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Research Papers

Assessment of Pedicle Bone Strength in the Lumbar Spine Using a “Smart” Ball Tip Probe

[+] Author and Article Information
Michael A. Tufaga, R. Trigg McClellan

Biomechanical Testing Facility,
UCSF/SFGH Orthopaedic Trauma Institute,
San Francisco, CA 94110
Department of Orthopaedic Surgery,
University of California San Francisco,
San Francisco, CA 94110

Glenn Diekmann

Department of Orthopaedic Surgery,
University of California San Francisco,
San Francisco, CA 94110

Jenni M. Buckley

Biomechanical Testing Facility,
UCSF/SFGH Orthopaedic Trauma Institute,
San Francisco, CA 94110
Department of Orthopaedic Surgery,
University of California San Francisco,
San Francisco, CA 94110
e-mail: jennibuckleyphd@gmail.com

Kathleen Mulligan

Department of Endocrinology,
University of California San Francisco,
San Francisco, CA 94110

Christopher Ames

Biomechanical Testing Facility,
UCSF/SFGH Orthopaedic Trauma Institute,
San Francisco, CA 94110
Department of Neurological Surgery,
University of California San Francisco,
San Francisco, CA 94110

1Corresponding author.Present address: Taylor Collaboration, St. Mary's Medical Center, 450 Stanyan St., San Francisco, CA 94117.

Manuscript received June 23, 2010; final manuscript received April 8, 2011; published online September 24, 2013. Assoc. Editor: Vijay Goel.

J. Med. Devices 7(4), 041002 (Sep 24, 2013) (4 pages) Paper No: MED-10-1062; doi: 10.1115/1.4004206 History: Received June 23, 2010; Revised April 08, 2011

Preoperative assessment of bone mineral density (BMD) via dual X-ray absorptiometry (DEXA) has been shown to help determine the need for screw augmentation, particularly in the pedicle. However, patients frequently present without preoperative DEXA scans, especially in trauma situations. We developed a “smart” probe for intra-operative assessment of local bone strength in the pedicle. A standard ball tip probe was instrumented with an in-line load cell and was used to test the bone strength of fresh-frozen human cadaveric lumbar spine. Local bone crush strength was assessed using the probe at five locations within the centrum and the pedicle. The contralateral pedicle was outfitted with a standard pedicle screw and insertion torque and pull-out force of the screw were measured. Correlation was found between PS-centrum (probe strength-centrum) and screw pull-out strength (adj. R2 = 0.42). A stepwise linear regression revealed that when considering BMD, insertion torque, and PS-centrum, pull-out strength could be predicted based on PS-centrum and insertion torque (adj. R2 = 0.51, p < 0.01, Fisher test). The results suggest that use of the new probe may be a more quantitative measurement of screw purchase when DEXA is unavailable, such as in spinal trauma cases. Furthermore, when used in conjunction with DEXA, the “smart” ball tip probe may improve assessment of the strength of pedicle screw purchase.

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Figures

Grahic Jump Location
Fig. 1

“Smart” ball tip probe with in-line load cell used to assess pedicle bone strength

Grahic Jump Location
Fig. 2

(Left) Five zones where bone crush strength was measured. (Right) Use of instrumented ball tip probe to assess cancellous bone strength.

Grahic Jump Location
Fig. 3

(Left) Mechanical testing setup for tangential fatigue and (right) coaxial destructive loading (pull-out)

Grahic Jump Location
Fig. 4

Correlation of bone crush strength in the centrum with level-specific T-score

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