This study showed that PVNS patients who received CR TKA had similar survival rates and functional outcomes as those patients diagnosed with knee OA who received CR TKA. In the average follow-up of 7.2 years, no infection, osteolysis, and knee instability were found in PVNS patients. In addition, there was no evidence of PVNS recurrence. However, the risk of postoperative stiff complications increased in patients with PVNS compared with the OA patients.
PVNS is a rare benign proliferative knee joint disease. Although the pathogenesis of PVNS remains unclear, some researchers believe that traumatic bleeding may be one of the causative factors [13,14]. In previously case report, localized pigmented villonodular synovitis presenting as recurrent dislocation of the patella . This is consistent with our study, in our study, 5 patients (29.4%) with PVNS had a history of patella dislocation or subluxation. Therefore, we believe that patella dislocation or subluxation may be a susceptible factor for knee PVNS. In the future clinical work, we should give sufficient attention to such patients.
Patients with PVNS who have symptomatic knee are usually treated. Because it is widely present in the anterior chamber of the knee joint, it can be treated by arthroscopic debridement for focal PVNS. However, diffuse PVNS require combined surgery, either through arthroscopy or open surgery [3,16]. If PVNS extends beyond the joint, an open surgery is required . Although open surgery can reduce the local recurrence rate of diffuse PVNS, these procedures may lead to a high incidence of knee stiffness after surgery . The ROM after TKA is associated with preoperative knee ROM , and in our study, we found that PVNS patients who underwent open synovectomy, the knee ROM was poor after CR TKA. In addition, we also found that the incidence of postoperative knee stiffness in patients with PVNS is higher than that of ordinary OA patients, even if they have not received other surgery before TKA, so we should pay more attention to the postoperative rehabilitation of patients with PVNS.
TKA is the most effective treatment for end-stage PVNS in patients who have severe OA due to PVNS progression, and the recurrence and revision rate of PVNS are lower than that with simple synovectomy . The long-term results of TKA treatment of PVNS have been well recorded, and some studies have reported excellent long-term survival of TKA in patients with PVNS [9,10]. However, it is still controversial to retain or substitute PCL in TKA when treated with PVNS patients. Due to limited case studies of knee PVNS, there is currently little data on the results of CR TKA in these patients.
During primary TKA, two principal designs are used: cruciate-retaining (CR)TKA and cruciate-substituting (PS) TKA. Compared with PS-TKA, the CR-TKA has been widely used because it improves the knee's ability to exercise, preserves the knee's proprioception, and increases the knee ROM and stability during knee extension and flexion [19,20]. Although PVNS and rheumatoid arthritis have different types of inflammation and mechanisms of joint destruction, they all produce chronic inflammation environment in joints, so the two have some comparability to some extent . Scott and his colleagues  pointed out that 95% of RA patients had complete PCL during TKA surgery and believed that PCL should be preserved during surgery to maximize femoral rollback. In addition, it has been informed that satisfactory clinical and radiological outcomes have been obtained in RA patients who were followed up for an average of 10.5 years with CR TKA . Miller  evaluated long-term outcomes of patients with RA who were followed up for 20 years after CR TKA. For any reason, the 20-year implant survival rate was 69%. They believe that PCL dysfunction is rarely the cause of surgical failure . This is consistent with our current research results. In our study, patients with PVNS who received CR TKA achieved excellent mid-term follow-up outcome.
PVNS most frequently affects the knee, although there are long-term follow-up studies and short-term complications in the previous literature on the evaluation of PVNS in arthroplasty, there is no clear middle-term follow-up control study. Previous TKA treatment in patients with PVNS was a minor cohort study, primarily to assess implant survival and function, and did not quantify the risk of postoperative complications, which may be due to fewer patients [9,25]. Although Houdek et al  did not compare the incidence of complications to the control group, their most common complication in this study was the loss of knee ROM, which was consistent with the increased risk of stiffness found in our research. According to previous reports, the risk of revision of PVNS patients is as high as 21%, which is significantly higher than the incidence of published primary TKA for OA [9,26]. In our research, only 1 patient underwent revision because of periprosthetic fracture. In addition, the implant survivalship without any correction for 7 years after CR TKA was 90.0% and we did not find any local recurrence, similar to the OA patients who received CR TKA. However, the revision rate may become inconsistent with additional long-term follow-up.
Radiotherapy and chemotherapy may be a viable option when surgery fails to eradicate PVNS or recurrence. Medium-dose external irradiation (30-35 Gy) combined with surgical resection can reduce the recurrence rate in patients with extensive or invasive diseases [27, 28]. In recent years, significant advances have been made in the treatment of diffuse PVNS [29,30]. Since PVNS often overexpress colony-stimulating factor 1 (csf1), receptor-targeted chemotherapeutic drugs (csf1r) may be an effective treatment [29,30]. Although these drugs were not used in patients in this series of studies, it is believed that young patients may consider using them to alleviate symptoms and delay TKA for as long as possible.
Our research has several limitations. First, this is a small sample retrospective study, which has its potential bias and weaknesses. A prospective study should be established to objectify these findings. Second, because the PVNS patients in our research were treated with CR TKA, we were unable to compare the efficacy of different prostheses, such as PS TKA, semi-constrained or rotating hinge prostheses.