TKR is one of the most common orthopedic surgical procedures. Because TKR involves extensive muscle and bone repair, it is known to be one of the most painful operations. Moreover, as prosthetic surgery is an elective procedure, patients delay and experience anxiety about surgery due to their experiences with severe pain. This leads to chronic fear of pain and negative thought patterns. Inadequate use of analgesics and poor pain management after surgery can cause delayed mobilization, increased risk of venous thrombosis, insufficient wound healing, prolonged hospital stay, unnecessary psychological distress, and lower patient satisfaction. Many patients experience psychological problems such as depression, anxiety, discouragement, negative feelings, crying, frustration, lack of motivation, fatigue, moodiness, irritability, despair, and helplessness. Patients have expressed a need for social or psychological support. In order for patients to avoid such pain-related complications, they must be taught how to cope with pain.

The severe pain, anxiety, and stress patients experience following TKR surgery may impact their postoperative recovery. Studies have shown that nonpharmacological interventions for postoperative pain reduce pain intensity and opioid use. One of these nonpharmacological methods is diaphragmatic breathing exercises (DBE). Also known as diaphragmatic breathing or deep breathing, DBE is an effective holistic mind-body training to cope with stress and psychosomatic conditions. DBE involves contraction of the diaphragm and expansion of the belly to deepen inhalation and exhalation, which consequently decreases breathing rate and maximizes blood gas concentrations.

DBE plays an important role in pain signaling, autonomic activation, emotional regulation, acid-base equilibrium, and anti-inflammatory processes. DBE has been shown to enhance emotions, effectively reduce anxiety and its symptoms, and alleviate negative emotions such as depression, stress, and anger. There are few studies in the literature on the effect of DBE on patients who underwent TKR. One of these is a semi-experimental study that evaluated the effect of relaxation intervention (breathing exercises and guided imagery) on patients who underwent TKR. They authors reported that relaxation techniques were effective in managing patients' pain and anxiety. Another study evaluating the effect of relaxation techniques and back massage in patients who underwent TKR and total hip replacement (THR) showed that these techniques reduced patients' pain and anxiety during bedrest. Investigated the effect of a DBE program in patients with OA and found that DBE had no effect in alleviating pain or improving physical function.

A few studies have used DBE to prevent pain and sensory tension in patients who underwent TKR. However, different relaxation techniques were used in addition to DBE in these studies. To the best of our knowledge, no studies have examined the efficacy of DBE alone. Given the rising incidence of TKR, it is imperative that we strengthen our arsenal of effective interventions against the pain and psychological problems associated with this procedure. The aim of the present study was to evaluate the effect of DBE on pain, anxiety, and depression in patients who underwent TKR.