REVIEW ARTICLE

Magnetic resonance imaging of anatomical variationsin the kneePart 2: Miscellaneous

Philippa Tyler & Abhijit Datir & Asif Saifuddin

Received: 25 December 2009 /Revised: 25 December 2009 /Accepted: 1 February 2010 /Published online: 11 March 2010# ISS 2010

Abstract Magnetic resonance imaging is the modality ofchoice for investigation of internal derangement of theknee. The reporting radiologist must be familiar with bothnormal anatomy and anatomical variants within the knee, inorder to avoid mis-diagnosis, over-investigation and unnec-essary intervention. This article reviews the recognisedanatomical variants of the non-ligamentous/musculotendi-nous structures of the knee, their anatomy, incidence andtypical appearances on MRI.

Keywords Normal variant . Knee anatomy .MRI

Introduction

Magnetic resonance imaging is the modality of choicefor the evaluation of internal derangement of the knee.

Improved technique with development of new sequencesand the greater availability of stronger magnets, com-bined with the excellent soft tissue contrast and multi-planar capabilities of MRI enables better visualisation ofsmaller structures within the knee. Normal variants ofthe knee are frequently encountered and a soundknowledge of both normal and variant anatomy allowsconfident diagnosis of these, thus avoiding mis-diagnosisand over-investigation, particularly in the pre-operativesetting.

This article reviews the non-ligamentous/musculotendi-nous anatomical variants seen on knee MRI.

Osseous variation

Patella

Bipartite patella

Bipartite patella is a common radiological finding, seen inapproximately 2% of individuals and refers to a secondaryossification centre that fails to unite with the main body ofthe patella. It is bilateral in approximately 40% of cases andis nine times more common in males than in females. Thebipartite patella is usually seen at 12 years of age and maypersist into adult life [1]. This variant is typically located atthe supero-lateral pole of the patella and is readily detectedusing all imaging modalities (Fig. 1). Three different typesof bipartite patella have been described, based on theposition of the bone fragment.

Type I Inferior pole of the patellaType II Lateral margin typeType III Supero-lateral type (most common)

P. Tyler (*) :A. SaifuddinDepartment of Radiology,The Royal National Orthopaedic Hospital NHS Trust,Brockley Hill,Stanmore, Middlesex HA7 4LP, UKe-mail: philippatyler@msn.com

P. TylerDepartment of Radiology, St Mary’s Hospital,Imperial College healthcare NHS Trust,London, UK

A. DatirDepartment of Radiology, Jackson Memorial Hospital,1611 NW 12th Avenue, West Wing-279,Miami, FL 33136, USA

A. SaifuddinThe Institute of Orthopaedics and Musculoskeletal Sciences,University College London,London, UK

Skeletal Radiol (2010) 39:1175–1186DOI 10.1007/s00256-010-0904-6

Although usually asymptomatic, it may sometimes beassociated with localised anterior knee pain [2]. In thesepatients, MRI reveals oedema and fluid at the interface ofthe bone fragment and native patella (Fig. 1b). This findingsuggests that the interface actually represents a chronicchondro-osseous tensile disruption, a finding supported bydevelopmental anatomy studies. Also, traumatic separationof bipartite patella is a known, but rare complication [3, 4].On MRI, the presence of intact hyaline cartilage overlyingthe defect (Fig. 1c) helps to differentiate this variant from apatellar fracture. Occasionally, there may be more than onebone fragment, qualifying for the term tripartite ormultipartite patella (Fig. 1d).

Dorsal defect of the patella

Dorsal defect of the patella (DDP) is a characteristic, well-defined defect in the supero-lateral aspect of the patella,occurring in approximately 1% of the population. Thepathogenesis of the DDP may be the same as that ofthe bipartite patella. Van Holsbeeck et al. [5] suggested thatthe initial abnormality is probably a traction lesion at theinsertion of the vastus lateralis muscle rather than ulcerationof articular cartilage. A possible relationship among

dysfunction of the quadriceps mechanism, patellar sublux-ation and the origin of the DDP has also been suggested.On MRI, the overlying articular cartilage appears intact andis often thickened to fill the defect (Fig. 2). The diameter ofthese lesions usually ranges from 4 to 26 mm with a meanof 9 mm [6]. Very rarely, this entity may be associated withpatellar hypoplasia and patello-femoral mal-alignment [7].

