Robert H. Chamberlain

Biomechanical assessment of anterior lumbar interbody fusion with an anterior lumbosacral fixation screw-plate: comparison to stand-alone anterior lumbar interbody fusion and anterior lumbar interbody fusion with pedicle screws in an unstable human cadaver model.

Study Design. Human lumbosacral cadaveric specimens were tested in an in vitro biomechanical flexibility experiment using physiologic loads in 5 sequential conditions. Objective. To determine the biomechanical differences between anterior lumbar interbody fusion (ALIF) using cylindrical threaded cages alone or supplemented with an anterior screw-plate or posterior pedicle screws-rods. Summary of Background Data. Clinically and biomechanically, stand-alone ALIF performs modestly in immobilizing the unstable spine. Pedicle screws improve fixation stiffness significantly, but supplementary anterior instrumentation has not been studied. Methods. There were 7 specimens tested: (1) intact, (2) after discectomy and facetectomy to induce moderate rotational and translational hypermobility, (3) with 2 parallel ALIF cages, (4) with cages plus a triangular anterior screw-plate, and (5) with cages plus pedicle screws-rods. Pure moments without preload induced flexion, extension, lateral bending, and axial rotation; linear shear forces induced anteroposterior translation. Angular and linear motions were measured stereophotogrammetrically, and range of motion (ROM) and stiffness were quantified. Results. Compared to the destabilized spine, interbody cages alone reduced ROM by 77% during flexion, 53% during extension, 60% during lateral bending, 69% during axial rotation, and 71% during anteroposterior shear (P < 0.001, analysis of variance/Fisher least significant difference). Addition of an anterior plate or pedicle screws-rods, respectively, further reduced ROM by 8% or 13% during flexion (P = 0.21), 21% or 28% during extension (P = 0.15), 5% or 25% during lateral bending (P = 0.04), 11% or 18% during axial rotation (P = 0.13), and 18% or 18% during anteroposterior shear (P = 0.17). Compared to stand-alone ALIF, both the anterior screw-plate and pedicle screw-rod fixation reduced vertebral ROM to less than 1.2° of rotation and less than 0.1 mm of translation. Conclusions. The anterior screw-plate and pedicle screws-rods both substantially reduced ROM and increased stiffness compared to stand-alone interbody cages. There was no significant difference in the amount by which the supplementary fixation devices limited flexion, extension, axial rotation, or anteroposterior shear; pedicle screws-rods better restricted lateral bending.

Biomechanical Analysis of Rigid Stabilization Techniques for Three-Column Injury in the Lower Cervical Spine

Study Design. Comparison of nondestructive multidirectional flexibility in groups of specimens receiving two different posterior instrumentation constructs with or without anterior plating. Objective. To compare stability after a three-column injury stabilized posteriorly by lateral mass screws-rods at C5–C6 and pedicle screws-rods at C7 (“LLP”) or by pedicle screws-rods at C5–C6–C7 (“PPP”), and to compare posterior, anterior, and combined anterior-posterior fixation. Summary of Background Data. Pedicle screws resist pullout better than lateral mass screws, but little research has compared the stability of pedicle screws to that of lateral mass screws used within constructs. Methods. Fourteen human cadaveric C4–T1 specimens were tested intact, posteriorly instrumented (7 LLP and 7 PPP), anteriorly instrumented, or with combined (anterior-posterior) instrumentation. Nonconstraining, nondestructive torques induced flexion, extension, lateral bending, and axial rotation while angular motion was recorded optically. Results. Posterior, anterior, and combined instrumentation each significantly improved stability (P < 0.05). Combined fixation provided significantly better stability than either anterior or posterior instrumentation alone. In no loading mode and in no testing condition was any parameter significantly different between LLP and PPP.Posterior instrumentation provided significantly better stability than anterior instrumentation. Conclusions. Anterior plate and posterior screw-rod fixation alone improve stability in a two-level, three-column cervical injury model. Combined fixation further improves stability. There is little discernible difference in immediate postoperative stability between posterior rod constructs combining lateral mass and pedicle screws and those using only pedicle screws.

Unilateral cervical facet dislocation: injury mechanism and biomechanical consequences.

