Screw loosening and cemented crowns that fall out are two of the most common complications we encounter in implant dentistry. But it can get a lot worse! Fractured components, cracked porcelain and acrylic, and implant failure are more rare but far more devastating.
Occlusion on natural teeth is often controversial and poorly understood, so imagine the confusion that exists about occlusion on implant restorations!
In a new series called “Implant Occlusion,” I’ll share my philosophies about designing occlusal schemes to preserve our restorations. Many of the restorative complications we encounter in implant dentistry can be traced back to poor occlusal designs.
First we must consider how implants and natural teeth are different.
In a system free from periodontal disease, a tooth is supported from occlusal forces by its periodontal ligament. All three dimensions of forces (apical-coronal, buccal-lingual, and mesial-distal) can press a tooth in a certain direction and will meet resistance from the ligament. Think of it like a cushion. The stress generated is distributed around the tooth for maximum protection. There is a little movement of the tooth in its socket, which is termed micro-motion.
When a tooth is acted upon by a force, the periodontal ligament helps cushion the blow by distributing the stress around the roots.
As a side note,when the tooth has lost attachment from active or past periodontal disease, the tooth loses the ability to defend itself from occlusion. When normal occlusal forces result in trauma to the periodontally-damaged tooth, we call this secondary occlusal trauma. When abnormal occlusal forces result in trauma to a tooth, we call this primary occlusal trauma.
Okay, back to implants. There is no periodontal ligament around an implant, so we lose that cushion effect. All three dimensions of forces will cause very, very small amounts of movement, termed micron-motion. Any give in the system is from the bending of bone. Instead of stress distribution, we see stress concentration.
Since there is no periodontal ligament, occlusal forces are concentrated in two areas around an implant: within the first few threads and the implant-abutment junction.
Numerous photo-elastic studies have revealed that stress is concentrated in two primary areas:
(1) The First Few Threads of the Implant
It has long been accepted for an implant to lose crestal bone to the first thread area and still be deemed to have successful osseointegration. Why do we see that crestal bone loss? Some will say that it is due to the stress concentration from occlusal forces. Others will say it is due to the microgap. I believe the latter, but more on that in another post. Either way, we should recognize that the first few threads of the implant is a stressed location. Where exactly this stress develops depends largely on the particular design of the implant you’re using.
Excessive stress in this area can lead to progressive crestal bone loss and to fracture of the implant.
(2) The Implant-Abutment Junction
The weakest link in a chain is the first to break. The implant-abutment junction is almost always the weakest link in our case. The abutment screw is torqued down to the manufacture’s specifications which aim to resist all three dimensions of occlusal forces. That little screw does a pretty good job!
Excessive stress in this area can lead to loosening or fracture of the abutment screw, loosening or fracture of the prosthetic screw (when one is used), and de-cementation of a cement-retained crown.
So now that we understand how natural teeth and implants react differently to occlusal forces and what some of the complications can be, we can begin to think about how we should design implant occlusion differently. In the next post, we’ll look at how cantilevers can be our best friend or our worst enemy.
If I’ve generated some interest about occlusion and the natural dentition, you may enjoy my e-book: “The TMJ and Occlusal Adjustment”