A biomechanical comparison of a novel two-loop suture technique and two sutures for laryngoplasty in the horse

David G. Wilson Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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 DVM, Diplomate ACVS
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Imma Roquet Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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Michelle L. Tucker Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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 DVM, PhD, Diplomate ACVS
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James L. Carmalt Department of Large Animal Clinical Sciences, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

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 MA, VetMB, MVetSc, PhD, FRCVS, Diplomate ABVP (Equine), Diplomate AVDC, Diplomate ACVSMR, Diplomate ACVS
DOI:
https://doi.org/10.2460/ajvr.22.11.0189
Volume/Issue:
Volume 84: Issue 5
Online Publication Date:
07 Mar 2023
Open access
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Abstract

OBJECTIVES

Evaluation of the strength of the novel suture technique by comparison with a 2-interrupted suture technique.

SAMPLE

40 equine larynges.

PROCEDURES

40 larynges were used; 16 laryngoplasties were performed using the currently accepted 2-suture technique and 16 using the novel suture technique. These specimens were subjected to a single cycle to failure. Eight specimens were used to compare the rima glottidis area achieved with 2 different techniques.

RESULTS

The mean force to failure, as well as the rima glottidis area of both constructs, were not significantly different. The cricoid width did not have a significant effect on the force to failure.

CLINICAL RELEVANCE

Our results suggest that both constructs are equally strong and can achieve a similar cross-sectional area of the rima glottidis. Laryngoplasty (“tie-back”) is currently the treatment of choice for horses with exercise intolerance due to recurrent laryngeal neuropathy. Failure to maintain the expected degree of arytenoid abduction post-surgery occurs in some horses. We believe this novel 2-loop pulley load-sharing suture technique can help achieve and, more importantly, maintain the desired degree of abduction during surgery.

Abstract

OBJECTIVES

Evaluation of the strength of the novel suture technique by comparison with a 2-interrupted suture technique.

SAMPLE

40 equine larynges.

PROCEDURES

40 larynges were used; 16 laryngoplasties were performed using the currently accepted 2-suture technique and 16 using the novel suture technique. These specimens were subjected to a single cycle to failure. Eight specimens were used to compare the rima glottidis area achieved with 2 different techniques.

RESULTS

The mean force to failure, as well as the rima glottidis area of both constructs, were not significantly different. The cricoid width did not have a significant effect on the force to failure.

CLINICAL RELEVANCE

Our results suggest that both constructs are equally strong and can achieve a similar cross-sectional area of the rima glottidis. Laryngoplasty (“tie-back”) is currently the treatment of choice for horses with exercise intolerance due to recurrent laryngeal neuropathy. Failure to maintain the expected degree of arytenoid abduction post-surgery occurs in some horses. We believe this novel 2-loop pulley load-sharing suture technique can help achieve and, more importantly, maintain the desired degree of abduction during surgery.

Recurrent laryngeal neuropathy (RLN) can affect any horse; however, the condition is most common in large-breed horses. The reported incidence in thoroughbred and draught breeds is between 2.6%1 to 8%2 and up to 35%,3 respectively. Upper airway dynamics are affected resulting in turbulent flow, some degree of respiratory stridor (roaring), and exercise intolerance.

To date, there are no effective medical treatment methods and, for the past 54 years, one of the most accepted surgical treatments has been the placement of a suture prosthesis to maintain abduction, mimicking the action of the cricoartenoideus dorsalis muscle during inspiration. Available follow-up information has shown success rates varying from 5% to 90%4–10; however, the realistic expectations range between 50% to 70%.6,11,12 There is some debate as to what degree of abduction should be targeted with the prosthesis. Some surgeons believe that the arytenoid should be pulled just past its resting position. Rakesh13 has suggested the goal should be to achieve 88% of the maximal cross-sectional area of the rima glottidis, which corresponds to a Dixon grade 2.14 The degree of abduction is generally confirmed intraoperatively under direct endoscopic visual observation. For reasons that are yet to be explained, it is common for the degree of abduction to be reduced either immediately after surgery or within weeks to months after surgery.13,15–17 Some suggested explanations include the pull-through of the suture from either the cricoid or the arytenoid cartilage.9,13,18–20 Some experimental studies suggest that various factors are implicated in abduction loss, such as the suture material, the configuration, the exact placement, and the technique used for the prosthesis application.9,13,19

