Imaging 15:217-241 (2003)
© 2003 The British Institute of Radiology
Imaging the knee
S Ostlere, FRCR
Nuffield Orthopaedic Centre and Oxford Radcliffe Hospital, Oxford, UK
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Summary
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- Plain films are adequate for most cases of uncomplicated arthropathies and trauma.
- For suspected internal derangement MRI is the optimum technique. Routine plain films are not indicated.
- Ultrasound is ideal for initial examination of periarticular swellings and tendon disorders
- MR or CT arthrography is the best method of assessing the post-operative meniscus
Disorders of the knee are responsible for a major source of referrals to the musculoskeletal radiologist. Most cases have suspected abnormalities within the joint either following an acute injury or a more insidious development of symptoms. Other common causes of referral are anterior knee pain, focal and diffuse swellings. MRI is the technique of choice for assessing the internal structures. CT arthrography is also an accepted technique but requires an intra-articular injection. Plain films are widely used in suspected skeletal trauma and athropathies but 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; focal masses; anterior knee pain; and the post-operative knee.
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Acute knee injury
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Plain films and MRI are the most useful investigations for imaging the acutely injured knee. Interpretation of plain films in skeletal trauma is discussed in Imaging trauma of the appendicular skeleton. Most knee injuries that are referred for MRI are due to rotational, valgus, varus or translation forces, or a combination of any of these. The pattern of abnormalities 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, with skiing being the most infamous culprit.
Imaging is useful in the acutely injured knee as clinical assessment may be difficult on account of pain and the presence of a haemarthrosis. A locked knee with restricted extension may be due to muscle spasm associated with ligamentous injury (pseudolocking) or a true block secondary to a displaced meniscal, or occasionally osteochondral, fragment. MRI is useful in differentiating these two entities and allows the early implementation of appropriate therapy, which is usually either arthroscopic removal of the cause of a physical block or early intensive physiotherapy [1].
Although plain films are often unhelpful there may be radiographic signs that indicate that there is a significant problem within the knee, such as a lipohaemarthrosis indicating an occult fracture or a lateral tibial capsular avulsion fracture (Segond lesion) which is associated with a high incidence of cruciate tears.
MRI is usually required independent of the plain film findings. MRI will provide an accurate picture of the state of the internal structures. The key to interpreting the scan is to be aware of the common patterns of injury. If one structure is disrupted then the reporter should carefully look for known associated injuries at other sites.
Acute meniscal tears
Meniscal tears can be broadly divided into vertical circumferential, 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.
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Figure 1. Basic types of meniscal tears: (a) vertical circumferential; (b) bucket handle tear; (c) vertical radial; (d) horizontal oblique.
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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 intercondylar 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 abnormally small (Figure 2) [2].
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Figure 2. Bucket handle tear. (a) Double posterior cruciate ligament (PCL) sign. The low signal meniscal fragment is seen lying in the intercondylar region (arrow) giving a double PCL appearance. (b) The peripheral portion of the meniscus is attenuated (arrow).
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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 third giving the appearance of an enlarged meniscal segment ("pseudohypertrophy") on some slices (Figure 3) [3].
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Figure 3. Pseudohypertrophy of the anterior third of the lateral 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.
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A flap tear is when a fragment is displaced from the superior or inferior surface of the meniscus. The displaced 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.
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Figure 4. Flap tear. The displaced meniscal fragment is seen lying in the posterior medial joint space on (a) sagittal and (b) coronal scans (arrows).
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Figure 5. Displaced meniscal tear. A fragment of the torn mid portion of the meniscus is seen to be displaced in the medial gutter (arrow).
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Undisplaced or minimally displaced meniscal tears are also seen in the acute setting. These are usually vertical circumferential tears which are unstable when of sufficient length. On MRI the tears are seen as linear high signal 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 is associated with an anterior cruciate ligament injury (see below). 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 normal meniscus [4].
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Figure 6. Vertical circumferential tear of the posterior third of the medial meniscus with minor displacement of the free edge (arrow).
