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.

Preclinical and Translational Studies in Small Ruminants (Sheep and Goat) as Models for Osteoporosis Research

PURPOSE OF THE REVIEW

This review summarizes research on the use of sheep and goats as large animal models of human osteoporosis for preclinical and translational studies.

RECENT FINDINGS

The most frequent osteoporotic sheep model used is the ovariectomized sheep with 12 months post-operatively or more and the combined treatment of ovariectomized sheep associated to calcium/vitamin D-deficient diet and glucocorticoid applications for 6 months, but other methods are also described, like pinealectomy or hypothalamic-pituitary disconnection in ovariectomized sheep. The goat model for osteoporosis research has been used in a very limited number of studies in osteoporosis research relative to sheep. These osteoporotic small ruminant models are applied for biomaterial research, bone augmentation, efficacy of implant fixation, fragility fracture-healing process improvement, or bone-defect repair studies in the osteopenic or osteoporotic bone. Sheep are a recognized large animal model for preclinical and translational studies in osteoporosis research and the goat to a lesser extent. Recently, the pathophysiological mechanism underlying induction of osteoporosis in glucocorticoid-treated ovariectomized aged sheep was clarified, being similar to what occurs in postmenopausal women with glucocorticoid-induced osteoporosis. It was also concluded that the receptor activator of NF-κB ligand was stimulated in the late progressive phase of the osteoporosis induced by steroids in sheep. The knowledge of the pathophysiological mechanisms at the cellular and molecular levels of the induction of osteoporosis in small ruminants, if identical to humans, will allow in the future, the use of these animal models with greater confidence in the preclinical and translational studies for osteoporosis research.

The Ovariectomized Ewe Model in the Evaluation of Biomaterials for Prosthetic Devices in Spinal Fixation

The effect of surgical ovariectomy on cancellous bone was investigated by comparing mechanical properties and microarchitectural characteristics of the lumbar vertebrae in ovariectomized and sham-operated ewes. Eighteen mongrel ewes, 4±1 years old, were randomly divided into three groups: 6 animals served as a control group (Baseline), 6 were bilaterally ovariectomized (OVX), and the others were used as a sham-operated group (SHAM). OVX and SHAM ewes were euthanized 24 months after surgery; the L5 vertebrae were processed for mechanical and histomorphometric analyses. Maximum load, maximum strength (p<0.0005) and elastic modulus (p <0.005) decreased by about 28% in the OVX group in comparison with the other groups. In the OVX group, vertebral cancellous bone volume, trabecular thickness and trabecular number decreased by about 32% (p<0.0005), 15% (p=0.001) and 20% (p=0.019), respectively. An overall decrease in the bone turnover rate of the OVX group was registered in terms of bone formation rate (p= 0.007) and activation frequency (p<0.0005). The variations observed in cancellous bone mechanics and histomorphometry would suggest the development of an osteopenic state in ewe vertebrae at 24 months. Such findings may be useful for future experimental investigations on biomaterials and prosthetic devices to be implanted in the osteopenic spine.

Tibial Implants: Biomechanical and Histomorphometric Studies of Hydroxyapatite-Coated and Uncoated Stainless Steel and Titanium Screws in Long-Term Ovariectomized Sheep

This study was designed to evaluate osteointegration of HA-coated and uncoated titanium and stainless steel screws in the cortical bone of long-term (24 months) ovariectomized sheep (OVX group), in comparison with Sham-aged sheep (control group). The screws were tested biomechanically (extraction torque) and histomorphometrically (affinity index: Al) 12 weeks after their implantation in tibial diaphyses. Tibial cortical bone parameters showed significant differences between the groups, showing a reduction of the selected parameters in the OVX group. ANOVA showed significant effects for both material and ovariectomy factors on obtained extraction torque (material: F=159.26, p < 0.0005; ovariectomy: F=20.04, p < 0.0005) and Al data (material: F=8.04, p < 0.001; ovariectomy: F=7.17, p < 0.05). In both groups the extraction torque for coated screws of both materials was significantly higher than for uncoated screws, and uncoated titanium had a better extraction torque than uncoated stainless steel. In the OVX group, the HA-coated stainless steel and titanium Al data were significantly higher than uncoated Al data. In conclusion, the biomechanical and histomorphological results obtained suggest employing HA-coated screws in the presence of osteopenic cortical bone.

