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CHAPTER 6 - Knee
From the upcoming book by Peter Simonian, MD


The knee is one of the most common sites of injury for the athlete in any sport, at any level or age. The general anatomy of the knee region is shown for reference in two views. It is made up of four bones: the femur or thigh bone, the patella or knee cap, the tibia or the top of the shin bone, and the fibula or small bone outside the tibia (FIGURE 5-1, 6-18). The motions of the knee are relatively simple including flexion or bending and extension or straightening with very small components of rotation and sliding. Problems can arise with limitations of these motions or stiffness as well as exaggerations of these motions or instability.


Tendon injuries about the knee are very common. These include both the patellar and quadriceps tendons which provide the transfer of power from the large and powerful quadriceps or thigh muscles. The quadriceps tendon resides above the patella or knee cap while the patellar tendon resides below the patella. The most common problem is inflammation resulting in tendonitis. This is also referred to as jumper’s knee when it involves the patellar tendon (FIGURE 6-19). This is one of the most common causes of anterior knee pain, or pain in the front of the knee; a problem that afflicts many different athletes.

A less common but more serious problem is rupture of either of these tendons. This usually occurs in middle-aged males as a result of forced quadriceps contraction. This problem requires surgical repair.

Another problem that can occur is degeneration of a portion of the tendon. This is usually treated conservatively but may require operative debridement when symptoms persist.

Contraction of the patellar tendon, as a result of severely limited knee motion, can pull the patella downwards resulting in what is called patella baja. This is a difficult problem that requires a combination of surgical releases and immediate prolonged rehabilitation to regain a functional degree of motion.


Ligaments are the key stabilizing structures around the knee joint. Basically, there are four key ligaments and one ligament area. The ACL and PCL which cross one another in a diagonal manner are inside the center of the joint (FIGURE 6-20). The MCL is located on the medial of the knee, and the LCL on the lateral side of the knee (FIGURE 6-21). These stabilize against varus and valgus or side-bending forces at the knee (FIGURE 1-2). The posterolateral knee structures represent the ligament area. Each ligament or ligament area behaves differently and all are not essential for daily function. However, the demand of athletics often necessitates stability of most of these structures.

The ACL or anterior cruciate ligament receives the most attention. It courses from the posterior area of lateral femoral notch to the anterior tibial spine, passing by the PCL which runs in the opposite configuration. The ACL keeps the tibia from moving anterior or frontwards relative to the femur (FIGURE 6-22, 6-23). It also controls a component of rotation. This is the ligament that typically is injured when a knee is "blown-out". One of the most common findings after injury is immediate swelling of the knee. The ACL is within the joint and bathed by the joint lubricating synovial fluid; therefore, healing of this ligament after rupture rarely occurs. Poor healing is the result of the limited blood supply in the region. Athletes who perform straight-line activities such as cycling, weightlifting, etc. can often tolerate the absence of this structure. Athletes for whom cutting is a crucial part of the sport, like basketball, ACL instability is often poorly tolerated and may cause the knee to subluxate or partially dislocate. When this occurs, the risk of cartilage damage or tear is significant. In this group of athletes, reconstruction of this ligament should be strongly considered. Reconstruction will serve two purposes; it will provide the athlete the needed stability to return to competition and protect the cartilage structures within the knee. An example of a surgically reconstructed ACL is shown later in the text.

The most common complication after ACL reconstructive surgery is limitation of motion. This complication can be minimized by insisting that the injured athlete wait until knee range of motion is re-established and swelling decreased before allowing surgical reconstruction. This can take anywhere from one week to two months depending on the situation.

Rehabilitation is recognized as a critical component in the treatment of the ACL-injured athlete and deserves special attention. The use of the kinetic chain concept in exercise following ACL reconstruction is important. The hip, knee, and ankle joints when taken together, comprise the three links of the lower extremity kinetic chain. Kinetic chain exercises like the squat recruit all three links in unison while exercises such as seated quadriceps extensions isolate one link of the chain. ACL strain is reduced during kinetic chain exercise by virtue of the orientation of the applied load and combined muscular contractions. Additionally, kinetic chain exercise through recruitment of all hip, knee, and ankle muscles in synchrony takes advantage of important training principles decribed later in the text. Exercising each muscle group in isolation may ultimately hamper complete recovery by interfering with the athlete’s coordination. Modifying the leg press and similar equipment will allow athletes to make better use of kinetic chain exercise and allow safe rehabilitation of the ACL-reconstructed knee.

