PubMed Health⌕ Search

Biomedical subjects

Lisa Ferrara

Publications and source records attributed to Lisa Ferrara.

8 recordsLinked to original sources
PubMed2006

The biomechanics of 1, 2, and 3 levels of vertebral augmentation with polymethylmethacrylate in multilevel spinal segments.

STUDY DESIGN: Experimental biomechanics of multilevel segments with 0, 1, 2, and 3 vertebral levels of polymethylmethacrylate augmentation. OBJECTIVE: To compare multilevel spinal segments with different numbers (0, 1, 2, and 3) of vertebral levels augmented with polymethylmethacrylate. SUMMARY OF BACKGROUND DATA: The stiffness and strength of single-level polymethylmethacrylate augmentations in individual and multilevel vertebrae treated by kyphoplasty and vertebroplasty have been studied, but the biomechanics of multilevel segments with more than 1 vertebral level augmented with polymethylmethacrylate are lacking, yet this is clinically relevant in multilevel compression fracture treatment. MATERIALS AND METHODS: A total of 48 multilevel segments (T3-T5, T6-T8, T9-T11, T12-L2, and L3-L5) from 12 spines with known bone mineral density (BMD) were allocated into 6 groups based on the number of vertebral levels augmented: 0 levels (n = 13), control group; 1 level (n = 7), group 2; 2 levels, groups 3, 4, and 5 (n = 7 in each); and 3 levels (n = 7), group 6. They were compressed to failure, disarticulated into individual vertebrae, and retested. Stiffness and strength were statistically analyzed using a univariate analysis of variance comparing the main effects, using least significant difference comparisons with 0.05 probability level. RESULTS: Strength was dependent on BMD (P < 0.001 multilevel segments, P < 0.001 individual vertebrae), with no differences among the 6 different augmentation groups, and no significant differences between augmented and nonaugmented individual vertebrae. Stiffness was dependent on BMD (P = 0.009 multilevel segments, P < 0.004 individual vertebrae), with no significant differences among the 6 different augmentation groups, and no significant differences between augmented and nonaugmented individual vertebrae. CONCLUSIONS: Multilevel segment biomechanics are dependent on BMD and not the pattern of augmentation, so the augmentation of fractured vertebrae can be extended to adjacent levels at risk for fracture to maintain stiffness and strength, thus preventing further fractures.

Aged↗
PubMed2005

Biomechanical comparison of transarticular facet screws to lateral mass plates in two-level instrumentations of the cervical spine.

STUDY DESIGN: In vitro biomechanical comparison of transarticular facet screws to lateral mass plates in two level instrumentations of the cervical spine. OBJECTIVE: Lateral mass plates are costly, and screw placement is difficult. Facet screws have never been tested as an alternative in the cervical spine. This biomechanical study compared cervical transarticular facet screws to lateral mass plates in two-level instrumentations of human cadaveric cervical spines. SUMMARY OF BACKGROUND DATA: Translaminar facet screws have been shown to have similar biomechanical performance to pedicle screw fixation in the lumbar spine, especially in flexion. They have proven to be fast, safe, and effective, with authors reporting 94% to 100% fusion rates in single-level lumbar fusions. However, a biomechanical comparison of transarticular facet screws to lateral mass plates in cervical spine instrumentations has not been reported. METHODS: Thirteen human cadaveric cervical motion segments (C2-C4, C5-C7) were tested before and after instrumentation, with either transarticular facet screws or lateral mass plates, in flexion, extension, lateral bending, and torsion. Specimens were subjected to six cycles under a load of 2 Nm. RESULTS: Both fixation systems significantly reduced range of motion (ROM) and increased stiffness compared with the intact state in flexion, extension, lateral bending, and torsion. There were also no significant differences between the facet screws and plates in any of the four directions. To compare the two systems, ROM of each was analyzed relative to the uninstrumented state. Flexion was 0.26 (or 26% of the intact state) for the transarticular facet screws versus 0.20 for the lateral mass plates (P = 0.34), extension was 0.10 versus 0.07 (P = 0.43), lateral bending was 0.17 versus 0.15 (P = 0.52), and torque was 0.25 versus 0.38 (P = 0.12). Load to failure testing failed to indicate any differences between the two methods of fixation because all the specimens failed elsewhere. CONCLUSION: This study proves that transarticular facet screws and lateral mass plates are equivalent in two-level instrumentations of the cervical spine. This is the first biomechanical study to test transarticular facet screws in this context.

Aged↗
PubMed2004

Stress relaxation of bone significantly affects the pull-out behavior of pedicle screws.

