GEOMETRY, PENETRATION FORCE, AND CUTTING PROFILE OF DIFFERENT 23-GAUGE TROCARS SYSTEMS FOR PARS PLANA VITRECTOMY

Background: To investigate the geometry, penetration force, and cutting profile of 23-gauge trocar systems for pars plana vitrectomy based on their grinding methods in a standardized laboratory setting. Methods: In this experimental study, Eleven different commercially available 23-gauge sclerotomy trocar systems were divided into 4 groups according to their needle grinding and deburring: "back" bevel, "spear" bevel, "lancet" bevel, and "spatula" bevel. The normative geometrical data of the trocar systems were systematically analyzed according to nomenclature ISO 7864 and ISO 9626. Force to penetrate a 0.4-mm thick polyurethane foil was measured by a Penetrometer, when the trocar needle was piercing, cutting, and sliding through the foil at different defined loading phases and plotted as a load-displacement diagram. Magnified images of the consecutive cut were taken under a microscope after the entire penetration through the foil. Three physicians used all trocar systems in a masked fashion on human sclera to evaluate the manual penetration force in 30 degrees and 90 degrees. Results: The mean outer diameter of the trocar systems was 0.630 +/- 0.009 mm, and the mean outer diameter of the trocars was 0.750 +/- 0.013 mm. The mean point length was 3.11 +/- 0.49 mm, and the mean length of the bevel was 1.46 +/- 0.23 mm. The primary bevel angle was 10.75 +/- 0.41 degrees, and the secondary bevel angle was 65.9 +/- 42.56 degrees. The piercing forces of the back bevel and spear-pointed trocars/needles were at the same level (0.087 +/- 0.028 N). The lancet-pointed needle had remarkable low piercing and cutting forces with 0.41 N (range, 0.35-0.47 N). The spatula bevel tip showed the highest penetration piercing force with 1.6 N (range, 1.59-1.73 N). The back bevel systems induced frequently triangular-shaped incisions, with two nearly rectangular cuts of short length. The spear bevels produced a regular characteristic linear cut. Especially, the lancet blade created straight cut with a linear wound apposition. Spatula trocar systems caused usually an arched accurate incision. The manual force to penetrate the human sclera in an angled and rectangular angle appeared in the surgeons hand lower with a back bevel, lancet, or spear tip, whereas higher with spatula bevel systems. Conclusion: Lancet and back bevel systems show less penetration force of inner needles than the spatula systems. The results of the penetration force experiments correlated well with the manual force on sclera.