Bioabsorbable suture which consists of a block copolymer

Abstract: This invention relates to a block copolymer which comprises a proportion of units having the formula: as one of said blocks, and another of said blocks comprising a proportion of units having the formula: and a proportion of units having the formula: said units of formula (II) and formula (III) being randomly combined. This invention relates further to a surgical article manufactured from this block copolymer. A method for preparing the bioabsorbable block copolymer as well as for preparing a bioabsorbable surgical article are also disclosed.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a copolymer, and more particularly to a surgical article manufactured from the copolymer and to a method of manufacturing the copolymer and surgical article. 2. Background of the Art Absorbable synthetic polymer sutures known in the prior art are usually manufactured, sold, and used as multifilament braids. The known absorbable polymers containing a glycolic acid ester linkage seem to be well suited for use in fabricating braided sutures. However, monofilament sutures fabricated from such polymers tend to be relatively stiff, particularly in the larger diameters. Yet, some surgeons prefer the suturing characteristics of a monofilament suture because of its smooth, continuous-surface. Thus, it has been recognized for some years that there is a need in surgery for flexible, absorbable, monofilament sutures which retain a safe and useful proportion of their strength for a relatively long period of time in vivo. To be fully useful as an absorbable suture it is essential that a monofilament or multifilament not only be absorbable and flexible but it must also be capable of a relatively long period of in vivo strength retention. An appropriate strength retention target for this type suture is considered to be about 35-70 days in vivo. U.S. Patent No. 4,429,080 to Casey et al discloses a triblock copolymer wherein the end blocks comprise polyglycolide, and the middle block comprises a glycolide/trimethylene carbonate copolymer. U.S. Patent 4,052,988 discloses polymers of p-dioxanone and copolymers with other monomers, i.e. lactide and/or glycolide, are briefly mentioned. A new polymer has been developed for use in the fabrication of absorbable monofilament or multifilament sutures. SUMMARY OF THE INVENTION Provided herein is a block copolymer for use in the fabrication of bioabsorbable sutures; and a method for making the same. The block copolymer is composed of glycolide as one block, and lactide/polydioxanone as the other block. The lactide/polydioxanone is first copolymerized at a first reaction temperature. The reaction temperature is then increased and glycolide is added to the reaction mixture. The resulting copolymer can be fabricated into both monofilament and multifilament absorbable sutures with advantageous flexibility and knot pull characteristics. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Bioabsorbable materials useful for fabricating surgical articles, such as sutures, surgical clips, etc., include homopolymers and copolymers of glycolide, lactide, 1,4-dioxanone, trimethylene carbonate, and caprolactone. The present invention relates to a bioabsorbable suture which consists of a block copolymer consisting of blocks A and B wherein: (i) block A consists of repeating units having the formula and (ii) block B consists of randomly combined repeating units having the formulae and wherein 65-75 weight% of the block copolymer comprises units of formula (I), 3-8 weight % of the block copolymer comprises units of formula (II) and 20-30 weight % of the block copolymer comprises units of formula (III). Also provided is a method for preparing a,bioabsorbable suture from a bioabsorbable block copolymer as defined above comprising the steps of: (a) polymerizing a mixture of lactide and 1,4-dioxanone at a first reaction temperature to create a copolymer thereof; (b) polymerizing glycolide with the lactide/1,4-dioxanone copolymer of step (a) at a second reaction temperature to create the bioabsorbable block copolymer; and (c) forming the resulting bioabsorbable block copolymer into the suture. The block copolymer thus used in the present invention has one block composed of units of glycolide: and another block composed of units of L-lactide: copolymerized with randomly intermingled units of 1,4-dioxanone: The block copolymer is preferably a diblock copolymer prepared by first copolymerizing the lactide monomer with 1,4-dioxanone and then polymerizing that copolymer with glycolide. Catalysts suitable for carrying out the polymerization include compounds of tin, aluminum, antimony, lead, boron, and titanium. A preferred