The Journal of Bone and Joint Surgery (American). 2005;87:2038-2051.
doi:10.2106/JBJS.D.02103
© 2005 The Journal of Bone and Joint Surgery, Inc.
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The Efficacy and Integrity of Shape Memory Alloy Staples and Bone Anchors with Ligament Tethers in the Fusionless Treatment of Experimental Scoliosis

John T. Braun, MD1, Ephraim Akyuz, MS2, James W. Ogilvie, MD1 and Kent N. Bachus, PhD2

1 Department of Orthopaedics, University Orthopaedic Center, University of Utah, School of Medicine, 590 Wakara Way, Salt Lake City, UT 84108. E-mail address for J.T. Braun: john.braun{at}hsc.utah.edu
2 Orthopaedic Research Laboratory, Departments of Orthopaedics and Bioengineering, University of Utah, Salt Lake City, UT 84112

Investigation performed at the Departments of Orthopaedics and Bioengineering, University of Utah, School of Medicine, Salt Lake City, Utah

In support of their research or preparation of this manuscript, one or more of the authors received grants or outside funding from Medtronic Sofamor Danek (Memphis, Tennessee). In addition, one or more of the authors received payments or other benefits or a commitment or agreement to provide such benefits from a commercial entity (Medtronic). No commercial entity paid or directed, or agreed to pay or direct, any benefits to any research fund, foundation, educational institution, or other charitable or nonprofit organization with which the authors are affiliated or associated.

Background: Scoliosis is a complex three-dimensional deformity with limited treatment options. Current treatments present potential problems that may be addressed with use of fusionless techniques for the correction of scoliosis. However, there are few data comparing the efficacy of different fusionless implant strategies in controlling scoliosis or on the integrity of rigid compared with flexible devices in an in vivo setting over time. The objective of this study was to compare the efficacy and integrity of rigid and flexible anterior thoracic tethers used to treat experimental scoliosis.

Methods: Experimental scoliosis was created in twenty-four Spanish Cross-X female goats and was subsequently treated with either anterior shape memory alloy staples or anterior ligament tethers attached to bone anchors. Serial radiographs were analyzed to determine the efficacy of the implants in controlling scoliosis progression as well as the integrity of the implants at study completion. After the goats were killed, the implants were analyzed with use of three quantitative indices of implant integrity and implant pullout testing.

Results: Over the treatment period, scoliosis progressed from 77.3° to 94.3° in the goats treated with staples and was corrected from 73.4° to 69.9° in the goats treated with bone anchors, with loosening of eighteen of forty-two staples (two of the eighteen dislodged) and evidence of drift in two of forty-nine anchors. Histologic sections revealed a consistent halo of fibrous tissue around the staple tines but well-fixed bone anchors at all sites. Pullout testing demonstrated that bone anchors had greater strength than staples initially and at the study completion, with an increase in bone anchor fixation over the course of the study.

Conclusions: In this scoliosis model, the flexible ligament tethers attached to bone anchors demonstrated greater efficacy and integrity than the more rigid shape memory alloy staples.

Clinical Relevance: Fusionless scoliosis surgery offers many theoretical advantages over instrumented arthrodesis. Improvements in fusionless implant design, with a specific focus on optimizing the fixation to bone and maximizing the tethering effect, may lead to better control of idiopathic scoliosis with lower rates of device loosening and failure.


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