Forty-five normal and inguinal cryptorchid stallions admitted for castration to The Large Animal Teaching Hospital, University of Copenhagen from September 2017 to May 2019 were included in the study. Horses were included in the study if they had normal descended testicles present in scrotum or if they were uni- or bilateral inguinal cryptorchids. Stallions were excluded from the study if they were abdominal cryptorchids or if they were to have more surgical procedures performed during the same anaesthetic period (e.g. arthroscopic fragment removal).
All horses were castrated using the same technique and the surgical wounds were sutured for primary closure. Horses were randomly assigned to be sutured with either a size 2–0 smooth, synthetic, absorbable monofilament suture of glycolide and e-caprolactone (Monocryl, Ethicon, Somerville, NJ) (SS) or a size 0 (corresponding to tensile strength 2–0)Footnote 2 bidirectional barbed, synthetic, absorbable, monofilament suture of glycolide and e-caprolactone (Quill, Surgical Specialties Corporation, Westwood, MA) (KBS).
In the selection of the SS suture for comparison with KBS, it was not possible to identify a complete match concerning needle profile. As Surgical Specialties Corporation specializes in knotless tissue closure devices,Footnote 3 the company does not market smooth counterparts to their knotless sutures, but lists Monocryl as a comparable suture.Footnote 4 Needles of both sutures were swaged, curved, and reverse cutting. It was not possible to find a 2–0 Monocryl suture with a needle of exactly the same length and curvature as the Quill suture, and the suture with the closest needle match was therefore chosen: a 24 mm, 3/8, reverse cutting needle (Monocryl) compared with a 26 mm, 1/2-circle, reverse cutting needle (Quill). The selected sutures were of similar material (both copolymers of glycolide and caprolactone) and colour (both undyed), but differed in size. The difference in suture size was deliberate. Size of sutures is determined by outer diameter of the suture. Barbs of KBS are created by cutting into the core of the suture, which effectively reduces the diameter of the suture . While other producers of unidirectional KBS (e.g. V-Loc, Medtronic, Minneapolis, MN) , have taken this into account and labeled their sutures accordingly (so that users of KBS select a suture size analogous to the size that would be selected if standard smooth suture material was used), this is not the case for Quill sutures. We thus upsized the KBS by one as recommended by the manufacturerFootnote 5 to effectively have similar size KBS and SS.
The barbed suture design eliminates the need to tie knots and further improve tissue apposition by a more even distribution of tension on the soft tissue [29, 30]. The barbs of the bidirectional suture is set in opposing directions from either side of the suture midpoint,Footnote 6 allowing the use of the suture in two directions. The bidirectional suture therefore has a needle attached to both ends of the suture. Unidirectional sutures, such as for instance V-Loc (Medtronic), has a pre-constructed loop end, which is used to anchor the suture in the tissue at the beginning of a suture line. This structure allows passage of the suture through tissue in a single direction only. In our study the bidirectional KBS suture was used to close the subcutaneous tissue with one end of the suture, whereas the other end of the bidirectional KBS was used to close the skin.
The study was approved by the Ethical and Administrative Committee of the Department of Veterinary Clinical Sciences, University of Copenhagen (reference number: 2017–002) and horse-owners of enrolled stallions signed an informed consent form.
Anaesthesia and perioperative treatment
Horses were sedated with 0.01 mg/kg detomidine intravenously (IV) (Orion Pharma, Espoo, Finland), 0.03 mg/kg acepromazine IV (Pharmaxim, Helsingborg, Sweden); within 5 min, sedation was supplemented with 0.03 mg/kg butorphanol IV (Merck, Kenilworth, NJ, USA). Anaesthesia was induced with 1.5 mg/kg zolazepam and tiletamin IV (Virbac, Carros, France). Anaesthesia was maintained with isoflurane (Virbac, Carros, France) in oxygen using intermittent positive pressure ventilation.
