Influence of Pedicle Screw Design on the Fatigue Fixation Strength in the Human Lumbar Spine

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Lawrence Arnold
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1 Influence of Pedicle Screw Design on the Fatigue Fixation Strength in the Human Lumbar Spine Rebecca Kueny, MSc. 1, Reza Danyali 1, Bartosz Nowak, PhD 2, Klaus Pueschel, Dr med. 3, Michael M. Morlock, PhD 1, Gerd Huber, PhD 1. 1 TUHH Hamburg University of Technology, Hamburg, Germany, 2 ulrich medical, Ulm, Germany, 3 University Medical Center Hamburg-Eppendorf, Hamburg, Germany. Disclosures: R. Kueny: 5; Johnson & Johnson. R. Danyali: None. B. Nowak: 3A; ulrich medical. K. Pueschel: None. M.M. Morlock: 6; DePuy Synthes. G. Huber: None. Introduction: Pedicle screw fixation has been shown to be safe and effective in promoting fusion and in restoring vertebral alignment and sagittal balance. However, after instrumentation loosening at the screw-bone interface can occur, especially in osteoporotic patients. This loosening has been shown to be a predominant failure mode and can often lead to non-fusion or the necessity for revision surgery. Alterations of the screw thread design have generally not been studied in terms of a physiologic fatigue force but only with a simple posterior pullout test which cannot give time dependent fatigue behavior. The aim of this study was to investigate whether an increase in the screw s outer surface area or the addition of posterior support would influence the fatigue strength of the screws. It was hypothesized that a greater outer surface area or an external support touching the posterior cortical shell may more evenly distribute the concentrated loading along the screw profile. This could in turn reduce the risk of screw fatigue loosening. Methods: The ex vivo experiment involved 14 human thoracolumbar spines (T12-L3) which were harvested from male donors (48.0 ± 3.8 years) and stored at -22 C after receiving ethics approval. A repeated measures test setup was used where within each vertebra one prototype screw design was tested against a commercially available reference screw (tangors, ulrich medical, Ulm, DE). Specimens were equally divided into four groups based on BMD, vertebral level, and testing order. The four prototype groups were: (1) Pitch1, (2) Pitch2, (3) krypton, and (4) Offset (N=8 each). Design Alterations: The screws all utilized the same major dimensions (length= 50 mm, diameter= 5.5 mm, Fig. 1A). The pitch of the reference screw was twice increased in 1 mm increments to increase the outer screw surface area. A 4 mm long posterior sleeve was added to the reference screw to add additional posterior support (Offset). The forth group utilized a widely varying thread design with a cylindrical core and triangular threads (krypton).

3 Instrumentation: The specimens were prepared by removing all soft tissue, creating a mold for clamping, and then instrumenting with screws. An orthopaedic surgeon utilized an alignment angled directly through the pedicle canal and parallel to the endplate in the sagittal plane. Test Setup: After instrumentation the specimens were exposed to a dynamic sinusoidal flexion-compression loading (0.5 Hz, 858 Bionix, MTS, Eden Prairie, MN, Fig. 1B) starting at ± 15 N and continuously increasing by 0.05 N every cycle until failure. Failure was defined as a 3.5 mm displacement in either the cranial or caudal directions. A motion capturing system (Vicon Motion Systems, LA, USA) was used to track the screw and vertebral body motions. Three spherical markers were attached to the vertebral body, three to the XY table, and three to define the screw axis (Fig. 1B). Data Analysis: Cycles to failure, removal torque, initial stiffness, and end stiffness were measures of fixation strength. BMD, age, pedicle and vertebral dimensions were used as anatomical variables. One specimen was replaced and retested due to a mechanical clamping failure. Results: Overall averages for the loosening parameters between the prototypes (N=32) and the reference (N=32) groups were similar with differences only ranging from % of the reference value. No significant differences in fixation strength were found for any of the prototype designs compared to the reference screws. Pitch1 was the only prototype group which consistently, slightly increased fixation strength than the reference group with benefits of 6.9% for cycles to failure (p=0.48, r=0.27) and 12.2% for removal torque (p=0.47, r=0.29, Tab. 1). The pitch2 group experienced a fixation reduction in terms of cycles to failure (-7.0%, p=0.37, r=0.34) and a slight increase in removal torque (1.7%, p=0.94, r=0.03) compared to the reference group. The krypton group showed a 3% (p=0.71, r=0.14) increase in cycles to failure compared to the reference group; however, removal torque decreased by 41.9% (p=0.16, r=0.52). The offset group exhibited a decrease in fixation for cycles to failure (- 1.4%, p=0.88, r=0.06) and no change in removal torque (0.0%, p=1.00, r=0.00) from the reference group.

The average volumetric bone mineral density (BMD) was 106.4 ± 14.1 mgcaha/cm3. BMD positively influenced the number of cycles to failure (p=0.04, r=0.24) and the removal torque (p=0.02, r=0.27).

4 The average volumetric bone mineral density (BMD) was ± 14.1 mgcaha/cm3. BMD positively influenced the number of cycles to failure (p=0.04, r=0.24) and the removal torque (p=0.02, r=0.27). An increase in the major pedicle diameter significantly decreased cycles to failure (p=0.01, r=-0.29) and removal torque (p=0.04, r=-0.23). Vertebral length positively correlated with cycles to failure (p=0.02, r=0.27). The average overhang for all screws was 12.3 ± 1.6 mm and negatively correlated with cycles to failure (p=0.02, r=-0.27). Discussion: Variation of the screw thread designs may provide substantial differences in the often tested pullout force [1]; however, these fixation gains did not translate to the more physiologic fatigue loading for the tested designs and setup. No significant differences were found for screw fixation for the prototypes compared to the reference screws. Although there was a large, non-significant 41.9% reduction of removal torque in the krypton group, there was also a 3.0% increase in cycles to failure. This pattern of results would be expected given that the krypton group has a cylindrical core design rather than the conical design of the reference group. Conical cores are designed to increase the screw press-fit and, therefore, the removal torque. If the observed small differences in fixation strength due to screw design reflect actual changes, it is unlikely that significant results could be obtained with the current test design due to the small sample size and large variance.

5 The influence of specimen characteristics, quantified in terms of BMD and major pedicle diameter, played a significant role screw fixation strength. A larger major pedicle diameter decreased both the cycles to failure and the removal torque of the screw; whereas, increasing BMD increased fixation. Significance: Increasing the thread pitch and adding a posterior support to pedicle screws was not able to significantly alter the fatigue fixation strength. The variation of specimen characteristics drove differences in fatigue fixation more than changes to the screw thread or the addition of posterior support. Acknowledgments: Funding from the Marie Curie ITN project SpineFX is kindly acknowledged. The authors would also like to thank ulrich medical for providing screws for testing. References: [1] Cho et al. (2010). JBJS, British Vol. 92, ORS 2014 Annual Meeting Poster No: 1592

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