Trimline Expectations

The Trimline is about retention but there are exceptions. There are several dental conditions which may serve to increase the retention to the point that it is undesirable and pose problems for aligner insertion and or removal. This is an area where clinicians need to be vigilant as the digital technician may not have all the information or the detail of information to see these conditions. It is important that the clinician requests a change in the Trimline and an explanation in the prescription. These include black triangles as the area of interdental papilla loss may be significant enough to cause problems with plastic engaging in these areas and causing problems. The clinician may choose to block off such areas with rope wax prior to scanning or the impression or ask the technician to digitally block out these areas.

Similarly, areas of recession and abfraction will need to be identified by the clinician and included in the prescription in order for the technician to provide a Trimline that does not engage these areas. If an aligner is designed so that it contacts the areas of exposed cementum, the patient may complain of sensitivity in these areas. Issues with the path of insertion or path of draw such as severely proclined teeth or malocclusions with teeth pointed in opposing directions are also exceptions. A clinician may opt for a lowered Trimline or alternatively accepting the Trimline as is and sectioning the aligner to aid in insertion and removal. Another situation is around the gingival contour of implants. The narrowing of the clinical crown to adapt to the head of the abutment may create excessive undercut and these areas will need to be requested to be blocked out.

Another situation with excess undercut is under some bridges and as with PVS impressions around bridges, this undercut will need to be blocked out. Low frenum may be irritated by the edge aligner resulting in abrasion and ulceration. Again, this needs to be identified by the clinician as the technician does not know from the photos if the frenum is mobile during function or not. Lastly, there are anatomic variations that result in very bulbous crowns or perhaps restorations that make them so, but the additional undercut may also produce excessive retention of the aligner.