Papers by Bryan Cunningham
Static and Cyclical Biomechanical Analysis of Pedicle Screw Spinal Constructs
Spine, Sep 1, 1993
Biomechanical evaluation of twelve different spinal devices in vitro employing pedicle screws was... more Biomechanical evaluation of twelve different spinal devices in vitro employing pedicle screws was performed using static (n = 5) and cyclical testing (n = 3) parameters. In general, the rank order of implant failures was similar between static and cyclical tests, performed at 600 N compressive load, 5 Hz, and 1 million cycles. The mean number of cycles to failure was higher for spinal instrumentation systems employing longitudinal rods than those using plates (ANOVA F = 16.94, P < .001). At 600 N, the compact Cotrel-Dubousset, TSRH, and ISOLA rod systems demonstrated mean cycles to failure ranging from 200,000 to 800,000 cycles. The remaining devices including Dyna-lok, Kirschner plate, and VSP devices had failures ranging from 50,000 to 210,000 cycles. Polyethylene cylinders representing vertebral bodies were used to eliminate the problems of biologic deterioration encountered with cadaveric spines (a full cyclical test to 1 million cycles required 56 hours), and thus to provide analysis of the weak portion of each spinal system. The failure of eleven of the twelve spinal systems occurred by fracture of a pedicle screw, most commonly at the junction of the upper screw thread and the collar (Kirschner, AO fixator, standard CD, ISOLA, and TSRH). However, in Dynalok and VSP systems, fracture of the threaded portion of the screw just posterior to the integral nuts was the most common screw fracture location. The compact CD system was the only spinal implant that consistently failed by fracture of the longitudinal spinal member (rod). The fatigue life of rod based systems was longer than plate based systems. These studies confirm the importance of anterior column load sharing (vertebral body, corpectomy bone graft) as the mean bending strength demonstrated by these implant systems was not inordinately high using this "worst case scenario" model.
A Prospective, Randomized, Multicenter Food and Drug Administration Investigational Device Exemption Study of Lumbar Total Disc Replacement With the CHARITÉ ™ Artificial Disc Versus Lumbar Fusion
Spine, Jul 1, 2005
Study Design. A prospective, randomized, multicenter, Food and Drug Administration-regulated Inve... more Study Design. A prospective, randomized, multicenter, Food and Drug Administration-regulated Investigational Device Exemption clinical trial. Objectives. The purpose of this study was to compare the safety and effectiveness of lumbar total disc replacement, using ...
Biomechanical and anatomical considerations in lumbar spinous process fixation—an in vitro human cadaveric model
The Spine Journal, Sep 1, 2014
Although multiple mechanisms of device attachment to the spinous processes exist, there is a pauc... more Although multiple mechanisms of device attachment to the spinous processes exist, there is a paucity of data regarding lumbar spinous process morphology and peak failure loads. Using an in vitro human cadaveric spine model, the primary objective of the present study was to compare the peak load and mechanisms of lumbar spinous process failure with variation in spinous process hole location and pullout direction. A secondary objective was to provide an in-depth characterization of spinous process morphology. Biomechanical and anatomical considerations in lumbar spinous process fixation using an in vitro human cadaveric model. A total of 12 intact lumbar spines were used in the current investigation. The vertebral segments (L1-L5) were randomly assigned to one of five treatment groups with variation in spinous process hole placement and pullout direction: (1) central hole placement with superior pullout (n=10), (2) central hole placement with inferior pullout (n=10), (3) inferior hole placement with inferior pullout (n=10), (4) superior hole placement with superior pullout (n=10), and (5) intact spinous process with superior pullout (n=14). A 4-mm diameter pin was placed through the hole followed by pullout testing using a material testing system. As well, the bone mineral density (BMD) (g/cm(3)) was measured for each segment. Data were quantified in terms of anatomical dimensions (mm), peak failure loads (newtons [N]), and fracture mechanisms, with linear regression analysis to identify relationships between anatomical and biomechanical data. Based on anatomical comparisons, there were significant differences between the anteroposterior and cephalocaudal dimensions of the L5 spinous process versus L1-L4 (p<.05). Statistical analysis of peak load at failure of the four reconstruction treatments and intact condition demonstrated no significant differences between treatments (range, 350-500 N) (p>.05). However, a significant linear correlation was observed between peak failure load and anteroposterior and cephalocaudal dimensions (p<.05). Correlation between BMD and peak spinous processes failure load was approaching statistical significance (p=.08). 30 of 54 specimens failed via direct pullout (plow through), whereas 8 of 54 specimens demonstrated spinous process fracture. The remaining cases failed via plow through followed by fracture of the spinous process (16 of 54; 29%). The present study demonstrated that variation in spinous process hole placement did not significantly influence failure load. However, there was a strong linear correlation between peak failure load and the anteroposterior and cephalocaudal anatomical dimensions. From a clinical standpoint, the findings of the present study indicate that attachment through the spinous process provides a viable alternative to attachment around the spinous processes. In addition, the anatomical dimensions of the lumbar spinous processes have a greater influence on biomechanical fixation than either hole location or BMD.
