The Anatomy of the Medial Compartment of the Knee

The medial (inner) compartment is an extremely significant part of the knee joint. Along with the lateral (outer) and patellofemoral (behind the kneecap) compartments, this area plays an influential role in your ability to do everyday activities like standing and walking. Many important structures are contained in this inner area.

Anatomy

The medial compartment of the knee includes everything within the inner half of the joint and is located where the tibia (shinbone) and femur (thigh bone) meet. The rounded end of the femur bone (medial femoral condyle) sits on a flattened area of the tibia bone called the medial tibial plateau.

In between the two bones lies a C-shaped piece of cartilage called the meniscus. Along with a similar section of the meniscus in the lateral compartment, this important structure helps absorb forces and spread them throughout the knee.

The ends of the femur and the tibia are also coated with a thin, shock-absorbing structure called articular cartilage. Both the meniscus and the articular cartilage provide crucial protection to the knee and allow the femur and tibia bones to smoothly slide on one another.

Surrounding the two bones is the joint capsule, which serves two purposes:

• The firm outer membrane is fibrous in quality and adds further stability to the joint itself.
• The inner synovial membrane produces a fluid called synovium (also known as joint fluid) that lubricates the area and helps the two bones slide effectively.

Muscles in the Medial Compartment

There are several different muscles that attach above or below the medial compartment of the knee. They include:

• Adductor magnus muscle: This muscle runs along the side of the leg and inserts near a boney bump called the adductor tubercle on the medial femoral condyle.
• Medial gastrocnemius muscle: This calf muscle travels up the lower leg from the Achilles tendon to a boney prominence below and slightly behind the adductor tubercle.
• Vastus medialis oblique (VMO): One of the four quadriceps muscles, the VMO runs down the leg on the inner part of the thigh and eventually joins the quadriceps tendon to insert into the superomedial (top and inside) border of the kneecap.
• Pes anserine bursa: On the anteromedial (front and inside) portion of the tibia, three muscles (from top to bottom: the sartorius, the gracilis, and the semitendinosus) insert one above the next and form the outer border of the pes anserine bursa. This fluid-filled sac separates the muscle tendons from the tibia bone and helps to reduce friction in the area.
• Semimembranosus: One of the three hamstring muscles, this structure originates from just below the buttocks and runs down the back of the leg. It inserts below the knee joint on the posterior (back) and posteromedial (back and inside) portions of the tibia.

Ligaments in the Medial Compartment

Multiple stabilizing ligaments also are present in the medial compartment of the knee. They include:

• Medial collateral ligament (MCL): The most prominent ligament in the medial compartment, this structure runs along the side of the knee from the medial femoral condyle to the tibia. It has both a superficial and a deep component, the deeper of which also inserts into the medial meniscus.
• Medial patellofemoral ligament (MPFL): This ligament originates on the medial femoral condyle (just in front of and slightly beneath the adductor tubercle) and inserts into the medial border of the patella.
• Posterior oblique ligament (POL): This deeper ligament runs from just above the adductor tubercle on the medial femoral condyle to the posterior portion of the tibia.
• Oblique popliteal ligament (OPL): This broad ligament spans from the adductor tubercle on the medial femoral condyle into multiple attachment locations including the semimembranosus tendon, the posteromedial tibia, the posteromedial joint capsule, and the posterior oblique ligament.

Function

Each of the anatomical structures in the medial compartment plays an important role:

• The rounded end of the medial femoral condyle sits on the relatively flat tibial plateau, which allows the two bones to roll, slide, and rotate slightly on one another. As a result of these joint movements, the knee is able to fully bend and straighten as you move.
• The meniscus helps improve the congruency of the joint and, along with the articular cartilage, assists in absorbing the stresses that are placed through the bones as you go about your day.
• Each of the muscles attaching near the medial compartment helps move the knee in one or multiple directions.
• The semimembranosus, semitendinosus, medial gastrocnemius, sartorius, and gracilis work to bend or flex the knee.
• The VMO extends or straightens the joint.
• The semimembranosus and semitendinosus assist with inward rotation and the gracilis and sartorius either inwardly or outwardly rotating the joint depending on the position of the leg.
• The adductor magnus muscle helps to bring the entire leg closer to the midline of the body.

The ligaments and capsule add stability to the joint and help prevent excessive movement. The individual ligaments and function include:

• Medial collateral ligament: This structure prevents inward buckling of the knee (called genu valgus) as well as the external or outward rotation of the tibia.
• Posterior oblique ligament: This ligament protects against inward knee movement, particularly when the joint is fully extended. The POL also helps prevent excessive posterior sliding of the tibia on the femur.
• Oblique popliteal ligament: This ligament stabilizes against excessive external rotation of the tibia and hyperextension of the knee.
• Medial patellofemoral ligament: This ligament provides medial stability to the kneecap and helps prevent lateral patellar subluxation or dislocation.

Associated Conditions

Because of the plethora of structures in the medial compartment, there are multiple different conditions that can affect this area. These include:

• Osteoarthritis (OA): This is more common on the inner knee because it is subjected to more of the forces associated with walking.
• Meniscus tear: This condition can occur gradually as a result of wear and tear or can come on more acutely (usually after an injury involving twisting or cutting). Symptoms of this condition can include pain, restricted knee range of motion, swelling, catching or locking of the joint, and giving way of the leg.
• Acute knee injury: While any of the structures may be affected, the most commonly injured is the MCL. Medial collateral ligament injuries typically occur following a twisting, pivoting, or cutting movement. Regardless of which muscle or ligament is affected, however, sprains and strains usually result in pain, swelling, pinpoint tenderness, and difficulty moving the knee. In more serious cases where a tear of the ligament or tendon occurs, the leg may become unstable or actually give way as you walk.

Treatment

Treatment options vary based on each situation but can include:

• The RICE principle (Rest-Ice-Compression-Elevation): This is common following a flare-up of pain in the medial knee or an injury to one of the structures in the area and is typically helpful in reducing some of the initial symptoms.
• NSAIDs or acetaminophen: These may be taken to help alleviate the acute pain. In some cases, a cortisone injection is also administered to address the pain and swelling associated with a meniscal tear or an osteoarthritis aggravation.
• Physical therapy: For most conditions—including OA, a medial meniscus tear, ligament sprains, and tendon strains—physical therapy (PT) is usually the first line of defense. Therapy can help restore flexibility in the joint and build up strength to support the damaged structure. PT can also assist in resolving any swelling that may be present.
• Surgery: If therapy is unsuccessful or if a more severe meniscal, tendon, or ligament tear is present, surgical intervention may be necessary. In the case of a ligament tear, this typically involves reconstructing the structure with a graft from another area of the body or from a cadaver. Following a tendon tear, a surgical repair of the muscle may be necessary to restore stability in the joint. Medial meniscus tears are most frequently addressed with a partial meniscectomy, which involves arthroscopically removing the damaged portion of cartilage. In some cases, a meniscal repair may also be possible, though this is dependent on the size and location of the tear.
• Partial or total knee replacement: In the case of advanced osteoarthritis, a partial or total knee replacement may need to be performed. This procedure involves removing the arthritic boney surfaces and replacing them with prosthetic components.