Preoperative predictors of chronic pain after laparoendoscopic groin hernia repair: A Swedish Hernia Registry study

Swedish healthcare is primarily tax-funded, with negligible patient co-payment, ensuring that surgery is accessible to virtually all residents.

This nationwide study included a consecutive, unselected cohort of patients who underwent laparoendoscopic groin hernia repairs over 76 months (> 6 years). Data were obtained from the SHR, a government-funded, non-profit registry that captures more than 95% of all groin hernia repairs in Sweden. The SHR provides systematic, continuously updated follow-up, refreshed weekly, until death, emigration, or patient opt-out [28]. Detailed descriptions of SHR data collection and validation processes have been published previously [29‐33].

The study was based on a prospective follow-up project that included all groin hernia repairs registered in the SHR between September 1, 2012, and December 31, 2018. All surviving patients were invited to complete a brief, patient-reported outcome questionnaire 1 year postoperatively [34]. To assess CPIP, it included the first item of the validated Short-form Inguinal Pain Questionnaire (sf-IPQ) [35]. Nearly all returned questionnaires were received by the SHR 12–15 months postoperatively.

This analysis used a comprehensive SHR dataset containing detailed patient and surgical information from September 1, 2012, with follow-up data (questionnaire responses and reoperations) through November 6, 2020. Patients who underwent bilateral repairs were excluded, as the single questionnaire did not distinguish between left- and right-sided pain. Consequently, only unilateral repairs were included.

The study endpoint, CPIP, was based on the patient’s reported maximum pain in the operated groin during the preceding week [35]. The 7-point Likert scale was dichotomized to classify outcomes as CPIP-positive or CPIP-negative (Table 1). A broader definition (CPIP-1) was used for the primary outcome to maximize statistical power, while a narrower definition (CPIP-2) served for sensitivity analysis.

The first author (BN) retrieved a complete, unrestricted, and unfiltered dataset from the SHR, including all recorded repairs and variables without prior selection or filtering. The data were manually reviewed for inconsistencies and implausible entries. Obvious errors were corrected when the true value could be confidently inferred; otherwise, values were recoded as missing.

Independent samples t-tests were used to compare baseline variables between respondents and non-respondents. For these comparisons, categorical variables with more than 2 levels were dichotomized.

The main analysis of CPIP risk factors employed multivariable logistic regression to estimate adjusted odds ratios (AORs) with 2-tailed 95% confidence intervals. All 18 candidate predicting variables were initially included, and backward stepwise selection was used to refine the model, retaining variables with P < 0.10. In the subsequent final model, statistical significance was set at P < 0.05.

Calculations were conducted using IBM^® SPSS^® Statistics, version 28 (IBM, Armonk, New York, USA).

During the inclusion period, the SHR recorded more than 100 000 hernia repairs in 87 668 patients [35].

Figure 1 shows the subsequent exclusions and the remaining cohort, stratified by sex. In almost all cases, the questionnaires returned were received by the SHR within 15 months.

A total of 82 centers contributed at least 1 repair that met the inclusion criteria, yielding 15 360 patients eligible for this study. Among these, 110 non-respondents who underwent subsequent ipsilateral repair within 15 months of surgery were reclassified as CPIP-positive respondents per study protocol and included in the main analysis. Including them, 10,525 (69%) were classified as questionnaire respondents and qualified for CPIP analysis.

Tables 2 and 3 compare the demographic and clinical features of respondents and non-respondents. Response rates varied significantly across 11 patient and surgical characteristics. Women, younger patients, those otherwise healthy (ASA 1), and smokers were less likely to respond. Lower response rates were also observed among patients who had primary repairs, emergency procedures, femoral hernias, small hernia defects, or treatment at low-volume surgical centers.

Figure 2 illustrates age-related response variability (top curve), and CPIP prevalence using both definitions: 3 027 of 10 525 (29%) for the primary definition (middle curve), and 1 614 of 10 415 (15%) for the secondary definition (bottom curve).

Bild vergrößern

An initial stepwise selection process eliminated 8 preoperative variables (smoking, diabetes, chronic obstructive pulmonary disease/asthma, bleeding diathesis, immunosuppression, collagen disorder, urgency, and groin side), and 1 intraoperative variable (surgeon level), none of which met the P < 0.10 inclusion threshold (Table 4).

The final model included 7 preoperative and 2 intraoperative risk factors (Table 5).

Female sex was associated with modestly increased risk.

