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Osteoporosis and biomaterial osteointegration
Biomedicine & Pharmacotherapy 58 (2004) 487–493 http://france.elsevier.com/direct/BIOPHA/
Dossier: Osteoporosis
Osteoporosis and biomaterial osteointegration M. Fini a,*, G. Giavaresi a, P. Torricelli a, V. Borsari a, R. Giardino a,b, A. Nicolini c, A. Carpi d a
Experimental Surgery Department, Research Institute Codivilla Putti, Rizzoli Orthopaedic Institute, Via di Barbiano, 1/10, 40136 Bologna, Italy b Chair of Surgical Pathophysiology, University of Bologna, Italy c Department of Internal Medicine, University of Pisa, Pisa, Italy d Department of Reproduction and Ageing, University of Pisa, Pisa, Italy Received 29 March 2004 Available online 11 September 2004
Abstract 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. © 2004 Elsevier SAS. All rights reserved. Keywords: Osteoporosis; Biomaterial; Osteointegration; Cell cultures; Animal models
1. Introduction The increase in life expectancy, at least in western countries, means that an increasing number of patients submitted to surgical operations of osteosynthesis and joint replacement are affected by age-related, post-menopausal and other forms of secondary osteoporosis due to systemic diseases or pharmacological therapies. Since the mean age of life expectancy is now over 80 years, the majority of people outlive the quality of their connective tissues [26]. This consideration is particularly important considering that the natural consequence of osteoporosis is the development of pathological fractures in various bones, which may require the implantation of biomaterials. The osteointegration of biomaterial depends not only on the properties of the implanted biomaterial but also on the * Corresponding author. E-mail address: [email protected] (M. Fini). 0753-3322/$ - see front matter © 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.biopha.2004.08.016
characteristics and regenerative capability of the host bone [37]. For this reason, researchers currently working on biomaterial evaluation place great importance on the different pathologies that are frequently present in orthopaedic patients and can negatively affect the success of surgical implants. Osteoporosis is undoubtedly one of the most frequently encountered diseases [13]. Even though much attention has been given to fracture prevention and new pharmacological and physical therapies aimed at maintaining a high level of bone mass, less attention has been directed at the study of orthopaedic biomaterial behaviour when implanted in osteoporotic bone. Bone regeneration is a complex process not yet completely understood, especially when osteoporosis is present [38]. Osteoporosis etiology (advanced age, hypogonadism, rheumatic diseases, alterations in thyroid and parathyroid function, malignancies) and therapies used to treat osteoporosis (estrogens, vitamin D, biphosphonates) can interfere with bone growth processes [38].
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Bone alterations due to age and estrogen deficiency are both structural and biological. Some experimental and clinical data suggest that osteoporosis negatively influences the healing processes after a fracture [31,38] and the osteointegration rate of prosthetic implants [6]. Models reproducing the osteoporotic condition can be useful to help understand the influence of the pathology on cell behaviour, bone regeneration and osteointegration processes, by increasing 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 ideal biomaterial should present the same biocompatibility and osteointegration characteristics when tested in healthy and pathological conditions. One of the main reasons for the still limited knowledge on bone regeneration and biomaterial osteointegration in the presence of osteoporosis may be the lack of idoneous and reliable models. It is recognized that it is extremely difficult to experimentally mimic, both in vitro and in vivo, the complex bone–biomaterial interactions and the decrease in osteoinductive capacity due to the loss of bioactive factors, bone morphogenetic proteins and mesenchymal staminal cells that accompany ageing and osteoporosis [14]. These alterations generally represent the main biological cause of implant failure irrespective of the high quality of the surgical device [4,9,23,28]. 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 an osteoporotic state are reported.
2. Bone regeneration and osteoporosis The principle mechanisms at the basis of osteointegration processes around implanted biomaterials are reported to be very similar to those occurring during fracture repair and involve a cascade of cellular and extracellular events [14]. In both osteointegration and fracture repair the healing process differs from that of soft tissues since it leads to regeneration of the damaged tissue after the implantation surgery or an injury. Bone is mainly composed of three kinds of cells (osteoblasts, osteocytes, osteoclasts), mineralized extracellular matrix (65%, hydroxyapatite and tricalciumphosphate), organic extracellular matrix (35%, collagen I and noncollagenous proteins such as osteocalcin, osteopontin and growth factors). Even if the growth factors of the organic extracellular matrix represent only about 1%, interest is being increasingly focused on this aspect in order to understand the pathophysyiological mechanisms of bone regeneration and osteointegration [7,12,49,51]. In fact, it is recognized that a predominant role is played by local factors such as cytokines and growth factors released in the lesion site [4].
