Glucocorticoid-treated sheep as a model for osteopenic trabecular bone in biomaterials research

Why Animal Experiments Are Still Indispensable in Bone Research: A Statement by the European Calcified Tissue Society

Major achievements in bone research have always relied on animal models and in vitro systems derived from patient and animal material. However, the use of animals in research has drawn intense ethical debate and the complete abolition of animal experimentation is demanded by fractions of the population. This phenomenon is enhanced by the reproducibility crisis in science and the advance of in vitro and in silico techniques. 3D culture, organ-on-a-chip, and computer models have improved enormously over the last few years. Nevertheless, the overall complexity of bone tissue cross-talk and the systemic and local regulation of bone physiology can often only be addressed in entire vertebrates. Powerful genetic methods such as conditional mutagenesis, lineage tracing, and modeling of the diseases enhanced the understanding of the entire skeletal system. In this review endorsed by the European Calcified Tissue Society (ECTS), a working group of investigators from Europe and the US provides an overview of the strengths and limitations of experimental animal models, including rodents, fish, and large animals, as well the potential and shortcomings of in vitro and in silico technologies in skeletal research. We propose that the proper combination of the right animal model for a specific hypothesis and state-of-the-art in vitro and/or in silico technology is essential to solving remaining important questions in bone research. This is crucial for executing most efficiently the 3R principles to reduce, refine, and replace animal experimentation, for enhancing our knowledge of skeletal biology, and for the treatment of bone diseases that affect a large part of society. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Osteoporosis Preclinical Research: A Systematic Review on Comparative Studies Using Ovariectomized Sheep

Sheep ovariectomy (OVX) alone or associated to steroid therapy, deficient diet, or hypothalamic–pituitary disconnection has proven to be of critical importance for osteoporosis research in orthopedics. However, the impact of specific variables, such as breed, age, diet, time after OVX, and other variables, should be monitored. Thus, the design of comparative studies is mandatory to minimize the impact of these variables or to recognize the presence of unwanted variables as well as to better characterize bone remodeling in this model. Herein, we conducted a systematic review of the last 10 years on PubMed, Scopus, and Web of Knowledge considering only studies on OVX sheep where a control group was present. Of the 123 records screened, 18 studies were included and analyzed. Results showed that (i) Merino sheep are the most exploited breed; (ii) 5–6 years of age is the most used time for inducing OVX; (iii) ventral midline laparotomy is the most common approach to induce OVX; (iv) OVX associated to steroid therapy is the most widely used osteoporosis model; and (v) success of OVX was mostly verified 12 months after surgery. In detail, starting from 12 months after OVX a significant decline in bone mineral density and in microarchitectural bone parameters as well as in biochemical markers were detected in all studies in comparison to control groups. Bone alteration was also site-specific on a pattern as follows: lumbar vertebra, femoral neck, and ribs. Before 12 months from OVX and starting from 3–5 months, microarchitectural bone changes and biochemical marker alterations were present when osteoporosis was induced by OVX associated to steroid therapy. In conclusion, OVX in sheep influence bone metabolism causing pronounced systemic bone loss and structural deterioration comparable to the situation found in osteoporosis patients. Data for treating osteoporosis patients are based not only on good planning and study design but also on a correct animal use that, as suggested by 3Rs principles and by ARRIVE guidelines, includes the use of control groups to be directly contrasted with the experimental group.

