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Pedicle Screw Designs in Spinal Surgery

Is There a Difference? A Biomechanical Study on Primary and Revision Pull-Out Strength

Seng, Wei Ren Daniel MBChB, MRCS (Glas); Chou, Siaw Meng BEng (Hons), PhD; Siddiqui, Shiblee S. MBBS, MS, Orthopaedics; Oh, Jacob Y.L. MBChB, FRCS Orth & Tr (Edinburgh)

doi: 10.1097/BRS.0000000000002789
BIOMECHANICS
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Study Design. An experimental laboratory-based biomechanical study.

Objective. The objective of this study was to evaluate, in a synthetic bone model, the difference in primary and revision pull-out strength using pedicle screws of different thread designs.

Summary of Background Data. Over the past few decades, there has been a growing interest in optimizing the screw pull-out strength using various screw designs (single-thread, mixed-thread, dual-thread). Although primary pull-out strength has been studied previously, little is known about revision pull-out strength of different pedicle screws.

Methods. The pull-out strengths of three different pedicle screw designs (single-thread, mixed-thread, dual-thread) were tested in standardized polyurethane foam in three sequences. Sequence 1: A 6.5 mm screw was inserted into the foam block and the primary pull-out strength measured. Sequence 2: A 6.5 mm screw was inserted, removed, and then reinserted into the same foam block. The revision pull-out strength was then measured. Sequence 3: A 6.5 mm screw was inserted, removed and a 7.5-mm screw of the same thread design was reinserted. The revision pull-out strength was then measured.

Results. The primary pull-out strength was similar across screw designs, although dual-thread screws showed higher primary pull-out strength (2628.8 N) compared to single-thread screws (2184.4 N, P < 0.05). For revision pull-out strength, the mixed-thread screws had significant reduction in revision pull-out strength of 18.6% (1890.2 N, P = 0.0173). Revision with a larger diameter screw improved the pull-out strength back to baseline. Single and dual-thread screws showed no significant reduction in revision pull-out strength.

Conclusion. The dual-threaded screws provided the strongest primary pull-out strength for spinal fixation. The mixed-thread screws, however, had the poorest revision pull-out strength, decreasing by 18.6% compared to other screw designs. In cases in which mixed-threaded screws have to be revised (at the index or revision surgery), surgeons should consider the use of larger diameter screws to improve the pull-out strength.

Level of Evidence: 5

This is a biomechanical study evaluating the primary and revision pull-out strength of three different compliance engineered pedicle screws in a synthetic bone model. We have found that the primary and revision pull-out strength is affected by the screw thread design.

Department of Orthopaedic Surgery, Tan Tock Seng Hospital, National Health Group, Nanyang, Singapore

School of Mechanical and Aerospace Engineering, College of Engineering, Nanyang Technological University, Nanyang, Singapore.

Address correspondence and reprint requests to Jacob Y.L. Oh, MBChB, FRCS Orth & Tr (Edinburgh), Department of Orthopaedic Surgery, Tan Tock Seng Hospital, 11 Jalan Tan Tock Seng, Singapore 308433; E-mail: Jacob_YL_OH@ttsh.com.sg

Received 17 April, 2018

Revised 15 June, 2018

Accepted 20 June, 2018

The manuscript submitted does not contain information about medical device(s)/drug(s).

No funds were received in support of this work.

No relevant financial activities outside the submitted work.

Copyright © 2018 Wolters Kluwer Health, Inc. All rights reserved.