Abstract:
A higher order polyamide monofilament suture exhibits improved properties such as greater resistance to creep, greater knot security and greater chemical stability in aqueous environments.

Description:
This is a divisional of U.S. application Ser. No. 08/126,679 filed Sep. 24, 1993, now U.S. Pat. No. 5,349,044 which is a continuation of U.S. application Ser. No. 07/828,005 filed Jan. 30, 1992, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a polyamide (nylon) monofilament suture and, more particularly, to such a suture manufactured from a higher order polyamide such as nylon 12. 
     Polyamide monofilament sutures are known. Illustrative of the known processes for the melt spinning of polyamide strands (filament,yarn, etc.) are those described in U.S. Pat. Nos. 2,212,772; 2,226,549; 3,091,016; 3,303,169; 3,345,445; 3,361,859; 3,379,810; 3,382,307; 3,577,500; 4,009,511; 4,374,797; 4,446,299; 4,461,740; 4,504,432; 4,504,545; 4,542,063; 4,578,421; 4,621,021; 4,624,816; 4,701,377; 4,758,472; 4,839,132; and, 4,859,389. 
     One important characteristic of a suture is the amount of effort typically required to straighten the suture upon its removal from the package in order to ready the suture for use. In the case of a polyamide monofilament suture, this effort appears to be related to the &#34;energy&#34; of the suture, i.e., the integrations of the stress-strain curve for the suture measured in kilograms, and is equivalent to the work expended in elongating the monofilament by a specified percentage of its original length. As the energy of a give size of polyamide monofilament suture is less so, too, the amount of effort required to straighten the suture prior to use is less. 
     Another important consideration in the manufacture and use of sutures is that a yarn may stretch when under tension for an extended period of time. Under the tension of a surgeon&#39;s knot used for wound closure, a suture may undergo stretching which will cause the ligature to loosen, a phenomenon referred to herein as &#34;creep&#34;. 
     Commonly owned copending U.S. patent application Ser. No. [203-452/1219] describes a process for manufacturing a polyamide monofilament suture from such nylons as nylon 6 and nylon 6,6 which exhibits reduced energy and/or an increased ability to retain a knot for a given suture when compared to prior polyamide sutures. 
     The present invention provides a suture exhibiting excellent dimensional stability, or resistance to creep, and high levels of strength retention and chemical stability when exposed to aqueous environments. The present invention further provides a suture exhibiting increased knot security or stability. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, there is provided a polyamide monofilament suture manufactured from a higher order polyamide, i.e, a fiber-forming polyamide homopolymer or copolymer possessing a substantial number of repeating units selected from the group consisting of units of the structure ##STR1## in which n is at least about 7 and units of the structure ##STR2## in which R 1  is an aliphatic group of from about 4 to about 20 carbon atoms, a cycloaliphatic group of from about 6 to about 20 carbon atoms or an aromatic group of from about 6 to about 20 carbon atoms and R 2  is an aliphatic, cycloaliphatic or aromatic group of from about 6 to about 20 carbon atoms, provided, R 1  contains at least 6 carbon atoms when R 2  is an aliphatic group of 6 carbon atoms or R 2  contains at least 8 carbon atoms when R 1  contains 4 carbon atoms. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a schematic illustration of apparatus which is suitable for manufacturing the high order polyamide monofilament suture of this invention; and, 
     FIG. 1B is a modification of the apparatus of FIG. 1A which is particularly suitable for the manufacture of polyamide monofilaments of smaller size, e.g., sizes 4/0 and smaller. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred higher order polyamides from which the suture of this invention can be manufactured contain at least about 30 mole percent, preferably at least about 50 mole percent and most preferably at least about 70 mole percent of units of the structure ##STR3## and/or the structure ##STR4## in which m, R 1  and R 2  have the above-designated meanings. 
     The higher order fiber-forming polyamides can be prepared by polymerizing aminocarboxylic acid and/or corresponding lactam or by polymerizing substantially equimolar proportions of diamine and dicarboxylic acid or functional derivative thereof such as an ester or acid chloride employing known and conventional techniques. Examples of the aforementioned aminocarboxylic acids or lactams thereof which are useful in preparing the higher order polyamides include those compounds containing at least about 8, preferably at least about 10 and most preferably at least about 12 carbon atoms. Particular examples of useful aminocarboxylic acids and lactams include capryllactam, 10 -aminodecanoic acid, 11-aminoundecanoic acid and laurolactam, and the like. 
     Diamines suitable for use in preparation of the higher order fiber-forming polyamides include the straight chain and branched chain alkyl, aryl and alkaryl diamines. Such diamines include, e.g., those represented by the general formula 
     
