The knee is a complex joint made up of different structures including bones, tendons, ligaments and muscles. They all work together to maintain normal function and provide stability to the knee during movement.
Having a well-functioning healthy knee is essential for our mobility and ability to participate in various activities. Understanding the anatomy of the knee enhances your ability to discuss and choose the right treatment procedure for knee problems with your doctor.
Bones of the Knee
The knee is a hinge joint made up of two bones, the thighbone (femur) and the shinbone (tibia). There are two round knobs at the end of the femur called femoral condyles which articulate with the flat surface of the tibia called the tibial plateau. The tibia plateau on the inside of the leg is called the medial tibial plateau, and on the outside of the leg it is called the lateral tibial plateau.
The two femoral condyles form a groove on the front (anterior) side of the knee called the patellofemoral groove. A small bone called the patella sits in this groove and forms the kneecap. It acts as a shield and protects the knee joint from direct trauma.
A fourth bone called the fibula is the other bone of the lower leg. This forms a small joint with the tibia. This joint has very little movement and is not considered a part of the main joint of the knee.
Articular Cartilage and Menisci of the Knee
Movement of the bones causes friction between the articulating surfaces. To reduce this friction, all articulating surfaces involved in movement are covered with a white, shiny, slippery layer called articular cartilage. The articulating surface of the femoral condyles, tibial plateaus and the back of the patella are covered with this cartilage. The cartilage provides a smooth surface that facilitates easy movement.
To further reduce friction between the articulating surfaces of the bones, the knee joint is lined by a synovial membrane which produces a thick clear fluid called synovial fluid. This fluid lubricates and nourishes the cartilage and bones inside the joint capsule.
Within the knee joint, between the femur and tibia, there are two C shaped cartilaginous structures called menisci. Menisci function to provide stability to the knee by spreading the weight of the upper body across the whole surface of the tibial plateau. The menisci help in load- bearing by preventing the weight from concentrating onto a small area, which could damage the articular cartilage. The menisci also act as a cushion between the femur and tibia by absorbing the shock produced by activities such as walking, running and jumping.
Ligaments of the Knee
Ligaments are tough bands of tissue that connect one bone to another bone. The ligaments of the knee function to stabilize the knee joint. There are two important groups of ligaments that hold the bones of the knee joint together, collateral ligaments and the cruciate ligament.
Collateral ligaments are present on either side of the knee. They function to prevent the knee from moving too far during side to side motion. The collateral ligament on the inside is called the medial collateral ligament (MCL) and the collateral ligament on the outside is called the lateral collateral ligament (LCL).
Cruciate ligaments, present inside the knee joint, control the back-and-forth motion of the knee. The cruciate ligament in the front of the knee is called anterior cruciate ligament or ACL and the cruciate ligament in the back of the knee is called posterior cruciate ligament or PCL.
Muscles of the Knee
Muscles: There are two major muscles, the quadriceps and the hamstrings, which enable movement of the knee joint. The quadriceps muscles are in the front of the thigh. When the quadriceps muscles contract, the knee straightens. The hamstrings are in the back of the thigh. When the hamstring muscles contract, the knee bends.
Tendons of the Knee
Tendons are structures that attach muscles to the bone. The quadriceps muscles of the knee meet just above the patella and attach to it through a tendon called the quadriceps tendon. The patella further attaches to the tibia through a tendon called the patella tendon. The quadriceps muscle, quadriceps tendon and patellar tendon all work together to straighten the knee. Similarly, the hamstring muscles at the back of the leg are attached to the knee joint with the hamstring tendon.
Pain, swelling and stiffness are the common symptoms of any damage or injury to the knee. If care is not taken during the initial phases of injury, it may lead to joint damage that may end up destroying your knee.
Arthritis is a general term covering numerous conditions where the joint surface or cartilage wears out. The joint surface is covered by a smooth articular surface that allows pain free movement in the joint. This surface can wear out for several reasons; often the definite cause is not known.
Osteonecrosis is a condition in which death of a section of bone occurs because of lack of blood supply to it. It is one of the most common causes of knee pain in older women. Women over the age of 60 years of age are commonly affected, three times more often than men.
Osteochondritis dissecans is a joint condition in which a piece of cartilage, along with a thin layer of the bone separates from the end of the bone because of inadequate blood supply. The separated fragments are sometimes called “joint mice”. These fragments may be localized, or may detach and fall into the joint space causing pain and joint instability.
A fracture is a condition in which there is a break in the continuity of the bone.
Causes of Knee Fractures
In younger individuals, knee fractures are caused from high energy injuries, as from a motor vehicle accident. In older people, the most common cause is weak and fragile bone.
Chondromalacia patella is a common condition characterized by softening, weakening and damage of the cartilage. The condition is most often seen among young athletes and older adults who have arthritis of the knee. It is especially seen in women.
