Biomechanical evaluation of stand-alone interbody fusion cages in the cervical spine.

STUDY DESIGN: An in vitro biomechanical investigation of the immediate stability in cervical reconstruction. OBJECTIVES: The purpose of this study was to compare the segmental stability afforded by the interbody fusion cage, the anterior locking plate, and the "gold standard" autograft. SUMMARY OF BACKGROUND DATA: Recently, interbody fusion cage devices have been developed and used for cervical reconstruction, but to the authors' knowledge no studies have investigated the biomechanical properties of the stand-alone interbody cage device in the cervical spine. METHODS: Using six human cervical specimens, nondestructive biomechanical testing were performed, including axial rotation (+/-1.5 Nm, 50 N preload), flexion/extension (+/-1.5 Nm) and lateral bending (+/-1.5 Nm) loading modes. After C4-C5 discectomy, each specimen was reconstructed in the following order: RABEA cage (cage), tricortical bone graft (autograft), cervical spine locking plate system (plate). Unconstrained three-dimensional segmental range of motion at C4-C5 and above and below were evaluated. RESULTS: In flexion/extension, the plate demonstrated significantly lower range of motion than did the cage and the autograft (P < 0.005), and the cage showed a significantly higher range of motion than did the intact spine (P < 0.05). Under axial rotation, the plate indicated a significantly lower range of motion than did all other groups (P < 0.05). No significant differences were indicated in lateral bending. Adjacent to C4-C5, an increased range of motion was observed. CONCLUSIONS: The increased motion adjacent to C4-C5 may provide an argument for acceleration of disc degeneration. From the biomechanical point of view, this study suggests that the cervical interbody fusion cage should be supplemented with additional external or internal supports to prevent excessive motion in flexion-extension.

The effects of hook pattern and kyphotic angulation on mechanical strength and apical rod strain in a long-segment posterior construct using a synthetic model.

STUDY DESIGN: Synthetic spine models were used to compare the effects of hook pattern and kyphotic angulation on stiffness and rod strain in long-segment posterior spinal constructs. OBJECTIVES: To examine the biomechanical effects of hook patterns and kyphotic angulation on long-segment posterior spinal constructs. SUMMARY OF BACKGROUND DATA: Kyphotic deformities managed by increasing rod diameter and hence construct stiffness have shown decreased postoperative loss of correction and hardware complications. The biomechanical effects of hook pattern and kyphosis are unknown. METHODS: Spine models of 0 degrees, 27 degrees 54 degrees sagittal contour, composed of polypropylene vertebral blocks and isoprene elastomer intervertebral spacers, representing T3-T12, were used for biomechanical testing of long-segment posterior spinal constructs. Models were instrumented with 6.35-mm titanium rods and one of the following hook configurations: 20-hook compression, 16-hook compression, 16-hook claw apex-empty,16-hook claw apex-full, or 8-hook claw. Construct stiffness and rod strain during axial compression were determined. RESULTS: The compression-hook patterns provided at least a 45% increase in construct stiffness (P = 0.013)and a 22% decrease in rod strain (P < 0.0001) compared with those obtained with the claw-hook pattern with the best biomechanical performance. When analyzing all five hook patterns, there was a 19% decrease in construct stiffness and 27% increase in rod strain when progressing from straight alignment to 27 degrees of sagittal contour (P < 0.0001). Progressing from straight alignment to 54 degrees decreased construct stiffness by 48% and increased rod strain by 55% (P < 0.0001). Construct stiffness was inversely correlated to rod strain in all five hook patterns (R2 = 0.82-0.98, P < 0.001). CONCLUSIONS: Using compressive-hook patterns and decreasing the kyphotic deformity significantly increases construct stiffness and decreases rod strain.

Biomechanical comparison of five different atlantoaxial posterior fixation techniques.

STUDY DESIGN: Five different reconstructions of the atlantoaxial complex were biomechanically compared in vitro in a nondestructive test. OBJECTIVES: To determine whether non-bone graft-dependent one-point fixation affords stability levels equivalent to three-point reconstructions. SUMMARY OF BACKGROUND DATA: Previous investigations have demonstrated that three-point fixation, using bilateral transarticular screws in combination with posterior wiring, provide the most effective resistance to minimize motion around C1-C2. However, placement of transarticular screws is technically demanding. Posterior wiring techniques affording one-point fixation have failure rates of approximately 15%, with failure considered to be secondary to structural bone graft failures. One-point, non-bone graft-dependent fixations have not been tested. METHODS: Eight human cervical specimens, C0-C3 were loaded nondestructively. Unconstrained three-dimensional segmental motion was measured. The reconstructions tested were two one-point fixations, one two-point fixation, and two three-point fixations. RESULTS: Under axial rotation two and three-point reconstructions provided better stiffness than the one-point reconstructions (P < 0.05). During flexion-extension, higher stiffness levels were observed in one- and three-point fixations when compared with the intact spine (P < 0.05). In lateral bending no significant differences were observed among the six groups, although the trend was that reconstructions including transarticular screws provided greater stability than one-point fixations. CONCLUSION: The current findings substantiate the use of three-point fixation as the treatment of choice for C1-C2 instability. [l: atlantoaxial fixation, biomechanics, cervical spine, instability, spinal instrumentation, transarticular screws]

Anterior vertebral screw strain with and without solid interspace support.

