Aakash Agarwal

Smaller Interval Distractions May Reduce Chances of Growth Rod Breakage Without Impeding Desired Spinal Growth: A Finite Element Study

BACKGROUND Growth rods allow regular distraction of the spine to compensate for growth. Traditionally such distractions are performed every 6 months via open surgery. However with the advent of minimally invasive techniques like magnetically controlled growing rods, the distractions can be performed non-surgically. This also implies that the interval of distraction could be changed or customized based on individual patients need. HYPOTHESIS In this study we have hypothesized that the distraction at shorter intervals reduces the stresses on the rods which in turn reduces the chance of rod failure. OBJECTIVE A finite element model of a juvenile spine was instrumented with growth rods and distractions were applied at different frequencies (2 months, 3 months, 6 months, and 12 months) for a period of two years to study the effects of frequency of distraction on maximum von Mises stresses on the rods for different loading conditions were studied. RESULTS The stresses on the rods were highest for 12-month distraction (2 distractions in 2 years) and lowest for 2-months distraction (12 distractions in 2 years). CONCLUSION It was found that the shorter intervals of distraction led to reduction of stresses on the rod for same spinal height gain in two years.

Biomechanical Evaluation of an Endplate-Conformed Polycaprolactone-Hydroxyapatite Intervertebral Fusion Graft and Its Comparison With a Typical Nonconformed Cortical Graft

In the thoracolumbar region, between 7% and 30% of spinal fusion failures are at risk for pseudarthrosis. From a biomechanical perspective, the nonconformity of the intervertebral graft to the endplate surface could contribute to pseudarthrosis, given suboptimal stress distributions. The objective of this study was to quantify the effect of endplate-graft conformation on endplate stress distribution, maximum Von Mises stress development, and stability. The study design used an experimentally validated finite element (FE) model of the L4-L5 functional spinal unit to simulate two types of interbody grafts (cortical bone and polycaprolactone (PCL)-hydroxyapatite (HA) graft), with and without endplate-conformed surfaces. Two case studies were completed. In Case Study I, the endplate-conformed grafts and nonconformed grafts were compared under without posterior instrumentation condition, while in Case Study II, the endplate-conformed and nonconformed grafts were compared with posterior instrumentation. In both case studies, the results suggested that the increased endplate-graft conformity reduced the maximum stress on the endplate, created uniform stress distribution on endplate surfaces, and reduced the range of motion of L4-L5 segments by increasing the contact surface area between the graft and the endplate. The stress distributions in the endplate suggest that the load sharing is greater with the endplate-conformed PCL-HA graft, which might reduce the graft subsidence possibility.

Distraction magnitude and frequency affects the outcome in juvenile idiopathic patients with growth rods: finite element study using a representative scoliotic spine model

BACKGROUND CONTEXT Growth rods are used to limit the progression of scoliosis without restraining the opportunity for the spine to grow. However, major complications like rod breakage, screw loosening, and altered sagittal contour have been encountered. OBJECTIVE To analyse the effect of the magnitude of distraction forces on the T1-S1 growth, maximum von Mises stresses on the rods, sagittal contours, and load at the pedicle screw-bone interface and quantify the maximum stresses on the rod for a period of 24 months using different frequencies of distraction in a representative scoliotic spine model. STUDY DESIGN A representative finite element model of a juvenile scoliotic spine was used to study the effect of magnitude and frequency of distraction on growth rods. METHODS A representative scoliotic model was developed and instrumented using proximal foundation, distal foundation, and rods. Part 1: simulation steps comprised 6 months of growth under various distraction forces to analyze effects of distraction force on the biomechanics of the spine and instrument. Part 2: simulation steps comprised 24 months of growth under various intervals of distraction to analyze effects of distraction interval on the propensity of rod fracture. RESULTS Part 1: an optimal distraction force exists for which the growth is sustained with minimum stress on the rod, lower loads at screw-bone interface, and unaltered sagittal contours. Part 2: the stresses on the rods were highest for 12-month distraction (2 distractions in 2 years) and lowest for 2-month distraction (12 distractions in 2 years). CONCLUSIONS The data and trend suggest that as the distraction forces vary so do the effects on spinal growth. The results of this study also signify the importance of shorter distraction period in reducing the stresses on the rods.

