I.INTRODUCTION
Postoperative bone healing after oral surgical procedures occurs uneventfully in most cases because of exceptional vascularity of head and neck structures when compared with other anatomic sites. However, host vascular and immune factors may affect the incidence of head and neck delayed healing and in certain patients, the normal process of osseous healing can be delayed1). Osteonecrosis(ONJ) is defined as non-vital bone tissue as a result of abnormal process of osseous healing, and is caused by several reasons such as infection, radiation, medication2). In most ONJ cases, a trigger factor is also present, and these trigger factors have an oral wound in common, for example due to tooth extractions, pressure denture sores, periodontal diseases, or surgical procedures such as implant placements3). An analysis of all patients with an osteonecrosis of the jaws in the years 2000–2004 revealed that 40% had an osteonecrosis due to some kind of odontogenic or surgical procedure; 28% suffered from osteoradionecrosis, 10% had a bisphosphonate related osteonecrosis of the jaws (BRONJ), and 8% had an osteomyelitis or sequester due to a trauma, and in 14%, no obvious reason was detected4). However, the reasons for osteonecrosis were bisphosphonates in 45%, odontogenic or surgically induced osteonecrosis in 32%, osteoradionecrosis in 17%, traumas in 1%, and in 4% the reason remained unclear in 2005-2012, and BRONJ turned out to be the most common reason for osteonecrosis of the jaws and is responsible for half of all osteonecroses3)s.
Although Osteomyelitis, osteoradionecrosis, MRONJ reveal a similar clinical symptom and radiographic finding, each disease represents a separate clinical progress and requires a different treatment approach. So it is very important to understand the pathophysiology of these diseases and to distinguish each disease. The aim of this study was to analyze the histological features of bone tissue obtained by patients affected by osteomyelitis, osteoradionecrosis, MRONJ and to study the differences in their pathophysiology which relates to separate mechanisms for each disease
II.Case report
Case 1.Osteomyelitis
In March 2015, a 36-year-old female visited the Department of oral and maxillofacial surgery of Yonsei University Dental Hospital with the chief complaint of nonhealing extraction socket. The patient had undergone extraction of the lower right second molar by a general dentist at 2 months ago. Initially, she was diagnosed with alveolar osteitis (dry socket) with respect to painful extraction socket and treated in other local clinic. However, pain did not improve and abnormal feeling of chin and lower lip was developed, and so she was referred to the Yonsei University Dental Hospital. Her medical history was zygoma reduction operation, 3 years ago. At oral examination, there were vestibular swelling and redness and tenderness on right lower extracted area with unhealed extraction socket. She had abnormal feeling and decreased sense on right lower lip and chin area. The panoramic radiography revealed irregular radiolucent areas with poorly defined borders invading the inferior alveolar canal. There was sequestra formation in nonhealing extraction sockets (Fig 1). She did not have the history of radiation or medication related to the bone disease. She was diagnosed as osteomyelitis or osteosarcoma provisionally. The biopsy at the extraction site was performed and the bone fragment was sent to the department of oral pathology. The histologic finding revealed much inflammatory cell and blood vessels. Necrotic bone as evidenced by empty osteocytic lacunae, and empty Haversian systems was demonstrated and the distinct reversal line was observed. She was diagnosed osteomyelitis (Fig 4A, B).
