imagining dpf

REVIEW

Imaging of patellofemoral disorders

D.A. Eliasa,*, L.M. Whiteb

Departments of aRadiology, King’s College Hospital, London, UK; and bDiagnostic Imaging, University ofToronto, Mount Sinai Hospital and the University Health Network, Toronto, Ontario, Canada

Received 12 June 2003; received in revised form 5 January 2004; accepted 13 January 2004

KEYWORDSPatellofemoral; Knee;

Computed tomography

(CT); Magnetic resonance

(MR); Conventional

radiographs

Anterior knee pain is a common symptom, which may have a large variety of causesincluding patellofemoral pathologies. Patellofemoral maltracking refers to dynamicabnormality of patellofemoral alignment and has been measured using plain film,computed tomography (CT) and magnetic resonance imaging (MRI) using static andkinematic techniques. Patellar dislocation is usually transient, but specific conventionalradiographic andMRI featuresmay provide evidence of prior acute or chronic dislocation.In addition, chondromalacia patellae, osteochondritis dissecans, patellofemoral osteo-arthritis, excessive lateral pressure syndrome, and bipartite patella have all beenimplicated in causing patellofemoral pain. The imaging and clinical features of theseprocesses are reviewed, highlighting the specific diagnostic features of each condition.q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.

Introduction

Patellofemoral disorders commonly cause anteriorknee pain and giving way. Anatomical featuresassociated with these disorders, and features ofjoint degeneration or prior trauma can be identifiedclinically and radiologically. However, the linkbetween anatomical or pathological findings andsymptoms is variable, and many surgical proceduresused for patellofemoral disorders have variableresults, especially at long-term follow-up.1 Thismay be in part because patellofemoral relationshipsare usually measured in the supine resting knee, butfailure occurs in the loaded, functioning joint.Methods for imaging the joint in both resting andfunctional states will be reviewed.

Anatomical and biomechanicalconsiderations

The posterior surface of the patella articulates withthe trochlear groove along the anterior surface of

the femoral condyles to form the patellofemoraljoint. The posterior patella has a narrower medialand a wider lateral facet. A variable, usually small,odd facet lies along the medial border of thepatella.

Bydisplacing the fulcrumofmotionof theextensormechanismanterior to the femur, the patellofemoralarticulation produces a mechanical advantageincreasing the force of the quadriceps muscles inextending the knee. Because of this, considerableforce is transmitted across the patellofemoral joint,whichmayvary fromhalf bodyweight duringwalking,up to25 timesbodyweight on lifting aweightwith theknees flexed at 908.2

In the fully extended knee the patella lies superiorto the trochlear cartilage. As the knee flexes to 308,the patella begins to engage with the trochlea.Between 30 and 908 of flexion, first the inferior andthen the superior patella cartilage articulates withthe trochlear cartilage. Beyond 1208, contact isreduced between the patella and trochlea such thatonly the small odd facet remains in contact with thefemur.Thedepthof the trochlear groove is importantinmaintaining the patella stabilized over the anteriorknee, but contact between the articular surfaces islimited during normal motion, and additional passiveand active mechanisms help maintain articularstability.

0009-9260/$ - see front matter q 2004 The Royal College of Radiologists. Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.crad.2004.01.004

Clinical Radiology (2004) 59, 543–557

*Guarantor and correspondent: D.A. Elias, Department ofRadiology, King’s College Hospital, Demark Hill, London SE5 9RS,UK. Tel.: þ11-44-20-7346-4599; fax: þ11-44-20-7346-3157.

E-mail address: [email protected]

Passive stabilizers

The patellar ligament and the medial and lateralpatellar retinacula form the passive stabilizers ofthe patella. The retinacula have deep and super-ficial layers.3,4 The superficial layers of the retina-cula attach to the patella and patellar ligament andextend to the fascia of sartorius medially and to thefascia of the iliotibial band laterally. The deeplayers contain thickenings that form ligaments,which provide significant stabilizing support to thepatella.

On the medial side, the medial patellofemoralligament (MPFL) a fascial thickening of the deeplayer of the medial retinaculum has been shown tobe the major passive restraint preventing lateralpatellar dislocation (Fig. 1).5 This ligament orig-inates between the adductor tubercle and themedial epicondyle of the femur and extendsforwards just deep to the inferior fibres of vastusmedialis to insert onto the superior two thirds ofthe medial patellar margin.4 –7 Inferior to the MPFL,the patellomeniscal and patellotibial ligamentsplay a more minor role in medial restraint of thepatella.

On the lateral side, the deep layer consists of thetransverse ligament, which extends from theiliotibial tract to the lateral patellar margin.Above this ligament lies the epicondylopatellarband and below it the patellotibial band.

