Popliteal (Baker) synovial cysts represent a distention of the gastrocnemius-semimembranosus bursa. The cyst contents likely originate from within the knee joint and traverse a relatively weak portion of the posteromedial knee capsule.² Most extend posterior (superficial), intermuscular, and inferior, but variants include progression lateral, proximal, or intramuscular. Affected patients have a wide age range, but the incidence increases sharply in individuals older than 50 years.³

As with many cystic masses, various theories exist as to the origin of popliteal cysts. Synovial outpouching through a relative capsular weakness, fluid extension through a valvular conduit into the gastrocnemius-semimembranosus bursa, and encapsulation of extravasated synovial fluid are proposed mechanisms.

Most popliteal cysts in adults arise in the setting of an intra-articular pathology, frequently a degenerative condition. The high incidence in patients with a tear of the posterior horn of the medial meniscus suggests these tears further decrease resistance to fluid egress.³

Patients may have a variety of symptoms, which often are most consistent with the underlying knee pathology. However, patients often attribute all symptoms to the visible, palpable mass. A patient with a true symptomatic or isolated popliteal cyst describes pain, swelling, and obstruction of flexion. Although exceedingly rare, neurovascular insults occurring secondary to Baker cysts include entrapment neuropathy, vascular compression, pseudothrombophlebitis, and compartment syndrome.⁴ A patient with acute rupture of the popliteal cyst will have distant swelling, bruising, and ecchymosis. There may be hemorrhage in the cyst, and pain can be severe. The patient often can recall the cyst before it ruptured. Compartment syndrome has been described following cyst rupture. Because a popliteal cyst can mimic a deep vein thrombosis, many are identified on ultrasonography ordered to evaluate for thrombus.

On MRI, the cyst origin is often visualized between the semimembranosus and medial head of gastrocnemius tendons. Low signal intensity on T1-weighted studies and high signal intensity on both T2-weighted and short tau inversion recovery studies characterize synovial cysts (Figure 2). Areas of increased signal on T1-weighted imaging may represent intracystic hemorrhage or protein-rich fluid. The cyst wall and internal septae are enhanced on images following contrast material administration. Loculated cysts often represent the two distinct components of the gastrocnemius-semimembranosus bursa.⁵ Loose bodies and calcifications can be seen within the cyst (Figure 3). Ruptured cysts will have significant edema and fluid tracking along the posterior calf.

The presence of an arthritic knee may incorrectly lead the clinician to conclude that a soft-tissue mass behind the knee is a benign cyst. Patients in the age group in which a popliteal cyst is most likely to develop are also in the age group where soft-tissue sarcoma most often occurs. The presence of a deep, firm, and proximal soft-tissue mass must raise suspicion that the mass is not a cyst, but a lesion warranting further evaluation (Figure 4).

Treatment of the cyst itself is often not warranted; management should focus on treating the underlying pathology. Ultrasonography in patients before and after undergoing total knee arthroplasty demonstrated that 85% of Baker cysts remained, but associated symptoms markedly diminished.⁶ Intra-articular injection and sclerotherapy can result in diminution of cyst volume. Reassurance and patient education are integral to popliteal cyst management. In the rare instances when the cyst is responsible for neurovascular compromise, excision with decompression of any compromised structure is indicated. Compartment syndrome secondary to an intact or ruptured cyst is treated emergently as to diminish morbidity associated with an untreated compartment syndrome. When the primary knee pathology is not addressed, the rate of cyst recurrence is high; in contrast, open cystectomy in combination with arthroscopic treatment of intra-articular pathology has yielded excellent results and a low rate of recurrence as discussed in a 2023 study.⁷

All-arthroscopic techniques for cystectomy can be used if the cyst remains symptomatic.^8,9 As with open surgery, when arthroscopic cystectomy is combined with procedures to address the underlying pathology, reported recurrence rates are low. An intracystic dye injection can help with arthroscopic visualization of the stalk.¹⁰

Popliteal cysts occur in pediatric patients much less frequently than in adult patients (Figure 5). In a review of 44
pediatric patients with popliteal cysts, more than 75% of the cysts did not show spontaneous resolution at 1 year.¹¹ However, a few patients had symptoms (excluding the presence of the mass itself as a symptom). An underlying musculoskeletal condition was identified in only one patient, whereas other studies have reported a stronger concurrence of associated joint or soft-tissue disorders.¹²

A definitive treatment algorithm for pediatric popliteal cyst is lacking. A 2021 study used a size of 3 cm as a surgical indication. Of the seven cysts smaller than that, five resolved with observation. The remaining two lesions increased in size and were treated surgically. Of the 10 cases treated surgically, three had recurrence.¹³

