US3424164A - Silk suture - Google Patents

Description

United States Patent 3,424,164 SILK SUTURE Alfred Bloch, Somerset, and Arthur S. Messores, North Brunswick, NJ, assignors to Ethicon, Inc., a corporation of New Jersey N0 Drawing. Filed May 20, 1966, Ser. No. 551,536 US. Cl. 128-3355 4 Claims Int. Cl. A611 17/00; D02g 3/40 ABSTRACT OF THE DISCLOSURE A non-capillary, braided silk suture is prepared by treating the suture with a solution of fibroin and drying at about 105 C. whereby the silk filaments are coated with fibroin and bonded together.

The present invention relates to surgical ligatures and sutures and more specifically to braided or twisted silk sutures that have been treated to reduce capillarity. Silk is a material widely used in surgery as a non-absorbable suture to sew wounds or as a ligature to tie up bleeding blood vessels. Silk so used is applied most frequently in form of a braided thread, that is, a central core strand surrounded by a tube-like braided structure. The central core sometimes is absent or filling the tube to a greater or lesser extent. The braided structure also varies in tightness, depending on the braiding technique applied. Another type of silk thread, namely twisted silk, is used less frequently in surgery. In it a plurality of silk fibers are twisted together. While the present invention is directed to the treatment of braided silk, the procedure to be described below is also applicable to twisted silk.

Braided silk per se is limp. When it is cut, the ends tend to broom; that is, the carriers of which the braid consists spread out, particularly upon pressure or friction such as applied when attempting to thread the braided strand through the eye of a needle. Threading may actually become impossible. Furthermore, braided silk exposed to liquids, solutions, or suspensions exercises a capillary or wick action. Through this capillary or wick action, it can imbibe blood or serous fluid when used in the suturing of a wound. If such tissue fluids are infested with microorganisms, the latter may be harbored within the interstices of the silk suture. It has been claimed that microorganisms captured within such silk structure are extremely resistant to destruction, wherefore such silk could present a danger for the surgical patient. Braided silk, which has not been treated to eliminate this wick effect, is called capillary silk and is classified as Type A Non-Absorbable Surgical Silk in the Pharmacopeia of the United States. Type B or non-capillary silk is treated in such a manner as to eliminate or reduce the aforementioned wick action.

Silk sutures may be treated in various Ways to produce a non-capillary silk. The methods known or previously described make use of some material foreign to silk, which is incorporated into the silk strand so that it adheres to the silk fibers and/or fills interfilamental voids. The method most commonly used in the trade is that of treating the silk with wax or wax compositions. This method has the advantage of rendering silk non-capillary and at the same time bonding the'filaments together so as to prevent brooming when the ends are cut. It also gives the silk a certain texture; that is, a certain stiffness combined with plasticity; which is usually called hand. Many surgeons prefer silk with a hand over limp silk, since the limp silk is more difficult to handle. Another method to render silk non-capillary is to treat it with water-repellent compounds, such as silicon compounds.

In order to attain these properties, as much as 20 per- Patented Jan. 28, 1969 cent to 30 percent by weight of wax composition may be incorporated into the silk. However, it is known that foreign materials incorporated into the silk to produce noncapillarity, such as waxes, may not be well tolerated by the host tissues. They are able to evoke marked inflammatory reactions and granuloma. Wax granuloma may form at a slow progressive rate about the suture material and in the case of a tendon repair cause difliculties with the function of the tendon after three to six months.

It has now been discovered that silk sutures and ligatures may be treated to obtain the desired properties of non-capillarity, bonding, and improved hand without adding thereto any foreign material that might have an adverse biological reaction on the host tissue.

In accordance with the present invention, silk filaments that have been braided or twisted to form a strand are treated with a solution or dispersion of silk to produce a silk strand of reduced capillarity and improved hand; the interstices of the silk structure being impregnated with a silk substance that coats the silk whereby the silk filaments are coated and bonded together.

