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Nuts, Bolts, & Duct Tape

The knee joint is a large and complex joint. In fact, the human knee joint is very interesting because humans are the only animals to stand with an erect posture and bear their body weight through the fully extended knee joint. While most people mistakenly believe the knee is a simple hinge joint this is not true, there is actually a large amount of rotational movement along with the traditional front-back motion. With bones, ligaments, and other body parts all contributing to the wide range of motion and the solid stability the knee joint can be broken down into its constituent parts and described but to fully understand the interactions of those parts a holistic view is necessary. The following explanation of the physiology of the knee will consist of three main categories: Bones of the Knee, The Menisci and Cartilage of the Knee, and The Ligaments, Capsule, and Synovial Membrane of the Knee. Within each of those categories the names, locations, and various attachments will be listed and referenced to pictures in the Physiology PhotoGallery. Also in each category, a description of the function of the knee will be given. Once the basic physiology is stated there will be a section looking at how the knee and all its parts might act together which will be followed by a Frequently Asked Questions area that will hopefully answer many questions that may come up along the way or in your own personal research. So let's get started!!!

Bones of the Knee

There are four bones involved with the knee joint: femur, patella, tibia, and fibula. Of these the femur and tibia are the most important to remember when dealing with the knee although the other two have their uses. Starting from the top of the knee and moving down there is the femur (thigh bone), patella (knee cap), tibia (shin bone), and the fibula.

Femur
The femur is the longest and the strongest bone in the body and the distal end is the end that is involved in the knee joint. At the distal end the femur splits into two prominences called condyles (photogallery). These condyles are known as the medial condyle and the lateral condyle. Each of these two condyles also has an anterior, lateral, posterior, and medial aspect (side). So for instance if someone were talking about the lateral condyle (the condyle furthest to the outside) and they wanted to describe some defect on the inside of that condyle they would refer to the area as the medial aspect of the the lateral condyle. Between the condyles there is a structure known as the trochlea which is just the result of there being space between the condyles.

Patella
The patella, commonly known as the knee cap, is a sesamoid bone that has two facets (photogallery) on the posterior aspect which interact with the femoral condyles fitting into an area known as the femoral trochlea. There is also an area for the patella tendon to attach on the posterior aspect of the distal end of the bone.

Tibia
This is a very important bone when describing the knee because it has so many things attached to it and is therefore crucial to proper joint function in so many ways. In relation to the knee the proximal end is the one referred to. Of primary importance are the two condylar notches located on the very top of the tibia into which the femoral condyles somewhat fit. These condylar notches are shallow, cup shaped depressions of roughly equal width but with the medial notch being longer than the lateral (important for the screw-home mechanism!). Due to fact that there is some space between the condylar notches and their upward slope towards each other, there is a space created called the intercondylar area. In this area, from the front to the rear, there is the anterior intercondylar area, lateral intercondylar tubercule, medial intercondylar tubercule, intercondylar eminence, and the posterior intercondylar area (photogallery). The purpose of all these different tibial features will become apparent shortly.

Fibula
The primary function in the knee of the fibula is the attachement of ligaments and tendons. It is located laterally next to the tibia and is shorter and thinner than the tibia.

 

The Menisci and Cartilage of the Knee

The menisci and cartilage are the components of the knee that are most involved in cushioning and articulation of the knee joint. Without these pieces the knee would and does rapidly grind itself into painful uselessness. Both the menisci and the cartilage are acellular materials composed primarily of collagen.

Menisci
There are two menisci in each knee joint, one in the lateral condylar notch and one in the medial condylar notch, summing to a total of four menisci. Both menisci are triangular in cross-section and they line the tibial-femoral articulating surfaces acting as cushions that fill in the imperfect fit between the femoral condyles and the tibial condylar notches. The medial meniscus is half-moon shaped while the lateral meniscus is almost a closed circle. Also, both menisci are attached to the tibia at both the proximal and distal ends of their structures. Along the outer rim of the menisci, the part of the triangle that is facing outwards from the knee joint, there is a tendonous attachment known as the the coronary ligaments that is flexible and there is also a limited vasular supply which extends approximately 15-35% into the menisci. This vacular supply allows for limited healing but damage further in on the menisci almost never heals. Compared to other primates humans have the largest relative menisci, probably as a direct result of their extensive role in the human knee. When thinking about the menisci and their physical properties it's helpful to think of it kind of like a wedge shaped piece of rubber curved into a rounded shape and wrapped in teflon then coated with PAM cooking spray. This gives an idea of the elastic ability of a meniscus and its low viscosity.

