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Summary

We present a surgical approach to treat posterior cruciate ligament cysts by an arthroscopic double posteromedial approach.

Abstract

Cruciate ligament cysts of the knee are a rare condition. Posterior cruciate ligament cysts of the knee are less common than anterior cruciate ligament cysts. In patients with asymptomatic isolated cruciate ligament cysts of the knee, conservative treatment is recommended. Symptomatic cruciate ligament cysts of the knee are mostly manifested as knee hyperflexion pain, straightening pain, knee discomfort after standing for a long time or walking for a long time, etc., which seriously affects the quality of life, surgical treatment can be performed. The surgical treatments can be divided into ultrasound-guided cyst puncture and fluid extraction procedure and arthroscopic cystectomy. Cysts are mostly lobulated with a multi-layer cyst wall, cyst fluid extraction does not remove the cyst wall completely but simply extracts cyst fluid, leading to a high recurrence rate. Arthroscopic surgery can completely remove the cyst wall with little trauma, a low recurrence rate, and fast postoperative recovery, so arthroscopic resection is the most common and preferred method of treatment. Since posterior cruciate ligament cysts mostly occur posterior to the ligament, we remove the cyst wall by adding a double posteromedial approach to the knee joint, and the cyst wall is removed under direct vision, which is simple to operate, the cyst wall is completely cleared, the trauma is small, the postoperative recovery is fast, and there is no recurrence. Here, 8 posterior cruciate ligament cysts were removed with complete postoperative symptom relief, no surgical complications, and no recurrence at 1-year follow-up.

Introduction

Joint cysts are cystic lesions, and the cyst fluid is a transparent jelly-like fluid that can be found in the ligament, meniscus, synovial membrane, and other parts of the knee joint^1,2. High mechanical stress can easily lead to cyst formation, which is why cysts are most common in the knee joint^3,4, and Baker's cysts are the most common type of cysts⁵. Cruciate ligament cysts of the knee are rare, occurring incidentally in 0.2% to 1.3% of cases scanned using knee magnetic resonance imaging (MRI) and in 0.6% of patients tested using knee arthroscopy^6,7. Posterior cruciate ligament cysts are rarer, with Brown and Dandy reporting that after performing knee arthroscopy on 6,500 patients, they found only 35 ligament cysts and only 6 from the posterior cruciate ligament⁵. Knee ligament cysts can occur regardless of sex or age but are more common in men aged 20 years to 40 years^4,8.

The cause of cruciate ligament cysts is unknown. For the non-invasive diagnosis of knee ligament cysts, MRI can clearly show the relationship between the size and position of the cyst and is the most accurate diagnostic method^4,6,9,10. Arthroscopic removal of cysts is the most effective and recommended treatment method^4,6,9,10. Arthroscopy can see the cyst site directly and completely remove the cyst wall, the recurrence rate is extremely low, and the patient recovers quickly after surgery^7,9,10. Posterior cruciate ligament cysts are mostly lobulated or multiloculated, most of which are located mainly behind the posterior cruciate ligament, and in 12.5% of patients, the cyst is mainly located anterior to the posterior cruciate ligament¹¹ (Figure 1A,B).

Many treatments can be used to treat posterior cruciate ligament cysts, such as ultrasound and computed tomography-guided joint paracentesis. However, several studies have shown that there is a potentially higher risk of recurrence because of which these procedures do not remove the cyst walls¹². Arthroscopic removal of cysts is the gold standard for the treatment of posterior cruciate ligament cysts. Arthroscopic surgery can completely remove the cyst wall, but it's hard to remove dorsal cruciate ligament cysts by the anterior approach alone. Abreu et al.¹² introduced an approach to the arthroscopic excision of PCL cysts using a trans-septal portal, and it's safe and effective. Tsai et al.¹³ reported that the trans-septal approach to the resection of posterior cruciate ligament cysts was successful in 15 patients, and there was no recurrence. The trans-septal approach requires the addition of a posterolateral approach, so it increases the risk of damaging the common peroneal nerve and popliteal neurovascular bundle. To avoid these risks, we present a surgical approach to treat posterior cruciate ligament cysts by an arthroscopic double posteromedial approach. Our approach does not require an additional posterolateral approach; there is no risk of injuring the common peroneal nerve. We use the double posteromedial approach, which keeps the entire surgical process on the medial side of the posterior septal, thus reducing the possibility of damaging the popliteal neurovascular bundle. The technique produces the same results while being safer than the trans-septal approach. This surgical approach is particularly suitable for the removal of cysts located on the dorsal side of the posterior cruciate ligament. The surgical approach is more advantageous if the cyst is compartmentalized or close to the tibial end.

