A new fracture model for "terrible triad" injuries of the elbow: influence of forearm rotation on injury patterns.

OBJECTIVE: The purpose of this study was to evaluate the influence of forearm rotation on failure patterns of the elbow under axial loads. METHODS: Fourteen upper extremities were resected mid-humerus and mounted on a custom apparatus, which allowed rotation of the ulna, radius, and humerus about a fixed wrist while loading in axial compression. Seven specimens were loaded to failure with the forearm in pronation and 7 in supination. RESULTS: Six of the 7 elbows axially loaded in pronation resulted in fractures of the radial head and coronoid with posterior dislocation (terrible triad). Six of the 7 elbows loaded in supination dislocated without fracture. One of the 7 elbows tested in supination had a terrible triad-type elbow injury. Five of the 6 specimens with ulna external rotation had damage to the lateral ligaments; all 8 specimens with internal rotation had damage to the medial ligaments. There were no significant differences in biomechanical parameters between pronation and supination. CONCLUSIONS: The forearm position during axial load was the primary determinant of fracture-dislocation pattern. When the forearm was pronated, a terrible triad injury pattern most often occurred. When the forearm was supinated, a dislocation without fracture most often occurred. In both forearm rotation positions, when the ulna internally rotated during failure, the medial structures were the first to be disrupted. When the ulna externally rotated, the lateral structures were the first to be disrupted. Understanding the pathomechanics of elbow dislocation may improve diagnosis and treatment of these injuries.

Development of a new model for rotator cuff pathology: the rabbit subscapularis muscle.

BACKGROUND AND PURPOSE: The New Zealand white rabbit subscapularis tendon passes under a bony arch to insert on the lesser tubercle of the humerus in a manner analogous to the supraspinatus tendon in humans. We assessed whether this unique anatomy may provide a new animal model of the shoulder to improve our understanding of rotator cuff pathology. METHODS: The dimensions of the rotator cuff insertions (subscapularis, supraspinatus, and infraspinatus) were measured on 10 fresh frozen cadaveric New Zealand white rabbit shoulders. Mechanical testing was performed on 8 fresh frozen subscapularis insertions (4 matched pairs). Video analysis of the gait cycle was performed on 2 live animals. RESULTS: The origins, insertions, and innervations of the rabbit rotator cuff musculature are analogous to those in humans. However, the rabbit acromion is a rudimentary structure with only the infraspinatus and teres minor muscles passing beneath. Furthermore, at the point where the infraspinatus passes under the arch, it is muscular rather than tendinous. The anterior aspect of the glenohumeral joint contains an additional bony tunnel with its boundaries being the tuberculum supraglenoidale laterally, the coracoideus process superiorly, the tuberculum infraglenoidale inferiorly, and the coracobrachialis muscle medially. The origin of the rabbit subscapularis muscle resides on the anterior scapula. The subscapularis tendon then traverses this bony tunnel prior to its insertion on the lesser tubercle of the humerus. Video analysis and anatomic dissections confirmed excursion of the subscapularis tendon within this bony tunnel throughout the gait cycle. The subscapularis footprint on the proximal humerus measured 6.8 mm (SD 0.29) x 2.5 mm (SD 0.17). Mechanical testing of the subscapularis tendon showed the stiffness to range from 57 to 117 N/mm (SD 23). Ultimate yield ranged from 88 to 215 N (SD 518). The elastic modulus of the rabbit tendon was 56 MPa. 6 of the 8 subscapularis tendons failed at the tendon mid-substance; the other 2 failed at the bony insertion. INTERPRETATION: The unique anatomic architecture and the mechanical characteristics of the rabbit subscapularis muscle provide an opportunity to improve our understanding of rotator cuff pathology.