Maturation of the Posterolateral Spinal Fusion and Its Effect on Load-Sharing of Spinal Instrumentation
Journal of Bone and Joint Surgery, American Volume, Nov 1, 1997
We investigated the temporal relationship among the biomechanical, radiographic, and histological... more We investigated the temporal relationship among the biomechanical, radiographic, and histological properties of a posterolateral spinal fusion mass to elucidate the changes in load-sharing of the spinal instrumentation and that of the fusion mass throughout the healing process. Destabilization of the posterior spinal column and transpedicular screw fixation at the segments between the third and fourth and the fifth and sixth lumbar vertebrae was performed in twenty-four sheep. A posterolateral spinal arthrodesis with use of autologous corticocancellous bone graft was done randomly at one of the two segments; the other segment (without bone graft) served as the instrumented control. Six animals each were killed at four, eight, twelve, and sixteen weeks postoperatively. Biomechanical testing showed that the posterolateral fusion mass had increased mechanical stiffness after the fourth week. The strain on the hardware, measured with use of rods instrumented with strain-gauges, decreased significantly (p < 0.01) beginning at eight weeks. Radiographically, three independent observations of each of the six animals at each time-period showed that, although all of the fusion masses were considered solid unions at sixteen weeks, bridging of trabecular bone was noted during only ten of eighteen observations at twelve weeks, three of eighteen observations at eight weeks, and none of eighteen observations at four weeks. Computerized tomography and histomorphometric analyses demonstrated that mineralization in the fusion mass increased in a linear fashion even after eight weeks. Histologically, the fusion mass consisted predominantly of woven bone at eight weeks; thereafter, it was gradually trabeculated. We found a great discrepancy between biomechanical stability and histological maturation of the posterolateral fusion mass. The biomechanical properties of a stable spinal fusion preceded the radiographic appearance of a solid fusion by at least eight weeks, suggesting that immature woven bone provided substantial stiffness to the fusion mass. The spinal instrumentation was subjected predominantly to bending stress rather than to axial stress, and the load-sharing of the spinal instrumentation decreased concurrently with the development of the spinal fusion.
Adjacent Level Intradiscal Pressure and Segmental Kinematics Following A Cervical Total Disc Arthroplasty
Spine, May 1, 2005
In vitro investigation of cervical adjacent level intradiscal pressures (IDPs) following a total ... more In vitro investigation of cervical adjacent level intradiscal pressures (IDPs) following a total disc replacement arthroplasty. The current in vitro study was undertaken to compare adjacent level IDPs and operative level kinematics following a cervical arthroplasty versus an arthrodesis procedure. Clinical data indicate the incidence of symptomatic transition syndrome to be as high as 3% annually following a cervical interbody arthrodesis. Recent developments in the motion preservation technology should, in theory, minimize transition syndrome at the adjacent levels. A total of 10 human cadaveric cervical spines were used in this investigation. Following intact analysis, all specimens were sequentially reconstructed at C5-C6 with 1) total disc replacement (TDR), 2) allograft dowel, and 3) allograft dowel + anterior cervical plate. Testing was performed in displacement control under axial rotation, flexion/extension, and lateral bending loading modes. IDPs were recorded at C4-C5 and C6-C7 whereas peak range of motion (ROM) and NZ were monitored at C5-C6 level. Similar IDPs were recorded between the intact condition and a TDR reconstruction at both adjacent levels under all loading modes (P > 0.05). However, the C4-C5 IDP values produced under flexion/extension testing for both arthrodesis treatments were significantly higher than the means obtained for the intact and disc replacement groups (P < 0.05). Similar intergroup differences were observed at the C6-C7 level; however, statistical significance was achieved during all three loading methods (P < 0.05). C5-C6 ROM analysis indicated a significantly lower ROM for both arthrodesis constructs compared with intact and TDR groups during flexion/extension testing (P < 0.05). No differences were recorded between the intact and the total disc replacement group under any loading conditions (P > 0.05). This is a first study to document that a cervical disc replacement arthroplasty procedure maintains adjacent level IDPs and reconstruction level kinematics near the preoperative values. Consequently, total disc replacement may provide an alternative to conventional surgical management of cervical discogenic pathology decreasing the incidence of symptomatic transition syndrome.
