Clavicular hook plate is designed according to the shape of human acromioclavicular joint, and the reduction of acromioclavicular joint is realized by lever principle. The risk of plate fracture after operation is low. Because of its simple operation and good curative effect, clavicular hook plate is widely used in clinic. However, a series of complications can occur after clavicular hook plate operation. (1) Failure of internal fixation and poor ligament repair; (2) Postoperative SIS symptoms such as periacromial pain, upper limb weakness and limited abduction of shoulder joint; (3) Subacromial osteolysis; (4) Clavicle stress fracture, acromial stress fracture.
The incidence of SIS after clavicular hook plate operation is 19% ~ 25%[8–9]. People have never stopped exploring the causes of SIS after clavicular hook plate operation. Macdonald et al. thought that the appearance of SIS after clavicular hook plate was related to the shape of acromion, and curved and hooked acromion were more likely to cause SIS than straight acromion. A number of studies [11–13] have shown that older patients after hook plate surgery may increase the risk of SIS, which may be related to shoulder tissue degeneration.Macdonald et al.found that the placement of clavicular hook plate would inevitably narrow the subacromial clearance, which may be related to the appearance of SIS. Elmaraghy et al.simulated clavicular hook plate implantation on cadavers and reached the same conclusion. It can be seen that there are many reasons for SIS after operation. We hope that the appropriate hook plate can be selected to reduce the possibility of SIS. Analysis reasons: when the type of hook plate used by the patient does not match with the patient itself, the difference between the depth of hook end and the height of acromion will be too large, resulting in the reduction of subacromial space. When the space occupied by the hook end is too much, the pain will be caused by the hook plate pressing the subacromial tissue, such as the acromial bursa and the long head of biceps brachii muscle.
Many scholars have put forward some suggestions on how to prevent the occurrence of SIS after hook plate operation. Li Kui et al.  suggested using shoulder arthroscopy to check whether the hook plate obviously occupies the acromion space during the operation, and then adjust the type of the hook plate. Li-Kun Hung et al. simulated the implantation process of clavicular hook plate at different hook angles (90°, 95°, 100°, 105° and 110°) for the treatment of acromioclavicular joint dislocation, and found clavicle hook plates with different hook angles It will cause different biomechanical behaviors of the clavicle and acromion. It is recommended to implant clavicle hook plates with different hook angles according to different shoulder joint shapes. Yin JP et al.conducted morphological studies on clavicular hook plate and acromioclavicular
joint, and found that the appearance of SIS was related to the mismatch between clavicular hook plate and acromioclavicular joint, suggesting personalized use of clavicular hook plate.However, due to the influence of intraoperative patient position, X-ray acromial imaging is not accurate , and it is difficult to promote the application of shoulder arthroscopy in the intraoperative assessment of the subacromial space clinically. As a result, it is difficult to measure the space occupied by hook plate in subacromial space, which can not provide suggestions for surgeons to choose hook plate type. It is difficult to design and use individualized clavicular hook steel plate because of the great individual difference in the shape of acromion. We think that the height of acromion can be measured by CT before operation, and the difference between the depth of hook end of hook plate and the height of acromion should be controlled within 6 mm in order to reduce the possibility of SIS after operation.
Preoperative imaging measurement of the height of the acromion is helpful to the selection of the type of hook plate used during the operation, but the data of the acromion may be inaccurate due to the presence of CT bone artifacts and cartilage. Therefore, it is better for the surgeon to measure the acromion height during the operation and select the appropriate clavicular hook plate. Our team developed a device to measure acromion height during surgery (Fig. 4).
After intraoperative exposure of acromioclavicular joint, the a side was placed at the distal end of the clavicle, and the B side was placed under the acromion. The height of the patient's acromion was measured by rotating the handle. The appropriate plate is then selected to reduce the presence of SIS after surgery. At present, the measuring instrument is in production, and its effect should be further studied after it is used in clinic.
To sum up, the appearance of SIS after clavicular hook plate operation may be related to the difference between the depth of the hook plate and the height of the acromion peak. When the difference is greater than 6 mm, it may be an influential factor for the appearance of SIS. The measurement of acromion height by imaging before operation can provide advice for the selection of hook plate model during operation, but there are defects. We expect the use of acromion height meter in clinical practice.