Introduction

The term “Osteology” was introduced in 1594 by Barthelemy Cabrol, a French anatomist, in his work “Alphabet Anatomic”. In 2000, the principles of “osteoimmunology” were introduced by Arron and Choi [1]:  the immune system is involved in bone synthesis and remodeling cascades, where it controls bone formation, regulates bone resorption and acts as a key factor in bone homeostasis [2,3]. Inversely, immune cells functions are influenced by the bone system [4]. It has been concluded that bone system and immune system are unified into a single entity: the osteo-immune system [5].
One of the consequences of an impaired immune response is the delay or failure of osseointegration [10]. Bone management cannot be achieved without an efficient immune response. Bone loss around implants (early or late) should be regarded as an immune mechanism.
Moreover immune cells and factors are the key regulators in wound healing and contribute to the early stages of angiogenesis. [6,7].

Inflammation: the key word

Osseointegration and wound healing begin with an immune reaction: inflammation. which is always beneficial through the activation of the cells repair genes.
In a physiological way, inflammation should end before the 5th day by the action of the antioxidants [8]. While inflammation is always beneficial during the first days, its persistence after the 5th day will create a harmful situation, with increase of oxidants production, leading to a deficient immune response. When the response is deficient, wound healing and bone formation are compromised, as osteoblasts are inhibited and osteoclasts activated. Thus, bone loss becomes mandatory, through the failure of bone remodeling. A defective immune system leads to a delayed or failed osseointegration, a rise in infections incidence and fibrosis. Fibrosis and connective tissue growth are consequences of a long-term inflammation [9].

Oxidants, antioxidants & oxidative stress

Oxidation-reduction (redox) reactions occur constantly in living creatures. Most take place inside the mitochondria, which continuously produce endogenous free radicals, also known as oxidants or ROS for Reactive Oxygen Species. ROS are a highly reactive group of molecules, by leakage in their outer shell of a single impaired electron. They are generated as by-products of aerobic metabolism [10,11]. At low concentrations, ROS are beneficial as they serve as signaling molecules to activate specific physiological pathways, controlling numerous life processes [12,13]. At higher concentration, they interfere with lipids, lipoproteins, proteins and nucleic acids, creating both molecular and cellular damages.

Besides, the human body balances oxidants with antioxidants. Antioxidants are low weight molecules able to counteract oxidation, either by preventing ROS formation, or scavenging/neutralizing ROS by providing the missing electron. Antioxidants can be produced endogenously or be provided by exogenous oral supplementation. One of the main benefit of antioxidants is their ability to down-regulate pro-inflammatory cytokines, reducing inflammation without side effects.

Antioxidants production is mostly controlled by Nrf2 (Nuclear Factor Erythroid-2-Related Factor 2), a transcription factor regulating over 500 human genes [14], which mainly controls damage repair processes and reduces inflammation by increasing antioxidants production. Recent studies have highlighted the crucial role of Nrf2 in osteoporosis, osteogenic stem cells differentiation and bone fracture healing [15,16]. Conversely, impaired Nrf2 nuclear translocation is associated with delayed cortical bone healing [17].

The term “oxidative stress” appeared in the medical literature in 1985 [18]. It is defined when oxidants and antioxidants levels are imbalanced and ROS production exceeds cellular antioxidant defenses [19,20]. Free radicals are produced in quantity under several conditions including tissue damage repair, pollutant exposure, unbalanced diet, anxiety, smoking or chronic inflammation. Oxidative damage to bio-macromolecules plays an active role in the etiology of a wide variety of acute and chronic diseases [21]. It plays a central role in the acceleration of the ageing process and in osteoporosis [22-24]. It has been proved to deteriorates the correct immune system functioning [25] and to increase the occurrence rate of numerous and diverse pathologies, such as cancer, arthritis, cardiovascular diseases, neurodegenerative troubles and may induce uncontrollable autoimmune illnesses [26-30].
Antioxidants, on the other hand, have opposite effects: they enhance mineralization processes and reduce the osteoclastic activity, either directly or by counteracting oxidants effects [31]. 

Oxidative Stress and Bone

Bone remodeling follows a complex cycle that lasts for approximately six months. Under normal circumstances, the cycle is regulated by cytokines, growth factors and hormones [32]. Elevated amounts of free radicals in osteoblasts inhibit their functions [33,34] and lead to apoptosis of both osteoblasts and osteocytes [35]. ROS also play a crucial role in osteoclast differentiation and function [36,37]. Excessive osteocytes apoptosis and increased osteoclastogenesis counteract osteogenesis and mineralization. As the bone remodeling turnover increases, bone loss is facilitated.

