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Imaging the knee

S OSTLERE, FRCR

Nuffield Orthopaedic Centre and Oxford Radcliffe Hospital, Oxford, UK

Disorders of the knee are responsible for a majorsource of referrals to the musculoskeletal radiologist. Mostcases have suspected abnormalities within the joint eitherfollowing an acute injury or a more insidious develop-ment of symptoms. Other common causes of referral areanterior knee pain, focal and diffuse swellings. MRI is thetechnique of choice for assessing the internal structures.CT arthrography is also an accepted technique butrequires an intra-articular injection. Plain films arewidely used in suspected skeletal trauma and athropathiesbut are inferior to MRI in most other conditions.Ultrasound in mainly used to differentiate cystic from

potentially malignant solid periarticular masses.The discussion is divided into five common clinical

subgroups: the acute injury; chronic dysfunction; focalmasses; anterior knee pain; and the post-operative knee.

Acute knee injury

Plain films and MRI are the most useful investigations forimaging the acutely injured knee. Interpretation of plainfilms in skeletal trauma is discussed in Imaging trauma of theappendicular skeleton. Most knee injuries that are referredfor MRI are due to rotational, valgus, varus or translationforces, or a combination of any of these. The pattern ofabnormalities seen on MRI depends on the mechanism of

injury and the position of the joint at the time of injury.Sporting activities account for the majority of cases, withskiing being the most infamous culprit.

Imaging is useful in the acutely injured knee as clinicalassessment may be difficult on account of pain and thepresence of a haemarthrosis. A locked knee with restrictedextension may be due to muscle spasm associated withligamentous injury (pseudolocking) or a true blocksecondary to a displaced meniscal, or occasionally osteo-chondral, fragment. MRI is useful in differentiating thesetwo entities and allows the early implementation of appro-priate therapy, which is usually either arthroscopic remo-val of the cause of a physical block or early intensivephysiotherapy [1].

Although plain films are often unhelpful there may beradiographic signs that indicate that there is a significantproblem within the knee, such as a lipohaemarthrosisindicating an occult fracture or a lateral tibial capsularavulsion fracture (Segond lesion) which is associated witha high incidence of cruciate tears.

MRI is usually required independent of the plain filmfindings. MRI will provide an accurate picture of the stateof the internal structures. The key to interpreting the scanis to be aware of the common patterns of injury. If onestructure is disrupted then the reporter should carefullylook for known associated injuries at other sites.

Acute meniscal tears

Meniscal tears can be broadly divided into vertical circum-

ferential, vertical radial and horizontal types (Figure 1).

Tears often have an oblique component and combinations

of the main types are not unusual. Generally speaking

vertical tears are traumatic and horizontal tears are

degenerative. Meniscal tears can be painful. Unstable

menisci and displace meniscal fragments cause mechanical

problems. Clinical correlation is essential as the incidence

of tears in the asymptomatic population is high.The most common meniscal lesion in the locked knee is

the bucket handle type tear which predominately involves

the medial meniscus. This lesion is a large circumferential

vertical tear of the meniscus with displacement of the free

internal portion into the intercondylar region. The MRI

signs are a low signal intensity mass lying in the inter-

condylar region. The posterior portion of the fragment

usually lies under the posterior cruciate ligament (PCL)

giving the double PCL sign. The peripheral meniscal

remnant will have an irregular edge and will appear abnor-

mally small (Figure 2) [2].

In the lateral compartment a typical lesion causing

locking is a displacement of the posterior third of the

meniscus into the anterior part of the compartment. The

fragment may rest adjacent to the intact anterior thirdgiving the appearance of an enlarged meniscal segment

(pseudohypertrophy) on some slices (Figure 3) [3].A flap tear is when a fragment is displaced from the

superior or inferior surface of the meniscus. The dis-

placed portion is usually easy to see on MRI (Figure 4).

Sometimes fragment of the mid portion of the medial

meniscus may be found lying on the medial aspect of the

medial tibial condyle giving a characteristic appearance

(Figure 5). Although the normal semimembranosus tendon

can mimic this lesion on a single slice, this pitfall is avoided

by inspecting the adjacent images.Undisplaced or minimally displaced meniscal tears are

also seen in the acute setting. These are usually verticalcircumferential tears which are unstable when of sufficientlength. On MRI the tears are seen as linear high signal

Summary

N Plain films are adequate for most cases of

uncomplicated arthropathies and trauma.

N For suspected internal derangement MRI is the

optimum technique. Routine plain films are not

indicated.

N Ultrasound is ideal for initial examination of

periarticular swellings and tendon disorders

N MRI or CT arthrography is the best method of

assessing the post-operative meniscus

Imaging, 15 (2003), 217241 E 2003 The British Institute of Radiology

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traversing the entire width of the meniscus (Figure 6).

When these tears lie close to the meniscocapsular junction

they are less conspicuous particularly when involving the

posterior third of the lateral meniscus. This latter lesion isassociated with an anterior cruciate ligament injury (seebelow). MRI is less sensitive in detecting meniscocapsular

junction tears (Figure 7). The diagnosis can be made with

confidence if fluid signal is seen behind the entire base of

the meniscus, but often this cannot be appreciated in the

undisplaced tear. Subluxation of the meniscus may occur,but this sign is less specific being also seen in the normalmeniscus [4].