Variations in patellar shape

The articular surface of the patella is divided into facets byseveral ridges. A major vertical ridge divides the medialand lateral facets, while a second vertical ridge near themedial border isolates a narrow strip known as the oddfacet. Wiberg [8] classified patellae into three types, basedon the position of the major vertical ridge:

Type I The medial and lateral facets are equal in size(10%) (Fig. 3a)

Type II The medial facet is smaller (approximately half)than the lateral facet (65%) (Fig. 3b)

Type III The medial facet is very small and consequentlyusually is steeply angled and convex, while thelateral facet is broad and concave (25%) (Fig. 3c)

Fig. 1 Bipartite patella.a Sagittal proton density-weighted (PDW) fast spin-echo(FSE), b coronal and c axialfat-suppressed PDW FSEimages showing a supero-lateralbipartite patella (arrows). Notethe intact articular cartilage(arrowhead in c) and theoedema adjacent to the fragment(arrowhead in b). Tripartitepatella. d Coronal PDW FSEimages showing two additionalpatellar fragments (arrows)

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It has been shown that in the dysplastic patello-femoralarticulation (Fig. 3d), the medial facet of the patellabecomes smaller in relation to the lateral facet, fromproximal to distal [9]. In particular, the Wiberg type IIIhas been associated with disorders of patellar instability[10]. MRI is needed to define clearly the cartilaginous andosseous morphology of the patella before surgery isconsidered for patients with patello-femoral dysplasia.

Double patella

Duplication of the patella is rare and usually occurs inassociation with multiple epiphyseal dysplasia. Ficat [11]divided patellar duplication into two types: frontal (oneanterior to the other) and horizontal (one superior to theother). The term “double-layered patellae” was coined byHodkinson to describe these duplications [12]. Furthermore,

Fig. 2 Dorsal defect of thepatella. a Coronal PDW FSEimage showing a round defect(arrow) in the supero-lateralaspect of the patella. b Axialfat-suppressed PDW FSE imageshowing the articular cartilagefilling the defect (arrow)

Fig. 3 Patellar types. a AxialPDW FSE image showing aType 1 patella with equal size ofthe lateral and medial facets. bAxial PDW FSE image showinga Type 2 patella with the lateralfacet being approximately twicethe width of the medial facet. cAxial fat-suppressed PDW FSEimage showing a Type 3 patellawith a very small medial facet. dSagittal PDW FSE imageshowing trochlear dysplasia(arrow) in association with theType 3 patella

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variations such as vertical duplication and duplication in thecoronal plane have been described in the literature [13].

Fabella and cyamella

The fabella is a sesamoid bone located in the lateral head ofthe gastrocnemius muscle/tendon (Fig. 4a, b). It is reportedin approximately 13–20% of patients and is frequentlybilateral [14, 15]. We have noted that in the absence of thefabella, the proximal lateral head of the gastrocnemiustendon appears focally thickened (Fig. 4c). A fabella isusually asymptomatic, but may occasionally cause symp-toms secondary to a fracture, dislocation, osteoarthritis,erosions or chondromalacia [14].

The cyamella is a sesamoid bone found at the muscu-lotendinous junction of the popliteus tendon, and is seen farless frequently than the fabella. Osseous sesamoid boneshave the same imaging characteristics as normal bone andcare must be taken not to mistake them for a loose bodywithin the knee joint.

Cortical desmoids

A cortical desmoid (synonyms: Bufkin lesion, corticalirregularity, periosteal desmoid, parosteal juxtacorticaldesmoid, avulsive cortical irregularity) is a fibro-osseouslesion of the posterior medial distal femoral metaphysis.The original reported incidence was 11.5% in males and3.6% in females, based on analysis of plain radiographs[16]. However, in a more recent study reviewing theimaging of 100 knees, 58 lesions were identified, 14 ofwhich were only visible on MRI [17]. They are bilateral inapproximately one third of cases.

Cortical desmoids are thought to be secondary torepetitive traction at the femoral insertion of the medialhead of the gastrocnemius or adductor magnus aponeu-rosis, the latter being situated slightly more medially onthe adductor tubercle. They are usually an asymptomaticincidental finding, but may be mistaken for bonetumours. Lesions have been reported in sites moreproximal to the usual meta-epiphyseal location andMRI investigation of these cases has revealed ananomalously high insertion of the medial head of thegastrocnemius [17]. Histologically, cortical desmoids arereactive fibrous or fibro-osseous lesions consisting ofdense collagenous fibrous tissue, mixed with smallamounts of cartilage and reactive bone [18].