Outcomes of Optimal Distraction Forces and Frequencies in Growth Rod Surgery for Different Types of Scoliotic Curves: An In Silico and In vitro Study

OBJECTIVE Analyze the effects of the distraction forces and frequencies on multiple representative scoliotic curves and to establish a relationship between high distraction forces and screw loosening. STUDY DESIGN Multiple representative finite-element models of a juvenile scoliotic spine were used to study the effects of the magnitude and frequency of distraction on growth rods. METHODS Simulation of 6 months of growth under various distraction forces to analyze the effects of distraction forces on the biomechanics of the scoliotic spine and growth rod instrumentation; simulation of 24 months of growth under various intervals of distraction to analyze the effects of the distraction interval on the propensity for rod fracture; in vitro study to assess screw loosening after 6 months. RESULTS For all scoliotic spine model instrumented with growth rods, an optimal distraction force existed at which normal T1-S1 growth was sustained, along with minimum stresses on the rods, the lowest load at the screw-bone interface, and the least alteration in the sagittal contour. The results followed similar trends for each model, with the numerical values of optimal distraction forces in proximity for all representative scoliotic spine models. The in vitro study proved that the pullout strength of pedicle screws reduced significantly after 6 months of fatigue at higher distraction forces (in comparison with optimal distraction forces). This corroborated the finite-element findings for lower loads at the screw-bone interface with optimal distraction forces. CONCLUSIONS This study concludes that the optimal distraction forces exists for all types of scoliotic curves that have been instrumented with growth rods, which exhibits reduction of stresses on the rods with frequent distractions. This study also links the second most common complication, screw loosening, with high distraction forces. Therefore, optimizing the biomechanical environment of the dual growth rods could drastically reduce the biomechanical complications associated with growth rods.

Harboring Contaminants in Repeatedly Reprocessed Pedicle Screws

Study Design: It consisted of evaluation of the pedicle screws for presence of residual nonmicrobial contaminants and tabulation of the minimum steps and time required for reprocessing implants as per guidelines and its comparison with actual practice. Objective: An evaluation of the nonmicrobial contaminants prevalent on the pedicle screws used for spine surgery and the underlying practice cause behind the source. Methods: The first component consisted of a random selection of 6 pedicle screws and its assessment using optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, and Fourier transform infrared spectroscopy. The second component consisted of review of implant reprocessing guidelines and its applicability. Results: Three types of contaminants were identified: corrosion, saccharide of unknown origin, and soap residue mixed with and were mostly present at the interfaces with low permeability. In addition, manufacturer’s guideline recommends 19 hours of reprocessing, whereas the real-time observation revealed a turnaround time of 1 hour 17 minutes. Conclusion: Repeatedly reprocessed pedicle screws host corrosion, carbohydrate, fat, and soap, which could be a cause of surgical site infection and inflammatory responses postsurgery. The cause behind it is the impracticality of repeated cleaning and inspection of such devices.

Efficacy of Intraoperative Implant Prophylaxis in Reducing Intraoperative Microbial Contamination

Study Design: A prospective single-center study. Objectives: Assess to what degree contamination of pedicle screws occur in standard intraoperative practice and if use of an impermeable guard could mitigate or reduce such an occurrence. Methods: Two groups of sterile prepackaged pedicle screws, one with an intraoperative guard (group 1) and the other without such a guard (group 2), each consisting of 5 samples distributed over 3 time points, were loaded onto the insertion device by the scrub tech and left on the sterile table. Approximately 20 minutes later, the lead surgeon who had just finished preparing the surgical site touches the pedicle screw. Then instead of implantation it was transferred to a sterile container using fresh clean gloves for bacterial and gene analysis. Guarded screw implies that even after unwrapping from the package, the screw carries an impermeable barrier along its entire length, which is only removed seconds prior to implantation. Results: The standard unguarded pedicle screws presented bioburden in the range of 105 to 107 (colony forming units/implant) with bacterial genus mostly consisting of Staphylococcus and Micrococcus, the 2 most common genera found in surgical site infection reports. The common species among them were Staphylococcus epidermis, Staphylococcus aureus, Micrococcus luteus, and Staphylococcus pettenkoferi, whereas the guarded pedicle screws showed no bioburden. Conclusions: Shielding the pedicle screws intraoperatively using a guard provides a superior level of asepsis than currently practiced. All unshielded pedicles screws were carrying bioburden of virulent bacterial species, which provides an opportunity for the development of postoperative infections.

