Multifilament and monofilament sutures are commonly employed in surgical procedures. Multifilament sutures were the first sutures developed from natural fibers such as silk. Multifilament sutures generally have a braided or twisted construction. These constructions provide sutures with significant flexibility and strength. However, multifilament sutures tend to abrade tissue as the sutures are passed through the tissue thereby damaging the tissue. The tissue drag generated by the suture abrasion also requires the surgeon to use more force to overcome the tissue's resistance to the suture. To reduce tissue drag and make it easier for surgeons to tie knots in multifilament sutures, sutures are often solution coated with a lubricious coating. This allows a knot to be tied and slid down easily without the "chatter" normally associated with and uncoated braided suture. Sutures are typically coated by immersing the suture in a coating solution containing a solvent and a lubricious material. Lubricious materials are commonly low molecular weight waxy aliphatic polyesters. Unfortunately the coating polymers also penetrate into the internal interstice of the suture so it is often necessary to pliabilize the sutures after coating to restore their flexibility. Although these coatings significantly reduce tissue drag, and make it easier to tie knots in these sutures, they do not completely solve the tissue abrasion and tie down issues.
Monofilament sutures provide some advantages over multifilament sutures. Monofilament sutures have a smooth uniform surface and therefore have less tissue drag. However, monofilaments tend to be stiffer and more difficult to tie secure knots than in equivalent multifilament sutures. To overcome these deficiencies in suture design several approaches have been tried.
Hunter et al. in U.S. Pat. No. 3,791,388 developed a multifilament suture that was impregnated with a binder and covered with a helical ribbon winding. Although this suture certainly improved the smoothness of multifilament sutures, the process for making this suture was slow and the binder tended to make the suture stiffer than was desirable. Schmitt et al. and Landi et al. in U.S. Pat. No. 3,982,543 and 4,043,344, respectively, described extruding a lubricious sheath over a multifilament core. Schmitt's approach was to use a wire coating technique like those described in U.S. Pat. Nos. 2,735,258; 2,401,291; and 2,824,485. These earlier patents describe how to make tennis strings from nylon yarns by applying multiple coats of a nylon containing solution to the nylon yarn and drying between separate application of the coating. This process provides a tough and wiry fiber well suited for tennis rackets. This process also tends to fill the interstices of the nylon yarn (see column 2 lines 20-24 of U.S. Pat. No. 2,824,485). Landi describes coating a suture by placing the suture in a split die. The die is raised to a temperature about 20.degree. C. above the melting point of a lubricious bioabsorbable coating copolymer that has been melted and the suture is slowly passed through the molten material in the die and collecting after the coating has solidified. Unfortunately both of these process will result in the coating being distributed into the interstices of the multifilament suture which will significantly stiffen the suture.
Kurtz et al. in WO 86/00020 also describes coating multifilament sutures with film-forming materials commonly used in the construction of absorbable and non-absorbable sutures. Kurtz indicates that the coating material will penetrate and fill the interstices of the core to increase the lateral strength of the core fibers and avoid the tendencies of these core fibers to abrade, kink or fibrillate. The coating will be added in an amount sufficient to not only fill all the interstices of the multifilament core, but also to coat the surface of the yarn or thread component. In a preferred embodiment the composite so formed is further coated by melt extruding a coating material onto the composite.
Unfortunately, the helical windings and filled interstical spaces described by the prior art patent applications and patents provide a suture that is too stiff and rigid to be used in place of multifilament sutures in many surgical applications. It is an object of the present invention to provide a desirable surgical suture that combines the flexibility of multifilament sutures with the strength of monofilament sutures.