Biomechanical Characteristics of Kissing Spine During Extension Using a Human Cadaveric Lumbar Spinal Model.

Introduction: Although kissing spine syndrome in the lumbar spinal region is a relatively common condition in older adults, no study examining its biomechanical characteristics has been reported. We hypothesized that kissing of the spinous processes during extension causes an increase in the flexural rigidity of the spine and significantly limits the deformation behavior of extension, which in turn might cause lower back pain. Methods: Three test models (human cadavers A, B, and C) were prepared by removing supraspinal/interspinous ligaments between L4 and L5. The dental resin was attached to the cephalocaudal spinous process so that the spinous processes between L4 and L5 were almost in contact with each other to simulate the condition of a kissing spine. The flexion-extension direction's torque-range-of-motion (torque-ROM) curve was generated with a six-axis material tester for biomechanical measurements. Results: In all three models, the maximum ROMs at the time of extension were smaller than those at the time of flexion, and no sudden increase in torque was observed during extension. Conclusion: The results indicated no apparent biomechanical effects of kissing between the spinous processes, suggesting that the contact between the spinous processes has little involvement in the onset of lower back pain.

Experimental Study of Failures of the Rigid Spinal Posterior Fixation System Under Compressive Load Conditions: A Cadaver Study.

Background Many studies have been conducted on the biomechanics of the spine to elucidate the fixation properties of spinal fusion surgery and the causes of instrumentation failure. Among these studies, there are some studies on load sharing in the spine and measurement using strain gauges and pressure gauges, but there is a lack of research on axial compressive loads. Methods Axial compressive load tests were performed on human cadaveric injured lumbar vertebrae fixed with pedicle screws (PS). Both the strain generated in the PS rod and the intradiscal pressure were measured. Subsequently, the stress generated in the PS rod and the load sharing of the spine and instrumentation were calculated. Results Even when only compressive load is applied, bending stress of more than 10 times the compression stress was generated in the rod, and the stress tended to concentrate on one rod. Rod deformation becomes kyphotic, in contrast to the lordotic deformation behavior of the lumbar spine. The stress shielding rate was approximately 40%, less than half. Conclusions This study obtained basic data useful for constructing and verifying numerical simulations that are effective for predicting and elucidating the causes of dislodgement and failure of spinal implants.

Biomechanical Problems Related to the Pedicle Screw System.

AIM: To assess biomechanical problems related to pedicle screw (PS) systems. MATERIAL AND METHODS: Functional spinal units (L3-4) of deer were evaluated using a 6-axis material testing machine. For the specimen models, we prepared an intact model, a damaged model, a PS model, and a crosslink model. We checked the range of motion (ROM) during bending and rotation tests. Eight directions were measured in the bending test: anterior, right-anterior, right, right-posterior, posterior, left-posterior, left, and left-anterior, and 2 directions were measured in the rotation test (right and left). RESULTS: ROMs of the PS model were smaller than those of the intact model in all directions. However, ROMs of the PS model in the rotation test were smaller than those of the damaged model and larger than those of the intact model. The stability of the crosslink model was better than that of the PS model during the bending test, but ROMs of the crosslink model were larger than those of the intact model during the rotation test. CONCLUSION: Excessive bending rigidity and rotational instability are the biomechanical problems related to PS systems. Based on these results, we speculate that one of the most significant causes of adjacent segment disease is excessive bending rigidity and one of the most important causes of instrumentation failure is rotational instability.

Biomechanical Stability of a Cross-Rod Connection with a Pedicle Screw System.

BACKGROUND Surgery with pedicle screw instrumentation does not provide sufficient torsional stability. This leads to pseudoarthrosis, loosening of the pedicle screws, and, ultimately, implant failure. MATERIAL AND METHODS Functional spinal units from 18 deer were evaluated using a 6-axis material testing machine. As specimen models, we prepared an intact model, a damaged model, a cross-rod model, and a cross-link model. We measured the range of motion (ROM) during bending and rotation tests. RESULTS The range of motions of cross-rod model were almost equal to those of cross-link model during the bending test. In the rotation test, the average ranges of motion of the intact, cross-rod, and cross-link models were 2.9°, 3.1°, and 3.9° during right rotation and 2.9°, 3.1°, and 4.1° during left rotation, respectively. The range of motions of the cross-rod model were significantly smaller than those of the cross-link model during the rotation test. The range of motions of the intact model were significantly smaller than those of the cross-link model during the rotation test, but there were no statistically significant differences between the range of motions of intact model and cross-rod model during the rotation test. CONCLUSIONS The stability of spinal fixation such as cross-rod model is equal to the fixation using the pedicle screw system during bending tests and equal to that of the intact spine during rotation tests.

