Pedicle Screw

To determine whether implant density impact three-dimensional deformity correction in posterior spinal fusion (PSF) without Ponte osteotomies (POs) for patients with Lenke 1 and 2 adolescent idiopathic scoliosis (AIS). Overview of Literature: Currently, the optimal pedicle screw (PS) density for flexible moderate-sized thoracic AIS curve correction is still controversial. There are limited data regarding the impact of implant density on three-dimensional correction in PSF without the use of PO for thoracic AIS surgery. Methods: A database of patients with AIS with Lenke 1 and 2 curves treated with PSF without PO and instrumented with PSs and ≥2-year follow-up was reviewed. The preoperative, immediate, and final follow-up postoperative radiographs were analyzed. The correlation between PS density and the following factors were determined: major curve correction (MCC), correction index (CI; MCC/ curve flexibility), kyphosis angle change, and rib index (RI) correction. Then, patients were divided into low-density (LD) and highdensity (HD) groups according to mean PS density for the entire cohort (1.5 PS per level). Demographics and radiographic and clinical outcomes were compared between groups. Results: The study included 99 patients with Lenke 1 and 23 patients with Lenke 2 AIS. The average MCC was 67.2%. There was no correlation between screw density and these parameters: MCC (r=0.10, p=0.26), CI (r=0.16, p=0.07), change in T2-T12 kyphosis angle (r=−0.13, p=0.14), and RI correction (r=−0.09, p=0.37). Demographic and preoperative radiographic parameters were similar between the LD and HD groups. At the latest follow-up, there were no differences between the two groups in regard to MCC, CI, change in T2-T12 kyphosis angle, RI correction, and Scoliosis Research Society-30 scores (all p>0.05). Conclusions: This study revealed no significant correlation between screw density and curve correction in any planes. HD construct may not provide better deformity correction in patients with flexible and moderate thoracic AIS undergoing PSF without PO.

The International Standardization Organization introduced standard 12189 for the preclinical evaluation of the mechanical
reliability of posterior stabilization devices. The well-known vertebrectomy model formalized in standard F1717 by
the American Society for Testing and Materials was modified with the introduction of a modular anterior support made
up of three calibrated springs, which allows to describe a more realistic scenario, closer to the effective clinical use, as
well to test even very flexible and dynamic posterior stabilization implants. Despite these important improvements, ISO
12189 received very little attention in the literature. The aim of the work is to provide a systematic procedure for the
assembly and validation of a finite element model capable of describing the experimental test according to ISO 12189.
The validated finite element model is able to catch very well the effective stiffness of the unassembled and assembled
constructs (percentage differences \2% and \10%, respectively). As concern the assembled construct, the experimentally
measured and predicted strains were found in a good agreement (R2 . 0.75, root mean square error \ 30%),
but the procedure without precompression lead to much better results (R2 . 0.96, root mean square error \ 10%).
Given the prediction errors of the assembled construct fall within the experimental range of repeatability, the finite element
model can be systematically implemented to support the mechanical design of a variety of spinal implants, to quantitatively
investigate the load-sharing mechanism, as well as to investigate the loading conditions set by ISO 12189
standard.

Background: Pedicle screw fixation devices are the predominant stabilization systems adopted for a wide variety of spinal defects. Accordingly, both pedicle screw design and bone quality are known as the main parameters affecting the fixation strength as measured by the pull-out force and insertion torque. The pull-out test method, which is recommended by the standards of the American Society for Testing and Materials (ASTM), is destructive. A non-destructive test method was proposed to evaluate the mechanical stability of the pedicle screw using modal analysis. Natural frequency (ω n) extracted from the modal analysis was then correlated with peak pull-out force (PPF) and peak insertion torque (PIT). Methods: Cylindrical pedicle screws with a conical core were inserted into two different polyurethane (PU) foams with densities of 0.16 and 0.32 g/cm3. The PIT and PPF were measured according to the well-established ASTM-F543 standard at three different insertion depths of 10, 20, and 30 mm. Modal analysis was carried out through recording time response of an accelerometer attached to the head of the screw impacted by a shock hammer. The effect of the insertion depth and foam density on the insertion torque, natural frequency, and pull-out force were quantified. Results: The maximum values of ω n , PIT, and PPT were obtained at 2,186 Hz, 123.75 N.cm, and 981.50 N, respectively, when the screw was inserted into the high-density foam at the depth of 30 mm. The minimum values were estimated at 332 Hz, 16 N.cm, and 127 N, respectively, within the low-density PU at the depth of 10 mm. The higher value of ωn was originated from higher bone screw stability and thus more fixation strength. According to the regression analysis outcomes, the natural frequency (ω n) was linearly dependent on the PIT (ω n =14 PIT) and also on the PPF (ω n =1.7 PPF). Coefficients of variation as the results of the modal analysis were significantly less than those in conventional methods (i.e. pull-out and insertion torque). Conclusion: The modal analysis was found to be a reliable, repeatable, and non-destructive method, which could be considered a prospective alternative to the destructive pull-out test that is limited to the in-vitro application only. The modal analysis could be applied to assess the stability of implantable screws, such as orthopedic and spinal screws.

Aim and Objective: Lower lumbar burst fractures are rare entity with most of the literature restricted to small case series. There is no clear cut consensus on the guidelines for management. Here we present functional results of different modalities of treatment of this rare type of spine injury. Material and Methods: The study was conducted at two tertiary care centres over a period of 9 years. Patients with lower lumbar burst fractures were evaluated for associated injuries, modality of treatment, Pain score (VAS) and neurological status (Frankel Grade) at the time of injury and at final follow up were recorded. The final functional outcome was evaluated using Smiley-Webster functional score. Results: A total of 34 patients with an average age of 37.3 years (24 males; 10 females) and a mean final follow up of 27.9 months were enrolled. 21 patients had L3, 8 had L4 and 5 had L5 burst fractures. 10 patients were managed conservatively by brace and 24 underwent surgical intervention. The pain score (VAS) improved from a mean of 8.5 at the time of injury to a mean of 1.6 at final follow up. Patients with neurological injury had on average improvement of one Frankel grade. 88% patients had excellent to good final functional outcome. Conclusion: Lower lumbar burst fractures are high velocity injuries with other non-spinal injuries being associated. Most of the patients have good functional outcome with both conservative as well as surgical intervention. Patients with complete cauda equina have poor outcome with respect to neurological recovery.