Introduction

The medial meniscus is a crescent-shaped, cartilaginous band found between the medial tibia and medial femur. The primary function is to decrease the amount of stress on the knee joint. The medial meniscus receives vascular supply via a capillary network formed by the medial, lateral, and middle geniculate arteries and receives its innervation from the posterior tibial, obturator, and femoral nerves. Discoid and ring-shaped variants of the medial meniscus have been discovered but are uncommon. Acute medial meniscus tears are common injuries in athletes and chronic, degenerative tears in the elderly population. Multiple variables determine the management strategy including age, mechanism, type of tear, symptoms, and goals of the individual. Surgical repair or meniscectomy is often the treatment. New developments in medial meniscus transplants are promising innovations for individuals suffering from medial meniscus-related injuries.

Structure and Function

The medial meniscus is a crescent-shaped fibrocartilaginous band that articulates inferiorly with the medial condyle of the tibia and superiorly with the medial condyle of the femur and occupies about 60% of the contact portion of the medial compartment.[1][2] The anterior horn attaches anterior to the ACL on the tibia, while the posterior horn attaches anteriorly to the PCL.[2] The anterior portion of the medial meniscus is anchored by the transverse ligament. Additionally, the medial meniscus attaches to the medial collateral ligament which limits its mobility. When compared to the lateral meniscus, the medial meniscus is more crescent-shaped, covers a smaller portion of the tibial plateau, and has decreased variety in size, shape, and thickness.[1]

The primary function of the meniscus is to distribute load across the knee joint which consequently decreases the stress placed on the joint.[2] The medial and lateral menisci also provide stabilization, shock absorption, nourishment, and proprioception across the knee joint. Additionally, they allow joint gliding and prevent knee hypertension. The medial meniscus has also been shown to protect the joint helping to prevent the development of osteoarthritis.[1] The importance of the meniscus in arthritis prevention was first noted in 1948 when complete meniscectomy demonstrated gradual development of cartilage deterioration.[3]

Embryology

The medial meniscus is derived from mesenchymal cells found within the intermediate layer of mesenchymal tissue and begins to take shape between weeks eight and ten of development.[4] In early development, medial meniscus cells are homogenous showing no differences in size and shape between each other. As maturation occurs, cells begin to differentiate and form distinctly different cells. Classification of cell types found in the meniscus is controversial, but examination has shown progenitor, fibroblast-like, and fibrochondrocyte (an intermediate cell type) cells exist in the meniscus.[1] As development continues, the cellularity of the meniscus decreases while collagen content increases.[5] The medial meniscus extracellular matrix is composed primarily of collagen type I, and some collagen type III and type V arranged into three distinct layers. The layer is mainly collagen type I. Cellularly it is fibroblast-like cells that communicate with other cells and the extracellular matrix.[4][5]  The middle layer mostly contains fibrochondrocyte cells and a large proportion of glycosaminoglycans (GAGs), proteoglycan aggregates that release water to provide lubrication and shock absorption.[4] Lastly, the superficial layer consists of progenitor cells that are speculated to have therapeutic and regenerative capabilities. These cells are CD34+ suggesting that they have a mesenchymal origin and have been shown to play a role in differentiation after injury.[1][5] Altogether, this unique arrangement of layers functions in concert to dissipate compressive stress into circumferential stress giving the meniscus its shock absorption-like properties.[4]

Blood Supply and Lymphatics

Blood supply is key to medial meniscal healing. The medial meniscus receives its vascular supply primarily from the medial, lateral, and middle geniculate arteries, branches of the popliteal artery, which is a continuation of the femoral artery.[4] The capillary network formed by these arteries supplies the superior and inferior portion of the medial meniscus. It is worth noting that only 10 to 30% of the medial meniscus receives a direct blood supply.[4][5][6] Additional supply is provided by radial branches of these blood vessels which insert directly into the anterior and posterior horns of the medial meniscus for nourishment.[2][6] The remaining portion of the meniscus is supplied via diffusion directly from the synovial fluid or mechanical motion.[2][4]  The absence of direct blood supply to the majority of the medial meniscus contributes to the challenges in healing a torn meniscus and post-operative recovery.

Nerves

The medial meniscus is part of the knee joint which is innervated by the articular branch of the posterior tibial nerve and the terminal branches of the obturator and femoral nerves. The nerve fibers join the vascular supply and innervate the joint capsule.[3] Additionally, various mechanoreceptors can be found in the anterior and posterior horns of the medial meniscus to help support movement and positional adaptation. Type I (Ruffini) mechanoreceptors are slow adapting and adjust to change in position and pressure. Type II (Pacinian) receptors, the most abundant receptor in the medial meniscus, are fast adapting and adapt to change in tension and joint acceleration. Type III (Golgi tendon organ) mechanoreceptors sense changes in the range of motion and are vital to neuron inhibition.[3][7]

