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 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 the 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.

The most common cause of chronic hip pain and disability is arthritis. Osteoarthritis, rheumatoid arthritis, and traumatic arthritis are the most common forms of this disease.

Osteoarthritis
This is an age-related wear and tear type of arthritis. It usually occurs in people 50 years of age and older and often in individuals with a family history of arthritis. 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. Rheumatoid arthritis is the most common type of a group of disorders termed inflammatory arthritis.

Posttraumatic arthritis
This can follow a serious hip injury or fracture. The cartilage may become damaged and lead to hip pain and stiffness over time.

Osteonecrosis
An injury to the hip, such as a dislocation or fracture, may limit the blood supply to the femoral head. This is called osteonecrosis (also sometimes referred to as avascular necrosis). The lack of blood may cause the surface of the bone to collapse, and arthritis will result. Some diseases can also cause osteonecrosis.

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 in life. This happens because the hip may not grow normally, and the joint surfaces are affected.

In a total hip replacement (also called total hip arthroplasty), the damaged bone and cartilage is removed and replaced with prosthetic components.

• The damaged femoral head is removed and replaced with a metal stem that is placed into the hollow centre of the femur. The femoral stem may be either cemented or "press fit" into the bone.
• A metal or ceramic ball is placed on the upper part of the stem. This ball replaces the damaged femoral head that was removed.
• The damaged cartilage surface of the socket (acetabulum) is removed and replaced with a metal socket. Screws or cement are sometimes used to hold t
he socket in place.
• A plastic, ceramic, or metal spacer is inserted between the new ball and the socket to allow for a smooth gliding surface.

(Left) The individual components of a total hip replacement. (Centre) The components merged into an implant. (Right) The implant as it fits into the hip.

When Surgery Is Recommended
There are several reasons why your doctor may recommend hip replacement surgery. People who benefit from hip replacement surgery often have:
• Hip pain that limits everyday activities, such as walking or bending
• Hip pain that continues while resting, either day or night
• Stiffness in a hip that limits the ability to move or lift the leg
• Inadequate pain relief from anti-inflammatory drugs, physical therapy, or walking supports

Realistic Expectations
An important factor in deciding whether to have hip replacement surgery is understanding what the procedure can and cannot do. Most people who undergo hip replacement surgery experience a dramatic reduction of hip pain and a significant improvement in their ability to perform the common activities of daily living.

With normal use and activity, the material between the head and the socket of every hip replacement implant begins to wear. Excessive activity or being overweight may speed up this normal wear and cause the hip replacement to loosen and become painful. Therefore, most surgeons advise against high-impact activities such as running, jogging, jumping, or other high-impact sports.

Realistic activities following total hip replacement include unlimited walking, swimming, golf, driving, hiking, biking, dancing, and other low-impact sports. With appropriate activity modification, hip replacements can last for many years.

Home Planning
Several modifications can make your home easier to navigate during your recovery. The following items may help with daily activities:

• Securely fastened safety bars or handrails in your shower or bath
• Secure handrails along all stairways
• A stable chair for your early recovery with a firm seat cushion (that allows your knees to remain lower than your hips), a firm back, and two arms
• A raised toilet seat
• A stable shower bench or chair for bathing
• A long-handled sponge and shower hose
• A dressing stick, a sock aid, and a long-handled shoehorn for putting on and taking off shoes and socks without excessively bending your new hip
• A reacher that will allow you to grab objects without excessive bending of your hips
• Firm pillows for your chairs, sofas, and car that enable you to sit with your knees lower than your hips
• Removal of all loose carpets and electrical cords from the areas where you walk in your home

You will either be admitted to the hospital on the day of your surgery

Anesthesia
Upon arrival at the hospital or surgery center, you will be evaluated by a member of the anesthesia team. The most common types of anesthesia are general anesthesia (you are put to sleep) or spinal, epidural, or regional nerve block anesthesia (you are awake but your body is numb from the waist down). The anesthesia team, with your input, will determine which type of anesthesia will be best for you.

Implant Components
Many different types of designs and materials are currently used in artificial hip joints. All of them consist of two basic components: the ball component (made of highly polished strong metal or ceramic material) and the socket component (a durable cup of plastic, ceramic, or metal, which may have an outer metal shell).

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 several factors, such as the quality and strength of your bone. A combination of a cemented stem and a non-cemented socket may also be used.

Your orthopaedic surgeon will choose the type of prosthesis that best meets your needs.

(Left) A standard non-cemented femoral component. (Centre) A close-up of this component showing the porous surface for bone ingrowth. (Right) The femoral component and the acetabular component working together.

(Left) The acetabular component shows the plastic (polyethylene) liner inside the metal shell. (Right) The porous surface of this acetabular component allows for bone ingrowth. The holes around the cup are used if screws are needed to hold the cup in place.

Procedure
The surgical procedure usually takes from 1 to 2 hours. Your orthopaedic surgeon will remove the damaged cartilage and bone and then position new metal, plastic, or ceramic implants to restore the alignment and function of your hip.

X-rays before and after total hip replacement. In this case, non-cemented components were used.

After surgery, you will be moved to the recovery room where you will remain for several hours while your recovery from anaesthesia is monitored. After you wake up, you will be taken to your hospital room.