Biomaterials in Spinal Fixation. An Experimental Animal Study to Improve the Performance

Different pedicle screws were biomechanically and morphologically studied and compared through the use of an animal model to determine their efficacy and resistance in spinal fixation. The principal objective was to compare biomechanical and histomorphological aspects of HA-coated screws to uncoated ones.

Fourty-eight cylindrical transpedicular self-tapping screws divided into three groups of sixteen each were employed; Group A: stainless steel screws; Group B: titanium screws; Group C: HA-coated titanium screws. The screws were implanted bilaterally and randomly into the L3, L4, and L5 pedicles of eight adult mongrel sheep. The final insertion torque was measured in all the implants. After one and four months, upon euthanization, four samples per group were extracted from the surrounding bone and the screw extraction torque was measured. The remaining samples were examined and processed for histological and histomorphological evaluations.

No differences were observed at one month among the extraction torque of the three groups. After four months the only significance between insertion and extraction values was for the HA group, i.e. p=0.001. Comparing the extraction torque values of the three groups after four months of healing, the HA-coated group showed a greater than twofold increase (p<0.0005). No differences were observed at one month among the percentages of bone-implant contact in the three groups. After four months the percentage was significant only for the C group (p<0.0005). At four months a correlation was found between the morphological and the biomechanical data of group C (p<0.0005).

The use of hydroxyapatite-coated screws could act as an effective method to improve the bone-implant interface, thus obtaining a strong fixation of the implant independently of the arthrodesis achieved with bone graft.

Laser Technology in Orthopedics: Preliminary Study on Low Power Laser Therapy to Improve the Bone-Biomaterial Interface

Low Power Laser (LPL) seems to enhance the healing of bone defects and fractures. The effect of LPL In other orthopedic areas such as osteointegration of implanted prosthetic bone devices is still unclear. In the present study, 12 rabbits were used to evaluate whether Ga-AI-As (780 nm) LPL stimulation has positive effects on osteointegration. Hydroxyapatite (HA) cylindrical nails were drilled into both distal femurs of rabbits. From postoperative day 1 and for 5 consecutive days, the left femura of all rabbits were given LPL treatment (Laser Group-LG) with the following parameters: 300 Joule/cm2 1 Watt, 300 Hertz, pulsating emission, 10 minutes. The right femura were sham-treated (Control Group-CG). At 4 and 8 weeks after implantation, histologic and histomorphometric investigations evaluated bone-biomaterial-contact. Histomorphometry showed a higher degree of osteointegration at the HA-bone interface in the LG Group at 4 (p<0.0005) and 8 weeks (p<0.001). These preliminary positive results seem to support the hypothesis that LPL treatment can be considered a good tool to enhance the bone-implant interface in orthopedic surgery.

Sandblasted Titanium Osteointegration in Young, Aged and Ovariectomized Sheep

To evaluate how aging and estrogen deficiency influence the success rate of Sandblasted Titanium (Ti/SA) implants, the osteointegration of Ti/SA rods was studied in the cortical and trabecular bone of 5 young, 5 aged and 5 ovariectomized (OVX) sheep.

The characterization of the host bone by transiliac biopsies of the iliac crest showed a progressive rarefaction of trabecular bone in aged and OVX animals when compared to young ones. A significant reduction, both in cortical and trabecular bone, of the osteointegration rate of Ti/SA rods in the presence of estrogen deficiency compared to young animals was observed, while only a minor reduction was observed in aged animals. These results were confirmed by the pushout test in cortical bone.

Bone quality affected the biological response of bone to Ti/SA implants in both trabecular and cortical bone; consequently, strategies to maximize the bone osteogenic properties of osteoporotic patients should be adopted.

