Insertional torque and pullout strength of pedicle screws with or without repositioning: a porcine study

Predicting pullout strength of pedicle screws in broken bones from X-ray images

Pedicle screw fixation is one of the most common procedures used in spinal fusion surgery. The screw loosening is a major concern, which may be caused by broken pedicles. In vitro pullout tests or insertion torque are the main approaches for assessing the stability of the screw; however, direct evidence was lacking for clinical human spines. Here, we aim to provide a model that can predict the pullout strengths of pedicle screws in various pedicle conditions from X-ray images. A weighted embedded bone volume (EBV) model is proposed for pullout strengths prediction by considering the bone heterogeneity and confinement of the screw. We showed that the pullout strength is proportional to the EBV for homogeneous bone and the weighted EBV for layered composite bone. The proposed weighted EBV model is validated with in vitro Sawbones® pullout experiments. The results show that the model has better accuracy than the simple EBV model, with a coefficient of determination of 0.94. The proposed weighted EBV model can help assess the stability of a pedicle screw in a broken pedicle by simply examining 2D X-ray images.

Factors Affecting Pedicle Screw Insertional Torque in Spine Deformity Surgery

STUDY DESIGN

Retrospective observational study of consecutive patients.

OBJECTIVE

We sought to: (1) clarify the key factors predominantly associated with the insertional torque of pedicle screws; (2) compare the optimal factors for pedicle screw insertion to obtain rigid screw fixation in patients with adult spinal deformity (ASD) and in those with adolescent idiopathic scoliosis (AIS); (3) determine the optimal screw/pedicle ratio (S/P) to obtain rigid pedicle screw fixation.

SUMMARY OF BACKGROUND DATA

Rigid pedicle screw fixation is mandatory to perform corrections for spinal deformities properly and to allow successful fusion after surgery. The fixation depends mainly on screw position accuracy and patient bone quality. Traditionally, spinal surgeons have decided the screw size, trajectory, and tapping size based on their intuition. Insertional torque has been indicated as useful to predict screw fixation strength, and is correlated with screw pullout strength and frequency of postoperative screw loosening.

METHODS

We compared insertion torque at L1-L3 levels of 324 screws in 68 patients with ASD and 58 screws in 32 patients with AIS. We assessed the association between screw/pedicle ratio and insertion torque by constructing a spline curve.

RESULTS

Pedicle and screw diameter correlated positively with insertion torque in patients with either ASD or AIS. The optimal screw/pedicle ratio to obtain rigid pedicle screw fixation in patients with ASD was close to, but less than one, and, by contrast, was about 1 to 1.25 in patients with AIS.

CONCLUSION

We propose the concept of an optimal S/P ratio for obtaining rigid pedicle screw fixation during spinal corrective surgery, which is different for patients with ASD and patients with AIS. The S/P ratio is useful for deciding the appropriate diameter screw for each case in preoperative planning.Level of Evidence: 4.

BIOMECHANICAL STUDY OF PEDICLE SCREW FIXATION STRENGTH: ASSOCIATION OF SCREW MALPOSITION AND SCREW INSERTION TORQUE

Pedicle screws have been widely used for the treatment of spinal diseases, but improper screw placement is not uncommon and may lead to neurovascular injuries and reduced screw fixation strength. This study aimed to investigate the feasibility of using real-time screw insertion torque monitoring to prevent screw penetration. Commercially available synthetic L4 vertebrae were divided in to seven test groups based on different screw placements. Screw insertion torque and maximal pullout strength were compared among groups. The results indicated that the insertion torque gradually increased when the screw tip was within vertebral cancellous bone without penetration. However, an instantaneous decrease of torque value was observed once the screw tip penetrated the cortex wall. When compared to the control group, higher pullout strength was found for the groups with medial cortex penetration. However, vertebrae with medial cortex penetration may lead to the concern of neurovascular damage. Meanwhile, lower pullout strength was found for the groups with lateral cortex penetration and end-plate penetration, which may lead to the concern of screw loosening. We concluded that pedicle screw penetration can be judged using real-time screw insertion torque monitoring during surgery, which may aid surgeons in avoiding neurovascular injury and reduction of screw fixation strength.

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

Objective

Redirecting of a laterally misplaced pedicle screw into the accurate position decreases the pull-out strength due to the reinsertion, lateral wall cortical perforation and widening of the pedicle hole. Thus, this biomechanical study was performed to quantitatively analyze the pullout strength of a redirected laterally misplaced pedicle screw into the accurate position.

Methods

Thirty pedicules of 15 bovine vertebrae were separated to 3 groups, according to the screw placement method: 1) standard flawless trajectory; 2) trajectory with lateral pedicle wall perforation; 3) trajectory with lateral wall perforation redirected to the standard trajectory. Samples were placed on a universal testing machine and pullout loads were measured. Kruskal-Wallis test was utilized within 95% confidence interval and p value <0.05 to test for the statistical significance.

Results

The mean pullout strength was 2891±654,2 N(1383-3814,5) in Group 1; 817,8±227,6 N(308,6-1144,9) in Group 2 and 2081,1±487,7 N(1583,5-2962,5) in Group 3. The results found out to be statistically significant (p<0.05). Inter-group comparisons revealed that lateral pedicle wall perforation significantly decreases the pullout strength (p<0.05) and redirection of the screw increases the strength (p<0.05), however it was still weaker than the screws with flawless standard trajectory but this was not statistically significant (p>0.05).

Conclusion

The results of this study confirm that pullout strength of pedicle screw decreases by approximately 71% when the lateral wall is perforated and decreases 28% after redirection to the accurate position.

Feasibility of Endoscopic Inspection of Pedicle Wall Integrity in a Live Surgery Model

Background

Perforations of the pedicle wall during cannulation can occur with experienced surgeons. Direct endoscopic visualization has not been used to inspect pedicles previously due to bone bleeding obscuring the camera visualization. The hypothesis of this study was that endoscopic visualization of pedicle wall integrity was technically feasible and would enable identification of clinically significant pedicle breaches.

Methods

A live porcine model was used. Eight lumbar pedicles were cannulated. Clinically significant breaches were created. An endoscope was introduced and was used to inspect the pedicles.

Results

All lumbar pedicles were endoscopically visible at a systolic pressure of 100 mm Hg. Clinically relevant anatomic structures and iatrogenic pathology, such as medial, lateral, and anterior breaches, were identified. There were no untoward events resulting from endoscopic inspection of the pedicle endosteal canal.

Conclusions

Endoscopic inspection of lumbar pedicles was safe and effective. The findings on endoscopic inspection corresponded with the ball-tip probe palpation techniques. Additional techniques, such as selection between 2 tracts, was possible with the endoscopic technique.