Protocol

The protocol follows the guidelines of the Ethics Committee of the Third Hospital of Hebei Medical University. Informed consent was obtained from the patients for including them and the data generated as a part of this study. Patients enrolled in this study were between the ages of 18-60. A total of eight patients were included in the study, five females and three males.

1. Preoperative preparation

1. Use the following inclusion and exclusion criteria for enrolling patients in this study.
   1. Use these inclusion criteria: Patients with a clear diagnosis of posterior cruciate ligament cyst by MRI of the knee; patients with significant knee pain, hyperextension pain, and other symptoms that may be caused by posterior cruciate ligament cyst or patients with combined meniscal injuries that require arthroscopic treatment; patients who have an interest in surgery because of the impact on their quality of life.
   2. Use these exclusion criteria: Patients with other serious medical conditions who cannot tolerate surgery; patients who refuse surgical treatment; patients with posterior cruciate ligament cyst diagnosed by MRI but without symptoms; patients with combined anterior cruciate ligament rupture or posterior cruciate ligament rupture.
2. Use general anesthesia or neuraxial anesthesia for all patients. Place the patient in the supine position on the surgical bed. Apply a tourniquet to the affected limb in the middle and upper part of the thigh for no more than 1 h. Disinfect the affected limb 2x with iodophor and lay the surgical sheet.

2. Establishment of arthroscopic approaches

1. Use a blood-repellent belt to expel blood from the affected limb. Tighten the blood-repellent belt from the end of the limb to the proximal end. Make each lap overlap by 1/3. Do not insert soft tissue between each turn. Drain blood from the soft tissues of the operated limb. Inflate the tourniquet by applying 50 kPa pressure/force.
2. Establish an anterolateral approach, 1 cm above the lateral knee line and 1 cm lateral to the patellar tendon margin. Make an incision of about 0.5 cm in size from the skin to the joint cavity using an 11G sharp blade.
3. Establish an anteromedial approach, 1 cm above the medial knee articular line and 1 cm medial to the patellar tendon margin. Make an incision of about 0.5 cm in size from the skin to the joint cavity using an 11G sharp blade.
4. Insert the arthroscope parallel to the tibial platform towards the intercondylar fossa in medial and lateral approaches, respectively. Explore the suprapatellar capsule, patellofemoral joint, medial trochanteric clearance, lateral trochanteric sulcus, anterior cruciate ligament, posterior cruciate ligament, medial meniscus, and lateral meniscus.
5. Partially remove the injured meniscus or suture the injured meniscus. Remove the joint-free body. Treat all injuries within the joint.
6. Establish a normal posteromedial approach as described below.
   1. Flex the knee at 90°. Insert the arthroscopy in the anteromedial approach to monitor the intercondylar fossa and place the switching rod in the anterolateral approach.
   2. Insert the exchange rod between the posterior cruciate ligament (PCL) and the medial femoral condyle into the posteromedial joint capsule under arthroscopic surveillance. There is usually a distinct sensation of slipping in and out.
   3. Rotate the lens to the medial posterior joint capsule. See the triangular area formed by the reverse fold of the joint capsule, the medial femoral condyle, and the medial meniscus.
   4. Turn off the operating room lights. Observe the translucent area of the posterior medial skin surface of the knee (Figure 2).
   5. Use a lumbar needle to assist positioning (Figure 3). Eccentric puncture into the joint capsule in the translucent area (usually 0.5-1 cm behind the posterior femoral condyle and proximal to the articular line).
   6. Make a small incision of about 0.5 cm in the skin using an 11G sharp blade. Insert a straight clamp into the joint capsule under arthroscopic surveillance. Establish a normal posteromedial approach.
7. Establish a high posteromedial approach as described below.
   1. Insert the arthroscopy into the posteromedial approach. Insert the lumbar needle in the direction of the joint, 2-3 cm proximal to the posteromedial approach.
   2. Make the lumbar needle penetrate into the joint capsule in the triangular space of the medial posterior condyle, medial meniscus, and posterior joint capsule.
   3. Make a small incision in the skin. Insert a straight clamp into the joint capsule under arthroscopic surveillance. Establish a high posteromedial approach.

