Knee Anatomy and its Associated Problems

The knee basically functions as a hinge, but in reality it is much more complicated than that. Although we see the hinge motion from the outside, on the inside the motion actually more closely resembles the swing arm of a four-bar linkage. Moreover, there is also a rotational component to knee motion as well. It is a marvelously designed joint but is actually more complex than the hip joint. It may not be surprising, then, that knee replacement did not evolve until after hip replacement.

Bones And Cartilage

The knee joins together the lower end of the femur and the upper end of the tibia (the large bone in the lower leg; the smaller one is the fibula). The patella (knee cap) also is in contact with the femur, where it glides up and down in a shallow trough called the trochlea.

The ends of the bones and the undersurface of the patella are covered with a layer of articular cartilage that is similar to the Teflon coating in a frying pan. The coating is normally about 1/8th inch thick, but over time, wear and tear and a number of diseases can lead to the loss of this cartilage coating.

When the cartilage is worn away, most patients experience significant and worsening pain from the "bone on bone" contact, and this process is osteoarthritis.

Trauma or injuries can also lead to loss of the articular cartilage. A common mechanism is a "dashboard" type injury in which a bent knee strikes the dash in a car accident, making a "divot" or pothole in the cartilage. This usually does not heal by itself, and over time, the resulting defect can lead to posttraumatic arthritis. Post-traumatic arthritis is also common with any fractures that occur in or close to the knee joint. It is common for patients who experience significant knee trauma to require knee replacement at a later date.

The knee joint is surrounded by a capsule. There is a tough membrane called the retinaculum that covers it along the front, going off either side of the patella. Sometimes this retinaculum can develop tears in an injury, and the patella may not track correctly afterwards as a result.

The Three Compartments Of The Knee

The knee joint has three compartments within it. Two of these are formed where the curved, camshaped end of the femur meets the tibia on both the inner (medial) and outer (lateral) side of the knee. The cruciate ligaments separate the medial and lateral compartments and run inside the intercondylar notch. The third compartment is formed by the patella and the trochlear groove of the femur that it glides up and down in (the patellofemoral compartment). There are pockets on the sides of the joint referred to as the medial and lateral gutters, which can sometimes harbor loose bodies and debris.

Knee problems can occur in any and all of these three compartments. Meniscal problems occur in the medial and lateral compartments, where most of the weightbearing also occurs. Problems with the undersurface of the patella (e.g., chondromalacia patellae) manifest in the patellofemoral compartment.

Sometimes arthritis and other conditions affect only one side of the knee, or affect it to a much greater degree than the rest of the knee, and therefore partial knee replacements and other procedures may focus on just that compartment. At other times, the term tricompartmental degenerative joint disease is used to describe arthritic changes affecting all three compartments, which may dictate a total knee replacement instead of a partial one. This is discussed in greater detail shortly.

Ligaments of The Knee

There are four primary ligaments around the knee that hold it together (there are actually more, but these are the most important ones). The anterior cruciate ligament (ACL) is in the center of the knee and keeps the tibia from sliding forward. It is commonly injured in sports injuries, and in young patients we reconstruct it with surgery. In older or more sedentary patients, an ACL rupture may often be left alone and/or treated with a brace and physical therapy to compensate with better muscle strength. The ACL is usually torn and shredded from chronic arthritic changes in many of the patients who undergo knee replacement.

The posterior cruciate ligament (PCL) is also in the center of the knee and runs opposite to the ACL, preventing the tibia from falling backwards relative to the femur. PCL injuries are not as common as ACL injuries, and the PCL is often intact at the time of knee replacement. In fact, many knee replacement designs make use of the PCL (more on this later), while others replace its function.

On the sides of the knee are the collateral ligaments, both the medial collateral ligament (MCL) and the lateral collateral ligament (LCL). These ligaments are responsible for keeping the knee from giving way from side to side. These are commonly sprained (especially the MCL), but even partial tears often heal without the need for surgery. A hinged brace may be used if the collateral ligaments are torn or injured, which keeps the knee from buckling from side to side.

