David N. Kunz

Posterior lumbar interbody fusion. A biomechanical comparison, including a new threaded cage.

Study Design In vitro biomechanical testing was performed on eight lumbar calf spines. Objectives To compare the initial stiffness of a standard method of posterior lumbar interbody fusion using structural autograft with the same procedure using additional posterior instrumentation. These constructs also were compared to a new titanium implant. Summary of Background Data Posterior lumbar interbody fusion is gaining wide acceptance for the treatment of segmental spinal instability, spondylolisthesis, and discogenic pain. Many methods have been described, including use of autograft or allograft bone, in either structural or nonstructural form, with or without additional fixation. A new threaded titanium interbody implant has been designed to increase initial stability while allowing bony ingrowth for fusion. Methods Eight lumbar calf spines were subjected to axial compression, sagittal moments (flexion‐extension), and axial torque while displacement was measured. Stiffness was calculated from the load displacement curves, for each construct under each load pattern. Results The posterior lumbar interbody fusion by bone graft alone was the least stiff construct of all modes tested. In two of eight specimens the bone graft dislodged posteriorly into the canal during torsional testing. The titanium interbody implant was similar in stiffness to the bone graft posterior lumbar interbody fusion with posterior instrumentation group in all three modes. They were both significantly stiffer than the normal spine, the destabilized spine, and the posterior lumbar interbody fusion by bone graft alone (P < 0.05). Conclusions In this model, the posterior lumbar interbody fusion with bone graft alone had less initial stiffness than that of the intact spine. The addition of posterior instrumentation or interbody implants can increase initial stiffness significantly.

Cervical stability after foraminotomy. A biomechanical in vitro analysis.

Laminectomy or facetectomy of the cervical spine, or both, may be needed for decompression of the spinal cord or of the nerve-roots. Acute stability of the cervical spine was tested after laminectomy and progressive staged foraminotomies in an in vitro model. Twelve cervical spines from human cadavera were used in the experiment. Biomechanical testing included the application of an axial load, the application of a flexion and extension moment, and the application of a torsional moment. Each specimen was tested intact, after laminectomy of the fifth cervical vertebra, and after progressive foraminotomy of the sixth cervical root. Foraminotomy was performed by resection of 25, 50, 75, and 100 per cent of the facet joint and capsule. Torsional stiffness decreased dramatically when more than 50 per cent of the facet had been resected. Statistically equivalent subsets were the intact specimen, laminectomy, 25 per cent facetectomy, and 50 per cent facetectomy in one subset, and 75 and 100 per cent facetectomy in the least-stiff subset. Flexion-moment testing showed that the posterior strain did not differ among three groups: the intact specimens, those that had been treated with laminectomy, and those that had been treated with a 25 per cent facetectomy. The 50 per cent facetectomy resulted in a 2.5 per cent increase in posterior strain, and the 75 or 100 per cent facetectomy, in a 25 per cent increase in posterior strain compared with the intact specimen. Segmental hypermobility of the cervical spine results if a foraminotomy involves resection of more than 50 per cent of the facet.(ABSTRACT TRUNCATED AT 250 WORDS)

Cervical stability after sequential capsule resection

A portion of the cervical facet joint must be resected to expose and decompress cervical nerve roots from a posterior approach. When posterior fusion is performed, it is common to remove the facet capsule only for the joints being fused. This study was performed to examine the effect of resection of the facet capsule alone, without disruption of the bony facet to determine what degree of facet-capsule resection leads to acute instability. Seven human cervical cadaveric spines were used in the experiment. Nondestructive biomechanical testing was performed in axial load, flexion, extension, and torsion. Each specimen was tested intact and after sequential resection of 25%, 50%, 75%, and 100% of the C5–6 facet capsules. Axial stiffness changed very little during the experiment. In torsion, the displacement increased 1% after a 25% capsule resection, 19% after a 50% resection, and 25% after a 75% or 100% resection. No gross subluxation was seen during the torsional test. In the flexion test, posterior displacement increased 4% after a 25% resection, 5% after a 50% resection, 32% after a 75% resection, and 22% after a 100% resection. There was a statistically increased displacement seen during the flexion test after 75% or 100% of capsule resection. Thus, significant hypermobility did occur during both torsion and flexion testing with greater than 50% resection of the facet capsules. Great care should be taken when exposing an unfused facet to limit facet-capsule resection to less than 50%. With resection of greater than 50% of the capsule, postoperative hypermobility can occur and may require stabilization.