STUDY DESIGN: This in vitro biomechanical study examines segmental anterior vertebral screw strain and solid rod construct stiffness with and without the addition of multilevel, threaded cortical bone dowels in a bovine model. OBJECTIVE: To determine whether strain at the bone-screw interface is higher at the end levels during physiologic range loading, and whether solid interspace support decreases segmental strain on the implant. SUMMARY OF BACKGROUND DATA: Anterior instrumentation provides greater correction and preserves distal motion segments. However, nonunion and implant failure are observed more frequently than with posterior segmental instrumentation, and when observed, loss of fixation occurs at the end levels. METHODS: Eight calf spines underwent mechanical testing in the following sequence: 1) intact condition, 2) anterior release with anterior solid rod and bicortical rib grafts, and 3) anterior release with anterior solid rod and threaded cortical bone dowels (L2-L5). Instrumented vertebral screws were used to assess strain within the vertebral body by the near cortex, whereas an anterior extensometer spanning the instrumented segments was used to measure segmental displacements to calculate construct stiffness. The protocol included axial compression (-400 N), right lateral bending (4 Nm (Newton-meter), away from the implant), and left lateral bending (4 Nm, toward the implant). Statistical analysis included a one-way analysis of variance and a Student-Newman-Keuls post hoc test. A pilot study was performed using four additional specimens loaded for 4000 cycles to investigate macroscopic loosening after fatigue loading. RESULTS: In lateral bending toward the implant, the strain was higher at both end levels, with no differences between the rib and dowel reconstructions. The stiffness values were greater than the intact values for both groups. In lateral bending away from the implant, the strain also was higher at both end screws, and the dowel group had less strain at these levels than the rib group. Both groups were stiffer than the intact condition, and the dowel group was stiffer than the rib group. Axial compressive strain also was higher at the end levels, but this difference did not reach statistical significance. The rib group did not reach intact stiffness values, whereas the dowel group was stiffer than the intact condition. The fatigue study showed gross loosening at one or both end levels in all cases. CONCLUSIONS: Higher strain was observed at the bone-screw interface in both end screws of an anterior solid rod construct during lateral bending, which correlates with the clinically observed failure location. This suggests that physiologic range loading may predispose to failure at the end levels. Disc space augmentation with solid implants increased construct stiffness in all three load paths and decreased strain at the end levels in lateral bending away from the implant. Future implant modifications should achieve better fixation at the end screws, and the current model provides a means to compare different strategies to decrease strain at these levels.

Biomechanical properties of anterior thoracolumbar multisegmental fixation: an analysis of construct stiffness and screw-rod strain.

STUDY DESIGN: Three types of anterior thoracolumbar multisegmental fixation were biomechanically compared in construct stiffness and rod-screw strain. OBJECTIVES: To investigate the effects of rod diameter and rod number on construct stiffness and rod-screw strain in anterior thoracolumbar multisegmental instrumentation. SUMMARY OF BACKGROUND DATA: No studies have been undertaken to investigate the biomechanical effects of rod diameter and rod number in thoracolumbar anterior instrumentation. METHODS: Ten fresh-frozen calf spines (T13-L5) were used. After intact analysis, a total discectomy and transection of the ALL and PLL were performed at L1-L2, L2-L3, and L3-L4 with intervertebral reconstruction using carbon fiber cages. Three types of anterior fixation were then performed at L1-L4: 1) 4.75-mm diameter single-rod, 2) 4.75-mm dual-rod, and 3) 6.35-mm single-rod systems. Single screws at each vertebra were used for single-rod and two screws for dual-rod fixation. These systems share the same basic design except rod diameter. Nondestructive biomechanical testing was performed and included compression, torsion, flexion-extension, and lateral bending. Construct stiffness and rod-screw strain of the three reconstructions were compared. RESULTS: The 6.35-mm single-rod fixation significantly improved construct stiffness compared with the 4.75-mm single rod fixation only under torsion (P < 0.05). The 4. 75-mm dual rod construct resulted in significantly higher stiffness than did both single-rod fixations (P < 0.05), except under compression. No statistical differences were observed in rod-screw strain between the two types of single rods, whereas dual-rod reconstruction exhibited less rod-screw strain (P < 0.05). CONCLUSIONS: For single-rod fixation, increased rod diameter neither markedly improved construct stiffness nor affected rod-screw strain, indicating the limitations of a single-rod system. In thoracolumbar anterior multisegmental instrumentation, the dual-rod fixation provides higher construct stiffness and less rod-screw strain compared with single-rod fixation.

