Risk Factors for postoperative shoulder pain of acromioclavicular joint dislocation treat with hook plate

Background: Postoperative shoulder pain was a most common complication after clavicular hook plate treatment for acromioclavicular joint dislocation. However, the researches on its risk factors were rare. The purpose of this study was to evaluate the incidences of postoperative shoulder pain of acromioclavicular joint dislocation treat with hook plate and to identify risk factors in patients with an acute acromioclavicular joint dislocation. Method: We retrospectively analyzed the prospectively collected data from 310 consecutive patients with AC joint dislocation between December 2014 and August 2019 at our institute. Patients rated the average intensity of shoulder pain using an 11-point numerical rating scale (NRS). The dependent variable was the presence of moderate-to-severe neck pain (NRS ≥4) at the last follow-up when the internal fixation will be removed. The independent variables included age, gender, body-mass index (BMI), smoking status, alcohol consumption, type of injury, Rockwood Classification, site of injury, operation time, injury-to-surgery, DHA, DH and AHP. Logistic regression analysis was performed to identify independent risk factors of moderate-to-severe shoulder pain of acromioclavicular joint dislocation treat with hook plate. Results: A total of 292 patients were included in the study and 18 cases were lost to follow-up. The follow-up rate was 94.1%. In all cases, there were 166 male cases and 126 female cases. Of these cases, the NRS < 4 group had 219 patients. Among them, there were 120 males and 99 females. 12 patients were lost to follow-up. There were 73 patients in NRS ≥ 4 group, 46 males and 27 females. 6 cases were lost to follow-up. The number of patient in NRS ≥ 4 group accounted for about 25.0% of the total cases. DH was the significant independent risk factor for postoperative shoulder pain of acromioclavicular joint dislocation treat with hook plate. DHA and AHP were also independent factors of postoperative shoulder pain, but they were all protective. Conclusions: DH was the significant independent risk factor for postoperative shoulder pain of acromioclavicular joint dislocation treat with hook plate. DHA and AHP were also independent factors of postoperative shoulder pain, but they were all protective. We should try to refer to these factors to avoid postoperative shoulder pain when performing clinical operations.


Introduction
Acromioclavicular joint dislocation is a common shoulder injury, accounting for about 9% of all shoulder injuries (1). Since Hachkenbruch first designed the clavicular hook plate and applied it to treat acromioclavicular joint dislocation, it has been gradually applied to the distal clavicle fracture and acromioclavicular joint dislocation in the world, and it was considered to be an ideal treatment (2,3). With the increase in usage, Some internal fixation complications gradually emerged, such as shoulder pain after internal fixation, subacromial impingement syndrome, subacromial erosion, failure of internal fixation, stress fracture(4). Especially postoperative shoulder pain, it was a common complication after clavicular hook plate treatment of acromioclavicular joint dislocation. At the same time, it was also a comprehensive manifestation of various complications, affecting shoulder joint activity and functional recovery and leading to a decline in quality of life. The literatures reported that the total incidence of postoperative Shoulder pain was about 14% (5, 6). "What is the cause of postoperative shoulder pain? How to prevent it? " became a big problem that needs to be solved urgently. A large number of scholars have conducted a lot of research and found that this was often related to the position of the hook, subacromial impingement syndrome and functional exercise (7, 8). However, there were no reports on definitive risk factors for postoperative shoulder pain after hook plating until now. The purpose of this study was to evaluate the incidences of postoperative shoulder pain of acromioclavicular joint dislocation treat with hook plate and to identify risk factors in patients with an acute acromioclavicular joint dislocation.

Patients
For this research, Acromioclavicular(AC) joint dislocation were classified using the method described by Rockwood et al(9). The inclusion criteria for the research were as follows: (1) acute, AC joint dislocation (grade III, IV, V, and VI ) determined according to the classification described by Rockwood et al; (2) no more than 14 days of trauma; (3) signed informed consent. The exclusion criteria were as follows: (1) other types (grade I, II) of AC joint dislocation; (2) no signed informed consent; (3) previous surgery on the shoulder; (4) history of AC joint dislocation or other shoulder trauma.
We retrospectively analyzed the prospectively collected data from 310 consecutive patients with AC joint dislocation between December 2014 and August 2019 at our institute. The research protocol was approved by the Institutional Review Board of the authors' institute. Written informed consent was obtained from all of the participants, and the research methods were carried out in accordance with approved guidelines.

Operation
Surgery was performed under brachial plexus anesthesia or general anesthesia.
Patients with supine position, and placed a pillow under the medial margin of the scapula of sick shoulder. After the routine disinfection and covering surgical position, took an incision to the arcuate of the shoulder from the distal end of the clavicle, followed by cutting the skin, subcutaneous and plastric muscles, separating the trapezius fascia to exposure the acromioclavicular joint. The dislocated lateral clavicle was reduced and temporarily stabilized by transarticular Kirschner wires from lateral acromion. The hook of the pre-curved clavicular hook plate was inserted below the acromion, the plate body was placed above the clavicle and the locking screws were fixed. Repaired the acromioclavicular joint capsule in the surgery, stitched the distal end of the clavicle angle and trapezius muscle. After intraoperative fluoroscopy and passive activities of the shoulder joint, rinsed and sutured the wound.

Dependent variable
Moderate-to-severe postoperative shoulder pain at the last follow-up when the internal fixation will be removed was presence as the dependent variable. Patients rated the average intensity of postoperative shoulder pain using an 11-point numerical rating scale (NRS) (0: no pain; 10: worst pain imaginable). The cut-off point between mild and moderate-to-severe postoperative pain intensity was set at NRS ≥ 4 (10). For logistic regression analysis, patients were categorized into two groups: (1) those who had moderate-to-severe neck pain (NRS ≥ 4); (2) those who had no or mild neck pain (NRS < 4).