         H.sub.2 N(CH.sub.2).sub.m NH.sub.2 
    
     wherein m is an integer of at least about 6 and preferably at least about 12. Suitable diamines include hexamethylene diamine, octamethylene diamine, trimethylhexamethylene diamine, m-phenylene diamene and m-xylylene diamine. 
     The useful dicarboxylic acids can be represented by the formula 
     
         HOOC--W--COOH 
    
     wherein W is a divalent aliphatic or aromatic group containing at least 6 carbon atoms. Suitable aliphatic acids include sebacic acid, suberic acid, azelaic acid, pimellic acid and adipic acid. Suitable aromatic acids include isophthalic acid and terephthalic acid. 
     Typical examples of the polyamide or nylon (as it is often called) resin used in the present invention are nylon 11, 12, 6/9, 6/12 and other higher order polyamides. Nylon 12 is an especially preferred fiber-forming polyamide resin from which the suture of this invention can be manufactured. In general, useful higher order polyamides will possess a number average molecular weight of at least about 10,000 and a melt flow index in g/10 min. of from about 1.5 to about 7.5. 
     Also included are block and random copolymers formed from one or more of the foregoing monomers and one or more other monomers copolymerizable therewith, e.g., lactones, alpha-hydroxy carboxylic acids and oligomers thereof, aminocarboxylic acids and/or corresponding lactams in addition to those mentioned, etc. Suitable lactones include gamma-butyrolactone and epsilon caprolactone. Suitable alpha-hydroxy carboxylic acids include hydroxybutanoic acid and hydroxycaproic acid. Useful higher order polyamides also include any of the foregoing homopolymers and copolymers blended with one or more compatible polymers, e.g., unlimited amounts of other polyamide homopolymers and/or polyamide copolymers, relatively minor amounts of polyester homopolymers and/or polyester copolymers, polyurethanes, etc. 
     A suitable process for the manufacture of the higher order polyamide monofilament suture of the present invention is disclosed and claimed in commonly assigned, concurrently filed U.S. patent application Serial No. [203-452/1219], the contents of which are incorporated by reference herein. The process comprises the operations of extruding the polyamide resin at an extrusion temperature of from about 180° to about 290° C. to provide a monofilament, stretching the solidified monofilament at a temperature of from about 40° to about 98° C. in water (or other suitable liquid medium) or at from about 50° to about 185° C. in air (or other suitable gaseous medium) at a stretch ratio of from about 3:1 to about 5:1 to provide a stretched monofilament and annealing (relaxing) the stretched monofilament at a temperature of from about 100° to about 240° C. to provide the finished suture, the annealing resulting in shrinkage of the stretched monofilament for a recovery to within about 80% to about 97% percent of the length of the monofilament prior to annealing. 
     FIG. 1A schematically illustrates a higher order polyamide monofilament suture manufacturing operation which is especially suitable for producing larger size sutures, e.g., those of sizes 3/0 and larger. Extruder unit 10 is of a known or conventional type and is equipped with controls for regulating the temperature of barrel 11 in various zones thereof, e.g., progressively higher temperatures in three consecutive zones A, B and C along the length of the barrel. Pellets or powder of polyamide resin are introduced to the extruder through hopper 12. Any of the polyamides which are useful for the formation of fibers can be used herein. Representative of such polyamides are polycaprolactam (nylon 6) and polyhexamethylene adipamide (nylon 6,6). Optionally, the polyamide sutured may be dyed. 
     Motor-driven metering pump 13 delivers extruded resin at a constant rate to spin pack 14 and thereafter through spinneret 15 possessing one or more orifices of desired diameter to provide a molten monofilament 16 which then enters quench bath 17, e.g., containing water, where the monofilament solidifies. The distance monofilament 16 travels after emerging from spinneret 15 to the point where it enters quench bath 17, i.e., the air gap, can vary and can advantageously be from about 0.5 to about 100 and preferably from about 1 to about 20 cm. If desired, a chimney (not shown), or shield, can be provided to reduce the length of air gap, e.g., to from 1 to 10 cm, thereby isolating monofilament 16 from contact by air currents which might otherwise affect the cooling of the monofilament in some unpredictable manner. In general, barrel zone A of the extruder can be maintained at a temperature of from about 215° to 225° C., zone B at from about 220° to 230° C. and zone C at from about 220° to about 235° C. Additional temperature parameters include: metering pump block 13 at from about 220° to about 235° C., spin pack 14 at from about 215° to about 235° C., spinneret 15 at from about 215° to about 235° C. and quench bath at from about 35 to about 45° C. 
