Magnetic resonance imaging (MRI) is the most powerful, accurate, noninvasive method for diagnosing meniscal tears. It is more accurate than physical examination and has influenced clinical practice and patient care by eliminating unnecessary diagnostic arthroscopies or by identifying an alternative diagnosis whose clinical presentation may mimic meniscal tears. MRI accurately depicts the anatomy and pathology affecting almost every joint in the body. Because MRI is highly accurate, most leading orthopedic surgeons prefer MRI to diagnostic arthroscopy.
The incidence of meniscal tears has been estimated to be about 60 per 100,000, but the true incidence is expected to be much higher. Jarraya et al found that more than 75% of patients with symptomatic osteoarthritis had a meniscal injury.
The International Society of Arthroscopy, Knee Surgery, and Orthopaedic Sports Medicine (ISAKOS) has classified meniscal tears based on parameters such as tear depth, tear width, radial location, tear pattern, tissue quality, tear length, and tissue excision.
The knee menisci were once thought to be functionless remnants of a leg muscle and expendable components of the knee. Much has been learned through laboratory investigation, clinical experience, and radiologic imaging. The meniscus is now known to play an important role in the complex biomechanics of the knee. For instance, it is involved in joint stability, load sharing and transmission, shock absorption,
and nutrition and lubrication of the articular cartilage.
Meniscal injuries are a common problem in sports; they are the most frequent injury to the knee joint. Such injuries are especially prevalent among competitive athletes, particularly those who play soccer, football, basketball, and (sometimes) baseball. In the past 25 years, the number of people participating in sports has greatly increased, resulting in a higher number of knee injuries.
(Meniscal tears are displayed in the images below.)
Coronal fat-saturated proton density–weighted image shows the popliteus recess containing joint fluid and located between the lateral aspect of the posterior horn of the lateral meniscus and the joint capsule. An extensive tear is present in the posterior horn of the medial meniscus (arrow). Note the normal oblique upward orientation of the posterior medial horn of the lateral meniscus.
Coronal fat-saturated proton density–weighted image shows the popliteus tendon originating from an undulation of the lateral femoral condyle. From there, it passes through the popliteus recess to insert on the proximal posterior tibial metaphysis. A radial tear (arrow) is present in the posterior horn of the medial meniscus.
Sagittal fat-saturated proton density–weighted image shows a full-thickness tear to the periphery of the anterior horn of the medial meniscus (MM). Tears in this location have a good likelihood of healing without surgical repair because they occur in the zone with a good blood supply to the meniscus. Also present is a partial thickness tear to the undersurface of the posterior horn of the MM.
Coronal fat-saturated proton density–weighted image shows irregularity to the upper (femoral) surface of the body of the lateral meniscus (LM, outer arrow), indicating fraying. Fraying usually occurs at the meniscal apex. Soft tissue densities (inner arrow) are present under the apex of the meniscus, indicating debris or a free meniscal fragment at this level. The body of the LM is unusually thick and longer than usual, indicating a discoid meniscus. The normal-sized medial meniscal body is present for comparison. Discoid menisci occur about 5 times more often here than in the LM, and they are more prone to injury.
Axial illustration of a full-thickness longitudinal tear of the posterior horn. The meniscus is viewed from above in (a), sagittal in (b), and coronal in (c). For Image 67-70, A = anterior, L = lateral, M = medial, and P = posterior.
Axial illustration of a full-thickness radial tear of the posterior horn. The meniscus is viewed from above.
Axial illustration of a full-thickness horizontal tear of the posterior horn. The meniscus is viewed from above.
Axial illustration of an oblique (parrot beak) tear of the posterior horn. The meniscus is viewed from above. In B, image 1 is most lateral, image 2 is middle, and image 3 is most medial. In C, image 1 is most anterior, image 2 is middle, and image 3 is most posterior.
