Kamran Aghayev

Designs and techniques that improve the pullout strength of pedicle screws in osteoporotic vertebrae: current status.

Osteoporosis is a medical condition affecting men and women of different age groups and populations. The compromised bone quality caused by this disease represents an important challenge when a surgical procedure (e.g., spinal fusion) is needed after failure of conservative treatments. Different pedicle screw designs and instrumentation techniques have been explored to enhance spinal device fixation in bone of compromised quality. These include alterations of screw thread design, optimization of pilot hole size for non-self-tapping screws, modification of the implants trajectory, and bone cement augmentation. While the true benefits and limitations of any procedure may not be realized until they are observed in a clinical setting, axial pullout tests, due in large part to their reproducibility and ease of execution, are commonly used to estimate the devices effectiveness by quantifying the change in force required to remove the screw from the body. The objective of this investigation is to provide an overview of the different pedicle screw designs and the associated surgical techniques either currently utilized or proposed to improve pullout strength in osteoporotic patients. Mechanical comparisons as well as potential advantages and disadvantages of each consideration are provided herein.

Biomechanical comparison of anterior cervical spine instrumentation techniques with and without supplemental posterior fusion after different corpectomy and discectomy combinations: Laboratory investigation.

OBJECT The objective of this study was to compare the stiffness and range of motion (ROM) of 4 cervical spine constructs and the intact condition. The 4 constructs consisted of 3-level anterior cervical discectomy with anterior plating, 1-level discectomy and 1-level corpectomy with anterior plating, 2-level corpectomy with anterior plating, and 2-level corpectomy with anterior plating and posterior fixation. METHODS Eight human cadaveric fresh-frozen cervical spines from C2-T2 were used. Three-dimensional motion analysis with an optical tracking device was used to determine motion following various reconstruction methods. The specimens were tested in the following conditions: 1) intact; 2) segmental construct with discectomies at C4-5, C5-6, and C6-7, with polyetheretherketone (PEEK) interbody cage and anterior plate; 3) segmental construct with discectomy at C6-7 and corpectomy of C-5, with PEEK interbody graft at the discectomy level and a titanium cage at the corpectomy level; 4) corpectomy at C-5 and C-6, with titanium cage and an anterior cervical plate; and 5) corpectomy at C-5 and C-6, with titanium cage and an anterior cervical plate, and posterior lateral mass screw-rod system from C-4 to C-7. All specimens underwent a pure moment application of 2 Nm with regards to flexion-extension, lateral bending, and axial rotation. RESULTS In all tested motions the statistical comparison was significant between the intact condition and the 2-level corpectomy with anterior plating and posterior fixation construct. All other statistical comparisons between the instrumented constructs were not statistically significant except between the 3-level discectomy with anterior plating and the 2-level corpectomy with anterior plating in axial rotation. There were no statistically significant differences between the 1-level discectomy and 1-level corpectomy with anterior plating and the 2-level corpectomy with anterior plating in any tested motion. There was also no statistical significance between the 3-level discectomy with anterior plating and the 2-level corpectomy with anterior plating and posterior fixation. CONCLUSIONS This study demonstrates that segmental plate fixation (3-level discectomy) affords the same stiffness and ROM as circumferential fusion in 2-level cervical spine corpectomy in the immediate postoperative setting. This obviates the need for staged circumferential procedures for multilevel cervical spondylotic myelopathy. Given that the posterior segmental instrumentation confers significant stability to a multilevel cervical corpectomy, the surgeon should strongly consider the placement of segmental posterior instrumentation to significantly improve the overall stability of the fusion construct after a 2-level cervical corpectomy.

Biomechanical analysis of an interspinous fusion device as a stand-alone and as supplemental fixation to posterior expandable interbody cages in the lumbar spine

