Lec # 06 - 08 Joint Prosthesis

outline Anatomy Total Joint Replacement Knee joint Causes Types of implants Components Knee implant fixations

Anatomy A joint is where the ends of two or more bones meet. There are different types of joints within the body. For example, the knee is considered a "hinge" joint, because of its ability to bend and straighten like a hinged door. The hip and shoulder are "ball-and-socket" joints, in which the rounded end of one bone fits into a cup-shaped area of another bone.

When Is Total Joint Replacement Recommended? Several conditions can cause joint pain and disability and lead patients to consider joint replacement surgery. In many cases, joint pain is caused by damage to the cartilage that lines the ends of the bones (articular cartilage)—either from arthritis, a fracture, or another condition. If nonsurgical treatments like medications, physical therapy, and changes to your everyday activities do not relieve your pain and disability, your doctor may recommend total joint replacement.

Total Joint Replacement Total joint replacement is a surgical procedure in which parts of an arthritic or damaged joint are removed and replaced with a metal, plastic or ceramic device called a prosthesis. The prosthesis is designed to replicate the movement of a normal, healthy joint. In 2011, almost 1 million total joint replacements were performed in the United States. Hip and knee replacements are the most commonly performed joint replacements, but replacement surgery can be performed on other joints, as well, including the ankle, wrist, shoulder, and elbow.

Lec # 06 Knee joint

anatomy The knee is the largest joint in the body and having healthy knees is required to perform most everyday activities. The knee is made up of the lower end of the thighbone (femur), the upper end of the shinbone (tibia), and the kneecap (patella). The ends of these three bones where they touch are covered with articular cartilage, a smooth substance that protects the bones and enables them to move easily. The menisci are located between the femur and tibia. These C-shaped wedges act as "shock absorbers" that cushion the joint. Large ligaments hold the femur and tibia together and provide stability. All remaining surfaces of the knee are covered by a thin lining called the synovial membrane. This membrane releases a fluid that lubricates the cartilage, reducing friction to nearly zero in a healthy knee. Normally, all of these components work in harmony. But disease or injury can disrupt this harmony, resulting in pain, muscle weakness, and reduced function.

Causes The most common cause of chronic knee pain and disability is arthritis. Although there are many types of arthritis, most knee pain is caused by just three types: osteoarthritis, rheumatoid arthritis, and post-traumatic arthritis. Osteoarthritis. This is an age-related "wear and tear" type of arthritis. It usually occurs in people 50 years of age and older, but may occur in younger people, too. The cartilage that cushions the bones of the knee softens and wears away. The bones then rub against one another, causing knee pain and stiffness. Rheumatoid arthritis. This is a disease in which the synovial membrane that surrounds the joint becomes inflamed and thickened. This chronic inflammation can damage the cartilage and eventually cause cartilage loss, pain, and stiffness. Post-traumatic arthritis. This can follow a serious knee injury. Fractures of the bones surrounding the knee or tears of the knee ligaments may damage the articular cartilage over time, causing knee pain and limiting knee function.

Types of Knee implants 1. Fixed Bearing Implants The most common knee replacement implant is referred to as a fixed-bearing implant. It is referred to as “fixed” because the polyethylene cushion of the tibial component is fixed firmly to the metal platform base. The femoral component then rolls over this cushion.

The fixed-bearing prostheses provide a good range of motion and just as long lasting as other implants for most patients. In some cases, excessive activity and/or extra weight can cause a fixed-bearing prosthesis to wear down more quickly. This wear can cause loosening of the implant, causing pain and joint failure .

2. Mobile Bearing Implants If you are younger, more active, and/or overweight, your doctor may recommend a rotating platform/mobile-bearing knee replacement. These implants are designed for potentially longer performance. The difference between a fixed-bearing implant and a mobile bearing implant is in the bearing surface. Fixed-bearing implants and mobile-bearing implants use the same three components.  In a mobile-bearing knee, a similar metal implant is inserted into the tibia, but the polyethylene tray is placed on a circular stem that allows slight rotation of the tray on the metal tibial platform during knee motion. This rotation allows patients a few degrees of greater rotation to the medial and lateral sides of their knee.

