Keywords
bioengineering - biomechanics - soft-tissue surgery - skin and soft-tissue reconstruction
- wound management
Introduction
The present study contributes to the existing body of knowledge by providing an analysis
of the mechanical properties of skin staples, which could potentially lead to improvements
in soft tissue and skin closure.
Surgical skin staples are the most commonly used staples in small animal surgery and
are considered the fastest and most cost-effective method for closing long skin incisions.[1]
[2]
[3]
[4] Apart from skin closure, skin staples have been used for closing flaps, securing
skin grafts, drains, dressings, or tubes, closing enterotomies and gastrotomies, and
performing intestinal anastomoses, gastropexies, and subcutaneous and fascial closures.[5]
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
Skin staplers of many different types are used for staple application into tissues.
Staples come in two sizes: regular (4.8–6.1 mm) and wide (6.5–7.0 mm).[5] Each staple is composed of a cross-member that is placed perpendicular to the incision,
two limbs that penetrate the skin, and after the firing through a stapler, two pointed
tips that are bending beneath the skin parallel to the cross-member.[5] When fired, the stapler anvil crimps the cross-member at two sites bringing the
limbs together and usually forming a rectangular or arcuate (triangular) configuration
allowing for proper good skin apposition.[1]
[5] Disadvantages of skin staples may include eversion of skin edges and rotation in
mobile regions or thin skin.[1]
[2]
[3]
[14] Patient movement can create or enhance distraction forces on the wound.[2]
[3]
Staple instability and pivoting may occur when thin skin wounds are undergoing translational
forces such as in the groin, axillary, or ventral abdominal regions; staples rotate
up to 90 degrees, perpendicular to the margins of the skin, becoming thereby inefficient.[1]
[2] Consequently, dehiscence may occur due to the skin stretching through a gap created
between the pointed staple limbs. The use of metallic staples, absorbable subcutaneous
staples, and polyglactin 910 sutures has been evaluated for closure of skin incisions
in eight experimental pigs.[14] Skin incisions closed with metal staples in one pig showed partial dehiscence in
the center of the incision as four metal staples had been pulled out of the skin edges.[14]
Previously, skin staplers of six manufacturers including Autosuture (United States
Surgical Corporation, United States), Appose (Sherwood-Davis & Geck, United States),
Precise (3M Medical – Surgical Division, United States), Proximate (Ethicon, Inc,
United States), Reflex (Richard-Allan Medical, United States), and Weck Visistat (Edward
Weck & Co, United States) were tested on canine cadavers[3]; it was found that all staplers except Proximate performed well on in vitro skin closures.[3] It was assumed that regional skin mobility may induce flexural or bending forces
that might lead to failure of formed staples and subsequent complications in wound
healing. No studies evaluating the flexural strength of different skin staples after
firing have been published.
The purpose of the present study was to evaluate the flexural deformation strength
of various staples postfiring and to compare the distance and alignment of the staple
tips providing insights into the mechanical resilience of different staple types.
We hypothesized that some staples may exhibit superior flexural strength characteristics
compared with other staples.
Materials and Methods
Using a material testing equipment, the flexural strength of different staple brands
postfiring was evaluated. Commercially available wide skin staples from nine different
manufacturers were tested. These included AD (Advan, Ningbo Advan Electrical Co Ltd,
China), AP (Appose, Covidien, United States), MAN (Manipler, B. Braun Surgical SA,
Spain), GIM (Gima, Ningbo Advan Electrical Co Ltd, China), LK (Leukoclip, Smith &
Nephew Medical Ltd, England), PRE (Precise, 3M Health Care, United States), PX (Proximate,
Ethicon Endo-Surgery, United States), HS (Henry Schein, United States), and VIS (Weck
Visistat, Teleflex, United States). For the evaluation of flexural strength, one stapler
of each type was fired, and six configured staples from each stapler were collected.
Each staple was then mounted sequentially in an Electroforce 3550 materials testing
machine (ΤΑ Instruments, United States). This was done by attaching the upper and
lower limbs of the construct to the upper and lower hooks in the middle of their limb's
length. The testing machine was set at a speed of 2 mm/min, and maximum force was
applied until the first flexural deformation occurred, which was recorded as a failure
([Fig. 1]). For the evaluation of staple tip alignment, and distance between the pointed tips,
two staplers of each type were fired six times. The fired staples were photographed
and examined under 4× magnification. All testing and examinations were performed by
the same investigator.
Fig. 1 Photograph of a fired stapler attached to the materials testing machine for evaluation.
The upper and lower hooks of the machine are mounted at the level of the limbs of
the staple. The first flexural deformation that occurred was recorded as a failure.
Statistical Analysis
Statistical analysis was performed using commercial software (IBM Statistics SPSS
27, United States). Data were evaluated for normality using the Shapiro–Wilk W test. Analysis of variance was used to compare the differences of mean maximum force
to flexural deformation among groups. Data were expressed as a mean ± standard deviation.
A p value less than 0.05 was considered significant.
Results
The maximum force to flexural deformation was greater for PRE (29.633 ± 7.8421 N),
GIM (27.483 ± 6.5637 N), and AD (27.283 ± 2.8708 N) than that for VIS (24.329 ± 1.0372 N),
HS (23.383 ± 5.2282 N), LK (22.288 ± 1.6915 N), AP (18.133 ± 1.2675 N), PX (16.200 ± 1.1541 N),
and MAN (14.067 ± 3.7393 N).