(a) (b)Figure 2. Bucket handle tear. (a) Double posterior cruciate ligament (PCL) sign. The low signal meniscal fragment is seen lying inthe intercondylar region (arrow) giving a double PCL appearance. (b) The peripheral portion of the meniscus is attenuated (arrow).

(a)

(c)

(b)

(d)

Figure 1. Basic types of meniscal tears: (a) vertical circumferential; (b) bucket handle tear; (c) vertical radial; (d) horizontal oblique.

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Figure 3. Pseudohypertrophy of the anterior third of thelateral meniscus. The posterior third of the meniscus has been

displaced anteriorly giving the impression of a large anterior

third of the meniscus (arrow). Note attenuated posterior third.

(a) (b)

Figure 4. Flap tear. The displaced meniscal fragment is seen lying in the posterior medial joint space on (a) sagittal and (b) coronalscans (arrows).

Figure 5. Displaced meniscal tear. A fragment of the torn midportion of the meniscus is seen to be displaced in the medial

gutter (arrow).

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Radial vertical tears extend from the free edge of themeniscus and are orientated in the radial plane, i.e. 90 tothe circumferential tears. They usually involve the lateralmeniscus often at the junction of the mid portion and theanterior or posterior thirds. On sagittal images the tearcommonly lies perpendicular to the scan plane and is seenas a break in the normal bow tie configuration of the

meniscus (Figure 8) [5]. On coronal images the tear isorientated along the scan plane and, as a result of partialvolume effect, is seen as an apparently normal shaped

meniscus containing diffuse increased signal (ghostmeniscus). Radial tears are also occasionally seen in themedial meniscus, particularly near its intercondylarattachment. Here the tear is best seen on the coronalimages. Because this is essentially an avulsion of theposterior horn of the meniscus from its attachment, themid portion of meniscus may be seen on coronal images tobe displaced medially beyond the edges of the joint. Thislatter finding is, however, non specific being also seen indegenerative joint disease. The ghost meniscus effect maybe seen on the sagittal images (Figure 9).

A congenital anomaly that predisposes the meniscus totears is the lateral discoid meniscus. The meniscus has adiscoid shape and covers the entire tibial articular surface.The abnormality is easily identified on MRI (Figure 10).The meniscus may tears in children and adolescents butmay remain asymptomatic into adulthood. The signifi-cance of the common incomplete discoid meniscus is lessclear. Discoid medial meniscus is an extremely rare entity.

Acute ligament tears

The diagnosis of an acute anterior cruciate ligament(ACL) tear has significant prognostic implications andmay be associated with long term disability and secondaryosteoarthritis. Although clinical evaluation for ACL insuf-

ficiency is quite accurate MRI is invariably performedprior to surgery to confirm the clinical findings andidentify associated lesions. The normal ligament is seen asa fairly loose collection of low signal fibres with a welldefined anterior border. On MRI, the most prominentfeature of an acute tear is an ill-defined mass representingfocal haemorrhage replacing the normal low signal linearligament (Figure 11). The tear usually involves the midportion. The MRI signs are less obvious when the liga-ment is avulsed at its femoral end as the ligament mayretain a fairly normal alignment [6]. Coronal and axialimages may help by showing discontinuity between theproximal ligament and the femoral condyle (Figure 12).There are a number of associated injuries involving other

structures which are quite specific for ACL tear. At the timeof injury there is usually valgus strain, external rotationand anterior translation of the tibia relative to the femoral

Figure 6. Vertical circumferential tear of the posterior third of themedial meniscus with minor displacement of the free edge (arrow).

Figure 7. Meniscocapsular separation. There is fluid seenbehind the meniscus. The meniscus is displaced anteriorly leav-

ing a bare area of tibia posteriorly (arrow).

Figure 8. Radial tear. The tear is seen as a discontinuity ofthe normal bow tie configuration of the mid portion of the

meniscus (arrow).

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condyle which frequently results in an impaction injury of

the posterior lip of the lateral tibial plateau against the

femoral condyle [7]. The typical appearance of this injuryon MRI is oedema in the posterior portion of the plateau,

occasionally accompanied by a small fracture of the pos-

terior lip, and a focal osteochondral impaction fracture of

the mid portion of the lateral femoral condyle (Figure 13).The latter may result in a loose body. The combination of

these two injuries is diagnostic of ACL tear. Similar bony

impaction may be seen on the medial side which is thoughtto be as result of rebound from the valgus force [8]. There

are often associated meniscal tears, the most characteristic

of which is a vertical circumferential tear of the posteriorthird of the lateral meniscus. This tear may be missed as

it lies close to the fluid-containing popliteus tendon sheath

which has been shown to result in difficulty in perceiving

the lesion [9]. It must be remembered that the menisco-

femoral ligament attaches to the posterior third of the

lateral meniscus in this region and can mimic a tear.

This pitfall can be avoided by tracing the course of thenormal meniscofemoral ligament on more medial images

(Figure 14) [10].MRI is less accurate in diagnosing partial tears of the

ACL. The arthroscopic diagnosis is also not straight

forward so the MRI signs have not been well established.Usually there are some intact fibres seen with some

increased signal seen in the posterior portion of the

ligament. Wavy or curved configuration may also be seen.There may be signs of minor anterior translation of the

tibia [11, 12].

