Spinal Balance/Alignment - Clinical Relevance and Biomechanics.

In the normal spine due to its curvature in various regions, C7 plumb line (C7PL) passes through the sacrum so that the head is centered over the pelvis-ball and socket hip joints and ankle joints. This configuration leads to the least muscular activities to maintain the spinal balance. For any reason like deformity, scoliosis, kyphosis, trauma, and/or surgery this optimal configuration gets disturbed requiring higher muscular activity to maintain the posture and balance. Several parameters like the thoracic kyphosis (TK), lumbar lordosis (LL), pelvic incidence (PI), sacral slope (SS), Hip- and leg position influence the sagittal balance and thus the optimal configuration of spinal alignment. Global sagittal imbalance is energy consuming and often painful compensatory mechanisms are developed, that in turn negatively influence the quality of life. This review looks at the clinical aspects of spinal imbalance, and the biomechanics of spinal balance as dictated by the deformities- ankylosing spondylitis, scoliosis and kyphosis; surgical corrections- pedicle subtraction osteotomies and long segment stabilizations and consequent postural complications like the proximal and distal junctional kyphosis. This review suggests several potential research topics as well.

Adjacent-Level Hypermobility and Instrumented-Level Fatigue Loosening With Titanium and PEEK Rods for a Pedicle Screw System: An In Vitro Study.

Adjacent-level disease is a common iatrogenic complication seen among patients undergoing spinal fusion for low back pain. This is attributed to the postsurgical differences in stiffness between the spinal levels, which result in abnormal forces, stress shielding, and hypermobility at the adjacent levels. In addition, as most patients undergoing these surgeries are osteoporotic, screw loosening at the index level is a complication that commonly accompanies adjacent-level disease. Recent studies indicate that a rod with lower rigidity than that of titanium may help to overcome these detrimental effects at the adjacent level. The present study was conducted in vitro using 12 L1-S1 specimens divided into groups of six, with each group instrumented with either titanium rods or PEEK (polyetheretherketone) rods. The test protocol included subjecting intact specimens to pure moments of 10 Nm in extension and flexion using an FS20 Biomechanical Spine Test System (Applied Test Systems) followed by hybrid moments on the instrumented specimens to achieve the same L1-S1 motion as that of the intact specimens. During the protocol's later phase, the L4-L5 units from each specimen were segmented for cyclic loading followed by postfatigue kinematic analysis to highlight the differences in motion pre- and postfatigue. The objectives included the in vitro comparison of (1) the adjacent-level motion before and after instrumentation with PEEK and titanium rods and (2) the pre- and postfatigue motion at the instrumented level with PEEK and titanium rods. The results showed that the adjacent levels above the instrumentation caused increased flexion and extension with both PEEK and titanium rods. The postfatigue kinematic data showed that the motion at the instrumented level (L4-L5) increased significantly in both flexion and extension compared to prefatigue motion in titanium groups. However, there was no significant difference in motion between the pre- and postfatigue data in the PEEK group.

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.

Biomechanics of the Sacroiliac Joint: Surgical Treatments.

BACKGROUND: Fixation is one of the most common surgical techniques for the treatment of chronic pain originating from the sacroiliac joint (SIJ). Many studies have investigated the clinical outcomes and biomechanics of various SIJ surgical procedures. However, the biomechanical literature points to several issues that need to be further explored, especially for the devices used in minimally invasive surgery of the SIJ. This study (part II) aims to assess biomechanical literature to understand the existing information as it relates to efficacies of the surgical techniques and the gaps in the knowledge base. Part I reviewed basic anatomy and mechanics of the SIJ joint, including difference between males and females, and causes of pain emanating from these joints. METHODS: A thorough literature review was performed pertaining to studies related to SIJ fixation techniques and the biomechanical outcomes of the surgical procedures. RESULTS: Fifty-five studies matched the search criteria and were considered for the review. These articles predominantly pertained to the biomechanical outcomes of the minimally invasive surgery with different instrumentation systems and surgical settings. CONCLUSIONS: The SIJ is one of the most overlooked sources of lower back pain. The joint is responsible for the pain in 15% to 30% of people suffering from lower back pain. Various studies have investigated the clinical outcomes of different surgical procedures intended to improve the pain and quality of life following surgery. The data show that these techniques are indeed effective. However, clinical studies have raised several issues, like optimal number and positioning of implants, unilateral versus bilateral placements, adjacent segment disease, implant designs, and optimal location of implants with respect to variations in bone density across the SIJ. Biomechanical studies using in vitro and in silico techniques have addressed some of these issues. Studies also point out the need for additional investigations for a better understanding of the underlying mechanics for the improved long-term surgical outcomes. Further long-term clinical follow-ups are essential as well. This review presents pertinent findings.