miR-29cb2 promotes angiogenesis and osteogenesis by inhibiting HIF-3α in bone

Coordination between osteogenesis and angiogenesis is required for bone homeostasis. Here, we show that miR-29cb2 is a bone-specific miRNA and plays critical roles on angiogenesis-osteogenesis coupling during bone remodeling. Mice with deletion of miR-29cb2 exhibit osteopenic phenotypes and osteoblast impairment, accompanied by pronounced decreases in specific H vessels. The decrease in bone miR-29cb2 was associated with pathological ovariectomy stimuli. Mechanistically, hypoxia-inducible factor (HIF)-3α, as a target for miR-29cb2, inhibits HIF-1α activity by competitively bonding with HIF-1β. Notably, miR-29cb2 in peripheral blood (PB) nearly is undetectable in sham and significantly increases in ovariectomy mice. Further evaluation from osteoporosis patients demonstrates similar signatures. ROC analysis shows miR-29cb2 in PB has higher sensitivity and specificity for diagnosing osteoporosis when compared with four clinical biomarkers. Collectively, these findings reveal that miR-29cb2 is essential for bone remodeling by inhibiting HIF-3α and elevated bone-specific miR-29cb2 in PB, which may be a promising biomarker for bone loss.

Bioinspired material architectures from bighorn sheep horncore velar bone for impact loading applications

Rocky Mountain bighorn sheep rams (Ovis canadensis canadensis) routinely conduct intraspecific combat where high energy cranial impacts are experienced. Previous studies have estimated cranial impact forces to be up to 3400 N during ramming, and prior finite element modeling studies showed the bony horncore stores 3 × more strain energy than the horn during impact. In the current study, the architecture of the porous bone within the horncore was quantified, mimicked, analyzed by finite element modeling, fabricated via additive manufacturing, and mechanically tested to determine the suitability of the novel bioinspired material architecture for use in running shoe midsoles. The iterative biomimicking design approach was able to tailor the mechanical behavior of the porous bone mimics. The approach produced 3D printed mimics that performed similarly to ethylene–vinyl acetate shoe materials in quasi-static loading. Furthermore, a quadratic relationship was discovered between impact force and stiffness in the porous bone mimics, which indicates a range of stiffness values that prevents impact force from becoming excessively high. These findings have implications for the design of novel bioinspired material architectures for minimizing impact force.

Analysis of microscopic bone properties in an osteoporotic sheep model: a combined biomechanics, FE and ToF-SIMS study

The present study deals with the characterization of bone quality in a sheep model of postmenopausal osteoporosis. Sheep were sham operated ( n = 7), ovariectomized ( n = 6), ovariectomized and treated with deficient diet ( n = 8) or ovariectomized, treated with deficient diet and glucocorticoid injections ( n = 7). The focus of the study is on the microscopic properties at tissue level. Microscopic mechanical properties of osteoporotic bone were evaluated by a combination of biomechanical testing and mathematical modelling. Sample stiffness and strength were determined by compression tests and finite-element analysis of stress states was conducted. From this, an averaged microscopic Young's modulus at tissue level was determined. Trabecular structure as well as mineral and collagen distribution in samples of sheep vertebrae were analysed by micro-computed tomography and time-of-flight secondary ion mass spectrometry. In the osteoporotic sheep model, a disturbed fibril structure in the triple treated group was observed, but bone loss only occurred in form of reduced trabecular number and thickness and cortical decline, while quality of the residual bone was preserved. The preserved bone tissue properties in the osteoporotic sheep model allowed for an estimation of bone strength which behaves similar to the human case.

Glucocorticoids affect bone mineral density and bone remodelling in OVX sheep: A pilot study

The aim of this study was to validate the combination of ovariectomy and glucocorticoid treatment in sheep as a large animal model for osteoporosis by measuring the concentration of specific biomarkers in the blood of the sheep and measuring bone loss over five months. Aged Merino ewes were randomly allocated into four groups: control, ovariectomy (OVX), and two OVX groups receiving glucocorticoids-one group once-monthly for five months (OVXG), and the other for two months followed by no treatment for three months (OVXG2). Parameters measured were biochemical markers of bone turnover, areal bone mineral density, volumetric bone mineral density, and total and trabecular bone parameters. Ovariectomy increased the concentrations of bone resorption marker C-terminal telopeptides of type 1 collagen (CTx-1) and bone turnover marker serum osteocalcin (OC) concentrations in the OVX group compared to control sheep. The combination of ovariectomy and glucocorticoid treatment increased the concentrations of CTx-1 and decreased serum OC concentrations in the OVXG group compared to OVXG2. Femur and lumbar spine bone density were lower in experimentally treated groups when compared with the control group. Total and trabecular _vBMD in the proximal tibia were significantly lower in the treatment groups when compared with the control group. A significant negative correlation between femoral bone density and CTx-1 was found. The results of this study suggest that the combination of OVX and glucocorticoids induces bone loss in a short period of time in sheep.

