Analysis of the mechanics of the thoracic spine in man. An experimental study of autopsy specimens

How Does the Rib Cage Affect the Biomechanical Properties of the Thoracic Spine? A Systematic Literature Review

The vast majority of previous experimental studies on the thoracic spine were performed without the entire rib cage, while significant contributive aspects regarding stability and motion behavior were shown in several other studies. The aim of this literature review was to pool and increase evidence on the effect of the rib cage on human thoracic spinal biomechanical characteristics by collating and interrelating previous experimental findings in order to support interpretations of in vitro and in silico studies disregarding the rib cage to create comparability and reproducibility for all studies including the rib cage and provide combined comparative data for future biomechanical studies on the thoracic spine. After a systematic literature search corresponding to PRISMA guidelines, eleven studies were included and quantitatively evaluated in this review. The combined data exhibited that the rib cage increases the thoracic spinal stability in all motion planes, primarily in axial rotation and predominantly in the upper thorax half, reducing thoracic spinal range of motion, neutral zone, and intradiscal pressure, while increasing thoracic spinal neutral and elastic zone stiffness, compression resistance, and, in a neutral position, the intradiscal pressure. In particular, the costosternal connection was found to be the primary stabilizer and an essential determinant for the kinematics of the overall thoracic spine, while the costotransverse and costovertebral joints predominantly reinforce the stability of the single thoracic spinal segments but do not alter thoracic spinal kinematics. Neutral zone and neutral zone stiffness were more affected by rib cage removal than the range of motion and elastic zone stiffness, thus also representing the essential parameters for destabilization of the thoracic spine. As a result, the rib cage and thoracic spine form a biomechanical entity that should not be separated. Therefore, usage of entire human non-degenerated thoracic spine and rib cage specimens together with pure moment application and sagittal curvature determination is recommended for future in vitro testing in order to ensure comparability, reproducibility, and quasi-physiological validity.

Economic access influences degenerative spine disease outcomes at rural Late Medieval Villamagna (Lazio, IT)

OBJECTIVES

Degenerative joint disease in the spine is heavily influenced by genetic, environmental, and epigenetic factors, as well as exacerbated by physical activity and injury. The objective of this study was to investigate the multivariate relationship between known predictors of degenerative joint disease in the spine, such as age and sex, with mortuary indicators of economic access such as grave inclusions, burial location, and burial type.

MATERIALS AND METHODS

The presence and severity of vertebral osteophytosis (VO) and vertebral osteoarthritis (VOA) was recorded for the vertebral columns of N = 106 adult individuals from the Late Medieval period at the rural monastery of San Pietro at Villamagna in Lazio, Italy (1300-1450 AD). Multiple skeletal indicators of degenerative joint disease, morphological sex, and age were compared with differences in mortuary treatment across four regions of the spine.

RESULTS

There are marked differences in severe joint disease outcome between groups with more and less economic access. Relative risk ratios suggest that males and females with less economic access have elevated risk for VO and VOA in specific spine regions, although this effect is reduced among females.

DISCUSSION

Current research on the consequences of economic and social inequality point to the important role of economic inequality in shaping disease outcomes. Our results suggest that biocultural effects of reduced economic access at the intraclass level may increase vulnerability to the downstream effects of risk exposure (e.g., biomechanical injure, physical activity, biochemical imbalance), and ultimately increase the risk and prevalence for severe degenerative disease outcomes in medieval Italy.

Thoracic spinal kinematics is affected by the grade of intervertebral disc degeneration, but not by the presence of the ribs: An in vitro study

BACKGROUND CONTEXT

Thoracic spinal three-dimensional kinematics is widely unknown. For the evaluation of surgical treatments and the complete validation of numerical models, however, kinematic data of the thoracic spine are essential.

PURPOSE

To identify possible effects of rib presence and grade of intervertebral disc degeneration on thoracic spinal kinematics including three-plane helical axes and instantaneous centers of rotation.

DESIGN/SETTING

Radiological grading of intervertebral disc degeneration and in vitro tests using n=8 human thoracic functional spinal units of the segmental levels T1-T2, T3-T4, T5-T6, T7-T8, T9-T10, and T11-T12, respectively, were performed with as well as without ribs to analyze the specific kinematic properties.