Three morphological types have been described—concave, convex and divergent, with the cortex appearingsplit and widened in the latter [17]. The concave type ismost commonly encountered and may resemble a fibrouscortical defect on plain radiographs.

A cortical desmoid is typically hypointense on T1- andintermediate signal to hyperintense on T2-weighted images(Fig. 5). Marginal sclerosis is seen as hypointensity on allsequences. Localised bone marrow oedema may occuracutely, but is not a feature of stable long-standing lesions.Enhancement following administration of IV gadoliniummay be seen (Fig. 5e). The differential diagnoses include afibrous cortical defect. A fibrous cortical defect typicallyerodes the cortex from within the bone, and has a tendencyto migrate proximally with time, whereas a corticaldesmoid disrupts the cortex from the outer surface andhas a constant position. However, both lesions have a peakincidence in childhood, and tend to disappear withincreasing age.

Fig. 4 Fabella. a Sagittal and b coronal PDW FSE images showing the fabella (arrows). c Sagittal PDW FSE image showing focal thickening of theproximal lateral gastrocnemius tendon (arrow) in the absence of the fabella

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Distal femoral variations

Distal femoral grooves

The distal femur may contain shallow grooves in theanterior articular (trochlear) surface, and the medial andlateral femoral condyles. The groove on the trochlearsurface is at the most proximal site of articulation withthe patella (Fig. 6a). The groove on the lateral femoralcondyle (Fig. 6b) is well-defined and extends antero-laterally to form a triangular depression into which thelateral meniscus rests during knee extension. The medialfemoral condylar groove (Figs. 6c, d) is confined to themedial aspect of the condyle and accommodates theanterior part of the medial meniscus on knee flexion [19,20]. Keats described these areas as normal variants, andtermed them the “transitional zone” [21]. They occur in upto 16% of knees, and their significance lies in the fact thatthey should not be misinterpreted as osteochondritisdissecans or a fracture.

Distal femoral cortical irregularity

Ossification anomalies of the distal femoral epiphysis arecommonly seen in children, frequently occur during periodsof accelerated skeletal growth and are usually asymptom-atic incidental findings. These variants usually resolvespontaneously over a period of several months, but maymimic stage 1 juvenile osteochondritis dissecans (OCD) onimaging. This may explain why childhood-onset “OCD” ismore frequently bilateral, has a different distribution withinthe distal femur and appears to have a more favourableprognosis than adult-onset OCD. Caffey et al. [22]examined the radiographic appearances of irregular ossifi-cation within the distal femoral condyle and identifiedareas of retarded provisional calcification within thecorresponding segment of the epiphysis, visible on plainradiographs as a radiolucent defect. An isolated focus ofcalcification may subsequently occur within the overlyingcartilage, leading to a radiographic appearance similar to anOCD. On MRI, these islets of calcification are of a similar

Fig. 5 Cortical desmoid. a Coronal PDW FSE, b axial T1W SE and csagittal PDW FSE images showing the cortical desmoid (arrows)located in the postero-medial distal femoral cortex at the site ofinsertion of the medial gastrocnemius (arrowheads in b, c). d Axial

fat-suppressed PDW FSE image showing the hyperintense signalintensity (SI) of the lesion (arrow). e Post-contrast axial T1W SEimage showing uniform enhancement (arrow)

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signal to that of normal sub-chondral bone, with noassociated bone marrow oedema (Fig. 7a). The radiolucentareas seen on plain radiographs are continuous with and ofthe same signal intensity as normal articular cartilage onMRI and can therefore be regarded as areas of uncalcifiedarticular cartilage [23].

By contrast, OCD is usually symptomatic, with bonemarrow oedema present within the calcified fragment andunderlying bone on MRI. Joint fluid may be presentbetween the fragment and adjacent bone, and the overlyingarticular cartilage is frequently disrupted. The calcifiedfragment may be non-viable and in this situation maybecome sclerotic.

Features more suggestive of a distal femoral corticalirregularity include: location in the infero-central orpostero-lateral femoral condyle, intact overlying cartilage,lack of bone marrow oedema and a residual cartilaginousmodel. Patterns of osseous variants include accessoryossification centres, spiculation of the distal femoral cortex(Fig. 7b), complete and incomplete “puzzle pieces” [24].

MRI remains the most accurate means of distinguishingbetween OCD and normal variants of ossification andshould be used in cases where the diagnosis is not clear onplain radiographs or CT.

