Evaluating shoulder integrity requires specialized orthopedic maneuvers to pinpoint the source of pain and dysfunction. The Empty Can Test and the Full Can Test are important diagnostic procedures designed to assess the strength and integrity of the supraspinatus tendon, a major component of the rotator cuff complex. Understanding the specific mechanics and biomechanical rationale behind each test is necessary to determine which provides the most accurate clinical information.
The Target Muscle: Understanding the Supraspinatus
The supraspinatus is one of the four muscles that make up the rotator cuff, originating from the scapula and inserting onto the greater tubercle of the humerus. Its primary function is to initiate the movement of the arm away from the body, known as abduction, for the first 15 to 20 degrees of range. Beyond this initial movement, the larger deltoid muscle takes over most of the force production.
The tendon of the supraspinatus travels through a narrow passageway in the shoulder known as the subacromial space. This anatomical position makes it highly susceptible to injury from repetitive friction or compression, often referred to as shoulder impingement. Pathology in this area is a frequent cause of shoulder pain, making the supraspinatus the most commonly damaged tendon within the rotator cuff.
Performing the Empty Can Test
The Empty Can Test, also known as the Jobe test, was historically the standard method for evaluating supraspinatus integrity. To perform this maneuver, the patient’s arm is positioned with 90 degrees of elevation in the scapular plane, about 30 degrees forward from the side of the body. The shoulder is then fully internally rotated so the thumb points toward the floor, mimicking the action of emptying a beverage can.
The clinician applies a downward force to the forearm, which the patient is instructed to resist. A positive result is indicated by the presence of pain or noticeable weakness compared to the unaffected arm. The drawback is that internal rotation causes the greater tuberosity to move beneath the acromion. This positioning can painfully compress the supraspinatus tendon, potentially leading to a false-positive result based on pain inhibition.
Performing the Full Can Test
The Full Can Test was developed as a modification to the original Empty Can Test to address its significant limitation of pain provocation. This test maintains the arm’s position at 90 degrees of elevation in the scapular plane, but the shoulder is externally rotated so the thumb points toward the ceiling. This position is sometimes referred to as neutral rotation.
By externally rotating the arm, the greater tuberosity moves away from the underside of the acromion. This biomechanical change effectively minimizes painful compression of the supraspinatus tendon within the subacromial space. The clinician applies a downward resistance, and the test is interpreted as positive if the patient experiences pain or exhibits significant weakness. This modification allows the clinician to better isolate the integrity of the muscle and tendon.
Clinical Rationale for Test Preference
The distinction between the two tests lies in their ability to isolate muscle pathology while minimizing painful impingement. Clinical studies have shown that supraspinatus muscle activity, measured by electromyography, is comparable between the Empty Can and Full Can positions. This means both positions are effective at engaging the target muscle.
The Full Can Test is the preferred choice among orthopedic specialists and physical therapists because it is significantly less provocative of pain. When a patient experiences less pain, they are less likely to exhibit guarding or pain-induced muscle weakness, which can obscure a true tear or strength deficit. By reducing the influence of pain, the Full Can Test provides a more accurate assessment of the supraspinatus muscle’s strength and structural integrity.
When weakness is the sole criterion for a positive result, the accuracy of both tests in detecting a torn supraspinatus tendon is considered similar, with the Full Can Test often showing slightly higher accuracy. Since the primary goal of the test is to evaluate the tendon’s ability to resist force, the modification to the Full Can position is a practical advancement. The consensus is that the Full Can Test is superior for clinical use because it yields comparable diagnostic value while greatly improving patient comfort.