METHODS

Specimens were loaded with pure moments of 5 Nm and constant loading rates of 1°/s in flexion/extension, lateral bending, and axial rotation. Optical motion tracking was performed to visualize helical axes and instantaneous centers of rotation on three-plane X-rays and to evaluate primary ranges of motion (ROMs) and coupled motions.

RESULTS

Motion segments with no or mild disc degeneration showed reproducible kinematics in all motion planes, whereas medium or severely degenerated specimens offered high variations and shifts of the rotational axes to the distal direction as well as lower ROM. Coupled motions were generally not detected.

CONCLUSIONS

With progressing disc degeneration, the rotational axes show higher variation and tend to shift in distal direction, especially in flexion/extension with a shift to the anterior direction, whereas rib resection does not affect thoracic spinal kinematics but its stability. Rib resections as part of spinal deformity treatment destabilize the thoracic spine, but do not alter its kinematics. Young and healthy discs, however, could be affected by surgical treatments of the thoracic spine regarding thoracic spinal kinematics.

An improved stiffness matrix model of the functional spinal unit for application to an improved understanding of pathological changes

The stiffness matrix is a useful way to describe the mechanical behaviour of the functional spinal unit, which is defined as the superior and inferior vertebrae, capsules and ligaments. This usefulness is extended by means of the concept of the "balance point". The balance point is the load application point where the coupling coefficients of the stiffness matrix are minimized. Theoretical considerations are used to demonstrate that the stiffness matrix varies with load point location and thus a single stiffness matrix does not fully characterize the motion segment as well as to derive the stiffness matrix at any one specified point from the stiffness matrix at some other specified point. Special characteristics of the stiffness matrix obtained by loading through the "balance point" were shown. Some possible advantages derived from mechanical testing using the "balance point" concept are discussed. This study validates an improved stiffness matrix model that enhances the understanding of pathological changes by setting the gold standard of the behaviour of a normal functional spinal unit.

Facet Joints of the Spine: Structure–Function Relationships, Problems and Treatments, and the Potential for Regeneration

The zygapophysial joint, a diarthrodial joint commonly referred to as the facet joint, plays a pivotal role in back pain, a condition that has been a leading cause of global disability since 1990. Along with the intervertebral disc, the facet joint supports spinal motion and aids in spinal stability. Highly susceptible to early development of osteoarthritis, the facet is responsible for a significant amount of pain in the low-back, mid-back, and neck regions. Current noninvasive treatments cannot offer long-term pain relief, while invasive treatments can relieve pain but fail to preserve joint functionality. This review presents an overview of the facet in terms of its anatomy, functional properties, problems, and current management strategies. Furthermore, this review introduces the potential for regeneration of the facet and particular engineering strategies that could be employed as a long-term treatment.

Challenging the Conventional Standard for Thoracic Spine Range of Motion: A Systematic Review

BACKGROUND

Segmental motion is a fundamental characteristic of the thoracic spine; however, studies of segmental ranges of motion have not been summarized or analyzed. The purpose of the present study was to present a summary of the literature on intact cadaveric thoracic spine segmental range of motion in each anatomical plane.

METHODS

A systematic MEDLINE search was performed with use of the terms "thoracic spine," "motion," and "cadaver." Reports that included data on the range of motion of intact thoracic human cadaveric spines were included. Independent variables included experimental details (e.g., specimen age), type of loading (e.g., pure moments), and applied moment. Dependent variables included the ranges of motion in flexion-extension, lateral bending, and axial rotation.

RESULTS

Thirty-three unique articles were identified and included. Twenty-three applied pure moments to thoracic spine specimens, with applied moments ranging from 1.5 to 8 Nm. Estimated segmental range of motion pooled means ranged from 1.9° to 3.8° in flexion-extension, from 2.1° to 4.4° in lateral bending, and from 2.4° to 5.2° in axial rotation. The sums of the range of motion pooled means (T1 to T12) were 28° in flexion-extension, 36° in lateral bending, and 45° in axial rotation.

CONCLUSIONS

The pooled ranges of motion were similar to reported in vivo motions but were considerably smaller in magnitude than the frequently referenced values reported prior to the widespread use of biomechanical testing standards. Improved reporting of biomechanical testing methods, as well as specimen health, may be beneficial for improving on these estimations of segmental cadaveric thoracic spine range of motion.

