A basic principle of surgery is selection of an optimal suture material.1 The size, type (absorbable or not absorbable), and nature (natural or synthetic) of suture material selected will differ depending on the nature of the tissue repaired and surgeon preference. In vitro studies2–13 conducted in several species have provided information on properties of suture materials after incubation in various tissues and body fluids. Choice of suture material for use in cattle is typically extrapolated from results obtained in other species.
Injury to the mammary glands is common in cattle.14 Obstruction of milk flow and teat lacerations require surgical intervention, which results in exposure of the suture line to milk. With proper surgical technique, noncomplicated thelotomy incisions heal rapidly with reported complication rates of 25% for cows with mastitis and an incidence of 6% for incisional dehiscence.15,16 There is a guarded prognosis for successful repair of teat lacerations, with a high incidence of dehiscence and fistula formation. Many factors predispose to dehiscence, including inadequate suture materials and inappropriate suture patterns. The unusual forces applied to teats during milking can also be a contributing factor. Because the suture line is exposed to milk in animals undergoing teat surgery, it is important to evaluate the effect of this fluid on suture materials and determine suture materials that should be used to avoid postsurgical complications.
In the face of infection (ie, mastitis), pH and the somatic cell count of milk increase as polymorphonuclear cells migrate into the mammary glands. Inflammation during mastitis compromises healing of incisions. In cattle, infection of the mammary glands is typically caused by gram-negative or -positive bacteria.17 Teat surgery may be performed in cattle with mastitis; however, in an effort to decrease the chance of complications, the effects of mastitis must be taken into consideration when choosing suture materials. In human and veterinary medicine, there is a significant loss of tensile strength and elasticity of suture materials that have been incubated in infected urine.7-9,12,13 Therefore, it is important to examine the effect of bacteria found in mastitic milk on suture properties and determine the best suture for use in cattle with mastitis to avoid post-surgical complications.
For the study reported here, we hypothesized that tensile properties (ie, breaking strength and elasticity) of suture materials would decrease more rapidly after incubation in milk and BCM, compared with results after incubation in PBSS. Our objectives were to determine the in vitro effects of incubation in PBSS, milk, and BCM on suture properties of 3 types of synthetic absorbable suture materials.
Materials and Methods
Sample population—Three types of synthetic absorbable suture materials were selected for use in the study. Materials were selected on the basis of the authors' clinical experiences. The synthetic absorbable suture materials were poliglecaprone 25,a polyglycolic acid,b and polydioxanone.c All sutures were of the same size (United States Pharmacopeia 3.0). Sutures of each type were obtained from the same manufacturer and lot number to minimize potential variation.
Incubation of suture materials—Three incubation media (PBSS [pH, 6.2], milk, and BCM) were used. Media were changed aseptically every 3 days to ensure sterility and consistency of bacteriologic counts.
Fresh milk was obtained from the bulk tank of the university dairy farm each week until the end of the study and pasteurized for use in the study. Somatic cell count of the milk was 443,000 cells/mL. Milk was pasteurized by heating to a temperature of 68°C for > 16 seconds.18 An electric heating element was used to achieve the pasteurization temperature. A commercial mercury thermometer was used to verify temperatures during the pasteurization process. To verify sterility of the milk, aerobic bacterial culture was performed, which yielded negative culture results after incubation for > 48 hours. Pasteurized milk was transferred into sterile containers, antimicrobials (gentamicin and ceftiofur) were added to milk without bacteria, and the final solution was stored at 4°C until use.
Streptococcus agalactiae isolated from a cow with mastitis was used to create BCM. Bacteria were cultured on a blood agar plate prior to each change of the medium. Purity of the culture was verified before it was diluted and added to the milk. A sterile loop was used to create a solution with a concentration of 108 bacteria/mL. A McFarland suspension solution for turbidity was used as the control sample. This BCM solution was prepared in 6 mL of sterile water. The solution was then added to 600 mL of milk to obtain a final dilution of 106 bacteria/mL. The final dilution was chosen on the basis of other in vitro studies7-9,12,13 in which investigators used bacteria-contaminated urine as a method for evaluating effects of cystitis on suture material. As a quality-control measure, 5 ML of the original solution was diluted until a suspension of 103 bacteria/mL was obtained. The final solution (106 bacteria/mL of milk) was cultured on blood agar plates for 48 hours for verification of bacteriologic counts. These procedures were repeated before every change of media.
Suture materials were removed from their respective packages (70 cm of suture material/package); each suture was cut into 2 equal pieces. Six pieces of the suture materials were inserted aseptically into each of several sterile containers. Containers were filled with 15 mL of medium (PBSS, milk, or BCM). A sterile plunger was inserted into each container to ensure that the sutures were submerged in the medium.
