Screw pull-out force is dependent on screw orientation in an anterior cervical plate construct

Short Plate with Screw Angle over 20 Degrees Improves the Radiologic Outcome in ACDF: Clinical Study

BACKGROUND

Anterior cervical discectomy and fusion surgery is a common procedure for degenerative cervical spine. This describes allospacer and implant-related outcomes, comparing medium plate-low screw angle and short plate-high screw angle techniques.

METHODS

From January 2016 to June 2019, 79 patients who underwent ACDF were prospectively enrolled. Patients were divided, depending on the plate-screw system used: medium plate-low screw angle (12.3 ± 2.5 to 13.2 ± 3.2 degrees), and short plate-high screw angle (22.8 ± 5.3 to 23.3 ± 4.7 degrees). Subsidence, ALOD, and sagittal cervical balance were analyzed using lateral cervical X-rays. NDI and VAS scores were also evaluated.

RESULTS

Age for medium plate-low-angled screw group is 58.0 ± 11.3 years, and 55.3 ± 12.0 in the short plate-high-angled screw group (p-value = 0.313). Groups were comparable in mean NDI (p-value = 0.347), VAS (p-value = 0.156), C2-C7 SVA, (p-value = 0.981), and lordosis angle (p-value = 0.836) at 1-year post-surgery. Subsidence was higher in the medium plate-low-angled screw than in the short plate-high-angled screw (25% and 8.5%, respectively, p-value = 0.045). ALOD is also more common in the medium plate group (p-value = 0.045).

CONCLUSION

Use of a short plate and insertion of high-angled screws (more than 20 degrees) has less chance of subsidence and occurrence of ALOD than the traditional technique of using medium plate and low angle.

Oblique Terminal Screw Placement: Does It Improve Fixation in Plate Osteosynthesis?

The technique of placing an oblique screw in the terminal hole of a plate to increase screw pullout strength is widely taught in the operating room. The origin of this technique is unclear; however, it may have been used simply as a means to identify radiographs and misinterpreted to have some biomechanical benefit. The objective of this study was to measure the structural effect of oblique terminal screw placement (OTSP) during plate osteosynthesis. Foam blocks and limited contact dynamic compression plates and screws were used along with a custom fixture device. The terminal screw was placed in either an oblique (30-degree angle outward) or perpendicular fashion. A load was applied perpendicular to the plate in cantilever bending until failure. The oblique screw construct was significantly weaker than the perpendicular screw construct (399N vs. 465N, P < 0.001), independent of the block of material used. Post hoc analysis showed that the screw angle (P < 0.001) was a significant determinant of the load required to cause screw pullout. OTSP led to a decrease in pullout strength compared with a perpendicular screw in a deformable foam medium similar in density to osteoporotic bone. In patients with poor bone quality, OTSP may create a suboptimal fracture fixation construct.

Effect of screw position on load transfer in lumbar pedicle screws: a non-idealized finite element analysis

Angled screw insertion has been advocated to enhance fixation strength during posterior spine fixation. Stresses on a pedicle screw and surrounding vertebral bone with different screw angles were studied by finite element analysis during simulated multidirectional loading. Correlations between screw-specific vertebral geometric parameters and stresses were studied. Angulations in both the sagittal and axial planes affected stresses on the cortical and cancellous bones and the screw. Pedicle screws pointing laterally (vs. straight or medially) in the axial plane during superior screw angulation may be advantageous in terms of reducing the risk of both screw loosening and screw breakage.

Radiologic changes of anterior cervical discectomy and fusion using allograft and plate augmentation: comparison of using fixed and variable type screw

Objective

To evaluate radiologic result of anterior cervical discectomy and fusion with allobone graft and plate augmentation, and the change of radiologic outcome between screw type and insertion angle.

Methods

Retrospective review of clinical and radiological data of 29 patients. Segmental angle, height and screw angles were measured and followed. The fusion rate was assessed by plain radiography and CT scans. We divided the patients into two groups according to screw type and angles. Group A: fixed screw, Group B: variable screw. Interscrew angle was measured between most upper and lower screws with Cobb's methods.

Results

Overall fusion rate was 86.2% on plain radiography. Fusion was also assessed by CT scan and Bridwell's grading system. There was no difference in fusion and subsidence rates between two groups. Subsidence was found in 5 patients (17.2%). Segmental lordotic angle was increased from preoperative status and maximized at the immediate postoperative period and then reduced at 1 year follow up. Segmental height showed similar increase and decrease values.

Conclusion

ACDF with allograft and plate showed favorable fusion rates, and the screw type and angle did not affect results of surgery.