     Entering quench bath 17, monofilament 16 is passed by driven roller 18 over rollers 19 and 20 and thereafter is wrapped around a first godet 21 provided with nip roll 22 to prevent slippage which might otherwise result from the subsequent stretching operation. Monofilament 16 passing from godet 21 is stretched, e.g., with stretch ratios on the order of from about 3:1 to about 5:1 and preferably from about 3.5:1 to about 4.5:1, to effect its orientation and thereby increase its tensile strength. In the stretching operation shown in FIG. 1A, generally suitable for larger size sutures, e.g., sizes 2/0 to 3/0, monofilament 16 is drawn through hot water draw bath 23 by means of second godet 24 which rotates at a higher speed than first godet 21 to provide the desired stretch ratio. The temperature of hot water draw bath 23 is advantageously from about 60° to about 98° C. and preferably is from about 80° to about 95° C. 
     In the alternative stretching operation shown in FIG. 1B, generally preferred for smaller sutures sizes, e.g., sizes 4/0 to 8/0, monofilament 16 is drawn by second godet 24&#39; through hot air oven chamber 23&#39; at a temperature of from about 100° to about 185° C. and preferably from about 150° to about 170° C. to provide the desired amount of stretch. Following the stretching operation shown in FIG. 1A or 1B, monofilament 16 is subjected to an on-line annealing (relaxation) operation as a result of which the monofilament shrinks. In the processes of FIGS. 1A and 1B, annealing (relaxation) is accomplished by driving monofilament 16 by third godet 26 through second hot air oven chamber 25 at a temperature of from about 120° to about 180° C. and preferably from about 140° to about 160° C. At these temperatures, monofilament 16 will generally recover to within about 85 to about 97 percent, and preferably to within about 95 percent, of its pre-annealed length to provide the finished suture. The third godet relates at a slower speed than the second godet thus relieving tension on the filament. 
     If desired, the suture can be dyed. If transportation in bulk is required after dyeing, it is preferable to collect the dyed monofilaments in skeins, not on spools, as the latter can cause damage to the suture (flattening). Logwood extract is a known dye for polyamide sutures and can be used herein. However, other dyes known to be suitable for incorporation in sutures can also be used herein, e.g., D&amp;C Green No. 5 and FD&amp;C Blue No. 2 as described in the Handbook of U.S. Colorants for Food, Drugs and Cosmetics, by Daniel M. Marrion, 1979. 
     It is believed that the superior physical and chemical properties of the polyamide sutures of the present invention are due at least in part to the increased carbon content in the structural units of the higher order polyamide resins used in their manufacture. This results in a decrease in the number of amide linkages in the resin. The hydrophilic amide linkage is not only susceptible to cleavage in an aqueous environment, it also favors hydrogen bonding and consequently an affinity for attracting water to the polymer. 
     In order that those skilled in the art may be better able to practice the present invention, the following examples are given as an illustration of the preparation and superior characteristics of the polyamide sutures of the present invention. It should be noted that the invention is not limited to the specific details embodied in the examples and further that all parts are be weight. 
     EXAMPLES 1-3 
     Table I below sets forth typical conditions for extruding, stretching and annealing (relaxing) various sizes of polyamide monofilament suture in accordance with this invention. All of the monofilament sutures were fabricated from AESNO (a commercially available nylon 12 from ATOCHEM North America, Inc.) having an inherent viscosity of about 1.6 dl/g and a moisture content of less than 0.08 weight percent, and a melt flow index of 3.4 g/10 min. as indicated by the manufacturer. 
     
