Surgical treatment of fragility sacrum fractureswith percutaneous sacroiliac (SI) screw fixation is associated with high failure rates. Turn-out is detected in up to 20% of the patients. The aim of this study was to evaluatea new screw-in-screw implant prototypefor fragility sacrum fracture fixation. Twenty-seven artificial hemi-pelvises were assigned to three groups (n=9) for instrumentation of an SIscrew, the new screw-in-screw implant prototype, ora trans-sacral screw. Prior to implantation, a vertical osteotomy was set in zone 1 after Denis. All specimens were biomechanically tested to failure in upright position. Validated setup and test protocol were used for complex axial and torsional loading applied through the S1 vertebral body to promote turn-out of the implants. Interfragmentary movements were captured via optical motion tracking. Screw motions were evaluated by means of triggered anteroposterior X-rays. Interfragmentary movements and implant motions were significantly higher for SIscrew fixation compared to both trans-sacral and screw-in-screw fixations. In addition, trans-sacral screw and screw-in-screw fixations revealed similar construct stability. Moreover, screw-in-screw fixation successfully prevented turn-out of the implant that remained during testing at 0° rotation for all specimens. From biomechanical perspective, fragility sacrum fracture fixation with the new screw-in-screw implant prototype provides higher stability than an SIscrew, being able to successfully prevent turn-out. Moreover, it combines the higher stability of trans-sacral screw fixation with the less risky operational procedure of SI screw fixation and can be considered as their alternative treatment option. This article is protected by copyright. All rights reserved.This article is protected by copyright. All rights reserved.