Trajectory of instantaneous axis of rotation in fixed lumbar spine with instrumentation.

BACKGROUND: Several studies showed instantaneous axis of rotation (IAR) in the intact spine. However, there has been no report on the trajectory of the IAR of a damaged spine or that of a fixed spine with instrumentation. It is the aim of this study to investigate the trajectory of the IAR of the lumbar spine using the vertebra of deer. METHODS: Functional spinal units (L5-6) from five deer were evaluated with six-axis material testing machine. As specimen models, we prepared a normal model, a damaged model, and a pedicle screw (PS) model. We measured the IAR during bending in the coronal and sagittal planes and axial rotation. In the bending test, four directions were measured: anterior, posterior, right, and left. In the rotation test, two directions were measured: right and left. RESULTS: The IAR of the normal model during bending moved in the bending direction. The IAR of the damaged model during bending moved in the bending direction, but the magnitude of displacement was bigger compared to that of the normal model. In the PS model, the IAR during bending test hardly moved. During rotation test, the IAR of the normal model and PS model located in the spinal canal, but the IAR of the damaged model located in the posterior part of the vertebral body. CONCLUSIONS: In this study, the IAR of damaged model was scattering and that of PS model was concentrating. This suggests that higher mechanical load applied to the dura tube and nerve roots in the damaged model and less mechanical load applied to that in the PS model.

Do the Position and Orientation of the Crosslink Influence the Stiffness of Spinal Instrumentation?

STUDY DESIGN: Biomechanical study of double-level pedicle screw constructs with or without crosslinks (CL) in an unstable model. OBJECTIVES: The purpose of this study is to investigate the optimal position and orientation of the CL. SUMMARY OF BACKGROUND DATA: Several reports have described biomechanical research on such CL, but no definite consensus has been reached regarding the effects. Very few studies have examined the position and orientation of the CL. The question of where and how the CL should be clinically set remains unanswered. METHODS: Ten cadaveric lumbar spines (L3-L5) of boars were used and 7 models were prepared by the sequential damage and spinal instrumentation of each specimen. Bending stiffness was measured in flexion, extension, lateral bending, and axial rotation for each model using 6-axis material tester under torque of 0 to ±3 N m. Results for each configuration were compared using analysis of variance and the Turkey-Kramer test. RESULTS: In flexion, extension, and lateral bending, 7 models showed similar stiffness with no significant differences. In axial rotation, stiffness increased significantly (P<0.05) in the cephalic, central, caudal, and oblique CL models in comparison with that of the no CL model, and stiffness of the horizontal 2 CL and oblique 2 CL models was significantly higher than that of cephalic, central, caudal, and oblique CL models (P<0.05). However, no significant differences in stiffness were seen between cephalic, central, and caudal CL models, between the central and oblique CL models, or between the horizontal and oblique 2 CL models. CONCLUSIONS: Concomitant use of CLs significantly increased axial rotational stiffness, even though stiffness in flexion, extension, and lateral bending was not increased. In addition, stiffness in axial rotation significantly improved with the use of 2 CLs instead of a single CL, and stiffness was unchanged by position and orientation of CL.

Biomechanical assessment of the effects of vertebral distraction-fusion techniques on the adjacent segment of canine cervical vertebrae.

OBJECTIVE To assess effects of vertebral distraction-fusion techniques at a treated segment (C5-C6) and an adjacent segment (C4-C5) of canine cervical vertebrae. SAMPLE Cervical vertebrae harvested from cadavers of 10 skeletally mature Beagles. PROCEDURES Three models (intact, titanium plate, and polymethylmethacrylate [PM MA]) for stabilization of the caudal region of the cervical vertebrae (C4 through C7) were applied to the C5-C6 vertebral segment sequentially on the same specimens. Biomechanical assessments with flexion-extension, lateral bending, and axial rotational tests were conducted after each procedure. Range of motion (ROM) for a torque load applied with a 6-axis material tester was measured at C4-5 and C5-6 and calculated by use of a 3-D video measurement system. RESULTS In both the plate and PMMA models, ROM significantly increased at C4-5 and significantly decreased at C5-6, compared with results for the intact model. The ROM at C5-6 was significantly lower for the plate model versus the PMMA model in lateral bending and for the PMMA model versus the plate model in axial rotation. Conversely, ROM at C4-5 was significantly higher in axial rotation for the PMMA model versus the plate model. No significant differences were identified in flexion-extension between the PMMA and plate models at either site. CONCLUSIONS AND CLINICAL RELEVANCE Results of this study suggested that vertebral distraction and fusion of canine vertebrae can change the mechanical environment at, and may cause disorders in, the adjacent segment. Additionally, findings suggested that effects on the adjacent segment differed on the basis of the fusion method used.