                       TABLE I______________________________________CONDITIONS OF MANUFACTURING VARIOUS SIZESOF HIGHER ORDER POLYAMIDE MONOFILAMENTSUTURE          Example          1      2        3          Suture SizeProcess Conditions            0        3/0      6/0______________________________________          Extrusion Operationextruder screw, rpm            24       10.8     3.4pump rpm         17.4     6.8      9.85driven roller, rpm            7        7        8barrel temp., °C., zone A            200      200      200barrel temp., °C., zone B            210      210      210barrel temp., °C., zone C            220      215      215clamp temp., °C.            215      215      215adapter temp., °C.            215      215      215pump temp., °C.            220      218      222block temp., °C.            220      218      218barrel melt temp., °C.            218      215      214pump melt temp., °C.            228      221      218spinneret melt temp., °C.            230      225      226barrel pressure, psi            1800     2150     2350pump pressure, psi            1100     1484     2650spinneret pressure, psi            4500     3228     2650pump size, cc per revolution            1.168    1.168    0.16diameter of spinneret orifices,            1.25     1.25     0.5mmno. of spinneret orifices            4        4        4air gap, cm      3        2        3quench bath temp., °C.            30       36       30depth of driven roller            38       37       38filtration, microns            12       12       12          Stretching (Orienting) Operationdraw bath temp, °C.            97       97       --first oven chamber temp, °C.            98       82       135first godet, mpm 10.3     10.3     11.6second godet, mpm            40.7     40.8     45.4draw ratio       4.1      4.1      4.1          Annealing (Relaxing) Operationsecond oven chamber temp,            98       98       98°C.third godet, mpm 39.0     38.9     43.4______________________________________ 
    
     COMPARATIVE EXAMPLE 1 
     Table II below sets forth typical conditions for extruding, stretching and annealing (relaxing) a size 3/0 polyamide 6 monofilament suture. In general, useful polyamides will possess a number average molecular weight of at least about 15,000 and a melt flow index in g/10 min of from about 3 to about 8 and preferred from about 4 to about 6. The suture was manufactured from Capron 8207F (Allied Signal Corp.), a polycaprolactone having a relative viscosity of 73.9 and a moisture content of 0.062 weight percent as indicated by the manufacturer. 
     
                       TABLE II______________________________________CONDITIONS OF MANUFACTORING A POLYAMIDE 6MONOFILAMENT SUTURE            Comparative Example            1            Suture SizeProcess Conditions            3/0______________________________________            Extrusion Operationextruder screw, rpm            22.0pump rpm         7.65driven roller, rpm            10barrel temp., °C., zone A            220barrel temp., °C., zone B            225barrel temp., °C., zone C            230clamp temp., °C.            225adapter temp., °C.            225pump temp., °C.            225block temp., °C.            225spinneret temp., °C.            230barrel melt temp., °C.            234pump melt temp., °C.            230spinneret melt temp., °C.            227______________________________________            Example            1            Suture SizeProcess Conditions            3/0______________________________________            Extrusion Operationbarrel pressure, psi            2650pump pressure, psi            2890spinneret pressure, psi            2730pump size, cc per revolution            1.168diameter of spinneret orifices,            1.25mmno. of spinneret orifices            4air gap, cm      2quench bath temp., °C.            45depth of driven roller            25filtration, microns            12            Stretching (Orienting) Operationdraw bath temp, °C.            80first oven chamber temp, °C.            --first godet, mpm 17.6second godet, mpm            70.1draw ratio       4.0            Annealing (Relaxing) Operationsecond oven chamber temp,            150°C.third godet, mpm 66.4______________________________________ 
    
