394 AJR:201, August 2013

al fibers (Fig. 1) is an appearance analogous to the classic “celery stalk” morphology of ACL mucoid degeneration [4]. Given the stability of the PCL on physical examination and the MRI appearance of the PCL being similar to mucoid degeneration in the ACL, we postulated that these MRI findings represent mucoid degen-eration of the PCL. Ultimately, findings from a physical examination performed of the index patient under anesthesia were normal and knee arthroscopy revealed an intact PCL with a bul-bous morphology and a diffuse yellowish hue, which was called “PCL mucoid degeneration” by the orthopedic surgeon.

The purpose of this study was to evaluate tram-track longitudinal intraligamentous PCL signal-intensity abnormalities, which is simi-lar to the MRI appearance of mucoid degenera-tion of the ACL. We also sought to differentiate this longitudinal intraligamentous PCL signal-intensity abnormality from PCL tears and to determine the incidence of coexistent ACL mu-coid degeneration, cruciate ganglia, and para-meniscal and intrameniscal cysts.

Materials and MethodsInstitutional review board approval and a waiv-

er of informed consent were obtained for this ret-rospective study. After our index case, we encoun-tered five additional knee MRI examinations that had findings similar to the index case. We then conducted a retrospective review of our institu-tion’s knee MRI database of records over an 8-year period (12,972 cases). We searched for the key-

Tram-Track Appearance of the Posterior Cruciate Ligament (PCL): Correlations With Mucoid Degeneration, Ligamentous Stability, and Differentiation From PCL Tears

J. Scott McMonagle1

Clyde A. Helms1

William E. Garrett, Jr.2

Emily N. Vinson1

McMonagle JS, Helms CA, Garrett WE Jr, Vinson EN

1Department of Radiology, Duke University Health System, Box 3808, Durham, NC 27710. Address correspondence to J. S. McMonagle (jscottmcm@gmail.com).

2Department of Orthopaedic Surgery, Duke University Health System, Durham, NC.

Musculoskeleta l Imaging • Cl in ica l Perspect ive

AJR 2013; 201:394–399

0361–803X/13/2012–394

© American Roentgen Ray Society

Although the MRI appearances of complete tears, partial tears, and chronic degeneration of the ante-rior cruciate ligament (ACL) have

been thoroughly described, comparable stud-ies in the literature about the posterior cruciate ligament (PCL) are lacking. In particular, the MRI appearance of ACL mucoid degeneration was first described in 1999 [1]. Since then, our understanding of ACL mucoid degeneration has evolved and is now well established [2–5]. Histologic evaluations of cruciate ligaments harvested from cadavers and from patients un-dergoing knee replacement surgery have long described a link between degenerative chang-es in the ACL and those in the PCL, particu-larly mucoid degeneration [6–8]. Despite the association of mucoid degeneration of the cru-ciate ligaments and the long-standing reports about the MRI appearance of ACL mucoid de-generation, only a few studies in the radiology literature describe the MRI appearance of mu-coid degeneration of the PCL [9, 10].

This discrepancy was highlighted at our in-stitution’s weekly sports medicine conference when an orthopedic surgeon showed a knee MRI examination that revealed diffuse PCL thickening and increased signal intensity—both of which are MRI findings associated with a PCL tear [11]. However, this index patient had normal results on a posterior drawer ex-amination. Homogeneous increased signal in-tensity that extends longitudinally throughout a thickened PCL with an intact rim of peripher-

Keywords: MRI, mucoid degeneration, posterior cruciate ligament

DOI:10.2214/AJR.11.7400

Received June 21, 2011; accepted after revision October 23, 2012.

Presented at the 2011 annual meeting of the American Roentgen Ray Society, Chicago, IL.

OBJECTIVE. The purpose of our study was to describe the MRI findings in the posterior cruciate ligament (PCL) analogous to mucoid degeneration in the anterior cruciate ligament (ACL); to correlate MRI findings in the PCL with ligamentous stability; to differentiate the PCL tram-track appearance from the appearance of PCL tears; and to emphasize the coexis-tence of PCL and ACL mucoid degeneration, cruciate ganglia, and meniscal cysts.

CONCLUSION. The tram-track PCL appearance commonly coexists with ACL mucoid degeneration; ganglia; and, less frequently, meniscal cysts. Both PCL tears and MRI findings suggestive of PCL mucoid degeneration show ligament thickening and increased PCL signal inten-sity. Tram-track PCLs are usually asymptomatic and typically have no ligamentous instability.