Active stabilizers

The four quadriceps muscles form the activestabilizers of the patella. The inferior portions ofthe vastus medialis and lateralis muscles form smallmuscle groups with a distinct oblique orientation oftheir fibres, the vastus medialis obliquus and thevastus lateralis obliquus muscles (Fig. 1). Theseprovide active restraining forces on the patella inthe medial and lateral directions, respectively. Thevastus medialis obliquus attaches to the distaladductor magnus tendon and inter-muscularseptum, and inserts onto the medial retinaculumand superomedial patellar border.5

Pathological processes

Anterior knee pain is a common clinical symptomwithmany potential causes, some of which relate tothe patellofemoral articulation (Table 1). Theterms used to describe patellofemoral disordersdiscussed below have been used rather loosely. Inparticular chondromalacia patellae has sometimesbeen used as a catch-all phrase to describepatellofemoral pain with or without documentedchondral abnormality. The term “patellofemoralpain syndrome” may be used in preference toinclude multiple conditions described in Table 1,each of which is associated with patellofemoralpain.8

Patellofemoral malalignment andmaltracking

Patellofemoral alignment refers to the staticrelationship between the patella and the trochleaat a given degree of knee flexion. Tracking refers to

Figure 1 Schematic diagram of the medial knee. Themedial patellofemoral ligament (MPFL) arises betweenthe adductor tubercle (the insertion of the adductormagnus tendon), and the medial epicondyle (the site oforigin of the medial collateral ligament). The ligamentthen runs forward just deep to the distal vastus medialisobliquus muscle to attach to the superior two thirds of themedial patella margin. Reprinted with permission fromRadiology.48

Table 1 Causes of anterior knee pain

Patellar malalignmentChondromalacia patellaePatellofemoral osteoarthritisExcessive lateral patella pressurePatella osteochondritis dissecansOsteochondral injuryPre-patella bursitisInfra-patella bursitisHoffa’s syndromeSynovial plicaPatella/quadriceps tendonosis/tearsOsgood-Schlatter diseaseSinding-Larsen-Johansson diseaseMeniscal tearsReflex sympathetic dystrophy

D.A. Elias, L.M. White544

dynamic patellofemoral alignment during kneemotion. Thus, malalignment and maltracking referto conditions in which there is an imbalance offorces on the patella that produce abnormalities ofalignment and tracking, respectively.9 This imbal-ance may result from a combination of variables inbony geometry, function of active and passive soft-tissue restraints and functional demands. The resultof patellofemoral malalignment and maltracking isunfavourable stresses and shearing forces thatexceed the physiological threshold of tissues andmay result in cartilage damage, degenerativechanges, strain of ligamentous structures, mechan-ical failure or patellar dislocation. Symptomstherefore include pain, apprehension and givingway.

Clear definition of patellofemoral malalignmentand maltracking is hindered by the fact that clinicaland radiological measures described are oftenabnormal in asymptomatic knees, and withindescribed normal ranges in symptomatic knees.Also measures of alignment will vary depending onthe degree of knee flexion, and may be verydifferent in the supine resting knee (in whichposition most clinicoradiological measurement isperformed) compared with the loaded walking knee(in which symptoms occur).

Imaging studies of patellofemoral trackingshould focus on the first 30–458 of flexion, asbeyond 458 most patellae engage fully with thetrochlear groove. In early flexion anatomicalfactors including an elevated patella (patellaalta), trochlear dysplasia and abnormalities of thesoft-tissue restraints of the patella have the mostpronounced effect in producing abnormal tracking.

The Q angle is formed between a line joining theanterior superior iliac spine and the centre of thepatella, and a line joining the centre of the patellaand the tibial tuberosity (Fig. 2). This angle ismeasured clinically, and is thought to indirectlyreflect the degree of valgus translational forceexerted upon the patella with contraction of theextensor mechanism of the knee. It may thereforehelp explain the tendency to lateral instability ofthe patella, and some studies have suggested thatthe Q angle may be increased in patients withpatellofemoral pain.10 Other authors have, how-ever, contradicted this finding,11 possibly becausein the fully extended knee, a malaligned patellamay be laterally translated, artificially reducing theQ angle measurement. The measurement maytherefore be more successfully performed at 308of knee flexion with the patella engaged in thetrochlea thus preventing this lateral displace-ment.12

