Anatomy of Meniscal Structures in the Knee:
The medial and lateral menisci are structures in the knee that act as shock absorbers. They help with the transmission of forces from the femur to the tibia by cushioning the articular cartilage, which is a Teflon-like coating over the surface of the knee joint. The menisci protect the cartilage from extreme forces across the knee joint during walking. 

Without the menesci, the knee?s cartilage surface is exposed to increased forces which cause advanced deterioration resulting in joint surface degeneration and arthritis.  

 The menisci are made of collagen and water materials called fibro-cartilage.   They are cup-shaped and stabilize the position of the round surface of the femur onto the flat surface of the tibia.  Each meniscus consists of a body, an anterior horn, and a posterior horn and is further divided into thirds, inner, middle and outer.  The outer third, closest to the periphery, is thickest while the inner third, closest to the inside of the knee, is the thinnest.  

The menisci receive their blood supply from the geniculate arteries of the knee so blood flows in a peripheral to midline direction.  The outer third of the meniscus has a much better blood supply than the inner third so tears have a much higher healing rate.

The ability of the menisci to protect the various positions of the knee during walking is due to the water content of the structure.  The higher the water content between collagen molecules, the more elastic the menisci are.  Menisci that contain less water are more brittle and more prone to tearing.  As we age, the water content decreases. 

Types of Menicus Injury:
Injury to a meniscus is categorized two ways:

• Acute Tears can occur at any age but are usually associated with a traumatic event involving a forceful twisting of the knee while it is bent. 
• Degenerative Tears occur more commonly in older people. Sixty percent of the population over age 65 probably has had some sort of degenerative meniscal tear.  The menisci of older people contain less water and are more brittle so tears can occur from simpler activities such as bending down or stumbling.

Results of Injury:
In the knee joint, the curved end of the femur contacts only a fraction of the flat surface of the tibia.  The menisci act as intermediary contacts between the femur and the tibia.  For this reason, forces transmitted down the femur through the knee are divided across a wider surface area of the tibia, which disperses and decreases the pressure.

The medial meniscus absorbs up to 55% of the load applied to the medial compartment of the knee and the lateral meniscus bears up to 75% of the lateral compartment load. While walking, forces on the knee increase by 2 to 4 times body weight.  These forces can increase by 4 to 8 times when running and jumping. 

Without the menisci, the forces are no longer distributed over a wide tibial surface area, but over the smaller, limited surface area of femoral tibial.  Loss of medial meniscus causes tibial contact to decrease 55-75% while loss of lateral meniscus decreases contact by 40-45%.  This leads to a 200-300% increase in contact pressure and can cause an early onset of degenerative arthritis of the knee.     

Symptoms:

• Acute tears are usually associated with a specific trauma among active people, most commonly a twisting injury. With an acute meniscal tear, swelling and tenderness along the knee joint line is usually present.

Other Symptoms include:

• Locking or clicking is a result of a torn piece of meniscus becoming trapped between the femur and tibia and is usually painful. 
• Instability is the feeling of ?giving way? of the knee both frequently and infrequently. With an acute meniscal tear swelling and tenderness along the knee joint line is usually present.
• Degenerative tears can occur through simple activities such as squatting during gardening, slipping on ice or a sudden increase in activity frequency (e.g. the weekend warrior). If there is a tear, even normal activities can become painful

Other symptoms include:

• Pain, swelling and knee joint line tenderness are classic signs. 
• Catching, locking, and instability are also frequent. 

Diagnosis:
The orthopaedist will first obtain a history to screen for any twisting event, ask for the date the pain began and about specific activities which cause pain.

In addition, the orthopaedist will:

• Push on the medial and lateral joint line of the knee to determine tenderness, which could be a meniscal tear.
• Place the knee through a full range of motion, hyper-extended and hyper-flexed, to assess for any pain and locking. 
• Perform the McMurry test, which involves flexing and extending the knee while internally and externally rotating the tibia on the femur to assess for pain or clicking. 
• Obtain X-Rays to find any fractures or arthritic changes (i.e., changes that demonstrate a narrowing of the cartilage between the femur and the knee.
• Perform an MRI (if diagnosis is still unclear)

Treatment:
Many factors are considered to develop a treatment plan for a meniscal injury including age, activity level, date of injury, symptoms and associated injuries of the knee.  In addition, size, location, and the type of tear help determine if it can be repaired with non-operative treatment versus surgery.

• Non-operative treatment is best for tears that are small and don?t cause mechanical locking of the knee.  These tears are observed for 8-12 weeks and if symptoms disappear, surgery is unnecessary.  Therapy includes activity restriction, ice and non-steroidal anti-inflammatory drugs (NSAIDS).
• Operative treatment is considered for patients who experience knee locking, instability and pain for more than 8-12 weeks or for patients who are high performance athletes.  If the anterior cruciate ligament is torn, the knee joint surface experiences increased sheer forces due to its increased motion.  In this instance, meniscal tears are treated operatively because the likelihood of natural healing is limited, (due to excessive motion).
• Types of Surgery
• Partial resection is for tears located in the inner 1/3 to 2/3 of the meniscus, which has a poor blood supply and low likelihood of healing.  Only the torn portion is resected, leaving the outer 1/3 intact to continue to function as a shock absorber for the knee.  Partial resection of the meniscus decreases early onset degenerative arthritis of the knee.
• Complete resection is performed only when absolutely necessary for catastrophic, complex tears that extend into the outer 1/3 of the meniscus, tears in the outer 1/3  that have undergone plastic deformity over time, or radial tears that extend into the outer third of the meniscus.  This incidence of early onset degenerative arthritis of the knee is high.
• Meniscal repair is considered for tears located in the outer third of the meniscus, specifically horizontal tears.  This is possible due to the increased blood supply in the outer 1/3 of the meniscus (red zone), which allows for more rapid healing.  It is also considered for large tears that would otherwise need a complete resection Repair is performed using vertically placed sutures, which join the torn edges of the meniscus together
• Meniscal Transplant involves taking a meniscus from a donor source (cadaver) and transplanting it into the affected knee after a sterilization procedure.  The long term results of this procedure are still being evaluated and only a limited number of surgeons perform this procedure on a routine basis.  This procedure is usually reserved for individuals who have undergone a catastrophic, irreparable meniscal tear that has led to complete resection. 

Recovery and Rehabilitation:
Non-Operative rehabilitation includes:

• Protective weight bearing with crutches from 3-7 days or until pain and swelling decreases enough to allow comfortable movement.
• If symptoms persist, surgery is considered.

Partial and complete resection rehabilitation includes:

• Protective weight bearing with crutches of the effected knee for 1-3 days. 
• Ice (cryo-cuff) and NSAIDS to decrease swelling. 
• An exercise bike for range of motion exercises. 
• And, after 3-7 days quadriceps strengthening exercises through physical therapy.
• Return to usual activities occurs at 1-6 weeks. 

Meniscal repair rehabilitation includes:

• Non-weightbearing for the first week after surgery. 
• At one week, full weight-bearing when walking, but with a brace to lock the knees, (discontinued after week 5)
• After 1 week, range of controlled motion and strengthening exercises
• Return to sports at 4-6 months after surgery. If symptoms persist, the meniscus must be re-evaluated.