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1 Department of Radiology, Thomas Jefferson University Hospital, 132 S 10th St.,
Main Bldg., Philadelphia, PA 19107.
2 Department of Orthopedics, Pennsylvania Hospital, 800 Spruce St.,
Philadelphia, PA 19107.
Received October 25, 2002;
accepted after revision November 6, 2003.
Address correspondence to D. Bergin.
Abstract
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MATERIALS AND METHODS. A database search of 4,221 knee MRI examinations over 2 years revealed 90 examinations with reported anterior cruciate ligament ganglion or mucoid degeneration. Imaging criteria for ligament ganglion included fluid signal in the ligament disproportionate to joint fluid showing mass effect on intact ligament bundles. The size, location, complexity, and degree of lobulation of ganglia were recorded. Criteria for mucoid degeneration included ligament bundles poorly seen on T1-weighted and proton density–weighted images but with both bundles seen as intact on T2-weighted images. Intraosseous cysts at the ligament attachments and presence of joint effusion were noted. Clinical assessment of ligament instability was recorded when available.
RESULTS. Of 74 examinations that met imaging criteria, 56 (76%) had discrete intraligamentous ganglia, 18 (24%) had mucoid degeneration, and 26 (35%) had features of both. Ganglia were located in the proximal ligament in 16 examinations (22%) and the distal ligament in 10 (14%) and involved the entire ligament in 30 (40%). Ganglia ranged in maximum diameter from 20 to 73 mm (mean, 31 mm). Complexity of ganglia was mild (41%), moderate (39.2%), or marked (19.8%). Intraosseous cysts were noted proximally in 48 examinations (65%) and distally in 20 (27%). Of 52 patients with accessible records, 48 had no clinical evidence of instability. Twelve patients who underwent arthroscopy had an intact anterior cruciate ligament at that time.
CONCLUSION. Anterior cruciate ligament ganglia and mucoid degeneration commonly coexist on MRI and are typically not associated with ligament instability.
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The purpose of this study was to describe the MRI features of anterior cruciate ligament ganglia and determine the incidence of coexistent mucoid degeneration of the ligament. We also sought to correlate MRI findings with clinical assessment of ligament instability.
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Unenhanced MRI of the knee was performed using a 1.5-T MR unit (Signa, General Electric Medical Systems) with a dedicated knee coil (Medrad), a 15- to 16-cm field of view, and a 4-mm section thickness with a 1-mm gap and a 256 x 192 matrix. Sequences used were axial T2 fast spin echo (TR range/TE range, 2,000–6,000/60–70; echo-train length, 8) with fat saturation, sagittal proton density (TR/TE range, 2,000/20–30; echo-train length, 4), sagittal T2 fast spin echo (TR range/TE range, 2,000–6,000/60–70; echo-train length, 8) with fat saturation, coronal T2 fast spin echo (2,000–6,000/60–70; echo-train length, 8) with fat saturation, and coronal T1 spin echo (400–800/8–16; echo-train length, 8). Eight patients who had indirect knee arthrography received 0.1 mmol/kg of body weight of gadopentetate dimeglumine (Magnevist, Berlex Laboratories) IV. Sequences performed after IV contrast material injection were sagittal T1 spin echo (400–800/8–16), sagittal T2 fast spin echo (2,000–6,000/60–70, echo-train length, 8) with fat saturation, coronal T1 (400–800/8–16), coronal T2 fast spin echo (2,000–6,000/60–70; echo-train length, 8) with fat saturation, and axial T2 fast spin echo (2,000–6,000/60–70; echo-train length, 8) with fat saturation.