In vitro biomechanical investigation of the stability and stress-shielding effect of lumbar interbody fusion devices.

OBJECT: Interbody fusion devices are rapidly gaining acceptance as a method of ensuring lumbar interbody arthrodesis. Although different types of devices have been developed, the comparative reconstruction stability remains controversial. It also remains unclear how different stress-shielded environments are created within the devices. Using a calf spine model, this study was designed to compare the construct stiffness afforded by 11 differently designed lumbar interbody fusion devices and to quantify their stress-shielding effects by measuring pressure within the devices. METHODS: Sixty-six lumbar specimens obtained from calves were subjected to anterior interbody reconstruction at L4-5 by using one of the following interbody fusion devices: four different threaded fusion cages (BAK device, BAK Proximity, Ray TFC, and Danek TIBFD), five different nonthreaded fusion devices (oval and circular Harms cages, Brantigan PLIF and ALIF cages, and InFix device); two different types of allograft (femoral ring and bone dowel) were used. Construct stiffness was evaluated in axial compression, torsion, flexion, and lateral bending. Prior to testing, a silicon elastomer was injected into the cages and intracage pressures were measured using pressure needle transducers. CONCLUSIONS: No statistical differences were observed in construct stiffness among the threaded cages and nonthreaded devices in most of the testing modalities. Threaded fusion cages demonstrated significantly lower intracage pressures compared with nonthreaded cages and structural allografts. Compared with nonthreaded cages and structural allografts, threaded fusion cages afforded equivalent reconstruction stiffness but provided more stress-shielded environment within the devices.

The biomechanical significance of anterior column support in a simulated single-level spinal fusion.

This study examines the biomechanical effects of interbody cages and variations in posterior rod diameter in a simulated single-level spinal fusion. A single-level spinal fusion model composed of polyethylene cylinders, posterior pedicular instrumentation, and variously positioned single or dual interbody cages was used for biomechanical testing. Constructs were tested under compressive flexural load, with measurement of stiffness, rod strain, cage strain, and intracage pressure. A strong linear correlation emerged between the mean construct stiffness and cage positioning within the sagittal plane that was inversely related to posterior rod strain. Two small titanium mesh cages were equivalent to one large cage. In a single-level spine model, the presence of and sagittal position of interbody cages significantly influences overall construct stiffness. Cage strain increased with more anterior positions and was inversely related to rod strain.

Augmentation of an anterior solid rod construct with threaded cortical bone dowels. A biomechanical study.

STUDY DESIGN: This static, nondestructive, in vitro biomechanical study examines anterior solid rod construct stiffness following the addition of multilevel, threaded cortical bone dowels in a bovine model. A comparison is made with a clinically relevant posterior construct with and without an anterior release. OBJECTIVES: To determine if the addition of solid, multilevel disc space implants will increase construct rigidity, while maintaining or enhancing anterior column length. SUMMARY OF BACKGROUND DATA: Anterior instrumentation for thoracolumbar and lumbar scoliosis has achieved greater correction and preserved distal motion segments; however, kyphosis over the instrumented segments and nonunion have been observed more frequently than with posterior segmental spinal instrumentation. METHOD: Fifteen calf spines underwent mechanical testing. Group A (n = 7) included anterior constructs: 1) intact, 2) anterior release/rod/rib graft (L2-L5), and 3) anterior release/rod/dowels (L2-L5). Group B (n = 8) included posterior constructs: 1) intact, 2) posterior rod without anterior release (T13-L5), 3) posterior rod (T13-L5)/anterior release/rib graft (L2-L5). The protocol included axial compression (-600 N), axial rotation (+7 Nm), flexion/extension (+7.5 Nm), and lateral bending (+7.5 Nm). An anterior extensometer measured segmental displacements to calculate construct stiffness. Lateral radiographs evaluated alignment for the anterior constructs. Statistical analysis involved a one way analysis of variance (ANOVA) and a Student-Newman-Keuls post hoc test. RESULTS: All reconstructions restored stiffness to intact values with the exception of the dowels alone in axial rotation. The rod/dowel construct was stiffer than all other groups in axial compression, flexion/extension, and lateral bending, with the exception of the posterior rod without discectomy, which was superior in flexion and statistically similar in extension, lateral bending, and axial rotation. The anterior construct with rib graft was equivalent to the posterior construct with rib graft in all modes of testing. The dowels created greater lordosis than the bicortical rib grafts. CONCLUSIONS: Disc space augmentation increased stiffness except in axial rotation, in which values were restored to the intact level. Stiffness was superior to a clinically relevant posterior instrumentation comparison group following anterior release, and was equivalent to a posterior construct without anterior release except in anterior flexion. In addition, the implants enhanced lordosis. Increased rigidity should improve rates of arthrodesis, while maintenance of sagittal alignment may prevent pathologic compensatory curves in adjacent spinal segments. Further research is required to determine the optimal method of achieving structural interspace support.