Immediately prior to induction of anaesthesia, horses received one dose of 22.000 IU/kg benzyl penicillin IV (Panpharma S.A., Luitré, France) and a dose of 1.1 mg/kg flunixin meglumine IV (Merck, Kenilworth, NJ, USA). During hospitalization horses were treated IV with 1.1 mg/kg flunixin meglumine 24 and 48 h after surgery and 1.1 mg/kg PO once after discharge at 48 h if discharge occurred before 48 h after surgery. Horses which were not vaccinated with tetanus routinely, were administered tetanus antitoxin (Colorado Serum Company, Inc., Denver, USA) subcutaneously and anti-Clostridium tetani toxoid (Pfizer, NY, USA) intramuscularly. Horses were confined to a box stall for 24 h post-surgery and then walked by hand two-three times daily for 15 min. Horses were discharged 24–72 h post-surgery depending on owners’ wishes. Owners were instructed to keep horses stall rested and hand walk them until 8 days post-surgery; hereafter horses were turned out in a small paddock for two additional weeks after which there were no restrictions in regard to turn out or ridden exercise. Owners were further instructed to daily look underneath their horse and monitor the surgical wounds and scrotal area for swelling, discharge from surgical wounds, and call the hospital /a veterinarian in case of presence of any of the above.
Horses were positioned in dorsal recumbency. The scrotal and inguinal area were aseptically prepared for surgery and draped. Ten mL of 2% lidocaine (AstraZeneca, Cambridge, UK) was injected in each testicle when possible, i.e. when the testicle was present in normal scrotal position and/or when it was easily identified in the inguinal canal. Castration was performed by the inguinal approach by exposing the superficial inguinal ring through a skin incision centred over the superficial inguinal ring. Castration was performed by the closed technique. After separation of the parietal tunic surrounding the testis from the scrotal ligament and fascia by digital dissection, the testis and parietal tunic were separated from surrounding fascia using a gauze swap while placing mild traction on the testicle and cord. A Sand’s non-cutting emasculator was applied to the cord for 10–30 s with the purpose of crushing it briefly at the area of subsequent ligature placement. One transfixating, circumferential ligature of size 2 polyfilament braided lactomer (Medtronic, Minnesota, USA) was applied in the crushing groove created by the Sand’s emasculator and a second ligature was applied 2–3 cm proximally before the cord was severed distal to the ligatures. For security in case a ligature should slip we standardly double ligate the cord during castration. The subcutaneous tissue, fascia and skin were closed in two layers as recommended  using either SS or KBS. The fascia beneath the skin and the subcutaneous tissue were closed in one layer and the skin was closed in another layer. Both sutures were applied in a simple continuous pattern in the deep layer, while the skin was closed intradermally. For the SS closure, 4 surgeon’s knots were used per incision with a minimum of 4 throws per knot. For the KBS the anchoring of the suture at the end of the suture line was performed by adding two additional stitches to the subcutaneous line and by backstitching in the dermal layer. Six different surgeons carried out the castrations.
Intraoperative variables and assessment of complications
Recorded variables were: 1) Length of the incision in cm as measured by a sterile ruler, 2) Closure time (seconds) measured from initial needle insertion until suture ends were cut off after dermal closure. Closure time was recorded for each side (right and left) and the average of the total time (right + left) spent on suturing each horse was used for statistical analyses. 3) Postoperative complications. Short-term complications were monitored daily during hospitalization until discharge or 72 h post-surgery, whichever occurred first. Clinical parameters (swelling, exudation, and dehiscence) were graded subjectively (none, minor, moderate or severe) based on presence in relation to the suture line and the surroundings of the suture line.
Post-discharge complications were assessed based on telephone interviews with the owners 2 months post-surgery. Those owners not available for contact at 2 months post-surgery were called repeatedly until contact or until 11 months after surgery, resulting in interviews performed between 2 and 11 months post-surgery. Owners were interviewed based on the discharge instructions (to daily look underneath their horse and monitor the surgical wounds and the scrotal area for swelling and/or discharge from the surgical wounds). Specifically horse-owners were asked if their horse had developed complications since discharge from the hospital, and where complications were present, owners were asked to describe them and report whether it had been necessary to request veterinary assistance to treat the complication(s).
The complications reported in the study are those related to SSIs and the suture line. Other short-term and post-discharge complications are reported in an additional file (see Additional file 1).
Differences in proportions of the categorical variables of suture line complications between 1) SS and KBS and 2) the six surgeons performing the castrations were compared using chi-square or Fisher’s exact test, as appropriate. The quantitative variables of time, age, and weight were compared between the two suture groups using two-sided student’s t-test (after test for variance equality). Significance threshold was set at P < 0.05. Data was analysed using SAS/STAT® software. The quantitative variables were all normally distributed, assessed by the Shapiro-Wilk normality test.