Bone defects, either fractures or lack thereof, are among the most common disorders encountered in the medical field. This condition can be brought about by numerous conditions such as injuries, cancer (and related treatments), surgery, infectious and autoimmune diseases, and even genetics. Variations with regards to cause of different bone defects also equates disparities between appropriate treatments. Decades of research have already contributed to the development of ever more advancing therapeutics seeking one vital purpose–guided bone regeneration (GBR). Although it is well known that bone can regenerate like any other tissue in the human body, bone requires a particular set of conditions to proceed. It follows an extended scheme of regeneration that involves several overlapping processes. ¹ Since bone tissue is the main support structure that influences stability of almost all surrounding soft tissue, it’s crucial that bone heals according to its form and associated function.

Stabilization of bone fractures is one of the most important considerations when addressing bone defects. Callus formation in broken bones progress slowly and only occurs within a very short range of distance. ¹ Minimizing the distance and mobility of the fractured bone tissue limits the occurrence of non-union. This has led medical and clinical scientists to develop different devices and techniques for fracture immobilization with the aim of GBR.

Aside from immobilization, another aspect to be considered is proper ossification. Bone is a hard tissue which contains high amounts of minerals compared to other soft tissues. This necessitates the presence of a mineral source to support calcification of the newly formed dense tissue. Hence, this led to medical and material scientists to examine biomaterials not only as immediate mineral source for tissue calcification but as proper tissue scaffolds mimicking the stability and structural form of bone tissue. Copious implantable materials for GBR based on natural sources (demineralized/acellular bone), synthetic materials (polymers/mineral-based/metals), and related composites have already been successfully utilized as viable medical products.

Biological mechanisms have also been elucidated with the aim of using cells and exploiting biomolecular pathways to improve previously developed techniques for GBR. The introduction of cell, protein, and genetic-based biomolecular analyses was accompanied by enormous interest in using biological molecules to promote augmented bone regeneration (ABR). Notwithstanding the advancements in GBR generated from recent investigations, the use of energy-based medical therapeutics such as pulsed electromagnetic field stimulation, ultrasound stimulation, and laser application for ABR has garnered traction lately.

Initial studies mainly utilized these aforementioned energy-based technologies for the purpose of observation and visualization of organs and tissues. Early investigations regarding the use of energy for tissue regeneration have been scarce and far in between. This could possibly be due to the persistent changes in understanding the fundamental nature of the different forms of energy. Unlike biomaterials and biologics, energy not only exists in different forms but also in a broad spectrum and is highly transferrable across various states of matter. This complicates not only the direct application of energy for therapeutic purposes but also imposes the promotion of proper safety precautions for both the practitioner and patient alike. In addition to this, technological advancements in the form of hardware capable of measuring, generating, and harnessing the potential of different forms of energy became a limiting factor in pursuing energy-based therapeutics early on. Nevertheless, the knowledge, policies, and machinery/devices have already reached the level of maturity enough to merit investigations several energy-based technologies for regenerative applications.

As previously mentioned, pulsed magnetic field stimulation (PMFs) is one of the energy-based therapy that is currently being investigated for ABR. Initial studies attributed the efficacy of PMFs to the two factors; the fast-tracked synthesis of bone matrix thru the induced weak electric currents; ² and the subsequent bone loss matrix downregulation. ^3,4 Later studies have revamped this theory by suggesting that PMFs boosts cell proliferation and step-up the osteogenic process. ^5,6 This led to the approval of PMFs for supplementing post-osteotomy recovery ⁷ and as treatment for both osteoarthritis and pseudoarthrosis. ^8,9

Low intensity pulsed ultrasound stimulation (LIPUs) similarly went through the same inquiries concerning its potential for ABR using animal models ¹⁰ and through clinical studies alike. ^11,12 A number of studies delving into the mechanism by which LIPUs leads to ABR had reported that ultrasound induction influences the c-fos, cyclooxygenase-2, prostaglandin-2, nitric oxide (NO), osteocalcin, and osteopontin levels in osteoblasts as a result of amplified calcium influx into the osteoblasts. ^13,14 This occurs through signal transduction across the cytoskeleton proteins, actin filaments, and extracellular matrix connected via adhesive contacts during LIPUs. Ultrasound stimuli travels between adherent cells setting off stretch-activated calcium channels; this in turn increases cell surface expression of integrins leading to formation of stress fibers via reorganization of actin cytoskeleton. ^15-17

Likewise, Low-level Laser therapy (LLLt); the use of non-ionizing light energy for regenerative medicine, has also been a prominent topic for ABR as of late. Compared to PMFs and LIPUs, application of LLLt for bone regeneration is a more recent and pervasive topic that picked up greater traction by the end of the 20th century. Based on current publication search, information regarding the use of PMFs and LIPUs dates back as far as mid 1970s and has relatively fewer number of publications compared to LLLt, this indicative of the major interest and relative greater potential of using lasers for bone regeneration therapy. Thus, this article will be a retrospect on select publications with notable developments concerning the use of LLLt for bone regeneration in the past decade.

