Evaluation of Spinous Process Tethering at the Proximal End of Rigid Constructs: In Vitro Range of Motion and Intradiscal Pressure at Instrumented and Adjacent Levels.

BACKGROUND: Adult spinal deformity surgery requires use of long thoracolumbar instrumentation, which is associated with risk of postoperative proximal junctional kyphosis (PJK). Tethering has been used in spinal surgery but not around the spinous process (SP) in the context of preventing PJK. METHODS: Researchers applied a nondestructive hybrid loading protocol to 7 T8-L2 cadaveric specimens in flexion-extension, lateral bending, and axial rotation (AR). A rigid construct (pedicle screws and rods) and 1- and 2-level SP constructs were tested, as was a hand-tie technique. SP tethering (SPT) constructs use clamps on both sides of the SP; SPT helix constructs use 1 clamp and wrap around the SP. RESULTS: All tether constructs showed greater motion at the instrumented level and less motion at adjacent levels compared to rigid constructs. In AR, 1- and 2-level SPT constructs restricted first instrumented level motion to a greater extent when compared with other tether constructs (P ≤ .05). Passing the band through the T10 SP did not produce significant biomechanical differences compared to passing it through the T9-T10 interspinous ligament (P > .05). Hand-tied constructs demonstrated more motion compared to tensioned constructs (P > .05). Intradiscal pressure results corroborated motion data. CONCLUSIONS: SPT at the proximal end of a rigid construct produced more favorable biomechanical outcomes at instrumented and adjacent levels than were seen with a completely rigid construct. Clinical research is needed to determine whether these methods reduce the risk of PJK among patients. LEVEL OF EVIDENCE: 3. CLINICAL RELEVANCE: This work sheds light on the biomechanical stability of proximal tethering constructs in an effort to enhance the surgeon's ability to reduce rates of proximal junctional kyphosis and failure in thoracolumbar spinal fusion surgery.

Effects of pre-contoured and in situ contoured rods on the mechanical strength and durability of posterior cervical instrumentation: a finite-element analysis and scanning electron microscopy investigation.

STUDY DESIGN: Finite-element analysis. OBJECTIVES: Intraoperative contouring of rods is a common procedure for spine surgeons to match the native curvature of the spine, but it may lead to premature weakening of the rod. This study investigated the effect of different bending methods on rod fatigue performance. Rod failure in the cervical spine is of clinical concern, particularly when spanning the cervicothoracic region and when considering corrective osteotomies for deformity correction and global spinal alignment. METHODS: Finite-element models were developed to simulate rod bending (3.5 mm D, 40 mm L) to achieve a 23° angle with 3 different bending methods: French single, multiple bending, and in situ bending. Simulations were conducted in 4 steps: rod bending, rod spring back, residual stress relaxation, and F1717 mechanical test simulation. RESULTS: French single bending resulted in the highest residual stress concentrations for both titanium (TiAlV) and cobalt chrome (CoCr) at 783 MPa and 507 MPa, respectively. During F1717 test simulation, the French single bent rod had its highest tensile stress in the middle, with 917 MPa and 623 MPa, respectively, for TiAlV and CoCr, compared to in situ (580 MPa and 586 MPa for TiAlV and CoCr) and the French multiple bent rod (765 MPa and 619 MPa for TiAlV and CoCr). The computational model found that CoCr rods made the construct least prone to deformation. CONCLUSIONS: French single bend with TiAlV rods put the construct at highest risk of failure. CoCr rods led to minimal physical changes in microstructure while showing evidence of flattening.

Biomechanical analysis of motion following sacroiliac joint fusion using lateral sacroiliac screws with or without lumbosacral instrumented fusion.

BACKGROUND: Sacroiliac joint hypermobility or aberrant mechanics may be a source of pain. The purpose of this study was to assess sacroiliac joint range of motion after simulated adjacent lumbosacral instrumented fusion, with or without sacroiliac joint fusion, with lateral sacroiliac screws. METHODS: In this in vitro biomechanical study, seven cadaveric specimens were tested on a six-degrees-of-freedom machine under load control. Left posterior sacroiliac joint ligaments were severed to maximize joint range of motion. Influence of lumbosacral instrumentation on sacroiliac joint motion, with or without fixation, was studied. FINDINGS: During flexion-extension in the setting of posterior sacroiliac joint injury and L5-S1 fixation, sacroiliac joint range of motion increased to 195% of intact. After fixation with lateral sacroiliac screws, average range of motion reduced to 144% of intact motion. Sacroiliac joint screws thus partially stabilized the joint and reduced motion. Use of 6 bilateral sacroiliac joint screws with L5-S1 screw and rod fixation in lateral bending and axial rotation yielded the greatest reduction in range of motion. Without lumbosacral fixation, baseline motion of the sacroiliac joint was reduced, and sacroiliac joint screw alone, using either 2, 3, or 6 screws, was able to restore motion at or below the level of an intact joint. INTERPRETATION: Sacroiliac joint ligament injury with existing lumbosacral fixation doubled sacroiliac joint range of motion, but thereafter, fixation with lateral sacroiliac screws decreased range of motion of the injured sacroiliac joint. Screw configuration played a minor role, but generally, 6 sacroiliac joint screws had the greatest motion reduction.