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Figure 7. Meniscocapsular separation. There is fluid seen behind the meniscus. The meniscus is displaced anteriorly leaving a "bare area" of tibia posteriorly (arrow).
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Radial vertical tears extend from the free edge of the meniscus and are orientated in the radial plane, i.e. 90° to the circumferential tears. They usually involve the lateral meniscus often at the junction of the mid portion and the anterior or posterior thirds. On sagittal images the tear commonly lies perpendicular to the scan plane and is seen as a break in the normal bow tie configuration of the meniscus (Figure 8) [5]. On coronal images the tear is orientated along the scan plane and, as a result of partial volume effect, is seen as an apparently normal shaped meniscus containing diffuse increased signal (ghost meniscus). Radial tears are also occasionally seen in the medial meniscus, particularly near its intercondylar attachment. Here the tear is best seen on the coronal images. Because this is essentially an avulsion of the posterior horn of the meniscus from its attachment, the mid portion of meniscus may be seen on coronal images to be displaced medially beyond the edges of the joint. This latter finding is, however, non specific being also seen in degenerative joint disease. The ghost meniscus effect may be seen on the sagittal images (Figure 9).
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Figure 8. Radial tear. The tear is seen as a discontinuity of the normal "bow tie" configuration of the mid portion of the meniscus (arrow).
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Figure 9. Radial type tear of the posterior third of the medial meniscus 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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A congenital anomaly that predisposes the meniscus to tears is the lateral discoid meniscus. The meniscus has a discoid 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 but may remain asymptomatic into adulthood. The significance of the common incomplete discoid meniscus is less clear. Discoid medial meniscus is an extremely rare entity.
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Figure 10. Discoid meniscus. (a) Coronal scan through the middle of the knee shows that the lateral meniscus extends across the entire joint compartment (arrow). (b) On sagittal scan the bow tie appearance of the meniscus persists into the centre of the joint compartment.
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Acute ligament tears
The diagnosis of an acute anterior cruciate ligament (ACL) tear has significant prognostic implications and may be associated with long term disability and secondary osteoarthritis. Although clinical evaluation for ACL insufficiency is quite accurate MRI is invariably performed prior to surgery to confirm the clinical findings and identify associated lesions. The normal ligament is seen as a fairly loose collection of low signal fibres with a well defined anterior border. On MRI, the most prominent feature of an acute tear is an ill-defined mass representing focal haemorrhage replacing the normal low signal linear ligament (Figure 11). The tear usually involves the mid portion. The MRI signs are less obvious when the ligament is avulsed at its femoral end as the ligament may retain a fairly normal alignment [6]. Coronal and axial images may help by showing discontinuity between the proximal 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 time of injury there is usually valgus strain, external rotation and anterior translation of the tibia relative to the femoral 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 injury on MRI is oedema in the posterior portion of the plateau, occasionally accompanied by a small fracture of the posterior 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 thought to 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 posterior third 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 meniscofemoral 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 the normal meniscofemoral ligament on more medial images (Figure 14) [10].
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Figure 11. Acute anterior cruciate ligament (ACL) tear. (a) Well defined straight fibres of the normal ACL (arrows). (b) In the acute ACL tear the normal ligament is replaced by a high signal heterogeneous mass (arrow).
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Figure 12. Acute anterior cruciate ligament (ACL) tear. (a) On the sagittal scan a curved ligament can be seen lying in the expected position of a normal ACL (arrow). (b) Coronal image showing avulsion of the ligament at its femoral attachment (arrow).
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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 fracture 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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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.
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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].
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 or forced 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].
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Figure 15. Acute posterior cruciate ligament tear. There is diffuse increased signal and widening of the ligament with focal disruption (arrow).
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Medial collateral ligament (MCL) injuries are common and usually partial. Any injury involving a valgus force is 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 the avulsion. 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).
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Figure 16. Acute medial collateral ligament tear. The proximal portion 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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Figure 17. Tear of the deep fibres of the medial collateral ligament (MCL). There is widening and increased signal deep to the superficial fibres of the MCL (arrow). There is some minor associated oedema in the adjacent portion of the femoral condyle.