McMonagle et al.MRI of the PCL

Musculoskeletal ImagingClinical Perspective

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words “PCL cyst,” “PCL signal,” and “abnormal PCL,” which yielded 61 MRI studies.

All imaging studies were performed on a 1.5-T unit (Signa, GE Healthcare) according to our stan-dard knee MRI protocol: sagittal fat-suppressed spin-echo proton density images (TR range/TE, 1500–2000/20) and sagittal, axial, and coronal fat-suppressed fast spin-echo (FSE) T2-weighted images (TR range/TE range, 2000–5000/57–80). The number of signals acquired was 2 for the FSE sequences and 1 for the proton density sequences. The echo-train length was 8–10 for the FSE se-quences. A slice thickness of 4 mm with a 0.4-mm interslice gap was used. The FOV was 16 cm, and the matrix size was 256 × 192.

These MRI examinations were retrospectively reviewed in consensus by two musculoskeletal ra-diologists with 5 and 33 years’ experience and one musculoskeletal radiology fellow. Based on our in-dex case, the imaging criteria of PCL mucoid de-generation included homogeneous longitudinal in-creased signal intensity in the PCL in all planes on proton density and T2 images with an adjacent well-defined intact rim of hypointense PCL fibers that give the PCL a tram-track appearance. In an attempt to prevent potential inclusion of injured PCLs, pa-tients were excluded if they had a clinical history of knee trauma or MRI findings indicative of prior knee injury. Of the 61 patients from our database search, 47 were excluded: 23 patients had a clini-cal history of acute trauma, 11 had MRI findings of an acute ACL tear, four had an ACL-deficient knee, and nine had bone contusions with an intact ACL.

Thus, there were 20 patients in the PCL mu-coid degeneration group: our index case, the five cases subsequently encountered in the reading room, and 14 examinations from the retrospec-tive search. Each case fulfilled the defined imag-ing criteria of PCL mucoid degeneration. There were eight women (40%) and 12 men (60%) with a mean age of 51.7 years (age range, 32–75 years). In an effort to differentiate MRI findings of PCL mucoid degeneration from those of PCL tears, we included for review an equal number of patients with surgically proven PCL tears. This group in-cluded seven women (35%) and 13 men (65%) with a mean age of 35.2 years (age range, 18–69 years).

Several different parameters for these 40 stud-ies—20 examinations that met the imaging crite-ria for PCL mucoid degeneration and 20 examina-tions of surgically proven PCL tears—were then retrospectively evaluated, again in consensus. Re-viewers were blinded to the preliminary consen-sus interpretations and therefore did not know if the study had been included as PCL mucoid degen-eration or as a surgically proven PCL tear. Using the imaging criteria of PCL mucoid degeneration, the reviewers first decided whether the study rep-

resented PCL mucoid degeneration or a PCL tear. The transverse anteroposterior diameter of the ver-tical portion of each PCL was measured on sagit-tal T2-weighted images. The PCL was evaluated for the presence or absence of complete fiber dis-ruption. The intrasubstance signal intensity of each PCL on proton density–weighted and T2-weighted sequences was graded as hyperintense (isointense to fluid), intermediate (increased signal intensity but less than signal intensity of joint fluid), or hy-pointense (normal PCL signal intensity). The rela-tive signal intensities on proton density– and T2 images were then compared with each other. Coex-istent MRI findings of ACL mucoid degeneration, ganglia involving or adjacent to the PCL, intrame-niscal cysts, parameniscal cysts, and intraosseous cysts at either the femoral or tibial PCL attach-ments were noted.

The electronic medical records of all patients were reviewed: Each patient’s presenting symp-toms, the clinical assessment of the PCL by an orthopedic specialist, and arthroscopic results, if available, were recorded.

