String Theory: an Examination of the Properties of “high Strength” Suture Materials

The evolution of arthroscopic shoulder surgery has lead to advanced developments in instrumentation and equipment. Pivotal to successful surgery are the high strength sutures. With all claiming high strength, we aimed to assess biophysical properties of each material via light microscopy, scanning electron microscope and mechanical testing. Peak load and stiffness were assessed using a knot free length secured with friction loops distracted at 10 mm/min using a Bionix MTS 1000kN load/test cell. Results were analysed by SPSS for windows New packets of #2 sutures were provided by Arthrex (Fibrewire), Depuy/Mitek (Orthocord), Linvatec (HiFi), and Smith & Nephew (Ultrabraid) for examination Fibrewire contains a UHMWPE central core within a braided polyester sleeve while Orthocord, does not have a structurally significant central core, and Hifi and Ultrabraid contain no central core. Fibrewire has a mean twist angle (MTA) of 20 degrees while the other three have a MTA of 35 degrees. HiFi, Orthocord and Ultrabraid have a large initial toe region as the braiding aligns itself with the applied load and achieved maximum strength and stiffness after significant deformation. In contrast, the central core and tighter braiding pattern in Fibrewire resulted in a stiffer suture early in loading (P<0.05). Ultrabraid had the highest ultimate strength in tensile testing at 264N followed by Fibrewire 238N, then HiFi at 215N and Orthocord at 212N. Fibrewire was significantly stiffer than HiFi, Orthocord, and Ultrabraid in the first 50N of testing All sutures provide strength well above those required for tissue repair on immediate testing. Our review of suture materials may provide more insight into the available sutures on the market. Further testing is required to interpret clinical implications including preloading and creep during knot tying. Introduction Suture materials form an integral part of arthroscopic shoulder surgery. With many suture materials available on the market, all claiming high strength, it may be confusing and difficult to distinguish one from another, and certainly to decide on which is superior. These materials are non-absorbable making it critical we understand what we are implanting into a patient and the properties of these implants. Many studies exist on knot performance using different knots and traditional suture materials. Monofilament sutures can lead to dehiscence and clinical failure due to knot slippage and/or loop elongation at low applied loads (1). This study examined the tensile and morphologic properties of the new so-called “high strength” sutures. Shoulder surgery has evolved from open to minimally invasive arthroscopic surgery. Most forms of surgery rely on suture materials. First generation braided multifilament non-absorbable sutures (ethibond / ticron) provided a suitable implant for open surgery. These sutures are based predominantly on polyester. Newer arthroscopic equipment (both implantable anchors and knot pushers) places higher loads on the suture. The first generation sutures were a common point of failure. The newer second generation high strength sutures are based on ultrahigh molecular weight polyethylene. The commercial form of this product is known as Dyneema. Dyneema has been used in many forms outside of the medical industry. Its biomechanical properties have lead to the rapidly expanding use throughout the world. It is manufactured through a gel-spinning process. It is capable of absorbing large amounts of energy and thus used in ballistics protection, from bullet-proof jackets to armored vehicles. Its strength is fifteen times stronger than steel yet is so light it floats on water, allowing its use in marine vessels. It has a high modulus of elasticity and is flexible. Its properties also include having superior wear and abrasion resistance. Second generation suture materials are composed of this Dyneema. Materials and Methods New samples of number 2 suture materials were opened and each examined straight from the packet. Materials were provided by Arthrex (Fibrewire), Depuy/Mitek (Orthocord), Linvatec (HiFi), and Smith & Nephew (Ultrabraid). Materials were characterized based on the macroscopic, microscopic , and electron microscopic appearances to define flaws and differences between materials. Each material was then loaded to failure (knot –free) in tension. Load deformation curves were analysed for ultimate strength and stiffness. Macroscopic/Microscopic Appearance Each material was examined under an Olympus stereozoom microscope both in transverse and longitudinal sections. Each was then photographed under x 4 and x 10 magnification. The photographs were then viewed in a windows picture viewer and the mean twist or mean braid angle measured (Fig 1) using an electronic goniometer. Cross-sections of each suture were also examined and photographed and the presence or lack of a central cord noted.(Figure 2) Electron Microscopic Appearance – Each material was sectioned and set in a liquid metal mould and photographed under an electron microscope under magnification of x 100, x 500 and x70 (transverse section only) (figure 3) Tensile Load Testing – Single strands of each material, knot-free, were loaded in tension to failure. A Bionix 858 MTS testing device with a 2 kN load cell was used to record load displacement curves. The material was secured using friction loops (3 wraps at each end) and then clamped past the friction loops so that no weakness could be created in the material with knots (Figure 4). The testing protocol used a constant 5cm length of material and was distracted to failure at a rate of 10mm/min.