(a) (b)

(c)

Figure 9. Radial type tear of the posterior third of the medialmeniscus close to its tibial attachment. (a) Coronal view shows

a tear of the meniscus near the tibial attachment (arrow).

(b) Sagittal scan along the plane of the tear shows a ghost-like

appearance of the meniscus (arrow). (c) Coronal image through

the middle of the knee shows medial displacement of the mid

portion of the meniscus (arrow).

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The normal PCL is a compact low signal intensity

structure that is easily identified on MRI. The ligament

may be partially or completely torn as result of a

hyperextension injury, an external rotation injury orforced posterior translation of the tibia (dashboard

injury). Partial and complete tears may occur. Tears usually

involve the mid portion of the ligament. The torn ligament is

widened and contains increased signal (Figure 15). Without

the presence of obvious discontinuity it can be difficult to

differentiate complete and partial tears [13].Medial collateral ligament (MCL) injuries are common

and usually partial. Any injury involving a valgus force is

(a) (b)

Figure 10. Discoid meniscus. (a) Coronal scan through the middle of the knee shows that the lateral meniscus extends across theentire joint compartment (arrow). (b) On sagittal scan the bow tie appearance of the meniscus persists into the centre of the joint

compartment.

(a) (b)

Figure 11. Acute anterior cruciate ligament (ACL) tear. (a) Well defined straight fibres of the normal ACL (arrows). (b) In theacute ACL tear the normal ligament is replaced by a high signal heterogeneous mass (arrow).

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(a) (b)

Figure 12. Acute anterior cruciate ligament (ACL) tear. (a) On the sagittal scan a curved ligament can be seen lying in the expectedposition of a normal ACL (arrow). (b) Coronal image showing avulsion of the ligament at its femoral attachment (arrow).

(a) (b)

Figure 13. Typical pattern of bony injury associated with anterior cruciate ligament tear. (a) Gradient echo T2 weighted sagittal

image showing evidence of trabecular trauma at the posterior portion of the lateral tibial plateau (arrow) and a small impaction frac-ture of the lateral femoral condyle (open arrow). (b) Short tau inversion recovery coronal image showing the small impaction fracture

of the lateral femoral condyle (arrow) with subchondral oedema. Note associated articular cartilage defect.

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likely to result in injury to the MCL. Isolated tears of the

MCL are treated conservatively and imaging is rarely

required. However, a torn MCL is often seen on MRI as

part of a more complex injury. The ligament is made up of

deep and superficial fibres. The ligament usually tears near

the femoral insertion (Figure 16). Minor bony oedema

may be seen in the adjacent condyle at the site of theavulsion. In minor injuries the tear may be confined to the

deep fibres and is seen on MRI as high signal on T2

weighted or short tau inversion recovery (STIR) images

deep to the superficial fibres (Figure 17).The valgus injury responsible for the MCL tear will

often result in an impaction injury on the lateral side of

the knee with subchondral oedema seen in the lateral

femoral condyle and tibial plateau. If the knee is in the

flexed position at the time of injury, the femoral sub-chondral oedema will lie in the posterior portion of the

condyle.

(a) (b)

Figure 14. (a) Typical vertical circumferential tear of the posterior third of the lateral meniscus associated with AC tear (arrow).(b) Normal attachment of a prominent meniscofemoral ligament (arrow) mimicking a tear.

Figure 15. Acute posterior cruciate ligament tear. There is dif-fuse increased signal and widening of the ligament with focal

disruption (arrow).

Figure 16. Acute medial collateral ligament tear. The proximalportion of the ligament is widened and contains increased

signal (arrows). There is associated soft tissue haemorrhage/

oedema medial to the ligament.

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Tears of the lateral collateral ligament are rarer.Although isolated injuries can occur, usually at its distalattachment to the fibula, tears are often associated withinjuries to other structures particularly those of the pos-terolateral corner injury (see below) (Figure 18) [14].

Patellar dislocation

Dislocation of the patella is another cause of post-traumatic haemarthrosis. The diagnosis if often notsuspected by clinician or patient. Dislocation may resultfrom a relatively innocuous twisting injury. MRI willshow a characteristic appearance with subcortical oedemaat the anterolateral aspect of the femoral condyle at thesite of patellar impaction. There may be correspondingoedema of the medial aspect of the patella or signs of amedial patellar osteochondral fracture with or withoutloose body. The medial retinaculum is usually torn(Figure 19) [15].

Hyperextension injury

Hyperextension injuries are associated with tears of theanterior third of the menisci and trabecular oedema in theanterior femoral condyle and tibial plateau. ACL, PCL,posterior capsule and popliteus muscle tears may bepresent. The posterior capsule is not a well definedstructure on MRI and the most obvious sign of a tearof this structure may be the presence of free fluid dissectingthe tissue planes posteriorly (Figure 20).

Posterolateral corner

Damage to the posterolateral structures is an importantinjury that may result from a variety of mechanisms.