The rib cage stabilizes the human thoracic spine: An in vitro study using stepwise reduction of rib cage structures

The stabilizing effect of the rib cage on the human thoracic spine is still not sufficiently analyzed. For a better understanding of this effect as well as the calibration and validation of numerical models of the thoracic spine, experimental biomechanics data is required. This study aimed to determine (1) the stabilizing effect of the single rib cage structures on the human thoracic spine as well as the effect of the rib cage on (2) the flexibility of the single motion segments and (3) coupled motion behavior of the thoracic spine. Six human thoracic spine specimens including the entire rib cage were loaded quasi-statically with pure moments of ± 2 Nm in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) using a custom-built spine tester. Motion analysis was performed using an optical motion tracking system during load application to determine range of motion (ROM) and neutral zone (NZ). Specimens were tested (1) in intact condition, (2) after removal of the intercostal muscles, (3) after median sternotomy, after removal of (4) the anterior rib cage up to the rib stumps, (5) the right sixth to eighth rib head, and (6) all rib heads. Significant (p < 0.05) increases of the ROM were found after dissecting the intercostal muscles (LB: + 22.4%, AR: + 22.6%), the anterior part of the rib cage (FE: + 21.1%, LB: + 10.9%, AR: + 72.5%), and all rib heads (AR: + 5.8%) relative to its previous condition. Compared to the intact condition, ROM and NZ increased significantly after removing the anterior part of the rib cage (FE: + 52.2%, + 45.6%; LB: + 42.0%, + 54.0%; AR: + 94.4%, + 187.8%). Median sternotomy (FE: + 11.9%, AR: + 21.9%) and partial costovertebral release (AR: + 11.7%) significantly increased the ROM relative to its previous condition. Removing the entire rib cage increased both monosegmental and coupled motion ROM, but did not alter the qualitative motion behavior. The rib cage has a strong effect on thoracic spine rigidity, especially in axial rotation by a factor of more than two, and should therefore be considered in clinical scenarios, in vitro, and in silico.

In vitro analysis of the segmental flexibility of the thoracic spine

Basic knowledge about the thoracic spinal flexibility is limited and to the authors' knowledge, no in vitro studies have examined the flexibility of every thoracic spinal segment under standardized experimental conditions using pure moments. In our in vitro study, 68 human thoracic functional spinal units including the costovertebral joints (at least n = 6 functional spinal units per segment from T1-T2 to T11-T12) were loaded with pure moments of ±7.5 Nm in flexion/extension, lateral bending, and axial rotation in a custom-built spine tester to analyze range of motion (ROM) and neutral zone (NZ). ROM and NZ showed symmetric motion behavior in all loading planes. In each loading direction, the segment T1-T2 exhibited the highest ROM. In flexion/extension, the whole thoracic region, with exception of T1-T2 (14°), had an average ROM between 6° and 8°. In lateral bending, the upper thoracic region (T1-T7) was, with an average ROM between 10° and 12°, more flexible than the lower thoracic region (T7-T12) with an average ROM between 8° and 9°. In axial rotation, the thoracic region offered the highest overall flexibility with an average ROM between 10° and 12° in the upper and middle thoracic spine (T1-T10) and between 7° and 8° in the lower thoracic spine (T10-T12), while a trend of continuous decrease of ROM could be observed in the lower thoracic region (T7-T12). Comparing these ROM values with those in literature, they agree that ROM is lowest in flexion/extension and highest in axial rotation, as well as decreasing in the lower segments in axial rotation. Differences were found in flexion/extension and lateral bending in the lower segments, where, in contrast to the literature, no increase of the ROM from superior to inferior segments was found. The data of this in vitro study could be used for the validation of numerical models and the design of further in vitro studies of the thoracic spine without the rib cage, the verification of animal models, as well as the interpretation of already published human in vitro data.

