One of the most common diagnoses we see in hand therapy is osteoarthritis of the 1st carpo-metacarpal joint where the metacarpal articulates with the trapezium. Also known as basal joint arthritis (CMC arthritis), this causes pain, joint deformity, loss of strength and loss of function. Patients come to us looking for relief of symptoms and a return to function.
In most of these cases an effective treatment strategy is a three-pronged approach:
- Activity modification
- CMC stabilization exercises
- Orthosis use to stabilize the basal joint.
For now, I want to focus on the CMC orthosis for basal joint arthritis and how it works to stabilize, protect and aid in grasp and functional hand use. To understand this, we first need to understand how osteoarthritis affects the thumb.
How Osteoarthritis Affects the Thumb
As cartilage breaks down in the saddle joint between the Trapezium and the 1st metacarpal, the joint loses internal stability. This creates pain with bone-on-bone friction. The ligaments also are now too laxed with the void left by the cartilage. This allows the metacarpo-phalangeal joint to radially sublux (Blue Arrows) out of position during pincer or power grasp usage. This happens when the Adductor Pollicis contracts (Red Arrows) and pulls the distal head of the metacarpal ulnarly.
Without the stability of taught ligaments to maintain approximation of the CMC joint, the proximal end cantilevers radially, away from the joint. This results in subluxation that can become a chronic position, called a ‘shoulder sign’ (shoulder sign thumb).
As the CMC joint subluxation worsens, the system tries to correct by stabilizing the 1st metacarpal against the second metacarpal with adduction. Unfortunately, this causes 2 negative outcomes:
- Further radial cantilever of the proximal head
- Tightening of the AP into a contracted position.
This subluxation pattern becomes more pronounced, painful and limiting to functional use. Without correction, this will continue to degrade the stability of the joint.
How CMC Orthosis Can Protect Against Joint Damage in the Hand
This is where orthosis use can be helpful in slowing this progression and protecting against further joint damage. A CMC short opponens orthosis is designed to protect the basal joint’s stability during grasp as well as helping maintain a functional opposition/abduction position at rest without furthering the subluxation.
Here are two primary ways orthosis can help protect from damage:
1. Maintain Approximation
The first mechanism of the short opponens splint is to maintain approximation of the CMC joint. Unlike other orthoses, this splint does not need to cross the joint to be effective. With simple radial support to the proximal metacarpal head the joint is approximated as the ulnar strap helps maintain contact and stability.
2. Maintain the Web Space
Second, the splint maintains the web space in a functional position with abduction and slight opposition. This is a better functional position for use without pain, but it also allows the AP to relax and lengthen. Decreased tension in the AP reduces the cantilever subluxation to the CMC joint.
3. Provide Mechanical Support
Finally, the stability of the thermoplastic around the CMC joint provides mechanical support during pincer and grip activities. For every 1 lb of force applied at the tips of a pincer grasp there is 12 lbs of force applied to the CMC joint. Using a Short Opponens orthosis provides an external support that the arthritic thumb lacks internally. The solid backing around the CMC prevents subluxation and shift during grip activities and pincer grasp.
The use of a short opponens orthosis provides light approximation of the CMC joint, improves functional grasp position for decreased AP tension and mechanical support to augment laxity in the joint capsule.
While use of an orthosis is only one aspect of treatment planning for basal joint osteoarthritis, it is a crucial aspect to improving the quality of life for the patient. If you want to see two options for how to make this important orthosis, check out our video with explanation on a version with and without a template.
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