Factors regulating the gripping force and stiffness of 25- and 27-gauge internal limiting membrane forceps

Abstract

This study aimed to identify the factors affecting the gripping force and stiffness of 25-gauge and 27-gauge (25G and 27G, respectively) internal limiting membrane (ILM) forceps and to compare the effect of these factors on various ILM forceps manufactured by different companies. This study evaluated 25G and 27G ILM forceps with two different types of tip shapes, Eckardt and Maxgrip, manufactured by Alcon (A), DORC (B), VitreQ (C), and Katalyst (D). The gripping force was defined as the force required to move the ILM forceps away from a thin paper by pulling the paper. Shaft stiffness was determined by measuring the shaft displacement under a known force. Multiple regression analysis revealed that the gripping force showed significant correlations with the gauge (P<0.001), type of shaft tip (Eckardt/Maxgrip) (P<0.001), and contact area of the tip (P<0.001). The shaft stiffness showed significant correlations with the gauge (P<0.001), length of the base (P<0.001), thickness of the metal of the shaft (P = 0.05), and lumen area of the shaft (P = 0.01). The gripping force and shaft stiffness differed for each product. Thus, vitreoretinal surgeons must select the appropriate type of ILM forceps based on their characteristics.

Fig 1. Schematic representation depicting the evaluation of the gripping force and shaft stiffness of the ILM forceps.

The gripping force was measured by pulling a 40-μm thin paper with each ILM forceps. The stiffness of the ILM forceps was evaluated by measuring shaft displacement under a force of 0.5 N at 20 mm from the shaft base. Shaft stiffness is proportional to the added force and cube of the distance from the base to the point of force, whereas it is inversely proportional to Young’s modulus and the moment of inertia of area. ILM, internal limiting membrane.

Fig 2. Schematic of the structure of the ILM forceps.

For the tip, the type of shaft tip (Ek or Mx) and the width of the contact area were measured. For the shaft, the lengths of the shaft and base, as well as the external and internal diameters of the shaft, were measured to determine the metal thickness and the lumen area. ILM, internal limiting membrane.

Fig 3. Comparisons of the gripping force and shaft stiffness among the 25G forceps.

For Companies A, C, and D, the gripping force of the Mx forceps was significantly higher than that of the Ek forceps (**p<0.01). No significant difference was observed for Company B. For Company A, the shaft stiffness of the Ek forceps was significantly lower than that of the Mx forceps (*p<0.05). No significant difference was observed for Companies B, C, and D. Ek: Eckardt type, Mx: Maxgrip type.

Fig 4. Comparisons of the gripping force and shaft stiffness among the 27G forceps.

The gripping force of the Mx forceps was significantly higher than that of the Ek forceps from all companies (**p<0.01, *p<0.05). For Companies A and B, the shaft stiffness of the Ek forceps was significantly lower than that of the Mx forceps (**p<0.01). No significant difference was observed for Companies C and D. Ek: Eckardt type, Mx: Maxgrip type.

Fig 5. Comparisons of the gripping force and shaft stiffness among the 25G Ek forceps from different companies.

(A) The gripping force of the forceps manufactured by Company B was significantly higher than that of the forceps manufactured by Company A (P<0.01). (B) The shaft stiffness of the forceps from Company C was greater than that of the forceps from Company A (P<0.01). Ek: Eckardt type.

Fig 6. Comparisons of the gripping force and shaft stiffness among the 27G Ek forceps from different companies.

The gripping force and shaft stiffness of the forceps manufactured by Company D were the highest. The gripping force and shaft stiffness of the forceps manufactured by Company C were both higher than those of the forceps manufactured by Companies A and B (P<0.01). Ek: Eckardt type.

Fig 7. Comparisons of the gripping force and shaft stiffness among the 25G Mx forceps from different companies.

(A) The forceps from Company A had a significantly higher gripping force than did the forceps from Companies C and D (P<0.01). (B) No significant differences in the shaft stiffness were observed among forceps from different companies. Mx: Maxgrip type.

Fig 8. Comparisons of the gripping force and shaft stiffness among the 27G Mx forceps from different companies.

(A) The forceps from Company A had the highest gripping force among forceps from all companies (P<0.01), whereas the forceps from Company C exhibiting the lowest gripping force (P<0.01). (B) The forceps from Company D had the highest shaft stiffness among forceps from all companies (P<0.01), whereas the forceps from Company A had the lowest shaft stiffness (P<0.01). Mx: Maxgrip type.

Fig 9. Comparisons among 25G Mx forceps from different companies.

Magnified photographs showing the cross-sections of the shafts and contact areas of the tips of the 25G Mx forceps, as viewed from the front and the side for forceps from each company. The metal thickness of the shaft and the structure of the contact area vary among forceps from different companies (A: Alcon; B: DORC; C: Katalyst; D: Vitre Q). Mx: Maxgrip type.

Fig 10. Comparisons among 25G Ek forceps from different companies.

Magnified photographs showing the cross-sections of the shafts and contact areas of the tips of the 25G Ek forceps, as viewed from the front and the side for forceps from each company. The metal thickness of the shaft and the structure of the contact area vary among forceps from different companies (A: Alcon; B: DORC; C: Katalyst; D: Vitre Q). Ek: Eckardt type.

Fig 11. Comparisons among 27G Mx forceps from different companies.

Magnified photographs showing the cross-sections of the shafts and the contact areas of the tips of the 27G Mx forceps, as viewed from the front and the side for forceps from each company. The metal thickness of the shaft and the structure of the contact area vary among forceps from different companies (A: Alcon; B: DORC; C: Katalyst; D: Vitre Q). Mx: Maxgrip type.

Fig 12. Comparisons among 27G Ek forceps from different companies.

Magnified photographs showing the cross-sections of the shafts and the contact areas of the tips of the 27G Ek forceps, as viewed from the front and the side for forceps from each company. The metal thickness of the shaft and the structure of the contact area vary among forceps from different companies (A: Alcon; B: DORC; C: Katalyst; D: Vitre Q). Ek: Eckardt type.