Iron dynamics in Al-Cu-Fe quasicrystals and approximants: Mössbauer and neutron experiments

We present new results on the iron dynamics in the icosahedral quasicrystal i-AlCuFe and two cubic approximants as well as the non-approximant Al-Cu-Fe cubic B2 phase. Conventional Mössbauer spectroscopy is used as well as, for the i-AlCuFe phase, high Doppler velocity Mössbauer spectroscopy and quasielastic neutron scattering for samples with different Fe isotope contents. We show that in the i-phase the Fe Lamb-Mössbauer recoilless fraction decreases below that predicted for lattice vibrations alone for temperatures above about 550 K. This decrease is correlated with the onset of a quasielastic signal seen in both Mössbauer and neutron backscattering spectroscopy, which indicates the presence above 550 K of Fe jump processes confined in a local cage. The timescale of the Fe jumps (660 ps at 1000 K) and their temperature dependence differ widely from those of Cu jumps in the same i-AlCuFe quasicrystal. From the temperature dependence of the quadrupole splitting of the (57)Fe Mössbauer spectrum, one can distinguish two kinds of Fe jumps, one starting at 550 K and the second above 800 K. In the two cubic approximants, a loss in the Fe recoilless fraction also occurs above 550 K, revealing the same kind of Fe dynamics as in the i-phase but the effect is smaller. On the other hand, no anomalous Fe dynamics (other than lattice vibrations) is detected in the B2-AlCuFe phase. Since the cubic approximants possess similar local configurations as the quasicrystal, we conclude that locally a Penrose tile description is appropriate. This shows that the detected Fe jumps can be interpreted in terms of phason-like local tiling flips.

Cartilage anlagen adapt in response to static deformation

Connective tissue adaptation, including the development of cartilaginous anlagen into bones, is widely believed to be related to dynamic, intermittent load and stress histories. Static stresses, on the other hand, are generally believed deleterious in tissue adaptation. Using serial MRI in a natural human experiment (manipulation and corrective casting of infant clubfoot), we have observed casting produces two effects: (1) the well recognized change in relative positions of the hindfoot anlagen; (2) a newly recognized immediate shape change in the anlagen. These changes seemingly enhance the rate of growth of the anlagen and of the ossific nucleus. The shape change or deformation in the anlagen would occur as a result of alterations in the magnitudes and directions of loading from soft tissue attachments and muscle activity and would necessarily be associated with changes in the stress states within the anlagen and, when present, the ossific nuclei. Given the known role of load and stress history in tissue adaptation, we presume the reduced stress histories influence the enhanced growth rates. These observations contradict some current theories of tissue adaptation since static, rather than dynamic stresses play a crucial role in accelerating the growth and development of anlagen in the infant clubfoot.

Modeling Deformation-Induced Fluid Flow in Cortical Bone?s Canalicular?Lacunar System

To explore the potential role that load-induced fluid flow plays as a mechano-transduction mechanism in bone adaptation, a lacunar-canalicular scale bone poroelasticity model is developed and implemented. The model uses micromechanics to homogenize the pericanalicular bone matrix, a system of straight circular cylinders in the bone matrix through which bone fluids can flow, as a locally anisotropic poroelastic medium. In this work, a simplified two-dimensional model of a periodic array of lacunae and their surrounding systems of canaliculi is used to quantify local fluid flow characteristics in the vicinity of a single lacuna. When the cortical bone model is loaded, microscale stress, and strain concentrations occur in the vicinity of individual lacunae and give rise to microscale spatial variations in the pore fluid pressure field. Furthermore, loading of the bone matrix containing canaliculi generates fluid pressures in the contained fluids. Consequently, loading of cortical bone induces fluid flow in the canaliculi and exchange of fluid between canaliculi and lacunae. For realistic bone morphology parameters, and a range of loading frequencies, fluid pressures and fluid-solid drag forces in the canalicular bone are computed and the associated energy dissipation in the models compared to that measured in physical in vitro experiments on human cortical bone. The proposed model indicates that deformation-induced fluid pressures in the lacunar-canalicular system have relaxation times on the order of milliseconds as opposed to the much shorter times (hundredths of milliseconds) associated with deformation-induced pressures in the Haversian system.