Biomechanical study of rotational micromovement of the pedicle screw.

BACKGROUND: In regard to the fixation using a pedicle screw (PS) and rod system, the mechanism from the onset of the clear zone up to the development of loosening of the pedicle screw is not completely clarified. The purpose of this study is to determine the cause of the pedicle screw loosening by performing a biomechanical study with three-dimensional movie analysis. METHODS: Ten PS fixation model of the lumbar spines (L3-4) of boar cadavers were used. The rotational angles of the L3 and L4 vertebral body and the screw at the time of applying a ±5 Nm load in the left anterior and right posterior flexion directions respectively were calculated based on those at the time of applying no load. The absolute value of the difference in the rotational angles between each vertebral body with left anterior flexion and right posterior flexion and the inserted screws was defined as rotational micromovement. RESULTS: In both the left anterior and right posterior flexion directions, there were significant differences (p < 0.05) in the rotational angles between the screw and the vertebral body for both the L3 and L4 vertebral bodies. CONCLUSION: Our biomechanical results showed that rotational micromovement occurred between the PS and the vertebral body, and repeated rotational micromovement might cause loosening of the screw or pullout of PS fixation.

A biomechanical comparison between cortical bone trajectory fixation and pedicle screw fixation.

PURPOSE: There have been several reports on the pullout strength of cortical bone trajectory (CBT) screws, but only one study has reviewed the stability of functional spine units using the CBT method. The purpose of this study was to compare vertebral stability after CBT fixation with that after pedicle screw (PS) fixation. METHODS: In this study, 20 lumbar spine (L5-6) specimens were assigned to two groups: the CBT model group that underwent CBT screw fixation (n = 10) and the PS model group that underwent pedicle screw fixation (n = 10). Using a six-axis material testing machine, bend and rotation tests were conducted on each model. The angular displacement from the time of no load to the time of maximum torque was defined as range of motion (ROM), and then, the mean ROM in the bend and rotation tests and the mean rate of relative change of ROM in both the bend and rotation tests were compared between the CBT and PS groups. RESULTS: There were no significant differences between the CBT and PS groups with regard to the mean ROMs and the mean rate of relative change of ROMs in both the bend and rotation tests. CONCLUSION: Intervertebral stability after CBT fixation was similar to that after PS fixation.

Biomechanical effects of pedicle screw fixation on adjacent segments.

Various biomechanical investigations have attempted to clarify the aetiology of adjacent segment disease (ASD). However, no biomechanical study has examined in detail the deformation behaviour of the adjacent segments when both pure torque and an angular displacement load are applied to the vertebrae along multiple segments. The purpose of this study is to investigate the biomechanical effects of pedicle screw fixation on adjacent segments. Ten cadaveric lumbar spines (L2-L5) of boars were used. Control and fusion models were prepared by disc damage and pedicle screw fixation of each specimen, and then, bending and rotation tests were performed using a six-axis material tester. In the biomechanical tests regulated by an angular displacement load, the range of motion (ROM) of the cranial and caudal adjacent segments in antero-posterior flexion and lateral bending was increased by about 20 % (p < 0.05), and the maximal torque in the fusion model was about threefold (p < 0.05) that in the control model. And in axial rotation, the ROM of cranial and caudal adjacent segments was increased by about 100 % (p < 0.001), and the maximal torque was about sixfold (p < 0.01) that in the control model. The ROM of adjacent segments was significantly increased after pedicle screw fixation as assessed by biomechanical tests regulated by an angular displacement load, but not in those regulated by torque. We present the results of biomechanical tests regulated by torque and angular displacement and show that the maximum torque of the fusion model was larger than that of the control model in the biomechanical test regulated by an angular displacement load, suggesting that mechanical stress on the segments adjacent to the fused segment is large. We think that ASD arises after spinal fusion surgery as a mechanism to compensate for the ROM lost due to excessive fusion by pedicle screw fixation, so that a large torque may be applied to adjacent segments within a physiologically possible range, and it might gradually lead to a degenerative intervertebral disc or progression of spondylolisthesis in the adjacent segments.