Suture materials were incubated at 37°C for up to 21 days. Media were changed aseptically every 3 days to ensure sterility and consistency of bacteriologic counts. At each change of media, a pooled sample of each of the solutions in which the sutures were incubated was obtained for testing. The pH was measured, and pooled solutions were submitted for aerobic bacteriologic culture.
Testing to determine breaking strength and elasticity—Tensile properties were evaluated for samples obtained before (day 0) and 7, 14, and 21 days after start of incubation. These time frames after start of incubation were selected on the basis of the expected intervals for healing of teat incisions.16 Testing was conducted in a laboratory controlled for temperature and relative humidity (20°C and 65% relative humidity).
On day 0, sutures were removed from their sterile packages, rinsed with deionized water, and tested. On days 7, 14, and 21, sutures were removed from the incubation media, rinsed with deionized water, and tested. Six pieces of each suture material for each medium were mechanically tested at each time point. A template was used to ensure suture loops were consistent in length.
After rinsing, a surgeon's throw followed by 4 square knots was used to join together the pieces of each type of suture material. All sutures were tied by the same investigator (SN) by use of a 2-hand tie technique. Suture was looped through cylinder clamps on a biomechanical distracting unitd for testing. A 25-kg load cell was used for testing. During elongation, a graph of the forces applied as a function of time was plotted by use of computer software.e
Elasticity of the suture was measured by use of the same computer software.e Suture was elongated at a rate of 60 mm/min until the breaking point was reached. Location of the point at which the suture loop broke was recorded. For the majority (85%) of the tests, the suture broke at the knot.
Statistical analysis—Statistical analysis was performed by use of statistical computer software.f Normality of the data was assessed. A 2-way ANOVA on repeated measures was performed.19 Variables evaluated were elasticity and breaking strength among time points and among media. Values of P < 0.05 were considered significant. Significant results were analyzed by use of a Bonferroni post hoc test.
Results
Poliglecaprone 25 suture—Breaking strength and elasticity of poliglecaprone 25 was significantly (P < 0.001) decreased after incubation for 7 days in all 3 media, compared with results for day 0 (Table 1). Breaking strength after incubation for 7 days in PBSS, milk, and BCM was 53%, 30%, and 27%, respectively, of the values for day 0. Elasticity after incubation for 7 days in PBSS, milk, and BCM was 71%, 45%, and 52%, respectively, of the values for day 0. After incubation in the 3 media for 14 or more days, tensile properties of poliglecaprone 25 could not be determined because suture integrity had degraded beyond the ability of the investigator to tie a knot without breaking the suture. Breaking strength and elasticity of poliglecaprone 25 were significantly (P = 0.01) decreased after incubation for 7 days in milk or BCM, compared with results after incubation in PBSS for 7 days.
Mean ± SD values for tensile properties of poliglecaprone 25 suture before (day 0) and after incubation for 7 or 14 days in PBSS, milk, and BCM.
| Variable | Day | PBSS | Milk | BCM |
|---|---|---|---|---|
| Breaking strength (N) | 0 | 49.13 ± 1.71 | 49.13 ± 1.71 | 49.13 ± 1.71 |
| 7* | 26.14 ± 1.90† | 14.71 ± 3.93† | 13.31 ± 6.04† | |
| 14 | ND | ND | ND | |
| Elasticity (mm) | 0 | 29.49 ± 1.39 | 29.49 ± 1.39 | 29.49 ± 1.39 |
| 7‡ | 22.69 ± 1.32† | 13.18 ± 0.97† | 15.24 ± 4.26† | |
| 14 | ND | ND | ND |
Within a row within a variable, values differ significantly (P ≤ 0.001) among media.
Within a column within a variable, value differs significantly (P ≤ 0.001) from the value for day 0.
Within a row within a variable, values differ significantly (P≤ 0.05) among media.
ND = Not determined because of loss of suture integrity.
Polyglycolic acid suture—Breaking strength and elasticity of polyglycolic acid suture were significantly (P < 0.001) decreased after incubation for 7 or 14 days in all 3 media, compared with results for day 0 (Table 2). Breaking strength in PBSS, milk, and BCM was 65%, 60%, and 64%, respectively, after incubation for 7 days and 14%, 15%, and 11%, respectively, after incubation for 14 days, relative to the values for day 0. Elasticity in PBSS, milk, and BCM was 69%, 54%, and 67%, respectively, after incubation for 7 days and 15%, 17%, and 16%, respectively, after incubation for 14 days, relative to values for day 0. After incubation in the 3 media for 21 days, tensile properties of polyglycolic acid suture could not be determined because suture integrity had degraded beyond the ability of the investigator to tie a knot without breaking the suture. Breaking strength for polyglycolic acid incubated in BCM was significantly decreased, compared with values for polyglycolic acid suture incubated in PBSS or milk.