Age, treated as a continuous variable, showed a weak inverse correlation with likelihood of CPIP (AOR 0.990, CI 0.988–0.993; P < 0.001). Due to non-linearity, age was analyzed post hoc in categories. Compared to patients 65 years or older, those younger than 45 years (AOR 1.36, CI 1.25–1.49, P < 0.001), and 45–64 years (AOR 1.34, CI 1.08–1.65, P = 0.008), were associated with higher CPIP risk. For more detailed analysis, the final model comprised 7 discrete age categories (Table 5).

Higher ASA grade and increasing BMI above normal weight (> 25 kg/m²), were each associated with progressively higher CPIP risks.

Recurrence repairs comprised 3 324 (22%) of the index procedures and were linked to an increased CPIP risk. Some may argue that including recurrent repairs would confound the results, and thereby disturb interpretation. To address this, we added a Supplementary Table where the outcomes are stratified by primary vs. recurrent repairs, demonstrating only minor differences in AORs (except for categories with very low numbers).

Given the similar risks associated with lateral (AOR 1.01, CI 0.91–1.13) and combined medial-lateral hernias (AOR 0.99, CI 0.80–1.22), these categories were merged with medial hernias as the reference group. Using this combined group as the reference, femoral hernias were associated with elevated risk.

Similarly, medium-size (EHS II) defects (AOR 0.99, CI 0.86–1.13) and large (EHS III) defects had comparable risk and were therefore merged for analysis. In comparison, small (EHS I) defects were associated with an increased CPIP risk.

Applying the stricter CPIP-2 definition (secondary outcome) reduced the overall prevalence by nearly half, attenuating statistical power. Nonetheless, the relative risk pattern remained consistent, as illustrated in Table 6 and the lower two curves of Fig. 2. No major discrepancies were observed between the main and sensitivity analyses.

The SHR, alongside the Danish Hernia Database, are worldwide the only registries of groin hernia repair with true nationwide coverage and lifelong individual follow-up, minimizing selection and attrition bias [31]. Sweden’s healthcare system ensures all residents have access to hernia repair, allowing the SHR data to reflect the entire adult population. The cohort analyzed comprised at least 95% of all unilateral laparoendoscopic groin hernia repairs performed in Sweden during the inclusion period. Annual validation of SHR data mitigates misclassification and supports internal validity [28]. The use of a validated patient-reported outcome measure, a fixed 1-year follow-up, and high response rate further enhance the robustness and generalizability of the results. A major advantage in this study was that the questionnaires were mailed, collected, registered, and assessed independently of the surgical centers. This minimized observer and input bias, thereby enhancing validity [31]. Additionally, the retrieved study database underwent thorough scrutiny to minimize data entry errors.

Despite these advantages, selection bias cannot be entirely excluded. The overall response rate (69%) was higher than in comparable registry-based studies [24, 38]. Yet, non-response bias remains a concern. Response rates varied across most variables. Younger and very elderly patients, women, those with higher BMI, and ASA 1, smokers, primary hernia, emergency repair, femoral hernia, and small (EHS I) defects were less likely to respond. While age- and sex-related response biases have been noted in a few previous studies, they are often only briefly mentioned [46, 47]. In a prior SHR study based on a different subset of the same larger cohort from which the present sample was drawn, telephone interviews with a random test sample of 119/8 005 (1.5%) non-respondents revealed a sixfold lower CPIP prevalence compared to respondents [34]. If generalizable, this suggests that both the present and previously reported risk estimates in, e.g., younger patients, may be exaggerated.

Although we adjusted for multiple covariates, some potentially relevant predictors—such as preoperative pain, socioeconomic status, and psychiatric history—were not recorded in the SHR during the study period [5, 12, 48]. While these factors may influence CPIP risk, they are unlikely to systematically vary across the included 18 variables in a way that would materially alter the findings.