The majority of researchers have focused their attention on a limited number of local factors such as interleukin 1 (IL-1), interleukin 6 (IL-6), tumor necrosis factor alfa (TNF-a), fibroblast growth factor (FGF), platelet derived growth factor (PDGF), transforming growth factor beta 1 (TGF-b1) and bone morphogenetic proteins (BMP) [2–4,7]. During the initial phases of reaction to trauma PDGF and TGF-b1 are released by activated platelets. Subsequently, macrophages and other inflammatory cells release FGF, PDGF, TGFb1 and probably also IL-1 and IL-6. During the first hours following the trauma numerous factors involved in lesion regeneration are therefore present and act on the bone, periosteal, medullar and adjacent soft tissue cells. Finally, it has now been demonstrated that mesenchymal staminal cells are the main instigators of regeneration processes, being present as early as 3 days after trauma and remaining at high levels of proliferation for the following days. In the ambit of naturally occurring bone regeneration processes, the presence of a biomaterial can greatly influence the early and late processes. Some authors have shown that superficial biomaterial topography can affect the rate of platelet activation, red cell aggregation, macrophage adhesion and activity, coagulum adhesion and retraction, vascularization, mesenchymal staminal cell and osteoblast adhesion, proliferation, synthetic activity and local factor composition and concentration [8,12,42,43]. Ageing and osteoporosis both modify cell proliferation, cell synthetic activity, cell reactivity to local factors, and mesenchymal staminal cell number [11,34,35,39,61]. These biological differences, in association with differences in biomechanical and microarchitectural bone properties, are involved in the reported increased failure risk of bone implanted biomaterial osteointegration in aged and osteoporotic bone. In fact, TGF-b1, IL-1, IL-6 are strictly involved in bone regeneration and biomaterial osteointegration processes. TGF-b1 is an important stimulator of osteointegration processes, while interleukins are present at the bone–biomaterial interface in the case of aseptical implant failure [1,2]. Some local factors have an important role in the pathogenesis of senile and post-menopausal bone loss because of their role as mediators of bone remodelling. The skeletal content of anabolic cytokines in bone tissue such as TGF-b1 seems to be significantly and negatively correlated with increasing age and post-menopausal osteoporosis [36]. On the other hand, the amount of IL-6, a catabolic cytokine involved in bone readsorption, increases in aged and post-menopausal osteoporotic patients [23,28]. Finally, the effect of IL-6 seems to be inhibited by estrogens and consequently, because of post-menopausal estrogen deficiency, the cytokine causes bone resorption and differentiation of osteoclast precursors in mature osteoclasts [45]. Bone mass decrease due to estrogen deficiency or to secondary osteoporosis has been correlated to osteoblast function and proliferation [10,50]. Taking into account basic and applied in vitro and in vivo research, the principle causes of increased failure risk of
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osteoporotic bone implanted biomaterials could be summarised as follows: (1) bone changes in structural and mechanical properties; (2) decreased number of mesenchymal staminal cells; (3) increase in osteoclast number and activity; (4) imbalance between anabolic and catabolic local factors acting on bone remodelling (growth factors and cytokines); (5) abnormal bone cell reactivity in proliferation rate and synthetic activity to systemic and local stimuli (hormones, growth factors and cytokines) and to mechanical stress.
3. In vitro studies Several authors have studied in vitro the behaviour of cells deriving from healthy and osteoporotic bone. Some of them have observed a decreased proliferation index of osteoblasts deriving from trabecular bone of osteoporotic women [35]. The reason for this decrease is still unclear but it could be due to a defective reactivity of cells to hormones and local factors. When studying in vitro cells deriving from healthy and osteoporotic patients, Lomri et al. [34] did not find any differences in cell reactivity to PTH, TGF-b1 and PGE2. Wong et al. observed that cells deriving from osteoporotic bone produce a significantly higher amount of alkaline phosphatase (ALP) if stimulated with vitamin D3. An abnormal behaviour of osteoporotic bone derived cells in comparison with healthy bone derived cells was suggested [61]. Neidlinger-Wilke et al. [39] studied the response to mechanical forces by investigating cell proliferation, ALP and TGFb1 synthesis. Cells deriving from healthy bone significantly increased proliferation rate and TGF-b1 production after the application of mechanical stress, while these effects were not observed in cell cultures deriving from osteoporotic bone. Torricelli et al. observed differences in proliferation rate, collagen I, osteocalcin (OC), TGF-b1 and IL-6 synthesis between healthy and osteoporotic human bone derived cells. These differences were significant after cell exposure to Vitamin D3 [54]. The same authors compared the in vitro behaviour of healthy and osteoporotic bone derived cells in rats. By measuring partial oxygen pressure in cultures they observed that cells derived from osteoporotic bone had an
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increased respiration rate causing a decrease of oxygen in the culture medium [16]. Regarding staminal mesenchymal cells, which are the main cells involved in bone regeneration, Rodriguez et al. [48] cultured cells from healthy and post-menopausal osteoporotic patients and observed in the latter a decreased growth rate, a minor response to mitogenic stimuli (i.e. Insulin-like growth factor I), and a decreased capacity of differentiation from mature osteoblasts as demonstrated by decreased ALP production and mineralization noduli formation in culture. Some authors used healthy and osteoporotic bone derived osteoblasts also for in vitro biomaterial testing [17,55–58] (Table 1). Torricelli et al. [57] using cells from healthy and osteoporotic rats, observed that a biological glass was not cytotoxic when tested in healthy bone derived osteoblasts, but caused a significant reduction of viability and OC level in osteoporotic bone derived cells [17]. The same authors compared the biological glass with titanium in cells deriving from healthy and osteoporotic sheep bone. Also in this case, osteoblasts derived from sheep osteoporotic bone showed an altered behaviour when cultured on the biological glass and in comparison with healthy bone derived cells, as far as Ca level, proliferation rate and osteocalcin production are concerned [56]. Other biomaterials tested under the same conditions, such as a nickel (Ni) reduced stainless steel and zirconia coated materials, behaved in a promising manner when tested in healthy and osteoporotic bone derived cells [55,58]. 4. In vivo studies Osteoporosis has been induced in various different laboratory animals using different methodologies: cortisonic administration, denervation, immobilization, absence of gravity, and surgical bilateral ovariectomy associated or not with low calcium diets. Bilateral ovariectomy is the method that best reproduces the clinical situation of post-menopausal osteoporosis and leads to the development of a systemic osteoporosis in both the peripheral and axial skeleton. The ovariectomized (OVX) rat is undoubtedly the animal most commonly used for studies on osteoporosis pathophysi-
Table 1 Main in vitro studies on biomaterials and osteoporotic bone derived osteoblasts Source of cells Osteoblasts from normal and OVX rats Osteoblasts from normal and OVX rats
Osteoblasts from normal and OVX sheep Osteoblasts from normal and OVX rats Osteoblasts from normal and OVX rats
Abbreviations are explained in the text.
