An injury, an inflammation, or a disease can lead to irreversible damage to the cartilage in the hip joint. This can cause severe pain and stiffness affecting everyday activity and leading to severe disability.
One of the most effective ways to regain functional range of motion of the hip joint allowing a return to active life style is a procedure called “hip arthroplasty or “hip replacement”. It involves removal of the damaged cartilage and bone and replacement of its function with high quality metal alloy and plastic or ceramic materials. Introduced over 50 years ago, this type of operation has evolved over the years and is considered today as one of the most effective operations that improves a person’s quality of life.
The normal hip joint:
Hip joint is a ball and socket joint. The head of the thigh bone, called the femur, forms the ball. This articulates to form a joint with the socket-like acetabulum in the pelvic or hip bone. The acetabulum is a concave shell covered by a layer of shining smooth tissue called “cartilage”. The head of the femur has matching spherical shape and is also covered by the cartilage. These two structures are surrounded or ‘encapsulated’ by a thick layer of fibrous tissue, called “capsule”, that provides major support to the hip joint. The space inside the capsuleis known as intra-articular space. It is filled with joint fluid, also known as “synovial fluid”, which provides lubrication and also nourishes the cartilage. The innermost layer of tissue, covering the capsule from the inside of the joint is called “synovial membrane”. Within the joint space, the head of the femur and the acetabulum are connected by strong vascularised structure, called “ligamentum teres” or “round ligament”. Its function is not yet well defined, but is probably one of assuring the stability of the hip joint and allowing adequate movement. To secure head stability within the acetabulum and to allow appropriate motion the acetabular edge has a thick layer of cartilage, called “labrum”.
The labrum can be damaged by trauma or abnormal development of the hip structure.
When is hip arthroplasty/replacement required?
When hip joint cartilage and underlying layer of bone are damaged to such an extent that they cannot regenerate, hip replacement is the procedure of choice.
Following pathological conditions can lead to this damage:
1. Osteoarthritis: This is the most common cause. In osteoarthritis, the cartilage is worn off due to underlying mechanical reason or due to unknown or idiopathic cause.
2. Avascular necrosis (AVN) or Osteonecrosis: This occurs when the initial damage is to blood circulation of the head of the femur, leading to bone death, or necrosis, with subsequent collapse of the head of the femur and cartilage destruction. Reasons for this condition are many - use of corticosteroids, alcohol consumption, certain metabolic disorders (Gaucher Disease, Sickle Cell Disease), decompression sickness, and more.
3. Posttraumatic Arthritis: This is when the hip joint is damaged as a result of trauma, as in case of hip joint fracture. It is more common among younger patients.
4. Inflammatory arthritis: In this condition, the cartilage is affected secondary to the disease of the synovial membrane. Rheumatoid arthritis is the best example of this type of condition.
How is the surgery performed?
The damaged cartilage and underlying thin layer of bone are replaced with artificial parts made out of high quality metal alloys and high molecular weight polyethylene or ceramic. On the side of the acetabulum, it is acetabular component or acetabular shell. It has a hemispherical shape, to match the shape of the native socket, and when inserted into the socket it is most of the time attached to the native bone by either its geometrical structure (press fit) with subsequent bone ingrowth into its rough and often porous surface, or by using an acrylic type of cement as filler. Occasionally screws are used to facilitate better stability of the shell within the patient’s native acetabulum.
The damaged cartilage on the head of the femur is replaced in two different ways. It is done by either resurfacing the head of the femur using so called resurfacing arthroplasty, also known as Birmingham resurfacing arthroplasty (BHR), while removing only superficial portion of the head of the femur and covering it with artificial implant made out of high quality metal alloy (Cobalt Chrome), or by replacing the entire head of the femur with either metal alloy or ceramic artificial head which is attached to the stem introduced into the femur. The stability of this part is secured by either press-fit mechanism or acrylic cement.
Surgical access to the patient’s hip joint is done in a different ways. Traditionally it is done through the backside of the joint, or so called “posterior approach”. More recently direct anteriorapproach has gained popularity. This involves exposure of the hip joint from the front. The patient’s recovery and rehabilitation are faster due to muscle sparing approach.
Prognosis of hip replacement surgery
Hip replacement surgery has a very high success rate. About 95% of patients will enjoy the benefits of the surgery for over 15 years. Longevity of the replaced hip also depends on proper patient education. Return to everyday activities and a variety of sports should be encouraged since it strengthens the muscles and improves the mobility of the joint.