ResultsThe tram-track appearance, our inclusion

criterion, reliably differentiated all 20 of the PCLs included as the mucoid degeneration group from the 20 PCLs with surgically proven tears. Of the 20 PCL tears, 19 (95%) showed amorphous or heterogeneously striated in-creased signal intensity without adjacent well-defined or intact peripheral fibers on both pro-ton density–weighted and T2-weighted images; these findings were easily differentiated from the tram-track appearance. Only on sagittal im-ages, one of the PCL tears (5%) showed linear, longitudinally oriented increased intraligamen-tous signal intensity that was somewhat similar

to the tram-track appearance (Fig. 2). However, although the peripheral rim of fibers appeared intact on sagittal images, the peripheral fibers were clearly discontinuous on both the coro-nal and axial images. Further, the longitudinal-ly oriented increased signal intensity through-out this PCL tear was heterogeneous and had a striated look, with multiple separate lines of increased signal intensity—as is often seen in PCL tears; it was not a single solid and homo-geneous streak of increased signal intensity, further differentiating it from the tram-track ap-pearance [11]. By contrast, all 20 PCLs in the mucoid degeneration group had a tram-track appearance created by homogeneous, longitu-dinal increased signal intensity with discrete borders surrounded by intact hypointense PCL fibers validated on all three imaging planes (Fig. 3). Further, evaluation in all three planes distinguished adjacent structures, such as the meniscofemoral ligaments, oriented perpendic-ular to the PCL that could erroneously appear as part of an intact peripheral rim if the PCL had been evaluated in only a single plane.

The tram-track appearance was seen throughout the entire length of 18 PCLs (90%). One of the tram-track PCLs (5%) had intraligamentous signal intensity that in-volved only the proximal half of the PCL, whereas another (5%) had intraligamentous increased signal intensity with an intact pe-ripheral rim of fibers that extended through the proximal and middle thirds of the PCL but that spared the distal third of the PCL.

In the 20 patients with MRI findings that met the imaging criteria for PCL mucoid de-generation, 19 PCLs (95%) measured 7 mm or more in anteroposterior diameter on sag-ittal T2-weighted images. The average PCL

Fig. 1—38-year-old man with posterior knee pain. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 4000/70) of posterior cruciate ligament (PCL) shows diffuse thickening and longitudinally increased signal intensity with peripheral rim of hypointense PCL fibers (arrows). At arthroscopy, PCL was intact, was bulbous in morphology, and had yellowish hue.

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thickness for PCL mucoid degeneration was 8.5 mm, with a range of 6–12 mm. Among the surgically proven PCL tears, 17 PCLs (85%) had a transverse anteroposterior di-ameter that measured 7 mm or more. PCL tears had an average PCL thickness of 8.8 mm and a range of 4–13 mm.

Ten of the 20 PCLs (50%) from the surgi-cally proven PCL tear group appeared com-pletely disrupted or avulsed. Partial fiber dis-continuity was seen in none of the PCL tear group. Four (40% of the total) of the ligaments were proximal tears with complete fiber dis-ruption from the femoral attachment, three (30%) tears were midsubstance, and three (30%) PCLs were completely disrupted from the tibial attachment; two of these three distal tears also involved small avulsed and retract-ed tibial cortical bone fragments. By contrast, conforming to the criteria of the tram-track appearance, PCLs in the mucoid degeneration group had no disruption of the PCL fibers.

Hyperintense signal intensity was seen more often in the PCL mucoid degeneration group than in PCL tears (Table 1). Seven PCLs in the mucoid degeneration group (35%) had hyperintense intrasubstance signal on proton density images, whereas only one PCL tear (5%) had hyperintense signal intensity on pro-ton density images. None of the PCLs in the mucoid degeneration group had hypointense signal intensity on proton density–weighted or T2-weighted images. By comparison, the PCL tear group had hypointense signal inten-sity on T2-weighted images in six of 20 PCLs (30%), and the majority of PCL tears, 14 of 20 (70%), showed intermediate signal inten-

sity. In all 20 of the PCL tears (100%), the signal intensity on proton density images was relatively higher than on T2-weighted imag-es. The majority of PCLs in the mucoid de-generation group, 14 of 20 (70%), also had relatively increased signal intensity on proton density images when compared with the sig-nal intensity on T2-weighted images.