Incompetence of the structures results in rotational insta-bility (allowing excessive external rotation of the tibia) andmay be responsible for the poor results following ACL

repair [16]. The major stabilizing structures in this regionare the popliteus tendon and muscle, the PCL, the lateralcollateral ligament (LCL) and several smaller structuresmaking up the arcuate complex, namely the arcuate,fabellofibular and popliteofibular ligaments. Of these threestructures the popliteofibular ligament is the only one seenconsistently on MRI [17]. The diagnosis in the acute phaseshould be suspected when disruption of more than one ofthese structures is encountered. Rupture of the PCL, LCL,

popliteal tendon and muscle and fibular avulsion by thearcuate complex can be accurately assessed on MRI

(Figure 21) [14]. Generalized oedema in the region of theposterolateral capsule may also be seen in the acute injury.

The usefulness of MRI in the management of chronicposterior corner injury has not been proven and manage-

ment is primarily based on clinical examination.

Chronic knee dysfunction

A common source of referral for imaging is the patientwith non-acute symptoms that point to a mechanicalproblem such as pain, intermittent locking and clicking.The purpose of imaging is primarily to select thosepatients who require arthroscopy. Patients with obviousclinical signs of meniscal tear do not usually requireimaging. The two main groups of patients are those who

have unstable meniscal tears and those with degenerativedisease. Only the former can expect a successful outcomefollowing arthroscopy.

Figure 17. Tear of the deep fibres of the medial collateral liga-ment (MCL). There is widening and increased signal deep to the

superficial fibres of the MCL (arrow). There is some minor asso-

ciated oedema in the adjacent portion of the femoral condyle.

Figure 18. Isolated tear of the lateral collateral ligament at thefibular insertion (arrow).

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Meniscal pathology

Residual traumatic meniscal tears, even of the bucket

handle type, may be found in patients scanned elec-tively, but the most common lesion encountered on thenon-acute scan is the horizontal oblique degenerativetype tear.

This lesion can affect any part of either meniscus but theposterior third of the medial meniscus is the most commonsite. A typical lesion is seen as linear horizontal obliquehigh signal extending to the inferior, or less commonly thesuperior, articular surface of the meniscus (Figure 22).There are often accompanying signs of early degenerativedisease such as articular cartilage defects, subchondral

oedema and cysts and small osteophytes. Signal that doesnot extend to an articular surface represents intrameniscal

degeneration. Care should be taken not to overcall tears, asin equivocal cases a tear is rarely found at arthroscopy [18].Degenerative type tears may be associated with a

perimeniscal cyst (see below) which is formed by fluidbeing pumped into the perimeniscal tissues through thetear. These cysts can in themselves be painful and mayneed arthroscopic decompression.

Ligament pathology

The torn ACL rarely heals spontaneously and undiag-nosed or neglected chronic tears are therefore not anuncommon finding on MRI. Signs of a chronic ACL tear

are non-visualization of the ligament or an abnormalhorizontal orientation of the ligament as it lies redundantin the intercondylar notch (Figure 23). The ligament may

become attached to the PCL. Secondary signs of ACL tearmay be present but rarely contribute to the diagnosis.

These signs include a vertically orientated PCL and anincreased distance between tangential parallel linesthrough the most posterior points of the lateral tibialplateau and femoral condyle (normal ,5 mm) [19, 20].These two signs are due to the anterior translation of thetibia. A focal old compression fracture of the lateralfemoral condyle is almost diagnostic of chronic ACL tear.

This must not be confused with the normal shallow sulcusseen in the midarticular position of the lateral condyle onthe sagittal scans. Chronic PCL tears are seen as dis-continuity of the ligament. The collateral ligaments tend to

heal spontaneously and chronic tears cannot usually bedetected on MRI unless there is some residual thickeningor ossification of the ligament.

Iliotibial band

The iliotibial band inserts on the anteromedial aspect ofthe medial tibial condyle. Iliotibial band friction syndromeis a specific condition usually seen in runners and cyclists

and is caused by repetitive flexion and impingement of theband against the lateral femoral condyle when the knee isflexed at around 30 degrees. There is focal tenderness overthe band and sometime a palpable creak as the kneeflexes. Imaging is usually not required. On MRI ill-definedoedema or less commonly a well-defined cyst, is seen

seated deep to the band (Figure 24) [21]. The band itselfusually appears normal. These abnormalities can beidentified on ultrasound [22].

(a) (b)

Figure 19. Recent patellar dislocation. (a) Sagittal short tau inversion recovery image shows oedema adjacent the lateral aspect ofthe lateral femoral condyle (arrow). (b) Axial image showing a large defect in the retropatellar cartilage (arrow). An articular carti-

lage fragment is embedded in the synovium of the lateral recess (open arrow). There is widening of the medial retinaculum (*).

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(a) (b)

(c)

Figure 20. Posterior capsule tear following hyperextensioninjury. (a) Sagittal image shows a tear of the posterior capsule

(arrow). (b) Coronal short tau inversion recovery image shows

fluid surrounding the popliteal vessels. (c) There is an asso-

ciated vertical tear of the anterior third of the lateral meniscus

(arrow).

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Bone and articular cartilage

Articular cartilage defects are a frequent finding onMRI of the knee. The sensitivity of MRI is relatively low

and is highly dependent on the sequences used. As cur-

rently specific treatment for articular cartilage defects isstill experimental, routine protocols are generally designed

for the assessment of the menisci, ligaments and bone.