Study Design. Human cadaveric cervical spines were subjected to nondestructive repeated-measures flexibility tests before, during, and after unilateral facet dislocation. Objectives. To assess the relative stability and kinematics of the spine in the normal condition, after unilateral facet lock without bony injury and after reducing (unlocking) the facet dislocation. Summary of Background Data. Experimental unilateral facet dislocation has been studied only when associated with extensive damage to disc, bone, or both. The relative stability of the normal, locked, and unlocked cervical spine has not been studied. Methods. Unilateral facet dislocation with minimal associated injury was created by slowly applying axial torque while specimens were bent laterally and flexed. Before and after injury, nondestructive torques were applied about each anatomic axis, while three-dimensional specimen motion was recorded stereophotogrammetrically. Results. Unilateral facet dislocation was created with a mean axial torque of 10.2 ± 2.7 Nm. After locking, range of motion, lax zone, and stiff zone were significantly reduced relative to normal (P < 0.05). After unlocking, the same parameters were significantly increased relative to normal. The position of the axis of rotation shifted when the facet was locked and the centrode elongated after the facet was unlocked. Conclusions. Unilateral facet dislocation without fracture can be created with moderate loads when axial torque is applied while the facet is distracted. The spine is stable while the articular masses are locked unilaterally; however, the motion segment becomes overtly unstable after the facet dislocation is reduced.

Hangman's fracture: a biomechanical comparison of stabilization techniques.

Study Design. In vitro biomechanical flexibility experiment studying 5 sequential conditions. Objective. To determine the biomechanical differences among 3 fixation techniques after a simulated hangman’s fracture. Summary of Background Data. Type II hangman’s fractures are often treated surgically with a C2–C3 anterior cervical discectomy, fusion, and plating. Other techniques include direct fixation with C2 pars interarticularis screws or posterior C2–C3 fixation connecting C2 pars screws to C3 lateral mass screws. Methods. Seven cadaveric specimens (Oc–C4) were tested intact, after a simulated hangman’s fracture, and after each fixation technique. Flexion, extension, lateral bending, and axial rotation were induced using nonconstraining torques while recording angular motions stereophotogrammetrically. Results. Direct screw fixation reduced motion an average of 61% ± 13% during lateral bending and axial rotation compared to the injured state (P < 0.007). However, instability remained during flexion and extension. Posterior C2–C3 rod fixation provided significantly greater rigidity than anterior plate fixation during lateral bending (P < 0.008) and axial rotation (P < 0.04). Conclusions. Direct fixation of the pars ineffectively limits flexion and extension after a Type II hangman’s fracture. If pars screw fixation can be achieved, posterior C2–C3 fixation more effectively stabilizes a hangman’s fracture than anterior cervical plating.

Biomechanics of stabilization after cervicothoracic compression-flexion injury.

Study Design. Biomechanical laboratory research. Objective. To determine whether anterior, posterior, or combined instrumentation provides the best stability for treating a cervicothoracic compression-flexion injury. Summary of Background Data. As the junction between the mobile cervical spine and rigid thoracic spine, the cervicothoracic junction poses unique challenges to the success of any fixation system spanning this region. Although posterior instrumentation is the preferred method of fixation in the unstable cervical spine, it is unknown whether this is the case across the unstable cervicothoracic junction. Methods. Flexion, extension, lateral bending, and axial rotation of cadaveric specimens were studied during application of nondestructive pure moments in a sequence of conditions: (1) intact, (2) after destabilization, (3) with posterior instrumentation from C6−T1 or T2, and (4) with corpectomy/graft and anterior alone or combined anterior/posterior instrumentation. Results. Compared to anterior instrumentation, posterior instrumentation allowed an 89% smaller range of motion (ROM) during lateral bending (P = 0.01) and 64% smaller ROM during axial rotation (P = 0.04). In most loading modes, combined instrumentation outperformed either anterior or posterior instrumentation alone. Most biomechanical measurements of stability improved when posterior instrumentation was extended from T1 to T2. Small and usually insignificant reductions in ROM averaging 15% were observed with C7 included in the posterior construct versus C7 excluded. Conclusions. Combined instrumentation provides a significant improvement in stability over either anterior or posterior instrumentation alone. Extension of the posterior instrumentation to include T2 improves stability at T1–T2 as well as rostral levels. Inclusion of C7 in theconstruct is largely inconsequential biomechanically.

Biomechanical effects of laminoplasty versus laminectomy: stenosis and stability.