In vitro Models to Test Orthopedic Biomaterials in View of Their Clinical Application in Osteoporotic Bone

The development of in vitro cell cultures, in association with in vivo experimentation, greatly improved the characterization of biomaterials for orthopedic devices before their clinical use. In recent years an increasing interest has arisen in the use of both pathological osteoblast cultures and animal models to perform in vitro and in vivo tests on biomaterial behavior. A growing number of prostheses, in fact, are implanted in osteoporotic patients, due to the increasing age of the population. Moreover, the presence of osteoporosis may affect bone-biomaterial osteointegration in these patients.

The present paper is a literature review and, after a short description of in vitro studies for characterization of osteoblasts derived from osteoporotic bone, the results of in vitro studies on biomaterial biocompatibility and osteointegration rate in the presence of osteoporotic bone derived osteoblast cultures are reported. Pathological cell culture models are able to demonstrate the different behavior of osteoblasts in response to biomaterials, when comparing normal and pathological conditions.

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.

In vivo preclinical evaluation of the influence of osteoporosis on the anchorage of different pedicle screw designs

We investigate the anchorage of pedicle screws with different surface treatments in osteoporotic bone. Eight ewes were divided into two groups of four animals each: four sheep underwent bilateral ovariectomy (OVX Group), whereas the operation was simulated in the remaining group (SHAM Group). Eighteen months after the first operation, the Dynesys(®) System was fitted to the sheep using pedicle screws with three different surface treatments: untreated, rough blasted (uncoated) and bioactive coated (bioactive). Uncoated screws showed a significantly higher bone ingrowth value compared with the untreated screws in the OVX group (9.3%, p < 0.005) and a significantly lower bone ingrowth value in the SHAM group (-11.0%, p < 0.05). Furthermore, the bioactive pedicle screws had a significant lower bone ingrowth value than the untreated screws in the SHAM group (-12.1%, p < 0.05). These results suggest that both tested surface treatments of pedicular screws may provide an advantage in terms of bone quality and osseointegration, when implanted in osteoporotic vertebrae.

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.

Osteointegration of titanium and hydroxyapatite rough surfaces in healthy and compromised cortical and trabecular bone: in vivo comparative study on young, aged, and estrogen-deficient sheep

The osteointegration rate of titanium (Ti; TI01) and duplex Ti plus HA (HT01) coating systems with high surface roughness was investigated in healthy, aged, and oestrogen-deficient sheep. After having evaluated the bone quality, TI01 and HT01 rods were implanted in the tibial diaphyses (two implants for each tibia) and epiphyses (1 implant for each tibia) of five young (YOUNG), five aged (AGED), and five aged and ovariectomized (OVX) sheep. The iliac crest trabecular bone volume (BV/TV) and number (Tb.N) in OVX sheep were respectively 33.5% and 28.5% lower than in YOUNG sheep (p < 0.005) and lower than in the AGED group (BV/TV, -17%; Tb.N, -13.5%; not significant); in the OVX group the trabecular separation was 77.9% higher than in YOUNG (p < 0.05) and 30.9% higher than in AGED animals. Lumbar vertebrae L5 bone mineral density was significantly lower in AGED (8.9%, p < 0.05) and OVX sheep (19.3%, p < 0.0005) when compared with YOUNG animals. Five samples of five sheep from each group were analyzed for each observation. At 3 months, in cortical bone both affinity index and pushout test results showed no significant differences between the two materials in each group of animals. In trabecular bone, the affinity index of HT01 was significantly higher than that of TI01 in each group of animals (YOUNG, 90.7%; AGED, 76.9%; OVX, 49.9%) with no significant differences between groups. In conclusion, the performance of TI01 and HT01 surfaces was high not only in YOUNG, but also in OVX animals and, therefore, they might be useful for aged and osteoporotic patients.