Meniscal variation

Several normal variants of the menisci have been describedin recent years including meniscal flounce, speckledanterior horn of the lateral meniscus, and Wrisberg variantof a discoid lateral meniscus. A thorough knowledge andfamiliarity of their appearances should help the radiologistto differentiate these variants from significant abnormali-ties, especially on MRI of the injured knee.

Meniscal flounce

Meniscal flounce is a wavy or folded appearance of theinner edge of the medial meniscus. It is considered to be a

Fig. 6 Distal femoral grooves. (a) Sagittal PDW FSE image showingthe shallow groove (arrow) in the trochlear surface of the anteriordistal femur. (b) Sagittal PDW FSE image showing the lateral distal

femoral groove (arrow). (c) Sagittal PDW FSE and (d) fat-suppressedcoronal PDW FSE images showing the medial distal femoral groove(arrows)

Fig. 7 Anomalous ossificationof the lateral femoral condyle.(a) Sagittal T2*W GE imageshowing a bony irregularity(arrow) deep to the intactposterior cartilage of the lateralfemoral condyle. (b) SagittalT2*W GE image showing‘spiculation’ of the posteroinfe-rior lateral femoral epiphysis(arrow)

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normal finding without any known significance, seen inpatients with ligamentous laxity in which sliding of the tibiaon the femur results in folding or buckling of the inner edgeof the meniscus. The meniscal flounce is thought to be atransient physiological distortion, and its degree can bechanged with meniscal location on the tibial plateau and theanatomical knee position. On MRI, it is usually observed ina neutral position and is slightly released or eliminated by amaximally flexed or extended position [25]. The meniscalflounce has been reported to be a rare phenomenon, beingseen in 0.2–6% of patients on MRI [26] compared witharthroscopy, because of the lack of external stress duringMRI studies.

Magnetic resonance imaging shows a wavy contour ofthe free edge on sagittal images and a truncated appearanceon coronal images (Fig. 8). It is important to recognise thisnormal variant as it may mimic a meniscal tear ordegeneration, particularly on coronal images owing to itstruncated appearance. A flounce does not predispose ameniscus to development of a tear and in the absence ofaltered signal intensity or other morphology indicative of atear, it should be regarded as a normal variant [27].

Discoid meniscus

Discoid meniscus refers to a thickened, disc-like meniscalshape instead of the normal semilunar configuration, with aminimal width of more than 15 mm in the coronal plane(Fig. 9a). It results from failure of the meniscus to perforatecentrally, occurs more commonly on the lateral side and isseen in approximately 3% of individuals [28, 29]. Variousclassifications of discoid menisci exist, including those byWatanabe et al. [30] and Hall [31]. Watanabe et al.developed an orthopaedic classification of three types:complete and incomplete types indicate the degree of

interposition between the femoral condyle and the tibialplateau; both of these types have an intact postero-lateralmenisco-tibial ligament. A third type of discoid meniscus,referred to as the Wrisberg ligament type, occurs morefrequently in children, although it can be seen in patients atany age. In this type, the posterior horn is not attached tothe capsule resulting in excessive mobility and subluxationinto the joint, causing pain [32]. Unlike the incidentaldiscoid meniscus, which should be asymptomatic unlesstorn, a Wrisberg variant can be a source of pain and requiresurgery. According to Hall’s classification scheme, discoidmenisci are classified by their shape as slab, biconcave,wedge, asymmetric anterior, forme fruste, or grossly torn.

The MRI diagnosis of discoid meniscus is defined ascontinuity of the anterior and posterior thirds of the meniscuson three or more consecutive 5-mm sagittal slices or, four ormore consecutive 4-mm sagittal slices [33]. Further, asdiscoid menisci may have increased supero-inferior height,an abnormally thickened bow-tie appearance should suggestdiscoid meniscus (Fig. 9b).

Although rare, many musculoskeletal abnormalitiesare associated with discoid meniscus, including highfibular head, fibular muscular defects, hypoplasia of thelateral femoral condyle, hypoplasia of the lateral tibialspine, lateral joint widening and abnormally shapedlateral malleolus. It is important to identify a discoidmeniscus because of the increased frequency of tears insuch menisci. MRI may help in pre-surgical planning fordiscoid menisci, especially to determine the presence of atear.