The authors believe that immediate loss of suture tension potentially could result from inappropriate tying of the knot. Specifically, as tension is applied to gain the desired abduction, the 2 strands of the suture are not parallel owing to a tendency to lift vertically on the suture during tightening rather than applying tension in a perfectly horizontal (axial) direction. This may be exacerbated by an inappropriate incision size or inadequate dissection around the larynx. As a result, the tied suture loop may represent a triangular shape, to a degree, and the natural parallel alignment that ensues after the release of the suture effectively results in a prosthesis that applies less than the desired tension.

Most surgeons place 2 interrupted prosthetic sutures during the surgical procedure to allow load sharing, and to increase the chance that at least 1 of the sutures is ideally positioned. A biomechanical evaluation of 6 different suture techniques confirmed that 2 sutures (1 placed dorsal to the spine of the muscular process of the arytenoid and 1 placed through the spine) provided the strongest construct.21 Moreover, 2-suture prostheses maximize the rima glottidis area.22,23 The concept of load sharing is laudable; however, it is unlikely that both interrupted sutures are under equal load at the time of initial placement. One explanation for the loss of abduction over time could be related to leaving soft tissue between the laryngeal cartilage and the prosthesis. Specifically, if the suture passes over the remnant of the cricoarytenoideus dorsalis and subsequent necrosis or muscular atrophy occurs, this would effectively loosen the tightest of the 2 sutures before load sharing comes into effect. Another possible explanation is the suture pulls through the cartilage at the sites of load, which would be particularly prevalent in 1 suture over another if unequal tension were created.

The authors believe there is a need to develop a prosthetic technique that will maintain better arytenoid cartilage abduction over time through increased knot security. Because 2 sutures are desirable, the senior author has developed a novel 2-loop pulley suture that allows immediate equivocal load distribution across the 4 points of contact in the arytenoid and cricoid cartilages. Maintenance of suture tension throughout knot tying should also be facilitated by utilizing a modified Miller’s knot. This knot type configuration has also demonstrated greater strength, resistance to bursting, and reduced suture breakage near the knot.24,25

The objectives of the current study were first to evaluate the utility of this novel prosthesis technique by comparing it to the standard 2-suture technique. Secondly, to assess the effects of both techniques on the rima glottidis area. The hypotheses were that the novel prosthesis technique would be stronger than or as strong as the standard 2-suture approach and that the rima glottidis areas achieved would not be significantly different.

Materials and Methods

Forty fresh equine larynges were collected from an abattoir. The soft tissues surrounding the actual laryngeal cartilage were left in situ. Normal saline-soaked gauze was used to wrap each specimen. Each larynx was placed in a sealed plastic bag and stored at −20 °C until testing.

In part 1, 32 specimens were used, equally divided into 2 groups (2 suture and modified Miller’s knot constructs; n = 16 each). No data could be found to approximate the pull-out strength of a modified Miller’s knot in the cartilage of an equine larynx. However, a post hoc power calculation suggested that to correctly determine that a statistically significant difference existed between groups (using an alpha of 95% and a power of 80%) we would have required 380 larynges per group.

This left 8 larynges to establish any potential differences in rima glottis size. Our results suggest that over 20,000 specimens per group would have been needed to prove a significant difference, based on our data.

Obviously, more specimens would have been preferable. However, given the very small (non-significant differences) between groups and the fact that we would have needed at least 380 larynges per group, we elected to test a manageable number as we could not source 760 larynges in the time available.

