Biomechanical testing sequelae relevant to spinal fusion and instrumentation

Biomechanical comparison of transpedicular versus intralaminar C2 fixation in C2-C6 subaxial constructs

STUDY DESIGN

Biomechanical study.

OBJECTIVE

To compare the relative rigidity of C2 transpedicular versus intralaminar fixation with and without offset connectors in C2-C6 subaxial constructs.

SUMMARY OF BACKGROUND DATA

Insufficient biomechanical data exists on C2 laminar fixation in subaxial constructs, and no study has considered C2-C6 subaxial constructs or the use of offset connectors.

METHODS

Six fresh-frozen cadaveric cervical spines underwent rigidity testing in the intact condition and after a destabilizing C3-C6 laminectomy. Specimens were instrumented with 20 mm pedicle and 20 mm intralaminar screws at C2, and with 14 mm lateral mass screws from C3-C6. In random order, three conditions (C2 pedicle screws, C2 laminar screws, and C2 laminar screws with offset connectors) were tested in flexion-extension, axial rotation, and lateral bending.

RESULTS

Laminar screws in C2-C6 constructs were equivalent to transpedicular fixation in flexion-extension (P = 0.985), were significantly more rigid than pedicle screws in axial rotation (P = 0.002), and were significantly less rigid than pedicle screws in lateral bending (P = 0.002). Laminar screw constructs were more rigid than the intact condition in all planes.

Improved biological performance of microarc-oxidized low-modulus Ti-24Nb-4Zr-7.9Sn alloy

Ti-24Nb-4Zr-7.9Sn (TNZS) is a newly developed beta-titanium alloy with low modulus and it has been considered as good material for dental or orthopedic implant. The purpose of the current study is to evaluate the effect of micro-arc oxidation (MAO) treatment on the biological performance of TNZS surface. The phases, morphology and chemical composition of the MAO-treated surface were characterized by X-ray diffraction, energy dispersive spectroscope and scanning electron microscopy analysis respectively. Then we tested the biocompatibility by examining the cell morphology and viability of osteoblast cells growing on MAO-TNZS surface. The bone binding strength of the specimens was evaluated by removal torque test after implantation in rabbit tibiae for 6 weeks. Compared with the none-treated titanium and TNZS specimens, MAO treated TNZS specimens showed a significant increase (p<0.05) in hydrophilicity, roughness, cell viability and removal torque forces. In summary, MAO treatment helps to form a porous surface with a biologically active bone-like apatite layer on TNZS specimens, which may improve the biological response of MAO-TNZS implants.

In vitro biomechanical comparison of transpedicular versus translaminar C-2 screw fixation in C2–3 instrumentation

Object

In instrumentation of the upper cervical spine, placement of pedicle screws into C-2 is generally safe, although there is the potential for injury to the vertebral arteries. Owing to this risk, translaminar screws into C-2 have been used. The aim of this study was to compare the stability of the in vitro cadaveric spine using C-2 laminar compared with C-2 pedicle screws in C2–3 instrumentation.

Methods

Eight fresh frozen human cadaveric cervical spines (C1–6) were potted at C1–2 and C5–6. Pure moments in increments of 0.3 Nm to a maximum of 1.5 Nm were applied in flexion, extension, right and left lateral bending, and right and left axial rotation. Each specimen was tested sequentially in three modes: 1) intact; 2) C2 pedicle screw–C3 lateral mass fixation; and 3) C2 laminar screw–C3 lateral mass fixation. The sequence of fixation testing was randomized. Motion was tracked with reflective markers attached to C-2 and C-3.

Results

Spinal levels with instrumentation showed significantly less motion than the intact spine in all directions and with all loads greater than 0.3 Nm (p < 0.05). Although there was no significant difference between C2 pedicle screw–C3 lateral mass fixation and C2 laminar screw–C3 lateral mass fixation, generally the former type of fixation was associated with less motion than the latter.

Conclusions

When pedicle screws in C-2 are contraindicated or inappropriate, laminar screws in C-2 offer a safe and acceptable option for posterior instrumentation.

Constructs incorporating intralaminar C2 screws provide rigid stability for atlantoaxial fixation

STUDY DESIGN

An in vitro biomechanical study of C1-C2 posterior fusion techniques using a cadaveric model.

OBJECTIVES

To investigate the acute stability afforded across the atlantoaxial segment by a novel technique that uses intralaminar screws in C2, and to compare these results to the stability obtained using a C2 pedicle fixation technique.