Natural silk, as found in the cocoon of the silkworm of commerce (Bombyx man) is composed of two major components; fibroin, the fibrous silk used as a textile fiber; and sericin, a non-fibrous protein. In silk technology, the sericin is removed by various ways and means. and only the fibroin is utilized for textile purposes. This present invention deals with the fibroin. For the purpose of clarity, the term silk shall be used in the following specification and claims to identify braided or twisted silk or strands of silk filaments. The term fibroin shall be used to identify a silk composition that has been dissolved or dispersed for use as a coating or impregnating material. It will be understood that the solutions or dispersions employed in carrying out the present invention contain fibroin and, therefore, are similar to silk filaments and strands from the standpoint of their chemical composition.

In the practice of the present invention, silk fibroin obtained from silk cocoons, virgin silk, or waste silk is dispersed or dissolved in a solvent. While many solvents are known for silk, a suitable solvent is aqueous lithium thiocyanate as this salt does not materially degrade the fibroin if the pH of the aqueous solution is buffered to a pH of about 7. Fifty percent by weight of lithium thiocyanate in water is a very efficient solvent, in which 20 percent to 25 percent by weight of fibroin can be readily dissolved. Such a solution cannot be used directly, however, to impregnate silk fibers, since it attacks them very rapidly. The lithium thiocyanate can be removed from the silk solution, for instance by dialysis. By exhaustive dialysis against water, a fi broin-in-water dispersion is obtained. This fibroin dispersion can be utilized to impregnate and coat the silk. However, a fibroin dispersion is labile in that is gels readily. Fibroin separates from the dispersion after standing for some time or through agitation. This property is undesirable for manufacturing purposes. An important facet of the present invention is that of stabilization of such fibroin solutions, One way of achieving this is to dialize the solution of fibroin in aqueous lithium thiocyanate against an equeous solution of a hydrogen-bond breaker urea or guanidine. The satisfactory removal of lithium thiocyanate from the fibroin solution may be demonstrated by a negative reaction with ferric chloride.

The dialysis of the fibroin solution to remove lithium thiocyanate is not necessary if the concentration of the lithium thiocyanate is reduced to no more than 20 percent by weight. However, the nature of the dilution is critical. Dilution of the fibroin solution with water or with a l-molar aqueous solution of urea results in the formation of a flocculent precipitate. The fibroin remains dispersed if a l-molar solution of calcium chloride is used as a diluent, but the fibroin solution assumes a milky, white appearance. Dioxane maybe used as a diluent to reduce the concentration of lithium thiocyanate in solution as may an aqueous solution of 50 parts acetamide dissolved in 50 parts of water. Dilution of the fibroin solution with aqueous acetamide is preferably carried out at pH 7 from the standpoint of optimum solution viscosity and minimum degradation of the fibroin.

Aqueous iithium bromide may be substituted for aqueous lithium thiocyanate as a solvent for fibroin. The lithium bromide solution so obtained is also stable after dilution with aqueous acetamide.

The present invention will appear more clearly from the following detailed examples which illustrate preferred embodiments of the invention idea.

EXAMPLE I The impregnation and coating of silk can be done by various methods, such as immersion or preferably by immersion under vacuum in order to obtain good penetration. After withdrawal from the fibroin solution, the silk strands are drained or wiped from excess fibroin and air dried with or without tension. The fibroin may be further fixed by various means, such as heating or fixation with precipitants, such as salts. These salts, as well as any urea present from the dialysis step, may be removed by bathing the fixed strands in water or a water-alcohol mixture. A fibroin solution containing 20 percent fibroin in 50 percent by weight of lithium thiocyanate is prepared. Aliquot portions of this solution are dialyzed in cellophane bags for 24 hours against urea solutions of varying urea concentrations. The dialysis is repeated a second time against fresh urea solutions of the respective concentrations. The complete removal of lithium thiocyanate is demonstrated by a negative reaction with ferric chloride of the fibroin solution within the cellophane :bag. The fibroin solutions, reduced to 10 percent concentrations during dialysis, are filtered through sintered glass filters under negative pressure and aliquots of each solution stored at room temperature (20 to 25 C.) and at 37 C.