Biomechanics of the human intervertebral disc: A review of testing techniques and results

Many experimental testing techniques have been adopted in order to provide an understanding of the biomechanics of the human intervertebral disc (IVD). The aim of this review article is to amalgamate results from these studies to provide readers with an overview of the studies conducted and their contribution to our current understanding of the biomechanics and function of the IVD. The overview is presented in a way that should prove useful to experimentalists and computational modellers. Mechanical properties of whole IVDs can be assessed conveniently by testing 'motion segments' comprising two vertebrae and the intervening IVD and ligaments. Neural arches should be removed if load-sharing between them and the disc is of no interest, and specimens containing more than two vertebrae are required to study 'adjacent level' effects. Mechanisms of injury (including endplate fracture and disc herniation) have been studied by applying complex loading at physiologically-relevant loading rates, whereas mechanical evaluations of surgical prostheses require slower application of standardised loading protocols. Results can be strongly influenced by the testing environment, preconditioning, loading rate, specimen age and degeneration, and spinal level. Component tissues of the disc (anulus fibrosus, nucleus pulposus, and cartilage endplates) have been studied to determine their material properties, but only the anulus has been thoroughly evaluated. Animal discs can be used as a model of human discs where uniform non-degenerate specimens are required, although differences in scale, age, and anatomy can lead to problems in interpretation.

The effects of forced breathing exercise on the lumbar stabilization in chronic low back pain patients

[Purpose] This study was conducted to investigate the effects of forced breathing exercise on the trunk functions of chronic low back pain patients. [Subjects and Methods] Twenty-four patients with chronic low back pain were randomly divided into groups of respiratory effort and trunk stabilization exercises. The exercises were performed for 45 minutes, 3 times per week for 6 weeks. Spinal stabilization was measured as the compensation of thesagittal angle joint in relation to the lumbar external load. [Results] After the intervention, the forced breathing and stabilization exercise groups showed a significant difference in lumbar spine stabilization between the first and second stress tests and the control group also showed a significant difference after the intervention. The M1 and M2 tests of lumbar spine stabilization revealed no significant differences between the groups. [Conclusion] The results of this research demonstrate that forced breathing exercise therapy is effective at improving the trunk stability and daily living activities of chronic low back pain patients.

A history of spine biomechanics. Focus on 20th century progress

The application of mechanical principles to problems of the spine dates to antiquity. Significant developments related to spinal anatomy and biomechanical behaviour made by Renaissance and post-Renaissance scholars through the end of the 19th century laid a strong foundation for the developments since that time. The objective of this article is to provide a historical overview of spine biomechanics with a focus on the developments in the 20th century. The topics of spine loading, spinal posture and stability, spinal kinematics, spinal injury, and surgical strategies were reviewed.

Biomechanics of Thoracolumbar Burst and Chance-Type Fractures during Fall from Height

Study Design In vitro biomechanical study. Objective To investigate the biomechanics of thoracolumbar burst and Chance-type fractures during fall from height. Methods Our model consisted of a three-vertebra human thoracolumbar specimen (n = 4) stabilized with muscle force replication and mounted within an impact dummy. Each specimen was subjected to a single fall from an average height of 2.1 m with average velocity at impact of 6.4 m/s. Biomechanical responses were determined using impact load data combined with high-speed movie analyses. Injuries to the middle vertebra of each spinal segment were evaluated using imaging and dissection. Results Average peak compressive forces occurred within 10 milliseconds of impact and reached 40.3 kN at the ground, 7.1 kN at the lower vertebra, and 3.6 kN at the upper vertebra. Subsequently, average peak flexion (55.0 degrees) and tensile forces (0.7 kN upper vertebra, 0.3 kN lower vertebra) occurred between 43.0 and 60.0 milliseconds. The middle vertebra of all specimens sustained pedicle and endplate fractures with comminution, bursting, and reduced height of its vertebral body. Chance-type fractures were observed consisting of a horizontal split fracture through the laminae and pedicles extending anteriorly through the vertebral body. Conclusions We hypothesize that the compression fractures of the pedicles and vertebral body together with burst fracture occurred at the time of peak spinal compression, 10 milliseconds. Subsequently, the onset of Chance-type fracture occurred at 20 milliseconds through the already fractured and weakened pedicles and vertebral body due to flexion-distraction and a forward shifting spinal axis of rotation.

Reliability of thoracic spine rotation range-of-motion measurements in healthy adults

CONTEXT

The reliability of clinical techniques to quantify thoracic spine rotation range of motion (ROM) has not been evaluated.

OBJECTIVE

To determine the intratester and intertester reliability of 5 thoracic rotation measurement techniques.

DESIGN

Descriptive laboratory study.

SETTING

University research laboratory.

PATIENTS OR OTHER PARTICIPANTS

Forty-six healthy volunteers (age = 23.6 ± 4.3 years, height = 171.0 ± 9.6 cm, mass = 71.4 ± 16.7 kg).

