Clinical manifestation and arthroscopic treatment of symptomatic posterior cruciate ligament cyst

Ganglion cyst of cruciate ligaments is a rare lesion. The reported morbidity of cruciate ligament cyst is 0.36% by MRI examination and 0.8% by arthroscopy [1]. The lesion is commonly seen in people aged 20–40 years old and involves more males than females [1‐3]. Compared with anterior cruciate ligament (ACL) cyst, posterior cruciate ligament (PCL) cyst is relatively seldom seen [3, 4]. Since not all PCL cysts are symptomatic, the reports of PCL cysts were hardly found until the wide utilization of MRI and arthroscopy two decades ago [5]. Most PCL cysts were incidentally detected by MRI or arthroscopy done for other knee lesions [2, 3, 6‐8] in that symptomatic PCL cyst causing knee discomfort and restricted movements were exceedingly rare [4, 7].

Most scholars consider that the etiology of PCL cyst may be associated with trauma and chronic injury [6‐9]. The main clinical manifestation includes knee pain, pain in extreme knee flexion, restricted movements, etc. [3]. Shetty et al. [7] have reported clinical manifestation of symptomatic PCL cysts as well as the process of arthroscopy-assisted operations. However, only a few studies have reported the surgical technique and outcomes of arthroscopic removal of PCL cyst. It is necessary to gain further insight into the entity of the disease, and the experience to diagnose and treat the lesion still needs accumulation. For this reason, we have retrospectively reviewed 11 cases of symptomatic PCL cyst treated with arthroscopy and systematically summarized the findings and experience in the aspects of clinical manifestation, MRI feature, arthroscopic operation, pathology, and the treatment outcome.

All the patients with PCL cysts who underwent arthroscopy operation from November 2012 to December 2014 at Zhongnan Hospital of Wuhan University were retrospectively reviewed, and the patients who met the following inclusion and exclusion criteria in Table 1 were included. Eleven patients including 8 males and 3 females with the mean age of 34.4 years (ranging from17 to 53 years) were selected according to the inclusion criteria, six of whom had cysts in their left knees and five in their right knees (Additional file 1). The duration of the symptoms ranged from 3 to 96 months.

After medical history collecting, thorough physical examinations, routine X-ray, and MRI examination of the knee were performed. Also, preoperative knee function was evaluated using International Knee Documentation Committee (IKDC) score. The mean follow-up period was 30.7 months (range from19 to 53 months).

Intra-articular ganglion cysts including PCL cyst are usually classified as “asymptomatic” and “symptomatic” [10, 11]. The cyst which is soley responsible for intermittent or chronic knee discomfort without any accompanied intra-articular lesions is defined as “symptomatic.” The cyst which has no clear etiology is difficult to be diagnosed clearly in time comparing with other intra-articular lesions or injuries in that it presents nonspecific clinical signs and symptoms. The results of this study showed that the most common manifestation of symptomatic PCL cyst was knee pain at flexion or in flexion-associated activities. Physical examination demonstrated that knee pain at hyperflexion and the resultantly reduced ROM appeared in almost all the patients. Literatures showed that the changes in the length and torsion of PCL induced by knee flexion might cause traction or compression on cysts, which may stimulate the nerve endings on synovium and result in limited ROM secondary to pain and abnormal sensation [1, 2, 12, 13]. Previous studies reported that the clinical manifestations of symptomatic PCL cyst might include arthralgia, joint swelling and effusion, and restricted ROM induced by pain [1‐4, 6‐8, 12, 14‐17]. Our results are similar to the above reports. And furthermore, the findings of this series of cases indicate that knee pain at flexion may be the specific clinical manifestation of symptomatic PCL cyst.

