Development and Internal Validation of a Nomogram for Predicting Acute Moderate-to-Severe Postoperative Pain After...

Acute moderate-to-severe postoperative pain is common after Achilles tendon surgery and may hinder early mobilization and the implementation of Enhanced Recovery After Surgery (ERAS) protocols. This study aimed to develop and internally validate a perioperative nomogram for predicting acute moderate-to-severe pain within the first 24 hours after Achilles tendon surgery using routinely available clinical variables.

We retrospectively analyzed 489 patients who underwent Achilles tendon surgery at a single center. Demographic, surgical, and perioperative analgesic data were extracted from electronic medical records. The primary outcome was moderate-to-severe postoperative pain, defined as any Numeric Rating Scale (NRS) score ≥ 4 within the first 24 postoperative hours. Candidate predictors were screened using Least Absolute Shrinkage and Selection Operator (LASSO) regression, followed by multivariable logistic regression to construct a parsimonious predictive model. The model was intended for perioperative use at or near the end of surgery, when the surgical approach and peripheral nerve block status are known. Model performance was evaluated using the Area Under the Curve (AUC), calibration plots, bootstrap internal validation, temporal split validation, and Decision Curve Analysis (DCA).

Among the 489 patients, 208 (42.5%) experienced moderate-to-severe postoperative pain. The final model included open surgery (OR 4.457, 95% CI 2.109-9.417), higher BMI (OR 1.059 per 1 kg/m², 95% CI 1.006-1.115), and no peripheral nerve block (OR 2.022, 95% CI 1.369-2.988). The apparent C-statistic was 0.652 (95% CI 0.603-0.700), and the bootstrap-corrected C-statistic was 0.648. Calibration was acceptable (optimism-corrected calibration slope 0.950; optimism-corrected calibration intercept -0.017; Hosmer-Lemeshow P = 0.162). In temporal split validation, the model achieved an AUC of 0.634 (95% CI 0.565-0.703). Decision curve analysis suggested potential net benefit across threshold probabilities from 0.11 to 0.56.

Open surgery and higher BMI were associated with increased odds of acute moderate-to-severe postoperative pain, whereas perioperative peripheral nerve block was associated with lower odds. The proposed 3-variable nomogram showed modest discrimination and acceptable internal calibration. It may serve as an adjunctive tool for perioperative risk stratification, but external validation and prospective evaluation are required before routine clinical use.