MAIN OUTCOME MEASURE(S)

We tested 5 thoracic rotation ROM techniques over 2 days: seated rotation (bar in back and front), half-kneeling rotation (bar in back and front), and lumbar-locked rotation. On day 1, 2 examiners obtained 2 sets of measurements (sessions 1, 2) to determine the within-session intertester reliability and within-day intratester reliability. A single examiner obtained measurements on day 2 (session 3) to determine the intratester reliability between days. Each technique was performed 3 times per side, and averages were used for data analysis. Reliability was determined using intraclass correlation coefficients, standard error of measurement (SEM), and minimal detectable change (MDC). Differences between raters during session 1 were determined using paired t tests.

RESULTS

Within-session intertester reliability estimates ranged from 0.85 to 0.94. Ranges for the SEM were 1.0° to 2.3° and for the MDC were 2.8° to 6.3°. No differences were seen between examiners during session 1 for seated rotation (bar in front, both sides), half-kneeling rotation (bar in front, left side), or the lumbar locked position (both sides) (all values of P > .05). Within-day intratester reliability estimates ranged from 0.86 to 0.95. Ranges for the SEM were 0.8° to 2.1° and for the MDC were 2.1 ° to 5.9°. Between-days intratester reliability estimates ranged from 0.84 to 0.91. Ranges for the SEM were 1.4° to 2.0° and for the MDC were 3.9° to 5.6°.

CONCLUSIONS

All techniques had good reliability and low levels of measurement error. The seated rotation, bar in front, and lumbar-locked rotation tests may be used reliably when more than 1 examiner is obtaining measurements.

En bloc control of deep and superficial thoracic muscles in sagittal loading and unloading of the trunk

External perturbation of the trunk via sudden loading and unloading is an established method to study control of spinal stability and postural equilibrium. As differential control of the deep and superficial lumbar multifidus occurs during predictable sagittal loading, we hypothesized that the deep and superficial components of the thoracic paraspinal muscles would also be differentially active during loading and unloading of the trunk. Variation in sagittal mobility between regions of the thorax and previous data of differences in control of the thoracic paraspinal muscles between regions in other tasks supported a hypothesis that there would be region-specific differences in responses to loading and unloading. This study used fine-wire electrodes to record electromyographic (EMG) activity from the right deep (multifidus/rotatores) and superficial (longissimus) muscles at T5, T8, and T11 in ten healthy subjects during predictable and unpredictable sudden loading and unloading of the trunk. EMG amplitude was calculated during 10 ms epochs for 50 ms before the onset of trunk perturbation and 150 ms after the perturbation. Contrary to our hypotheses, deep and superficial thoracic paraspinal muscles were similarly active (loading: p=0.470; unloading: p=0.137) and similarly affected by the degree of predictability at all levels. Thus, deep and superficial thoracic paraspinal muscles are recruited en bloc during sagittal plane trunk perturbations. This contrasts previous findings of differential control between the deep and superficial thoracic paraspinal muscles during rotational tasks, and provides evidence that discrete control of thoracic paraspinal muscle fascicles is specific to the direction of forces applied to the trunk.

Thoracic spine extension mobility in young adults: influence of subject position and spinal curvature

STUDY DESIGN

Cross-sectional study.

OBJECTIVES

To examine extension mobility of the thoracic spine in young, asymptomatic adults, with particular reference to the influence of subject position and magnitude of the thoracic kyphosis.

BACKGROUND

Impairment of thoracic extension motion is commonly associated with mechanical pain disorders in this region of the spine. Knowledge of normal thoracic mobility and the factors that may influence this motion is important in the evaluation and management of thoracic pain disorders.

METHODS

In 40 asymptomatic adults, the total and regional thoracic extension range of motion was measured using 2-dimensional photographic image analysis. Extension mobility was measured in standing, sitting, prone, and 4-point kneeling. The association between the magnitude of the habitual thoracic kyphosis and extension mobility was also examined.

RESULTS

When measured from the habitual standing position, the mean range of flexion was 11.5° (3.7°) and mean extension range was 8.7° (3.7°). Thoracic extension was significantly greater in unloaded positions compared to loaded positions (P<.001). The standing thoracic kyphosis angle was significantly correlated with the end range thoracic extension angle in all positions (r = 0.63-0.79, P<.001). There was a poor correlation between the thoracic kyphosis angle and thoracic extension range of motion in all positions (r = 0.11-0.34, P>.06).

