Popliteal cysts (also called Baker’s cysts) are most frequently characterized by the enlargement of the gastrocnemius-semi-membranosus bursa in the posteromedial region of the knee. They were first described by Adams¹⁾ in 1840, and Baker²⁾ described the association of popliteal cysts with intra-articular lesions in 1877. The majority of primary popliteal cysts without communication to the joint are found in children without associated knee disorders³⁾. The prevalence of communication between the gastrocnemius-semimembranosus bursa and the knee joint cavity in adults varies in relation to the type of study. Handy⁴⁾ reported that a connection was observed in 30%–50% of cadaveric dissections, 55% of open surgical excisions, 37% of knee diagnostic arthroscopies, and 50% of arthrograms of normal knee joints, even without a popliteal cyst. Several studies on the pathogenesis of popliteal cysts have shown that they are associated with intra-articular pathology and valvular mechanism^5–7). Thus, popliteal cysts are almost never an isolated pathology in adult knees and are almost always associated with another pathology of the knee joint such as a meniscal tear and osteoarthritis^8–10). Moreover, the presence of a valve and the existence of an effusion create unidirectional flow of synovial fluid from the articular cavity to the cyst, which is one of the fundamental factors responsible for the formation and persistence of cysts⁶⁾. Although incidentally detected asymptomatic popliteal cysts do not require treatment, large cysts can cause popliteal pain or limit the range of motion (ROM), hence the target of surgical intervention¹¹⁾.

Many methods of surgical resection have been suggested. The recurrence rate seems to be high (up to 63%) after a simple open resection^9,12). One important reason is the intra-articular pathology that caused the knee effusion is not treated by an open procedure. Also, it is difficult to obtain a tight closure over the opening after open excision, and a large open wound appears necessary¹³⁾. Since Sansone and De Ponti⁶⁾ suggested changes in treatment strategies in 1999, the focus has been shifted toward treatment of the associated intra-articular pathology with use of the least invasive manner possible. An arthroscopic approach to a popliteal cyst could effectively manage not only the cyst by arthroscopic decompression but also the associated intra-articular pathology¹⁴⁾.

Recent advances in arthroscopic techniques make it possible to effectively address the intra-articular pathology, particularly the orifice of the cyst and the cyst itself, which has a valvular mechanism¹⁵⁾. Resecting the inner wall and septa inside the cyst while correcting the valvular mechanism has produced excellent results in previous papers^6,16–19). An arthroscopic procedure without cystectomy also produced satisfactory clinical outcomes without intra- or postoperative complications^7,11,14,20). Interestingly, none of the published studies directly compared clinical outcomes of arthroscopic valve debridement with and without cystectomy. There was a previous systematic review reported by Zhou et al.²¹⁾, which investigated open surgical excision procedures without comparison of complication and recurrence rates. The purpose of this systematic review and meta-analysis was to compare clinical outcomes and complication and recurrence rates after arthroscopic treatment of popliteal cysts with and without cystectomy.

We searched PubMed/MEDLINE, EMBASE, KoreaMed, and Cochrane Library to conduct a systematic review of literature from January 1967 to August 2016. The detailed search strategy is presented in Table 1. This search yielded 2,065 hits: 908 from PubMed/MEDLINE, 1,118 from EMBASE, 10 from Cochrane and 29 from KoreaMed. Duplicated studies, nonhuman studies, studies of extra knee joint cysts, conservative treatment and open surgery studies were excluded, leaving 33 articles for literature review. All the references in the identified studies were manually checked to detect any additional published data, and they were included if they met the inclusion criteria. Table 2 summarizes the inclusion and exclusion criteria. On the basis of the exclusion criteria (Table 2) and review of the abstracts and full texts, nine studies were ultimately included in the analysis (Fig. 1).

Two authors independently reviewed the full text of the nine articles. The methodological quality of each study included in the analysis was evaluated using the Coleman methodology score (CMS)²²⁾, which contains 10 criteria and produces a final score ranging from 0 to 100 points. Each author scored the methodological quality of the studies twice; disagreement was resolved by consensus among the authors. All data were extracted into an Excel spreadsheet ver. 2016 (Microsoft, Redmond, WA, USA). The extracted data included details of the study design, study period, patient demographics (sex, mean age, and mean duration of follow-up), pre- and postoperative functional scores, surgical technique, and complication rate. All statistical analyses were performed with RevMan version 5.3 software (The Cochrane Collaboration, Copenhagen, Denmark) and Stata version 14.2 static software (StataCorp., College Station, TX, USA).

