Convex-concave rules and shoulder mobilizations

One of the staples of physical therapy and kinesiology foundations includes the convex-concave rules of joint motion. It states that when a convex surface moves on a concave surface, the convex surface rolls one way and glides the opposite direction. If a concave surface moves on a convex surface, the concave surface rolls and glides in the same direction. This can be crucial to understand for both joint mobility assessment and manual treatment.

That being said, the convex-concave rules do not always apply to joint kinematics or mobilizations. For example, when reading about the shoulder mechanics in The Athlete's Shoulder, many studies were cited saying, there was a gross movement in the rolling direction typically for the various shoulder movements (I apologize for the lack of specific citation, but I read the book several years ago and do not own it). For example, external rotation exhibited a net posterior translation. That doesn't mean that opposite gliding motion isn't necessary, but that we can't always assume that if a patient has restricted shoulder motion one way, we should mobilize the capsule the opposite way. In Johnson et al's study, shoulder ER mobility improved greater with posterior mobilization of the capsule compared to anterior mobilization. According to convex-concave rules, anterior mobilization should be the choice. Now something to be aware of in the study, is that the patient population was of patients with Frozen Shoulder. It is possible that these patients respond differently to mobilizations and that the above study doesn't always apply.

Clinically, what does this mean? We shouldn't always expect that the convex-concave rules will apply. What I recommend is when assessing a joint, look for restrictions and lack of stability in each plane. For example, note in a shoulder if there is increased laxity (or even potential subluxation) in one direction and increased stiffness in another. Mobilize the hypomobile direction if you find one. In a case like adhesive capsulitis, the patient will likely benefit from mobilization in any direction.

-Chris

Reference:

Johnson AJ1, Godges JJ, Zimmerman GJ, Ounanian LL. (2007). The effect of anterior versus posterior glide joint mobilization on external rotation range of motion in patients with shoulder adhesive capsulitis. J Orthop Sports Phys Ther. 2007 Mar;37(3):88-99.

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