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 includes 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]

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]

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]

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] 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, who 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 an ACL, MCL, and lateral meniscus tears.[19]