Osseointegration of osteoporotic bone implants: Role of stem cells, Silica and Strontium - A concise review

Abstract

Osteoporotic fracture treatment has become a skeletal reconstructive challenge due to accelerated bone turnover and impaired bone regeneration potential. Poor osseointegration ability of the osteoporotic bone usually results in implant pull out and failure. Adoption of conventional bone fracture treatment strategies like autografts and allografts have limited applications in such pathological conditions. Hence biomaterials functionalised with therapeutic ions or drugs may be adopted to aid the delivery of therapeutic factors at the defect site to promote bone healing and implant integration, towards functional restoration of the fractured bone. This concise review narrates on improving the osseointegration ability of biomaterials using functional ions like Silica and Strontium. Silica based bone substitutes are known to promote bone healing in non pathological conditions. Further, Strontium based drugs show significant effects in the prevention and treatment of osteoporotic bones. In addition, stem cell therapy has become the focus of orthopaedic research attributed to its ability to restore and accelerate the bone healing process, but the clinical application of stem cells in osteoporotic condition is scarce. Present review suggests a novel strategy of combining the therapeutic potential of functional ions like Silica, Strontium and stem cells within a single implant unit to facilitate osseointegration and osteogenesis, so as to reduce the chances of implant rejection/pull out and encourage osteoporotic bone re-union.

1. Introduction

Osteoporosis is a condition of skeletal fragility characterized by weak bone mass and increased fracture susceptibility.¹⁹ Osteoporotic bones suffer a reduction in bone mineral density (BMD) as a consequence of the increased bone turnover by the osteoclast cells. With increase in bone turnover, crystallinity of Hydroxyapatite (HA) phase in the bone increases and there is a decrease in the acid phosphate level.⁷ These micro architectural changes lead to decrease in the cross linking efficiency and decreased tolerance of the bone towards mechanical stress, thereby pre-disposing the bone to fragility fractures. In most cases such fractures do not heal and increases the incidence of further fractures in the vicinity of bone by 2–3 times. The increasing number of elderly population prone to fragility fractures enlarges the socioeconomic burden of osteoporotic fractures.

2. Osteoporotic bone healing pathology

Osteoporotic fracture healing has become an orthopaedic challenge because of the impaired healing ability and poor osseointegration ability. Compositional difference in vitamin D and non availability of growth factors contributes to delayed regeneration potential. Estrogen also plays a dominant role by stimulating RANK L production which in turn leads to the activation of osteoclasts and bone resorption (Venken et al., 2008). In men, testosterone plays a crucial role in maintaining BMD. Decline in Parathyroid hormone (PTH) also results in decreased Calcium (Ca) absorption and there by decrease in BMD.

During fracture repair Mesenchymal Stem Cells (MSCs) residing in the bone marrow are recruited to the fracture site and their interaction with the residing cells along with the local niche determines the bone healing efficacy.¹² In adult or young individuals red marrow forms the major part of bone marrow. But in osteoporotic condition, marrow becomes populated with adipose cells resulting in white marrow. A decrease in the proliferation and osteogenic differentiation potential of MSCs residing in the bone marrow micro-environment has also been reported with the onset of osteoporosis.⁴

3. Osteoporotic fixation strategies - improving osteogenesis and osseointegration

High rate of implant failure have been reported in osteoporotic patients. Clinical reports suggest that inherent weak bone architecture and compromised osteointegrative ability is the major cause of increased implant pull out and non-union.⁴⁰ Osteoporosis also affects previously osseointegrated implant leading to implant pull out, as evident from the implantation studies in rat osteoporotic models.²⁶ Research on osteoporotic fracture fixation have so far focused mainly on regulating bone turnover by systemic drug administration along with implant fixation. But its application is limited attributed to the high dose and prolonged drug administration period required. Implants used in osteoporotic fractures should not only provide a mechanical support but also need to address the underlying pathology of the weak bone to favour bone regeneration. Therefore therapies that can aid osseointegration, osteogenesis with the simultaneous ability to regulate bone remodeling ability may prove to be beneficial for osteoporotic bone reconstruction.