SUMMARY OF BACKGROUND DATA

There are numerous techniques available for rigidly coupling C1 and C2. It has been shown that screw techniques provide higher acute stability than wiring practices. However, many of these methods that use screw fixation in C2 can be technically difficult, especially in cases in which there is an aberrant vertebral artery course or if the C2 pedicle is not large enough to accommodate the instrumentation. A novel technique that uses intralaminar screws in C2 with C1 pedicle screws and bilateral longitudinal rods has been recently developed in an effort to overcome many of these issues. To date, there are no published reports as to whether this new technique provides equivalent (or better) fixation to the currently accepted methods.

METHODS

Six fresh-frozen human cadaveric cervical spines (C0-C4) were used in this study. Specimens were tested in their intact condition after destabilization via odontoidectomy, and after implantation of 3 different fixation constructs: (1) the Harms technique, 2 pedicle screws in C2, (2) a single C2 pedicle screw and a single C2 intralaminar screw, and (3) a construct having bilateral intralaminar C2 screws. Pure moment loading in flexion/extension, lateral bending, and axial rotation was applied to the occiput. Subsequent relative intervertebral rotations were determined using a 3 camera system. Range of motion for the intact, destabilized, and 3 fixation scenarios was determined, and statistical analysis was performed using one-way analysis of variance Fisher least-significant-difference post hoc test for multiple comparisons.

RESULTS

The data indicate that odontoidectomy significantly increased C1-C2 motion in flexion/extension and lateral bending. All 3 fixation techniques significantly reduced motion compared to the intact and destabilized cases. There were no statistically significant differences between the C2 intralaminar and pedicle screw techniques.

CONCLUSIONS

The results clearly indicate the potential of the intralaminar screw technique to provide stability that is equivalent to methods currently used. Given the serious complications that can follow vertebral artery injury and the decreased likelihood of injury by avoiding placement of C2 pedicle screw(s) and C1-C2 transarticular screw(s), strong consideration should be given to using a construct that incorporates C2 intralaminar screw(s).

Intervertebral disc replacement maintains cervical spine kinetics

STUDY DESIGN

An in vitro biomechanical study of C4-C5 intervertebral disc replacement using a cadaveric model.

OBJECTIVES

To investigate the degree of motion afforded by a ball-and-socket cervical intervertebral disc prosthesis design.

SUMMARY OF BACKGROUND DATA

Intervertebral disc prostheses designs attempt to restore or maintain cervical disc motion after anterior cervical discectomy and reduce the likelihood of accelerated degeneration in adjacent discs by maintaining normal motion at the affected disc level. Surprisingly, the actual kinetic and biomechanical effects that cervical disc arthroplasty imparts on the spine have not been widely reported. Accordingly, we investigated what effect implanting a cervical disc prosthesis has on the range of motion at the affected level as well as how it changes the coupled motion patterns at the level of implantation.

METHODS

Six fresh-frozen human cadaveric cervical spines (C2-C7) were used in this study. We evaluated two different spinal conditions: intact and after disc replacement at C4-C5. Compression (using the follower load concept) and pure moment loading were applied to the specimen. Range of motion was measured using an optical tracking system. Statistical differences between the intact and replaced condition range of motion was determined using analysis of variance with post hoc comparisons (alpha = 0.05).

RESULTS

The data indicate that the intervertebral disc prosthesis approximated the intact motion in all three rotation planes at the affected level. Finally, changes in cervical coupled rotations, specifically lateral bending during axial rotation loading and axial rotation during lateral bending loading, were not statistically significant between the two tested conditions.

CONCLUSIONS

Our data demonstrate that a ball-and-socket design can replicate physiologic motion at the affected and adjacent levels. More importantly, the data indicate that motion coupling, which is most dramatic in the cervical spine and plays an important biomechanical role, is maintained.

Mechanical performance of the new posterior spinal implant: effect of materials, connecting plate, and pedicle screw design

STUDY DESIGN

A newly designed spinal implant was tested to evaluate multicycle stiffness and fatigue resistance.

OBJECTIVES

To investigate the effect of different materials, connecting plate, and pedicle screw design on the mechanical performance of the spinal implant.

