Why do temporary crowns break?

Temporary crowns are used to protect the teeth between appointments. They cover exposed dentine to avoid dentine sensitivity, protect, fragile margins, hold the teeth in spatial relationship to adjacent teeth to avoid tooth tipping and overeruption. They are also of utmost importance in full mouth cases as they allow diagnosis of occlusal design problems. Early failure of temporary crowns can be an important indicator of insufficienies in preparation design or parafunctional forces in the patient's mouth. Failure of temporary crowns can be a common occurrence and can happen for multiple reasons:

1. Flat preps: i.e not following the curvature of the cusps. if you flatten the cusps only they will be underprepped at the fissure area and the temporaries will be too thin (Figure 1).  Sink your bur in at an angle that matches the incline of the cusps to ensure there is sufficient occlusal reduction (Figure 2).
2. Insufficient reduction in the secondary plane. The secondary plane of reduction will ensure that there is sufficient reduction for thickness of temps at the cusp- facial lingual surface. This is located on the buccal incline of the buccal cusps of the lower teeth and palatal incline of the palatal cusps of the upper teeth. The secondary plane of reduction is important for incisors as the incisal and facial thirds are the most cosmetically important areas and are often underprepped. With insufficient reduction, the technician will either overbulk the surface or there will be show through of the tooth due to thin porcelain leading to a brown area in the final restoration.
3. Undercuts on prep, Undercut of interproximal contour of adjacent teeth, different paths of insertion from anterior to posterior. The will cause issues when you are trying to remove the temporaries in the putty matrix and will break if you force them out. Ensure that your preparation has no undercuts, consider making full arch temporaries in sections to account for differential paths of removal.
4. Rough preparation surfaces result in a surface that is too retentive to the bisacryl material. This may lead to bonding or locking of the temporary crown to the tooth or core. These issues are very unlikely to occur with a smooth prep surface. Ensure there is lubrication over the prep or the prep has been fully cured under glycerin to prevent bonding of the temporary material to the tooth or core. Use a fine grit diamond bur for polishing the restoration. This will not cut tooth well so it can only be used if you are happy with the prep. If the prep is still ugly, use a rough diamond bur. Perform a final polish with a polishing cup under water spray.
5. Temporary crown too thin due to insufficient buildup in mockup. In some cases where the final prosthesis will be thin, the mockup result in temporary restorations that are too thin to survive. You sometimes need to trim the mockup putty or overbuild the waxup to make temporaries that are thicker.
6. Insufficient reduction in preparation: This will result in temporary crowns that are tooth this. Use burs that are the same width that you want in occlusal reduction. For axial reductions, use a bur that is twice the desired width of the prep and sink the bur in halfway. If you use a bur that is the same width as the prep you will end up with a J prep all the way around where there is a small lip at the margin. This poses issues as the intraoral scanner won't pick up the fine margin and if poured up in stone it will break in processing and the crown won't seat. In the initial preparation stage, we tend to make square preps with insufficient reduction at the line angles between the interproximal and buccal/lingual surfaces. Refining involves further reduction to round these angles. In general, soflex discs are very good at making corners round.

Figure 1: The bur used for reduction was held too horizontally which has resulted in a flat prep, either this will result in over preparation at the cuspal areas or underpreparation at the fissure areas. The temporary crown will be too thin at the fissure area  and will often fracture in the midline of the prep.

Figure 2: You must ensure that the bur is held at an angle in occlusal reduction to ensure there is uniform reduction of all cusp inclines.

Figure 3: Ensure the secondary plane of reduction is performed to ensure enough thickness of material in the outer aspects of the cusps.