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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 subchondral oedema will lie in the posterior portion of the condyle.
Tears of the lateral collateral ligament are rarer. Although isolated injuries can occur, usually at its distal attachment to the fibula, tears are often associated with injuries to other structures particularly those of the posterolateral corner injury (see below) (Figure 18) [14].
Patellar dislocation
Dislocation of the patella is another cause of post-traumatic haemarthrosis. The diagnosis if often not suspected by clinician or patient. Dislocation may result from a relatively innocuous twisting injury. MRI will show a characteristic appearance with subcortical oedema at the anterolateral aspect of the femoral condyle at the site of patellar impaction. There may be corresponding oedema of the medial aspect of the patella or signs of a medial patellar osteochondral fracture with or without loose body. The medial retinaculum is usually torn (Figure 19) [15].
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Figure 19. Recent patellar dislocation. (a) Sagittal short tau inversion recovery image shows oedema adjacent the lateral aspect of the lateral femoral condyle (arrow). (b) Axial image showing a large defect in the retropatellar cartilage (arrow). An articular cartilage fragment is embedded in the synovium of the lateral recess (open arrow). There is widening of the medial retinaculum (*).
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Hyperextension injury
Hyperextension injuries are associated with tears of the anterior third of the menisci and trabecular oedema in the anterior femoral condyle and tibial plateau. ACL, PCL, posterior capsule and popliteus muscle tears may be present. The posterior capsule is not a well defined structure on MRI and the most obvious sign of a tear of this structure may be the presence of free fluid dissecting the tissue planes posteriorly (Figure 20).
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Figure 20. Posterior capsule tear following hyperextension injury. (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 associated vertical tear of the anterior third of the lateral meniscus (arrow).
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Posterolateral corner
Damage to the posterolateral structures is an important injury that may result from a variety of mechanisms. Incompetence of the structures results in rotational instability (allowing excessive external rotation of the tibia) and may be responsible for the poor results following ACL repair [16]. The major stabilizing structures in this region are the popliteus tendon and muscle, the PCL, the lateral collateral ligament (LCL) and several smaller structures making up the arcuate complex, namely the arcuate, fabellofibular and popliteofibular ligaments. Of these three structures the popliteofibular ligament is the only one seen consistently on MRI [17]. The diagnosis in the acute phase should be suspected when disruption of more than one of these structures is encountered. Rupture of the PCL, LCL, popliteal tendon and muscle and fibular avulsion by the arcuate complex can be accurately assessed on MRI (Figure 21) [14]. Generalized oedema in the region of the posterolateral capsule may also be seen in the acute injury. The usefulness of MRI in the management of chronic posterior corner injury has not been proven and management is primarily based on clinical examination.
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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.
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Chronic knee dysfunction
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A common source of referral for imaging is the patient with non-acute symptoms that point to a mechanical problem such as pain, intermittent locking and clicking. The purpose of imaging is primarily to select those patients who require arthroscopy. Patients with obvious clinical signs of meniscal tear do not usually require imaging. The two main groups of patients are those who have unstable meniscal tears and those with degenerative disease. Only the former can expect a successful outcome following arthroscopy.
Meniscal pathology
Residual traumatic meniscal tears, even of the bucket handle type, may be found in patients scanned electively, but the most common lesion encountered on the non-acute scan is the horizontal oblique degenerative type tear.
This lesion can affect any part of either meniscus but the posterior third of the medial meniscus is the most common site. A typical lesion is seen as linear horizontal oblique high signal extending to the inferior, or less commonly the superior, articular surface of the meniscus (Figure 22). There are often accompanying signs of early degenerative disease such as articular cartilage defects, subchondral oedema and cysts and small osteophytes. Signal that does not extend to an articular surface represents intrameniscal degeneration. Care should be taken not to overcall tears, as in equivocal cases a tear is rarely found at arthroscopy [18].
