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Experimental data suggest that lumbar torsion contributes to lumbar disc degenerative changes, such as instability, spondylolisthesis and spinal canal stenosis. However, some basic mechanical characteristics of the lumbar spine under torsional loading have not yet been reported in detail. For example, the function of the facet joints under combined mechanical loads such as torsion with superimposed flexion or extension postures is an area of interest about which little biomechanical data have been reported. In this study, the kinematic response to axial torsion with superimposed axial compression (200 N), compression-flexion (3 and 6 Nm) and compression-extension (3 and 6 Nm) was investigated in 10 cadaveric lumbar functional spinal units. Range of motion (ROM), and helical axes of motion (HAM), were analyzed. There was no difference in ROM between no preload, pure compressive and flexion-compression preload conditions. The ROM was significantly reduced by both extension-compression preload conditions (11% reduction for 3 Nm and 19% reduction for 6 Nm of extension) compared to the pure compressive preload. For no preload, the average HAM position in the transverse plane of the intervertebral disc was near the posteriormost part of the disc and located laterally on the side contralateral to the applied torsional moment. In the transverse plane, the HAM position showed a discrete trend towards the posterior part of the specimens during extension. Kinematic data were visualized using computer animation techniques and CT-based reconstructions of the respective specimens. This information may be used for identifying and characterizing physiologic and pathologic motion and for specifying conservative and surgical treatment concepts and, thus, may find application to identifying indications for spinal fusion or in evaluating the effect of future semi-flexible instrumentation.

Study Design. In vitro biomechanical testing of thoracic spine specimens using a atandardized three-dimensional spine flexibility protocol. Objectives. To compare the mechanical stabllity of the intrapeoicular and extrapedicular technique for pedicle screw placement. The hypothesis was that extrapedicular screw placement provides an equally rigld construct. Summary of Background Data. Pedicle screws provide rigld fixation of instabilities in the lumbar and lumbosacral spine. Anatomic considerations and the potential risk. of neurologic complications are the main teasons to hesitate using pedicle screws in the thoracic spine. Extrapedicular fixation would allow safer insertion due to an increased distance to the spinal canal. Methods. Twsive human cadaveric thoracic spines (six intra-, six extrapedicular) were instrumented with the USS system, using computed tomography-based computer navigation to ensure acurate placement. The specimens were tested in flexion-extension, torsion, and fateral bending, The ROM was measured using an optoeiectronic system, and the two methods were compared before and after implantation of the USS construct and before and after fatigue testing of the construct. Results. The ROM of the instrumented spine was reduced to less than 50% that of its original ROM. There were no statistically significant differences in the ROM reduction between the intra- and the extrapedicular technique. Cyclic fatiguing of the construct did not significantly increase the ROM. Conclusions. The extrapedicular technique provides a construct for stabilization of the thoracic spine that is as rigid as the conventional intrapedicular technique, but has the adyantage of a safer surglcal screw insertion.

Presently, there is little consensus about how, or even if, axial preload should be incorporated in spine flexibility tests in order to simulate the compressive loads naturally present in vivo. Some preload application methods are suspected of producing unwanted “artefact” forces as the specimen rotates and, in doing so, influencing the resulting kinematics. The objective of this study was to quantitatively compare four distinct types of preload which have roots in contemporary experimental practice. The specific quantities compared were the reaction moments and forces resulting at the intervertebral disc and specimen kinematics. The preload types incorporated increasing amounts of caudal constraint on the preload application vector ranging from an unconstrained dead-load arrangement to an apparatus that allowed the vector to follow rotations of the specimen. Six human cadaveric spine segments were tested (1-L1/L2, 3-L2/L3, 1-L3/L4 and 1-L4/L5). Pure moments were applied to the specimens with each of the four different types of compressive preload. Kinematic response was measured using an opto-electronic motion analysis system. A six-axis load cell was used to measure reaction forces and moments. Artefact reaction moments and shear forces were significantly affected by preload application method and magnitude. Unconstrained preload methods produced high artefact moments and low artefact shear forces while more constrained methods did the opposite. A mechanical trade-off is suggested by our results, whereby unwanted moment can only be prevented at the cost of shear force production. When comparing spine flexibility studies, caution should be exercised to ensure preload was applied in a similar manner for all studies. Unwanted moments or forces induced as a result of preload application method may render the comparison of two seemingly similar studies inappropriate.

