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.

Figure 1

Two different screw insertion techniques (fluoroscopy).

Figure 2

A typical fatigue load-displacement curve. Stiffness damage (d_{s_N}= d_N- d₁), creep damage (d_{c_N}), and total damage (d_{t_N}= d_{s_N}+ d_{c_N}).

Figure 3

The response curve of the averaged damage vs. cycle. Black lines are for the angled and gray lines are for the straight. ─ : total damage, --- : creep damage, and –··– : stiffness damage. Creep dominated the fatigue damage caused by screw insertion.