Low-level laser/light therapy entails exposure of cells and tissues to light energy ranging within the red and near-infrared (600-1070 nm) wavelength of the electromagnetic spectrum. ¹⁸ The presence of chromophores like hemoglobin and melanin, which have effective absorption below 600 nm, can be efficiently utilized to treat superficial tissues while longer wavelength ranging from 780-950 nm are used for addressing deeper tissues like bone. ¹⁸ This energy-based therapy is delivered in lower power density need to generate heat thus the term “cold laser” became associated with LLLt. ¹⁹

In fact, a recent publication concerning naming scheme for the said procedure have led to the promotion of “Photobiomodulation therapy” (PBMt) as an alternative term for LLLt. ²⁰ This was to improve consolidation of published articles making use of low-level/low-power laser/light treatments that do not induce ionizing or thermal effects on cells and tissues nonetheless stimulate certain biological response. The term “photobiostimulation” was derived from the photochemical effect resulting from the absorption of light energy promoting certain chemical changes that affect biological processes in living organisms. Thus, for the remainder of this article, LLLt will be referred to as PBMt.

Although abundant studies have been conducted regarding the use of PBMt for regenerative purposes, the mechanism by which it enhances the recovery of damaged tissue is yet to be fully elucidated. Based on existing literature, improvement on bone regeneration through PBMt is attributed to at least two modes of action. The first mechanism is ascribed to the absorption of the light energy by the respiratory chain terminal enzyme-cytochrome C oxidase leading to the increase of mitochondrial membrane potential which consequently elevates adenosine triphosphate, cyclic adenosine monophosphate, NO, and reactive oxygen species levels. ²¹ This increased mitochondrial activity causes interactions with the cell nucleus that translating to modifications in gene expression related to dense tissue formation and ossification. ^21,22 Through this effect, PBMt has been known to influence not only cell viability but also the expression of collagen 1, osteocalcin, bone morphogenetic protein-2, alkaline phosphatase, bone sialoprotein in osteoblasts. ^23-25 The second mechanism relates to the effect of PBMt on macrophage polarization where in the same alteration in mitochondrial activity affects the proinflammatory and anti-inflammatory protein expressions in macrophage cells. Studies have shown that PBMt is capable favoring the transformation of macrophages into M2 phenotype which alleviate inflammation and promotes tissue repair and remodeling. ^1,26

Current literature regarding the use of PBMt for bone regeneration is mainly sourced from orthodontic applications reporting the suitability of the procedure for reducing pain and inflammation, ^26,27 periodontitis management, ^26,28 and bone remodeling during tooth movement. ^29,30 However, independent studies are still being conducted regarding the use of PBMt for regenerating critical sized bone defects in animal models, ^31-33 enhancing results of distraction osteogenesis, ^34,35 regenerating irradiated bone, ³⁶ repair bone lesions, ³⁷ manage osteoporosis, ^38,39 and resolve osteonecrosis. ^40-42

Studies regarding the use of PBMt in conjunction with other energy-based therapy have also been conducted for ABR to a limited extent. The concept of combined energy-based therapeutics mimics the trend in biomedical material development in which combinations thereof or composites are tested to determine possible advantages within the same context. For the most part, PBMt is paired with LIPUs and more commonly compared with PMFs. This is probably due to the fact that PBMt and PMFs are both electromagnetic radiations that both exists in a spectrum although at drastically different wavelengths. On the other hand, LIPUs is high frequency vibrations that travel across tissues and does not carry photon energy, making it a suitable complementary physical stimulation for tissues and cells.

The combination of LIPUs and PBMt have been studied as cumulative alternating treatment for improving bone consolidation during distraction osteogenesis, ⁴³ remodeling bone during tooth movement ⁴⁴ and post-osteotomy tibial bone regeneration. ⁴⁵ These studies primarily concentrate on the synergistic effect of both treatments with LIPUs improving cell infiltration and viability; and PBMt promoting osteogenesis and reducing inflammation.

Aside from the combined application of PBMt with either LIPUs or PMFs, it has also been tested out together with synthetic and biologically derived implantable materials for bone regeneration. This was brought about by the initial success from the use of PBMt for treating disease and wound in different tissue types. Since the start of the 21st century, PBMt has been applied as a supplementary procedure in some studies that makes use of biomaterials, drugs, grafts, and cell seeded tissue scaffolds for bone regeneration.