The PCL or posterior cruciate ligament courses from the medial, anterior femoral notch to the posterior tibia, passing by the ACL. The PCL keeps the tibia from moving in a posterior or backwards direction relative to the femur (FIGURE 6-24), and injury to it is less common. Like the ACL, because of the location, the PCL will typically not heal after disruption. The PCL can be injured by itself or in conjunction with other knee ligaments. At times, athletes may not be aware of isolated injury to this structure and function well despite disruption of the PCL. It is uncertain if injury of this ligament without reconstruction will increase the chance of developing osteoarthritis of the knee. When the PCL injury is part of a multi-ligament injury to the knee, reconstruction should be considered.

The MCL or medial collateral ligament also known as the tibial collateral ligament is another commonly injured ligament; however, the response to injury is very different compared to the ACL. This ligament will almost always heal on its own because it is outside the joint, not exposed to the synovial lubricating fluid, where the blood supply is much better. Therefore, surgical repair or reconstruction is rarely needed. Treatment in a hinged knee brace is usually adequate. This type of injury typically occurs after stress is applied to the lateral region of the knee resulting in the opening of the medial region of the knee and subsequent injury to the MCL. This could be referred to as a valgus stress (FIGURE 1-2). The MCL is commonly injured in conjunction with the ACL or when multiple ligaments are damaged. In fact, there is a common combination of injury to three structures, the ACL, MCL, and medial meniscus; this combination of injuries is relatively common and known as the "terrible triad". In these cases, surgical reconstruction of this ligament may be needed.

The LCL or lateral collateral ligament also known as the fibular collateral ligament is less commonly injured and resides on the lateral region of the knee opposite in location to the MCL. A force applied to the medial knee will result in an opening to the lateral knee and damage to the ligament. This could be referred to as a varus stress (FIGURE 1-2). This ligament is often injured in conjunction with other ligaments in the area. Usually reconstruction is needed because of the multiple structures injured.

The posterolateral corner is a region of the knee composed of several structures, located between the PCL and the LCL. This area provides primarily rotational stability (FIGURE 6-25). Injury to the posterolateral corner region is relatively rare. Many times, damage to the posterolateral corner may be associated with peroneal nerve injury, which is located in the same area of the knee. This nerve injury wil be discussed in the Nerve section below. Reconstruction of this area is complicated, and the operating surgeon should have experience with this relatively rare injury. Typically, the earlier the reconstruction of this area is done after injury, the better the result.


There are several types of cartilage in the human body with very different characteristics. In the knee there are primarily two types of cartilage. The most familiar is the meniscus. This type of cartilage is almost rubbery in consistency and is able to absorb forces through the knee similar to shock absorbers. There are two menisci, each crescent shaped; one is in the medial knee and the other is located in the lateral knee (FIGURE 2-3). These menisci are vulnerable to two basic types of tears, traumatic and degenerative. Traumatic types are caused by acute twisting injuries at any age, and degenerative tears are those that occur after years of wear with no specific trauma. There is also a rare meniscal shape called discoid that does not have the characteristic crescent shape, but instead have a more hemispherical shape. The discoid shaped meniscus is present in some children making the menisci vulnerable to injury..

When menisci tear, symptoms of clicking and locking as well as pain at the joint-line can develop. In cases where these symptoms develop, arthroscopic surgery is usually indicated to resolve the problem. The torn cartilage can either be removed or repaired using many different techniques. If at all possible, repair should be attempted to preserve the meniscus. If that is not possible, a minimal amount of unstable cartilage should be removed. In this way, the majority of remaining meniscal tissue can still absorb force within the knee. In years past, the entire meniscus was removed for any symptomatic meniscal tear. This radical treatment resulted in premature knee arthritis in many of these patients.

Only certain meniscal tears are amenable to repair. In general, degenerative tears are usually not repairable. Success of repair is dependent on the blood supply. Only the outer or peripheral third is vascular; therefore, most tears in this region will predictably heal. Tears in this region are usually repaired while tears in the inner third are almost always removed. Tears in the central third are in a gray zone, and there are some new techniques to encourage vascularization or blood supply to improve healing in this marginal area (FIGURE 6-26). Some arthroscopic surgical illustrations are shown at the end of this chapter.