RadiographsThe standard radiographic series for assessment ofpatellofemoral relationships includes an anteropos-terior, a lateral and an axial view. Several tech-niques have been described for the axial view of thepatellofemoral joint (Fig. 3). The most widelyaccepted techniques are performed with thepatient supine. The feet are placed either on13 oroff14 the end of the examination table and theradiograph is taken with 20–458of knee flexion. Inorder to attempt a more physiological assessmentof patellofemoral alignment, a technique forperforming a standing axial view has been advo-cated.2

Wiberg15 described three patellar types based onmorphology on the axial view (Fig. 4). However, thevalue of such classifications in predicting patellainstability is unproven. Additionally, as patellarconfiguration appears to change from level to levelon cross-sectional patellar imaging, the validity ofmorphological assessment on axial radiographsappears questionable. Trochlear depth may bemeasured using the sulcus angle (Fig. 5a). Thismeasurement has been shown to be relativelyinsensitive to the angulation between the beamand the femur with a normal range of 126–1508.14,16

Measurements of lateral patellar displacementinclude the congruence angle,14 and lateral dis-placement.13 Lateral tilt may be measured using

Figure 2 The Q angle is formed between a line joiningthe anterior superior iliac spine (ASIS) and the centre ofthe patella, and a line joining the centre of the patellaand the tibial tuberosity.

Imaging of patellofemoral disorders 545

the lateral patellofemoral angle (Fig. 5).17 Abnorm-alities to these measurements may representmalalignment, but in mild degrees of knee flexion,abnormal measurements of patellar tilt and sub-luxation may be seen in asymptomatic patients withnormal patellofemoral biomechanics. Recent worksuggests that apparent discrepancies betweenradiographic measures of the patellofemoral articu-lation and clinical symptoms may be resolved whenmeasurements are performed on weight-bearing,axial views.18

The lateral view of the knee allows assessment ofthe vertical position of the patella. A high-ridingpatella (patella alta) is associated with lateralpatellar dislocation and subluxation, chondro-malacia patellae, patellar ligament rupture andSinding-Larsen-Johansson disease. A low-ridingpatella (patella baja) is seen in quadriceps tendonrupture, neuromuscular disorders, achondroplasia,and after surgical advancement of the tibialtuberosity. For the Insall-Salvati method of patellar

position measurement, the patellar ligament lengthis divided by the maximal diagonal length of thepatella on the lateral radiograph (Fig. 6a).19 Thenormal ratio is approximately 1, and a ratio of,0.8is considered to show patella baja, whilst a ratio of.1.2 is indicative of patella alta. A modification ofthis index, which is less sensitive to variation inpatella morphology, is calculated as the distancebetween the inferior articular surface of the patellaand the patellar ligament insertion divided by thelength of the patella articular surface (Fig. 6b).20

For the modified index, patella alta is defined as aratio of .2.

On a true lateral radiograph with the posteriorborders of the femoral condyles overlapping, anassessment may be made of patella tilt andtrochlear groove depth (Fig. 7).21,22 Measurementof patellar tilt on lateral radiographs with the kneein full extension, has been shown to have a greatersensitivity for patellofemoral pain and prior dislo-cation than measures of patellar tilt on the axialview.22 This is because patellar tilt may becomeless pronounced at the greater degrees of flexionrequired for performing an axial radiograph.23

Measurement of trochlear groove depth on lateralradiographs of the knee additionally allows forassessment of the critical proximal portion of thegroove at which site dysplasia reduces patello-femoral engagement during early flexion. In con-trast, only the more distal trochlear groove istypically seen on axial radiographs.

Where there is no patellar tilt (designated gradeI) the median ridge of the patella lies posterior tothe lateral patellar facet. Therefore, on a truelateral radiograph of the knee the median ridge andlateral facet form two separate borders that appearslightly concave.22 With mild patellar tilt (grade II)the median ridge and lateral facet lie in the samecoronal plane, so that on the true lateral radiographonly one border is seen. With further tilt (grade III),the lateral facet projects posterior to the medianridge and appears convex. The depth of thetrochlear groove may be measured on the lateralradiograph of the knee, 1 cm distal to its upperlimit. A depth of ,5 mm is considered dysplastic.

