ABSTRACT: Ex vivo biomechanical testing of human cadaveric thoracic spine segments.
To determine whether a hybrid construct, using a combination of pedicle screws (PSs) and lamina hooks, was equivalent to a PS construct, in a short-segment thoracic spine fixation model.
Comparisons have been made among PS, lamina hook, and hybrid screw-hook constructs, but these have generally been in long-segment scoliosis correction. In this study, we compared the hybrid and screw-only constructs in a short-segment thoracic fixation.
For pullout testing, matched specimens were used for PS (n = 8) and hybrid (n = 8) constructs. Construct stiffness, and the force required for construct failure, were measured. Dynamic testing was carried out on specimens in the PS (n = 7) and hybrid (n = 7) groups in compression, flexion, extension, and left and right lateral bending. Each group was tested intact, after instrumentation, and after corpectomy.
When compared with the hybrid group, a significantly greater force was required for construct failure in the PS group, and these PS constructs were significantly stiffer. No differences were found between groups in dynamic testing.
A construct employing PSs is significantly stiffer and more resistant to pullout failure than a hook-screw hybrid construct.
Spine 02/2008; 33(2):173-7. · 2.08 Impact Factor
ABSTRACT: A preliminary in vitro biomechanical study was conducted to determine if the pressure at a bone graft-mortise interface and the load transmitted along a ventral cervical plate could be used as parameters to assess fusion status.
An interbody bone graft and a ventral plate were placed at the C3-4 motion segment in six fresh cadaveric goat spines. Polymethylmethacrylate (PMMA) was used to simulate early bone fusion at the bone graft site. The loads along the plate and the simultaneous pressures induced at the graft-endplate interfaces were monitored during simulated stages of bone healing. Each specimen was nondestructively tested in compression loading while the pressures and loads at the graft site were recorded continuously. Each specimen was tested under five conditions (Disc, Graft, Plate, PMMA, and Removal).
The pressure at the interface of the bone graft and vertebral endplate did not change significantly with the addition of the ventral plate. The interface pressure and segmental stiffness did increase following PMMA augmentation of the bone graft (simulating an intermediate phase of bone fusion). The load transmitted along the ventral plate in compression increased after the addition of the bone graft, but decreased after PMMA augmentation. Thus, there was an increase in pressure at the graft-endplate interface and a decrease in load transferred along the ventral plate after the simulation of bone fusion. Upon removal of the ventral plate, the simulated fusion bore most of the axial load, thus explaining a further increase in graft site pressure.
These observations support the notions of load sharing and the redistribution of loads occurring during and after bone graft incorporation. In the clinical setting, these parameters may be useful in the assessment of fusion after spine surgery. Although feasibility has been demonstrated in this preliminary study, further research is needed.
Journal of Neurosurgery Spine 12/2007; 7(5):542-8. · 1.53 Impact Factor
ABSTRACT: Pedicle screw pullout testing in osteoporotic and control human cadaveric vertebrae, comparing augmented and control vertebrae.
To compare the pullout strengths of pedicle screws fixed in osteoporotic vertebrae using polymethyl methacrylate delivered by 2 augmentation techniques, a standard transpedicular approach and kyphoplasty type approach.
Pedicle screw instrumentation of the osteoporotic spine carries an increased risk of screw loosening, pullout, and fixation failure. Osteoporosis is often cited as a contraindication for pedicle screw fixation. Augmentation of the vertebral pedicle and body using polymethyl methacrylate may improve fixation strength and construct survival in the osteoporotic vertebrae. While the utility of polymethyl methacrylate has been demonstrated for salvage of screws that have been pulled out, the effect of the cement technique on pullout strength in osteoporotic vertebrae has not been previously studied.
Thirteen osteoporotic and 9 healthy human lumbar vertebrae were tested. All specimens were instrumented with pedicle screws using a uniform technique. Osteoporotic pedicles were augmented with polymethyl methacrylate using either a kyphoplasty type technique or a transpedicular augmentation technique. Screws were tested in a paired testing array, randomly assigning the augmentation techniques to opposite sides of each vertebra. Pullout to failure was performed either primarily or after a 5000-cycle tangential fatigue conditioning exposure. After testing, following screw removal, specimens were cut in the axial plane through the center of the vertebral body to inspect the cement distribution.
Pedicle screws placed in osteoporotic vertebrae had higher pullout loads when augmented with the kyphoplasty technique compared to transpedicular augmentation (1414 +/- 338 versus 756 +/- 300 N, respectively; P < 0.001). An unpaired t test showed that fatigued pedicle screws in osteoporotic vertebrae augmented by kyphoplasty showed higher pullout resistance than those placed in healthy control vertebrae (P = 0.002). Both kyphoplasty type augmentation (P = 0.007) and transpedicular augmentation (P = 0.02) increased pullout loads compared to pedicle screws placed in nonaugmented osteoporotic vertebrae when tested after fatigue cycling.