Biomaterials Ti6Al4V two biological glasses (AP 40 and RKKP) Two biological glasses (AP 40 and RKKP) Ti6Al4V two biological glasses (AP 40 and RKKP) Ni reduced SSt ZrO2 and Al2O3 uncoated and coated with a biological glass (RKKP)
Main results Significant decrease of MTT and OC levels in osteoporotic bone derived cells cultured with AP 40 AP 40 caused a significant decrease of cell viability and proliferation and OC level in osteopenic bone derived cells not observed in normal bone derived cells Altered morphology, marked decrease of cell proliferation and OC level in osteopenic bone derived cells cultured with AP 40 No differences in cell behaviour between normal and osteopenic bone derived cells No differences in cell behaviour were observed in normal bone derived cells while RKKP-coated materials improved ALP levels in osteopenic bone derived cells
Refs. [17] [57]
[56] [55] [58]
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Table 2 Main in vivo tests on biomaterial osteointegration in osteoporotic bone Animal model Normal and OVX rats
Normal and OVX rats
Biomaterials Ti6Al4V, HA, ZrO2, Al2O3, two biological glasses (AP 40 and RKKP) Ti6Al4V, two biological glasses (AP 40 and RKKP) HA, Ti6Al4V
Rats submitted to sciatic nerve resection
HA, Al2O3
Normal and OVX rats
Normal, OVX and OVX plus neurectomi- HA, Ti6Al4V sed rats Normal and OVX rats
HA
Normal and submitted to OVX and OVX plus low Ca diet rabbits Normal, OVX and OVX plus low Ca diet rabbits Normal and steroid induced osteoporotic rabbits Normal and OVX sheep
Ti
Normal and OVX sheep
Normal and OVX sheep Normal and OVX sheep
Ti Ti Uncoated and HA coated CoCr alloy Uncoated and HA coated SSt
Uncoated and HA coated Ti6Al4V Uncoated and HA coated SSt and Ti6Al4V
Main results Refs. Significant decrease of the osteointegration rate for AP 40, ZrO2 [15] and Ti6Al4V in the OVX rats All biomaterials decreased the osteointegration rate in OVX rats; AP 40 osteointegrated only when implanted in normal rats Both biomaterials decreased the osteointegration rate in OVX rats (the decrease was significant for HA) Superior affinity of bone affected by disuse osteoporosis to HA compared with Al2O3 Ti6Al4V higher osteointegration rate in normal bone than in osteopenic bone; higher osteointegration rate of HA also in OVX plus neurectomised rats Slight decrease in bone contact in OVX rat cortical bone; significant decrease in bone contact in OVX rat cancellous bone Local GH administration ameliorated bone growth and mineralization around implants Formation of new bone around implants was delayed in osteopenic rabbits Significant lower mechanical attachment of Ti implanted in tibial bone but not in the mandible of osteopenic rabbits Improvement of biological fixation of HA coated implants more pronounced in the OVX sheep Significant decrease of biological osteointegration rate for uncoated SSt in OVX sheep; significant decrease of extraction torque for both uncoated and HA coated SSt in OVX sheep Significant decrease of biological and mechanical fixation for both biomaterials in OVX sheep femur Significant decrease of bone-to implant contact of uncoated materials versus HA-coated ones in OVX sheep; significant decrease of mechanical attachment of uncoated materials versus HA coated ones and of uncoated Ti6Al4V versus uncoated SSt in OVX sheep
[17] [18] [24] [25]
[44] [59] [37] [20] [60] [41]
[46] [47]
Abbreviations are explained in the text.
ology, diagnosis and therapy [29,38]. As shown in Table 2, some authors have also used the rat model for studies on biomaterial osteointegration in osteoporotic bone [15,17,18,24,25,44]. Because of the small dimension of the animal, cylindrical nails with a diameter of 2 mm are generally positioned in the trabecular bone of the distal femur or the proximal tibia [15,17,18,24,25,44]. Fini et al. [15] implanted nails made of a titanium alloy (Ti6Al4V), hydroxyapatite (HA), zirconia (ZrO2), alumina (Al2O3) and two biological glasses (AP 40 and RKKP) in the distal femurs of healthy and OVX rats. Histomorphometric analysis showed that the osteointegration rate of AP 40, ZrO2 and Ti6Al4V was significantly decreased in OVX rats compared to normal animals. When comparing the two different biological glasses and Ti6Al4V in the distal femur of healthy and OVX rats, 2 months after implantation, histomorphometric analysis showed the development of osteoporosis in the implanted site and a decrease in the osteointegration rate for the tested biomaterials in OVX rats as compared to healthy rats. The authors underline that one of the two biological glasses had an osteointegration index of about 55% in healthy rats and of only 4.1% in OVX rats [17]. The same researchers subsequently utilized the same animal model to study the behaviour of HA and Ti6Al4V. Both materials presented a decreased osteointegration rate in the presence of osteoporosis,
the decrease being significant in the case of HA [18]. Hayashi et al. [24] implanted nails made of Al2O3 and HA in the proximal tibia of healthy and osteoporotic rats and observed that HA implants maintained the same osteointegration rate in the presence of osteoporosis, while the Al2O3 nails significantly decreased the osteointegration index when implanted in osteoporotic rarefacted bone. The same authors subsequently compared HA and Ti6Al4V nails implanted in the proximal tibia of healthy, OVX and OVX plus sciatic nerve injured rats. The osteointegration rate of Ti6Al4V in healthy bone was higher than in osteoporotic bone. On the contrary, in the case of HA implants a good osteointegration level in the absence of a fibrous capsule was maintained in OVX animals also submitted to sciatic nerve injury [25]. Pan et al. [44] implanted HA coated nails in the tibial metaphysis of healthy and osteoporotic rats and observed a significant reduction of trabecular bone volume around implants, while the decrease was less evident in the cortical bone. Studies on small-size animals like the rat are correct from an ethical and scientific point of view and are acceptable in the first phases of a research, but because of differences due to rat bone quality as compared to human beings and of the small dimension of the animal that allows only miniaturized implants differing from clinically used implants, it is necessary in a second phase of the in vivo research to adopt a