3. Exposing and removing posterior cruciate ligament cysts

1. Insert the arthroscopy in the high posteromedial approach. Insert the shaver in the normal posteromedial approach (Figure 4).
2. Remove synovial tissue between the posterior cruciate ligament and the joint capsule using a shaver.
3. Find the cyst, which is a translucent, raised synovial membrane around the posterior cruciate ligament (Figure 1C). Remove the cyst wall and see yellow and translucent fluid flowing out of the cyst wall, visualize compartment-like tissue in the cyst.
4. Remove the posterior cyst wall completely. Avoid injury to the posterior cruciate ligament and posterior blood vessels and nerves and explore the posterior cruciate ligament without injury.
5. Insert the arthroscopy in the anterolateral approach. See a portion of the cyst at the femoral terminus of the posterior cruciate ligament.
6. Remove the anterior cyst wall and see yellow and translucent fluid flowing out of the cyst wall. Remove the cyst wall completely.
7. Remove the synovial tissue in the V-shaped space between the anterior cruciate ligament and the posterior cruciate ligament. Probe the posterior cruciate ligament and check for damage to the posterior cruciate ligament.
8. Insert the arthroscopy in the medial patellar approach. Check the V-shaped space between the anterior cruciate ligament and the posterior cruciate ligament for residual cysts.
9. Insert the arthroscopy in the posteromedial approach. Check for residual cysts.

4. Closure of the incision

1. Insert the arthroscopic sheath in the anterolateral approach. Squeeze the suprapatellar capsule and completely drain the intra-articular fluid. Check knee extension and flexion activities from 0° to 120°.
2. Suture the incision with silk thread No. 4.

5. Postoperative rehabilitation

1. Quadriceps exercise: Ask the patient to perform this exercise in the supine position. Straighten the affected limb, hook up the toes, slowly lift the leg upward to a height of about 15 cm on the heel, stay for 3 seconds, and then slowly lower it down.
2. Joint release training: Ask the patients to perform this as they sit at the bedside and extend and flex the knee joint by themselves.
3. Ask the patient to get out of bed on the second day after surgery and move appropriately, and gradually return to normal activities.
4. Change the dressing every 3 days after surgery and remove the stitches 14 days after surgery.
5. Ask the patient to perform non-confrontational activities such as jogging in the first month after surgery and resume normal activities in the third month after surgery.

6. Postoperative care and follow-up

1. Ask the patient to lie flat without the pillow for 6 hours after surgery. The patient was hospitalized for approximately 3 days.
2. At 3 months after surgery, check whether the patient's range of motion returns to normal and 12 months after surgery, perform an MRI examination of the affected knee combined with arthroscopy to check whether there is discomfort in the affected knee and whether the cyst has recurred.

Representative Results

All eight patients were successfully operated on without any complications. Seven of the 8 patients had isolated posterior cruciate ligament cysts, and 1 patient had medial meniscal injury. The main symptoms of all patients before surgery are knee hyperflexion pain, inability to squat freely, pain, and discomfort in the back of the knee after standing for a long time or walking for a long time. After surgery, all symptoms were relieved and disappeared (Table 1).

Of the 7 patients with simple posterior cruciate ligament cysts, 1 year after surgery, no special discomfort was found. The postoperative knee range of motion significantly improved compared with the preoperative period, the patient could squat freely, and no special discomfort in the knee joint after standing for a long time or walking for a long time was found (Table 2). For the single patient with medial meniscal injury, a partial resection of the medial meniscus and 2°-3° degeneration of medial cartilage was performed during surgery. The follow-up 1 year after surgery found occasional mild knee joint discomfort, the knee range of motion significantly improved compared with the preoperative period, and the patient could squat freely. Mild pain on the inside of the knee after prolonged walking is considered to be a symptom caused by mild osteoarthritis. Repeat knee MRI shows mild cartilage degeneration. Knee discomfort was not associated with a posterior cruciate ligament cyst.

Posterior cruciate ligament cysts were seen on preoperative MRI in all patients (Figure 1A,D). All patients had an MRI 1 year after surgery and no recurrence was found (Figure 1B,E). In all patients, cysts can be seen during surgery (Figure 1C), and the cyst wall was completely removed during surgery, which means the success of all surgeries (Figure 1F).

Figure 1: Preoperative and postoperative MRI and arthroscopic images of cysts. (A) Sagittal MRI of the knee: posterior cyst of the posterior cruciate ligament (indicated by the red arrow). (B) Posterior cruciate ligament cyst recurrence was not seen on sagittal MRI of the knee 1 year after surgery (indicated by red arrow). (C) Posteromedial approach, arthroscopic view of posterior cruciate ligament cyst (indicated by red arrow). (D) MRI of the coronal position of the knee: anterior cyst of the posterior cruciate ligament (indicated by the red arrow); posterior cruciate ligament cyst with compartment-like structures visible within the cyst. (E) No recurrence of posterior cruciate ligament cyst on knee coronal MRI 1 year after surgery (indicated by red arrow). (F) Arthroscopic resection of posterior cruciate ligament cyst wall (indicated by red arrow). Please click here to view a larger version of this figure.