Part 1–Biomechanical evaluation of suture constructs

Thirty-two specimens were used to compare the novel suture technique with the standard 2-suture approach. The larynges were removed from the freezer 12 hours before dissection, preparation, and mounting. All the soft tissues, including intrinsic and extrinsic musculature, were removed. The cricoid and the left arytenoid cartilage were isolated and the ventral one-third of the cricoid cartilage was removed. Two #8 wood screws were placed in each of the cartilages (Figure 1) for testing purposes. A custom-made Teflon mold was used to insert the specimens in liquid plastic (SmoothCast 300Q®, Smooth-On Inc.) for testing purposes. The mold was filled with liquid plastic and the left arytenoid and the cricoid cartilages were partially submerged in it while it was solidifying. To allow prosthesis application (Figure 2), approximately 2 cm of each cartilage (enough to allow adequate abduction of the arytenoid cartilage) was exposed above the plastic material. Once the plastic became completely solid (about 4 minutes after pouring it into the mold), the resultant plastic blocks with the larynges embedded in them were removed from the mold and submerged in normal saline until testing, 18 to 24 hours later.

Figure 1
Figure 1

Two #8 wood screws (arrows) in the arytenoid cartilage (A) while placement of 1 (arrowhead) of 2 wood screws in the cricoid cartilage (C) is being demonstrated. Note that the left side of the larynx is shown, held from the ventral aspect, and that the rostral portion is on the right side of the image.

Citation: American Journal of Veterinary Research 84, 5; 10.2460/ajvr.22.11.0189

Figure 2
Figure 2

The solidified plastic mold with the dorsal aspect of the cricoid cartilage (C) and the muscular process of the arytenoid cartilage (A) visible. Approximately 1 cm of the dorsal aspect of each specimen protruded from the plastic surface. The dotted line denotes the region of division of the plastic block for testing. The cartilages were not cut in this process.

Citation: American Journal of Veterinary Research 84, 5; 10.2460/ajvr.22.11.0189

Each specimen was assigned a number from 1 to 32, and before testing each plastic block was divided at the level of the cricoarytenoideus joint (CAJ) with a band saw in a transverse plane, parallel to the CAJ. After transection, left-sided laryngoplasties were performed using #5 Polyester Suture (Ethibond, Johnson & Johnson Medical Products) with a swaged trochar point needle. The specimens were paired according to their assigned number (ie, larynx number 1 with number 2, number 3 with number 4). A coin toss was used to determine which of the suture patterns was used in the first larynx of each pair; the other pattern was used in the corresponding larynx of a pair.

Suture bites in the cricoid were 0.5 and 1 cm lateral to the dorsal spine and included 1 cm of cartilage. One-centimeter bites of the muscular process of arytenoid were through the spine and approximately 0.5 cm dorsal to the spine. Consistency of the suture placement was maintained by measuring to ensure bites of 1 cm ± 2 mm. For interrupted sutures, the dorsally placed and the more ventally placed sutures were tied using 5 throws. Before tying the 2-loop pulley suture strands were played through their respective bites to ensure that they moved freely. Two loop pulley sutures were tied using a modified Miller’s knot configuration with a total of 5 throws (Figure 3). Tying the knot involved passing 1 suture end beneath the most superficial suture strand. That suture strand binds the knot while four additional throws are placed. Suture clamping was required for interrupted sutures but not for 2-loop pulley sutures. All laryngoplasties were performed by the same experienced board-certified surgeon (D.G.W.).

Figure 3
Figure 3
Figure 3

(a) Drawing of larynx showing suture in modified Miller’s knot configuration. (b) Diagram showing the construction of the modified Miller’s knot with the bites through the arytenoid and cricoid cartilages. The numbers represent the order of the bites taken, the light blue strands are not involved in knot construction and the interrupted dark blue paths represent where the suture is passing under the suture overlying it in that area.

Citation: American Journal of Veterinary Research 84, 5; 10.2460/ajvr.22.11.0189

The rostral to caudal length of each cricoid was measured from a dorsal plane and recorded to allow evaluation of the effect of larynx size on suture holding power. The constructs were then mounted on a material-testing machine (Instron 1122, Instron Co.; Figure 4). A single cycle to failure was used at a previously reported distraction rate of 100 mm/min, while the procedure was video recorded.21 Load in Newtons (N) and displacement in millimeters (mm) were recorded at 10 Hz. Load-displacement curves were generated for each construct. Failure load was defined as the first decrease in load. Mode of failure was determined by examination of the construct after testing and by reviewing the video recording.

