Axial Rotation Range Of Motion Research Articles

The influence of tracking marker locations on three-dimensional wrist kinematics

ObjectivesTo determine the influence of tracking marker locations on wrist kinematics during free movements and the golf swing, with the intention of recommending a solution that generates meaningful three-dimensional wrist kinematics. DesignRepeated measures. MethodsSix participants performed free movements of flexion/extension, radial/ulnar deviation and forearm supination/pronation, with a further sixteen participants performing golf drives. A passive motion capture system tracked four different marker sets located on participants’ hand and forearm segments. Variables of peak angle and range of motion were used to compare marker sets during free movements and angles at the top of the backswing and impact were compared during the golf swing. ResultsWrist marker set had a large (η2≥0.557) and often significant (p≤0.051) effect on the variables measured during free movements, and a mixed (η2≥0.108, p≤0.198) effect on wrist angles during the golf swing. Wrist axial rotation range of motion during free forearm supination/pronation revealed the greatest difference between marker sets (∼42°). The large values generated by two of the marker sets for this rotation appeared to influence the values of flexion/extension and radial/ulnar deviation during the golf swing. ConclusionsThe location of markers used to measure wrist kinematics can have a large effect on the angles generated. A solution of two markers located at the distal end of the forearm and one at the proximal, appears to minimise values of wrist axial rotation during free forearm supination/pronation and, consequently, produce more meaningful three-dimensional wrist kinematics.

Biomechanical Study of a Novel, Expandable, Non-Metallic and Radiolucent CF/PEEK Vertebral Body Replacement (VBR).

Vertebral body replacement is well-established to stabilize vertebral injuries due to trauma or cancer. Spinal implants are mainly manufactured by metallic alloys; which leads to artifacts in radiological diagnostics; as well as in radiotherapy. The purpose of this study was to evaluate the biomechanical data of a novel carbon fiber reinforced polyetheretherketone (CF/PEEK) vertebral body replacement (VBR). Six thoracolumbar specimens were tested in a six degrees of freedom spine tester. In all tested specimens CF/PEEK pedicle screws were used. Two different rods (CF/PEEK versus titanium) with/without cross connectors and two different VBRs (CF/PEEK prototype versus titanium) were tested. In lateral bending and flexion/extension; range of motion (ROM) was significantly reduced in all instrumented states. In axial rotation; the CF/PEEK combination (rods and VBR) resulted in the highest ROM; whereas titanium rods with titanium VBR resulted in the lowest ROM. Two cross connectors reduced ROM in axial rotation for all instrumentations independently of VBR or rod material. All instrumented states in all planes of motion showed a significantly reduced ROM. No significant differences were detected between the VBR materials in all planes of motion. Less rigid CF/PEEK rods in combination with the CF/PEEK VBR without cross connectors showed the smallest reduction in ROM. Independently of VBR and rod material; two cross connectors significantly reduced ROM in axial rotation. Compared to titanium rods; the use of CF/PEEK rods results in higher ROM. The stiffness of rod material has more influence on the ROM than the stiffness of VBR material.

294. Coupled rotation patterns in cervical axial rotation can change when the head is kept level

BACKGROUND CONTEXT Axial rotation of the cervical spine with the cranium held level occurs during certain daily activities such as looking over one's shoulder when walking or driving. These activities require rotation from the suboccipital and subaxial cervical spine. The literature on cervical spine axial rotation and coupled rotations has been obtained from experiments where the segmental axis of rotation is aligned with the global axis of rotation. In looking over one's shoulder, the segmental rotation axes may not be aligned with the global and primarily vertical rotation axis due to curvature of the spine. Furthermore, fusion may limit the amount of rotation that can be obtained from the subaxial spine and reduce the facility with which the daily activities are performed. PURPOSE (1) To define the rotation of the intact subaxial spine during horizontal axial rotation of the cranium, and (2) To measure the effect of C5-C7 fusion on the rotation of the subaxial spine during horizontal axial rotation of the cranium. STUDY DESIGN/SETTING Biomechanical experiment. PATIENT SAMPLE Five C0-T1 cadaveric spine specimens (38-54 years old, 3 female and 2 male). OUTCOME MEASURES Rotation of the subaxial cervical spine in response to applied axial rotation moment. METHODS A left and right axial rotation moment of 1Nm was applied to specimens mounted in an apparatus that held the cranium in an orientation consistent with horizontal gaze (horizontal Frankfort line). The axial rotation axis passed through the midpoint between the two external auditory meatuses. Rotations of the individual vertebrae before and after C5-C7 fusion using lateral mass screws and posterior rods were measured using an optoelectronic motion measurement system. RESULTS C5-C7 fusion reduced the C3-T1 axial rotation range of motion from a mean of 31±5° to 22±5°. In the nonfused levels, the axial rotation range of motion changed less than half a degree. Holding the cranium level (horizontal gaze) changes the segmental and overall coupled rotation behavior of the cervical spine during axial rotation. In 3 specimens, left axial rotation induced both right lateral bending in the lower segments and left lateral bending in upper segments, and vice versa with right axial rotation. In the three specimens in which both left and right lateral bending was induced by axial rotation, there was a 0.1° to 3.7° reduction in the coupled C3-C4 lateral bending range of motion. In the other two specimen, C3-C4 lateral bending range of motion increased by 0.8° to 1.8°. C5-C7 fusion changed the amount but not the direction of coupled lateral bending of unfused segments. CONCLUSIONS Without the horizontal gaze constraint, right axial rotation has been shown to induce right lateral bending in the cervical spine. There are various activities of daily living, such as looking over one's shoulder, that involve holding the gaze horizontal while axially rotating the cervical spine. In this biomechanical experiment, a constrained horizontal axial rotation resulted in specimen-specific coupled rotation of the subaxial cervical spine. In three of the specimen, axial rotation induced left and right segmental lateral bending regardless of the axial rotation direction.This specimen-specific difference in the coupled lateral bending and in the ability to perform constrained axial rotation after C5-C7 fusion leads us to wonder whether there may also be a patient-specific difference. This difference would have clinical implications for patients who are candidates for C5-C7 fusion. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Biomechanical analysis of pedicle screw density in posterior spine instrumentation