Local Treatment of Osteoporotic Sheep Vertebral Body With Calcium Sulfate for Decreasing the Potential Fracture Risk: Microstructural and Biomechanical Evaluations

STUDY DESIGN

In this study, calcium sulfate (CS) was injected through pedicle into the osteoporotic vertebral body in vivo in sheep, and micro-computed tomography analysis, histologic observation, and biomechanical test were performed.

OBJECTIVE

To investigate the improvement on microstructure and biomechanical performance of lumbar vertebrae augmented with CS in osteoporotic sheep.

SUMMARY OF BACKGROUND DATA

The present treatments for osteoporosis relies on systemic medications intended to increase the bone mineral density (BMD). Although effective, these time-consuming medications provide little protection from fracture in the "early period" after initiation of therapy. In this regard, the strategy of local treatment is to target specific areas of the skeletal system that are prone to osteoporotic fractures. However, there is little or no research focused on local treatment of osteoporotic vertebrae with CS.

METHODS

Eight female sheep were induced to osteoporosis with bilateral ovariectomy and methylprednisolone administration for 12 months. After successful establishment of an osteoporotic model, lumbar vertebrae (L1-L4) in every sheep were randomly divided into 2 groups: CS group and control group (2 vertebrae in each group in every sheep). CS was injected into the vertebral body transpedicularly in the CS group and no treatments were performed in the control group. Three months later, all sheep were killed and all L1-L4 vertebrae were harvested. Thereafter, microstructure and biomechanical performance of the cancellous bone of the vertebral body were assessed through micro-computed tomography analysis, histologic observation, and biomechanical test, respectively.

RESULTS

After a 12-month induction with ovariectomy and methylprednisolone administration, the mean BMD of the sheep lumbar vertebrae significantly decreased (>25%) compared with the value before induction, which demonstrated successful establishment of osteoporosis. Three months after injection of CS, CS was completely degraded without any remains in bone tissue and the quality of bone tissue (amount and density of the bone tissue) in the CS group was significantly higher than that in the control group. The ultimate load, stiffness, and energy absorption in the CS group were all significantly higher than those in the control group.

CONCLUSIONS

The preliminary data suggest that local injection of CS can significantly improve the amount, density, and biomechanical performance of the bone trabeculae in osteoporotic vertebra. The local injection of CS could also be used as a new method to improve the physical microstructure and augment the mechanical properties in "high-risk" vertebral bodies, decreasing the potential fracture risk of patients with osteoporosis. The strict inclusion and exclusion criteria should be performed before treatment.

Can we induce osteoporosis in animals comparable to the human situation?

Osteoporosis is a chronic systemic bone disease of growing relevance due to the on-going demographic change. Since the underlying regulatory mechanisms of this critical illness are still not fully understood and treatment options are not satisfactorily resolved, there is still a great need for osteoporosis research in general and animal models in particular. Ovariectomized rodents are standard animal models for postmenopausal osteoporosis and highly attractive due to the possibility to specifically modify their genetic background. However, some aspects can only be addressed in large animal models; such as metaphyseal fracture healing and advancement of orthopedic implants. Among other large animal models sheep in particular have been proven invaluable for osteoporosis research in this context. In conclusion, today we are able to influence the bone metabolism in animals causing a more or less pronounced systemic bone loss and structural deterioration comparable to the situation found in patients suffering from osteoporosis. However, there is no perfect model for osteoporosis, but a variety of models appropriate for answering specific questions. Though, the appropriateness of an animal model is not only defined in regard to the similarity to human physiology and the disease itself, but also in regard to acquisition, housing requirements, handling, costs, and particularly ethical concerns and animal welfare.