A novel technique for sacropelvic fixation using image-guided sacroiliac screws: a case series and biomechanical study.

In this study, we sought to assess the safety and accuracy of sacropelvic fixation performed with image-guided sacroiliac screw placement using postoperative computed tomography and X-rays. The sacroiliac screws were placed with navigation in five patients. Intact specimens were mounted onto a six-degrees-of-freedom spine motion simulator. Long lumbosacral constructs using bilateral sacroiliac screws and bilateral S1 pedicle and iliac screws were tested in seven cadaveric spines. Nine sacroiliac screws were well-placed under an image guidance system (IGS); one was placed poorly without IGS with no symptoms. Both fixation techniques significantly reduced range of motion (P<0.05) at L5-S1. The research concluded that rigid lumbosacral fixation can be achieved with sacroiliac screws, and image guidance improves its safety and accuracy. This new technique of image-guided sacroiliac screw insertion should prove useful in many types of fusion to the sacrum, particularly for patients with poor bone quality, complicated anatomy, infection, previous failed fusion and iliac harvesting.

Anterior Fixation of Floating Facet Fractures in the Cervical Spine: A Prospective Case Series and Biomechanical Analysis.

BACKGROUND: Unilateral fractures involving complete separation of the lateral mass from the vertebra and lamina (floating lateral mass fractures) are a unique subset of cervical spine fractures. These injuries are at significant risk for displacement without operative fixation. Posterior fixation has proven to facilitate adequate fusion. However, there are few data supporting the clinical success of single-level anterior fixation. METHODS: Biomechanical evaluation of floating lateral mass fractures and a consecutive case series of patients with rotationally unstable floating lateral mass fractures treated with anterior fixation using an integrated cage-screw device with anterior plating (ICSD) was performed. The study comprised 7 fresh human cadaver cervical spines (C2-C7), and 11 patients with floating lateral mass fractures. Segmental flexibility testing evaluating axial rotation, flexion/extension, and lateral bending was performed in a cadaveric model after 2 types of single-level anterior fixation and 1 type of 2-level posterior fixation. Eleven patients with a floating lateral mass fracture of the cervical spine underwent anterior fixation with an ICSD. Radiographs and clinical outcomes were retrospectively reviewed. RESULTS: Compared with the intact condition, posterior instrumentation significantly (P < .05) reduced range of motion (ROM) in all 3 planes; anterior fixation with cervical plate and interbody spacer significantly reduced ROM in lateral bending only; and the ICSD significantly reduced ROM in flexion/extension and lateral bending. In the clinical arm, there were no long-term complications, subsidence >2 mm, failure of fixation, reoperation, pseudoarthrosis, or listhesis at final follow-up. CONCLUSIONS: The addition of 2 screws placed through a cervical cage can improve anterior fixation in a human cadaveric model of floating lateral mass fractures. Early clinical results demonstrate a low complication rate and a high rate of healing with single-level anterior fixation using this technique.

In Vitro Biomechanical and Fluoroscopic Study of a Continuously Expandable Interbody Spacer Concerning Its Role in Insertion Force and Segmental Kinematics.

STUDY DESIGN: In vitro cadaveric study. PURPOSE: To compare biomechanical performance, trial and implant insertion, and disc distraction during implant placement, when two interbody devices, an in situ continuously expandable spacer (CES) and a traditional static spacer (SS), were used for transforaminal lumbar interbody fusion. OVERVIEW OF LITERATURE: Severe degenerative disc diseases necessitate surgical management via large spacers to increase the disc space for implants. Next-generation interbody devices that expand in situ minimize insertion forces, optimize fit between vertebral endplates, and limit nerve root retraction. However, the literature lacks characterization of insertion forces as well as details on other parameters of expandable and static spacers. METHODS: Ten cadaveric segments (L5-S1) were divided into two groups (n=5) and implanted with either CES or SS. Each specimen experienced unconstrained pure moment of ±6 Nm in flexion-extension, lateral bending, and axial rotation to assess the contribution of CES and SS implants in biomechanical performance. Radiographic analysis was performed during trial and implant insertion to measure distraction during spacer insertion at the posterior, central, and anterior disc regions. Pressure sensors measured the force of trial and implant insertion. RESULTS: Biomechanical analysis showed no significant differences between CES and SS in all planes of motion. A total 2.6±0.9 strikes were required for expandable spacer trials insertion and 2.6±0.5 strikes for CES insertion. A total of 8.4±3.8 strikes were required to insert SS trials and 4.2±1.6 strikes for SS insertion. The total force per surgery was 330 N for CES and 635 N for SS. Fluoroscopic analysis revealed a significant reduction in distraction during implant insertion at the posterior and anterior disc regions (CES, 0.58 and 0.14 mm; SS, 1.04 and 0.78 mm, respectively). CONCLUSIONS: Results from the three study arms reveal the potential use of expandable spacers. During implant insertion, CESs provided similar stability, required less insertion force, and significantly reduced over-distraction of the annulus compared with SS.