Background: Posterior spinal fusion using pedicle screw instrumentation is the current standard for surgical management of adolescent idiopathic scoliosis (AIS). Decreasing the density of pedicle screw instrumentation is a method of decreasing costs associated with the procedure. Minimal biomechanical data exist evaluating pedicle screw density, and the optimal screw density has not been determined conclusively. This study evaluated the effects of decreasing pedicle screw density using in vitro robotic biomechanical testing of porcine thoracic spines. Methods: Eight porcine thoracic spines were dissected to T3-T15. Spines were instrumented with bilateral pedicle screws at levels T4-14 and were attached to a robotic system for biomechanical testing. Full instrumentation (100%) and subsequent constructs of decreased pedicle screw densities (82%, 64%, and 64% with crosslinks [64%+CL]) and noninstrumented spines were tested for total range of motion (ROM) in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) using three-dimensional segmental spinal kinematics with an optical tracking system. Results: No significant differences in ROM were noted in FE, LB, or AR when comparing full instrumentation (100%) to constructs of decreasing pedicle screw density (82% and 64%). The addition of crosslinks in the 64%+CL construct decreased ROM in AR (P=0.0634) as compared to the 64% construct. Conclusions: Decreasing pedicle screw density in biomechanical testing of nondeformed porcine thoracic spines does not result in any significant changes to ROM when comparing constructs with greater than 60% density. The addition of crosslinks may confer added stability to AR in patients with decreased density constructs.

A Biomechanical Evaluation of a Next-Generation Integrated and Modular ACDF Device Possessing Full-Plate, Half-Plate, and No-Profile Fixation Iterations

Study Design:In vitro biomechanical study.Objectives:The objective of this in vitro biomechanical range-of-motion (ROM) study was to evaluate spinal segmental stability following fixation with a novel anterior cervical discectomy and fusion (ACDF) device (“novel device”) that possesses integrated and modular no-profile, half-plate, and full-plate fixation capabilities.Methods:Human cadaveric (n = 18, C3-T1) specimens were divided into 3 groups (n = 6/group). Each group would receive one novel device iteration. Specimen terminal ends were potted. Each specimen was first tested in an intact state, followed by anterior discectomy (C5/C6) and iterative instrumentation. Testing order: (1) novel device (group 1, no-profile; group 2, half-plate; group 3, full-plate); (2) novel device (all groups) with lateral mass screws (LMS); (3) traditional ACDF plate + cage; (4) traditional ACDF plate + cage + LMS. A 2 N·m moment was applied in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) via a kinematic testing machine. Segmental ROM was tracked and normalized to intact conditions. Comparative statistical analyses were performed.Results:Key findings: (1) the novel half- and full-plate constructs provided comparable reduction in FE and LB ROM to that of traditional plated ACDF (P ≥ .05); (2) the novel full-plate construct significantly exceeded all other anterior-only constructs (P ≤ .05) in AR ROM reduction; and (3) the novel half-plate construct significantly exceeded the no-profile construct in FE (P < .05).Conclusions:The novel ACDF device may be a versatile alternative to traditional no-profile and independent plating techniques, as it provides comparable ROM reduction in all principle motion directions, across all device iterations.

Validation of an In Vivo Medical Image-Based Young Human Lumbar Spine Finite Element Model.