Pullout strength of bone-patellar tendon-bone allograft bone plugs: a comparison of cadaver tibia and rigid polyurethane foam

PURPOSE

To compare the load-to-failure pullout strength of bone-patellar tendon-bone (BPTB) allografts in human cadaver tibias and rigid polyurethane foam blocks.

METHODS

Twenty BPTB allografts were trimmed creating 25 mm × 10 mm × 10 mm tibial plugs. Ten-millimeter tunnels were drilled in 10 human cadaver tibias and 10 rigid polyurethane foam blocks. The BPTB anterior cruciate ligament allografts were inserted into these tunnels and secured with metal interference screws, with placement of 10 of each type in each material. After preloading (10 N), cyclic loading (500 cycles, 10 to 150 N at 200 mm/min) and load-to-failure testing (200 mm/min) were performed. The endpoints were ultimate failure load, cyclic loading elongation, and failure mode.

RESULTS

No difference in ultimate failure load existed between grafts inserted into rigid polyurethane foam blocks (705 N) and those in cadaver tibias (669 N) (P = .69). The mean rigid polyurethane foam block elongation (0.211 mm) was less than that in tibial bone (0.470 mm) (P = .038), with a smaller standard deviation (0.07 mm for foam) than tibial bone (0.34 mm).

CONCLUSIONS

All BPTB grafts successfully completed 500 cycles. The rigid polyurethane foam block showed less variation in test results than human cadaver tibias.

CLINICAL RELEVANCE

Rigid polyurethane foam blocks provide an acceptable substitute for human cadaver bone tibia for biomechanical testing of BPTB allografts and offer near-equivalent results.

Pullout of a lumbar plate with varying screw lengths

Background

Screw length pertains to stability in various orthopedic fixation devices. There is little or no information on the relationship between plate pullout strength and screw length in anterior lumbar interbody fusion (ALIF) plate constructs in the literature. Such a description may prove useful, especially in the treatment of osteoporotic patients where maximizing construct stability is of utmost importance. Our purpose is to describe the influence of screw length on ALIF plate stability in severely and mildly osteoporotic bone foam models.

Methods

Testing was performed on polyurethane foam blocks with densities of 0.08 g/cm³ and 0.16 g/cm³. Four-screw, single-level ALIF plate constructs were secured to the polyurethane foam blocks by use of sets of self-tapping cancellous bone screws that were 20, 24, 28, 32, and 36 mm in length and 6.0 mm in diameter. Plates were pulled out at 1 mm/min to failure, as defined by consistently decreasing load despite increasing displacement.

Results

Pullout loads in 0.08-g/cm³ foam for 20-, 24-, 28-, 32-, and 36-mm screws averaged 303, 388, 479, 586, and 708 N, respectively, increasing at a mean of 25.2 N/mm. In 0.16-g/cm³ foam, pullout loads for 20-, 24-, 28-, 32-, and 36-mm screws averaged 1004, 1335, 1569, 1907, and 2162 N, respectively, increasing at a mean of 72.2 N/mm.

Conclusions

The use of longer screws in ALIF plate installation is expected to increase construct stability. Stabilization from screw length in osteoporotic patients, however, is limited.

Motion generated in the unstable cervical spine during the application and removal of cervical immobilization collars

BACKGROUND

Many studies have compared the restriction of motion that immobilization collars provide to the injured victim. No previous investigation has assessed the amount of motion that is generated during the fitting and removal process. The purpose of this study was to compare the three-dimensional motion generated when one-piece and two-piece cervical collars are applied and removed from cadavers intact and with unstable cervical spine injuries.

METHODS

Five fresh, lightly embalmed cadavers were tested three times each with either a one-piece or two-piece cervical collar in the supine position. Testing was performed in the intact state, following creation of a global ligamentous instability at C5-C6. The amount of angular motion resulting from the collar application and removal was measured using a Fastrak, three-dimensional, electromagnetic motion analysis device (Polhemus Inc., Colchester, VT). The measurements recorded in this investigation included maximum values for flexion/extension, axial rotation, medial/lateral flexion, anterior/posterior displacement, axial distraction, and medial/lateral displacement at the level of instability.

RESULTS

There was statistically more motion observed with application or removal of either collar following the creation of a global instability. During application, there was a statistically significant difference in flexion/extension between the one-piece (1.8 degrees) and two-piece (2.6 degrees) collars, p = 0.009. There was also a statistically significant difference in anterior/posterior translation between the one-piece (3.6 mm) and two-piece (3.4 mm) collars, p = 0.015. The maximum angulation and displacement during the application of either collar was 3.4 degrees and 4.4 mm. Statistical analysis revealed no significant differences between the one-piece and two-piece collars during the removal process. The maximum angulation and displacement during removal of either collar type was 1.6 degrees and 2.9 mm.