Micromechanically based poroelastic modeling of fluid flow in Haversian bone

To explore the hypothesis that load-induced fluid flow in bone is a mechano-transduction mechanism in bone adaptation, unit cell micro-mechanical techniques are used to relate the microstructure of Haversian cortical bone to its effective poroelastic properties. Computational poroelastic models are then applied to compute in vitro Haversian fluid flows in a prismatic specimen of cortical bone during harmonic bending excitations over the frequency range of 10(0) to 10(6) Hz. At each frequency considered, the steady state harmonic response of the poroelastic bone specimen is computed using complex frequency-domain finite element analysis. At the higher frequencies considered, the breakdown of Poisueille flow in Haversian canals is modeled by introduction of a complex fluid viscosity. Peak bone fluid pressures are found to increase linearly with loading frequency in proportion to peak bone stress up to frequencies of approximately 10 kHz. Haversian fluid shear stresses are found to increase linearly with excitation frequency and loading magnitude up until the breakdown of Poisueille flow. Tan delta values associated with the energy dissipated by load-induced fluid flow are also compared with values measured experimentally in a concurrent broadband spectral analysis of bone. The computational models indicate that fluid shear stresses and fluid pressures in the Haversian system could, under physiologically realistic loading, easily reach the level of a few Pascals, which have been shown in other works to elicit cell responses in vitro.

Primary adult human bone cells do not respond to tissue (continuum) level strains

Bone adapts to its mechanical environment, and, since the late 1800s, investigators have presumed that this adaptation relates to strain magnitude. Indeed, overwhelming evidence supports the view that either strain or some strain-related quantity stimulates bone adaptation or remodeling. Virtually all investigators, implicitly or explicitly, assume that the level of strain magnitude responsible for bone adaptation is that measured by strain gauges in vivo (i.e., 100-2500 microstrain) and that bone cells are directly deformed by strained matrix. We present evidence that bone cell deformation in this range does not cause bone adaptation. First, bone cells in vitro typically do not respond to average (continuum) levels of strain magnitude. Second, bone cells in vitro do respond to fluid flow-induced shear stresses in these ostensible physiological ranges. Third, in vivo strain magnitudes presumed to stimulate remodeling reflect only averages, and not local peaks, which are 2-15 times higher. Thus, we hypothesize that sensing cells do not respond to levels of strain presumed to be physiological.

Venous thromboembolic disease management patterns in total hip arthroplasty and total knee arthroplasty patients: a survey of the AAHKS membership

The American Association of Hip and Knee Surgeons (AAHKS) distributed a survey to its members exploring practice patterns implemented to prevent venous thromboembolic disease (VTED) in patients undergoing total hip arthroplasty (THA) and total knee arthroplasty (TKA). Of 720 (33%) members, 236 responded. Prophylaxis was prescribed for 100% of patients during the course of hospitalization for THA and TKA. Warfarin was the commonest pharmacologic treatment used for THA (66%) and TKA (59%) patients. Low-molecular-weight heparin was used in 16% of THA patients and 18% of TKA patients. The most commonly employed mechanical modality was pneumatic devices in THA (51%) and TKA (50%). Universal acceptance of the need for prophylaxis administration for patients undergoing THA and TKA is shown. The method and duration remain highly variable; although the survey illustrates such variation, it suggests there is no one best method of prophylaxis.

A broadband viscoelastic spectroscopic study of bovine bone: implications for fluid flow

To explore the hypothesis that mechanical excitation-induced fluid flow and/or fluid pressure are potential mechanical transduction mechanisms in bone adaptation, a complementary experimental and analytical modeling effort has been undertaken. Experimentally, viscoelastic tan delta properties of saturated cortical bovine bone were measured in both torsion and bending, and significant tan delta values in the 10(0)-10(5) Hz range were observed, although the nature of the damping is not consistent with a fluid pressure hypothesis. Analytically, micromechanically based poroelasticity models were exercised to quantify energy dissipation associated with load-induced fluid flow in large scale channels. The modeling results indicate that significant damping due to fluid flow occurs only above 1 MHz frequencies. Together, the experimental and analytical results indicate that at excitation frequencies presumed to be physiological (1-100 Hz), mechanical loading of bone generates extremely small pore fluid pressures, making the hypothesized fluid-pressure transduction mechanism upon osteocytes untenable.