Osteoimmunology

Osteoimmunology is based on the strong connections found between the bone and immune systems [38].  Immune and bone cells have a conjoined heritage in stem cells: they share signaling pathways and influence each other permanently [39]. Immune system controls bone and its resorption and thus acts as a key factor in bone homeostasis [2,3].

In periodontium, microbial dysbiosis alone is not sufficient to trigger periodontitis. Immune cells, by activating periodontal ligament cells, enable the expression of molecules including RANKL, and thus osteoclastic activity [40]. Indeed, activation of non-specific immune defenses will provoke an inflammatory reaction. T-lymphocytes have been identified as the cells responsible for bone renewal, both in periodontitis and in other pathologies [41,42]. They actively engage macrophages and then RANKL via osteoclasts. Patients with periodontitis, compared with healthy patients, have T-lymphocytes overexpressing RANKL and TNF alpha: this overexpression is responsible for osteoclastic activity and associated bone loss [43].

Neutrophils also play a role in the production of free radicals, in RANKL expression and in the progression of chronic inflammation, not just acute inflammation as has long been recognized [44].
Conversely, immune cells and factors are also key regulators of wound healing and contribute to the early stages of angiogenesis [45,46].

After a bone fracture, immune cells, especially macrophages, are found throughout the healing process, organizing the body’s defense against pathogens [47]. Fracture healing starts through an inflammatory reaction, induced by the release of leukocyte inflammatory cytokines, predominantly angiogenic [48]. After angiogenesis onset, the granulation tissue, also named “soft bone callus”, is formed [49]. However, two conditions are required for this cascade of events to occur: a sufficient amount of antioxidants, and a proper activation of repair genes [50].

Oxidative stress and immune deffiency in oral surgery

Inflammation is still the key phenomenon. Oxidation systematically follows long term inflammation, which can result from both local or systemic conditions. 
Elevated ROS levels, either associated to metabolic diseases, chronic ischemia or inflammation, damage immune cells and thus compromise immune functions, affecting bone formation and maintenance.

Oxidative Stress following systemic inflammation

Globally, chronic inflammatory diseases (autoimmune diseases, diabetes, allergy, obesity, Human Immunodeficiency Virus ect..) are responsible of systematic oxidative stress.

- Diabetes, as a chronic inflammatory disease [51], induces oxidative stress through hyperglycemia [52]. Diabetics healing issues, periodontal fragility and systemic complications (including nephropathy, neuropathy, angiopathy) are related to their oxidation level [53].

- Tobacco smoke: its contact with tissues such as skin, lung or oral tissues, breaks down antioxidants [54]. Smoke exposure impairs the oxidant/antioxidant balance in oral tissue and raises the release of pro-inflammatory cytokines [55]. Resulting inflammatory response and oxidative stress in peri-implant tissue induces delayed wound healing and increased risk of peri-implantitis.

- Penicillin allergy, as any form of allergy is an immune dysfunction, in which patients don’t recruit enough neutrophils to ward off pathogens. On the other hand, activated T-lymphocytes continuously release pro-inflammatory cytokines [56]. Resulting chronic inflammation and limited prophylaxis engender a three-to-four fold increased risk of surgical site infection, compared to non-allergic patients [57]. Indeed, alternatives to amoxicillin do not show an equivalent efficiency [58]. 
Moreover, allergic patients are often deficient in vitamin D, and thus in chronic oxidative stress [59].
        
- Vitamin D deficiency is also a major factor in ROS production. Vitamin D is in reality an hormone, synthesized mainly after UV radiations on skin. Normal serum level range is between 30 to 100ng/mL (or 75 to 250nmol/mL) [60]. Vitamin D levels influence cell growth and stimulates antioxidants production [61]. By promoting Nrf2 accumulation and activation, vitamin D enhances the expression of numerous gene regulating immunity and bone growth and remodeling [62,63]. It has also been shown to have antimicrobial and anti-inflammatory properties [64,65]. Vitamin D deficiency is widespread. Identified risk factors are : age, dark skin, depression, obesity, smoking, diabetes, allergy, kidney insufficiency. Vitamin D deficiency significantly impact the osseointegration and success rate of dental implants therapies and bone grafts outcomes [66-69].
        
- Hypercholesterolemia. LDL cholesterol is an oxidant, known to promote artherosclerosis.
Studies have reported the association between elevated LDL and alterations of the functions of bone-forming osteoblasts and osteoporosis [70, 71,72]. 
Its accumulation into osteoblasts slows down bone metabolism and synthesis, driving stem cells to produce fat cells. Bone becomes fatty, taking a yellowish color.
In contrast, High-Density Lipoprotein (HDL) cholesterol acts as an antioxidant [73-75].

