Anatomy of the Knee Joint

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The knee is one of the largest joints in the body and a frequent site of injury, such as patellar dislocations, ACL, and meniscal tears. Both traumatic and degenerative injury can lead to cartilage defects. Depending on the degree of damage and patient age, the defects can be treated with cartilage transplants or joint replacement surgery.

Knee Joint Anatomy
Anatomy of the Knee Joint

Bones of the Knee Joint

Femur (Thigh Bone)

The femoral condyles are the two rounded prominences at the end of the femur; they are called the medial and the lateral femoral condyle, respectively. The motions of the condyles include rocking, gliding and rotating. Any abnormal surface structure or cartilage damage can lead to cartilage breakdown and arthritis (loss of cartilage padding).

Tibia (Shin Bone)

The Tibia meets the Femur at the knee in two areas on which the Femur rides. This area called the Tibial Plateau is divided into a medial (inside of your knee) and a lateral (outside) part.

Patella (Knee Cap)

The Patella is a bone that lies within the quadriceps tendon. It rides in the shallow groove over the front part of the Femur called the Trochlea. The Patella acts as a lever arm to help the quadriceps muscle extend the knee.

Several bones meet to form the knee joint; it consists of the femur, tibia, and patella. These bones are held together by ligaments, which connect two bones to each other, and tendons, which connect a muscle to a bone.

 

Cartilage of the Knee Joint

Articular Cartilage, also called Hyaline Cartilage, covers the joint surfaces where the femur, tibia, and patella articulate with each other. This glistening white substance has the consistency of firm rubber but has very low friction to allow sliding motion with almost no resistance. With normal joint fluid for lubrication, the surface is more slippery than ice-on-ice and allows smooth and easy knee joint motion for decades.

Hyaline cartilage is a tissue composed of Type II collagen and other special molecules, including glycosaminoglycans (GAG), which help to attract water into the cartilage, giving it viscoelasticity to dampen shock and distribute forces to the bone underneath. Embedded in this matrix are chondrocytes, living cells that initially produced the matrix, and now maintain it.

Cartilage Defects

Cartilage Defect
Cartilage Defect

Damage to the articular cartilage can occur for several reasons:

  • trauma, such as car accidents or falls;
  • injuries, such as patellar dislocations or ACL tears;
  • joint abnormalities, such as osteochondritis dissecans (OCD);
  • previous surgery, such as removal of meniscal tissue;
  • repetitive injury over time.

Articular cartilage damage is common, being present in over 60% of knees that undergo arthroscopy. Unfortunately, articular cartilage has a very low capacity for healing, since it does not contain blood vessels. Both superficial and full-thickness cartilage injuries may lead to progressive damage, similar to a pothole increasing in size with time, and can eventually end in osteoarthritis.  The larger the initial cartilage injury, the faster the potential progression of arthritis.

Appearance of Cartilage Defect during Arthroscopy

Condyle Defect
Cartilage defect of the medial femoral condyle during arthroscopy (left) and during open surgery for ACI (right)

 

Osteoarthritis

Degenerative joint disease, unlike an isolated chondral defect, involves the entire joint although it may have originally started locally as a focal cartilage defect. Over time, the cartilage defect enlarges and is worn down to the underlying bone. Increased pressure on the bone causes it to remodel and become stiffer and harder. Osteophytes (bone spurs) form and the joint capsule becomes thicker and inflamed. The end result is a permanent dysfunction of the joint.

This degenerative process is complex with a combination of abnormal mechanical forces and destructive enzymes. There still are a number of unknown questions regarding the rate of progression, but what is known is that once the degenerative process is initiated it is difficult to stop. That is the reason why it is so important to prevent or halt this negative cycle before painful symptoms significantly restrict your level of activity and hamper your quality of life. With end-stage osteoarthritis, your only course of treatment then becomes a total joint replacement.

 

Meniscus

Meniscus cartilage is the other type of cartilage in the knee and is made of a substance called fibrocartilage. There are two of these C-shaped pads in each knee, one on the medial (inside) part, and one on the lateral (outside) part. They are attached to the tibia and lie between the tibial plateau and the femoral condyle to cushion the forces of walking, jumping and running. They accomplish this by distributing joint forces over a larger area of the joint — transferring force from the curved femoral condylar margins to the flatter tibial plateaus. If the meniscal cartilage is injured, it might have to be removed or repaired arthroscopically. If the entire meniscus is lost due to injury and/or surgery, the articular cartilage of the joint is exposed to much higher stresses and starts to break down, resulting in arthritis.

Normal Meniscus
Normal meniscus on MRI (left) and during arthroscopy (right)

 

Torn Meniscus
Torn meniscus on MRI (left) and during arthroscopy (left)