Since MRI examination could sensitively and specifically identify the anatomic and morphologic interrelation of the synovial tissues with their surrounding structures without invasion, it has been suggested as the primary choice for diagnosing cystic lesions of the knees [18‐20]. The cyst showed by MRI exhibits well-defined and homogeneous low-signal intensity on T1W, high-signal intensity both on T2W, and fat-suppressed T2-weighted SE, while the cystic wall manifests middle or low signal in T2W [1‐3, 7, 8, 12, 17, 21‐23]. The PCL cysts are usually single or multiloculated ones who are separated by soft tissue [8]. The MRI pictures in our series of cases were in accordance with these imaging features. The precise location of the PCL cyst is important for planning an arthroscopic approach. At present, posteromedial or posterolateral portals are not routine approach and the posterior compartment is not visualized regularly. Once the cyst is located posterior to PCL, it would be difficult to found the cyst for subsequent diagnosis. Therefore, the preoperative MRI scan is essential.

Although the pathogenesis of ganglion cysts of cruciate ligament is still unknown, most literature showed that trauma and chronic injuries played a key role in cyst formation [2, 3, 6, 7]. Bui-Mansfield et al. [18] found that 50% of the cruciate ganglion cysts had intra-articular lesion and degeneration, indicating that the cysts were associated with trauma and slight chronic injuries. Shetty et al. [7] reported that most cysts contained yellowish viscous fluid, while only a few cysts had bloody liquid. Lunhao et al. [6] explained that the cysts with bloody viscous liquid were associated with trauma, which resulted in a lesion of the PCL, synovial hernia, and formation of the hematoma. Along with the course of extension, the synovial tissue encases the hematoma and form a cyst gradually. In the early period, the content is bloody liquid and the hematoma would be absorbed and organized. The size of cyst increases as the synovial tissue keeps secreting mucus. At this stage, the contents turned to yellowish viscous fluid due to the breakdown of hemoglobin, while the cysts with transparent liquid contents coincided with a chronic injury. In the current study, 5 of the 11 patients had a definite history of trauma. The others memorized their experience of antecedent knee trauma or repeated slight knee injuries when questioned in detail. The findings in our study provided further evidence to support that the etiology of PCL cysts is closely associated with trauma and chronic injury. Pathologic examination of HE staining revealed inflammatory cell infiltration, widespread thick bundles of collagen, fibroplasia, and massive capillary proliferation in cyst wall. The inner surface of the cyst was covered by synoviocytes. Mucoid and lipoid degeneration, ruptured bundles of collagen (Fig. 3), were also found. All these indicated that the formation of PCL cyst might be associated with trauma which subsequently induced a synovial hernia and the degeneration of ligament secondary to trauma and chronic injury.

Treatment of ganglion cysts of cruciate ligaments mainly includes arthroscopic resection, ultrasound-, and CT-guided joint paracentesis [6, 7, 24]. However, postoperative recurrence occurred more often after paracentesis alone than after a complete resection [24]. The reason is that the cystic wall will persist and it may recur as the aspirating hole acts as a one-way valve and the synovium continues secreting mucus after paracentesis. While arthroscopy could excise the cyst completely and manage other lesions at the same time, the recurrence is rare after a complete excision [7, 9, 16, 24]. In the present study, all cases were performed the arthroscopic excision of the PCL cysts, and no recurrence was observed on postoperative MRI images at final follow-up. The double posteromedial approach for the arthroscopic resection of PCL cyst in this study, which was different from the posterior trans-septal portal [9], had a good clinical outcome.

The present study is the first to systematically describe the findings of the symptoms, imaging diagnosis, etiology, pathologic changes, and arthroscopic treatments of PCL cyst. There were still several limitations of our study, such as the small number of patients, the retrospective design, and the lack of comparative control group, which need to be further improved in future research.

In conclusion, the etiology of symptomatic PCL cyst may be associated with trauma and chronic injuries. The knee pain at flexion could be considered as a typical manifestation of symptomatic PCL cyst. MRI examination is an ideal method for diagnosing PCL cyst in time. And arthroscopic resection of PCL cysts would lead to a good outcome for patients without any recurrence.