When Convex-Concave Rule Meets Clinical Practice

The convex-concave rule was developed by Freddie Kaltenborn. It has been taught for decades as the way to explain the arthrokinematics of a joint. It identifies that when the concave joint surface moves on the convex surface, rolling and gliding of the two surfaces occur in the same direction. Thus, if the concave joint surface rolls posterior, it will also glide posterior. However, if the convex joint surface moves on the concave surface, rolling and gliding occur in the opposite direction, i.e. if the convex surface rolls posterior, it will glide anterior on the concave surface.

Simple right? Well, several researchers have questioned if the rule applies to clinical practice. This can be challenging to assess since we are talking about millimeters of motion. Until recently when the Mobil-Aider Arthrometer was developed to assess linear translation of 14 motions across five different joints (shoulder, elbow, wrist, knee, & ankle), arthrokinematics could not be objectively measured. More information about the Mobil-Aider, as well as the supporting research can be found at….. https://mobil-aider.com/

But let’s explore the concave-convex rule relative to clinical practice. Shoulder internal (IR) and external (ER) rotation is a good place to start. Based on the convex-concave rule, ER should be accompanied by rolling posterior and gliding anterior. In other words, the convex humerus rolls posterior and glides anterior on the concave glenoid fossa. However, a study by Johnson et al (2007) compared the impact of both an anterior glide and a posterior glide of the shoulder on ER range of motion (ROM). The results demonstrated a 31 degree increase in ER ROM with a posterior glide as compared to just a 3 degrees increase with anterior glides. So how does that make sense? An anterior glide should occur with ER.

Sigel (2020) questioned if ER arthrokinematics involved spinning instead of rolling and gliding. Although capsular tightness was not assessed in the study, Neumann (2012) suggested the efficacy of the posterior glide could be related to capsular tightness. This tightness would skew the humeral head anterior and limit the amount of anterior translation available. A posterior glide could centralize the humeral head in the glenoid fossa. This centralization could unload the anterior capsule to provide more linear translation in the anterior direction for more ER. This research demonstrates the strong relationship between arthrokinematics and osteokinematics.

The take home message is the convex-concave rule is a wonderful guiding principle but the assessment of the joint limitation is paramount. Neumann states we “should not mobilize a pathological joint according to a rule, but treat pathological clinical findings, which are in correlation with the patient’s complaints.” In assessing posterior capsular restrictions of the shoulder, we must be sure to perform the technique correctly. Reinold (2015) discussed the importance of knowing anatomy of the shoulder and appreciating the need to angle the posterior glide laterally to avoid hitting the posterior glenoid rim. Performing a posterior glide in the sagittal plane (not angled) could result in one thinking the glenoid rim endfeel is posterior capsule tightness. This would be a false representation of the arthrokinematics. Care should be taken to mobilize a joint with respect to the anatomic orientation.

In summary, the convex-concave rule has merit and is a good starting point. A thorough assessment of arthrokinematics and osteokinematics are prudent to determine the course of intervention.

If you would like to learn more about how the Mobil-Aider orthopedic measurement device can help you to quantify joint mobility, please visit: https://mobil-aider.com/

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