Speckled anterior horn of the lateral meniscus

Speckled anterior horn of the lateral meniscus is a frequentfinding on MRI caused by fibres of the anterior cruciate

Fig. 8 Meniscal flounce. a Sagittal fat-suppressed PDW FSE and b coronal PDW FSE images showing the “wavy” inner margin of the medialmeniscus (arrows). c Sagittal T2W FSE image showing a lateral meniscal flounce (arrow)

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ligament (ACL) inserting into the meniscus [34]. Increasedsignal intensity at the anterior horn of the lateral meniscusnear the central attachment site appears speckled or spottyand is most obvious on T1- and proton-density weightedimages (Fig. 10). This is likely to be explained by theanatomical nature of the attachment site, which is a junctionof fibrocartilage (i.e. meniscus) and collagen (i.e. ligament).It is important to be aware of this signal intensity on MRI toavoid an erroneous diagnosis of a tear of the anterior hornof the lateral meniscus. Tears are unusual in this location,accounting for only 2% of all meniscal tears and 6% oflateral meniscal tears.

Meniscal ossicles

Meniscal ossicles are commonest in the young malepopulation and are usually located in the posterior horn ofthe medial meniscus [35]. Although the exact aetiology is

unknown, they are likely to be developmental or post-traumatic. Histologically, the ossicles consist of bonemarrow and cancellous bone contained by cortex coveredwith hyaline cartilage [36]. The possible explanation for themore frequent location in the posterior horn of the medialmeniscus may be the strong tibial attachment of themeniscus with resultant reduced motion compared withthat of the lateral meniscus. When symptomatic, it maypresent with diffuse knee pain and a sensation of locking.

On conventional radiography, it may be difficult todifferentiate meniscal ossicles from osteochondral loosebodies (Fig. 11a). This distinction is clinically important asosteochondral loose bodies are often treated surgicallywhereas meniscal ossicles are treated surgically only ifsymptomatic. On MRI, meniscal ossicles are well margin-ated and demonstrate the characteristic signal intensity (SI)of normal bone marrow: high SI on T1W/PDW (Fig. 11b)and low SI on fast spin-echo T2W images with fat

Fig. 9 Discoid meniscus.a Coronal PDW FSE imageshowing a partial lateral discoidmeniscus (arrow) with a mini-mal coronal diameter of over15 mm. b Sagittal PDW FSEimage showing absence of thenormal “bow-tie” appearance ofthe lateral meniscus (arrow)

Fig. 10 Speckled anterior hornof the lateral meniscus.a Sagittal PDW FSE imageshowing a spotty appearance tothe anterior horn of the lateralmeniscus (arrow). b CoronalPDW FSE image showinginsertion of distal fibres of theanterior cruciate ligament (ACL;arrow) into the central portionof the anterior horn of the lateralmeniscus

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suppression. On the other hand, loose bodies usually showlow SI on T1W images, allowing the differentiation frommeniscal ossicles.

Meniscal shape deformities

The discoid meniscus is the most common meniscalanatomical variant, but ring-shaped, double-layered, hypo-plastic and partial or complete absence of the menisci mayalso occur [14]. Meniscal variants are often asymptomatic,but may present with mechanical symptoms requiringtreatment. All meniscal anatomical variations occur morefrequently in the East Asian population and most common-ly involve the lateral meniscus. Excluding the discoidlateral meniscus, the incidence of meniscal malformationshas been estimated at approximately 0.3% [37].

Only a few case reports of a ring-like meniscus exist inthe literature, with most of these being incidental findings atarthroscopy. These morphological variations may simulatea meniscal fragment in the intercondylar notch, giving theappearance of a bucket-handle meniscal tear [38].

An abnormal band of the lateral meniscus is narrowerthan the underlying normal lateral meniscus and is free andmobile, except at its attachments to the posterior horn andmiddle segments of the adjacent meniscus [39].

A double-layered meniscus lies proximal and parallel tothe normal underlying meniscus, and is not freely mobile.The double-layered meniscus is thicker than the abnormalband, and has peripheral attachments to the surroundingjoint capsule [39]. Double-layered, ring-like and abnormalbands of menisci have the same signal characteristics asnormal menisci and they may give the impression ofmeniscal tears on MRI. However, meniscal tears, includingbucket-handle tears, more commonly involve the medialmeniscus, while meniscal anatomical variants more fre-quently occur in the lateral meniscus.

Plicae

Synovial plicae are thin folds of vascularised synovialtissue found within the lining of the knee joint. Theyrepresent persistent remnants of the synovial membrane thatdivided the foetal knee into three compartments and haveno known function in adults.