However, articular cartilage defects are painful and arthro-

scopic treatment by removing unstable fragments is

beneficial in some cases. On MRI full thickness defects

are best seen on sequences that have high contrast between

fluid and articular cartilage such as STIR, T2 fast spin

echo and some more specialized gradient echo techniques

(Figure 25). Although the commonly used proton density

with fat suppression sequence is less sensitive in identifying

defects it will, like other fat suppression techniques, readily

identify the frequent accompanying subchondral oedema

in the adjacent bone. Specialized sequences can be used when

optimum assessment of the articular cartilage is required

such as in research trials. Spoiled gradient recalled acquisi-

tion in steady state (SPGR) with fat suppression is a

popular choice, the sequence providing good contrast and

high resolution thin slices [23].Bony abnormalities are frequently seen on MRI.

Subchondral bone oedema is commonly associated

with trauma in the absence of an identifiable fracture.The abnormality is probable due to oedema and hae-

morrhage secondary to trabecular microfractures [24].

The abnormality may persist for months. Subchondral

oedema and small cysts are seen in association with

articular cartilage defects. MRI is a useful tool in

patients with a clinical suspicion of fracture but normal

plain films. The most common occult fractures are of the

lateral tibial plateau and avulsion fractures at the distal

end of the ACL or PCL (Figure 26). MRI is sensitive in

identifying insufficiency or stress fracture which are seen

as low signal lines surrounded by oedema. The most

common lesion seen around the knee is an insufficiency

fracture of the medial tibial plateau. This lesion isusually associated with osteoporosis and is therefore seen

in the older age group (Figure 27). Another cause for

(a) (b)

Figure 21. Posterolateral corner injury. (a) Short tau inversion recovery (STIR) coronal scan showing oedema in the tip of the fibula(arrow) due to avulsion injury by the arcuate complex. The popliteofibular ligament is well demonstrated (open arrow). (b) STIR

coronal scan in a different case with acute rupture anterior cruciate ligament. There is high signal medial to the collateral ligament

in keeping with tear of the popliteofibular ligament.

Figure 22. Typical degenerative horizontal oblique type tear ofthe posterior third of the medial meniscus extending to the

inferior articular surface (arrow).

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medial pain in the elderly is the specific condition of

spontaneous osteonecrosis of the knee (SONK) which

involves the subchondral bone of the medial femoral

condyle (Figure 28). The MRI appearances are specific,

with extensive oedema involving most of the medial

femoral condyle and a focal low signal intensity sub-

chondral lesion with some flattened at the weight bearing

portion of the condyle [25]. On plain films, linear sub-

chondral gas may be seen in addition to sclerosis and

focal flattening of the condyle.

Figure 23. Chronic anterior cruciate ligament tear. The liga-

ment is lying on the floor of the intercondylar region (arrows).

(a) (b)

Figure 25. Articular cartilage defects. (a) Sagittal volume acquisition gradient echo (DESS) sequence showing small defect in thearticular cartilage of the medial femoral condyle. (b) Axial T2 weighted fast spin echo image showing full thickness defect involving

the medial facet of the patella (arrow).

Figure 24. Iliotibial band friction syndrome. There is some

increased signal (arrow) related to the deep surface of the ilio-tibial band.

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Osteochondritis dissecans is the name given to focal

subchondral osteonecrosis that typically occurs on the

lateral aspect of the medial femoral condyle in children

and adolescents. The lateral condyle and the patella are

other favoured sites. The lesion can usually be seen on

plain films, but MR is more sensitive. The articular

cartilage may be disrupted or intact and in old lesions the

defect may be filled with cartilage. MRI is useful to stage

the lesion but in general symptomatic patients will

proceed to arthroscopy for excision of an unstable lesion

or drilling to promote revascularization. The most reliable

Figure 26. Occult tibial plateau fracture. Sagittal proton den-sity fat suppression image clearly shows fracture line (arrows).

Figure 27. Insufficiency fracture secondary to rheumatoid associated osteoporosis. (a) Coronal short tau inversion recovery imageshowing extensive oedema in the medial tibial plateau. Note marginal erosion (arrow). (b) More anterior slice shows the typical low

signal line surrounded by oedema (arrow).

Figure 28. Spontaneous osteonecrosis of the knee. Coronalshort tau inversion recovery image showing a focal low signal

subchondral lesion in the medial femoral condyle (arrow) sur-

rounded by extensive oedema. There is some flattening of the

articular surface.

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signs of an unstable fragment are high signal between thefragment and bone and a visible defect in the overlyingarticular cartilage (Figure 29) [26].

Osteonecrosis is sometimes encountered in the femur ortibia usually in patients with known risk factors.These lesions are not symptomatic unless they involvethe subarticular bone when the articular surface may

collapse. MRI will show the typical geographic lesioncontaining fat and bordered by a low signal intensity line.The border may show the typical double line sign (parallel

high and low signal intensity lines) on T2 weighted images(Figure 30).

Arthropathies

Generalized arthropathies such as rheumatoid

arthritis are covered in Imaging in rheumatology.Monoarthropathies unrelated to degenerative or trau-matic conditions are uncommon and require investiga-tion. The knee is the most common site for a few rareconditions. Diffuse pigmented villonodular synovitis

presents as swelling and pain related to recurrenthaemarthroses. In the early stages plain films arenormal and MRI is required to confirm the diagnosis.Synovial hypertrophy returning low signal intensity on

T2 weighted images is seen. This is best appreciated ongradient echo images (Figure 31). The condition mayresult in bone erosion and premature osteoarthritis. Thedifferential diagnosis included other causes of recurrent

(a) (b)

Figure 30. Osteonecrosis. (a) Coronal T1 weighted and (b) short tau inversion recovery images showing the typical geographic lesionscontaining fat signal (arrows). The femoral infarct has breached the articular surface.