Menisci

Within the space between the femur and tibia is another cartilage structure called the meniscus. It is a C-shaped gasket made of the same type of cartilage as your ears or nose, and there is such a C shaped segment in each side of the knee (the medial and lateral menisci). It functions as a shock absorber and "chock block" within the knee. It normally does not spread all the way across the knee joint (when it does, this is called a discoid meniscus, which is often problematic and sometimes requires surgery). Instead, it fits around the rim of the knee.

For many years the meniscus was considered "the appendix of the knee," and painful tears were typically treated by removing the entire meniscus, which worked great... for a while. Patients later developed significant arthritis without the stabilizing influence of the meniscus. Today, these tears are treated with arthroscopic surgery instead of complete resection (more on that in a later chapter).

If the meniscus tears, this often leads to pain and clicking in the knee on the affected side.Tears can occur as the result of a twisting injury, or some types simply occur as a wear and tear process, like a rug that has been walked on too many times and develops a frayed spot. If the tear is big enough, the torn flap can flip into the joint space, catching like a door with a piece of carpet stuck in the hinge.This can cause buckling, and catching, and arthroscopic surgery may be needed to correct the problem.

Loose bodies can occur and locking, also cause similar mechanical problems. These can arise from bits of bone or cartilage that break free and rattle about inside the knee joint. At times, these loose bodies can cause pain that may move around in position and cause locking of the knee when they become caught between the bones of the joint.

Alignment of The Knee (Varus/Valgus)

The overall mechanical alignment of the knee is important. Although some patients are born with a natural curve in the knee, many arthritis patients develop worsening deformity as one side of the knee wears out faster than the other. If the inner side (medial compartment) wears out first - as is the case for the majority of knee arthritis patients - the knee begins to become progressively more "bow-legged," a deformity known as varus. If the outside compartment (lateral compartment) wears out first, then the knee becomes progressively "knock-kneed," a deformity known as valgus.

Sometimes the patella does not track correctly in its groove, or may even "jump out" and dislocate frequently. This is usually treated conservatively with a brace and physical therapy, but if it persists there are several surgeries described to change the tracking of the patella.

Side to side deformities like varus and valgus are best assessed with standing x-rays, which show the alignment. The gap normally seen on an x-ray is actually the cartilage, which does not appear on x-ray, and it can be assessed by its absence when the gap disappears or is smaller than it should be with weightbearing x-rays.

Varus knee deformity (bow-legged).		Valgus knee deformity (knock-kneed).

Extensor Mechanism

The muscle groups in the thigh (notably the quadriceps) attach to the upper end of the patella, and below the patella is a tough tendon that links the patella to the front of the tibia. This functions like a pulley, and when the muscles in the thigh contract, the tibia is extended and the knee straightens.

Any disruption of the extensor mechanism, either by injury or surgery, can be devastating (more on this in the complications section of the book) - it is like cutting the strings of a marionette, and the knee cannot be extended. Quadriceps tears, patella fractures (with a gap between the fragments), and patella tendon ruptures are all usually repaired surgically because of the importance of this entire assembly in moving the knee backwards and forwards.

Nerves and Blood Vessels

The nerve and blood vessel anatomy around the knee is somewhat simpler than the hip. The major structures (e.g., the popliteal artery and the tibial nerve) run along the very back of the knee, which is usually left alone in most joint replacement surgeries. It is possible to injure the popliteal artery during surgery, but it is exceedingly uncommon (typically seen mostly with complex revision surgery when it does occur). Most surgical approaches to the knee approach from the front of the knee for this reason. There are surgical approaches to the back of the knee (known as the popliteal fossa), but these are usually only used for specific cases such as tumor resection.

There are some arteries that surround the knee known as the geniculate arteries. These may commonly be encountered during surgery and are usually cauterized.

The peroneal nerve passes around the head of the fibula. It is uncommon for this to be injured with surgery, but pressure from a retractor or from swelling and blood within the joint can sometimes injure the nerve, leading to the very uncommon occurrence of a foot drop after surgery. When it does occur, foot-drop usually resolves on its own within months to a year in most cases, but may require use of an orthosis to keep the foot from dragging.

Please remember the information on this site is for educational purposes only and should not be used to make a decision on a condition or a procedure. All decisions should be made in conjunction with your surgeon and your primary care provider.