Subacromial Erosion After Hook Plate Fixation in Acute Acromioclavicular Joint Dislocation

Objectives: To determine the incidence of subacromial erosion, perform quantitative analysis, and identify risk factors after locking hook plate fixation for acute acromioclavicular joint injury. Design: A retrospective case series study. Setting: A single tertiary university hospital. Patients/Participants: The study was conducted on 35 patients who had acute acromioclavicular joint injury. Intervention: Patients underwent the locking hook plate fixation. Main Outcome Measurements: The computed tomography (CT) was conducted to measure the subacromial erosion. The acromioclavicular slope (AC slope) of the unaffected side, the acromion-hook angle, the acromioclavicular anteroposterior distance (AC-AP distance), and the preoperative acromioclavicular interval of the affected side were analyzed to identify the risk factors of subacromial erosion. Results: According to the CT findings, subacromial erosion was found in all cases, and the mean value was 5.0 mm, which is 53% of the entire acromion thickness. The AC slope (B = −0.159, P < 0.001) and AC-AP distance (B = 0.233, P = 0.004) were found to have a significant influence on postoperative subacromial erosion. The AC slope showed a negative correlation with the amount of erosion, whereas the AC-AP distance showed a positive correlation with erosion. Conclusion: The CT findings revealed that subacromial erosion occurred in all cases, and the mean erosion depth was about 50% of the acromial thickness. If the preoperative AC slope of the unaffected side was more acute and the AC-AP distance was larger, the incidence of subacromial erosion was higher. Level of Evidence: Therapeutic Level IV. See Instructions for Authors for a complete description of levels of evidence.


INTRODUCTION
The acromioclavicular joint (ACJ) injury is a common injury that accounts for about 9% of the total shoulder girdle injuries. About half of the ACJ injuries occur in young people in their 20s, and the incidence among males is 5 times higher than among females. 1 The most common injury mechanism of the ACJ injury is considered as an external force applied directly from above the acromion process, which usually occurs when falling with the upper limb in an adducted position. 2 Among ACJ injuries, incomplete injuries that belong to Rockwood type I and II have been reported to show good results with nonsurgical treatment. For the higher grade injuries, such as injuries graded to Rockwood type IV ; VI, surgical treatment showed satisfactory outcomes. 3,4 However, there is still much controversy over optimal treatments for the injuries that belong to Rockwood type III. [4][5][6] Recent biomechanical studies have suggested that restoring acromioclavicular distance as closely as possible to a preinjury state is important for maintaining the superoinferior and anteroposterior (AP) stability of ACJ. 2 A variety of surgical treatments for ACJ injuries have been reported to date. Of these surgical treatments, the locking hook plate, which is a technique to promote the natural healing of the injured ligament through ACJ fixation, has been used widely as a surgical treatment for acute ACJ injuries because of a simple surgical technique, a short operation time, and satisfactory clinical results. 7,8 Di Francesco et al 7 reported midterm results of the use of hook plate in Rockwood type III and V ACJ injuries. In that study, the authors addressed that an acceptable joint alignment was achieved in all the patients with the healing of coracoclavicular ligament in 88% of patients. On the other hand, subacromial erosion is a typical complication of the hook plate that can occur because of its characteristics of forming nonrigid fixation, which may lead to acromial fracture. [9][10][11][12] It has been reported that 16%-26% of those patients who underwent hook plate fixation had subacromial erosion. 13,14 However, because the studies up to now determined the presence of erosion only by with visible radiolucency around the hook on the plain radiographs, the incidence rate may be altered if other imaging modalities, such as computed tomography (CT) scan, were used.
The purpose of this study was to evaluate the clinical and radiological outcomes after locking hook plate fixation for acute ACJ injury and to conduct quantitative analysis of the true incidence and erosion of subacromial erosion through the CT evaluation. In addition, the study also tried to identify the preoperative and postoperative risk factors of erosion. The authors of the study hypothesized that the hook plate might cause a much higher incidence of subacromial erosion than reported in the previous literatures.

Ethics Approval
This study was retrospective in nature, and final approval of informed consent exemption by the institutional review board was obtained (KHUH IRB 2019-04-079).

Patient Selection
This study retrospectively examined those patients who underwent hook plate fixation for acute ACJ injury at our hospital from January 2011 to October 2015. The inclusion criteria were as follows: (1) ACJ injury of Rockwood type III or higher, (2) operation within 2 weeks from the date of injury, (3) coracoclavicular ligament disruption confirmed by magnetic resonance imaging, (4) follow-up of more than 1 year after surgery, and (5) CT scan before implant removal. The exclusion criteria were as follows: (1) ACJ injury of Rockwood type I or II, (2) the patients who had operation after 2 weeks of injury, (3) the patients with concomitant other part fracture around shoulder including distal clavicle fracture, and (4) loss to follow-up. During this period, ACJ injury of Rockwood type I and II had conservative treatment with arm sling application for 2 weeks. For ACJ injury of Rockwood type III or higher, all the patients received hook plate insertion. Finally, a total of 35 patients were selected for the study based on these criteria.