Mean ± SD values for tensile properties of polyglycolic acid suture before (day 0) and after incubation for 7, 14, or 21 days in PBSS, milk, and BCM.
| Variable | Day | PBSS | Milk | BCM |
|---|---|---|---|---|
| Breaking strength (N) | 0 | 48.25 ± 3.22 | 48.25 ± 3.22 | 48.25 ± 3.22 |
| 7‡ | 31.55 ± 2.58† | 28.83 ± 1.81† | 30.90 ± 2.32† | |
| 14§ | 6.53±1.08† | 7.53 ± 0.82† | 5.38 ± 0.65† | |
| 21 | ND | ND | ND | |
| Elasticity (mm) | 0 | 15.10 ± 1.16 | 15.10 ± 1.16 | 15.10 ± 1.16 |
| 7‡ | 8.87 ± 0.34† | 8.09 ± 0.48† | 10.12 ± 2.18† | |
| 14‡ | 2.32 ± 0.57† | 2.64 ± 0.47† | 2.39 ± 0.47† | |
| 21 | ND | ND | ND |
Within a row within a variable, values differ significantly (P = 0.01) among media.
See Table 1 for remainder of key.
Polydioxanone suture—Breaking strength and elasticity were significantly decreased after incubation in all media at day 21, compared with values for day 0 (Table 3). Breaking strength after incubation for 21 days in PBSS, milk, and BCM was 83%, 82%, and 86%, respectively, of the value for day 0. Elasticity after incubation for 21 days in PBSS, milk, and BCM was 76%, 83%, and 87%, respectively, of the value for day 0. Tensile properties of polydioxanone suture did not differ significantly among the media at any time point.
Mean ± SD values for tensile properties of polydioxanone suture before (day 0) and after incubation for 7, 14, or 21 days in PBSS, milk, and BCM.
| Variable | Day | PBSS | Milk | BCM |
|---|---|---|---|---|
| Breaking strength (N) | 0 | 46.45 ± 4.99 | 46.45 ± 4.99 | 46.45 ± 4.99 |
| 7 | 41.10 ± 2.96 | 43.62 ± 4.61 | 42.37 ± 4.44 | |
| 14 | 39.71 ± 1.90∥ | 43.25 ± 2.94 | 42.43 ± 5.54 | |
| 21 | 38.35 ± 1.21∥ | 38.24 ± 3.53∥ | 40.08 ± 3.08¶ | |
| Elasticity (mm) | 0 | 29.27 ± 3.64 | 29.27 ± 3.64 | 29.27 ± 3.64 |
| 7 | 24.90 ± 1.96 | 27.90 ± 3.23 | 27.17 ± 1.96 | |
| 14 | 24.23 ± 1.18∥ | 26.86 ± 1.67 | 26.52 ± 3.93 | |
| 21 | 22.17 ± 0.66† | 24.29 ± 2.84∥ | 25.39 ± 1.68¶ |
Within a column within a variable, value differs significantly (P = 0.01) from the value for day 0.
Within a column within a variable, value differs significantly (P ≤ 0.05) from the value for day 0.
See Table 1 for remainder of key.
Discussion
In the study reported here, incubation in milk had a significant effect on breaking strength and elasticity of suture materials. There was a significant, rapid decrease in breaking strength and elasticity by 7 days after initiation of incubation for poliglecaprone 25, a synthetic monofilament suture. Incubation of poliglecaprone 25 in PBSS resulted in significantly higher breaking strength and elasticity, compared with results for that suture material when incubated in milk and BCM. Therefore, we assume that milk components increase the degradation rate of the material. The mechanism by which there is more rapid degradation is unknown but may be related to pH or milk constituents. Because poliglecaprone 25 is designed to be a suture material with a rapid absorption rate, the use of poliglecaprone 25 in teat surgery in cattle is not recommended.1 Rapid degradation of poliglecaprone combined with the apparent increase in degradation caused by milk may result in incisional dehiscence. In vivo studies would be needed to determine whether it can be used in teats of affected cattle.
