Monoblock or modular? Impact of stem design and conicity angle on long-term implant survival in revision total hip arthroplasty: a 20-year follow-up registry study on 3647 implants

Introduction: Revision total hip arthroplasty (rTHA) often requires femoral stem revision due to aseptic loosening, instability, or fractures. Long, tapered conical stems are preferred for stability and bone loss management. Recent design changes, like increased conicity angles, aim to enhance fixation and reduce subsidence. Monoblock and modular stems offer distinct pros and cons, but their long-term outcomes remain debated. This study evaluates the long-term survival of monoblock versus modular conical stems, with a focus on conicity angles (2° vs. 3°), using 20 years of registry data. Methods: A retrospective review was conducted using the Emilia Romagna Registry of Orthopedic Prosthetic Implants (RIPO) from 2000 to 2021. A total of 3,647 non-cemented conical stems used in rTHA were analyzed: 32.4% monoblock and 67.6% modular. Kaplan–Meier survival analysis assessed implant longevity, stratified by stem design and conicity angle. Results: Use of modular stems increased from 24% to over 83% during the study period. Overall failure rate was 6.3–5.1% for monoblock stems (mainly due to aseptic loosening) and 6.9% for modular stems (primarily due to instability). Monoblock stems had superior 20-year survival (93.7%) compared to modular (86.8%, p = 0.009). Among modular stems, those with 2° conicity had significantly better 15-year survival (91.9%) than 3° designs (88.0%, p = 0.001). No significant difference was observed between conicity angles in monoblock stems. Conclusion: Monoblock stems provide better long-term survival in rTHA. Modular designs, while offering intraoperative flexibility, carry higher revision risk, especially with greater conicity. Stem selection should balance design features with patient needs and surgical expertise.