Under pressure
A masked Dr. Stacy Anderson (left) conducts a surgical procedure on an equine patient. Photo: Michael Raine.
When your horse is undergoing major abdominal surgery for a condition such as colic the last thing you want to worry about is whether the surgical incision site may open up during recovery or in the early post-operative period.
It can happen, but thankfully, acute incisional bursting (or dehiscence) following abdominal surgery is extremely rare in horses.
“Acute incisional bursting is often associated with falling during recovery. The intra-abdominal pressure is so high that it causes the incision to open,” explains Dr. Stacy Anderson, an equine surgery resident at the Western College of Veterinary Medicine (WCVM). “Unfortunately, humane euthanasia is often performed when this happens.”
During her combined Master of Veterinary Science-large animal surgical residency program, Anderson focused her research on the effect of surgical technique and suture type on maximum incisional bursting strength.
“I wanted to look at the strength of the suture used to close abdominal incisions in horses,” she says. “The likelihood of acute dehiscence (incision bursting) is very low, but using a model that causes the incision to burst is the best way to test how strong the materials are that were used to close the incision.”
A celiotomy involves making an incision into the abdominal cavity. Anderson investigated two types of celiotomies: ventral midline (VM) celiotomy and right ventral paramedian (RVP) celiotomy.
“Traditionally, most equine surgeons use the VM approach where an incision is made through the midline of the abdomen,” says Anderson. “However, I did an internship at a practice where they preferred to use the RVP technique.”
With an RVP celiotomy, surgeons make an incision about five to 10 centimetres to the right of midline. Sometimes, if a horse has had previous surgery with a VM incision, the surgeon may opt for the RVP approach.
Anderson gained interest in comparing these surgical techniques while doing a retrospective study that investigated the occurrence of incisional complications and associated risk factors using a RVP celiotomy.
“Initially, my project was only going to focus on the comparison between the two celiotomy techniques, but then I decided to add another variable – suture type.” says Anderson. She worked on this project with her residency supervisor, Dr. Joe Bracamonte, an assistant professor in large animal surgery at the WCVM.
Equine surgeons in the college’s Veterinary Medical Centre (VMC) use a large diameter suture material to close colic surgeries. “As far as I know, we use a suture that’s larger than what is commonly used elsewhere in the world,” says Anderson.
The study, funded through Bracamonte’s research program, began in August 2010. First, Anderson created a 25-centimetre (cm) VM or RVP celiotomy in 20 fresh equine cadavers. A 200 litre (L) balloon-like bladder was inserted into the abdomen of each cadaver before closing the incision with either seven polydioxanone (7PD) or two polyglactin 910 (2PG) suture.
“7PD is 1 millimetre (mm) in diameter and it’s the suture we use in the VMC,” says Anderson. “The 2PG suture is only 0.5 mm in diameter and it’s more commonly used by equine surgeons throughout North America.”
Next, an air compressor inflated the bladder until the construct failed either along the incision line or the body wall. An instrument recorded the pressure (bursting strength) at which the celiotomy construct burst open.
Anderson noted the age and body weight of each horse, the celiotomy type and suture type used, the mode of failure (whether it failed at the abdominal wall or along the suture) and the bursting strength.
“I found that the bursting strength didn’t differ between the two celiotomy types,” says Anderson, adding that the surgical approach also had no effect on mode of failure.
However, when the effect of age was taken into consideration, RVP celiotomies had a significantly lower mean bursting strength compared to VM celiotomies.
“With increasing age, RVP celiotomies come apart at a lower pressure,” explains Anderson. “In humans, muscle degeneration occurs with age. I’m thinking that a similar phenomenon occurs in animals.”
As for suture type, there were remarkable differences noted between the 7PD and 2PG sutures. Anderson discovered that six of the 10 celiotomies closed with 2PG failed at the suture line, while all of the celiotomies closed with 7PD remained intact.
“The 7PD suture didn’t fail, it was the muscle layer of the abdominal wall that eventually came apart, which suggests this construct is similar in strength, if not stronger, than the abdominal wall itself,” says Anderson, noting that this has never been demonstrated before.
The data from Anderson’s study will prove useful for surgeons when choosing a suture type for the closure of an abdominal incision in horses. “I hope it’ll get surgeons thinking about other types of suture versus what’s traditionally used,” she says. “Equine surgeons should consider using the strongest suture and method to close celiotomies, especially when it comes to older horses.”
To confirm her findings, Anderson proposes that in-vivo studies using live horses be performed. “The stronger the suture, the more likely it’ll support the abdominal wall of a live patient while it’s healing,” she says. “But to know for sure, we need to do further research.”
Robyn Thrasher of Edmonton, Alta., is a second-year veterinary student at the WCVM. Robyn produced stories about the veterinary college’s clinical services, research program and its researchers as part of her summer job in research communications.
