Magnetic resonance imaging of degenerative changes of the posterior cruciate ligament

Introduction

Some patients, for anatomic or biomechanical reasons, are predisposed to degenerative changes in the knee. Some of these changes, such as osteoarthritis, are widely described and well-known by orthopedic doctors, sports doctors, physiatrists, and radiologists; others, such as mucoid (or myxoid) degeneration of the posterior cruciate ligament (PCL), have only a few descriptions and are poorly understood (1–3).

Mucoid degeneration of cruciate ligaments has been considered as uncommon since its first imaging description in 1999 (4). Mucoid degeneration of the anterior cruciate ligament (ACL) has been increasingly reported and nowadays represents a well-known entity (5–8). On the other hand, the counterpart of the PCL is considered more rare, and most recent papers dealing with this topic are based on case reports (1–3).

Mucoid degeneration is believed to share a common origin with intra-articular ganglia (or ganglion cysts). In fact, although the etiology of mucoid degeneration and ganglia is not clearly elucidated, an association between these lesions has been proven in the ACL as well as in the PCL, thus reflecting a continuum of degenerative changes (9,10). In particular, the pathogenesis of mucoid degeneration has been attributed to mucinous or myxoid degeneration of connective tissue, as suggested by previous reports with histological correlation (1,3). The most widely held physiological explanation attributes ganglia formation to mucoid degeneration of collagen and connective tissue (11–13).

Clinically, mucoid degeneration of the PCL may produce knee pain, but more often it is asymptomatic or associated with additional findings that are the primary cause of pain. It typically occurs in middle-aged people and its prevalence was previously reported as 0.1% (10); however, since mucoid degeneration is often asymptomatic, if one is not aware of the diagnosis it may be missed and then underestimated. On the other hand, intra-articular ganglia are widely described and well-known lesions, which represent a recognized cause of knee pain (13–15). Their prevalence is 0.2–1.9% on imaging or arthroscopy (11,12). In both types of degenerative changes, an appropriate diagnosis is needed in order not to deprive the patients of correct treatment, which may be either conservative or surgical (2,16).

The objectives of this study were: (i) to evaluate the prevalence of degenerative changes of the PCL, such as mucoid degeneration and ganglia, in knee magnetic resonance imaging (MRI); and (ii) to assess the confidence of radiologists with these degenerative changes.

Material and Methods

Results

A series of 762 patients (459 men, 303 women, 41 ± 16 years, age range, 8–85 years) underwent an MRI examination of the knee, at our institution, from 1 July 2013 through 31 June 2015: 366 examinations were performed with the 1.5 T unit, 396 with the 0.26 T unit. Twenty-four patients with a history of knee trauma as well as 46 individuals with MRI findings suggestive of prior knee injury (ACL tear in eight patients; PCL tear in one patient; bone contusion with no other gross abnormalities in 37 patients) were excluded. A total of 692 patients (412 men, 280 women, 42 ± 16 years; age range, 8–80 years) entered this study. The radiologists and the radiology fellow in consensus identified mucoid degeneration in 34 patients (4.9%) (Figs 1–3), ganglia in 14 patients (2.0%) (Fig. 4), and both a mucoid degeneration and a ganglion in four patients (0.6%) (Fig. 3). OPEN IN VIEWER

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Fig. 2.

Mucoid degeneration of the PCL in a 54-year-old man. Sagittal proton density fat-suppressed image (a) and coronal proton density fat-suppressed image (b) show diffuse thickening and longitudinally increased signal intensity with peripheral hypointense rim of the PCL fibers (arrows).

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Fig. 3.

Mucoid degeneration of the PCL associated with a ganglion in a 67-year-old woman. Sagittal proton density fat-suppressed images show proximal thickening of the PCL fibers (arrows) (a) with normal aspect of the distal fibers (b). In contact with the PCL, a ganglion is present (open arrow).

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Fig. 4.

Ganglion of the PCL in a 22-year-old man. Sagittal proton density fat-suppressed image (a) and coronal proton density fat-suppressed image (b) show a well-delineated lobulated cystic lesion within the PCL (arrows).

The majority of patients (28 out of 34; 82.4%) with mucoid degeneration were symptomatic; however, 24 of these patients (85.7%) had additional MRI findings that were likely the primary cause of knee pain, and the others suffered from posterior knee pain for several years, without difficulty in flexion and extension of the knee. No patient had ligamentous instability on physical examination. The patients with mucoid degeneration and a ganglion had posterior knee pain.

Diffuse thickening was present in seven patients (Fig. 1), partial thickening in 16 (four associated with a ganglion) (Fig. 3), and longitudinal intraligamentous PCL signal-intensity abnormalities, the so-called “tram-track appearance” of the PCL, in 15 (Fig. 2). In all cases, there was increased signal intensity on fluid-sensitive sequences.

In the previous reports, only three cases of PCL mucoid degeneration (7.9%) were described with tram-track appearance (Fig. 2). In the reports of the patients with degeneration and ganglia, only the ganglia were described (Fig. 3).

Twelve out of 18 patients with ganglia were symptomatic with posterior knee pain. All were correctly diagnosed in the previous reports as cystic fluid lesions on fluid-sensitive sequences (Fig. 4).

There was no statistically significant difference for age (42 ± 17 years versus 42 ± 11 years; P = 0.99) or sex (41.3% versus 30.8% of women; P = 0.14) between patients with and without degenerative changes of the PCL, respectively.