The standardized creation of a lumbar spine vertebral compression fracture in a sheep osteoporosis model induced by ovariectomy, corticosteroid therapy and calcium/phosphorus/vitamin D-deficient diet

INTRODUCTION

Vertebral compression fractures (VCFs) are one of the most common injuries in the aging population presenting with an annual incidence of 1.4 million new cases in Europe. Current treatment strategies focus on cement-associated solutions (kyphoplasty/vertebroplasty techniques). Specific cement-associated problems as leakage, embolism and the adjacent fracture disease are reported adding to open questions like general fracture healing properties of the osteoporotic spine. In order to analyze those queries animal models are of great interest; however, both technical difficulties in the induction of experimental osteoporosis in animal as well as the lack of a standardized fracture model impede current and future in vivo studies. This study introduces a standardized animal model of an osteoporotic VCF type A3.1 that may enable further in-depth analysis of the afore mentioned topics.

MATERIAL AND METHODS

Twenty-four 5-year-old female Merino sheep (mean body weight: 67 kg; range 57-79) were ovariectomized (OP1) and underwent 5.5 months of weekly corticosteroid injections (dexamethasone and dexamethasone-sodium-phosphate), adding to a calcium/phosphorus/vitamin D-deficient diet. Osteoporosis induction was documented by pQCT and micro-CT BMD (bone mineral density) as well as 3D histomorphometric analysis postoperatively of the sheep distal radius and spine. Non osteoporotic sheep served as controls. Induction of a VCF of the second lumbar vertebra was performed via a mini-lumbotomy surgical approach with a standardized manual compression mode (OP2).

RESULTS

PQCT analysis revealed osteoporosis of the distal radius with significantly reduced BMD values (0.19 g/cm(3), range 0.13-0.22 vs. 0.27 g/cm(3), range 0.23-0.32). Micro-CT documented significant lowering of BMD values for the second lumbar vertebrae (0.11 g/cm(3), range 0.10-0.12) in comparison to the control group (0.14 g/cm(3), range 0.12-0.17). An incomplete burst fracture type A3.1 was achieved in all cases and resulted in a significant decrease in body angle and vertebral height (KA 4.9°, range: 2-12; SI 4.5%, range: 2-12). With OP1, one minor complication (lesion of small bowel) occurred, while no complications occurred with OP2.

CONCLUSIONS

A suitable spinal fracture model for creation of VCFs in osteoporotic sheep was developed. The technique may promote the development of improved surgical solutions for VCF treatment in the experimental and clinical setting.

A reversal phase arrest uncoupling the bone formation and resorption contributes to the bone loss in glucocorticoid treated ovariectomised aged sheep

Large animals as sheep are often used as models for human osteoporosis. Our aim was therefore to determine how glucocorticoid treatment of ovariectomised sheep affects the cancellous bone, determining the cellular events within the bone remodelling process that contributes to their bone loss. Twenty female sheep were assigned for two groups; an untreated control group and an ovariectomised group treated with glucocorticoids (0.6 mg/kg/day, 5 times weekly) for 7 months. At 7 months the glucocorticoid-treated ovariectomised sheep showed a significant change in the bone microstructure revealed by a decreased trabecular bone volume and thickness compared to the control sheep. The treatment led to a temporary elevation of the bone resorption marker CTX (c-terminal collagen telopeptide), while the bone formation marker osteocalcin remained suppressed all 7 months. Histomorphometrically, the treated sheep had a complete absence of osteoid surfaces, and a 5-fold increase in the extent of eroded/reversal surfaces after 7 months. Most of these reversal surfaces were actually arrested reversal surfaces, defined as reversal surfaces without the presence of neighbouring osteoid surfaces or osteoclasts, which is classically observed next to active reversal surfaces. As in humans, these arrested reversal surfaces had compared to active reversal surfaces a reduced canopy coverage, a significantly decreased cell density, and a decreased immunoreactivity for the osteoblastic markers osterix, runx2 and smooth muscle actin in the mononuclear reversal cells colonising the surfaces. In conclusion, glucocorticoid treatment of ovariectomised sheep induced a significant bone loss, caused by an arrest of the reversal phase, resulting in an uncoupling of the bone formation and resorption during the reversal phase, as recently demonstrated in postmenopausal women with glucocorticoid-induced osteoporosis. This supports the relevance of the sheep model to the pathophysiology of glucocorticoid-induced osteoporosis in postmenopausal women, making it a relevant preclinical model for orthopaedic implant and biomaterial research.