MRI examinations were reviewed by two musculoskeletal radiologists in consensus. Criteria for anterior cruciate ligament ganglia were fluid signal in the substance of the ligament having at least two of the following three criteria: mass effect on anterior cruciate ligament fibers, lobulated margins, and anterior cruciate ligament fluid disproportionate to joint fluid. Criteria for mucoid degeneration were anterior cruciate ligament fibers poorly seen on T1-weighted or proton density sequences, but seen on T2-weighted sequences. The ligament is of increased signal on T1- and T2-weighted sequences [7]. For both anterior cruciate ligament ganglia and mucoid degeneration, both bundles of the ligament had to be seen as intact from origin to insertion to exclude partial tears. These imaging criteria for anterior cruciate ligament ganglion and mucoid degeneration were based on prior reports with MRI and histology correlation [6–9].
The size and location of the anterior cruciate ligament ganglion within the ligament were noted. The complexity of the anterior cruciate ligament ganglion was graded, depending on the number of internal septations, as mild, two or fewer internal thin septations; moderate, three to five internal septations; and marked, greater than five internal septations. Lobulation of anterior cruciate ligament ganglion cyst margin was graded as mild, simple contour; moderate, fewer than three lobulations; and marked, greater than three lobulations. Presence and size of intraosseous cysts at the femoral and tibial attachments of the anterior cruciate ligament were recorded. Joint effusion, when present, was graded as small, moderate, or large.
Knee MRI examinations were excluded that did not meet criteria for anterior cruciate ligament ganglia or mucoid degeneration. Examinations were also excluded of patients who had a history of trauma to prevent potential inclusion of an injured anterior cruciate ligament. Patient symptoms, clinical assessment of anterior cruciate ligament by an orthopedic specialist, as well as arthroscopic reports, were obtained from patient records when available. Twelve patients had arthroscopic surgery. Stability of the anterior cruciate ligament was assessed by an orthopedist using Lachman's test. This is the most reliable clinical test to evaluate anterior cruciate ligament rupture and stability. With the knee held in 10–20° of flexion, the proximal tibia is pulled anteriorly and posteriorly. Anterior translation indicates ligament laxity. Clinical records were not available in 22 cases.
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The study population consisted of 36 men and 38 women with a mean age of 42 years (range, 19–66 years). Fifty-six patients (76%) had discrete intraligamentous ganglia. Eighteen patients (24%) had features consistent with anterior cruciate ligament mucoid degeneration only. Twenty-six patients (35%) had MRI features of both anterior cruciate ligament ganglia and anterior cruciate ligament mucoid degeneration (Fig. 1A, 1B, 1C). The mean age of patients with discrete anterior cruciate ligament ganglia only was 39 years (range, 19–60 years). The mean age of patients with coincident anterior cruciate ligament ganglia and mucoid degeneration was 45 years (range, 30–66 years). The mean age of patients with mucoid degeneration only was 43 years (range, 22–66 years).
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Anterior cruciate ligament ganglia ranged in diameter from 20 to 73 mm (mean, 31 mm). Ganglia were located in the proximal half of the anterior cruciate ligament in 16 examinations (16%), were located in the distal ligament in 10 (14%) and involved the entire ligament in 30 (41%). Complexity and lobulation of ganglia on MRI varied from mild to marked (Table 1). Discrete intraosseous cysts were identified in the proximal tibia in 48 patients (65%) and in the distal femur in 20 (27%). The mean diameter of intraosseous cysts at the femoral insertion was 4 mm (range, 2–20 mm) and at the tibial insertion was 5 mm (range, 2–22 mm). Fifty-one knees with discrete anterior cruciate ligament ganglion cysts had joint effusions (Table 1). Of eight examinations that were performed after the administration of IV gadolinium, five showed diffuse enhancement (Fig. 2A, 2B) of the anterior cruciate ligament ganglion and three showed rim enhancement only.
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On review of 43 accessible charts of 56 patients with anterior cruciate ligament ganglia, presenting symptoms varied greatly including knee pain, a clicking or popping sensation, and swelling (Table 2). Forty patients with anterior cruciate ligament ganglia on MRI had negative findings on a Lachman's test at clinical examination. Three patients with anterior cruciate ligament ganglia on MRI had an equivocal Lachman's test on clinical examination and subsequently had intact anterior cruciate ligament confirmed at arthroscopy. Thirteen patients with anterior cruciate ligament ganglia on MRI were lost to clinical follow-up.