Mathematical models of the human spine can be used to investigate spinal biomechanics without the difficulties, limitations, and ethical concerns associated with physical experimentation. Validation of such models is necessary to ensure that the modeled system behavior accurately represents the physics of the actual system. The goal of this work was to validate a medical image-based nonlinear lumbosacral spine finite element model of a healthy 20-yr-old female subject under physiological moments. Range of motion (ROM), facet joint forces (FJF), and intradiscal pressure (IDP) were compared with experimental values and validated finite element models from the literature. The finite element model presented in this work was in good agreement with published experimental studies and finite element models under pure moments. For applied moments of 7.5 N·m, the ROM in flexion-extension, axial rotation, and lateral bending were 39 deg, 16 deg, and 28 deg, respectively. Excellent agreement was observed between the finite element model and experimental data for IDP under pure compressive loading. The predicted FJFs were lower than those of the experimental results and validated finite element models for extension and torsion, likely due to the nondegenerate properties chosen for the intervertebral disks and morphology of the young female spine. This work is the first to validate a computational lumbar spine model of a young female subject. This model will serve as a valuable tool for predicting orthopedic spinal injuries, studying the effect of intervertebral disk replacements using advanced biomaterials, and investigating soft tissue degeneration.

Novel use of telescoping growth rods in treatment of early onset scoliosis: An in vivo and in vitro study in a porcine model.

IntroductionTreatment of early‐onset scoliosis (EOS) can be difficult. Various forms of growing rods exist to correct deformity while delaying definitive spinal fusion. The disadvantage of traditional growing rods is need for repeated surgical lengthening procedures. Telescoping growth rods (TelGR) are a prototype new, guided growth technology with a rod mechanism that allows spontaneous longitudinal growth over time without manual lengthening. We hypothesized that the TelGR system will permit unrestricted growth with limited complications through 12 weeks in vivo, and that the range of motion (RoM) in each of three directions and stiffness of the TelGR system would not be significantly different than the rigid rod system in vitro.Materials and Methods In vivo: Six immature pigs were surgically implanted with TelGR with cephalad fixation at T6‐7 and caudal fixation at T14‐L1. Radiographs of the involved vertebral segments were measured postoperatively and after 12 weeks. In vitro: A robotic testing system was utilized for flexibility tests in flexion‐extension (FE), lateral bending (LB), and axial rotation (AR) of eight immature porcine specimens (T3‐T15). Testing was performed on both dual rigid rods and bilateral TelGR with instrumentation at T4‐5 and T13‐14.Results In vivo: Over the 12‐week period, the rod length of the TelGR increased an average of 65 mm. In vitro: TelGR demonstrated significantly increased motion in LB and AR RoM compared with rigid rods. No difference was noted in FE RoM.DiscussionThe in vivo results in this study showed expected skeletal growth with spines instrumented with TelGR. In vitro findings of increased RoM in AR and LB suggest that the TelGR system may be less rigid than traditional growing rods. Treatment with TelGR might, if proven efficacious in the clinical setting, decrease the need for repeated surgical intervention compared with traditional growing rods. This study adds to the limited body of biomechanical evidence examining guided growth technology.

Biomechanical Study of Novel Unilateral Fixation Combining Unilateral Pedicle and Contralateral Translaminar Screws in the Subaxial Cervical Spine

In several situations, the stability of the subaxial cervical spine is damaged and involves the lateral mass of 1 side; in these cases, a pedicle screw (PS) or lateral mass screw (LMS) may not be suitable for placement on the affected side. Therefore, salvage short-segment fixation with satisfactory stability is needed when bilateral fixation is not feasible. Seven fresh-frozen human cervical spine specimens were used to test the 3-plane range of motion (ROM) of the C4-C5 segment. Quasistatic 2-Nm flexibility testing was performed in the following sequence: 1) intact; 2) destabilization (using 3-column injury models) treated with bilateral mass screws (BMSs); 3) destabilization treated with a unilateral PS combined with a contralateral translaminar screw (UPS+CTLS); and 4) destabilization treated with bilateral PSs (BPSs). Then, a pullout strength test was performed for the PSs, LMSs, and translaminar screws (TLSs) using 7 isolated C4 and C5 vertebrae. The UPS+CTLS group showed no significant difference from the BMS group in the 3-plane ROM or from the BPS group in the axial rotation or flexion-extension ROM but showed a significantly greater lateral bending ROM than did the BPS group. The pullout strength test showed that both C4 and C5 TLSs had strength similar to that of LMSs but poorer than that of PSs. Fixation with the hybrid UPS+CTLS construct performed as well as BMS fixation in our biomechanical tests and may play a clinical role when BPS or BMS placement is not feasible for short-segment fixation.

In Vitro Biomechanical Evaluation of a Novel, Minimally Invasive, Sacroiliac Joint Fixation Device.