Case 2.Osteoradionecrosis
In December 2014, a 76-year-old male visited the Department of oral and maxillofacial surgery of Yonsei University Dental Hospital with a chief complaint of discomfort in left lower posterior edentulous jaw. Discomfort was developed after using the lower complete denture for 2-3years. He was diagnosed larynx cancer, 16 years ago and radiotherapy was done after cancer resection in head and neck area. His medication at the time of presentation at the dental hospital included antihypertensive drugs only. At oral examination, ulceration with slight bony exposure and gingival redness was observed in left posterior edentulous area. He was diagnosed provisionally traumatic ulcer or osteonecrosis left lower jaw, and denture area apposing ulcer was trimmed and antibiotic was described. In February 2014 (2 months after) he revisited with complaint of pus discharge and pain on the same area. At oral examination, fistula with bony exposure and pus discharge was revealed and tenderness on palpation was present on left posterior edentulous area. The panoramic radiography revealed the radiopaque area with unclear boundary separated the surrounding bone (Fig 2). He was diagnosed as osteoradionecrosis provisionally and the sequestrectomy was performed. The bone fragment was sent to the department of oral pathology, and extended homogenous regions of complete bone necrosis (empty osteocytic lacunae, and empty Haversian systems) were identified. Histologic findings showed distinct reversal line within necrotic bone. Some inflammatory cells and little vessels were observed. He was diagnosed osteoradionecrosis based on the irradiation history (Fig 4C, D).
Case 3.: Medication related osteonecrosis of the jaws
In April 2015, a 81-year-old female visited the Department of oral and maxillofacial surgery of Yonsei University Dental Hospital for an extraction socket evaluation. The patient had undergone extraction of the upper right second molar due to periodontitis by a general dentist at 2 months ago. Her medical history included osteoporosis which had been diagnosed 10 years ago and she were taking alendronate (FosamaxR) At oral examination, bone suspected sequestrum in extraction socket was exposed and gingival inflammation was obseved in upper right second molar area(extraction site). The panoramic radiography revealed nonhealing extraction socket and a mixed radiopaque-radiolucent lesion with relative clear boundary in upper right second molar area (Fig 3). She was diagnosed MRONJ(BRONJ) provisionally and seqestrectomy was performed. The bone fragment was sent to the department of oral pathology, and the lack of nucleoli in bone lacunae and necrosis of bone marrow was demonstrated. No inflammatory cell and no blood vessels are found in bone marrow. Unclear reversal line and many bacterial colonies were observed. He was diagnosed MRONJ based on the medication history (Fig 4E, F).
III.Discussion
Histologic examination of osteomyelitis, osteoradionecrosis and MRONJ revealed necrotic bone as evidenced by empty osteocytic lacunae, and empty Haversian systems in all cases. However, the gross pattern of necrosis differed between all three diseases. The histological findings of osteomyelitis and osteoradionecrosis showed the distinct reversal line within necrotic bones, but the MRONJ showed unclear reversal line. Although the much inflammatory cells in the marrow space were identified in osteomyelitis, and some inflammatory cells in osteoradionecrosis, no inflammatory cell are found in bone marrow of MRONJ. Many blood vessels were detected in osteomyelitis specimen although there was an absence of blood vessels in osteoradionecrosis and MRONJ (Fig 4A-F).
First, the osteomyelitis and osteoradionecrosis showed the necrotic bones with distinct reversal line of active remodeling, but the MRONJ show unclear reversal line.
Bone is a dynamic tissue that is constantly formed and resorbed in response to changes in mechanical loading, serum calcium levels and in response to paracrine and endocrine factors. The dynamic nature of the bone is achieved by a process called “remodeling”. Remodeling commences with signals that initiate osteoclast formation followed by osteoclast-mediated bone resorption, a reversal period, and then a long period of bone matrix formation mediated by osteoblasts, followed by mineralization of the matrix10). During the reversal period, bone resorption transitions to bone formation, and a dark line is seen which separates the new bone from old bone, this is called the reversal line. The reversal line demarcates the cessation of osteoclast activity from the commencement of osteoblast activity at a remodeling site in bone and contains regulatory signals that, in part control osteoblast activity and plays a role in bone remodeling11). So, from our histology finding, we elucidated that bone remodeling for bone repair was going on in osteomyelitis and osteoradionecrosis, but was not gone in MRONJ.