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Eight patients with anterior cruciate ligament ganglia on MRI subsequently had arthroscopy and all had an intact ligament. Anterior cruciate ligament ganglia detected on MRI were not seen at arthroscopy. The standard arthroscopic approach was used in each case, using anterior portals. These ganglia on MRI had a mean diameter of 22 mm (range, 11–57 mm) and involved the entire ligament (n = 4) or the proximal (n = 2) or distal (n = 2) half of the ligament. A 12-mm anterior cruciate ligament ganglion cyst diagnosed on MRI was aspirated blindly at arthroscopy, using MRI as guidance. This patient, who otherwise had normal MRI findings, had presented with posterior knee pain of unknown cause. This patient reported resolution of symptoms after arthroscopy. Findings at arthroscopy in this group included medial meniscus tear (n = 4), lateral meniscus tear (n = 2), patellofemoral osteoarthritis (n = 3), and medial compartment osteoarthritis (n = 3).
Mucoid degeneration of the anterior cruciate ligament was seen in 44 MRI examinations. Twenty-six had mucoid degeneration coincident with anterior cruciate ligament ganglia. In 41 examinations (93%), mucoid degeneration involved the entire anterior cruciate ligament. In three examinations (7%), mucoid degeneration involved the distal half of the anterior cruciate ligament only. Associated joint effusions were small in 22 (50%) examinations, moderate in nine (20%), and large in one (20%). On 11 (25%) examinations, no appreciable joint effusion was present. Intraosseous cysts were noted at the tibial insertion on 31 examinations (70%) and at the femoral origin in 17 studies (39%) (Figs. 3A and 3B). The mean diameter of intraosseous cysts was 3.5 mm (range, 1–13 mm) at the tibial insertion and 4 mm (range, 1–20 mm) at the femoral origin. Two patients with MRI features of mucoid degeneration who had indirect arthrograms showed diffuse enhancement of the ligament.
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Review of accessible records of nine of 18 patients with mucoid degeneration of the anterior cruciate ligament on MRI revealed symptoms including knee pain, "giving way," and swelling (n = 6). Eight patients had a negative anterior drawer test on physical examination. One patient who had a positive Lachman's test on clinical examination had an intact ligament at arthroscopy. Clinical correlation was not available in the remaining nine patients in this group.
Of 18 patients with only mucoid degeneration on MRI, four had an intact anterior cruciate ligament confirmed at arthroscopy. Findings at arthroscopy in this group included medial meniscus tear (n = 2) and medial compartment osteoarthritis (n = 2).
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Clinical presentation associated with ganglion cysts of the anterior cruciate ligament is variable in the literature [5]. Most reported cases are incidental findings without contributory symptoms [5, 9]. Some case reports have ascribed symptoms, including pain at the medial joint line, mechanical locking, clicking and swelling, to the presence of these cysts [10–13]. Knee pain was the most common symptom of patients in our series. This, however, was a nonspecific symptom and was attributed to other intraarticular causes in most patients. Calvisi et al. [14] reported five cases of anterior cruciate ligament ganglion cysts discovered incidentally during arthroscopy; three were isolated findings, and two were associated with additional intraarticular causes. Of 12 patients in our study who had arthroscopy, meniscal tears and chondromalacia were the most frequent diagnoses. Although all anterior cruciate ligaments were confirmed intact at arthroscopy, intrasubstance ganglia and mucoid degeneration of the anterior cruciate ligament were not described on arthroscopy reports. This is in keeping with prior literature reports describing how anterior cruciate ligament ganglion cysts and mucoid degeneration identified on MRI are frequently not discernible at arthroscopy using the standard anterior portal approach. Anterior cruciate ligament ganglia and mucoid degeneration are definitively detected at arthroscopy by probing the ligament using MRI for guidance, and commonly necessitate a posterior portal approach [7–9]. MRI is better than arthroscopy at delineating those entities that do not cause any appreciable abnormality of the ligament surface [15, 16]. Thus, MRI may be used preoperatively to guide the orthopedist to aspirate the symptomatic ganglion that is not directly seen at arthroscopy, as described with one patient in our series.
