The primary findings of the study were that patients undergoing revision arthroscopy for hip SC presented favorable clinical outcomes at minimum of 2-year follow-up, although the postoperative PROs, rate of achieving MCID, and PASS were lower compared to patients undergoing primary arthroscopy.

In the present study, primary arthroscopic treatment for hip SC demonstrated significant improvement of the PROs, which was consistent to previous studies [4,5,6,7,8]. Although some of the studies reported cases of recurrence, the clinical outcomes of this specific population were not reported and compared. To our expectation, the significant improvement of all of the PROs were also observed in the revision group, which suggested that revision arthroscopy could achieve favorable therapeutical effect. However, we found that patients in the revision group presented inferior postoperative PROs compared to the primary group. Similarly, these patients demonstrated lower rates of achieving MCID and PASS. The challenges associated with performing complete synovectomy and removing loose bodies during revision arthroscopy may exert a certain impact on the outcomes. Although arthroscopy has advantages of less invasion, reduced complications, better perioperative care, and fast rehabilitation compared to open surgery, it cannot provide easy access to the acetabular fossa and the posterior proximal aspect of the femur that are commonly affected by SC [5]. In our clinical practice, the management of revision arthroscopy treatment is challenged by the severe adhesion, elevated tension, and insertion of residual loose bodies into the erosive bone in these patients. It was reported that inadequate removal of synovium and loose bodies is one of the most common reasons for recurrence [3]. Therefore, we have developed and applied a standardized protocol in order to maximize the removal of loose bodies and synovium [15]. An extensive capsulotomy was performed for all patients to enhance accessibility. However, the accessibility is still limited and presents challenges, particularly in managing the posterior capsular and the region of obturator externus. Therefore, for the case presented persistent hip pain and residual loose bodies following revision arthroscopy in the present study, a subsequent open surgery was performed. The extensive capsulotomy also held the potential risk of residual loose bodies leaking [30]. Therefore, capsular repair was performed for all patients.

Among the 12 patients in the revision group, only 1 patient underwent second revision surgery and finally conversion to THA. Similarly, de SA et al. [3] conducted a systematical review and found a relatively satisfactory survivorship, with 1 out of 9 patients undergoing second revision surgery. However, caution should be exercised when interpreting the results of survivability due to the limited sample size and follow-up period.

In the present study, all of the demographic characteristics and radiographic parameters were comparable between the two groups. Relevant factors of recurrence have been preliminarily investigated in previous studies. Boyer et al. [9] and Zini et al. [4] suggested that increasing age may be a predictor to recurrence, while the similar results were not observed in other studies [6, 8]. The different exclusion criteria of OA may contribute to the difference, as Boyer et al. did not mention this aspect, and Zini et al. excluded patients with Kellgren-Lawrence grade > 2.

In addition to baseline factors, it is important to consider that various activities of SC may also exert an influence. Milgram et al. [17] classified SC into 3 stages. In Stage 1, metaplasia occurs in the synovial membrane without the presence of loose bodies. In Stage 2, progressive metaplasia results in the detachment of partially surrounded loose bodies by a synovial membrane. In Stage 3, metaplastic activity of the synovial membrane is restrained and multiple loose bodies can be observed within the joint. Boyer et al. [9] categorized patients into an active group (stage 1 and 2) and a quiescent group (stage 3), and compared the clinical outcomes. However, the recurrence rate was comparable between the two groups. Similar results were also found in the present study, as there was no significant difference in the proportions of Milgram stage 2 and 3 between patients who underwent revision and primary arthroscopy. In our clinical practice, loose bodies and pathological synovium containing gelatinous material were frequently observed under arthroscopy in a majority of patients from the revision group. Furthermore, we observed that revision surgery was performed in 8 out of 12 patients (66.7%) who presented radiographic recurrence within a 2-year period following primary arthroscopy. Although the correlation between various pathological features and recurrence remains unclear, we recommend surgeons to perform regular follow-up, particularly during the initial 2 years following primary surgery for these patients.

Interestingly, we observed a high prevalence of cam-type impingement concomitant to SC. FAIS is an abnormal contact between the proximal aspect of the femur and the acetabulum, which causes similar symptoms to SC such as hip pain and joint stiffness [2, 5]. Although idiopathic anatomic abnormalities of the femoral neck and the acetabulum are the most common causes of the cam- and the pincer-type impingement respectively, various etiologies have been proposed as causes of secondary FAIS. Concurrent presentation of SC and FAIS was observed in a subset of patients, as evidenced by radiography and arthroscopy [4, 5, 31]. Abolghasemian et al. [5] suggested that synovial inflammation at an early stage of SC and subsequent interposition of loose bodies was supposed to be the potential mechanisms underlying FAIS. Generally, residual cam-impingement was an important factor related to persistent hip pain and the result of revision surgery [32]. In the present study, a relative high rate of patients underwent femoroplasty during the revision process. However, no significant difference was found in Alpha angle and LCEA between the two groups, which suggested that impingement may not be the primary factor contributing to revision surgery rather than SC. We proposed that the cam-type impingement may be a subsequent condition of SC, while this topic still requires further study with dynamic radiographic examinations. Nevertheless, patients should be informed the potential risk of concomitant FAIS, and the necessity of more extensive surgery for treating both SC and FAIS.

Overall, the present study provided compelling evidence through the utilization of multiple validated PROs, facilitating comprehensive assessment of hip function across various dimensions. The results revealed that patients could expect significant improvements in their clinical outcomes following revision arthroscopy for treating hip SC. However, it is crucial for surgeons to provide critical context to these patients, as they may not experience the same level of satisfaction with their postoperative results comparing to primary arthroscopy.

Several limitations in the present study must be acknowledged. Due to the retrospective nature of the study, selection and recall bias were inevitable. The sample size was relatively small and under power due to the rarity of SC. A prior power analysis was performed given the matching ratio to be 1:4, MCID of mHHS to be 8, and the standard deviation to be 10. A total of 80 patients were required to achieve a power of 0.8 when alpha was set as 0.05. Caution should be exercised when interpreter the results of comparison of clinical outcomes and achievement of clinical thresholds between the two groups, as there was a risk of beta error. Further study with a larger sample size and longer follow-up period was required in the future. The surgical technique applied in the present study necessitates extensive experience. Caution should be exercised when generalizing the outcomes to the entire population who underwent revision arthroscopy for hip SC. In the revision group, only 5 patients underwent primary arthroscopy at our institute. Therefore, we only collected preoperative and intraoperative data before and during the revision arthroscopy for this group. Although we considered SC as the most prominent factor for persistent pain and requirement of surgery in this cohort, over half of patients presented concomitant FAIS. Therefore, further study should include patients with isolated SC to eliminate the effect of FAIS.