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further experimental model using animals with a higher phylogenetic level [3,12]. Also according to the “International Standard Organization” [27] for orthopaedic biomaterial biocompatibility and osteointegration evaluation the rat is not considered as an experimental model for skeletal implantation and only rabbits, sheep and dogs are suggested. Bilateral ovariectomy associated with a low-calcium diet (0.07% calcium content) in rabbits was utilized by Tresguerres et al. [59] to evaluate the local effect of growth hormone (GH) on metallic biomaterial osteointegration in the osteoporotic tibia. Results showed that local GH administration in the implanted site could ameliorate the periosteal and cortical reaction and mineralization of osteoid tissue 14 days after implantation [59]. Bilateral ovariectomy plus low calcium and phosphorus diet (0.15% calcium, 0.59% phosphorus content) was used by Mori et al. [37] to study the bone–implant interface in osteoporotic rabbits. One month after ovariectomy and diet administration, and when tibia bone mineral density (BMD) showed a 12% decrease as compared to healthy rabbits, a metallic implant was inserted in the distal tibia. In healthy rabbits, new bone formation was evident around implants at 15 days and osteointegration processes were completed at 8 weeks. In the osteoporotic animals, however, bone formation around implants was delayed, even though at 12 weeks a good bone–biomaterial contact was observed. Following the experience of these authors, it is possible to deduce that the presence of osteoporosis can alter tissue reparative processes around implants, even though osteointegration has sometimes been achieved also in the presence of this pathology [37]. Fujimoto et al. used a model of osteoporosis secondary to corticosteroid administration in rabbits where metallic implants were inserted in both the proximal tibia and jaw. Mechanical osteointegration measurement evidenced a significant decrease in extraction torque values in the case of tibial implants in prednisolone treated rabbits, while no differences were observable between the two animal groups as far as the jaw implants were concerned [20]. Before 1994 the sheep was seldom used in experimental studies regarding osteoporosis or other skeletal pathologies. As this large size animal model offers many advantages, it has increasingly been used in orthopaedic scientific research over the last 10 years. Recent research has suggested that the sheep is a promising model for osteoporosis studies and, because of its dimension and bone characteristics, is also suitable for the evaluation of biomaterials [5,40,52]. Bone mineral loss has been observed in sheep after bilateral ovariectomy and it has also been reported that trabecular sheep bone is qualitatively similar to the human bone, thus permitting more reliable researches on bone remodelling [30]. To this regard, Torricelli et al. [53] compared in vitro osteoblast proliferation and synthetic activity of osteoblasts deriving from trabecular bone of humans, rats and sheep and observed that sheep osteoblasts were more similar to human osteoblasts as far as viability and osteocalcin, IL-6 and TGFb1 production are concerned.
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After bilateral ovariectomy a decrease in trabecular and cortical bone mass has been observed in the vertebral bodies, iliac crest and femural and tibial diaphysis of sheep [19,21,22]. The development of an osteoporotic state was observed 12 months after ovariectomy and was even more evident at 24 months [19]. In detail, at 24 months a significant decrease in bone volume (BV/TV), trabecular thickness (Tb.Th) and trabecular number (Tb.N) associated with a significant increase in trabecular separation (Tb.Sp) were observed in the vertebral body [22]. In the vertebral pedicular bone of sheep, 24 months after ovariectomy, both BV/TV and Tb.Th were significantly decreased in comparison with sham-operated sheep [41]. Also histomorphometric cortical bone parameters measured in diaphyseal cortical bone of the tibia of sheep 24 months after ovariectomy showed significant differences in bone cross-sectional area, medullary area, periosteal and endocortical perimeter [46,47]. Densitometric examination (DEXA) showed a BMD decrease of about 24% (P < 0.0005) in the lumbar column at the same experimental time [41]. Unfortunately, while bilateral ovariectomy certainly offers a very similar model to the clinical situation of postmenopausal osteoporosis, if not associated with a lowcalcium diet or the administration of corticosteroids a particularly long time is necessary for osteoporosis to develop [19,32,33]. Turner et al. [60] studied HA osteointegration in the distal femur of OVX sheep and observed a better osteointegration rate of a ceramic material in comparison with a metallic uncoated material 6 weeks after implantation. To our knowledge no other studies have been performed on biomaterial osteointegration on OVX sheep. Recently, the present authors performed experimental studies in 24-month OVX sheep to study vertebral pedicular and diaphyseal cortical bone fixation of HA-coated and uncoated metallic materials (stainless steel and a titanium alloy Ti6Al4V) [41,46,47]. The “Affinity Index” of uncoated screws implanted in vertebral pedicular bone of osteoporotic sheep was seen to be significantly decreased (–35%), while high values were observed for HA-coated screws also in osteoporotic animals [41]. In the case of cortical bone implants, areas where bone was not in direct contact with the screw surfaces were observed in osteoporotic animals. HA-coated Ti6Al4V screws showed a significantly higher osteointegration rate than uncoated screws in both healthy and OVX sheep. The same phenomenon was observed in the case of stainless steel screws where HA coating significantly increased osteointegration rate in osteoporotic sheep. In OVX animals the presence of HA caused a significantly higher osteointegration rate of both SSt and Ti6Al4V in comparison with uncoated Ti6Al4V and stainless steel [46,47]. From a biomechanical point of view, in pedicular vertebral bone, the uncoated screws evidenced the worst osteointegration rate in OVX animals (–22% in comparison with healthy animals). The presence of HA increased mechanical fixation significantly
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in both healthy and OVX animals, but a significant decrease in fixation was observed for HA-coated screws in OVX animals [41]. In cortical bone mechanical osteointegration of Ti6Al4V was significantly higher than that of SST in both healthy and osteoporotic bone [46,47].
[8]
[9]
[10]
5. Discussion and conclusion Osteoporosis seems to greatly affect the biological and mechanical fixation of many biomaterials already in clinical use for fracture fixation and joint prostheses. Even though a large number of experimental in vitro and in vivo studies have been carried out on orthopaedic biomaterial evaluation, almost all of them were performed on healthy cells and animals with an ideal bone metabolism and in the absence of metabolic and degenerative pathologies. The transferability of these research results to the clinical situation can be difficult due to the substantial difference between the relative experimental conditions (i.e. the osteogenenetic capacity of animal models and implanted patients). 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. Bone-derived cells from patients and animals with the most frequent orthopaedic pathologies, and the use of pathological animals, represent a further “bridge” to the clinical situation [14]. The available data on the different behaviour of the same biomaterials when tested under healthy and pathological conditions support this approach. For these purposes, knowledge of the adopted models and the preliminary characterization of cell behaviour and animal bone quantity and quality in healthy and osteoporotic conditions are necessary.
