This study showed that the patients with PVNS who underwent CR TKA had similar survival rate and functional outcomes as the patients with OA who underwent CR TKA. In the minimum five years follow-up , no infection, osteolysis, and knee instability were found in the patients with PVNS. Furthermore, there was no evidence of the PVNS recurrence. However, these patients should pay more attention to the occurrence of postoperative stiffness complication.
The 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 a previously case report, the localized pigmented villonodular synovitis presenting as recurrent dislocation of the patella . This is consistent with our study, in our study, five patients (29.4%) with PVNS had a history of patella dislocation or subluxation. And, we appropriately increased the external rotation osteotomy of the femur in surgery for those patients. In the postoperative follow-up, these patients achieved good postoperative clinical outcomes, no abnormal patella track and only one patient with anterior knee pain was found. Therefore, we believed that the patella dislocation or subluxation may be a susceptible factor for the knee PVNS. In the future, we should pay more attention to these patients.
The local knee PVNS is widely present in the anterior chamber of the knee, and the patients who have symptomatic are usually treated with arthroscopic debridement. However, the diffuse PVNS require combined surgery, either through arthroscopy or open surgery [3,16]. If the 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 the patients with PVNS 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 was higher than the patients with OA, even if they have not received other surgery before TKA, so we should pay more attention to the postoperative rehabilitation of patients with PVNS.
It is generally known that 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 with 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 the PCL in TKA when treated with the patients with PVNS, and due to limited case studies of the knee PVNS, there is currently little data on the results of CR TKA in these patients.
During primary TKA, two principal designs are used: CR TKA and posterior-stabilized (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 the PVNS and the rheumatoid arthritis have different types of inflammation and mechanisms of joint destruction, they all produce chronic inflammation environment in joints, so the two diseases have some comparability to some extent . Scott and his colleagues  pointed out that 95% of rheumatoid arthritis patients had complete PCL during TKA surgery and believed that the 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 the rheumatoid arthritis patients who were followed up for an average of 10.5 years with CR TKA . Miller  evaluated long-term outcomes of patients with rheumatoid arthritis who were followed up for 20 years after CR TKA, for any reason, the 20-year implant survival rate was 69%. They believe that the PCL dysfunction is rarely the cause of surgical failure . This is consistent with our present study. In our study, the patients with PVNS who underwent CR TKA achieved excellent mid-term follow-up outcomes.
The PVNS most frequently affects the knee, although there were long-term follow-up studies and short-term complications in the previous literature with the evaluation of PVNS in arthroplasty, there was no clear middle-term follow-up control study. The 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 the fewer patients [9,25]. Although Houdek et al  did not compare the incidence of complications to the control group, their most common complication in their study was the loss of knee ROM, which was similar with our study. According to the previous reports, the revision rate of the patients with PVNS was as high as 21%, which was significantly higher than the incidence of published primary TKA for OA [9,26]. In our tudy, only one 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. These clinical outcomes were similar to the patients with OA who underwent 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 study, it was believed that the young patients should consider using them to alleviate symptoms and delay TKA for as long as possible.
Our study had several limitations. First, this was a small sample retrospective study, which had its potential bias and weaknesses. A prospective study should be established to objectify these findings. Second, because the patients with PVNS in our study 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.