Cartilage
Cartilage in the knee covers the femoral condyles, the tibial condylar notches, and the patellar facets, the exact areas that will be moving against one another. A classic description of cartilage is avascular, aneural, and alymphatic. It regenerates due to chondrocytes on the bottom layer near the bone that continuously send new cartilage up to the articulating surface. Because cartilage has no blood supply the chondrocytes recieve nutrients through diffusion with the synovial fluid. The average thinkness of cartilage on the tibia is 2.92 mm, on the femur 1.99 mm, and on the patella 3.33 mm (Ateshian, et. al.), but this can vary depending upon a persons activity level (more activity=more cartilage) and age (older=less cartilage). Compared to a menicus, cartilage is very hard and not very elastic. Just think of taking a thigh and a drumstick from a piece of a chicken and rubbing the bones against each other, the ends are hard and slide past one another easily.

The Ligaments, Capsule, and Synovial Membrane of the Knee

The ligaments of the knee provide stability to the hinge motion of the femoraltibial joint and also allow for some rotational range of motion due to their flexibility. There are four main ligaments: anterior cruciate ligament, posterior cruciate ligament, lateral collateral ligament, and the medial collateral ligament. All ligaments are acellular and composed almost entirely from collagen. The capsule is more of a shrink wrap around the knee and the ligaments, it holds everything in like it was a tight sleeve while the synovial membrane is kind of like a wall paper to the capsule and secretes various goodies into the knee joint.

Anterior Cruciate Ligament (ACL)
The tibial attachment of the ACL is in the anterior intercondylar area and from here it extends upwards, backwards, laterally to attach to the medial aspect of the lateral femoral condyle(photo gallery). It is about 33 mm long and 11 mm in diameter with a tensile strength from 2200 N to 2500 N (Chhabra, et. al. pages 203 and 206). There are two bundles of fibers in the ACL, anterior and posterior, the anterior are taut in in flexion and the posterior are taut in extension (cite book). A large part of the purpose of the ACL is to control the gliding of the tibia on the femur during extension and flexion. However, another important function of the ACL is its role in the screw-home mechanism (more on this later).

Posterior Cruciate Ligament (PCL)
Attaching in the posterior intercondylar area of the tibia and then travelling upwards, forwards, and medially to attach on the anterior end of the lateral aspect of the medial femoral condyle the PCL is usually 38 mm in length and 13 mm in diameter (Chhabra, et. al. page 203) with a tensile strength higher than that of the ACL but still widely disputed. The PCL helps prevent hyperextension and hyperflexion and controls medial mobility of the tibia.

Lateral Collateral Ligament (LCL)
This ligament attaches on the lateral aspect of the lateral femoral condyle (on a structure called an epicondyle) and attaches to the proximal end of the fibula. This is the main function of the fibula pertaining to the knee. The LCL looks very much like a pencil in length and width and acts like a cord providing tension and strength against varus movement of lower leg. It is relaxed in flexion and is unlikely to be hurt because it is protected by the other leg.