Oxidative Stress following local inflammation

- Inflammatory soft tissues : In patients with thin phenotype, the junctional epithelium is permeable to bacteria, and pathogens can penetrate and spread to the periodontium, inducing long-term inflammation [76]. Long term inflammation and the resulting underlying bone oxidation generate resorption, either in native or grafted bone.
Indeed, in grafted bone, when submitted to inflammatory soft tissues or to periodontium contamination during the healing period, fibrosis sets in [77]. 

- Foreign body reaction is a common consequence of biomaterials use. They have the potential to induce adverse immune reactions, resulting in excessive inflammation, impaired healing, fibrotic encapsulation, tissue destruction, or even isolation and rejection of medical devices [78]. 
Biomaterials foreign body reaction depends on their biocompatibility [79], and thus can be categorized in two types, based on their immunogenicity: 
Human, allogenic or porcine bone have low immunogenicity resulting in low inflammation and oxidation processes.
On the other hand, other xenogeneic substitutes (such as bovine) have good biological activity, but the elimination of immunogenic reactions while retaining osteogenic abilities is challenging : inflammation persists for a long time, leading to fibrosis and connective tissue formation [80]. Synthetic materials are subject to degradation, which exposes a fresh surface to oxidants released by macrophages and foreign body giant cells. The persistence of the foreign body reaction, which occurs at the interface between the tissue and the biomaterial over time, suggests a continuous oxidation process, albeit at low levels [81].

- Chronic hypoxia is another source of oxidant production. Tissues usually become hypoxic by losing their vasculature when exposed to overpressure or tension [82].
Cortical bone blood supply is organized by the soft tissue through the periosteum. Therefore, any soft tissue tension will lead to the underlying bone hypoxia and condemn it to resorb [83].
Soft tissue traction can occur with inconsistent flap release after bone augmentations, inadapted suture technique or lack of keratinized attached gingiva [84].
Implant placement, with high primary stability, might as well create bone compression. Cancellous bone is flexible, thus few stress is induced even if implants are placed with high torque. In contrast, cortical bone is a rigid tissue with a reduced blood supply. Consequently, pressure will cause hypoxia and oxidation especially if the cortical bone is thick [85]. This is the mechanism of marginal bone loss/ 
Regarding grafted bone, bone matrix becomes stiff after few months. Mechanically, the regenerated bone has lower flexibility and behaves as a cortical bone. When it is submitted to pressure, it will resorb.

Review of solutions to improve immune response lower oxidative stress

In the field of oral surgery, better understanding of osteoimmunology permits guidelines and protocols proposal to enhance bone formation, reduce complications or failures and to achieve long term stability. 
The key word is anti-inflammation. The strategy is to reduce or stop inflammatory mechanisms and sources.

Systemic improvement of immune response

Antioxidation is one of the ways to enhance immune system and to reduce inflammation length.

- External antioxidants or nutraceuticals have demonstrated their ability to decrease oxidative damages. Oral antioxidants alone have shown a significant reduction of up to 40% in oxidative stress blood serum markers, especially in smokers, patients at higher risk [88].
Exogeneous antioxidants can be classified in two groups: those with Nrf2 activation functions and those with direct antioxidative properties:
It had been proven that Nrf2 activators hold the potential to control the inflammatory-driven bone loss [89,90]. Among those activators, we can find vitamin D, vitamin C, melatonin, and zinc. [91-98].
Other nutraceuticals can be prescribed to patients, to increase endogenous antioxidants production and thus down-regulates pro-inflammatory cytokines, such as probiotics, vitamin K2, vitamin E, vitamin B6, copper, magnesium, and fatty acids [99-104].

Micronutrients and nutraceuticals could enhance peri-implant wound, bone healing and stability, and therefore dental implant osseointegration and long-term success [105]. Their supplementation have to be considered in bone reconstructions, especially on patients with systemic disorders (diabetes, auto immune or inflammatory diseases) and should start at least ten to fifteen days before surgery. 

- Reduction of serum LDL cholesterol: hypercholesterolemia has been identified as a risk factor for dental implants stability and must be treated before surgery if LDL serum levels exceed 1.4g/l [106]. Treatment can be achieved through administration of a combination of cholesterol-lowering medications improvement of lifestyle (exercise, low-saturated-fat diet, not smoking, drastic reduction of alcohol and red meat consumption).