Implant Retention or Removal for Management of Surgical Site Infection After Spinal Surgery.

STUDY DESIGN: A literature review. OBJECTIVE: To summarize the implant removal rate, common bacterial organisms found, time of onset, ratio of superficial to deep infection, and regurgitating the prevalence among all the retrospective and prospective studies on management and characterization of surgical site infections (SSIs). METHODS: PubMed was searched for articles published between 2000 and 2018 on the management or characterization of SSIs after spinal surgery. Only prospective and retrospective studies were included. RESULTS: A total of 49 articles were found relevant to the objective. These studies highlighted the importance of implant removal to avoid recurrence of SSI. The common organisms detected were methicillin-resistant Staphylococcus aureus, methicillin-resistant Staphylococcus epidermis, Staphylococcus epidermis, Staphylococcus aureus, and Propionibacterium acnes, with prevalence of 1% to 15%. A major proportion of all were deep SSI, with minority reporting on late-onset SSI. CONCLUSION: Long-term antibiotics administration, and continuous irrigation and debridement were common suggestion among the authors; however, the key measure undertaken or implied by most authors to avoid risk of recurrence was removal or replacement of implants for late-onset SSI.

Biomechanics of the Sacroiliac Joint: Anatomy, Function, Biomechanics, Sexual Dimorphism, and Causes of Pain.

BACKGROUND: The sacroiliac joints (SIJs), the largest axial joints in the body, sit in between the sacrum and pelvic bones on either side. They connect the spine to the pelvis and thus facilitate load transfer from the lumbar spine to the lower extremities. The majority of low back pain (LBP) is perceived to originate from the lumbar spine; however, another likely source of LBP that is mostly overlooked is the SIJ. This study (Parts I and II) aims to evaluate the clinical and biomechanical literature to understand the anatomy, biomechanics, sexual dimorphism, and causes and mechanics of pain of the SIJ leading to conservative and surgical treatment options using instrumentation. Part II concludes with the mechanics of the devices used in minimal surgical procedures for the SIJ. METHODS: A thorough review of the literature was performed to analyze studies related to normal SIJ mechanics, as well as the effects of sex and pain on SIJ mechanics. RESULTS: A total of 65 studies were selected related to anatomy, biomechanical function of the SIJ, and structures that surround the joints. These studies discussed the effects of various parameters, gender, and existence of common physiological disorders on the biomechanics of the SIJ. CONCLUSIONS: The SIJ lies between the sacrum and the ilium and connects the spine to the pelvic bones. The SIJ transfers large bending moments and compression loads to lower extremities. However, the joint does not have as much stability of its own against the shear loads but resists shear due the tight wedging of the sacrum between hip bones on either side and the band of ligaments spanning the sacrum and the hip bones. Due to these, sacrum does not exhibit much motion with respect to the ilium. The SIJ range of motion in flexion-extension is about 3°, followed by axial rotation (about 1.5°), and lateral bending (about 0.8°). The sacrum of the female pelvis is wider, more uneven, less curved, and more backward tilted, compared to the male sacrum. Moreover, women exhibit higher mobility, stresses/loads, and pelvis ligament strains compared to male SIJs. Sacroiliac pain can be due to, but not limited to, hypo- or hypermobility, extraneous compression or shearing forces, micro- or macro-fractures, soft tissue injury, inflammation, pregnancy, adjacent segment disease, leg length discrepancy, and prior lumbar fusion. These effects are well discussed in this review. This review leads to Part II, in which the literature on mechanics of the treatment options is reviewed and synthesized.

A Multicenter Trial Demonstrating Presence or Absence of Bacterial Contamination at the Screw-Bone Interface Owing to Absence or Presence of Pedicle Screw Guard, Respectively, During Spinal Fusion.