The mechanisms of bisphosphonate are osteoclast impairment and cellular death (osteoclast apoptosis) which prevents bone renewal, resulting in necrotic mineralized trabecular and cortical bone. Although the pathophysiology has not been fully elucidated, theories currently discussed regarding BRONJ onset describe reduced bone remodeling, the antiangiogenic effect, a negative impact on the bone-covering soft tissues, and the relative ease of jaw bone contamination with oral bacteria that might contribute to the development of BRONJ3,12). Our histologic finding of unclear reversal line and absence of blood vessel in MRONJ (BRONJ) patient are consistent with the mechanism of BRONJ (reduced bone remodeling, the antiangiogenic effect).
Second, the inflammatory cells in the marrow space were identified in osteomyelitis and osteonecrosis, no inflammatory cells are found in bone marrow of MRONJ. This means that suppression of immune functions may occur more characteristically in MRONJ compared with osteomyelitis and osteoradionecrosis.
Monocytes and macrophages belong to the innate immune system representing the first line of defense against microorganisms, initiating local inflammatory reaction. They are essential cells in chronic inflammatory diseases and also in fighting infectious agents. To perform their functions, monocytes need to be activated, either via inflammatory cytokines produced by the adaptive immune system or via direct stimulation by bacterial products13). Hoefert et al analyzed the function of macrophages in BRONJ in contrast to patients with osteoradionecrosis and chronic osteomyelitis of the jaws. The results indicated a different expression of CD14 and CD68 markers of monocytes/macrophages in BRONJ as compared to other jaw infections, and patients receiving bisphosphonate medication without BRONJ showed no differences to other controls13). This could be a sign of macrophage immunosuppression by bisphosphonate, and it is consistent with histologic findings of our case. Cytotoxic and migration inhibitory effects of bisphosphonates on macrophages have been described as early as 1986. Migration inhibition of bisphosphonates appreared to be more sensitive indicator than cytotoxicity.
Infection, inflammation or injury triggers a recruitment of monocytes out of the blood circulation into the affected area. Reaching those infected areas in the jaw, they get exposed to the high bisphosphonate concentrations. Since bisphosphonate do not affect neutrophils, the risk of systemic infection is minimal. However, the reduced function or presence of monocytes and macrophages could become critical for the development of a local infection, BRONJ13).
A small number of inflammatory cells were demonstrated in our histology of osteoreadionecrosis patient. It means that a there was small local defense in the radiated bone. The aetiology of osteoradionecrosis is described that after radiation, a formation of hypoxic, hypocellular and hypovascular tissue appears9). The evidence of nearly the same ratio of CD68/CD14 macrophages like in the osteomyelitis group indicated that the macrophage have the normal immune function. However, the main problematic issue of macrophages in osteoradionecrosis is the reduced vascular system to migrate into the affected bone. Once reached the region of interest, hypoxic conditions may have an influence13).
Histologic evidence of osteomyelitis indicates an intense inflammation by microorganism and much blood vessel in the marrow space. It suggested that microorganism provoke an inflammatory response and cause blood vessel thrombosis. By that and liberated toxins, a possible anaerobic environment is created, and this local environment compromises local innate immunity even with normal and regular attracted macrophages6).
Third, many blood vessels was detected only in osteomyelitis although there was an absence of blood vessels in osteoradionecrosis and MRONJ. In osteoradionecrosis, there was direct injury to all cell populations including endothelial cell of blood vessel in the field of radiation7). This hypovascularity makes difficult inflammatory cell migration and induced osteonecrosis. Bisphosphonate have been found to inhibit endothelial function in vitro and in vivo. These antiangiogenic effects are made responsible for the ischemia of the jawbone12,15).
In the near future, the distribution of osteonecrosis might change because of other medications. These agents are denosumab, bevacizumab, and sunitinib, and are antibodies against either RANKL, VEGF, or a tyrosin kinase receptor. Due to the shorter half-life of those antibodies, it might be easier to influence the bone remodeling3). Our case report of MRONJ was only bisphosphonate related ONJ. To the understand of pathophysiology of MRONJ, histologic study of osteonecrosis related to other medications was needed.