[11]
[12] [13] [14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
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Dossier: Osteoporosis
Osteoporosis and biomaterial osteointegration M. Fini a,*, G. Giavaresi a, P. Torricelli a, V. Borsari a, R. Giardino a,b, A. Nicolini c, A. Carpi d a
Experimental Surgery Department, Research Institute Codivilla Putti, Rizzoli Orthopaedic Institute, Via di Barbiano, 1/10, 40136 Bologna, Italy b Chair of Surgical Pathophysiology, University of Bologna, Italy c Department of Internal Medicine, University of Pisa, Pisa, Italy d Department of Reproduction and Ageing, University of Pisa, Pisa, Italy Received 29 March 2004 Available online 11 September 2004
Abstract 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. © 2004 Elsevier SAS. All rights reserved. Keywords: Osteoporosis; Biomaterial; Osteointegration; Cell cultures; Animal models
1. Introduction The increase in life expectancy, at least in western countries, means that an increasing number of patients submitted to surgical operations of osteosynthesis and joint replacement are affected by age-related, post-menopausal and other forms of secondary osteoporosis due to systemic diseases or pharmacological therapies. Since the mean age of life expectancy is now over 80 years, the majority of people outlive the quality of their connective tissues [26]. This consideration is particularly important considering that the natural consequence of osteoporosis is the development of pathological fractures in various bones, which may require the implantation of biomaterials. The osteointegration of biomaterial depends not only on the properties of the implanted biomaterial but also on the * Corresponding author. E-mail address: [email protected] (M. Fini). 0753-3322/$ - see front matter © 2004 Elsevier SAS. All rights reserved. doi:10.1016/j.biopha.2004.08.016
characteristics and regenerative capability of the host bone [37]. For this reason, researchers currently working on biomaterial evaluation place great importance on the different pathologies that are frequently present in orthopaedic patients and can negatively affect the success of surgical implants. Osteoporosis is undoubtedly one of the most frequently encountered diseases [13]. Even though much attention has been given to fracture prevention and new pharmacological and physical therapies aimed at maintaining a high level of bone mass, less attention has been directed at the study of orthopaedic biomaterial behaviour when implanted in osteoporotic bone. Bone regeneration is a complex process not yet completely understood, especially when osteoporosis is present [38]. Osteoporosis etiology (advanced age, hypogonadism, rheumatic diseases, alterations in thyroid and parathyroid function, malignancies) and therapies used to treat osteoporosis (estrogens, vitamin D, biphosphonates) can interfere with bone growth processes [38].
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Bone alterations due to age and estrogen deficiency are both structural and biological. Some experimental and clinical data suggest that osteoporosis negatively influences the healing processes after a fracture [31,38] and the osteointegration rate of prosthetic implants [6]. Models reproducing the osteoporotic condition can be useful to help understand the influence of the pathology on cell behaviour, bone regeneration and osteointegration processes, by increasing 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 ideal biomaterial should present the same biocompatibility and osteointegration characteristics when tested in healthy and pathological conditions. One of the main reasons for the still limited knowledge on bone regeneration and biomaterial osteointegration in the presence of osteoporosis may be the lack of idoneous and reliable models. It is recognized that it is extremely difficult to experimentally mimic, both in vitro and in vivo, the complex bone–biomaterial interactions and the decrease in osteoinductive capacity due to the loss of bioactive factors, bone morphogenetic proteins and mesenchymal staminal cells that accompany ageing and osteoporosis [14]. These alterations generally represent the main biological cause of implant failure irrespective of the high quality of the surgical device [4,9,23,28]. 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 an osteoporotic state are reported.
2. Bone regeneration and osteoporosis The principle mechanisms at the basis of osteointegration processes around implanted biomaterials are reported to be very similar to those occurring during fracture repair and involve a cascade of cellular and extracellular events [14]. In both osteointegration and fracture repair the healing process differs from that of soft tissues since it leads to regeneration of the damaged tissue after the implantation surgery or an injury. Bone is mainly composed of three kinds of cells (osteoblasts, osteocytes, osteoclasts), mineralized extracellular matrix (65%, hydroxyapatite and tricalciumphosphate), organic extracellular matrix (35%, collagen I and noncollagenous proteins such as osteocalcin, osteopontin and growth factors). Even if the growth factors of the organic extracellular matrix represent only about 1%, interest is being increasingly focused on this aspect in order to understand the pathophysyiological mechanisms of bone regeneration and osteointegration [7,12,49,51]. In fact, it is recognized that a predominant role is played by local factors such as cytokines and growth factors released in the lesion site [4].
