions used [35]. These insights also require reevaluation of the traditionally advised orientation of the femoral component. When regarding the “safe zone” for combined anteversion of 25°–50°, a widespread variation in results can be observed in the literatures. The recommendations and “safe zones” used to date fail to predict or explain the majority of dislocations [36]. In this study, the control group had a combined anteversion angle > 44°, while the posterior dislocated THA was within the “safe zone”. It cannot be concluded that THA with low or high combined anteversion is prone to dislocation.
In contrast to that of the femoral component, the influence of acetabular component orientation on the risk of dislocation has been extensively investigated. Traditionally, a “safe zone” between 10° and 15° anteversion of the neck of the femoral stem in the transverse plane has been used as a guideline for placement of the femoral component [37]. Other studies reported substantially higher anteversion values. One study described wide variations in femoral anteversion in the standing position. Before surgery, more than 80% of patients had values outside of the “safe zone”—a rate that increased to 85% after THA [38]. In another study, similar variations in native femoral anteversion were observed (–15º to 30°) [39]. Whether femoral component anteversion affects hip joint stability remains controversial. One study claimed that there is no evidence on the optimal femoral component anteversion or for the “safe zone” of 10° to 15° [36], although some studies have demonstrated that low femoral anteversion is associated with a higher risk of posterior dislocation and that patients with high femoral anteversion were at risk for anterior dislocation [40, 41].
According to our results, the horizontal and vertical hip centers of rotation of the acetabular component remained unchanged after the procedure (p = 0.148 and 0.573, respectively) without inward or upward shifts of the socket, which would not decrease the offset of the hip or substantially increase the risk of bone-on-bone impingement [42]. In our cohort, the dislocation rate was 13.73%. However, a significantly decreased abduction angle and insufficient stem antetorsion was observed in patients with dislocation vs. controls. Our results suggest that the influence of femoral component anteversion on hip stability has been underestimated. We hypothesize that patients with certain pelvic dynamics are at a higher risk of THA instability. Other authors have suggested that patients may develop instability despite optimal component orientation because they have abnormal spinopelvic dynamics characterized by restricted pelvic tilt from the standing to sitting position [43]. In our cohort, dislocation usually occurred when patients tried to pick up an object from the ground with deep flexion or internal rotation of the flexed hip joint and knees. Likewise, noncompliance is more prevalent in these patient populations, and such actions are not strictly avoided [44]. During these movements, component malpositioning and insufficient stem antetorsion may have shifted the neck of the prosthesis posterosuperiorly, increasing the workload on the short external rotator. In such cases, inadequate tissue tension and joint hyperlaxity cannot stabilize the femoral head in the acetabulum. Combined with a lower abduction of the acetabular component, this may result in increased contact between the neck of the prosthesis and the superior margin of the cup component, leading to primary impingement and posterior dislocation of the femoral head.
This study had some limitations, including its small sample size and retrospective design. The monocentric nature of the data and lack of randomization may have also resulted in selection bias. Dislocation after THA is a multifactorial problem, and this study addressed only parts of this issue. Some parameters were measured on plain AP radiographs, the accuracy of which is inevitably affected by radiological positioning and interobserver variability. Further, we did not evaluate the integrity of the posterior soft tissues using MRI or ultrasound, and the variable-magnifying effect of the soft tissue may have resulted in overestimation of the measurements. However, despite these limitations, we believe that our study provides new insight into the risk factors for hip dislocation in patients undergoing primary cementless THA for FNF.