CONCLUSION

When measured from the habitual standing position, thoracic extension range of motion in young individuals is small and poorly correlated with the magnitude of the standing thoracic kyphosis. Unloaded positions (4-point kneeling and prone), compared to positions that load the spine (standing and sitting), appear to promote a greater range of thoracic extension motion.

Segmental vertebral rotation in early scoliosis

In order to investigate the development of the vertebral axial rotation in patients with early scoliosis, the vertebral rotation angle (VRA) was quantified on the basis of 132 anteroposterior radiographs obtained from patients with diagnosed or suspected scoliosis. The rotation was measured in the apical vertebra and in the two suprajacent and two subjacent vertebrae. The radiographic material was divided into a control reference group and three scoliotic groups with varying Cobb angle from 4 degrees up to 30 degrees. In the reference group a slight vertebral rotation was significantly more often seen to the right. In the scoliotic groups, the rotation was most pronounced in the apical segments. The mean VRA toward the convex side was significantly increased in the vertebrae just suprajacent to the apex in curves with a Cobb angle of 8 degrees-15 degrees and in the cranial four vertebrae in curves with a Cobb angle of 16 degrees-30 degrees. Atypical vertebral rotation to the opposite side of the major curve was observed in 12.8% of the cases. There was a significant positive correlation between the VRA and the Cobb angle. These results show that a slight VRA to the right is a common feature in the normal spine, and that the VRA increases with progressive lateral deviation of the spine. It is concluded that the coronal plane deformity in early idiopathic scoliosis is accompanied and probably coupled to vertebral rotation in the horizontal plane.

Acceleration injury of the cervical spine by hypertranslation of the head. Part I. Effect of normal translation of the head on cervical spine motion: a radiological study

This paper starts from the concept that acceleration injury of the cervical spine is caused by hypertranslation of the head with respect to the trunk, and not by hyperflexion or hyperextension. This first part of the paper studies the effect of normal head translation upon cervical spine posture and motion. Lateral radiographs of the neck in chin-out and chin-in positions reveal that this translation produces maximal motion at the cranio-vertebral junction C0-2, from full extension in chin-out position to full flexion in chin-in position. Motion decreases from C2-3 downward. Below C6 the direction of motion is reversed. The normal range of head translation is small, notably with a fixed thoracic spine. The hypothesis is developed that hypertranslation of the head will almost immediately result in damaging hyperflexion or hyperextension of the craniovertebral junction.

Interrater Reliability of a Passive Physiological Intervertebral Motion Test in the Mid-Thoracic Spine

OBJECTIVE

To examine the interrater reliability of a passive physiological intervertebral motion (PPIM) test of a mid-thoracic spine motion segment.

METHODS

Nineteen males and 22 females with a mean age of 22.7 years (range, 19-40 years) and no known spinal pathologies were tested independently by 3 certified manual therapy instructors. Investigators performed 3-dimensional segmental mobility testing at a preselected thoracic motion segment. Interrater reliability was assessed with Cohen's kappa statistics, using 3 pairwise comparisons for determination of the direction of lateral flexion leading to the greatest amount of segmental rotation.

RESULTS

Percent agreement ranges were 63.4% to 82.5%, with kappa scores ranging from 0.27 to 0.65.

CONCLUSION

The PPIM testing demonstrated fair to substantial interrater reliability. A majority of females (91%) demonstrated greatest segmental PPIM motion in contralateral rotation with lateral flexion, whereas a majority of males (90%) demonstrated greatest segmental PPIM motion in ipsilateral rotation with lateral flexion. These findings are applicable to asymptomatic subjects of the same age category. Interrater reliability of 3-dimensional PPIM testing is fair to substantial for assessing passive segmental mobility of the mid-thoracic spine.