Sheep model for osteoporosis: sustainability and biomechanical relevance of low turnover osteoporosis induced by hypothalamic-pituitary disconnection

Hypothalamo-pituitary disconnection (HPD) leads to low bone turnover and osteoporosis in sheep. To determine the sustainability of bone loss and its biomechanical relevance, we studied HPD-sheep 24 months after surgery (HPD + OVX-24) in comparison to untreated control (Control), ovariectomized sheep (OVX), and sheep 12 months after HPD (HPD + OVX-12). We performed histomorphometric, HR-pQCT, and qBEI analyses, as well as biomechanical testing of all ewes studied. Twenty-four months after HPD, histomorphometric analyses of the iliac crest showed a significant reduction of BV/TV by 60% in comparison to Control. Cortical thickness of the femora measured by HR-pQCT did not change between 12 and 24 months after HPD but remained decreased by 30%. These structural changes were caused by a persisting depression of osteoblast and osteoclast cellular activity. Biomechanical testing of the femora showed a significant reduction of bending strength, whereas calcium content and distribution was found to be unchanged. In conclusion, HPD surgery leads to a persisting low turnover status with negative turnover balance in sheep followed by dramatic cortical and trabecular bone loss with consequent biomechanical impairment.

In vivo assessment of local effects after application of bone screws delivering bisphosphonates into a compromised cancellous bone site

BACKGROUND

The primary stability of cancellous screw is difficult to obtain in bone of compromised quality and failure of screw fixation is common. To overcome this problem, it is proposed to locally deliver bisphosphonate from the screw. An in vivo validation of the increase in fixation of the cancellous screw is then needed in compromised bone.

METHODS

In this study, we used an overdrilling procedure, which enables consistent modeling of reduced screw stability comparable to compromised cancellous bone. Forty eight adult NZW rabbits were used in this study and all animals underwent bilateral femur implantation. One leg was implanted with the screw containing the bisphosphonate (biocoated group) while the other was used as control (control group) with the screw only. Mechanical testing and micro-CT imaging were used to assess the effect of local drug delivery of Zoledronate on screws fixation at 5 time points.

FINDINGS

At the early time points (1, 5, and 10 days), no significant difference could be seen between the biocoated and control groups. At 6 weeks, the bone volume fraction was significantly higher in the trabecular region of the biocoated group. However, this increase did not have a significant effect on the pull-out force. At the last time point, 11 weeks, both the bone volume fraction and the pull-out force were significantly higher in the biocoated group.

INTERPRETATION

The results of this study suggest that, in compromised bone, local delivery of bisphosphonate enhances the stability of bone screws.

Glucocorticoid induced osteopenia in cancellous bone of sheep: validation of large animal model for spine fusion and biomaterial research

STUDY DESIGN

Glucocorticoid with low calcium and phosphorus intake induces osteopenia in cancellous bone of sheep.

OBJECTIVE

To validate a large animal model for spine fusion and biomaterial research.

SUMMARY OF BACKGROUND DATA

A variety of ovariectomized animals has been used to study osteoporosis. Most experimental spine fusions were based on normal animals, and there is a great need for suitable large animal models with adequate bone size that closely resemble osteoporosis in humans.