SUMMARY OF THE BACKGROUND DATA

The addition of cross-linkages did not significantly increase implant compression/flexion stiffness, but accelerated fatigue failure at the rod junctions. Both Ti-6Al-4V spinal implants and the 316L stainless-steel counterparts have been used extensively for clinical cases; however, design factors establishing the proposed superiority of the Ti-6Al-4V implant for fatigue resistance have not, as yet, been extensively studied.

METHODS

Twenty implants with connecting plates (two materials by two screw designs by five implants) and five implants without connecting plates were assembled to UHMWPE blocks and cyclically loaded from 60 N to 600 N at a frequency of 5 Hz.

RESULTS

Failure sites for the tested prototypes were at the cephalic screw hubs or rod-plate junctions. All Ti-6Al-4V implants demonstrated reduced stiffness compared to the structurally identical 316L analogs. The use of connecting plates raised the stiffness of the 316L prototypes without cross-links. However, elimination of the connecting plate avoided stress concentration at the rod/plate junctions and increased fatigue life. The Ti-6Al-4V new system with the minimal notch effect at the screw hubs achieved greater fatigue resistance than its 316L counterpart. By contrast, enlargement of the inner-hub diameter resulted in greater gains for fatigue resistance than for stiffness, especially for Ti-6Al-4V variants.

CONCLUSIONS

Although Ti-6Al-4V was superior to 316L for endurance-limit properties, structural design of the Ti-6Al-4V implant dramatically affects fatigue resistance. This may explain the differences between existing studies and the current report, comparing fatigue life for implants made from these two materials. Our results reveal that Ti-6Al-4V must be carefully treated because of sensitivity to notch, with special consideration given to screw-hub design.

The pylon concept of pelvic anchorage for spinal instrumentation in the human cadaver

STUDY DESIGN

Human cadavera morphometric analysis of the iliac columns and biomechanical implant testing of traditional Galveston technique compared to intrailiac instrumentation of the entire iliac column.

OBJECTIVES

To describe the anatomy of the iliac columns and to evaluate the strength in forward flexion of a large implant spanning the entire column length compared to standard Galveston technique.

SUMMARY OF BACKGROUND DATA

We have observed substantial and straight columns of bone in the pelvis, connecting the acetabula to the sacrum, which may allow for improved spinopelvic instrumentation.

METHODS

Twenty adult cadaveric pelves were used. Each specimen was oriented in the computed tomography scanner to obtain a cross-section of the iliac columns, which begin from 2 cm caudal to the posterior iliac spines and end above the acetabula at the anterior inferior iliac spines. Two different instrumentation techniques were used. Standard Galveston pelvic fixation with paired 6.25-mm diameter rods extending 8 cm into the pelvis (Group 1) was compared to paired 8-mm diameter, 15-cm long custom implants, placed within the length of the entire iliac columns and connected to 6.25-mm spinal rods (Group 2). Both constructs had two rigid cross-links connecting the rods. Testing in forward flexion was performed for each construct with the MTS model 881 at 5 N/sec until failure occurred.

RESULTS

The rectangular shaped iliac columns averaged 15.2 (SD 0.8) cm in length, 2.5 (SD 0.3) cm in width and were consistently straight. The iliac column orientation as viewed in the transverse plane was 22 degrees laterally directed from the midsagittal plane. For the Galveston technique, failure with a flexion force occurred at a mean of 682 (SD 217) N. The iliac column implants failed at a mean of 2153 (SD 1370) N (P < 0.004).

CONCLUSION

The human adult pelvis has substantial and straight columns of bone extending from 2 cm below the posterior iliac spine, traversing above the sciatic notch, and ending at the anterior iliac spine. The shape resembles a weight-bearing long bone such as the tibia. Analogous to the architectural pylon, in this cadaver model, large implant instrumentation of the entire length of these pelvic columns provides at least three times stronger anchorage for spinal instrumentation compared to standard Galveston technique.

Simulation of bone adaptive remodeling using a stochastic process as loading history

In this simulation study for bone adaptive remodeling, loading conditions are described as stochastic processes to catch the unpredictable characteristics of daily physical activities, which are observed to be closely related with bone adaptive remodeling. This will not only eliminate the necessity of arbitrary choices for loading conditions, but also generate greater flexibility for simulations of bone adaptive remodeling. The sensitivity of simulation outcomes to the parameters in the simulation algorithm was examined by applying stochastic loading conditions on finite element models of simplified spine structures. In this way, the limitations induced by simplifying loading conditions into constant or cyclic loads can be avoided and, potentially, more clinical observations could be accommodated when more comprehensive finite element models are available.

