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Meniscal Anatomy/Meniscal Injuries Print

Arthroscopic treatment of meniscal injuries is one of the most common orthopedic surgical procedures. Although our knowledge and understanding of the anatomy function, and treatment of meniscal pathology has evolved, meniscal tears continue to cause significant symptoms and long-term impairment of the knee. Therefore, it is important for the treating physician to have a full understanding of the anatomy and biomechanics of menisci, symptoms of meniscal tears, findings at arthroscopy, techniques for surgical treatment and outcome measures for reporting results.

Anatomy

Figure 1 (ANTERIOR MENISCAL PICTURE)

The menisci are semi-lunar shaped structures on the medial and lateral sides of the knee. They are composed of collagen and cells of either fibroblast or chondrocyte origin. Seventy-five percent of the weight of the menisci consist of water. The organic matrix is composed of three-quarters collagen, with type 1 collagen predominating. The collagen fibers are oriented in a characteristic fashion. The most superficial fibers are oriented radially. Most of the collagen fibers, however, are found in the deep layer and are arranged in a circumferential orientation, which follow the periphery. The radial fibers are woven between the circumferential fibers, which help to provide structural integrity. The arrangement of fibers enables them to resist the hoop stresses that are produced in the meniscus during weight bearing.

 

Figure 2

In cross section, the menisci are triangular, being thicker at the periphery and tapering to a thin free edge centrally. The superior surfaces are concave to accommodate the convexity of the femoral condyles (Fig. 2). The medial meniscus is semilunar in shape and is thinner and narrower anteriorly. The posterior horn is thicker and wider, averaging approximately 10.6 mm. The anterior and posterior horns are attached to the intercondylar eminence with an additional slip from the posterior horn attaching to the posterior cruciate ligament. The peripheral circumference is firmly attached to the capsule by the coronary ligaments. The medial meniscus is also firmly attached to the posterior oblique ligament. The medial meniscus covers approximately 64% of the medial tibial plateau. The lateral meniscus covers approximately 84% of the lateral tibial plateau. It is more circular than the medial meniscus and is also more uniform in width (average 12 to 13 mm).

The anterior and posterior horns of the lateral meniscus also attach to the intercondylar eminence, but in closer proximity to the anterior cruciate ligament than the medial meniscus. The peripheral attachment of the lateral meniscus to the capsule is thinner and looser than on the medial side. In addition, there is no attachment in the region of the popliteal hiatus, and there is no attachment of the lateral meniscus to the lateral collateral ligament.


Vascular Anatomy

The lateral, medial and middle genculate arteries form a parameniscal capillary plexus along the entire rim of the menisci: The blood supply consists of vessels that are circumferential with radial branches that end in small capillary loops. Prenatally, the vessels transverse the entire meniscus, but the vascularity decreases because of vertical load bearing.

In the adult, the capillary loop extends no deeper than 10 to 25% width of the lateral meniscus and 10 to 30% of the medial meniscus (Fig. 2 & 3). The lateral meniscus in front of the popliteus tendon is avascular. The vascular synovial tissue is important in rneniscal healing. Rasping these tissues can stimulate vascular overgrowth into the menisci and cause migration of pluripotential mesenchymal cells that play role in healing after meniscal repair.


Biomechanics of the Menisci

Load Transmission: The medial meniscus translates 50% of the load and the lateral meniscus transits 70% of the load in extension. At 90 degrees of flexion the lateral meniscus transmits 85% of the load. The ability of the menisci to transmit load comes from the shape of the menisci, which bridge the incongruity between the spherical femoral condyles and flat tibial plateau.

Partial meniscectomy of the inner 1/3 of the menisci decreases contact area by 10% and increases peak load by 65%. A total meniscectomy decreases contact area by 75% and increases peak load by 235%. Fg. 4 demonstrates wear in the knee caused by loss of the meniscus.

 

Shock Absorption

The intact menisci can dissipate forces because they are biphasic structures. The solid phase consists of proteoglyphans and collagen and the liquid phase is water. They function under compression similar to articular cartilage. The unique architecture allows transmission of vertical forces into tangential and radial forces as the menisci are loaded. In order for the meniscus to provide a protective function, they must have structural continuity throughout. Loss of the menisci results in a 20% reduction in shock absorption,


Joint Lubrication

The biphasic composition of the meniscus is also important for joint lubrication. Water is forced out of the menisci and into the joint space with compression and the water is reabsorbed when the load is removed. This sponge phenomenon not only dissipates force but also circulates cellular nutrients. There is a 20% increase in coefficient of friction after meniscectomy.


Joint Stability

The medial meniscus plays an important function in joint stability. Markolf in 1981 found that after medial meniscectomy there is an increase in AP translation at 90 degrees of 1.82 mm in varus valgus translation and flexion is increased 1.36 mm. In the ACL deficient knee, elasticity after medial meniscectomy is increased 18-20% in extension and 58% at 90 degrees of flexion. In addition, in situ graft forces in an ACL replacement graft are increased by 33 to 50% after a medial meniscectomy.


Acute Meniscal Tear

A patient with a recent meniscal tear typically presents with complaints of pain and swelling. Physical examination often reveals tenderness over the joint line, a decreased range of motion, and an effusion. Figure 5 demonstrates an acute meniscal tear.