Operative Technique and Rehabilitation
All surgeries were performed by one senior author, who has more than 25 years of practice in shoulder surgeries. The operation was performed in a beach-chair position under general anesthesia; a longitudinal incision was performed along the long axis of the clavicle centered on the ACJ, and then the upper part of the dislocated joint was exposed. The senior surgeon inspected the coracoclavicular ligament and visualized the disruption, cleaned out the traumatic zone of injury/hematoma. Reduction was performed by pressing the distal clavicle with using wide and flat osteotome and K-wires were inserted longitudinally and temporarily to the ACJ. After inserting a locking hook plate (3.5 mm LCP Clavicle Hook Plates; Synthes, Solothurn, Switzerland) into the proper position, we checked whether ACJ was reduced in C-arm. All plates were 5-hole plates, 85 mm in length including 26 mm of hook length. Hook depth was determined based on the restoration of the position of distal clavicle as closely as possible to the unaffected side. The hook was perpendicular to the plate, and it was not bended to align to the undersurface of the acromion. Then, 2.8 mm drill bit was used to predrill the screw hole, and we fixed the hook plate with 2 lateral bicortical 3.5-mm locking screws and 3 medial bicortical 3.5mm nonlocking screws.
Patients put on shoulder slings and were immobilized for up to 2 weeks after operation. After that, they started with passive motion and the range of motion (ROM) was gradually increased. Abduction and elevation above 90 degrees were allowed. Patients were instructed to refrain from engaging in excessive physical activities, such as playing tennis or climbing mountain, or returning to weight bearing other than ordinary daily activities until implant removal, and the ROM was exercised to a tolerable level, and extreme motion was avoided. After removal of the plate, passive ROM exercises, active ROM exercises, and any physical activities were allowed.

Preoperative and Postoperative Evaluations
Clinical evaluation was performed before surgery and in each follow-up examination. Pain was measured using the Visual Analog Scale (VAS) scores. The ROM of the shoulder joint was assessed by measuring forward flexion, external rotation at side, internal rotation to posterior, and abduction. The study measured changes in shoulder function using the University of California at Los Angeles (UCLA) score as a comprehensive clinical evaluation. The clinical evaluations were performed blindly by one of the author who is an orthopaedic surgeon.

Radiologic Evaluations
The panoramic unloaded AP radiographs of the affected and contralateral unaffected ACJ in neutral rotation were obtained with patients in a standing position immediately before operation, after operation, and in each follow-up examination including the last one. For AP radiographs, patients were placed in the upright position, and the arms were placed along the sides of their body. The central ray of the beam was angled 15 degrees cephalad to midclavicle. The source of image receptor distance was 100 cm. 15 To evaluate the postoperative reduction quality, the following radiologic factors were measured ( Fig. 1): (1) acromioclavicular interval (ACI), (2) coracoclavicular distance (CCD), and (3) acromioclavicular distance (ACD).
ACI was defined as the perpendicular distance between the clavicle distal end and the medial edge of the acromion; CCD was defined as the perpendicular distance between the upper border of the coracoid process and the inferior cortex of the clavicle; ACD was defined as the perpendicular distance between the line passing the upper border of acromion and the line parallel to the upper border of the lateral part of the clavicle. All radiographs were analyzed by 2 authors who reached a consensus. Reduction loss was decided by comparing ACI, CCD, and ACD in the immediately postoperative and the final follow-up radiographs. Radiologic results of contralateral unaffected side and affected side at the final follow-up examination were compared to determine whether anatomical reduction was appropriate.
The CT scan was performed to all the patients who had hook plate insertion for AC separation. It was performed just before implant removal for quantitative analysis of subacromial erosion, and the length of a vertical line drawn from the undersurface of acromion to the upper end of the erosion was taken as the depth of erosion and measured in a sagittal and coronal view, respectively. The largest erosion depth measured in each view was recorded as the final erosion depth (Fig. 2). Plain radiograph assessments are performed only in 2-dimensional views, and there would be some limitations with accurately measuring the depth of the subacromial erosion (Fig. 3). Therefore, only CT scans were used for measuring the depth of the subacromial erosion.
The following radiologic factors on plain AP radiographs or CT images were additionally measured to determine the risk factors of subacromial erosion occurring after hook plate fixation (Fig. 4): (1) acromioclavicular slope (AC slope) of the unaffected side, (2) acromion-hook angle (AH angle) of the affected side, (3) acromioclavicular AP distance (AC-AP distance) of the affected side, and (4) preoperative ACI of the affected side.
The AC slope was defined as the angle formed between the upper surface of the acromial process and the upper surface of the distal clavicle on the plain AP radiograph. The AH angle was defined as the angle formed between the upper surface of the acromial process and the hook of the locking hook plate on a plain AP radiograph. The AC-AP distance, which was measured in the axial view of the CT, was defined as the distance between the 2 lines tangent to the anterior margin of the acromial process and the anterior margin of the distal clavicle. In addition, the preoperative ACI of the affected side was included as a risk factor and measured together with the aforementioned 3 items. Radiologic evaluations were performed by 2 fellowship-trained orthopaedic surgeons.

