Original Report MR Imaging of Infrapatellar Plica Injury OBJECTIVE. CONCLUSION. Materials and Methods

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1 R. Lee Cothran 1 Philip M. McGuire 1,2 Clyde A. Helms 1 Nancy M. Major 1 David E. Attarian 3 Received October 25, 2001; accepted after revision October 11, Department of Radiology, Box 3808, Duke University Medical Center, Durham, NC Address correspondence to R. L. Cothran. 2 Present address: Radiology Alliance, P.A., th Ave. N., Ste. 602, Nashville, TN Division of Orthopaedic Surgery, Duke University Medical Center, Durham, NC AJR 2003;180: X/03/ American Roentgen Ray Society Original Report MR Imaging of Infrapatellar Plica Injury OBJECTIVE. Injury to the infrapatellar plica (ligamentum mucosum) has not been previously described in the radiology literature to our knowledge. This article shows the MR imaging appearance of injury to the infrapatellar plica. CONCLUSION. Injury to the infrapatellar plica is uncommon but should be considered as a potential source of knee pain, especially if no other evidence indicates internal derangement. MR imaging can reveal a typical appearance for infrapatellar plica injury. T he plicae of the knee are normal synovial folds that are remnants from the embryologic development of the knee [1]. At arthroscopy a normal infrapatellar plica has a variable appearance. It is a thin, pliable fold of synovial tissue with elastic and areolar components. The infrapatellar plica may be a complete septum or may be partially attached to the anterior cruciate ligament. The infrapatellar plica may be split, fenestrated, or absent [2, 3]. Plica syndrome is a well-known clinical entity whereby the normally thin, pliable synovial plica becomes abnormally thickened, edematous, or fibrotic, leading to clinical symptoms. This syndrome usually involves the mediopatellar plica or, sometimes, the suprapatellar plica [4]. Traditionally, the infrapatellar plica has been thought to be incidental and not a source of symptoms [2]. However, some orthopedic surgeons at our institution have told us that they have encountered abnormal infrapatellar plicae, sometimes thickened, fibrotic, or acutely ruptured with hemarthrosis (Feagin J, personal communication). We retrospectively reviewed the preoperative MR images from one of these patients and noted a high T2 signal along the course of the infrapatellar plica. This finding led us to perform a database search for other patients who had signal abnormality reported in this region on knee MR imaging. The purpose of this article is to show the MR imaging appearance of abnormal infrapatellar plicae and to clinically correlate these findings with the patients symptoms and arthroscopy findings. Materials and Methods Our MR imaging database was retrospectively searched for MR imaging knee examinations performed between January 1995 and April 2001, in which signal abnormality was reported in the approximate region of the infrapatellar plica. Of 5237 MR imaging studies of the knee, nine knees were interpreted prospectively as having curvilinear signal abnormality associated with the infrapatellar plica. The prospective interpretation was performed in the scope of daily dictation during this period by one of six musculoskeletal radiologists. The cases prospectively interpreted as containing a high T2 signal in the region of the infrapatellar plica were then reexamined in a retrospective manner by two musculoskeletal radiologists; confirmation of the abnormal signal in the region of the infrapatellar plica was by consensus. An additional case was submitted by an orthopedic surgeon after arthroscopy revealed an abnormally thickened, hypertrophic infrapatellar plica. These images revealed abnormal signal along the infrapatellar plica on retrospective review but were not prospectively interpreted as having an abnormal infrapatellar plica. The MR imaging findings were correlated with clinic notes, arthroscopic reports (five knees in four patients), and discussion with the orthopedic surgeons. All patients were imaged on a 1.5-T MR im- AJR:180, May

2 Cothran et al. aging system (Signa; General Electric Medical Systems, Milwaukee, WI) with a routine knee protocol including sagittal fat-suppressed spin-echo proton density weighted images (TR/TE, 20/2000) and sagittal, axial, and coronal fast spin-echo T2weighted images with fat suppression (TR range/ TE range, /65 75). The sagittal fast spin-echo T2-weighted images with fat suppression were the primary images used to evaluate the infrapatellar plica. The signal abnormality could also be seen on the fast spin-echo T2-weighted axial and coronal images and on the proton density weighted sagittal sequence, but the sagittal T2-weighted sequence best showed the signal abnormality. The infrapatellar plica was considered abnormal when a high T2 signal extended along the course of the