TABLE I Urea concentration against which silk fibroin solution Observed gelation time is dialyzed At room temperature At 37 C.

3 days 1 day. 0.1 molar 3 days. 3 days. 0.5 molar 7 days--. 3 days. 1.0 molar 3 weeks. 7 days. 4.0 molar 8 weeks 6 weeks.

Two parts of fibroin fibers obtained from cleaned silk cocoons are dissolved in 10 parts of 50 percent by weight LiSCN. The fibroin solution is placed into a cellophane bag and dialyzed against 1000 parts of 4-molar urea solution for 24 hours. The solution is then dialyzed a second time against 100 parts of 4 molar urea for hours. At this time, the fibroin solution gives a negative reaction for the SCN-ion with FeCl The fibroin solution is filtered through a sintered glass filter under negative pressure and 4 stored at 25 C. A coil of braided silk, U.S.P. size #1, is immersed in the fibroin solution, air dried, and then heated at 110 C. for 1.5 hours. The silk strand so obtained is non-capillary.

EXAMPLE III Two and one-half parts of silk fibroin obtained from silk cocoons are dissolved in 10- parts of 50 percent LiSC-N. The fibroin solution is dialyzed against 1000 parts of 1 molar urea for 24 hours; the dialysis is repeated with another 1000 parts of 1 molar urea for 24 hours. The fibroin solution is filtered as in Example 11. Five feet of braided silk, 'U.S.P. size #1, is immersed in 5 milliliters of the fibroin solution, and the whole placed under vacuum for 2 hours. The impregnated silk coil is drained, air dried for about 20 hours, and then heated for 2 /2 hours at 105 C. The dried sample is immersed for 3 hours at room temperature in 50 percent by volume aqueous isopropanol to remove urea from the silk. The sample is non-capillary, bonded so as not to broom when out, and has a hand markedly different from the untreated control.

EXAMPLE IV Twenty parts of waste silk fibers are dissolved in parts of 50 percent LiSCN. The fibroin solution is dialyzed against 2000 parts 1 molar urea. The dialysis step is repeated. The dialyzed fibroin solution contains 10.5 percent fibroin. A coil of braided silk is immersed in an aliquot of the fibroin solution under vacuum for 2.5 hours. The silk is withdrawn and the excess is wiped off with a towel. The silk strands are dried under two different conditions.

(A) A 10 yard strand is dried without tension.

(B) A 10 yard strand is dried under 50 grams tension.

Both silk strands are heated at C. for 3 hours and immersed in 50 percent by volume aqueous isopropanol for 3 hours at room temperature.

Drying under tension results in a shrinkage or shortening of 0.4 percent; without tension of 1.7 percent. Both are acceptable. Physical properties, such as tensile strength, knotting, and knot-holding power are excellent. The samples are non-capillary.

EXAMPLE V Silk strands are treated by the procedure discussed in Example III, except that the tfibroin solution is dialyzed against a 1 percent solution of a quaternary ammonium salt, instead of against a 1 molar urea solution. The treatment resulted in non-capillary silk.

EXAMPLE W Silk strands are treated by the procedure discussed in Example III, except that the fibroin solution is dialyzed against 0.1 molar solution. The treatment resulted in non-capillary silk.

EXAMPLE VII Size 3-0 black, braided, unwaxed, silk sutures are impregnated by the following method: twenty parts of degummed silk cocoons are solubilized overnight in 100 parts of a 50 percent LiSCN solution (i.e., 50 parts LiSCN and 50 parts H O) at pH 7.0. The fibroin concentrate is then dialyzed against 4000 parts of 1 molar urea at room temperature for a period of 24 hours. Following dialysis, the fibroin solution is checked with ferric chloride for complete salt removal and then filtered through a coarse sintered glass filter under vacuum.

