CMC Arthritis Orthosis in the Hand: Mechanics and Purpose

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:

  1. Activity modification
  2. CMC stabilization exercises
  3. 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

cmc orthosis

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:

  1. Further radial cantilever of the proximal head
  2. 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.
cmc orthosis

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.

cmc orthosis

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.


  1. Kathie on April 15, 2019 at 6:22 pm

    Well done Josh! Perfectly worded!!!

    • Josh MacDonald on April 15, 2019 at 9:17 pm


  2. Stephanie on April 16, 2019 at 4:56 pm

    This is a very helpful explanation of CMC Arthritis! What is the recommended splint wearing schedule (Night/day, duration, etc.)? Thank you!

    • Josh MacDonald on April 16, 2019 at 8:21 pm

      I recommend wearing when active. It isn’t needed when sleeping or when at rest (e.g. watching T.V.). But wearing it as often as possible during any level of activity will relieve pain and protect the joint.

  3. Firdevs. Kul on July 10, 2019 at 3:05 am

    Josh, It is very benefit explanations for CMC Arthritis well done Thanks. PT. Firdevs

    • Josh MacDonald on July 10, 2019 at 6:25 am

      I’m glad you found it helpful.

  4. Sandra L Reed on November 2, 2020 at 5:48 am

    Where can I buy one of these?

  5. Aaron Kubistek on November 2, 2020 at 5:56 am

    Clinical Pearl
    When making a spica: Just after you placed the material on the patients hand place a small rubber bouncy ball between the index finger and thumb.
    This correctly aligns the thumb and allows you to see where you may need to trim.
    Patients find this method much more comfortable

  6. Kathy on November 2, 2020 at 2:46 pm

    Well written and great info for OT who does not see hands all the time.

    • Nancy I on November 3, 2020 at 8:55 pm

      Even long time practicing hand therapist can learn from articles like this I never would’ve thought to put a small bouncy ball in between the thumb and the index. Perfect idea

      • Miranda Materi on November 8, 2020 at 4:16 pm

        Thanks for commenting!! It is so fun being a part of a community that has a culture of sharing.

    • Miranda Materi on November 8, 2020 at 4:16 pm

      Thank you!

  7. Marlene on November 6, 2020 at 1:23 pm

    I am very petite & need one of these! I’m an active OT. I have been using elastic tape cut in half lengthwise and making my own but with COVID, that splint would be so much nicer!

    I appreciate any assistance!
    Thank you

    • Miranda Materi on November 8, 2020 at 4:16 pm

      The elastic tape is an excellent idea. A custom splint works well or you could try try the Metagrip push splints as well. They are low profile and aren’t as restrictive for work

  8. Kathrin Sauer on November 12, 2020 at 1:18 am

    Hello dear Miranda,

    a great article and a great orthosis, what is the cut plan? ICh am from Germany and an enthusiastic reading hand therapist.


  9. Krista on November 16, 2020 at 8:31 am

    I would love to see a pattern for the orthosis if you have it available. Or even a quick tutorial on how to make. I make thumb spica’s different than this but this is less cumbersome and in the palmar surface. Thanks for all the good information.

    • Miranda Materi on December 2, 2020 at 7:08 pm

      Hi Krista !
      If you go to our IG page @handtherapyacademy we provide tutorial on orthosis fabrication for free. If you want a more detailed explanation we have a monthly membership.;

  10. Sarita Streng on November 30, 2020 at 10:54 am

    I like the way you described this. Thanks.

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