in- frapatellar plica. Only those five knees in four patients who had been examined arthroscopically at the time of retrospective review are included in the results and discussion. Results On MR imaging, a normal infrapatellar plica is best appreciated on sagittal images as a thin, low-signal, curvilinear structure coursing between the intercondylar notch and the inferior pole of the patella or adjacent fat [5] (Fig. 1). The infrapatellar plica is normally isointense to other ligaments on all sequences [5, 6]. On MR imaging, all five knees had a high T2 signal associated with the infrapatellar plica that extended into the Hoffa fat pad, suggesting injury to the plica. The signal abnormality was typically curvilinear, but two knees also had a globular component. The studies included one male and three females one patient had bilateral infrapatellar plica abnormalities (Fig. 2). All patients were young, ranging in age from 14 to 38 years (average age, 24.8 years). A history of prior sports-related trauma was noted for four of the five knees. Return to the sporting activity before injury is known to have occurred in three of these four knees (two of Fig. 1. Normal infrapatellar plica. A, Schematic drawing of knee in sagittal section through intercondylar notch shows infrapatellar plica (black arrow ) extending from inferior pole of patella (P) or immediately adjacent fat, through Hoffa s fat pad, to intercondylar notch of femur anterior to anterior cruciate ligament (white arrow ). B, Sagittal fast spin-echo T2-weighted MR image (TR/TE, 4000/72) with fat suppression through intercondylar notch shows normal infrapatellar plica as thin, linear low-signal-intensity structure (black arrow ) in Hoffa s fat with more prominent intercondylar component (straight white arrows) lying anterior to anterior cruciate ligament (curved white arrows), proximal attachment in intercondylar portion of femur, and distal visualized portion attaching to prominent transverse ligament. C, Sagittal fast spin-echo T2-weighted MR image (4000/72) with fat suppression through intercondylar notch shows partially resorbed or less prominent intercondylar infrapatellar plica (arrows), with portion in Hoffa s fat as only visible component on MR image. A B 1444 C AJR:180, May 2003

3 MR Imaging of the Knee A B Fig year-old woman soccer player with anterior knee pain. A, Sagittal fast spin-echo T2-weighted MR image (TR/TE, 4000/69) with fat suppression through intercondylar notch shows curvilinear high T2 signal along course of infrapatellar plica (arrow ). Fluid signal immediately anterior to anterior cruciate ligament in intercondylar notch may be related to infrapatellar plica avulsion or may simply represent joint fluid. Other findings on MR imaging included discoid lateral meniscus and mediopatellar plica (not shown). At arthroscopy (not shown), infrapatellar plica was thickened and avulsed from its femoral attachment, and redundant infrapatellar plica interfered with full extension. Infrapatellar plica was resected; after surgery, patient was asymptomatic and resumed playing soccer. B, One year later patient injured her contralateral knee and suffered bucket-handle tear of discoid lateral meniscus. Sagittal fast spin-echo T2-weighted MR image (4000/70) with fat suppression through intercondylar notch shows fluid signal along course of infrapatellar plica (white arrows), which was interpreted as injury to infrapatellar plica. Infrapatellar plica was arthroscopically resected, and meniscus was débrided. Fragment from bucket-handle meniscus tear can be seen in posterior joint (black arrow ). these knees were in a single individual). The knee without a known history of prior sports trauma did have a history of prior arthroscopic surgery for cartilage débridement. Three of the five knees exhibited a limitation to extension without history of prior surgery. In two of these cases, redundant tissue was seen anterior to the anterior cruciate ligament (Fig. 3). This tissue was resected at arthroscopy, which confirmed infrapatellar plica injury in four of the five knees. Four of five knees had other findings that were addressed at arthroscopy (Table 1). mal tissue may persist as a plica, especially in the supra-, infra-, and mediopatellar regions. These synovial folds usually partially or totally regress, and they infrequently cause symptoms in the adult. Rarely, the suprapatellar or infrapatellar plicae may persist in their entirety, which leads to persistent complete compartmentalization of the joint [1]. The plicae are usually of no clinical significance, with a normal plica in the adult being a thin, pliable asymptomatic synovial fold containing abundant elastic and areolar tissue. The presence of elastic tissue allows changes in shape and length as the plica glides over bony structures. Occasionally, however, the plicae may give rise to clinical