It is important to differentiate selection bias inherent to study design from the clinical selection that occurs in routine practice. This distinction is particularly relevant when interpreting results related to preoperative groin pain, BMI, and ASA. Preoperative groin pain, a known risk factor for CPIP, is also a primary indication for hernia repair [2, 3, 5, 12, 40, 46, 49]. While hernia surgery aims to alleviate discomfort, it may not always accomplish that and can sometimes even inflict new pain [50]. Because preoperative symptoms were not recorded, distinguishing between pre-existing and surgery-induced pain was not feasible [51]. However, we presume that preoperative pain prevalence was evenly distributed across most subgroups, and therefore minimally impacted the estimated AORs, except for categories where the level of preoperative pain could have been a major determinant when selecting patients for surgery. Since groin hernias are rarely incapacitating or life-threatening, surgical indications are typically relative and based on shared decision-making. For example, obese or frail individuals face increased anesthetic and surgical risks [52‐55]. Among such patients, physicians may be more likely to recommend conservative management for those with negligible discomfort and offer surgery to those with more intense pain [3, 50]. Consequently, CPIP estimates for overweight, obese, and high-ASA patients may potentially be overestimated in non-randomized studies, warranting cautious interpretation of CPIP risk in these categories.

In contrast to many healthcare systems where residence, ethnicity, or socioeconomic status influence access to surgical care, Sweden’s universal healthcare minimizes such disparities. Accordingly, these variables are unlikely to confound the findings, and are not collected in the SHR.

The seemingly high CPIP prevalence in this study should not be interpreted as inherent to the laparoendoscopic approach but rather reflects the chosen CPIP definition and its purpose. Multivariable models with numerous predictors can result in small subgroups and an increased risk of type II errors. To maximize power, we used a broad CPIP definition, yielding a 29% prevalence at 1 year. While about half of these cases involved fairly mild pain (IPQ level 3), the primary focus was not on prevalence but on relative differences in risk, expressed as odds ratios. Sensitivity analysis using a stricter CPIP definition confirmed the robustness of these risk estimates, although confidence intervals widened due to reduced statistical power.

The results confirm previous observations that women are at higher CPIP risk, extending this finding to laparoendoscopic repair [12, 17, 30]. This aligns with modern pain research, which highlights sex-related biological differences in pain perception. Historically, medical and biological research has been male-centric—focusing on male cells, animals, and humans— leaving female pain mechanisms underexplored. However, contemporary studies have established that female sex per se is an independent risk factor for chronic pain in general, likely due to genetic and biological influences [56, 57]. In this context, the modest increase in CPIP risk among women (AOR 1.15) is perhaps within or even below expected ranges, and not implying inferior surgical care.

The finding that younger age was associated with higher CPIP risk corroborates several previous studies [2, 3, 12, 20, 24, 25, 30, 46]. However, age also influenced response rates, potentially confounding the results—an issue rarely addressed in CPIP research. In contrast, a Dutch TEP study with near-complete follow-up at 1 year found no significant CPIP difference between young (18–30 years) vs. older (≥ 31 years) patients [49]. Moreover, we used an IPQ item, which—like many patient-reported outcome measures—assesses pain severity by using limitations in physical activity as proxy. Aasvang and Kehlet hypothesized that interpretations of “daily activities” vary by age. Younger individuals, engaging in more strenuous activities, may experience greater functional impairment from equivalent pain [2]. The above reflections imply a need for cautious interpretation of CPIP risk in younger populations.

The strong positive association between higher BMI and CPIP is consistent with previous research [20, 24, 25, 30]. How BMI affects CPIP risk may be multi-factorial. Adiposity increases surgical complexity and chances of complications [53‐55]. Reasonably, an elevated CPIP risk follows. Conversely, patient selection may exaggerate the CPIP risk in overweight and obese individuals. Despite a higher hernia and recurrence risk, obese individuals are under-represented among surgical patients [58]. Excess subcutaneous inguinal fat can obscure symptoms and delay diagnosis of hernia [53, 54]. In addition, surgeons may be more reluctant to recommend repair for obese individuals with mild symptoms, due to the added surgical challenge [53]. Consequently, obese patients who undergo surgery likely have more severe preoperative symptoms on average than their non-operated counterparts, which introduces confounding by indication.

The high percentage (22%) of index repairs being recurrent reflects international guidelines recommending laparoscopic reoperation after open repair [3]. Recurrent repairs were associated with increased CPIP risk, which is reasonable and consistent with previous studies [2, 5, 25, 46].

Finally, small defects (EHS I) were associated with higher CPIP risk, mirroring reports from the Herniamed registry [20, 24]. However, patients with small hernias are more likely to undergo surgery if they experience pain, whereas those with minimal or no discomfort may not seek medical attention until the defect enlarges to an EHS II or III [50]. This selection effect may, at least partially, explain the observed association

Although surgical method and center volume influenced CPIP risk, these variables were primarily included to optimize model adjustment and are therefore beyond the scope of this discussion.