Unlike MRI, clinical examination cannot demonstrate the location, shape, or length of a meniscal tear. These factors are important in treatment decisions. Large, blinded prospective studies with diverse patient populations suggest that for experienced clinicians, the sensitivity of physical examination for detecting meniscal tears is 70-90%.
The negative predictive value (NPV) is no greater than 67%; therefore, about one third of meniscal tears are missed with clinical screening alone. In situations of multiple knee lesions, the accuracy of clinical examination in diagnosing meniscal tears decreases to 30%.
The clinical diagnosis of meniscal tears becomes more difficult and unreliable in the presence of concurrent acute ligamentous injuries of the knee. With acute anterior cruciate ligament (ACL) tears, the sensitivity for diagnosing medial meniscus (MM) tears is 45% and 58% for lateral meniscus (LM) tears. The sensitivity of joint line tenderness for diagnosing meniscal tears is 75%. The sensitivity of the Apley grinding test for meniscal tears is about 45%. The sensitivity of the Payr test for diagnosing meniscal tears is about 40%.
Specificity also decreases, most likely due to the presence of tibial and femoral bone bruises that frequently accompany acute ACL tears. Pain from these bone injuries can cause joint-line tenderness, a finding that otherwise suggests the presence of a meniscal tear.
Advantages of MRI
In many cases, MRI results lead to changes in the proposed management. One study determined that about one third of all diagnostic arthroscopies need not be performed if MRI is used; another study showed that the use of MRI prevented 51% of diagnostic arthroscopic procedures; and a third study showed that with the use of MRI, the morbidity associated with arthroscopy was avoided.
MRIs show many of the essential characteristics of meniscal tears critical to management, such as their location, shape, length, and depth. In this way, MRI helps to make an accurate assessment of stability and of the likelihood of tear propagation, and it enables one to determine whether the meniscal tear can be repaired. It is advantageous to know ahead of time if a given meniscal tear can be repaired, because the additional equipment, surgical assistants, and time needed for repair can be anticipated. Patients also benefit from knowing early on whether surgery is necessary. The recovery time for meniscal repair is longer than that for partial meniscectomy (PM). Patients may want to time surgery to fit in with their other obligations.
When combined with clinical data, such as the patient’s age, athletic requirements, and physical findings (eg, possible associated ligamentous injuries), a treatment plan may be developed by assessing the need for and timing of surgery and by determining the type of surgery (meniscal debridement, rasping, repair, partial or total resection, or meniscal transplantation). MRI may be used to identify other injuries, such as ligament tears, especially anterior cruciate ligament (ACL) tears, the presence of which may also influence the decision whether to perform surgery.
With MRI, physicians may obtain images in several planes, providing multiple perspectives on meniscal and ligamentous injuries. Other advantages include the following:
MRI does not expose the patient to ionizing radiation
MRI does not normally involve the intravenous administration of contrast material, the use of which is associated with a small but definite number of adverse effects
MRI does not require joint manipulation
MRI is painless and can be performed in less than 35 minutes
MRI does not require the intra-articular injection of iodinated radiographic contrast material, which is needed for arthrography
Arthrography has been supplanted by MRI except for patients who are too large to fit into the MRI unit or for patients who have contraindications to MRI (eg, intracranial aneurysm clips, orbital metallic foreign bodies are present). Magnetic resonance arthrography is used to evaluate residual or recurrent meniscal tears after meniscal surgery. The detection of residual or recurrent meniscal tears following meniscectomy or meniscal repair is difficult with conventional MR images.
Plain radiography is extremely limited in the assessment of meniscal tears. Radiographs may be obtained to rule out unsuspected lesions, such as osteochondritis dissecans and loose bodies. In the presence of a discoid meniscus (DM), radiographs may show widening of the medial or lateral joint compartments; hypoplasia of the lateral femoral condyle related to the increased size of the LM; a high fibular head; cupping of the lateral tibial plateau; or a squared-off lateral femoral condyle.