Cell alignment is induced by cyclic changes in cell length: studies of cells grown in cyclically stretched substrates

Many types of cells, when grown on the surface of a cyclically stretched substrate, align away from the stretch direction. Although cell alignment has been described as an avoidance response to stretch, the specific deformation signal that causes a cell population to become aligned has not been identified. Planar surface deformation is characterized by three strains: two normal strains describe the length changes of two initially perpendicular lines and one shear strain describes the change in the angle between the two lines. The present study was designed to determine which, if any, of the three strains was the signal for cell alignment. Human fibroblasts and osteoblasts were grown in deformable, rectangular, silicone culture dishes coated with ProNectin, a biosynthetic polymer containing the RGD ligand of fibronectin. 24 h after plating the cells, the dishes were cyclically stretched at 1 Hz to peak dish stretches of 0% (control), 4%, 8%, and 12%. After 24 h of stretching, the cells were fixed, stained, and their orientations measured. The cell orientation distribution was determined by calculating the percent of cells whose orientation was within each of eighteen 5 degrees angular intervals. We found that the alignment response was primarily driven by the substrate strain which tended to lengthen the cell (axial strain). We also found that for each cell type there was an axial strain limit above which few cells were found. The axial strain limit for fibroblasts, 4.2 +/- 0.4%, (mean +/- 95% confidence), was lower than for osteoblasts, 6.4 +/- 0.6%. We suggest that the fibroblasts are more responsive to stretch because of their more highly developed actin cytoskeleton.

Primary human bone cultures from older patients do not respond at continuum levels of in vivo strain magnitudes

Osteoporosis is characterized by excessive loss of bone mass, while exercise is believed to maintain or enhance bone mass. Since exercise marginally affects osteoporosis, we wondered whether bone cells from osteoporotic patients would fail to respond to strain. Primary human bone-like cultures were obtained from females over age 60 with hip arthroplasty procedures performed for either osteoporotic fracture (n = 8) or non-osteoporotic osteoarthrosis (n = 5). Cultures (96,000 cell/cm2) were strained in rectangular optically clear silastic wells. Three periods of uniaxial substratum strain (1000 micro-strain, 1 Hz, 10,000 cycles, sine wave) were provided every 24 h using a four-point bending, computer-controlled device. Results at a frequency of 1 Hz were compared to cultures exposed to 20 Hz with bone cells derived from one osteoarthritic subject. Alterations in protein level expression of bone-related proteins were determined using a semi-quantitative confocal approach along with enzyme (alkaline phosphatase) activity and enzyme mRNA copy number using cRNA RT-PCR. Strain did not alter levels of bone-related protein levels, enzyme activity, or steady state copy number per cell in response to strain in either group. Strained cultures from osteoporotic patients exhibited little variation from unstrained controls, while individual cultures from osteoarthritic patients exhibited increases in one protein or the other. The results suggest that bone cells from older individuals may not be responsive to continuum levels of strain anticipated with vigorous activity.

Mathematical modelling of stress in the hip during gait

A mathematical model is developed for calculating the contact stress distribution in the hip for a known resultant hip force and characteristic geometrical parameters. Using a relatively simple single nonlinear algebraic equation, the model can be readily applied in clinical practice to estimate the stress distribution in the most frequent body positions of everyday activities. This is demonstrated by analyzing the data on the resultant hip force obtained from laboratory observations where a stance period of gait is considered.

Toward an understanding of implant occlusion and strain adaptive bone modeling and remodeling

STATEMENT OF PROBLEM

Dental implant failure rates for osseointegration are greater in the highly atro-phic maxilla. Presuming higher failure rates relate to strain-driven adaptation, an enhanced understanding of formative bone response to loading (modeling) and maintenance of an integrated state (remodeling) should improve treatment.

PURPOSE

To understand the role of occlusal loading on long-term osseointegration in areas of compromised cancellous bone, a review of the salient features of adaptive bone modeling and remodeling is presented with an emphasis on cancellous bone responses.

CONCLUSIONS

The ability for dental implants to maintain a long-term stable interface in the maxilla lies in the ability of trabecular bone to maintain adequate local material (strength) and architectural (connectivity) properties. In this discussion, an emphasis has been placed on understanding how trabecular bone can respond to the mastication-induced loading environment on an implant.