Basic research on a cylindrical implant made of shape-memory alloy for the treatment of long bone fracture.

The internal fixing materials made from shape-memory alloys (SMAs) have recently been reported for long bone fracture. We present a new internal fixation technique using a cylindrical SMAs implant in a rat femoral fracture healing. The implant was designed in a shape to circumferentially fix the fractured bone using resilient SMA claws. To evaluate the fixing ability of the implant, three-point bending and rotation tests were performed. Fifteen female Wister rats were treated surgically as an experimental model. All rats were killed at 16 weeks postoperatively, and the radiological and histological evaluations were performed. In biomechanical test, the good fixation ability of the implant was demonstrated. In animal model, no cases of postoperative infection or death were encountered and postoperative gait was stable in all cases. Radiological examination at 16 weeks postoperatively demonstrated the implant firmly fixed to the fractured part, endosteal healing, and no callus formation in all cases. In Histological evaluation, bone union in all cases was characterized by endochondral ossification from within the medullary cavity. In conclusion, our cylindrical SMA implant provided good fixation in biomechanical tests, and achieved bone union in all 15 rats. If a larger size is designed in the future, our implant will be a clinically applicable, useful fixing material for fracture of the human long bones.

Biomechanical study of the lumbar spine using a unilateral pedicle screw fixation system.

Lumbar fusion combined with unilateral pedicle screw fixation has received favourable clinical reports. However, there are very few reports about the biomechanical properties of this system. The purpose of this study was to compare the biomechanics of a unilateral pedicle screw system with a bilateral system. Two fresh lumbar vertebral columns from human cadavers were used. Seven models were prepared by the sequential damage and spinal instrumentation of each specimen. Bending and rotation tests were performed to clarify the range of motion for each model using a 6-axis material tester that we have developed. We showed that the unilateral pedicle screw system offers only uneven fixation. This results in dispersion of rigidity depending on the direction of bending and rotation. The bilateral pedicle screw system, however, allows excellent fixation in all directions.

Clinical application of a handy intraoperative measurement device for lumbar segmental instability.

We describe the development of a new device that permits handy intraoperative measurement of lumbar segmental instability. The subjects comprised 80 patients with lumbar degenerative disease. Relationships between preoperative radiological assessments and extended distance as measured using our new device were investigated. Mean extended distance measured using the device was 3.7 +/- 1.9 mm. Correlation coefficients between angular motion and extended distance, and translational motion and extended distance were 0.76 and 0.66, respectively, revealing significant positive relationships between these values (p < 0.01 each). The correlation coefficient between the intervertebral endplate angle on the flexion film and extended distance was -0.78, showing a significant negative relationship (p < 0.01). In conclusion, the device for intraoperative measurement of lumbar segmental instability that we have developed appears to permit simple measurement of intervertebral instability and provides operators with valuable information for selecting operative methods of spinal fusion or instrumentation.

Changes in water content of intervertebral discs and paravertebral muscles before and after bed rest.

BACKGROUND: Although low back pain can be principally produced or increased during action, it may also be induced or enhanced in the morning after bed rest. During bed rest, tissue edema (increased water content) may occur. In this study, we measured the changes in water content in the intervertebral disc and the paravertebral muscle before and after bed rest using a magnetization transfer magnetic resonance imaging (MT-MRI) technique that permits measuring water content in tissues. METHODS: A total of 20 student volunteers were enrolled in this study. MT-MRI evaluation was performed before and after bed rest. To measure water content in the intervertebral disc and paravertebral muscle, two MRI sequences were performed using MT pulse-off and MT pulse-on. Based on the two images obtained, the equivalent cross-relaxation rate (ECR) was calculated. RESULTS: The ECR for intervertebral discs was significantly lower after bed rest than before bed rest (P < 0.01). The ECR for paravertebral equivalent cross-relaxation rate muscles was significantly higher after bed rest than before bed rest (P < 0.05). CONCLUSION: We obtained results indicating that after bed rest the water content in the intervertebral disc and the paravertebral muscle was increased and decreased, respectively.

Effects of barium concentration on the radiopacity and biomechanics of bone cement: experimental study.