Independent variables
Patient characteristics Age at the time of surgery, sex, body-mass index (BMI), smoking status and alcohol consumption were included in the patient characteristics. According to the smoking status, the patients were categorized into two groups: current smokers and current non-smokers. The current non-smokers included the past smokers or those who never smoked (11). As for the definition of alcohol consumption, the standard we set was Alcohol > 15 drinks/week. A standard drink is 12 oz of 5% alcohol beer, 8 oz of 7% malt liquor, 5 oz of 12% alcohol wine or 1.5 oz of 40% alcohol liquor (12).

Surgical factors
The first was the operation time, which was specifically from the beginning of the skin incision to the final suture of the skin. The second was the time from injury to surgery which calculated in days.

Hook plate factors
Regarding the hook plate factors, this research included a total of three items. (1) The hook depth (DH), it specifically referred to the distance between the connection point between the hook body and the hook tip and the cortical bone below at the distal of the clavicle (Fig. 1). (2) The hook plate angle (AHP), the angle with hook plate body and hook tip (13) (Fig. 2). (3) The distance between hook and acromion articular surface (DHA), its definition was the closest distance from the acromial articular surface to the hook body (Fig. 3).

Statistical analysis
Means and standard deviations (SDs) were used to describe distributions for continuous variables, and proportions were used to summarize categorical variables. One-way ANOVA tests were used to compare the means of continuous and ordinal variables; values of 0.05 represent a statistically significant difference. For categorical variables, the Pearson chi-square analysis was used, and a P value of 0.05 represented a statistically significant difference. Binary logistic regression analysis was performed to account for confounding significant variables. P < 0.05 was considered significant. Odds ratios and 95% confidence intervals were calculated for each of the risk factors included in the logistic regression models.
Analyses were performed using SPSS version 21 (IBM).

General results
A total of 292 patients were included in the study and 18 cases were lost to followup. The follow-up rate was 94.1%. In all cases, there were 166 male cases and 126 female cases. Of these cases, the NRS < 4 group had 219 patients. Among them, there were 120 males and 99 females. 12 patients were lost to follow-up. There were 73 patients in NRS ≥ 4 group, 46 males and 27 females. 6 cases were lost to follow-up. The number of patient in NRS ≥ 4 group accounted for about 25.0% of the total cases.

Risk Factors for postoperative shoulder pain
According to relevant literature reports (13)(14)(15)(16)(17) and the clinical experience of the research team, this study included a total of 13 potential risk factors Age, Gender, body-mass index (BMI), smoking status, alcohol consumption, Type of injury, Rockwood Classification, Site of injury, Operation time, Injury-to-surgery, DHA, DH and AHP (Table 1). Age was divided into 3 categories: less than or equal to 30 years old, more than 30 years old and less than 60 years old, more than or equal to 60 years old. There were 43,148 and 28 patients in each age level of NRS < 4 group,  (Table 2).

Discussion
In this study, the incidence of postoperative shoulder pain was 25%, which was significantly higher than the 14% reported in the literatures (5, 6). Some studies had also occurred in a similar situation to this study. The incidence of postoperative shoulder pain was often higher and they were all above 20% (18) did not follow the criteria of "with and without". Instead, from a clinical perspective,"whether or not the treatment was needed " (10) we divided the postoperative shoulder pain into two categories: the NRS for mild pain and the NRS for moderate-to-severe pain (19). The existing literature provided variable cut-off points for identifying moderate-to-severe pain, ranging from NRS ≥ 3 to NRS ≥ 6, depending on differences in diagnosis and methods of analysis but most studies had Defining cut-off points of NRS ≥ 4 10).
According to the results of the final regression analysis, DH was the most significant independent risk factor for postoperative shoulder pain. DH was defined as the distance between the junction of the hook body and the hook tip and the subclavian cortex. The greater the DH, the deeper it was in the joint. The junction of the hook body and the hook tip was also the radian of the clavicle hook from straight to flat.
The radian was often located above the supraspinatus fossa of scapula. If the hook was too deep or the subacromial space was too small, it may cause oppression to the supraspinatus muscle and shoulder pain. The report of the anatomical study also found (20) that the radian of the hook was located above the humeral head. In 15 corpse specimens, 9 cadaveric specimens were found that the clavicle hook touched the muscular abdomen of the supraspinatus muscle. Another literature reported that some scholars performed shoulder arthroscopy on 12 patients with shoulder pain and shoulder limitation. 11 cases (92%) had rotator cuff compression (7). All these showed that the repression of supraspinatus muscle by the radian of hook had a close relationship with shoulder pain, and it coincided with the finding of this study that DH is an independent risk factor for postoperative shoulder pain.
DHA and AHP were also independent factors of postoperative shoulder pain, but they were all protective. DHA was the distance from the hook body to the acromion articular surface. Related literatures reported (6) that if the DHA was too small, the hook body easily came into contact with the shoulder and caused friction and pain.
In general, the gap between the hook body and acromion articular surface was greater than 5 mm, we could avoid rubbing when the shoulder joint moves. The AHP was the angle of the clavicle hook. The smaller the angle, the easier the hook tip will be upturned. This often resulted in surface contact between the tip of the hook and the acromion became point contact. The stress was highly concentrated on the contact point of the tip of the hook and the acromion, which was more likely to cause the occurrence of bone erosion and pain. Some reports also suggested that    Figure 1 The schematic diagram of the hook depth Make two middle lines along the hook body and the  The abridged general view of DHA Take the two points at the maximum distance above and b