Simulating the Long-Term Biomechanical Performance of Cortical Screws: Can a more MIS Screw Option Provide Equivalent Stability as Traditional Pedicle Screws?

Presented at SMISS Annual Forum 2014
By Kris Radcliff MD
With Brandon Bucklen PhD, Omar Elkassabany , Yiwei Cai , Noelle Klocke MS, Jonathan Harris MS, John Hao ,

Disclosures: Kris Radcliff MD A; Depuy, Medtronic, Paradigm. B; Globus Medical, Inc., Depuy. F; Globus Medical, Inc. Brandon Bucklen PhD E; Globus Medical Inc., Omar Elkassabany E; Globus Medical Inc., Yiwei Cai E; Globus Medical Inc., Noelle Klocke MS E; Globus Medical, Inc., Jonathan Harris MS E; Globus Medical, Inc., John Hao E; Globus Medical, Inc.,

Introduction:
Cortical screws allow more of a midline, muscle-sparing trajectory to traditional pedicle screws and are designed to engage in the pedicles' dense cortical bone, theoretically providing equivalent immediate stabilization (as previously shown). However, it is unknown how these shorter screws perform during the first few months following surgery, and whether they continue to provide similar stabilization.

Aims/Objectives:
To assess, in a clinically relevant lumbar laminectomy model with interbody spacers, the longer term biomechanical rigidity of cortical screws compared to traditional pedicle screws following simulated in vivo spine wear.

Methods:
Fresh-frozen cadaveric lumbar spines (n=8, L1-S) were thawed and carefully denuded of soft tissues, and potted at L1 and S (Bondo, Bondo Corp, Atlanta, GA). Intact range of motion (ROM) data was collected using Optotrak Certus software (NDI, Inc. Waterloo, Canada) during flexion-extension (FE), lateral bending (LB), and axial rotation (AR) on a six degree-of-freedom spine motion simulator (7.0Nm, 1.5°/sec). Next, laminectomies were performed (L3-L5). Then, static interbody spacers (Globus Medical, Inc., Audubon, PA) were placed snugly following L3-4 and L4-5 discectomy. Finally, each specimen received posterior stabilization, consisting of titanium rods and either 5.0/6.0mm cortical screws (25-35mm length) or 5.5mm (30-55mm length) pedicle screws (Globus Medical, Inc., Audubon, PA). ROM testing was performed before (PreFat) and after fatigue (PostFat). The L1-S PreFat ROM became the specimen-specific, displacement parameter for fatigue loading (18,000 cycles, 0.75 Hz) on a MTS 858 Mini Bionix (MTS Corporation, Minneapolis, MN). Absolute differences in normalized motion (to intact) were evaluated using a Student t-test (ΔROM=PostFat-PreFat, α<0.05).

Results:
Average relative motion between L3-L5 increased slightly from the PreFat values in most loading modes, the largest being FE in the pedicle group (ΔFE: 3.68±3.15 % Intact); only LB decreased slightly in the pedicle group (ΔLB: -0.59±1.36 % Intact). However, all modes of motion were statistically equivalent between the screw groups for all PreFat, PostFat, and ΔROM values (p>0.05).

Conclusions:
Cortical screws with a static interbody spacer provided statistically equivalent biomechanical stabilization as compared to traditional pedicle screws within a laminectomy model even after 18,000 wear cycles simulating several weeks of activity. Thus, the smaller incision needed to place cortical screws may be advantageous for soft-tissue tissue recovery, while still providing similar stability as more laterally-placed pedicle screws. While more specimens could confirm these trends, this experiment demonstrated that there appears to be no immediate concern for the long-term performance of cortical screws in this model.