In the study reported here, there was a significant decrease in breaking strength and elasticity of polyglycolic acid, a synthetic multifilament suture material, after incubation in all media for 14 days; bacterial contamination of milk exacerbated this effect. In principle, it is not desirable to use multifilament suture material when there is infection. Polyglycolic acid is coated with polycaprolate to decrease the rate of degradation, but degradation was more severe in milk contaminated with S agalactiae in this study. On the basis of the results of this study, when a cow has mastitis caused by S agalactiae, polyglycolic acid should not be used. The increased degradation rate of polyglycolic acid by bacteria can lead to incisional dehiscence. Additional studies are needed to evaluate the effects of other types of bacteria found in clinical and subclinical mastitis on tensile properties of suture materials.
Polydioxanone, a synthetic monofilament suture material, retained the highest breaking strength and elasticity throughout the study, compared with results for the other suture materials. The type of medium did not significantly affect degradation rate of polydioxanone. However, breaking strength and elasticity were lower, but not significantly different, after incubation of polydioxanone in PBSS. It is possible that milk constituents protected polydioxanone from hydrolysis by acting as a coating agent. Additional studies to evaluate polydioxanone by use of scanning electron microscopy after incubation in milk may help determine the effect of the milk on this suture material. On the basis of results of this study, polydioxanone would seem to be the suture material most suitable for use in teat surgery. Of particular clinical interest was that the tensile properties of polydioxanone were similar when incubated in milk with or without contaminating bacteria.
Good surgical technique and selection of the optimal suture material should decrease the chance of postoperative complications when performing teat surgery in cows. Teat incisions are expected to heal during a period of 21 days.16 On the basis of results of the study reported here, polydioxanone would appear to be the best of the 3 suture materials tested for use in teat surgery because it is a synthetic, absorbable, monofilament material that retains useful breaking strength despite effects of bacteria. Polyglycolic acid and poliglecaprone 25 sutures would seem to be less desirable for use in teat surgery because these materials were degraded too severely and rapidly, especially when considered in context of the expected healing time of 21 days.
We chose to evaluate a testing loop, rather than a single thread. Size of the suture material used (United States Pharmacopeia 3.0) made it difficult to affix or clamp the ends of the suture without causing damage or having them slip. The suture loop was also chosen because it most closely represents a clinical situation.20–22 A template was used to efficiently create uniform loops, but there may have been variation among the knots tied by the investigator. Although it has been proposed that a jig with weights attached to a needle driver can ensure consistency in knots, the fragility of our materials made its use impractical.20 Techniques for hand ties with gentle tying of knots prevented damage to the suture materials. Multiple low-weight axial loading for cycling the loop before testing would have been necessary to ensure uniform tightness of knots. To minimize variation, only 1 investigator (SN) tied the knots. A graph of forces applied over time was plotted for each suture loop tested. Each graph was reviewed for irregular decrease of force during elongation of the materials. A sudden decrease in force, which is believed to be secondary to slippage of knots for monofilament suture materials, was detected in < 5% of the graphs. With multifilament sutures, a decrease of force can be secondary to breaking of threads within the suture as well as slippage of knots.
Degradation of suture material progressed to a point at which it was impossible to tie a knot without damaging the sutures by day 14 of incubation for poliglecaprone 25 and day 21 of incubation for polyglycolic acid. Therefore, no measurements were obtained for those suture materials at those time points. We believe that the breaking strength at those time points would have been close to zero. Tying the loop before sutures were incubated may have allowed testing of the sutures at those time points. We chose not to tie the sutures before incubation in an attempt to provide subjective assessment of suture handling properties. Although objective testing would have been preferred, we believe that the results reported here were accurate in that the sutures had lost integrity beyond any ability to resist tensile testing.
The study reported here is an initial step in the evaluation of interactions between suture materials and healing of teats. Additional studies should evaluate cycling fatigue of the material in an attempt to mimic the approximately 500 changes of pressure on an incision during milking by machine. Bursting strength should be evaluated for various suture patterns and sizes of tissue bites to determine optimal surgical techniques. Research is needed because incisional dehiscence is a catastrophic event following teat surgery that can result in loss of the gland for milk production or culling of the affected cow from the herd.
ABBREVIATION
| BCM | Bacteria-contaminated milk |
Monocryl, Ethicon, Johnson & Johnson, Somerville, NJ.
Dexon II, Syneture, Norwalk, Conn.
PDS II, Ethicon, Johnson & Johnson, Somerville, NJ.
Instron 1322, Instron Industrial Products, Grove City, Pa.
Universal test program (UTP-III) 1993, Interlaken Technology, Chaska, Minn.
PRISM, version 4, GraphPad Software Inc, San Diego, Calif.
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