The majority of researchers have focused their attention on a limited number of local factors such as interleukin 1 (IL-1), interleukin 6 (IL-6), tumor necrosis factor alfa (TNF-a), fibroblast growth factor (FGF), platelet derived growth factor (PDGF), transforming growth factor beta 1 (TGF-b1) and bone morphogenetic proteins (BMP) [2–4,7]. During the initial phases of reaction to trauma PDGF and TGF-b1 are released by activated platelets. Subsequently, macrophages and other inflammatory cells release FGF, PDGF, TGFb1 and probably also IL-1 and IL-6. During the first hours following the trauma numerous factors involved in lesion regeneration are therefore present and act on the bone, periosteal, medullar and adjacent soft tissue cells. Finally, it has now been demonstrated that mesenchymal staminal cells are the main instigators of regeneration processes, being present as early as 3 days after trauma and remaining at high levels of proliferation for the following days. In the ambit of naturally occurring bone regeneration processes, the presence of a biomaterial can greatly influence the early and late processes. Some authors have shown that superficial biomaterial topography can affect the rate of platelet activation, red cell aggregation, macrophage adhesion and activity, coagulum adhesion and retraction, vascularization, mesenchymal staminal cell and osteoblast adhesion, proliferation, synthetic activity and local factor composition and concentration [8,12,42,43]. Ageing and osteoporosis both modify cell proliferation, cell synthetic activity, cell reactivity to local factors, and mesenchymal staminal cell number [11,34,35,39,61]. These biological differences, in association with differences in biomechanical and microarchitectural bone properties, are involved in the reported increased failure risk of bone implanted biomaterial osteointegration in aged and osteoporotic bone. In fact, TGF-b1, IL-1, IL-6 are strictly involved in bone regeneration and biomaterial osteointegration processes. TGF-b1 is an important stimulator of osteointegration processes, while interleukins are present at the bone–biomaterial interface in the case of aseptical implant failure [1,2]. Some local factors have an important role in the pathogenesis of senile and post-menopausal bone loss because of their role as mediators of bone remodelling. The skeletal content of anabolic cytokines in bone tissue such as TGF-b1 seems to be significantly and negatively correlated with increasing age and post-menopausal osteoporosis [36]. On the other hand, the amount of IL-6, a catabolic cytokine involved in bone readsorption, increases in aged and post-menopausal osteoporotic patients [23,28]. Finally, the effect of IL-6 seems to be inhibited by estrogens and consequently, because of post-menopausal estrogen deficiency, the cytokine causes bone resorption and differentiation of osteoclast precursors in mature osteoclasts [45]. Bone mass decrease due to estrogen deficiency or to secondary osteoporosis has been correlated to osteoblast function and proliferation [10,50]. Taking into account basic and applied in vitro and in vivo research, the principle causes of increased failure risk of
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osteoporotic bone implanted biomaterials could be summarised as follows: (1) bone changes in structural and mechanical properties; (2) decreased number of mesenchymal staminal cells; (3) increase in osteoclast number and activity; (4) imbalance between anabolic and catabolic local factors acting on bone remodelling (growth factors and cytokines); (5) abnormal bone cell reactivity in proliferation rate and synthetic activity to systemic and local stimuli (hormones, growth factors and cytokines) and to mechanical stress.
3. In vitro studies Several authors have studied in vitro the behaviour of cells deriving from healthy and osteoporotic bone. Some of them have observed a decreased proliferation index of osteoblasts deriving from trabecular bone of osteoporotic women [35]. The reason for this decrease is still unclear but it could be due to a defective reactivity of cells to hormones and local factors. When studying in vitro cells deriving from healthy and osteoporotic patients, Lomri et al. [34] did not find any differences in cell reactivity to PTH, TGF-b1 and PGE2. Wong et al. observed that cells deriving from osteoporotic bone produce a significantly higher amount of alkaline phosphatase (ALP) if stimulated with vitamin D3. An abnormal behaviour of osteoporotic bone derived cells in comparison with healthy bone derived cells was suggested [61]. Neidlinger-Wilke et al. [39] studied the response to mechanical forces by investigating cell proliferation, ALP and TGFb1 synthesis. Cells deriving from healthy bone significantly increased proliferation rate and TGF-b1 production after the application of mechanical stress, while these effects were not observed in cell cultures deriving from osteoporotic bone. Torricelli et al. observed differences in proliferation rate, collagen I, osteocalcin (OC), TGF-b1 and IL-6 synthesis between healthy and osteoporotic human bone derived cells. These differences were significant after cell exposure to Vitamin D3 [54]. The same authors compared the in vitro behaviour of healthy and osteoporotic bone derived cells in rats. By measuring partial oxygen pressure in cultures they observed that cells derived from osteoporotic bone had an
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increased respiration rate causing a decrease of oxygen in the culture medium [16]. Regarding staminal mesenchymal cells, which are the main cells involved in bone regeneration, Rodriguez et al. [48] cultured cells from healthy and post-menopausal osteoporotic patients and observed in the latter a decreased growth rate, a minor response to mitogenic stimuli (i.e. Insulin-like growth factor I), and a decreased capacity of differentiation from mature osteoblasts as demonstrated by decreased ALP production and mineralization noduli formation in culture. Some authors used healthy and osteoporotic bone derived osteoblasts also for in vitro biomaterial testing [17,55–58] (Table 1). Torricelli et al. [57] using cells from healthy and osteoporotic rats, observed that a biological glass was not cytotoxic when tested in healthy bone derived osteoblasts, but caused a significant reduction of viability and OC level in osteoporotic bone derived cells [17]. The same authors compared the biological glass with titanium in cells deriving from healthy and osteoporotic sheep bone. Also in this case, osteoblasts derived from sheep osteoporotic bone showed an altered behaviour when cultured on the biological glass and in comparison with healthy bone derived cells, as far as Ca level, proliferation rate and osteocalcin production are concerned [56]. Other biomaterials tested under the same conditions, such as a nickel (Ni) reduced stainless steel and zirconia coated materials, behaved in a promising manner when tested in healthy and osteoporotic bone derived cells [55,58]. 4. In vivo studies Osteoporosis has been induced in various different laboratory animals using different methodologies: cortisonic administration, denervation, immobilization, absence of gravity, and surgical bilateral ovariectomy associated or not with low calcium diets. Bilateral ovariectomy is the method that best reproduces the clinical situation of post-menopausal osteoporosis and leads to the development of a systemic osteoporosis in both the peripheral and axial skeleton. The ovariectomized (OVX) rat is undoubtedly the animal most commonly used for studies on osteoporosis pathophysi-
Table 1 Main in vitro studies on biomaterials and osteoporotic bone derived osteoblasts Source of cells Osteoblasts from normal and OVX rats Osteoblasts from normal and OVX rats
Osteoblasts from normal and OVX sheep Osteoblasts from normal and OVX rats Osteoblasts from normal and OVX rats
Abbreviations are explained in the text.