STUDY DESIGN: A prospective multicenter study. OBJECTIVE: The objective of this study was to assess bacterial contamination in current practices of pedicle screw handling and comparing it to a novel method of using an intraoperative, sterile implant guard for screws. SUMMARY OF BACKGROUND DATA: Postoperative infections occur at the higher end of 2%-13%, as cited in the literature, and are underestimated due to various reasons in such publications. Despite concerns associated with vancomycin application immediately before closure, it is theoretically impossible to irrigate the screw-bone interface postimplantation. Consequently, any contamination of pedicle screw before implantation is permanent, and has the potential to cause deep-bone infection, or hardware loosening due to encapsulation of biofilm between the bone and the screw. Therefore, continued vigilance and effective preventive measures should be undertaken if available. MATERIALS AND METHODS: Two groups of presterile individually-packaged pedicle screws, one incased in a sterile, protective guard (group 1: G) and the other without such a guard (group 2: NG), 31 samples in each group were distributed over 28 spinal fusion surgeries at 5 independent hospitals groups. Each were loaded onto the insertion device by the scrub tech and left on the sterile table. Twenty minutes later, the lead surgeon who had just finished preparing the surgical site, handles the pedicle screw, to check the fit with the insertion device. Then, instead of implantation, it was transferred to a sterile container using fresh sterile gloves for bacterial analysis. RESULTS: The standard unguarded pedicle screws presented bioburden in the range of 10 to 10 colonies forming units per screw, whereas the guarded pedicle screws showed no bioburden. CONCLUSION: Standard, current, handling of pedicle screws leads to bacterial contamination, which can be avoided if the screws are sterilely prepackaged with an intraoperative guard (preinstalled).

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.

Optimal satellite rod constructs to mitigate rod failure following pedicle subtraction osteotomy (PSO): a finite element study.

BACKGROUND CONTEXT: Pedicle subtraction osteotomy (PSO) is a challenging restoration technique for sagittal imbalance and is associated with significant complications. One of the major complications is rod fracture and there exists a need for a biomechanical assessment of this complication for various instrumentation configurations. PURPOSE: To evaluate and compare the global range of motion (ROM), rod stress distribution, and the forces on the pedicle subtraction site in various instrumentation configurations using finite element analysis. STUDY DESIGN/SETTING: A computational biomechanical analysis. METHODS: A previously validated osseoligamentous three-dimensional spinopelvic finite element model (T10-pelvis) was used to develop a 30° PSO at the L3 level. In addition to the standard bilateral cobalt chromium primary rod instrumentation of the PSO model, various multirod configurations including constructs with medially, laterally, and posteriorly affixed satellite rods and the short-rod technique were assessed in spinal physiological motions. T10-S1 global ROM, maximum von Mises stress on the rods and at the PSO level, factor of safety (yield stress of the rod material/maximum actual stress in the rod) and the load acting across the PSO site were compared between various instrumentation configurations. The higher the factor of safety the lesser the chances of rod failure. RESULTS: Among all multirod constructs, posteriorly affixed satellite rod construct showed the greatest motion reduction compared to the standard bilateral rod configuration followed by medially and laterally affixed satellite rod constructs. Compared to the standard bilateral rod configuration, recessed short-rod technique resulted in 4% to 49% reduction in T10-S1 ROM recorded in extension and lateral bending motions, respectively, while the axial rotation motion increased by approximately 31%. Considering the maximum stress values on the rods, the recessed short-rod technique showed the greatest factor of safety (FOS = 4.1) followed by posteriorly (FOS = 3.9), medially (FOS = 3), laterally affixed satellite rod constructs (FOS = 2.8), and finally the standard bilateral rod construct (FOS = 2.7). By adding satellite rods, the maximum von Mises stress at the PSO level of the rods also reduced significantly and at this level resulted in the greatest FOS in the posteriorly affixed satellite rod construct. Compared to the standard bilateral rod construct, the load magnitude acting on the osteotomy site decreased by 11%, 16%, and 37% in the laterally, medially, and posteriorly affixed satellite rod constructs, respectively, and did not change with the short-rod technique. CONCLUSIONS: Adding satellite rods increases the rigidity of the construct, which results in an increase in the stability and the reduction of the global ROM. Additionally, having satellite rods reduces the stress on the primary rods at the PSO level and shifts the stresses from this PSO region to areas adjacent to the side-by-side connectors. The data suggest a significant benefit in supplementing medial over lateral satellite rods at the PSO by reducing stress on the primary rods. Except the recessed short-rod technique, all other multirod constructs decrease the magnitude of the load acting across the osteotomy region, which could cause a delayed or non-union at the PSO site. CLINICAL SIGNIFICANCE: The study evaluates the mechanical performance of various satellite rod instrumentation configurations following PSO to predict the risk factors for rod fracture and thereby mitigate the rate of clinically relevant failures.

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.

Quantitative Characteristics of Consecutive Lengthening Episodes in Early-onset Scoliosis (EOS) Patients With Dual Growth Rods.