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Figure 22. Typical degenerative horizontal oblique type tear of the posterior third of the medial meniscus extending to the inferior articular surface (arrow).
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Degenerative type tears may be associated with a perimeniscal cyst (see below) which is formed by fluid being pumped into the perimeniscal tissues through the tear. These cysts can in themselves be painful and may need arthroscopic decompression.
Ligament pathology
The torn ACL rarely heals spontaneously and undiagnosed or neglected chronic tears are therefore not an uncommon finding on MRI. Signs of a chronic ACL tear are non-visualization of the ligament or an abnormal horizontal orientation of the ligament as it lies redundant in the intercondylar notch (Figure 23). The ligament may become attached to the PCL. Secondary signs of ACL tear may be present but rarely contribute to the diagnosis. These signs include a vertically orientated PCL and an increased distance between tangential parallel lines through the most posterior points of the lateral tibial plateau and femoral condyle (normal <5 mm) [19, 20]. These two signs are due to the anterior translation of the tibia. A focal old compression fracture of the lateral femoral condyle is almost diagnostic of chronic ACL tear. This must not be confused with the normal shallow sulcus seen in the midarticular position of the lateral condyle on the sagittal scans. Chronic PCL tears are seen as discontinuity of the ligament. The collateral ligaments tend to heal spontaneously and chronic tears cannot usually be detected on MRI unless there is some residual thickening or ossification of the ligament.
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Figure 23. Chronic anterior cruciate ligament tear. The ligament is lying on the floor of the intercondylar region (arrows).
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Iliotibial band
The iliotibial band inserts on the anteromedial aspect of the medial tibial condyle. Iliotibial band friction syndrome is a specific condition usually seen in runners and cyclists and is caused by repetitive flexion and impingement of the band against the lateral femoral condyle when the knee is flexed at around 30 degrees. There is focal tenderness over the band and sometime a palpable "creak" as the knee flexes. Imaging is usually not required. On MRI ill-defined oedema or less commonly a well-defined cyst, is seen seated deep to the band (Figure 24) [21]. The band itself usually appears normal. These abnormalities can be identified on ultrasound [22].
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Figure 24. Iliotibial band friction syndrome. There is some increased signal (arrow) related to the deep surface of the iliotibial band.
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Bone and articular cartilage
Articular cartilage defects are a frequent finding on MRI of the knee. The sensitivity of MRI is relatively low and is highly dependent on the sequences used. As currently specific treatment for articular cartilage defects is still experimental, routine protocols are generally designed for the assessment of the menisci, ligaments and bone. However, articular cartilage defects are painful and arthroscopic 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 acquisition in steady state (SPGR) with fat suppression is a popular choice, the sequence providing good contrast and high resolution thin slices [23].
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Figure 25. Articular cartilage defects. (a) Sagittal volume acquisition gradient echo (DESS) sequence showing small defect in the articular 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).
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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 haemorrhage 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 is usually associated with osteoporosis and is therefore seen in the older age group (Figure 27). Another cause for 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 subchondral lesion with some flattened at the weight bearing portion of the condyle [25]. On plain films, linear subchondral gas may be seen in addition to sclerosis and focal flattening of the condyle.
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Figure 26. Occult tibial plateau fracture. Sagittal proton density fat suppression image clearly shows fracture line (arrows).
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Figure 27. Insufficiency fracture secondary to rheumatoid associated osteoporosis. (a) Coronal short tau inversion recovery image showing 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).
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Figure 28. Spontaneous osteonecrosis of the knee. Coronal short tau inversion recovery image showing a focal low signal subchondral lesion in the medial femoral condyle (arrow) surrounded by extensive oedema. There is some flattening of the articular surface.
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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 signs of an unstable fragment are high signal between the fragment and bone and a visible defect in the overlying articular cartilage (Figure 29) [26].
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Figure 29. Osteochondritis dissecans of the medial femoral condyle. Coronal short tau inversion recovery image. There is high signal line surrounding the low signal necrotic fragment (arrow) indicating instability.
