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I t p a y s t o b e A B M P C e r t i f i e d : w w w. a b m p . c o m / g o / c e r t i f i e d c e n t r a l 101 2 The glenoid labrum. Image is from 3D4Medical's Essential Anatomy 3 application, available on the App Store. There is no visible sign of trauma in Emily's shoulder region, so functional evaluation will be important for determining the tissues most likely injured. 3 Nerves of the brachial plexus, stretched during lateral flexion to the opposite side. Image is from 3D4Medical's Essential Anatomy 3 application, available on the App Store. Glenoid labrum Nerves of the brachial plexus The shoulder joint lacks bony stability, so the rotator cuff muscles are key to providing that stability. In Emily's injury scenario, a sudden traction force was applied to the upper extremity, placing an enormous pulling load on the rotator cuff muscles. Compounding the problem is the fact that Emily was in a relaxed state at the time, and thus the muscles were not taut or prepared for the pulling force, which means their ability to withstand the traction load was greatly reduced. Emily also likely sustained compression trauma in the shoulder joint when she fell. If she landed on her elbow or outstretched hand, it is likely she jammed the humeral head into the glenoid fossa as she landed on the ground. The sudden compressive load on the shoulder joint could have thrust the humeral head against the underside of the acromion process and seriously pinched subacromial tissues, such as the supraspinatus tendon, subacromial bursa, or upper margins of the glenohumeral joint capsule (Image 1). We should also consider that this potential sudden compressive load could have jammed the humeral head against the glenoid labrum (Image 2). A sudden compressive injury, like falling on an outstretched arm, is a common mechanism for producing injury to the glenoid labrum. There is a good possibility that the pain Emily is feeling down her upper extremity is coming from rotator cuff damage. However, she reports the pain as more like a sharp and burning sensation, which usually points to some type of neurological injury. Most nerve injuries in the upper extremity, such as thoracic outlet syndrome and carpal tunnel syndrome, involve nerve compression. But the mechanics of this particular injury situation don't indicate the likelihood of nerve compression. She could have developed an acute nerve compression by slamming her hand on the ground when she fell. However, she is also reporting sensations in her neck and upper extremity, and this is unlikely to result from nerve compression at the wrist. When we consider the biomechanics of her injury more closely, another possibility emerges. Emily reported that she was in a relaxed state and looking in the opposite direction of the dog when he lunged at the squirrel. The dog took off to her right and suddenly jerked the leash, which would have pulled her upper torso to the right, making her head rapidly flex to the left laterally. This sudden and rapid movement to her right is similar to what happens in lateral whiplash in an automobile accident when a person is hit from the left.