3.1. Drugs and osteoporotic fracture fixation

Majority of the osteoporotic drugs exert their role by either inhibiting bone resorption or favouring bone formation, thereby stabilizing the fractured bone site. Bisphosphonates form the major class of drugs that regulate the hyper activity of the osteoclasts. Other drugs include selective estrogen receptor modulators (SERMs), calcitonin, bisphosphonates, strontium ranelate and denosumab, all of which function as anti resorbers.^5,15 In contrast to the catabolic drugs, anabolic drugs that favour bone formation include PTH analog and teriparatide. But for the drugs to be effective long term systemic administration is required. Long term administration of most of these drugs have been often associated with side effects such as osteonecrosis, incidence of atypical fractures, thromboembolism and increased cardiovascular risk.³⁹ So a better strategy may be to deliver these drugs locally at the defect site with the aid of implants.

3.2. Hydroxyapatite based scaffolds

Hydroxyapatite based scaffolds is considered as an optimum scaffold material for orthopaedic applications because of its chemical similarity to human bone and excellent bio-activity.²⁵ Implant coating with HA has proven to improve osseointegration ability of inert metals. Self setting calcium phosphate cements (CPC) have also been used extensively as bone void fillers in osteoporotic fractures.¹³ Even though HA is highly osteoconductive, it is devoid of any intrinsic osteoinductive property and hence incorporation of growth factors or cells may prove to be beneficial. HA based scaffold may be also tailored with ions like Magnesium , Zinc, Silica, Flourine, Chlorine, Strontium etc as it possess variations in its lattice and morphology.³³ Therefore incorporation of additional bone regulatory agents/biologics with anti-resorptive and osteogenic ability may help in molding HA as a better scaffold to suit osteoporotic applications.⁴⁵

3.2.1. Incorporation of strontium in HA

Strontium (Sr) has gained interest for osteoporotic applications as Sr ions enhance osteoblast proliferation and simultaneously reduce osteoclast activity.²⁹ Sr prevents the formation of ruffled border by the pre-osteoclast cells, thus preventing it from maturation and limits its resorption ability.⁶ Sr also up regulates Ca sensing receptor activity by raising Ca concentration in the micro environment, thereby promoting osteogenesis by the osteoblast cells.⁸ Improved osteogenic ability (ALP activity) of Sr doped calcium phosphate scaffolds has been observed using rat OB sarcoma cells.³⁶ Beneficial effects of Sr incorporated HA on bone mass at the bone implant interphase has also been reported in healthy rabbit segmental bone defect model.³¹ Studies suggest a daily intake of 2mg/per kg body weight of strontium ranelate to treat osteoporosis systemically.⁵¹ Recently Strontium ranelate has reported to exhibit adverse affects including skin rashes and deep vein thrombosis in osteoporotic patients¹⁶ and hence disapproved for systemic treatment. But recent studies have proven that local delivery of Sr at very low dose at the fracture site aid osteoporotic bone healing^24,43 proving the clinical therapeutic potential. As very low dose is used, less side effects are expected compared to oral administration. We have already proven the safety and efficacy of 10%Sr loaded HA implants in rat⁹ and sheep¹⁰ osteoporotic models respectively.

Accepted low dose of strontium is less than 4 mmol Sr/kg/day.²⁹ On the contrary Quade et al. reported that accumulated Sr release over the course of 28 days reached 141.2 μg (∼27 μgmg⁻¹) from Sr50 and 266.1 μg (∼35 μgmg⁻¹) from Sr100, respectively and it has an osteo-anabolic effect.³⁷ 1–10% is the suggested optimum concentration for osteoporotic applications so as to improve in vitro osteoblast differentiation.⁴⁹ Chemical similarity between Sr and Ca helps in the development of Sr incorporated Hydroxyapatite (SrHA) scaffolds. 10% SrHA scaffold release 0.11 mM Sr ions/mg sufficient enough to exert therapeutic role in osteoporotic bone healing.⁹ 0.1 mM Sr is proven not to inhibit osteoclastogenesis but significantly attenuate osteoclastic substrate resorption to favor bone remodeling.⁴¹ Lower single-dose local administration of bone morphogenetic protein-2 and Sr modified CPC had an additive effect on bone healing in osteoporosis rats.⁴³ Since HA is osteoconductive in nature, Sr incorporation in HA would help in developing a HA scaffold with improved osteointegrative ability. Simultaneously incorporating the anti-resorptive ability of Sr to suit osteoporotic applications.

