Guidelines for cortical screw versus pedicle screw selection from a fatigued decompressive lumbar laminectomy model show similar stability and less bone mineral density dependency

Highlights

• Cortical and pedicle screws were compared before and after in vivo wear simulation.

• Wear measured in a biomechanical model simulating multilevel decompression/fusion.

• Intact motion was collected for bone density-matched specimen pairs.

• No differences in motion were observed between cortical and pedicle trajectories.

• Cortical screws had significantly less pullout resistance.

Abstract

Background

Traditional pedicle screws are the gold standard for lumbar spine fixation; however, cortical screws along the midline cortical bone trajectory may be advantageous when lumbar decompression is required. While biomechanic investigation of both techniques exists, cortical screw performance in a multi-level lumbar laminectomy and fusion model is unknown. Furthermore, longer-term viability of cortical screws following cyclic fatigue has not been investigated.

Methods

Fourteen human specimens (L1–S1) were divided into cortical and pedicle screw treatment groups. Motion was captured for the following conditions: intact, bilateral posterior fixation (L3–L5), fixation with laminectomy at L3–L5, fixation with laminectomy and transforaminal lumbar interbody fusion at L3–L5 both prior to, and following, simulated in vivo fatigue. Following fatigue, screw pullout force was collected and “effective shear stress” [pullout force/screw surface area] (N/mm²) was calculated; comparisons and correlations were performed.

Findings

In flexion-extension and lateral bending, all operative constructs significantly reduced motion compared to intact (P < 0.05), regardless of pedicle or cortical screws; only posterior fixation with and without laminectomy significantly reduced motion in axial rotation (P < 0.05). Pedicle screws significantly increased average pullout strength (944.2 N vs. 690.2 N, P < 0.05), but not the “effective shear stress” (1.01 N/mm² vs. 1.1 N/mm², P > 0.05).

Interpretation

In a posterior laminectomy and fusion model, cortical screws provided equivalent stability to pedicle screw fixation, yet had significantly lower screw pullout force. No differences in “effective shear stress” warrant further investigation of the effect of screw length/diameter in the aforementioned screw trajectories.

Introduction

Posterior fixation is essential for treatment of a variety of destabilizing pathologies of the spine. Screw design and operative technique influence fixation strength - the outer diameter determines pullout strength and the inner determines fatigue strength; correct placement within the cortical bone of the pedicle helps to ensure rigid fixation (Cho et al., 2010). While insertion of traditional pedicle screws (PS) using the Weinstein approach is recommended to improve stability, tissue dissection and visualization of the lateral and inferior corners of the superior articular facet is required (Weinstein et al., 1988).

Alternatively, Santoni et al. (2009) proposed an alternative cortical bone trajectory (CBT) in an effort to maximize screw purchase. The CBT approach aims to engage the denser cortical bone at the pars, inferior and superior cortices of the pedicle isthmus, and junction of the superior margin of pedicle and superior endplates (Kojima et al., 2015; Mai et al., 2016; Santoni et al., 2009). More recently, cortical screws (CS), with different thread pitch geometries, have been specifically designed to optimize screw-bone contact along the CBT (Reline® MAS Midline, Nuvasive, San Diego, CA, USA; SOLERA™, Medtronic, Memphis, TN, USA; Creo MCS™, Globus Medical Inc., Audubon, PA, USA).

As opposed to the Weinstein approach, the more medial insertion point of the CBT minimizes exposure lateral to the facet, damage to the paravertebral muscles during dissection and interoperative distraction, and potentially spares the neurovascular bundle to the erector spinae muscles (Crawford 3rd et al., 2019; Lee et al., 2015; Mizuno et al., 2014; Rodriguez et al., 2014). Furthermore, the medialized CBT approach may be advantageous when stabilizing the spine following “midline” decompression and arthrodesis; CS are inserted within the exposure required for laminectomy, thus avoiding the dissection lateral to the facet joints required for PS. Recent clinical investigations find that the midline approach with CBT screw placement significantly reduces intraoperative time, blood loss, radiological muscle damage, and shorter length of stay (Crawford 3rd et al., 2019; Elmekaty et al., 2018; Hoffman et al., 2019; Lee et al., 2015; Lee and Shin, 2018; Marengo et al., 2018).

While previous in vitro studies have examined the CBT approach and CS or PS fixation, investigations have largely been limited to screw insertion torque, toggling, and pullout (Akpolat et al., 2016; Baluch et al., 2014; Li et al., 2018; Matsukawa et al., 2015; Matsukawa et al., 2016; Sansur et al., 2016; Santoni et al., 2009; Ueno et al., 2015; Wray et al., 2015). Differences between CS and PS fixation have largely been inconclusive, as some studies observed improved resistance to toggling (Baluch et al., 2014; Li et al., 2018) and pullout forces (Akpolat et al., 2016; Ueno et al., 2015), while others have reported biomechanical equivalency (Sansur et al., 2016; Santoni et al., 2009; Weinstein et al., 1988; Wray et al., 2015); Akpolat et al. (2016) found that PS fixation significantly improved resistance against toggling and higher pullout force compared to CS fixation (Akpolat et al., 2016). Both Oshino et al. (2015) and Perez-Orribo et al. (2013) have established kinematic equivalency between CS and PS fixation in animal and human cadaveric studies, respectively (Oshino et al., 2015; Perez-Orribo et al., 2013). Nevertheless, iatrogenic destabilization such as multilevel indirect decompression and laminectomy has not been characterized. Furthermore, the longer-term performance of CS and PS fixation within an operative construct is unknown. Therefore, the purpose of this study was to assess the performance of CS and PS within a multilevel decompressive laminectomy model, and to determine whether a CS construct provides resistance to bone-screw interface failure equivalent to that provided by traditional PS, even after simulated in vivo fatigue.