Figure 4
Figure 4

A final construct mounted on the material-testing machine. Inset: The split plastic block with the dorsal cricoid (C) and arytenoid (A) cartilages, demonstrating gapping associated with applied tension to the block construct.

Citation: American Journal of Veterinary Research 84, 5; 10.2460/ajvr.22.11.0189

Part 2–Comparison of suture construct rima glottidis area

Eight larynges were used to compare the rima glottidis area obtained with the 2 different suture techniques. All the soft tissues, with the exception of the intrinsic musculature, were removed. To establish abduction on the right side of the larynx, laryngoplasty was completed in all specimens using the 2-loop pulley suture technique and the same suture material as above. Left-sided laryngoplasties were performed using either a 2-loop pulley technique followed by a 2-suture technique or vice versa (determined by a coin toss). Suture placement was as described in part 1. To ensure consistent suture tension, a custom-designed tensiometer was used to ensure consistent suture tension and facilitated repeatable degrees of abduction within, and between, specimens (Figure 5). A force of 17.65 N produced a repeatable degree of abduction, approximating a Dixon grade II. Two-loop pulley sutures were pulled tight (18.16 N–18.91 N) and secured by clamping the first throw of a surgeon’s knot with Carmalt forceps. Sutures in the 2-suture technique were individually tightened (17.65 N–19.91 N) and secured by clamping the first throw of a surgeon’s knot with Carmalt forceps. All left-sided laryngoplasties were performed by the same surgeon (D.G.W.).

Figure 5
Figure 5

Custom designed tensiometer used to ensure consistent suture tension and the repeatability of the degree of abduction between different specimens.

Citation: American Journal of Veterinary Research 84, 5; 10.2460/ajvr.22.11.0189

To allow determination of rima glottides area, a ruler was held in the plane of the rima glottidis and a digital photograph was taken (Lumix DMC-FS62 Digital Camera Panasonic). At this point, the second suture technique was used and a second photograph was taken. The rima glottidis was manually traced using computer software (Image-Pro Plus 7.0, Media Cybernetics Inc.) and the rima glottidis area was calculated. Each measurement was performed in triplicate.

Statistical analysis

Descriptive statistics were applied to the data, which were also examined for normality distribution using a Shapiro-Wilks test using commercially available software (SAS v/9.0 for Windows, SAS Institute). A Student 2-sample T-test was used to compare the force at failure (N) of the 2 different constructs and to examine the relationship between the suture construct and area.

Linear regression was used to determine whether there was a significant effect of cricoid width and to examine the effect of suture load on rima glottis area. The significance level was set at P ≤ .05.

Results

Thirty-two larynges were used for the first biomechanical comparison. One construct failed by muscular process pull-out from the potting material and was excluded from the final data analysis. The overall mean force at failure (n = 31) was 189.35 N (SD 36.7 N). The overall mean width of the cricoid cartilage was 6.72 cm (±0.73 cm). The mean force at failure for 2-suture constructs was 193.5N (±29.9 N) and was not significantly different (P = .5) from 2-loop pulley constructs that had a mean failure force of 184.9 N (±45.6 N). There was no significant effect of cricoid width on force at failure (P = .51, adjusted r2 = −0.02). None of the sutures broke. Failure was always via pull-out from the respective cartilages. Twenty-three constructs failed by muscular process pull-out and 5 failed by cricoid pull-out. In 3 larynges, 1 suture pulled out of the cricoid and the other pulled out of the muscular process of the arytenoid cartilage.

Eight specimens were used to compare the rima glottidis areas between constructs. The mean rima glottidis area obtained was 22.4 cm2 (± SD 5.7, range 12.0–32.9). There was no significant effect of the suture construct on the area (P = 0.94). The 2-loop pulley suture had a mean area of 22.3 cm2 (±5.1) compared with the mean of 2-sutures, which was 22.5 cm2 (±6.6).

