SEDL / STP / STP1431-EB / STP11134S

Clinical Relevance of Pull-out Strength Testing of Pedicle Screws

Dawson, JM
Director of Spine Biomechanics Research, Centerpulse Spine Tech Inc., Minneapolis, MN

Boschert, P
Director of Spine Biomechanics Research, Centerpulse Spine Tech Inc., Minneapolis, MN

Macenski, MM
Director of Spine Biomechanics Research, Centerpulse Spine Tech Inc., Minneapolis, MN

Rand, N
Senior orthopedic surgeonAdjunct Professor, Assuta HospitalVanderbilt University, Tel-AvivNashville, TN

Pages: 10    Published: Jan 2003

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Abstract

Pull-out strength testing is described in many published manuscripts and is specially treated in ASTM F1692-96, “Standard Test Method for Determining Axial Pull-Out Strength of Medical Bone Screws.” Extensive biomechanics are reported on the importance of variables like hole preparation and cement augmentation. In this study, the pull-out strengths of variously-shaped, titanium pedicle screws were tested and the literature was searched for cited incidences of pull-out. The screws were extracted from polyurethane foam that modeled weak, moderate, and strong cancellous bone. Testing revealed significantly different pull-out strengths depending upon screw design and substrate strength. However, the literature search yielded few citations of screw pull-out. Screw loosening and hardware failure are sometimes mentioned in clinical reports but rarely is pull-out documented as a failure mechanism. Standardized testing is useful in comparing various designs but may have little relationship to in vivo performance.