Elsevier

The Spine Journal

Volume 9, Issue 5, May 2009, Pages 404-410
The Spine Journal

Technical Review
Pullout strength of pedicle screws augmented with particulate calcium phosphate: An experimental study

https://doi.org/10.1016/j.spinee.2008.07.001Get rights and content

Abstract

Background context

Pressure-injected and in situ curing bone cements have been studied as alternatives in augmenting lumbar pedicle screw fixation but are frequently found to leak outside the confines of the target vertebra.

Purpose

The objective is set to determine the mechanical efficacy of a porous granular/particulate calcium phosphate (CP) bone augmentation product (Skelite) applied manually without pressurized injection in this application.

Study design/setting

The biomechanical analysis compared the axial pullout strength and insertional torque of augmented and nonaugmented pedicle screws in cellular polyurethane foams.

Methods

The insertion torque and pullout strength of 6.5-mm pedicle screws inserted (via 3.5-mm pilot holes) into polyurethane blocks mimicking the porosity of cancellous bone were measured. New pilot holes were then packed with granular particles of Skelite and retested. Last, those blocks initially tested to failure without augmentation were packed with Skelite and retested. Measurements were performed for polyurethane densities of 0.16 and 0.32 g/cc (corresponding to the porosity of osteoporotic and normal bone) and strain rates of 0.5 and 5 mm/min.

Results

Peak pullout force averaged 2132.5 ± 119.3 N and 1840.1 ± 216.7 N in high density samples without and with augmentation and 688.2 ± 91.4 N and 861.6 ± 74.5 N in low density samples without and with augmentation. After failure, approximately 50% and 77% of the peak pullout force of original high and low density samples was regained by augmentation. Statistical analysis revealed significant (p < .0001) correlation between the addition of CP, peak pullout resistance, and insertion torque.

Conclusion

Granular CP augmentation improved the pullout strength in both failed (pulledout) samples and low density (porosity of osteoporotic cancellous bone) polyurethane blocks.

Introduction

Pedicle screw fixation is a routine tool in spine stabilization. Screws provide rigid bony fixation points on which internal fixation devices can be mounted. The mechanical performance of pedicle screws will depend on the biomechanical characteristics of the bone-screw interface and is influenced by physical properties of the screw including, length, pitch, diameter, thread, and shaft design [1], [2], [3], [4], [5], [6], [7]. It has also been reported that low bone density in patients with osteoporotic bone can adversely affect the mechanical strength of bone–screw interface [8], [9], [10].

Axial pullout tests have been extensively used to evaluate the fixation stability of pedicle screws in cadaver and animal bones [1], [3], [9], [11], [12], [13], [14] and in artificial materials with properties resembling those of bone [3], [15], [16].

Researchers have found that conical core screws demonstrate higher pullout strength and screw insertion torque than cylindrical core screws [17]. In clinical pilot studies, it was found that coating of pedicle screws with plasma-sprayed hydroxyapatite increases pullout strength and reduces the risk of loosening [18], [19].

Different methods can be used to enhance the short- and long-term stability of the screws inserted in the osteoporotic bone. The injection of in situ curing of CP cement [20], [21] and polymethylmethacrylate (PMMA) [22] into the screw hole is the common option in enhancing pedicle screw fixation strength. The special ability of the CP in the fixation application is to speed natural tissue healing and then be replaced by the patient's own bone tissue. The presence of new bone growth in the insertion area of CP suggested that the osteoconductivity characteristics of the CP enhance osteogenesis.

The present study investigates for the first time, the effect of adding granular particles of CP to the polyurethane blocks on the pullout strength of pedicle screws. We have investigated the effect of CP–based Skelite (a synthetic bone graft analog produced by Millenium-Biologix, Kingston, Ontario, Canada) on the mechanical stability of pedicle screws in cellular polyurethane foam, as a preliminary study before possible clinical evaluation in osteoporotic bone.

Section snippets

Preparation of specimens

Fig. 1 presents the polyurethane test blocks and the pedicle screw used in the present study. Test samples of 4×4×4 cm extracted from 14×18×4 cm cellular polyurethane blocks (Sawbones, Pacific Research Corporation, Vashon, Washington, USA) were used as biomechanical testing material. Blocks with two different densities were used. The lower density of 0.16 g/cc has cell size of 0.5 to 2.0 mm and Young Modulus of 23 MPa (approximating human osteoporotic cancellous bone), whereas the higher density of

Results

All samples were loaded to failure with the Instron machine (Fig. 2). Load-displacement curves were recorded and analyzed for each individual specimen. The screws were then individually examined to ensure the structure was completely preserved.

Typical load-displacement curves, respectively, for high and low density polyurethane, in original and failed samples with or without CP augmentation, are presented in Fig. 3. These data illustrate a linear elastic behavior of the specimens up to peak

Discussions

Despite considerable evolution and advance in technology and technique because pedicle screws were first introduced to clinical spine practice by Roy-Camille et al. in the 1960s [23], screw failure through pullout or loosening remain a clinical problem of importance [26], [27]. Vertebral augmentation with PMMA is an attractive option in improving screw fixation [28], very analogous to vertebroplasty, but one where potential for cement leakage beyond the confines of the target bone [22] has

Conclusions

Manual packing of pedicle screw pilot holes with granular/particulate CP has the potential to augment screw purchase in osteoporotic bone and to partially reconstitute the purchase of screws the threads of which are stripped intraoperatively. However, application of this technique to normal bone probably will reduce screw pullout strength. It was also shown that low bone density can adversely affect the pullout strength and insertional torque for the standard 6.5-mm pedicle screw.

Acknowledgments

The authors thank [BLINDED] for the supply of granular calcium phosphate, pedicle screws, and instruments for use in this research.

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    This research was supported by funding from the Government of Ontario (Materials and Manufacturing Ontario [MMO] grant #IA90250).

    FDA device/drug status: not applicable.

    Author disclosures: none.

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