Sacroiliac (SI) joint pathology may result in low-back pain, which causes substantial disability. Treatment failure with operative management of SI pain may be related to incomplete fusion of the joint and to fixation failure. The objective of this study was to evaluate the initial biomechanical stability of SI joint fixation with a novel implantable device in an in vitro human cadaveric model. The right and left sides of 3 cadaveric L4-pelvis specimens were tested (1) intact, (2) destabilized, and (3) instrumented with an implantable SI joint fixation device using a simulated single-stance load condition. Right-leg and left-leg stance data were grouped together for a sample size of 6, and angular range of motion (ROM) was determined during application of flexion-extension, lateral bending, and axial rotation bending moments to a limit of 7.5 Nm. Following intact testing, destabilization by severing the posterior SI joint capsule and ligaments and the pubic symphysis reliably produced a significantly destabilized joint with the mean angular ROM more than doubling in flexion-extension and lateral bending and more than tripling in axial rotation (P ≤ .003) compared to the intact condition. Instrumentation with the SI screw fixation device significantly reduced mean joint ROM compared to the destabilized condition in all 3 anatomic planes tested (P < .001). When compared to the intact condition, the SI-instrumented condition significantly reduced lateral bending (P = .01) and had a similar ROM in flexion-extension (P = .14) and axial rotation (P = .85). Instrumentation with the SI screw fixation device significantly reduced mean joint ROM compared to the destabilized condition, with similar ROM in flexion-extension and axial rotation, and it significantly reduced ROM in lateral bending compared to that for the intact joint. The ROM values observed with the instrumented condition were comparable to levels of mobility considered favorable for spinal fusion.

Biomechanical properties of novel transpedicular transdiscal screw fixation with interbody arthrodesis technique in lumbar spine: A finite element study

PurposeThe purpose of this study was to investigate finite element biomechanical properties of the novel transpedicular transdiscal (TPTD) screw fixation with interbody arthrodesis technique in lumbar spine. MethodsAn L4–L5 finite element model was established and validated. Then, two fixation models, TPTD screw system and bilateral pedicle screw system (BPSS), were established on the validated L4–L5 finite element model. The inferior surface of the L5 vertebra was set immobilised, and moment of 7.5 Nm was applied on the L4 vertebra to test the range of motion (ROM) and stress at flexion, extension, lateral bending and axial rotation. ResultsThe intact model was validated for prediction accuracy by comparing two previously published studies. Both of TPTD and BPSS fixation models displayed decreased motion at L4–L5. The ROMs of six moments of flexion, extension, left lateral bending, right lateral bending, left axial rotation and right axial rotation in TPTD model were 1.92, 2.12, 1.10, 1.11, 0.90 and 0.87°, respectively; in BPSS model, they were 1.48, 0.42, 0.35, 0.38, 0.74 and 0.75°, respectively. The screws' peak stress of above six moments in TPTD model was 182.58, 272.75, 133.01, 137.36, 155.48 and 150.50 MPa, respectively; and in BPSS model, it was 103.16, 129.74, 120.28, 134.62, 180.84 and 169.76 MPa, respectively. ConclusionBoth BPSS and TPTD can provide stable biomechanical properties for lumbar spine. The decreased ROM of flexion, extension and lateral bending was slightly more in BPSS model than in TPTD model, but TPTD model had similar ROM of axial rotation with BPSS model. The screws' peak stress of TPTD screw focused on the L4–L5 intervertebral space region, and more caution should be put at this site for the fatigue breakage. The translational potential of this articleOur finite element study provides the biomechanical properties of novel TPTD screw fixation, and promotes this novel transpedicular transdiscal screw fixation with interbody arthrodesis technique be used clinically.

Biomechanical evaluation of interbody fixation with secondary augmentation: lateral lumbar interbody fusion versus posterior lumbar interbody fusion.

Many approaches to the lumbar spine have been developed for interbody fusion. The biomechanical profile of each interbody fusion device is determined by the anatomical approach and the type of supplemental internal fixation. Lateral lumbar interbody fusion (LLIF) was developed as a minimally invasive technique for introducing hardware with higher profiles and wider widths, compared with that for the posterior lumbar interbody fusion (PLIF) approach. However, the biomechanics of the interbody fusion construct used in the LLIF approach have not been rigorously evaluated, especially in the presence of secondary augmentation. Spinal stability of 21 cadaveric lumbar specimens was compared using standard nondestructive flexibility studies [mean range of motion (ROM), lax zone (LZ), stiff zone (SZ) in flexion-extension, lateral bending, and axial rotation]. Non-paired comparisons were made among four conditions: (I) intact; (II) with unilateral interbody + bilateral pedicle screws (BPS) using the LLIF approach (referred to as the LLIF construct); (III) with bilateral interbody + BPS using the PLIF approach (referred to as the PLIF construct); and (IV) with no lumbar interbody fusion (LIF) + BPS (referred to as the no-LIF construct). With bilateral pedicle screw-rod fixation, stability was equivalent between PLIF and LLIF constructs in lateral bending and flexion-extension. PLIF and LLIF constructs had similar biomechanical profiles, with a trend toward less ROM in axial rotation for the LLIF construct. LLIF and PLIF constructs had similar stabilizing effects.