The cause of anterior cruciate ganglion cysts remains controversial. In 1924, Caan [1] first described the anterior cruciate ligament ganglion during a routine dissection. Currently, two theories relating to the pathogenesis of ganglion cysts exist, although the true cause is still unknown [17–19]. The first theory attributes the presence of ganglion cysts to being a product of mucinous degeneration of the connective tissue [8, 19]. The second theory considers it a cause of herniation of synovial tissue through a defect in the joint capsule or tendon sheath, similar to those of wrist joint origin [17, 20]. For both theories, the relationship to previous trauma is uncertain and has not been documented. Levine [4] presented a case of a 23-year-old woman who sustained a twisting knee injury followed by a recurrence of swelling, pain, and locking. Surgical exploration showed a degenerative cyst in the intact anterior cruciate ligament. The investigator proposed that the ganglion formed because of a congenital abnormality or a previous injury [4]. Patients with a history of acute trauma were excluded from our study, suggesting that these ganglion cysts may be developmental or degenerative in etiology.
Reports of clinical assessment of the anterior cruciate ligament (e.g., the Lachman's test) in patients with anterior cruciate ligament ganglia are generally equivocal or negative [3, 5, 13]. In our population, 48 patients with accessible records who had anterior cruciate ligament ganglia or mucoid degeneration on MRI had no evidence of ligament insufficiency on clinical examination. Three patients with equivocal and one with a positive Lachman's test on clinical examination were subsequently found to have an intact anterior cruciate ligament at arthroscopy.
Discrete intraosseous ganglia were observed in 66% of studies with intrasubstance ligament ganglia and 77% of patients with mucoid degeneration. Intraosseous ganglia have been reported to be histologically identical to soft-tissue ganglia [21–23]. They generally have no identifiable communication with the articular surface or joint cavity [21]. Pope et al. [22] described four cases of intraosseous ganglia consisting largely of myxoid material. In that study, intraosseous ganglia were identified at the femoral and tibial attachments of the anterior cruciate ligament in patients with anterior cruciate ligament intrasubstance ganglia as well as mucoid degeneration. Kaatee et al. [17] described how long-standing pressure of a ganglion cyst against an adjacent hard surface may produce an indentation through gradual erosion of the bone, creating an intraosseous ganglion. The high incidence of intraosseous ganglia in patients with either mucoid degeneration of the anterior cruciate ligament or intrasubstance anterior cruciate ligament ganglia suggests that these two entities may share a similar pathogenesis.
This study is limited by lack of clinical correlation in 22 patients. Because it is a retrospective study, mild cases of mucoid degeneration may not have been initially reported, lessening the true incidence of this entity in our study. The number of anterior cruciate ligament ganglia identified, however, suggests that they may have a higher incidence than previously reported [3–5]. This study corroborates findings of previous authors that anterior cruciate ganglia are typically asymptomatic [5–8]. As noted in previous studies, patient symptoms are generally attributed to other abnormalities found on MRI or at arthroscopy [3, 5–8]. This is the first large study to confirm that ganglia are not associated with ligament instability. This study also shows how anterior cruciate ligament ganglion cysts commonly occur in association with MRI features of mucoid degeneration and that these entities are typically not associated with ligament insufficiency or patient symptoms. Both anterior cruciate ligament ganglion cysts and mucoid degeneration have a high association with intraosseous cysts at the femoral and tibial attachments. These findings lend credence to the theory that anterior cruciate ligament ganglia may be part of a degenerative process and that these two entities may represent different manifestations of this continuum.
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