Our knowledge and under- standing of the anatomy and function of the meniscus has evolved significantly over the past few decades.
This understanding combined with advances in arthroscopic surgery, have dramatically changed our surgical philosophy.
Open total meniscectomy is no longer acceptable treatment, Commonly accepted treatment of meniscal disorders now include arthroscopic partial meniscectomy, as well as meniscal repair. Efforts are now increasingly directed at meniscal preservation and even restoration
The menisci are two crescentic fibrocartilaginous structures made predominantly of Type 1 collagen. The collagen bundles are arranged in a circumferential pattern that is optimal for absorption of compressive loads. Radial fibres are woven between the circumferential fibres, which help to provide structural integrity.
The arrangement of the collagen fibres enables them to resist the hoop stresses that are produced at the meniscus during weight bearing.
The menisci are triangular in cross section, the peripheral border of each meniscus being thick, convex, and attached to the capsule of the joint and the opposite border tapering to a thin, free edge. The posterior horn of the medial meniscus is larger than the anterior horn, whereas the anterior and posterior horns of the lateral menisci are typically of similar size. The proximal surfaces of the menisci are concave and in contact with the femoral condyles; the distal surfaces are flat and rest on the tibial plateau. The medial meniscus covers approximately 64% of the medial tibial plateau. The lateral meniscus covers approximately 84% of the lateral tibial plateau. Embryologically, the menisci form from mesenchymal tissue and appear as distinct structures by the eighth to tenth week of gestational development. Initially highly cellular, the perinatal meniscus also has an abundance of blood vessels. Progressive and gradual changes occur from birth to mid-adolescence, consisting of decreasing cellularity, decreasing vascularity, and increasing collagen content.
The medial meniscus is firmly attached to the posterior intercondylar fossa of the tibia directly anterior to the PCL insertion.
The lateral meniscus anterior horn is attached to the intercondylar fossa, directly anterior to the lateral tibial tubercle and adjacent to the ACL. The posterior horn is attached to the intercondylar fossa directly posterior to the lateral tibial tubercle and anterior to the posterior horn of the medial meniscus. the meniscofemoral ligaments, connect the posterior horn of the lateral meniscus to the intercondylar wall of the medial femoral condyle.
Peripherally, the medial meniscus is continuously attached to the capsule of the knee. The tibial attachment of the meniscus, sometimes known as the coronary ligament, attaches to the tibial margin a few millimeters distal to the articular surface. The anterior horn of the medial meniscus has the largest insertion site surface area (61.4 mm2), and the posterior horn of the lateral meniscus has the smallest (28.5 mm2).
The anterior attachment is more variable (4 Types described by Ohkoshi) –
- Type I insertions were located in the flat intercondylar region of the tibial plateau;
- Type II occurred on the downward slope from the medial articular plateau to the intercondylar region;
- Type III occurred on the anterior slope of the tibial plateau; there was no firm bony insertion of the anterior horn in type IV.
- The occurrence for type I was 59% (20 of 34); type II, 24% (8 of 34); type III, 15% (5 of 34); Type IV, 3% (1 of 34). The variance in insertion patterns may have clinical applications for patients with atypical anterior knee pain and for performing meniscal allograft. Type III and type IV insertions may be unable to resist peripheral extrusion of the loaded meniscus, placing it at risk for anterior subluxation and causing anterior knee pain in specific cases. Awareness of these patterns may be valuable in medial meniscus harvest and transplantation.
The ligament of Humphry passes anterior to the PCL, whereas the ligament of Wrisberg passes posterior to the PCL. The attachment of the lateral meniscus is interrupted by the popliteal hiatus through which passes the popliteus tendon.
Because the lateral meniscus is not as extensively attached to the capsule as the medial meniscus is, it is more mobile and may displace up to 11mm with knee flexion. The controlled mobility of the lateral meniscus, which is guided by the popliteal tendon and meniscofemoral ligament attachments, may explain why meniscal injuries occur less frequently on the lateral side.
- Load transmission.They transmit 50% of compressive load and increase contact area. As load is applied, the menisci will tend to extrude from between the articular surfaces of the femur and tibia. In order to resist this tendency, circumferential tension is developed along the collagen fibers of the meniscus as hoop stresses. The circumferential continuity of the peripheral rim of the meniscus is integral to meniscal function. Partial meniscectomy, or bucket-handle tearing, will still preserve meniscal function so long as the peripheral rim is intact. Conversely, if a radial tear extends to the periphery and interrupts the continuity of the meniscus, the load-transmitting properties of the meniscus are lost. Under axial femoral compressive loads, the peak contact stress and maximum shear stress in the articular cartilage increased 200% more after a lateral than a medial meniscectomy. These increased stresses could partly explain the higher cartilage degeneration observed after a lateral meniscectomy.
- Increase articular conformity and contact area. The tibial femoral contact area decreased by up to 75% in postmeniscectomy knees as demonstrated by Baratz and Mengator.
- Distribute synovial fluid
- Provide joint stability. The medial meniscus especially controls antero-posterior translation.
- Prevention of soft-tissue impingement during joint motion.
- Proprioception. Total or subtotal meniscectomy is a one way ticket to knee osteoarthritis.
Classic Fairbank’s signs postmeniscectomy – squaring of femoral condyle, joint space narrowing and osteophytes.
Thompson (1991) studied meniscal motion with 3-dimensional MRI and cinematic MRI. Medial meniscal excursion was approximately 5.1 mm, and lateral meniscal excursion, 11.2 mm. The posterior horn excursion has been noted to be less than that of the anterior horn, both medially and laterally. DePalma has demonstrated that most lateral meniscal motion occurs after 5 to 10 degrees of flexion, whereas most medial meniscal displacement occurs after 17 to 20 degrees of flexion. The posterior oblique ligament is firmly attached to the posterior medial meniscus, thereby limiting its displacement and rotation. This possibly accounts for the increased risk of injury to the medial meniscus. Conversely, the relatively increased mobility of the lateral meniscus is also responsible for the more frequent occurrence of injuries on the medial side.
The meniscus in a neonate is entirely vascular. By the age of 10, the meniscal vascularity is similar to the adult meniscus. Studies by Arnoczky and Warren have demonstrated that only the outer 10% to 25% of the lateral meniscus and 10% to 30% of the medial meniscus are vascular.These vessels are derived from the middle, medial, and lateral geniculate arteries. The inner two thirds of the meniscus is avascular and is nourished by the synovial fluid through diffusion.
Arnoczky SP, Warren RF: Microvasculature of the human meniscus. Am J Sports Med 10:90-95, 1982. The microvascular anatomy of the medial and lateral menisci of the human knee was investigated in 20 cadaver specimens by histology and tissue clearing (Spalteholz) techniques. It was found that the menisci are supplied by branches of the lateral, medial, and middle genicular arteries. A perimeniscal capillary plexus originating in the capsular and synovial tissues of the joint supplies the peripheral 10-25% of the menisci. A peripheral, vascular, synovial fringe extends a short distance over both the femoral and tibial surfaces of the menisci but does not contribute any vessels to the meniscal stroma. The posterolateral aspect of the lateral meniscus adjacent to the popliteal tendon is devoid of penetrating peripheral vessels as well as a synovial fringe. The anterior and posterior horn attachments of the menisci are covered with vascular synovial tissue and appear to have a good blood supply.
The anterior and posterior horns are the most richly innervated, and the body innervation follows the pattern along the periphery.
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