Novel posterior artificial atlanto-odontoid joint for atlantoaxial instability: a biomechanical study.

OBJECTIVE Atlantoaxial instability is usually corrected by anterior and/or posterior C1-2 fusion. However, fusion can lead to considerable loss of movement at the C1-2 level, which can adversely impact a patient's quality of life. In this study, the authors investigated the stability and function of a novel posterior artificial atlanto-odontoid joint (NPAAJ) by using cadaveric cervical spines. METHODS The Oc-C7 regions from 10 cadaveric spines were used for anteroposterior (AP) translation and range of motion (ROM) tests while intact and after destabilization, NPAAJ implantation, and double-rod fixation. RESULTS The mean AP C1-2 translational distances in the intact, destabilization, and double-rod groups were 6.53 ± 1.07 mm, 11.54 ± 1.59 mm, and 3.24 ± 0.99 mm, respectively, and the AP translational distance in the NPAAJ group was significantly different from that in the intact group (p < 0.05). The AP translational distance in the NPAAJ group was not significantly different from that in the double-rod group (p = 0.24). The mean flexion, extension, and axial rotation ROM values of the NPAAJ group were 9.87° ± 0.91°, 8.75° ± 0.99°, and 61.93° ± 2.93°, respectively, and these were lower than the corresponding values in the intact group (p < 0.05). The mean lateral bending ROM in the NPAAJ group (9.26° ± 0.86°) was not significantly different from that in the intact group (p = 0.23), and the flexion, extension, and rotation ranges in the NPAAJ group were 79.5%, 85.2%, and 82.3%, respectively, of those in the intact group. CONCLUSIONS Use of NPAAJ for correction of atlantoaxial instability disorders caused by congenital odontoid dysplasia, odontoid fracture nonunion, and C-1 transverse ligament disruption (IA, IB, and IIB) may restore the stability and preserve most of the ROM of C1-2. Additionally, the NPAAJ may prevent soft tissue from embedding within the joint. However, additional studies should be performed before the NPAAJ is used clinically.

Biomechanics of unilateral and bilateral sacroiliac joint stabilization: laboratory investigation.

OBJECTIVE Bilateral symptoms have been reported in 8%-35% of patients with sacroiliac (SI) joint dysfunction. Stabilization of a single SI joint may significantly alter the stresses on the contralateral SI joint. If the contralateral SI joint stresses are significantly increased, degeneration may occur; alternatively, if the stresses are significantly reduced, bilateral stabilization may be unnecessary for patients with bilateral symptoms. The biomechanical effects of 1) unilateral stabilization on the contralateral SI joint and 2) bilateral stabilization on both SI joints are currently unknown. The objectives of this study were to characterize bilateral SI joint range of motion (ROM) and evaluate and compare the biomechanical effects of unilateral and bilateral implant placement for SI joint fusion. METHODS A lumbopelvic model (L5-pelvis) was used to test the ROM of both SI joints in 8 cadavers. A single-leg stance setup was used to load the lumbar spine and measure the ROM of each SI joint in flexion-extension, lateral bending, and axial rotation. Both joints were tested 1) while intact, 2) after unilateral stabilization, and 3) after bilateral stabilization. Stabilization consisted of lateral transiliac placement of 3 triangular titanium plasma-sprayed (TPS) implants. RESULTS Intact testing showed that during single-leg stance the contralateral SI joint had less ROM in flexion-extension (27%), lateral bending (32%), and axial rotation (69%) than the loaded joint. Unilateral stabilization resulted in significant reduction of flexion-extension ROM (46%) on the treated side; no significant ROM changes were observed for the nontreated side. Bilateral stabilization resulted in significant reduction of flexion-extension ROM of the primary (45%) and secondary (75%) SI joints. CONCLUSIONS This study demonstrated that during single-leg loading the ROMs for the stance (loaded) and swing (unloaded) SI joints are significantly different. Unilateral stabilization for SI joint dysfunction significantly reduces the ROM of the treated side, but does not significantly reduce the ROM of the nontreated contralateral SI joint. Bilateral stabilization is necessary to significantly reduce the ROM for both SI joints.

Shi's Daoyin Therapy for Neck Pain: A Randomized Controlled Trial.

