It has taken me 10 years to figure out why some people complain that the WristWidget® slips over the ulna. It isn't common but it is important. This blog is dedicated to my sister: the dreaded slippage of the WristWidget®.
This is also for the rare patient that sends the message, “My WristWidget® slips down my arm.” I hope to help you understand your “unique wrist” and the challenges that they pose.
Anatomical Variations of the Human Wrist.
Not every human body is the same. Surprise! There are all kinds of “anatomical variations” in the nervous system, boney structures, and tendons. Not all Ulna heads are the same. Over the past 10 years, I have looked at so many Ulna bones! The variations are fascinating to me.
I would like to thank Dr. Elisabet Hagert, Dr. Harvey Chin, and Dr. Steven L. Moran for describing this so beautifully in “Anatomy of the Distal Radioulnar Joint and UlnoCarpal Complex”
The joint between the ulna and radius at the wrist is not really a joint. It is a fairly recent evolutionary joint. Imagine the hips where the ball and socket are deep and defined. The long bone of the upper leg “femur”, sits into its socket with great stability. Next is the shoulder. The shoulder has fantastic mobility but limited stability. The bones aren't solid in place. The joint gains is stability primarily by muscles! Next is the elbow. The elbow is the stiffest joint in the body (although the pip joint in the finger is close) actually having 3 joints in one. This is why the elbow has such a high propensity for stiffness when immobilized. The wrist joint developed rotation later in our evolution for swinging and eating. The radius rotates around the ulna creating what appears to be, a very young ball and socket joint. It has a long way to go.
Nonetheless, imagine a very shallow bowl. Then imagine a very small ball. The ball of the ulna sits in the bowl of the radius. Some bowls are shallow (Type A: flat face sigmoid), some are deep (Type B ‘Ski slope sigmoid), some shaped like the letter C (Type C: C type sigmoid). Some bowls have an odd “s” shape. (Type D: “s” type sigmoid)
Type A 42%.
Type C - 30%
Type B - 14%
Type D- 14%
I find it fascinating that when the wrist is in neutral, only 60% of the cartilaginous surface of the radius and ulna make contact. In the end ranges of supination and pronation, only 10% of the surfaces touch. The stability of the DRUJ in the extreme end range motions are dependent on ligaments AND muscles. This is important for all of your weightlifters!
I am always asking people for their X-rays to see if there is a correlation between types and fitting. The positioning of the ulna into the radius relies on the ligamentous structures within and surrounding the joint. If one or several of the ligaments are torn, the ulna changes appearance significantly.
There is another important anatomical consideration. The space between the distal ulna and the distal wrist crease (last crease on your wrist before it turns into your hand) varies.
The most common gap is 8 mm. Some people have only a 3 mm space to work with. If one puts too much compression here with too narrow of a strap, the ulna nerve goes numb. This is not good. Sometimes the color changes- not ok.
I have been working on this problem for more than10 years and want your stories, your xrays, and your solutions. I am not entirely happy with this “fix” but I will get it eventually. I might consider pulling my sewing machine out and making custom sewn versions. Unfortunately, my vision has suffered from sewing too long, so it would likely take me weeks to make one :)