The other type of cartilage is called articular. This type of cartilage is present in all joints. This cartilage lines the ends bones at the joint surfaces and is much stiffer in consistency; this provides an essentially frictionless or slippery surface for the joints. This surface thickness is variable but averages about 3mm (FIGURE 5-3). Damage to this surface is currently irreversible. When damage involves the majority of the surface, it is referred to as osteoarthritis. There are several alternative treatments for this problem. When it has progressed and the athlete is very symptomatic, an artificial knee replacement can be considered. However, artificial knee replacements are not designed to withstand impact loading which occurs in the majority of sports. Therefore, an athlete will likely have to change his or her lifestyle to some degree after knee replacement. If this change in lifestyle would decrease the athlete’s knee pain without having had the artificial knee replacement, then a change in lifestyle may be the appropriate solution rather than the artificial knee replacement. No person should be eager to have an artificial total knee replacement, especially if they are still very active. This is because the life of an artificial total knee replacement is currently limited to 10-20 years depending upon the situation. A revision total knee replacement after the first one wears out is typically never as good and the surgery is much more complicated. The end of this chapter reviews the actual total knee replacement surgery.

There are three compartments within the knee; the medial or inside, the lateral or outside, and the patellofemoral area underneath the knee cap. When articular cartilage is worn in only the medial or lateral region of the knee sparing the other compartments, an osteotomy can be performed. The surgical technique is illustrated. Typically, the medial compartment is worn and the patient tends to have a varus or "bowed knee" alignment (FIGURE 1-2). An osteotomy means that the bone is cut above or below the knee joint and a wedge of bone is removed; this changes the alignment of the lower extremity decreasing the pressure on the arthritic compartment of the knee and increasing pressure on the uninvolved compartment. This is a very good option for someone who would like to remain active because no artificial joints are involved. There are no simple solutions to treat isolated patellofemoal arthritis. Many surgical treatments in the past have not been satisfactory. There are some surgeries designed to reduce pressure under the knee cap, which have been moderately successful. Often times a total knee replacement is the only lasting solution for severe patellofemoral arthritis.

When there are small focal or isolated defects in this articular cartilage (FIGURE 6-27), there are several new treatment options. However, many of these treatment lack long-term follow-up; so it is difficult to make any predictions. These treatments include harvesting cartilage cells from the patient at a first surgery, then reproducing them outside the body, and finally returning to the operating room several weeks later to place these reproduced cartilage cells into the defect. A piece of the periosteum or outer skin of the bone is also harvested and sewn over the defect and acts as a cover to hold in the cartilage cells in the defect. Another option is to remove small plugs of articular cartilage from areas of the knee that do not bear weight. These plugs are then placed into the defect. This technique is illustrated later in the text. The most simple solution is to make small holes in the bone underneath the cartilage defect which will encourage cartilage healing. The only problem to this solution is that the cartilage that forms is different than normal articular cartilage and is more like scar tissue.


The bursae are small sacks that allow tissues to move against one another. They occur mainly in areas of maximal motion, such as joints, where there is very little tissue between the bone and skin. The area about the patella or knee cap fits the description of an area of multiple bursae. These bursae can become inflamed by many things; however the most common cause is from direct compression of these areas. This occurs most commonly from kneeling over long periods of time. Bursitis of the anterior knee or front of the knee is one of the most common causes of knee pain. Symptoms nearly always resolve by avoiding kneeling activities and other activities that initially caused the inflammation.


There are several problems that can occur with the bones about the knee, including fractures, dislocations, bone contusions or bruises, and avascular necrosis (AVN) which is bone death due to a limited blood supply.

Fractures of bones about the knee are relatively common in sporting activities. The most common fracture involves the tibial plateau, which represents the top portion of the tibia or shin bone. This bone represents the main portion of the lower half of the knee joint. The problem with this fracture is that it nearly always extends into the knee joint disrupting the articular cartilage. It is extremely important to make all attempts at reestablishing this articular surface to decrease the chance of developing osteoarthritis. This usually requires surgery with plate and screw fixation; sometimes a bonegraft is required. The bonegraft is typically taken from a small area of the iliac wing of the pelvis and placed into the area of missing bone at the fracture site. This technique is illustrated later in the chapter. Sometimes this surgery can be done with the assistance of arthroscopy.

Fractures can also occur to the distal femur or the end of the thigh bone, which makes up the upper portion of the knee. These distal femur fractures are not as common because these injuries typically take much more energy to occur. Distal femur fractures also can involve the articular surface and all attempts must be made to re-establish the articular surface surgically with plates and screws.