A cell strain system for small homogeneous strain applications

A cell culture system has been developed that enables application of well characterized, homogeneously distributed cyclic strains to monolayer cell cultures. Optically clear silicone culture dishes atop Plexiglas base plates are deformed by four-point bending of flexible silicone culture wells driven in user specified strain cycle patterns using computer controlled electromagnetic linear actuators. Cyclic mechano-transduction can be induced in amplitudes of 0 to 3000 mustrain, in frequencies of 0 to 30 Hz and in any specified strain cycle pattern. The cell culture system, which contains six simultaneously driven culture wells, has been mechanically characterized by holographic interferometry, laser displacement sensor recordings of the dish surfaces, strain gauge monitoring of the base plates, and finite element modeling of the dishes on the base plates. The standard deviation of the strain amplitudes among the six simultaneously stimulated culture wells is less than 5%. The cell culture system allows accurate generation of small magnitudes of well characterized, homogeneous strain, easy handling of the culture wells, flexible setting of cyclic strain pattern parameters, simultaneous stimulation of 6 culture wells, and light microscopic observation of the cell cultures.

Quantifying osteoarthrotic hip incongruence. An approach to optimizing osteotomies

Osteoarthrosis of the hip may be treated by osteotomy, but surgeons report variable results, and there is no consensus regarding which method to use in choosing the type of osteotomy. The authors defined three biomechanical measures of hip incongruence (characteristic point locus, joint space, and contact region) and developed a two-dimensional frontal plane model to compute joint incongruence over the joint range of motion during normal activities of daily living. The preoperative measures were calculated for 38 patients who had undergone osteotomy at least 5 years earlier. The authors calculated the measures throughout a functional range of motion after 13 stimulated varus or valgus osteotomies. A logistic regression analysis determined which, if any, of the three measures, in conjunction with other clinical variables, correctly predicted outcome. The average values for the characteristic point locus, joint space, and contact region measures ranged from 0.260 cm to 2.127 cm, 0.963 cm2 to 9.327 cm2, and 0.063 cm to 4.230 cm, respectively. Unimodal behavior between two of the three measures (joint space and contact region) and osteotomy angle were observed, suggesting these two would be the most useful in predicting an optimal osteotomy. The most significant independent variable predicting clinical outcome was the joint space measure. This supports the potential of an optimization approach for determining the best angle for a hip osteotomy.

Pelvic muscle and acetabular contact forces during gait

Locations, magnitudes, and directions of pelvic muscle and acetabular contact forces are important to model the effects of abnormal conditions (e.g., deformity, surgery) of the hip accurately. Such data have not been reported previously. We computed the three-dimensional locations of all pelvic muscle and acetabular contact forces during level gait. The approach first required computation of the intersegmental joint resultant forces and moments using limb displacement history, foot-floor forces, and estimated limb inertial properties from one subject. The intersegmental resultant moments were then distributed to the muscles using a 47-element muscle model and a non-linear optimization scheme. Muscle forces were vectorally subtracted from the intersegmental resultants to compute the acetabular contact forces. While the peak joint force magnitudes are similar to those reported previously for the femur, the directions of pelvic contact forces and muscle forces varied considerably over the gait cycle. These variations in contact force directions and three-dimensional forces could be as important as the contact force magnitudes in performing experimental or theoretical studies of loads and stresses in the periacetabular region.

What do tissues and cells know of mechanics?

Many tissues and cells adapt to their mechanical environment, i.e. the stresses and strains to which the tissues are exposed during daily activities. Simple mechanical explanations of such adaptation naturally provide appeal, yet fail to predict accurately tissue appearance and behaviour. Continuum mechanics, the field from which our concepts of stress and strain arise, assumes a solid continuous structure, an assumption that does not apply at cell level. Merely correlating a given stress or strain magnitude with tissue behaviour does not per se account for the time over which responses occur. Tissues undoubtedly 'temporally process' mechanical signals, responding to some portion of the mechanical environment, and ignoring others. Further, the deformation that cells experience will depend upon their nonrigid connections to the matrix, so that the strains in the tissue may not be those experienced by the cell (and those causing the adaptation). Relatively simple mechanical models incorporating temporal features may adequately predict tissue adaptation, but the noncontinuum nature of tissues suggests that such models cannot adequately explain either tissue adaptation or the responses leading to adaptation.

