Results

Outcomes after surgeries on Friday, 13th in comparison to other Fridays

Between 2009 and 2019, 945 arthroplasties (259 THA, 686 TKA) were performed on 21 Fridays falling on the 13th. Five-year cumulative incidence rates for aseptic and septic revisions did not differ significantly between Friday the 13th and other Fridays (Fig. 1). Adjusted hazard ratios confirmed no significant differences: aseptic revision HR 0.80 (95% CI 0.44–1.47, p = 0.488), septic revision HR 0.83 (95% CI 0.31–2.25, p = 0.726), and mortality HR 1.00 (95% CI 0.83–1.19, p = 0.965) (Table 1). The mortality rates did not differ significantly between surgeries performed on Friday the 13th and those on other days (Fig. 2). Additionally, the distribution of procedures performed on Friday the 13th (with a mean of 12.3 ± 4.6 THA and 20.3 ± 4.8 TKA procedures) did not significantly differ from procedures performed on other Fridays (with a mean of 11.5 ± 4.2 THA and 19.4 ± 6.1 TKA surgeries) (p = 0.386) (Fig. 3). Moreover, the patient cohorts operated on Friday the 13th and other Fridays did not show statistically significant differences in terms of age and comorbidities, except for a higher proportion of patients using NSAIDs in the other Friday cohort.

Fig. 1: Revision rates after primary hip or knee arthroplasty performed on Fridays, the 13th (left panel) and revision rates after primary THA or TKA performed on other Fridays (right panel). Aseptic revisions are shown in lilac and septic revisions in light pink

Fig. 2: Survival of patients after primary hip or knee arthroplasty performed on Fridays, the 13th and other Fridays

Outcome | HR | 95% CI | p-value
Aseptic revision | 0.80 | 0.44–1.47 | 0.488
Septic revision | 0.83 | 0.31–2.25 | 0.726
Mortality | 1.00 | 0.83–1.19 | 0.965

Fig. 3: Distribution of primary hip or knee arthroplasty procedures performed on Fridays between 2009 through 2019. Red dots indicate Fridays falling on the 13th