Correction to: Compressive loading of the murine tibia reveals site-specific micro-scale differences in adaptation and maturation rates of bone.

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Compressive loading of the murine tibia reveals site-specific micro-scale differences in adaptation and maturation rates of bone.

Loading increases bone mass and strength in a site-specific manner; however, possible effects of loading on bone matrix composition have not been evaluated. Site-specific structural and material properties of mouse bone were analyzed on the macro- and micro/molecular scale in the presence and absence of axial loading. The response of bone to load is heterogeneous, adapting at molecular, micro-, and macro-levels. INTRODUCTION: Osteoporosis is a degenerative disease resulting in reduced bone mineral density, structure, and strength. The overall aim was to explore the hypothesis that changes in loading environment result in site-specific adaptations at molecular/micro- and macro-scale in mouse bone. METHODS: Right tibiae of adult mice were subjected to well-defined cyclic axial loading for 2 weeks; left tibiae were used as physiologically loaded controls. The bones were analyzed with µCT (structure), reference point indentation (material properties), Raman spectroscopy (chemical), and small-angle X-ray scattering (mineral crystallization and structure). RESULTS: The cranial and caudal sites of tibiae are structurally and biochemically different within control bones. In response to loading, cranial and caudal sites increase in cortical thickness with reduced mineralization (-14 and -3%, p < 0.01, respectively) and crystallinity (-1.4 and -0.3%, p < 0.05, respectively). Along the length of the loaded bones, collagen content becomes more heterogeneous on the caudal site and the mineral/collagen increases distally at both sites. CONCLUSION: Bone structure and composition are heterogeneous, finely tuned, adaptive, and site-specifically responsive at the micro-scale to maintain optimal function. Manipulation of this heterogeneity may affect bone strength, relative to specific applied loads.

Moderate physical exercise results in increased cell activity in articular cartilage of the knee joint in rats.

BACKGROUND: Moderate exercise regimens have shown minor positive effects on matrix turnover in articular cartilage (AC), while effects at cellular level, e.g. proliferation, are scarcely described. AIM: The aim of this study was to investigate the effects of moderate exercise on cell proliferation and recruitment of cells possibly active in regeneration in different regions of cartilage in the rat knee joint. METHODS: Eighteen rats were orally given 5-bromo-2-deoxyuridine (BrdU) for 14 days for in vivo DNA labeling. Nine rats underwent treadmill training for 50 min/day, 5 days/week (exercise group), and 9 rats served as controls (no exercise). Animals were sacrificed after 14, 56 and 105 days, and knee joints were harvested. BrdU+ cells were visualized immunohistochemically (IHC) and counted in AC, posterior stem cell niche (PN), potential migration route (PMR; area between PN and the AC border), potential migration area (PMA; region between PN and AC including PN) and epiphyseal cartilage plate (EP) of the tibia and femur. RESULTS: Compared to controls, in the exercise group BrdU+ cells/mm(2) were increased on days 14 (p = 0.022) and 105 (p = 0.045) in AC of the tibia and on day 105 (p = 0.014) in AC of the femur. BrdU+ cell numbers were increased in the PMR region of the tibia on days 14 (p = 0.023) and 105 (p = 0.0018) and in the PMR region of the femur on day 105 (p = 0.0099) as well as in the PMA region of the tibia (p = 0.0008) and femur (p = 0.0080) on day 105. No significant differences in BrdU+ cells/mm(2) were seen in PN or EP between the groups at any time point. Regarding collagen 2A1 expression and proteoglycan accumulation, no significant differences between groups were detected. CONCLUSIONS: The results indicate increased cell activity in AC in response to physical exercise and may help to understand the complexity of AC regeneration in the normal mammal knee joint.

Biocompatibility and resorption of a radiopaque premixed calcium phosphate cement.

