Haemorrhagic synovial cyst of the posterior cruciate ligament: A case report☆

1. Introduction 

Intraarticular ganglion cysts associated with the cruciate ligaments of the knee joint have been previously reported [1]. We describe here a case of haemorrhagic synovial cyst arising from the posterior cruciate ligament. Haemorrhagic synovial cyst arising from the posterior cruciate ligament in the knee may become symptomatic. The patient presented with complaints of knee pain, clicking sensation and terminal restriction of flexion and extension movements. The cyst extended mainly into the posterior compartment of the knee joint and it was treated arthroscopically with use of the posterior–posterior triangulation technique [2]. Haemorrhagic synovial cyst, though rare, should be included in the differential diagnosis of symptomatic swellings arising from the cruciate ligaments of the knee joint.

2. Case report 

A 24-year-old male martial arts instructor presented with complaints of progressive pain in his right knee joint since two months duration. He reported a trivial incidence of twisting injury to his right knee joint three months previously during his routine training session. This resulted in mild pain and swelling of the knee joint that was relieved in a week. He was able to carry out his routine activities and vocation during this period. His knee pain reappeared after a month and gradually progressed. The pain was aggravated on extreme positions of knee flexion and extension. He walked with a limp and he preferred keeping the knee joint in a flexed position. It was difficult for him to climb stairs and run. He noticed mild swelling and a painful clicking sensation during flexion/extension movements of the knee joint for the past one month. Although the patient was able to perform his activities of daily living with some difficulty, he was unable to perform his martial arts training. Physical examination showed mild knee joint effusion. There was painful limitation of passive flexion beyond 90 degrees and extension at the terminal 10 degrees. Active movements showed a flexion of 80 degrees limited by pain and re striction of terminal 10 degrees of extension. There was no evidence of quadriceps atrophy, no joint line tenderness, no features of ligamentous laxity and the tests for cruciate and collateral ligament damage were within normal limits. There was no history of any prior knee pathology.

Plain radiographs of the knee joint and the routine hematological and immunological investigations were normal. Analgesics were prescribed, but they offered no significant relief. Magnetic resonance imaging (MRI) of the right knee joint showed a well demarcated multilocular cystic mass surrounding the proximal half of the PCL, and this mass mainly extended to the posterior compartment of the knee joint (Fig. 1). Under epidural anesthesia, we re-examined the knee joint and found a full range of joint movement. On arthroscopy, we found a well-defined reddish-white cystic mass filling the intercondylar notch and projecting anteriorly between the cruciate ligaments (Fig. 2A). The surface of the mass was relatively elastic on probing. It was found to arise from the posterolateral surface of the proximal half of the PCL and extended to the posterior and anterior compartments of the knee joint. We then approached through the V-shaped space between the two cruciate ligaments to decompress the posterior extension of the cyst as described by Lakdawala et al. [3]. We decompressed the cyst using an arthroscopic shaver; this revealed a dull red-colored fluid leaking out of the cyst (Fig. 2B). However, we were not sure about the completeness of removing the cystic mass via our anterior approach. Therefore, we utilized the direct posterior–posterior triangulation technique to visualize the posterior extent of the cystic mass (Fig. 3A). With the knee flexed at 90 degrees, an arthroscopic beam was introduced into the deep part of the lateral gutter area. The lateral skin incision was put under arthroscopic illumination and an obturator was introduced into the posterolateral compartment of the knee joint. After examining the posterolateral compartment, the obturator was pushed in close approximation with the posterior femoral cortex through the loose areolar tissue behind the PCL to pierce the posterior septum to the posteromedial compartment and posteromedial portal was made. Motorized shaver was introduced through the posteromedial portal. The remaining portion of the cystic mass was identified. It arose from the posterior aspect of the PCL. The cystic mass was completely cleared off from the PCL and the posterior septum area (Fig. 3B). The main body of the PCL was found to be intact and no repair was required. The cyst wall was sent for histopathological examination. We first performed hematoxylin and eosin (H and E) staining that revealed a true synovial cell lining and numerous small sized capillaries in the cystic wall. This pointed towards the diagnosis of synovial cyst (Fig. 4A). To further evaluate the presence of the narrow lumen capillary vessels in the cystic wall, we performed CD34 immunostaining using the mouse anti-human CD34 antibody (Immunotech, Marseille Cedex, France) at a 1:20 dilution. We observed that much of the capillary vasculature expressed CD34 positivity in the cystic wall; this finding favored the diagnosis of synovial cyst (Fig. 4B). MRI scanning was repeated after one year of follow up, and it showed no signs of recurrence of the cystic mass lesion (Fig. 5). Six months after surgery, knee pain was fully resolved but there was terminal restriction of flexion by 20 degrees. At the end of two years follow up, patient regained full range of knee movement and got back to his routine vocation of martial arts instructor.