The patella or knee cap is the third bone that can be fractured at the knee. This usually occurs from a direct blow to the front of the knee. It can also occur from a forced contraction of the quadriceps or thigh muscles. The undersurface of the patella is also covered by articular cartilage and again must be re-established with surgery to decrease the chance of arthritis at this patellofemoral joint.

In children, epiphyseal or growth plate fractures are more common than ligament injuries (FIGURE 4-1). These fractures may not be obvious. Therefore, stress radiographs may be required to find the area of injury. This means that small forces are applied to the leg in different directions while the radiographs are taken. If injury is present, the growth plate will open slightly. A more detailed description of epiphyseal injuries can be found later in the text.

Dislocations can also occur of either the knee cap (FIGURE 6-28) or the entire knee joint, meaning the femur dislocates from the tibia. Patellar or knee cap dislocations are more common and typically occur in the adolescent female with loose joints, and may be related to the increased laxity of their tissues and the relative valgus of the female knee (FIGURE 3-2). These dislocations can be of varying degree with some cases only resulting in only an abnormal tilt of the patella (FIGURE 6-28). True patellar dislocations will typically reduce themselves back into place spontaneously. At times a small push will be required to reduce the patella back into position. The articular cartilage surface on the undersurface of the patella can be damaged with dislocations. If a free fragment of bone or cartilage is present, arthroscopic removal should be done.

Dislocation of the knee joint is a much more devastating injury. Again this typically requires a large force. For this to occur, multiple ligaments are typically disrupted. There is also a significant risk of damage to the surrounding nerves and vessels. Despite the best attempts at reconstruction of the muliple ligament injuries, a degree of permanent disability is usually the end result.

Any of these injuries whether fracture or dislocation, can break through the skin. When this occurs the fracture is considered open or compound. This requires emergent surgery to decrease the chance of infection.

Bone bruises or contusions can occur at the knee joint with any trauma. Bone contusions are most commonly recognized to occur with ACL injuries. The significance of these injuries is still poorly understood.

The blood supply to the bones of the knee can also be compromised resulting in death and subsequent collapse of the articular bone. This condition is again called avascular necrosis (AVN). It is uncertain why this occurs but it has been related to several conditions. The use of oral corticosteroids like cortisone is probably the most common relation to this condition. This medication is used for asthma and other inflammatory conditions and discussed in more detail later in the text.

The patella or knee cap can also be subject to stresses resulting in pain without anatomic malalignment, history of trauma, or patellar instability. This pain is caused from increased pressure between the knee cap or patella and the end of the femur. Frequently, the pain occurs when sitting for prolonged periods of time with the knee flexed or bent. Patellar pain occurs most commonly with sporting events. This condition has been called patellofemoral stress syndrome and is almost always treated conservatively. The exercise program should focus primarily on stretching with some strengthening. If the conservative program fails, surgical management may be considered. A surgical release of the tight tissues can be done arthroscopically to relieve the pressure on the patella. The attachment of the patellar tendon on the tibia can also be surgically elevated to help relieve the pressure. These surgical procedures require continuation of the exercise program to be successful.


Muscle contusions or bruises and strains can occur to the muscles around the knee. Typically, this occurs to the thigh muscles.Hematoma to the thigh muscles can also occur. In rare circumstances, this injured muscle can become ossified or turn to bone. This can result in severe limitation of knee motion. This problem will often require a combination of surgical release and prolonged rehabilitation to regain motion.

Muscle strains often involve the hamstring muscles in the posterior thigh region. These injuries can take a long period to heal and can be very frustrating to the athlete.


There are two main nerves that cross the knee joint. These nerves supply the sensation and muscle control to all the tissues below the knee joint. The main nerve is located behind the knee and is called the tibial nerve. There is also a nerve on the outside of the knee called the peroneal nerve. This nerve supplies control to the muscles on the outside of the leg and sensation to the top of the foot.

The most common injury that can damage the nerve is dislocation of the knee joint. Nerve injuries can also occur with severe fractures about the knee. There are several types of knee nerve injuries including stretch injuries, contusions or bruises to the nerve from direct blows, and lacerations. Many stretch injuries and contusions will heal on their own with time. When the nerve is cut, it should be repaired with the understanding that it will never function the same as it did prior to injury. An EMG electrical conduction study which is used to diagnose carpal tunnel syndrome in the wrist can help determine the amount of nerve function that will return after nerve injury around the knee.


