Internal Derangement of the Knee: Meniscal Injuries

Meniscal Tears

The two major MRI criteria for the diagnosis of a meniscal tear are intrameniscal signal intensity that extends to the meniscal surface and abnormal meniscal morphology. Early MRI evaluation of the menisci relied on a descriptive grading system for intrameniscal signal developed by Stoller and colleagues. This grading system describes three grades of intrameniscal signal intensity ( Table 26-1 ).

Although grades I and II are considered to represent abnormal intrameniscal signal intensity, they do not represent true tears because there is no extension to the articular surface. The surgeon will not be able to see these abnormalities at arthroscopy. A true tear extends to the articular surface of the meniscus (grade III intrameniscal signal intensity). The accuracy of MRI for the diagnosis of surgically proven meniscal tears increases with the number of sequential MR images on which increased intrameniscal signal intensity is visualized contacting the articular surface of the meniscus (grade III signal intensity). Using short-TE sequences (TE = 20), De Smet and colleagues found a low frequency (9%) of tears seen in menisci without surfacing intrameniscal signal, a moderate frequency (56% medial and 30% lateral) in menisci with surfacing intrameniscal signal visualized on one image, and a high frequency (90%) of tears in menisci with surfacing intrameniscal signal seen on more than one contiguous MRI slice. The authors interpret the finding of signal contacting the surface of the meniscus on only one image as a possible tear. They also surmised that a meniscal tear is unlikely if signal intensity does not unequivocally extend to the articular surface of the meniscus on MRI examination, with such a change in signal intensity likely representative of intrasubstance or myxoid degeneration, which is presumably a result of aging and is not a source of symptoms.

If one uses appropriate diagnostic criteria, the current sensitivity and specificity of MRI of the menisci for diagnosing meniscal tears range from 90% to 95%. Although the presence of signal in contact with the meniscal surface is a sensitive indicator of a meniscal tear, false-positive findings still occur using arthroscopic surgery as a standard for comparison. De Smet and colleagues reported a false-positive rate of 20% in the lateral menisci and 11% in the medial menisci. They suggest possible causes that include interval healing of the tears. A second possible cause of such false-positive cases is that some meniscal tears may be missed at arthroscopy. This is possible because the inferior surface of the medial meniscus is the most difficult region to visualize directly at arthroscopy.

In a study by Justice and Quinn, errors in MRI evaluation of the medial meniscus were equally divided between false-negative and false-positive diagnoses. Sixty percent of errors in the medial meniscus that were missed at MRI tended to be small and treated with conservative measures. In 19% of false-positive cases, the inner third of the meniscus was frayed at arthroscopy but not frankly torn, and they suggest that differentiation between MRI appearances of meniscal fraying and tearing is sometimes impossible.

There is variability in reports of errors in diagnosing lateral meniscal tears. Some studies found that diagnostic errors tended to be false-negative findings, whereas other studies have shown an overall trend toward increased false-positive errors. It has been suggested that overall there is a trend toward a decrease in the total number of false-positive diagnoses due to better recognition of normal anatomic and postsurgical variations in this region.

Most meniscal tears occur in the posterior one third of the meniscus. However, De Smet and colleagues found that 2% of medial and 16% of lateral meniscal tears illustrated MRI findings of signal contacting the surface in only the anterior two thirds of the meniscus. They stated that although most tears will involve the posterior one third of a meniscus, tears outside of this region, particularly in the lateral meniscus, are not uncommon.

Sagittal images were shown to be more important for the diagnosis of a meniscal tear compared with coronal images. De Smet and coworkers demonstrated that 2% of medial meniscal tears and 4% of lateral meniscal tears can be diagnosed on only coronal MR images, whereas 31% of medial meniscal tears and 45% of lateral meniscal tears will be seen on sagittal MR acquisitions only. However, the combination of the two planes is useful and is suggested for meniscal tear interpretation.

The sensitivity for meniscal tears decreases if there is an associated anterior cruciate ligament tear. Meniscal tears associated with anterior cruciate ligament injury tend to occur in the posterior horn of the lateral meniscus or in the periphery of either the medial or lateral menisci. Tears in the posterior horn of the lateral meniscus can be difficult to diagnose because there are several pitfalls that can occur in this region (see later discussion). When an anterior cruciate ligament tear is present, close attention should be paid to these regions of the menisci to not miss a tear.

As previously mentioned, the menisci are thick peripherally and thin centrally. Sagittal images through the peripheral menisci should demonstrate a “bow tie” configuration. There should be two contiguous images of the body of the meniscus using 4-mm slice thickness. More centrally, the anterior and posterior horns of the menisci appear as two triangles ( Fig. 26-3 ). The posterior horn of the medial meniscus is larger than the anterior horn, and the anterior and posterior horns of the lateral meniscus are approximately equal in size ( Figs. 26-4 and 26-5 ). The posterior horn of either meniscus should never be smaller than the anterior horn.