Medial Collateral Ligament (MCL)
The MCL is a very important ligament in the knee and more complex in attachment than the other three ligaments. This ligament consists of three layers: superficial, deep, and posterior. By far the largest and most crucial of the three layers the superficial layer. It is a strong (about twice the tensile strength of the ACL), flat, triangular band of collagen that arises from the medial femoral condyle to the medial aspect of the tibia. The base of the triangle is the line from the femoral attachment to the tibial insertion with the point oriented along the line of the medial meniscus going posteriorly. When the knee is flexed the anterior edge of the MCL is taut and the posterior edge is relaxed. The deep layer is attached to the capsule and is associated with the medial meniscus, this is important in relation to MCL/menicus injuries. Finally, the posterior layer consists of oblique fibers which blend into the posterior capsule. As the principal protector from valgus forces and rotary stress and lacking protection of another leg like the LCL benefits from. In sports such as football or soccer and even in industries such as mining the MCL is often torn when a force of sufficient magnitude and energy is applied against the outside of the knee while the lower leg is planted or fixed. If the knee has a slight bend and is struck on the lateral side it naturally bends and the ligaments are not injured but when the leg is hyperextended the knee cannot unlock in time with the result that the MCL is torn (I.S. Smillie). Often meniscal tears and ACL ruptures will be associated with an MCL injury. Interestingly the MCL is the only ligament that has enough of a vascular supply that it can heal a partial tear without surgery.

Capsule
The shrinkwrap sleeve of the knee, the fibrous capsule is a tough, somewhat elastic covering that orginates just above the cartilage on the femur and terminates just below the outside edge of the menisci on the tibia. The MCL is attached to the outside of the capsule by its deep fibers and the LCL is not attached to the capsule at all. Both the ACL and PCL are contained within the capsule. When thinking about the capsule try to imagine a neoprene sleeve, not only does it keep certain substances in but also keeps other substances out while providing additional stability to the joint.

Synovial Membrane
This membrane is composed of vacularized connective tissue that roughly lines the capsule. Its femoral attachment is just below where the capsule originates and the tibial origin is just above the capsule attachment. Unlike the capsule, however, the synovial membrane is not very elastic or resistant to mechanical damage. The tissue overcomes this lack by being pleated, having extra folds of membrane that can be extended when the knee joint rotates through its range of motion. This also has the added benefit of increased surface area from which to secrete various substances into the joint space. Secretion of these substances, synovial fluid, is the principal job of the synovial membrane because it is from this synovial that much of the cells within the joint derive nutrients and oxygen and the joint derives its lubrication. The fluid also has some phagocytic ability that aids in microbial defense and cellular clean up in the joint. What the synovial fluid does not contain is: red blood cells, clotting factors, or hemoglobin.

 

The BIG Picture
Now that basic parts of the knee are established it's time to look at the knee as a functional whole. When the knee flexes or extends there is rolling and sliding of the femur over the tibia. This is possible because the cartilage along with the synovial fluid create such a low coefficient of friction that it is like ice flowing over ice. In order for the system to have any stability there are the ligaments mostly the MCL and LCL protecting against varus and valgus movement and the ACL, PCL, and geometry of the fit between the femoral condyles and the tibial condylar notches. But without the synovial fluid the all the components composed of collagen would quickly break down from lack of metabolic inputs. Yet the soft membrane is easily damaged without protection. Enter the capsule, strong enough for stability yet dense enough to protect the synovial membrane. What about the menisci in all this? Well the menisci have so many important functions their roles in the functions of the knee is saved for last.

1) Weight bearing: both menisci receive around 50% of the load when in a cushioning role although the lateral meniscus sometimes bears more.
2) Shock absorbing: cartilage does not like sudden compressive forces, this damages and degrades cartilage very quickly. The menisci are able to handle compressive loads much better due to their cross-hatch construction which allows them to handle loads from multiple axis.
3)Stabilizing: as mentioned before the fit between the condyles and condylar notches is imperfect, this discrepancy is made up by the menisci which increase the surface contact area of the femoral cartilage. This directly correlates with reduced wear and increased longevity of the articulating surfaces in the knee (Chhabra, et. al. page 134).
4)Synovial protection: the synovial membrane wraps around the outside of much of the meniscus and without the meniscus there the membrane would get sucked into the articulating area of the joint and pinched.
5)Rotational facilitating: also mentioned before is the fact that the menisci are primarily attached at their distal and proximal ends to the tibia. As such they have a tremendous amount of play along the rest of their unattached length and are able to move as the femoral condyles either push or pull them. This has direct import in the screw-home mechanism.