Biomechanical evaluation of five different occipito-atlanto-axial fixation techniques.

STUDY DESIGN: The stabilizing effects of five different occipitocervical fixations were compared. OBJECTIVES: To evaluate the construct stability provided by five different occipito-atlanto-axial fixation techniques. SUMMARY OF BACKGROUND DATA: Few studies have addressed occipitocervical reconstruction stability and no studies to data have investigated anterior-posterior translational stiffness. METHODS: A total of 21 human cadaveric spines were used. After testing intact spines (CO-C2), a type II dens fracture was created and five different reconstructions were performed: 1) occipital and sublaminar wiring/rectangular rod, 2) occipital screws and C2 lamina claw hooks/rod, 3) occipital screws, foramen magnum screws, and C1-C2 transarticular screws/rod, 4) occipital screws and C1-C2 transarticular screws/Y-plate, and 5) occipital screws and C2 pedicle screws/rod. Biomechanical testing parameters included axial rotation, flexion/extension, lateral bending, and anterior-posterior translation. RESULTS: Pedicle screw fixation demonstrated the highest stiffness among the five reconstructions (P < 0.05). The two types of transarticular screw methods provided greater stability than hook or wiring reconstructions (P < 0.05). The C2 claw hook technique resulted in greater stability than sublaminar wiring fixation in anterior-posterior translation (P < 0.05). However, the wiring procedure did not significantly increase the stiffness levels beyond the intact condition under anterior-posterior translation and lateral bending (P > 0.05). DISCUSSION: C2 transpedicular and C1-C2 transarticular screws significantly increased the stabilizing effect compared to sublaminar wiring and lamina hooks. The improved stability afforded by C2 pedicular and C1-C2 transarticular screws offer many potential advantages including a high rate of bony union, early ambulation, and easy nursing care. CONCLUSION: Occipitocervical reconstruction techniques using C1-C2 transarticular screws or C2 pedicle screws offer biomechanical advantages compared to sublaminar wiring or lamina hooks. Pedicle screw fixation exhibited the highest construct stiffness among the five reconstructions.

Does spinal kyphotic deformity influence the biomechanical characteristics of the adjacent motion segments? An in vivo animal model.

STUDY DESIGN: In an in vivo sheep model, the effects of spinal fusion and kyphotic deformity on the neighboring motion segments were analyzed. OBJECTIVES: To investigate the effects of spinal fusion and kyphotic deformity on the adjacent motion segment. SUMMARY OF BACKGROUND DATA: The in vivo effects of kyphotic deformity on the neighboring motion segments have not been investigated in any studies. METHODS: Eighteen sheep were equally randomized into three groups based on surgical procedure: L3-L5 in situ posterolateral fusion (n = 6) L3-L5 kyphotic posterolateral fusion (n = 6), and surgical exposure alone (n = 6). After a 16-week survival period, the adjacent motion segment changes were analyzed radiographically, biomechanically, and histologically. RESULTS: The kyphosis group showed 5.0 degrees +/- 2.6 degrees and 1.7 degrees +/- 1.8 degrees compensatory hyperlordosis at L2-L3 and L5-L6, respectively, compared with surgical exposure and in situ posterolateral fusion, the kyphotic posterolateral fusion significantly influenced cranial adjacent motion segment biomechanics by inducing more stiffness in the posterior ligamentous complex (P < 0.05) and increasing lamina strain under flexion-extension loading (P < 0.05). Results of histologic analysis showed significant degenerative changes of the L2-L3 facet joints in the kyphosis group. CONCLUSIONS: It is inferred that in the kyphosis group, compensatory hyperlordosis at the cranial adjacent level leads to lordotic contracture of the posterior ligamentous complex. The increased lamina strain, exhibited by the in situ group under flexion-extension, was further increased in the kyphosis group, indicating higher load transmission through the posterior column. Significant degenerative changes of the cephalad adjacent facet joints observed in the kyphosis group served to corroborate the biomechanical data. These results indicate that a kyphotic deformity may lead to facet joint contracture and facet arthritis and may serve as the origin of low back pain at the cranial adjacent level.