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Osteonecrosis is sometimes encountered in the femur or tibia usually in patients with known risk factors. These lesions are not symptomatic unless they involve the subarticular bone when the articular surface may collapse. MRI will show the typical geographic lesion containing 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).
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Figure 30. Osteonecrosis. (a) Coronal T1 weighted and (b) short tau inversion recovery images showing the typical geographic lesions containing fat signal (arrows). The femoral infarct has breached the articular surface.
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Arthropathies
Generalized arthropathies such as rheumatoid arthritis are covered in Imaging in rheumatology. Monoarthropathies unrelated to degenerative or traumatic conditions are uncommon and require investigation. The knee is the most common site for a few rare conditions. Diffuse pigmented villonodular synovitis presents as swelling and pain related to recurrent haemarthroses. In the early stages plain films are normal and MRI is required to confirm the diagnosis. Synovial hypertrophy returning low signal intensity on T2 weighted images is seen. This is best appreciated on gradient echo images (Figure 31). The condition may result in bone erosion and premature osteoarthritis. The differential diagnosis included other causes of recurrent haemarthrosis such as bleeding disorders, usually haemophilia, and synovial haemangioma. Haemophilia can be excluded by talking to the patient. If a synovial haemangioma is the cause of the haemarthrosis the tumour should be visible on MRI.
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Figure 31. Pigmented villonodular synovitis. Gradient echo T2 weighted image showing extensive synovial hypertrophy returning low signal intensity (arrows).
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Another rare condition is synovial osteochondromatosis. 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].
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Figure 32. Lipoma arborescens. (a) T1 weighted and (b) short tau inversion recovery axial scans through the suprapatellar pouch showing fatty synovial villous mass projecting into the joint.
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Focal masses
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Focal swellings around the knee are common. Most focal masses are found to be cystic. Ultrasound is a good preliminary test to exclude the rarer, potentially malignant solid lesions. Although ultrasound alone is often sufficient if a lesion is found to be cystic, MRI is usually required if the diagnosis of meniscal cyst is being entertained.
Periarticular cystic lesions are frequently encountered on MRI performed for other reasons such suspected internal derangement.
Solid lesions
The most important solid lesion to diagnose is synovial sarcoma. The knee is a common site for this rare malignancy 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 ultrasound 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.
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Figure 33. Synovial sarcoma. (a) Plain radiograph shows a calcified soft tissue mass on the lateral aspect of the knee. (b) T2 weighted axial image shows a mass containing extensive low signal representing the tumour calcification (arrows).
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Synovial osteochondromatosis is chondral metaplasia resulting 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 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).
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Figure 34. Synovial osteochonromatosis. (a) Plain film showing multiple ossified foci behind the knee. (b) On ultrasound the popliteal cyst is filled with solid tumour containing calcific foci.
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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 Hoffa's fat pad. On MRI the mass will be seen to contain some loss of signal intensity on T2 weighted images representing a combination of fibrosis and scattered haemosiderin deposits. Excision is usually curative (Figure 35).
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Figure 35. Focal pigmented villonodular synovitis. T2 weighted image showing focal lesion in Hoffa's fat pad (arrow). There are scattered low signal foci representing fibrosis and haemosiderin deposits.
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Bony lesions may be a cause of a focal swelling around the knee. Osteochondromata are common. Imaging beyond the plain film is used in symptomatic lesions particularly to exclude malignant transformation of the cartilage cap. The posterior aspect of the distal femur is a typical location for a parosteal osteogenic sarcoma (see Evaluation of focal bone lesions).
Cystic lesions
The most common cystic lesions encountered are bursae, synovial cysts, ganglions or meniscal cysts.
Bursae
Bursae 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 soft tissues where there is abnormal repetitive pressure. Normal bursae are found in predictable places. Bursae become visible on imaging when they develop an effusion due to inflammatory synovitis or trauma. The common symptomatic bursae are those of the pre-patellar bursa, infrapatellar bursa, pes anserinus and semimembranosus bursae (Figure 36). A small bursa is sometimes seen between the deep and superficial fibres of the MCL. Bursae can also be involved in other synovial conditions such as synovial ostechondromatosis.
