Anterior vertebral body screw pullout testing with the hollow modular anchorage system--a comparative in vitro study.

Pullout of implants at the proximal and distal ends of multilevel constructs represents a common spinal surgery problem. One goal concerning the development of new spinal implants is to achieve stable fixation together with the least invasive approach to the spinal column. This biomechanical study measures the influence of different modes of implantation and different screw designs, including a new monocortical system, on the maximum pullout strength of screws inserted ventrolaterally into calf vertebrae. The force pullout of eight different groups were tested and compared. Included were three bicortical used single screws (USS, Zielke-VDS, single KASS). To further increase pullout strength either a second screw (KASS) or a pullout-resistant nut can be added (USS with pullout nut). A completely new concept of anchorage represents the Hollow Modular Anchorage System (MACS-HMA). This hollow titanium implant has an increased outside diameter and is designed for monocortical use. Additionally two screw systems suitable for bicortical use were tested in monocortical mode of anchorage (USS, single KASS). We selected seven vertebrae equal in mean size and bone mineral density for each of the eight groups. The vertebral body and implant were connected to both ends of a servohydraulic testing machine. Displacement controlled distraction was applied until failure at the metal-bone-interface occurred. The maximum axial pullout force was recorded. Mean BMD was 312 +/- 55 mg CaHA/ml in cancellous bone and 498 +/- 98 mg CaHA/ml in cortical bone. The highest resistance to pullout found, measured 4.2 kN (KASS) and 4.0 kN (USS with pullout nut). The mean pullout strength of Zielke-VDS was 2.1 kN, of single KASS 2.5 kN, of MACS-HMA 2.6 kN and of USS 3.2 kN. There was no statistically significant difference (t-test, p > 0.05) between bicortical screws and the new monocortical implant. For the strongest fixation at the proximal or distal end of long spinal constructs the addition of a second screw or a pullout-resistant nut behind the opposite cortex offers even stronger fixation.

[The MACS-HMA hollow screw. An alternative possibility for stable implant anchoring in the vertebral body also for long fusions]. / Die MACS-HMA-Hohltonne. Eine alternative Möglichkeit der stabilen Implantatverankerung im Wirbelkörper auch für langstreckige Fusionen.

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.