Objective To compare the immediate and short term effectiveness of Shi's Daoyin therapy (DT) rather than the Melbourne Protocol (MP) in terms of pain, mobility, and isometric strength of cervical muscles in nonacute nonspecific neck pain patients. Material and Methods A total of 114 nonacute nonspecific neck pain patients aged 20~50 years were recruited and randomly assigned to be treated by either Shi's DT or the MP. 56 cases and 54 cases received treatment for 3 weeks and were evaluated before and after intervention and at 3-week follow-up in Shi's DT group and MP group, respectively. The outcome measures were Chinese version of the Neck Disability Index (NDI), cervical range of motion (ROM), maximal voluntary isometric force (MVIF), and pain intensity (Numeric Pain Rating Scale, NPRS). Results All outcomes of both groups showed statistically significant improvements after the intervention and at 3-week follow-up (P < 0.05), while no statistically significant difference was found in NDI between groups. When followed up after 3 weeks, the ROM in axial rotation was significantly greater in the Shi's DT group (P < 0.05), and the NPRS in the Shi's DT group was significantly lower than the MP group (P < 0.05). At the end of the treatment period, the MVIF in lateral bending in the Shi's DT group had a lower value (P = 0.044) than in the MP group, but there was no significant difference in flexion and extension between the two groups. Conclusions Both Shi's DT and MP groups demonstrated an obvious reduction in pain intensity and improvements in neck mobility after a short term follow-up period. The improvement of Shi's DT in disability and pain during functional activities is generally similar to that of the MP for the treatment of nonacute nonspecific neck pain.

Does Resection of the Posterior Longitudinal Ligament Affect the Stability of Cervical Disc Arthroplasty?

The need for posterior longitudinal ligament (PLL) resection during cervical total disc arthroplasty (TDA) has been debated. The purpose of this laboratory study was to investigate the effect of PLL resection on cervical kinematics after TDA. Eight cadaveric cervical spine specimens were tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) to moments of ±1.5 Nm. After testing the intact condition, anterior C5-C6 cervical discectomy was performed followed by PLL resection and implantation of a compressible, 6-degrees-of-freedom disc prosthesis (M6-C, Spinal Kinetics Inc, Sunnyvale, California). Next, a second prosthesis was implanted at C6-C7 with PLL intact. Finally, the C6-C7 PLL was resected while the disc prosthesis remained in place. Segmental range of motion (ROM) and stiffness in the high flexibility zone around the neutral posture were analyzed using repeated measures ANOVA. At C5-C6, following TDA and PLL resection, FE, LB, and AR ROMs decreased significantly. Anterior and posterior disc height, segmental lordosis, and flexion stiffness increased significantly. At C6-C7, TDA with the PLL intact resulted in a significant increase in anterior disc height and segmental lordosis with no change in posterior disc height. FE, LB, and AR ROMs all decreased significantly, while flexion stiffness increased significantly compared to intact. PLL resection at C6-C7 did not result in a notable change compared to TDA with PLL intact. At the same level, flexion stiffness decreased following PLL resection compared to TDA with a value closer to intact. Two-level TDA (C5-C7) with PLL resection did not result in a loss of segmental stability. PLL resection did not significantly affect motion segment kinematics following cervical TDA using a prosthesis with inherent stiffness. Motion segment stiffness loss after PLL resection can be compensated for by a TDA design that can provide resistance to angular motion.

Anterior Transdiscal Axial Screw Fixation for Subaxial Cervical Spine: A Biomechanical Study.

To evaluate stability of anterior transdiscal axial screw (ATAS) fixation for anterior instrumentation and to compare with standard anterior cervical decompression and fusion and plate (ACDFP) fixation in human subaxial cervical spine. Flexibility tests were conducted on 7 cadaveric specimens (C5-T1) in an intact and injured state and instrumented with ACDFP fixation, ATAS fixation, and ACDFP plus ATAS fixation at the C6-7 segment after section of the anterior and posterior longitudinal ligaments and discectomy. A pure moment of ±2.0 N-m was applied to the specimen in flexion-extension, lateral bending, and axial rotation. Range of motion (ROM) and neutral zone were calculated for the C6-7 segment. ROM was reduced significantly compared with the intact or injured condition for 3 configurations under all motions. ATAS fixation resulted in similar ROM in C6-7 compared with ACDFP fixation in flexion (2.7° vs. 2.6°, P=0.419), extension (2.7° vs. 2.1°, P= 0.152), and lateral bending (4.6° vs. 4.2°, P= 0.295) but larger ROM in axial rotation (6.1° vs. 5.3°, P= 0.014). When combined with an anterior plate, ATAS fixation reduced ROM to 1.2° in flexion, 1.1° in extension, 3.3° in lateral bending, and 3.8° in axial rotation, which were significantly smaller than ACDFP or ATAS fixation alone. ATAS fixation is a biomechanically effective alternative or supplemental method of anterior fixation in subaxial cervical spine.