Meniscal cysts are uncommon and may arise from the lateral or medial meniscus. Histologic evaluation of meniscal cysts demonstrates both synovial lining and dense fibrous tissue in the cyst wall, components of both synovial cysts and ganglia. Most meniscal cysts are identified in association with meniscal tears and likely represent a synovial cyst formed by excursion of intra-articular fluid through the tear. A review of 102 meniscal repairs found a 13.7% incidence of meniscal cyst formation after meniscal repair postulating that the microtrauma from the meniscal suture needle results in either a conduit for synovial fluid egress or meniscal degeneration and subsequent cystic formation.¹⁴

Isolated symptomatic meniscal cysts are uncommon. If a cyst extends to the meniscal periphery, it can appear as a tender, palpable joint line mass. The patient may experience a sense of tightness in the affected knee even while retaining a normal range of motion. MRI is both sensitive and specific for identifying meniscal cysts. Many of these cysts are multilocular and have a low-intensity signal on T1-weighted imaging and a high-intensity signal on T2-weighted imaging (Figure 6). Hemorrhage into a cyst, with subsequent deposition of proteinaceous material, can result in a heterogeneous appearance. A heterogeneous cyst in the absence of a meniscal tear can mimic both benign and malignant tumors.¹⁵

Asymptomatic and symptomatic lesions can be safely observed. However, lesions with a heterogeneous MRI appearance and without an associated meniscal tear should be followed closely, and biopsy may be necessary to rule out neoplasm.

Decompression can be performed during meniscal surgery for a patient with an associated tear. An isolated, symptomatic meniscal cyst can be excised arthroscopically, but an open approach may be warranted for a large lesion. Little is known about the recurrence rate of meniscal cysts, but according to recent studies recurrence is less likely when the underlying pathology is treated.^16,17

Acromioclavicular cysts are fluid collections over the acromioclavicular joint and superior shoulder. They may be striking in size and are indicative of an underlying degenerative shoulder girdle pathology. Two types of acromioclavicular cysts have been described based on etiology. Type 1 cysts originate from the acromioclavicular joint proper and communicate with the underlying subacromial bursa. The more common type 2 cysts result from fluid extravasation from a degenerative glenohumeral joint through an incompetent rotator cuff.¹⁸

On physical examination, the patients have a large supraclavicular, transilluminating mass without a thrill or bruit. Physical examination often demonstrates signs of underlying rotator cuff and/or joint pathology. In evaluating the MRI studies of a shoulder with a cyst, it is important to look for the presence and extent of rotator cuff injury. The pathognomonic geyser sign represents the conduit through which glenohumeral synovial fluid traverses the acromioclavicular joint into the cephalad cystic space.^18,19

As with all secondary findings, treatment of the underlying pathology provides the best chance for definitive cyst resolution. Observation and reassurance are an acceptable treatment because the cyst is not a de facto danger to the patient. Physical therapy, medication, and injections may safely be used to manage the underlying condition. Cyst aspiration may provide transient resolution of the mass itself, but a recurrence is likely without management of the underlying condition. Open excision often is performed with a reconstruction or salvage procedure for the underlying pathology. With a type 1 cyst, distal clavicle excision, bursectomy, and cyst removal may be indicated after unsuccessful nonsurgical treatment. Management of a type 2 cyst may involve surgical reconstruction to treat an irreparable rotator cuff tear or advanced glenohumeral degenerative arthropathy and is outside the scope of this update.

Spinoglenoid and suprascapular (pseudo)cysts are paralabral ganglia with the potential to compress the suprascapular nerve.²⁰ The high prevalence of superior labral anterior to posterior (SLAP) lesions in patients with spinoglenoid cysts has led to the proposed cyst etiology of being a one-way valve for synovial fluid egress. Open and arthroscopic decompression have been described with a growing trend toward arthroscopy.²¹ Addressing associated pathologies such as SLAP lesions is outside the scope of this update.

Although pain and weakness are often presenting symptoms of a paralabral ganglia, large lesions can extend superficially and present as a mass (Figure 7).

Despite the high incidence of wrist ganglia, uncertainties remain regarding their etiology, capability to cause wrist pain, and management.²² Wrist ganglia can be dorsal or volar (palmar). Volar ganglia are most commonly identified over the scapholunate interval but may be present at the interval between the radial artery and the flexor carpi radialis tendon, adjacent to the scaphoid tubercle, within the anatomic snuffbox, or more distally over the hand. Ulnar-side lesions can occur.