Cross-sectional imagingA number of studies have shown that computedtomography (CT) and magnetic resonance imaging(MRI) of the patellofemoral joint can effectivelydemonstrate abnormalities of patellar tracking inpatients with patellofemoral symptoms.24 –31 Thesestudies have demonstrated that in many patientswith patellofemoral symptoms and patella sub-luxation at less than 308 of flexion, the patellacentralizes in the trochlear groove as flexion

Figure 3 Techniques for performing the axial radio-graph of the patella. The prone technique (a) requiresknee flexion .908, and therefore eliminates subluxationin most patients with tracking abnormality. Supinetechniques are more valuable for assessment of patellaalignment and include those of Laurin et al.13 (b) with theknee flexed at 208, and Merchant et al.14 (c) with the kneeflexed at 458. The Merchant technique may be performedwith the beam direction reversed (d), which eliminatesthe need for a special cassette holder. To perform aweight-bearing axial view (e) a specially designed kneesupport is required, but this may provide a morephysiologic assessment, of patellofemoral alignment.2

Figure 4 Anatomic variations in patellar shape.15 Type1 patellae have concave medial and lateral facetsapproximately equal in size. Type 2 also have concavefacets, but the medial facet is smaller than the lateral.Type 3 have a small convex medial facet.


increases beyond this point (Fig. 8). Conversely,asymptomatic knees often show some degree ofphysiological patellar tilt or subluxation at fullextension. Imaging over the range of 5–308 offlexion is therefore of greatest value in discriminat-ing normal from abnormal tracking and alignment.

For CT or MRI examination patients may liesupine or prone. A passive supine examination maybe performed with the knee at 0, 10, 20 and 308 offlexion. For a patient with an equal leg and thighlength of 450 mm, this requires raising the back ofthe knee by 0, 40, 80 and 115 mm from the table.32

Alternatively, various devices are described toallow passive placement of the joint in fixeddegrees of early flexion. At each position axialimages are performed through the patellofemoraljoint. Prone examination requires a cut out in thetable top below the knee to prevent artificialpatella displacement by the table.

In order to assess patellofemoral relationshipsmore physiologically, rapid techniques able tocapture images during active knee motion aredescribed. With CT this was initially performedusing electron beam imaging,33 but more recently atechnique using a helical CT machine to perform acontinuous 10 s exposure with no table movementhas been described.25,26 The images are acquired atmid-trochlea level, and during the exposure theknee is actively flexed and extended with the thighstrapped down at the end of the examination table.A weighted boot may be used to increase quad-riceps loading. With MRI, fast imaging sequences,such as spoiled gradient recalled acquisition in thesteady state (GRASS) have been used,28 withsequences triggered manually or using motionsensitive detectors. In supine MR studies, activemotion appears to accentuate the measured differ-ences in patellar tracking between painful and

asymptomatic knees.31 This may be reproduced inprone studies by using a device for quadricepsloading,29 and is presumably because quadricepsdysfunction may contribute to maltracking, a factornot assessed in passive studies.

For evaluation of subluxation and tilt on cross-sectional patellofemoral studies some authorsadvocate a purely qualitative assessment andevaluation of a series of axial images at a singlelevel combined as a cine loop allows trackingabnormalities to be readily identified.27,34 Othershave adapted various plain radiographic measure-ments,23 –25,30,31,33,35 including the sulcus angle,lateral patellofemoral angle, congruence angle andlateral patella displacement. The trochlea–tuber-cle distance is an additional measurement per-formed on cross-sectional images, which—similarlyto the Q angle—is a measurement of the lateral pullon the patella. For this measurement axial imagesthrough the apex of the intercondylar notch and thetibial tubercle are overlapped, and the horizontaldistance between the apex of the notch and thetubercle is measured. A tubercle lying more than2 cm lateral to the apex of the notch has beenshown to be specific, but is poorly sensitive formaltracking.34

Post-operative imagingSurgery for correction of patellar maltrackingincludes procedures for bony and soft-tissuerealignment. Several forms of osteotomy transferthe tibial tuberosity to a more medial position so asto reduce the Q angle and reduce the lateraltranslational stress on the patella. The Hauserprocedure36 was an early example, which has nowfallen out of favour. This osteotomy transfers thetibial tuberosity not only medially, but alsoposteriorly, causing a significant increase in

Figure 5 Measurements of patellofemoral relationships on the axial radiograph. (a) A line is drawn from the lowestpoint of the intercondylar sulcus, B, to the highest points of the lateral and medial femoral condyles, A and C. The anglebetween lines AB and BC is the sulcus angle (normal range 126–1508). To measure the congruence angle (curved arrow),the sulcus angle is bisected to produce a reference line, and the angle is measured between this reference and a linejoining the apex of the sulcus, B, and the lowest point of the patellar articular surface, D. In the normal knee, point Dshould lie no more than 168 lateral to the bisected sulcus angle.14 (b) Lateral patellar displacement is measured bydrawing a line joining the summits of the medial and lateral femoral condyles and dropping a perpendicular to this at thelevel of the summit of the medial condyle. The distance of the medial margin of the patella from this perpendicular ismeasured (arrowheads). In the normal knee the medial patellar margin should lie no more than 1 mm lateral to theperpendicular.13 (c) The lateral patellofemoral angle (curved arrow) is the angle between a line joining the apices of thefemoral condyles and a line joining the limits of the lateral patellar facet. The angle is taken to be normal when it openslaterally, and abnormal when it opens medially.13