OBJECT In this paper the authors evaluate through in vitro biomechanical testing the performance of an interspinous fusion device as a stand-alone device, after lumbar decompression surgery, and as supplemental fixation to expandable cages in a posterior lumbar interbody fusion (PLIF) construct. METHODS Nine L3-4 human cadaveric spines were biomechanically tested under the following conditions: 1) intact/control; 2) L3-4 left hemilaminotomy with partial discectomy (injury); 3) interspinous spacer (ISS); 4) bilateral pedicle screw system (BPSS); 5) bilateral hemilaminectomy, discectomy, and expandable posterior interbody cages with ISS (PLIF-ISS); and 6) PLIF-BPSS. Each test consisted of 100 N of axial preload with ± 7.5 Nm of torque in flexion-extension, right/left lateral bending, and right/left axial rotation. Significant changes in range of motion (ROM), neutral zone stiffness (NZS), elastic zone stiffness (EZS), and energy loss (EL) were explored among conditions using nonparametric Friedman test and Wilcoxon signed-rank comparisons (p ≤ 0.05). RESULTS The injury increased ROM in flexion (p = 0.01), left bending (p = 0.03), and right/left rotation (p < 0.01) and also decreased NZS in flexion (p = 0.01) and extension (p < 0.01). Both the ISS and BPSS reduced flexion-extension ROM and increased flexion-extension stiffness (NZS and EZS) with respect to the injury and intact conditions (p < 0.05), but the ISS condition provided greater resistance than BPSS in extension for ROM, NZS, and EZS (p < 0.01). The BPSS increased the rigidity (ROM, NZS, and EZS) of the intact model in lateral bending and axial rotation (p ≤ 0.01), except in EZS for left rotation (p = 0.23, Friedman test). The incorporation of posterior cages marginally increased (p = 0.05) the EZS of the BPSS construct in flexion but these interbody devices provided significant stability to the ISS construct in lateral bending and axial rotation for ROM (p = 0.02), in lateral bending for NZS (p = 0.02), and in flexion/axial rotation for EZS (p ≤ 0.03); however, both PLIF constructs demonstrated equivalent ROM and stiffness (p ≥ 0.16), except in lateral bending where the PLIF-BPSS was more stable (p = 0.02). In terms of EL, the injury increased EL in flexion-extension (p = 0.02), the ISS increased EL for lateral bending and axial rotation (p ≤ 0.03), and the BPSS decreased EL in lateral bending (p = 0.02), with respect to the intact condition. The PLIF-ISS decreased lateral bending EL with respect to the ISS condition (p = 0.02), but not enough to be smaller or, at least, equivalent, to that of the PLIF-BPSS construct (p = 0.02). CONCLUSIONS The ISS may be a suitable device to provide immediate flexion-extension balance after a unilateral laminotomy, but the BPSS provides greater immediate stability in lateral bending and axial rotation motions. Both PLIF constructs performed equivalently in flexion-extension and axial rotation, but the PLIF-BPSS construct is more resistant to lateral bending motions. Further biomechanical and clinical evidence is required to strongly support the recommendation of a stand-alone interspinous fusion device or as supplemental fixation to expandable posterior interbody cages.

Mini-open lateral retroperitoneal lumbar spine approach using psoas muscle retraction technique. Technical report and initial results on six patients

PurposeThe main aim of this paper was to report reproducible method of lumbar spine access via a lateral retroperitoneal route.MethodsThe authors conducted a retrospective analysis of the technical aspects and clinical outcomes of six patients who underwent lateral multilevel retroperitoneal interbody fusion with psoas muscle retraction technique. The main goal was to develop a simple and reproducible technique to avoid injury to the lumbar plexus.ResultsSix patients were operated at 15 levels using psoas muscle retraction technique. All patients reported improvement in back pain and radiculopathy after the surgery. The only procedure-related transient complication was weakness and pain on hip flexion that resolved by the first follow-up visit.ConclusionsPsoas retraction technique is a reliable technique for lateral access to the lumbar spine and may avoid some of the complications related to traditional minimally invasive transpsoas approach.

Role of vertebral augmentation procedures in the management of vertebral compression fractures in cancer patients.

Purpose of reviewTo review the current status of vertebral augmentation procedures (VAPs) in the management of symptomatic vertebral compression fractures (VCFs) in cancer patients. Recent findingsThe natural history of VCFs in the cancer setting is presumably different from the one seen with osteoporotic fractures. Factors contributing to the poor outcome with conservative treatment in cancer patients include continued bone loss due to tumor invasion, poor nutritional status, immobilization, prolonged steroid use, gonadal ablation, chemotherapy and radiotherapy. VAPs have been shown by retrospective and prospective randomized studies to be effective in treating symptomatic VCFs. Advantages of VAPs include immediate pain relief, avoiding delays in chemoradiation, outpatient care in the majority of cases, biopsy, vertebral height restoration, and potential antitumor effect of bone cement. Results from the prospective randomized Cancer Fracture Evaluation (CAFÉ) trial show superiority of balloon kyphoplasty (BKP) over conservative management in cancer patients with VCFs with similar rate of adverse events between treatment arms. Additionally, the field is still evolving with advances such as combination with radiosurgery and spinal radiofrequency ablation (RFA), use of kyphoplasty without a balloon and highly viscous cement to prevent leakage. SummaryVAPs are well tolerated and effective methods to provide palliative care for cancer patients with VCFs and should be offered to symptomatic patients.

