Stability, Deformity and Fixation of the Floating Shoulder: A Cadaveric Biomechanical Study.

Abstract

Floating shoulder injuries cause instability and deformity due to disruptions of the scapula, clavicle and superior shoulder suspensory complex (SSSC) ligaments. Resulting deformity of the glenopolar angle (GPA) has not previously been established, nor has the impact on stability and deformity when surgical fixation is performed. This study sought to quantify stability and deformity for multiple injury patterns, and the improvement to these parameters provided by clavicle and coracoclavicular (CC) ligament fixation. Fourteen cadaveric specimen upper extremities were used, which included the entire upper extremity, scapula, clavicle and cranial-most ribs. After being mounted upright, a scapular neck fracture was created, followed by either a midshaft clavicle fracture or sectioning of the acromioclavicular (AC) and coracoacromial (CA) ligaments. Subsequent sectioning of the other structure(s) followed by the CC ligaments was then performed. In all specimens the clavicle was then plated, followed by a CC ligament repair. At each step, a radiograph in the AP plane of the scapula was taken to measure GPA, and displacement of the glenoid fragment using radiopaque markers placed in the scapula. These radiographs were taken both unloaded and with a 100N applied medializing force. When evaluating deformity related to sectioning, the GPA was reduced when the CC ligaments were sectioned compared to an isolated scapula fracture (p=0.022) and compared to a combined scapula and clavicle fracture (p=0.037). For stability, displacement with a 100N force was significantly increased when the CC ligaments were sectioned compared to an isolated scapula fracture (p=0.027). In cases of an ipsilateral scapula neck and clavicle fracture with intact ligaments, fixation of the clavicle alone provided a statistically significant improvement in GPA (p=0.002); but not in reduction of displacement (p=0.061). In cases of an ipsilateral scapula neck and clavicle fracture with concomitant disruption of the CA, AC and CC ligaments, GPA was improved by clavicle fixation (p<0.001), and increasingly so by subsequent CC ligament repair (p<0.001). Displacement was also improved in these two states respectively (p<0.001, p=0.008). This biomechanical study confirmed the importance of the AC, CA and CC ligaments in conferring stability in SCCC injuries. Disruption of the CC ligaments created significant deformity of the GPA and instability with a medializing force. Clinical treatment should consider the integrity of these ligaments and their repair in conjunction with clavicle fixation, knowing that this combination should restore a biomechanical state equivalent to an isolated scapula fracture.