Regeneration of a Spinal Ligament after Total Lumbar Disk Arthroplasty in Primates
Cells Tissues Organs, 2009
Total disk arthroplasty (TDA) is a new procedure that replaces the intervertebral disk space with... more Total disk arthroplasty (TDA) is a new procedure that replaces the intervertebral disk space with an artificial motion segment and necessitates the resection of the anterior longitudinal ligament (ALL). We assessed whether a collagen-based graft made from porcine small-intestine submucosa (SIS) can be used as a regenerative scaffold to restore the function and structure of the ALL in the lumbar spine. A total of 10 mature male baboons underwent TDA at L5–L6 using one of two treatments: (1) TDA only (n = 5) or (2) TDA combined with SIS (n = 5). Six months postoperatively, mock revision surgery was performed to assess tissue adhesions followed by non-destructive multidirectional flexibility testing of the spinal segment. The vertebral segments were then processed for histology. The tissue adhesion score was 2.8 ± 0.8 in the TDA only group and 1.8 ± 1.4 in the TDA-SIS group (p = 0.2). Segmental range of motion and the length of the neutral zone were similar in both groups. Histology showed that the SIS scaffold led to an organized ligamentous structure with a significantly (p = 0.027) higher thickness (2.18 ± 0.25 mm) compared to the connective tissue structure in the TDA-only group (1.66 ± 0.33 mm). We concluded that using a SIS bioscaffold after TDA did not lead to increased great vessel adhesion while its use facilitated the formation of highly organized ligamentous tissues. However, the SIS- induced and newly formed ligamentous tissue anterior to the spinal segment did not lead to a measurable limitation of spinal extension.
Comparative fixation methods of cervical disc arthroplasty versus conventional methods of anterior cervical arthrodesis: serration, teeth, keels, or screws?
Journal of neurosurgery, Feb 1, 2010
Object Using a synthetic vertebral model, the authors quantified the comparative fixation strengt... more Object Using a synthetic vertebral model, the authors quantified the comparative fixation strengths and failure mechanisms of 6 cervical disc arthroplasty devices versus 2 conventional methods of cervical arthrodesis, highlighting biomechanical advantages of prosthetic endplate fixation properties. Methods Eight cervical implant configurations were evaluated in the current investigation: 1) PCM Low Profile; 2) PCM V-Teeth; 3) PCM Modular Flange; 4) PCM Fixed Flange; 5) Prestige LP; 6) Kineflex/C disc; 7) anterior cervical plate + interbody cage; and 8) tricortical iliac crest. All PCM treatments contained a serrated implant surface (0.4 mm). The PCM V-Teeth and Prestige contained 2 additional rows of teeth, which were 1 mm and 2 mm high, respectively. The PCM Modular and Fixed Flanged devices and anterior cervical plate were augmented with 4 vertebral screws. Eight pullout tests were performed for each of the 8 conditions by using a synthetic fixation model consisting of solid rigid polyurethane foam blocks. Biomechanical testing was conducted using an 858 Bionix test system configured with an unconstrained testing platform. Implants were positioned between testing blocks, using a compressive preload of −267 N. Tensile load-to-failure testing was performed at 2.5 mm/second, with quantification of peak load at failure (in Newtons), implant surface area (in square millimeters), and failure mechanisms. Results The mean loads at failure for the 8 implants were as follows: 257.4 ± 28.54 for the PCM Low Profile; 308.8 ± 15.31 for PCM V-Teeth; 496.36 ± 40.01 for PCM Modular Flange; 528.03± 127.8 for PCM Fixed Flange; 306.4 ± 31.3 for Prestige LP; 286.9 ± 18.4 for Kineflex/C disc; 635.53 ± 112.62 for anterior cervical plate + interbody cage; and 161.61 ± 16.58 for tricortical iliac crest. The anterior plate exhibited the highest load at failure compared with all other treatments (p < 0.05). The PCM Modular and Fixed Flange PCM constructs in which screw fixation was used exhibited higher pullout loads than all other treatments except the anterior plate (p < 0.05). The PCM VTeeth and Prestige and Kineflex/C implants exhibited higher pullout loads than the PCM Low Profile and tricortical iliac crest (p < 0.05). Tricortical iliac crest exhibited the lowest pullout strength, which was different from all other treatments (p < 0.05). The surface area of endplate contact, measuring 300 mm2 (PCM treatments), 275 mm2 (Prestige LP), 250 mm2 (Kineflex/C disc), 180 mm2 (plate + cage), and 235 mm2 (tricortical iliac crest), did not correlate with pullout strength (p > 0.05). The PCM, Prestige, and Kineflex constructs, which did not use screw fixation, all failed by direct pullout. Screw fixation devices, including anterior plates, led to test block fracture, and tricortical iliac crest failed by direct pullout. Conclusions These results demonstrate a continuum of fixation strength based on prosthetic endplate design. Disc arthroplasty constructs implanted using vertebral body screw fixation exhibited the highest pullout strength. Prosthetic endplates containing toothed ridges (≥ 1 mm) or keels placed second in fixation strength, whereas endplates containing serrated edges exhibited the lowest fixation strength. All treatments exhibited greater fixation strength than conventional tricortical iliac crest. The current study offers insights into the benefits of various prosthetic endplate designs, which may potentially improve acute fixation following cervical disc arthroplasty.