Unstable knee can be caused by sudden twisting of the knee, tears of the meniscus, ligament or capsule, osteoarthritis of the knee (wear and tear of the cushioning cartilage tissue between bones) and sports injuries. When these tissues get injured, the patella or kneecap can move out of its groove in the knee joint and lead to instability.
Meniscus tear is the commonest knee injury in athletes, especially those involved in contact sports. A sudden bend or twist in your knee causes the meniscus to tear. This is a traumatic meniscus tear. Elderly people are more prone to degenerative meniscal tears as the cartilage wears out and weakens with age. The two wedge-shaped cartilage pieces present between the thighbone and the shinbone are called meniscus. They stabilize the knee joint and act as “shock absorbers”.
The lateral meniscus is the outer meniscus of the knee joint that gives a cushioning effect during weight bearing activities. Lateral meniscus syndrome is characterized by an injury caused by the tearing of the cartilage tissue or a rare case of a congenital abnormality called a discoid meniscus, which results in knee pain.
Medial meniscal injuries are usually considered as either traumatic or degenerative. Whilst degenerate tears may present with a gradual history of increasing symptoms, traumatic injuries will usually occur as the knee is extended and rotated from a flexed position against resistance. This may occur as a single event during a sporting endeavor or during a period of unaccustomed squatting such as laying flooring or playing with children. The most commonly injured area is the posterior horn.
Dislocation of the patella occurs when the patella moves out of the patellofemoral groove, (called as trochlea) onto a bony head of the femur. If the kneecap partially comes out of the groove, it is called as subluxation and if the kneecap completely comes out, it is called as dislocation (luxation). Patella dislocation is commonly observed in young athletes between 15 and 20 years and commonly affects women because of the wider pelvis creates lateral pull on the patella.
Patellar dislocation occurs when the kneecap slides out of the trochlea. When dislocation of the patella occurs on more than one occasion, it is referred to as recurrent patellar dislocation. The risk of further dislocation increases to almost 70% to 80% after two episodes of dislocation.
Quadriceps tendon rupture most commonly occurs in middle-aged people who participate in sports which involve jumping and running. Quadriceps tear occur by fall, direct blow to the leg and when you land on your leg awkwardly from a jump.
Knee Arthroscopy is a common surgical procedure performed using an arthroscope, a viewing instrument, to consider the knee joint to diagnose or treat a knee problem. It is a relatively safe procedure and most the patient’s discharge from the hospital on the same day of surgery.
Total knee replacement is a surgical treatment for knee arthritis. Over the years, minimally invasive knee replacement surgical techniques have been developed to lessen tissue trauma and improve patient outcomes. This minimally invasive approach involves much smaller incisions than the usual 10-12-inch incisions used in the traditional knee replacement and spares the quadriceps muscle and tendon, which control bending of the knee, from being cut to access the knee joint.
Total knee replacement, also called total knee arthroplasty, is a surgical procedure in which the worn out or damaged surfaces of the knee joint are removed and replaced with artificial parts.
Unicompartmental knee replacement is a minimally invasive surgery in which only the damaged compartment of the knee is replaced with an implant. It is also called a partial knee replacement.
Revision knee replacement surgery involves replacing part or all your previous knee prosthesis with a new prosthesis. Although total knee replacement surgery is successful, sometimes the procedure can fail due to various reasons and require a second revision surgery.
Computer-navigation provides your surgeon with real time 3-D images of your mapped knee and the surgical instruments during surgery. The data for the images is provided by the infrared sensors fixed to the bones of the knee and the surgical instruments. Their position is tracked by an infrared camera placed above the surgical table connected to the computer. The computer than generates the real-time images with the help of the appropriate software to guide the surgeon to precisely resurface and cut the bones of the knee and fix the implant precisely and accurately according to the pre-operative surgical plan.
Total knee replacement is the surgical treatment for knee arthritis, where the damaged knee is removed and replaced with an artificial knee implant. Traditionally performed as an inpatient procedure, total knee replacement surgery is now being conducted on an outpatient basis, allowing patients to go home the same day of the surgery.
Knee implants are artificial devices that form the essential parts of the knee during a knee replacement surgery. The knee implants vary by size, shape, and material. Implants are made of biocompatible materials that are accepted by the body without producing any rejection response. Implants can be made of metal alloys, ceramics, or plastics, and can be joined to the bone. The metals used include stainless steel, titanium, and cobalt chrome; whereas, the plastic used is polyethylene.
The surgical repair of the completely torn ligament involves reconstruction of the torn ligament using a tissue graft taken from another part of the body or from a donor. The damaged ligament is replaced by the graft and fixed to the femur and tibia using metallic screws. Gradually, over a period of a few months, the graft heals.