Anterior cervical discectomy, fusion, and plating : a comparative animal study

Thirty-five goats were used as animal models for three-level anterior cervical discectomy and fusion. The goats were divided equally into five experimental groups: Group I, three-level anterior cervical discectomy without fusion; Group IIa, three-level discectomy with autogenous bone (Smith-Robinson technique); Group IIb, autogenous bone grafting plus anterior plate application; Group III, three-level discectomy and fusions with fresh frozen allograft bone; Group IIIa, graft only; and Group IIIb, anterior plating. The goats were killed, and spines were removed and analyzed for evidence of fusion. Radiographic union was judged to have occurred in 0% of Group I disc spaces, 48% of Group IIa, 52% of Group IIb, and 38% of Groups IIIa and IIIb. Histologic fusion was judged to have occurred in 0% of Group I specimens, 38% of Group IIa, 45% of Group IIb, 0% of Group IIIa, and 19% of Group IIIb. The histologic fusion rate was significantly higher in Groups IIa and IIb than in Groups I and IIIa. There was no statistically significant increase in the histologic fusion rate between goats with anterior cervical plating and goats without plating. Biomechanically, the spines in Groups IIb and IIIb, ie, those with anterior plates applied, were stiffer in axial load, torsion, and flexion/extension. Peri-end-plate vascularity was significantly diminished in those groups that had anterior plates applied. Qualitative analysis of fluorochrome labels showed that autografts revascularized more rapidly than did allografts in both the nonplated and plated groups. Histomorphometric analysis failed to reveal any significant device-related osteopenia in those vertebrae spanned by the anterior plate. We found that although autograft bone led to a significantly higher rate of union than did allograft bone, the addition of anterior plate fixation did not significantly increase union rate. Biomechanical rigidity was significantly increased in all modes of testing by the use of an anterior plate. The decreased vascular response seen in spines that underwent plating may be responsible for the lack of increased union. The increased rigidity found with anterior plating supports its use in traumatic conditions. The failure, however, to increase significantly the union rate in this model fails to lend support to the use of anterior cervical plating for degenerative discectomy and fusion procedures.

Anterior cervical discectomy and fusion using a porous hydroxyapatite bone graft substitute.

Objectives. This study analyzed the use of a coral hydroxyapatite bone substitute for use in ACDF both with and without an anterior cervical plate. Study Design. The healing of multilevel anterior cervical fusions was tested using a goat model. Comparisons were drawn with histologic, radiographic, and bio‐mechanical test data. Methods. Forty‐nine mature alpine goats had three‐level anterior discectomies performed. Seven treatment groups of seven goats each were used; Group I with no fusion, Group IIa having tricortical iliac crest autograft, Group IIb having autograft plus an anterior plate, Group Ilia having tricortical iliac crest fresh‐frozen allograft, Group IIIb having allograft plus an anterior plate, Group IVa having rectangular‐shaped implants of porous hydroxyapatite, and Group IVb having ProOsteon 500 implants with an anterior cervical plate. Results. Histologically, at 12 weeks 48% of the ProOsteon (Interpore, Irvine, CA) implants were rated as incorporated, 10% as possessing a fibrous gap, 29% as collapsed, and 14% as extruded. Anterior cervical plating improved the results with 71% of the implants showing good incorporation, 24% with collapse, and 5% with a fibrous gap. These histologic results compare favorably with autogenous bone and are improved over allograft bone. Fluorochrome analysis showed that none of the implants had complete turnover with host bone, but that all possessed peripheral creeping substitution with cutting cones of new bone formation at 12 weeks. Biomechanically, the spines using the ProOsteon implant were less stiff in torsion than autograft, but equal in stiffness to allograft. Flexion‐extension neutral zone stiffness was lower in the ProOsteon implant group than either allograft or autograft. Conclusions. The use of a coral‐based hydroxyapatite bone graft substitute for anterior cervical fusions led to significant rates of implant collapse at 12 weeks but showed excellent biologic compatibility with good early creeping substitution of the implant by host bone. The concomitant use of an anterior cervical plate with the implant prevents extrusion.

Mechanical evaluation of cross-link designs in rigid pedicle screw systems.