Discussion

The PCL is the strongest ligament of the knee. It arises from the anterolateral surface of the medial femoral condyle and reaches the posterior intercondylar area of the tibia. The PCL consists of two functional bundles: the anterolateral and the posteromedial. It is the primary restraint to posterior tibial translation relative to the femur (17,19). On MRI, the normal PCL appears as a broad curvilinear band of low-signal intensity on fluid-sensitive-sequences and it is usually 6 mm or less in antero-posterior diameter (10,20). An antero-posterior diameter of 7 mm or more and increased intrasubstance signal intensity in the PCL are suggestive of a tear (17). However, thickening and increased signal intensity do not always correspond to a tear, as mucoid degeneration also appears as increased signal on fluid-sensitive images and may be associated with an antero-posterior diameter greater than 6 mm (10). As a result, neither increased PCL signal intensity nor ligament thickness reliably discriminates PCL degenerative changes from tears, thus additional criteria are needed. In particular, preceding episodes of trauma or MRI findings indicative of a prior knee injury are possible indicators of a tear, while ligament bundles intact from their origin to insertion are highly suggestive of mucoid degeneration. In our study, after exclusion of individuals presenting with knee trauma or MRI findings suggestive of prior injuries, a total of 52 patients had PCL degenerative changes, mostly being cases of mucoid degeneration. Absence of ligamentous instability in patients with PCL mucoid degeneration, assessed through the posterior drawer test that is highly sensitive and specific for PCL injuries (21), supported the validity of our exclusion criteria.

In the present study, 38 individuals out of 692 (5.5%) presented with a mucoid degeneration of the PCL. This result does not agree with the only retrospective study available in the literature attempting to evaluate the prevalence of PCL mucoid degeneration, which was reported as 0.1% (10). This work was, however, exclusively based on a database search in which key words describing PCL abnormalities were used: since mucoid degeneration of the PCL has received poor emphasis in the literature and is not mentioned even in the major textbooks, it may be difficult for general radiologists to achieve this diagnosis. Second, in this previous study only the “tram-track appearance” was considered as imaging criterion of PCL mucoid degeneration, although PCL thickening was also described in association with such intra-ligamentous signal intensity change. In the present study, the imaging criteria of mucoid degeneration of the PCL included both intra-ligamentous signal abnormalities resembling a “tram track” and thickening in PCL antero-posterior diameter, either partial or diffuse, which was further described in previous case reports (1–3). Importantly, three-plane evaluation allowed us to distinguish the meniscofemoral ligaments, which are adjacent to the PCL and may be erroneously interpreted as part of its peripheral rim in only one plane.

Mucoid degeneration of the PCL and intraligamentous ganglia may be easily distinguished on the basis of signal intensity, which in mucoid degeneration is not frankly cystic and not as bright as the synovial fluid on fluid-sensitive sequences (6). In our patients with PCL mucoid degeneration, four individuals also had an intra-articular ganglion. This result well correlates with the literature, as the coexistence of mucoid degeneration and ganglia in the PCL as well in the ACL has already been described and a common pathogenesis is thus hypothesized (9,10). In our study, 14 individuals out of 652 (2.0%) were affected with a PCL ganglion that was not associated with mucoid degeneration. These data agree with the literature, as intra-articular ganglia are regarded as uncommon lesions with a prevalence of 0.2–1.9% on imaging or arthroscopy (11,12). Occurrence in middle-aged people in our patients affected with degenerative changes of the PCL also well correlates with the previous studies (10,12,14,18).

All PCL ganglia were correctly diagnosed in the previous reports at our institute, thus reflecting an optimal confidence of radiologists with these entities. Contrastingly, only a few cases of PCL mucoid degeneration (7.9%) were previously described. This may be explained by the fact that, unlike mucoid degeneration, ganglia associated to the PCL are well-known and easily recognizable, since they appear as well defined, smooth-walled, lobulated, or multiloculated lesions and show very high signal on fluid-sensitive sequences (14,22). Mucoid degeneration of the PCL is often missed by radiologists or misdiagnosed as a PCL tear, as reported above, thus confirming that it is still poorly understood.

Some limitations should be considered in this study. First, it was performed in a single center by musculoskeletal radiologists who had similar experience. Second, the diagnosis of mucoid degeneration or ganglia was not surgically or arthroscopically confirmed. However, in daily clinical practice most patients with PCL mucoid degeneration will not demand surgical or arthroscopic intervention, as they usually do not have signs of ligamentous instability. More importantly, MRI is regarded as superior to arthroscopy in diagnosing intrasubstance ligamentous abnormalities (6). The finding of a clearly abnormal PCL strictly following the above-described imaging criteria and not associated with knee instability, prior knee trauma or injury should represent the key to a correct diagnosis.

In conclusion, mucoid degeneration and ganglia are likely to represent a continuum of degenerative changes of the PCL. Ganglia are well described in the literature and very easily diagnosed by radiologists; on the other hand, mucoid degeneration lacks appropriate references in the literature, but it is more common than previously assumed and underestimated by radiologists. Mucoid degeneration of the PCL should be diagnosed whenever the ligament is intact and thickened, with increased signal intensity on fluid-sensitive sequences, in patients with no history of trauma or knee injury and no clinical signs of ligamentous instability.