Biomechanical comparison of single- and two-level cervical arthroplasty versus arthrodesis: effect on adjacent-level spinal kinematics
The Spine Journal, Apr 1, 2010
The use of motion-preserving spinal implants versus conventional arthrodesis instrumentation syst... more The use of motion-preserving spinal implants versus conventional arthrodesis instrumentation systems, which stabilize operative segments, necessitates improved understanding of their effect on spinal kinematics and the biomechanically optimal method for surgical reconstruction. The primary objective of this study was to measure operative- and adjacent-level kinematics after single- and two-level cervical arthroplasty and compare them with those after anterior cervical arthrodesis. A secondary objective was to locate the centers of intervertebral rotation at the operative and adjacent levels after arthroplasty and compare them to those after arthrodesis. This biomechanical study used an in vitro human cadaveric model to compare the multidirectional flexibility kinematics of single- versus two-level cervical disc arthroplasty reconstructions. Eight cadaveric cervical spines (C2-T2) were biomechanically evaluated between Levels C4 and T1 in the intact condition and under the following reconstructions: single-level arthroplasty (C6-C7) using porous coated motion (PCM) device; single-level arthrodesis (C6-C7) using interbody cage with anterior plate; two-level arthroplasty (C5-C7) using PCM devices; two-level hybrid treatment of arthroplasty (C5-C6) using PCM device and arthrodesis (C6-C7) using cage/plate; and two-level arthrodesis (C5-C7) using cage/plate. Multidirectional flexibility testing used the Panjabi hybrid testing protocol, including pure moments for the intact condition with overall spinal motion replicated under displacement control for subsequent reconstructions. Unconstrained intact moments of +/-3.0 Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM) and neutral zone. The calculated centers of intervertebral rotation were compared for all intervertebral levels under flexion-extension conditions. Axial rotation loading demonstrated a significant decrease in the C6-C7 ROM for the single-level arthrodesis group compared with the intact spine and the single-level arthroplasty group (p<.05). No differences were observed between the intact and single-level arthroplasty groups (p>.05). For the two-level hybrid treatment group, the C5-C6 ROM significantly increased compared with the intact, single-level arthroplasty, and two-level arthrodesis groups (p<.05). Moreover, a significant increase was observed in the adjacent-level (C7-T1) ROM for the two-level arthrodesis group compared with all other treatment groups (p<.05). Under flexion-extension, no differences were observed in C6-C7 ROM between the intact spine and single-level arthroplasty groups (p>.05). However, as expected, the single-level arthrodesis and two-level hybrid treatment groups demonstrated a decreased ROM at C6-C7 versus the intact spine and arthroplasty treatments (p<.05). In terms of adjacent-level effects, two-level arthrodesis (C5-C7) led to increased ROM in the inferior level (C7-T1) in axial rotation and flexion-extension compared with the intact spine and all other treatment groups (p<0.05). Lateral bending loading conditions demonstrated no significant difference among the treatment groups (p>.05). In flexion-extension, the centers of intervertebral rotation for the intact spine and single-level arthroplasty groups were localized in the central to posterior one-third of the inferior vertebral body for each motion segment: C5-C6, C6-C7, and C7-T1. The single-level arthrodesis group produced more diffuse centers of rotation, particularly at the operative (C6-C7) and inferior adjacent levels (C7-T1). This study highlights the biomechanical effects of single- and two-level cervical arthroplasty versus single- and two-level arthrodesis on four functional spinal levels (C4-T1). Operative-level ROM was preserved with single- and two-level arthroplasty under all loading modes. The distal adjacent level (C7-T1) demonstrated the greatest increase among the four levels in ROM compared with the intact condition after two-level arthrodesis. These kinematic findings were corroborated by changes in the adjacent-level centers of rotation after arthrodesis and…
An In Vitro Human Cadaveric Study Investigating the Biomechanical Properties of the Thoracic Spine
Spine, Feb 1, 2002
An in vitro human cadaveric study comparing the effects of anterior and posterior sequential dest... more An in vitro human cadaveric study comparing the effects of anterior and posterior sequential destabilization conditions on thoracic functional unit mechanics was studied. To investigate the biomechanical properties of the human thoracic spine. Few studies have addressed the mechanical role of the costovertebral joints under torsion in the stability of the human thoracic spine. Sixteen functional spinal units with intact costovertebral joints were obtained from six human cadavers and randomized into two groups based on destabilization procedures: Group 1, anterior to posterior sequential resection; and Group 2, posterior to anterior sequential destabilization. Biomechanical testing was performed after each destabilization procedure, and the range of motion under maximum load was calculated. Group 1: Under flexion-extension, lateral bending, and axial rotation loading, discectomy increased the range of motion by 193%, 74%, and 111%, respectively. Moreover, subsequent right rib head resection further increased the range of motion by 81%, 84%, and 72%, respectively. Group 2: Under all loading conditions laminectomy + medial facetectomy resulted in a 22-30% increase in range of motion. Subsequent total facetectomy led to an additional 15-28% increase in range of motion. The rib head joints serve as stabilizing structures to the human thoracic spine in the sagittal, coronal, and transverse planes. In anterior scoliosis surgery additional rib head resection after discectomy may achieve greater curve and rib hump correction. The lateral portion of the facet joints plays an important role in providing spinal stability and should be preserved to minimize postoperative kyphotic deformity and segmental instability when performing decompressive wide laminectomy.