The force-displacement curves of the staples AD, AP, GIM, LK, PX, and HS displayed
four phases: an initial increase in flexural deformation, a decrease, another increase,
and a final decrease. In contrast, the staples from MAN and VIS showed five phases:
an increase in flexural deformation, a decrease, a plateau, another increase, and
a final decrease. The curve for the PRE included an increase, followed by a decrease,
a plateau, and a final decrease. These load-displacement curves translate the uniaxial
displacement of the staples' two ends to bending, representing the flexural strength
of the nine different staples ([Fig. 2]).
Fig. 2 Load displacement curves of nine different staples underwent flexural strength bending.
Data were shown as mean ± standard deviation (SD).
The PRE, AP, and AD staples fired had their pointed tips met, whereas MAN, LK, GIM,
HS, PX, and VIS showed a gap between pointed tips before applying any force. PX staples
also showed malalignment between their pointed tips, whereas all the other staples
had their pointed tips well aligned before applying any force.
Discussion
The study meant to contribute to the existing knowledge on staple characteristics
by providing an analysis of the mechanical properties of skin staples; findings might
help choose the most appropriate staple materials for improvements in soft tissue
and skin closure.
The skin staplers tested were those most commonly used in small animal practices in
Greece. Some types of staplers lock their pointed tips in proximity, whereas a gap
between their tips is evident in others.[1] This gap between the tips may also allow increased postoperative tissue swelling
with less vascular compromise,[1] and this may be seen as an advantage. This increased gap may also enhance a trend
for staple rotation up to 90 degrees during increased skin tension.[1] Metallic staples and skin sutures have been used for closure of subdermal plexus
flaps in 97 clinical dogs.[6] Wound dehiscence was observed in 35% of all dogs in this study. However, the sample
size of dogs in this study was insufficient to determine whether the skin closure
method influences the incidence of dehiscence. However, in another experimental study
in 10 dogs, the use of metallic staples, tissue glue, and intradermal sutures has
been assessed for closure of skin incisions. Wound dehiscence was observed in one
dog and staple loss was noted in another three dogs with metal staples.[4] An ex vivo study in cadaveric tissues evaluated the holding power of different skin staples
that were placed in skin incisions followed by rubbing the closed incisions to mimic
body movement; it was found that among others Appose staples were rated the highest,
showing no rotation; Weck Visistat staples rotated in less than 10%, Precise rotated
in 25%, and Proximate showed 50% rotation under similar ex vivo conditions.[3] In our study, AP and PRE staples were found to close completely without gapping
tips, assuming that they would exhibit less tendency to rotate than the other staple
types such as Proximate, which we found malaligned.
The nonopening of the limbs of the formed staple ensures its secure fixation, less
rotation, and good apposition of skin edges as local skin mobility, body movements,
and increased incisional tension may result in the deformation of the formed staple.
When staple deformation occurs, it can be assumed that the secure fixation of the
staples was reduced and that rotational movements became more likely, resulting in
incorrect apposition of the edges and impairment of wound healing ([Fig. 3]). Reasons for staple rotation may include staple placement too high or loosely failing
to be engaged properly in the skin, not placing the staple in the center of the incision
line, improper apposition of the incision, or leaving a gap at the incision edges.
The mechanical strength of a stapled wound depends rather on the summary of the resilience
of several staples and rarely on the failure of one single staple. PRE, GIM, and AD
staples in our study showed greater force to flexural deformation than the other staple
types. In a recent ex vivo canine study, the tensile strength of simple interrupted, cruciate intradermal, and
subdermal suture patterns used for closure of skin incisions was compared. The simple
interrupted and cruciate patterns exhibited significantly higher mean tensile strength
at skin-edge separation and suture-line failure than the intradermal and subdermal
patterns.[15] Although no reliable comparisons can be made between these findings and those of
our study, the maximum force of flexural deformation of ours was less than the tensile
strength of external or internal suture patterns.
Fig. 3 A formed staple postfiring (left) underwent flexural or bending forces that might
lead to failure (right).
This experimental setup study presented here has limitations: one is the creation
of flexural deformation by a force applied in a single direction only; in a clinical
setting, forces are acting over time at different angles and directions. Another is
bending and/or closure of staples will be different when fired into tissues as compared
with just firing “into air.” These ex vivo experimental results cannot be easily applied to real situations, and our experimental
setup was unable to test the theory that staples rotate instead of opening up.
Conclusions
Using material testing equipment, the study objective was to evaluate different staple
types in terms of flexural deformation strength and determine the distance and alignment
of the staple pointed tips. Staples manufactured by PRE, GIM, and AD were more resilient
to flexural deformation than those of the AP, MAN, VIS, LK, PX, and HS types. The
PRE, AP, and AD staples, when fired, had their pointed tips closed, whereas the MAN,
LK, GIM, HS, PX, and VIS staples showed a gap between pointed tips. PX-type staples
also showed malalignment between their tips, whereas all the other staples had a complete
alignment of their pointed tips. Based on these observations, the PRE, GIM, and AD
staples yielded more resilience to deformity in one direction than the other staple
types.