Figure 6 Measurements of patellar height on a lateralradiograph of the knee of a 14-year-old male withrecurrent patellar dislocation. (a) For the Insall-Salvatimethod the patellar ligament length is divided by themaximal diagonal length of the patella on the lateralradiograph.19 The ratio here is 1.5 (.1.2 indicates patellaalta). (b) A modified index, which is less sensitive to

variation in patella morphology, is calculated as thedistance between the inferior articular surface of thepatella and the patellar ligament insertion divided bythe length of the patella articular surface.20 The ratio ismeasured at 2.2 (.2 indicates patella alta).

Figure 7 Patellofemoral measurements on the lateralradiograph.21,22 (a) Axial diagrams depicting grades I, IIand III patellofemoral alignment. (b) Diagrams represent-ing the corresponding appearance of the patellar on atrue lateral radiograph of the knee for grades I, II and IIIalignment. In grade I alignment (normal) the median ridgeof the patella (open arrow) lies posterior to the lateralfacet (curved arrow). On a lateral radiograph the medianridge and lateral facet form two separate borders whichappear slightly concave. With mild patellar tilt (grade II)the median ridge and lateral facet line up on the lateralviews so that only one border is seen. With further tilt(grade III), the lateral facet projects posterior to themedian ridge and appears convex. (c) Normal lateralradiograph of the knee. The depth of the trochlear groovemay be measured 1 cm distal to its upper limit (arrows).Less than 5 mm is considered dysplastic. Patella align-ment is grade I. Note that these measurements may onlybe made on a true lateral radiograph in which theposterior borders of the femoral condyles virtuallyoverlap.


pressure within the medial patellofemoral joint,and resultant complications of patellofemoralosteoarthritis37 (Fig. 9). Therefore realignmentosteotomies that transfer the tibial tuberositymedially but not posteriorly are preferred (such asin the Roux-Elmslie-Trillat procedure).38 Wherepatella alta is felt to be significant in causing lateengagement of the patella with the trochleaduring knee flexion, tibial tuberosity advancementhas been advocated. In patients with excessivefemoral anteversion or external tibial torsion,rotational osteotomies can be performed toreduce lateral translational stress on the patella.In patients with significant lateral patellar tilt alateral retinacular release is sometimes per-formed. This can be identified as a verticaldisruption in the lateral retinacular fibres onMRI. Repair or reconstruction of the medialligaments may be performed where there hasbeen prior traumatic disruption. Multiple realign-ment procedures may be combined in a singleoperation. For example the Roux-Elmslie-Trillatprocedure includes lateral release, medial capsu-lar tightening and medial and distal displacementof the tibial tuberosity.

Lateral patella dislocation

Lateral patellar dislocation is a common cause ofacute traumatic haemarthrosis in young active

patients, usually occurring during sporting activi-ties.39 However, patellar dislocation is usually

Figure 8 Supine CT examination of both knees at 0, 10,30 and 458 of knee flexion in a 14-year-old male patientwith right-sided anterior knee pain and a history ofrecurrent left patella dislocations. The right knee showsno subluxation, but there is lateral patella tilt at 10 and308 which normalizes by 458 of flexion. The left kneeshows osteochondral irregularity to the medial patellawith a small separated adjacent bony fragment (arrow-head) as well as an osteochondral fragment at the lateralfemoral condyle (arrow), all consistent with prior patellardislocation. There is a joint effusion. There is patellar tiltand lateral subluxation at 10 and 308 which revert to mildmedial subluxation at 458. Both knees show shallowfemoral trochlear grooves.

Figure 9 Medial patellofemoral osteoarthritis 15 yearsafter tibial tuberosity transfer for patellar instability. (a)Sagittal fat-saturated T2-weighted image demonstratessurgical changes at the tibial tuberosity, as well asmarrow oedema of the inferior patella. (b) Axial fat-saturated T2-weighted image demonstrates medial tilt,as well as cartilage loss and marrow oedema at the medialpatella and trochlea. Medialization of the tibial tuberos-ity has increased stresses in the medial patellofemoralcompartment leading to osteoarthritis.


transient, with patients frequently unaware that ithas occurred. Additionally, full examination of theacutely swollen painful knee may be difficult. Forthese reasons patellar dislocation has been found tobe initially clinically unsuspected in as many as 45–73% of cases,40,41 and MRI findings are sometimesthe first indication of the diagnosis.

