Effect of thread pitch on pull-out strength of laparoscopic myoma screws

The purpose of this study was to evaluate the effect of thread-pitch on pull-out strength and bending strength of buttress-thread screws designed for laparoscopic myoma extraction.

The ultimate failure load of four 5-mm diameter buttress-thread screws with 3-, 4-, 5-, and 6-mm thread pitch, 40 mm in thread-length were examined on fresh myoma specimens. The myoma tissue at each traction site was evaluated histologically to determine its density. The critical minimal pull-out strength based on moderate-density myoma group was estimated. The bending strength was also determined for each screw.

A wide range of ultimate failure loads with a mean +/- SE of 129.3 +/- 5.5 N (range, 30.4-255.7 N) for all screws and tissue densities was recorded. In moderate-density myomas, the mean ultimate failure loads decreased linearly with increasing thread-pitch from 3 mm (148.0 +/- 9.5 N) to 6 mm (119.8 +/- 9.4 N) (test for trend: P < 0.05). Based on the criterion of a minimum pull-out strength of at least 50 N in not less than 95% of tractions in medium-density myomas, the 3-mm and 5-mm pitch screws were found to have acceptable properties. The 5-mm pitch screw had less thread-turn than the 3-mm pitch for the same thread-length and would need less application time. The bending strength also decreased with increasing thread pitch from 3 to 5 mm, then became stable at around 15 N.

The pull-out strength of soft tissue buttress-thread screws decreased linearly with increasing thread-pitch. Thread-pitch should be considered when designing laparoscopic myoma-screws.