Normal and osteopenic bone‐derived osteoblast response to a biomimetic gelatin–calcium phosphate bone cement

We have recently developed a new calcium phosphate bone cement enriched with gelatin (GEL-CP), which exhibits improved mechanical properties with respect to the control cement (C-CP). In a previous study, we demonstrated the good response of osteoblast-like cells to the new biomimetic bone cement. Herein, we extend the investigation to primary culture of osteoblasts derived from healthy and pathological bones. Osteoblasts derived from normal (N-OB) and osteopenic (O-OB) sheep bones were cultured on samples of GEL-CP, and their behavior was compared with that of cells cultured on C-CP as control. Cell morphology, proliferation, and differentiation were evaluated at 3 and 7 days. SEM analysis revealed that both N-OB and O-OB showed a normal morphology when cultured on GEL-CP. Biological tests demonstrated that the gelatin-enriched cement improves osteoblasts' activity and differentiation of O-OB cultures, with respect to the control samples. The data indicate that the new composite cement positively stimulates alkaline phosphatase activity, collagen type I, and osteocalcin production, not only in N-OB, but also in O-OB culture. The improvement due to the presence of gelatin suggests that the biomimetic composite material could be successfully applied as bone substitute also in the presence of osteopenic bone.

An investigation of the effects of hydroxyapatite coatings on the fixation strength of cortical screws

Hydroxyapatite (HA) are commonly applied to orthopaedic implants for acceleration of osteointegration and so overcoming the loosening problems such as in cortical screws. Electrophoretic deposition (EPD) of hydroxyapatite was applied for coating of cortical screws in this work. The effects of hydroxyapatite-coated and uncoated cortical screws on insertion and extraction torque were investigated through in vivo experiments. Three groups of screws were undertaken: first group with no coating, second group coated with HA and the third group coated with HA+interlayer, a synthetic calcium silicate compound. Five sheep were operated, and 60 cortical (20 x 3) screws from those of groups were implanted in cortical femurs to observe the effect of HA and interlayer on screws. Results show that as an alternative to plasma spray coating method, the EPD process enables to produce a quick, easy, cheap and uniform adjustable coating layer. Also from biomechanical and SEM examinations, HA coating by EPD method of cortical screws resulted in extremely improved fixation with reduced risk of loosening problem.

Osteoporosis and biomaterial osteointegration

Biomaterial osteointegration depends not only on the properties of the implanted material but also on the characteristics and regenerative capability of the host bone. For this reason, researchers involved in biomaterial evaluation now place great importance on the various pathologies often present in orthopaedic patients which can negatively affect the success of surgical implants. Osteoporosis is undoubtedly one of the most frequently encountered of such diseases. Models reproducing the osteoporotic condition can be useful to understand the influence of the pathology on cell behaviour, bone regeneration and osteointegration processes, thus increasing our basic knowledge and allowing the development of surgical techniques and implant biomaterials more suitable for use in the surgical treatment of fractures in osteoporotic patients. The present paper is a literature review and, after a short description of how the presence of osteoporosis could influence bone regenerative processes, the results of the main studies on biomaterial biocompatibility and osteointegration both in vitro and in vivo in the presence of osteoporotic condition are reported. Both cell cultures and animal models are able to demonstrate the different response of bone to biomaterials by comparing healthy and pathological conditions. The use of pathological bone-derived cells and pathological animals is therefore recommended to test candidate orthopaedic materials.

Long-term effects of dietary anion-cation balance on acid-base status and bone morphology in reproducing ewes

The beneficial effects of anionic salts on calcium metabolism have been shown by supplementing rations with such salts during the last 3 weeks of pre-partum. However, there are few reports on the effects of anionic salts supplementation for periods of 4 weeks or longer on acid-base status, mineral metabolism and bone morphology. This study was conducted to evaluate the effects of the long-term dietary supplementation of anionic salts on the acid-base status, plasma minerals concentrations and bone morphology in sheep. Twenty-seven twin-bearing sheep were assigned to two experimental groups and a control group, depending on dietary cation-anion difference (DCAD) (+272.6, -88.9 and + 164.5 mEq/kg DM, respectively). Sheep assigned to each dietary treatment received their respective rations beginning 6 weeks prepartum and continuing until 12 days post-partum. Diets containing anionic salts induced a mild metabolic hyperchloraemic acidosis from 1 week pre-partum to 2 days post-partum that was completely compensated by non-respiratory mechanisms. These changes on acid-base status were accompanied by an increase of plasma ionized calcium levels. Plasma total calcium, phosphorus and magnesium concentrations were not affected by dietary treatment. Parathyroid hormone concentrations were related to the concentration of ionized calcium of plasma and were higher in sheep fed the cationic diet. Plasma osteocalcin levels were increased in sheep fed the anionic diet and cortical bone remodelling occurred in all the animals during late pregnancy in light and electron microscopy observation, but was particularly evident in the sheep fed the anionic diet. Bone turnover might be stimulated because of the role of the bone in buffering systemic acidosis. The data suggest that anionic salts ameliorated calcium metabolism around parturition by increasing bone resorption and the concentration of ionised calcium in plasma, possibly mediated by a mild hyperchloraemic metabolic acidosis induced by the salts.