Pedicle screw augmentation with polymethyl methacrylate improves the initial fixation strength and fatigue strength of instrumentation in osteoporotic vertebrae. Pedicle screws augmented using the kyphoplasty technique had significantly greater pullout strength than those augmented with transpedicular augmentation technique and those placed in healthy control vertebrae with no augmentation.
Spine 06/2007; 32(10):1077-83. · 2.08 Impact Factor
ABSTRACT: In vitro biomechanics.
To determine if osteoporotic vertebral compression fracture (VCF) augmentation increases adjacent level load transfer.
Osteoporotic VCF subsequent to augmentation may result from disease progression or increased adjacent level load transfer, or both.
There were 11 T3-T7 and 10 T8-T12 divided by lumbar bone mineral density into a normal group (No. 1; n = 11) and an osteoporotic group (No. 2; n = 10). Strain and centrum stress were measured on T4 and T6 (T3-T7), and T9 and T11 (T8-T12) during tests in the intact state, following a centrum defect, during and after an augmented VCF at T5 or T10, and during a subsequent VCF. Stiffness and strength were compared: between groups 1 and 2; among intact, defect, and augmented VCF states; and between the initial and subsequent VCF.
Group 1 was stiffer than 2 in compression (P = 0.01) and flexion (P = 0.07), with no difference in adjacent level load transfer (strain P = 0.72, centrum stress P = 0.36) or strength (P = 0.07). The centrum defect reduced compressive stiffness from the intact (P = 0.001), which was partially restored following VCF augmentation (P = 0.006). There were no differences in flexion stiffness (P > or = 0.14). Adjacent level load transfer in flexion exceeded that in compression (strain P = 0.001, centrum stress P = 0.19). Initial and subsequent VCF occurred at similar forces (P = 0.26) with higher adjacent level load at subsequent (strain and centrum stress P = 0.04).
Augmentation of multilevel spinal segments with VCF produced by combined compression, flexion, and a centrum defect normalizes adjacent level load transfer at physiologic loads. In both normal and osteoporotic spinal segments, as loads approach those of the initial VCF, protection from augmentation is lost, and subsequent adjacent level VCFs occur from extreme loading, and not the augmentation process.
Spine 10/2006; 31(21):E790-7. · 2.08 Impact Factor
ABSTRACT: Osteoporosis frequently leads to vertebral compression fractures. Percutaneous cement augmentation, one recent technique, may alter the biomechanics of the vertebral body and spinal segment. These alterations reportedly predispose the spinal segment to additional vertebral compression fractures. We investigated the changes in segment stiffness and strength after polymethylmethacrylate augmentation. Twelve thoracic segments consisting of five vertebral bodies were divided into two groups, a pure moment group (Group 1) and an eccentric compression group (Group 2). Baseline measurements of stiffness were taken on each segment followed by the creation of an initial vertebral compression fracture during which stiffness and strength were measured. After augmentation, stiffness was again measured. Finally, a second vertebral compression fracture was created measuring stiffness and strength again. Augmentation did not alter stiffness before and after augmentation in either group. Augmentation also did not result in any difference in strength measured at subsequent fracture when compared with strength measured at initial fracture in either group. The augmentation of vertebral compression fractures by kyphoplasty does not alter the stiffness or the strength of the multilevel segments and eccentric compression in contrast to pure moments leads to a lower strength during mechanical testing.
Clinical Orthopaedics and Related Research 03/2006; 443:124-30. · 2.53 Impact Factor
ABSTRACT: Pedicle screw instrumentation of the thoracic spine remains technically challenging. Transverse process and costotransverse screw fixation techniques have been described as alternatives to pedicle screw fixation (PSF). In this study, the authors introduce thoracic transfacet PSF and compare its experimental biomechanical results with those of standard PSF in short-term cyclic loading in cadaveric thoracic specimens.
Specimens were tested intact for six cycles at compressive loads of 250 N offset by 1 cm along appropriate axes to induce flexion, extension, and left and right lateral bending. The specimens were then fixed with either a pedicle screw/rod construct or transfacet pedicle screws and retested in the same fashion. After this sequence, specimens were loaded until failure in flexion mode at a rate of 5 mm/minute was observed. Both fixation constructs provided significantly greater stiffnesses than that demonstrated when the specimen was intact (p < 0.05, two-way analysis of variance). Additionally, the two constructs were statistically equivalent in terms of stiffness and load-to-failure values (p < 0.05, two-tailed nonpaired t-test). The only difference observed was that the low midthoracic region (T7-9) was biomechanically weaker than the upper midthoracic and lower thoracic areas in flexion after the destabilization and instrumentation-augmented stabilization procedures.