Figure 2: Light transmission phenomena on the surface of the skin. Flex the knee at 90° and insert the arthroscopy through the anterolateral approach into the posteromedial joint capsule between the PCL and the medial femoral condyle, and light transmission can be observed on the posteromedial skin surface of the knee. Please click here to view a larger version of this figure.

Figure 3: Establishing a posteromedial approach. At 0.5-1 cm each behind the posterior femoral condyle and the proximal end of the joint line, touch the needlepoint of the posteromedial approach on the posteromedial skin of the joint and puncture the lumbar needle toward the posteromedial capsule, and there is water flowing out. Please click here to view a larger version of this figure.

Figure 4: The position of the arthroscope and shaver in the double posteromedial approach. Use the same method to create an additional posteromedial approach 2-3 cm proximal to the posteromedial approach, forming a double posteromedial approach. The arthroscopy and shaver are placed, respectively. Please click here to view a larger version of this figure.

	Knee hyperflexion pain	Inability to squat freely	Pain and discomfort in the back of the knee
Pre-surgery	8 out of 8	8 out of 8	8 out of 8
Post-surgery	0 out of 8	0 out of 8	0 out of 8

Table 1: Comparison of the symptoms of pre-surgery and post-surgery. The patient's symptoms improved significantly after surgery.

Independent posterior cruciate ligament cyst	posterior cruciate ligament cysts with medial meniscal injury	Recovery time	relapse
7 out of 8	1 out of 8	3 months	0 out of 8

Table 2: Types of the patients. None of the patients had any recurrence after surgery.

Discussion

Posterior cruciate ligament cyst is a rare disease. Knee ligament cysts are usually discovered during MRI or knee arthroscopy exams. The causes of knee ligament cyst formation are varied, including post-traumatic formation, synovial tissue hernia formation during embryogenesis, and mesenchymal stem cell proliferation and formation. Recently, trauma and tissue stimulation have been recognized by most experts^2,7,14.

Most patients with posterior cruciate ligament cysts are asymptomatic and can be treated conservatively. In simple posterior cruciate ligament cysts, clinical symptoms are mostly related to the location or size of the cysts and are non-specific^11,14. As the posterior ligament cyst enlarges, in addition to the pain symptoms of the knee joint, knee noose, hyperextension or hyperflexion pain, and reduced range of motion of the knee joint is observed^8,11,14. The main methods of post-treatment of cruciate ligament cysts include arthroscopic resection and ultrasound-guided cyst puncture sac aspiration.

Jawish et al. found that cruciate ligament cysts treated with puncture and suction of cystic fluid showed relapse or poor results and required secondary posterior resection surgery or arthroscopic surgery¹⁵. The cyst shape is leaf-shaped or fusiform and may contain internal septations or be multilovary^11,16,17 (Figure 1A,D). A cyst's formation requires an intact cyst wall in which the sac fluid is encased. However, simply extracting the fluid without removing the cyst wall results in the puncture site acting as a one-way valve. Due to the influence of a one-way valve or small puncture hole, the fluid in the cyst does not flow directly and freely between the joint cavity and the cyst cavity. This causes synovial fluid to accumulate in the cyst cavity, leading to the recurrence of the cyst after it is drained. This is the primary reason why cysts often reoccur after puncture and fluid suction. Arthroscopic surgery can completely remove the cyst wall, eliminate the cyst cavity, and cannot form a one-way flap, thereby avoiding synovial fluid from entering the cyst cavity in one direction, and the cyst will not recur^{8,13,18,19,20}. Cyst fluid aspiration does not remove the cyst wall, only removes the synovial fluid in the cyst cavity, while arthroscopic surgery removes the cyst wall to release cystic fluid and can remove all the divided cyst walls under direct vision, completely preventing recurrence.

Some scholars have reported that arthroscopic cruciate ligament cysts after septal resection can completely remove cysts^12,13. However, the addition of a posterolateral incision may increase the risk of injury to the common peroneal nerve, which may lead to foot dropping after surgery. Although the posterior cruciate ligament is not prone to degeneration because it receives both subchondral and synovial nutrients^{21,22,23,24,25}, and the posterior cruciate ligament has vegetative posterior cruciate ligament vessels in the posterior synovial membrane, the septal approach may damage the blood vessels of the vegetative posterior cruciate ligament, and so the postoperative posterior cruciate ligament has poor blood supply, resulting in posterior cruciate ligament degeneration. Although the post-cruciate ligament cyst is treated with this surgical method, there is no recurrence after surgery, but the partial normal nutrient supply structure of the synovial membrane is damaged²⁴.

