Ronald P. McCabe

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.

Degradation of bone structural properties by accumulation and coalescence of microcracks

Failure of bone adaptation to protect the skeleton from fatigue fracture is common, and site-specific accumulation and coalescence of microcracking in regions of high strain during cyclic loading is considered a key factor that decreases the resistance of whole bones to fracture. We investigated the effect of cyclic fatigue loading on the monotonic structural properties of the rat ulna during accumulation and coalescence of microcracks. Cyclic end-loading of the ulna was performed at 4 Hz ex vivo at an initial peak strain of -6000 muepsilon to 20% loss of stiffness (n = 7) or 40% loss of stiffness (n = 7) bilaterally. A 0% loss of stiffness monotonically loaded control group (n = 7) was also included. Volumetric bone mineral density (vBMD), ultimate strength (F(u)), stiffness (S), and energy-to-failure (U) were determined in one ulna and in the contralateral ulna vBMD, cortical bone area (B.Ar), maximum and minimum second moments of inertia (I(MAX) and I(MIN)), microcrack density (Cr.Dn), microcrack mean length (Cr.Le), and microcrack surface density (Cr.S.Dn) were determined. In two additional groups of rats, cyclic end-loading of the ulna was also performed ex vivo unilaterally to 20% loss of stiffness (n = 10) and 40% loss of stiffness (n = 10) and then vBMD, F(u), S, U, B.Ar, I(MAX), and I(MIN) were determined bilaterally. Fatigue loading had incremental degradative effects on ulna structural properties. This decreased resistance to fracture was associated with accumulation and coalescence of branching arrays of microcracks within the cortex of the ulna. Microcracking was most prominent in the middiaphysis and corresponded to the region of the bone that fractured during monotonic structural testing. Fatigue loading influenced the relationship between bone cross-sectional geometry and vBMD and ulna structural properties. At 40% loss of stiffness, F(u), S, and U were all significantly correlated with cross-sectional bone geometry and vBMD, whereas this was not the case at 20% loss of stiffness and with the 0% loss of stiffness monotonic control ulnae. We also found a biologically significant individual animal effect. Larger ulnae required a higher number of load cycles for fatigue to develop, retained higher strength, and accumulated a greater amount of microcracking at the end of the cyclic fatigue testing. Small increases in bone size and density can substantially improve the resistance of whole bones to fracture as microcracking accumulates and coalesces during cyclic fatigue loading.

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.

Evaluation of a new method to create a standardized muscle stretch injury

Herein we describe a new test system to produce a standardized partial muscle-tendon junction (MTJ) stretch injury. In anesthetized rabbits the tibialis anterior (TA) muscle-tendon unit is unilaterally shortened using a custom designed clamp roller system. An angular displacement (average velocity of 450 degrees x s[-1]) is applied about the foot to plantarflex the ankle 90 degrees while the lower extremity is fixed. During ankle rotation the TA muscle is tetanically stimulated to generate an eccentric stretch injury at the MTJ. Forty-eight hours after injury, isometric torque deficit (injured/sham) was measured. Two groups of animals (N = 6 in each group) were tested with the only difference between the two groups being the initial tendon shortening. In Group 1 (tendon shortening = 1.2 cm. N = 6) the torque deficit was 36.7+/-5.9% (mean+/-SD). In Group 2 (tendon shortening = 1.5 cm. N = 6) the torque deficit was 58.7+/-7.4% (mean+/-SD). No order effect was suggested by the data (P = 0.6062), but the difference in torque deficit between the two groups was highly significant (P = 0.0001). For all tests in which the tendon was temporarily shortened before muscle stimulation and stretch (N = 12) there was a visible hematoma at the MTJ similar to the injury that is common in athletic injuries. Histological evaluation 48 h after injury revealed both fiber tearing and inflammation at the MTJ. In addition, there was focal fiber damage in the muscle belly for both groups. The damage and inflammatory process, however, were more severe in the group with greater initial tendon shortening.

A Multi-Degree of Freedom System for Biomechanical Testing

A system is described that allows axial, torsional, and bending testing of biomechanical specimens. The system uses electric motors under closed loop control in its grips allowing application of pure bending moments. These grips attach to an axial/torsional testing system. Thus, it provides simultaneous closed loop control of all three degrees of freedom (D.O.F), so that under any given test condition either the loads or the displacements for each D.O.F. can be maintained at zero, selected constant values, or simultaneously controlled. This enables the expedient evaluation of the mechanical behavior of biological structures under complex loadings or simple loadings (one D.O.F.) with no artificially induced constraints in the other two D.O.F.s due to specimen mounting.

A biomechanical analysis of prophylactic fixation for pathological fractures of the distal third of the humerus.

Twenty-four matched pairs of fresh-frozen humeri from human cadavera were divided randomly into four groups, in order to determine the most biomechanically desirable construct for the prophylactic fixation of impending fractures of the distal third of the humerus. Group I comprised intact humeri and matched humeri in which a 50 per cent lateral, semicylindrical cortical defect of the distal third had been created, resulting in a reproducible model of an impending fracture due to a lytic defect involving 50 per cent cortical disruption at the distal end of the humeral medullary canal. In Group II, such a lateral defect was created in both the right and the left, matched humeri. Group III was composed of humeri in which the defect had been fixed prophylactically with a single plate and the contralateral humeri, which had been treated with double-plating. Group IV comprised specimens in which the defect had been fixed with double-plating as well as those fixed with Rush rods. The fixation of each specimen in Groups III and IV was supplemented with bone cement. Each specimen was tested in torsion to failure, and the resulting peak torque, torsional stiffness, and total energy absorbed were analyzed for each group. The Group-I specimens that had a defect had a significantly lower (p < 0.05) peak torque, torsional stiffness, and total energy absorbed than the intact specimens; all of the specimens with a defect failed at the defect, and all of the intact specimens failed proximally. In Group II, there was a high side-to-side association with respect to peak torque, torsional stiffness, and total energy absorbed.(ABSTRACT TRUNCATED AT 250 WORDS)