Upsala Journal of Medical Sciences, Mar 6, 2015
Purpose. Anterior plate fixation has been reported to provide satisfactory results in cervical sp... more Purpose. Anterior plate fixation has been reported to provide satisfactory results in cervical spine distractive flexion (DF) injuries stages 1 and 2, but will result in a substantial failure rate in more unstable stage 3 and above. The aim of this investigation was to determine the biomechanical properties of different fixation techniques in a DF-3 injury model where all structures responsible for the posterior tension band mechanism are torn. Methods. The multidirectional three-dimensional stiffness of the subaxial cervical spine was measured in eight cadaveric specimens with a simulated DF-3 injury at C5-C6, stabilized with four different fixation techniques: anterior plate alone, anterior plate combined with posterior wire, transarticular facet screws, and a pedicle screw-rod construct, respectively. Results. The anterior plate alone did not improve stability compared to the intact spine condition, thus allowing considerable range of motion around all three cardinal axes (p > 0.05). The anterior plate combined with posterior wire technique improved flexion-extension stiffness (p = 0.023), but not in axial rotation and lateral bending. When the anterior plate was combined with transarticular facet screws or with a pedicle screws-rod instrumentation, the stability improved in flexion-extension, lateral bending, and in axial rotation (p < 0.05). Conclusions. These findings imply that the use of anterior fixation alone is insufficient for fixation of the highly unstable DF-3 injury. In these situations, the use of anterior fixation combined with a competent posterior tension band reconstruction (e.g. transarticular screws or a posterior pedicle screws-rod device) improves segmental stability.
The Effects of Rigid Spinal Instrumentation and Solid Bony Fusion on Spinal Kinematics
Spine, Apr 1, 1998
Spinal kinematics after the implementation of rigid spinal instrumentation or the achievement of ... more Spinal kinematics after the implementation of rigid spinal instrumentation or the achievement of a solid fusion was studied using a sheep posterolateral spinal arthrodesis model. To investigate the effects of rigid spinal instrumentation or solid fusion on spinal kinematic parameters. Numerous studies have attempted to define spinal instability in terms of kinematics. Recent in vitro studies have documented the neutral zone, or a measure of spinal laxity, as more sensitive to spinal instability than the range of motion. Seven skeletally mature sheep underwent a single-level posterolateral lumbar arthrodesis using autologous bone graft augmented with transpedicular screw fixation. The animals were killed 4 months after surgery. The identical surgical procedures were performed in seven sheep cadaveric spines, which served as acute postoperative controls. Each functional spinal unit was tested biomechanically before and after hardware removal. The experimental control groups consisted of destabilized spines and spines that underwent transpedicular screw fixation alone, whereas the fusion groups included spines that underwent posterolateral fusion alone or posterolateral fusion with instrumentation. Rigid instrumentation and solid fusion significantly decreased the neutral zone and range of motion in all testing modes. In axial rotation and lateral bending, solid fusion reduced the range of motion significantly more than transpedicular screw fixation alone. However, in all testing modes, the neutral zones showed no statistical difference between transpedicular screw fixation alone and fusion groups. The range of motion was an equivalent or better indicator of fixation or fusion stability compared with the neutral zone. Moreover, the immediate postoperative fixation stability, even if using transpedicular screw fixation, was less than the stability present after a solid fusion.
The Use of Interbody Cage Devices for Spinal Deformity: A Biomechanical Perspective
Clinical Orthopaedics and Related Research, 2002
Spinal instrumentation has revolutionized the treatment of spinal deformities and offers a pletho... more Spinal instrumentation has revolutionized the treatment of spinal deformities and offers a plethora of techniques and designs to surgically treat deformity conditions. The authors address the biomechanical properties afforded by various posterior and anterior spinal instrumentation mechanisms, with and without intervertebral reconstruction, and the principles associated with optimal reconstruction techniques. The integration of multiple strategies can improve anterior and posterior construct stiffness in the treatment of spinal deformities. Structural interbody support probably is the best method to minimize longitudinal rod and screw-bone interface strain. Moreover, anterior load-bearing structural grafts and interbody devices have been shown to increase construct stiffness, decrease the incidence posterior implant failure, permit the use of smaller diameter longitudinal rods, and may enhance the rate of successful spinal arthrodesis. From a biomechanical standpoint, treatment of medium to high-grade spondylolisthesis with stand-alone interbody or transvertebral cages, in the absence of supplemental posterior fixation, is contraindicated. Collectively, the included studies reinforce the principles of load sharing between the anterior and posterior spinal columns and affirm the biomechanical dominance of anterior column support in circumferential spinal arthrodesis.
