In general, the fabrication of overdentures should be a straightforward process. If we try and follow a simple set of rules during the diagnoses and treatment planning process, these overdenture cases can become more fun for the restorative dentist and produce a better result for our patients.
There are five guidelines to follow when fabricating overdentures: 1) determine if the prosthesis needs to be retentive or supportive, 2) establish a path of draw (parallelism), 3) determine whether you will need a resilient or rigid attachment, 4) evaluate patients ability to perform oral hygiene adequately, and 5) establish a flat plane of occlusion. Let's look at each of these more closely. The key to all of these overdenture cases is to start by articulating the case and carefully working up the patient just as you would if it were a full denture case. To determine the issue of whether the overdenture will require a retentive vs. supportive prosthesis, begin by having the patient place their existing prosthesis in place. If this prosthesis moves when the patient bites on cotton rolls, or if it loosens after time when adhesive is placed, or loosens upon sneezing, the overdenture needs to be retentive. In retentive cases two implant fixtures can work. On the other hand, if their existing prosthesis hurts them upon biting or they are experiencing chronic soreness, the new overdenture will have to be supportive in nature. This means that at least three implants will be required and that these implants should be splinted together.
If implant placement is going to result in difficulty achieving a path of draw for your attachments, then you will have two choices: either the attachments you have chosen need to be able to allow for this amount of angulation adjustment or you will need to consider fabricating a bar. Correcting differences in angulation greater than 100? are easily handled by fabricating a bar. Angulation problems that are between 70? and 100? should be corrected with a resilient attachment like a Brident, ERA or Straumann ball/anchor attachments. If the angulation problem is between 30? and 70?, this can be corrected with a rigid (metal) attachment (e.g. "O" rings). Smaller angulation problems of less than 30? can be easily handled with ERA attachments.
The issue of resilient vs. rigid attachments can be solved by knowing the quality of bone and number of implants your surgeon is placing for you. Resilient attachments will be necessary when you will be utilizing fewer, short and thinner implants in a poor quality of bone (Type III & IV). Rigid attachments can be used when more, longer and wider diameter implants are utilized in stronger bone (Type I & II).
When assessing the issue of the patients' ability to perform homecare, it is important to create a situation that will allow patients easy accessibility around whatever attachment you have created. In general the bars are more difficult to clean around, especially for our elderly patients. One indication about the patients' ability to perform adequate oral hygiene can be evaluated while the implants are healing. If, at subsequent post-operative/follow-up visits, you note plaque and calculus gathering around the healing screws/abutments then you may want to consider individual attachments which allow ease of cleansibility.
Finally building the ideal occlusion into your overdenture cases will afford you the greatest opportunity for long-term predictability with you overdenture cases. This translates to a flat occlusion which allows for total freedom in all excursive movements. There should be no cuspid rise in these cases with occlusion to be rebuilt just as in the traditional denture case.
Here are some answers to commonly asked questions I get regarding overdenture cases:
How do I know when I should consider a case that should be for an overdenture? Generally there are two reasons to consider fabricating an overdenture: 1) problems in the horizontal dimension & 2) loss of vertical dimension as the mandible resorbs down and forward (the patient looks like they are over closed or class III).
What if I need to keep teeth as well as implants to incorporate in the overdenture? Teeth mean that you will want to consider utilizing resilient attachments and not use a bar.
Where can I go to learn more about restoring my overdenture cases? Two of the more comprehensive articles written about overdenture restorations are: Journal of Prosthetic Dentistry, 1/98 Zarb & Stern and Journal of Oral and Maxillofacial Implants 7/98, Vol 13, #4 Zarb.
Are there any easy to use, affordable overdenture attachments out there for me to use? Yes, check out the relatively new "Locator" from Zest.
What are some of the DOs and DON'Ts regarding Hader bars? Hader bars are a resilient attachment (not rigid) that allow rotation around the bar (a posterior to anterior rotation of the denture). They can be very effective in utilization with shorter or thinner implants in poor quality of bone. The prefabricated bars are best. The lab waxes and solders to it and this often results in the passive fit that is crucial to success of the case. When delivering the bar have your assistant screwing down one side while you are simultaneously screwing down the other side to secure a passive fit of the bar. You can verify this with GC fit checker. You need to have at least 12mm of interocclusal distance to consider utilizing a Hader bar. Fabricating a broad, flat bar will ensure that you are gaining support from the bar for your overdenture.
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