Suture Biomechanics and Static Facial Suspension（3）
METHODS
Six samples each of 0, 2-0, and 3-0 polypropylene, polybutilate-coated polyester (PBCP) (Ethibond Excel; Johnson & Johnson), and polyester impregnated with PTFE (PIP) (Tevdek; Teleflex,Inc, Limerick, Pa) were tested to failure using testing conditions similar to those in a previous study by our group of expanded PTFE and human acellular dermis.2 A materials testing machine (Mini Bionix; MTS Systems Corporation, Eden Prairie, Minn) was used to apply tension to single loops of each suture tied with a single knot consisting of 8 square knots. The suture loops were tied around two 13-mm-diameter polished brass disks, which, when moved apart by the testing machine, applied tension to the loops. The knot was always positioned on the upper disk to roughly simulate the position where it normally sits on the lateral orbital rim (Figure 3). The gauge length of suture between disk axes was 44 mm. Based on measurement of an SFS suture explanted from a patient after failure, actual gauge length in vivo can be as little as half that value. This smaller length is consistent with the 22-mm gauge length used in the previous study of expanded PTFE and human acellular dermis.2 Doubling the gauge length in the present study was necessary to allow adequate space for the materials testing machine to function properly with the brass disks and suture loops in place. Changing the gauge length does not alter load to failure data; stiffness and elongation change in inverse proportion to sample length and can be adjusted easily for cross-comparison between studies. The testing machine was used to increase axial loading at a rate of 20 mm/min until each sample failed.
Figure 3. The Mini Bionix testing apparatus (MTS Systems Corporation, Eden Prairie, Minn) with suture loop in place and knot positioned on the upper disk to roughly simulate positioning in vivo on the lateral orbital rim.
The failure site of suture samples was always near the knot but never on the disks, suggesting that there were no variables introduced by the disks (eg, irregular surface or friction) that induced suture failure. This is consistent with findings in failed SFS sutures explanted from patients that demonstrated breakage near the knot (unpublished observations). The mean maximum load to failure was calculated for each suture type.
Stiffness was calculated as the slope of the curve when load was plotted as a function of elongation. This curve was linear for monofilament polypropylene samples, and stiffness could be calculated. Polybutilate-coated polyester and PIP are polyfilament braided sutures; these yielded curves that were reproducible but not linear. Therefore, slope was always calculated for PBCP and PIP by using the initial segment of the curve, where maximum stiffness was present. In addition, we calculated elongation in millimeters at specific load values of 20 and 30 N as an alternative method for comparing the amount of stretch that occurred among the groups having linear and nonlinear stiffness. This calculation allowed us to quantify the amount of overcorrection required for each suture during surgery to compensate for stretching that occurs at loads likely to be encountered clinically. Means were obtained and statistical analysis was performed using the analysis of variance between groups and Bonferroni-Dunn tests (α = .05).
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