Moonlight surgery: no influence of moon phase or Friday 13th on outcomes of total knee arthroplasty.

Objectives: The purpose of the present study was to investigate associations between revision-free survival and functional scores of total knee arthroplasty (TKA) and moon phase on the day of surgery, as well as operations performed on a Friday 13th.

Participants: The data of all patients that received TKA between 2003 and 2019 were extracted from the Tyrol arthroplasty registry. Patients that had undergone previous total or partial knee arthroplasty as well as patients that had missing pre- or post-operative WOMAC were excluded. Patients were allocated to one of the following four groups according to moon phase on the day of surgery: new, waxing, full and waning. Patients operated on a Friday 13th were also identified and compared to patients operated on any other days/dates. A total of 5923 patients met the inclusion criteria, with mean age of 69 ± 9 years, and comprising 62% women.

Results: There were no significant differences in revision-free survival among the four moon phase groups (p = 0.479), and no significant differences in preoperative and postoperative total WOMAC (p = 0.260, p = 0.122), There were no significant differences in revision-free survival patients operated on Friday 13th vs. other days/dates (p = 0.440). The preoperative total WOMAC was significantly worse for patients operated on a Friday 13th (p = 0.013), which was observed in the pain (p = 0.032) and function (p = 0.010) subscales. There were no significant differences in postoperative total WOMAC at 1 year follow-up (p = 0.122).

Conclusions: Neither moon phase on the day of surgery nor Friday 13th were associated with revision-free survival or clinical scores of TKA. Patients operated on a Friday 13th had significantly worse preoperative total WOMAC but similar postoperative total WOMAC at 1-year follow-up. These findings could help reassure patients that TKA renders consistent outcomes regardless of the preoperative pain or function, and in spite of bad omens or moon phases.