Muscles

Although not directly articulating with musculature, the medial meniscus plays a vital role in stabilizing and cushioning the medial knee during movement. The knee has two primary movements: flexion and extension; control of these two movements are by two separate groups of muscles. Extension of the knee is controlled by the quadriceps muscles which include the vastus lateralis, vastus intermedius, rectus femoris, and vastus medialis, whereas flexion of the knee is acted upon by the hamstring muscles: the semitendinosus, semimembranosus, and bicep femoris.[8][9]

Physiologic Variants

Variations in medial meniscus shape are much less common than variations found in the lateral meniscus. Commonly, the medial meniscus is a crescent-shaped structure. However, instances of a discoid-shaped medial meniscus exist. This thicker, disc-shaped medial meniscus is an uncommon condition typically unilateral affecting one knee but can occur bilaterally. Individuals with a discoid medial meniscus often present with medial knee pain. Recommendations are that symptomatic patients undergo partial meniscectomy and reshaping, while asymptomatic patients preserve their meniscus.[10][11][12] The ring-shaped medial meniscus is an even rarer finding. Gines-Cespedosa and Monllau, documented a patient presenting with knee locking, pain, and swelling refractory to conservative treatment. During arthroscopy, this patient demonstrated a ring-shaped medial meniscus which received a partial meniscectomy.[13]

Surgical Considerations

Total vs. Partial Meniscectomy

The goal of surgical treatment of meniscal tears is to relieve pain, prevent degeneration of the knee joint, and allow improvement of activities of daily living. Total meniscectomy was the preferred surgical treatment until the 1970s when it became apparent that complete removal of the meniscus might lead to increased arthritic changes. There has been a shift toward a partial meniscectomy and arthroscopic repair to avoid degenerative changes that previously occurred with complete meniscectomy. The basis for surgical candidacy relies on the type of meniscal injury because of the limited blood supply the meniscus receives. Flaps, radial tears, and degenerative tears are not surgical candidates. Interestingly, concomitant ACL and meniscal repair have a 90% success rate at 2-year follow-up compared to 50 to 65% success rate of isolated meniscal repair. The current hypothesis is that the bleeding caused by surgery promotes improved healing.[2]

Arthroscopic Repair

Arthroscopic meniscus repair is favorable among the younger, athletic population and has demonstrated clinically significant results. Arthroscopic repair involves utilizing sutures to stitch together torn portions of the meniscus. A study done by Nakayama et al. demonstrated that 14 out of 18 medial meniscus cases returned to play at 5.5 months.[14] Mintzer et al. demonstrated that 25 out of 26 patients who had undergone arthroscopic meniscal repair under the age of 17 years old showed clinical healing at a 2-year follow-up.[15] Interestingly, Logan et al. showed that lateral meniscus repairs had a higher success rate than medial meniscus repairs; however, their results show meniscal repair is successful in the medium and long-term with most elite athletes returning to the preinjury level of activity.[16]

Transplantation

Allograft transplantation has also found use in patients who have undergone partial or complete medial meniscectomy. Indications for transplant include pain at the medial meniscal compartment with a complete meniscal tear. Patients are expected to make a full return to painless activity. Synthetic implants are another alternative for patients who have had a partial meniscectomy. The goal with implants is to create a regenerated meniscus using an absorbable model.[2]

Clinical Significance

Younger Population

Meniscal tears in the younger population generally occur with athletics and acute injury. Patients present with sharp, medial knee pain after experiencing a "pop" when changing direction, cutting or pivoting, or through direct trauma from another person.[3] Anterior cruciate ligament (ACL) tears often accompany these injuries (>80%).  Patients often complain of medial swelling and locking, and depending on acuity may present with joint line tenderness and joint effusion.[1] A physical exam of a medial meniscus tear yields a positive McMurray rotation test and a positive Thessaly test with catching or locking of the joint moving medially. Unless accompanied by an ACL tear, Lachman and/or Anterior Drawer tests will both be negative. Confirmatory diagnosis is made using MRI while the gold standard for assessment is arthroscopy.[3]

Older Population

Meniscal tears in the older population are due to long-term degeneration instead of acute injury. Patients generally present with medial joint swelling, pain, and locking and often show radiographic evidence of osteoarthritis. MRI is a confirmatory test. There has been much discussion regarding meniscus surgery in the older population. Concerns with decreased meniscus vascularization and possible worsening osteoarthritis, present issues when discussing surgical candidacy. Despite this, complete or partial meniscectomy, depending on the amount of damage, is currently the preferred method of intervention among some surgeons.[1] There is a growing shift towards either non-operative management or total knee replacement in older patients with atraumatic, degenerative meniscus changes. Arthroscopic debridement of the medial meniscus in the setting of radiographic osteoarthritis showed little benefit at one year compared to corticosteroid injection.[17]Unhappy Triad of the Knee

The original "Unhappy Triad," first noted in the 1950s by Dr. O'Donoghue, showed that athletes who suffered both ACL and MCL tears have a high index of suspicion for medial meniscus injuries. The original O'Donoghue study demonstrated that 77% of his ACL-MCL injuries had an additional medial meniscus tear.[18] However, subsequent work by Shelbourne shows that the O'Donoghue triad should instead be in the category of ACL, MCL, and lateral meniscus tears.[19]