Post Placement and Use of Multiple Posts
When to use a post
Posts are designed to provide retention for restorations when there is insufficient remaining tooth structure to retain the core. Studies have shown that a single properly placed post is adequate to retain a core. This article will not deal with the controversy of optimal post length but will focus on post placement.
There is still much misinformation about the function of the post. Post placement does NOT strengthen teeth. Posts weaken teeth due to the need to machine the post space to reasonably accommodate what is in many cases a prefabricated post. "Screw" posts cause even more stress on the root and are aptly named. Cast posts (while well fitting) have recently undergone criticism for their association with high rates of root fracture in anterior teeth.
POST PLACEMENT DECISIONS
As an endodontist, my primary concern is producing predictable successful endodontics. However, there are occasions when endodontic treatment is not successful. Conventional retreatment has a reasonably good rate of success, though obviously not as high as initial conventional treatment. Depending on the individual case and circumstance, I often tell patients that non-surgical retreatment prognosis is about 75-80% successful. Those cases that cannot be treated successfully in a non-surgical manner must be managed surgically. Because no treatment mode is 100% successful I believe that when endodontic treatment is performed, it should always acknowledge the possibility that retreatment may be necessary in the future. Every effort should be made to (1) allow for retreatment with the least damage to the tooth and to (2) minimize the possibility of the need for surgical intervention. This means preparing posts spaces in canals that have the highest rate of success. Where possible we should avoid post placement in canals where surgical retreatment may result in severe compromise of tooth structure .
This philosophy may conflict with some of the prosthodontic opinions that appear to favor placement of posts to minimize occlusal load or to best take advantage of remaining tooth structure. It has always been my opinion that the three most important factors that ultimately determine the disposition of a case are ( in order) (1) the Periodontal condition (2) the Endodontic condition (3) the restoration. Without adequate periodontal health, the endodontic and restorative results are meaningless. Similarly, in a periodontally healthy tooth without endodontic success, the restoration of the tooth is also irrelevant, since the tooth is not comfortable. Lastly, where both the periodontal and endodontic conditions are favorable, the restorative treatment usually determines success. It also has the most flexibility. It is this area of post placement during restoration that I wish to discuss.
The palatal root should be used, mainly because it generally has the greatest root thickness, is relatively straight in the coronal third, has a high level of endodontic success and runs less risk of fracture should the post need to be removed.
Post selection in Maxillary Molars must always bear in mind that palatal roots invariably curve to the buccal aspect, especially in the apical third. Improper depth and size of post placement in these canals runs the risk of palatal perforation when placed too deep or strip perforation of the buccal aspect when too thick a post is used. This is extremely difficult to detect radiographically because the post may look perfectly centered in the canal. The only indication may be a furca radiolucency or pocketing, draining sinus and/or percussive sensitivity. In some rare situations, it may be preferable to use the DB root, which is often straighter. Post diameter must be conservative in that case.
Under no circumstances should the MB root be used because of its curvature and high risk of perforation. Post placement in MB1 can also affect nonsurgical retreatment attempts when MB2 is missed (especially when the canals are joined). Failure to treat MB2 is one of the most common reasons for failure of conventional maxillary molar endodontics.
Where two roots are present, Maxillary premolars should have a post placed in the buccal canal. The reasoning is that in the event of treatment failure it may be possible to conservatively retreat the palatal canal. Care must be exercised in first premolars because of the mesial concavity. A high percentage of these teeth become endodontically involved because of mesial marginal ridge cracks that extend down the mesial root face. Non-passive Post placement can exacerbate cracks in this area. Placement of posts in the palatal root often means resecting a perfectly good buccal root to gain access to the failing palatal root. Should the posted buccal canal need surgical access, the buccal root is easier to locate and prepare. In that case the healthy palatal root length is conserved. A similar rule can be used in the case of Mandibular teeth with buccal and lingual canals.