Testing the daily stress stimulus theory of bone adaptation with natural and experimentally controlled strain histories

Theories of bone adaptation generally consider that a departure in some feature of the normal homeostatic mechanical stimulus governs mechanical adaptation. Specifically, the 'daily stress stimulus' theory commonly used in computational models of bone adaptation suggests that the mechanical stimulus arises from a synthesis of the peak magnitudes from each loading event during a day. In this study, the homeostatic daily strain history of the adult turkey ulna was established by categorizing and counting the natural wing activities of adult male turkeys over a full 24h period. Strain signals were recorded in vivo for each activity type at three mid-diaphysis sites using stacked rosette strain gages. Following surgical isolation and transverse metaphyseal pinning of the ulnae, additional strain signals were recorded during controlled axial and torsional loading regimens associated with documented maintenance, loss, or addition of bone mass. When the present data were incorporated into the daily stress stimulus formulation, the theory did not consistently discriminate maintenance versus formation regimens, i.e., some maintenance regimens were associated with a substantially higher daily stimulus than some regimens causing bone formation.

Popliteus function in ACL-deficient patients

Anterior cruciate ligament (ACL) injuries commonly result in anterolateral rotary instability and a 'pivot shift' phenomenon. Since popliteus muscle stimulation causes a pivot shift, some postulate the popliteus muscle plays a role in causing pivot shifts. To see if patients with pivot shifts exhibited excessive popliteus muscle activity, we studied fine-wire EMGs of the popliteus in 16 normal subjects and 10 ACL-deficient subjects. Subjects performed six activities (level walking and jogging, ascending walking and jogging, and descending walking and jogging). Except for minor timing differences in ascending treadmill and ascending jogging, the signals were similar for injured and uninjured limbs; similar variance ratios suggested similar pattern variability. Thus, we observed only minor popliteus EMG signal differences in this group of patients. We conclude that the popliteus muscle does not actively contribute to instability in the studied activities.

Sensory innervation of the cat knee articular capsule and cruciate ligament visualised using anterogradely transported wheat germ agglutinin-horseradish peroxidase

Wheat germ agglutinin-horseradish peroxidase conjugate (WGA-HRP) was injected into the dorsal root ganglia (L5-S1) of the cat and used as an anterograde tracer substance for intra-axonal labelling of peripheral nerve endings in joint capsule and cranial (anterior) cruciate ligament (CCL). We believed that the high specificity of WGA-HRP for neural tissue along with the high visibility of its reaction product could help resolve controversies concerning the sensory innervation of the cruciate ligaments. Substantial amounts of WGA-HRP were transported in tibial nerve axons to the level of the knee. However, using standard HRP histochemistry we found that the capsular tissue and ligament synovia disintegrated during the incubation reaction. This problem was avoided by air drying the tissue slices on glass slides prior to reaction. Abundant labelling occurred in the posterior capsule with dense filling of axons and terminal endings. Sensory endings displayed features consistent with Ruffini endings and pacinian corpuscles. Sensory endings were located throughout the CCL in its sagittal plane, in the subsynovial layers and between collagen fascicles. In each CCL we observed 5-17 ovoid and elongated endings with dense terminal arborisations. These endings were between 100 and 150 microns long, were encapsulated, and gave rise to 1 or 2 axons. Large (up to 1.5 mm in maximum extent) elongated regions of dense, inhomogeneous labelling were found in the body of several CCLs. These resembled Golgi tendon-like endings, with the exception of their large size. We conclude that anterograde transport of HRP to the knee is a useful technique for labelling mechanoreceptors and axons in knee tissue. However, recently developed immunohistochemical analysis of peripheral tissue using protein gene product 9.5 appears to be the method of choice and should be employed for further study of human and animal cruciate ligament innervation.

