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.
To learn more about the Knee and the services we provide or to make an appointment, please call us at 203.598.0700.