Biomaterials Ti6Al4V two biological glasses (AP 40 and RKKP) Two biological glasses (AP 40 and RKKP) Ti6Al4V two biological glasses (AP 40 and RKKP) Ni reduced SSt ZrO2 and Al2O3 uncoated and coated with a biological glass (RKKP)
Main results Significant decrease of MTT and OC levels in osteoporotic bone derived cells cultured with AP 40 AP 40 caused a significant decrease of cell viability and proliferation and OC level in osteopenic bone derived cells not observed in normal bone derived cells Altered morphology, marked decrease of cell proliferation and OC level in osteopenic bone derived cells cultured with AP 40 No differences in cell behaviour between normal and osteopenic bone derived cells No differences in cell behaviour were observed in normal bone derived cells while RKKP-coated materials improved ALP levels in osteopenic bone derived cells
Refs. [17] [57]
[56] [55] [58]
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Table 2 Main in vivo tests on biomaterial osteointegration in osteoporotic bone Animal model Normal and OVX rats
Normal and OVX rats
Biomaterials Ti6Al4V, HA, ZrO2, Al2O3, two biological glasses (AP 40 and RKKP) Ti6Al4V, two biological glasses (AP 40 and RKKP) HA, Ti6Al4V
Rats submitted to sciatic nerve resection
HA, Al2O3
Normal and OVX rats
Normal, OVX and OVX plus neurectomi- HA, Ti6Al4V sed rats Normal and OVX rats
HA
Normal and submitted to OVX and OVX plus low Ca diet rabbits Normal, OVX and OVX plus low Ca diet rabbits Normal and steroid induced osteoporotic rabbits Normal and OVX sheep
Ti
Normal and OVX sheep
Normal and OVX sheep Normal and OVX sheep
Ti Ti Uncoated and HA coated CoCr alloy Uncoated and HA coated SSt
Uncoated and HA coated Ti6Al4V Uncoated and HA coated SSt and Ti6Al4V
Main results Refs. Significant decrease of the osteointegration rate for AP 40, ZrO2 [15] and Ti6Al4V in the OVX rats All biomaterials decreased the osteointegration rate in OVX rats; AP 40 osteointegrated only when implanted in normal rats Both biomaterials decreased the osteointegration rate in OVX rats (the decrease was significant for HA) Superior affinity of bone affected by disuse osteoporosis to HA compared with Al2O3 Ti6Al4V higher osteointegration rate in normal bone than in osteopenic bone; higher osteointegration rate of HA also in OVX plus neurectomised rats Slight decrease in bone contact in OVX rat cortical bone; significant decrease in bone contact in OVX rat cancellous bone Local GH administration ameliorated bone growth and mineralization around implants Formation of new bone around implants was delayed in osteopenic rabbits Significant lower mechanical attachment of Ti implanted in tibial bone but not in the mandible of osteopenic rabbits Improvement of biological fixation of HA coated implants more pronounced in the OVX sheep Significant decrease of biological osteointegration rate for uncoated SSt in OVX sheep; significant decrease of extraction torque for both uncoated and HA coated SSt in OVX sheep Significant decrease of biological and mechanical fixation for both biomaterials in OVX sheep femur Significant decrease of bone-to implant contact of uncoated materials versus HA-coated ones in OVX sheep; significant decrease of mechanical attachment of uncoated materials versus HA coated ones and of uncoated Ti6Al4V versus uncoated SSt in OVX sheep
[17] [18] [24] [25]
[44] [59] [37] [20] [60] [41]
[46] [47]
Abbreviations are explained in the text.
ology, diagnosis and therapy [29,38]. As shown in Table 2, some authors have also used the rat model for studies on biomaterial osteointegration in osteoporotic bone [15,17,18,24,25,44]. Because of the small dimension of the animal, cylindrical nails with a diameter of 2 mm are generally positioned in the trabecular bone of the distal femur or the proximal tibia [15,17,18,24,25,44]. Fini et al. [15] implanted nails made of a titanium alloy (Ti6Al4V), hydroxyapatite (HA), zirconia (ZrO2), alumina (Al2O3) and two biological glasses (AP 40 and RKKP) in the distal femurs of healthy and OVX rats. Histomorphometric analysis showed that the osteointegration rate of AP 40, ZrO2 and Ti6Al4V was significantly decreased in OVX rats compared to normal animals. When comparing the two different biological glasses and Ti6Al4V in the distal femur of healthy and OVX rats, 2 months after implantation, histomorphometric analysis showed the development of osteoporosis in the implanted site and a decrease in the osteointegration rate for the tested biomaterials in OVX rats as compared to healthy rats. The authors underline that one of the two biological glasses had an osteointegration index of about 55% in healthy rats and of only 4.1% in OVX rats [17]. The same researchers subsequently utilized the same animal model to study the behaviour of HA and Ti6Al4V. Both materials presented a decreased osteointegration rate in the presence of osteoporosis,
the decrease being significant in the case of HA [18]. Hayashi et al. [24] implanted nails made of Al2O3 and HA in the proximal tibia of healthy and osteoporotic rats and observed that HA implants maintained the same osteointegration rate in the presence of osteoporosis, while the Al2O3 nails significantly decreased the osteointegration index when implanted in osteoporotic rarefacted bone. The same authors subsequently compared HA and Ti6Al4V nails implanted in the proximal tibia of healthy, OVX and OVX plus sciatic nerve injured rats. The osteointegration rate of Ti6Al4V in healthy bone was higher than in osteoporotic bone. On the contrary, in the case of HA implants a good osteointegration level in the absence of a fibrous capsule was maintained in OVX animals also submitted to sciatic nerve injury [25]. Pan et al. [44] implanted HA coated nails in the tibial metaphysis of healthy and osteoporotic rats and observed a significant reduction of trabecular bone volume around implants, while the decrease was less evident in the cortical bone. Studies on small-size animals like the rat are correct from an ethical and scientific point of view and are acceptable in the first phases of a research, but because of differences due to rat bone quality as compared to human beings and of the small dimension of the animal that allows only miniaturized implants differing from clinically used implants, it is necessary in a second phase of the in vivo research to adopt a