Comparative analysis of posterior fusion constructs as treatments for middle and posterior column injuries: an in vitro biomechanical investigation.

BACKGROUND Titanium pedicle screw-rod instrumentation is considered a standard treatment for spinal instability; however, the advantages of cobalt-chromium over titanium is generating interest in orthopedic practice. The aim of this study was to compare titanium versus cobalt-chromium rods in posterior fusion through in vitro biomechanical testing. METHODS Posterior and middle column injuries were simulated at L3-L5 in six cadaveric L1-S1 human spines and different pedicle screw constructs were implanted. Specimens were subjected to flexibility tests and range of motion, intradiscal pressure and axial rotation energy loss were statistically compared among five conditions: intact, titanium rods (with and without transverse connectors) and cobalt-chromium rods (with and without transverse connectors). FINDINGS All fusion constructs significantly (P<0.01) decreased range of motion in flexion-extension and lateral bending with respect to intact, but no significant differences (P>0.05) were observed in axial rotation among all conditions. Intradiscal pressure significantly increased (P≤0.01) after fusion, except for the cobalt-chrome conditions in extension (P≥0.06), and no significant differences (P>0.99) were found among fixation constructs. In terms of energy loss, differences became significant P≤0.05 between the cobalt-chrome with transverse connector condition with respect to the cobalt-chrome and titanium conditions. INTERPRETATION There is not enough evidence to support that the cobalt-chrome rods performed biomechanically different than the titanium rods. The inclusion of the transverse connector only increased stability for the cobalt-chromium construct in axial rotation.

Robotics in Neurosurgery: Evolution, Current Challenges, and Compromises

BACKGROUND Advances in technology have pushed the boundaries of neurosurgery. Surgeons play a major role in the neurosurgical field, but robotic systems challenge the current status quo. Robotic-assisted surgery has revolutionized several surgical fields, yet robotic-assisted neurosurgery is limited by available technology. METHODS The literature on the current robotic systems in neurosurgery and the challenges and compromises of robotic design are reviewed and discussed. RESULTS Several robotic systems are currently in use, but the application of these systems is limited in the field of neurosurgery. Most robotic systems are suited to assist in stereotactic procedures. Current research and development teams focus on robotic-assisted microsurgery and minimally invasive surgery. The tasks of miniaturizing the current tools and maximizing control challenge manufacturers and hinder progress. Furthermore, loss of haptic feedback, proprioception, and visualization increase the time it takes for users to master robotic systems. CONCLUSIONS Robotic-assisted surgery is a promising field in neurosurgery, but improvements and breakthroughs in minimally invasive and endoscopic robotic-assisted surgical systems must occur before robotic assistance becomes commonplace in the neurosurgical field.

Spinal Neoplastic Instability: Biomechanics and Current Management Options

BACKGROUND Often the spine is afflicted from primary or metastatic neoplastic disease, which can lead to instability. Instability can cause deformity, pain, and spinal cord compression and is an indication for surgery. Although overt instability is uniformly agreed upon, it is sometimes difficult for specialists to agree on subtle degrees of instability due to lack of objective criteria. METHODS In this article, treatment options and the spine instability neoplastic system are discussed and the neoplastic instability literature is reviewed. RESULTS The Spinal Instability Neoplastic Score helps specialists determine whether instability is present and when surgery may be indicated. However, other parameters such as spinal cord compression and extent of disease dictate whether surgery is the most appropriate option. A wide range of fusion techniques exists, each one tailored to the location of the lesion and goals for surgery. CONCLUSIONS To optimize results, expert knowledge on the techniques and patient selection is of importance. Furthermore, a multidisciplinary approach is required because treatment of neoplastic disease is multimodal.

