In the current biomechanical study, we compared the fixation strength of a conventional two-screw configuration to the same construct augmented with a cerclage wire for the treatment of humerus split type GT fracture. The result was in accordance with our hypothesis: the addition of a cerclage wire to the two-screw construct significantly decreased the fracture displacement in 100 N and 200 N traction forces, and also significantly increased the failure loading comparison to the conventional two-screw configuration.
Anatomic studies have shown that the GT is located 8 ± 3.2 mm inferior to the most superior aspect of the humeral head, lateral to the humeral head and posterolateral to the lesser tuberosity [25]. After GT fracture, the force vectors of the teres minor and the lower infraspinatus cause posterior displacement; meanwhile, the supraspinatus and upper infraspinatus result in superior displacement of the GT fragment [26]. When the displaced tuberosity is pulled posteriorly and superiorly, it may block the external rotation and abduction respectively [27, 28]. In addition, alteration of the rotator cuff attachment can lead to weakness of the rotator cuff and abnormal shoulder biomechanics. Previous biomechanical studies have indicated that 0.5 cm and 1.0 cm of superior displacement of the GT increases the necessary deltoid muscle force required for shoulder abduction by 16 and 27%, respectively, while a posterosuperior displacement of 1.0 cm increases the deltoid force by 29% [29]. Thus, Park et al. suggested that the fracture should be repaired if the displacement is more than 5 mm in young active patients, while fractures with 3 mm of displacement should be reduced in heavy laborers and athletes who are involved in overhead activity [30]. Our results showed that two-screw construct augmented with a cerclage wire had less than 3 mm displacement under 200 N load which was strong enough to bear rehabilitation interventions [31]. The two-screw construct augmented with a cerclage wire, average failure load more than 400 N, is expected to tolerate the maximal load from the supraspinatus of ~ 302 N [32].
40% of GT fractures are split fracture in which the fragment is large and the fracture line is parallel to the humeral shaft beginning proximally at the junction of the RC footprint and humeral head cartilage and extending distally and laterally to the level of the surgical neck [16]. Three methods of fixation have been described for split-type fractures: double-row suture-bridge, interfragmentary compression screws, or a small locking plate augmented with sutures through the RC tendon [17]. Suture anchor fixation have been described as an effective technique and a previous study, which compared the biomechanical strength of suture anchors and two-screw fixation in the management of split type GT fractures, showed that the suture anchor constructs were stronger than the fixation constructs using screws [24]. Nevertheless, the current results showed that the addition of a cerclage wire to the two-screw construct had the comparable biomechanical strength to the suture anchor constructs.
Open reduction with internal fixation (ORIF) and arthroscopic-assisted reduction with internal fixation (ARIF) can be employed to repair split-type fractures [33,34,35]. Some surgeons advocate plate and screw fixation for split GT fracture, and biomechanical study reveals that locking plate fixation provides the strongest and stiffest biomechanical fixation for split type greater tuberosity fractures [36, 37]. However, there remain concerns of more deltoid muscle dissection, axillary nerve injury [38], and subacromial impingement after osteosynthesis with plate and screws [39]. Previous studies have reported good functional recovery in treating isolated GT fractures with screw fixation [17,18,19]. A biomechanical analysis has also shown that strong fixation for isolated fractures of the greater tuberosity can be achieved by two cancellous screws [40]. Compared with plate fixation, there are advantages of screw fixation, including the less invasive approach, less intraoperative blood loss, lower risk for axillary nerve injury, and good shoulder function recovery [41]. Despite these advantages, screw fixation of a split GT fragment cannot always be performed due to insufficient bone stock, comminuted fracture, or osteoporotic bone. Moreover, screw fixation of a fragile GT fragment might lead to further comminution [42, 43]. Therefore, surgeons should exercise caution when using a screw fixation for GT fractures in older and osteoporotic bones.
French et al. suggested that the screws tightened by the wire loop provide a compressive force to counteract the varus force in supracondylar fractures of elbow [44]. Previous biomechanical analysis has also demonstrated that the addition of a cerclage wire provides substantial improvement in mechanical performance regarding fixation of femoral neck fractures when compared to the conventional inverted triangle triple screws construct [20]. The current results revealed that the two-screw construct augmented with cerclage wire significantly increased the biomechanical strength. This result may be attributed to the increased stability established by eliminating individual screw toggling. The washer played a role as the wire holder since the augmented wire was passed through the space between the screw and the washer. After screw insertion, the wire was twisted and tightened to connect the washers firmly. The augmented wire-screws construct was likely to serve as a single synthesized construct. Meanwhile, the stability of a femoral neck fracture was improved by adding an interlocking plate to three cannulated screws in the cadaveric biomechanical study [45]. The authors supposed that the synthesized construct comprising cerclage wire with cannulated screws had a similar function as the interlocking plate. More specifically, the entire construct reduced the motion between the screw and bone, and improved the strength of the femoral neck as well as GT fracture fixation.
Besides, tightening of the augmented wire might apply a distractive force on the screws, which provided the preload to the distracted screws. The force was directed against the subchondral bone which made the screw capture the humeral head firmly (Fig. 5). It could optimize the force to counteract the tension of the rotator cuff and reduce the micromotions. A previous study has proved that reduced micromotions might improve endosteal healing and sprouting angiogenesis [46]. These biological advantages may fasten the fracture healing in the early stage and theoretically decrease the risk of displacement, failure of the bone-implant construct, and mal-unions or non-unions. To our knowledge, this is the first study comparing the biomechanical strength of a screw-only construct and a screw-configuration construct augmented with a cerclage wire for the fixation of split type GT fractures. It provides support from a mechanical perspective for the treatment of isolated GT fracture of humerus.
This study has several limitations. Absolute standardization is difficult to realize because of the natural variations between cadaveric humeri regarding external and internal bone factors. Further, our results imply only the immediate postoperative strength because no healing can occur after the fixation. Fractures in the study were produced using a smooth saw, rather than the jagged features typically present at the interface between bone fragments in the clinical situation. Since actual forces on the humeral head are a combination of compression, torsion, and shear, the forces on the fixation construct may vary with different abduction angles. The loading pattern in this study was 0 degrees of abduction with a uniaxial direction. In addition, the model used here excludes some relevant forces, such as those of the infraspinatus and teres minor. These other forces may affect the clinical relevance of GT fractures. Therefore, our model, as with all models, may not represent the precise clinical situation. Last, although only one experienced orthopedic surgeon (CL Lin) performed all fixation constructs to maintain consistency, confounder might still exist. However, each of fixation construct in our study was completed manually to simulate clinical environment. Accordingly, future studies evaluating the strength of these constructs in different loading modes are needed.
In conclusion, this biomechanical cadaveric study demonstrated that a fixation construct augmented with a cerclage wire has superior biomechanical performance than a conventional two-screw configuration with respect to fixation of humerus split type GT fracture. It provides support from a mechanical perspective for the future clinical application of a cerclage wire.