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Bone Densitometry Within Titanium Lumbar Interbody Fusion Cages: A Computed Tomography Feasibility Study

Robertson, Douglas D. MD, PhD*†; Sharma, Gulshan B. PhD*†; Gilbertson, Lars G. PhD; Kang, James D. MD§

doi: 10.1097/BRS.0b013e3181b61e00

Study Design. The use of lumbar fusion cages to augment interbody arthrodesis has become popular. Previous studies have shown that titanium cages may obscure information within and around the cage, confounding assessments of fusion. Densitometry within cages, if possible, could be an important outcome variable for comparing different cage augmented fusion treatments.

Objective. The purpose of this feasibility study was to (1) validate CT-based (computer tomography-based) density evaluations within titanium cages in a phantom model, and (2) demonstrate that intrametal cage lucencies can be produced by CT reconstruction artifacts and do not necessarily represent bone fragmentation or fibrous and cartilaginous tissue.

Methods. An anthropomorphic lumbar spine phantom was used. Dual titanium cages were inserted into 6 anterior-middle column replicas, and the cages were filled with 10, 100, or 400 mg/cc hydoxyapatiite equivalent material. The column replicas were composed of either fibrous/cartilaginous tissue equivalent material or trabecular bone equivalent material. Column replicas were placed within the phantom and imaged with CT. Direct axial and reformatted sagittal and coronal images were evaluated for artifact. Region of interest analysis was performed for intracage density.

Results. Intracage CT values calculated from axial, sagittal, and coronal images were directly related to the true CT values of the materials (r = 0.99, P < 0.0001). CT was used to distinguish between fibrous tissue and bone within cages. CT values within cages overestimated the intracage materials' true values (paired t test, P < 0.001). Intracage CT values were correctable and could be converted to bone mineral content. Intracage CT values were not affected by adjacent anterior-middle column material differences (fibrous/cartilaginous tissue vs. trabecular bone) (paired t test, P > 0.05). Lucent streaking within and adjacent to cages was similar to clinical images.

Conclusion. In this phantom-based study, densitometry was performed within titanium cages, even in the presence of CT artifact. These results support further pursuit of this potential assessment tool.

Computed tomography artifacts within titanium cages create lucencies which may simulate graft fragmentation and lack of fusion. However, even in the presence of these artifacts, bone densitometry is possible within titanium cages. On the basis of these findings, we think that computed tomography has the potential for improving assessments and comparisons of titanium cage augmented arthrodesis.

From the *Department of Bioengineering, University of Pittsburgh, Pittsburgh, PA; †Cylab, Carnegie Mellon University, Pittsburgh, PA; ‡Cleveland Clinic Spine Institute, Cleveland Clinic, Cleveland, OH; and §Department of Orthopaedic Surgery, University of Pittsburgh Medical Center, Pittsburgh, PA.

Acknowledgment date: December 8, 2008. Revision date: April 21, 2009. Acceptance date: April 22, 2009.

The device(s)/drug(s) is/are FDA-approved or approved by corresponding national agency for this indication.

No funds were received in support of this work. No benefits in any form have been or will be received from a commercial party related directly or indirectly to the subject of this manuscript.

Address correspondence and reprint requests to Douglas D. Robertson, MD, PhD, Innovas-MIBL, 403 Landon Gate, Pittsburgh, PA 15238; E-mail:

© 2009 Lippincott Williams & Wilkins, Inc.