Current TMJ Implant Options

The world of alloplastic implant options is small you might say.  There are three options on the market for your surgeon to choose from:
1. TMJ Implants
2. TMJ Concepts
3. Biomet/Lorenz

To start, there are several criteria that are considered when designing an implant for use in the TMJ.  This is done to ensure that a situation like the Vitek implant disaster does not happen again, and takes into account what was learned with that experience, as well as what can be applied from hip and knee replacement implants that have been around for decades.  I found an excellent review article in the McGill Journal of Medicine that explains implant design in detail.
(Sinno H, Youssef T, Giladino M, Bobyn D. Engineering Alloplastic Temporomandibular Joint Replacements. MJM 2010 13(1): 63-72.

The following is a summary of the key factors in implant design.

1. Biocompatibility.
The implant has to affix well to the bone to which it is attached, while causing the least amount of damage to the surrounding tissues as possible.

2. Modulus, or stiffness of the material.
The more flexible the implant material, the more it can share the load with the bone it is affixed to. When bone is stressed with a load, the more bone density and strength are preserved in the long run.

3. Stiffness.
This depends on both the flexibility of the material and the shape and thickness of the implant.  The thicker the implant, even if it is made of a flexible material, the stiffer it will be.  This is the reason why, when you look at pictures of the implants, they show little notches in the metal parts.  Putting these notches in, or making it hollow, or very porous, helps to reduce the stiffness of the implant.

4. Stress Shielding.
Similar to, and perhaps a summary of modulus and stiffness, when 2 materials have a uniform load applied to both, the stiffer material will absorb most of that load.  So when an implant is attached to bone, it will absorb the loading and shield bone from the stress.  As mentioned above, this will cause the bone to lose density and strength in the long term.

5. Notch Sensitivity.
This was the hardest for to me to understand, but when an implant is made with notches in it to reduce it's stiffness, or perhaps to encourage the bone to grow into the notches and rigidly fix the implant, those notches are a point of weakness.  So notch sensitivity is is how sensitive the material is to fracture due to the presence of the notches.

6. Modularity.
This enables the use of different materials for different parts of the implant. Materials that are best for wear resistance can be used for bearing components. Materials that are less stiff can be used for load carrying and fixation to reduce the stress shielding and bone resorption that results.

7. Materials.
The same materials that are used in hip/knee replacements are what are used in TMJ implants.  Cobalt-Chromium (Co-Cr) and titanium alloys are useful in the stems and bodies of the implants. There are also a variety of options for porous fixation surfaces.  A new development is the use of highly cross-linked polyethylene, which is a very wear resistant plastic - ultra high molecular weight polyethylene (UHMWPE) that is also being used widely in hip and knee replacements.
Stainless steel is not used, because it can corrode - so it's not biocompatible for the TMJ.

A few notes on Co-Cr, titanium alloys and UHMWPE:
Co-Cr is very hard and abrasion resistant, but can cause significant stress sheilding.
Titanium alloys are more flexible than Co-Cr, but does not have good wear properties.  It is also notch sensitive, so it can't be impregnated with beads or wire to promote bony ingrowth.  It can, however, be roughened at the surface, or coated.
UHMWPE has had long term success in orthopaedic surgery, but is subject to wear after repeated loading.

Comparison of current implants:

The condylar portion of all implants is made with Co-Cr alloy.  The Biomet/Lorenz implants have a plasma spray titanium coating on the bone-contacting surface to help bone fixation. The Co-Cr bearing has high hardness and strength. 

Attached to the condyle is the ramus (below the condyle on the jaw).  In TMJ Implant and Biomet, the ramus is made of Co-Cr alloy.  TMJ concepts uses titanium alloy. The authors of the study thought that since the ramus is a thinner bone, using a titanium alloy would distribute the load better, and reduce stress sheilding.

The ball and socket portion of the implant is the fossa.  The TMJ implants uses metal-on-metal bearings made of Co-Cr.  Both TMJ Concepts and Biomet use UHMWPE.

Overall, the study suggests that Alloplastic TMJ reconstruction devices have shown a lot of predictability and reliability.  Choosing Alloplastic over Autogenous reconstruction provides a few benefits, including custom made implants that allow for immediate jaw function (in most cases, no wiring shut) after surgery. 

Of course, everyone in this "business" knows that these implants are not meant to last forever, and will need replacement at some point in time.  The current suggestion is about 20 years of life, since they are relatively new and every year that goes by, another one lasts another year longer.

It's sure no picnic to sift through all of this information...so I hope it's somewhat helpful in learning more about TMJ implants!