Glucocorticoid treatment of ovariectomized sheep affects mineral density, structure, and mechanical properties of cancellous bone

Thus far, orthopedic research lacks a suitable animal model of osteoporosis. In OVX sheep, 6 months of steroid exposure reduced bone density and mechanical competence. Bone properties and bone formation did not recover for another 6 months. Therefore, steroid-treated OVX sheep may serve as a large animal model for osteopenic bone.

INTRODUCTION

The purpose of this study was to explore the effects of glucocorticoid treatment on cancellous bone density, microarchitecture, biomechanics, and formation of new bone.

MATERIALS AND METHODS

Sixteen ovariectomized merino sheep received either a 6-month glucocorticoid treatment (GLU; 0.45 mg/kg methylprednisolone) or were left untreated (control). Cancellous bone biopsy specimens from the tibia were harvested 6 months after ovariectomy. After 12 months, the animals were killed, and biopsy specimens were obtained from the contralateral tibia and the lumbar spine. All biopsy specimens were scanned for apparent bone mineral density by peripheral quantitative computed tomography (pQCT) and tested mechanically in uniaxial compression. Three-dimensional bone reconstructions were obtained by microcomputed tomography. Formation of new bone was analyzed using histologies of the femoral condyles.

RESULTS

After 6 months, mineral density (-19%) and mechanical competence (-45%) were reduced by glucocorticoid treatment (p < 0.1). BV/TV (-21%; p < 0.01) and trabecular thickness (-20%; p = 0.01) declined, whereas BS/BV increased (24%; p = 0.01). After 12 months, mineral density (-33%) and mechanical properties (-55%) were reduced even more profoundly (p < 0.05). Also, the structural parameters (BS/BV and Tb.Th.) still seemed to be affected by glucocorticoid treatment (p < 0.05). New bone formation, assessed by measurement of osteoid surface, was markedly reduced (-63%, p < 0.1) by glucocorticoid treatment. The differences between groups were generally more pronounced at the tibia and the femur than at the spine.

CONCLUSION

The effects of short-term high-dose steroid administration on bone mineral in this animal model were comparable with those observed in humans after long-term corticoid treatment. Reduction in bone quality and bone formation rate persisted after the cessation of steroid administration. Glucocorticoid treatment of ovariectomized sheep may therefore serve as a large animal model for steroid-induced osteopenia.

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

The purpose of this study was to determine the alterations in ovine trabecular bone induced by a combination of ovariectomy and steroid treatment. Twenty-four female skeletally mature Merino sheep were randomly assigned to ovariectomy alone (OVX), ovariectomy combined with glucocorticoid treatment for 6 months (OVX + GC), or no treatment (control). Biopsies of trabecular bone were harvested 6 and 12 months after the beginning of the study from the proximal tibia. The biopsies were scanned for apparent bone mineral density by quantitative computed tomography and were mechanically tested. Three-dimensional bone reconstructions were obtained by micro-computed tomography. Trabecular bone from the OVX + GC animals had a markedly reduced apparent bone mineral density (27% less than control), bone volume (34%), and elastic modulus (36%) at 6 months. At 12 months, the reductions in apparent bone mineral density (33%), bone volume (37%), and elastic modulus (62%) appeared to be even more pronounced. Ovariectomy alone did not result in a perceptible reduction in any parameter. The combination of ovariectomy and glucocorticoid treatment in sheep resulted in a successful induction of substantial loss of trabecular bone and thus may serve as a large-animal model for osteopenic trabecular bone for the development and testing of orthopedic implants and techniques under osteoporotic conditions.