Biomechanical comparison of an interspinous fusion device and bilateral pedicle screw system as additional fixation for lateral lumbar interbody fusion

BACKGROUND This investigation compares an interspinous fusion device with posterior pedicle screw system in a lateral lumbar interbody lumbar fusion. METHODS We biomechanically tested six cadaveric lumbar segments (L1-L2) under an axial preload of 50N and torque of 5Nm in flexion-extension, lateral bending and axial rotation directions. We quantified range of motion, neutral zone/elastic zone stiffness in the following conditions: intact, lateral discectomy, lateral cage, cage with interspinous fusion, and cage with pedicle screws. FINDINGS A complete lateral discectomy and annulectomy increased motion in all directions compared to all other conditions. The lateral cage reduced motion in lateral bending and flexion/extension with respect to the intact and discectomy conditions, but had minimal effect on extension stiffness. Posterior instrumentation reduced motion, excluding interspinous augmentation in axial rotation with respect to the cage condition. Interspinous fusion significantly increased flexion and extension stiffness, while pedicle screws increased flexion/extension and lateral bending stiffness, with respect to the cage condition. Both posterior augmentations performed equivalently throughout the tests except in lateral bending stiffness where pedicle screws were stiffer in the neutral zone. INTERPRETATION A lateral discectomy and annulectomy generates immediate instability. Stand-alone lateral cages restore a limited amount of immediate stability, but posterior supplemental fixation increases stability. Both augmentations are similar in a single level lateral fusion in-vitro model, but pedicle screws are more equipped for coronal stability. An interspinous fusion is a less invasive alternative than pedicle screws and is potentially a conservative option for various interbody cage scenarios.

In vitro evaluation of a lateral expandable cage and its comparison with a static device for lumbar interbody fusion: a biomechanical investigation

OBJECT Through in vitro biomechanical testing, the authors compared the performance of a vertically expandable lateral lumbar interbody cage (EC) under two different torque-controlled expansions (1.5 and 3.0 Nm) and with respect to an equivalent lateral lumbar static cage (SC) with and without pedicle screw fixation. METHODS Eleven cadaveric human L2-3 segments were evaluated under the following conditions: 1) intact; 2) discectomy; 3) EC under 1.50 Nm of torque expansion (EC-1.5Nm); 4) EC under 3.00 Nm of torque expansion (EC-3.0Nm); 5) SC; and 6) SC with a bilateral pedicle screw system (SC+BPSS). Load-displacement behavior was evaluated for each condition using a combination of 100 N of axial preload and 7.5 Nm of torque in flexion and extension (FE), lateral bending (LB), and axial rotation (AR). Range of motion (ROM), neutral zone stiffness (NZS), and elastic zone stiffness (EZS) were statistically compared among conditions using post hoc Wilcoxon signed-rank comparisons after Friedman tests, with a significance level of 0.05. Additionally, any cage height difference between interbody devices was evaluated. When radiographic subsidence was observed, the specimens data were not considered for the analysis. RESULTS The final cage height in the EC-1.5Nm condition (12.1 ± 0.9 mm) was smaller (p < 0.001) than that in the EC-3.0Nm (13.9 ± 1.1 mm) and SC (13.4 ± 0.8 mm) conditions. All instrumentation reduced (p < 0.01) ROM with respect to the injury and increased (p ≤ 0.01) NZS in flexion, extension, and LB as well as EZS in flexion, LB, and AR. When comparing the torque expansions, the EC-3.0Nm condition had smaller (p < 0.01) FE and AR ROM and greater (p ≤ 0.04) flexion NZS, extension EZS, and AR EZS. The SC condition performed equivalently (p ≥ 0.10) to both EC conditions in terms of ROM, NZS, and EZS, except for EZS in AR, in which a marginal (p = 0.05) difference was observed with respect to the EC-3.0Nm condition. The SC+BPSS was the most rigid construct in terms of ROM and stiffness, except for 1) LB ROM, in which it was comparable (p = 0.08) with that of the EC-1.5Nm condition; 2) AR NZS, in which it was comparable (p > 0.66, Friedman test) with that of all other constructs; and 3) AR EZS, in which it was comparable with that of the EC-1.5Nm (p = 0.56) and SC (p = 0.08) conditions. CONCLUSIONS A 3.0-Nm torque expansion of a lateral interbody cage provides greater immediate stability in FE and AR than a 1.5-Nm torque expansion. Moreover, the expandable device provides stability comparable with that of an equivalent (in size, shape, and bone-interface material) SC. Specifically, the SC+BPSS construct was the most stable in FE motion. Even though an EC may seem a better option given the minimal tissue disruption during its implantation, there may be a greater chance of endplate collapse by over-distracting the disc space because of the minimal haptic feedback from the expansion.