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Pullout of a lumbar plate with varying screw lengths

Daniel Kyle Palmer, BS, David Rios, BS, Wyzscx Merfil Patacxil, MS, Paul A. Williams, MS, Wayne K. Cheng, MD, Serkan İnceoğlu, PhD

Orthopaedic Biomechanics Laboratory, Department of Orthopaedic Surgery, Loma Linda University, Loma Linda, CA

Abstract 

Background

Screw length pertains to stability in various orthopedic fixation devices. There is little or no information on the relationship between plate pullout strength and screw length in anterior lumbar interbody fusion (ALIF) plate constructs in the literature. Such a description may prove useful, especially in the treatment of osteoporotic patients where maximizing construct stability is of utmost importance. Our purpose is to describe the influence of screw length on ALIF plate stability in severely and mildly osteoporotic bone foam models.

Methods

Testing was performed on polyurethane foam blocks with densities of 0.08 g/cm3 and 0.16 g/cm3. Four-screw, single-level ALIF plate constructs were secured to the polyurethane foam blocks by use of sets of self-tapping cancellous bone screws that were 20, 24, 28, 32, and 36 mm in length and 6.0 mm in diameter. Plates were pulled out at 1 mm/min to failure, as defined by consistently decreasing load despite increasing displacement.

Results

Pullout loads in 0.08-g/cm3 foam for 20-, 24-, 28-, 32-, and 36-mm screws averaged 303, 388, 479, 586, and 708 N, respectively, increasing at a mean of 25.2 N/mm. In 0.16-g/cm3 foam, pullout loads for 20-, 24-, 28-, 32-, and 36-mm screws averaged 1004, 1335, 1569, 1907, and 2162 N, respectively, increasing at a mean of 72.2 N/mm.

Conclusions

The use of longer screws in ALIF plate installation is expected to increase construct stability. Stabilization from screw length in osteoporotic patients, however, is limited.

keywords: 
ALIF, lumbar spine, Anterior plate, Pullout, Polyurethane foam, Screw length
Volume 6
doi: 
10.1016/j.ijsp.2011.12.002