     COMPARATIVE EXAMPLE 2 
     A size 3/0 nylon 6 suture was prepared in the matter of Comparative Example 1 except that the suture was dyed with black logwood extract. 
     The physical properties of the higher order polyamide monofilament sutures of this invention and those of the comparative examples were measured at 73° F. and 50 relative humidity. Measurements of percent elongation, tensile strength and energy were carried out employing an Instron Corporation (Canton, Mass.) Tensile Tester, model no. 4301, equipped with yarn grips and operated with a gauge length of 127 mm and a crosshead speed of 127 mm/min. 
     The physical properties of the sutures and the procedures employed for their measurement are set forth in Table III as follows: 
     
                       TABLE III______________________________________PROCEDURES FOR MEASURING PHYSICALPROPERTIES OF POLYAMIDE MONOFILAMENTSUTURESPhysical Property         Test Procedure______________________________________knot pull, tensile         U.S.P. XXI, tensile strength, suturesstrength, kg  (881)straight pull, kg         ASTM D-2256, Instron Corporationelongation at break, %         ASTM D-2256tensile strength, kg/mm.sup.2         ASTM D-2256, Instron Corporation         Series IX Automated Materials Testing         System 1.03A0-5% and 0-10% strain         ASTM D-2256, Instron Corporationenergies, kg-mm         Series IX Automated Materials Testing         System 1.03Aknot security A 2 cm loop is tied with a surgeon&#39;s         square knot (1 = 1 = 1 = 1) securing         the throws at 20% of the USP XXII         knot strength for 2/0 nonabsorbable         sutures (n = 10 loops per group). The         loop is placed next to a cloth-wrapped         mandrel rotating at .5 rpm. The fixtures         are secured to allow contact of the         cloth material against the fourth throw         or, top throw, of each knot. The cloth         wrapping is moistened with 37° C.         water prior to the test and is period-         ically remoistened during the test. Each         pass of the cloth across the knot (for a         total of 100 passes), the knot is inspected         for top throw security. For a knot to be         considered secure, the 3 mm ears must         not come undone and there must be no         relaxation of the knot or loss of the         fourth throw.melt flow     ASTM D-1238______________________________________ 
    
     Table IV below sets forth,, the physical properties of the size 3/0 polyamide 12 suture of the present invention as compared to those properties of the polyamide 6 suture employing the physical testing procedure set forth in Table II. 
     
                       TABLE IV______________________________________          Comp.    Comp.Physical Property          Ex. 1    Ex. 2    Ex. 1 Ex. 2______________________________________Wet strength retention for          80%      81%      105%  99%knot-pullWet strength retention for          94%      83%      100%  100%straight-pullDiameter change          +5%       0%       +1%  -1%Modulus change -49%     -32%      -5%  -6%diameter (mm)  0.230    0.236    0.242 0.244Wet diameter (mm)          0.241    0.236    0.245 0.242knot-pull (kg) 1.78     1.63     1.48  1.63Wet knot-pull (kg)          1.42     1.32     1.55  1.61Straight-pull (kg)          2.09     2.01     2.07  1.83wet straight-pull (kg)          1.97     1.68     2.09  1.85Energy 0-5% (kg-mm)          1.04     1.30     2.50  2.18Wet Energy 0-5%          0.24     0.42     2.69  1.70(kg-mm)Energy 0-10% (kg-mm)          6.56     3.70     10.93 8.53Wet Energy 0-10%          3.55     2.71     11.00 7.51(kg-mm)______________________________________ 
    
     As demonstrated in Table IV, the strength retention for both straight pull and knot-pull of the nylon 12 suture when exposed to an aqueous medium containing large quantities of water is much greater than that of a corresponding nylon 6. 
     The yarn of Example 2 and Comparative Example 2 were placed under 1.8 g/den constant tension in a water bath at ambient temperature. At the time intervals listed below, the percent elongation, or &#34;creep&#34; was measured Table V further illustrates the physical stability of the nylon 12 suture. 
     
                       TABLE V______________________________________      Nylon 6         Nylon 12Creep, %   Comparative Example 2                      Example 2______________________________________ 5 min     3.3%            1.1%10 min     4.4%            1.1%30 min     6.1%            1.7%45 min     7.2%            1.7%18 hours   13.2%           2.9%______________________________________ 
    
     Table V further illustrates the physical stability of the nylon 12 suture. 
     Obviously, other modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that changes may be made in particular embodiments of the invention described which are within the full intended scope of the invention as defined by the claims.