Disc Related and Non-Disc Related Disorders of the Thoracic Spine

Different anatomical structures and pathophysiological functions can be responsible for lumbar pain, each producing a distinctive clinical profile. Pain can arise from the intervertebral disc, either acutely as a primary disc related disorder, or as result of the degradation associated with chronic internal disc disruption. In either case, greatest pain provocation will be associated with movements and functions in the sagittal plane. Lumbar pain can also arise from afflictions within the zygapophyseal joint mechanism, as a result of synovitis or chondropathy. Either of these conditions will produce the greatest pain provocation during three-dimensional movements, due to maximal stress to either the synovium or joint cartilage. Finally, patients can experience different symptoms associated with irritation to the dural sleeve, dorsal root ganglion, or chemically irritated lumbar nerve root. Differential diagnosis of these conditions requires a thorough examination and provides information that can assist the clinician in selecting appropriate management strategies.

The clinical relevance of biomechanics

Most neurologists are familiar with biomechanics but may be unsure of the relevance of this field to their practice. Actually those involved in musculoskeletal problems are undoubtedly using biomechanical principles. This article is limited to the spine, but the basic principles of biomechanics are applicable to other parts of the body. In this article, we describe the spine and trunk as a biomechanical organ, the biomechanical principles behind back injuries and their importance, the role of biomechanical issues in pain, the utility of clinical tests based on biomechanical principles, the effects of aging, and the future directions in spine biomechanical research.

Endoscopic discectomy increases thoracic spine flexibility as effectively as open discectomy. A mechanical study in a porcine model

STUDY DESIGN

Two surgical techniques for anterior discectomy were compared biomechanically. The surgical procedures were performed in live, anesthetized, skeletally immature pigs. Spine flexibility was measured in vitro.

OBJECTIVE

To determine whether endoscopic techniques for discectomy are as effective as open procedures in increasing spine flexibility.

SUMMARY OF BACKGROUND DATA

Although studies have verified that discectomy increases spine flexibility, no study has confirmed whether endoscopic techniques increase flexibility as effectively as standard thoracotomy, which is a substantially different procedure.

METHODS

The intervertebral disc between vertebrae T8 and T9 was resected from 30 live, anesthetized, adolescent pigs. In 15 pigs, the chest was opened via thoracotomy of the eighth rib, and the disc was excised. In the other 15 pigs, the disc was removed endoscopically. These motion segments and six intact controls were tested mechanically in side bending, flexion-extension, and axial rotation.

RESULTS

No statistically significant differences in flexibility were found between open and endoscopic groups in any loading direction. The statistical power to detect a 20% difference between surgical groups was > or = 95%.

CONCLUSIONS

Endoscopic and open techniques were equally effective in increasing spine flexibility. Because endoscopy may reduce surgical morbidity compared with open discectomy, these results support the use of endoscopy for the surgical correction of scoliosis before instrumentation.

Biomechanical effects of transthoracic microdiscectomy

STUDY DESIGN

Nondestructive flexibility testing was performed to quantify biomechanical parameters of human cadaveric thoracic spines before and after microdiscectomy.

OBJECTIVES

To assess the biomechanical differences between the normal thoracic spine and the thoracic spine after microdiscectomy and to determine whether microdiscectomy results in spinal instability.

SUMMARY OF BACKGROUND DATA

Previous studies have investigated thoracic disc properties and the biomechanical effects of thoracic ligament or bone trauma. No studies were found assessing the effects of thoracic discectomy.

METHODS

Eight motion segments (T4-T5 to T11-T12) from five human cadaveric thoracic spines were studied before and after microdiscectomy. Three-dimensional motion was recorded in response to nondestructive, nonconstraining pure moments. Parameters measured included the neutral zone, elastic zone, range of motion, rotational flexibility, and instantaneous axis of rotation.

RESULTS

The neutral zone, elastic zone, and range of motion increased a small but significant (average P = 0.02 for range-of-motion increase) amount in all directions after thoracic microdiscectomy (mean bilateral range of motion increase, 2.1 degrees; range, 0.5-4.2 degrees). Flexibility increased slightly during lateral bending and flexion. The instantaneous axis of rotation location usually did not change, but sometimes shifted slightly away from the discectomy site after microdiscectomy.

CONCLUSIONS

Thoracic microdiscectomy had small effects on the immediate mechanics and kinematics of the thoracic spine and did not overtly destabilize the motion segments.