In nine of the 20 patients with PCLs with a tram-track appearance (45%), coexistent MRI findings of ACL mucoid degeneration were also present (Fig. 4). A coincident gan-glion exerted mass effect on, emanated from, or was immediately adjacent to the PCL in 16 (80%) of the PCLs in the mucoid degen-eration group (Fig. 5). Intrameniscal cysts were present in five knees in the PCL mu-coid degeneration group (25%), three in the lateral meniscus and two in the medial me-niscus, and parameniscal cysts were noted in two patients (10%), one adjacent to the later-

al meniscus posterior horn and one adjacent to the medial meniscus posterior body. In-traosseous cysts were noted in four of the 20 patients (20%) in the PCL mucoid degenera-tion group: Two were adjacent to the tibial at-tachment of the PCL and two were adjacent to the femoral attachment. Coexistent ACL mucoid degeneration was not seen in any of the patients with surgically proven PCL tears (0%). A ganglion was adjacent to the PCL in four of the 20 (20%) PCL tears. Two intrame-niscal cysts (10%) and one parameniscal cyst (5%) were present in the PCL tear group.

Review of each patient’s electronic med-ical record showed that knee pain was the most common presenting symptom, seen in all 20 patients with tram-track PCLs. How-ever, 18 of these 20 patients (90%) had addi-tional MRI findings that were likely the pri-mary cause of knee pain. All 20 patients with a tram-track PCL appearance (100%) had a

AFig. 2—54-year-old man with severe knee pain after motor vehicle crash.A, Sagittal fat-suppressed proton density–weighted MR image (TR/TE, 2000/20) shows thickening and multiple linear, longitudinally oriented intrasubstance striations of posterior cruciate ligament (PCL) and few peripheral hypointense PCL fibers (arrows).B and C, Axial (4000/65) (B) and coronal (4000/60) (C) fat-suppressed T2-weighted images show abnormal thickening and multiple heterogeneous linear striations of intermediate signal intensity throughout PCL, in contrast to homogeneous and solid intrasubstance signal intensity of mucoid degeneration. Additionally, peripheral rim of hypointense fibers (arrows) is not intact on axial or coronal images. At surgery, this PCL was described as torn.

CB

TABLE 1: Posterior Cruciate Ligament (PCL) Intrasubstance Signal Intensity

MRI Sequence

PCL Intrasubstance Signal Intensity

Hyperintensea Intermediateb Hypointensec

PCL mucoid degeneration

Proton density 7 (35) 13 (65) 0 (0)

FSE T2 4 (20) 16 (80) 0 (0)

PCL tears

Proton density 1 (5) 18 (90) 1 (5)

FSE T2 0 (0) 14 (70) 6 (30)

Note—Data presented are no. (%) of patients. FSE T2 = fast spin-echo T2-weighted. aIsointense to fluid.bIncreased signal intensity but less than signal intensity of joint fluid.cNormal PCL signal intensity.

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negative posterior drawer test on physical ex-amination by an orthopedic specialist. Two patients (10%) in this group also had poste-rior knee pain with knee flexion beyond 90° and one patient (5%) had posterior midline knee tenderness to palpation.

Of the 20 patients with a tram-track PCL appearance, six (30%) had subsequent ar-throscopy; all six patients had no ligamentous instability during their examination under an-esthesia. Five of the PCLs were described as normal and intact, whereas the sixth PCL, our index case, was intact but also was bul-bous with a diffuse yellowish hue.

DiscussionThe true incidence of PCL mucoid degen-

eration is difficult to assess because it is, as in our patient population, typically asymp-tomatic and incidentally noted. As a point of reference, Bergin et al. [2] reported that the incidence of mucoid degeneration in the ACL is 1.0% (44/4221). The incidence of

PCL mucoid degeneration in our retrospec-tive study population was 0.1% (14/12,972).

The normal appearance of the PCL on MRI is that of a broad curvilinear band of low signal intensity on proton density–weighted and T2-weighted images that is usually 6 mm or less in anteroposterior di-ameter. In 2008, Rodriguez et al. [11] re-ported two MRI findings that are highly suggestive of a torn PCL: an anteroposteri-or diameter of 7 mm or more and increased intrasubstance signal intensity in the PCL, particularly with PCLs showing relative-ly higher signal intensity on proton density images than T2-weighted images. However, PCL thickening and increased signal intensi-ty do not always equate to a functionally torn PCL: 19 of the 20 PCLs that met the imaging criteria for mucoid degeneration (95%) were greater than 7 mm in anteroposterior diame-ter and 14 (70%) PCLs in the mucoid degen-eration group had signal intensity that was relatively higher on proton density–weight-

ed images than on T2-weighted images. As a result, neither PCL thickness nor relatively higher signal intensity is a reliable discrim-inator and additional imaging criteria that differentiate PCL degenerative changes from PCL tears are requisite. The tram-track ap-pearance was the best MRI finding for dis-tinguishing between PCL mucoid degenera-tion and PCL tears in this small series.