A finite element investigation of upper cervical instrumentation

STUDY DESIGN

The finite element technique was used to predict changes in biomechanics that accompany the application of a novel instrumentation system designed for use in the upper cervical spine.

OBJECTIVE

To determine alterations in joint loading, kinematics, and instrumentation stresses in the craniovertebral junction after application of a novel instrumentation system. Specifically, this design was used to assess the changes in these parameters brought about by two different cervical anchor types: C2 pedicle versus C2-C1 transarticular screws, and unilateral versus bilateral instrumentation.

SUMMARY OF BACKGROUND DATA

Arthrodesis procedures can be difficult to obtain in the highly mobile craniovertebral junction. Solid fusion is most likely achieved when motion is eliminated. Biomechanical studies have shown that C1-C2 transarticular screws provide good stability in craniovertebral constructs; however, implantation of these screws is accompanied by risk of vertebral artery injury. A novel instrumentation system that can be used with transarticular screws or with C2 pedicle screws has been developed. This design also allows for unilateral or bilateral implantation. However, the authors are unaware of any reports to date on the changes in joint loading or instrumentation stresses that are associated with the choice of C2 anchor or unilateral/bilateral use.

METHODS

A ligamentous, nonlinear, sliding contact, three-dimensional finite element model of the C0-C1-C2 complex and a novel instrumentation system was developed. Validation of the model has been previously reported. Finite element models representing combinations of cervical anchor type (C1-C2 transarticular screws vs. C2 pedicle screws) and unilateral versus bilateral instrumentation were evaluated. All models were subjected to compression with pure moments in either flexion, extension, or lateral bending. Kinematic reductions with respect to the intact (uninjured and without instrumentation) case caused by instrumentation use were reported. Changes in loading profiles through the right and left C0-C1 and C1-C2 facets, transverse ligament-dens, and dens-anterior ring of C1 articulations were calculated by the finite element model. Maximum von Mises stresses within the instrumentation were predicted for each model variant and loading scenario.

RESULTS

Bilateral instrumentation provided greater motion reductions than the unilateral instrumentation. When used bilaterally, C2 pedicle screws approximate the kinematic reductions and instrumentation stresses (except in lateral bending) that are seen with C1-C2 transarticular screws. The finite element model predicted that the maximum stress was always in the region in which the plate transformed into the rod.

CONCLUSIONS

To the best of the authors' knowledge, this is the first report of predicting changes in loading in the upper cervical spine caused by instrumentation. The most significant conclusion that can be drawn from the finite element model predictions is that C2 pedicle screw fixation provides the same relative stability and instrumentation stresses as C1-C2 transarticular screw use. C2 pedicle screws can be a good alternative to C2-C1 transarticular screws when bilateral instrumentation is applied.

Differential biomechanical effects of injury and wiring at C1-C2

STUDY DESIGN

An in vitro study compared the biomechanics of the upper cervical spine among three groups of cadaveric specimens, each with a different source of instability: transverse-alar-apical ligament disruptions, odontoid fractures, or odontoidectomies. The responses of the three groups were again compared after a uniform posterior cable and graft fixation was applied to the specimens.

OBJECTIVES

To quantify and compare the effects of different injuries on atlantoaxial stability and to determine whether a single fixation technique effectively treats each injury.

SUMMARY OF BACKGROUND DATA

Previous biomechanical studies of atlantoaxial instability have been focused on mechanisms of injury or on comparison among fixation types.

METHODS

Cables and pulleys applied torques to human cadaveric C0-C6 specimens quasistatically while an optical system tracked three-dimensional angular and translational motion at C0-C1 and C1-C2. Specimens were tested immediately after injury, after posterior cable and graft fixation, and after 6000 cycles of fatigue.

RESULTS

Odontoidectomies increased C1-C2 angular and translational range of motion significantly more than odontoid fractures or ligament disruptions, especially during flexion-extension. Odontoid fractures produced a slightly larger increase in C1-C2 angular range of motion than ligament disruptions but a smaller increase in C0-C1 range of motion. The different injuries affected the lax zone and the position of C1-C2 axis of rotation differently. Restabilization by posterior cable and graft reduced motion only moderately for each injury type. All three fixated injuries were susceptible to loosening from fatigue.

CONCLUSION

The three different injuries produce different spinal biomechanical responses. To best promote fusion, posterior cable and graft fixation should be used with an adjunctive stabilizing technique to treat all three injuries.