We designed this biomechanical experiment based on the anatomical characteristics of the greater tuberosity and rotator cuff as well as the biomechanical characteristics of fragment displacement to compare the fixation strength of two commonly used internal fixation methods to treat split-type greater tuberosity fracture in two traction directions of the supraspinatus and infraspinatus-teres minor muscles. The supraspinatus plays a major role within the range of shoulder abduction less than 15°, while the infraspinatus and teres minor muscles are mainly involved in the external rotation of the shoulder joint [13]. The displacement of the greater tuberosity fragment results in the instability of the rotator cuff and dysfunction of the shoulder joint. Therefore, we should pay attention to the fixation strength in both directions of shoulder abduction and external rotation. In recent years, researches on this type of fracture have focused primarily on fixation strength in the direction of supraspinatus traction. A study by Seppel et al. [14] utilizing 12 cadaver bones showed no statistical difference in the fixation strength of single-row and double-row anchors in the treatment of split-type greater tuberosity fracture. Lin et al. [15] compared the fixation methods of double-row suture anchor, double-row suture-bridge, and double-screw fixation in the direction of supraspinatus traction. When loaded to the 3-mm displacement, the fixation strength of double-row suture-bridge was stronger than that of other two fixation methods. When loaded to 5-mm displacement and failure, there was no statistical difference between double-row suture-bridge and double-row suture anchor, which were stronger than that of double-screw fixation. However, no studies have been reported on the fixation strength in the direction of infraspinatus-teres minor traction. The external rotation induced by infraspinatus and teres minor has not been paid much attention in clinic due to long-term immobilization and early limitations of external rotation of the shoulder joint after operation. To avoid shoulder joint stiffness resulting from long-term immobilization, early rehabilitation is essential but relies on the rigid fixation in both directions.
The double-row suture-bridge has been widely used in clinic, which can effectively resist the traction of the supraspinatus and prevent the postoperative redisplacement of the greater tuberosity fragment [16, 17]. However, this technique is more suitable for avulsion fractures of rotator cuff tendon attachment as well as rotator cuff tear [18]. For split-type greater tuberosity fracture with a large fragment, the suture anchor fixation is an elastic method, which may have shortcomings such as poor strength, anchor pulling out, rotator cuff cutting, etc., and its efficacy is controversial [19, 20]. In our study, the fixation strength of double-row suture-bridge technique has poor fixation strength to resist rotation in the direction of infraspinatus-teres minor traction. Therefore, it is not suitable for this type of fracture.
The double-screw technique has a strong advantage in the fixation of cancellous bone fracture. With cancellous lag screws, the fracture was rigidly fixed with compression. In this experiment, we found that the fixation strength of double screws was equal to that of double-row suture anchor in the supraspinatus traction direction, but greater than that of double-row suture anchor in the infraspinatus-teres minor traction direction. Moreover, double-screw fixation can be completed under arthroscopy with minimal incisions, or with a percutaneous method to reduce further damage to soft tissue and blood supply [21, 22, 23]. In recent years, absorbable screws have been introduced, which can reduce the burden of secondary surgery, avoid metal reaction, and reduce the phenomenon of stress shielding [24].
However, double-screw fixation also has defects, and the fixation strength is less than that of locking plate or tension band, and under continuous supraspinatus traction, the fracture fragment is easily displaced, making the internal fixation ineffective [25, 26]. Double-screw fixation with washers increases the risk of acromial impingement. Furthermore, for patients with osteoporosis or comminuted fragments, it is impossible to obtain instability [27].
Clinically, various mechanical factors affect the greater tuberosity of the humerus. The major limitation of this experiment is that the in vitro biomechanical experiments can only simulate the instantaneous fixation strength but not dynamic mechanical changes in the process of shoulder movement, which requires further investigation.