STUDY DESIGN: A prospective single-center study. OBJECTIVE: The aim of this study was to record the characteristic forces and lengths observed during distraction episodes in early-onset scoliosis (EOS), and analyze their interdependencies on the key variability among the patients. SUMMARY OF BACKGROUND DATA: The goal of the growing-rod technique is to achieve deformity correction alongside maintaining growth of the spine. The deformity correction is achieved during the initial surgery, but follow-up distraction episodes are necessary to maintain the growth. The key variables, under the control of a surgeon, that affect the growth are the applied distraction forces and the distraction lengths. Since the advent of dual growth rod technique, there have been many studies exploring the relationship between these and the actual growth. However, there is sparse evidence on the actual magnitude of distraction forces, and none on its association with patient's parameters such as sex, age, and deformity. METHODS: In a consecutive series of 47 patients implanted with dual growth rods, the distraction forces (in N) and the lengths (in mm) achieved during each distraction episode and compared against the episode-specific demographics. The values obtained from each side, that is, concave and convex sides, were averaged to calculate the mean. Statistical analysis was performed using t-distribution because for each normalized time points (distraction episode). RESULTS: In cumulative, the distraction force increased by an amount of 268%, with 120% increase in the early stages (distractions episodes 1-6) and 68% increase in the later stages (distractions episodes 6-11), whereas the cumulative decrease in the length over 11 distractions episodes was 47%, with 34% and 20% in the early and later stages, respectively. The study does not identify any significant trend with respect to sex, age, and deformity. CONCLUSION: The distraction force and the length increased and decreased respectively with every consecutive distraction episode, with no correlation to sex, age, extent of deformity, or the extent of correction. LEVEL OF EVIDENCE: 5.

Patient-specific Distraction Regimen to Avoid Growth-rod Failure.

STUDY DESIGN: A finite element study to establish the relationship between patient's 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 Cobb's 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.

A Paradigm Shift Toward Terminally Sterilized Devices.

Given the complexity of the sterilization process, and the risk involved in absence of strict adherence to the protocol described by the medical device manufacturers, terminally sterilized devices are emerging and being promoted in the field of medical practices. The characteristics associated with conventional reprocessing are demanding logistics, costs of delay, operations and adverse events, and unacceptable liability. Demanding logistics were a result of decoupled staff between the operating room and sterilize processing department, understaffed and high-volume processing with an additional burden due to inventory management and inefficient training. Other costs arose from upkeep, delay in operating room, and surgical-site infections. Liability arose from the repeatedly use of an unquantifiable process thus adding uncertainties, limited shelf life of the reprocessed implants, contingency of flash sterilization and introduction of newer technology with higher demand on cleaning performances. In contrast, terminally sterilized single-use devices do not carry any of the aforementioned-characteristics, deeming it to be the simplest solution to the current conundrum. This review serves to provide an evaluation of logistics, costs, and potential adverse effects, both directly and indirectly, associated with current practices in the sterile processing department, and also describes as to how the use of terminally sterilized devices can help circumvent those.

Sex Specific Sacroiliac Joint Biomechanics During Standing Upright: A Finite Element Study.

STUDY DESIGN: The comparison of sacroiliac joint (SIJ) angular motions, pelvis ligaments strain, load sharing, and stress distribution across the joint for male and female spine-pelvis-femur models using finite element analysis. OBJECTIVE: To quantify biomechanical parameters at SIJ for all motions for both male and female models. SUMMARY OF BACKGROUND DATA: SIJ has been recognized as a main source of pain in 13% to 30% of patients with low back pain. It is shown that the SIJ rotation and translation in different planes are not exceeding 2° to 3° and 2 mm, respectively. Due to limitation of in vivo and in vitro studies, it is difficult to quantify certain biomechanical parameters such as load-sharing and stress distribution across the joint. Finite element analysis is a useful tool which can be utilized to understand the biomechanics of the SIJ. METHODS: The validated finite element models of a male and a female lumbar spine-pelvis-femur were developed from computer tomography (CT) scans. The models were used to simulate spine physiological motions. The range of motion, ligament strains, load sharing, and stress distribution across the left and right SIJs were compared between male and female models. RESULTS: Motions data at SIJs demonstrated that female model experienced 86% higher mobility in flexion, 264% in extension, 143% in left bending, and 228% in right bending compared with the male model. The stresses and loads on SIJs were higher on the female model compared with the male model. Female model ligaments underwent larger strains compared with the male model ligaments. CONCLUSION: Female SIJ had higher mobility, stresses, loads, and pelvis ligament strains compared with the male SIJ which led to higher stress across the joint, especially on the sacrum under identical loading conditions. This could be a possible reason for higher incidence of SIJ pain and pelvic stress fracture in females. LEVEL OF EVIDENCE: N/A.

