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Implants Used in Orthopaedic Surgery

Implants used in Orthopaedic Surgery by Dr. Derrick Lousaing

Did you know that the materials used in implant manufacture for orthopaedic surgery play an important role in implant fixation? There are several factors which influence the choice of implants used by your Orthopaedic Surgeon. They include rigidity, corrosion characteristics, biocompatibility, tissue receptivity and surface morphology.

Biocompatibility is the prime requisite in orthopaedic implants. It is important that the implant does not adversely interact with the physiological environment in which it is placed and vice versa. Cell and tissue response to bio-materials is partly dependent on the choice of materials, the method of manufacture and the surface characteristics of finishing that is recognized under regulatory bodies.

The surface structure and arrangement affects the stability within the skeleton or within the surrounding cement or bone mantle in which the implant is placed. There are three categories of materials which are used in orthopaedic implants:

1. Metals and Metal Alloys
Metal implants are used extensively and mainly are iron, cobalt, chromium, titanium, and tantalum. The mechanical, biological and physical properties of the materials play a significant role in longevity. The most commonly used orthopaedic metal is stainless steel which accounts for 60% of implants and this is usually 316L stainless steel which has a low-carbon content.

The alloys are various combinations of cobalt, chromium, titanium and tantalum. Each has its own inherent properties and it is up to the manufacturer to assure and reduce the effects of debris formation, corrosion and failure by improper manufacturing processes. This stresses the fact that if metals are used in the body that do not meet regulatory criteria, longevity is severely compromised. In this light, patients should ask their doctors whether the implants used are approved and in what territories and the track record of its use and longevity after implantation.

2. Polymers
Polymers are formed by linking a large number of base elements monomers through a chemical reaction. The most commonly used are ultra-high molecular weight polymers (UHMWP) and high-density polymers. The two most important concerns with the use of polymers in the body are “creep” and progressive wear. Creep is a slow temporary-dependent deformation that takes place under load. Wear is debris degenerated from frictional loading.

There are multiple mechanisms to decrease this process. The most recent and accepted method is the use of antioxidants and vacuum preparation which has decreased both important factors. Here again, regulatory manufactured and international branded products are reliable in this regard.

3. Ceramics
The ceramics that are used in orthopaedics are mainly the aluminium oxide, zirconium oxide ceramics and the calcium phosphate. These materials are very resistant to compression but weak under tension and shear and are therefore brittle. Ceramics are used to actually form implants and have been very successful in that regard with low wear characteristics. Calcium phosphate is used to coat metal implants and improve bone in-growth and integration.

Metal allergy may be more of an arguable myth. Poor implant techniques and poor product selection may be responsible for an adverse reaction in the surrounding rather than a true metal allergy.

Apart from sound technical surgical expertise, lack of regulation has allowed unregulated implants to make their Way freely in to our orthopaedic environment with the consequential compromise to outcomes and longevity. This is particularly important in joint replacement surgery. The patients should therefore demand from their surgeon the information regarding implants used in the body and the origin in order to make an informed and cost conscious choice.