METHODS

Eighteen female skeletal mature sheep were randomly allocated into 3 groups, 6 each. Group 1 (GC-1) received prednisolone (GC) treatment (0.60 mg/kg/day, 5 times weekly) for 7 months. Group 2 (GC-2) received the same treatment as GC-1 for 7 months followed by 3 months without treatment. Group 3 was left untreated and served as the controls. All sheep received restricted diet with low calcium and phosphorus during experiment. After killing the animals, cancellous bone specimens from the vertebra, femurs, and tibias were micro-CT scanned and tested mechanically. Serum biomarkers were determined.

RESULTS

In lumbar vertebra, the GC treatment resulted in significant decrease of cancellous bone volume fraction and trabecular thickness, and bone strength. However, the microarchitecture and bone strength of GC-2 recovered to a similar level of the controls. A similar trend of microarchitectural changes was also observed in the distal femur and proximal tibia of both GC treated sheep. The bone formation marker serum-osteocalcin was largely reduced in GC-1 compared to the controls, but recovered with a rebound increase at month 10 in GC-2.

CONCLUSION

The current investigation demonstrates that the changes in microarchitecture and mechanical properties were comparable with those observed in humans after long-term GC treatment. A prolonged GC treatment is needed for a long-term observation to keep osteopenic bone. This model resembles long-term glucocorticoid treated osteoporotic model, and is useful in preclinical studies.

Microarchitectural adaptations in aging and osteoarthrotic subchondral bone issues

The human skeleton optimizes its microarchitecture by elaborate adaptations to mechanical loading during development and growth. The mechanisms for adaptation involve a multistep process of cellular mechanotransduction stimulating bone modelling, and remodeling resulting in either bone formation or resorption. This process causes appropriate microarchitectural changes tending to adjust and improve the bone structure to its prevailing mechanical environment. Normal individual reaches peak bone mass at age between 25 and 30 years, and thereafter bone mass declines with age in both genders. The bone loss is accompanied by microarchitectural deterioration resulting in reduced mechanical strength likely leading to fragility fractures. With aging, inevitable bone loss occurs, which is frequently the cause of osteoporosis; and inevitable bone and joint degeneration happens, which often results in osteoarthrosis. These diseases are among the major health care problems in terms of socio-economic costs. The overall goals of the current series of studies were to investigate the age-related and osteoarthrosis (OA) related changes in the 3-D microarchitectural properties, mechanical properties, collagen and mineral quality of subchondral cancellous and cortical bone tissues. The studies included mainly two parts. For human subjects: aging- (I–IV) and early OArelated (V–VI) changes in cancellous bone properties were assessed. For OA guinea pig models (VII–IX), three topics were studied: firstly, the spontaneous, age-related development of guinea pig OA; secondly, the potential effects of hyaluronan on OA subchondral bone tissues; and thirdly, the effects on OA progression of an increase in subchondral bone density by inhibition of bone remodeling with a bisphosphonate. These investigations aimed to obtain more insight into the age-related and OA-related subchondral bone adaptations.

Review of nonprimate, large animal models for osteoporosis research

Large animal models are required for preclinical prevention and intervention studies related to osteoporosis research. The challenging aspect of this requirement is that no single animal model exactly mimics the progression of this human-specific chronic condition. There are pros and cons associated with the skeletal, hormonal, and metabolic conditions of each species that influence their relevance and applicability to human physiology. Of all larger mammalian species, nonhuman primates (NHPs) are preeminent in terms of replicating important aspects of human physiology. However, NHPs are very expensive, putting them out of reach of the vast majority of researchers. Practical, cost-effective alternatives to NHPs are sought after among ungulate (porcine, caprine, and ovine) and canine species that are the focus of this review. The overriding caveat to using large lower-order species is to take the time in advance to understand and appreciate the limitations and strengths of each animal model. Under these circumstances, experiments can be strategically designed to optimize the potential of an animal to develop the cardinal features of postmenopausal bone loss and/or yield information of relevance to treatment.

