#248 Pedicle Screw Re-insertion Using Previous Pilot Hole and Trajectory Does Not Reduce Fixation Strength

General Session: What's New in Biologics and Biomechanics

Presented by: R. Tracey

Author(s):

R.W. Tracey (1)
D.G. Kang (1)
R.A. Lehman (1)
A.J. Bevevino (1)
M. Donahue (1)
R. Gaume (1)
D. Ambati (1)
A.E. Dmitriev (1)

(1) Walter Reed National Military Medical Center, Orthopaedic Surgery, Bethesda, MD, USA

Abstract

Introduction: During pedicle screw instrumentation, a low current reading (<6-10mA) with intraoperative evoked electromyogram (EMG) stimulation of a pedicle screw warrants complete removal in order to palpate the tract to reassess for pedicle wall violation. On many occasions no violation is found, and the same screw is re-inserted along the same trajectory without additional redirection. Previous studies have reported significantly decreased insertional torque during this reinsertion, however fixation strength has never been evaluated biomechanically.

Methods: Thirty-one (n=31) thoracic and nine (n=9) lumbar individual fresh-frozen human cadaveric vertebral levels were evaluated. Each level was instrumented bilaterally with 5.5mm (thoracic) and 6.5mm (lumbar) titanium polyaxial pedicle screws. A paired comparison was performed for each level, and randomized between control and the test group with screw re-insertion, which was performed by completely removing the pedicle screw, palpating the tract, and then re-inserting along the same trajectory. Screw insertional torque (IT) was measured with each revolution, and peak IT reported in inch-pounds (in-lb). Screws were tensile loaded to failure “in line” with the screw axis, and pullout strength (POS) measured in Newtons (N).

Results:

Thoracic Re-insertion: There was no significant difference detected for pedicle screw POS between re-inserted (RI) and control screws (732±307 N versus 742±320 N, respectively; p=0.78). We also found no significant difference in IT between the initial test screw (INI) (7.28±3.51 in-lb) and control (7.69±4.45 in-lb) (p=0.33). However, IT for RI screws (5.14±4.18 in-lb) was significantly decreased compared to INI and control screws (29% decrease, p=0.00; 33% decrease, p=0.00, respectively).

Lumbar Re-insertion:There were similar findings for lumbar pedicle screws, with no significant difference for pedicle screw POS between RI and control screws (943±344N versus 803±422N; p=0.09), as well as a significant IT decrease between RI and control screws (6.38±4.61 in-lb versus 9.56±3.84 in-lb; p=0.04).

Correlation Analysis: Test group screws in both the thoracic and lumbar spine had significant, strong correlations between initial screw IT and pullout strength (r=0.79, p=0.00; r=0.93, p=0.00). There was a moderate correlation between re-insertion IT and pullout strength in the thoracic spine (r=0.56, p=0.00), but no significant correlation for the lumbar spine (r=0.218; p=0.57).

Discussion: Despite a significant reduction in pedicle screw IT with re-insertion along a previous tract, there was no significant difference in pedicle screw pullout strength; which is the most clinically significant aspect of immediate stability. Therefore, when the surgeon must completely remove a pedicle screw for tract inspection, re-insertion along the same trajectory may be performed without significantly compromising screw fixation strength.