Watertown Chiropractic - Shoulder Injuries and Auto Accidents

Shoulder Injuries in Motor Vehicle Collisions

There are four chief classes of glenohumeral trauma in low-to-moderate velocity motor vehicle collisions (MVC). They are (a) blunt force, (b) compressive, (c) distractive, and (d) inertial. More complex mechanisms are seen in unrestrained occupants, roll-over crashes, and in complex collisions involving multiple impacts and multiple impulses.

Blunt force can be directed to any point on the shoulder, but these are typically laterally oriented for drivers and outboard passengers, most commonly the result of contact with the vehicle's interior hard points, such as the door or B-pillar. Compressive injuries occur to both drivers and passengers when bracing against a frontal crash. As far as I am aware, the etiology of the distractive injury was not appreciated until crash testing at my institute revealed the mechanism. When a driver grips the steering wheel forcefully in a rear impact MVC, the car will be accelerated forward beneath him. This can result in a substantial distractive force within the joint and the rotator cuff tendon. Using the impulse-momentum theorem, the forces within the joint will calculate to be surprisingly high due to the brief duration of the load.

Inertial force injuries, as the name implies, are related to differential inertial effects due to differential impulse delivery. In essence, when the occupant is in the normal seated posture, a rear impact MVC will first accelerate the torso. The upper extremities, which each have substantial mass and weigh 6-9 lb in an adult, are not immediately accelerated by the seat back. Thus, they lag behind the torso's acceleration and this results in a posterior shear force that will tend to produce either a Bankart type lesion or an anterior labroligamentous periosteal sleeve avulsion (ALPSA). As the arm's inertia catches up to the torso's, the torso is then abruptly arrested by the emergency locking retractor and shoulder harness, and the arm's forward momentum then produces an anterior shear effect across the glenoid labrum. This can produce humeral avulsion of the glenohumeral ligaments-the so-called HAGL lesion. Heavier persons are likely to be a greater risk than persons of slight build. In distractive and inertial loading, rotator cuff injuries are also possible. It is also not uncommon to see impingement syndromes develop in the aftermath of whiplash injuries, although the etiology is beyond the scope of this newsletter.