Ibandronate treatment reverses glucocorticoid-induced loss of bone mineral density and strength in minipigs

The Göttingen minipig is one of the few large animal models that show glucocorticoid (GC)-induced bone loss. We investigated whether GC-induced loss of bone mineral density (BMD) and bone strength in minipigs can be recovered by treatment with the bisphosphonate ibandronate (IBN). 40 primiparous sows were allocated to 4 groups when they were 30 months old: GC treatment for 8 months (GC8), for 15 months (GC15), GC treatment for 15 months plus IBN treatment for months 8-15 (GC&IBN), and a control group without GC treatment. Prednisolone was given at a daily oral dose of 1 mg/kg body weight for 8 weeks and thereafter 0.5 mg/kg body weight. IBN was administered intramuscularly and intermittently with an integral dose of 2.0 mg/kg body weight. BMD of the lumbar spine (L1-3) was assessed in vivo by Quantitative Computed Tomography (QCT) at months 0, 8, and 15. Blood and urine samples were obtained every 2-3 months. After sacrificing the animals lumbar vertebrae L4 were tested mechanically (Young's modulus and ultimate stress). Histomorphometry was performed on L2 and mineral content determined in ashed specimens of T12 and L4. In the GC&IBN group, the GC associated losses in BMD of -10.5%+/-1.9% (mean+/-standard error of the mean, p<0.001) during the first 8 months were more than recovered during the following 7 months of IBN treatment (+14.8%+/-1.2%, p<0.0001). This increase was significantly larger (p<0.0001) than the insignificant +2.1%+/-1.2% change in group GC15. At month 15, the difference between groups GC&IBN and GC15 was 22% (p<0.01) for BMD, 48% (p<0.05) for Young's modulus, and 31% (p<0.14) for ultimate stress; bone-specific alkaline phosphatase showed trends to lower values (p<0.2) while deoxypyridinoline was comparable. This minipig study demonstrates that GC-induced impairment of bone strength can be effectively and consistently treated by IBN. GC&IBN associated alterations in BMD and bone turnover markers can be monitored in vivo using QCT of the spine and by biochemical analyses, reflecting the changes in bone strength.

Bone formation in a long bone defect model using a platelet-rich plasma-loaded collagen scaffold

Platelet-rich plasma (PRP), a platelet concentrate made of autogenous blood, has been used in recent years to improve bone defect healing particularly in maxillofacial reconstructions. The aim of the present study was to assess the effect of PRP on new bone formation in a critical diaphyseal long bone defect. A critical size defect (2.5 cm) in the tibial diaphysis of 16 sheep was supplied either with autogenous PRP in a collagen carrier or with collagen alone (controls). Platelets were enriched about 3.5 fold compared to normal blood in the PRP. After 12 weeks, the explanted bone specimens were quantitatively assessed by X-ray, computed tomography (CT), biomechanical testing and histological evaluation. Bone volume, mineral density, mechanical rigidity and histology of the newly formed bone in the defect did not differ significantly between the PRP treated and the control group, and no effect of PRP upon bone formation was observed. It was suggested that PRP does not enhance new bone formation in a critical size defect with a low regenerative potential. Such bone defects might require more potent stimuli, e.g. combinations of functional biomaterials or autografts, precursor cells or osteoinductive growth factors.

Inhibition of cortical and cancellous bone formation in glucocorticoid-treated OVX sheep

INTRODUCTION

Little is known about the effects of steroids on cortical bone. The purpose of this study was to explore the effects of glucocorticoid treatment on cortical bone density, strength, and formation of new bone in ovariectomized sheep and to compare it to cancellous bone.

MATERIALS AND METHODS

Sixteen ovariectomized merino sheep either received a 6-month glucocorticoid treatment (GLU; 0.45 mg/kg/day Methylprednisolone s.c.) or were left untreated (control). After 2 and 4 months, newly formed bone was labeled by the administration of calceingreen and Tetracycline-hydrochloride. After 12 months, the animals were sacrificed and cortical specimens were obtained from the tibial diaphysis. Cancellous bone specimens were harvested from the proximal tibia. All specimens were scanned for apparent BMD by pQCT and tested mechanically. Formation of new bone was analyzed using histological slices of the femoral condyles and cross-sections of the mid-diaphysis of the tibia.

