MRI Web Clinic — January 2005

Posterior Root Tear of the Medial Meniscus
William N. Snearly, M.D.

Clinical History: 53 year old female with 2-3 weeks of knee pain and instability. Fat suppressed proton density coronal and sagittal images are provided. What are the findings? What is your diagnosis?

1a
1b
Figure 1:

Fat suppressed proton density coronal and sagittal images

Findings

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2b
Figure 2:

(2a) A proton density-weighted coronal image demonstrates fluid signal extending through the posterior meniscal root medially (arrow). Extensive degenerative signal is present within the posterior horn of the meniscus. (2b) A proton density-weighted sagittal image demonstrates focal absence of the posterior meniscal root with fluid signal between the posterior cruciate ligament and the posterior tibial eminence (arrow).

Diagnosis

Posterior root tear of the medial meniscus.

Discussion

Tears of the meniscal root can be easily overlooked on MR examinations, but like radial tears of the meniscus, root tears may have serious biomechanical consequences. Important functions of the meniscus include load distribution, force absorption, proprioception, lubrication, and stabilization. The meniscus is responsible for supporting between 40% and 70% of the axial load across the knee; the remainder is transmitted by direct contact of articular cartilage.1 Any alteration in the meniscus that reduces its ability to resist hoop stress will therefore lead to altered biomechanics, with increased load across the articular cartilage.

The meniscal roots represent the attachment sites of the menisci to the medial tibial eminence. The posterior meniscal root of the medial meniscus attaches immediately anterior to the posterior cruciate ligament (PCL). The posterior meniscus should be seen on every sagittal image up to the one that shows the PCL (figure 3a). In the coronal plane, the posterior meniscal root is horizontally oriented and extends to attach at the medial tibial eminence (figure 4a).

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Figure 3:

(3a) A proton density-weighted sagittal image obtained at the level of the PCL demonstrates a normal posterior medial meniscal root - (arrows). Compare this image with figure B, where fluid signal is seen between the PCL and the tibial eminence.

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Figure 4:

(4a) A proton density-weighted coronal image obtained at the level of the PCL insertion. Notice that the posterior root of the medial meniscus extends horizontally to attach adjacent to the PCL insertion (arrow). Compare this image with figure A where a fluid-filled gap is seen between the PCL insertion and the posterior horn of the medial meniscus.

Avulsions or tears of the meniscal root are relatively uncommon, occurring in 2.2% – 9.8% of patients undergoing MRI of the knee.2,3,4 However, it is important that these tears be diagnosed as several complications of meniscal root tear have been described, including meniscal extrusion,1,5 secondary osteoarthritis,3,4,5 and subchondral insufficiency fracture.6

The meniscus is considered “extruded” when it extends beyond the margin of the tibia (figure 5a).1 Meniscal extrusions of greater than 3mm are associated with tears of the posterior meniscal root.1,5 Meniscal extrusion is caused by disruption of the collagen fibers within the meniscus that resist hoop stress. The resultant alteration of normal meniscal biomechanics allows meniscal extrusion, and may be responsible for the accelerated osteoarthritis reported in these patients.

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Figure 5:

(5a) Figure 5a is from another patient with a medial meniscal root tear. As in the initial case, fluid signal is seen within the posterior meniscal root (arrow). The medial meniscal body is extruded several millimeters beyond the margin of the tibial plateau (arrowheads). Such meniscal extrusion is associated with the development of osteoarthritis.

Large radial tears or root avulsions undermine the ability of the meniscus to maintain hoop stress, and as a result the contact forces across the joint may increase to levels that are present following total meniscectomy.6 In addition to accelerated osteoarthritis, this increased loading of the subchondral bone may lead to insufficiency fracture (figure 6a). Yao, et. al. found subchondral insufficiency fractures to have a predilection for the medial joint compartment and to be associated with meniscal tears, particularly radial tears and root tears. They also found that the affected patients were older than other patients with meniscal tears.6 Perhaps the combination of decreased bone density in older patients and the increased contact forces associated with radial or root tears of the meniscus leads to insufficiency fracture.

The incidence of insufficiency fracture on MRI is low, with only 1.3% of patients having a fracture in one study. Of those, 60% were in the medial femoral condyle, 20% in the medial tibial plateau, 16% in the lateral femoral condyle, and 4% in both the medial femoral condyle and the medial tibial plateau. In 76% of the cases, a meniscal tear was identified in the same compartment as the fracture.6

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Figure 6:

(6a) A coronal proton density-weighted image in another patient with a meniscal root tear (not shown) demonstrates crescentic low signal in the subchondral regions of both the medial femoral condyle and the medial tibial plateau(arrows) with extensive associated marrow edema. The appearance is compatible with subchondral insufficiency fractures. Notice that the medial meniscus is extruded several millimeters beyond the margin of the tibia (arrowheads).

Conclusion

Tears or avulsions of the posterior root of the medial meniscus are an infrequent finding on MRI examinations of the knee. However, it is important to diagnose these tears as several complications can result, including meniscal extrusion, secondary osteoarthritis, and subchondral insufficiency fracture. Careful examination of the images with attention to the normal meniscal anatomy is required for the detection of meniscal root tears.

References

1 Costa CR, Morrison WB, Carrino JA. Medial meniscus extrusion on knee MRI: Is extent associated with severity of degeneration or type of tear” AJR 2004; 183:17-23.

2 Armfield D, Akhtar U, et. al. MRI of Posterior medial meniscal root avulsion. Presented at the Radiologic Society of North America; Chicago, IL Dec. 2002.

3 Stanczak J, Boutin R, Yao L> Derangements of the meniscal root: Another complication of obesity” Presented at the Radiologic Society of North America; Chicago, IL Dec. 2002.

4 Pessis E, Bach F, et. al. Normal and pathologic features of the posterior meniscal root: Study with CT arthrography and MRI. Presented at the Radiologic Society of North America; Chicago, IL Dec. 2003.

5 Lerer DB, Umans HR, Hu MX, Jones MH. The role of meniscal root pathology and radial meniscal tear in medial meniscal extrusion. Skeletal Radiology 2004; 33:569-574.

6 Yao L, Stanczak J, Boutin RD. Presumptive subarticular stress reactions of the knee: MRI detection and association with meniscal tear patterns. Skeletal Radiology 2004; 33:260-264.

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