Final answer:
To obtain a lateral projection of the upper humerus when a patient cannot abduct their arm, the patient is positioned prone with arms and legs close to the body.
Step-by-step explanation:
The lateral projection of the upper humerus on patients who are unable to abduct their arm is obtained when a person is positioned prone with their upper limbs, including palms, touching their sides, and lower limbs touching at sides. This allows for the appropriate imaging without requiring arm abduction, which may be impossible or painful for some patients due to injury or surgery. In this prone position, projection of the humerus can be accomplished without superior rotation of the scapula, which is involved in arm abduction and could cause discomfort or be physically impossible for individuals with certain conditions.