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Figure 36. Semimembranosus bursitis. Axial short tau inversion recovery image showing distended bursa at the pes anserinus (arrow).
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Synovial cysts
Synovial cysts communicate with the joint. By far the most common encountered at the knee is the popliteal (Baker's) 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 gastrocnemius and semimembranosus (Figure 37).
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Figure 37. Ultrasound showing typical configuration of a popliteal cyst with a neck communicating with the joint between the medial head of gastrocnemius (MG) and semimembranosus (SM).
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Perimeniscal cysts
Perimeniscal cysts are formed by fluid being pumped through a meniscal tear into the perimeniscal tissue (Figure 38). They occur in both lateral and medial menisci and may be painful. On the lateral side they tend to form a mass at the site of the meniscal tear as the overlying LCL is not bound to the meniscus. On the medial side the cyst has a tendency to migrate along the tightly bound tissue planes and emerge as palpable masses some distance from its origin [28]. On ultrasound the cyst can sometimes be traced to a meniscal tear but MRI is usually required to establish the diagnosis. Even on MRI it may occasionally be difficult to demonstrate the communication as the connecting stalk may be tenuous. The treatment is to decompress the cysts arthroscopically.
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Figure 38. Meniscal cyst. Cornonal T2 weighted image showing a meniscal cyst (arrow) communicating with a horizontal tear of the mid portion of the lateral meniscus.
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Ganglion
A cyst that does not fall into one of above categories is usually labelled a ganglion. A ganglion is due to myxoid degeneration of a structure such as a tendon, ligament or joint capsule. They differ histologically from synovial cysts but a small proportion will communicate with the joint. The main body of the lesion is often connected to its origin by a stalk which may be demonstrated on imaging (Figure 39). Ganglions may cause symptoms other than swelling. Lesions arising from the cruciate ligaments prevent full knee flexion (Figure 40). They may be successfully 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].
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Figure 39. Ganglion arising from the patellar tendon. Ultrasound image showing a typical anechoic lesion (arrow). The lesion is connected to the patellar tendon via a stalk (open arrow).
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Figure 40. Anterior cruciate ligament (ACL) ganglion. (a) Sagittal gradient echo T2 weighted image showing a ganglion intimately related to the ACL (arrow). (b) Short tau inversion recovery coronal image showing some of the ACL fibres being separated by the cyst (arrow).
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Figure 41. Ganglion of the common peroneal nerve in patient with 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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Imaging of cysts
Ultrasound is an excellent technique for confirming that a lesion is indeed cystic. Cysts are either anechoic or may contain variable amounts of echoes particularly in those 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 be cautious 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.
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Anterior knee pain
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Anterior knee pain is a common complaint particularly in adolescents and young females. The cause is often not apparent clinically and most patients are treated conservatively without resorting to imaging. Most cases are related to the patellofemoral joint. The main purpose of imaging is to exclude other specific condition such as osteochondral defect or patellar tendinopathy and to identify any abnormal morphology of the patellofemoral joint that may be amenable to surgery. Plain films provide limited information. MRI is sensitive for excluding significant unexpected pathology and is can be used to assess the patellofemoral anatomy and patella subluxation. Ultrasound is useful in diagnosing suspected patellar tendinosis.
Patellar tendinosis (jumper's knee)
This condition which affects athletic young adults is an overuse syndrome of the proximal patellar tendon possibly associated with impingement of the tendon on the inferior pole 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 directly adjacent 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 high signal on T2 weighted and STIR images (Figure 42).
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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 (P=patella).
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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 seen just lateral to the inferior pole of the patella due to impingement of this portion of the tendon on the lateral condyle.
Osgood Schlatter's disease
In this condition there is fragmentation of the tibial tubercle due to chronic repetitive trauma. The condition is typically 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 will confirm the diagnosis, provide reassurance and negate the need for plain films. Fragmentation of the tubercle and inflammatory changes in the adjacent tendon are typical findings (Figure 43). The pathology is also well demonstrated on MRI.