Analogous to intrameniscal cysts, which are isointense to fluid in only 8.4% of the cases [12], the majority of PCL intrasubstance signal intensity, particularly in our tram-track PCL group, was not fluid signal intensity. Instead, PCLs in both study groups were usually in-termediate on proton density and T2-weight-ed imaging, 65% and 80% for PCL mucoid de-generation versus 90% and 70% for PCL tears, respectively. PCLs in the mucoid degeneration group were more likely to have intrasubstance signal intensity isointense to fluid on proton density images, 35%, as compared with only 5% of PCL tears. Relative signal intensities,

AFig. 4—65-year-old man with anterior knee pain and normal findings on anterior and posterior drawer clinical examinations.A–C, Consecutive coronal fat-suppressed T2-weighted MR images (TR/TE, 3700/50) show increased signal intensity and thickening of both anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL); these findings are consistent with ACL (black arrows) and PCL (white arrows) mucoid degeneration.

CB

BA CFig. 3—52-year-old woman with chronic left knee pain with stable posterior cruciate ligament (PCL) on clinical examination.A and B, Axial (TR/TE, 4000/65) (A), coronal (4000/68) (B), and sagittal (4000/68) (C) fat-suppressed T2-weighted MR images show typical appearance of PCL mucoid degeneration (arrows) with increased intraligamentous signal intensity and peripheral rim of intact hypointense PCL fibers. C, Sagittal image shows anterior meniscofemoral ligament (arrowhead) that is adjacent to but separate from intact peripheral PCL rim (arrows).

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however, cannot reliably distinguish between PCL mucoid degeneration and PCL tears, in-creasing the importance of additional MRI criteria such as the tram-track appearance.

Bergin et al. [2] reported that ACL mucoid degeneration involved the entire ACL in 93% of cases; similarly, the majority of our PCLs with a tram-track appearance, 18 of 20 (90%), involved the entire length of the PCL. One of the patients in our series had undergone subse-quent knee MRI 2 and 6 years after the initial MRI examination. The transverse and longitu-dinal dimensions of the hyperintense intralig-amentous signal intensity enlarged with each subsequent examination (Fig. 6), suggesting that just as degenerative changes of osteoar-thritis progress over time, so too does the ex-tent of PCL mucoid degeneration proceed.

The posterior drawer examination is the most accurate clinical test, with a reported sensitivity of 90% and specificity of 99% for orthopedic surgeons with fellowship training in sports medicine [13]. However, the accu-racy of the posterior drawer test significant-ly decreases with distracting injuries, when even subtle internal or external rotation of the tibia is present, or even when the knee is not precisely at 90° of flexion. The positive like-lihood ratio of the posterior drawer test also significantly decreases for less experienced providers and those without dedicated training [13]. Thus, physical examination of the PCL is highly valuable for assessing the functional competency of the ligament, but it is not with-out limitations. All of the patients in our PCL mucoid degeneration group had physical ex-

aminations that included a posterior drawer test. Further, the results were reported as ei-ther “normal” or “no laxity” for all 20 PCLs that had a tram-track appearance. All of the PCLs in the surgically proven PCL tear group, by contrast, had been preoperatively noted to have grade II or grade III PCL laxity.

Investigators have reported that the major-ity of PCL tears (62%) involve stretching de-formation and, as a result, do not have com-plete fiber avulsion or disruption [11]. In the group with a surgically proven PCL tear, 10 of the 20 PCLs (50%) showed fiber disruption. Consequently, although fiber disruption indi-cates a torn PCL, visualization of intact PCL fibers poorly correlates with functional compe-tence of the PCL. By comparison, all 20 PCLs (100%) that had a tram-track appearance with an intact peripheral rim of PCL fibers on all im-aging planes (i.e., the inclusion criteria for this study) had a mechanically intact PCL.