3.2.2. Incorporation of Silica in HA

Silicon (Si) is a trace element found in the bone and is found to play an important role in bone in-growth and osseointegration. Studies on varying the Si concentration showed that orthosilicic acid at physiological concentrations (0–50 μM) could stimulate differentiation of human osteoblast-like cells.³⁸ Si coated surfaces induced a much earlier bone nodule formation in vitro indicating its improved bone bonding ability.⁵⁰ Ability of Si to differentiate osteoprogenitors to mature osteoblast and thereby favour bone regeneration and osseointegration has also been reported.⁴⁴ The proposed mechanism of improved bioactivity is attributed to the generation of elemental silicon which directly stimulates the differentiation and proliferation of bone forming osteoblasts.³⁵

Addition of dopants like Si into tricalcium phosphate ceramics is reported to induce osteoinductive properties in HA based scaffolds.¹⁷ Si incorporation in HA is reported to increase the dissolution property and thereby promotes bone apposition in vivo.²¹ A recent study has signified the improved biocompatibility and osteoinductivity of Silica and HA incorporated PLGA composite in rabbit bone defect model.²⁰ Recent study by Ishikawa et al., reported that Silica incorporated scaffolds exhibited osseointegration through the apparent recruitment of mesenchymal stem cells which differentiated into osteoblasts.²³ Together with the increased dissolution property, osteoconductive and osteointegrative ability - Silica incorporation in HA may aid osteoporotic bone defect healing and bone-implant bonding.

Recent studies have also reported the inhibitory role of Si at a higher concentration (200–500 μM) on RAW_264.7 cells – which are the osteoclast precursor cells,.¹⁴ Formation of very few osteoclast cells with distorted morphology on 1.2% HASi, post 14 days of culture substantiates the inhibitory effect of Si on osteoclast cells.¹⁸ However, not much is known about the mechanism and further research may be warranted in this front.

3.3. Stem cells and osteoporosis

Majority of studies on the pathophysiology of osteoporosis have focused on regulating the hyperactivity of osteoclast cells and very few studies have explored the differential metabolic activity of the bone forming - osteoblast and osteoprogenitor cells. There is a proportional relationship between the number of circulating osteoprogenitor cells and BMD,³⁴ which depicted the therapeutic potential of the local delivery of osteogenically induced cells at the fracture healing site to aid osteogenesis.

Only recently, the potential application of stem cells in osteoporotic bone healing has been explored and hence only very few studies reported the possibility of stem cell therapy for osteoporotic applications. Stem cell therapy in osteoporotic fractures is expected to augment BMD and reduce fracture susceptibility by increasing the number and proliferation potential of resident stem cells.⁴² The significance of mode of infusion of stem cells has been reported in the therapy for osteoarthritis.¹ But it has also been documented that systemic administration of autologous or allogenic stem cells in single or multiple doses do not have much significant influence in improving the bone density in osteoporotic rat models.²² The study also suggested localised delivery of stem cells at the defect site or usage of reprogrammed stem cells to have more profound effect in vivo.

Stem cells can be procured from sources like bone marrow, adipose, umbilical cord, embryo etc. Recent studies have shown that umbilical cord blood derived stem cells (UCBSCs) and circulating stem cells hold distinct therapeutical potential in osteoporotic applications as they can be easily harvested without much morbidity, possess less immunogenic potential and exhibit strong differentiation potential.⁵² Application of human UCBSCs could effectively prevent OVX-mediated bone loss in nude mice.³ Stem cells derived from dental pulp (DPSCs) and from that of exfoliated teeth have also proved to have strong application in orthopaedics,² but the feasibility of their application in osteoporotic condition has to be validated.