As the patella dislocates laterally, there may beinjury to the medial soft-tissue restraints of thepatella. During spontaneous relocation, the medialpatella impacts upon the anterior aspect of thelateral femoral condyle resulting in osteochondralfractures and potential impaction deformity of theinferomedial patella, analogous to the Hill-Sachslesion of the humeral head, which follows anteriordislocation of the glenohumeral joint.

On conventional radiographs the patella is rarelyseen to be dislocated (Fig. 10). There may,however, be patella tilt and or subluxation. Ahaemarthrosis may be present. Osteochondralfractures of the medial patella or anterior lateralfemoral condyle may be seen,42 –44 and there maybe a separated intra-articular fragment. However,the plain radiographic findings of patellofemoraldislocation are often subtle.

A constellation offindings have been described atMRI, characteristic for previous lateral patellardislocation, and which may be helpful diagnosti-cally.40,41,45,46 These include joint effusions,injuries to the medial retinaculum, and contusionor osteochondral injury of the anterolateral portionof the lateral femoral condyle and of the medialpatella. Signal changes are also described withinthe superior portion of Hoffa’s fat pad.47 Bonecontusions occur at characteristic locations, at theanterior portion of the lateral femoral condyle(anterior and superior to the typical site ofcontusion occurring with anterior cruciate ligamentinjury), and at the medial patellar margin infer-iorly. In one study, a concave impaction deformityof the inferomedial patella, identified on MRI on atleast two consecutive slices, had a sensitivity of 44%and a specificity of 100% for previous lateral patelladislocation (Fig. 11).48

Injuries to the medial retinaculum are identifiedafter lateral patella dislocation at its midsubstanceand patella attachment, and this appears assurrounding oedema, or fibre irregularity or dis-continuity. Emphasis on the biomechanical import-ance of the MPFL is noted in some recent surgicalstudies, some with MRI correlation, which haveidentified injury to the ligament in as many as 94–100% of acute patella dislocators at open oper-ation.6,49 –51 Arthroscopically, such injury may berelatively occult as the MPFL is an extra-capsularstructure. Almost all of the MPFL injuries in these

studies occurred at or close to its femoral attach-ment, and it is this portion of the ligament that isbest identified on axial MR images (Figs. 12 and13).52 Identification of these injuries may beimportant as several authors have advocatedMPFL repair to prevent recurrent patellofemoralinstability and dislocation.49,53 –55

Evidence of injury to the vastus medialis obliquusmuscle, after lateral patellar dislocation has beennoted on MRI as oedema about its distal fibres and

Figure 10 Knee radiographs after lateral patellar dis-location. (a) Anteroposterior radiograph of the kneeshowing a laterally dislocated patella. The patella usuallyspontaneously reduces and this appearance is rare. (b)Axial radiograph of the patella in a different patient. Thepatella is reduced, but note the osteochondral fragmentadjacent to the medial patella and the small concavedefect at the medial patellar margin. These occur due toimpaction of the medial patella upon the anterior aspectof the lateral femoral condyle at the time of dislocation.


possibly stripping of its distal attachment to theadductor tubercle (Fig. 14).48 –50

Excessive lateral pressure syndrome

Excessive lateral pressure syndrome refers to acondition in which lateral patellar tilt is dominantwith little or no subluxation.56 This condition occursin adolescents and adults often presenting withpatellofemoral pain. Imaging studies may revealcartilage loss, sclerosis and cystic change of thelateral patella and trochlea (Fig. 15). Dynamicstudies may show lateral patella tilt, which tends toincrease with increasing flexion, but with littlesubluxation.27

Chondromalacia patellae

The term chondromalacia patellae is properlyapplied to a syndrome of anterior knee pain,typically in young patients, with initial pathologicalchanges of cartilage softening, swelling andoedema.15,57 Proposed causes include trauma,chronic stress, patellofemoral instability and ana-tomic variations in bony morphology. Chondroma-lacia patellae may be reversible, or may progress todevelop patellofemoral osteoarthritis.58

Cartilage lesions typically occur along the medialpatella at the ridge between the medial and the oddfacets, but may occur anywhere along the patellararticular surface. A variety of classifications forcartilage disease have been proposed. In onesystem, grade 1 represents cartilage softening and

Figure 11 Axial gradient echo MRI image of the knee 3weeks after acute transient lateral patellar dislocationdemonstrates a concave impaction deformity (smallwhite arrows) of the medial patella. There is a contusion(asterisk) at the lateral femoral condyle. Note thecomplete tear (open white arrow) at the patellarinsertion of the medial patellar retinaculum. Reprintedwith permission from Radiology.48