Favorable results using fibrin sealants in vascular surgery and soft tissue reconstruction have prompted investigation of these biologic adhesives for orthopedic applications. One important recent application was as a sealant for periosteal grafts applied to defects in articular surfaces, a procedure that contained injected chondrocytes cultured in vitro. The low and variable adhesive strength of autologous fibrin substances prompted our investigation of allogeneic fibrin. An in vitro test method was developed to investigate the use of a fibrin sealant for attaching periosteal (bovine) patches to articular cartilage (bovine). Dermis-dermis (porcine) adhesion also was evaluated. In tests of the periosteum-to-cartilage bond performed in a physiological environment, we determined the effects of the following variables on the adhesive shear strength: set time, source of fibrinogen (bovine versus human), and fibrinogen concentration. A specially designed test rig was developed to avoid nonshear force components. Adhesive shear strength increased with fibrin set time for both fibrinogen concentrations and sources (p

A generic three-dimensional finite-element model of the upper half of the femur containing a cemented femoral stem of a total hip arthroplasty was developed to study those factors influencing cement strains near the tip of a cemented femoral component. This generic model was verified through another three-dimensional finite-element model that had been created based on the precise geometry of a cadaver femur implanted with a contemporary cemented femoral component. This cadaveric femoral reconstruction had been created with strain gauges embedded in the cement mantle and was then loaded under conditions simulating single leg stance and stairclimbing. By use of the cement strains measured experimentally in the cadaver femur, and comparison of them with those obtained from the finite-element model of that cadaver femur, it was possible to establish proper material properties, boundary conditions, and loading conditions for the generic model. The generic model was then modified parametrically to determine those factors that influence the strains occurring within the cement mantle near the tip of a cemented femoral component. These models suggest that the single factor that most adversely influenced peak strains at or near the tip of the prosthesis was a thin cement mantle. This effect was present both when the cement mantle was reduced in thickness and when a similar effect occurred by virtue of a varus or valgus placement of the stem. Factors that decreased the peak cement strains near the tip of the femoral stem included a more flexible stem and thicker cement mantles. This effect of a more flexible stem could be obtained by changing the modulus of the metal implant, by uniformly reducing the thickness of the stem, or by tapering the stem within the same bone geometry. Thicker cement mantles reduced both the axial and the shear strains occurring at the tip of the prosthesis. The presence or absence of a hole in the tip of the prosthesis per se, as for a centralizer, had no significant effect on the peak cement strains seen around the tip of the prosthesis; however, truncating the tip of the prosthesis from a hemisphere to a flat profile, which resulted in a sharp corner at the tip of the prosthesis, produced a 35% increase in cement strains at the tip as a result of a stress concentration effect. Thus, the common way of modifying the tip to have a hole for a centralizer, which involved truncating the tip, increased the cement strains occurring near the tip of the prosthesis.

Physical forces applied to connective tissues may cause significant changes in cell metabolism and gene expression. Theoretical investigations indicate that mechanical loading histories beginning very early in skeletal development may guide endochondral ossification patterns and the initial architectural construction of bones. Developmental patterns and structures of bones can be emulated using mathematical algorithms or "rules of construction" which relate developmental processes to tissue stress (or strain) histories. Skeletal forms and tissues are well-designed for their mechanical function primarily because their histomorphological construction has been guided by mechanical loading during growth and development. Construction rules of developmental mechanics can also be used to describe many of the histological and morphological adaptations of mature skeletal tissues to changes in customary physical activity. Over many generations, changes in the heritable genetic information occurs by mutation and genetic variability. The range of skeletal forms that are possible in evolution due to such variations, however, is constrained by the developmental rules of construction that reflect biophysical processes associated with the tissue mechanical loading.

In a companion paper, we presented a time-dependent theory for bone modeling and remodeling in response to a daily loading history. This paper represents a preliminary attempt to use the theory to determine the distribution of bone density within the adult proximal femur under an assumed normal loading history. Subsequent functional adaptation of the internal structure due to changes in the loading history are then determined. Throughout this preliminary study, the external geometry of the proximal femur is considered to be fixed, i.e., changes in the external shape are neither stimulated nor allowed. Linear and trilinear (dead-zone nonlinearity) rate remodeling laws were compared. Computer emulations using two-dimensional finite element models were successful in creating a normal-appearing distribution of bone tissue when remodeling was initiated from a solid structure of homogeneous bone density. Subsequent reduction in the loading history caused regional bone atrophy. Reinstatement of the normal loading history caused a generalized increase in bone mass but resulted in a slightly different bone distribution than was calculated for a constant loading history. These results demonstrate the utility of the remodeling theory and are consistent with the hypothesis that similar stress-related phenomena are responsible for both normal morphogenesis and functional adaptiation in response to changes in the bone loading.

A time-dependent approach for emulating bone modeling and remodeling in response to the daily loading history is presented. We postulate that genotype, systemic metabolic conditions, and local tissue interactions establish the level of local tissue mechanical stimulation (attractor state) appropriate for the maintenance of bone tissue. The net daily rate of apposition or resorption on a bone surface is determined by the difference between the actual stimulus and the tissue attractor state and can be modulated by other biologic factors. In calculating the net change in local bone apparent density, the technique takes into account the bone surface area available for osteoblastic and osteoclastic activity. Endosteal, periosteal, haversian, and cancellous bone modeling and remodeling are thereby treated in a consistent, unified fashion.