There are several vessels that cross the knee joint. The most important vessels are located directly behind the knee and named the popliteal artery and vein. If the artery is damaged, it must be repaired in a very short period of time to prevent death of the tissues below the knee. Again the most common way these vessels are injured is with dislocations and severe fractures about the knee.

The veins can also form clots of blood called deep venous thrombosis (DVT). This can occur with prolonged immobilization, like a long plane flight. This can also occur after certain surgeries, including joint replacement of the hip or knee. The problem is not with the clot itself, but the potential for the clot to move or embolize to another crucial organ like the lungs. This can result in sudden death and is called a pulmonary embolus. Therefore, these clots in the veins must be treated very seriously. The actual prevention and treatment of these clots in the lower extremity veins is still controversial. A more detailed description of this phenomenon is in the Vessel section of the text.

Conservative Treatment

Conventional Surgery

When articular cartilage has completely been worn, knee joint replacement is successful (FIGURE 9-10). Again, durability is an issue, and total knee replacement should be the last resort. It is designed to relieve pain and improve routine mobilization. Total knee replacement should only be considered for a patient not planning to engage in future activities that will place high impact forces on the artificial knee.

These artificial knees are made out of metal on the femoral side and high density plastic on the tibial and patellar or knee cap surfaces. Sometimes a metal backing is used on the tibial side. There are essentially two total knee designs. One that preserves the PCL and one that does not. Both types have demonstrated excellent outcomes. The most important factor is probably the surgeon’s experience for performing one type over the other. Again, there is some controversy whether to use cement or not. However, most surgeons currently use a cemented type of knee replacement.

The postoperative course is similar to hip replacement surgery. Hospitalization will be approximately five days. Some surgeons may encourage the patient to donate blood prior to surgery. This is because there is a chance that a blood transfusion will be needed after the surgery. In the early postoperative period, steps are taken to prevent blood clots from forming in the legs. This can include compressive devices on the lower extremities to encourage blood flow, and/or the use of blood thinners. Recovery will continue beyond six months following surgery. The rehabilitation will focus on regaining a functional range of knee motion. The most important goal for a good result is to make sure the knee comes out straight into full extension. Studies have shown that at least 110 degrees of flexion are needed for good function.

Fracture treatment osteotomies, as well as collateral ligament reconstructions are done in an open fashion. Similarly all nerve and vessel repair surgey is done in open fashion.

Arthroscopic Surgery

The arthroscope has been used most successfully in the knee. The arthroscope can be used to make the diagnosis of difficult problems. It can be used to perform all types of meniscus surgery (FIGURE 10-2) from meniscus removal, to meniscus repair (FIGURE 10-3), to replacement with an allograft meniscus.

An allograft is tissue from a deceased donor (FIGURE 10-4); these tissues are rigorously tested by accredited tissue banks throughout the country. There are some sources that provide allograft tissue that are not as credible, and it is worth inquiring where the surgeon obtains their tissue allografts. Another question regarding allograft tissue is disease transmission. The best chance for reducing disease transmission or infection is comprehensive screening of the donor and proper handling of the tissue. There are also specific treatments for the allograft tissue that can prevent infection and disease transmission. However, these treatments can decrease the strength of the tissue and are not always desirable.

The ACL (FIGURE 10-5) and PCL can be reconstructed with the aid of the arthroscope. The other knee ligaments require open surgery for reconstruction. Reconstruction of knee ligaments requires a choice of either autograft or allograft replacement tissue. Autograft tissue is taken from the same person, whereas allograft tissue, as mentioned, is taken from a deceased donor. The same issues of potential disease transmission, although extremely unlikely, must be considered when using allografts for reconstruction of the knee ligaments. Another consideration is that allograft tissue does not seem to heal as well as an autograft. The advantage to the use of allograft tissue substitutes is that no donor tissue needs to be taken from a normal structure of the injured athlete. With autograft harvest, pain and potential complications can occur at the donor site.

There are circumstances where the use of allograft tissue is the only reasonable choice. In cases where a revision surgery is done or multiple ligaments must be reconstructed there may be no autograft tissues available. In the case of revision surgery, the autograft tissue may have been used for the first surgery. In the case of multiple ligament reconstructions often required after knee dislocation, there are usually not enough autograft tissues available. Another option in these complicated circumstances is to take autograft tissue from the uninjured knee; this option has both advantages and disadvantage


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