The shoulder joint is the most unstable joint in the human body with the largest range of motion, and its stability depends on the integrity of the surrounding soft tissues, especially the rotator cuff. Therefore, it is important not only to place the prosthesis in the right position, but also to maintain the balance of the soft tissues around the shoulder, otherwise symptomatic subluxation or total dislocation of the shoulder joint and subacromial dynamic impingement will occur. The incidence of postoperative instability has been reported to be 0% to 22%, accounting for 38% of all total shoulder arthroplasty complications. The anterior drawer test and abduction-external rotation of the shoulder are used to check anterior stability, the posterior drawer test and anterior flexion-internal rotation of the shoulder are used to check posterior stability, and the Sulcus test is used to check inferior stability. The following factors are associated with anterior instability: the sum of the posterior tilt of the shoulder pelvis and humeral prosthesis is less than 350-450, anterior deltoid dysfunction, subscapularis tear, and posterior capsule tightness. Because anterior deltoid dysfunction can cause significant instability that is difficult to correct, every effort should be made to avoid damage to the deltoid muscle during surgery. Precautions should be taken not to cut off the starting point of the deltoid muscle when approaching through the deltoid pectoralis major and to keep in mind the location of the axillary nerve during exposure to avoid injury. Clinically, a simple lack of posterior tilt of the prosthesis does not result in significant instability unless there is a combined rotator cuff tear or rostral arch injury, whereas a simple subscapularis rupture produces postoperative anterior instability of the affected shoulder. Poor operator surgical technique, poor soft tissue quality, oversized prosthesis, and improper postoperative physical therapy are thought to be associated with this. In addition, the eccentric distance (offset) of the humeral prosthesis is also related to the function and integrity of the subscapularis. The use of thick pads for the glenoid prosthesis or large size humeral prosthesis increases the eccentric distance, increases the tension of the subscapularis after suturing, and can lead to structural impingement signs under the acromion. Posterior joint capsule overtightening is another cause of anterior instability, and internal rotation of the affected shoulder will force the humeral head to move forward. Therefore, if the sliding distance of the humeral head prosthesis on the shoulder pelvis is less than l/2 of its diameter during the intraoperative posterior drawer test, the release of the posterior capsule should be considered. 2. Posterior instability The most common cause of posterior instability is excessive posterior tilt of the prosthesis. In patients with chronic osteoarthritis, limited external rotation and axillary radiographs suggesting humeral head subluxation indicate eccentric wear of the posterior shoulder pelvis. A preoperative bilateral CT scan of the shoulder joint can show the degree of wear more clearly and help the surgeon to correctly locate the center of the shoulder pelvis and the direction of filing. Smaller posterior shoulder pelvis defects can be corrected by filing down the anterior shoulder pelvis or reducing the posterior tilt of the humeral prosthesis, while larger defects need to be filled with a larger prosthesis or bone graft. Patients with old posterior shoulder dislocation often develop anterior shoulder soft tissue contracture and posterior capsule laxity secondary to posterior instability. Therefore, the goal of soft tissue balancing in these patients is to achieve 400 external rotation, with the humeral head prosthesis sliding over the shoulder pelvis no more than 1/2 its diameter in neutral position. after releasing the anterior soft tissues to balance with the posterior structures, the use of a large prosthesis to move the center of rotation outward will ensure shoulder stability. Appropriately reducing the posterior tilt of the humeral prosthesis both deflects the humeral head away from the direction of dislocation and increases the offset distance during internal rotation of the prosthesis, thereby straining the posterior joint capsule and improving the stability of the shoulder joint. If posterior instability still exists after completing the above operation, posterior capsular tightening is feasible. 3. Underlying instability Low placement of the humeral prosthesis can cause relaxation of the deltoid and rotator cuff, which in turn can lead to instability and secondary impingement signs below the shoulder joint. In a normal shoulder joint, the humeral head can be moved down a distance of half the height of the shoulder pelvis. Since the humeral prosthesis is placed in the medullary cavity, it should not move further down than this, otherwise normal tissue tension cannot be maintained. Rotator cuff injury The incidence of rotator cuff injury is 1%-14%, accounting for the second most common complication rate of total shoulder arthroplasty. The continuous upward movement of the humeral head prosthesis after surgery suggests thinning of the supraspinatus muscle, rotator cuff rupture, or an imbalance in force between the strong deltoid muscle and the weak rotator cuff. In most patients with postoperative symptoms of chronic rotator cuff injury, close observation is possible. Non-steroidal anti-inflammatory drugs, hot compresses, and strengthening exercises for the deltoid, rotator cuff and scapularis muscles are often effective. Surgery should only be considered if the patient has significant symptoms, significant functional impairment, or acute postoperative trauma. Intraoperative injury to the rotator cuff can be avoided by performing an osteotomy of the humeral head (at least the posterior portion of the humeral head) under direct vision using an osteotome; also avoid osteotomies that are too low or outward (injury to the superior rotator cuff), or osteotomies that are too posterior to the humeral head (injury to the posterior rotator cuff). If there is a rotator cuff tear, it should be repaired as much as possible. If there is a preoperative impingement