Similar to patients with ACL mucoid degen-eration who rarely complain of knee instability and typically have a negative Lachman sign [2], patients with PCL mucoid degeneration typi-cally do not have ligamentous instability. All six tram-track PCL patients who subsequent-ly underwent surgery also had a normal pos-terior drawer examination under anesthesia. Additionally, all six patients had an intact lig-ament at arthroscopy; in one patient, the PCL also had a bulbous appearance and yellow-ish hue. The arthroscopic appearance of these tram-track PCLs resembles the two possible arthroscopic appearances of ACL mucoid de-generation: normal or bulbous with a yellow-ish hue [14]. Although no specimens were obtained in our PCL mucoid degeneration

Fig. 5—59-year-old man who presented with 6-week history of posterior knee pain and fullness and no history of trauma. Sagittal fat-suppressed T2-weighted MR image (TR/TE, 3700/50) shows complex ganglion (arrow) that exerts mass effect on adjacent proximal posterior cruciate ligament (PCL). Longitudinal tram-track appearance of PCL mucoid degeneration is also seen. PCL ganglion was aspirated at surgery and PCL was described as normal and intact.

CA BFig. 6—37-year-old female triathlete with patellofemoral pain.A, Sagittal T2-weighted MR image (TR/TE, 3300/70) at initial presentation shows tram-track appearance (arrow) involving only proximal half of posterior cruciate ligament (PCL) (transverse dimension, 4.3 mm; longitudinal dimension, 12.0 mm). Patient did not have meniscofemoral ligament.B, Sagittal T2-weighted MR image (4000/60) obtained 2 years after A shows intraligamentous tram-track appearance (arrow). PCL has increased in transverse dimension (5.5 mm) and longitudinal dimension (13.6 mm) since initial examination.C, Most recent sagittal T2-weighted MR image (3400/63) obtained 6 years after A shows continued increase in transverse (7.5 mm) and longitudinal (18.2 mm) size of tram-track appearance (arrow) relative to both earlier studies. At arthroscopy, PCL was reported as normal.

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group, Shoji et al. [10] described a PCL with MRI findings identical to our group that was ar-throscopically excised and revealed mucoid de-generation at histologic examination.

The cause of cruciate ligament mucoid de-generation remains controversial. Bergin et al. [2] described the coexistence of ACL mu-coid degeneration with ACL ganglia and in-traosseous cysts. The coincident association between ganglia and PCL mucoid degenera-tion, seen in 16 of our patients (80%), sug-gests that these two entities may also share a common pathogenesis in the PCL. Some in-vestigators have suggested that ganglia are a product of connective tissue mucoid degen-eration [15, 16], whereas others theorize that a congenital abnormality or previous injury facilitates and possibly predisposes cruci-ate ligaments to degenerative ganglion for-mation [17]. Nine of our patients (45%) had MRI findings of both ACL and PCL mu-coid degeneration, further supporting a com-mon pathogenesis among degeneration of the ACL and PCL, similar to what has previous-ly been reported in cadavers and in patients undergoing knee replacement surgery [6–8]. Seven meniscal cysts (35%)—five intrame-niscal cysts and two parameniscal cysts—were seen in our tram-track PCL group. Be-cause meniscal cysts are reportedly found in 8% of knee MRI studies [12], the increased prevalence of meniscal cysts in our tram-track PCL patients suggests a common de-generative pathway or at least that some patients, for anatomic or biomechanical rea-sons, are more predisposed to soft-tissue de-generative changes throughout the knee.

This study was limited by its retrospective and observational design. In an attempt to ex-clude injured PCLs, we chose to exclude pa-tients with a preceding episode of trauma or with MRI findings indicative of a prior knee injury. Our assumption of no prior knee in-jury relies heavily on each patient’s reported clinical history; obviously we cannot guaran-tee that each patient with tram-track PCL find-ings had absolutely no prior or remote histo-ry of knee trauma. Potential selection bias was unavoidable given the inclusion criteria for the PCL tram-track group. Partial PCL tears, which might have imaging features similar to or overlapping with those described with mu-

coid degeneration, were also potentially ex-cluded from the study. Mild or nonspecif-ic repetitive trauma and even healed partial tears have been espoused as potential causes of mucoid degeneration in the ACL [15]; simi-lar causality for PCL mucoid degeneration can be suggested and, to date, have not been de-finitively refuted. McIntyre et al. [3] described ACL mucoid degeneration that progressed to an atraumatic complete ACL tear. Significant selection bias was also present in our surgical-ly proven PCL tear group because progression to surgery implied clinical instability of the PCL. Because most PCL tears are not treat-ed surgically, there is uncertain clinical sig-nificance of distinguishing between PCL tears and PCL mucoid degeneration. This study is further limited by lack of direct histologic cor-relation; five of the six tram-track PCLs that proceeded to arthroscopy had a normal ap-pearance, similar to arthroscopic findings in cases of ACL mucoid degeneration.

