Generality
The hip prosthesis is necessary when the joint, which joins the femur and the acetabulum, is seriously damaged. In such circumstances, the application of the prosthesis aims to restore normal joint mobility, which would otherwise be irremediably compromised.
Figure: the classic model of hip prosthesis. It replaces the entire joint and a large part of the femur. From the site: hcitalia.it
The operation is invasive and requires careful rehabilitation, but the results are more than satisfactory: the patient, in fact, returns to lead a normal life, without particular limits.
There are different models of hip prostheses: what varies are the material used and the methods of application. Medical research aims to extend the life of the prostheses, to preserve them from the wear of time and to reduce the invasiveness of the intervention.
Brief anatomical reference: the hip
The term hip identifies both the anatomical region of the human body that connects the trunk to the lower limbs, and the joint that resides in this position. The hip joint, or more simply the hip, is made up of a "skeletal scaffold held together. from different muscles and ligaments.
The bones, which form the hip joint, are:
- The root (or proximal part) of the femur, consisting of the head and underlying neck
- The acetabulum, a cavity inside which the head of the femur is positioned
To reduce friction and impact shocks, the hip joint is surrounded by synovial fluid and cartilage. If it were without it, the bone surfaces would deteriorate, due to the continuous rubbing between them.
The hip is fundamental, as it allows the man to assume an upright position, to walk, to run and so on.
When is it necessary to intervene?
Like any joint, the hip can also be damaged. When this happens, the first therapeutic measure consists of conservative treatment (rehabilitation, physiotherapy and painkillers). However, if the extent of the damage is significant or chronic, it should be seriously considering the possibility of undergoing hip replacement surgery. In these cases, persistent pain and the inability to carry out the easier daily activities (standing, walking, driving, etc.) convince the patient to have surgery.
The hip prosthesis replaces the natural joint, which is no longer functional.
MOST FREQUENT CAUSES OF JOINT DAMAGE
The most common causes, which determine damage to the hip joint, are three:
- Osteoarthritis. They are the most common osteoarthritis, characterized by the consumption (by continuous rubbing) of the articular cartilage. For this reason, they are also called "wear-and-tear osteoarthritis". The patient, usually elderly, experiences pain and motor difficulties.
- Rheumatoid arthritis. It is an autoimmune disease, in which the immune system, instead of defending the body from infections, "turns" against it. The consequences are the joints: they become stiff, painful and swollen.
- Bone fracture. Hip fractures are one of the most common bone fractures in older people. Spontaneous healing is sometimes not enough to restore full joint mobility.
OTHER CAUSES
At the origin of a hip damage, there may also be other causes, less common than the previous ones. One of these is septic arthritis, which is a "bacterial inflammation of the joint". Another is avascular necrosis, due to "alcohol abuse." Another is Paget's disease of bone, which impairs growth and bone turnover. The bones become more fragile and are at ongoing risk of fracture.
Finally, there are bone tumors and congenital dysplasia of the hip. The latter, in particular, is characterized by an anomalous arrangement of the articular bone elements, which affects the mobility of the joint. The disorder is present from birth. and it has sometimes disabling effects.
WHO UNDERGOES THE INTERVENTION?
The individuals, who undergo the most hip prosthesis surgery, are the elderly aged between 60 and 80 years. This is in line with what has just been said, regarding the main causes of joint damage in the hip. In fact, osteoarthritis, rheumatoid arthritis and hip fractures are typical pathological conditions of old age.
Intervention on young adults and children is rare. The most frequent cause in these situations is congenital dysplasia of the hip.
WHAT ARE THE BENEFITS OF THE INTERVENTION?
Hip prosthesis surgery aims to achieve the following objectives and benefits:
- Pain relief
- Improvement of joint mobility
- Improvement of the motor skills of the operated individual
- Significant improvement in the quality of life
How is the operation performed? The procedure
A brief introduction must be made. There are three types of hip prosthesis surgery. However, in this article, only one type will be dealt with in detail, namely the one that involves the replacement of the entire hip joint and a large part of the femur. The reason is simple: although it is the most invasive procedure, it provides the best results.
The other two methods will be briefly illustrated in the chapter dedicated to alternative interventions.
ANESTHESIA
Hip replacement surgery is usually performed under general anesthesia. However, it is also possible to opt for an epidural anesthesia, in which only the lower part of the body is insensitive to pain. Those who choose this second option are not conscious anyway, as they must take strong sedatives.
THE REAL PROCEDURE
After the anesthesia has taken place, the actual operation begins. The procedure lasts between 60 and 90 minutes and can be divided into three key moments:
- Incision of the hip
- Removal of the damaged joint
- Replacement with an "artificial joint
When the damaged joint is removed, the upper part of the femur (head, neck and a piece of the body) and the portion of the acetabulum, in which the femur itself is housed, are removed.
Figure: the various parts that make up a hip prosthesis. The insert, that is the part contained in the cup, is used to house the head and to allow the latter to move without friction and friction. From the site: pathologieortopediche.com
Only at this point is the replacement of the hip with a metal alloy prosthesis. The surgeon begins by fixing an artificial cavity to the pelvis, which acts as an acetabulum. This cavity is called a prosthetic cup (or cup). applies the so-called prosthetic stem. One end of the stem is made to weld to the remaining femur; the other end has a head, very similar to that of the femur, which fits perfectly inside the prosthetic cup.
Finally, to firmly fix the cup and the head of the stem, there are two solutions: either you apply acrylic cement (a kind of glue), or you use a pressure mechanism.
USED MATERIALS
The hip prosthesis is made of different materials. The stem and the cup are of a metal alloy, while the insert and the head, on the other hand, can be made of plastic or ceramic as well as metal. The materials of construction affect the duration and wear of the prosthesis.
The following table shows the possible materials with which to build the various parts of the prosthesis, and the possible associations between head and insert.
Polyethylene
Ceramic
Metal
Chromium - cobalt - molybdenum alloy
Ceramic
Metal / polyethylene
Ceramic / polyethylene
Ceramic / ceramic
Metal / metal
There are two strategies for welding the stem to the femur.
The first involves the use of an adhesive, acrylic cement. The weld created is very strong, so much so that the removal of the prosthesis, when it is worn, is difficult and becomes a problem. This option is known as a cemented prosthesis.
The second strategy, on the other hand, consists in creating, first of all, a housing, in the femur, perfect for the stem; and, then, in "inserting the latter" with a pressure mechanism. When you opt for this type of welding (known as cementless prosthesis), you use prostheses, whose stem has very small holes. The holes allow the bone to grow inside, further anchoring the prosthesis. The advantages of the cementless prosthesis are the easy removal.
CHOICE OF THE PROSTHESIS
There are more than 60 different models of hip prostheses. However, less than ten are actually used. The choice of the most appropriate prosthesis is up to the surgeon, who from time to time makes various considerations relating to:
- Age of the patient
- Body weight and fragility of some materials (ceramic)
- Possible allergies of the patient to the materials (metals) of the prosthesis
- Sex
- Basic pathology
Given its importance, the age of the patient deserves special attention. An elderly patient has no particular needs: the prosthesis may not be the longest-lived and not even of the non-cemented type. This is because it is unlikely that you will undergo a second surgery to replace the worn prosthesis.
Conversely, however, a young patient needs a durable and possibly uncemented prosthesis. In this way, in addition to postponing replacement operations for as long as possible, it is also made easier to carry out.
Post-operative course, risks of the "intervention, alternative interventions"