 

Anatomy of the Knee, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, Figure 7
Figure 5
Anatomy of the Knee, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, Figure 8
Figure 6
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Figure 7: MRI of Meniscal Tear

Anatomy of the Knee, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, Figure 10

Figure 8
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Figure 9

Anatomy of the Knee, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, Figure 12

Figure 12

meniscal-injuries-of-the-knee-dr-allen-f-anderson-nashville-orthopaedic-surgery-sports-medicine-figure-12a

Figure 12A

meniscal-injuries-of-the-knee-dr-allen-f-anderson-nashville-orthopaedic-surgery-sports-medicine-figure-12b

Figure 12B
meniscal injuries of the knee, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, figure 13b
Figure 13a
meniscal injuries of the knee, Dr. Allen F. Anderson, nashville, orthopaedic surgery, sports medicine, figure 13a
Figure 13b

Chronic Meniscal Tear

A patient who has had a meniscal tear for several months or longer typically presents with intermittent pain, catching or locking symptoms. Physical examination also reveals tenderness over the joint line, pain with forced hyperflexion, and occasionally muscle atrophy and an effusion.

Figure 6 demonstrates a chronic meniscal tear.

 

Meniscal tears can be either traumatic or degenerative in nature. Meniscal tears are uncommon in persons under 10 years of age, but become increasingly common during and after adolescence. Degenerative tears can be found in as much as 60 percent of the population over age 65. The majority of these tears, however, are asymptomatic and occur in association with degenerative joint disease. The changing patterns of meniscal injury with chronological age most likely correlate with alteration in collagen fiber orientation with aging, as well as increasing intrasubstance degeneration.

MRI may be used to confirm the clinical diagnosis. The accuracy rates of MRI range from 92 to 98%. MRI may also be used to evaluate meniscal degeneration. A grading system has been developed to describe abnormal intrameniscal signal: Grade 1 is oval or globular in appearance and does not communicate with any meniscal surface. Grade 2 signal is more linear, but similarly does not communicate with the articular surfaces. Grade3 signals within the meniscus are linear and communicate with either superior or inferior articular surfaces. Grades 1 and 2 signals are consistent with intrasubstance myxoid degeneration, whereas grade 3 signal is consistent with a tear (Fig. 7).

Meniscal tears are classified on length, depth, location, and tear pattern.

 

Tear length

Tear length indicates the length of the meniscal tear that reaches the surface of the meniscus. It does to include contained tears (MRI grade II) that do not reach the surface of the meniscus.

 

Tear depth

Tear depth - mirrors the MRI classification of 0 to 3. A tear depth of 3A is a partial tear that extends through either the superior or inferior surface. A horizontal tear may also be a 3A partial tear. A 3B tear is a complete tear that extends through both the superior and inferior surfaces of the meniscus.

 

Location

The ISAKOS meniscal documentation committee adopted location classifications.  One describes the locations of the tear as front (anterior), middle and back (posterior).  The other system describes where the tear is relative to the width of the meniscus. (Fig. 8)  Zone 1 includes tears at the menisco-synovial junction and tears with a rim width of less than 3 mm. Zone 2 tears have a rim width of 3 to less than 5 mm, while zone 3 tears have a rim width of 5 rom or greater.

The committee discouraged the use of the terms red-red, red-white and white-white to describe the zone of meniscal tears. The vascular supply of the menisci varies and, therefore, cannot be precisely determined arthroscopically by meniscal tear rim width.

Tear Pattern

Longitudinal- Vertical

The longitudinal-vertical tear as pictured may be located anywhere along the meniscus. The extension of this tear may result in a bucket handle tear.

Figure 9 demonstrates the meniscal tear patterns.

 

Horizontal Tear

The horizontal tear begins at the inner margin of the meniscus and extends toward the capsule.

Radial Tear

This drawing demonstrates the meniscal tear patterns (Figure 10).

The radial tear also begins at the inner margin and extends towards the capsule. This type of tear is typically located at the junction of the middle and posterior thirds of the lateral meniscus. These tears may extend completely through the meniscal rim, transecting the meniscus. Figure 11 demonstrates a radial tear.

Flap Tear

Figure 12A demonstrates a flap tear. A flap tear may be either a vertical flap tear or a horizontal flap tear. The vertical flap tear extends through both the inferior and superior surfaces of the meniscus. The horizontal flap tear is an extension of the horizontal tear. Either the inferior or superior surface the meniscus may remain intact in a horizontal flap tear.

Fib. 12B shows arthroscopic excision of the torn meniscus.

Complex Tear

Figure 13 shows probing a complex meniscal tear and 13B after excision. The complex patterns demonstrate tearing of several planes.

Discoid

The discoid meniscus is a congenital variance, that usually occurs laterally. Watanabe classified this abnormality into three types. The incomplete discoid type is larger than a normal meniscus and it has normal attachments. The complete type covers the entire tibial plateau, but also maintains normal attachment. The third type of discoid meniscus lacks a posterior capsular attachment and is more often symptomatic than the other two types.

 
© Allen F. Anderson, M.D. 2017