Calcium phosphate cements (CPC) are used as bone void filler in various orthopedic indications; however, there are some major drawbacks regarding mixing, transfer, and injection of traditional CPC. By using glycerol as mixing liquid, a premixed calcium phosphate cement (pCPC), some of these difficulties can be overcome. In the treatment of vertebral fractures the handling characteristics need to be excellent including a high radio-opacity for optimal control during injection. The aim of this study is to evaluate a radiopaque pCPC regarding its resorption behavior and biocompatibility in vivo. pCPC and a water-based CPC were injected into a Ø 4-mm drilled femur defect in rabbits. The rabbits were sacrificed after 2 and 12 weeks. Cross sections of the defects were evaluated using histology, electron microscopy, and immunohistochemical analysis. Signs of inflammation were evaluated both locally and systemically. The results showed a higher bone formation in the pCPC compared to the water-based CPC after 2 weeks by expression of RUNX-2. After 12 weeks most of the cement had been resorbed in both groups. Both materials were considered to have a high biocompatibility since no marked immunological response was induced and extensive bone ingrowth was observed. The conclusion from the study was that pCPC with ZrO(2) radiopacifier is a promising alternative regarding bone replacement material and may be suggested for treatment of, for example, vertebral fractures based on its high biocompatibility, fast bone ingrowth, and good handling properties.

Investigation of different cell types and gel carriers for cell-based intervertebral disc therapy, in vitro and in vivo studies.

Biological treatment options for the repair of intervertebral disc damage have been suggested for patients with chronic low back pain. The aim of this study was to investigate possible cell types and gel carriers for use in the regenerative treatment of degenerative intervertebral discs (IVD). In vitro: human mesenchymal cells (hMSCs), IVD cells (hDCs), and chondrocytes (hCs) were cultivated in three gel types: hyaluronan gel (Durolane®), hydrogel (Puramatrix®), and tissue-glue gel (TISSEEL®) in chondrogenic differentiation media for 9 days. Cell proliferation and proteoglycan accumulation were evaluated with microscopy and histology. In vivo: hMSCs or hCs and hyaluronan gel were co-injected into injured IVDs of six minipigs. Animals were sacrificed at 3 or 6 months. Transplanted cells were traced with anti-human antibodies. IVD appearance was visualized by MRI, immunohistochemistry, and histology. Hyaluronan gel induced the highest cell proliferation in vitro for all cell types. Xenotransplanted hMSCs and hCs survived in porcine IVDs for 6 months and produced collagen II in all six animals. Six months after transplantation of cell/gel, pronounced endplate changes indicating severe IVD degeneration were observed at MRI in 1/3 hC/gel, 1/3 hMSCs/gel and 1/3 gel only injected IVDs at MRI and 1/3 hMSC/gel, 3/3 hC/gel, 2/3 gel and 1/3 injured IVDs showed positive staining for bone mineralization. In 1 of 3 discs receiving hC/gel, in 1 of 3 receiving hMSCs/gel, and in 1 of 3 discs receiving gel alone. Injected IVDs on MRI results in 1 of 3 hMSC/gel, in 3 of 3 hC/gel, in 2 of 3 gel, and in 1 of 3 injured IVDs animals showed positive staining for bone mineralization. The investigated hyaluronan gel carrier is not suitable for use in cell therapy of injured/degenerated IVDs. The high cell proliferation observed in vitro in the hyaluronan could have been a negative factor in vivo, since most cell/gel transplanted IVDs showed degenerative changes at MRI and positive bone mineralization staining. However, this xenotransplantation model is valuable for evaluating possible cell therapy strategies for human degenerated IVDs.

Premixed acidic calcium phosphate cement: characterization of strength and microstructure.

By using a premixed calcium phosphate cement (CPC), the handling properties of the cement are drastically improved, which is a challenge for traditional injectable CPCs. Previously premixed cements have been based on apatitic cements. In this article, acidic cement has been developed and evaluated. Monocalcium phosphate monohydrate and beta-tricalcium phosphate were mixed with glycerol to form a paste. As the paste does not contain water, no setting reaction starts and thus the working time is indefinite. Powder/liquid ratios (P/L) of 2.25, 3.5 and 4.75 were evaluated. Setting time (ST) and compressive strength (CS) were measured after 1 day, 1 week and 4 weeks in phosphate buffered saline (PBS) solution, and the corresponding microstructure was evaluated using electron microscopy and X-ray diffraction. The ST started when the cements were placed in PBS and ranged from 28 to 75 min, higher P/L gave a lower ST. Higher P/L also gave a higher CS, which ranged from 2 to 16 MPa. The microstructure mainly consisted of monetite, 1-5 microm in grain size. After 4 weeks in PBS, the strength increased. As acidic cements are resorbed faster in vivo, this cement should allow faster bone regeneration than apatitic cements. Premixed cements show a great handling benefit when compared with normal CPCs and can be formulated with similar ST and mechanical properties.