It can be difficult to determine exact muscle intervals. When we are operating on the radial head, there is usually associated injury to the lateral collateral ligament and common digit and wrist muscle origins. Along with this damage there is usually a rent in the fascia that can be opened and extended distally. This usually lies in the interval between the extensor carpi radialis brevis and extensor digitorum communis (Kaplan interval). The associated ligament and muscle injury will make the rest of the exposure very easy.

The Kaplan interval can also be identified by elevating the origin of the extensor carpi radialis brevis from the supracondylar ridge of the distal humerus, elevating the brachialis from the anterior humerus, then continuing distally until the joint is entered and the capitellum is visualized. Elevating these muscles is necessary to exposure the coronoid from the lateral side. Split the common wrist and digital extensor musculature at the point that divides the capitellum in half anterior/posterior.