69 Implant loss and associated risk factors following implant-based breast reconstruction Mann-Whitney U test, chi-square tests, or Fisher’s exact test (in the case of small cell counts). Univariate logistic regression, using individual breasts as the unit of analysis, was performed to determine the association between patient or surgical risk factors and implant loss, providing odds ratios (ORs) with 95% confidence intervals (CIs) and p-values. Cases with missing data on risk factors were excluded from the analysis. Multivariate mixed-effects logistic regression was used to adjust for confounders and correct for clustered data of patients who underwent bilateral mastectomies and therefore contributed two breasts to the analysis. All pre- and perioperative variables were considered potential confounders (obesity, age, bra size, comorbidities, smoking status, tumor type, year of operation, nipplesparing procedure, sentinel node dissection, type of reconstruction, neoadjuvant chemotherapy, bilateral operation, and radiotherapy). In addition, the patients were divided into subgroups (TE and DTI) before repeating the analysis. Significant univariate risk factors were inserted into a multivariate logistic regression model, and backward stepwise selection was performed to develop a practical risk model. Risk factors with p-values less than 0.05 were retained in the risk model. A maximum of four risk factors were included, based on the number of implant loss events. The multicollinearity of the individual risk factors was tested before introducing them to the logistic regression model. Surgeon’s volume was not included in the risk model, as this factor cannot be generalized to other practices. The predicted and observed risk of implant loss was computed for each risk factor (accumulating from zero to four). Continuous data are presented as median (range) and categorical variables as frequency and percentages. RESULTS Study population A total of 297 implant-based breast reconstructions were performed among 225 patients during the study period. Follow-up time varied from 1–4 years. The patients had a median age of 50 years (range: 22–72 years) and a median BMI of 24.3 (range: 16.5–44.1). In 27.6% of the patients, the bra cup size was larger than C, and in 6.2%, the American Society of Anesthesiologists score was three or more. Of the patients, 14.7% were active smokers. The median operative time was 137 minutes (range: 36–300 minutes). In 50.8% of the implant-based breast reconstructions, the underlying cause was invasive carcinoma, while in 18.5%, the underlying cause was ductal carcinoma in situ. In 29.0% of the reconstructions, a prophylactic mastectomy was performed. The median weight of the resected specimen was 397 grams (range: 39–1300 grams). In 40.1%, a nipple-preserving mastectomy was performed. In 79.8%, a TE was placed, and in 20.2%, a DTI reconstruction was performed. Most implants (94.6%) were placed in the subpectoral pocket, and postoperative radiotherapy was administered in 19.9% of the breast reconstructions. 5
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