In selected thoracic surgical procedures, transfacet PSF may, after analysis of long-term biomechanical data, potentially become a reasonable alternative to conventional PSF.
Journal of Neurosurgery Spine 09/2005; 3(3):224-9. · 1.53 Impact Factor
ABSTRACT: Different atlantoaxial fusion techniques are used for instability. Transarticular screws are biomechanically superior to wiring techniques and equivalent to C1 lateral mass to C2 pedicle (C1LM-C2P) fixation. Recently, C1 lateral mass to C2 laminar (C1LM-C2L) fixation has been shown to have flexibility similar to C1LM-C2P fixation in flexion, extension, lateral bending, and axial rotation.
Compare the stiffness of C1LM-C2P with C1LM-C2L screw rod fixation.
In vitro biomechanical study.
Stiffness in flexion/extension, lateral bending, axial rotation, and anterior-posterior (AP) translation.
Eight fresh-frozen human cadaveric cervical spines (C1-C3) were tested intact and, after a type II odontoid fracture, were instrumented and tested with two fixation constructs: C1LM-C2P screws and C1LM-C2L screws. The testing involved flexion, extension, lateral bending, AP translation, and axial rotation. Stiffness was measured and compared with a repeated-measures analysis.
C1LM-C2P was significantly stiffer than the intact in AP translation (p<.001), lateral bending (p=.001), and axial rotation (p=.002) and equivalent in flexion/extension (p=.09). C1LM-C2L was significantly stiffer than the intact in AP translation (p<.01) and axial rotation (p<.004) and equivalent in lateral bending (p<.71) and flexion/extension (p=.22). C1LM-C2P was stiffer than C1LM-C2L in right/left lateral bending (p<.001) and axial rotation (p=.009) and equivalent in AP translation (p=.06) and flexion/extension (p=.74).
C1LM-C2P fixation is equivalent to C1LM-C2L fixation in flexion/extension and AP translation and superior in lateral bending and axial rotation.
The Spine Journal 7(6):682-8. · 3.29 Impact Factor
ABSTRACT: As the population ages, the number of individuals undergoing pharmacotherapy to prevent or treat osteoporosis is increasing. Drugs of the bisphosphonate family prevent bone resorption, as does calcitonin, though by different mechanisms. Bisphosphonates are deposited in bone, preventing resorption by osteoclasts. Calcitonin is a direct inhibitor of osteoclasts, but is not itself incorporated in bone. The same late middle-aged and elderly patients who are being treated for osteoporosis frequently come to spine fusion. Bone remodelling is a vital part of graft incorporation. Interventions that interfere with remodelling may have a detrimental effect on the rate, time course, and strength of the fusion mass.
To delineate the effects of these anti-osteoporosis medications on the fusion process.
Randomized, prospective, double-blind, animal model.
Posterolateral arthrodesis was performed at L5/6 in 60 skeletally mature 4.0-4.5 kg New Zealand white rabbits, using 3 cc of autologous iliac crest graft per side. Rabbits were randomized to one of three groups: PAM--pamidronate 1.2 mg subcutaneously 3 times/week for 4 weeks preoperatively, then 0.6 mg/day via miniosmotic pump for 4 weeks postoperatively; CAL--calcitonin 14 IU/day via pump for 4 weeks postoperatively only; CON--no drug intervention. All animals were killed 5 weeks after surgery. Fusion, defined by absolute lack of intersegmental motion, was assessed by manual palpation by two spine surgeons. Where there was disagreement, a third surgeon made the final determination. Stiffness and peak load to failure were determined by mechanical testing of each operated motion segment, and normalized to the adjacent, unoperated level.
Four rabbits excluded (1 each: death; euthanasia for hind-limb palsy; infection; incorrect level). Number fused at 5 weeks: CON 10/18 (56%), PAM 7/19 (37%), CAL 13/19 (68%). Fisher exact test showed no significant differences between groups. Analysis of variance (ANOVA) showed no significant differences in mechanical testing between CAL and CON, but PAM specimens had significantly less peak load than CON or CAL animals (p<.01) and were less stiff than CON (p<.01) or CAL (p<.05) animals.
Though one must be careful in extrapolating animal data to humans, this study suggests that calcitonin is not detrimental to spine fusion. Pamidronate, however, does lead to a mechanically less robust fusion. Based on this study, there is no evidence to support a recommendation to stop antiresorptive therapy for osteoporosis in the spine fusion patient.
The Spine Journal 5(5):542-7. · 3.29 Impact Factor