Synergistic effects of bisphosphonate and calcium phosphate nanoparticles on peri-implant bone responses in osteoporotic rats

Abstract

The prevalence of osteoporosis will increase within the next decades due to the aging world population, which can affect the bone healing response to dental and orthopedic implants. Consequently, local drug targeting of peri-implant bone has been proposed as a strategy for the enhancement of bone-implant integration in osteoporotic conditions. In the present study, an established in-vivo femoral condyle implantation model in osteoporotic and healthy bone is used to analyze the osteogenic capacity of titanium implants coated with bisphosphonate (BP)-loaded calcium phosphate nanoparticles (nCaP) under compromised medical conditions. After 4 weeks of implantation, peri-implant bone volume (%BV; by μCT) and bone area (%BA; by histomorphometry) were significantly increased within a distance of 500 μm from implant surfaces functionalized with BP compared to control implants in osteoporotic and healthy conditions. Interestingly, the deposition of nCaP/BP coatings onto implant surfaces increased both peri-implant bone contact (%BIC) and volume (%BV) compared to the deposition of nCaP or BP coatings individually, in osteoporotic and healthy conditions. The results of real-time PCR revealed similar osteogenic gene expression levels to all implant surfaces at 4-weeks post-implantation. In conclusion, simultaneous targeting of bone formation (by nCaP) and bone resorption (by BP) using nCaP/BP surface coatings represents an effective strategy for synergistically improvement of bone-implant integration, especially in osteoporotic conditions.

Introduction

The use of bone implants in dental or orthopedic rehabilitation is generally successful as reflected by 10-year survival rates of 95% in healthy patients [1]. In the clinic, however, the aging patient population challenges the use of implants due to general as well as oral health issues in modern societies. For instance, osteoporosis, a common systemic bone disease, develops with age and is more prevalent in women and men aged above 50 years [2]. The prevalence of osteoporosis has been reported to increase up to 70% in patients at 80 years old. Worldwide, osteoporosis affects approximately 200 million people and the national osteoporosis foundation (NOF) estimates that over 40 million people in the USA already have osteoporosis or are at high risk in 2020 [3], [4]. Osteoporosis is characterized by a severe decrease in bone mass and alteration of trabecular bone microstructure due to an imbalance between bone resorption (by osteoclasts) and bone formation (by osteoblasts) [5]. Further, it has been shown that bone healing in osteoporosis is impaired and the biological activity of bone cells is negatively influenced [6]. For bone implant treatment in osteoporotic patients, the osteoporotic condition impedes primary stability, biological fixation and final osseointegration [7]. As such, the application of bone implants in osteoporotic patients remains a clinical challenge in dental and orthopedic surgery.

For successful implant osseointegration, the bone-implant interface has to interact optimally with the bone tissue in the implant vicinity. For medically healthy patients, new bone (i.e. woven bone) is formed directly in contact with the implant surface by osteoblastic cells, where after it transforms into mature bone [8]. This interaction can be improved by implant-related factors, such as implant design, surgical technique, and osteophilicity of the implant surface [9]. Because the implant surface directly interacts with bone tissue, a variety of surface modifications have been explored [10]. The currently available surface modifications aim to combine advantages of physical properties (e.g. roughness) with bioactive cues (i.e. bone-bonding) to improve implant integration [11]. Over the past two decades, calcium phosphate (CaP) coatings have demonstrated to favor the healing response to the implant surface and hence to enhance peri-implant bone formation [12], [13]. The presence of a CaP coating at the implant surface is anticipated to facilitate colonization by mesenchymal precursor cells and upregulate specific gene expression in the vicinity of the implant [14]. Recently, it was shown that electrostatic spray deposition (ESD) enables the functionalization of implant surfaces with CaP nanoparticles (nCaP) that mimic the mineral component of natural bone [15]. An additional advantage of ESD is that it enables the deposition of nCaP in combination with organic biomolecules such as collagen proteins, growth factors, peptides, or other therapeutic agents [16]. As a consequence, a novel generation of therapeutic implant coatings can be synthesized that instruct bone cells by releasing drug molecules locally around the implant surface, especially for compromised conditions such as osteoporosis [17], [18]. In clinical practice, the deposition of nCaP in combination with therapeutic implant coatings might have a dual effect during bone-implant integration, and their concomitant use may offer a simultaneous targeting of peri-implant bone anabolic/catabolic processes. Particularly bisphosphonates (BPs) are appealing for such purpose, as their therapeutic function to inhibit osteoclast proliferation and activity can be exploited for local effects in the vicinity of an implant surface toward improved implant fixation [19]. In view of this, the idea for therapeutic nCaP/BP coatings for bone implants represents an appealing approach to improve bone responses and implant integration in osteoporotic conditions.

The present study aimed to evaluate the efficacy of an ESD-derived nCaP/BP coating on peri-implant bone response in osteoporotic as well as healthy conditions using an established rat femoral condyle implantation model [20], [21]. At 4 weeks post-implantation, histological, histomorphometrical, and microcomputed tomography (μCT) were performed. In addition to conventional histological analysis, we also performed real-time polymerase chain reaction (RT-PCR) after 4 weeks of healing to evaluate osteogenic gene expression in the peri-implant bone in osteoporotic and healthy conditions.