Can Cortical Screws Provide the Same Resistance to Catastrophic Hardware Failure as Traditional Pedicle Screws following Simulated Fatigue: A Biomechanical Investigation

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:
Instrumentation failure remains an enduring problem within spine surgery. Designs seek to improve the bone-implant interface whilst sparing tissue damage. Cortical screws are gaining traction as a posterior stabilization option, and are designed to optimize the posterior fixation attainable via a more midline surgical approach, thus reducing some of the soft tissue injury observed with traditional pedicle screws. However, the longer term performance and ability to resist catastrophic failure/pullout has not been investigated.

Aims/Objectives:
Determine whether pedicle screws and cortical screws have an equivalent pullout strength following simulated in vivo fatiguing within a lumbar decompression cadaveric model.

Methods:
Fresh-frozen cadaveric lumbar spines (n=8, L1-S) were thawed, carefully denuded, and potted at L1 and S (Bondo, Bondo Corp, Atlanta, GA). Laminectomies were performed from L3-L5, and static TLIF interbody cages (Globus Medical, Inc., Audubon, PA) were placed following L3-4/L4-5 discectomy. 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). Average DEXA bone mineral density scores were matched (±0.1) between the screw groups’ pairs. The maximum flexion between L1-S, as observed on a six degree-of-freedom spine motion simulator (7.0 N-m,1.5°/sec), became the specimen-specific, displacement parameter for fatigue loading (18,000 cycles, 0.75 Hz) on an MTS 858 Mini Bionix (MTS Corporation, Minneapolis, MN). After fatigue, the specimens were secured with a secondary potting procedure, leaving the screw heads accessible and without disturbing the trajectories. The screw heads were then securely connected to the MTS, and screw pullout test was performed along the major axis of the screw (5mm/min). The ultimate load-to-failure of the bone-screw interface was recorded and compared between the two groups (Student t-test, p<0.05).

Results:
Average DEXA scores for both cortical and pedicle groups were -1.6±1.1. There were 19 bone-screw interface failures (a.k.a. screw pullout) in the cortical screw group, and 21 in the pedicle group; the other failure modes were vertebral body fracture (screw remained in the pedicle, n=4) and tulip-screw interface failure (n=4). Average pullout load-to-failure was 710.9±346.3N for the cortical screw group, which was significantly lower than 1138.9±349.2N for the pedicle screw group (p<0.01).

Conclusions:
Following several months of simulated wear fatigue in osteopenic specimens, cortical screws had less resistance to pullout strength versus the traditional pedicle screws. Future directions should focus on case-dependencies that suggest certain vertebral morphometry or bone quality may be favorable for cortical screw instrumentation.