[The MACS-HMA hollow screw. An alternative possibility for stable implant anchoring in the vertebral body also for long fusions].

Our biomechanical in vitro tests compared the stability of the MACS HMA (Aesculap, Tuttlingen) implants to three established systems. The MACS HMA is a modular system consisting of porous hollow titanium screws with an outer diameter of 12 mm for monocortical use. We report the preliminary results of MACS HMA used for correction of scoliosis. All other implant systems used are designed with bicortical screws; one is for application with two screws/two rods and one uses a pullout-resistant nut behind the opposite cortex to increase stability. Significantly increased motion in the craniocaudal direction was recognized for bicortical standard screws (20 +/- 17 microns) compared to MACS HMA (10 +/- 11 microns) or the dual screw system (12 +/- 6 microns). Two-tailed t-test showed significantly higher stability for the dual screw system (4.2 kN) and the system with pullout-resistant nut (4.0 kN) compared to all other systems (p < 0.025 or higher). Bicortical implants (2.1-3.2 kN) and MACS HMA (2.6 kN) did not reveal significant differences in pullout strength. All biomechanical tests and in vivo use demonstrated favorable performance of MACS HMA implants.