Figure 12 Appearance of the normal MPFL on MRI. (a)Axial gradient echo MR image of the knee immediatelyinferior to the adductor tubercle demonstrates a normalfemoral origin of the MPFL (open arrow). The distal vastusmedialis obliquus muscle (arrowhead) lies anteriorly. (b)Axial gradient echo MR image just inferior to (a)demonstrates the proximal origin of the medial collateralligament (open arrowhead). Note that the medial patellarretinaculum (open arrow) shows a bilaminar appearance(see text) Reprinted with permission from Radiology.48


swelling, grade 2 represents cartilage fragmenta-tion or fissuring of up to 1.3 cm in diameter, grade 3represents fragmentation or fissuring of.1.3 cm indiameter, and grade 4 represents cartilage lossdown to subchondral bone.59

Conventional radiographs are insensitive for theassessment of cartilage lesions, but will show jointspace loss when cartilage loss is extensive, as wellas associated changes of sclerosis and cystic changein the underlying subchondral bone. Arthrographycombined with axial imaging of the patella mayshow imbibition of contrast into areas of chondro-malacia, but again sensitivity is low. CT arthro-graphy after the intra-articular injection of air oriodinated contrast may successfully demonstratefocal areas of cartilage irregularity or loss. Scinti-graphy may show abnormality in chondromalacia,but usually only when changes in the subchondralbone are present.

MRI holds promise in having the unique ability topotentially identify cartilage defects, as well asinternal derangement of the cartilage layer beforegross morphological cartilage loss. Images are bestperformed in the axial plane. Numerous MRItechniques have been investigated for imaging ofpatella cartilage. Normal cartilage may havevariable signal characteristics dependent upon thesequences used, and a bi- or tri-laminar appearancehas variably been reported.60,61 The appearance ofnormal cartilage may be affected by truncation,chemical shift and magic angle artefacts, as well as

Figure 13 Axial fat-saturated T2-weighted image of theknee 4 days after acute transient lateral patellardislocation. There is complete disruption of the medialpatellofemoral ligament from its femoral attachment(thin white arrow). Note the concave impaction deform-ity of the inferomedial patella (black arrow) with marrowcontusion. There is a joint effusion and prominentoedema along the lateral patellar retinaculum. Note thesite of origin of the medial collateral ligament (openwhite arrow).

Figure 14 Coronal spin-echo T2-weighted MR image ofthe knee 3 weeks after acute transient lateral patellardislocation demonstrates high T2 signal (white arrows)surrounding the distal vastus medialis obliquus muscle(VMO), consistent with oedema.

Figure 15 Axial fat-saturated T2-weighted MR image ofthe knee in a patient with findings of excessive lateralpressure syndrome. There is marked lateral patellar tiltbut little subluxation and there is full-thickness cartilageloss and marrow oedema confined to the lateral patellafacet. Note the normal cartilage thickness at the medialpatella (white arrows).


its anatomical and chemical structure. Unsurpris-ingly therefore, the reported accuracy of MRI forcartilage lesions varies widely in the literature,with sensitivities reported of 31%62 up to 100%.63

The accuracy of MRI for early chondromalaciawithout cartilage loss is unproven.

For successful cartilage imaging, an MRIsequence must demonstrate good contrast at theinterfaces between cartilage and subchondral boneand between cartilage and joint fluid. Fast spin-echo intermediate-weighted or T2-weightedimages with fat saturation fulfil these criteria,with black subchondral bone, intermediate signalcartilage and high signal joint fluid, and have beenshown to be accurate in the assessment of cartilagelesions (Fig. 16).64 These sequences additionallydemonstrate oedema and subchondral cysticchange in the underlying subchondral bone. Avariety of two and three-dimensional gradient-recalled echo techniques have also been success-fully used. Three-dimensional fat-suppressedspoiled gradient-recalled echo (SPGR) sequenceshave been shown to be accurate, but requirerelatively lengthy examination times.61 MR arthro-graphy, using intra-articular administration ofsaline or dilute gadolinium has also been advo-cated. Many other techniques have been exten-sively studied but a discussion of these is beyondthe scope of this article. Magnetization transfer,diffusion-weighted imaging, ultra-short TE

sequences and spectroscopic imaging may prove

to be valuable techniques for imaging articularcartilage in the future.

Patellofemoral osteoarthritis

Involvement of the patellofemoral compartment inosteoarthritis of the knee is common. Typically thelateral patellofemoral joint is involved in conjunctionwith either the lateral or medial femorotibial joint.Medial patellofemoral disease may occur in associ-ation with medial femorotibial osteoarthritis, butisolated involvement of the medial patellofemoraljoint is distinctly unusual. Where osteoarthritisappears to predominate in the patellofemoral com-partment with sparing of the femorotibial joint, andparticularly where subchondral cysts are a dominantfeature, the possibility of calcium pyrophosphatedeposition disease should be considered.