In summary, although the pathogenesis of cruciate ligament mucoid degeneration re-mains unknown, it is important that MRI find-ings suggestive of PCL mucoid degeneration be differentiated from chronic or acute interstitial, partial, and complete PCL tears given their re-spective and important differences in ligamen-tous stability and the practical implications for clinical management. Clinical correlation in our study population indicates that the tram-track PCL appearance is usually asymptomatic and typically has no ligamentous laxity. The tram-track PCL appearance further informs clinical decision-making by allowing the radiologist to discriminate between mechanically unsta-ble torn PCLs and mechanically intact PCLs. Coexistent MRI findings suggest associations between PCL and ACL mucoid degeneration, ganglia, and potentially even meniscal cysts.

References 1. Kumar A, Bickerstaff DR, Grimwood JS, Suvarna

SK. Mucoid cystic degeneration of the cruciate

ligament. J Bone Joint Surg Br 1999; 81:304–305

2. Bergin D, Morrison WB, Carrino JA, Nallamshet-

ty SN, Bartolozzi AR. Anterior cruciate ligament

ganglia and mucoid degeneration: coexistence and

clinical correlation. AJR 2004; 182:1283–1287

3. McIntyre J, Moelleken S, Tirman P. Mucoid degen-

eration of the anterior cruciate ligament mistaken for

ligamentous tears. Skeletal Radiol 2001; 30:312–315

4. Papadopoulou P. The celery stalk sign. Radiology

2007; 245:916–917

5. Roberts CC, Towers JD, Spangehl MJ, Carrino JA,

Morrison WB. Advanced MR imaging of the cru-

ciate ligaments. Magn Reson Imaging Clin N Am

2007; 15:73–86

6. Stubbs G, Dahlstrom J, Papantoniou P, Cherian M.

Correlation between macroscopic changes of ar-

throsis and the posterior cruciate ligament histolo-

gy in the osteoarthritic knee. ANZ J Surg 2005;

75:1036–1040

7. Allain J, Goutallier D, Voisin MC. Macroscopic

and histological assessments of the cruciate liga-

ments in arthrosis of the knee. Acta Orthop Scand

2001; 72:266–269

8. Hodler J, Haghighi P, Trudell D, Resnick D. The

cruciate ligaments of the knee: correlation between

MR appearance and gross and histologic findings

in cadaveric specimens. AJR 1992; 159:357–360

9. Viana SL, Fernandes JL, Mendonca JL, Freitas FM.

Diffuse intrasubstance signal abnormalities of the

posterior cruciate ligament: the counterpart of the

mucoid degeneration of the anterior cruciate liga-

ment? A case series. JBR-BTR 2008; 91:245–248

10. Shoji T, Fujimoto E, Sasashige Y. Mucoid degen-

eration of the posterior cruciate ligament: a case re-

port. Knee Surg Sports Traumatol Arthrosc 2010;

18:130–133

11. Rodriguez W Jr, Vinson EN, Helms CA, Toth AP.

MRI appearance of posterior cruciate ligament

tears. AJR 2008; 191:1031

12. Anderson JJ, Connor GF, Helms CA. New observa-

tions on meniscal cysts. Skeletal Radiol 2010;

39:1187–1191

13. Rubinstein RA Jr, Shelbourne KD, McCarroll JR,

VanMeter CD, Rettig AC. The accuracy of the clinical

examination in the setting of posterior cruciate liga-

ment injuries. Am J Sports Med 1994; 22:550–557

14. Makino A, Pascual-Garrido C, Rolon A, Isola M,

Muscolo DL. Mucoid degeneration of the anterior

cruciate ligament: MRI, clinical, intraoperative,

and histological findings. Knee Surg Sports Trau-

matol Arthrosc 2011; 19:408–411

15. Fealy S, Kenter K, Dines JS, Warren RF. Mucoid

degeneration of the anterior cruciate ligament. Ar-

throscopy 2001; 17:E37

16. Liu SH, Osti L, Mirzayan R. Ganglion cysts of the

anterior cruciate ligament: a case report and review

of the literature. Arthroscopy 1994; 10:110–112

17. Levine J. A ganglion of the anterior cruciate liga-

ment. Surgery 1948; 24:836–840

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