The data in this study were gathered from 4 hospitals serving 1,600,000 inhabitants in western Sweden: Sahlgrenska University Hospital, Göteborg; NU Hospital Group, Trollhättan; Skaraborg Hospital, Skövde and Södra Älvsborg Hospital, Borås. All patients who underwent primary or secondary laparotomy through midline abdominal incisions for vascular procedures or laparotomies with drainage or lavage, procedures on the small bowel, the colon or the rectum between January 1, 2010 and December 31, 2010 were included. The patients were identified using codes from the Nordic Medico-Statistical Committee (NOMESCO) Classification of Surgical Procedures version 1.9. Exclusion criteria were trauma surgery, no initial closure of the abdominal wall and patients with primary mesh inlay at the midline abdominal incision. To conform with the hypothesis we excluded the patients where a documented suture quota <3.5 was stated in the operative report (n = 4), since such a low ratio cannot be considered clinically acceptable (Fig. 1).
A clinical record form (CRF) was constructed and used for data extraction from medical records regarding suture technique (specified ratio ≥3.5:1 or unspecified ratio, hereafter referred to as specified group and unspecified group), emergency or elective procedure, demography, co-morbidity, type of surgery, suture technique, surgical wound complications, reoperations and mortality. Skin preparation was by washing with antibacterial agents 1–3 times before surgery and local skin disinfection in the operation theatre was performed according to standard practice. Factors that were not possible to ascertain retrospectively were suture type, how many patients were given antibiotics pre- or post-operation, length of surgery or the surgeon’s experience level. Wound complications of interest were wound infection, wound dehiscence and incisional hernia. Data were extracted in 2014, by one of the authors (SW).
Each patient was followed from the index operation in 2010 until the time of the review of the medical records in 2014, rendering a follow-up time with a median of forty-one months (range 0–58). The end of the follow-up was defined by one of the following: the time of review of the medical record, a renewed operation with midline abdominal incision, death of the patient or if the patient was lost to follow-up.
Definition of endpoints
Wound dehiscence was defined as a complete disruption of the wound including the fascia closure after the index operation or by a significant gap between the edges of the fascia necessitating reoperation. Incisional hernia was defined as documentation of hernia in the medical records or re-operation for this condition. Registration was based on clinical findings and did not depend on a CAT scan having been done. Timing of the occurrence of wound dehiscence and incisional hernia was retrieved from the medical records. The definition used for wound infection was based on two factors: records noting that the patient was treated with antibiotics for wound infection or if there was a note in the medical record of purulent discharge from the wound, irrespective of positive bacteriologic cultures or treatment with antibiotics.
To assess the relationship between suture technique and occurrence of reported wound dehiscence and incisional hernia, as well as the significance of the previously documented risk factors in the studied cohort, a Cox proportional hazards model  was used after checking the validity of model assumptions . Risk factors found to have an eligible contribution, defined as having a p-value for the Wald test <.20, were simultaneously included in a multiple Cox regression analysis. As our primary objective was to evaluate the significance of suture technique, this risk factor was included in all analyses. Statistical analyses were performed using SAS 9.3 (SAS Institute Incorporated, Cary, NC, USA) and R (R Development Core Team. A language and environment for statistical computing. Vienna, Austria: R Foundation for Statistical Computing; 2005).