Study Design. In vitro human cadaveric study simultaneously quantifying sagittal plane flexibility and spinal canal stenosis. Objective. To compare biomechanical stability and the change in cross-sectional area during flexion and extension after laminectomy and open-door laminoplasty. Summary of Background Data. Spinal canal stenosis has been quantified in vitro but has not been quantified in studies of laminectomy or laminoplasty. Methods. Cadaveric specimens were loaded in physiologic-range flexion and extension using nonconstraining pure moments while recording segmental angles optoelectronically. Custom flexible tubing was placed within the spinal canal, and water was continuously pumped through the tubing while measuring upstream pressure. Spinal canal cross-sectional area correlated to water pressure, allowing continuous monitoring of the smallest cross-sectional area of the canal. Specimens were tested (1) normal, (2) after modeling stenosis by inserting hemispherical wooden beads in the spinal canal at 3 levels, (3) after open-door laminoplasty at 5 levels, and (4) after expanding laminoplasty to laminectomy. Results. Range of motion (ROM) in the normal, stenotic, and laminoplasty conditions did not differ significantly. However, laminectomy increased ROM significantly more than other conditions. ROM after laminectomy was 13% greater than after laminoplasty. After modeling stenosis, the cross-sectional area decreased to 52% ± 12% of normal. Laminoplasty restored the cross-sectional area to 70% ± 12% of normal whereas laminectomy restored cross-sectional area to 101% ± 4% of normal. Among all conditions, areas differed significantly except normal versus laminectomy. Conclusion. Laminoplasty leaves the spine in a significantly more stable condition than laminectomy. However, laminoplasty failed to relieve stenosis completely. In this study, stenosis was modeled as about 50% occlusion of the spinal canal. The degree of stenosis should be considered in clinical decisions of whether laminectomy or laminoplasty is more appropriate.

Unilateral cervical facet dislocation: biomechanics of fixation.

Study Design. Unilateral facet dislocation was created in human cadaveric cervical spines. Specimens were sequentially instrumented with posterior or anterior screws and plates, and studied biomechanically. Objective. To determine the biomechanical differences between anterior and posterior fixation for stabilization of a reduced unilateral cervical facet dislocation. Summary of Background Data. Although previous studies have compared anterior to posterior instrumentation, no data exist on the biomechanics of either type of stabilization after this particular injury. Methods. In 6 human cadaveric cervical spine segments, a reproducible unilateral facet dislocation was created and then unlocked (reduced). Nondestructive torques were applied to specimens that were intact, injured-reduced, fixated using posterior nonlocking lateral mass plates, and fixated using a bone graft plus an anterior nonlocking plate. Flexion, extension, lateral bending, and axial rotation were measured stereophotogrammetrically. Results. Lateral mass plating was more effective than anterior plating in limiting motion after reduction of a unilateral facet dislocation. Averaged, over all loading directions, lateral mass plates reduced the range of motion to 17% of normal; anterior plates reduced range of motion to 89% of normal. In all loading directions, lateral mass plates performed significantly better than anterior plates (P < 0.05, paired Student t-tests). Conclusions. Anterior and posterior plating effectively stabilized a reduced unilateral facet dislocation. Lateral mass fixation provided better immobilization than anterior graft and plate.

Biomechanical Comparison of Anterior Versus Posterior Lumbar Threaded Interbody Fusion Cages

Study Design. Biomechanical flexibility tests were performed in specimens receiving anterior lumbar interbody fixation or posterior lumbar interbody fixation using dual threaded cages. Objectives. To determine differences in stability between anterior lumbar interbody fixation and posterior lumbar interbody fixation immediately after surgery and after fatigue. Summary of Background Data. No direct biomechanical comparison of lumbar fixation with threaded anterior lumbar interbody fixation or posterior lumbar interbody fixation cages has been performed previously. Methods. Sixteen anterior lumbar interbody fixation specimens and 16 posterior lumbar interbody fixation specimens underwent nondestructive biomechanical testing. Flexibility was assessed during applied flexion, extension, lateral bending, axial rotation, and anteroposterior shear before and after fixation and fatigue. After testing, specimens were dissected, and the quality of fixation was graded. Results. Variability in angular range of motion after fixation was greater than normal interspecimen variability by 89% after anterior lumbar interbody fixation and by 117% after posterior lumbar interbody fixation. During flexion–extension and lateral bending, posterior lumbar interbody fixation allowed a mean of 60% smaller neutral zones than anterior lumbar interbody fixation (P < 0.05, nonpaired Student t test). During axial rotation, anterior lumbar interbody fixation allowed 15% less range of motion than posterior lumbar interbody fixation unless facets were kept intact with posterior lumbar interbody fixation (6 of 16 specimens), in which case anterior lumbar interbody fixation allowed 41% greater range of motion than posterior lumbar interbody fixation. During anteroposterior shear, both anterior lumbar interbody fixation and posterior lumbar interbody fixation restrained range of motion to within 50% of normal. Anterior lumbar interbody fixation loosened, on average, 130% more with fatigue than posterior lumbar interbody fixation during anteroposterior shear. Conclusions. Both anterior lumbar interbody fixation and posterior lumbar interbody fixation provided inconsistent stability. Therefore, stand-alone anterior lumbar interbody fixation or posterior lumbar interbody fixation may often be ineffective clinically. During all modes of loading except axial rotation, posterior lumbar interbody fixation performed slightly better than anterior lumbar interbody fixation, perhaps due to deeper hole preparation and destruction of anterior stabilizers necessary for anterior lumbar interbody fixation. Avoiding resection of facets during posterior lumbar interbody fixation led to significantly better performance during axial rotation.

