Generality
Hip prosthesis is necessary when the joint, which joins the femur and the acetabulum, suffers serious damage. In such circumstances, the application of the prosthesis aims to restore normal joint mobility, which would otherwise be irretrievably compromised.
Figure: the classical model of hip prosthesis. It replaces the entire joint and most of the femur. From the site: hcitalia.it
The intervention 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 of realization and the modalities of application. Medical research aims to extend the life of the prosthesis, to preserve it from the wear of time and to reduce the invasiveness of the intervention.
Short 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 composed of a skeletal scaffold held together by different muscles and ligaments.
The bones, which form the hip joint, are:
- The root (or proximal part) of the femur, composed of the head and the underlying neck
- The acetabulum, a cavity in which the femoral head is positioned
To reduce friction and impact shocks, the hip joint is surrounded by synovial fluid and cartilage . If it did not, bone surfaces would deteriorate due to continuous rubbing between them.
The hip is fundamental, since it allows the man to assume the standing position, to walk, to run etc.
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, the possibility of undergoing hip replacement surgery should be seriously considered. In these cases, it is the persistent pain and the inability to perform the easiest daily activities (standing, walking, driving etc.) to convince the patient to undergo surgery.
The hip prosthesis replaces the natural joint, which is no longer functional.
MOST CAUSES OF ARTICULAR DAMAGE
The most common causes, which cause damage to the hip joint, are three:
- Osteoarthritis . They are the most common arthrosis, characterized by the consumption (by continuous rubbing) of the articular cartilage. For this reason, they are also called "wear arthrosis". 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. Joints pay the consequences: they become stiff, painful and swollen.
- Bone fracture . The hip fracture is one of the most common bone fractures among older people. Spontaneous healing is sometimes not sufficient to restore full joint mobility.
OTHER CAUSES
At the origin of 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 bone disease, which alters growth and bone turnover. The bones become more fragile and are at continuous 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 bony articular elements, which affects the mobile ability of the joint. The disorder is present from birth and has sometimes disabling effects.
WHO DO YOU INTERVENT?
The individuals who undergo most hip replacement surgery are the elderly between the ages of 60 and 80. This is in line with what has just been said regarding the main causes of joint damage to the hip. In fact, osteoarthritis, rheumatoid arthritis and hip fractures are pathological circumstances typical of old age.
The 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 INTERVENTION?
The hip replacement procedure aims to achieve the following goals and benefits:
- Pain reduction
- Improvement of joint mobility
- Improvement of the motor skills of the operated individual
- Significant improvement in the quality of life
How is the surgery performed? The procedure
It is necessary to make a brief introduction. There are three types of hip replacement surgery. However, in this article, only one typology will be dealt with in detail, namely that which provides for the replacement of the entire hip joint and a good part of the femur. The reason is simple: although it is the most invasive intervention, it provides the best results.
The other two methods will be briefly illustrated in the chapter dedicated to alternative interventions.
ANESTHESIA
The hip replacement procedure 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. Whoever chooses this second option, however, is not conscious, since he must take strong sedatives.
THE TRUE AND OWN PROCEDURE
Once anesthesia has occurred, the actual operation begins. The procedure lasts between 60 and 90 minutes and can be divided into three key moments:
- Hip incision
- Damaged joint removal
- 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, serves to house the head and allow it to move without friction and friction. From the site: patologieortopediche.com
Only at this point, we move on to hip replacement with a metal alloy prosthesis. The surgeon begins by attaching an artificial cavity to the pelvis, which acts as an acetabulum. This cavity is called a prosthetic cup (or cup ). Next, apply 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 sits perfectly inside the prosthetic cup.
Finally, to fix the cup and the head of the stem firmly, there are two solutions: either acrylic cement is applied (a kind of glue), or a pressure mechanism is used.
USED MATERIALS
The hip prosthesis is made of different materials. The stem and the cup are of a metal alloy; the insert and the head, on the other hand, can be made not only of metal but also of plastic or ceramic. The construction materials affect the durability and wear of the prosthesis.
The following table shows the possible materials, with which to construct the various parts of the prosthesis, and the possible associations between head and insert.
| Parts of the prosthesis | Materials |
| Cup | Titanium |
| Insert | Polyethylene Ceramic Metal |
| Stem | Titanium alloy |
| Head | Chromium - cobalt - molybdenum alloy Ceramic |
| Possible associations between head / insert | Metal / polyethylene Ceramics / polyethylene Ceramics / Pottery Metal / metal |
There are two strategies for welding the stem to the femur.
The first involves the use of an adhesive, acrylic cement . The welding created is very strong, 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, instead, 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 opting for this type of welding (known as a cementless prosthesis ), prostheses are used, whose stem has very small holes. The holes allow the bone to grow inside, anchoring the prosthesis more. The advantages of the cementless prosthesis consist in easy removal.
CHOICE OF 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 rests with the surgeon, who from time to time makes various considerations relating to:
- Age of the patient
- Body weight and fragility of some materials (ceramic)
- Any patient allergies to the materials (metals) of the prosthesis
- Sex
- Basic pathology
Given its importance, the patient 's age deserves special attention. An elderly patient has no particular needs: the prosthesis may not even be of the longest and not even the non-cemented type. This is because it is difficult to undergo a second operation to replace the worn prosthesis.
On the contrary, instead, a young patient needs a durable and, possibly, non-cemented prosthesis. In this way, in addition to delaying the replacement operation as much as possible, it is also easier to carry out.
Post-operative course, Risks of the intervention, Alternative interventions »
