Abstract Background Surgeon at primary total hip arthroplasty sometimes cannot achieve sufficient cementless acetabular press-fit fixation and must resort to other fixation methods. Despite a ...predominant use of cementless cups, this issue is not fully clarified, therefore we performed a large retrospective study to: 1) identify risk factors related to patient or implant or surgeon for unsuccessful intraoperative press-fit, 2) check for correlation between surgeons’ volume of operated cases and the press-fit success rate. Hypothesis Unsuccessful intra-operative press-fit more often occurs in older female patients, particular implants, due to learning curve and low-volume surgeons. Materials and Methods Retrospective observational cohort of prospectively collected intraoperative data (2009-2016) included all primary total hip arthroplasty patients with implant brands that offered acetabular press-fit fixation only. Press-fit was considered successful if acetabulum was of the same implant brand as the femoral component without additional screws or cement. Logistic regression models for unsuccessful acetabular press-fit included patients’ gender/age/operated side, implant, surgeon, approach (posterior n = 1206, direct-leteral n = 871) and surgery date (i.e. learning curve). Results In 2,077 patients (mean 65.5 years, 1,093 females, 1,163 right hips) were implanted three different implant brands (973 ABG-II™-Stryker, 646 EcoFit™Implantcast, 458 Procotyl™L-Wright) by eight surgeons. Their unsuccessful press-fit fixation rates ranged from 3.5 % to 23.7 %. Older age (odds ratio 1.01 (95% CI 0.99 - 1.02)), female gender (2.87 (95% CI 2.11 - 3.91)), right side (1.44 (95% CI 1.08 - 1.92)), surgery date (0.90 (95% CI 1.08 - 1.92)) and particular implants were significant risk factors only in three surgeons with less successful surgical technique (higher rates of unsuccessful press-fit with Procotyl™-L and EcoFit™ (p= 0.01)). Direct-lateral hip approach had a lower rate of unsuccessful press-fit than posterior hip approach (p < 0.01), but there was no correlation between surgeons’ volume and rate of successful press-fit (Spearman's rho = 0.10, p = 0.82). Subcohort of 961 patients with 5-7-year follow-up indicated higher early/late cup revision rates with unsuccessful press-fit. Discussion Success of press-fit fixation depends entirely on the surgeon and surgical approach. With proper operative technique, the unsuccessful press-fit fixation rate should be below 5 % and the impact of patients’ characteristics or implants on press-fit fixation is then insignificant. Findings of huge variability in operative technique between surgeons of the presented study emphasize the need for surgeon-specific data stratification in arthroplasty studies and indicate the possibility of false attribution of clinically observed phenomena to patient-related factors in pooled data of large centers or hip arthroplasty registers. Level of evidence Level III, retrospective observational case control study
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