Imaging of the Knee

X-Rays

Radiographs are considered the most appropriate first imaging modality in the work up of both traumatic and nontraumatic knee pain. Radiographs are used primarily to evaluate osseous structures and are useful in the assessment for fracture, dislocation, AVN or infarction, arthritis, infection and tumor. Soft tissue injuries and abnormalities, effusions and popliteal cysts can sometimes be seen or inferred on plain film but are better defined by other modalities. The standard radiographic evaluation of the knee includes two views, an AP and a lateral view. A weight bearing view of the knee should be included if osteoarthritis is a concern. If there is patellar pain, then a sunrise view should be added. (24,25)

Ultrasound

Ultrasound of the knee is not typically performed although there are a few instances where it may be helpful. The main use of ultrasound of the knee is to evaluate the popliteal vessels for thrombus, aneurysm and flow. In the setting of knee dislocation, MRA or conventional angiography is preferred for evaluation of popliteal artery injury. Ultrasound of the knee is sometimes used to determine whether a palpable lesion in the posterior knee is a popliteal cyst. Ultrasound is very good at determining the cystic nature of this lesion however it does little toward understanding the cause of the popliteal cyst, which is usually internal derangement (meniscus or ligament injury) for which MRI is the study of choice.

Nuclear Medicine

Nuclear medicine is not often used in the work up of knee pain.

CT

CT scan of the knee is typically performed for secondary evaluation of fractures and osseous tumors. It provides greater detail than radiographs and is helpful in preoperative planning. CT may also be preformed to evaluate for loose osteochondral bodies in the joint. CT arthrogram can be used in the place of MRI for the evaluation of internal derangement when the patient is unable to have or tolerate an MRI. It is not the most appropriate study for internal derangement as it is invasive, uses ionizing radiation and has a slightly lower sensitivity than MRI. CT scan is also used to guide biopsies and aspirations of the skeletal system.

Arthrography

Arthrography of the knee is a technique which has been supplanted by MRI. It was invasive, difficult to perform and used ionizing radiation. Occasionally, an MRI with intraarticular contrast is performed in the setting of a postoperative patient with recurrent knee pain. CT arthrogram, as mentioned above, is used in patients who cannot have or tolerate MRI.

MRI

Magnetic resonance imaging is used to evaluate both osseous and soft tissue structures of the knee. CT and MRI are both capable of providing high resolution scans but the superior soft tissue contrast of MRI ­ the ability to distinguish different types of soft tissue based on their signal characteristics - and the ability to scan in multiple planes sets MRI apart. CT uses x-ray attenuation values to distinguish soft tissues whereas soft tissue contrast in MRI is related to the proton resonance within the tissues and their microenvironments. Because the knee has many components (ligaments, tendons, menisci, bones, fluid) it is necessary to use different sequences to adequately demonstrate each of these varied structures. The clinical indication for obtaining MRI helps to determine which pulse sequences and imaging planes will be obtained and this is why clinical history is so important to the radiologist. MR has many advantages however the disadvantages of MRI are that it cannot be used by all patients due to contraindications such as pacemakers or to claustrophobia and the scan is susceptible to artifact from metal and motion.

Indications for MRI

MRI is very useful modality for a number of clinical situations. MR is the study of choice in the assessment of internal derangement of the knee including ligament or meniscus tear, tendon injury, effusions and cartilage injury (24). MRI is also indicated to evaluate the painful postoperative knee for such problems as ACL graft integrity, arthrofibrosis and recurrent meniscal tears. MRI arthrography can be used in the postoperative setting to evaluate for new or recurrent tears in the partially resected meniscus. Studies in postoperative patients have shown that although MR arthrography is a more invasive study, it is also more sensitive for meniscal tears than MR alone (10, 33). MRI is also extremely helpful for finding occult or stress fractures which are not be apparent on radiographs. In the evaluation of soft tissue and osseous tumors MRI is indispensable as it describes in great detail the lesion’s size, location, extension and internal character. These parameters help to determine if resection is possible and whether limb salvage surgery can be performed. Infection in and around the knee can be imaged by MR to determine, for example, whether osteomyelitis or necrotizing fasciitis is present. MRI is useful when early osteonecrosis is suspected because MRI findings precede plain film manifestations of disease. In late osteonecrosis, MR imaging is not useful and plain films are all that is needed for work-up. MRI is also not useful in the setting of degenerative osteoarthritis and the preoperative work up for knee arthroplasty. In this setting, radiographs alone are sufficient. It is hoped that in the future the need for arthroplasty will decrease as cartilage repair and maintenance techniques improve. MRI techniques for cartilage imaging are improving and through this MRI may then become more helpful in the imaging of osteoarthritis (17). The American College of Radiology’s appropriateness committee has described situations in which MRI is not indicated and these are when plain films are diagnostic for severe osteoarthritis, inflammatory arthritis, stress fracture, advanced osteonecrosis and reflex sympathetic dystrophy. In these situations, MR is not indicated because it would not alter the treatment plan(25).

