The effect of numbers and locations of retentive holes placed on master casts on reducing the polymerization distortion of the maxillary complete denture

Polymerization shrinkage is one of the disadvantages of Polymethyl Methacrylate, resulting in distortion of resin denture bases. One of the methods, designed to reduce this distortion, is mechanical anchoring to the cast. This study aimed to evaluate the effect of numbers and locations of retentive holes on reducing polymerization distortion. This was an experimental study, including 48 similar master casts in 4 groups, fabricated during the study. A power analysis using a power factor of 0.8 and a significance level of 0.01 determined the sample size of 12 casts per each test group. Groups were assigned as: A (control); B (one anchoring hole in midline along the posterior border); C (additionally, two extra anchoring holes in lateral palate, between midline extending to ridge crest); D (two more anchoring holes along the midline, within 15 mm and 20 mm from the posterior border(. An Olympus-SZX9 measuring microscope was used for the gap measurement across the denture base and cast interface. Variables were the gap distance between the denture base and the cast in midline, in PPSA and the mean gap in right and left lateral palate in PPSA. The data were analyzed with one-way ANOVA followed by the Post-Hoc test. Variables were significantly different in four studied groups (p=0.000) and the gap distance between the denture base and the cast in mid-palate was less affected than lateral palate in anchorage technique. In general, anchoring significantly reduced mean gap distances (p=0.000). Generally, anchoring during processing, improved adaptation of the denture base to the master cast. Mechanical anchorage between denture base and cast significantly reduced the gap distances in lateral and mid-palate; and this decrease was significantly affected by numbers and locations of anchoring holes.