Post-Operative Loss of Lordosis Associated with Polyaxial Pedicle Screw Rotational Slippage

Presented at SMISS Annual Forum 2016
By James Marino MD
With Robert K. Eastlack MD, Evan Obrien MD, James Koziol PhD,

Disclosures: James Marino MD D; Trinity Orthopedics, LLC majority ownership. Robert Eastlack MD None, Evan Obrien MD None, James Koziol PhD None,

Introduction

Restoration (i.e. intraoperative position) and maintenance (i.e. during the postoperative healing phase of instrumented fusion) of lordosis in instrumented lumbar fusions is a primary surgical objective. Many if not most instrumented fusions are performed with polyaxial pedicle screws, which are known to mechanically fail under bending loads at the polyaxial mechanism (Fogel et al, Stanford et al, Shroerlucke et al). The clinical presence of polyaxial mechanism failure or “rotational slippage” has yet to be clinically demonstrated. This study of twenty-seven consecutive patients treated by a fellowship trained spine surgeon with an ALIF procedure supplemented by percutaneous polyaxial pedicle screw fixation, provides evidence of loss of lordosis in the early postoperative period (prior to fusion) associated with proportional polyaxial screw rotational slippage. 

Aims/Objectives

To measure the changes in lordosis of lumbar fused segments and any associated changes in the angle subtended by the average or common pedicle screw axes of the upper and lower instrumented vertebrae. 

Methods

A retrospective analysis of lordotic measurements and measurements of the average or “common” pedicle screw divergence angles of twenty-seven patients undergoing lumbar fusions, were obtained in all twenty-seven patients, using digitally photographed intraoperative and postoperative lateral radiographs. Digitized goniometric measurements were obtained using free online available software (Screen Scales Softpedia for Windows). 

Results

More than 44% of the patients demonstrated twenty percent or more loss of the lordosis obtained at the time of surgery and one in every five patients lost greater than 30% of the intraoperative or immediate postoperative lordosis measured in the fused segment(s). There was a high correlation (“r”) between the loss of lordosis and a reduction in the “common” pedicle screw divergence angle (r = 0.950, 95% CI .893 - .977). The average loss of lordosis per construct was 6 degrees (range 0-20) and the average loss of pedicle screw divergence angle was 4 degrees (range 0-12). The concordance coefficient for intra-observer measurements of lordosis was .931 (95% CI, .781-.981) . Loss of lordosis was no more common in the patients treated with BMP-2 (33% of the patients) versus those not treated with BMP-2. Loss of lordosis was no more prevalent in measurements taken of radiographs obtained greater than six months postoperative than those taken less than six months postoperative. 

Conclusions

This patient series conclusively demonstrated a nearly linear relationship between loss of lordosis and rotational slippage of polyaxial screws in the postoperative healing period.