The overall mean peak suture load was 18.52 N (±0.31 N, range 17.65 N–19.14 N). The 2-loop pulley suture had 1 peak load while each of the sutures in 2-suture constructs had a distinctly different loading shape, likely representing the load to failure of 1 suture being surpassed followed by that of the other (Figure 6). Suture load was not significantly different between constructs, 2-loop pulley 18.53 N (±0.27 N) compared with the double suture with 18.46 N (±0.38 N), P = 0.61. Examination of the 2-suture constructs did not reveal any significant difference in the load between the axial and abaxial sutures (P = 0.87), axial suture 18.47 N (±0.22 N) vs abaxial suture 18.44 N (±0.51 N).

Figure 6
Figure 6

Load-deformation curves showing failure of 2 loop pulley suture and 2 suture constructs.

Citation: American Journal of Veterinary Research 84, 5; 10.2460/ajvr.22.11.0189

Discussion

One of the currently accepted surgical techniques for laryngoplasty is the placement of 2 interrupted sutures with a goal of maximizing rima glottidis area,22,23 load distribution and increasing the chance that at least 1 of the sutures is ideally positioned. A biomechanical evaluation of 6 different suture techniques confirmed that 2 sutures (1 placed dorsal to the spine of the muscular process of the arytenoid and 1 placed through the spine) provided the strongest construct.21 While the concept of even load distribution is laudable, it is unlikely that both interrupted sutures are under equal load at the time of initial placement. Our objective was to determine if there were any differences in the construct strength between the current standard 2-suture technique and a novel 2-loop pulley technique. Our results suggest that both techniques achieve similar pull-out strength. This novel 2-loop pulley technique does remain secure during additional throw placement. One author has suggested half-hitch formation could contribute to suture loosening. The configuration of the modified Miller’s knot should prevent that from happening.26 When used in the clinical setting (J.L.C. and D.G.W. personal experience), this novel technique allows maintenance of abduction during knot tying without the need to clamp the suture.

There was no significant difference in the rima glottidis area obtained between the suture techniques, suggesting that the novel technique can achieve the desired degree of abduction with similar force application when performing a laryngoplasty in a clinical setting. In an idealized test model, the tension required to achieve abduction with the 2-loop pulley suture would be half that required with a single-looped suture. The fact that we needed similar distraction forces to achieve abduction can be explained by the fact that the suture bites in the 2-loop pulley technique in the current application are not aligned linearly. Rather, there is some angulation resulting in a rather narrow cruciate formation. An additional factor contributing to the tension required to gain abduction with the 2-loop pulley technique is the tissue drag of the braided polyester suture material.

A study published by 2 of the authors (J.L.C. and D.G.W.) to test 6 different suture configurations, used clamps to mount the constructs into the material-testing machine. In that study, clamp failure occurred in about 33% of all specimens (41 out of a total of 121 larynxes).21 In an effort to improve on the previous methodology, we chose to embed the cartilages in liquid plastic to allow true measurement of suture pullout.27 Just 1 out of 32 constructs failed at the potting material-cartilage attachment.

The breed, age, and weight of the horses used were not available for inclusion; however, there is currently no evidence in the literature supporting the need to know the horses’ age to be able to test the different laryngoplasties.6,12,18,28 In an effort to control for larynx size, the cricoid width was measured. Surprisingly, no significant difference was found in force pull-out between larger and smaller larynges. This could be explained by the small number of specimens that were larger or smaller than the average in our population. Alternatively, the holding power of the cartilage may be the limiting factor. The result may have been different if the suture bite size had been increased in larger horses.

The current single cycle to failure study may not accurately mimic the failure scenario in vivo, where cyclic loading quite likely is an important factor. Recognizing this limitation, from a purely mechanical perspective, the possible advantage of an immediate load-sharing suture would have to be evaluated in an in vivo study. In conclusion, our results suggest that both techniques described for laryngoplasty procedures are equally strong and achieve similar rima glottides areas. A weakness of the current study is the small number of constructs tested. Based on the biomechanical information, the clinician could select either construct.

