Choosing the right hip prosthesis

A hip prosthesis consists of three main components:

• The femoral stem: the patient’s femoral head is removed, the femoral canal is reamed to insert it. The femoral stem can be cemented or not cemented (press-fit technique), this depends on the patient’s age, morphology, bone specificity and the surgeon’s habits.
• The femoral head: a spherical head made of metal, polymer or ceramic is placed at the upper end of the femoral stem to replace the old damaged femoral head that has been removed.
• The acetabular prosthesis (or cup): the damaged cartilage of the acetabular roof where the old femoral head was located is removed. It is replaced by a conical acetabular prosthesis. Screws or cement can be used to hold it in place. Inside this cup shape is a plastic, ceramic or metal insert, which will be in contact with the femoral head of the prosthesis.

Hip prostheses can be differentiated according to the materials used to make them. Currently, these materials are divided into three types:

• Metal: certain metals such as stainless steel, cobalt-chrome or titanium are used to make the femoral stem.
• Polymers: Polyethylene, which is a very hard plastic and the most commonly used in the world, is the main material used in this category. It is an inert and perfectly biocompatible substance that was introduced in orthopedics in the 1960s as a component of the cemented acetabulum prosthesis. Today this material is still used for some patients but the disadvantage is the risk of plastic wear over time and therefore a shorter prosthesis lifespan. However, this risk remains mitigated because some patients have been able to keep this type of prosthesis for 30 years and others for only a few years.
• Ceramics: Ceramics are a good alternative because of their strength. A titanium cup can be used with a ceramic insert.

The movable area between the femoral head and the cup forms what we call friction torque. It is the most fragile part of the prosthesis, particularly in terms of wear. There are four possible pairings:

• Ceramic-polyethylene
• Ceramic-ceramic
• Metal-polyethylene
• Metal-metal

Each friction pair has advantages and disadvantages, and it is up to the orthopedic surgeon to choose the most appropriate one based on several criteria such as the patient’s age, physical activity, bone specificity, etc.

It should be noted that, in general, the use of metal on metal implants is not recommended. Some of the companies that manufactured them decided in 2010/2011 to stop marketing them, and in the interests of patients, to recall those not being used. The problem stems from the friction between the different elements of the implant that can detach small metal particles that then enter the bloodstream. In the hip joint, these small particles can cause allergic reactions leading to localized pain and lesions.

As we saw in question 1, prostheses can be attached to the femur or acetabulum either by surgical cement or by secondary bone regrowth (non-cemented or press-fit technique). It is common to associate a cemented femoral stem with an uncemented cup.

Zimmer cemented hip prosthesis

The characteristics of each of these techniques are as follows:

DePuy Synthes uncemented prosthesis

• By surgical cement: the surgical cementused is an acrylic polymer. It hardens in 15 minutes during the operation and its fixation is immediately solid.
• By non-cemented or press-fit technique: cementless prostheses (stem or cup) stabilize after six to twelve weeks thanks to the phenomenon of bone regrowth. To promote thisbone regrowth, the surface of the prosthesis is often coated with a thin layer of hydroxyapatite, a mineral component of bone. The adjacent bone then identifies hydroxyapatite as one of its components and quickly grows back onto the osteoconductive coating of the prosthesis. Hydroxyapatite can be chemically manufactured.

In recent years, there has been an improvement in the lifespan of prostheses: the proportion of prostheses that are still functional after ten years of use is around 99% in patients under 50 years of age.
In older, and therefore more sedentary, patients, similar figures can also be observed.
Hip replacement is therefore still possible for patients of all ages.

The lifetime of the prosthesis mainly depends on the following factors:

• A patient’s age, BMI and degree of activity
• The diameter of the prosthetic head
• The type of friction torque (“hard-soft” or “hard-hard”)

In the latter case, it is important to note that the longevity of a prosthesis is very much due to its composition. The main advantages of the “hard-hard” pairing (when the femoral head and prosthetic cups are both made of metal or ceramic) are a very low wear rate and the ability to use wider femoral heads, which limits the risk of dislocation. It should be noted that with metal-metal and ceramic-ceramic pairings, there is a risk of dispersal of debris in the tissues surrounding the prosthesis (see question 2). Although ceramic-ceramic prostheses suffer less breakage and are more resistant to erosion caused by friction than metal-metal pairings, their use still calls for caution.

In addition to the risks inherent to any surgical intervention (anesthetic risks, nosocomial diseases), complications can occur:

• Risk of dislocation: this is a major complication for the patient and the risk varies with time. It is particularly high during the first few months after surgery and then decreases once the first year has passed. It then slowly increases again over time. Several factors can cause dislocation. They can be related to the patient, surgery and implants, or post-operative follow-up. Following a first dislocation episode, the risk of recurrence is significantly higher.
• Risk of infection: any surgical procedure involves an infectious risk, but this risk is increased when a prosthesis is inserted because a foreign body is introduced into the body. The immune system is then diverted and a localized immunodeficiency zone can be created. Bacteria that would normally have no chance of surviving then manages to grow on this foreign body. This risk of infection can be more prevalent in the elderly because their immune defenses are less effective. Other factors such as obesity, which complicates interventions, or diabetes, which reduces immunity, as well as tobacco, can increase the risk of infection.
• Risk of allergic reaction: there is a risk of allergic reaction related to certain materials the prosthesis is made of.
• Risk of revision surgery: malfunction, wear or rupture of the implant may require revision surgery.