Effects of leg length discrepancies on the forces at the hip joint

The authors questioned whether leg length discrepancies of the magnitude ordinarily seen after total hip reconstruction (<2 cm) would substantially alter hip joint forces. Using conventional gait analysis techniques to ascertain intersegmental resultant hip forces and moments, the authors used lifts to simulate leg length discrepancies of 2.3, 3.5, and 6.5 cm in 7 normal subjects. The 2.3-cm lift produced no changes. On the side of the lift (long limb), the 3.5- and 6.5-cm lifts modestly decreased mean peak intersegmental resultant hip forces by 6% and 12%, respectively, but not moments. The changes were, however, variable, with a few subjects showing increases and the rest showing decreases in selected forces or moments. On the side opposite to the lift (short limb), the 3.5- and 6.5-cm lifts increased mean peak intersegmental resultant hip forces by 2% to 12%, but not moments except in 1 case (8%). It is concluded that leg length discrepancies of the sort commonly seen after total hip reconstruction would likely cause no substantial changes in hip forces.

The sensitivity of joint afferents to knee translation

The cruciate ligaments contain mechanoreceptors which putatively contribute to knee function and dysfunction. However, the interpretation of studies showing neural responses to traction loads applied to the cat cranial cruciate ligament (CCL analogous to the anterior cruciate ligament in humans) depends upon demonstrating that non-CCL periarticular receptors are not stimulated. We assessed the capability to rigidly fix the knee against traction loads applied to the feline CCL. The tibia and femur were fixed either with clamps or Steinmann pins. Motion of the bones was monitored with liquid metal strain gages (LMSG) and the activity of the posterior articular nerve (PAN) was recorded while traction loads of up to 20-30 N were applied to the CCL. Joint afferents recorded from the PAN were insensitive to the CCL loads in the rigidly fixed preparation. Motion of the proximal tibia and distal femur was less than 100 micrometers for both methods of fixation, with neither method demonstrating more rigid fixation. In contrast, we observed vigorous discharges with focused light pressure on the capsule and under conditions allowing 200-500 micrometers of tibial displacement on the femur. This suggests that clinically undetectable instability may give rise to aberrant mechanoreceptor activity contributing to dysfunction.

Effects of anisotropy and material axis registration on computed stress and strain distributions in the turkey ulna

Finite element stress and strain distributions were studied parametrically for a curved long bone using several common material simplifications. A new technique is presented whereby local material axes conforming to local surface topology were automatically computed. Linearly elastic stress/strain solutions were evaluated as a function of the manner in which principal material directions are defined. The simplifications inherent in assumptions of local isotropy or globally registered transverse isotropy led to appreciably different solutions, particularly for some of the lesser-magnitude components of the strain tensor.

Resection arthroplasty of the hip

Forty-four patients with 46 resection arthroplasties performed for various indications were followed an average of 8.0 years. Pain relief was good or excellent in 77% and eradication of infection was achieved in all but one. Completeness of cement removal at the time of resection arthroplasty did not affect the results or control of infection. Patients with a history of prior septic total hip arthroplasty had results similar to those for patients with other indications for resection arthroplasty. A comprehensive review of the literature supports these results.

Cellular deformation reversibly depresses RT-PCR detectable levels of bone-related mRNA

Osteoblastic cells respond to mechanical stimuli with alterations in proliferation and/or phenotypic expression. In some cases, these responses occur within only a few applications of stimuli (i.e. 'cycle-dependent trigger response') rather than in a dose-dependent manner. To explore potential mechanisms of the cycle dependent trigger response, we raised the following questions: (1) Does strain of bone cells alter gene expression; if so, how quickly does it occur and how long does it last? (2) Are alterations in message level strain magnitude dependent? (3) Are alterations in steady-state message levels cycle dependent? Cultures were evaluated for osteocalcin mRNA one week following a daily stretch application at four stretch magnitudes and four cycle numbers and compared to nonstretched controls. Steady state mRNA message was ascertained prior to and at 10, 20, 30, 60, 120, 180, and 240 min following initiation of stretch. Following mRNA isolation, first strand cDNA synthesis was performed and fluorometrically quantitated. A reverse transcriptase based PCR (RT-PCR) approach allowed assessment of osteocalcin mRNA levels from microcultures (50,000 cells per 10 microliters culture or 5000 cells mm2) of rat calvarial osteoblasts. Optimized PCR was performed using primers to the bone specific protein, osteocalcin (OC) and two 'housekeeping' genes, beta-actin and GAP-DH. PCR products were separated on 4% agarose gels and band intensities digitized with relative quantitation based on internal standards in each gel. The lowest magnitude of stretch (- 1 KPa) at 1800 cycles per day reproducibly depressed message for osteocalcin, but not beta-actin when assayed immediately following the cessation of strain application. By three hours following the initiation of stretch, message levels returned to control values. At the time of stretch cessation, the 1800 cycle stretch regimen diminished (p < 0.0001) steady-state osteocalcin message independently of the four stretch magnitudes. Stretch for 300 cycles failed to depress (p = 0.05) osteocalcin message cultures at any time, but 600 cycles depressed message by 30 min. By one and two hours, cultures stretch 600, 900, and 1800 cycles showed similar levels of message depression. Four hours following the initiation of stretch, message levels returning to nonstrained levels in all groups. We conclude that alterations in cell response to strain are in part mediated by gene expression, that alterations last 3-4 h in this system, and that the message mechanism itself exhibits a trigger-response dependency to cycle number.