CONCLUSIONS

There were statistically significant differences in motion between the one-piece and two-piece collars during the application process, but it was only 1.2 degrees in flexion/extension and 0.2 mm in anterior/posterior translation. Overall, the greatest amount of angulation and displacement observed during collar application was 3.4 degrees and 4.4 mm. Although the exact amount of motion that could be deleterious to a cervical spine-injured patient is unknown, collars can be placed and removed with manual in-line stabilization without large displacements. Only trained practitioners should do so and with great care given that some motion in all planes does occur during the process.

Pullout analysis of a lumbar plate with varying screw orientations: experimental and computational analyses

STUDY DESIGN

Experimental and finite element analysis of anterior lumbar interbody fixation (ALIF) plate pullout.

OBJECTIVE

The objective of this study was to determine the effect of screw angle and orientation on ALIF plate pullout strength.

SUMMARY OF BACKGROUND DATA

It has been thought that angling the screws in an ALIF plate leads to better fixation strength; however, a few studies are published on this question, which produced conflicting results.

METHODS

Using custom guides, screws were configured in 9 different orientations to affix ALIF plates to polyurethane foam blocks. Pullout tests were performed at a rate of 1 mm/min. In addition, finite element analyses were performed on a 2-dimensional screw-block model to gain insight into the internal stress during pullout.

RESULTS

The pullout load was the greatest, with screws positioned 12° outward sagittaly and 6° inward coronally (936 ± 72 N). This orientation was statistically greater than the orientation with the lowest pullout load (812 ± 45 N, P < 0.05); however, no group was statistically different than placing the screws straight in (868 ± 86 N, P > 0.05). Finite elements analysis showed some gain in pullout strength at 12° followed by some loss at greater angles. As the screw insertion angle increased, stress levels elevated within the block even in the regions away from the screw.

CONCLUSION

Significant difference was found between certain screw-angle configurations; however, when compared with simply placing the screws straight in, the difference was never more than 8%. This implies that there is greater freedom in the angle and placement of screws than previously thought. Our results show that there is little change in fixation strength when placing the screw in a different direction.

Comparison of 4 airway devices on cervical spine alignment in a cadaver model with global ligamentous instability at C5-C6

STUDY DESIGN

Human cadaveric study using various intubation devices in a cervical spine instability model.

OBJECTIVE

We sought to evaluate various intubation techniques and determine which device results in the least cervical motion in the setting of a global ligamentous instability model.

SUMMARY OF BACKGROUND DATA

Many patients presenting with a cervical spine injury have other injuries that may require rapid airway management with endotracheal intubation. Secondary neurologic injuries may occur in these patients because of further displacement at the level of injury, vascular insult, or systemic decrease in oxygen delivery. The most appropriate technique for achieving endotracheal intubation in the patient with a cervical spine injury remains controversial.

METHODS

A global ligamentous instability at the C5-C6 vertebral level was created in lightly embalmed cadavers. An electromagnetic motion analysis device (Liberty; Polhemus, Colchester, VT) was used to assess the amount of angular and linear translation in 3 planes during intubation trials with each of 4 devices (Airtraq laryngoscope, lighted stylet, intubating LMA, and Macintosh laryngoscope). The angular motions measured were flexion-extension, axial rotation, and lateral bending. Linear translation was measured in the medial-lateral (ML), axial, and anteroposterior planes. Intubation was performed by either an emergency medical technician or by a board-certified attending anesthesiologist. Both time to intubate as well as failure to intubate (after 3 attempts) were recorded.

RESULTS

There was no significant difference shown with regards to time to successfully intubate using the various devices. It was shown that the highest failure-to-intubate rate occurred with use of the intubating LMA (ILMA) (23%) versus 0% for the others. In flexion/extension, we were able to demonstrate that the Lightwand (P = 0.005) and Airtraq (P = 0.019) resulted in significantly less angular motion than the Macintosh blade. In anterior/posterior translation, the Lightwand (P = 0.005), Airtraq (P = 0.024), and ILMA (P = 0.021) all caused significantly less linear motion than the Macintosh blade. In axial rotation, the Lightwand (P = 0.017) and Airtraq (P = 0.022) resulted in significantly less angular motion than the Macintosh blade. In axial translation (P = 0.037) and lateral bending (P = 0.003), the Lightwand caused significantly less motion than the Macintosh blade.