 

FAQ

What are the largest risk factors for knee problems?
What is the screw-home mechanism?
Is the knee structually sound?
How can you tell if there is a tear of a ligament in the knee?
Do all injuries require surgery?
What are the uncommon circumstances that could injure the LCL?
Why are women more susceptible to knee injuries than men?
How great are the loads on the knee joint?
How is the patella important?
What happens if there is an infection in the knee joint under the capsule?
Is the knee able to respond to use/disuse?
What is "Dashboard Knee" (I.S. Smillie)?
Why is immunosuppression not necessary in an allogeneic transplant?
Does a transplanted ligament regenerate, does is just wear out, or is it able to last a lifetime without regeneration?

Q)What are the largest risk factors for knee problems?

A)In regards to traumatic knee injuries contact sports are the number one risk factor. Being overweight and being out of shape are the two other main risk factors in sustaining a knee injury, either traumatic or chronic.


Q)What is the screw-home mechanism?

A) This is the name given to the action when a knee reaches full extension and just as the knee locks into hyperextension the knee rotates in an outward direction just a few degrees. This has the effect of putting most of the weight onto the cartilage, menisci, and bones of the joint while giving a rest to the muscles of the thigh and calf. What allows for this is that the medial femoral condyle is slightly curved and the medial condylar notch is slightly longer than the lateral condylar notch. The ACL and to some extent the PCL are important in guiding this movement of the bones.


Q) Is the knee structually sound?

A) The knee is actually a pretty resistant structure and can actually withstand alot of impact either suddenly and/or chronically if it is in a good orientation which usually means slightly bent. Condidering the demands placed upon human knee's day in and day out they hold up well. Many injuries to the knee are traceable to bad technique or decisions (Shen, Washington Post) such as athletes that do not stay on their toes, weekend warriors, and 'running like a girl'. In light of these facts many schools and athletic programs are implementing training methods that aim to reduce the bad techniques in athletes (Shen, Washington Post) and increase facilities maintainence programs to control problems like gopher holes.


Q) How can you tell if there is a tear of a ligament in the knee?

A) Generally there is some pain (degree varies) and a popping noise. An interesting paradox is that the more complete a tear the less pain there is (I.S. Smillie). In my case (complete LCL rupture with slight tear of the capsule) there was only a pop that was audible up to ten yards away and no pain for about one hour, only later did the intense pain set in due to the swelling that built up.


Q) Do all injuries require surgery?

A) No (Chhabra, et. al. page 217). Depending on the degree of injury, the structure injured, and the patients probable future activity level surgery may or may not be beneficial. For instance there are many people, even competitive athletes, that return to their normal pre-injury activity level with a complete ACL tear and no reconstructive surgery.


Q) What are the uncommon circumstances that could injure the LCL?

A) In order to injure the LCL the lower leg must experience significant varus force while the femur is fixed. This is difficult because there is a whole other leg keeping most potential hazards (i.e. linebackers, cars, runaway baby carriages) from directly contacting the knee is such manner and instead the protecting leg is hit causing an MCL rupture. But there are ways that the LCL is injured and for wrestlers this is not an uncommon injury because of the dangerous and improperly applied leg holds and take downs that are used.


Q) Why are women more susceptible to knee injuries than men?

A) Even in today's politically correct atmosphere there still remain some differences between men and women. Although anyone can sustain a knee injury women are at an increased risk for ligament tears. When women go through their monthly hormonal cycles the level of estrogen in their bloodstream increases, increased estrogen can lead to looser joint that can in turn get out of alignment making a tear more likely. Women also tend to have smaller ACL's than men. Lastly, females have a tendency to move in a fashion that puts more stress on their knees. Often females move in a more upright position instead of a crouched position (as males do) when participating in physical activities and women rely on their quadricepts more than their hamstrings (Shen, Washington Post).


Q) How great are the loads on the knee joint?

A) During level walking the load can increase to up to three times normal body weight and up to four times body weight when climbing stairs (Chhabra, et. al. page 208). Incredibly when jumping the load can increase seven to ten times body weight (Shen, Washington Post)!


Q) How is the patella important?

