I. Overview/Epidemiology
A. Meniscus function
1. The meniscus provides stability, absorbs shock, increases articular congruity, aids in lubrication, prevents synovial impingement, and limits flexion/extension extremes.
2. In addition, the most important function of the meniscus is load-sharing across the knee joint, which it accomplishes by increasing contact area and decreasing contact stress.
B. Epidemiology of meniscus injuries
1. Meniscus injuries are among the most common injuries seen in orthopaedic practices.
2. The incidence of acute meniscus tears is 61 cases per 100,000 people per year.
3. Arthroscopic partial meniscectomy is one of the most common orthopaedic procedures.
II. Pathoanatomy
A. General information
1. The menisci are crescent-shaped, with a triangular appearance on cross-section.
2. The fibers of the menisci have a circumferential orientation with radial tie fibers presenting longitudinal splits.
3. The lateral meniscus covers 84% of the condyle surface; it is 12 to 13 mm wide and 3 to 5 mm thick.
4. The medial meniscus is wider in diameter than the lateral meniscus; it covers 64% of the condyle surface and is 10 mm wide and 3 to 5 mm thick.
B. Biochemistry
1. The menisci are composed of 65% to 75% water.
2. The extracellular matrix is predominantly composed of type I collagen. Types II, III, V, and VI are also identified.
3. Proteoglycans make up 1% of the dry weight.
C. Vascularity
1. The meniscal vascular supply arises from the superior medial and lateral, inferior medial and lateral, and middle genicular arteries.
2. Although controversy exists, it appears that 50% of the meniscus is vascularized at birth, whereas only 10% to 25% of the meniscus is vascularized in the adult.
3. The vascularity of the meniscus impacts the ability of meniscal repairs to heal.
4. The vascularity has been divided into three regions or zones (
Table 1).
D. Biomechanics
1. In extension, as much as 50% of the load is absorbed by the meniscus, with the percentage of load-sharing increasing to 90% at 90° of knee flexion.
2. Beyond 90° of flexion, most of the force is transmitted to the posterior horns of the menisci.
3. The lateral meniscus provides more biomechanical support to the joint than does the medial meniscus.
4. Contact stress increases two to three times that of normal when the meniscus is removed.
a. Biomechanical studies of partial meniscectomy have demonstrated increasing contact stresses with increasing loss of meniscal tissue.
[Table 1. Vascular Zones of the Meniscus]
b. Removal of the inner third of the meniscus results in a 10% decrease in contact area and a 65% increase in contact stress.
III. Evaluation
A. History
1. Meniscus tears are unusual in patients younger than age 10 years.
2. Most meniscus tears in adolescents and young adults occur with a twisting injury or with a change in direction.
3. Middle-aged and older adults can sustain meniscus tears from squatting or falling.
4. With an acute meniscus tear, an effusion may develop slowly several hours after injury. This differs from an anterior cruciate ligament (ACL) injury, where swelling develops rapidly within the first few hours.
5. Patients with meniscus injuries localize pain to the joint line or posterior knee and describe mechanical symptoms of locking or catching.
6. Chronic meniscus tears demonstrate intermittent effusions with mechanical symptoms.
B. Physical examination
1. Small joint effusions and joint line tenderness with palpation are common findings with meniscus tears.
2. Manipulative maneuvers, including the McMurray and Apley tests, may produce a palpable or audible click with localized tenderness, but they are not specific for meniscal pathology.
3. Range of motion is typically normal, but longitudinal bucket-handle tears may block full extension of the knee joint.
C. Imaging
1. Standard knee radiographs should be obtained for evaluating for bone injuries or abnormalities.
2. A weight-bearing radiograph is necessary to evaluate for osteoarthritis.
a. This may include a weight-bearing AP or 45° PA flexion view.
b. A right-to-left difference of ≥2 mm represents a significant difference that will be verified by articular cartilage chondrosis at the time of arthroscopy.
3. MRI remains the noninvasive diagnostic procedure of choice for confirming meniscal pathology.
a. A grade III signal, which is a signal that reaches the articular surface of the meniscus, indicates a tear (
Figure 1).
[Figure 1. The grading scale for meniscus tears on MRI. Grade 0 is a normal meniscus. Grade I has globular increase signal within the meniscus that does not extend to the surface. Grade II has linear increase signal within the meniscus that does not extend to the surface. Grade III has increase signal that abuts the free edge of the meniscus, indicating a meniscal tear.]
b. MRI has demonstrated a high negative predictive value for meniscus tears.
c. A well-performed MRI of a knee with no meniscal pathology will rarely demonstrate a meniscus tear.
D. Differential diagnosis
1. Prior to MRI, several large studies demonstrated accuracy of the clinical diagnosis of meniscus tears to be 70% to 75%.
2. The differential for meniscus tears includes intraarticular and extra-articular diagnoses.
a. Intra-articular possibilities include osteochondritis dissecans, medial patella plica, patellofemoral pain syndromes, loose bodies, pigmented villonodular synovitis, inflammatory arthropathies, and osteonecrosis.
[
Table 2. Classification of Meniscus Tears]
b. Extra-articular possibilities include collateral ligament injuries, slipped capital femoral epiphysis, bone or soft-tissue tumors, osteomyelitis, synovial cyst, pes or medial collateral ligament bursitis, injury, reflex sympathetic dystrophy, lumbar radiculopathy, iliotibial band friction, and stress fracture.
IV. Classification
A. Meniscus tears (Table 2,
Figure 2)
B. Discoid meniscus
1. Discoid meniscus is a larger-than-normal meniscus.
2. Discoid meniscus is rare, but it more commonly affects the lateral meniscus (1.4% to 15%) (
Figure 3) than the medial meniscus (<1%).