A comparison between an unconstrained and a partially constrained system for in vitro biomechanical testing of the L5-S1 spinal unit was conducted. The objective was to compare the compliance and the coupling of the L5-S1 unit measured with an unconstrained and a partially constrained test for the three major physiological motions of the human spine. Very few studies have compared unconstrained and partially constrained testing systems using the same cadaveric functional spinal units (FSUs). Seven human L5-S1 units were therefore tested on both a pneumatic, unconstrained, and a servohydraulic, partially constrained system. Each FSU was tested along three motions: flexion-extension (FE), lateral bending (LB) and axial rotation (AR). The obtained kinematics on both systems is not equivalent, except for the FE case, where both motions are similar. The directions of coupled motions were similar for both tests, but their magnitudes were smaller in the partially constrained configuration. The use of a partially constrained system to characterize LB and AR of the lumbosacral FSU decreased significantly the measured stiffness of the segment. The unconstrained system is today's "gold standard" for the characterization of FSUs. The selected partially constrained method seems also to be an appropriate way to characterize FSUs for specific applications. Care should be taken using the latter method when the coupled motions are important.

Intervertebral cages in the lumbar spine represent an advancement in spinal fusion to relieve low back pain. Different implant designs require different endplate preparations, but the question of to what extent preservation of the bony endplate might be necessary remains unanswered. In this study the effects of endplate properties and their distribution on stresses in a lumbar functional spinal unit were investigated using finite-element analyses. Three-dimensional finite-element models of L2-L3 with and without a cage were used. An anterior approach for a monobloc, box-shaped cage was modelled. The results showed that inserting a cage increased the maximum von Mises stress and changed the load distribution in the adjacent structures. A harder endplate led to increased concentration of the stress peaks and high stresses were propagated further into the vertebral body, into areas that would usually not experience such stresses. This may cause structural changes and provide an explanation for the damage occurring to the underlying bone, as well as for the subsequent subsidence of the cage. Stress distributions were similar for the two endplate preparation techniques of complete endplate preservation and partial endplate removal from the centre. It can be concluded that cages should be designed such that they rely on the strong peripheral part of the endplate for support and offer a large volume for the graft. Furthermore, the adjacent vertebrae should be assessed to ensure that they show sufficient density in the peripheral regions to tolerate the altered load transfer following cage insertion until an adequate adaptation to the new loading situation is produced by the remodelling process.

Intervertebral cages in the lumbar spine have been an advancement in spinal fusion to relieve low back pain. Even though initial stability is accepted as a requirement for fusion, there are other factors. The load transfer and its effect on the tissues adjacent to the cage may also play an essential role, which is not easily detectable with experimental tests. In this study the effects of an intervertebral cage insertion on a lumbar functional spinal unit were investigated using finite element analyses. The influences of cage material, cancellous bone density and spinal loading for the stresses in a functional spinal unit were evaluated. Three-dimensional (3D) finite element models of L2-L3 were developed for this purpose. An anterior approach for a monobloc, box-shaped cage was modelled. Models with cage were compared to the corresponding intact ones. The results showed that inserting a cage increased the maximum von Mises stress and changed the load transfer in the adjacent structures. Varying the cage material or the loading conditions had a much smaller influence than varying the cancellous bone density. The denser the cancellous bone, the more the stress was concentrated underneath the cage, while the remaining regions were unloaded. This study showed that the density of the underlying cancellous bone is a more important factor for the biomechanical behaviour of a motion segment stabilized with a cage, and its eventual clinical success, than the cage material or the applied load. Inserting an intervertebral cage markedly changed the load transfer. The altered stress distribution may trigger bone remodelling and explain damage of the underlying vertebrae

Interdisciplinary communication of three-dimensional kinematic data arising from in vitro biomechanical tests is challenging. Complex kinematic representations such as the helical axes of motion (HAM) add to the challenge. The difficulty increases further when other quantities (i.e. load or tissue strain data) are combined with the kinematic data. The objectives of this study were to develop a method to graphically replay and animate in vitro biomechanical tests including HAM data. This will allow intuitive interpretation of kinematic and other data independent of the viewer's area of expertise. The value of this method was verified with a biomechanical test investigating load-sharing of the cervical spine. Three 3.0 mm aluminium spheres were glued to each of the two vertebrae from a C2-3 segment of a human cervical spine. Before the biomechanical tests, CT scans were made of the specimen (slice thickness=1.0 mm and slice spacing=1.5 mm). The specimens were subjected to right axial torsion moments (2.0 Nm). Strain rosettes mounted to the anterior surface of the C3 vertebral body and bilaterally beneath the facet joints on C3 were used to estimate the force flow through the specimen. The locations of the aluminium spheres were digitised using a space pointer and the motion analysis system. Kinematics were measured using an optoelectronic motion analysis system. HAMs were calculated to describe the specimen kinematics. The digitised aluminium sphere locations were used to match the CT and biomechanical test data (RMS errors between the CT and experimental points were less than 1.0 mm). The biomechanical tests were "replayed" by animating reconstructed CT models in accordance with the recorded experimental kinematics, using custom software. The animated test replays allowed intuitive analysis of the kinematic data in relation to the strain data. This technique improves the ability of experts from disparate backgrounds to interpret and discuss this type of biomechanical data.