symptoms. The mediopatellar plica is most commonly symptomatic, becoming thickened, edematous, and eventually fibrotic. This change in morphology may lead to a snapping sound as the plica moves over a bony protuberance. Associated mechanical or Discussion Some debate exists as to the origin of the plicae of the knee. A common theory is that the plicae are inconstant synovial remnants from the embryologic development of the knee. The knee joint is believed to be originally composed of three compartments during embryologic development: medial, lateral, and suprapatellar. A recent embryologic study suggests that in the first trimester, multiple cavities exist, which coalesce to form larger cavities, and by 10.5 weeks the knee joint consists of a single cavity with synovial lining. After coalescence of the small cavities, a small amount of mesenchy- AJR:180, May 2003 Fig year-old woman with skiing injury. Sagittal fast spin-echo T2-weighted MR image (TR/TE, 4000/67) with fat suppression through knee 6 months after injury shows fluid signal along course of infrapatellar plica (arrows) interpreted as torn anterior cruciate ligament with associated rupture of infrapatellar plica. At arthroscopy 2 months later (not shown), scar tissue was found in expected position of infrapatellar plica, suggesting that plica had been injured. 1445

4 Cothran et al. TABLE 1 a 1 yr later. Patients Histories, Imaging Findings, and Surgical Impressions of Infrapatellar Plicae Patient No. 1 F 18 Soccer goalie, mechanical knee symptoms Sex Age History Imaging Findings Surgical Impression inflammatory synovitis may also occur [2]. The suprapatellar plica may cause symptoms if it is imperforate or has a one-way communication causing fluid to become loculated in the most superior aspect of the suprapatellar pouch cephalad to the suprapatellar plica. The infrapatellar plica, or ligamentum mucosum, is a vestigial remnant of the embryonic tissue and is typically not thought to be a source of clinical symptoms. The infrapatellar plica has a narrow femoral attachment in the intercondylar notch of the femur. The infrapatellar plica attaches just anterior to the anterior cruciate ligament and parallels that ligament for a short distance. The infrapatellar plica then curves gently upward to attach to the infrapatellar fat pad or inferior pole of the patella. The alar folds continue laterally to cover the Hoffa fat pad [4, 5]. On occasion, partial attachment to the anterior cruciate ligament or anterior horn lateral meniscus may be seen [3]. Discoid lateral meniscus, mediopatellar plica 1 a F 19 Soccer injury Discoid lateral meniscus, bucket-handle tear 2 M 14 Remote baseball injury, Contusion to anterior tibia popping, limited extension 3 F 29 Known chondral injury, previous arthroscopy Patellar chondral abnormality 4 F 38 Skiing accident Anterior cruciate ligament tear Not mentioned, presumed normal The prevalence of plicae reported in the literature varies according to the method of detection; plicae are easier to see and thus more commonly reported on arthroscopy than on arthrography. On double-contrast arthrography, the infrapatellar plica was detected in only 10% of knees [7]. On cadaveric dissection, the infrapatellar plica was seen in 65% of patients [5]. On arthroscopy, the infrapatellar plica has been reported as being frequently present [5]. On MR imaging, the infrapatellar plica is best seen on sagittal images, with a curvilinear appearance as it courses in an anteroposterior orientation [3 5]. An injured or diseased infrapatellar plica is suggested when a significant amount of curvilinear high T2 signal is seen along the expected course of the infrapatellar plica, or if a markedly thickened plica is visualized. Patel et al. [8] reported seeing a horizontal cleft in the infrapatellar fat pad on MR imaging in 90% of knees, with the infrapatellar Fig year-old male collegiate basketball player with bilateral anterior knee pain. Sagittal fast spin-echo T2-weighted MR image (TR/TE, 4000/73) with fat suppression through left intercondylar notch shows curvilinear high signal intensity along course of infrapatellar plica (arrows). Knee was otherwise normal. plica forming the roof of the cleft. This cleft did not account for the abnormal signal along the infrapatellar plica in the cases we have described, because the signal in our series was more extensive along the course of the infrapatellar plica, and slightly cephalad in location relative to the horizontal cleft. To our knowledge, abnormalities of the infrapatellar plica have not been described in the radiology literature. The orthopedic literature contains a report of two cases, with the infrapatellar plica described as thickened, fibrotic, impinging on the notch, and preventing full extension. Resection led to improved range of motion [9]. Also found in the orthopedic literature is an anatomic and