Pullout strength of misplaced pedicle screws in the thoracic and lumbar vertebrae - A cadaveric study

Abstract

Background:The objective of this cadaveric study was to analyze the effects of iatrogenic pedicle perforations from screw misplacement on the mean pullout strength of lower thoracic and lumbar pedicle screws. We also investigated the effect of bone mineral density (BMD), diameter of pedicle screws, and the region of spine on the pullout strength of pedicle screws.Materials and Methods:Sixty fresh human cadaveric vertebrae (D10–L2) were harvested. Dual-energy X-ray absorptiometry (DEXA) scan of vertebrae was done for BMD. Titanium pedicle screws of different diameters (5.2 and 6.2 mm) were inserted in the thoracic and lumbar segments after dividing the specimens into three groups: a) standard pedicle screw (no cortical perforation); b) screw with medial cortical perforation; and c) screw with lateral cortical perforation. Finally, pullout load of pedicle screws was recorded using INSTRON Universal Testing Machine.Results:Compared with standard placement, medially misplaced screws had 9.4% greater mean pullout strength and laterally misplaced screws had 47.3% lesser mean pullout strength. The pullout strength of the 6.2 mm pedicle screws was 33% greater than that of the 5.2 mm pedicle screws. The pullout load of pedicle screws in lumbar vertebra was 13.9% greater than that in the thoracic vertebra (P = 0.105), but it was not statistically significant. There was no significant difference between pullout loads of vertebra with different BMD (P = 0.901).Conclusion:The mean pullout strength was less with lateral misplaced pedicle screws while medial misplaced pedicle screw had more pullout strength. The pullout load of 6.2 mm screws was greater than that of 5.2 mm pedicle screws. No significant correlation was found between bone mineral densities and the pullout strength of vertebra. Similarly, the pullout load of screw placed in thoracic and lumbar vertebrae was not significantly different.

Risk Factors for Clinically Relevant Loosening of Percutaneous Pedicle Screws

“…Numerous biomechanical studies in vitro have attempted to clarify crucial factors for the anchorage capacity of pedicle screws, focusing on bone density, insertion depth, misplacement, and insertion angle. However, these studies have inherent limitations associated with a cadaveric study and/or bone models, in which it is difficult to mimic the actual clinical situation, such as the influence of adjacent vertebral musculature and ligaments, and the wide variations in the material properties of bone, living tissues, and postoperative loading [4][5][6]9,[16][17][18][19][20][21][22] . Therefore, the strengths and clinical relevance of this study are that it showed that a lower axial trajectory and a high screw pull-out force during rod connection might be crucial risk factors for PPS loosening in clinical situations.…”

“…Posterior pedicle screw instrumentation has become the most frequently used surgical technique to treat spinal disorders 1) Recently, a minimally invasive spine surgery technique, percutaneous pedicle screw (PPS) placement, has been growing in popularity with the concept of an internal splint to achieve stability of the thoracolumbar spine in a variety of indications, such as vertebral fractures, vertebral osteomyelitis, and metastatic spinal tumors 2) However, poor screw-to-bone fixation or local high strains at the bonescrew interface might cause clinical problems such as screw loosening 3) Numerous biomechanical studies of pedicle screw pull-out strength have been conducted using human cadaveric spine specimens, animal spine models, or bone models [4][5][6][7][8][9] However, these studies have remained limited because it is difficult to accommodate the wide variety of actual clinical factors.…”

Effection of monoplanar pedicle screw on facet joint degeneration in thoracolumbar vertebral fractures

“…To the best of our knowledge, this is the first study to examine both the accuracy and rate of facet joint violation for MPPS. Facet joints violation by pedicle screw accelerates the degeneration but also weakens the pullout strength [ 26 ]. Subsequently, we concluded that implanting the pedicle screw by the open approach has lower VR than by the percutaneous approach.…”

Quantitative comparison of a laterally misplaced pedicle screw with a re-directed screw. How much pull-out strength is lost?

“…George et al 25 reported that pedicle fractures decreases the pullout strength by 11% compared to intact pedicles. In a cadaveric study, Saraf et al, 26 biomechanically tested the misplaced pedicles and found that lateral misplacement reduces 47.3% of the pullout strength. Similar with latter study, we found a significant decrease in pullout strength in case of lateral wall perforation, however the magnitude was found to be greater in our study reaching 71%.…”