Philip Nasser

Relationship Between Bone Morphology and Bone Quality in Male Tibias: Implications for Stress Fracture Risk†

Biomechanical properties were assessed from the tibias of 17 adult males 17‐46 years of age. Tissue‐level mechanical properties varied with bone size. Narrower tibias were comprised of tissue that was more brittle and more prone to accumulating damage compared with tissue from wider tibias.

The Role of Interstitial Fluid Flow in the Remodeling Response to Fatigue Loading

Load‐induced fluid flow enhances molecular transport through bone tissue and relates to areas of bone resorption and apposition. Remodeling activity is highly coordinated and necessitates a means for cellular communication via intracellular and extracellular means. Osteocytes, osteoblasts, and osteoclasts, which reside in disparate locations within the tissue, communicate intracellularly via the cellular syncytium and extracellularly via the pericellular fluid space of the lacunocanalicular system. Both of these communications systems are physically disrupted by microdamage incurred during fatigue loading of bone. The purpose of this study was to develop an analytical model to understand the role of interstitial fluid flow in the remodeling response to fatigue loading. Adequate transport was assumed a prerequisite for maintenance of cell viability in bone. Diffusive and convective transport were simulated through the lacunocanalicular network in a healthy undamaged state as well as in a damaged state after fatigue loading. The model predicts that fatigue damage impedes transport from the blood supply, depleting the concentration of molecular entities in and downstream from areas of damage. Furthermore, the presence of microcracks alters the distribution of molecular entities between individual lacunae. These effects were confirmed by the results of an in vivo pilot study in which fluorescent, flow‐visualizing agents pooled within microcracks and were absent from areas surrounding microcracks, corresponding to areas deprived of fluid flow. Loss of osteocyte viability is coupled to targeting and initiation of new remodeling activity. Taken as a whole, these data suggest a link between interstitial fluid flow, mass transport, maintenance of osteocyte viability, and modulation of remodeling activity.

Serum IGF-1 Determines Skeletal Strength by Regulating Subperiosteal Expansion and Trait Interactions†‡

Strong correlations between serum IGF‐1 levels and fracture risk indicate that IGF‐1 plays a critical role in regulating bone strength. However, the mechanism by which serum IGF‐1 regulates bone structure and fracture resistance remains obscure and cannot be determined using conventional approaches. Previous analysis of adult liver‐specific IGF‐1–deficient (LID) mice, which exhibit 75% reductions in serum IGF‐1 levels, showed reductions in periosteal circumference, femoral cross‐sectional area, cortical thickness, and total volumetric BMD. Understanding the developmental sequences and the resultant anatomical changes that led to this adult phenotype is the key for understanding the complex relationship between serum IGF‐1 levels and fracture risk. Here, we identified a unique developmental pattern of morphological and compositional traits that contribute to bone strength. We show that reduced bone strength associated with low levels of IGF‐1 in serum (LID mice) result in impaired subperiosteal expansion combined with impaired endosteal apposition and lack of compensatory changes in mineralization throughout growth and aging. We show that serum IGF‐1 affects cellular activity differently depending on the cortical surface. Last, we show that chronic reductions in serum IGF‐1 indirectly affect bone strength through its effect on the marrow myeloid progenitor cell population. We conclude that serum IGF‐1 not only regulates bone size, shape, and composition during ontogeny, but it plays a more fundamental role—that of regulating an individuals ability to adapt its bone structure to mechanical loads during growth and development.

Long-Term Disuse Osteoporosis Seems Less Sensitive to Bisphosphonate Treatment Than Other Osteoporosis†

We sought to determine whether risedronate can preserve cortical bone mass and mechanical properties during long‐term disuse in dogs, assessed by histomorphometry and biomechanics on metacarpal diaphyses. Risedronate slowed cortical thinning and partially preserved mechanical properties, but it was unable to suppress bone loss to the degree seen in other osteoporoses.

Noninvasive fatigue fracture model of the rat ulna.

