Ruey-Mo Lin

Rate-related fatigue injury of vertebral disc under axial cyclic loading in a porcine body-disc-body unit

OBJECTIVE: Cyclic loading tests were performed on fresh mature porcine lumbar spines to investigate the mechanical responses at different loading rates, morphological changes in the disc and to produce the clinically relevant injuries of porcine intervertebral disc. DESIGN: Two-segment units of fresh porcine spine with all the posterior elements removed were used for a disc stress analysis and morphological observation during axial cyclic loading. BACKGROUND: The repetitive loading of the spine has been implicated as a risk factor in developing low back disorders. However, few studies have discussed morphologic changes in vertebral disc due to fatigue stress and no study has investigated the relationship between disc strength and bone mineral density. METHODS: 21 body-disc-body specimens divided into three groups were subjected to cyclic loading at test speeds of 0.5, 5 and 20 mm min(-1). The correlation between residual stress of specimen and bone mineral density was investigated and the disc morphological changes were observed through the middle cross section of the intervertebral disc using a zoom stereo microscope. RESULTS: There was no visible crack observed following testing in the 0.5 mm min(-1) test group. However, one disc in the 5 mm min(-1) and two discs in the 20 mm min(-1) test group exhibited disc herniation. The residual stress was found to be positively and linearly correlated with bone mineral density and decreased as the loading rates increased. CONCLUSION: Faster loading rate generates greater stress decay, and disc herniation is more likely to occur under higher loading rate conditions. The most common site of disc herniation is the posterolateral area which is in agreement with that of the human spine.

Geometric and morphological changes of the intervertebral disc under fatigue testing.

OBJECTIVE To understand the injury mechanism of the intervertebral disc at different loading rates and to explore the anatomic and histological changes of intervertebral discs. DESIGN Fresh porcine lumbar spines were used for fatigue testing to study the morphological changes of the intervertebral disc. BACKGROUND Intervertebral disc problem is one of the most common causes that lead to low back pain. Slow repetitive loading was considered to be the critical factor of spine and disc injuries. METHODS Twenty-four lumbar functional units were subjected to cyclic loading at three different loading rates. The geometric measurements and magnetic resonance image observations were conducted for the comprehension of morphological changes. The detail observation was taken through a stereomicroscope. RESULTS There was no significance in geometric changes between different loading rates. For magnetic resonance imagings, morphological changes included the changes of nucleus pulposus shape, bulge of anterior and posterior longitudinal ligaments, and dehydration in annulus fibrosus. CONCLUSION The morphological changes of intervertebral disc were revealed in certain kinds of lesions. The results imply that fatigue failure and degeneration or instability are strongly linked. The correlation of magnetic resonance imaging and anatomic observation showed a high correspondence in the comparison of shape and position of the nucleus pulpasus. RELEVANCE The changes of geometric measurements and relationship between anatomic observation and magnetic resonance imaging finding had been analyzed. It could help in understanding the mechanism of triggering cause in the early stage of disc degeneration.

Distribution and regional strength of trabecular bone in the porcine lumbar spine.

OBJECTIVE: The regional strength and distribution of trabecular bone in the porcine lumbar spine were examined to understand the mechanical responses in quadrupedal vertebrae. DESIGN: The cancellous bone columns prepared from different regions of porcine lumbar vertebrae were subjected to axial compression to investigate the regional strength related to disc structure. Some vertebrae were also serially sectioned sagittally and transversely to observe the trabecular patterns. BACKGROUND: Animal spines were often used in biomechanical testing and the results were said to be similar to those of humans. However, none of them are truly like human bipedal locomotion. The understanding of regional variations in compressive properties may help us to explain the similarity and interpret the data of animal experiments. METHODS: The dried, defatted cancellous bone columns were subjected to uniaxial compression of different regions at a deformation rate of 5 mm/min. The mechanical properties in different regions were compared. The main trabecular orientation was also observed using a zoom stereo microscope. RESULTS: The ultimate strength of the posterior column tended to be larger than that of the anterior column. The ratio of bony strength overlying the nucleus pulposus to that overlying the annulus fibrosis was constant and averaged 1.16. The main trabecular struts were noted to be parallel to the spinal canal and cross-bridged by thinner trabeculae. CONCLUSIONS: The trabecular morphology and regional mechanical properties of the porcine spine were comparable to those of other similar studies on the human spine. Its significance needed further study.

Biomechanical properties of muscle-tendon unit under high-speed passive stretch.