The initial fixation strength of pedicle screws is commonly tested using a standard pull-out test with load applied at a constant rate. This method overlooks the cyclic nature of in situ loading responsible for clinical failure. This study was undertaken to determine the effects of stress relaxation properties at the bone-screw interface on screw fixation strength. Pedicle screws were inserted into calf lumbar vertebrae using a paired testing array. After embedding and mounting in a custom fixture, axial pull-out tests were performed at the rates of 1, 5, and 25 mm/min. For each vertebra, one screw was pulled at a continuous rate. The other screw was pulled at increments of 0.5 mm, at the same rate, with 1000 s pause between increments. Peak load, energy-to-failure, displacement-to-failure, and stiffness were calculated for each screw pull-out test. Two-way ANOVA showed that the standard pull-out method yielded significantly higher peak loads (p < 0.05) at faster pull-out rates and higher stiffnesses (p < 0.05) at all rates compared to the stress relaxation pull-out protocol. These results suggest that the stress relaxation properties of bone significantly affect the pull-out behavior of pedicle screws, reducing the peak load and stiffness values observed during testing. This mode of testing may provide a better biomechanical model of screw pull-out failure and a more accurate estimate of initial fixation strength.

Animals↗
PubMed2002

Pedicle morphometry in the upper thoracic spine: limits to safe screw placement in older patients.

STUDY DESIGN: An anatomic study of pedicle dimensions and orientation was performed for upper thoracic vertebrae from elderly human subjects. OBJECTIVES: To quantify dimensions of thoracic pedicles, and to determine the potential for safe transpedicular screw fixation in the upper thoracic spine. SUMMARY OF BACKGROUND DATA: Clinical and anatomic reports support thoracic pedicle fixation as a safe, effective alternative to hook fixation in both normal and osteoporotic bone. Much available data, however, pertains to young, robust patients, thoracolumbar segments, and mixed placement techniques. METHODS: For this study, T1-T6 vertebrae from 18 human cadavers were separated into individual vertebrae. Examiners measured each vertebra to determine medial-lateral pedicle width, cranial-caudal pedicle height, and coaxial depth from lamina to anterior vertebral cortex. Mean values were derived from repeated measures compared by level and side. RESULTS: Dimension a varied with individual and level, but not between the left and right pedicles. Pedicle diameter uniformly diminished as specimens proceeded caudally from T2. Findings showed that 25% of T1 pedicles, 17% of T2 pedicles, and 42% of T3 pedicles were narrower than 5.5 mm. At T4 61% of pedicles were too small, at T5 67% were too small, and at T6 75% were too small to accept a 5.5-mm screw. Dimension b remained relatively constant. Pedicles became increasingly narrow and oblong in the T4-T6 cross sections. Dimension c increased consistently from T1 to T6. Safe screw lengths ranged from 30 mm at T1 and T2, to 35 mm at T4 -T5, to 40 mm at T5 and T6. CONCLUSIONS: Even the largest patients had some pedicles that could not accommodate the smallest standard pedicle screw, and more than one half of the pedicles average patients were too small. Transpedicular screw placement is not safe in these patients. Proper placement must avoid penetration of the medial pedicle wall.

Aged↗
PubMedDate not supplied

Pedicle screw fixation strength: pullout versus insertional torque.