Fatigue damage occurs in response to repeated cyclic loading and has been observed in situ in cortical bone of humans and other animals. When microcracks accumulate and coalesce, failure ensues and is referred to as fatigue fracture. Experimental study of fatigue fracture healing is inherently difficult due to the lack of noninvasive models. In this study, we hypothesized that repeated cyclic loading of the rat ulna results in a fatigue fracture. The aim of the study was to develop a noninvasive long bone fatigue fracture model that induces failure through accumulation and coalescence of microdamage and replicates the morphology of a clinical fracture. Using modified end‐load bending, right ulnae of adult Sprague‐Dawley rats were cyclically loaded in vivo to fatigue failure based on increased bone compliance, which reflects changes in bone stiffness due to microdamage. Preterminal tracer studies with 0.8% Procion Red solution were conducted according to protocols described previously to evaluate perfusion of the vasculature as well as the lacunocanalicular system at different time points during healing. Eighteen of the 20 animals loaded sustained a fatigue fracture of the medial ulna, i.e. through the compressive cortex. In all cases, the fracture was closed and non‐displaced. No disruption to the periosteum or intramedullary vasculature was observed. The loading regime did not produce soft tissue trauma; in addition, no haematoma was observed in association with application of load. Healing proceeded via proliferative woven bone formation, followed by consolidation within 42 days postfracture. In sum, a noninvasive long bone fatigue fracture model was developed that lends itself for the study of internal remodeling of periosteal woven bone during fracture healing and has obvious applications for the study of fatigue fracture etiology.

Biological co-adaptation of morphological and composition traits contributes to mechanical functionality and skeletal fragility.

A path analysis was conducted to determine whether functional interactions exist among morphological, compositional, and microstructural traits for young adult human tibias. Data provided evidence that bone traits are co‐adapted during ontogeny so that the sets of traits together satisfy physiological loading demands. However, certain sets of traits are expected to perform poorly under extreme load conditions.

Physiological loading of joints prevents cartilage degradation through CITED2

Both overuse and disuse of joints up‐regulate matrix metalloproteinases (MMPs) in articular cartilage and cause tissue degradation;however, moderate (physiological) loading maintains cartilage integrity. Here, we test whether CBP/p300‐interacting transacti‐vator with ED‐rich tail 2 (CITED2), a mechanosensitive transcriptional coregulator, mediates this chondropro‐tective effect of moderate mechanical loading. In vivo, hind‐limb immobilization of Sprague‐Dawley rats up‐regulates MMP‐1 and causes rapid, histologically detectable articular cartilage degradation. One hour of daily passive joint motion prevents these changes and up‐regulates articular cartilage CITED2. In vitro, moderate (2.5 MPa, 1 Hz) intermittent hydrostatic pressure (IHP) treatment suppresses basal MMP‐1 expression and up‐regulates CITED2 in human chondro‐cytes, whereas high IHP (10 MPa) down‐regulates CITED2 and increases MMP‐1. Competitive binding and transcription assays demonstrate that CITED2 suppresses MMP‐1 expression by competing with MMP transactivator, Ets‐1 for its coactivator p300. Furthermore, CITED2 up‐regulation in vitro requires the p38δ isoform, which is specifically phosphorylated by moderate IHP. Together, these studies identify a novel regulatory pathway involving CITED2 and p38δ, which may be critical for the maintenance of articular cartilage integrity under normal physical activity levels.—Leong, D. J., Li, Y. H., Gu, X. I., Sun, L., Zhou, Z., Nasser, P., Laudier, D. M., Iqbal, J., Majeska, R. J., Schaffler, M. B., Goldring, M. B., Cardoso, L., Zaidi, M., Sun, H. B. Physiological loading of joints prevents cartilage degradation through CITED2. FASEB J. 25, 182–191 (2011). www.fasebj.org

Gap junction impedance, tissue dielectrics and thermal noise limits for electromagnetic field bioeffects

