Increasing emphasis has been placed on the importance of enhancing functional outcome after inguinal hernia surgery. Endoscopic totally extraperitoneal (TEP) inguinal hernioplasty has been proven to cause less postoperative pain than open inguinal hernia repair [9, 10]. However, pain remains the most common complaint reported by patients after TEP.

Adequate postoperative analgesia is pivotal to achieving an optimal surgical outcome and maximizing patient satisfaction [4]. Inadequate pain control will lead to an increased incidence of nausea, delayed recovery, prolonged hospital stay, and unanticipated admissions after day surgery [13]. Various studies have been conducted to optimize postoperative analgesia after TEP [8, 12, 17]. However, the factors that determine the severity of pain after TEP remain unknown. The identification of factors predictive of acute pain would enable analgesic regimens to be adjusted accordingly. Therefore, the present study was undertaken to evaluate the impact of various perioperative clinical factors on the severity of postoperative pain in patients who undergo TEP.

Materials and methods

Patients

Between November 1999 and December 2002, all patients who underwent TEP performed by a single surgeon (H.L.) were recruited for this study. Demographic features, perioperative details, and follow-up data were collected prospectively and entered into a computer database. Univariate and multivariate analyses were performed to identify the independent factors affecting the degree of pain.

Surgical technique

All TEP were performed under general anesthesia. The operative techniques for conventional and needlescopic TEP have been previously described [7, 8]. Prior to February 2001, a Prolene mesh measuring 10 × 14 cm2 was routinely anchored in place with an endo-stapler (Multifire Endo Hernia 0°, US Surgical Corporation, Norwalk, CT, USA). Thereafter, stapling of the mesh was performed only when the diameter of the hernial defect was >4 cm or when overlap of the defect by the mesh was inadequate (<3 cm). All skin incisions were infiltrated with 10 ml 0.5% bupivacaine before wound closure.

Postoperative assessment

Severity of postoperative pain at rest and on coughing was assessed daily after the operation, using a linear analogue pain score, on a scale from 0 to 10. The analgesic regimen was standardized with oral compound analgesic, proproxyphene 50 mg and acetaminophen 325 mg, 4 times daily and diclofenac sodium SR 100 mg daily at the patient’s request. Patients were allowed to resume a normal diet and normal activities immediately after the operation. Patients who underwent ambulatory TEP were discharged from the day surgery unit on the same day after separate assessments by the anesthetist and the surgeon. While the patient was staying in the hospital, pain score assessment was performed by the surgeon. On discharge, all patients were taught to complete a pain score chart at home for daily documentation of pain score at rest and on coughing. The pain score chart was collected at the hernia clinic by the surgeon (H.L.) 1 week after discharge. All postoperative morbidities were recorded.

Statistical analysis

Clinical factors (n = 11) that were studied included age (≤65 vs >65 yrs), sex (male vs female), operative time (≤60 vs >60 min), unilateral vs bilateral TEP, day case vs inpatient procedure, stapling vs nonstapling of mesh, direct vs indirect inguinal hernia, primary vs recurrent inguinal hernia, needlescopic vs conventional TEP, as well as whether the patient had a history of previous open inguinal hernia repair and postoperative development of seroma. To reduce multiple comparisons, mean daily pain scores at rest and on coughing were computed—i.e., the sum of the daily pain score divided by 5. Statistical comparisons of mean daily pain score at rest and on coughing were analyzed by Student t-test with respect to the studied parameters. Significant factors were then entered into multivariate linear regression analysis to identify independent factors predictive of postoperative pain. Statistical analysis was performed with the help of computer software (SPSS/PC+ 9.0; SPSS, Chicago, IL, USA). A p value of <0.05 was considered statistically significant. However, owing to the multiple comparisons, exact p values could not be given for each studied variable on every postoperative day.

Results

Patient demographics and overall pain score

A total of 509 patients were recruited in the present study. There were 491 men and 18 women. The mean age of the study population was 64 ± 13.5 (SD) years. The numbers of attempted unilateral and bilateral TEP were 387 and 122, respectively, giving a total of 631 hernioplasties. Conversions to open repair and transabdominal preperitoneal (TAPP) hernioplasty were required in two patients (0.3%) and one patient (0.2%), respectively. The overall pain score on coughing was significantly higher than the pain score at rest from postoperative days 1 to 5 (p < 0.05) (Fig. 1). A total of 102 patients (20%) did not take any analgesics after the operation.

Figure 1
figure 1

Overall postoperative pain scores at rest and on coughing (p < 0.05; Student t-test). The bar represents mean ± 2 SEM.

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Clinical factors influencing pain score at rest

Significant factors associated with a higher mean daily pain score at rest during the postoperative period included age ≤65 years (Fig. 2), female sex (Fig. 3), unilateral TEP, and day case TEP (Table 1). The mean ages of the male (64 ± 13.2 years) and female (58 ± 19.6 years) patients were comparable. However, the patients who underwent day case TEP (55 ± 13.1 years) and unilateral TEP (63 ± 14.0 years) were significantly younger than those who had inpatient (66 ± 12.7 years) and bilateral TEP (67 ± 10.9 years), respectively (p < 0.05). After the effect of age was adjusted for by selecting an age-matched cohort for the two variables on the respective days, the differences in pain score became insignificant.