BACKGROUND CONTEXT: Researchers studying early pedicle screw designs have suggested that pullout strength and insertional torque are correlated. For surgeons using pedicle screws, insertional torque is widely believed to be a good predictor of pullout strength and initial stability of the screw and construct. How appropriate is this assumption when applied to new screw and thread designs? PURPOSE: This study investigated the correlation between insertional torque and pullout strength of three different pedicle screw designs, with different insertional torque characteristics. We hypothesized that a significant increase in insertional torque would indicate a commensurate increase in pullout strength. STUDY DESIGN: Biomechanical analysis of instrumented vertebral specimens. METHODS: Calf lumbar vertebra were prepared and instrumented with one of three pedicle screws. Pilot hole preparation was standardized and coaxial orientation was confirmed by direct inspection. Screws did not penetrate the pedicle cortex or abut or penetrate the anterior vertebral cortex. Any specimen with pedicle wall breach was discarded. The pedicles were instrumented with one of three screws: 1) 7.5 x 40 mm conical, asymmetric progressive thread (Xia; Stryker Spine, Allendale, NJ), 2) 7.5 x 40 mm conical with traditional V-shaped thread (Osteonics, Stryker Spine, Allendale, NJ)) or 3) 6.5 x 40 mm cylindrical with V thread (Osteonics, cylindrical). Paired testing allowed individual screws to be directly compared with a contralateral "control." Insertional torque and peak torque values were recorded for each rotation up to full insertion. Pullout testing was conducted at a rate of 1 mm/minute. Load-displacement data were recorded at 20 Hz. Stiffness was considered the slope of the most linear part of the curve before the yield point. RESULTS: Peak loads for 7.5 conical Xia screws measured 1,783+/-589.1 N compared with 1,943.0+/-625.8 N for 7.5 conical Osteonics screws and 1,641.0+/-356.7 N for 6.5 cylindrical Osteonics screws. The peak insertional torque values were 6.7+/-1.9 Nm (158% greater than control), 4.5+/-1.1 Nm (73% greater than control) and 2.6+/-0.7 Nm, respectively. Insertional torques for Xia screws were significantly greater than conical (p=.001) and cylindrical Osteonics screws (p<.0001), and insertional torques for Osteonics conical screws were significantly greater than those of cylindrical screws (p<.0001). Although pullout loads for the conical Osteonics screws were consistently higher than either the Xia or Osteonics cylindrical screws, the differences were not significant (p>.05). There was no significant correlation between pullout strength and insertional torque (p>.05). CONCLUSIONS: This unexpected result is best explained by the progressively narrowing flutes of the Xia screw, which compact the trabeculae into a smaller volume as the screw nears full insertion. The trapezoidal threads also increase contact with the cortical surface area and compress trabeculae toward the cortex, thus creating greater friction and higher torque values. This increase in torque did not translate into a commensurate increase in pullout strength, where trabeculae fail in shear.

Animals↗
PubMedDate not supplied

The effects of the antiresorptive agents calcitonin and pamidronate on spine fusion in a rabbit model.

BACKGROUND CONTEXT: 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. PURPOSE: To delineate the effects of these anti-osteoporosis medications on the fusion process. STUDY DESIGN: Randomized, prospective, double-blind, animal model. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Animals↗
PubMedDate not supplied

A biomechanical assessment of disc pressures in the lumbosacral spine in response to external unloading forces.

BACKGROUND CONTEXT: Axial back pain affects a large percentage of the population. Often aggravated by weight-bearing activity, these patients frequently have associated degenerative or post-traumatic lumbar disc disease. Aquatherapy is frequently used to transition patients from less activity limited by pain to greater activity by reducing weight-bearing load of the lumbar spine. Development of a means to permit patients similar spinal unloading while active during normal daily living would have the potential to promote similar effects. PURPOSE: The purpose of this study is to measure internal disc pressure at L4/L5 in response to forces exerted by an external vest. The study hypothesis anticipated an unloading of the lumbar spine during upright posture, as measured by intradicsal pressure at the L4/5 disc, correlating with external forces provided to the trunk by the device. STUDY DESIGN: A controlled experimental study of spine biomechanical loading was undertaken using isolated cadaver torsos obtained from an approved tissue source. Ages ranged at death with a mean of 65+/-6 years. METHODS: The distractive force created by inflating a set of pneumatic lifters within vests for treatment of low back pain were calibrated in a materials testing machine. Effects of inflation on the disc pressures within the lumbar spine then were tested. A microscopic pressure sensor (Samba, Gothenburg, Sweden) was placed into the nucleus of the L4/L5 disc of six isolated cadaver torsos (1 female, 5 male) using a 15-gauge spinal needle under direct fluoroscopic visualization. The pressure sensor was 0.42 mm in diameter, and had a calibrated response range of 0-7500 mm Hg. A pneumatically actuated lumbar vest was fit snugly to the torso. Each torso was supported in an upright, weight-bearing position for testing. The vest was inflated while the internal disc pressure was monitored and recorded. The data were analyzed to test for correlation between the amount of external unloading force provided by the vest and the intradiscal pressure measured in vitro. RESULTS: Application of external loads between the pelvis and ribcage by the vest demonstrated a maximum mean reduction of internal disc pressure at L4/L5 of 25% when the vest was inflated to a level producing approximately 400 N of effective load. The reduction in disc pressure was significantly different compared with baseline (upright, weight-bearing disc pressure without the vest) for all distraction settings (p<.01) except for the very lowest setting which was significant only at p=.025. CONCLUSIONS: Spinal unloading with an externally applied vest with adequate surface interface is effective in reducing intradiscal pressures. Ambulatory reduction of pressure would permit beneficial reduction of loads and permit patients with weight-bearing intolerance a better quality of life.

Aged↗