Osteoblasts cultured from osteoporotic bone: a comparative investigation on human and animal-derived cells

In vitro studies on pathophysiology and innovative treatments of many orthopaedic diseases, based on the investigations of cells from pathologic skeletal tissues, greatly improve basic knowledge of osteoporosis. Primary osteoblast (OB) cultures derived from osteopenic bone from different species (human, rat, sheep) were compared to assess the differences that should be taken into account when performing in vitro biocompatibiliy tests or investigating pharmacological and physical treatments. Primary OB were isolated from osteopenic patients and animals by well-established methods and their metabolism was assessed with or without 1,25(OH)2D3. The greatest significant differences were observed between rat and human cells both under basal conditions and after 1,25(OH)2D3 stimulation. In addition, the response to 1,25(OH)2D3 stimulation of OBs from osteopenic rats was significantly different from that of human and sheep OB cultures, in terms of NO, OC, IL-6, and TGF-beta1. Cells derived from osteopenic sheep behaved much more similarly to those from humans, except for a significant difference in terms of TGF-beta1 observed both under basal conditions and after stimulation.

Biological assessment of the bone-screw interface after insertion of uncoated and hydroxyapatite-coated pedicular screws in the osteopenic sheep

The sheep seems to be a promising model of osteoporosis and biomaterial osteointegration in osteopenic bone. The long-term ovariectomized sheep model was used for the biological investigation of bone healing around uncoated and hydroxyapatite (HA)-coated pedicle screws in osteopenic bone. Four sheep were ovariectomized and four sheep were sham-operated. Twenty-four months after surgery, the animals were implanted with uncoated and HA-coated stainless steel screws in the lumbar vertebral pedicles. Four months later, bone-to-implant contact, bone ingrowth, and bone hardness were measured around screws. Uncoated stainless steel presented significantly (p < 0.0005) lower bone-to-implant contact in healthy and osteopenic bone compared with HA-coated stainless steel. HA significantly improved bone ingrowth in healthy bone (p < 0.05) compared with uncoated stainless steel. Osteopenia significantly (p < 0.05) reduced the area of bone ingrowth around the screw threads for both types of implants. In the inner thread area, bone microhardness significantly increased (p < 0.05) in HA-coated surface versus uncoated for healthy and osteopenic bone. HA coating significantly enhances bone-to-implant contact also in osteopenic bone in comparison with uncoated stainless steel surfaces. Bone ingrowth and mineralization are ameliorated by the osteoconductive HA coating. However, osteopenia seems to greatly influence bone ingrowth processes around the implanted screws regardless of the characteristics of the material surface.

Mechanical and histomorphometric evaluations of titanium implants with different surface treatments inserted in sheep cortical bone

Improvement of the implant-bone interface is still an open problem and the interest in chemical modification of implant surfaces for cementless fixation has grown steadily over the past decade. Mechanical and histomorphometric investigations were performed at different times on implants inserted into sheep femoral cortical bone to compare the in vivo osseointegration of titanium screws ( X 3.5 x 7 mm length) with different surface treatments. After 8 weeks of implantation, the push-out force of anodized and hydrothermally treated implants (ANODIC) was significantly higher than that of machined implants (MACH) (36%, p<0.0005), whereas a decrease of 39% was observed for acid-etched implants (HF) when compared to other surface treatments. After 12 weeks of implantation, the push-out force values of HF implants were still significantly lower than those observed for MACH (-19%, p<0.01) and hydroxyapatite vacuum plasma-sprayed implants (HAVPS, -25%, p<0.0005), and the highest push-out force was found in HAVPS (p<0.001) implants. After 8 and 12 weeks of implantation, the AI of HF implants was significantly (p<0.05) lower ( approximately -25%) than that of MACH, HAVPS and ANODIC implants. In conclusion, results appear to confirm that there are no specific differences between ANODIC and HAVPS implants in terms of behavior. Moreover, although MACH implants show some surface contaminating agents, they appear to ensure good osseointegration within 12 weeks both mechanically and histomorphometrically, as do ANODIC and HAVPS implants. However, further studies are required to investigate bone hardness and mineralization around implants.