Incorporating ligament laxity in a finite element model for the upper cervical spine

Background ContextPredicting physiological range of motion (ROM) using a finite element (FE) model of the upper cervical spine requires the incorporation of ligament laxity. The effect of ligament laxity can be observed only on a macro level of joint motion and is lost once ligaments have been dissected and preconditioned for experimental testing. As a result, although ligament laxity values are recognized to exist, specific values are not directly available in the literature for use in FE models. PurposeThe purpose of the current study is to propose an optimization process that can be used to determine a set of ligament laxity values for upper cervical spine FE models. Furthermore, an FE model that includes ligament laxity is applied, and the resulting ROM values are compared with experimental data for physiological ROM, as well as experimental data for the increase in ROM when a Type II odontoid fracture is introduced. Design/SettingThe upper cervical spine FE model was adapted from a 50th percentile male full-body model developed with the Global Human Body Models Consortium (GHBMC). FE modeling was performed in LS-DYNA and LS-OPT (Livermore Software Technology Group) was used for ligament laxity optimization. MethodsOrdinate-based curve matching was used to minimize the mean squared error (MSE) between computed load-rotation curves and experimental load-rotation curves under flexion, extension, and axial rotation with pure moment loads from 0 to 3.5 Nm. Lateral bending was excluded from the optimization because the upper cervical spine was considered to be primarily responsible for flexion, extension, and axial rotation. Based on recommendations from the literature, four varying inputs representing laxity in select ligaments were optimized to minimize the MSE. Funding was provided by the Natural Sciences and Engineering Research Council of Canada as well as GHMBC. The present study was funded by the Natural Sciences and Engineering Research Council of Canada to support the work of one graduate student. There are no conflicts of interest to be reported. ResultsThe MSE was reduced to 0.28 in the FE model with optimized ligament laxity compared with an MSE 0f 4.16 in the FE model without laxity. In all load cases, incorporating ligament laxity improved the agreement between the ROM of the FE model and the ROM of the experimental data. The ROM for axial rotation and extension was within one standard deviation of the experimental data. The ROM for flexion and lateral bending was outside one standard deviation of the experimental data, but a compromise was required to use one set of ligament laxity values to achieve a best fit to all load cases. Atlanto-occipital motion was compared as a ratio to overall ROM, and only in extension did the inclusion of ligament laxity not improve the agreement. After a Type II odontoid fracture was incorporated into the model, the increase in ROM was consistent with experimental data from the literature. ConclusionsThe optimization approach used in this study provided values for ligament laxities that, when incorporated into the FE model, generally improved the ROM response when compared with experimental data. Successfully modeling a Type II odontoid fracture showcased the robustness of the FE model, which can now be used in future biomechanics studies.

Treatment of thoracolumbar burst fractures by short-segment pedicle screw fixation using a combination of two additional pedicle screws and vertebroplasty at the level of the fracture: a finite element analysis

BackgroundTraditional one-above and one-below four-screw posterior short-segment instrumentation is used for unstable thoracolumbar burst fractures. However, this method has a high rate of implant failure and early loss of reduction. The purpose of this study was to use finite element (FE) analysis to determine the effect of treating thoracolumbar burst fractures by short-segment pedicle screw fixation using a combination of two additional pedicle screws and vertebroplasty at the level of the fracture.MethodsAn intact T11-L1 spine FE model was created from the computed tomography images of a male subject. Four fixation models with posterior fusion devices (pedicle screws, rods, cross-link) were established to simulate an unstable thoracolumbar fracture with different fusion surgeries: short-segment fixation with: 1) a link (S-L); 2) intermediate bilateral screws (S-I); 3) a link and calcium sulfate cement (S-L-C); 4) intermediate bilateral screws and calcium sulfate cement (S-I-C). Different loading conditions (flexion, extension, lateral bending, and axial rotation) were applied on the models and analyzed with a FE package. The range of motion (ROM), and the maximum value and distribution of the implant stress, and the stress in the facet joint, were compared between the intact and fixation models.ResultsThe ROM in flexion, extension, axial rotation, and lateral bending was the smallest in the S-I-C model, followed by the S-I, S-L-C, and S-L models. Maximum von Mises stress values were larger under lateral bending and axial rotation loadings than under flexion and extension loading. High stress was concentrated at the crosslink and rod junctions. Maximal von Mises stress on the superior vertebral body for all loading conditions was larger than that on the inferior vertebral body. The maximal von Mises stress of the pedicle screws during all states of motion were 265.3 MPa in S-L fixation, 192.9 MPa in S-I fixation, 258.4 MPa in S-L-C fixation, and 162.3 MPa in S-I-C fixation.ConclusionsShort-segment fixation with two intermediate pedicle screws together with calcium sulfate cement at the fractured vertebrae may provide a stiffer construct and less von Mises stress of the pedicle screws and rods as compared to other types of short-segment fixation.

Biomechanical Stability of a Stand-Alone Interbody Spacer in Two-Level and Hybrid Cervical Fusion Constructs.