RESULTS

The intracortical formation of new bone in glucocorticoid-treated sheep was 70% lower after 2 months and 80% lower after 4 months. Six months after the termination of the steroid administration, the active bone area was 20% lower than in the control group. Cortical width and cortical bone area were reduced by 7-8% and marrow area increased by 8% in steroid-treated animals compared to control animals. Neither cortical apparent bone mineral density nor biomechanical parameters of cortical bone specimens differed between the groups. Cancellous bone formation in steroid-treated animals declined by 68% after 2 months and by 90% after 4 months. After 1 year, cancellous bone formation was reduced by 38% compared to control. The apparent cancellous bone mineral density and cancellous bone compressive stiffness were reduced by 34% and 55%, respectively.

CONCLUSION

A six-month glucocorticoid treatment of ovariectomized sheep resulted in a substantial reduction of bone formation both in cortical and in cancellous bone and reduced cortical width by increased endosteal resorption. Beyond changes in cancellous bone, impaired cortical bone remodeling may add to the increased fracture risk at the hip or the distal radius in patients treated with steroids.

Biocompatibility and functionality of the degradable polymer alkylene bis(dilactoyl)-methacrylate for screw augmentationin vivo

Recently, a new degradable polymer has been developed on the basis of alkylene bis(dilactoyl)-methacrylate as an alternative material for screw augmentation. The polymer has been investigated in vitro and in a short-term experiment in rabbits exhibiting promising results. The aim of the present study was to investigate its long-term biocompatibility and mechanical functionality in a large animal model. The polymer was used for screw augmentation in the cancellous bone of the femoral condyle and tibia epiphysis of 12 sheep and was compared to polymethylmethacrylate (PMMA) augmented and nonaugmented screws. After an implantation period of 6 months, bone, regional lymph nodes, and several organs were histologically evaluated. The mechanical efficacy was investigated by a biomechanical pullout test. A lot of mononuclear macrophages and multinuclear foreign body giant cells with incorporated polymer particles indicate strong inflammatory reactions. Large osteolysis zones with osteoclasts were found in the surrounding polymer. The polymer was fragmented but not substantially degraded. Polymer particles were also found in the regional lymph nodes. Lung, liver, kidney, and spleen did not show any pathological signs. The pullout force of screws augmented with the new polymer was significantly reduced in comparison to PMMA augmented and nonaugmented screws, respectively. It was concluded that the material has poor biocompatibility and cannot be recommended for clinical application as screw augmentation material.

Influence of the stiffness of bone defect implants on the mechanical conditions at the interface--a finite element analysis with contact

The study focused on the influence of the implant material stiffness on stress distribution and micromotion at the interface of bone defect implants. We hypothesized that a low-stiffness implant with a modulus closer to that of the surrounding trabecular bone would yield a more homogeneous stress distribution and less micromotion at the interface with the bony bed. To prove this hypothesis we generated a three-dimensional, non-linear, anisotropic finite element (FE) model. The FE model corresponded to a previously developed animal model in sheep. A prismatic implant filled a standardized defect in the load-bearing area of the trabecular bone beneath the tibial plateau. The interface was described by face-to-face contact elements, which allow press fits, friction, sliding, and gapping. We assumed a physiological load condition and calculated contact pressures, shear stresses, and shear movements at the interface for two implants of different stiffness (titanium: E=110GPa; composite: E=2.2GPa). The FE model showed that the stress distribution was more homogeneous for the low-stiffness implant. The maximum pressure for the composite implant (2.1 MPa) was lower than for the titanium implant (5.6 MPa). Contrary to our hypothesis, we found more micromotion for the composite (up to 6 microm) than for the titanium implant (up to 4.5 microm). However, for both implants peak stresses and micromotion were in a range that predicts adequate conditions for the osseointegration. This was confirmed by the histological results from the animal studies.