Recently, several studies have demonstrated the use of the clavicular hook plate as a priority for distal clavicle fracture, especially for Neer type II fracture, with respect to the achievement of rigid fixation and a high rate of fracture union [3,9]. Karduna et al. proved that the hook plate can provide stronger anti-deformation capacity than conventional fixation such as tension band wire . However, other researchers have believed that the hook plate can exert adverse effects on subacromial tissues, including subacromial impingement, acromial osteolysis and rotator cuff tear. One cadaver study revealed that the placement of the implant should be repositioned based on the different types of the acromion, because the wrong position may cause subacromial impingement . Another cadaver study also indicated that the designs of the hook plate in fact cannot keep pace with the changes of acromioclavicular joint morphology . All the above suggested that appropriate selection of the clavicle hook plate correlated with prognosis closely. Only by combining the three factors of characteristics of hook plate (plate length, hook depth and hook angle), fracture pattern and acromioclavicular joint morphology into an organic whole, can the best therapeutic effect be realized.
The literature has still lacked a clear understanding of the relationship between characteristics of hook plate and acromioclavicular joint morphology. To date, almost all studies focused on the importance of acromion sagittal angle, and neglected distal clavicle–acromion coronal angle [12,19,20]. Furthermore, conventional distal clavicle–acromion angle (measured from the central axis of the distal clavicle and acromion) was insufficient for practical requirements according to our experience. Therefore, we performed the current study to determine whether large lateral angle of acromion correlated with the severity of postoperative pain in treating distal clavicle fractures with a clavicular hook plate.
Type selection of clavicular hook plate:
Currently, clavicular hook plates still cannot match the anatomy of the distal clavicle and acromion perfectly . For remedying the situation, a number of alternative characteristics of hook plate during the procedure, such as different plate length, hook depth and hook angle have been recommended. However, for some rare acromial morphology like large lateral angle of acromion, these characteristics cannot meet our demands.
In our study, only a plate equipped with a depth of 15 mm and small angle can barely accommodate the acromion with a large lateral angle. Interestingly, as the angle of implant increased, the resistance of reduction increased, which cannot be weakened by reshaping the plate. In our experience, this phenomenon was reflected obviously when α > 30° and β < 70°. Lee and Shih et al. investigated the mechanics of the plate length and hook depth using finite element analysis (FEA) method and found that the stress on the acromion and clavicle was smaller when using a hook plate with greater length and depth [22,23]. Hung et al. also used FEA to investigate the impacts of different hook angles. They found that a larger hook angle of implant exerted a larger load on the acromion because the larger hook angle made the contact position between the hook plate and acromion more proximal . This theory seemed to contradict the phenomenon we encountered. On the contrary, the contact position between the hook plate and acromion was in fact away from the proximal when using a larger depth and angle in large lateral angle of acromion. (Fig 4) Therefore, the hook plate was forced to be attached to the distal and proximal part of clavicle simultaneously, leading to excessive stress.
Paradoxically, a small angle and depth of hook plate may induce subacromial impingement more easily. Previous study has shown that the proximal clavicular hook was the main part compressing the supraspinatus tendon . When using a small angle, the corner of the proximal hook was fallen lower, resulting in a larger contact area and force on the supraspinatus. Furthermore, as the distal clavicle–acromion coronal angle increased, the risk of proximal hook slippage may increase . Therefore, at least in theory, we suggest that other schemes have to be taken into account when the α angle exceeds 10°. Whereas, from the observation in clinical practice, the α angle exceeding 30° may be a clear indication.
Lateral angle of acromion and impingement:
In our research, all patients complained of a variety of pain related to impingement, especially when their arms were lifted over their head. In addition, seven patients who had shoulder impingement symptoms scored lower JOA scores and had poor satisfaction. At the end of this study, the impingements had developed into rotator cuff tear in these seven cases for an overall incidence of 77.8%! In our supplementary cases, we have not found such a high rate of RCL, though the incidence had a tendency to increase with an increase in α angle.
Other investigators have also reported subacromial impingement and rotator cuff tear associated with implantation of a clavicular hook plate [9–11,13,25–30]. (Table 3) Of particular concerns were two articles. One study reported that incidence of subacromial shoulder impingement and rotator cuff lesion was calculated by dynamic sonographic evaluation, reaching as high as 37.5% and 15% . Another one investigated 12 patients treated with hook plate by arthroscopic evaluation, and 91.7% of them developed signs of impingement . Compared with the above, there was a far higher incidence of pain and rotator cuff tear in our research. We considered that the stress on supraspinatus tendon increased on account of the large lateral angle of acromion. Almost all studies have neglected the effect of distal clavicle–acromion coronal angle in the treatment of distal clavicular fracture, which was a significant factor impacting the outcome of the hook plate in our study.
To the best of our knowledge, the only solution to postoperative persistent pain was removal of the implant as soon as bony union has occurred. Leu et al. demonstrated that the impingement problems can disappeared within 8 weeks after removal . Of our patients, the mean time of removal was 6.7 months, which was longer than the other studies. A shorter interval between union and plate removal might provide a key factor for preventing further development of impingement. As evidenced by our supplementary research, RCL did not occur for those undergoing removal surgery within half years after operation in group A, B and C. In addition, early excessive mobility of the acromioclavicular joint may be another reason for developing a rotator cuff lesion in patients with a large lateral angle of acromion. According to a study by Kashii et al. , patients should avoid forward flexion or adduction greater than 90° and internal rotation of the shoulder behind the back until the hook plate is removed.
A limitation of this study was the small number of patients with a large lateral angle acromion. Further studies with a larger number of cases are needed. Another limitation of this study was that we did not carry out a comprehensive analysis for the all types of distal clavicle fracture. Besides, we cannot be certain that there is a variation of this angle between left and right sides in a single individual. Similarly, while one may assume the impingement from the plate cause the rotator cuff tear this study, as it is designed, cannot establish causation. In order to establish causation the patients each would have had to have an ultrasound or MRI before fracture fixation or the injury. But it is unlikely to perfect interrelated examinations before the injury. Lastly, a separate analysis of the hook plate was incomplete. Instead, a comprehensive accessment was available only by comparing a variety of procedures.