Kinematic efficacy of supplemental anterior lumbar interbody fusion at lumbosacral levels in thoracolumbosacral deformity correction with and without pedicle subtraction osteotomy at L3: an in vitro cadaveric study.

PURPOSE: Pedicle subtraction osteotomy (PSO) is performed to treat rigid, sagittal spinal deformities, but high rates of implant failure are reported. Anterior lumbar interbody fusion has been proposed to reduce this risk, but biomechanical investigation is lacking. The goal of this study was to quantify the (1) destabilizing effects of a lumbar osteotomy and (2) contribution of anterior lumbar interbody fusion (ALIF) at the lumbosacral junction as recommended in literature. METHODS: Fourteen fresh human thoracolumbosacral spines (T12-S1) were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Bilateral pedicle screws/rods (BPS) were inserted at T12-S1, cross connectors (CC) at T12-L1 and L5-S1, and anterior interbody spacers (S) at L4-5 and L5-S1. In one group, PSO was performed in seven specimens at L3. All specimens were sequentially tested in (1) Intact; (2) BPS; (3) BPS + CC; (4) BPS + S; and (5) BPS + S + CC; a second group of seven spines were tested in the same sequence without PSO. Mixed-model ANOVA with repeated measures was performed (p ≤ 0.05). RESULTS: At the osteotomy site (L2-L4), in FE, BPS, BPS + CC, BPS + S, BPS + CC + S reduced motion to 11.2, 12.9, 10.9, and 11.4%, respectively, with significance only found in BPS and BPS + S construction (p ≤ 0.05). All constructs significantly reduced motion across L2-L4 in the absence of PSO, across all loading modes (p ≤ 0.05). PSO significantly destabilized L2-L4 axial rotational stability, regardless of operative construction (p ≤ 0.05). Across L4-S1 and L2-S1, all instrumented constructs significantly reduced motion, in both PSO- and non-PSO groups, during all loading modes (p ≤ 0.05). CONCLUSIONS: These findings suggest anterior interbody fusion minimally immobilizes motion segments, and interbody devices may primarily act to maintain disc height. Additionally, lumbar osteotomy destabilizes axial rotational stability at the osteotomy site, potentially further increasing mechanical demand on posterior instrumentation. Clinical studies are needed to assess the impact of this treatment strategy.

Biomechanical Investigation of a Novel Revision Device in an Osteoporotic Model: Pullout Strength of Pedicle Screw Anchor Versus Larger Screw Diameter.

STUDY DESIGN: In vitro cadaveric biomechanical study. OBJECTIVE: To assess revision pullout strength of novel anchored screws (AS) versus conventional larger diameter traditional pedicle screws (TPS) in an osteoporotic model. SUMMARY OF BACKGROUND DATA: Pedicle screws are the most ubiquitous method of treating spinal pathologies requiring lumbar fusion. Although these screws are effective in providing 3-column stabilization of the spine, revision surgeries are occasionally necessary, particularly for geriatric and osteoporotic populations. Innovative technologies should be tested to ensure continued improvement in revision techniques. METHODS: For 4 specimens at L2-L5 (T-score=-3.6±0.54), 6.5-mm-diameter TPS were inserted into left and right pedicles and were pulled out; revision screws were then inserted. Polyether-ether-ketone anchors, designed to expand around a 6.5-mm screw, were inserted into all left pedicles. On the contralateral side, 7.5-mm-diameter TPS were inserted at L2-L3, and 8.5-mm-diameter TPS at L4-L5. Pullout testing was performed at 10 mm/min. The maximum pullout strength and insertion forces were recorded. RESULTS: The initial average pullout force (6.5-mm screw) was 837 N (±329 N) and 642 N (±318 N) in L2-L3 and L4-L5 left pedicles, and 705 N (±451 N) and 779 N (±378 N) in L2-L3 and L4-L5 right pedicles, respectively. Comparison of revision pullout forces versus initial pullout forces revealed the following: 87% and 63% for AS in L2-L3 and L4-L5 left pedicles, respectively; 56% for 7.5-mm and 93% for 8.5-mm TPS in L2-L3 and L4-L5 right pedicles, respectively. CONCLUSIONS: Anchor sleeves with 6.5-mm-diameter pedicle screws provided markedly higher resistance to screw pullout than 7.5-mm-diameter revision screws and fixation statistically equivalent to 8.5-mm-diameter screws, possibly because of medial-lateral expansion within the vertebral space and/or convex filling of the pedicle. AS results had the lowest SD, indicating minimal variability in bone-screw purchase.

An In Vitro Biomechanical Study Evaluating Cervical Extension Plates for Stabilizing Degenerated Adjacent Levels.

