How to Choose the Right Hip Implant

A hip prosthesis is an implantable medical device that consists of three parts: the femoral stem, the femoral head and the acetabular cup. These three parts replace the damaged hip joint, restoring mobility and relieving pain for the patient.

01.What are the components of a hip prosthesis?

The hip prosthesis consists of three main components:

The femoral stem

after removing the patient's femoral head, the patient's femoral canal is reamed and the femoral stem is inserted. The femoral stem may be cemented or uncemented (press fit technique) depending on the patient's age, morphology, bone idiosyncrasies and the physician's habits.

Femoral Head

A spherical head made of metal, polymer or ceramic is placed on the upper end of the femoral stem to replace the old damaged femoral head that has been removed.

Acetabular prosthesis (or cup prosthesis)

the damaged cartilage from the top of the acetabulum, where the old femoral head was located, is removed. In its place is a tapered acetabular prosthesis. Screws or cement can be used to hold it in place. Inside this cup is a plastic, ceramic or metal inlay that will make contact with the prosthetic femoral head.

02. What are the main materials used for hip prostheses?

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

Metals

Certain metals, such as stainless steel, cobalt-chromium alloy or titanium are used to make femoral stems.

Polymers

polyethylene, a very hard plastic and the most commonly used material in the world. It is an inert and very biocompatible substance that was introduced into orthopedics in the 1960s as a component of cemented acetabular prostheses. Today, this material is still used in some patients, but the downside is that over time, there is a risk that the prosthesis will wear out of the plastic, and therefore the life of the prosthesis will be shortened. However, this risk can still be minimized as some patients can keep this prosthesis for up to 30 years and others for only a few years.

▲Photo: PROCOTYL® L Acetabular Cup (Minimally Invasive Orthopaedic Products: Compatible with Delta Ceramic Liners and A- Class Highly Cross-Linked Polyethylene Liners)

The area of movement between the femoral head and the femoral cup creates what we call the friction moment. It is the weakest part of the prosthesis, especially in terms of wear and tear. There are four possible pairings:

-Ceramic-polyethylene

-Ceramic-ceramic

-Metal-polyethylene

-Metal-metal

Each friction pair has advantages and disadvantages, and the orthopedic surgeon will choose the most appropriate friction combination based on several criteria, including the patient's age, physical activity, and bone specificity.

It is important to note that metal prostheses are generally not recommended. Some companies producing such implants decided to stop selling them in 2010-2011, and for the benefit of patients, decided to recall those implants that were not used. The problem stems from friction between the different elements of the implant, and this friction can dislodge tiny metal particles that then enter the bloodstream. In the hip joint, these small particles can cause an allergic reaction, leading to localized pain and lesions.

03 . What are the main methods of fixation of hip prostheses?

Prostheses can be fixed to the femur or acetabulum by surgical cementing or secondary bone regeneration (uncemented or compression techniques). Commonly, a cemented femoral stem is associated with an uncemented femoral cup. The characteristics of this technique are described below:

Surgical bone cementing technique is used

the bone cement used is an acrylic polymer. It hardens within 15 minutes during the procedure and sets immediately after fixation.

Uncemented or press-fit techniques are used

Uncemented prostheses (prosthetic rods or cups) stabilize after six to twelve weeks due to the phenomenon of bone regeneration. To promote bone regeneration, the surface of the prosthesis is usually coated with a thin layer of hydroxyapatite, a mineral component of bone. Adjacent bone recognizes hydroxyapatite as one of its components and then rapidly grows out of the bony layer of the prosthesis. Hydroxyapatite can be manufactured chemically.

04. How long does a hip prosthesis last?

The service life of prostheses has increased in recent years: in patients under 50 years of age, the proportion of patients whose prostheses are still functioning after ten years of use is approximately 99%.

Similar figures can be observed in older and therefore sedentary patients. Therefore, hip replacement surgery can be performed in patients of all ages.

The service life of the prosthesis depends mainly on the following factors:

-the patient's age, body mass index and activity level

-The diameter of the prosthetic head

-the type of friction moment

In the latter case, it is important to note that the longevity of the prosthesis depends to a large extent on the composition of the prosthesis. When both the femoral head and the prosthetic cup are made of metal or ceramic, the main advantages are the very low wear rate and the possibility of using a wider femoral head, limiting the risk of dislocation. It is important to note that there is a risk of dispersion of debris in the tissue surrounding the prosthesis when metal-to-metal and ceramic-to-ceramic prostheses are paired. Although ceramic-ceramic prostheses break less than metal-metal prostheses and are more resistant to frictional erosion than metal-metal pairs, they should still be used with caution.

05. What are the risks associated with hip prostheses?

In addition to the risks inherent in any surgical intervention (anesthesia risks, hospital-acquired diseases), complications can occur:

Risk of dislocation

this is the main complication in patients and the risk varies over time. It is particularly high in the first months after surgery and decreases after the first year. It then slowly increases again over time. There are several factors that can lead to dislocation, which may be related to the patient, the surgery and implants, or postoperative follow-up. The risk of recurrence increases significantly after the first episode of dislocation.

Risk of infection

any surgical procedure carries a risk of infection, and when a prosthesis is implanted, this risk increases as the foreign body enters the body. In this way, the immune system is diverted and a localized area of immunodeficiency is created. Bacteria that normally have no chance of surviving can then grow on this foreign body. This risk of infection may be more likely in older people because they have poorer immune defenses. Other factors, such as obesity, which complicates interventions, or diabetes, which lowers immunity, and smoking, can increase the risk of infection.

Risk of allergic reactions

some of the materials used in prostheses have the potential to cause allergic reactions.

Risk of revision surgery

Failure, wear and tear, or rupture of the prosthesis may require revision surgery.