Implant Prophylaxis: The Next Best Practice Toward Asepsis in Spine Surgery

Study Design: A literature review. Objectives: An evaluation of the contaminants prevalent on implants used for surgery and the aseptic methods being employed against them. Methods: PubMed was searched for articles published between 2000 and 2017 for studies evaluating the contaminants present on spine implants, and associated pre- and intraoperative implant processing and handling methodology suggested to avoid them. Systematic reviews, observational studies, bench-top studies, and expert opinions were included. Results: Eleven studies were identified whose major focus was the asepsis of implants to reduce the incidence of surgical site infection incidences during surgery. These studies measured the colony forming units of bacteria on sterilized implants and/or gloves from the surgeon, scrub nurse, and assistants, as well as reductions of surgical site infection rates in spine surgery due to changes in implant handling techniques. Additionally, the search included assessments of endotoxins and carbohydrates present on reprocessed implants. The suggested changes to surgical practice based on these studies included handling implants with only fresh gloves, keeping implants covered until the immediate time of use, reducing operating room traffic, avoiding reprocessing of implants (ie, providing terminally sterilized implants), and avoiding touching the implants altogether. Conclusions: Both reprocessing (preoperative) and handling (intraoperative) of implants seem to lead to contamination of sterilized implants. Using a terminally sterilized device may mitigate reprocessing (preoperative implant prophylaxis), whereas the use of fresh gloves for handling each implant and/or a permanent shielding technique (intraoperative implant prophylaxis) could potentially avoid recontamination at the theatre.

Patient-specific Distraction Regimen to Avoid Growth-rod Failure

Study Design. A finite element study to establish the relationship between patients curve flexibility (determined using curve correction under gravity) in juvenile idiopathic scoliosis and the required distraction frequency to avoid growth rod fracture, as a function of time. Objective. To perform a parametric analysis using a juvenile scoliotic spine model (single mid-thoracic curve with the apex at the eighth thoracic vertebra) and establish the relationship between curve flexibility (determined using curve correction under gravity) and the distraction interval that allows a higher factor of safety for the growth rods. Summary of Background Data. Previous studies have shown that frequent distraction with smaller magnitude of distractions are less likely to result in rod failure. However there has not been any methodology or a chart provided to apply this knowledge on to the individual patients that undergo the treatment. This study aims to fill in that gap. Method. The parametric study was performed by varying the material properties of the disc, hence altering the axial stiffness of the scoliotic spine model. The stresses on the rod were found to increase with increased axial stiffness of the spine, and this resulted in the increase of required optimal frequency to achieve a factor of safety of two for growth rods. Results. A relationship between the percentage correction in Cobbs angle due to gravity alone, and the required distraction interval for limiting the maximum von Mises stress to 255 MPa on the growth rods was established. The distraction interval required to limit the stresses to the selected nominal value reduces with increase in stiffness of the spine. Furthermore, the appropriate distraction interval reduces for each model as the spine becomes stiffer with time (autofusion). This points to the fact the optimal distraction frequency is a time-dependent variable that must be achieved to keep the maximum von Mises stress under the specified factor of safety. Conclusion. The current study demonstrates the possibility of translating fundamental information from finite element modeling to the clinical arena, for mitigating the occurrence of growth rod fracture, that is, establishing a relationship between optimal distraction interval and curve flexibility (determined using curve correction under gravity). Level of Evidence: N/A

Letter to the Editor concerning “Rod fracture and lengthening intervals in traditional growing rods: is there a relationship?” by P. Hosseini et al. Eur Spine J (2016). doi:10.1007/s00586-016-4786-8

We read with great interest the article by Hosseini et al., published in the European Spine Journal [1]. The premise of their research was to test the conclusion of our previously published article(s), in a clinical scenario [2–5]. The article was well written but their interpretation of our work was erroneous. We highlighted the importance of shorter intervals of distraction, with low magnitudes of distraction forces (shown in Table 3 of our article [2]), as a way to reduce stresses on the rods. This reduction in stresses would in turn reduce the propensity of rod breakage (Fig. 1), which are very often seen with growth rod constructs. They however, presented a correlation between the ‘‘different distraction intervals’’ and the ‘‘incidences of rod breakage’’, without factoring in the actual magnitude of forces that were applied during the procedures (Fig. 2) [1]. Their conclusion mistakenly assumes that the distractions simulated by us were of same magnitudes, and only differed in the interval of application. This misunderstanding deters further research onto using shorter intervals (through smaller magnitude as shown in Fig. 1) of distraction (via non-invasive distraction methodology like magnetic growth rods) [1]. We commend them for their initiative to test our hypothesis; however, as the ‘‘treatment group: shorter intervals of distraction’’ is a new technique (only possible due to the advent of non-invasive magnetic growth rods), there are no retrospective data on it. We recently have

Changes in Endplate Thickness and Apparent Density After Disc Herniation

Endplates play a major role in intervertebral disc (IVD) health, but the opposite could also be true with changes in IVD affecting the morphological features of endplates. We here hypothesize that changes in biomechanical environment due to disc herniation will change the thickness and apparent density of the endplates at the index level.Copyright