STUDY DESIGN: To evaluate the biomechanical stability of 2 extender plates in a human cervical cadaveric model. OBJECTIVES: To evaluate 2 extender plates, placed adjacent to initially implanted plates and to compare their biomechanical stability with traditional techniques. SUMMARY OF BACKGROUND DATA: Traditionally, adjacent level degeneration is surgically treated by removing the previously implanted plate and extending the instrumentation to the new degenerated level. The exposure needed to remove the previously implanted plate may be extensive. To overcome these complications, cervical extension plates, which add-on to the initially implanted plate, were developed. MATERIALS AND METHODS: Fourteen fresh-frozen human cadaver cervical spines (C2-C7) were divided into 2 groups of 7 for a series of constructs to be tested. In group 1, an extender plate, which attaches to its own primary plate, was tested. In group 2, a universal extender plate, which can be placed adjacent to any previously implanted plate, was tested. The specimens prepared were mounted on a 6-degree-of-freedom spine simulator and were sequentially tested in the following order: (1) intact; (2) single-level plate; (3) single-level plate with extender plates; and (4) 2-level plate. An unconstrained pure moment of ±1.5 N m was used in flexion-extension, lateral bending, and axial rotation. RESULTS: All instrumented constructs significantly reduced the range of motion compared with the intact condition. In both the groups, single-level plates with adjacent extender plates demonstrated stability comparable to their respective 2-level plates in all loading modes. CONCLUSIONS: Extender plates give surgeons the opportunity to treat adjacent levels without removing the primary implants, which may reduce the overall risk of damage to vital neurovascular structures. From this cadaveric biomechanical model, both types of extender plates prove to be viable options for treating adjacent level degeneration.

Range of motion after thoracolumbar corpectomy: evaluation of analogous constructs with a novel low-profile anterior dual-rod system and a traditional dual-rod system.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVES: To compare the biomechanical stability of traditional and low-profile thorocolumbar anterior instrumentation after a corpectomy with cross-connectors. Dual-rod anterior thoracolumbar lateral plates (ATLP) have been used clinically to stabilize the thorocolumbar spine. METHODS: The stability of a low-profile dual-rod system (LP DRS) and a traditional dual-rod system (DRS) was compared using a calf spine model. Two groups of seven specimens were tested intact and then in the following order: (1) ATLP with two cross-connectors and spacer; (2) ATLP with one cross-connector and spacer; (3) ATLP with spacer. Data were normalized to intact (100 %) and statistical analysis was used to determine between-group significances. RESULTS: Both constructs reduced motion compared to intact in flexion-extension and lateral bending. Axial rotation motion became unstable after the corpectomy and motion was greater than intact, even with two cross-connectors with both systems. Relative to their respective intact groups, LP DRS significantly reduced motion compared to analogous DRS in flexion-extension. The addition of cross-connectors reduced motion in all loading modes. CONCLUSIONS: The LP DRS provides 7.5 mm of reduced height with similar biomechanical performance. The reduced height may be beneficiary by reduced irritation and impingement on adjacent structures.

Finding the right fit: studying the biomechanics of under-tapping with varying thread depths and pitches.

BACKGROUND CONTEXT: Compromise of pedicle screw purchase is a concern in maintaining rigid spinal fixation, especially with osteoporosis. Little consistency exists among various tapping techniques. Pedicle screws are often prepared with taps of a smaller diameter, which can further exacerbate inconsistency. PURPOSE: The objective of this study was to determine whether a mismatch between tap thread depth (D) and thread pitch (P) and screw D and P affects fixation when under-tapping in osteoporotic bone. STUDY DESIGN: This study is a polyurethane foam block biomechanical analysis. MATERIALS AND METHODS: A foam block osteoporotic bone model was used to compare pullout strength of pedicle screws with a 5.3 nominal diameter tap of varying D's and P's. Blocks were sorted into seven groups: (1) probe only; (2) 0.5-mm D, 1.5-mm P tap; (3) 0.5-mm D, 2.0-mm P tap; (4) 0.75-mm D, 2.0-mm P tap; (5) 0.75-mm D, 2.5-mm P tap; (6) 0.75-mm D, 3.0-mm P tap; and (7) 1.0-mm D, 2.5-mm P tap. A pedicle screw, 6.5 mm in diameter and 40 mm in length, was inserted to a depth of 40 mm. Axial pullout testing was performed at a rate of 5 mm/min on 10 blocks from each group. RESULTS: No significant difference was noted between groups under axial pullout testing. The mode of failure in the probe-only group was block fracture, occurring in 50% of cases. Among the other six groups, only one screw failed because of block fracture. The other 59 failed because of screw pullout. CONCLUSIONS: In an osteoporotic bone model, changing the D or P of the tap has no statistically significant effect on axial pullout. Osteoporotic bone might render tap features marginal. Our findings indicate that changing the characteristics of the tap D and P does not help with pullout strength in an osteoporotic model. The high rate of fracture in the probe-only group might imply the potential benefit of tapping to prevent catastrophic failure of bone.