An in vivo study of the primary and coupled rotations of the thoracic spine

OBJECTIVE: To provide preliminary data on three-dimensional thoracic spine kinematics measured in vivo. DESIGN: This study measured the three planes of thoracic spine motion in normal subjects using an external measuring device. BACKGROUND: Few studies have investigated the primary and associated coupled rotations in the thoracic spine in vivo. Most knowledge of motion characteristics comes from in vitro studies which have limitations. There is a lack of agreement on the patterns of thoracic coupled motion especially that between lateral flexion and axial rotation. METHODS: Thoracic motion was examined in 60 normal subjects (30 males, 30 females) aged 18-24 years. The primary and coupled rotations of the thoracic regions T(1-4), T(4-8), T(8-12) were measured using a 3 SPACE Fastrak system. RESULTS: The three thoracic regions displayed the characteristic variations in range and distribution of primary rotations previously described. The pattern of coupled motion varied between subjects but an ipsilateral pattern predominated between lateral flexion and axial rotation in the middle and lower thoracic regions while the upper thoracic region was found to exhibit either a contralateral or ipsilateral pattern. Gender did not influence results. CONCLUSIONS: The pattern of coupled motion in the thoracic spine demonstrated some variability between subjects in vivo. Lateral flexion and axial rotation were strongly coupled with overall, their relationship being predominantly ipsilateral.

The development of adolescent idiopathic scoliosis

There are many conflicting actiological theories for adolescent idiopathic scoliosis. We present a simple new model of scoliosis and a mechanism by which it is initiated and progresses. This mechanism provides a final common pathway for the multiple aetiological factors. A simple model of the spine, incorporating its fundamental mechanical features, was constructed. The model consisted of interconnected anterior compression and posterior tension columns. It allowed normal spinal movements, with flexion limited by the posterior column and rotation centered around the anterior column. It also allowed deformities to develop. The ends of the model were fixed in the position of the vertebrae they represented. Overgrowth of the anterior column relative to the posterior column caused the model to take up the shape of an idiopathic scoliosis. The greater the overgrowth, the more marked the deformity. Normally anterior and posterior column growth are coupled. During the growth spurt the thoracic kyphosis flattens indicating that anterior growth temporarily exceeds posterior growth. If this over-growth is marked a scoliosis will develop, as demonstrated by the model. Once this occurs the coupling is lost, anterior growth further outstrips posterior growth and the deformity progresses. Not all scolioses worsen, as the tendency to progress is balanced by neuromuscular factors and remodelling. Factors that increase the growth rate, induce asymmetry or decrease the inherent stability of the spine all encourage the development and progression of a scoliosis. This explains the complex biomechanics of scoliosis and provides a final common pathway by which the multiple aetiological factors can induce idiopathic scoliosis. It has important implications for the understanding and treatment of this condition.

The influence of spine geometry on the coupling between lateral bending and axial rotation

A biomechanical model study was undertaken to examine to what extent facet joint orientation and inclination of the motion segments influence the coupling between lateral bending and axial rotation that occurs in the human spine. The results show that there is an opposite influence of the inclination of the motion segments and the orientation of the facet joints on the axial rotation associated with lateral bending. As one moves in a rostrocaudal direction along the spine there is a decrease of the axial rotation associated with lateral bending. At the upper part of the thoracic spine there is a reinforcement of the axial rotation due to the ventrally inclined orientation of the vertebrae. In the lower thoracic and upper lumbar spine the axial rotation during lateral bending will oppose that due to the dorsally inclined orientation of the vertebrae. In a flexed position of the spine, this coupling is stronger than in an extended spine.

Anatomicomechanical study for the hydraulic line of a thermal left ventricular assist system

A thermal left ventricular assist system currently under development consists of two separate major components, i.e., a pump/actuator module and an engine/thermal battery module. The possible implantation site of the engine/battery module is tentatively determined to be the iliac fossa, which requires a flexible interconnecting line to the pump/actuator module. A quantitative and biomechanical study was done on the effect of the implanted hydraulic line on the bendability of the torso. It revealed that the semirigid interconnecting line would not severely restrict the movement of the patient provided it has the proper prebend configuration. The cadaver fitting study proved the anatomical feasibility of the retroperitoneal iliac fossa as the implant site of the Thermal Ventricular Assist System 8 (TVAS 8) engine/battery module. The location and configuration of the interconnecting line of the TVAS 8 were determined using biomechanical analysis so that the restriction of the body movement due to the line could be minimized. The main route of the line was the left side of the trunk, and the line is W shaped in configuration. The line elongation is the most critical resistive factor and accordingly there may still be rigidity during lateral flexion.