catalyst is stannous octoate. Other catalysts include stannous chloride, dibutyl tin dilaurate, dibutyl tin diacetate, dibutyl tin dichloride, stannic chloride pentahydrate, aluminum isopropoxide, antimony trioxide, stannic fluoride, stannous citrate, stannous acetate, antimony trifluoride, tin tetraisopropoxide, lead oxide, tetraisopropyl titanate, titanium acetyl acetonate, tetraoctylene glycol titanate, boron trifluoride etherate, and aluminum trichloride. The composition range of the monomer units of the block copolymer in terms of weight % of the final block copolymer is set forth below in Table 1. Block Copolymer Composition (weight per cent) Monomer unit Range glycolide 65% to 75% lactide 3% to 8% 1,4-dioxanone 20% to 30% In the examples below a preferred composition of the block copolymer of the present invention includes 70 wt% of glycolide, 25.5 wt.% 1,4-dioxanone, and 4.5 wt.% lactide. The polymerization reaction may be carried out at a temperature of from 100°C to 250°C. The first step is the polymerization of the lactide and 1,4-dioxanone, which is carried out at a temperature of between about 100°C and 150°C and preferably about 120°C. This reaction temperature is maintained until the polymerization is substantially completed, i.e., about 30 minutes. The second step comprises raising the reaction temperature to between about 200°C and 250°C, and adding glycolide preferably when the temperature has reached about 200°C. The glycolide will then copolymerize with the lactide/dioxanone copolymer to create a separate block in a diblock copolymer. The resulting copolymer may then be subjected to further processing such as extruding, drawing, relaxing, etc. The area for use of the present invention is in the fabrication of sterile synthetic absorbable sutures, wherein glycolide is employed as the predominant monomer. The sutures may be either multifilament or monofilament. Absorbable monofilament sutures fabricated from such copolymers have been found to be useful in that they are more flexible and more resistant to in vivo strength loss than corresponding size monofilament sutures fabricated from a polymer containing only glycolic acid ester linkage. The sutures are fabricated from the copolymer using conventionally employed forming procedures, such as extrusion, and subsequently sterilized. The resulting sutures are employed in a conventional manner. Surgical sutures fabricated from the polymer of the present invention display good flexibility and knot pull strength. The following examples illustrate procedures which are useful in conjunction with the practice of the present invention. EXAMPLE 1 A conventional polymerization reactor was preheated to a temperature of 120°C. Quantities of 510 grams of 1,4-dioxanone, and 90 grams of L-lactide were added to the reactor with 0.2 grams of stannous octoate catalyst. The reactor was held at 120°C. for 60 hours until copolymerization was substantially completed. A sample of the polymer, designated as Sample 1, was taken at this point. Then the reactor temperature was gradually increased. When the reactor temperature reached 180°C, Sample 2 was taken of the polymer. When the temperature reached 200°C, 1400 grams of glycolide was added to create a block copolymer having at least one glycolide block and at least one lactide/dioxanone block. When the reactor temperature reached 220°C the polymer was stirred for 10 minutes. The polymer was then extruded and Sample 3 was taken. Samples 1, 2, and 3 were tested for inherent viscosity (dl/g), and enthalpy change (ΔH) in calories/gram. The enthalpy was measured on a differential scanning calorimeter which measured the specific heat of the sample over a range of temperatures with a scan rate of 20°C/min. The change in enthalpy is an indication of the extent of crystallinity. The inherent viscosity of a polymer is an indicator of its molecular weight, and was measured for the above-mentioned samples in accordance with standard measuring techniques and equipment known to those skilled in the art. Table 2 below sets forth inherent viscosity and enthalpy data for samples 1, 2, and 3. Sample Inherent Viscosity Enthalpy (ΔH) 1 0.61 11.1 @ 59°C 2 0.61 10.63 @ 332°C 3 0.90 14.99 @ 211°C The above data indicate that sample 3, which was taken from the polymer after the glycolide was copolymerized with the lactide/dioxanone copolymer, exhibited a higher inherent viscosity and, therefore, a higher molecular weight. This indicates copolymerization of the glycolide occurred. Also shown is a greater ΔH, which indicates a greater degree of crystallinity. These physical properties, i.e., higher molecular weight and greater crystallinity, indicate that the material is suitable for fabrication into a