Table 1 Univariate analyses of 11 clinical factors influencing postoperative pain at rest
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Figure 2
figure 2

Daily postoperative pain scores at rest in young (≤65 years) and old (>65 years) patients. The bar represents mean ± SEM.

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Figure 3
figure 3

Daily postoperative pain scores at rest in male and female patients. The bar represents mean ± SEM.

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On multivariate regression analyses, independent factors found to influence mean pain score at rest included age (p < 0.001; β = −0.422, 95% confidence interval [CI]: −0.558, −0.287) and gender (p < 0.001; β = 0.984, 95% CI: 0.614, 1.356).

Clinical factors influencing pain on coughing

Significant factors associated with a higher mean pain score on coughing included age ≤65 years (Fig. 4), female sex (Fig. 5), and prosthetic stapling (Fig. 6) during the postoperative period (Table 2). Comparison of the mean ages between male (64 ± 13.2 years) and female (58 ± 19.6 years) patients, and between patients with (65 ± 12.8 years) and without stapling (64 ± 14.2 years) of the mesh showed no significant differences.

Table 2 Univariate analyses of 11 clinical factors influencing postoperative pain on coughing
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Figure 4
figure 4

Daily postoperative pain scores on coughing in young (≤65 years) and old (>65 years) patients (p < 0.05). The bar represents mean ± SEM.

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Figure 5
figure 5

Daily postoperative pain scores at rest in male and female Patients. The bar represents mean ± SEM.

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Figure 6
figure 6

Daily postoperative pain scores on coughing in patients with and without stapling of the mesh. The bar represents mean ± SEM.

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On multivariate regression analysis, age (p =0.001; β= −0.561, 95% CI: −0.878, −0.245), Sex (p = 0.006; β = 1.228, 95% CI: 0.361, 2.095), and prosthetic stapling (p = 0.001, β = 0.521, 95% CI: 0.204, 0.837) were shown to be independent factors influencing postoperative mean pain score on coughing.

Discussion

Among all the variables studied in the present study, age was the most significant predictive factor of acute pain after TEP. Similar findings have been reported after open repair of inguinal hernia [2, 6], abdominal hysterectomy [15], and thoracic and abdominal operations [1]. In general, when given equal doses of analgesics, elderly patients attained better pain relief than young patients [11].

Hypotheses that have been proposed to explain the age-related differences in pain perception include a higher pain threshold, a diminished number of nerve cells, and a slower clearance of analgesic from the body in elderly individuals [11, 15]. In addition, owing to the higher prevalence of diseases and chronic health problems in old age, elderly patients may have learned to cope with postoperative pain better than younger people. Callesen et al. [2] attributed the elevated pain in young patients to their higher level of activity and more critical expectations for the postoperative course. To avoid potential bias, clinical trials comparing postoperative pain after different types of hernia repair should therefore ensure that patient demographics among the treatment groups are comparable.

A significant difference between men and women in postoperative pain score was observed in the present study. A number of clinical trials have proven the existence of sex-related differences in pain perception, susceptibility to pain-related diseases, and the efficacy of analgesics [3, 5, 14]. Women were more likely that men to report temporary or persistent pain, occurring at increased frequency and duration [18]. The mechanism underlying the greater pain sensitivity of women remains unclear, but it has been attributed to biological, hormonal, psychological, and physiological differences [16]. Ellermeier and Westphal [3] demonstrated sex-related differences in autonomic responses, such as papillary reflexes, to the same painful stimuli, indicating that there are different degrees of pain sensitivity for men and women.

Patients who had prosthetic stapling reported a significantly higher pain score on coughing, but their pain scores at rest were equivalent to those of patients without stapling. Coughing instantaneously elevates intraabdominal pressure and stretches the abdominal wall. The resultant traction force transmitted via the staples onto the abdominal wall might account for the increased pain on coughing. Avoidance of prosthetic stapling might therefore help to reduce pain on coughing or movement. This finding may have implications for the future practice of TEP.

Other clinical factors, such as hernia pathology and seroma, had no impact on the degree of postoperative pain. Despite the use of smaller trocars, needlescopic TEP failed to bestow any benefit in terms of reducing postoperative pain. However, future randomized trials on a larger scale are required to confirm this finding.

In conclusion, age and sex were the most significant predictive factors of pain after TEP. Prosthetic stapling was also associated with a significantly higher pain score on coughing. Adjustment of the pain relief regimen in accordance with the patient’s age and sex may help to optimize postoperative analgesia and enhance recovery. To reduce postoperative pain on coughing, prosthetic stapling should be avoided whenever feasible.