Introduction and background:

The surgical extraction of wisdom teeth under local anesthesia is one of the most common oral surgical procedures (1). Ten million third molars (wisdom teeth) are extracted from approximately 5 million people in the United States of America each year at an annual cost of over $3 billion. Additionally, more than 11 million patient-days of "standard discomfort or disability" were reported due to surgical removal of wisdom teeth, with an average of 4.9 lost work days per procedure (2, 3). Most of wisdom teeth surgical extractions require bone removal (4). Bone removal is associated with a broad range of postoperative discomfort and complications (e.g., pain, swelling, bruising, and malaise). Most of these complications are initiated by the inflammatory process from traumatic injury to the bone during the surgery. Currently, postoperative pain management is limited to acetaminophen, opioids and NSAIDs (5, 6). However, these drugs are all problematics. Acetaminophen is not effective in managing severe pain (6). Opioids and NSAID are effective in pain management, but opioids can cause constipation and addiction (7, 8), While NSAIDs may delay bone healing (9, 10). Dentists and maxillofacial surgeons all over the world prefer to prescribe NSAID after this type of surgery (11). The mechanism of action of NSAIDs is the reversible inhibition of the enzyme cyclooxygenase (COX), which is believed to be responsible for the synthesis of prostaglandins (PGs) (10). Prostaglandins play a major role in inflammatory and nociceptive processes (12). Two isoforms of COX, namely, COX-1 and COX-2, are responsible for the synthesis of PG from arachidonic acid (AA). Both have essential roles in the inflammatory process after bone surgery, but COX-1 is involved more in the integrity of the gastrointestinal tract and renal tract tissue, while COX-2 is mainly involved in the inflammatory and healing process later (13). NSAIDs are either nonselective (inhibiting both COX-1 and COX-2) or selective (inhibiting COX-2 only). Ibuprofen is a peripherally acting analgesic with potent anti-inflammatory action that works through reversible and balanced COX-1/ COX-2 inhibition, which provides faster and more effective analgesia than many other NSAIDs without any apparent increased safety risk (14). Ibuprofen is routinely used in the treatment of moderate to severe acute pain in the general population, including dantal pain (15-17). Results from Cochrane systematic reviews demonstrate that higher doses and frequencies of NSAID use are associated with better pain control after dental (wisdom tooth extraction) and non-dental surgeries (9, 18, 19). Increasing the dose and frequency of NSAIDs was associated with an increased risk of adverse effects. Despite the promising results associated with the use of different NSAIDs for pain control after wisdom tooth surgeries, patients still report pain and other discomfort indicators such as swelling and trismus, especially during the first three days after the procedure (20). These symptoms can affect patient's life, including activities of daily living and even quality of life (3, 20-22). Animal studies suggest that NSAID affect bone healing outcomes such as biomechanical measuments and number of bones with non-unions (23-35). Some authors have even compared the effect of NSAIDs on fracture healing with those of other pharmacological agents such as steroids (36, 37). Høgevold et al, demonstrated that short-term administration of some anti-inflammatories, such as indomethacin, inhibits fracture healing, while methylprednisolone does not (36). Bhattacharyya et al. reported that patients receiving NSAIDs within 90 days after fracture had a 3.7-fold risk of nonunion, while the risk for opioid users was 1.6-fold (38). Clinical trials in humans pose some challenges, especially the need to control confounders such as smoking, diabetes, obesity and other factors that are associated with the healing of long-bone fractures (6, 39). Four retrospective studies suggested that patients using NSAIDs after fracture had a higher incidence of non-union compared to those that did not use NSAIDs (38, 40-42). All living organisms possess a circadian rhythm that anticipates the response to changes during the 24-hour cycle (43). The circadian system in mammals is composed of a central clock within the suprachiasmatic nuclei and a peripheral clock inside all cells. The circadian clock is controlled through a feedback loop featuring a heterodimer of two core clock gene products: circadian locomotor output cycles kaput (Clock) and brain and muscle Arnt-like protein-1 (Bmal1). Clock and Bmal1 drive the expression of two inhibitors, cryptochrome (Cry) and period (Per) (44). This molecular clock modulates the immune response and bone healing process (44). The circadian rhythms in the expression of various cytokines by immune cells are presented in Table S1. Macrophage activity, leukocyte recruitment, and proinflammtory mediators such as interleukin-1β (IL-1β), IL-6, and IL-12 increased at the beginning of the daily activity phase.

During the active phase, there are also increased levels of Toll-like receptors TLR9 and TLR4, which upregulate the expression of CCL2, CXCL1, and CCL5, leading to leukocyte recruitment and potential tissue damage at the injured site (43-46)(Fig.1.). On the other hand, anti-inflammatory mediators and other growth and angiogenesis factors, such as vascular endothelial growth factor (VEGF), peak during the resting phase (44, 47, 48) (Table S2.). The 24-hour circadian rhythm oscillation occur in bone tissue during growth (49), formation, resorption (50, 51), and endochondral ossification during fracture bone healing (51). Experimental studies in rodents and humans reveal that disruption of sleep and circadian rhythm impair bone formation (52). All bone cells such as osteoblasts, osteoclasts, and chondrocytes express clock genes that influence bone volume regulation, such as Per or Cry (53, 54). The circadian clock also affects pain, with sensitivity peaking during the active phase (55). Part of the pain response oscillation could be explained by changes in COX-1 and COX-2 activity throughout the day (56), especially after an injury or insult (57). These variations may contribute to the clinical evidence of circadian and circannual variations in the pharmacokinetics effects of NSAID. Specifically, maximum absorption and effectiveness are achieved when the drug is administered during the active phase (58-62). We performed experimental studies to estimate the extent to which the timing of NSAID doses can affect animal pain behaviour and recovery after bone fracture surgery. We used a validated model of fracture healing in young female mice. The results showed that administration of NSAIDs during the active phase after tibia fracture surgery, compared to administration during the resting phase, significantly attenuated the pain response and improved the mechanical and histomorphometric properties of the healing callus. Recent evidence suggests that the timing of skin wounds or surgery affects healing and postoperative recovery after cardiovascular surgeries (63, 64). The chronotherapeutic use of anti-inflammatory medications after oral surgeries is of clinical importance, especially with increasing evidence regarding the clinical efficacy of this approach in medicine. No clinical studies have investigated the effectiveness of this approach after minor oral surgeries. Expected contribution and research question The overarching aim of this study is to contribute evidence regarding the efficacy of NSAID chronotherapy in patient recovery from wisdom tooth surgery. This is an important aim considering that the outcomes of this investigation can be used for other medical interventions involving bone healing. The research question is as follows: among patients undergoing surgical extraction of wisdom teeth, to what extent does a morning doses of ibuprofen 400 mg, in comparison to a routine dose schedule of three times per day, decrease patients' pain and improve wound healing when taken for three days after surgery .