clinical study that describes a torn infrapatellar plica as the only abnormality in three of 57 patients with posttraumatic hemarthrosis and a clinically stable joint [10]. That same article also describes the presence of a small artery in the infrapatellar synovial fold in five of 12 cadaveric knee dissections [10]. lesions of Hoffa s fat pad as seen on MR imaging have been described previously, including Hoffa s disease or Hoffa s syndrome [11]. In addition, the clinical and histologic findings have been reported in cases of Hoffa s disease, in which there is injury to, or hemorrhage within, Hoffa s fat pad, usually related to trauma. This injury then results in impingement of portions of the fat in extension, which results in pain and functional impairment. Chronically, such an injury may result in patellar crepitus [12]. Magi et al. [12] also state in their conclusion, It is a wellknown fact that many cases of Hoffa disease have been misdiagnosed and incorrectly treated as meniscal syndromes. Possibly the abnormal signal we are seeing around the infrapatellar plica is related to Hoffa s disease or Hoffa s syndrome. Infrapatellar plica injury of an acute or chronic nature would likely be associated with injury to Hoffa s fat pad. In fact, Smillie [13] writes of the infrapatellar plica: The clinical importance of the fold, apart from a source of haemorrhage if divided, is that by anchoring the fat pad it may limit expansion in a forward direction when swelling occurs and thus may be responsible for the compression of the synovial membrane. Reports of the MR imaging appearance of Hoffa s disease are relatively few, but imaging findings have tended to be more dramatic than the findings in our series, which leads us to suggest that, if they are related, the findings in our series may represent a less uncommon or more subtle imaging presentation of what was 1446 AJR:180, May 2003

5 MR Imaging of the Knee once thought to be a relatively uncommon clinical problem. We believe that abnormal increased signal along the infrapatellar plica at MR imaging may be indicative of direct trauma to the plica or to inflammation of the plica that is possibly related to Hoffa s disease or syndrome. Also, an acute rupture of the infrapatellar plica may mimic an anterior cruciate ligament rupture with a painful pop and hemarthrosis, with MR imaging showing the anterior cruciate ligament to be intact and a high T2 signal along the course of the infrapatellar plica. However, because abnormality of the infrapatellar plica appears to be uncommon in isolation, we recommend close correlation with the clinical symptoms and exclusion of other internal derangement before suggesting injury to the infrapatellar plica as a sole cause for knee symptoms. We have seen isolated abnormal signal in the infrapatellar plica in a collegiate basketball player with no other abnormalities detected on MR imaging (Fig. 4). Examination by an experienced orthopedic surgeon confirmed the anterior location of the pain, and an abnormal or injured infrapatellar plica was thought by the examiner to be the probable cause after review of the MR images. In summary, injury to the infrapatellar plica should be considered as a potential cause of knee pain or hemarthrosis, particularly in patients with signal abnormality of the infrapatellar plica and no other evidence of internal derangement. References 1. Ogata S, Uhthoff H. The development of synovial plicae in human knee joints: an embryologic study. Arthroscopy 1990;6: Hardaker W, Whipple T, Basset F. Diagnosis and treatment of the plica syndrome of the knee. J Bone Joint Surg Am 1980;62: Kim S, Min B, Kim H. Arthroscopic anatomy of the infrapatellar plica. Arthroscopy 1996;12: Apple JS, Martinez S, Hardaker WT, Daffner RH, Gehweiler JA. Synovial plicae of the knee. Skeletal Radiol 1982;7: Kosarek FJ, Helms CA. The MR appearance of the infrapatellar plica. AJR 1999;172: Boles CA, Martin DF. Synovial plicae in the knee. AJR 2001;177: Brody GA, Pavlov H, Warren RF, Ghelman B. Plica synovialis infrapatellaris: arthrographic sign of anterior cruciate ligament disruption. AJR 1983;140: Patel SJ, Kaplan PA, Dussault RG, Kahler DM. Anatomy and clinical significance of the horizontal cleft in the infrapatellar fat pad of the knee: MR imaging. AJR 1998;170: Kim S, Choe W. Pathological infrapatellar plica: a report of two cases and literature review. Arthroscopy 1996;12: Kohn D, Deiler S, Rudert M. Arterial blood supply of the infrapatellar fat pad: anatomy and clinical consequences. Arch Orthop Trauma Surg 1995;114: Jacobson JA, Lenchik L, Ruhoy MK, Schweitzer ME, Resnick D. MR imaging of the infrapatellar fat pad of Hoffa. RadioGraphics 1997;17: Magi M, Branca A, Bucca C, Langerame V. Hoffa disease. Ital J Orthop Trauma 1991;17: Smillie I. Diseases of the knee joint, 2nd ed. New York: Churchill Livingstone, 1980:162 AJR:180, May