Biomechanical Analysis of Rotational Motions After Disc Arthroplasty
Spine, Sep 1, 2006
An anatomic and biomechanical bench-top basic scientific comparative analysis to determine the ap... more An anatomic and biomechanical bench-top basic scientific comparative analysis to determine the appropriateness of total disc replacement (TDR) in a lumbar spine with scoliotic tendencies. Only limited data are currently available studying the application of disc replacement adjacent to scoliosis fusions. Theoretically, motion preservation should help delay the continuum of lumbar degeneration adjacent to scoliosis fusions and rotationally unstable lumbar segments. As a tertiary referral center for failed TDR, we noticed an alarming number of lumbar spinal rotational iatrogenic instability patterns but none occurring in the cervical spine. It is appropriate to analyze the bench-top rotational stability of disc replacement to predict whether this new technology is feasible for a larger prospective clinical study in the treatment of degenerative scoliosis. Measurements were taken from 60 human specimens from the Hamann-Todd Osteological Collection: 1) to determine the rotational arc of influence (AOI) = the angle formed from the center of axial rotation to the outermost extent of the facet joints; and 2) to determine the relative anatomic size discrepancy between the left and right facets proportionately with the cross-sectional area of the intervertebral disc = facet/endplate ratio (FER). Biomechanical testing was performed using fresh frozen human cadaveric spines with the following conditions to determine the rotational stability: 1) intact; 2) resection of ALL, anulus, disc, and PLL simulating the preparation for a TDR; 3) a more radical anular resection; 4) entire 360 degrees anular resection; and 4) insertion of the respective unconstrained-type disc replacement. Using a 6 degrees of freedom spine simulator, unconstrained pure moments of +/-8.0 Nm (lumbar) and +/-3.0 Nm (cervical) were used for axial rotation with quantification of the operative level range of motion and neutral zone, with data normalized to the intact spine condition. There were anatomic limitations in the lumbar spine that make it less desirable to apply uncon-strained disc replacements; indeed, the spine was at risk for iatrogenic lumbar scoliosis. The anulus fibrosis, anterior longitudinal ligament, and the posterior longitudinal ligament are critical structures in preventing iatrogenic scoliosis. The lumbar facet joints are more posteriorly located and are smaller relative to the intervertebral disc, compared with this association in the cervical spine. Because the facet capsular ligaments are mechanically less effective with lower tensile strength in the lumbar spine, multiple-level arthroplasty tends to accentuate scoliotic tendencies; this is independent of prosthetic design and surgical technique. Implantation of the lumbar TDR never restored the motion segment back to the rotational stability of the intact segment achieving a range of 120% to 140% rotational range of motion compared with the intact condition. This rotational instability proved to be additive as a two-level lumbar TDR resulted in between 240% and 260% increase in rotational instability compared with the intact condition. The neutral zone of the intact cervical spine was restored even using an unconstrained cervical TDR. The greater inherent rotational constraints of the cervical spine make it more amenable to stable multilevel arthroplasty compared with the lumbar spine.
Biomechanical Evaluation of Lumbosacral Reconstruction Techniques for Spondylolisthesis
Spine, Nov 1, 2002
Biomechanical evaluation of lumbosacral fixation using a porcine model. The primary objective of ... more Biomechanical evaluation of lumbosacral fixation using a porcine model. The primary objective of the current study was to quantify and compare the changes in lumbosacral range of motion produced by four different methods of surgical stabilization, as well as comparing the relative effects of the four constructs in destructive testing. The lumbosacral junction continues to be a difficult region to obtain a successful spinal arthrodesis and is one of the primary regions for construct failure. Twenty-four fresh-frozen porcine lumbosacral spines were used in this investigation. Following intact analysis, the specimens were radically destabilized at the lumbosacral junction and randomized into four treatment groups based on reconstruction: 1) L7-S1 pedicle screws alone (n = 6); 2) L7-S1 pedicle screws and interbody cage (titanium mesh) (n = 6); 3) L7-S1 pedicle screws and iliac screws (n = 6); and 4) L7-S1 pedicle/iliac screws and interbody cage (n = 6). Nondestructive, multidirectional flexibility analyses included four loading methods and fatigue component followed by a destructive flexural load to failure. Lumbosacral peak range of motion (millimeters or degrees) and ultimate failure load (Nm) of the four reconstruction techniques were statistically compared using a one-way analysis of variance combined with Fisher&amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;#39;s PLSD. 1) Axial compression: There were no differences in lumbosacral flexibility among the four treatment groups (P &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;gt; 0.05). 2) Axial rotation: Iliac screw constructs, with or without cages, decreased flexibility at the lumbosacral junction compared with pedicle screws alone (P &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 0.05). However, interbody cage reconstructions did not significantly reduce motion. 3) Flexion-extension: Iliac screws with interbody cages reduced segmental motion about the lumbosacral junction, which was significantly different from the remaining treatments (P &amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;amp;lt; 0.05). 4) Lateral bending: The iliac screw constructs afforded significantly less lumbosacral motion compared with bothpedicle screw constructs (with or without cages). With destructive testing, pullout for L7-S1…