OBJECTIVE The purpose of this study was to investigate the strain injury mechanisms of the Achilles muscle-tendon unit during high-speed passive stretch. DESIGN The high-speed traction device consisted of an impactor which dropped freely to hit one end of a lever, transferring the impact energy to traction energy at the other end. A muscle-tendon unit was attached to the other end of the lever via a force link, and the elongation was recorded with a high-speed camera. BACKGROUND The muscle-tendon unit is thought to act viscoelastically. It is generally strain rate dependent, exhibiting higher tensile stress at faster strain rates. However, previous studies of passive stretch in muscle-tendon units usually employed low strain rates. METHODS 16 fresh Achilles muscle-tendon units were subjected to passive stretch at a test speed of 310 cm s(-1). The history of elongation and the traction force of the muscle-tendon unit during the elongation process were analyzed. RESULTS The muscle-tendon units exhibited highly nonlinear mechanical behavior. Most of the elongation occurred in muscle and resulted in structural failure. Failure was not found in the tendon or muscle-tendon junction. Muscle fibers during stretching reached their maximum mechanical strength and then progressively ruptured. CONCLUSION The strain rate is an important factor in strain injuries of the muscle-tendon unit due to passive stretch. The muscle is a good energy absorber; the rupture process can absorb a great deal of external energy and prevent complete failure of the muscle, while also protecting bone and joints. RELEVANCE The study of muscle-tendon unit under high-speed stretch could help us to understand the mechanism of strain injuries over passive stretch in real-life situations.

Strength of internal fixation for calcaneal fractures

OBJECTIVE: To compare the strength of two types of fixation method for calcaneal fractures. DESIGN: A biomechanical testing examined the stability of 12 fractured calcaneal specimens fixed with two different methods. BACKGROUND: Though anatomic reduction and internal fixation for the treatment of intra-articular fractures of the calcaneus has become popular, biomechanical data on the fixation strength is lacking. METHODS: Twenty fresh frozen specimens of amputated human legs were impacted by a 20 kg weight dropped from a 155 cm height to create calcaneal fractures. Twelve specimens which demonstrated a longitudinal and a transverse primary fracture lines were selected for open reduction and internal fixation. Group 1: a lateral buttress plate and parallel screws placed in the latero-medial direction were used. Group 2: a longitudinal screw was added in addition to the fixation used in group 1. Biomechanical testing was performed by applying a tibial shank load until the internal fixation failed. RESULTS: All mechanical failures of the reconstructed calcaneus occurred through the transverse primary fracture line. The average failure load was 805+/-356 N in group 1 and 2905+/-910 N in group 2 (Wilcoxon p<0.05). CONCLUSIONS: A longitudinal transfixing screw could significantly improve the stability of the transverse primary fracture line in calcaneal fractures.

Differences in mechanical response between fractured and non-fractured spines under high-speed impact.

OBJECTIVE: The differences in mechanical response between fractured and non-fractured spines were investigated using a porcine spine impact model. DESIGN: Ten three-vertebrae segments (C3-C5) of porcine spine were subjected to a single impact to study the trauma mechanism. Small steel balls glued to the vertebra and a high-speed camera were used to observe the deformation of vertebral body and disc during impact. After trauma, the episodes of fractured specimens were compared with those of non-fractured specimens. BACKGROUND: Experimental trauma models using the spines of mature animals have rarely been evaluated. Finding a well-controlled, reproducible protocol based on an easily accessible specimen was therefore important. These models will be promising if clinical fractures can be produced. METHODS: All of the specimens were subjected to high-speed flexion-compression loading. The impact to the load cell and the operation of the high-speed camera were synchronized. The force-time sequence and disc deformation curve were recorded. The results from fractured and non-fractured spines were then compared. RESULTS: There were three burst fractures, four pedicle fractures, one facet joint fracture, one compression fracture and one fracture-dislocation. All of these fractures were similar to clinical fractures. Compared to non-fractured specimens, the fractured specimens had lower maximal force and longer reaction time. The characteristic steep decline in the middle region of the force-time curve was also consistently noted in the fractured spines. CONCLUSIONS: Spinal fractures similar to those found clinically were successfully produced in porcine spines. The characteristics of the mechanical responses observed should be helpful in the interpretation of events which occur during impact.

