Intraoperative lateral wall breach simulation in the cadaveric spine and the impact of thread designs of screws on pullout strength in the osteoporotic thoracic vertebrae: A biomechanical study in human cadavers.

OBJECTIVE: This study aimed (1) to simulate pedicle screw pullout after intraoperative external wall perforation and (2) to assess restoration strength with different thread designs in the pedicle screw instrumentation for osteoporotic thoracic vertebrae. METHODS: Twenty fresh-frozen human cadaveric thoracic vertebra bodies were prepared and divided into 4 groups: group 1, 5.5 mm × 45 mm polyaxial single thread pedicle screws (PASTS); group 2, after wall injury 5.5 mm × 45 mm PASTS; group 3, 6.5 mm × 45 mm PASTS after wall injury; and group 4: 6.5 mm × 45 mm polyaxial mixed-threaded screws after wall injury. While group 1 was the control group, groups 2, 3, and 4 were used as study groups after the lateral wall breach. All prepared screw units were placed on a universal pullout measurement testing device. RESULTS: The mean bone mineral density for 20 thoracic vertebrae was 0.57 ± 0.12 g/cm2 (range 0.53-0.6 g/cm2 ). The mean pullout strength was 474.90 Newtons (N) for group 1, 412.85 N for group 2, 475.4 N for group 3, and 630.74N for group 4. The lateral wall breach caused a 14.1 % decrease in average pullout strength compared with the initial screw pullout. Mixed (double)-threaded screws increased pullout strength compared to 6.5 mm screws (P=.036) Conclusion: Using a 1 mm thicker polyaxial pedicle screw or mixed (double)-threaded pedicle screw seems to increase pullout strength; however, this was statistically significant only for group 4. In the thoracic spine, the redirection possibility of the pedicle screw is limited, and augmentation with cement will not be appropriate due to the risk of wall injury-related leakage. Therefore, care should be taken to avoid violating the lateral cortex by using appropriate pedicle entry points and trajectories.

Direct vertebral rotation significantly decreases the pullout strength of the pedicle screw: a biomechanical study in adult cadavers.

The pullout strength of the pedicle screws after direct vertebral rotation (DVR) maneuver is not known. This biomechanical study was performed to quantitatively analyze the pullout strength of a pedicle screw after DVR maneuver using human cadaveric vertebrae. Thoracic vertebral bodies from three cadavers were harvested and stripped of soft tissues. Thirty pedicles of 15 vertebrae were separated into two groups after bone mineral density measurements. Polyaxial 5.5 mm pedicle screws with appropriate length were inserted with a freehand technique for each pedicle. One Kirschner wire was inserted to the anterior part of each vertebral corpus the half depth of each corpus was embedded into PVC pipes using polyester paste. In the DVR group, each screw was pulled horizontally with 2 kg (~20 N) load over a screwdriver rigidly attached to the screw, and a DVR maneuver was simulated. The control group did not load with a DVR maneuver. Samples were placed on a universal testing machine and pullout loads were measured. The Mann-Whitney U test was utilized, and the P value <0.05 was considered as statistically significant. In the DVR group, the mean pullout strength was 183.35 N (SD ± 100.12), and in the control group, the mean pullout strength was 279.95 N (SD ± 76.26). Intergroup comparisons revealed that DVR maneuver significantly decreases the pullout strength (P = 0.012). The results of this study confirm that the pullout strength of pedicle screw significantly decreases by approximately 35% when DVR maneuver is applied.

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.

A new multipartite plate system for anterior cervical spine surgery; finite element analysis.

BACKGROUND: There are numerous available plates, almost all of which are compact one-piece plates. During the placement of relatively long plates in the treatment of multi-level cervical pathologies, instrument related complications might appear. In order to overcome this potential problem, a novel 'articulated plate system' is designed. We aimed to delineate finite element analysis and mechanical evaluations. MATERIAL AND METHODS: A new plate system consisting of multi partite structure for anterior cervical stabilization was designed. Segmental plates were designed for application onto the ventral surface of the vertebral body. Plates differed from 9 to13 mm in length. There are rods at one end and hooks at the other end. Terminal points consisted of either hooks or rods at one end but the other ends are blind. Finite element and mechanical tests of the construct were performed applying bending, axial loading, and distraction forces. RESULTS: Finite element and mechanical testing results yielded the cut off values for functional failure and breakage of the system. CONCLUSIONS: The articulated system proved to be mechanically safe and it lets extension of the system on either side as needed. Ease of application needs further verification via a cadaveric study.

Comparison of fixation techniques in Vancouver type AG periprosthetic femoral fracture: a biomechanical study.

OBJECTIVE: The purpose of this study was to biomechanically compare cable, trochanteric grip plate, and locking plate techniques in Vancouver type AG fracture model in an in vitro test environment. METHODS: Fifteen pieces of fourth-generation synthetic femora were separated into 3 groups of 5 models each. A greater trochanteric fracture model was created after femoral stem implantation. Group 1 was fixated with only cable, Group 2 with trochanteric grip plate, and Group 3 with locking plate. Horizontal stiffness, axial stiffness, and failure loads were compared between the groups. RESULTS: In horizontal compression tests, Group 3 had the highest values, but the only statistically significant difference was between the locking plate group and cable group. Axial distraction test results showed that mean stiffness of Group 1 was 94.6±9.44 N/mm, that of Group 2 was 174.8±28.64 N/mm, and that of Group 3 was 185.6±71.64 N/mm. While locking plate versus cable fixation and grip plate fixation versus cable fixation showed statistically significant differences (p<0.05), comparison of locking plate versus grip plate fixation showed no statistically significant difference (p>0.05). In axial failure load test, Group 3 had the highest results. The only significant difference was between the locking plate and cable groups (p<0.05). CONCLUSION: In Vancouver type AG fractures stable fixation may be achieved with grip plate fixation and locking plates, with the former ensuring more stable osteosynthesis.

Characteristics of multi-layer coating formed on commercially pure titanium for biomedical applications.

An innovative multi-layer coating comprising a bioactive compound layer (consisting of hydroxyapatite and calcium titanate) with an underlying titanium oxide layer (in the form of anatase and rutile) has been developed on Grade 4 quality commercially pure titanium via a single step micro-arc oxidation process. Deposition of a multi-layer coating on titanium enhanced the bioactivity, while providing antibacterial characteristics as compared its untreated state. Furthermore, introduction of silver (4.6wt.%) into the multi-layer coating during micro-arc oxidation process imposed superior antibacterial efficiency without sacrificing the bioactivity.