fiber. EXAMPLE 2 The block copolymer of Example 1 was extruded, ground and dried at from 20 to 120°C at a pressure of less than 10 torr. The resulting material was then formed by extrusion into a monofilament with an Instron rheometer (Dc=0.0301"; Lc=1.001") at a temperature of 203°C. The extrusion speed was 3 inches per minute. The monofilament was then dried at room temperature overnight. The monofilament was then drawn at a 4.5x draw ratio in an oven at 60°C by being passed around two godgets. The first godget providing a linear tangential velocity of 10 feet per minute and the second godget providing a linear tangential velocity of 45 feet per minute. The drawn monofilament was then tested for straight pull and knot pull strength. An Instron Series lX Automated Materials Testing System v4.03e was employed with the following parameters and conditions: Interface type 1011 series; Sample rate = 20.00 pts/sec.; crosshead speed = 2.0 inches/min.; humidity = 50%; temperature = 73°F. Specimens of the monofilament were tested and the results of the testing are set forth below in Table 3. (Specimen diameter = 0.0073 inches/0.1854 mm) STRAIGHT PULL KNOT-PULL Avg. load @ maximum load (lbs.) 3.111 2.780 Avg. Elongation at maximum load % 27.59 22.470 Avg. Young's Modulus (kpsi) 541.8 ---- EXAMPLE 3 The filament of Example 2 was then subjected to controlled shrinkage or relaxation in accordance with the following method. The monofilament was placed in a hot air oven at a temperature of 75°C for 10 minutes. The length of the filament before treatment was 91.8cm. After treatment, the filament length was 82.6cm. Thus, the observed shrinkage was 10%. After relaxation, the filament was tested for inherent viscosity and mechanical properties in accordance with the methods as described in Examples 1 and 2. The results of the testing are set forth below in Table 4. Post shrinkage data Inherent viscosity 0.79/0.82 Diameter 0.209mm (.00823 inches) Avg. load at maximum load (knot pull) 1.24 kg (2.728 lbs.) Avg. load at maximum load (straight pull) 1.45 kg (3.20 lbs.) Elongation at maximum load (straight pull) 39% Young's Modulus (straight pull) 513.2 kpsi The data of the above examples indicate that the copolymer of the present invention is advantageous for the manufacture of a monofilament suture. The block copolymer described above is bioabsorbable and, as seen from the Young's modulus value, is flexible, strong, and possesses advantageous handling characteristics.

Claims: A bioabsorbable suture which consists of a block copolymer consisting of blocks A and B wherein: (i) block A consists of repeating units having the formula and (ii) block B consists of randomly combined repeating units having the formulae and wherein 65-75 weight % of the block copolymer comprises units of formula (I), 3-8 weight % of the block copolymer comprises units of formula (II), and 20-30 weight % of the block copolymer comprises units of formula (III). A suture according to Claim 1, wherein the block copolymer is a diblock copolymer. A suture according to Claim 2, which is a monofilament suture. A method for preparing a bioabsorbable suture according to any of claims 1 to 3 from a bioabsorbable block copolymer, comprising the steps of: (a) polymerizing a mixture of lactide and 1,4-dioxanone at a first reaction temperature to create a copolymer thereof; (b) polymerizing glycolide with the lactide/1,4-dioxanone copolymer of step (a) at a second reaction temperature to create the bioabsorbable block copolymer and (c) forming the resulting bioabsorbable block copolymer into the suture. A method according to Claim 4, wherein said first reaction temperature is 100-150°C. A method according to Claim 4 or 5, wherein said second reaction temperature is 200-250°C. A method according to any of Claims 4 to 6, wherein step (a) is carried out in the presence of a polymerization catalyst. A method according to Claim 7, wherein said catalyst is stannous chloride, dibutyl tin dilaurate, dibutyl tin diacetate, dibutyl tin dichloride, stannic chloride pentahydrate, aluminum isopropoxide, antimony trioxide, stannic fluoride, stannous citrate, stannous acetate, antimony trifluoride, tin tetraisopropoxide, lead oxide, tetraisopropyl titanate, titanium acetyl acetonate, tetraoctylene glycol titanate, boron trifluoride etherate or aluminum trichloride. A method according to any of Claims 4-8, wherein said forming step includes the step of extruding the block copolymer into a filament.

Inventors: KAPLAN DONALD S, KENNEDY JOHN, MUTH ROSS R, KAPLAN, DONALD S., KENNEDY, JOHN, MUTH, ROSS R.
Applicants: UNITED STATES SURGICAL CORP, UNITED STATES SURGICAL CORPORATION
Application year: 1992
Main classification: C08G63
IPC codes: C08G63, A61L17