Biomechanical characteristics of pedicle screws in osteoporotic vertebrae—comparing a new cadaver corpectomy model and pure pull‐out testing

Abstract

Currently, evaluation of the stability of spinal instrumentations often focuses on simple pull-out or cyclic loading. However, the loading characteristics and the specimen alignment rarely simulate physiological loading conditions, or the clinical situation itself. The purpose of this study was to develop an alternative setup and parameters to compare static and dynamic characteristics of pedicle screws at the bone-implant interface in lumbar osteoporotic cadavers. A corpectomy model development was based on ASTM-1717 standard, allowing a deflection of the cranial and caudal element under loading. Twelve human osteoporotic vertebrae (L1-L4) were analyzed for morphological CT-data and T-Score. For group A (n ¼ 6) loads were simulated as in vivo measurements during walking, representing 2 months postoperatively. A subsequent pull-out was performed. Group B (n ¼ 6) was tested with pure pull-out. Screw loosening at the tip/head was optically measured and analyzed with respect to clinical patterns. Correlations between CT-data, T-Score, and in vitro parameters were determined. For group A, the subsidence for the head/tip was measured towards the upper/lower endplate, resulting in visible deflections. The progress of the subsidence was greatest within the first and last cycles until failure. The predominant patterns were pure rotation and toggling. However, the pull-out between groups was not significantly different. Pedicle-angle and cyclic-subsidence correlated with R ¼ 0.806/0.794. T-Score and pull-out correlated only in group A. With the corpectomy setup, clinically observed wipe effects and a loss of correction could be simulated. The presented parameters facilitate analysis of the complex changing load distributions and interactions between the left and right bone-implant interface. ß

Risk Factors for Clinically Relevant Loosening of Percutaneous Pedicle Screws

“…Numerous biomechanical studies in vitro have attempted to clarify crucial factors for the anchorage capacity of pedicle screws, focusing on bone density, insertion depth, misplacement, and insertion angle. However, these studies have inherent limitations associated with a cadaveric study and/or bone models, in which it is difficult to mimic the actual clinical situation, such as the influence of adjacent vertebral musculature and ligaments, and the wide variations in the material properties of bone, living tissues, and postoperative loading [4][5][6]9,[16][17][18][19][20][21][22] . Therefore, the strengths and clinical relevance of this study are that it showed that a lower axial trajectory and a high screw pull-out force during rod connection might be crucial risk factors for PPS loosening in clinical situations.…”

Complex biomechanical properties of non-augmented and augmented pedicle screws in human vertebrae with reduced bone density

“…As reported elsewhere, we developed a method to apply quasi-physiological loading and detect screw movement [11]. To our knowledge, the effect of cement augmentation in an in vivo physiological setup focusing on the observed screw movement has not been investigated to date.…”

Reduced cement volume does not affect screw stability in augmented pedicle screws

“…However, they were not able to draw this conclusion based on a cyclic fatigue test, which is a more physiological testing method for screw failure testing [23,30]. especially since withdrawing of the screws posteriorly through the pedicle has clinically rarely been reported [12,23,27,31].…”