The radiographic features of patellofemoralosteoarthritis include joint space loss, subchondralsclerosis and cysts and osteophytes at the posteriormargins of the patella. On MRI, cartilage loss andsubchondral sclerosis, oedema and cystic changesare identified at the patellar and trochlea surfaces.

Osteochondritis dissecans

Osteochondritis dissecans of the patella is muchless common than osteochondritis of the femoralcondyles. It is usually unilateral and typically occursin males between 15 and 20 years old. The conditionis thought to be traumatic in origin, and there isoften a history of onset of pain during knee flexionwhilst lifting a weight.65,66

Osteochondritis dissecans of the patella almostnever affects the superior pole and is mostcommonly seen along the medial patellar facet.On conventional radiographs there is an osseousdefect and a separated bony fragment or fragmentsmay be seen. MRI may be helpful in determining thestability of the osteochondral fragment.67 Signs ofinstability on T2-weighted images include thepresence of a high signal intensity line betweenthe fragment and the host bone, articular fractureindicated by fluid signal traversing the subchondralbone plate, a focal osteochondral defect filled withjoint fluid, and a 5 mm or larger fluid-filled cystdeep to the lesion.68

Differentiation of osteochondritis dissecans fromchondromalacia patellae may be difficult in somecases, but in general, the latter condition isconfined to the overlying cartilage, whilst osteo-chondritis involves a fragment of subchondral bone(Fig. 17).

Figure 16 Axial fat-saturated T2-weighted MR image ofthe knee in a patient with cartilage loss at the medialpatellar facet consistent with chondromalacia patellae.Note the cartilage flap (black arrow) and the jointeffusion.


Dorsal defect of the patella

A dorsal defect of the patella is a well-defined focaldefect in the subchondral bone that is confined tothe superolateral aspect of the patella. On arthro-graphy and MRI the overlying articular cartilageappears intact and indeed thickens to fill the defect(Fig. 18). This lesion is usually asymptomatic andappears to represent a variant of ossification,possibly within the spectrum of the bipartitepatella. All of these features serve to differentiatethis lesion from osteochondritis dissecans.69,70

Bipartite patella

Bipartite patella occurs in approximately 2% ofindividuals and is much more common in males(Fig. 19). It is usually a bilateral finding. With rareexceptions this anomaly occurs at the superolateralaspect of the patella. The lesion is usually asympto-matic, but can be associated with localized anteriorknee pain.71 Histologically there may be evidenceof chondro-osseous tensile failure at the junctionwith the host bone similar to that seen in Osgood-Schlatter disease and Sinding-Larsen-Johansson. Ithas been suggested therefore that the lesion mayrelate to stress at the site of insertion of vastus

lateralis,69 challenging the previously held beliefthat the lesion simply represented a normal variantof ossification of no clinical significance.

The presence of increased activity on Tc99m bonescintigraphy has been correlated with painfulbipartite patellae and may represent evidence ofabnormal stress.72

Figure 17 Axial fat-saturated T2-weighted MR image ofthe knee in a patient with osteochondritis dissecans ofthe lateral patellar facet. There is focal full-thicknesscartilage loss, as well as loss of a fragment of subchondralbone, as evidenced by loss of the black stripe represent-ing the subchondral bone plate within the lesion. Deep tothe lesion there is marrow oedema.

Figure 18 Dorsal defect of the patella. (a) Sagittal T1-weighted MR image shows a defect in the subchondralbone of the superior patella. (b) Axial fat-saturated T2-weighted MR image confirms the superolateral defect ofthe subchondral bone. Note that the overlying cartilage isthickened over the defect to produce a near normalarticular surface.


Conclusion

Anterior knee pain is a common symptom with alarge variety of possible causes including pathol-ogies related to the patellofemoral joint. Many ofthese pathologies have been rather loosely definedand remain incompletely understood because of the

complex nature of patellofemoral biomechanicalfunction and dysfunction. Imaging techniques havecontributed to the further understanding of theseprocesses, but the goal of imaging under trueconditions of everyday physiological function andbiomechanical loading has yet to be realized. In themeantime, optimal patient management is bestinformed when imaging protocols are appropriatelytailored to highlight patellofemoral pathology andinterpreted in the context of a current under-standing of the various pathological processesoccurring in the patellofemoral articulation.

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imagining dpf

Documents

patellofemoral pain

patellofemoral maltracking

posterior patella

bipartite patella

superior patella cartilage

patellofemoral osteoarthritis

extended knee

knee flexes