In vitro analysis of circumferential joint replacement, including bilateral facet joint replacement with lateral lumber disc prosthesis: a parametric investigation of disc sizing.

PURPOSE: Lateral lumbar disc prosthesis (LLDP) is an innovative device used to restore motion in select patients through a lateral retroperitoneal approach. No in vitro biomechanical studies have been published. Further, the potential for in toto circumferential joint restoration when use of this anterior disc is combined with facet replacement remains unqualified but signifies a potentially interesting clinical direction. METHODS: Researchers conducted a biomechanical feasibility study of an LLDP designed to investigate parameters of disc sizing used with bilateral facet joint replacement in a cadaveric model. Tested constructs at L4-L5 included (1) intact, (2) LLDP, (3) LLDP + wide discectomy, (4) LLDP + bilateral facetectomy, and (5) LLDP + bilateral facet joint replacement (BFJR). Investigators tested instrumented constructs (2-5) with an LLDP at compact-fit and lax-fit heights and used raw data to perform statistical analysis by repeated measures analysis of variance (ANOVA), along with Student-Newman-Keuls post hoc analysis (p ≤ 0.05). RESULTS: Increased height of the LLDP resulted in significantly less motion compared with intact. Widening the discectomy while using lax-fit sizing led to motion similar to intact in flexion-extension. As expected, motion was greater with lax-fit height than with compact-fit height in all loading modes and constructs, as is noted with a widened discectomy. The L4-L5 center of rotation was maintained regardless of placement of the LLDP. CONCLUSIONS: After bilateral facetectomy, reconstruction of the three-joint complex achieved by combining the LLDP with BFJR may provide a viable alternative to current clinical treatment regimens.

In vitro biomechanical study of pedicle screw pull-out strength based on different screw path preparation techniques.

BACKGROUND: Poor screw-to-bone fixation is a clinical problem that can lead to screw loosening. Under-tapping (UT) the pedicle screw has been evaluated biomechanically in the past. The objective of the study was to determine if pedicle preparation with a sequential tapping technique will alter the screw-to-bone fixation strength using a stress relaxation testing loading protocol. MATERIALS AND METHODS: Three thoracolumbar calf spines were instrumented with pedicle screws that were either probed, UT, standard-tapped (ST), or sequential tapped to prepare the pedicle screw track and a stress relaxation protocol was used to determine pull-out strength. The maximum torque required for pedicle screw insertion and pull-out strength was reported. A one-way ANOVA and Tukeys post-hoc test were used to determine statistical significance. RESULTS: The pedicle screw insertion torques for the probed, UT, ST and sequentially tapped (SQT) techniques were 5.09 (±1.08) Nm, 5.39 (±1.61) Nm, 2.93 (±0.43) Nm, and 3.54 (±0.67) Nm, respectively. There is a significant difference between probed compared to ST (P ≤ 0.05), as well as UT compared to both ST and SQT (P ≤ 0.05). The pull-out strength for pedicle screws for the probed, UT, ST and SQT techniques was 2443 (±782) N, 2353(±918) N, 2474 (±521) N, and 2146 (±582) N, respectively, with no significant difference (P ≥ 0.05) between techniques. CONCLUSIONS: The ST technique resulted in the highest pull-out strength while the SQT technique resulted in the lowest. However, there was no significant difference in the pull-out strength for the various preparation techniques and there was no correlation between insertion torque and pull-out strength. This suggests that other factors such as bone density may have a greater influence on pull-out strength.

Indirect decompression and vertebral body endplate strength after lateral interbody spacer impaction: cadaveric and foam-block models.

OBJECTIVE The lateral transpsoas approach to the lumbar spine is a well-defined procedure for the management of discogenic spinal pathology necessitating surgical intervention. Intervertebral device subsidence is a postoperative clinical risk that can lead to recurrence of symptomatic pathology and the need for surgical reintervention. The current study was designed to investigate static versus expandable lateral intervertebral spacers in indirect decompression for preserving vertebral body endplate strength. METHODS Using a cadaveric biomechanical study and a foam-block vertebral body model, researchers compared vertebral body endplate strength and distraction potential. Fourteen lumbar motion segments (7 L2-3 and 7 L4-5 specimens) were distributed evenly between static and expandable spacer groups. In each specimen discectomy was followed by trialing and spacer impaction. Motion segments were axially sectioned through the disc, and a metal stamp was used to apply a compressive load to superior and inferior vertebral bodies to quantify endplate strength. A paired, 2-sample for means t-test was performed to determine statistically significant differences between groups (p ≤ 0.05). A foam-block endplate model was used to control simulated disc tension when a spacer with 2- and 3-mm desired distraction was inserted. One-way ANOVA and a post hoc Student Newman-Keuls test were performed (p ≤ 0.05) to determine differences in distraction. RESULTS Both static and expandable spacers restored intact neural foramen and disc heights after device implantation (p > 0.05). Maximum peak loads at endplate failure for static and expandable spacers were 1764 N (± 966 N) and 2284 N (± 949 N), respectively (p ≤ 0.05). The expandable spacer consistently produced greater desired distraction than was created by the static spacer in the foam-block model (p ≤ 0.05). Distraction created by fully expanding the spacer was significantly greater than the predetermined goals of 2 mm and 3 mm (p ≤ 0.05). CONCLUSIONS The current investigation shows that increased trialing required for a static spacer may lead to additional iatrogenic endplate damage, resulting in less distraction and increased propensity for postoperative implant subsidence secondary to endplate disruption.