Biomechanical Analysis of a Resorbable Anterior Cervical Graft Containment Plate

Study Design. A series of in vitro experiments were performed in human cadaveric cervical spines in the normal condition, after discectomy and graft, and instrumented with MacroPore resorbable anterior plates (MacroPore, San Diego, CA). Flexibility, graft containment, and load-to-failure were studied. Objective. To assess the stability, strength, and resistance to graft extrusion provided by the resorbable plate after discectomy and grafting. Summary of Background Data. Metallic plates are known to improve stability after discectomy and graft. Resorbable plates have not been evaluated regarding stability offered or ability to contain the graft. Methods. Specimens were loaded using nonconstraining, nondestructive torques to induce flexion, extension, lateral bending, and axial rotation. One and 2-level specimens were studied: (1) normal, (2) after discectomy and graft, (3) after resorbable plating with 2 screws per vertebra, and (4) after resorbable plating with 3 screws per vertebra. All specimens were loaded to failure after completing nondestructive tests. Additional 1-level specimens with and without resorbable plate were tested for graft containment using anterior shear force directly on the graft. Results. The 1-level resorbable plate did not limit motion significantly better than grafted but unplated specimens. However, 2-level resorbable plates allowed significantly less motion than grafted but unplated specimens during all loading modes (P < 0.05). Specimens with resorbable plates resisted graft extrusion significantly better than unplated specimens. With 1-level resorbable plates, 2 or 3 screws per vertebra provided equivalent stability; in 2-level plates, 3 screws provided significantly better stability. Comparison to previous 1-level metallic plate data revealed a significant difference in motion only during flexion. Conclusions. The 1-level resorbable plate does not increase stability compared to grafted but unplated specimens and provides less stability than a metal plate, especially during flexion. However, the resorbable platesubstantially improves resistance to graft extrusion. The2-level resorbable plate significantly reduces motion compared to grafted but unplated specimens. When applying a 2-level plate, 3 screws per vertebra are recommended. In a 1-level plate, 2 or 3 screws per vertebra are equivalent.

Biomechanical analysis of a newly designed bioabsorbable anterior cervical plate. Invited submission from the joint section meeting on disorders of the spine and peripheral nerves, March 2005.

OBJECT The authors present a biomechanical analysis of a newly designed bioabsorbable anterior cervical plate (ACP) for the treatment of one-level cervical degenerative disc disease. They studied anterior cervical discectomy and fusion (ACDF) in a human cadaveric model, comparing the stability of the cervical spine after placement of the bioabsorbable fusion plate, a bioabsorbable mesh, and a more traditional metallic ACP. METHODS Seven human cadaveric specimens underwent a C6-7 fibular graft-assisted ACDF placement. A one-level resorbable ACP was then placed and secured with bioabsorbable screws. Flexibility testing was performed on both intact and instrumented specimens using a servohydraulic system to create flexion-extension, lateral bending, and axial rotation motions. After data analysis, three parameters were calculated: angular range of motion, lax zone, and stiff zone. The results were compared with those obtained in a previous study of a resorbable fusion mesh and with those acquired using metallic fusion ACPs. For all parameters studied, the resorbable plate consistently conferred greater stability than the resorbable mesh. Moreover, it offered comparable stability with that of metallic fusion ACPs. CONCLUSIONS Bioabsorbable plates provide better stability than resorbable mesh. Although the results of this study do not necessarily indicate that a resorbable plate confers equivalent stability to a metal plate, the resorbable ACP certainly yielded better results than the resorbable mesh. Bioabsorbable fusion ACPs should therefore be considered as alternatives to metal plates when a graft containment device is required.