The Stability of Reconstruction Methods After Thoracolumbar Total Spondylectomy
Spine, Aug 1, 1999
After total spondylectomy, five types of spinal reconstruction techniques were compared biomechan... more After total spondylectomy, five types of spinal reconstruction techniques were compared biomechanically. To evaluate the stability provided by five reconstruction methods after total spondylectomy. Total spondylectomy presents a worst-case scenario for spinal reconstruction. However, few investigators have biomechanically investigated spinal reconstruction stability after total spondylectomy. Eight human cadaveric spines (T11-L5) were used. After intact analysis, a total spondylectomy was performed at L2 and reconstructed using Harms titanium mesh (Depuy-Motech, Warsaw, IN) as an anterior strut. Anterior, posterior, or circumferential instrumentation techniques were then performed using the Kaneda SR and ISOLA pedicle screw systems (AcroMed Corp., Cleveland, OH) as follows: 1) anterior instrumentation at L1-L3 with multisegmental posterior instrumentation at T12-L4 (AMP), 2) anterior instrumentation at L1-L3 with short posterior instrumentation at L1-L3 (ASP), 3) anterior instrumentation at L1-L3 (A), 4) multilevel posterior instrumentation at T12-L4 (MP), and 5) short posterior instrumentation at L1-L3 (SP). Nondestructive biomechanical testing was performed under axial compression, flexion-extension, and lateral bending loading modes. Only circumferential instrumentation techniques (AMP, ASP) exhibited higher stiffness than the intact spine in all loading modes (P &lt; 0.05). Short circumferential fixation provided more stability than did multilevel posterior instrumentation (P &lt; 0.05). Multilevel posterior fixation provided more stiffness than did short posterior and anterior instrumentation alone (P &lt; 0.05). Only circumferential fixation techniques provide more stability than the intact spine in all testing modes. Short circumferential instrumentation provides more stability than multilevel posterior instrumentation alone and requires fewer levels of spinal fusion.
Biomechanical evaluation of total disc arthroplasty
The Spine Journal, Sep 1, 2002
Purpose of study: This in-vitro biomechanical study was undertaken to compare the multidirectiona... more Purpose of study: This in-vitro biomechanical study was undertaken to compare the multidirectional flexibility properties and map the instantaneous axis of rotation (IAR) of total disc arthroplasty versus conventional threaded fusion cages and cages augmented with transpedicular fixation for single-level spinal reconstruction.Methods used: A total of eight human cadaveric lumbosacral spines (L3 to sacrum) were used in this investigation and biomechanically evaluated under the following L4–L5 reconstruction conditions: 1) intact spine; 2) SB Charitè Disc Prosthesis; 3) BAK cages; 4) BAK cages + ISOLA pedicle screw/rod fixation (anteroposterior). The superior (L3–L4) and inferior (L5–S1) intervertebral levels remained unconstrained to quantify adjacent level properties. Multidirectional flexibility testing included pure, unconstrained moments (±8 Nm) in axial rotation, flexion-extension and lateral bending, with quantification of the operative and adjacent level range of motion (ROM) and neutral zone (NZ), which were normalized to the intact condition. Stepwise flexion-extension radiographs were digitized and used to calculate the intervertebral IAR for the operative and adjacent levels.of findings: The SB Charitè prosthesis indicated an average percentage increase in axial rotation ROM by 44% compared with the intact condition (p.05), whereas the BAK and anteroposterior stabilization groups resulted in significant decreases in ROM (BAK=57%, anteroposterior=93%) (p<.01) when compared with the intact and SB Charitè conditions. Based on flexion-extension radiographs, the intervertebral centers of rotation were in the posterior 33% of the operative intervertebral disc only for the SB Charitè reconstruction, with definitive evidence of physiologic intervertebral translation (6 mm).Relationship between findings and existing knowledge: The SB Charitè restored motion to the level of the intact segment in flexion-extension and lateral bending and increased motion in axial rotation. The anterior annular resection necessary for device implantation and unconstrained design of the prosthesis account for this change in rotation. The normal flexion-extension axis of rotation of a motion segment is an ellipse rather than a single point. Only disk replacement rather than pedicle instrumentation or BAK interbody instrumentation preserves the normal mapping of segmental motion at the operative level and the adjacent vertebral spinal levels.Overall significance of findings: Total disk replacement is currently the only method of restoring mobility to the degenerative lumbar disc. This serves as the first comprehensive biomechanical study comparing the operative level multidirectional flexibility characteristics between total disc arthroplasty, interbody arthrodesis cages and pedicle screw spinal instrumentation.Disclosures: Device or drug: SB Charitè Prosthesis. Status: Investigational. Device or drug: BAK/L cages. Status: Approved. Device or drug: Isola Pedicle screws and rods. Status: approved.Conflict of interest: Ryan W. Cunningham, grant research support: Waldemar Link Inc.
Does Spinal Instrumentation Influence the Healing Process of Posterolateral Spinal Fusion?