sign, acromioplasty should be performed at the same time, and the rehabilitation process should be decided according to the intraoperative repair. Cofield et al. reported that 10 years after total shoulder arthroplasty, the revision rate was about 11%, and the loosening of the shoulder pelvis prosthesis was the main cause; Torchia et al. reported that the average follow-up after Neer-type total shoulder arthroplasty was 12.2 years, and the shoulder pelvis loosening rate was 5.6%. The shoulder pelvis bed that fits the prosthesis better transmits the load placed on the prosthesis, thereby reducing abnormal stresses that can lead to wear or loosening of the prosthesis. The use of a spherical file with a neutral core along the anatomic axis of the shoulder pelvis reduces the repetitive adjustment and bed distortion caused by manual filing after cartilage scraping and improves the tilt of the shoulder pelvis. Translucent bands around the shoulder pelvis prosthesis were associated with osteoporosis and poor hemostasis of the bone bed, and using modern bone cement techniques, only 1 of 38 patients had translucent bands exceeding 50% of the cement-prosthesis interface. Pulsatile irrigation, thorough hemostasis with gauze or sponges dipped in thrombin, and maintenance of pressure after placement of the prosthesis are the key points of the technique. Intraoperative fractures Intraoperative fractures, mainly rib fractures, account for about 2% of all complications. The incidence is higher in patients with rheumatoid arthritis due to osteoporosis. Careful visualization and precise prosthesis placement techniques are essential to reduce intraoperative fractures. Intraoperative forceful external rotation of the upper arm dislocates the humeral head and can cause spiral fractures of the humeral stem, so the soft tissues anterior to the joint must be thoroughly released before dislocation and a bone hook is used at the humeral neck to assist in dislocation. In external rotation of the shoulder, the posterior humeral head is held against the shoulder pelvis, and in internal rotation, the insertion of a shoe horn helps to remove the humeral head and reduce the tension on the posterior capsule to facilitate pulling on the humeral head to expose the shoulder pelvis. This is particularly important in patients with osteoarthritis who have a deformed shoulder pelvis due to eccentric wear: in a normal shoulder pelvis, the axis passes through the center of the pelvis and is perpendicular to the articular surface, at the midpoint of the line connecting the upper and lower scapular feet (crura) at the level of the scapular neck. It can be used as a reference mark for intraoperative positioning. After shoulder arthroplasty, the following range of motion should be achieved: 140-160 supination, 40-60 neutral external rotation of the upper arm, 90 abduction, 70 internal rotation, and extreme posterior extension. Postoperative limitation of range of motion is often due to inadequate soft tissue release or overfilling of the joint. The range of motion can be increased by releasing the soft tissues during surgery: Z-plasty of the subscapularis and the coronal surface of the anterior capsule can help improve neutral external rotation of the upper arm; releasing the posterior inferior capsule can improve supination and supination rotation; releasing the rostro-humeral ligament can help increase forward flexion, posterior extension and external rotation; releasing the posterior capsule can improve internal rotation, internal retraction and supination; releasing the posterior capsule can improve internal rotation and supination. Release of the posterior capsule may improve internal rotation, adduction, and supination; even release of the pectoralis major muscle to increase external rotation when these methods do not work. Overfilling of the joint may be due to the large size of the prosthesis, or it may be due to improper placement of the prosthesis. To reestablish a normal humeral head height, the humeral prosthesis should be about 5 mm higher than the greater tuberosity, so the humeral osteotomy surface should be close to the medial surface of the supraspinatus stop, otherwise the prosthesis will be positioned high, causing excessive tension in the joint capsule and restricting supination, and causing frequent impingement of the rotator cuff tendons around the humeral head under the rostral arch. In addition, the prosthesis must be in a neutral position in the medullary cavity. When the forearm is draped over the body, the shoulder joint is incongruously filled and the greater tuberosity is abnormally protruded, resulting in rotator cuff laxity, glenohumeral instability and dynamic impingement, which affects the function of the shoulder joint. VI. Nerve injury : The incidence of nerve injury after shoulder arthroplasty is low. Lynch et al. reviewed 417 patients with total shoulder arthroplasty and a total of 18 cases had nerve injury, 13 of which were brachial plexus injuries. The authors identified a long incision (deltoid pectoralis major approach) and the use of aminoglutethimide (for rheumatoid arthritis) as risk factors for injury. He believes that intraoperative exposure with the upper arm in an abducted 90 position or in external rotation and posterior extension can strain the brachial plexus and cause nerve injury. Of course, the prerequisite for avoiding nerve injury is to be familiar with the anatomical relationship of the shoulder joint: the axillary nerve penetrates into the quadrilateral foramen at the inferior border of the subscapularis muscle, and the external rotation of the humerus can increase the distance between the subscapularis muscle dissection and the axillary nerve, which is conducive to the protection of the axillary nerve; the myocutaneous nerve can enter the rostro-humeral muscle within 5 cm from the root of the rostral process, and excessive freeing of the joint tendon must be avoided after cutting the rostral process. The incidence of heterotopic ossification and infection are 24% and 0.8%, respectively, and the preventive measures are the same as those for other arthroplasty; shoulder pelvis wear and central displacement are complications specific to humeral head replacement, and total shoulder revision can eliminate the symptoms.