Characterisation of the neurosensory elements of the feline cranial cruciate ligament

The identification and distribution of mechanoreceptors in the cranial cruciate ligament of the cat (analogous to the anterior cruciate ligament in other species) was studied histologically using a modified celloidin embedding technique to achieve serial sectioning of bone-ligament-bone preparations with gold chloride staining. We identified distinctive large elongated structures situated between the collagen bundles of the ligament (resembling endings described as Freeman and Wyke type III; also termed Golgi tendon receptors). These endings were found near the middle of the ligament well away from the bone-ligament junction. Axons seen entering only one end of each type III ending helped to confirm its neural basis. While we saw structures resembling types I and II endings (i.e. Ruffini and pacinian endings, respectively) in individual sections, serial sections failed to reveal convincing evidence of their existence. Such structures almost always appeared to be vascular in nature on adjacent sections, with vessels entering and exiting. We conclude that serial sections are critical to interpreting the presence or absence of mechanoreceptors.

Proliferative and phenotypic responses of bone-like cells to mechanical deformation

Limited in vivo and in vitro experiments suggest that bone and bone-like cells respond to mechanical signals in a trigger-like rather than a dose-response fashion; i.e., they fail to respond until they have been stimulated with some given number of cycles of loading, and then once they respond, additional cycles produce little or no effect. To explore this notion, rat calvaria-derived osteoblast-like cells and the cell line MC3T3-E1 were plated at a high cell density (5,000 cells/mm2) on silicone membranes coated with type-I collagen and were allowed to attach for 24 hours. The membranes then were exposed to vacuum pressure (-1 kPa, 0.5 Hz) on a daily basis, and cultures were assayed every 2 days for 2 weeks. The proliferation of nontransformed cells increased 7-fold with as few as four daily cycles but not with one cycle per day. Furthermore, 1,800 cycles of vacuum did not result in a greater response than four cycles per day. We observed inverse phenotypic responses: the expression of osteocalcin was depressed compared with controls in the cultures of osteoblast-like cells that were strained with as few as four cycles per day. Alkaline phosphatase activity was depressed in the cultures of both the osteoblast-like cells and the MC3T3-E1 cells exposed to low vacuum pressures (-1 kPa) with four daily cycles of vacuum pressure. Increasing the vacuum magnitude did not affect the occurrence of a "trigger response" between one and four cycles of vacuum application.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional geometric and structural symmetry of the turkey ulna

Structural models of long-bone preparations usually assume left-right symmetry of contralateral bones under normal (baseline) conditions. To obtain insight on how this assumption affects the detection of subtle changes (as from functional adaptation), we formally examined the three-dimensional geometric and structural symmetry of paired long bones, using contemporary image reconstruction and stress analysis techniques. Nine pairs of ulnae from normal male turkeys were reconstructed computationally from serial transverse images obtained by either (a) mechanical sectioning and digital photographic imaging or (b) computed tomography. Computed tomography scans allowed greater precision in reconstruction than did digitally imaged photographs. Left-right comparisons of parameters of geometric symmetry (from computed tomography reconstructions) revealed average differences in whole bone volume and whole bone principal moments of inertia of 3.6 and 3.0%, respectively. Differences in bone curvature were indexed as noncolinearity of left compared with (mirrored) right centroidal axes, giving a disparity of 0.7 +/- 0.3 mm. Within the longitudinal central 20% of the diaphysis (the customary region for histomorphometry), average left-right differences in cross-sectional area and area principal moments of inertia for computed tomography images were 4.7 and 5.0%, respectively. The overlap of longitudinally paired cross sections of the mid-diaphysis, aligned at common centroids and oriented in the respective principal inertial directions, was greatest (as much as 95%) in the central 20% of the diaphysis. Paired three-dimensional finite element models demonstrated nearly identical left and right stress/strain fields throughout the ulnar diaphyses for both compressive and torsional loading. Our data suggest that the assumption of contralateral geometric symmetry in long bones should be judged in the context of the specific attribute of symmetry under consideration; however, we conclude that for purposes of finite element modeling the assumption of symmetry is reasonable.