Statistical Analysis
To compare the mean values, the data were analyzed using the normality test in the Kolmogorov-Smirnov test. All the variables showed the normal distribution. The paired t test was performed to assess the difference between the preoperative and postoperative results. Comparisons between the 2 groups were performed using the x 2 test for comparing proportions between groups and the independent t test for mean difference between groups. A Spearman correlation test was performed to assess the correlation between subacromial erosion and patients' factors. The multivariate regression analysis was used to identify the risk factors for the subacromial erosion. The intraclass correlation coefficient (ICC) was calculated to assess the consistency between observers for all radiologic measurements. ICC of less than 0.01 was determined to be poor; 0.01-0.2, slight; greater than 0.2-0.4, fair; greater than 0.4-0.6, moderate; greater than 0.6-0.8, substantial; and greater than 0.8-1.0, almost perfect. 16 Statistical analyses were conducted using IBM SPSS Statistics, version 20.0 (SPSS, Chicago, IL), and P , 0.05 was considered statistically significant.

Clinical Outcomes
According to the VAS scores, the mean pain level before operation was measured at 7.6 points, which improved significantly by 0.9 points right after the hook plate removal and by 0.8 points at the final follow-up examination (P , 0.001). The ROM of the affected side showed a slight limitation at the final follow-up when compared with the unaffected side. Statically significant differences were seen only in the forward flexion and the internal rotation posterior but the differences were not so large (P = 0.004 and 0.005) ( Table 2). The mean UCLA score at the final follow-up examination was 30.1 6 5.2 points, which is a significant improvement when compared with the mean UCLA score before operation that was 12.2 6 3.8 (P = 0.003). The subgroup analysis of UCLA score was described in Table 3.

Radiologic Outcomes
The ICCs of immediate postoperative and the final follow-up evaluations were almost perfect and noted in Table 4.
For reduction quality, 3 factors (ACD, CCD, and ACI) showed that the affected side was well restored right after surgery without any significant difference from the unaffected side. At the final follow-up examination after the removal of the hook plate, all 3 factors showed a significant reduction loss compared with the uninjured side, but the difference was as modest as 1.6 ; 2 mm (Table 5).
Subacromial erosion was found in all cases in the CT images. However, only 57.1% (20/35) showed subacromial erosion on plain radiographs (Fig. 3). The mean erosion depth was 4.7 6 2.4 mm in the coronal view and 4.7 6 2.2 mm in the sagittal view, which corresponded to 47% and 53% of the total acromion thickness, respectively. Seventeen cases or 49% of the entire patient group showed erosion with a size of more than 50% of the total acromion thicknesses. The ICCs of erosion depth were classified as almost perfect, with intraobserver and interobserver ICCs of 0.91 and 0.89, respectively (Table 4).

Risk Factor Analysis for the Subacromial Erosion
The ICCs of 4 radiologic factors were almost perfect and noted in Table 4. According to the results of correlation analysis of 4 radiologic factors measured to identify risk factors of subacromial erosion, all of them showed significant correlations with postoperative erosion. A negative correlation was found in the AC slope (r = 20.491, P = 0.003), whereas positive correlations were found in the AH angle (r = 0.579, P , 0.001), AC-AP distance (r = 20.436, P = 0.009), and preoperative ACI (r = 0.341, P = 0.045). In the final multivariate regression analysis of these 3 factors, 2 factors such as AC slope (B = 20.159, P , 0.001) and AH angle (B = 0.233, P = 0.004) were found to have significant influence on postoperative subacromial erosion. There was no correlation between the Rockwood type of ACJ injury (P = 0.304) and the patients' age with the amount of erosion (P = 0.853).

Complications
The most common postoperative complication was subacromial erosion which was found in all patients on CT scan. Among them, 4 cases (11%) showed that only a thin cortical shell remained as a bone above the hook and 2 cases (6%) had acromial fracture. Amount of erosion were not significantly related to clinical outcomes such as VAS scores and UCLA scores (P = 0.218 and P = 0.342, respectively). There was significant reduction loss compared with the uninjured side after implant removal at the final follow-up. However, there was no ACJ instability or upward displacement of clavicle over 3 mm compared with the unaffected shoulder after implant removal.