Because of this mobility, mobile-bearing knee implants do require more support from the ligaments surrounding the knee. If the soft tissues are not strong enough, though, the knee is more likely to dislocate. Mobile-bearing implants may also cost a bit more than fixed-bearing implants.

Knee implant components Typical total knee replacement implants have three basic components: femoral, tibial and patellar. The femoral component is generally made of metal and curves around the end of the femur (your thighbone). There is a groove down the center of this part of the implant which allows the patella (kneecap) to move up and down as the knee bends and straightens. The tibial component is a flat metal platform with a polyethylene (plastic) insert or spacer. This component varies in structure, depending on which type of surgery is performed. The patellar implant is a dome-shaped piece of polyethylene that mimics the kneecap. This implant is used in some knee replacements; not in others.

Types of Knee Implant Fixation The other big difference between types of knee replacements is fixation. There are cemented, cementless, and hybrid (combination of cemented and cementless) designs. Cemented prostheses utilize a special kind of bone cement that helps hold the components of the artificial joint in place. The majority of knee replacements are generally cemented. Cemented knee replacements have been used successfully in all types of knee replacement patients. Historically, some younger, more active patients had problems with loosening cement but the material is now much improved.

2. Cement less implant designs become fixed by bone growth into the surface of the implant. Most implant surfaces are textured or coated with a porous material so that the new bone actually grows into the surface of the implant. Screws or pegs may also be used to stabilize the implant until bone ingrowth occurs. Recovery does generally take a bit longer as the bone grows into the prosthesis. Cement less implants are not usually a viable for patients with osteoporosis, as the existing bone must be in good shape in order to grow into the implant. 3. Some surgeons also use a hybrid of the two methods of fixation, though this is more common in hip replacement surgery.

Lec # 07 Hip joint

anatomy The hip is one of the body's largest joints. It is a ball-and-socket joint. The socket is formed by the acetabulum, which is part of the large pelvis bone. The ball is the femoral head, which is the upper end of the femur (thighbone). The bone surfaces of the ball and socket are covered with articular cartilage, a smooth tissue that cushions the ends of the bones and enables them to move easily. A thin tissue called synovial membrane surrounds the hip joint. In a healthy hip, this membrane makes a small amount of fluid that lubricates the cartilage and eliminates almost all friction during hip movement. Bands of tissue called ligaments (the hip capsule) connect the ball to the socket and provide stability to the joint.

Common Causes The most common cause of disability is arthritis. Osteoarthritis, rheumatoid arthritis, and traumatic arthritis are the most common forms of this disease. Osteoarthritis. The cartilage cushioning the bones of the hip wears away. The bones then rub against each other, causing hip pain and stiffness. Osteoarthritis may also be caused or accelerated by subtle irregularities in how the hip developed in childhood.

Rheumatoid arthritis. This is an autoimmune disease in which the synovial membrane becomes inflamed and thickened. This chronic inflammation can damage the cartilage, leading to pain and stiffness. Post-traumatic arthritis. This can follow a serious hip injury or fracture. The cartilage may become damaged and lead to hip pain and stiffness over time.

Avascular necrosis. An injury to the hip, such as a dislocation or fracture, may limit the blood supply to the femoral head. This is called avascular necrosis. The lack of blood may cause the surface of the bone to collapse, and arthritis will result. Childhood hip disease. Some infants and children have hip problems. Even though the problems are successfully treated during childhood, they may still cause arthritis later on in life. This happens because the hip may not grow normally, and the joint surfaces are affected.

Types of Hip implants Hip implants are medical devices intended to restore mobility and relieve pain usually associated with arthritis and other hip diseases or injuries. Factors that influence the longevity of the device include the patient’s age, sex, weight, diagnosis, activity level, conditions of the surgery, and the type of implant chosen. There are currently five types of total hip replacement devices available with different bearing surfaces. These are: Metal-on-Polyethylene: The ball is made of metal and the socket is made of plastic (polyethylene) or has a plastic lining. Ceramic-on-Polyethylene: The ball is made of ceramic and the socket is made of plastic (polyethylene) or has a plastic lining. Metal-on-Metal: The ball and socket are both made of metal. Ceramic-on-Ceramic: The ball is made of ceramic and the socket has a ceramic lining. Ceramic-on-Metal: The ball is made of ceramic and the socket has a metal lining.