- Among immunomodulatory antibiotics, azithromycin is a well-known antibiotic from the macrolide family, discovered in 1980 and which immunomodulatory functions were published in 1987. It is retrieved in high concentrations in phagocytic cells and fibroblasts, enabling activity against microorganisms and pathogens in infected sites [107].
Aside from its antimicrobial effects, azithromycin exerts anti-inflammatory activities and may be of therapeutic benefit in various inflammatory disorders by inhibiting oxidative stress, inflammation and apoptosis. It improves epithelial barrier dysfunctions and promotes connective tissue remodeling [108]. When administered as a pre-operative prophylaxis, a single dose of azithromycin generates higher active substance concentrations in periodontal tissues and greater inhibition of inflammatory mediators at peri-implant wound sites, compared to amoxicillin [109].

- Autophagy describes cell regeneration under fasting conditions. This concept brought the Nobel prize to Christian de Duve in 1974 [110] and then recently to Yoshinori Ohsumi in 2016. When fasting, cells undergo autophagy, forming unusually large vacuoles that function as "cell dumps", collecting and recycling unwanted substances [111]. Autophagy improves both innate and adaptative immune responses and accelerates wound healing through angiogenesis.
Intermittent fasting, lasting at least 16 hours a day (by skipping the breakfast for example) represents a straightforward and effective way to harness the benefits of autophagy for our patients.

- Physical exercise: It is known that physical exercise improves global health through the improvement of immune system. Indeed, exercise permits the mobilization of both hematopoietic and mesenchymal stem cells into several tissue such as bone marrow, fat and synovia. These cells are the main immune cells. Hematopoietic stem cells produce the white cells, responsible of the adaptive and innate immune systems [112]. Mesenchymal stem cells regulate both innate and adaptive immune systems.
Walking is the most efficient type of physical exercise. However, it must be done at least 30 minutes per day and at a sustained pace (around 3 steps/second) [113].

Local improvement of immune response 

Surgical management should be performed to avoid local oxidation.
- Growth factors and PRF are used routinely in oral surgery and various medical fields. Platelet-rich fibrin (PRF), prepared without anticoagulant, represents the simplest and most efficient method, as it combines platelets, leukocytes and a fibrin matrix [114]. PRF induces angiogenesis and anti-inflammation pathways; possesses osteogenic properties and inhibits osteoclastogenesis [115,116]. Recent studies indicate that the main mechanism of action of PRF lies in its antioxidant promotion and thus improves local immune responses [117,118]. 
Additionally, PRF enables constitution of a sticky bone graft. By reducing the mobility of the graft (induced by muscle activity), it avoids inflammation coming from the bone callus fragmentation. 
Finally, the fibrin matrix can be used to perform an open wound technique, where wound closure is voluntarily not achieved and PRF membranes are used to fill the gap between soft tissue edges.
It promotes a secondary wound healing cascade and an increased width of keratinized mucosa.

- Low inflammatory biomaterials: to avoid inflammation and its deleterious consequences, oral surgeons should prefer the use of low inflammatory biomaterials, such as human or porcine bone, over alloplastic or other xenogeneic bone grafts [80]. 

- Implant placement should avoid contact between the crestal cortical bone and the implant neck, by either overdrilling the cortical bone or placing implants subcrestally. New implant designs tend to present narrower neck and facilitate pressure less positioning near the crestal bone. Similarly, when dealing with grafted bone at re-entry, the same attention and protocols must be applied. During implant placement in a dense bone graft, a reduced torque and crestal overdrilling should be performed, to avoid graft stress caused by implant pressure.

- Tension free sutures and flap closure must be applied on every occasion, to avoid ischemia. Flap release is a critical step of the surgery and requires careful consideration to guarantee a tensionless situation. Wound closure and suture technique should respect PASS principles as described by Wang: primary closure, angiogenesis, space creation/maintenance and stability [119]. Bone grafts outcomes rely the application of such principles.

Conclusion

Osteoimmunology significantly widened the knowledge on bone health and diseases, shedding light on the intricate interplay between bone and immune system. Researchers hope that the interdisciplinary nature of osteoimmunology will lead to major discoveries in bone regeneration and the development of targeted therapies for bone diseases.
Osteoimmune supportive protocols, described in this article, include biologic patient preparation, use of azithromycin, Platelet-Rich-Fibrin low inflammatory biomaterials, and surgical behaviors such as pression and tension awareness. Applying these protocols could decrease the incidence of peri-implantitis and increase dental implants success rate.
Osteoimmunology have provided valuable insights into periodontitis pathogenesis and may offer a new perspective on the understanding of peri-implantitis, as they both present similar symptoms: chronic inflammation of the periodontal tissue and subsequent destruction of the alveolar bone around teeth or implants.
However, the lack of randomized clinical studies or such protocols in the current scientific literature, underscores the necessity for further scientific research and validation. Additional studies must analyze the impact of nutraceuticals on the prevention of periimplantitis.