An initial biomechanical investigation of fusionless anterior tether constructs for controlled scoliosis correction.

BACKGROUND CONTEXT: Conservative treatment for adolescent idiopathic scoliosis is often unsuccessful and requires surgical intervention. Theoretically, anterior fusionless surgery can achieve correction as the patient grows to skeletal maturity. PURPOSE: The objective of the present study was to determine differences in range of motion (ROM) between multiple anterior tether constructs and tensioning techniques. Coronal plane Cobb angles were evaluated. STUDY DESIGN/SETTING: This is a cadaveric biomechanical study. METHODS: Cadaveric spines underwent biomechanical testing to investigate two factors relevant to anterior tether reconstruction: (1) effect of fixation at the T4, superior, and T12, inferior, levels (S-I), as opposed to fixation at all T4-T12 continuous levels (Cont.); and (2) tensioning of the tether sequentially (SEQ T) or only at terminal points (T). Reconstructions were conducted at Cont., and ROM and coronal plane Cobb angles were measured. Rigid rods (R) were used as control for the tether. Funding for the present study was provided by Globus Medical, Inc., and three of five authors are employees of Globus Medical, Inc. RESULTS: Normalized lateral bending ROM for intact was 100(±33)%. The S-I R construct reduced motion to 39(±8)%. Tethering at terminal points resulted in ROM for S-I T and S-I No T of 61(±21)% and 70(±17)%, respectively. Screws placed at every level resulted in motion of 28(±9)% for the Cont. R construct, and a stepwise increase in motion to 44(±15)%, 47(±18)%, and 71(±19)%, respectively, for Cont. SEQ T, Cont. T, and Cont. No T. These relative trends were the same in all loading modes. Average change in overall coronal plane Cobb angle from intact was 4.6(±3.2)° and 9.9(±5.5)° for Cont. T and Cont. SEQ T constructs, respectively. CONCLUSIONS: Tensioned tether constructs allowed greater ROM than rigid constructs, and no significant difference in ROM was noted between tensioning techniques. Sequential tensioning can produce greater correction with no biomechanical advantage.

Short Segment Spinal Instrumentation With Index Vertebra Pedicle Screw Placement for Pathologies Involving the Anterior and Middle Vertebral Column Is as Effective as Long Segment Stabilization With Cage Reconstruction: A Biomechanical Study.

STUDY DESIGN: An in vitro, cadaveric biomechanical study. OBJECTIVE: The aim of the present study was to compare single-segment posterior instrumentation and fracture-level screws with single/multilevel posterior fixation and corpectomy in a simulated, unstable burst fracture model. SUMMARY OF BACKGROUND DATA: The optimal extent of instrumentation for surgical cases of non-neoplastic vertebral body pathologies remains uncertain. Although several clinical studies demonstrate advantages of short segment instrumentation with index-level screws over more extensive corpectomy and anterior-posterior techniques, a comprehensive biomechanical comparison of these techniques is currently lacking. METHODS: Six bovine spines (T11-L5) were tested in flexion, extension, lateral bending (LB), and axial rotation (AR) following simulated burst fracture at L2. Posterior instrumentation included 1 level above/below (1LF) and 2 levels above/below fracture level (2LF), intermediate or index screws at fracture level (FF), and cross-connectors above/below fracture level (CC). Anterior corpectomy devices included expandable corpectomy spacers with/without integrated screws, ACDi and ACD, respectively FORTIFY-Integrated/FORTIFY; Globus Medical, Inc., PA. Constructs were tested in the following order: (1) Intact; (2) 1LF; (3) 1LF and CC; (4) 1LF and FF; (5) 1LF, CC, and FF; (6) 2LF; (7) 2LF and CC; (8) 2LF and FF; (9) 2LF, CC, and FF; (10) 2LF and ACD; (11) 2LF, ACD, and CC; (12) 1LF and ACDi; (13) 1LF, ACDi, and CC. RESULTS: During flexion, all constructs except 1LF reduced motion relative to intact (P ≤ 0.05). Anterior support was most stable, but no differences were found between constructs (P ≥ 0.05). Every construct reduced motion in extension, though no differences were found between constructs and intact (P ≥ 0.05). During LB, all constructs reduced motion relative to intact (P ≤ 0.05); 2LF constructs further reduced motion (P ≤ 0.05). No construct returned AR motion to intact, with significant increases in 1LF and ACDi, 2LF and ACD, and 2LF, ACD, and CC (P ≤ 0.05). Cross-connectors and fracture screws reinforced each other in posterior-only constructs, providing maximum stability (P ≥ 0.05). CONCLUSIONS: This biomechanical comparison study found no significant superiority of combined anterior-posterior constructs over short segment fracture screw fixation, only multilevel posterior instrumentation with and without anterior support, providing increased stability in LB. Biomechanical equivalency suggests that short segment fracture screw intervention may provide appropriate stabilization for non-neoplastic pathologies involving the anterior and middle vertebral columns. LEVEL OF EVIDENCE: 2.