Size 30 black, braided silk is loosely wound on a perforated spool to allow permeation. The spool is placed in a reservoir, and the urea solution is added to a height suflicient to completely cover the spool. The silk reservoir with spool and fibroin solution is placed in a vacuum chamber overnight with 30 inches of vacuum maintained.

The silk strand is then drawn mechanically from the fibroin bath through an orifice which controls the amount Diameter mils 9.8 Straight tensile strength lbs 4.6 Knot tensile strength lbs 2.9 Wet knot tensile strength lbs 2.3

The structure so obtained is non-capillary, non-brooming, and has excellent hand and bonding characteristics.

EXAMP-IJE VIII Size 3-0 black, braided, unwaxed, silk sutures are triple coated by the process described above in Example VII. The suture following 3-fold impregnation shows the following physical properties:

Average diameter mils 10 Average straight tensile strength lbs 4.8 Average knot tensile strength lbs 3.2 Average wet knot tensile strength lbs 2.3

The final strand is about 2 percent heavier indicating that about 2 percent by weight fibroin is picked up by the strands. The suture exhibits non-capillarity, nonbrooming, and excellent hand and bonding characteristics.

EXAMPLE IX Using the impregnating method and fibroin solution described in Example VII, sizes 2-0, 3-0, 4-0, and 6-0 black, braided, unwaxed, silk sutures are impregnated with fibroin. Single impregnation of the sutures of various sizes resulted in the following physical characteristics.

Size Test All sutures are found to be non-capillary, and all exhibit excellent bonding and hand characteristics.

EXAMPLE X Twenty parts of degummed silk cocoons are solubilized overnight in 100 parts of a 50 percent LiSCN solution (i.e., 50 parts LiSCN and 50 parts H 0) at pH 7.0. After solubilization, the fibroin concentrate is diluted with a 50 percent aqueous acetamide solution by using two parts of the acetamide solution to one part fibroin-LiSCN concentrate. The solution is filtered through cheesecloth (6 layers).

Spools with loosely-wound, non-waxed, black, braided, silk sutures, size 0 and 3-0, are impregnated by the method described in Example VII. All samples prove to be noncapillary and exhibit excellent hand and non-brooming characteristics.

EXAMPLE XI Size 30, black, braided, unwaxed silk is impregnated with a fibroin solution utilizing LiBr instead of LiSCN as the solubilizing agent. The degummed silk cocoon is solubilized in LiBr by placing 20 parts silk in parts of 10 molar LiBr which has been adjusted to pH 7.0. Following solubilization, the solution is diluted with aqueous acetamide and applied to silk sutures as described in Example X. Physical studies on the impregnated suture give the following results:

Diameter, mils. 10.0 Straight pull, lbs. 4.5 Knot pull, lbs. 3.0 Wet knot, lbs. 2.2

The suture is non-capillary and exhibits excellent physical characteristics such as knot holding, lack of slippage, non-brooming, and good hand qualities.

What is claimed is:

1. A surgical silk suture comprising a plurality of silk filaments, that together form a strand, said silk filaments being coated with fibroin and characterize-d by non-capillarity, non-brooming, improved knot retention, and good hand qualities.-

2. The suture of claim 1 wherein the filaments are bonded together by the fibroin.

3. The suture of claim 1 wherein the fibroin partially fills the interstices present betwen the individual filaments that form the strand.

4. The suture of claim 1 wherein the fibroin amounts to about 2 percent by weight of the strand.

References Cited UNITED STATES PATENTS 1,992,202 2/1935 Furmam et a1. 106-161 1,989,005 1/1935 Fink et al. 106-161 2,193,188 3/1940 Bradley 128-3355 2,357,503 9/1944 Cidonio 57-157 3,187,752 6/1965 Glick 128-3355 DALTON L. TRULUCK, Primary Examiner.

J. YASKO, Assistant Examiner.

US. Cl. X.R.