Subacromial osteolysis and rotator cuff injury caused by the hook are serious complications caused by clavicle hook plates, and there are many reports of these complications [1–11]. An inappropriate hook depth and incongruency between the hook and undersurface of the acromion have been reported as causes of subacromial osteolysis [20–23].
If the hook depth is too short, the pressure applied to the acromion by the hook will increase and cause subacromial osteolysis [22, 23]. Lee et al. used finite element analysis (FEA) and found that deeper implantation of the hook reduces the force applied to the acromion by the hook [22]. However, if the hook depth is too long, the hook will make contact the rotator cuff, causing impingement syndrome or rotator cuff injury [1, 3, 5, 8, 11]. It is important to select the appropriate hook depth to prevent these complications.
According to previous reports about the morphology of the acromioclavicular joint, there are many patients whose acromion is inclined downward with respect to the distal end of the clavicle in the coronal plane [6, 24]. The hook portion of the clavicle hook plate forms a right angle and does not match the shape of the acromion. Therefore, when the clavicle hook plate is installed on a patient whose acromion is inclined downward, only the tip of the hook comes into contact with the acromion. Shen et al. reported that bending of the hook is necessary to increase the contact area between the hook and the acromion because an increased acromioclavicular angle promotes osteolysis [6]. Li et al. compared clavicle hook plates with hook angles of 0 and 15 degrees and reported that patients treated with the 15-degree hook plate had less severe pain and better postoperative recovery than those treated with the 0-degree clavicle hook plate [25]. Hung et al. reported that the more the hook was inclined downward, the more proximal the contact position between the hook and the acromion [20]. Based on the above results, we hypothesized that subacromial osteolysis can be reduced by bending the hook downward.
Since the angle between the acromion and the distal end of the clavicle varies greatly, the appropriate bending angle of the hook varies from case to case. In our study, there were individual differences in the hook inclination angle, which ranged from 2 to 23 degrees and was directed downward. It is necessary to determine the appropriate hook bending angle for each patient. Hyun et al. reported that subacromial osteolysis can be reduced by bending the hook using a fluoroscopic view obtained by directing the X-ray beam from the anterior to posterior direction at approximately 30 to 40 degrees caudally [4]. However, Lee et al. reported that arthroscopy is necessary because fluoroscopic assessments cannot be used to accurately evaluate the presence of impingement of the hook and the rotator cuff [2]. In our study, even when the hook was bent arthroscopically such that the hook was parallel to the acromion, there were some cases in which the hook and the acromion did not appear parallel in the fluoroscopic view. The reason for the difficulty in determining the proper fluoroscopic direction is that the position of the hook relative to the acromion differs depending on the patient. ElMaraghy et al. reported that the posterior hook implantation angle varied widely, ranging from 0˚ to 67˚, from the midline toward the posterior aspect of the acromion [26]. According to the above results, arthroscopy was more useful than fluoroscopy in bending the hook accurately such that it was parallel with the acromion.
The width of the hook of the Synthes clavicle hook plate is 5.2 mm. To avoid hook breakage, the surgical technique manual provided by the manufacturer recommends not bending the hook more than 10 to 15 degrees. However, the hook of the HOYA HTS clavicular plate is bent at the base of the hook (arrow in Fig. 1A) using a dedicated bender. The width of the hook tip is 5.2 mm, but the width of the portion to be bent is 7.46 mm; therefore, it is difficult to break the hook, even when it is bent. In the mechanical tests conducted by the manufacturer, the hook of the HOYA HTS clavicular plate did not break, even when the hook was bent 17.5 degrees downward and a force of 200 N was applied to the hook from the top 1,000,000 times. Since bending the hook back and forth many times may cause the hook to break, we did not bend it in the reverse direction.
In previous reports, X-ray or computed tomography (CT) was commonly used to evaluate subacromial osteolysis [2, 4, 6–10]. However, it is difficult to accurately evaluate subacromial osteolysis. The radiolucent area cannot be evaluated accurately when the hook and the radiolucent area overlap in X-rays or hook artifacts appear in CT scans. In our study, there were patients in which osteolysis was revealed after plate removal, even though osteolysis was not visible before plate removal. We quantitatively assessed the subacromial bony defects by performing subacromial arthroscopy at plate removal. Using arthroscopy, the effect of hook bending on subacromial osteolysis could be accurately evaluated.
Rotator cuff injury is also a serious complication that is caused by the hook [1, 3, 5, 11]. Gu et al. reported that the rotator cuff and shoulder joint capsule can impinge with the hook if the hook is placed deeply or the subacromial space is small [1]. Lee et al. arthroscopically confirmed the presence of impingement of the rotator cuff and the hook, and a plate with a different hook depth was used or the hook portion was bent if impingement was identified [2]. We predicted that rotator cuff injury can be prevented by bending the hook to be parallel to the undersurface of the acromion. However, the degree of rotator cuff injury in the bending group was not different from that in the nonbinding group. Deng et al. reported that since most hook plates do not match the shape of the shoulder exactly, the hook depth is longer than the thickness of the acromion, then subacromial space is narrowed by the hook [27]. We considered that some degree of rotator cuff injury was inevitable when using the clavicle hook plate. Fortunately, although partial-thickness rotator cuff tears were observed, there were no patients with complete rotator cuff tears. There are reports showing symptom improvement without special treatment after plate removal, even in patients who exhibited pain due to the hook [1, 2, 5, 17]. We considered that localized injury of the rotator cuff surface can be naturally repaired by plate removal. Additional studies are needed to accurately determine the complications caused by the hook.
There were several limitations to this study. Our study was retrospective and nonrandomized, and the sample size was small. Furthermore, different plates were used for the bending group and the nonbending group. Therefore, the difference in the degree of subacromial osteolysis may be affected by factors other than hook bending.