The role of in vivo mechanical loading histories in normal skeletogenesis is related to the process of adaptive, stress-regulated bone remodeling in the adult. The results of many previous computer models for endochondral ossification and bone modeling and remodeling are reviewed. These studies support the view that simple stress-related mathematical algorithms or "construction rules" can be used to emulate normal skeletal development and architectural construction. Such mathematical rules presumably represent the net result of biophysical phenomena influencing cell metabolism and biosynthetic activity. These rules are also successful in describing the adaptation of adult bone to changes in tissue stresses. The findings suggest that stress-related functional adaptation in mature bones may be merely the adult manifestation of the same mechanical construction rules that guide and constrain normal development.

Using a three-dimensional finite element model of a plated long bone, we studied the influence of screw tightness, sliding frictional interfaces, and loading magnitude on the stresses within the plated bone. The model incorporated frictional interface elements that allowed stress-free separation under tensile loading to occur between the plate and bone and between the screw heads and the plate. The applied loading simulated both static preloads created by tightening the screws that secure the plate to the bone and physiologic loads created by activity. Initial screw tightening with plate application created regions of bone hydrostatic compressive stress that may be partly responsible for ischemia under the plate. The inclusion of frictional interfaces resulted in a nonlinear relationship between physiologic load and bone strain that was dependent on screw tightness. This nonlinear response correlated well with the results of previous in vitro studies showing that slippage between the plate and the bone can occur at physiologic load levels. The results showed that the effect of such slippage can be at least as important as plate meterial, rigidity, and placement in determining the degree of stress shielding. The results also indicated that previous plated bone models that assumed tight interfaces may have overestimated the extent of mechanical stress shielding.

A theory relating bone maintenance to mechanical loading history has been applied to successfully predict the distribution of bone density and trabecular orientation in the adult proximal femur. The loading history was simulated by determining the stress fields in a two-dimensional finite element model exposed to various discrete loading cases and making assumptions about the relative number of loading cycles associated with each load case. The total stimulus to bone maintenance was then calculated by a linear superposition of the stimulus of each loading case. Based on the calculated total stimulus, the apparent density and material properties of each element were changed and the stress solutions were again determined. Using this iterative technique, the bone apparent density and orientation characteristics were predicted. The results indicate that the trabecular morphology of the femur can only be explained by considering the joint loadings from multiple directions. Contrary to the ‘trajectorial theory’ promoted by Wolff (The Law of Bone Remodelling, 1892), trabecular orientations predicted from our multiple-load analyses are not necessarily perpendicular and do not correspond to the principal stress directions of any one loading condition. Our predicted orientations correspond better to the drawing of bone trabecular morphology by von Meyer (Archs Anat. Physiol. wiss. Med.34, 615–628, 1867) than to the classic drawing by Wolff and suggest that further study of the trajectorial theory is warranted.

In recent years much attention has been focused on the problem of refracture after the removal of internal fixation plates. Two concepts that have been implicated as contributory causes of cortical bone osteopenia or thinning which may result in refracture are: 1) mechanical stress shielding; and 2) vascular insufficiency. The first of these two possibilities has received considerable attention and is the focus of the present study.

A new theory is introduced to describe some of the influences of mechanical stresses on chondroosseous biology. It is proposed that degeneration and ossification is a normal process for all cartilage in the appendicular skeleton, which is accelerated by intermittently applied shear stresses (or strain energy), and inhibited or prevented by intermittently applied hydrostatic pressure. These concepts were applied using finite element computer models in an effort to predict the ossification pattern of the prenatal and postnatal femoral anlage. The theoretical calculations successfully predicted the key features of skeletal morphogenesis including the development of the primary ossification site, a tubular diaphysis and marrow cavity, metaphyseal and epiphyseal trabecular bone, the location and geometry of the growth plate, the appearance and location of the secondary ossific nucleus, and the existence and thickness distribution of articular cartilage. The results suggest that degenerative joint disease in immobilized or nonload-bearing mature joints may be a manifestation of the final stage in the ossification of the anlage. In nonfunctional joints, the absence or reduction of intermittent hydrostatic pressure in the articular cartilage permits cartilage degeneration and the progressive advance of the ossification front toward the joint surface until the articular cartilage has been ossified.