Out of all the sources, the highest osteogenic regeneration potential is expected from the adult MSCs; especially bone marrow derived mesenchymal stem cells (BMSCs). It has been reported that Platelet Rich Plasma (PRP), when combined with MSCs significantly improved bone healing in rat osteoporotic model by 42 days,⁴⁶ supported by the up regulation in the expression of RUNX2, OSX, and OPN, which are directly involved in osteogenesis. But the differentiation potential and the overall number of BMSCs has found to be inversely proportional with advancing age and osteoporosis. Conversely, adipose derived mesenchymal stem cells (ADMSCs) are emerging as a more reliable source, as ADMSCs can be easily isolated in abundance without much morbidity and produce larger yield in vitro. The in vitro expansion time required is also less compared to BMSCs.²⁷ Various studies have evaluated the functionality of ADMSCs from young and aged mice and found aging to exert minimum effect on in vitro proliferation and differentiation ability.¹¹ ADMSC transplantation has promoted osteogenesis and improved bone mineral density in osteoporotic rabbit model by 12 weeks.⁴⁷ Osteogenically differentiated cells create a cellular environment at the implant site, not only for promoting new bone formation but also for recruiting more native stem cells through paracrine secretions at the implant site to hasten the bone healing process.

3.4. Tissue engineered functionalised scaffolds – next generation scaffolds for osteoporotic bones

Research in orthopaedic reconstructive surgery has been focused on the development of alternatives that exactly mimic the natural bone in terms of function, structure and cellular environment. Knowing the pathophysiology of osteoporotic fractures it would be legitimate to assume that stem cells when seeded on the HA scaffolds invoke in them an osteoinductive nature, which is lacking and very much essential for osteoporotic bone regeneration. Stem cells differentiate along the osteogenic lineage when seeded on hydroxyapatite containing scaffolds and the clinical application of tissue engineered hydroxyapatite based scaffolds are reported.³⁰ Our previous studies have demonstrated the osteogenic efficacy of in-house developed ceramic scaffolds loaded with osteogenically differentiated MSCs in rat rabbit^31,53 and goat.³²

Stem cell therapy for osteoporosis itself is in the infant state and hence not many studies have explored the potential of tissue engineered HA scaffolds for osteoporotic applications. Delivery of mesenchymal stem cells (MSCs) from PLGA microspheres enhanced bone regeneration in trabecular bone defects in OVX rats.⁴⁸ A very recent study proved the osteogenic efficacy of HA scaffolds loaded with BMSCs genetically modified for osteoprotegerin (OPG). OPG delivery at the defect site regulated osteoclast resorption and simultaneously BMSCs on HA scaffolds promoted osseointegration and bone regeneration in rat osteoporotic model.²⁸ Our group could successfully prove the therapeutic potential of HA seeded with ADMSCs for promoting osteoporotic bone healing in sheep bone defect model¹⁰. Understanding the clinical significance of Silica, Strontium or stem cells, adopting a multifactorial approach of seeding MSCs on HA scaffolds functionalised with Si or Sr or both as a single implant prove to be more beneficial for osteoporotic bone healing. This “Combination Product” may open up new research avenues in the field of osteoporotic skeletal reconstructions.

4. Conclusions

The key research challenges in osteoporotic fracture healing are - osseointegration of implants and impaired host bone regeneration potential. While adopting the tissue engineering strategy, the cellular part of the scaffold helps in bone regeneration and the functionalised scaffold would assist in improving osseointegration, thus maintaining the contour and aesthetics of the fractured bone. Tissue engineering based approach may be particularly relevant for aged osteoporotic patients for whom the number and renewal capability of osteoprogenitors are low. The concept of localised delivery of combination therapies at the defect site may find enormous clinical applicationsas well as beneficial in enhancing functional recovery of fractured osteoporotic bones.

Declaration of conflicting interests

The author(s) declare no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.