PURPOSE: This study was undertaken to evaluate the changes in the radiopacity and mechanics of polymethylmethacrylate (PMMA) bone cement with the addition of barium. MATERIALS AND METHODS: Barium sulfate powder was added to a PMMA bone cement with an initial 10% barium concentration. The changes in radiopacity and strength were evaluated by testing cement blocks containing four barium concentrations (10%, 20%, 30%, 40%). Radiopacity was evaluated by measuring the computed tomography (CT) values of the bone cement, and strength was evaluated by compressive, three-point bending, and impact load tests. RESULTS: CT values increased in proportion to the barium concentration. The compressive load test showed that cement with a 40% barium concentration was significantly more fragile than cement with lower barium concentrations. The three-point bending load test showed that the cement became more fragile in proportion to the barium concentration. The impact load test showed that cement with 30% and 40% barium concentrations was significantly more fragile than cement with 10% and 20% barium concentrations. CONCLUSION: Radiopacity is increased and strength is reduced by adding increasing concentrations of barium powder to bone cement. The results of the present study suggest that adding barium permits the radiopacity and strength of bone cement to be adjusted in clinical practice.

Tadpole system as new lumbar spinal instrumentation.

BACKGROUND: There have been reports of serious complications associated with pedicle screw fixation, including nerve root injuries caused by accidental screw insertion. We have developed a new system of lumbar spinal instrumentation that we call Tadpole system. The purposes of this report were to show the results of a biomechanical study and the short-term outcome of a clinical study, as well as to determine the usefulness of this system. METHODS: The Tadpole system lumbar spinal fusion is a hook-and-rod system according to which the spine is stabilized using 2 sets of 2 spinous processes each that are held in place by 4 hooks tandemly connected to a rod. The biomechanical study was done using 5 human lumbar cadaveric spines, and the range of motion (ROM) was examined in a non-treatment model, an injured model, a pedicle screw fixation model and a Tadpole system model. For the short-term clinical study the Tadpole system was used in 31 patients, and the factors analyzed were operation time, time required for spinal instrumentation, amount of intraoperative bleeding, postoperative improvement rate of the Japanese Orthopaedic Association (JOA) score for lumbar spinal disorders, instrumentation failure, spinous process fracture, spinal fluid leakage, nerve root injury, postoperative infection, and bone fusion 2 years after the operation. RESULTS: The ROM in the Tadpole system model was slightly bigger than that in the pedicle screw fixation model, but smaller than that in the normal control model. These biomechanical data indicated that the Tadpole system provided fairly good stability. The mean operation time was 79 min, the mean time required for spinal instrumentation was 8 min, and the mean amount of intraoperative bleeding was 340 mL. The mean postoperative improvement rate of JOA score was 70.9 +/- 24.8%. Instrumentation failure (dislocation of a hook) occurred in one patient, and none of the patients developed spinous process fracture, spinal fluid leakage, nerve root injury, or postoperative infection. Two years after the operation, bone union was confirmed in 29 of the 31 patients (93.5%). CONCLUSION: We conclude that this system is a useful, easy-to-use and safe spinal instrumentation technique for lumbar fusion surgery.

Equivalent cross-relaxation rate imaging for sentinel lymph node biopsy in breast carcinoma.

Sentinel lymph node biopsy (SLNB) is an important technique for detecting axillary lymph node metastasis in breast carcinoma patients. However, false-negative results are a problem. Equivalent cross-relaxation rate (ECR) imaging (ECRI) is a measurement method that can be used to quantitatively evaluate a change in the structural organization of lymph nodes by magnetic resonance imaging (MRI). We performed axillary ECRI in an attempt to decrease the false-negative results of SLNB. Regions without metastases showed a higher ECR value. On the other hand, regions with metastases showed a lower ECR value. The ECR images were compared with macroscopic histology images in which the presence or absence of axillary lymph node metastasis could be evaluated. ECRI is a potentially useful method for evaluating the efficacy of SLNB.

Equivalent cross relaxation rate image for decreasing a false negative case of sentinel lymph node biopsy.

In the breast carcinamas, sentinel lymph node biopsy (SLNB) attracts attention as technique to be settled by axillary lymph node metastasis, but existence of a false negative case is a problem. Equivalent cross relaxation rate image (ECRI) is the measurement method that we can evaluate a change of organization structure quantitatively by magnetic resonance imaging. We executed axillary ECRI as a purpose with decreasing a false negative case of SLNB. ECRI related to a macroscopic image, and it was possible to evaluate yes or no of axillary lymph node metastasis. ECRI is the useful method to evaluate adaptability of SLNB.