Revision strategies for salvaging or improving failed cylindrical cages.

STUDY DESIGN: This is a review of 20 patients who experienced failure of threaded interbody fusion cages and underwent surgical correction. OBJECTIVE: To review the causes and possible treatment strategies for failed cylindrical cages. SUMMARY OF BACKGROUND DATA: Intraoperative complications have been described in the past; however, management of the postoperative patient with failure of interbody fusion devices has not been described. METHODS: In 20 patients with failed threaded titanium fusion cages (18 Bagby and Kuslich Devices [BAK; Sulzer-Spine Tech, Minneapolis, MN], 2 Ray Threaded Fusion Cages [Ray TFC; Surgical Dynamics, Norwalk, CT) who underwent revision surgery, all had failure before successful arthrodesis was achieved. Eight of the original titanium cages had been inserted anteriorly (7 laparoscopically), and 12 had been inserted for posterior interbody lumbar fusion. Before the revision surgery, five of the implants were thought to be solid by the referring surgeon, but pseudarthrosis was clearly present in all. In addition, 14 other explanted BAK devices were subjected to undecalcified histologic preparation, quantitative histomorphometry, and histopathologic analysis. RESULTS: The average length of time before revision surgery (implant duration) was 31.8 weeks (range, 1-156 weeks). The most common revision procedure was posterior exploration of the symptomatic nerve root with foraminotomy for unrecognized lateral recess stenosis (11 cases) or excision of iatrogenically herniated intervertebral disc fragments (4 cases). However, four cages inserted through posterior exposure during an interbody lumbar fusion procedure had to be removed because of migration into the spinal canal. In nine cases posterior pedicle screw instrumentation was necessary in addition to posterolateral fusion using iliac crest bone grafting. CONCLUSIONS: All 20 cages failed because of surgical technique rather than an intrinsic defect in fusion cage technology. The factors associated with failure of the original insertion procedure were failure to achieve adequate distraction of the anulus fibrosis; undersized cages, especially when placed through the posterior interbody lumbar fusion approach; cerebrospinal fluid leakage or pseudomeningocele; Type 2 diabetes mellitus; the use of local bone graft rather than iliac crest inside the cage; anterior insertion in an excessively lateral position resulting in symptoms of a far lateral disc herniation; and failure to identify the spinal midline during an anterior approach.

The stability of reconstruction methods after thoracolumbar total spondylectomy. An in vitro investigation.

STUDY DESIGN: After total spondylectomy, five types of spinal reconstruction techniques were compared biomechanically. OBJECTIVES: To evaluate the stability provided by five reconstruction methods after total spondylectomy. SUMMARY OF BACKGROUND DATA: Total spondylectomy presents a worst-case scenario for spinal reconstruction. However, few investigators have biomechanically investigated spinal reconstruction stability after total spondylectomy. METHODS: 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. RESULTS: Only circumferential instrumentation techniques (AMP, ASP) exhibited higher stiffness than the intact spine in all loading modes (P < 0.05). Short circumferential fixation provided more stability than did multilevel posterior instrumentation (P < 0.05). Multilevel posterior fixation provided more stiffness than did short posterior and anterior instrumentation alone (P < 0.05). CONCLUSIONS: 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.

Static and fatigue biomechanical properties of anterior thoracolumbar instrumentation systems. A synthetic testing model.

STUDY DESIGN: A mechanical testing standard for anterior thoracolumbar instrumentation systems was introduced, using a synthetic model. Twelve recent instrumentation systems were tested in static and fatigue modes. OBJECTIVES: To establish the testing standard for anterior thoracolumbar instrumentation systems using a synthetic model and to evaluate the static and fatigue biomechanical properties of 12 anterior thoracolumbar instrumentation systems. SUMMARY OF BACKGROUND DATA: Although numerous studies have been performed to evaluate the biomechanics of anterior spinal instrumentation using a cadaveric or animal tissue, problems of specimen variation, lack of reproducibility, and inability to perform fatigue testing have been pointed out. In no studies has a precise synthetic testing standard for anterior thoracolumbar instrumentation systems been described. METHODS: An ultra-high-molecular-weight polyethylene cylinder was designed according to the anatomic dimensions of the vertebral body. Two cylinders spanned by spinal instrumentation simulated a total corpectomy defect, and a compressive lateral bending load was applied. The instrumentation assembly was precisely standardized. The static destructive and fatigue tests up to 2 million cycles at three load levels were conducted, followed by the failure mode analysis. Twelve anterior instrumentation systems, consisting of five plate and seven rod systems were compared in stiffness, bending strength, and cycles to failure. RESULTS: Static and fatigue test parameters both demonstrated highly significant differences between devices. The stiffness ranged from 280.5 kN/m in the Synthes plate (Synthes, Paoli, PA) to 67.9 kN/m in the Z-plate ATL (SofamorDanek, Memphis, TN). The Synthes plate and Kaneda SR titanium (AcroMed, Cleveland, OH) formed the highest subset in bending strength of 1516.1 N and 1209.9 N, respectively, whereas the Z-plate showed the lowest value of 407.3 N. There were no substantial differences between plate and rod devices. In fatigue, only three systems: Synthes plate, Kaneda SR titanium, and Olerud plate (Nord Opedic AB, Sweden) withstood 2 million cycles at 600 N. The failure mode analysis demonstrated plate or bolt fractures in plate systems and rod fractures in rod systems. CONCLUSIONS: The biomechanical testing standard for anterior thoracolumbar instrumentation systems was successfully designed. It provided a repeatable and consistent experimental condition and controlling dimensional and surgical factors. The comparison of 12 instrumentation systems highlights the importance of mechanically balanced device design without a weak link in the development of instrumentation.