Local anatomy around the posterior medial passage of the knee is relatively complex. According to our positioning method, when the knee joint is flexed by 90°, the great saphenous vein and saphenous nerve are 2-3 cm anterior to it. The goosefoot tendon part is distal to its distal end, and the channel pierces part of the sartorius tendon above the gracilis tendon 0.5-1 cm. After the skin is poked, a slight peel and entry into the joint capsule using straight forceps bluntly minimizes collateral damage to the above structures. Since important vascular nerve structures are anterior or posterior to the high posteromedial approach, the high posteromedial approach does not increase the risk of vascular nerve injury. The double posteromedial approach removes the cyst wall under direct vision, does not destroy the posterior septal synovial tissue, preserves the blood vessels of the postoperative posterior cruciate ligament, and retains sufficient nutritional support for the recovery of the postoperative posterior cruciate ligament.

In patients with posterior cruciate ligament cysts of the knee, the main cyst portion is mostly behind the posterior cruciate ligament. In 3 out of 4 patients, the major cyst part originates posteriorly behind the posterior cruciate ligament. A double posteromedial approach was added, arthroscopic observation was inserted into the high posteromedial approach, and a shaver was inserted into the conventional posteromedial approach to remove the cyst. It has the advantages of easy observation and operation for the excision of the posterior cruciate ligament's posterior cyst.

In our surgical approach, the establishment of a double-posteromedial approach is the most critical step. When the normal posteromedial approach is established, patients often experience narrowing of the intercondylar fossa, which makes it difficult for the exchange rod to pass through the anterolateral approach through the intercondylar fossa to the posteromedial compartment, so we cannot establish the posteromedial approach under arthroscopic supervision. In this case, in patients with severe intercondylar fossa stenosis, we need to have the intercondylar fossa formed before inserting the exchange rod. For patients with relatively narrow intercondylar fossa, we can add the anterior median approach, through which an exchange rod is inserted through the intercondylar fossa into the posteromedial compartment. The establishment of a high posteromedial approach is also very important. If the high posteromedial approach is too close to the normal posteromedial approach, arthroscopy and shaver strike can have an impact on the operation. If the posteromedial approach is anterior or posterior, we may not completely remove the cyst because of an unclear visual field when we remove the cyst. Therefore, it is important to establish a suitable position for a double posteromedial approach under arthroscopic monitoring.

Our surgical method also has its drawbacks, and establishing a double posteromedial approach carries the risk of damaging the saphenous vein and saphenous nerve. Although the approach is a safe distance from the saphenous vein and saphenous nerve, since we need to obtain the best cyst field of view and operating space in the posteromedial approach, we need to adjust the optimal position of the posteromedial approach according to different cyst locations, which may increase the risk of injury to the saphenous vein and saphenous nerve. Due to the limited space of the posteromedial compartment, the establishment of two suitable positions of double approach on the basis of limited space increases the technical difficulty and requires higher surgical techniques of the operator. The arthroscopic double posteromedial approach is a safe and effective technique for PCL cyst resection. For the removal of dorsal cysts of the posterior cruciate ligament, especially those near the tibial end or compartmentalized cysts, this surgical approach offers significant advantages.

Materials

Arthroscopic sheath	smith&nephew	72200829	6mm
Arthroscopy	smith&nephew	72202087	30 mm x 4 mm
Beam guide	smith&nephew	72204925	5 mm x 3.6 m
Beam guide-arthroscopy end connector	smith&nephew	2143
Beam guide-panel connector	smith&nephew	2147
Blood-repellent belt	selani	tpe15100	15 cm x 1 m
Blunt puncture cone	smith&nephew	4356	4 mm
Camera	smith&nephew	72200561	NTSC/PAL
Coupler	smith&nephew	72200315
DYONICS POWER II	smith&nephew	72200873	100-24VAC, 50/60Hz
DYONICS POWERMAX ELITE	smith&nephew	72200616
Endoscopic camera system	smith&nephew	72201919	560P NTSC/PAL
HD monitor	smith&nephew	LB500031	27 inch
Hook probe	smith&nephew	3312
Incisor plus platinum shaver	smith&nephew	72202531	4.5 mm
Lumbar needle  AN-E/S	tuoren	AN-E/S	1.6 mm x 80 mm
Micropunch,teardrop,left	smith&nephew	7207602
Micropunch,teardrop,right	smith&nephew	7207601
Micropunch,teardrop,straight	smith&nephew	7207600
Pitbull Jr. Grasper	smith&nephew	14845