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further experimental model using animals with a higher phylogenetic level [3,12]. Also according to the “International Standard Organization” [27] for orthopaedic biomaterial biocompatibility and osteointegration evaluation the rat is not considered as an experimental model for skeletal implantation and only rabbits, sheep and dogs are suggested. Bilateral ovariectomy associated with a low-calcium diet (0.07% calcium content) in rabbits was utilized by Tresguerres et al. [59] to evaluate the local effect of growth hormone (GH) on metallic biomaterial osteointegration in the osteoporotic tibia. Results showed that local GH administration in the implanted site could ameliorate the periosteal and cortical reaction and mineralization of osteoid tissue 14 days after implantation [59]. Bilateral ovariectomy plus low calcium and phosphorus diet (0.15% calcium, 0.59% phosphorus content) was used by Mori et al. [37] to study the bone–implant interface in osteoporotic rabbits. One month after ovariectomy and diet administration, and when tibia bone mineral density (BMD) showed a 12% decrease as compared to healthy rabbits, a metallic implant was inserted in the distal tibia. In healthy rabbits, new bone formation was evident around implants at 15 days and osteointegration processes were completed at 8 weeks. In the osteoporotic animals, however, bone formation around implants was delayed, even though at 12 weeks a good bone–biomaterial contact was observed. Following the experience of these authors, it is possible to deduce that the presence of osteoporosis can alter tissue reparative processes around implants, even though osteointegration has sometimes been achieved also in the presence of this pathology [37]. Fujimoto et al. used a model of osteoporosis secondary to corticosteroid administration in rabbits where metallic implants were inserted in both the proximal tibia and jaw. Mechanical osteointegration measurement evidenced a significant decrease in extraction torque values in the case of tibial implants in prednisolone treated rabbits, while no differences were observable between the two animal groups as far as the jaw implants were concerned [20]. Before 1994 the sheep was seldom used in experimental studies regarding osteoporosis or other skeletal pathologies. As this large size animal model offers many advantages, it has increasingly been used in orthopaedic scientific research over the last 10 years. Recent research has suggested that the sheep is a promising model for osteoporosis studies and, because of its dimension and bone characteristics, is also suitable for the evaluation of biomaterials [5,40,52]. Bone mineral loss has been observed in sheep after bilateral ovariectomy and it has also been reported that trabecular sheep bone is qualitatively similar to the human bone, thus permitting more reliable researches on bone remodelling [30]. To this regard, Torricelli et al. [53] compared in vitro osteoblast proliferation and synthetic activity of osteoblasts deriving from trabecular bone of humans, rats and sheep and observed that sheep osteoblasts were more similar to human osteoblasts as far as viability and osteocalcin, IL-6 and TGFb1 production are concerned.
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After bilateral ovariectomy a decrease in trabecular and cortical bone mass has been observed in the vertebral bodies, iliac crest and femural and tibial diaphysis of sheep [19,21,22]. The development of an osteoporotic state was observed 12 months after ovariectomy and was even more evident at 24 months [19]. In detail, at 24 months a significant decrease in bone volume (BV/TV), trabecular thickness (Tb.Th) and trabecular number (Tb.N) associated with a significant increase in trabecular separation (Tb.Sp) were observed in the vertebral body [22]. In the vertebral pedicular bone of sheep, 24 months after ovariectomy, both BV/TV and Tb.Th were significantly decreased in comparison with sham-operated sheep [41]. Also histomorphometric cortical bone parameters measured in diaphyseal cortical bone of the tibia of sheep 24 months after ovariectomy showed significant differences in bone cross-sectional area, medullary area, periosteal and endocortical perimeter [46,47]. Densitometric examination (DEXA) showed a BMD decrease of about 24% (P < 0.0005) in the lumbar column at the same experimental time [41]. Unfortunately, while bilateral ovariectomy certainly offers a very similar model to the clinical situation of postmenopausal osteoporosis, if not associated with a lowcalcium diet or the administration of corticosteroids a particularly long time is necessary for osteoporosis to develop [19,32,33]. Turner et al. [60] studied HA osteointegration in the distal femur of OVX sheep and observed a better osteointegration rate of a ceramic material in comparison with a metallic uncoated material 6 weeks after implantation. To our knowledge no other studies have been performed on biomaterial osteointegration on OVX sheep. Recently, the present authors performed experimental studies in 24-month OVX sheep to study vertebral pedicular and diaphyseal cortical bone fixation of HA-coated and uncoated metallic materials (stainless steel and a titanium alloy Ti6Al4V) [41,46,47]. The “Affinity Index” of uncoated screws implanted in vertebral pedicular bone of osteoporotic sheep was seen to be significantly decreased (–35%), while high values were observed for HA-coated screws also in osteoporotic animals [41]. In the case of cortical bone implants, areas where bone was not in direct contact with the screw surfaces were observed in osteoporotic animals. HA-coated Ti6Al4V screws showed a significantly higher osteointegration rate than uncoated screws in both healthy and OVX sheep. The same phenomenon was observed in the case of stainless steel screws where HA coating significantly increased osteointegration rate in osteoporotic sheep. In OVX animals the presence of HA caused a significantly higher osteointegration rate of both SSt and Ti6Al4V in comparison with uncoated Ti6Al4V and stainless steel [46,47]. From a biomechanical point of view, in pedicular vertebral bone, the uncoated screws evidenced the worst osteointegration rate in OVX animals (–22% in comparison with healthy animals). The presence of HA increased mechanical fixation significantly
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in both healthy and OVX animals, but a significant decrease in fixation was observed for HA-coated screws in OVX animals [41]. In cortical bone mechanical osteointegration of Ti6Al4V was significantly higher than that of SST in both healthy and osteoporotic bone [46,47].
[8]
[9]
[10]
5. Discussion and conclusion Osteoporosis seems to greatly affect the biological and mechanical fixation of many biomaterials already in clinical use for fracture fixation and joint prostheses. Even though a large number of experimental in vitro and in vivo studies have been carried out on orthopaedic biomaterial evaluation, almost all of them were performed on healthy cells and animals with an ideal bone metabolism and in the absence of metabolic and degenerative pathologies. The transferability of these research results to the clinical situation can be difficult due to the substantial difference between the relative experimental conditions (i.e. the osteogenenetic capacity of animal models and implanted patients). 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. Bone-derived cells from patients and animals with the most frequent orthopaedic pathologies, and the use of pathological animals, represent a further “bridge” to the clinical situation [14]. The available data on the different behaviour of the same biomaterials when tested under healthy and pathological conditions support this approach. For these purposes, knowledge of the adopted models and the preliminary characterization of cell behaviour and animal bone quantity and quality in healthy and osteoporotic conditions are necessary.
[11]
[12] [13] [14]
[15]
[16]
[17]
[18]
[19]
[20]
[21]
[22]
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