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.

Does Spanning a Lateral Lumbar Interbody Cage Across the Vertebral Ring Apophysis Increase Loads Required for Failure and Mitigate Endplate Violation.

STUDY DESIGN: Randomized Biomechanical Cadaveric Study-Level II. OBJECTIVE: We aimed to elucidate that placing lateral lumbar interbody cages that span the stronger ring apophysis will require increasing loads for failure, decreasing rates of subsidence, regardless of bone density or endplate integrity. SUMMARY OF BACKGROUND DATA: There are several reports regarding the rates and grades of cage subsidence when utilizing the lateral lumbar interbody fusion technique. However, there is limited data on how spanning the lateral cage across the ring apophysis can prevent it. METHODS: Eight fresh-frozen human spines (L1-L5) were utilized. Each vertebra was placed with their endplates horizontal in an MTS actuator. A total of 40 specimens were randomized into Groups:Load displacement data was collected at 5 Hz until failure. RESULTS: Longer cages spanning the ring apophysis provided more strength in compression with less subsidence relative to shorter cages, regardless of endplate integrity.Longer cages, spanning the ring apophysis, resting on intact endplates (G2) had a significant (P < 0.05) increase in strength and less subsidence when compared with the smaller cage group resting on intact endplates (G1) (P = 0.003).Longer cages spanning the ring apophysis of intact endplates (G2) showed a significant (P < 0.05) increase in strength and resistance to subsidence when compared with similar length cages resting on decorticated endplates (G4) (P = 0.028). CONCLUSION: Spanning the ring apophysis increased the load to failure by 40% with intact endplates and by 30% with decorticated endplates in this osteoporotic cadaveric model. Larger cages that span the endplate ring apophysis could improve the compressive strength and decrease subsidence at the operative level despite endplate violation or osteoporosis. LEVEL OF EVIDENCE: 2.

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.

Development of nanosilica bonded monetite cement from egg shells.

This work represents further effort from our group in developing monetite based calcium phosphate cements (CPC). These cements start with a calcium phosphate powder (MW-CPC) that is manufactured using microwave irradiation. Due to the robustness of the cement production process, we report that the starting materials can be derived from egg shells, a waste product from the poultry industry. The CPC were prepared with MW-CPC and aqueous setting solution. Results showed that the CPC hardened after mixing powdered cement with water for about 12.5±1 min. The compressive strength after 24h of incubation was approximately 8.45±1.29 MPa. In addition, adding colloidal nanosilica to CPC can accelerate the cement hardening (10±1 min) process by about 2.5 min and improve compressive strength (20.16±4.39 MPa), which is more than double the original strength. The interaction between nanosilica and CPC was monitored using an environmental scanning electron microscope (ESEM). While hardening, nanosilica can bond to the CPC crystal network for stabilization. The physical and biological studies performed on both cements suggest that they can potentially be used in orthopedics.

Effect of Distraction Force on Growth and Biomechanics of the Spine: A Finite Element Study on Normal Juvenile Spine With Dual Growth Rod Instrumentation.

BACKGROUND: Growth rods are used to limit the progression of scoliosis without restraining the opportunity for the spine to grow. The growth is sustained by consecutive distraction at intervals of 6 months. The optimal distraction force for a scoliotic patient is not defined adequately and rod breakage, screw loosening, stimulation of growth and altered sagittal contour has been observed. HYPOTHESIS: The hypothesis of this study is that for every patient with dual growth rods treatment there exists a distraction force that will sustain the growth of that patient's spine equal to normal growth with minimum changes in sagittal contours, results in lower von Mises stresses on the rods and minimum force at the pedicle screw-bone interface at 6 months. OBJECTIVE: In this finite element study, we undertook an objective to identify the effect of magnitude of distraction forces on the T1-S1 growth, maximum von Mises stresses on the rods, sagittal contours, and the load at the pedicle screw-bone interface. RESULTS: 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. Another observation was that higher distraction forces (in the given range) didn't produce stresses on rod or load on screw that could result in failure of the implant. CONCLUSION: Restoration of sagittal contour along with height restoration could guide the clinical practice, for the given range of distraction force.