Spine, Jun 1, 1999
An in vivo sheep model was used to investigate the effect of spinal instrumentation on the healin... more An in vivo sheep model was used to investigate the effect of spinal instrumentation on the healing process of posterolateral spinal fusion. To examine the role of spinal instrumentation during the healing process of posterolateral fusion. In long bone fractures, internal fixation improves the union rate but does not accelerate the healing process. Spinal instrumentation also improves the fusion rate in spinal arthrodesis. However, it remains unclear whether the use of spinal instrumentation expedites the healing process of spinal fusion. Sixteen sheep underwent posterolateral spinal arthrodeses at L2-L3 and L4-L5 using equal amounts of autologous bone. One of those segments was selected randomly to be augmented with transpedicular screw fixation (Texas Scottish Rite Hospital spinal system). The animals were killed at 8 weeks or 16 weeks after surgery. Fusion status was evaluated by biomechanical testing, manual palpation, plain radiography, computed tomography, and histology. Instrumented fusion segments demonstrated significantly higher stiffness than did uninstrumented fusions at 8 weeks after surgery. Radiographic assessment and manual palpation showed that the use of spinal instrumentation improved the fusion rate at 8 weeks (47% versus 38% in radiographs, 86% versus 57% in manual palpation). Histologically, the instrumented fusions consisted of more woven bone than the uninstrumented fusions at 8 weeks after surgery. The 16-week-old fusion mass was diagnosed biomechanically, radiographically, and histologically as solid, regardless of pedicle screw augmentation. The current study&#39;s results demonstrated that spinal instrumentation creates a stable mechanical environment to enhance the early bone healing of spinal fusion.
Basic scientific considerations in total disc arthroplasty
The Spine Journal, Nov 1, 2004
Total disc arthroplasty serves as the next frontier in the surgical management of intervertebral ... more Total disc arthroplasty serves as the next frontier in the surgical management of intervertebral discogenic pathology. As we move from an era of interbody spinal arthrodesis to one in which segmental motion is preserved, this promising new technology offers increasing clinical and research challenges in the areas of spinal kinematics, histologic osseointegration at the prosthetic-bone interface and the effects of particulate wear debris. The primary focus of this paper is to provide a methodologic basis to investigate the spinal kinematics, histologic osseointegration and particulate wear debris after total disc arthroplasty by using in vitro and in vivo models. Part I: Using an in vitro cadaveric model, multidirectional flexibility testing evaluated the functional unit kinematics under the following L4-L5 reconstruction conditions: 1) intact spine, 2) Charite disc prosthesis, 3) BAK cages, 4) BAK cages+ISOLA pedicle screw or rod fixation (anteroposterior). Part II: A total of 27 mature baboons (n=27, Papio cynocephalus) underwent L5-L6 total disk replacement procedures to investigate the biomechanical, histochemical and biologic ingrowth characteristics of two different lumbar disc prostheses (AcroFlex and Charite) for total disc arthroplasty. Functional spinal unit fusion status was assessed by using radiographic analysis, biomechanical testing, undecalcified histopathologic and histomorphometric analyses. Part III: Using a total of 50 New Zealand white rabbits, this investigation served to quantify the neural and systemic tissue histopathologic response, after epidural application of four different types of spinal instrumentation particulate wear debris: 1) sham (control) (n=10), 2) stainless steel 316LVM (n=10), 3) titanium alloy Ti-6AL-4V (n=10), 4) cobalt chrome alloy (n=10) and 5) ultrahigh molecular weight polyethylene (UHMWPE) (n=10). In vitro multidirectional flexibility testing demonstrates the operative and adjacent level motion-preserving properties of total disc arthroplasty versus interbody arthrodesis cages and pedicle screw spinal instrumentation. To this end, disc replacement preserves the normal centrode or locus of intervertebral rotation at the operative and adjacent intervertebral spinal levels compared with conventional stabilization implants. On the basis of nonhuman primate modeling in the current studies, porous titanium interface surfaces afforded the greatest percentage of trabecular ingrowth at the prosthesis-end plate interface. In vivo segmental motion under multidirectional testing was preserved with the Charite device and slightly diminished with the AcroFlex implants. The porous ingrowth coverage at the bone-metal interface was more favorable for total disk replacement (range, 40% to 50%) compared with that reported for cementless total joint components in the appendicular skeleton (range, 10% to 30%). Direct epidural application of spinal instrumentation particulate wear debris elicits a chronic histiocytic reaction localized primarily within the epidural fibrous layers. Moreover, particles have the capacity to diffuse intrathecally, eliciting a macrophage and cytokine response within the epidural tissues, cerebrospinal fluid and spinal cord itself. Overall, on the basis of the postoperative time periods evaluated, no evidence was observed of an acute neural or systemic histopathologic response to the materials included in the current project. The implementation of dynamic spinal stabilization systems for fusionless correction of spinal deformity, dynamic posterior stabilization and total disc arthroplasty necessitates improved understanding with regard to spinal kinematics, patterns and mechanisms of histologic osseointegration and the neurohistopathologic response to particulate wear debris. Collectively, the current studies provide a methodologic basis to comprehensively evaluate these three areas.
A Prospective Randomized U.S. Fda Study of the Charité Disc Replacement – a Radiographic Outcome Analysis of 276 Consecutive Patients
Orthopaedic Proceedings, Apr 1, 2004
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