Characterization of bone defect repair in young and aged rat femur induced by xenogenic demineralized bone matrix

BACKGROUND

The osteoinductive effect of some biomaterials could be affected by those systemic conditions typical of old age. The aim of the present paper was to assess the effects of age on the healing of bone defects treated with demineralized bone matrix (DBM).

METHODS

The study was conducted in young (3 month old) and aged (18 month old) rats to assess the efficacy of DBM in the treatment of osseous defects in bone with limited repair capacities. A standard bone defect was created in the distal femoral condyles of male Wistar rats: the left condyle was filled with rabbit DBM granules, while the right condyle was left empty (control). Histological and microhardness analyses were performed at 30 and 45 days after implant surgery.

RESULTS

After implantation of xenogenic DBM, bone healing areas of the aged and young groups showed a significant increase in the formation of newly mineralized bone relative to controls. Measurements of trabecular thickness on day 45 revealed no differences between newly formed and preexisting bone in the young group, while control values were lower. Microhardness measurements demonstrated that newly mineralized bone, either induced by DBM or not, and preexisting bone were comparable in terms of trabecular hardness after 45 days.

CONCLUSIONS

In conclusion, xenogenic DBM seems to be effective in bone defect healing, since it increases mineralized tissue volume. In both DBM-filled and empty sites, age seems to have a detrimental effect on the volume of new bone formation but no influence on bone maturation.

Pedicular fixation in the osteoporotic spine: a pilot in vivo study on long-term ovariectomized sheep

Spinal instrumentation success is greatly affected by the presence of osteoporosis. To date, however, no data exist on in vivo investigations on biomaterial and surgical techniques in the osteoporotic spine. In the present study 24 uncoated and 24 HA-coated screws were implanted in the L3, L4 and L5 pedicles of eight sheep (four ovariectomized, OVX Group; four sham-operated, Control Group). At four months, uncoated screws showed a significant decrease of about -22% in the extraction torque of the OVX Group as compared to the Control Group (p < 0.005). The extraction torque of HA-coated screws significantly (p < 0.0005) improved in both groups when compared to that of uncoated screws and showed increases ranging from 133% to 157%. Pedicle trabecular bone of OVX sheep showed a significant decrease in BV/TV (-30%; p < 0.05) and Tb.Th (-33%; p < 0.01). The affinity index (AI) results revealed significant (p < 0.0005) differences between uncoated and HA-coated screws for both groups: values were lower for uncoated than HA-coated screws by about -35%. A significant difference was also found for the AI data of uncoated screws between the OVX and Control Groups (-13%, p < 0.005). The current findings have demonstrated that long-term ovariectomized sheep can be used to study in vivo osteointegration in the osteoporotic spine. The HA coating has proven to improve bone purchase and bone-screw interface strength in healthy and osteopenic animals.

Human osteoblast cultures from osteoporotic and healthy bone: biochemical markers and cytokine expression in basal conditions and in response to 1,25(OH)2D3

The development of methods to culture bone cells has enhanced in vitro studies and allowed researchers to investigate bone cell metabolism in healthy tissue and in various different bone diseases. Greater knowledge of cultures of pathologic bone tissue-derived osteoblasts may be helpful in performing in vitro experiments that test biomaterials and therapies to be used in the orthopedic field, since this kind of approach better reflects the conditions of clinical relevance to many patients. In the present study primary cultures of human osteoblastic cells were isolated from donors with osteoporosis (HOB, Human Osteopenic Bone) and their respective controls (HNB, Human Normal Bone). They were then characterized in baseline conditions and after stimulation with 10(-9) M 1,25(OH2)D3. Specific biochemical markers of bone cells and cytokines involved in bone turnover were evaluated to assess cell metabolism and any possible differences between osteoblasts derived from healthy and osteopenic bone tissue. Under baseline conditions, HNB and HOB in vitro cultures showed some differences in proliferation (MTT test), PICP, OC and IL-6. The HNB response to 1,25-(OH2)D3 stimulation differed significantly from that of the HOB cultures but only with regard to the MTT test, and ALP and PICP levels; the other selected parameters showed a similar behavior for both cultures. The current findings should be taken into account when cultures derived from human bone are used for in vitro experiments.