Does spinal instrumentation influence the healing process of posterolateral spinal fusion? An in vivo animal model.

STUDY DESIGN: An in vivo sheep model was used to investigate the effect of spinal instrumentation on the healing process of posterolateral spinal fusion. OBJECTIVES: To examine the role of spinal instrumentation during the healing process of posterolateral fusion. SUMMARY OF BACKGROUND DATA: 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. METHODS: 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. RESULTS: 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. CONCLUSION: The current study's results demonstrated that spinal instrumentation creates a stable mechanical environment to enhance the early bone healing of spinal fusion.

Osteogenic protein versus autologous interbody arthrodesis in the sheep thoracic spine. A comparative endoscopic study using the Bagby and Kuslich interbody fusion device.

STUDY DESIGN: Using an in vivo interbody arthrodesis model, the efficacy of the Bagby and Kuslich (BAK) device packed with recombinant human osteogenic protein-1 (rhOP-1) was evaluated. OBJECTIVES: To compare the efficacy of osteogenic protein with that of autograft for interbody arthrodesis, with fusion success based on biomechanical, histologic, and radiographic analyses. SUMMARY OF BACKGROUND DATA: The use of recombinant human bone morphogenetic proteins (rhBMPs) as osteoinductive bone graft substitutes or expanders has recently gained considerable research interest, particularly when applied in posterolateral arthrodesis. However, whether these results can be extrapolated to a successful interbody spinal arthrodesis remains uncertain. METHODS: Twelve sheep underwent a multilevel thoracic spinal decompression by thoracoscopic approach. Three noncontiguous destabilization sites (T5-T6, T7-T8, T9-T10) were prepared and randomly treated as follows. Control group treatments were nonsurgical, destabilization alone, and empty BAK. Experimental groups were treated with autograft alone, BAK device packed with autograft, or BAK device packed with rhOP-1. Four months after surgery, interbody fusion status was quantified by biomechanical testing, computed tomography, microradiography, and histomorphometry. RESULTS: Results of biomechanical analysis showed statistically higher segmental stiffness levels when comparing the control and experimental groups with four of the five testing methods (P < 0.05). Computed tomography and microradiography characterized destabilization alone as producing one fusion in six preparations; the empty BAK, two in six;, autograft alone, four in eight; BAK with autograft, five in eight; and BAK with rhOP-1 group, six in eight-all evidenced by woven trabecular bone spanning the fusion sites. Histomorphometry yielded significantly more trabecular bone formation at the fusion sites in the three experimental groups than in the two control groups (P < 0.05). CONCLUSIONS: Interbody spinal fusions showing biomechanical and histomorphometric equivalency to autologous fusions have been achieved with rhOP-1. The functional unit stability and histologic osteointegration evidenced by the BAK/rhOP-1 complex shows this interbody arthrodesis technique to be a viable alternative toconventional autologous iliac crest, thereby obviating the need for an iliac crest donor site and associated patient morbidity.

Biomechanical analysis of anterior versus circumferential spinal reconstruction for various anatomic stages of tumor lesions.