Acknowledgment

The authors would like to express appreciation to Mr. Hans Steinmetz from the College of Engineering, University of Saskatchewan for his assistance with the biomechanical testing in this process, including the design of fixtures, specifically, the construction of the strain measuring device.

The authors declare that there were no conflicts of interest.

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Contributor Notes

Corresponding author: David G. Wilson (david.wilson@usask.ca)
Cover American Journal of Veterinary Research
American Journal of Veterinary Research
Publication Date:
01 May 2023
  • Figure 1
    View in gallery
    Figure 1

    Two #8 wood screws (arrows) in the arytenoid cartilage (A) while placement of 1 (arrowhead) of 2 wood screws in the cricoid cartilage (C) is being demonstrated. Note that the left side of the larynx is shown, held from the ventral aspect, and that the rostral portion is on the right side of the image.

  • Figure 2
    View in gallery
    Figure 2

    The solidified plastic mold with the dorsal aspect of the cricoid cartilage (C) and the muscular process of the arytenoid cartilage (A) visible. Approximately 1 cm of the dorsal aspect of each specimen protruded from the plastic surface. The dotted line denotes the region of division of the plastic block for testing. The cartilages were not cut in this process.

  • Figure 3 Figure 3
    View in gallery
    Figure 3

    (a) Drawing of larynx showing suture in modified Miller’s knot configuration. (b) Diagram showing the construction of the modified Miller’s knot with the bites through the arytenoid and cricoid cartilages. The numbers represent the order of the bites taken, the light blue strands are not involved in knot construction and the interrupted dark blue paths represent where the suture is passing under the suture overlying it in that area.

  • Figure 4
    View in gallery
    Figure 4

    A final construct mounted on the material-testing machine. Inset: The split plastic block with the dorsal cricoid (C) and arytenoid (A) cartilages, demonstrating gapping associated with applied tension to the block construct.

  • Figure 5
    View in gallery
    Figure 5

    Custom designed tensiometer used to ensure consistent suture tension and the repeatability of the degree of abduction between different specimens.

  • Figure 6
    View in gallery
    Figure 6

    Load-deformation curves showing failure of 2 loop pulley suture and 2 suture constructs.

Figure 1

Two #8 wood screws (arrows) in the arytenoid cartilage (A) while placement of 1 (arrowhead) of 2 wood screws in the cricoid cartilage (C) is being demonstrated. Note that the left side of the larynx is shown, held from the ventral aspect, and that the rostral portion is on the right side of the image.
Figure 2

The solidified plastic mold with the dorsal aspect of the cricoid cartilage (C) and the muscular process of the arytenoid cartilage (A) visible. Approximately 1 cm of the dorsal aspect of each specimen protruded from the plastic surface. The dotted line denotes the region of division of the plastic block for testing. The cartilages were not cut in this process.
Figure 3

(a) Drawing of larynx showing suture in modified Miller’s knot configuration. (b) Diagram showing the construction of the modified Miller’s knot with the bites through the arytenoid and cricoid cartilages. The numbers represent the order of the bites taken, the light blue strands are not involved in knot construction and the interrupted dark blue paths represent where the suture is passing under the suture overlying it in that area.
Figure 3

(a) Drawing of larynx showing suture in modified Miller’s knot configuration. (b) Diagram showing the construction of the modified Miller’s knot with the bites through the arytenoid and cricoid cartilages. The numbers represent the order of the bites taken, the light blue strands are not involved in knot construction and the interrupted dark blue paths represent where the suture is passing under the suture overlying it in that area.
Figure 4

A final construct mounted on the material-testing machine. Inset: The split plastic block with the dorsal cricoid (C) and arytenoid (A) cartilages, demonstrating gapping associated with applied tension to the block construct.
Figure 5

Custom designed tensiometer used to ensure consistent suture tension and the repeatability of the degree of abduction between different specimens.
Figure 6

Load-deformation curves showing failure of 2 loop pulley suture and 2 suture constructs.
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