The Rauschning and Lindgren (RL) knee score²³⁾ was used to evaluate the treatment outcome and efficacy in all nine studies. The parameters considered in the RL criteria are subjective symptomatology related to the presence of a popliteal cyst (pain and a posterior sense of tension in the popliteal fossa) and the clinical importance of posterior swelling or reduction in the ROM. All included studies were evaluated by using the following scale⁶⁾: grade 0, absence of swelling and pain, no limitation in ROM; grade 1, light swelling or sense of posterior tension after intense activity, minimal limitation of ROM; grade 2, swelling and pain after normal activity, ROM limitation less than 20°; and grade 3, swelling and pain even when resting, ROM limitation more than 20°.

We used a forest plot for the heterogeneity evaluation performed using the p-values and I² values. Then, on the basis of the heterogeneity evaluation and the number of studies included, we investigated the odds ratio (OR) of the presence of a grade 2 or higher RL functional evaluation, which indicates an unsuccessful clinical score²⁴⁾, in each arthroscopic treatment group.

Characteristics of the included studies, such as pathology associated with popliteal cysts and demographic/baseline characteristics of patients are summarized in Tables 3–5. In five of the nine studies, arthroscopic cystectomy was performed after debridement of communicating valves^{7,13,14,25,26)}; overall, 186 patients underwent arthroscopic cystectomy after valve debridement, and the minimum follow-up was 13 months. Three studies reported the outcomes of arthroscopic valve debridement without cystectomy^6,11,27); overall, 103 patients underwent that procedure, and the minimum follow-up was 22.9 months. In one study, the communicating valve was closed arthroscopically without cystectomy²⁸⁾ in 22 patients, and the mean follow-up was 2 years.

The mean CMS score of the included studies was 67.33 points (range, 55 to 78 points). The part A mean CMS scores were 4.11 points for study size, 4 points for mean follow-up, 10 points for the number of different surgical procedures, 0 point for the type of study, 5 points for diagnostic certainty, 5 points for the description of surgical procedures, and 5.55 points for the description of postoperative rehabilitation. The part B mean scores of the CMS were 7.67 points for outcome criteria, 11 points for the procedure for assessing outcomes, and 15 points for the description of the patient selection process. The mean CMS score for each criterion and the detailed CMS score for each study are shown in Tables 6 and 7, respectively.

According to the RL functional evaluation criteria, RL grade 0 or 1 at final follow-up was used to define successful treatment²⁹⁾. Most patients in all nine studies showed improvement in the RL grade after arthroscopic treatment. At final follow-up, the knees were rated as normal or nearly normal in 99.5% of patients (185 out of 186) who underwent combined arthroscopic valve debridement with cystectomy and in 92.0% of patients (115 out of 125) who underwent arthroscopic treatment without cystectomy. We evaluated the OR of RL grade 2 or higher (indicating an unsuccessful clinical outcome)²⁴⁾ after pooling the data from studies with cystectomy and those without cystectomy. The forest plot shows that the OR was 122.05 in the cystectomy group (95% confidence interval [CI], 33.84–440.23, p<0.001; heterogeneity I²=0%, p=0.81) and 58.12 in the non-cystectomy group (95% CI, 9.00–375.26, p<0.001; heterogeneity I²=77%, p=0.005) (Fig. 2). The OR for all included studies (cystectomy and non-cystectomy groups together) was 78.31 (95% CI, 25.29–242.43, p<0.001; heterogeneity I²=54%, p=0.03). The results of the test for subgroup differences of the total studies were I²=0% and p=0.52.

The rates of recurrence and complications are shown in Table 3. Overall, 0% (0 out of 186) of the cystectomy group and 8.0% (10 out of 125) of the non-cystectomy group had a recurrent cyst after treatment. The postoperative complication rate was 6.5% (12 out of 186) in the cystectomy group and 1.6% (2 out of 125) in the non-cystectomy group. The reported complications included hematoma formation (cystectomy group, 2.7%; non-cystectomy group, 0%), portal site infection and discomfort (cystectomy group, 1.6%; non-cystectomy group, 0%), persistent pain and swelling (cystectomy group, 1.0%; non-cystectomy group, 1.6%), and technical error during cyst debridement (cystectomy group, 1.0%; non-cystectomy group, 0%). The effect size for complications was 0.16 with cystectomy (95% CI, −0.02–0.35, I²=0%, p<0.001) and 0.03 without cystectomy (95% CI, −0.01–0.06, I²=0%, p<0.001) (Fig. 3). The heterogeneity of complications between the groups was p=0.159, and the overall effect size was 0.05 (95% CI, 0.00–0.10, I²=51.91%, p=0.08).