Accuracy and Clinical Utility of MRI

MRI of the knee in the clinical setting has been utilized since the 1980’s. There is now extensive data on the accuracy, sensitivity and specificity of MR for internal derangement of the knee. Most studies report results for high field magnets (1.5 tesla) but there are some studies which use data from low field magnets (<= 0.35 Tesla). As expected, the results from the low field magnets are lower (5,10). The overall diagnostic accuracy of MRI for patients undergoing arthroscopy is 97% (36). For individual injuries, the current expected MRI sensitivity, specificity and accuracy for medial meniscal tears are 93-98%, 84-91% and 90-98%, for lateral meniscal tears, 80-96%, 91-99% and 89-98% and for ACL tears, 71-100%, 88-98% and 79-98% (2, 4-9, 12-14, 18, 23, 24). In a few studies (7,30,35), the results of clinical exams by orthopedists for ACL tears and the results of MRI exams for ACL tears were of similar sensitivity which raised the question of whether MR was beneficial in this setting. Some studies, predominantly in the radiology literature, felt that MRI was worthwhile in this setting because of the additional information provided, such as the status of menisci, the status of other ligaments and the presence of osseous injury (21). Other studies, predominantly in the orthopedic literature, felt that MRI was “an expensive and unnecessary diagnostic test” given the accuracy of clinical exam for ACL tear (30). The radiologic and orthopedic literature agree that MRI is most useful in the clinical decision making process when the clinical exam is unclear, when the pathology involves menisci or when there are multiple injuries present. MRI is helpful in the setting of suspected meniscal tear because of the lower accuracy of the clinical exam. In one study, the accuracy and positive predictive value of MRI was found to be nearly twice that of clinical examination for meniscal tears (20). However, the most important aspect of MRI in the setting of meniscal tears may be its high negative predictive value (4,19). A high negative predictive value can alter management and avoid unnecessary surgery. As Rangger et al stated, “The use of magnetic resonance imaging in establishing diagnosis of disorders of the knee joint altered treatment in a significant proportion of patients. Magnetic resonance imaging should be done before arthroscopy of the knee in all cases in which the clinical diagnosis has been reduced to a suspected meniscal injury” (28). MR can also be helpful in the clinical situation when there are multiple suspected injuries. In this situation, the accuracy of both the clinical exam and MR declines, but MRI decreases to approximately 60% whereas the clinical exam accuracy decreases to 28 - 30% (22,40). MR offers a more accurate description of injuries which may alter surgical approach or management.

Clinical Impact and Cost Effectiveness of MRI

Recent studies have evaluated the cost effectiveness of MRI and its impact on clinical practice. Alioto et al found that initial treatment recommendations were beneficially altered in 18% of their population solely because of MRI results (1). Trieshmann et al in an article in Arthroscopy found that after combining the MRI and clinical information, surgical decision-making was altered in 27% of patients and stated “The data further show that MRI is a cost effective technique for avoiding unnecessary surgery and affects patient outcome by improving surgical decision making.”(36). Munshi found that prospective use of MRI could have prevented 22% of arthroscopic procedures (21). One study indicates that diagnostic arthroscopies could be reduced to 5% through the use of MRI (20). Bui-Mansfield et al found that 42% of their patients could have been spared surgery on the basis of MR imaging, with cost savings of $680 per patient (2). Ruwe et al found that due to MRI findings, 51% of patients avoided diagnostic arthroscopy with a net savings of $103,700 for 103 patients or about $1000 per patient (32). A study by Weinstable et al found that about $80,000 could have been saved by scanning all patients and decreasing the rate of arthroscopy (39). Mackenzie found a significant shift away from surgical management after MRI and that MRI not only significantly influenced the surgeon’s diagnosis and management plans but also found an improvement in patients health related quality of life (15). Maurer stated that MR imaging affects the diagnosis and management of acute knee injury by decreasing the number of arthroscopy procedures, improving clinician diagnostic certainty and assisting in management decisions (16). Bryan et al studied the impact of MRI on chronic knee pain in England and found that the use of MRI did not increase overall costs to the system, was not associated with worse outcomes and avoided surgery in a significant proportion of patients (3). In summary, MR imaging impacts clinical practice by increasing diagnostic certainty which then may alter patient management. In situations where patient management was altered, it often changed from surgical to non-surgical management. This was found to be a cost-saving approach to knee pain.

Ordering Practice

MRIs of the knee are ordered by orthopedic and non orthopedic physicians. There are two studies which evaluated the ordering practices of these two groups of physicians and the studies have conflicting conclusions. The study by Sherman et al demonstrated that non-orthopedic practitioners had significantly more normal knee MRI examination results when compared to orthopedic specialists (34). However, a study by Uppal et al found no significant difference in the occurrence of clinically important internal derangement of the knee at MR imaging between patients referred by orthopedic surgeons and those referred by other physicians (37). As mentioned above, MRI has a positive impact on clinical decision making in that it increased diagnostic certainty. Diagnostic certainty can help the primary care physician to determine when it is appropriate to treat the patient conservatively and when it is appropriate to refer the patient to an orthopedist. Robling et al studied the use of MRI by general practitioners and found that 24% of patients were managed in primary care following MRI when they otherwise would have been referred to a specialist and when referrals were made, the general practitioners felt enabled to reinforce the request (29).

 

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