DISCUSSION
In the past, transarticular fixation using Kirschner wire or Steinmann pin was widely performed and showed satisfactory clinical results. However, as complications such as wire breakage or migration have been reported recently, it is rarely performed now. 17,18 Hook plate has been actively used as an ACJ fixation device recently to obtain good clinical and radiological results. Koukakis et al 19 reported an excellent functional outcome after hook plate fixation in patients with ACJ injury. Especially, because it does not require a high level of surgical skill, it can be performed successfully by a relatively less-experienced surgeon without any difficulty. Unlike the previously used Wolter plate, hook plate fixation does not require drilling into acromion, which makes the operation easy to perform. However, complications have been reported to occur because of the use of mobile hook. The subacromial erosion described in this study is a typical complication caused by mobile hook, and extra care and precaution need to be taken, because it may lead to pathologic acromial fracture in a severe case.
The most notable finding in this study was that subacromial erosion was found in all cases of hook plate in the CT images. The previous hook plate studies reported that subacromial erosion occurred as a postoperative complication, but because they used a plain radiograph, they reported a considerably lower incidence rate (16%-26%) than what the authors of this study have found out. 13,14 In our study, the incidence rate of subacromial erosion was 57.1%, which was higher than those 2 studies. However, those 2 studies were on the lateral end fracture of the clavicle with using hook plate for reduction, meaning that there would be less tension on the ACJ compared with that of ACJ injury in our study. Another study by Liu and Yang 20 reported that the subacromial erosion rate after hook plate insertion was 58.0%, which was comparable with our study. In our study, there was discrepancy of the subacromial erosion rate between the plain radiographs and CT images. In the analysis of acromial morphology in case of hook plate fixation, Yoon et al 21 reported that there was a wide individual variation in the posterior acromial process when the hook was being fixed, which made it difficult to represent it as an average model. It means that it is not easy to judge subacromial erosion on plain radiograph. Therefore, the authors used the CT to determine the occurrence of erosion before the hook plate removal and found that subacromial erosion occurred in all cases. This incidence rate was higher than the ones reported by the previous studies using plain radiographs, so the clinician should pay more attention to complications. It was found that the erosion of a size of more than 50% of the acromial thickness occurred in about half of all patients. In addition, it was found that 6 cases or 17% of the entire patient group, which was considered to be a significantly high percentage, had acromial fracture or impending fracture where erosion was severe to the extent that only the acromion cortical shell remained. Many previous studies recommended that the retention after hook plate fixation should last for 3-6 months before it was removed. 12,22,23 In our study, the implant retention lasted for 3.2 months in average before it was removed, which was less than other studies. However, 17% of the patients had severe subacromial erosion or fracture, which is a high rate, and it means that if the implant retention period lasts longer for the better healing of ligament, it can increase the incidence rate of acromial fracture.
The authors tried to identify risk factors for subacromial erosion among various preoperative and postoperative radiologic factors. Finally, both the preoperative AC slope of the unaffected side and the postoperative AC-AP distance of the affected side were found to be significant risk factors. The AC slope was negatively correlated with the amount of erosion, which means that the amount of erosion was increased as the slope was decreased to form a sharp angle. If the AC slope becomes smaller, the hook located on the acromial undersurface will have a more pointed contact, which leads to a greater stress on the tip of the hook, resulting in more erosion. The AC-AP distance was positively correlated with the amount of erosion, meaning that more distal erosion may occur if the distal clavicle is reduced more in backward than the acromion. Because this is related to AP stability after fixation of the hook plate, it is believed that the displacement of a hook plate in the superoinferior direction is controllable but the displacement in the AP direction is difficult to control. In other words, if the AP instability is in a serious condition due to a severe soft tissue and ligament damage around the ACJ, the hook plate alone is not sufficient to restore the affected side to the original anatomy, and the resulting erosion may be more serious. The limitations of this study are as follows: First, it was a retrospective study and thus had all the disadvantages associated with retrospective models, including selection bias. Second, we were not able to measure the depth of the subacromial erosion from plain radiographs. It would be more informative if it were able to compare the depth of the subacromial erosion between plain radiographs and CT scan. Third, because of the small number of patients, we could not do subgroup analysis of patients' factors which can affect the amount of subacromial erosion, such as osteoporosis, diabetes, kidney disease, and smoking history. Finally, because all the patients were allowed to do abduction and elevation above 90 degrees after 2 weeks of immobilization, we could not know if the restriction of the abduction and elevation for longer period would have different amount of erosion.

CONCLUSIONS
The CT scan images showed that subacromial erosion occurred in all cases and about half of the patient group had erosion of more than 50% of the total acromial thickness. It was found that if the preoperative AC slope of the unaffected side became more acute, and if the AC-AP distance was larger, there was more subacromial erosion.