Study Design. This study was designed to evaluate the biomechanical performance of 5 different cross‐link brands to determine which design characteristics are biomechanically desirable. Methods. The Cotrel‐Dubousset, Isola, Puno Winter Byrd, Rogozinski, and Texas Scottish Rite Hospital sys tems were assembled to vertebral models according to the manufacturers specifications. Three constructs were tested for each brand of instrumentation: without cross links, with one cross‐link, and with two cross‐links. Four modes of loading: axial, torsional, flexion‐extension, and lateral‐flexion were used. Load‐displacement curves were plotted. The stiffness was calculated from the slope of these curves. Objectives. Five different rigid pedicle screw systems were tested to determine: 1) what are the characteristics of cross‐link design that are most effective in limiting torsional motion; 2) whether two cross‐links are more effective than one; and 3) whether cross‐linkage in creases the construct stiffness in lateral bending. Summary of Background Data. Cross‐linkage has been shown to increase the torsional stiffness of rod and screw constructs. Increased construct stiffness has been correlated with higher fusion rates. Results. Increases in axial, flexion‐extension, or lat eral‐flexion stiffness, with the addition of one or two cross‐links, were not statistically significant. In torsional loading, increases in stiffness within brands were statis tically significant in every case. The average increase was 44% with one added cross‐link and 26% with two. The magnitude of the increase in torsional stiffness was compared with the cross‐sectional area of the respec tive cross‐link. Greater stiffness correlated with larger cross‐sectional area (r = 0.81 for one cross‐link, and r = 0.60 for two). Conclusion. The use of cross‐linkage in spinal fusion increases torsional stiffness in pedicle screw and hook constructs. This study 1) confirmed the effectiveness of cross‐linkage in limiting torsional motion and showed the superiority of two cross‐links to one cross‐link in limiting torsional motion, 2) showed that increase of torsional stiffness of a cross‐linked construct is propor tional to the cross‐sectional area of the cross‐link, and 3) demonstrated that cross‐links do not increases stiffness in the lateral flexion mode.

Anterior cervical discectomy and fusion : a comparison of techniques in an animal model

An animal model for three–level anterior carvical discectomy and fusion was established in the goat. Twenty–one goats underwent surgery, with seven goats in each of three experimental groups. In Group I, all seven goats underwent three–level anterior cervical discectomy without fusion. In Group II, each of the seven goats had a three–level anterior cervical discectimy with autogenous bone performed according to the Smith–Robinson technique. In Group III, fresh–frozen allograft bone was used for each of the three–level discectomy and fusion. Each goat was then killed after 12 weeks. Analysis consisted of radiographic review, fluorochrome labelling, biomechanical rigidity and flexion and extension, axial compressive load, and torsion. Histologic analysis was also performed for evidence of fusion and vertebral body histomorphometric analysis. The analysis of results showed that radiographic union was judged to have occurred in 0 of 21 Group l disc spaces, 10 of 21 Group II disc spaces, and 8 of 21 group III disc spaces. Histologic fusion was judged to have occurred in 0 of 21 Group I goats, 10 of 21 Group II goats, and 0 of 21 Groups III goats. The histologic fusion rate was significantly higher in Group II that either Group I or Group III. Biomechanically, the spines that had autogenous bone grafting (Group II) were significantly stiffer in compressive axial load and in extension. Both Group II and Group III were stiffer in fiexion than Group I. An evaluation of the peri–endplate vascularity showed that the vascularity measured 10.4% in Group I, 16.7% in Group II, and 8.5% in Group III. This was significantly Greater in Group II than in either Group I or Group III. In addition, qualitative analysis of the fluorochroem labelling showed that the autografts had revascularized by 6 weeks in all specimens, whereas only 1 of 21 allograft specimens had revasularized by 6 weeks. By 9 weeks, the allografts had revascularized. In this model using three–level anterior cervical discectomies and fusions, the authors found that autograft bone led to a significantly higher rate of union that did allogtaft bone. We believe that the increased rate of union with autograft bone is partly due to an increased vascular respinse to this bone. This study supports the use of autogenous iliac crest bone grafting over allograft bone grafting for cervical spine fusions.