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Figure 43. Osgood Schlatter's disease. Sagittal ultrasound image showing fragmentation of the tibial tubercle (arrow) and widening of the distal patellar tendon (open arrow).
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Chondromalacia patellae
This term refers to degeneration of the articular cartilage and is associated with anterior knee pain in the young. The grading spans from softening of the cartilage to full thickness defects. The minor grades are common in the young and are difficult to detect on MRI. Macroscopic defects may be seen if the appropriate sequence are performed in the appropriate plane. There is little practical advantage in making an imaging diagnosis of early chondromalacia (Figure 44).
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Figure 44. Chondromalacia patellae. T2 weighted axial image. Minor retropatellar articular cartilage defects are seen.
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Abnormal patellofemoral morphology
There are three main anatomical variants that predispose to symptoms: a) an abnormally laterally placed tibial tubercle; b) an abnormally shallow trochlear groove; and c) a high lying patella (patellar alta). These adverse factors often coexist resulting in lateral subluxation or tilt of the patella on knee extension. Operative procedures include release of the lateral retinaculum and medial transfer of the tibial tubercle.
Plain films have been superseded by cross-sectional imaging for the assessment of patellofemoral morphology in many centres. Subluxation tends to occur as the knee extends beyond 30 degrees of flexion, a position that is difficult to evaluate on skyline plain films. The depth of the trochlear groove can be assessed on a true lateral radiograph but CT and MRI will give a more complete picture of the morphology (Figure 45). The degree of laterisation of the tibial tubercle can be assessed by comparing 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 dynamic or pseudodynamic tracking studies using CT or MRI. In pseudodynamic studies a video is compiled of multiple axial images of the knee in progressive extension from around 40 degrees flexion to full extension. True dynamic studies can be obtained with the use of rapid acquisition axial MRI while the knee is actively extended against resistance (Figure 46) [32] .
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Figure 45. Reconstructed axial MRI through the proximal part of the trochlear groove. The articular surface has an abnormaly flat configuration.
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Figure 46. Patellar subluxation. Two images from a dynamic tracking study. (a) Normal position of the patellae with the knees flexed at 40°. (b) With the knees in the extended position there is bilateral subluxation of both patellae.
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Although the role of imaging in these patients is not fully established, quantitative analysis of the anatomy and subjective assessment of subluxation helps guide management.
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Post-operative knee
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Imaging following knee replacement is discussed in Imaging of prosthetic joints. The other main indications for imaging the post-operative knee are suspected ACL grafts disruption and suspected recurrent meniscal tears following partial menisectomy.
MRI signs of re-rupture of an ACL graft are similar to those for the native ligament. Gradient echo images should be avoided as these exacerbate metal artefact. On spin echo images the normal graft can usually be clearly identified as a low signal structure (Figure 47). Impingement of the graft on the anterior edge of the intercondylar roof may be seen if the tibial tunnel is lying in an excessively anterior position. Focal anterior arthrofibrosis (Cyclops lesion) may occur causing a block to extension. This lesion is seen as a low signal intensity mass on MRI [33].
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Figure 47. Anterior cruciate ligament graft. T2 weighted image showing (a) intact ligament (b) acute tear.
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The accuracy of MRI for the diagnosis of tears following 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 significant meniscal tears if there has been resection of more than 25% of the meniscus [34] . The tear will be outlined by the high signal gadolinium on T1 weighted images (Figure 48). CT arthrogram is also an accurate method of assessing the post-operative meniscus.
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Figure 48. Recurrent meniscal tear. (a) Proton density with fat suppression shows high signal lesion within the meniscal remnant. This could represent a simple tear (b) T1 weighted fat suppression MR following intraarticular injection of dilute solution of gadolinium showing recurrent tear (arrow).
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Answers to multiple-choice questionnaire: Musculoskeletal imaging [from Imaging 15(4)]
Imaging,
December 1, 2004;
16(2):
191 - 192.
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Summary
Acute knee injury