3. Discoid menisci are classified into three subtypes:
a. Type I (incomplete)
b. Type II (complete)
c. Type III (Wrisberg variant): Lacks posterior attachment to the tibia; may be repaired
[Figure 2. Illustrations of common meniscus tears.]
V. Treatment
A. Nonsurgical
1. Management is predicated on symptoms.
2. Not all meniscus tears cause symptoms, and many symptomatic tears become asymptomatic.
3. Tear types that commonly may be managed non-surgically include:
a. Stable longitudinal tears <10 mm in length with <3 to 5 mm displacement
b. Degenerative tears associated with significant osteoarthritis
c. Short (<3 mm in length) radial tears
d. Stable partial tears
4. Nonsurgical management can include ice, nonsteroidal anti-inflammatory drugs, or physical therapy for range of motion and general strengthening of the lower extremities.
[Figure 3. Illustration of the classification system for lateral discoid menisci: type I (complete), type II (incomplete), and type III (Wrisberg variant). Type III discoid meniscus has no posterior attachment to the tibia. The only posterior attachment is through the ligament of Wrisberg toward the medial femoral condyle.]
B. Surgical
1. Excision
a. Indications—Arthroscopic partial meniscectomy is indicated for radial, oblique, flap, horizontal cleavage, and complex tears, as well as for tears located in the white-white avascular zone.
b. Arthroscopic meniscectomy has been demonstrated to decrease surgical morbidity and improve function.
c. Surgical procedures—The goal of arthroscopic partial meniscectomy is to debride degenerative or torn meniscal tissue, leaving a stable contoured rim and preserving as much tissue as possible.
d. Studies have demonstrated >80% satisfactory function at minimum 5-year follow-up, despite a 50% finding of Fairbanks radiographic changes (osteophytes, flattening of femoral condyles, and joint space narrowing).
e. Studies also have demonstrated that degenerative changes and a decrease in function occur more quickly in patients who have undergone arthroscopic lateral meniscectomy. This is probably due to the increased biomechanical protective effect of the lateral meniscus.
f. Factors that seem to predict better long-term function following arthroscopic partial meniscectomy include age younger than 40 years, normal lower extremity alignment, minimal arthritic changes noted at the time of arthroscopy, and a single fragment tear.
2. Repair—Once the function of the meniscus was understood, repair gained importance.
a. Indications—Tear types appropriate for repair include vertical longitudinal tears in the vascular zone of the meniscus and displaced bucket-handle tears that remain in good condition once they are reduced.
b. Surgical procedures—The four potential meniscal repair techniques are open, arthroscopic inside-out, arthroscopic outside-in, and arthroscopic all-inside.
i. Open repair is usually reserved for peripheral tears in the posterior horn approached through a capsular incision.
ii. Arthroscopic inside-out repairs are performed using absorbable or nonabsorbable sutures placed using zone-appropriate cannulas; the sutures are retrieved and tied through a small capsular incision.
iii. The outside-in technique is usually reserved for anterior horn tears. It involves placing a suture through a needle placed across the tear. The suture is then retrieved and tied outside the knee through an arthroscopic portal with the knot pulled into the knee to reduce the tear when tied over the capsule.
iv. All-inside repairs involve absorbable stents or sutures tied to stents placed through arthroscopic portals. All-inside repairs may offer decreased neurovascular risk.
c. Mechanical studies have investigated many of these devices and have demonstrated reasonable loads to failure, but no device improves on the load to failure of vertically placed sutures.
d. Clinical success rates for all meniscus repair techniques in stable knees are reasonable, ranging from 70% to 95%.
i. Second-look arthroscopy has shown lower rates of success, ranging from 45% to 91%.
ii. Ligamentously unstable knees decrease the success rate of meniscus repair to 30% to 70%.
iii. Several studies have demonstrated meniscus repair success greater than 90% when performed in conjunction with ACL reconstruction.
e. Complications include failure to heal the tear, knee stiffness, and potential damage to the articular surface from mechanical devices used to repair the tear.
3. Transplantation
a. Indications—Typically, meniscal allograft transplantation has been reserved for the symptomatic patient after partial or total meniscectomy has failed.
b. Contraindications include uncorrected lower extremity malalignment, uncorrected ligamentous instability, and significant chondral changes in the treated compartment.
c. Meniscal allograft transplantation, although lacking long-term results, has been performed for more than 10 years.
d. Technically, grafts have performed better when placed with a bone block or plug.
e. A variety of meniscal scaffold options continue to be investigated, but they have not yet undergone widespread human clinical use or investigation.
Top Testing Facts
1. The menisci are composed of 65% to 75% water.
2. The extracellular matrix of the meniscus is predominantly composed of type I collagen.
3. Only the peripheral third of the meniscus is vascularized. The vascularity of the meniscus decreases with advancing age.
4. In extension, as much as 50% of the load is absorbed by the meniscus, with the percentage of load-sharing increasing to 90% at 90° of knee flexion.
5. The lateral meniscus provides more biomechanical support to the joint than does the medial meniscus.
6. Partial meniscectomy causes increased contact pressures on the articular cartilage.
7. Horizontal cleavage tears occur more frequently in the older population and may be associated with meniscal cysts.
8. Factors that seem to predict better long-term function following arthroscopic partial meniscectomy include age younger than 40 years, normal lower extremity alignment, minimal arthritic changes noted at the time of arthroscopy, and a single fragment tear.
9. The strongest meniscal repair construct is vertically placed sutures.
10. Ligamentously unstable knees decrease the success rate of meniscal repair from 70% to 95% to 30% to 70%.
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