Final answer:
For the inferosuperior axial projection of the humerus, the arm should be abducted with the elbow flexed at 90 degrees, and the posterior humeral shaft should align with the image receptor, considering both superior and inferior rotations of the scapula.
Step-by-step explanation:
The proper position of the humerus for the inferosuperior axial projection involves the upper extremity being abducted and the elbow flexed at a 90-degree angle. This projection requires the posterior shaft of the humerus to be aligned with the image receptor. When positioning, attention must be paid to the anatomical landmarks such as the infraglenoid tubercle of the scapula, ensuring that the posterior humeral shaft distal to the radial groove is in line with the detector. This proper alignment allows for an accurate representation of the humerus, avoiding overlap with other structures and optimizing the usefulness of the radiographic projection.
During the movement for positioning, the superior rotation of the scapula is also important. As the arm is abducted, the scapula must superiorly rotate to allow the greater tubercle of the humerus to clear the acromion. Additionally, inferior rotation of the scapula occurs during limb adduction, which involves the downward motion of the glenoid cavity with upward movement of the medial end of the scapular spine. Both superior and inferior rotations are vital for the full range of motion required for this and various other projections.