A) Patients who have undergone a pallectomy (removal of the patella) are not prevented from playing sports because they can't run but rather because the can't stop (I.S. Smillie). The patella is really a sesamoid bone that is inserted in the patellar ligament that helps to increase the distance of the ligament from the axis of rotation thus providing greater leverage to the quadricepts muscle (up to 30% greater) (Chhabra, et. al. page 216). On the back of the patella there are two facets with a ridge between them, this ridge fits into what is known as the femoral trochlea, the area between the two femoral condyles. Four ligaments, one from each compass direction, keeps the patella in the trochlea all of which creates the "Q angle".


Q) What happens if there is an infection in the knee joint under the capsule?

A) In the first place this is not as likely as might be thought. Even though the synovial fluid is filled with a lot of nutrients and is oxygen rich this mixture is tailored to the needs of the cells it normally serves, the chondrocytes. The chondrocytes have a very low metabolic rate thus reducing there need for too rich of a medium. Within the fluid itself there are phagocytic cells such as macrophages that normally help clean up the dead cells in the joint yet they are capable of defense of the joint. If all else fails there is always the option of antibiotic therapy since antibiotics can freely difuse through the capsule and the synovial membrane. Patients get into trouble when an infection goes too long undiagnosed and treated appropriately because bacteria will adhere to the bone surfaces (osteomyelytis) and form a biofilm which is largely impenetrable to antibiotics and the immune system (Chhabra, et. al. page 101). At this point there are only two options either removal of the affected area and everything below it by amputation or going in and manually scraping away the bacterial colony and implanting antibiotic laced paste or antibiotic eluting beads (Chhabra, et. al. page 108).


Q) Is the knee able to respond to use/disuse?

A) The knee, like most biological systems, does respond to the amount of use it recieves. In athletes it has been observed than the cartilage is thicker than in comparable non-active people (Ateshi, et. al.). There is also increased fluid production from the synovial membrane when there is more movement, thus increasing lubrication within the joint.


Q) What is "Dashboard Knee" (I.S. Smillie)?

A) Dashboard Knee is the name given to the phenomenom of injury to the knee joint, especially the PCL, in a car accident due to a flexed knee coming into abrupt contact with the dashboard. This contact forces the proximal end of the tibia to push back further than it normally would and consequently the PCL which is providing tension to the flexed knee is streched beyond its capacity and ruptures. The incidence of this type of injury can be dramatically reduced by the use of seat belts.


Q) Why is immunosuppression not necessary in an allogeneic transplant?

A) Ligaments are composed of collagen which is an acellular material. Patient 1's immune system views Patient 2s transplanted ligament as a piece of collagen, which is chemically the same between every organism, that has no cells and thus no class I MHC presentation so there is no hyper, acute, or chronic rejection.


Q) Does a transplanted ligament regenerate, does is just wear out, or is it able to last a lifetime without regeneration?

A) The ligament may be made of collagen but that collagen is deposited there by fibroblasts. The donor fibroblasts are killed before transplantation and then, once the transplant is complete, the recipient fibroblasts colonize the ligament and maintain it throughout life.

Sources

First and foremost I would like to thank Dr. Michael J. Hulstyn at the Rhode Island Hospital not only for his generous help and clear explanations but also for his excellent care and truly geniune friendship during the coarse of my surgeries as a result of my April 2003 injury.

1) G.A. Ateshian, L.J. Soslowsky, V.C. Mow, 1991, Quantitation of Articular Surface Topography & Cartilage Thickness in Knee Joints using Stereophotogrammetry, Journal of Biomechanics.

2) A. Chhabra, L.I. Katolik, R. Pavlovich Jr., B. J. Cole, M.D. Miller, 2003, Orthopaedic Approaches to Sports Medicine, Chicago Press.

3) L. Ombregt, P. Bisschop, H. J. ter Veer, 2003, A System of Orthopaedic Medicine, Churchill Livingstone.

4) I.S. Smillie, 1978, Injuries of the Knee Joint, Churchill Livingstone

5) Fern Shen, 2002, Know Your Knees, Washinton Post