STUDY DESIGN: Spinal reconstruction procedures for metastasis evaluated biomechanically using human cadaver specimens. OBJECTIVE: To investigate the stiffness of anterior versus circumferential spinal reconstructions for different anatomic stages of tumor lesions. SUMMARY OF BACKGROUND DATA: Metastatic tumors predominantly involve the vertebral bodies. Although anterior instrumentation and strut grafts provide excellent stability, it remains unclear to what extent vertebral destruction requires anterior reconstructions alone versus combined anterior and posterior procedures. METHODS: Ten human cadaveric thoracolumbar spines were used. The L1 vertebral body and posterior elements were resected sequentially based on Weinstein's anatomic zone classification for tumor lesions. Anterior reconstruction was performed between T12 and L2 using an iliac strut graft and the Kaneda SR system (AcroMed, Cleveland, OH). For circumferential reconstruction, the Cotrel-Dubousset hook and rod system was combined with the anterior reconstruction procedure. Experimental groups included the intact condition and five reconstruction stages: anterior reconstructions for corpectomy, subtotal and total spondylectomies, and circumferential reconstructions for subtotal and total spondylectomies. Nondestructive biomechanical testing was performed under four different loading modes. RESULTS: All the reconstruction groups except anterior instrumentation alone for total spondylectomy returned stiffness to a level equivalent or higher to that of the intact spine. There were no statistical differences observed between anterior and circumferential reconstruction for subtotal spondylectomy. Anterior instrumentation alone for total spondylectomy did not restore stiffness to the intact level, and demonstrated significantly lower stiffness than that of circumferential reconstruction. CONCLUSIONS: For corpectomy or subtotal spondylectomy, anterior reconstruction alone can provide stiffness equivalent to circumferential reconstruction. However, total spondylectomy significantly reduces the anterior reconstruction stiffness, suggesting the need for combined anterior and posterior procedures.

A comparative mechanical analysis of the pointe shoe toe box. An in vitro study.

Dancing en pointe requires the ballerina to stand on her toes, which are protected only by the pointe shoe toe box. This protection diminishes when the toe box loses its structural integrity. The objectives of this study were 1) to quantify the comparative structural static and fatigue properties of the pointe shoe toe box, and 2) to evaluate the preferred shoe characteristics as determined by a survey of local dancers. Five different pointe shoes (Capezio, Freed, Gaynor Minden, Leo's, and Grishko) were evaluated to quantify the static stiffness, static strength, and fatigue properties (cycles to failure) of the shoes. Under axial loading conditions, the Leo's shoe demonstrated the highest stiffness level, and the Freed shoe exhibited the least strength. Under vertical loading conditions, the Leo's and Freed shoes demonstrated the highest stiffness levels, and the Gaynor Minden and Freed shoes exhibited the highest strength. Fatigue testing highlighted the greatest differences among the five shoes, with the Gaynor Minden demonstrating the highest fatigue life. Dancers rated the top five shoe characteristics, in order of importance, as fit, comfort, box/platform shape, vamp shape, and durability and indicated that the "best" shoe is one that "feels right" and permits artistic maneuvers, not necessarily the strongest or most durable shoe.

Stability of posterior spinal instrumentation and its effects on adjacent motion segments in the lumbosacral spine.

STUDY DESIGN: An in vitro biomechanical analysis of three anterior instability patterns was performed using calf lumbosacral spines. Stiffness of the constructs was compared, and segmental motion analyses were performed. OBJECTIVES: To clarify the factors that alter the stability of the spinal instrumentation and to evaluate the influence of instrumentation on the residual intact motion segments. SUMMARY OF BACKGROUND DATA: Recently, many adverse effects have been reported in fusion augmented with rigid instrumentation. Only few reports are available regarding biomechanical effects of stability provided by spinal instrumentation and its effects on residual adjacent motion segments in the lumbar-lumbosacral spine. METHODS: Eighteen calf lumbosacral spine specimens were divided into three groups according to instability patterns--one-level, two-level, and three-level disc dissections. Six constructs were cyclically tested in rotation, flexion-extension, and lateral bending of intact spines, of destabilized spine, and of spines with four segmental posterior instrumentation systems used to extend the levels of instability (Cotrel-Dubousset compression hook and three transpedicular screw fixation systems). During each test, stiffness values and segmental displacements were measured. RESULTS: The rigidity of the instrumented construct increased as the fixation range became more extensive. Although application of the instrumentation effectively reduced the segmental motion of the destabilized vertebral level, the motion at the destabilized level tended to increase as the number of unstable vertebral levels increased, and the fixation range of the instrumentation became more extensive. Instrumented constructs produced higher segmental displacement values at the upper residual intact motion segment when compared with those of the intact spine. In contrast, the instrumented constructs decreased their segmental displacement values at the lower residual intact motion segment with higher magnitude of the translational (shear) motion taking place compared with the intact spine in flexion-extension and lateral bending. These changes in the motion pattern became more distinct as the fixation range became more extensive. CONCLUSIONS: As segmental spinal instrumentation progresses from one level to three levels, the overall torsional and flexural rigidity of the system increases. However, segmental displacement at the site of simulated instability becomes more obvious. Application of segmental instrumentation changes the motion pattern of the residual intact motion segments, and the changes in the motion pattern become more distinct as the fixation range becomes more extensive and as the rigidity of the construct increases.