Differences of lumbosacral kinematics between degenerative and induced spondylolisthetic spine

OBJECTIVE To investigate the differences of lumbosacral kinematics between degenerative and induced spondylolisthetic subjects. DESIGN Translations and angulations of spondylolisthetic spine from L1-L2 to L5-S1 were documented by taking X-ray films at flexion, standing and extension positions. BACKGROUND The unstable mechanism of spondylolisthesis leads to lower back pain. It is important to determine the kinematics and the process of spondylolisthesis. METHODS Nineteen subjects with spondylolisthesis participated in this research, seven subjects with diagnosis of degenerative and 12 with induced spondylolisthesis, were taken lateral radiographs at three positions including flexion, standing and extension. RESULTS The differences of angulation among three positions (flexion, standing, and extension) at different levels were statistically significant (P<0.05) in both spondylolisthetic groups. The differences of translation among three different positions in induced spondylolisthetic group had a statistical significance (P<0.05) except at the level of L5-S1 (P>0.05). CONCLUSIONS Segmental total translation and angulation at each level of induced spondylolisthetic spine were greater than those of degenerative spondylolisthetic spine except L5-S1 level, which illustrated the evolution of spondylolisthesis from unstable to less unstable. RELEVANCE The results showed induced spondylolisthesis may link to degenerative spondylolisthesis. It provided essential knowledge to detect the evolution of degenerative spondylolisthesis clinically earlier.

Radiographic and balance characteristics for patient with osteoporotic vertebral fracture

Abstract Osteoporosis is a common health issue and leads to many fractures as well as balance problems. The radiographic and balance characteristics for different types of spinal deformity caused by osteoporosis are still unknown. The purposes of this study were to investigate the differences in radiographic and balance characteristics among different types of spinal deformities (lower kyphosis and whole kyphosis) and normal spines. Sagittal anterior‐posterior and medial‐lateral radiographic views were taken for radiographic analysis, while forceplate data at zero, three, six, and nine minutes of standing were employed for balance testing. There were significant differences in radiographic findings and balance performances between osteoporotic subjects and normal individuals. The lower kyphotic group revealed significantly greater spinal curvatures such as T‐Cobbs (50.9±18.3 degrees), L‐Cobbs (33.4±22.0 degrees), and SI angles (33.4±22.0 degrees) than those of the whole kyphotic group. However, significantly greater C7‐S1 distance (6.9±3.5.0 cm) and C7‐Lapex distance (6.2±3.9 cm) were found in the whole kyphotic group. For balance performance, the sway area of normal, lower kyphotic and whole kyphotic groups are 37.4±19.1 mm2, 71.9±71.2 mm2 and 119.1±114.8 mm2, respectively. The study suggested balance performance could be predicted by spinal lateral radiographies for patients with osteoporotic vertebral fractures.

Biomechanical properties of muscle-tendon unit under fast passive stretch

This paper presents a new procedure to evaluate the injury mechanism of muscle-tendon unit. This preliminary study used an impactor with variable mass that dropped freely from a height of 50 cm to impact one end of a lever; this lever jig could immediately change compression energy to traction energy. The traction force history of the specimen was recorded during the traction. At the same time a high-speed camera recorded the stretch event. The results showed that a muscle-tendon unit has a highly nonlinear mechanical behaviour. Muscle fibers reached their maximum stretch force and completed failure step by step. This effect is important as the ability of muscles to function as energy absorbers can prevent rupture entirely to themselves at any one time and can protect bones and joins.

Regional mechanical properties in porcine lumbar vertebrae trabeculae

A IISTR ACT Thc morphology and comprcssivc mechanical propcrtics of cancellous trabeculae in porcine lumbar vertebrae were examined on the basis of anatomical rcgions. The main trabecular struts wcre notcd to be parallel to the spinal canal and cross-bridged by the thinner. horizontal trabeculae. The wabcculae were homogcncously distributcd evcn in the centcr of the body. For mcchnnical testing. eight cancellous bone columns in two layers wcre prepared from each vcrtebrae. In cach layer, the anterior columns were under the annulus fibrosis and thc postcrior colurnns werc under thc nuclcus pulposus. There were no significant differcnces bctwcen dicse two laycrs in trabcculrv bpnc dcnsity, elastic modulus, yicld strcngth and ultimntc strcngth. Ncithcr wcrc bctwccn thc right and IcR pairs. Howcvcr. tlic ultimatc strcngth of tllc postcrior columns tcndcd to be Iargcr than those of antcrior columns, although the diffcrcnces wcre not statistically significant (p = 0.0679). The ratio of strcngth of bone ovcrlying thc nuclcus to tIi:11 ovcrlying tlic annulus was ratlicr constant and avcragcd I . 16. Thc charactcristics of rcgional mechanical propcrtics was elucidatcd by the orientation of the main trabecular platc. In conclusion, the homogeneous dcnsity o l uabcculnc and its mechanical rcspnse to disc made thc porcinc lumbar spine idcal for axial loading models. [