Figure 29. Osteochondritis dissecans of the medial femoralcondyle. Coronal short tau inversion recovery image. There is

high signal line surrounding the low signal necrotic fragment

(arrow) indicating instability.

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haemarthrosis such as bleeding disorders, usually hae-

mophilia, and synovial haemangioma. Haemophilia can be

excluded by talking to the patient. If a synovial hae-

mangioma is the cause of the haemarthrosis the tumour

should be visible on MRI.Another rare condition is synovial osteochondromato-

sis. This focal metaplasia usually presents as a focal mass

commonly in the popliteal fossa (see below).Lipoma arborescens is a vary rare condition that

predominately involves the suprapatellar pouch of the

knee. The synovium is infiltrated with fat and forms

villous-like lesions or mass that project into a joint

effusion. Although the aetiology of the condition is not

known, there is often other knee pathology present such as

degenerative disease or rheumatoid arthritis (Figure 32)

[27].

Focal massesFocal swellings around the knee are common. Most

focal masses are found to be cystic. Ultrasound is agood preliminary test to exclude the rarer, potentiallymalignant solid lesions. Although ultrasound alone isoften sufficient if a lesion is found to be cystic, MRI isusually required if the diagnosis of meniscal cyst is beingentertained.

Periarticular cystic lesions are frequently encountered onMRI performed for other reasons such suspected internalderangement.

Solid lesionsThe most important solid lesion to diagnose is synovial

sarcoma. The knee is a common site for this rare malig-

nancy which affects young adults. The diagnosis is often

delayed as periarticular masses are common and not

always investigated on an urgent basis. On ultrasound

a non specific solid vascular periarticular mass is seen.

Calcification is common and may be detected on ultra-

sound and plain film. The MRI features are also non

specific, with the lesion returning high signal on T2 and

STIR images. The calcification may be discernable as low

signal foci within the lesion (Figure 33). Occasionally the

lesion will lie in an intra-articular position.

Synovial osteochondromatosis is chondral metaplasiaresulting in focal synovial mass as well as intra-articular

loose bodies. The lesion often calcifies giving a typical

appearance on plain film. MRI is used to evaluate

(a) (b)

Figure 32. Lipoma arborescens. (a) T1 weighted and (b) short tau inversion recovery axial scans through the suprapatellar pouchshowing fatty synovial villous mass projecting into the joint.

Figure 31. Pigmented villonodular synovitis. Gradient echo T2weighted image showing extensive synovial hypertrophy return-

ing low signal intensity (arrows).

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the intra-articular extent of the lesion. On MRI the

metaplastic cartilage typically returns high, relatively

homogeneous signal on T2 weighted images with multiple

foci of low signal intensity representing calcification.

Ultrasound shows a poorly vascular solid lesion with

calcific foci. Plain film is useful to confirm the presence of

and characterize the calcification. The lesion is benign and

treatment is excision (Figure 34).The nodular form of pigmented villonodular synovitis is

another synovial based lesion which may present as a mass

or with symptoms of internal derangement. A common site

for this lesion is Hoffas fat pad. On MRI the mass will be

(a) (b)

Figure 33. Synovial sarcoma. (a) Plain radiograph shows a calcified soft tissue mass on the lateral aspect of the knee. (b) T2weighted axial image shows a mass containing extensive low signal representing the tumour calcification (arrows).

(a) (b)

Figure 34. Synovial osteochonromatosis. (a) Plain film showing multiple ossified foci behind the knee. (b) On ultrasound the popli-teal cyst is filled with solid tumour containing calcific foci.

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seen to contain some loss of signal intensity on T2weighted images representing a combination of fibrosis

and scattered haemosiderin deposits. Excision is usually

curative (Figure 35).Bony lesions may be a cause of a focal swelling around the

knee. Osteochondromata are common. Imaging beyond theplain film is used in symptomaticlesions particularly to exclude

malignant transformation of the cartilage cap. The posterioraspect of the distal femur is a typical location for a parostealosteogenic sarcoma (see Evaluation of focal bone lesions).

Cystic lesions

The most common cystic lesions encountered are bursae,synovial cysts, ganglions or meniscal cysts.

BursaeBursae are normal synovial lined structures that can be

affected by the same conditions as synovial lined joints.Bursae may also be developmental, forming within the softtissues where there is abnormal repetitive pressure. Normalbursae are found in predictable places. Bursae become visibleon imaging when they develop an effusion due to inflamma-

tory synovitis or trauma. The common symptomatic bursaeare those of the pre-patellar bursa, infrapatellar bursa, pesanserinus and semimembranosus bursae (Figure 36). A smallbursa is sometimes seen between the deep and superficial fibresof the MCL. Bursae can also be involved in other synovialconditions such as synovial ostechondromatosis.

Synovial cystsSynovial cysts communicate with the joint. By far the

most common encountered at the knee is the popliteal

(Bakers) cyst. This is a normal extension of the synovium

which can become distended due to any chronic condition

of the joint. The cyst has a typical diagnosic configuration

on ultrasound and MRI with the neck emerging from

between the tendons of the medial head of gastrocnemiusand semimembranosus (Figure 37).