The role of anteromedial foraminotomy and the uncovertebral joints in the stability of the cervical spine. A biomechanical study.

STUDY DESIGN: The biomechanical role of the cervical uncovertebral joint was investigated using human cadaveric spines. Sequential resection of cervical uncovertebral joints, including clinical anteromedial foraminotomy, was conducted, followed by biomechanical testing after each stage of resection. OBJECTIVES: To clarify the biomechanical role of uncovertebral joints and clinical anteromedial foraminotomy in the cervical spine and their effects on interbody bone graft stability. SUMMARY OF BACKGROUND DATA: Although the biomechanical role of the cervical uncovertebral joints has been considered to be that of a guiding mechanism in flexion and extension and a limiting mechanism in posterior translation and lateral bending, there have been no studies quantifying this role. According to results in quantitative anatomic studies, anatomic variations exist in uncovertebral joints, depending on the vertebral level, articular angulation, and relative height of the joints. METHODS: Fourteen human functional spinal units at C3-C4 and C6-C7 underwent sequential uncovertebral joint resection, with each stage of resection followed by biomechanical testing. The uncovertebral joint was divided anatomically into three parts on each side: the posterior foraminal part, the posterior half, and the anterior half. The loading modes included torsion, flexion, extension, and lateral bending. A simulated anterior bone graft construct was also tested after each uncovertebral joint resection procedure. RESULTS: Significant changes in stability were observed after sequential uncovertebral joint resection in all loading modes (P < 0.05). The biomechanical contribution of uncovertebral joints decreased in the following order: the posterior foraminal part, the posterior half, and the anterior half. Unilateral and bilateral foraminotomy most affected the stability of the functional spinal unit during extension, causing a 30% and 36% decrease in stiffness of the functional spinal unit, respectively. The effect was less in torsion and lateral bending. After sequential resection, there was a statistically significant difference between decreases in torsional stiffness at C3-C4 and C6-C7 (P < 0.05). The stiffness of the simulated bone graft construct decreased progressively during flexion and lateral bending after each foraminotomy (P < 0.05). Increased bone graft height of 79% returned stability to the preforaminotomy level. CONCLUSIONS: This is the first study to quantitate the biomechanical role of uncovertebral joints in cervical segmental stability and the effect at each intervertebral level. The effect differs because of anatomic variations in uncovertebral joints. The major biomechanical function of uncovertebral joints includes the regulation of extension and lateral bending motion, followed by torsion, which is mainly provided by the posterior uncovertebral joints. This study highlights the clinical assessment of additional segmental instability attributed to destruction of the uncovertebral joints during surgical procedures or by neoplastic lesions.

Video-assisted thoracoscopic surgery versus open thoracotomy for anterior thoracic spinal fusion. A comparative radiographic, biomechanical, and histologic analysis in a sheep model.

STUDY DESIGN: In this in vivo investigation, a sheep model was used to compare the efficacy of a video-assisted thoracoscopic approach and a traditional thoracotomy in promoting a successful interbody spinal arthrodesis. OBJECTIVES: To compare the incidence of successful anterior spinal arthrodesis among three stabilization techniques-iliac crest, Bagby and Kuslich device, and Z-plate--performed using a video-assisted thoracoscopic approach and conventional open thoracotomy approaches. SUMMARY OF BACKGROUND DATA: A clinical outcome study on open versus endoscopic spinal fusion is not yet available. Moreover, no basic scientific investigations have been conducted to determine whether the success of an endoscopic arthrodesis is comparable to that of a conventional open procedure. METHODS: Fourteen Western Crossbred sheep underwent three identical destabilization procedures at T5-T6, T7-T8, and T9-T10, in which the anterior and middle osteoligamentous columns of the spine were resected, followed by three randomized reconstruction procedures using iliac autograft alone, and Z-plate stabilization with iliac autograft. In seven sheep, the entire destabilization-reconstruction procedure was performed using a video-assisted thoracoscopic surgical approach. In the remaining seven, the procedure was performed by conventional open thoracotomy. RESULTS: Histomorphometric and biomechanical evaluation demonstrated that the video-assisted thoracoscopic approach and open thoracotomy arthrodesis had comparable bone formation and biomechanical properties (P > 0.05). However, the Z-plate fusions, as a group, demonstrated increased flexion-extension stiffness properties and trabecular bone formation compared with the autograft and Bagby and Kuslich device fusions (P < 0.05). CONCLUSIONS: Thoracic interbody spinal fusions performed by thoracoscopy have demonstrated histologic, biomechanical, and radiographic equivalence to those performed by a thoracotomy approach. However, in the endoscopy group, intraoperative complications causing longer operative times, higher estimated blood loss, and increased animal morbidity indicated a substantial learning curve associated with the adoption of this surgical technique.