Study Design:In vitro human cadaveric biomechanical analysis.Objective:To evaluate the segmental stability of a stand-alone spacer (SAS) device compared with the traditional anterior cervical plate (ACP) construct in the setting of a 2-level cervical fusion construct or as a hybrid construct adjacent to a previous 1-level ACP construct.Methods:Twelve human cadaveric cervical spines (C2-T1) were nondestructively tested with a custom 6-degree-of-freedom spine simulator under axial rotation (AR), flexion-extension (FE), and lateral bending (LB) at 1.5 N m loads. After intact analysis, each specimen underwent instrumentation and testing in the following 3 configurations, with each specimen randomized to the order of construct: (A) C5-7 SAS; (B) C5-6 ACP, and C6-7 SAS (hybrid); (C) C5-7 ACP. Full range of motion (ROM) data at C5-C7 was obtained and analyzed by each loading modality utilizing mean comparisons with repeated measures analysis of variance with Sidak correction for multiple comparisons.Results:Compared with the intact specimen, all tested constructs had significantly increased segmental stability at C5-C7 in AR and FE ROM, with no difference in LB ROM. At C5-C6, all test constructs again had increased segmental stability in FE ROM compared with intact (10.9° ± 4.4° Intact vs SAS 6.6° ± 3.2°, P < .001; vs.Hybrid 2.9° ± 2.0°, P = .005; vs ACP 2.1° ± 1.4°, P < .001), but had no difference in AR and LB ROM. Analysis of C6-C7 ROM demonstrated all test groups had significantly greater segmental stability in FE ROM compared with intact (9.6° ± 2.7° Intact vs SAS 5.0° ± 3.0°, P = .018; vs Hybrid 5.0° ± 2.7°, P = .018; vs ACP 4.4° ± 5.2°, P = .005). Only the hybrid and 2-level ACP constructs had increased stability at C6-C7 in AR ROM compared with intact, with no difference for all test groups in LB ROM. Comparison between test constructs demonstrated no difference in C5-C7 and C6-C7 segmental stability in all planes of motion. However, at C5-C6 comparison between test constructs found the 2-level SAS had significantly less segmental stability compared to the hybrid (6.6° ± 3.2° vs 2.9° ± 2.0°, P = .025) and ACP (6.6° ± 3.2° vs 2.1° ± 1.4°, P = .004).Conclusions:Our study found the currently tested SAS device may be a reasonable option as part of a 2-level hybrid construct, when used below an adjacent 1-level ACP, but should be used with careful consideration as a 2-level SAS construct. Consequences of decreased segmental stability in FE are unknown; however, optimal immediate fixation stability is an important surgical principle to avoid loss of fixation, segmental kyphosis, interbody graft subsidence, and pseudarthrosis.

Guidance value of TOI classification for treatment of traumatic T-type atlantoaxial dislocation

Objective To investigate the guidance value of TOI classification in treating traumatic T-type atlantoaxial dislocation (ADD). Methods A retrospective case series study was made on 32 cases of traumatic TOI T-type ADD treated between January 2012 and December 2015. There were 19 males and 13 females, aged (38.4±14.7)years. Fifteen cases of T1-type underwent external fixation or internal fixation without fusion, while 17 cases of T2-type underwent internal fixation with fusion. Symon-Lavender clinical standard, Japanese orthopedic association score (JOA), visual analogue scale (VAS), atlas-dens interval (ADI) and space available for the cord (SAC) were used to evaluate the therapeutic effect. Results Patients were followed up for 6-54 months (mean, 32.4 months). At final follow-up, ADI was decreased to (2.3±1.4)mm from preoperative (5.6±1.6)mm, but SAC was increased to (15.4±1.9)mm from preoperative (12.0±2.9)mm (P<0.01). At final follow-up, cervical axial rotation range of motion was 102°-154° in T1-type cases and 57°- 93° in T2-type cases. Range of motion for atlantoaxial joint was preserved in T1-type cases, but lost in T2-type cases. According to the Symon-Lavender clinical standard, there were 14 cases of mild disability, nine moderate disability, eight severe disability and one extremely severe disability before operation; there were 21 cases of mild disability, nine moderate disability and two severe disability at last follow-up. Significant difference was observed in the grades according to the Symon-Lavender clinical standard before operation and at last follow-up (P<0.05). At last follow-up, JOA score was increased to (14.6±2.9)points from preoperative (9.9±3.2)points, and VAS was decreased to (2.7±1.3)points from preoperative (6.0±1.6)points (P<0.01). Conclusions By using TOI classification, reconstruction of stability and improved neurological function can be achieved in treatment of traumatic T-type atlantoaxial dislocation. Non-fusion treatment of T1-type atlantoaxial dislocation can preserve range of motion for atlantoaxial joint. Key words: Atlanto-axial joint; Dislocations; Classification