Perimeniscal cystsPerimeniscal cysts are formed by fluid being pumped

through a meniscal tear into the perimeniscal tissue(Figure 38). They occur in both lateral and medial menisciand may be painful. On the lateral side they tend to form amass at the site of the meniscal tear as the overlying LCLis not bound to the meniscus. On the medial side the cysthas a tendency to migrate along the tightly bound tissueplanes and emerge as palpable masses some distance fromits origin [28]. On ultrasound the cyst can sometimes betraced to a meniscal tear but MRI is usually required to

establish the diagnosis. Even on MRI it may occasionallybe difficult to demonstrate the communication as theconnecting stalk may be tenuous. The treatment is to

decompress the cysts arthroscopically.

Figure 35. Focal pigmented villonodular synovitis. T2 weightedimage showing focal lesion in Hoffas fat pad (arrow). There

are scattered low signal foci representing fibrosis and haemosi-

derin deposits.

Figure 36. Semimembranosus bursitis. Axial short tau inversionrecovery image showing distended bursa at the pes anserinus

(arrow).

Figure 37. Ultrasound showing typical configuration of a pop-

liteal cyst with a neck communicating with the joint betweenthe medial head of gastrocnemius (MG) and semimembranosus

(SM).

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GanglionA cyst that does not fall into one of above categories is

usually labelled a ganglion. A ganglion is due to myxoiddegeneration of a structure such as a tendon, ligament orjoint capsule. They differ histologically from synovialcysts but a small proportion will communicate with the

joint. The main body of the lesion is often connected to itsorigin by a stalk which may be demonstrated on imaging

(Figure 39). Ganglions may cause symptoms other thanswelling. Lesions arising from the cruciate ligamentsprevent full knee flexion (Figure 40). They may besuccessfully treated by ultrasound or CT guided aspiration

[29]. A ganglion involving of the common peroneal nerve

is a well recognised condition. The cyst, which is thought

to arise from the proximal tibiofibular joint, dissects along

the nerve resulting in foot drop (Figure 41) [30].

(a) (b)

Figure 40. Anterior cruciate ligament (ACL) ganglion. (a) Sagittal gradient echo T2 weighted image showing a ganglion intimately relatedto the ACL (arrow). (b) Short tau inversion recovery coronal image showing some of the ACL fibres being separated by the cyst (arrow).

Figure 39. Ganglion arising from the patellar tendon. Ultrasoundimage showing a typical anechoic lesion (arrow). The lesion is con-

nected to the patellar tendon via a stalk (open arrow).

Figure 38. Meniscal cyst. Cornonal T2 weighted image show-ing a meniscal cyst (arrow) communicating with a horizontal

tear of the mid portion of the lateral meniscus.

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Imaging of cystsUltrasound is an excellent technique for confirming that

a lesion is indeed cystic. Cysts are either anechoic or may

contain variable amounts of echoes particularly inthose lesions related to inflammatory conditions such as

infection. The demonstration of floating mobile echoes will

prove that the lesion is cystic. In synovial lined cysts

solid elements representing synovial hypertrophy will be

seen in inflammatory conditions. Non compressibility

and hypervasculatrity will differentiate synovium from

fluid. Ultrasound is also an excellent tool for guiding

aspiration.Simple cysts on MRI are seen as high signal intensity

on T2 and low signal intensity on T1 weighted images.

MRI without contrast is inferior to ultrasound in

differentiating inflamed synovium from fluid but can

detect blood products in haematomas. One should becautious of making the diagnosis of a cystic lesion on

unenhanced MRI as solid neoplasms can have similar

signal characteristics. Ultrasound is an easy way of

confirming the cystic nature of a lesion and should be

performed if there is any doubt.

Anterior knee pain

Anterior knee pain is a common complaint particularly in

adolescents and young females. The cause is often notapparent clinically and most patients are treated conserva-tively without resorting to imaging. Most cases are related to

the patellofemoral joint. The main purpose of imaging is toexclude other specific condition such as osteochondral defector patellar tendinopathy and to identify any abnormalmorphology of the patellofemoral joint that may be amen-able to surgery. Plain films provide limited information.MRI is sensitive for excluding significant unexpected

pathology and is can be used to assess the patellofemoralanatomy and patella subluxation. Ultrasound is useful indiagnosing suspected patellar tendinosis.

(a)

(c)

(b)

(c)

Figure 41. Ganglion of the common peroneal nerve in patientwith foot drop. (a) Ultrasound shows widened hypoechoic

mass (arrow) along the course of the nerve at the fibula neck

(F). (b) The lesion is seen as high signal on short tau inversion

recovery coronal MRI (arrow). (c) Axial T2 image show high

signal in anterior compartment indicating denervation of tibialis

anterior.

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Patellar tendinosis (jumpers knee)

This condition which affects athletic young adults is anoveruse syndrome of the proximal patellar tendon possiblyassociated with impingement of the tendon on the inferiorpole of the patella. Diagnosis is usually suspected clinically.

Ultrasound is the easiest method of confirming the diagnosis.The condition typically involves the under surface of the

most central part of proximal portion of the tendon directlyadjacent to the inferior pole of the patella. On ultrasound

this portion of the tendon is hypoechoic and widened.Hypervascularity is often seen in and around the lesion [31].