Biomechanical Analysis of a Novel Pedicle Screw Anchor Designed for the Osteoporotic Population.

OBJECTIVE: The biomechanical study was performed to investigate the effect of a novel pedicle screw anchor in increasing the pullout strength of pedicle screws. METHODS: Ten lumbar vertebral bodies with a weighted average T-score of -2.13 were used. Pedicle screws of 4.5 mm diameter and 25 mm length were inserted in to one pedicle randomly and matched with an anchor in the corresponding pedicle. Fatigue testing was performed by applying an axial load of ±200N to the screw tulip, along the axis of the rod, at a rate of 0.5 Hz for 1,000 cycles. After fatigue loading was completed, all screws underwent axial pullout testing at a rate of 0.1 mm/sec until failure. A paired two sample for means t-test was performed to determine a significant difference between the two groups (p ≤ 0.05). RESULTS: Following fatigue testing, the axial displacement at the 1,000 cycle point for the anchor and non-anchor group was 1.4 ± 0.7mm and 2.9 ± 1.2mm, respectively. The anchor group had significantly lower axial displacement compared to the non-anchor group (p ≤ 0.01). The group with the anchor reached an average maximum load of 702 ± 373N. The average yield load for the non-anchor group was 421 ± 293N. The anchor group yield load was significantly greater than the non-anchor group (p ≤ 0.01). CONCLUSIONS: A novel anchor for standard pedicle screws resulted in significantly less axial movement during fatigue and a greater failure force compared a screw with no anchor. The anchor may provide a stronger bone-to-screw interface, than a non-anchor screw, without the complications of cement augmentation.

In vitro biomechanics of the craniocervical junction-a sequential sectioning of its stabilizing structures.

BACKGROUND CONTEXT: Occipitocervical dislocations involve translations of the craniocervical joints. The relative contributions of each ligament to overall stability and the effects of the occipitoatlantal joint capsules on the pathologic translation are unknown. Although incidences of occipitocervical dislocations are rare after blunt trauma, they are usually fatal. When patients do survive these dislocations, the proper diagnosis is difficult, which in turn may increase the fatality rate. A biomechanical model may provide a greater pathologic understanding of craniocervical subluxation. PURPOSE: The purpose of the study is to build an in vitro biomechanical model to determine which stabilizing ligament(s) of the craniocervical junction are most important in restraining rotation and translations during these rotations. This may guide clinical diagnosis, which could assist in treatment options. STUDY DESIGN/SETTING: The study design includes a biomechanical cadaveric test. METHODS: Seven cadaveric specimens were tested using a 6-degree-of-freedom spine simulator under the following conditions: intact, clivus/alar removal (CR), transverse ligament destruction (TLD), occipitoatlantal (OA) joint capsulotomyoccipitoatlantal (OA) joint capsulotomy (C0-C1 JC), and C1-C2 joint capsulotomy (C1-C2 JC). Flexion-extension (FE), lateral bending (LB), and axial rotation (AR) were applied (2.5 Nm) to a C0-C2 segment, whereas anterior-posterior (AP) and cranial-caudal (CC) translations were recorded. Average motions were normalized to intact (100%) for each joint. RESULTS: Increases in C0-C1 angular and translational motions occurred after TLD and C0-C1 JC. At the atlantoaxial joint, there were significant (p<.05) increases from intact in FE (TLD=154%, C0-C1 JC=174%) and in AR (TLD=178%, C0-C1 JC=224%). Anterior-posterior translation during applied LB increased significantly after TLD (248% intact). Cranial-caudal translation during applied FE increased significantly after TLD (188%) and C0-C1 JC (361%). Increases in C1-C2 angular motion occurred after TLD and C1-C2 JC and in translation after CR and TLD. At the C1-C2 joint, there were significant increases from intact in FE (TLD=172%, C1-C2 JC=160%) and in LB (TLD=286%, C1-C2 JC=332%); in AR, there were no statistical differences. Anterior-posterior translation increased significantly after CR (280%). Cranial-caudal translation also increased significantly after CR (205%) and TLD (298%) during LB. CONCLUSIONS: Transverse and alar ligaments appear to be the main stabilizers of the craniocervical junction. The vertical structures attached to the clivus and OA joint capsules function as secondary stabilizers. Craniocervical dislocations seem to affect FE and lateral bending the most, whereas increased translation seems to occur primarily in the AP and CC directions. Models of craniocervical trauma should section all three restraining structures for the future studies.

