358 - Medial-to-Lateral Trajectory Pedicle Screws (Cortical/Pars Screws) – A...

#358 Medial-to-Lateral Trajectory Pedicle Screws (Cortical/Pars Screws) – A Biomechanical Analysis Using a Novel Screw Design

General Session: Advocacy of MIS

Presented by: G. Deol

Author(s):

G.S. Deol (1)
N. Khanna (2)
B.A. Heiges (3)
Z.A. Dooley (4)
A.W.L. Turner (4)

(1) University of North Carolina School of Medicine, Department of Orthopaedics, Raleigh, NC, USA
(2) Indiana University School of Medicine, Department of Orthopaedic Surgery, Munster, IN, USA
() 3Optim Orthopaedics, Savannah, GA, USA
(4) NuVasive, Inc., San Diego, CA, USA

Abstract

Introduction: Pedicle fixation has been the gold standard for posterior lumbar instrumentation. Open pedicle fixation necessitates dissection lateral to the pars and facets, denervating musculature, and increasing morbidity. Recently, cortical or pars screws with a medial-to-lateral rather than lateral-to-medial trajectory have gained popularity. Prior biomechanical studies have shown equivalent pullout and toggle characteristics compared to traditional pedicle screws, but a comparison of the stability of the cortical screws to conventional screws in a 1-level construct has not previously been conducted. Testing was performed using a newly developed “tri-zone” screw with cortical-cancellous-cortical thread pitch utilizing the medial-to-lateral cortical screw trajectory.

Method: 6 human cadaveric spines were dissected and cleaned of musculature. Each spine was tested intact for baseline ROM using a multi-directional testing protocol to ± 7.5 Nm, in flexion-extension, lateral bending, and axial rotation. Medial facetectomies were performed at each instrumented level. No interbody devices were used in order to isolate the rigidity of the fixation and eliminate variability associated with disc space preparation and cage sizing. Each spine was then instrumented with either the conventional or medial-lateral screw trajectory construct at L2-L3, and with the other screw trajectory at L4-L5. The screw trajectory constructs were alternated between specimens. Multi-directional testing was repeated. Range of motion (ROM) was assessed using an optoelectronic motion capture system and results were normalized to baseline.

Results: In flexion-extension, the 1-level medial-to-lateral trajectory screw construct provided more stability compared with the conventional trajectory screw construct (M-L: 12.4% of intact ROM vs conv: 17.1%), which was a statistically significant difference (p = 0.029). In lateral bending and axial rotation, there were no significant differences between the screw trajectories (p = 0.161 and p = 0.148, respectively), however the medial-to-lateral screw construct tended to be more stable in axial rotation (M-L: 50.7% intact ROM vs conv: 63.1%), while the conventional construct was more stable in lateral bending (conv: 21.7% intact ROM vs M-L: 28.2%).

Conclusion: Cortical or pars fixation with a medial-to-lateral screw trajectory provides fixation that is biomechanically comparable to traditional pedicle fixation. Additionally, the new screw design fully optimizes the cortical to cancellous transitions along this trajectory. Given similar biomechanical characteristics to traditional pedicle fixation and less tissue disruption which may directly translate to decreased patient morbidity from surgery, this alternative fixation method has multiple minimally invasive clinical applications.