Tying the knot: Mayo Clinic team tests novel surgical knot to improve open repairs

Whether at work or the family home, knots are never far from the mind of Chunfeng Zhao, M.D., a specialist in the Department of Orthopedic Surgery at Mayo Clinic in Rochester, Minn., and a professor of orthopedics at Mayo Clinic College of Medicine. "My wife will be getting ready for a run and I will say to her, 'Be sure your shoes are tied and your knots are strong — you can't run your best race if they come untied,' " he says. Dr. Zhao has been fully engaged in orthopedic research at Mayo Clinic since 1997 after practicing orthopedic surgery for 15 years in China, where he was trained.

The importance of knots

Artist's drawing of six knots tested Left to right, the knots the Mayo team tested: (a) 3-throw square knot, (b) 4-throw square knot, (c) 1-throw surgeon's knot with a 2-throw square knot, (d) 1-throw surgeon's knot with a 3-throw square knot, (e) 5-throw square knot, (f) Mayo's TSOL knot.

As a researcher with a clinical background, Dr. Zhao knows how important a surgical knot will be. "One single knot unraveling can cause an entire surgery to fail, such as a tendon repair in which only a single stitch is used to repair a broken tendon. We observed this in 100 percent of the repairs in our recent study," Dr. Zhao explains. "The surgical knots or square knots that are made every day in the OR may not be as strong as we thought, or we are just not making the knot right."

In a 1984 study, of the 25 surgeons in the study, most believed they were tying a square knot, when in fact, 80 percent were tying sliding knots. Slippage and unraveling also are common sources of error. In high-tension closures, such as flexor tendon repairs, reports cite knot unraveling occurring 71 to 86 percent of the time, depending on technique.

To remedy these trends and safeguard patient care, Dr. Zhao with his Mayo orthopedic colleagues subjected a novel surgical knot to rigorous biomechanical testing: the two-strand overhand locking (TSOL) knot. Their report appears in the June 2013 issue of The Journal of Bone & Joint Surgery.

Unraveling knot failure

Knots and their holding strength have a history as old as civilization. As early as the first century A.D., knot strength was a topic of medical discussion. In his essay explaining how to tie 16 knots commonly used in surgical and orthopedic procedures, the Greek physician Heraklas described the "square" or "reef" knot still in use today. Knot failure is most concerning when it happens inside the body for internal tissue repairs. "If a knot fails externally, for skin closure, it's undesirable, but you might have 10 sutures in place and can stand to lose one without affecting wound healing," Dr. Zhao says. "But an internal failure, such as to a tendon repair, is serious."

Adds Mayo Clinic orthopedic surgeon and Dr. Zhao's collaborator, Peter C. Amadio, M.D., "You need a very secure repair with high tensile strength to hold the tendon, especially during rehabilitation." Increasing the number of throws is not an option in flexor tendon repairs because it creates bulk that can interfere with healing.

Biomechanical testing

Mayo's biomechanical knot testing apparatus This custom-made device made by Mayo orthopedic biomechanics experts relies on a two-rod system to help make a single test loop. First, suture material is wrapped around a 30-mm rod, then the small rod is removed so the knot is secure around the larger rod. Weights are then attached to test a given knot's holding power.

The fundamental importance of a secure knot to successful flexor tendon repair motivated the team to review more than 50 flexor tendon repair techniques. The team also tested knots in Mayo's Biomechanics Laboratory on a variety of custom-designed machines. The phenomenon of knot unraveling emerged as the common source of problems.

In recent years, suture materials have gotten stronger. What should be an advantage — increased strength to prevent breakage — actually contributes to knot failure because the stronger materials tend to be low friction and therefore slippery. Explains Mayo Clinic Biomechanics Lab Director Kai-Nan An, Ph.D., "In the past, the suture materials were not that strong, so the weak link of the repair was suture failure. Now the suture materials are much stronger, but tend to be slippery, so the weak link of the repair has shifted, and is now the knot unraveling."

A new twist: TSOL

To overcome the unraveling problem, researchers experimented with tying knots and testing them for failure force. Says Dr. Zhao: "The idea of the TSOL knot stemmed from experiments in which we needed to tie a suture as a cable to pull a tendon for a thousand cycles of motion under a high load. One knot never failed during our testing. I didn't know the name of it. I don't remember learning it. I just know it because I've done it thousands of times. I always teach it to my fellows who are doing similar experiments, saying, 'Make this knot. It won't fail you.' " Mayo's test results show that the failure strength for the TSOL knot was much greater than that of five other knot groups tested.

Dr. Zhao considers the findings to be "just one more tool. It is not practice-changing, but it is a help, especially for tricky repairs like the flexor tendon where you have room to put in only one or two stitches. Those knots have to stick for the best clinical outcome to occur."

For more information

Zhao C, et al. Beyond the square knot: A novel knotting technique for surgical use. The Journal of Bone & Joint Surgery. 2013;95:1020.