Biomechanical analysis of differing pedicle screw insertion angles

Abstract

Background

Pedicle screw fixation to stabilize lumbar spinal fusion has become the gold standard for posterior stabilization. A significant percentage of surgical candidates are classified as obese or morbidly obese. For these patients, the depth of the incisions and soft tissue makes it extremely difficult to insert pedicle screws along the pedicle axis. As such, the pedicle screws can only be inserted in a much more sagittal axis. However, biomechanical stability of the angled screw insertion has been controversial. We hypothesized that the straight or parallel screw was a more stable construct compared to the angled or axially inserted screw when subjected to caudal cyclic loading.

Methods

We obtained 12 fresh frozen lumbar vertebrae from L3 to L5 from five cadavers. Schantz screws (6.0 mm) were inserted into each pedicle, one angled and along the axis of the pedicle and the other parallel to the spinous process. Fluoroscopic imaging was used to guide insertion. Each screw was then subjected to caudal cyclic loads of 50 N for 2000 cycles at 2 Hz. Analysis of initial damage, initial rate of damage, and total damage during cyclic loading was undertaken.

Findings

Average total fatigue damage for straight screws measured 0.398 ± 0.38 mm, and 0.689 ± 0.96 mm for angled screws. Statistical analysis for total fatigue damage ratio of angled to straight screws revealed that a significant stability was achieved in straight-screw construct (P < 0.03).

Interpretation

This study showed that straight screw insertion results in a more stable pedicle-screw construct. The angled screw insertion technique resulted in more scattered values of damage indicating that the outcome from the angled screw fixation is less predictable. This validates the use of this technique to implant pedicle screws across the axis of the pedicle (parallel to the mid sagittal line) rather than along the axis, and has broad implications in instrumented posterior lumbar spinal surgery.

Introduction

Pedicle screw fixation has become the mainstay of fixation for stabilization of the posterior lumbar spine. Originally described by Boucher in 1959, Roy-Camille popularized this technique in Europe in the 1960s, and his spinal plating system has been called the “predecessor of most modern pedicular screw–plate fixation systems” (Boucher, 1959, Roy-Camille et al., 1976, Roy-Camille, 1992). Pedicle screw fixation is now readily accepted for treatment of fractures, tumors, and degenerative disease. Loosening due to fatigue loading and screw breakage are commonly cited reasons for failure, and numerous studies have been conducted to determine which factors are most important in determining biomechanical stability of the pedicle screw (Esses and Bednar, 1989, Willet et al., 1993, Zdeblick et al., 1993). To date, biomechanical studies have for the most part examined pullout failure of the screw as the endpoint to determine stability (Barber et al., 1998, Law et al., 1993, Yerby et al., 1997). Even those few reports that used a cyclic loading model utilized a displacement control rather than load control mechanism to determine relative stability (Barber et al., 1998, Law et al., 1993, Soshi et al., 1991). While parameters studied included using bigger screws, drilling or probing the pilot hole, tapped and untapped screws, coupling, angular insertion, and augmentation with bushings and polymethylmethacrylate, fatigue failure based on the clinical scenario has rarely been reported.

Morphometric anatomic studies have determined that pedicles flare out laterally from the upper to lower lumbar spine (Ruland et al., 1991, Zindrick, 1991). Transverse pedicle angles of the lower lumbar spine range from 8.0–23.5° at L3 (mean 14.4°) to 19.0–44.0° at L5 (mean 29.8°) (Zindrick et al., 1987). Some studies have suggested that convergent screws are a stronger construct, and have recommended that screws be inserted axially within the lumbar pedicle (Barber et al., 1998). On the basis of these studies and those testing pullout strength (Law et al., 1993, Yerby et al., 1997), screw insertion technique along the axis of the pedicle has been described as superior, with increasing angular distance from the vertebral midline at lower levels of the lumbar spine (Cook et al., 2000). However, the patient population subjected to surgery includes a significant number of obese or morbidly obese individuals who present a challenge in exposure for pedicular insertion of screws. The incisions are deep and approaching the axial pedicle along its axis is difficult. As an alternative to pedicular insertion, in the technique described by Roy-Camille in 1976 and 1992, pedicle screws are inserted in a vertical fashion, crossing the axis of the pedicle rather than proceeding in line with it. While attempts have been made to determine the stability of these screws, most studies utilize a displacement control to determine pullout strength at the bone–screw interface, rather than examining them dynamically at sub-failure forces to determine relative stability based on amount of screw toggle acquired during fatigue testing (Barber et al., 1998, Brantley et al., 1994, Soshi et al., 1991).

This study was designed to determine the stability of pedicle screws that were inserted by both straight and angled techniques. Cyclic, sub-failure load control was used to simulate in vivo loading. The displacement of each screw was measured and compared with the contralateral screw that was inserted by the differing technique. Based on this work, the fatigue stability of the two different types of screw insertion technique was examined by answering two research questions: (1) is the rate of damage different between the two screw insertion methods? (2) is there a difference in stiffness and creep damage between the two methods?