ABSTRACT
CONTENTS 2.1 Introduction ..................................................................................................15 2.2 Molecular Chain Design for Tailored Properties ...................................16 2.3 Composition and Properties of Typical Copolymers and
Sutures Thereof ............................................................................................17 2.3.1 Copolymers for Monofilament Sutures.......................................18 2.3.2 Copolymers for Braided Sutures ..................................................19 2.3.3 Effect of Composition on Properties of Segmented
Polymers and Their Braided Sutures...........................................21 2.4 Conclusion and Perspective on the Future .............................................23 References ............................................................................................... 23
Since the first development of polyglycolide as an absorbable suture, the majority of the absorbable products pertained to soft tissue repair and were based primarily on polymers with short to moderate strength retention profiles in the biological environment. However, interest in using absorbable systems in orthopedics justified the search for absorbable polymers that can be used in the production of devices with prolonged strength retention, due to the slow healing rate of bones as compared to soft tissues. Meanwhile, a special need for sutures with prolonged breaking strength retention (BSR) profiles for soft tissue repair has been voiced by a number of surgeons, particularly those who repair slow-healing soft tissues, as in the case of geriatric patients or patients with compromised wounds. For investigators, including the author, who are interested in fibers with prolonged BSR, it is
well recognized that absorbable fibers suitable for constructing biomedical constructs, as in certain surgical sutures and meshes as well as prosthetic tendons and ligaments, must be based on polymers that meet certain requirements. In effect, these polymers are expected to have:
• High molecular weight • A high degree of crystallinity • Minimum or no monomeric species
These requirements were claimed to have been fulfilled by the l-lactide/ glycolide copolymers described by Benicewicz et al. as well as Kennedy and Liu.1-3 However, in certain high load-bearing applications where a prosthetic fibrous construct experiences cyclic stresses and is expected to maintain a substantial fraction of its initial strength for several weeks postoperatively, additional requirements are imposed. Typical examples of such constructs are surgical meshes for hernia repair and prosthetic tendons and ligaments. These additional requirements are expected to be associated with having a high degree of toughness, as measured in terms of the work-to-break, without compromising significantly their high tensile strength, high Young’s modulus, low stretchability, and high yield strength. Such requirements also are expected to be associated with a polymeric chain exhibiting higher hydrolytic stability than those containing primarily glycolate sequences. Unfortunately, until recently, the available literature on absorbable polymers provided conflicting teachings that may be applied towards meeting the aforementioned additional requirements. To increase toughness, more flexible I-caprolactonebased sequences in polyglycolide chain have been introduced successfully in the production of low modulus sutures, but with compromised strength.4,5 A similar situation is encountered in the copolymer of glycolide and trimethylene carbonate.6 Interestingly, fibers made of these two types of copolymers do display a lower propensity to hydrolysis than polyglycolide, but their strength loss profiles remain unsuitable for long-term, load-bearing applications. This provided the incentive to develop the high lactide copolymer discussed in the rest of this chapter.