Relatively recent publications applying PBMt on bone implants focuses on both the stimulatory effect on the bone defect and the photothermal effect on the implant material. Case in point, the use of magnetic particles (strontium hexaferrite) ⁴⁶ and hydrogenated black titanium oxide coating ⁴⁷ both improve bone regeneration and simultaneously ablate cancer cells through hyperthermia upon exposure to near-infrared laser. Under this context the researchers makes use of a non-ionizing light energy to promote bone tissue repair whilst killing tumor cells with heated implanted biomaterial. The use of black phosphorous together with PBMt as bone implants has also been investigated. Subsequent publications have reported the usability of near-infrared light not only as stimulation for tissue repair but also as a photothermal release mechanism for strontium chloride ⁴⁸ and microsphere drug carriers ⁴⁹ for bone regeneration.

Osteoporosis treatment through use of simvastatin and alendronate have also been tested together with PBMt where in reports indicated enhanced cell viability and bone formation. ^50,51 Similarly, studies about post-surgery PBMt of bone defects implanted with autogenous bone grafts, ⁵² non-autogenous bone ^53,54 implants, and fibrin sealants ^55,56 confirm the benefits of the said treatment in terms of bone regeneration and implant integration.

The use of biologics such as stem cells ^39,57 and platelet concentrate alongside PBMt were also explored for ABR in independent studies. The observed increased bone regeneration benefited from PBMt enhancing stem cell viability whilst improving anti-inflammatory response in animals treated with platelet concentrate respectively.

Low-level laser/light therapy entails exposure of cells and tissues to light energy ranging within the red and near-infrared (600-1070 nm) wavelength of the electromagnetic spectrum. ¹⁸ The presence of chromophores like hemoglobin and melanin, which have effective absorption below 600 nm, can be efficiently utilized to treat superficial tissues while longer wavelength ranging from 780-950 nm are used for addressing deeper tissues like bone. ¹⁸ This energy-based therapy is delivered in lower power density need to generate heat thus the term “cold laser” became associated with LLLt. ¹⁹

In fact, a recent publication concerning naming scheme for the said procedure have led to the promotion of “Photobiomodulation therapy” (PBMt) as an alternative term for LLLt. ²⁰ This was to improve consolidation of published articles making use of low-level/low-power laser/light treatments that do not induce ionizing or thermal effects on cells and tissues nonetheless stimulate certain biological response. The term “photobiostimulation” was derived from the photochemical effect resulting from the absorption of light energy promoting certain chemical changes that affect biological processes in living organisms. Thus, for the remainder of this article, LLLt will be referred to as PBMt.

Although abundant studies have been conducted regarding the use of PBMt for regenerative purposes, the mechanism by which it enhances the recovery of damaged tissue is yet to be fully elucidated. Based on existing literature, improvement on bone regeneration through PBMt is attributed to at least two modes of action. The first mechanism is ascribed to the absorption of the light energy by the respiratory chain terminal enzyme-cytochrome C oxidase leading to the increase of mitochondrial membrane potential which consequently elevates adenosine triphosphate, cyclic adenosine monophosphate, NO, and reactive oxygen species levels. ²¹ This increased mitochondrial activity causes interactions with the cell nucleus that translating to modifications in gene expression related to dense tissue formation and ossification. ^21,22 Through this effect, PBMt has been known to influence not only cell viability but also the expression of collagen 1, osteocalcin, bone morphogenetic protein-2, alkaline phosphatase, bone sialoprotein in osteoblasts. ^23-25 The second mechanism relates to the effect of PBMt on macrophage polarization where in the same alteration in mitochondrial activity affects the proinflammatory and anti-inflammatory protein expressions in macrophage cells. Studies have shown that PBMt is capable favoring the transformation of macrophages into M2 phenotype which alleviate inflammation and promotes tissue repair and remodeling. ^1,26

Current literature regarding the use of PBMt for bone regeneration is mainly sourced from orthodontic applications reporting the suitability of the procedure for reducing pain and inflammation, ^26,27 periodontitis management, ^26,28 and bone remodeling during tooth movement. ^29,30 However, independent studies are still being conducted regarding the use of PBMt for regenerating critical sized bone defects in animal models, ^31-33 enhancing results of distraction osteogenesis, ^34,35 regenerating irradiated bone, ³⁶ repair bone lesions, ³⁷ manage osteoporosis, ^38,39 and resolve osteonecrosis. ^40-42