The Perio Perspective: Root Fractures
The Perio Perspective
“Things fall apart; the centre cannot hold.”
WB Yeats, The Second Coming
Our job is to save teeth. This means preserving gingiva, alveolar bone, periodontal ligament, cementum, enamel and dentin. But nothing lasts forever. So our second duty is to restore lost tissues. In doing so, one of our challenges is that restoring lost tissue can require strategic removal of more tissue. An example is osseous surgery, which is the most predictable method for eliminating periodontal pockets, but which results in gingival recession. Other examples include essentially all restorative and endodontic procedures, which preserve the involved teeth but reduce their structural integrity.
At a recent Spear Education seminar, Dr. Greggory Kinzer explored the reasons for loss of endodontically treated teeth. The primary causes are vertical root fracture and failure of the post and core and crown. Why do these happen?
One reason is NOT drying of the tooth due to endodontic treatment, as many of us learned in the past. This does not occur to any significant degree. The true culprit is loss of tooth structure. Several studies have concluded that preservation of structure should be a primary focus, and that vertical root fracture is best avoided using root canal therapy with conservative access preps and minimal instrumentation of the pulp chamber and canals. This is one reason for the recent adoption of Gentle Wave technology by many endodontists.
When crowning teeth, a critical determinant of success is ferrule length. Ferrule is the term for the amount of natural tooth structure coronal to the margin. It does not include restorative materials or buildups, which can be used to increase crown retention but do not add strength. Studies have shown that a minimal ferrule height of 1.5 to 2 mm is essential to avoid failure of the restoration. Since the biologic width averages 2 mm, this means that at least 4 mm of supracrestal tooth structure is ideal. The most important areas of the tooth for ferrule length are the ones under tension during function – the palatal surfaces in the maxilla and the buccal surfaces in the mandible. A chief rationale for crown lengthening surgery is to establish sufficient supracrestal tooth structure. During surgery, biologic shaping of the tooth’s emergence profile can often shift the crown margin coronally to minimize the amount of bone resection. Anyone who performs crown lengthening surgery should be adept at biologic shaping, whose principles have been most clearly presented by Dr. Danny Melker:
The minimal residual root length following crown lengthening is 9 mm. When this cannot be preserved, or when 4 mm of supracrestal tooth structure cannot be achieved, an implant may be a more predictable option.
A second parameter is thickness of axial walls of roots. There are no studies that set a specific dimension, but any procedure that thins these areas will increase the risk of fracture. For example, post spaces should be kept to a minimum.
On the topic of post and cores, there are a few principles to follow. First is to not place a post if there is sufficient tooth structure to retain the restoration. This often includes anterior teeth devitalized by trauma but not fractured. Second is that the post material should be stiff if the ferrule length is marginal. Examples of stiff post materials include zirconia, gold, and stainless steel. Fiber posts are stiff if they are bonded to place, since the bonding material has no flexibility. Dr. Kinzer prefers bonded posts instead of cemented posts when ferrule length is less than ideal. Finally, regardless of post placement, cuspal coverage is indicated whenever the endodontically treated tooth has a large pre-existing restoration.
We are of course not endodontists or restorative dentists, but we thought that this information was valuable. As the tide turns back toward salvaging teeth when possible, rather than reflexively extracting them and placing implants, we advocate any measures that can achieve this.