CONCLUSION

In a cadaver model of C5-C6 instability, the greatest amount of motion was caused by the most commonly used intubation device, the Macintosh blade. Intubation with the Lightwand resulted in significantly less motion in all tested parameters (other than ML translation) as compared with the Macintosh blade. It should also be noted that the Airtraq caused less motion than the Macintoshblade in 3 of the 6 tested planes. There were no significant differences in failure rate or the amount of time it took to successfully intubate in comparing these techniques. We therefore recommend the use of the Lightwand, followed by the Airtraq, in the setting of a presumed unstable cervical spine injury over the Macintosh laryngoscope.

The effects of screw orientation in severely osteoporotic bone: a comparison with locked plating

BACKGROUND

Techniques such as varying screw insertion angles and the use of locked plating have been shown to improve the strength of fixation in bone. The effects of these methods is less clearly understood in bone of exceedingly poor quality.

METHODS

Forty plate-bone constructs were assembled and divided into four groups of ten. Perpendicularly placed screws were placed in one group, convergently placed crossing screws were placed in a second group, an oblique end screw was placed in a third group, and a fourth group utilized perpendicularly placed locking screws in a locking plate. All test subjects were mounted and loaded in cantilever bending to the point of failure. Stiffness, initial load to failure, and maximal load tolerated were all analyzed.

FINDINGS

All four groups demonstrated evidence of failure at similar loads (21.8-26.1N). The locked group was able to tolerate significantly higher loads overall (37.3N, P=.044). All three non-locked groups demonstrated similar failure patterns and load to failure. Locking constructs demonstrated a distinctly different failure pattern. No significant differences were detected with regard to screw orientation and load to failure. The group with an oblique end screw was significantly less stiff than the other three constructs (P=.017).

INTERPRETATION

In a severely osteoporotic model, failure in cantilever bending at low forces will take place regardless of fixation methods used. The mechanism of failure is different in locked constructs compared to traditional constructs. The added benefit of oblique screw placement observed in healthy bone is not observed in osteoporotic bone.

Surgery in the cervicothoracic junction with an anterior low suprasternal approach alone or combined with manubriotomy and sternotomy: an approach selection method based on the cervicothoracic angle

Object

The authors propose an easy MR imaging method to measure and categorize individual anatomical variations within the cervicothoracic junction (CTJ). Furthermore, they propose guidelines for selection of the appropriate approach based on this new categorization system.

Methods

In the midsagittal section of the cervicothoracic MR imaging studies obtained in 95 Chinese patients, a triangle was drawn among 3 points: the suprasternal notch (SSN), the midpoint of the anterior border of the C7/T1 intervertebral disc, and the corresponding anterior border in the CTJ at the level of the SSN. The angle above the SSN was specified as the cervicothoracic angle (CTA). The spatial position between the brachiocephalic vein (BCV), the aortic arch, and the CTA was also measured. Based on these measurements involving the CTA, 3 different patient-specific categorizations are proposed to assist surgeons with selection of the appropriate anterior approach to the CTJ. Three categories of operative approach based on whether the most caudal part of the lesion site was above, within, or below the area of the CTA were classified. The patients were divided into long- or short-necked groups based on whether their own CTA was greater than (long necked) or less than (short necked) the average CTA. Finally, a left BCV was called superiorly located when it coursed above the manubrium. The method was evaluated in 21 patients with spinal bone tumors in the CTJ to illustrate the measurement of both the CTA and the great vessels, and corresponding approach selections.

Results

In this series of 95 patients, the most common vertebra above the SSN was T-3, especially the upper one-third of T-3. The mean CTA was 47.64°. The left BCV was superior to the manubrium in 21.1% of the 95 cases, and 93.6% of the left BCVs were at the T-2 and T-3 levels. Type A and most Type B lesions could be addressed via a low suprasternal approach, or this approach combined with manubriotomy, if necessary. Type C lesions falling below the CTA will need alternative exposure techniques, including manubriotomy, sternotomy, lateral extracavitary, or thoracotomy. The spinal levels that could be exposed in the long-necked CTJ group were always 1 or 2 vertebral levels lower than those in the short-necked CTJ group during the anterior low suprasternal approach without the manubriotomy.

Conclusions

Imaging of the thoracic manubrium should be routinely included on MR imaging studies obtained in the CTJ. It is important for the surgeon to understand the pertinent anatomy of the individual patients and to determine the feasible surgical approaches after evaluating the CTA and vascular factors preoperatively. An anterior low suprasternal approach, or this approach combined with manubriotomy, is applicable in most of the cases in the CTJ. It should be cautioned that preoperatively unrecognized variations of the left BCV above the SSN might result in potential intraoperative trauma during an anterior approach.