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Fig. 1. Preoperative MRI of the knee joint demonstrated a well-defined, multiloculated cystic mass showing homogenous high signal intensity on the fat suppressed T2-weighted sagittal (A) and coronal (B) images. This mass was located adjacent to the proximal segment of the PCL, and its anterior extension occupied the intercondylar region.

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Fig. 2. A: The synovial cyst is seen extending anterior to the PCL through the anterolateral portal. The intact ACL is seen adjacent to it. B: Reddish-brown contents that leaked out of the cyst following decompression visualized through the posteromedial portal.

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Fig. 3. A: Schematic illustration of the posterior–posterior triangulation technique. It shows the arthroscope introduced through the posterolateral portal and shaver from the posteromedial portal for the complete removal of the synovial cyst arising from the proximal half of the PCL. B: The intact PCL observed after removal of the cyst seen through the anterolateral portal.

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Fig. 4. A: Haematoxylin and eosin (H and E) staining of the specimen showed true synovial cells lining the cyst wall with much proliferation of the underlying capillary-sized vasculature. B: CD 34 immunostaining delineates the endothelial cell-lined capillaries in the cyst wall.

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Fig. 5. Postoperative follow up MRI of the knee joint showing the intact PCL without any signs of recurrence of the cyst.

3. Discussion 

Intraarticular synovial cysts of the knee joint have not been reported earlier. A few reported cases of synovial cysts in the knee joint include those arising at the chondral fracture site of the medial femoral condyle [4], those arising from an arthroscopic portal site [5] and the ones that developed following ACL reconstruction [6]. These were basically ganglion cysts.

Unlike the extraarticular synovial cysts (e.g., Baker's cyst), which are believed to result from the leakage of the synovial fluid through a tear in the capsule of the knee joint, the etiology and pathogenesis of intraarticular synovial cysts are unknown. But we presume that injury is the most likely etiology. The trauma sustained by our patient during his training session may have been the initiating factor for the origin of the cyst. The dull red color of the cystic contents of our case might be the indicator of the traumatic hemorrhage that was caused by rupture of the vessels in the lining synovium of the PCL.

As could be expected, the synovial cysts arising from the posterior aspect of the PCL could impinge between the PCL and the intercondylar roof, and so this can limit the terminal degree of flexion; if it extends anteriorly, it can limit the final range of the extension by a similar mechanism. We believe that pain could have been produced by the stretching of the nerve endings in the lining synovial membrane that get stimulated upon compression of the cyst during the terminal range of the movement and during change of posture. On arthroscopic examination, we could see the cyst projecting anteriorly in front of the PCL and was lying medial to the ACL. We attempted to completely decompress the cyst through the anterior portals, and we tried to decompress the posterior extent of the cystic mass anteriorly through the V-shaped space between the two cruciate ligaments [3]. We examined the posterior compartment of the knee joint using the posterior–posterior triangulation technique for confirming total removal of the cystic mass [2], [7]. The posterior extension of the cyst was clearly visualized and it was removed using the technique of posterior triangulation; this posterior extension would have been missed if the arthroscopy was limited to only the anterior approach.

The differential diagnosis of a synovial cyst includes ganglion, meniscal cyst, parameniscal cyst, pigmented villonodular synovitis, synovial sarcoma, malignant fibrous histiocytomas and lipoma. Pathological examination showed that the cyst was lined with true synovial cells and it contained a lot of narrow lumen vasculature in the cyst wall, which is uncommon in ganglion cysts. Areas with myxoid change and fibrous tissue in the cyst wall were absent. We demonstrated the density of the vasculature by performing CD 34 immunostaining of the endothelial cells in the vessel wall. Synovial cyst usually contains clear, watery synovial fluid that is unlike the thick, glassy, mucinous content of the ganglion. The dull red colored watery contents of the cyst in our case supports the presence of synovial cyst. We would like to suggest that the posterior cruciate ligament synovial cyst should be included in the differentiate diagnosis of cystic lesions that arise from the posterior compartment of the knee joint.