Abstract The model presented in this study quantitatively examines the effect of gap junctions and gap junction impedance on electromagnetic field (EMF) dosimetry in a tissue target. A simple linear distributed-parameter electrical model evaluates the effect of tissue structure on the thermal threshold (signal-to-thermal-noise ratio) for detectable induced transmembrane voltage. Analysis of the angular frequency response of the array model, using a membrane impedance which includes ion binding and coupled surface chemical reaction kinetics, suggests that the frequency range, over which maximum detectable induced transmembrane voltage could be achieved, is orders of magnitude lower than that for a single cell. Gap junction impedance has negligible effect on both the frequency response and the increased transmembrane voltage due to a cell array unless its value becomes as high as that of an artificial bilayer lipid membrane. This results in a threshold for induced electric field bioeffects of approximately 10 μV cm−1 at the target site for a 1–10 mm cell array. Physiological variations in gap junction impedance appear to have little effect on this threshold. Thus, cells in gap junction contact in developing, repairing or resting state tissue structures would be expected to be able to detect significantly weaker EMF signals than isolated single cells. The lowered frequency response of a cell array reinforces the suggestion that the spectral density of the input signal should be adjusted to the bandpass of the detector pathway for dose-efficient and selective EMF bioeffects.

The Effects of Core Suture Purchase on the Biomechanical Characteristics of a Multistrand Locking Flexor Tendon Repair: A Cadaveric Study

PURPOSE To determine the effects of suture purchase on work of flexion (WOF), 2-mm gap force, and load to failure on the combination cross-locked cruciate-interlocking horizontal mattress (CLC-IHM) flexor tendon repair in zone II. METHODS A total of 33 fresh-frozen cadaveric fingers were mounted in a custom jig, and the flexor digitorum profundus of each finger was fixed to the mobile arm of a tensile strength machine. Initial measurements of WOF were obtained. Each tendon was repaired with the CLC core suture, randomly assigned to placement of 3, 5, 7 or 10 mm from the cut edge of the tendon, and completed with the IHM circumferential suture. After the repair was completed, measurements of WOF were repeated. Each finger was cycled 1000 times. After each 250 cycles, gapping was recorded, and WOF was measured again. Change in WOF (WOF after repair - WOF of intact tendon) was calculated. Tendons were then dissected from the fingers and linearly tested for 2-mm gap force and ultimate load to failure. RESULTS The group repaired at 10 mm had the lowest percent increase in WOF (5.2%), the highest 2-mm gap force (89.8 N), and the highest ultimate load to failure (111.5 N). The group repaired at 3 mm had the highest percent increase in WOF (22.1%), the lowest 2-mm gap force (54.6 N), and the lowest ultimate load to failure (84.6 N). CONCLUSIONS A 10-mm suture purchase is the recommended distance for optimal performance for the CLC-IHM combination repair method. This method with a 10-mm suture purchase has a low increase in WOF, high strength, and high resistance to gapping, and it should be strong enough to tolerate early motion.

Fourier Transform Infrared Imaging Microspectroscopy and Tissue-Level Mechanical Testing Reveal Intraspecies Variation in Mouse Bone Mineral and Matrix Composition

Fracture susceptibility is heritable and dependent upon bone morphology and quality. However, studies of bone quality are typically overshadowed by emphasis on bone geometry and bone mineral density. Given that differences in mineral and matrix composition exist in a variety of species, we hypothesized that genetic variation in bone quality and tissue-level mechanical properties would also exist within species. Sixteen-week-old female A/J, C57BL/6J (B6), and C3H/HeJ (C3H) inbred mouse femora were analyzed using Fourier transform infrared imaging and tissue-level mechanical testing for variation in mineral composition, mineral maturity, collagen cross-link ratio, and tissue-level mechanical properties. A/J femora had an increased mineral-to-matrix ratio compared to B6. The C3H mineral-to-matrix ratio was intermediate of A/J and B6. C3H femora had reduced acid phosphate and carbonate levels and an increased collagen cross-link ratio compared to A/J and B6. Modulus values paralleled mineral-to-matrix values, with A/J femora being the most stiff, B6 being the least stiff, and C3H having intermediate stiffness. In addition, work-to-failure varied among the strains, with the highly mineralized and brittle A/J femora performing the least amount of work-to-failure. Inbred mice are therefore able to differentially modulate the composition of their bone mineral and the maturity of their bone matrix in conjunction with tissue-level mechanical properties. These results suggest that specific combinations of bone quality and morphological traits are genetically regulated such that mechanically functional bones can be constructed in different ways.