In the case of Mandibular molars, posting of the mesial roots should be avoided at all costs. This root often has a concavity on the distal aspect that may not be readily apparent in a radiograph. Estimation of root thickness in this area can be difficult when using films. This is one of the most common reasons for post-treatment furca involvement in the case of apparently successful but restored endodontic treatment.
Maxillary Anteriors and Cuspids
These teeth represent minimal risk because of their relatively round shape, reasonable dentin thickness and symmetry. Most problems that occur in these teeth are related to improper post size selection and angulation of post placement. Fortunately, a high percentage of these perforations are labial and can often be surgically corrected if they do not cause concurrent periodontal problems. Because of their relatively small roots, post size selection in lateral incisors is especially critical.
Mandibular anteriors are often figure 8 shaped in cross section. Many have two canals. Serious consideration must be given as to the risks of post placement in these thin roots. Post size selection is critical. Clinicians must be supremely confident in the endodontic treatment before proceeding with post placement since placement of a post often commits the tooth to surgical correction if the endodontic treatment fails.
Endodontic Retreatment Before Post Placement: - Yes or No ?
Before proceeding with placement of any post, a critical analysis of the Endodontic treatment must be performed. Periapical radiography must examine for any apical or lateral lesions. It must also examine the quality, density and position of the Endodontic filing. Lateral canals that may be affected by post preparation must also be considered. Where a demonstrably filled lateral canal is present, every attempt should be made to avoid breaking the Endodontic seal by post preparation. The post preparation should be shortened to a level coronal to the accessory canal or another canal should be selected for post placement. It is important to remember that once a post has been placed, the case will require disassembly retreatment or surgery if the endodontics fails. When in doubt, retreat before post placement.
Method of Post Space Preparation
The safest method for post preparation initially involves removal of the gutta percha with a heat carrier. Small handpiece driven Gates Glidden burs can be used to cleanse gutta percha remnants from the canal walls. Under no circumstances should large diameter burs (such as Peeso reamers and higher speed twist drills) contact the entire circumference of the canals since this can risk strip perforation. Final preparation of the post space is most safely performed with hand driven reamers or hand driven para-post drills.
Although Endodontists have recently had very promising results in sealing perforations with the new Mineral Trioxide Aggregate (MTA), the material is quite expensive to use and not always successful. The best method of ensuring success is avoiding these thin areas of tooth with post drills and understanding root anatomy.
Unconventional Post Use
Posts can be abused. Posts should not be used "through" an access opening made in crowns or as a method of uniting previously carious roots to existing prosthetics. Placement of a post in an intact natural crown is simply malpractice but is surprisingly common. At best it represents a misunderstanding of the concept of post use. At worst, it unnecessarily obligates any retreatment to a surgical mode.
How Many Posts? - Financial Implications
With the development of better core materials, higher levels of dentin bond strength and better seal, the need for multiple posts has diminished. Placement of multiple posts runs a high risk of perforation and fracture. This is especially hazardous in the case of the MB root of maxillary molars, mesial roots of mandibular molars and mandibular incisors. Unfortunately, insurance reimbursement is often based on the number of posts placed, so there is an obvious financial incentive for the restoring dentist to place the maximum number of posts. Dentists must resist the urge to place posts in this manner, regardless of the financial consequences. Aside from providing little additional retention, use of posts in this way risks perforation, increases the risk of fracture and makes endodontic retreatment and disassembly much more complicated.
Posts are an excellent adjunct to the restorative armamentarium. However, they should be used only when insufficient core material is present to allow for retention of the restoration. Proper post selection takes into account all anatomic variables that influence the size and shape of the post.
One of the Endodontist's biggest problems is the post perforation. It is even more frustrating when the case has been successfully treated but iatrogenically damaged by improper post selection or placement. No clinician Endodontically treats a tooth with the expectation that it will fail. However post placement should always take into consideration the possible need for non-surgical or surgical Endodontic retreatment.