Implant Components Many different types of designs and materials are currently used in artificial hip joints. All of them consist of two basic components: A cup : A cup (made of stainless steel or titanium) that is placed in the patient’s acetabulum.  In the cup, a liner (made of polyethylene or ceramic) is impacted and serves as an interface between the cup and the replacement femoral head.

A femoral implant comprised of :  A round head (made of stainless steel or titanium) that is placed on the neck of the stem and articulates with the liner.  A neck (cone-shaped) that can be fixed on the stem or a modular neck. When the femoral stem can accommodate a modular neck, the surgeon can choose between different necks that can be adapted to the shape of the femur.  A femoral stem (made of stainless steel or titanium) inserted in the femur.

The prosthetic components may be "press fit" into the bone to allow your bone to grow onto the components or they may be cemented into place. The decision to press fit or to cement the components is based on a number of factors, such as the quality and strength of your bone.

Shoulder joint

Although shoulder joint replacement is less common than knee or hip replacement, it is just as successful in relieving joint pain. Shoulder replacement surgery was first performed in the United States in the 1950s to treat severe shoulder fractures. Over the years, shoulder joint replacement has come to be used for many other painful conditions of the shoulder, such as different forms of arthritis.

Anatomy Your shoulder is made up of three bones: your upper arm bone (humerus), your shoulder blade (scapula), and your collarbone (clavicle). The shoulder is a ball-and-socket joint: The ball, or head, of your upper arm bone fits into a shallow socket in your shoulder blade. This socket is called the glenoid. The surfaces of the bones where they touch are covered with articular cartilage, a smooth substance that protects the bones and enables them to move easily. A thin, smooth tissue called synovial membrane covers all remaining surfaces inside the shoulder joint. In a healthy shoulder, this membrane makes a small amount of fluid that lubricates the cartilage and eliminates almost any friction in your shoulder. The muscles and tendons that surround the shoulder provide stability and support.

Causes One of the most common cause of shoulder joint pain is arthritis. The most common types of arthritis are: Osteoarthritis (OA) Post-traumatic arthritis Rheumatoid arthritis (RA)

Components The artificial shoulder joint can have either two or three parts, depending on the type of surgery required. The humeral component (metal) is implanted in the humerus. The humeral head component (metal) replaces the humeral head at the top of the humerus. The glenoid component (plastic) replaces the surface of the glenoid socket.

Total Shoulder Replacement The typical total shoulder replacement involves replacing the arthritic joint surfaces with a highly polished metal ball attached to a stem, and a plastic socket. These components come in various sizes. They may be either cemented or "press fit" into the bone. If the bone is of good quality, your surgeon may choose to use a non-cemented (press-fit) humeral component. If the bone is soft, the humeral component may be implanted with bone cement. In most cases, an all-plastic glenoid (socket) component is implanted with bone cement.

Elbow joint

Anatomy The elbow is a hinge joint which is made up of three bones: The humerus (upper arm bone) The ulna (forearm bone on the pinky finger side) The radius (forearm bone on the thumb side) The surfaces of the bones where they meet to form the elbow joint are covered with articular cartilage, a smooth substance that protects the bones and enables them to move easily. A thin, smooth tissue called synovial membrane covers all remaining surfaces inside the elbow joint. In a healthy elbow, this membrane makes a small amount of fluid that lubricates the cartilage and eliminates almost any friction as you bend and rotate your arm. Muscles, ligaments, and tendons hold the elbow joint together.

components In total elbow replacement surgery, the damaged parts of the humerus and ulna are replaced with artificial components. The artificial elbow joint is made up of a metal and plastic hinge with two metal stems. The stems fit inside the hollow part of the bone called the canal. Hinge allows the two pieces of the new joint to glide easily against each other as you move your elbow. The hinge allows the elbow to bend and straighten smoothly.

The metal replacement parts are made of chrome-cobalt alloy or titanium and there is a liner made of polyethylene (plastic). The bone cement is made of polymethylmethacrylate (acrylic, a type of plastic).

fixations There are two different ways to hold the artificial elbow in place. A cemented prosthesis uses a special type of epoxy cement to glue it to the bone. An uncemented prosthesis has a fine mesh of holes on the surface. Over time, the bone grows into the mesh, anchoring the prosthesis to the bone.

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