Anterior Instrumentation of the Thoracolumbar Spine: A Biomechanical Comparison

Study Design. To evaluate the fatigue strength and stiffness of four anterior thoracolumbar fixation devices using a corpectomy model without load-sharing bone graft to test the devices under the worst case scenario of instability. Objectives. To gain a more thorough understanding of the biomechanical qualities of anterior fixation devices to improve clinical application and design. Summary of Background Data. For many surgeons, the anterior approach has become the treatment of choice for patients with compression of the spinal cord, whether it is caused by trauma, tumor, or infection. When stabilization is needed, anterior fixation devices have been advocated for many years to avoid the additional approach required for posterior fixation. Many of these devices, however, have an unacceptably high rate of hardware failure. Recently, several new devices for anterior fixation have been marketed with purported advantages in fatigue life and ease of use. Methods. Four implants, the Synthes Anterior Thoracolumbar Locking Plate, the Kaneda device, a Texas Scottish Rite Hospital anterior construct, and the Z-Plate were attached to vertebral models and tested for stiffness in multiple planes on a modified Materials Testing System machine. They then were fatigued to failure on an Instron testing machine. Results. The Anterior Thoracolumbar Locking Plate was the stiffest in axial compression, lateral flexion, and torsion. The Texas Scottish Rite Hospital anterior construct was the least stiff in flexion-extension, with no significant differences in the stiffness of the anterior thoracolumbar locking plate, that of the Kaneda device, and that of the Z-Plate. Fatigue life exceeded 80,000 cycles for the anterior thoracolumbar locking plate and averaged 26,472 cycles for the Z-Plate, 6915 cycles for the Texas Scottish Rite Hospital construct, and 4419 cycles for the Kaneda device. Conclusions. The significantly greater fatigue life of the Anterior Thoracolumbar Locking Plate and the Z-Plate may predict a lower incidence of hardware failure than with previous anterior devices. This has been confirmed in preliminary clinical studies with the Z-Plate. Further clinical studies are needed to show if these lower failure rates will continue over a long-term period.

Biomechanical evaluation of occipitocervical fixation devices.

Human cadaveric occipitocervical specimens were implanted with three types of instrumentation. The devices were tested biomechanically under three modes of loading to determine the stiffness of spinal constructs and the failure mechanisms of the constructs under extreme flexion. The devices tested were the AXIS Fixation System (with custom plate), the Y-Plate, and the Luque rectangle. No significant differences in stiffness among the devices were found under compression and flexion. The stiffnesses of the plate systems were statistically higher than the Luque rectangle in extension and torsion. In extreme flexion, the plate systems failed by fracture of the C2 pedicles. Modern plate systems, for occipitocervical fixation, provide more stiffness and stability than traditional wiring techniques. This study provides surgeons with information on the relative merits of modern plate and screw systems compared with traditional rod and wire constructs.

A biomechanical comparison evaluating the use of intermediate screws and cross-linkage in lumbar pedicle fixation.

In lumbar fusion, controversy remains regarding the effectiveness of cross-linking and the necessity of placing pedicle screws at the intermediate levels of the segment to be fused. The purpose of this study is to evaluate the stiffness of various rod/screw constructs used to instrument a three-level fusion with specific emphasis on the effect of cross-linking and the intermediate pedicle screws. Nine lumbar calf spines were mounted at L1 and L5. Pedicle screws (TSRH, Danek, Memphis, TN) were then placed bilaterally in the L2, L3, and L4 pedicles. Random sequence testing of the following constructs was then conducted: TSRH rods connected bilaterally to the L2 and L4 pedicles with and without a cross-link, and rods connected bilaterally at the L2, L3, and L4 levels with and without a cross-link. The tests were conducted on a modified MTS testing machine (MTS, Minneapolis, MN) and consisted of cyclic application of axial load, torsion, and flexion and extension. The tests yielded axial, sagittal, and torsional stiffness values. Statistical analysis was performed using log transformation and Fischers test of least significant difference. In axial testing the use of additional screws in the intermediate pedicles increased stiffness an average of 160% (p = .007). The addition of a cross-link did not increase stiffness with axial loading. In flexion testing the six-screw construct was 84% stiffer when compared with the four-screw construct (p = 0.0001). There was no significant change in flexion stiffness with addition of cross-links. In torsional testing the six-screw construct was 38% stiffer than the four-screw construct (p = 0.042).(ABSTRACT TRUNCATED AT 250 WORDS)