Oblique or Horizontal Tears

Oblique or horizontal tears are the most common morphologic pattern of meniscal tearing and often extend to the undersurface of the meniscus, commonly involving the posterior horn of the medial meniscus ( Figs. 26-6 and 26-7 ). These oblique or horizontal tears are usually degenerative and occur in older patients rather than being caused by trauma.

Vertical Longitudinal Tears and Bucket Handle Tears

Vertical longitudinal tears occur along the length of the meniscus, oriented approximately parallel to the collagen bundles ( Figs. 26-8 and 26-9 ). In the setting of extensive vertical longitudinal tears with connection of the tear fragment to the anterior and posterior meniscal horns, the inner aspect of torn meniscal tissue can displace centrally into the intercondylar aspect of the joint, forming a ring of meniscal tissue referred to as a displaced bucket handle tear ( Fig. 26-10 ), and there is often tearing of the meniscal remnant ( Fig. 26-11 ).

The normal meniscus should be seen on two consecutive, sagittal, 4-mm-thick images, illustrating continuity between the anterior and posterior horns and meniscal body (bow tie configuration). With the central segment of a bucket handle meniscal tear displaced, the peripheral aspect of the meniscus with an intact bow tie appearance is seen on only one sagittal image, and this is called the absent bow tie sign. Displaced bucket handle fragments may become displaced centrally into the intercondylar notch, located below the posterior cruciate ligament and can cause the appearance of a double posterior cruciate ligament sign on sagittal MRI ( Fig. 26-12 ).

The anterior or posterior limb of the meniscal tissue can also tear, causing discontinuity in the ring of meniscal tissue; and therefore it is important to assess the meniscal tissue using all imaging planes to follow the course of the meniscus.

The displaced meniscal tissue can also flip anteriorly and be located adjacent to the anterior horn of the meniscus. This is best assessed on sagittal images where the two meniscal fragments are seen adjacent to each other (although it can also be seen on axial images) ( Fig. 26-13 ).

Radial and Parrot Beak Tears

Radial tears are common. These are a type of vertical tear and are also known as free-edge tears. These tears can cause a small defect in the bow tie configuration of the meniscus on sagittal images and can be seen as a blunted appearance to the normal triangular configuration of the meniscus on sagittal or coronal images ( Figs. 26-14 and 26-15 ).

These tears are most common along the free edge of the meniscal body, in the middle third of the lateral meniscus. Tears at the posterior meniscal root are not uncommon and are often radial-type tears. ( Fig. 26-16 ).

A parrot beak tear has a radial tear configuration with a small contiguous portion that extends along the longitudinal orientation of the meniscus ( Figs. 26-17 and 26-18 ).

Peripheral Tears

The peripheral one third of the meniscus is considered the red zone and has more vascularity compared with the inner two thirds of the meniscus. Because of the increased vascularity, these tears can be treated by meniscal repair. Some surgeons may choose not to treat these tears but to watch and wait, with the hope that they may heal on their own.

The stability of a meniscal tear is important for determining whether surgical intervention is necessary. Vande Berg and colleagues assessed four MRI criteria for the recognition of instability of meniscal lesions; and, in a retrospective study of 50 patients using arthroscopy as a gold standard, they found MRI to have a sensitivity and specificity of 82%, positive predictive value of 90%, and negative predictive value of 70% for the determination of meniscal tear stability.

The four MRI criteria of instability of a meniscal tear included the following:

• 1.

  Tear cleft is visualized on more than two 4-mm-thick sagittal and three 3-mm-thick coronal images—corresponding to a lesion greater than 10 mm.

• 2.

  Tear complexity—if more than one cleavage plane or more than one lesion pattern (contour irregularity, peripheral meniscal separation, and meniscal tear) was found in the same meniscal area, the lesion is more likely to be unstable.

• 3.

  Fluid-like signal intensity within the meniscus on T2-weighted images suggests that the torn edges of the meniscus are moderately separated and therefore unstable with accumulation of intrameniscal fluid in the cleavage plane. This sign is highly specific but has poor sensitivity for a meniscal lesion.

• 4.

  A displaced meniscal fragment is direct evidence of an unstable lesion.

Peripheral tears can be overlooked in the presence of an anterior cruciate ligament tear. Medial peripheral tears (and meniscocapsular injuries) are reported to occur more frequently when a bone contusion is seen at the posterior medial tibial plateau. This is likely due to a shearing mechanism at the time of injury from tension placed on these structures during subluxation and rotation of the femur on the tibia to the anterior cruciate ligament. As well, the posterior horn of the medial meniscus may be transiently entrapped and impacted between the medial femoral condyle and medial tibial plateau at the time of the osseous contusive injury related to a relocation of the knee after a pivot shift translation of the joint at the time of the injury.