Many studies have reported that conservative management of popliteal cysts results in a high rate of cyst persistence³⁰⁾; however, an open surgical excision cannot be considered as a definitive solution in most patients because of the high rate of recurrence^8,23). Therefore, treatments for popliteal cysts focus on the management of intra-articular lesions and communication between the cyst and the articular cavity, as well as cystectomy, although the operative techniques differ from study to study.

The current literature on the treatment of popliteal cysts does not support one therapeutic method over another because of the lack of high-quality studies. Several papers showed superior outcome scores for arthroscopic valve debridement with or without cystectomy, but those findings are not consistent across papers. Our aim in this study was to review the available literature to compare clinical outcomes and complication and recurrence rates after arthroscopic treatment of popliteal cysts with and without cystectomy. Two important facts emerged from this systematic review and meta-analysis. First, no direct comparative study has been done between arthroscopic valve debridement and the use of cystectomy as a surgical intervention for popliteal cysts. Second, the total recurrence rate was lower, although the total complication rate was higher, in the cystectomy group than in the non-cystectomy group.

Cystectomy was performed in five of the studies discussed in this report^{7,13,14,25,26)}; patients in the other four studies did not undergo cystectomy^6,11,27,28). We assessed RL scores before and after surgery, and our statistical analyses found I²=0% and Z=7.56. The pooled results showed that the OR of unsuccessful clinical outcomes (RL grade ≥2) was greater in the cystectomy group than in the non-cystectomy group with 122.05 (p<0.001) and 58.12 (p<0.001), respectively. Similar results were shown in a previous systematic review/meta-analysis by Zhou et al.²¹⁾. They found a success rate of 98.2% after cyst wall resection and the rate was 94.7% without cyst wall resection; however, their review included open surgical excision procedures.

In the studies reviewed here, the recurrence rate was 0% in the cystectomy group and 8.0% in the non-cystectomy group. Despite the homogeneity and low recurrence rate of the cystectomy group, for now, we can only infer that cystectomy might benefit patients with popliteal cysts because of the small number of patients recruited and the presence of heterogeneity in the non-cystectomy group. Furthermore, the OR in the non-cystectomy group was as high as 58.12. Therefore, cystectomy might not significantly increase the chance of achieving a good clinical result. The cystectomy group also had a relatively high rate of complications (6.5% vs. 1.6%); however, there was no severe or permanent complication (Table 3) and it is difficult to conclude that the accompanying cystectomy was entirely responsible for such complications. It is necessary to investigate whether the complications are truly related to the resection of the inner wall and septa inside the cyst because the procedure accompanied correction of the valvular mechanism.

In all nine studies, the investigators reported on the incidence of concomitant intra-articular lesions in patients with popliteal cysts, and injuries to the medial meniscus and articular cartilage were the most common concomitant lesions in the joint (Table 4). These results indicate that more effective treatment methods for medial meniscal injuries and articular cartilage injuries are needed to further improve the therapeutic efficacy of treatments for popliteal cysts. In addition, some of the reviewed studies described reasons for treatment failure^6,11,14,28): knee arthritis, chondromalacia of the posterior tibial plateau, and severe chondral defects. These data not only confirm the importance of managing intra-articular lesions during surgery but also indicate that the currently available methods used to treat severe arthritis and cartilage defects are unsatisfactory.

This systematic review and meta-analysis has several limitations. First, it is based on low quality studies. Studies with only level IV evidence were included in our analysis because our literature search did not return any high quality studies. The current literature on the treatment of popliteal cysts was limited to retrospective case series where the superiority of one treatment strategy over another could not be evaluated. Second, one study²⁸⁾ performed communication valve closure surgery in the non-cystectomy group; the decreased homogeneity of the treatment procedure in the non-cystectomy group might have affected the results of the study. Therefore, high quality studies are needed to confirm the efficacy of various arthroscopic surgical interventions regardless of concurrent cystectomy in the treatment of popliteal cysts. Third, some sources of selection bias, including different follow-up periods and different intervals from injury to surgery, were present in the reviewed studies. Further research needs to be conducted, particularly studies that directly compare the cystectomy group and non-cystectomy group, to draw a definitive conclusion.

All the currently available studies on the arthroscopic management of popliteal cysts showed satisfactory outcomes in both the cystectomy and non-cystectomy groups. Neither procedure showed clear superiority to the other. Arthroscopic cystectomy after management of intra-articular lesions was associated with a low recurrence rate but a relatively high incidence of complications. However, it is unclear whether the cystectomy procedure alone was responsible for the high complication rate. It is necessary to evaluate whether such complications are truly related to the resection of the inner wall and septa inside the cyst, not to the correction of the valvular mechanism.

No potential conflict of interest relevant to this article was reported.