The sensitivity of joint afferents to knee translation

The cruciate ligaments contain mechanoreceptors which putatively contribute to knee function and dysfunction. However, the interpretation of studies showing neural responses to traction loads applied to the cat cranial cruciate ligament (CCL-analogous to the anterior cruciate ligament in humans) depends upon demonstrating that non-CCL periarticular receptors are not stimulated. We assessed the capability to rigidly fix the knee against traction loads applied to the feline CCL. The tibia and femur were fixed either with clamps or Steinmann pins. Motion of the bones was monitored with liquid metal strain gauges (LMSG) and the activity of the posterior articular nerve (PAN) was recorded while traction loads of up to 20-30 N were applied to the CCL. Joint afferents recorded from the PAN were insensitive to the CCL loads in the rigidly fixed preparation. Motion of the proximal tibia and distal femur was less than 100 micrometers for both methods of fixation, with neither method demonstrating more rigid fixation. In contrast, we observed vigorous discharges with focused light pressure on the capsule and under conditions allowing 200-500 micrometers of tibial displacement on the femur. This suggests that clinically undetectable instability may give rise to aberrant mechanoreceptor activity contributing to dysfunction.

Prevalence of previously undetected osteonecrosis of the femoral head in renal transplant recipients

PURPOSE

To determine the prevalence of osteonecrosis (ON) of the femoral head in renal transplant recipients and determine the natural history of previously undetected lesions.

MATERIALS AND METHODS

The hips of 132 renal transplant recipients were examined with magnetic resonance (MR) imaging, with radiographs obtained only in the patients with positive MR images.

RESULTS

Ten patients (15 hips) were considered to have ON (prevalence, 7.6%). Eleven of the hips had Ficat stage 0 disease (asymptomatic, preradiographic) and were followed up with serial radiography and MR imaging. Over an average follow-up of 22 months, only one of these lesions showed progression. The other 10 hips did not show progression at MR imaging or radiography.

CONCLUSION

Previously undetected ON may have a benign course in many cases. Although a substantial number of patients have previously unsuspected disease, further study is necessary to assess the need for "prophylactic" surgery in such cases.

The effects of signal conditioning on the statistical analyses of gait EMG

Ensemble averaged EMG profiles generated for leg muscles during gait have been used to clinically assess disease or injury. Several of the methods that have been reported for conditioning gait EMG signals were compared using data collected from clinically normal subjects walking on a treadmill. Specifically investigated were the effects of filtering and the quantity of data averaged upon several statistical tests that measure the variability of, or differences between, EMG profiles. Our results suggest that the variance ratio (VR) provides a reasonable test of data variability because of its modest sensitivity to both the degree of filtering and the amount of data averaged. They also suggest that of the comparison statistics: Pearson's r, the Kolmogorov-Smirnov T test and the ANOVA F ratio, the T test was the most reliable in detecting differences between given profiles for all test conditions. However, recognition of this ability of the T test must be tempered by the knowledge that while obvious EMG signal differences did exist, observable functional differences in gait did not. The relationship between statistically similar/dissimilar EMG patterns and clinically functional/dysfunctional gait patterns needs to be established. In addition, since all of the test statistics studied were affected to some degree by filtering and averaging, care should be used when comparing statistical results from separate studies unless it is known that the studies were conducted under similar conditions, including data processing. To that end, we recommend that at least 20 strides be used in the averaging process since the statistics we tested have reached or are asymptotically approaching their final values by this point.