Plicae have an incidence of 50–90%, with 11% of kneescontaining all three main plicae [40, 41]. The commonestare the supra-patellar and infra-patellar plicae, with themedio-patellar plica most likely to become symptomatic.MR imaging reveals plicae as low signal intensity bandswithin hyperintense joint fluid, which are best seen on

Fig. 11 Meniscal ossicle.a Lateral radiograph showing abone fragment (arrow) in theposterior aspect of the knee.b Sagittal PDW FSE imageshowing the bone fragment(arrow) lying within theposterior horn of the medialmeniscus and having the SIcharacteristics of marrow

Fig. 12 Supra-patellar plica. Sagittal PDW FSE image showing thesupra-patellar plica (arrow) as a thin hypointense band postero-superior to the patella

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gradient echo T2-weighted, T2-fat-suppressed and protondensity-weighted images.

The supra-patellar plica is located at the border betweenthe supra-patellar bursa and the knee joint cavity [42], andfollows an oblique path from the synovium at the level ofthe anterior femoral metaphysis, to insert above the patellaposterior to the quadriceps tendon. Zidorn classified thesupra-patellar septum into four types, with Type 3 repre-senting the supra-patellar plica:

Type 1 The knee joint and the supra-patellar bursa arecompletely separated by a septum

Type 2 The supra-patellar bursa and the knee joint commu-nicate via fenestrations (porta) within the septum

Type 3 A remaining fold of plica exists, usually in amedial location

Type 4 Complete involution of the septum [43]

The supra-patellar plica is best visualised in the sagittalplane, where it is seen as a low-signal band supero-posterior to the patella (Fig. 12) [44]. It may impinge on

Fig. 13 Infra-patellar plica. Sagittal T1W SE image showing theinfra-patellar plica (arrow) as a thin, hypointense band runningthrough the infra-patellar fat pad

Fig. 14 Mediopatellar plica. aAxial fat-suppressed PDW FSEimage showing a Type Bmediopatellar plica (arrow). bSagittal, c axial and d coronalfat-suppressed PDW FSEimages showing a Type Cmediopatellar plica (arrows)extending to the cartilage overthe trochlear. e Axialfat-suppressed PDW FSE imageshowing a Type D mediopatellar plica with a centralfenestration (arrow)

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the articular cartilage of the supero-medial angle of thetrochlear on knee flexion, giving rise to superior knee painexacerbated by prolonged sitting and climbing stairs.

The infra-patellar plica is the most common plica in theknee. It has a narrow proximal attachment anteriorly withinthe intercondylar notch, and runs anteriorly through theinfra-patellar fat pad to its distal attachment at the inferiorpole of the patella (Fig. 13). It is easily identified on MRimaging, being seen as a low-signal intensity structurerunning anterior and parallel to the ACL on sagittal images.It may contain a vertical septum, a fenestration, be split orbipartite, and may be separate from the ACL. Care must betaken not to misinterpret a normal infra-patellar plica as aloose body, an ACL (in an ACL-deficient knee), or focalnodular synovitis [42].

The mediopatellar plica (medial plica) runs obliquelyfrom the medial wall of the knee joint to insert into thesynovium overlying the infra-patellar fat pad [42]. Themediopatellar plica usually has a unique attachment, but isoccasionally connected to the supra-patellar plica. Fourtypes of mediopatellar plica have been described:

Type A A cord-like elevation of the synovial wallType B Has a shelf-like appearance and does not cover the

anterior surface of the medial femoral condyle(Fig. 14a)

Type C Similar to Type B, but larger and extendingmedially to cover the medial femoral condyle(Fig. 14b–d)

Type D The plica has a central fenestration (Fig. 14e)

Types C and D are most likely to cause symptoms, as theplica may become trapped between the medial femoralcondyle and the patella. In this situation, the plica canbecome hard and thickened, and may result in internaldamage to the knee joint.

The medio-patellar plica is best visualised on T2-weighted axial and sagittal images, and is seen as a lowsignal intensity structure on all sequences. Intra-articularfluid secondary to an effusion or arthrogram assists in thevisualisation of plicae, but in an over-distended knee, aType C plica may be misinterpreted as a Type B.

The lateral patellar plica is the least common type ofplica, present in 1–3% of the population. It originates in thelateral wall of the knee joint, above the popliteal hiatus andattaches to the infra-patellar fat pad. It is a fine, low-intensity structure running longitudinally within the lateralrecess of the knee joint.

Conclusions

Normal anatomical variants are frequently encountered onMRI, and may be present in over a quarter of all patients

[13]. A sound knowledge of both normal and variantanatomy is required in order to avoid unnecessary over-investigation and mis-diagnosis.

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