Friday, September 28, 2012

Kinematic analysis of dynamic shoulder motion in patients with reverse total shoulder arthroplasty JSES

JSES has published Kinematic analysis of dynamic shoulder motion in patients with reverse total shoulder arthroplasty.

This important article reminds us that apparent shoulder motion, i.e. the motion of the arm relative to the thorax, is comprised of two components, humeroscapular and scapulothoracic. These to combine to yield the humerothoracic motion we discussed in yesterday's post. The article on reverse total shoulders referenced above uses an electromagnetic tracking system to differentiate humeroscapular (glenohumeral) motion from scapulothoracic motion. In 17 patients with reverse total shoulders, they found that, although humeroscapular motion exceeded scapulothoracic motion, the percent contribution of scapulothoracic motion to total humerothoracic motion was greater in patients with reverse total shoulders than in normal subjects. Since they found that humeroscapular motion in reverse total shoulders was 1.6 to 2 times the scapulothoracic motion, we can conclude that scapulothoracic motion provides about 1/3 of the total humerothoracic motion. The authors make a important recommendation: "scapulothoracic joint function...should be carefully assessed before proceeding with the operation [reverse total shoulder]."

So, how does one do this? As pointed out yesterday, one way is to use sensors connected to pins drilled into the humerus and scapula of volunteers to make sure that the movement of soft tissues on the bones did not affect the position of surface mounted sensors. To seen an example, look at minute 6:31 of this video

Another way as suggested in Practical Evaluation and Management of the Shoulder. Chapter 2. is to recognize that the position and motion of the scapula can be observed clinically through the use of four easily palpable landmarks:

By observing the patient's ability to actively move the scapula and comparing this ability to their unaffected shoulder, the surgeon can assess the scapulothoracic function necessary for a functioning reverse total shoulder.

The recognition that we can, using our fingers, define the position of the scapula while examining the shoulder gives us the ability to more precisely define humerothoracic (or glenohumeral) motion using scapular landmarks:

These data, whether obtained by our fingers or by electromagnetic tracking, can be recorded on a humeroscapular global diagram:


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