On MRI the affected portion of the tendon returns highsignal on T2 weighted and STIR images (Figure 42).

Less commonly tendinosis affects the distal end of the

tendon or the entire tendon. The condition is sometimes an

unexpected finding picked up on MRI performed for

anterior knee pain.

Occasionally a focal proximal abnormality is seenjust lateral to the inferior pole of the patella due to

impingement of this portion of the tendon on the lateral

condyle.

Osgood Schlatters disease

In this condition there is fragmentation of the tibialtubercle due to chronic repetitive trauma. The condition istypically seen in athletic adolescent males and is primarily

a clinical diagnosis as the patients will have a chronic

painful bump at the tibial tubercle. Ultrasound willconfirm the diagnosis, provide reassurance and negate

the need for plain films. Fragmentation of the tubercle and

inflammatory changes in the adjacent tendon are typicalfindings (Figure 43). The pathology is also well demon-

strated on MRI.

Figure 42. Patellar tendinosis. (a) T2 weighted sagittal image showing high signal lesion in the deep portion of the tendon (arrow).(b) Sagittal ultrasound of the proximal patellar tendon showing a widened tendon with reduction in reflectivity (arrows). There is

marked increased vascularity seen in the region (P5patella).

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Chondromalacia patellae

This term refers to degeneration of the articularcartilage and is associated with anterior knee pain in theyoung. The grading spans from softening of the cartilageto full thickness defects. The minor grades are common in

the young and are difficult to detect on MRI. Macroscopicdefects may be seen if the appropriate sequence areperformed in the appropriate plane. There is little practical

advantage in making an imaging diagnosis of earlychondromalacia (Figure 44).

Abnormal patellofemoral morphology

There are three main anatomical variants that predis-pose to symptoms: a) an abnormally laterally placed tibialtubercle; b) an abnormally shallow trochlear groove; and

c) a high lying patella (patellar alta). These adverse factorsoften coexist resulting in lateral subluxation or tilt of the

patella on knee extension. Operative procedures includerelease of the lateral retinaculum and medial transfer ofthe tibial tubercle.

Plain films have been superseded by cross-sectionalimaging for the assessment of patellofemoral morphologyin many centres. Subluxation tends to occur as the kneeextends beyond 30 degrees of flexion, a position that is

difficult to evaluate on skyline plain films. The depth ofthe trochlear groove can be assessed on a true lateralradiograph but CT and MRI will give a more complete

picture of the morphology (Figure 45). The degree oflaterisation of the tibial tubercle can be assessed bycomparing the relative position of the tubercle with the

deepest point of the trochlear groove in the sagittal plane.The tendency for subluxation can be assessed on dynamicor pseudodynamic tracking studies using CT or MRI. Inpseudodynamic studies a video is compiled of multipleaxial images of the knee in progressive extension fromaround 40 degrees flexion to full extension. True dynamicstudies can be obtained with the use of rapid acquisitionaxial MRI while the knee is actively extended against

resistance (Figure 46) [32] .Although the role of imaging in these patients is

not fully established, quantitative analysis of the anatomyand subjective assessment of subluxation helps guidemanagement.

Post-operative knee

Imaging following knee replacement is discussed inImaging of prosthetic joints. The other main indicationsfor imaging the post-operative knee are suspected ACLgrafts disruption and suspected recurrent meniscal tearsfollowing partial menisectomy.

MRI signs of re-rupture of an ACL graft are similar tothose for the native ligament. Gradient echo images should

be avoided as these exacerbate metal artefact. On spinecho images the normal graft can usually be clearlyidentified as a low signal structure (Figure 47). Impingement

of the graft on the anterior edge of the intercondylar roofmay be seen if the tibial tunnel is lying in an excessivelyanterior position. Focal anterior arthrofibrosis (Cyclops

Figure 44. Chondromalacia patellae. T2 weighted axial image.Minor retropatellar articular cartilage defects are seen.

Figure 43. Osgood Schlatters disease. Sagittal ultrasoundimage showing fragmentation of the tibial tubercle (arrow) and

widening of the distal patellar tendon (open arrow).

Figure 45. Reconstructed axial MRI through the proximalpart of the trochlear groove. The articular surface has an

abnormaly flat configuration.

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lesion) may occur causing a block to extension. This lesion

is seen as a low signal intensity mass on MRI [33].

The accuracy of MRI for the diagnosis of tearsfollowing partial menisectomy is less than that for the

untouched meniscus. Linear high signal extending to the

articular surface and irregularity of the meniscal surface

are common findings in stable untorn meniscal remnants.

MR arthrograhy is more accurate in identifying significantmeniscal tears if there has been resection of more than 25%

of the meniscus [34] . The tear will be outlined by the high

(a) (b)

Figure 47. Anterior cruciate ligament graft. T2 weighted image showing (a) intact ligament (b) acute tear.

(a) (b)

Figure 46. Patellar subluxation. Two images from a dynamic tracking study. (a) Normal position of the patellae with the kneesflexed at 40 . (b) With the knees in the extended position there is bilateral subluxation of both patellae.

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signal gadolinium on T1 weighted images (Figure 48). CTarthrogram is also an accurate method of assessing thepost-operative meniscus.

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