Stabilization of the craniocervical junction after an internal dislocation injury: an in vitro study.

BACKGROUND CONTEXT: Reconstructive surgeries at the occipitocervical (OC) junction have been studied in treating degenerative conditions. There is a paucity of data for optimal fixation for a traumatically unstable OC joint. In clinical OC dislocations, segmental fixation may be impossible because of vertebral artery injury or fracture. Segmental fixation of the occiput, C1, and C2 demonstrated maximum biomechanical stability in fixation of an unstable craniocervical dislocation. A biomechanical study comparing various points of cervical posterior screw fixation after recreating traumatic injury would illuminate relative advantages between the various techniques. PURPOSE: To determine the rigidity lost, if any, of segmental C0-C2 posterior screw fixation versus fixation skipping C1 at the OC junction, with or without a cross-connector. STUDY DESIGN: This study is a cadaveric biomechanical investigation. METHODS: Intervertebral motions and translations were recorded in seven specimens under conditions in the following order: intact, OC dislocation model with complete disruption of the cruciate ligaments, alar ligaments, and occipitoatlantal/atlantoaxial capsules (injury), segmental posterior fixation (SPF) with posterior instrumentation (ELLIPSE; Globus Medical, Inc., Audubon, PA, USA) at occiput, C1, and C2 levels, endpoint fixation (EPF) with posterior instrumentation at occiput and C1 level skipping C1, and endpoint fixation with a cross-connector (EPFC). Motion was applied through a custom spine simulator with a pure moment load of 2.5 Nm and measured with motion capture markers attached to occiput (C0), anterior C1 ring, and C2. Flexion-extension (FE), lateral bending (LB), axial rotation (AR), and cranial-caudal (CC) motions were recorded in terms of C0-C2. Results were reported as a percentage of injured motion (injury=100%), unless otherwise stated. RESULTS: The injury significantly increased the motion to 165%, 263%, and 130%, during FE, LB, and AR, respectively, of intact. The CC translations increased to 164%, 254%, and 121% during FE, LB, AR, respectively, of intact. Segmental posterior fixation significantly reduced motion to 7%, 8%, and 1%, during FE, LB, and AR, respectively, of injury. Endpoint fixation significantly increased motion in FE, resulting in 12%, 6%, and 4% during FE, LB, and AR, respectively, of injury when compared with SPF. The EPFC construct decreased the motion compared with its counterpart to 8.6%, 5.7%, and 3.2% during FE, LB, and AR, respectively. CONCLUSIONS: All fixation constructs significantly reduced motion in all loading modes and CC translations, compared with intact and injury. The construct with the greatest stability against craniocervical dislocation included SPF with instrumentation at the occiput, C1, and C2. By skipping C1 using the EPF, FE and cephalad-caudal translations significantly increased compared with posterior fixation at every level. The addition of a cross-connector increased the stability but was not statistically significant.

In vitro biomechanical study to quantify range of motion, intradiscal pressure, and facet force of 3-level dynamic stabilization constructs with decreased stiffness.

STUDY DESIGN: An in vitro biomechanical study. OBJECTIVE: To perform in vitro biomechanical testing on a lumbar spine using a 6-degree-of-freedom machine. To compare the range of motion (ROM), intradiscal pressure, and facet force of different 3-level dynamic stabilization constructs with traditional rigid constructs. To determine the effect of decreasing the stiffness of the dynamic construct on the various parameters. SUMMARY OF BACKGROUND DATA: Dynamic stabilization systems are a surgical option that may minimize the development of adjacent segment disease. METHODS: Seven T12-S1 specimens were tested at ± 7.5 Nm in flexion-extension, lateral bending, and axial rotation. The testing sequence was (1) intact, (2) intact with facet sensors, (3) L3-S1 rigid (3R), (4) L3-L4 dynamic and L4-S1 rigid (1D-2R A), (5) L3-L5 dynamic and L5-S1 rigid (2D-1R A), and (6) L3-S1 dynamic (3D A). Constructs 1D-2R A, 2D-1R A, and 3D A were tested again with the specialized designs of B and C of decreased stiffness. ROM, intradiscal pressure, and facet force were measured. RESULTS: In all loading modes there was a trend of increasing motion with decreased stiffness. Significant differences were seen with more dynamic stabilization levels but no significance was seen with only decreasing the stiffness. 3R facet force at the caudal instrumented level significantly decreased compared with intact and dynamic stabilization constructs during axial rotation. CONCLUSION: Biomechanical testing resulted in a trend of increased ROM across instrumented levels as the stiffness was decreased. Dynamic stabilization increased the ROM across instrumented levels compared with rigid rods. These results suggest that decreasing the stiffness of the construct may lessen the probability of adjacent-level disease. Although the specialized devices are not commercially available, clinical data would be necessary for a clearer understanding of adjacent level effects and to confirm the in vitro biomechanical findings. LEVEL OF EVIDENCE: N/A.