The Empty Can Test, also known as the Jobe test, and the Full Can Test are physical maneuvers clinicians use to assess shoulder health. These procedures evaluate the integrity of the rotator cuff, focusing primarily on the supraspinatus muscle and its tendon. By placing the arm in distinct positions and applying resistance, a clinician can provoke symptoms or reveal weakness, guiding the initial diagnosis of a potential injury.
Understanding Supraspinatus Function
The supraspinatus is one of four muscles that make up the rotator cuff, a group responsible for stabilizing the shoulder joint. This small, triangular muscle is located in the upper part of the shoulder blade (scapula) in a depression called the supraspinous fossa. Its tendon travels laterally, passing under a bony arch of the shoulder and attaching to the upper arm bone, the humerus.
The primary function of the supraspinatus is to initiate arm abduction, the motion of lifting the arm away from the body. It is responsible for the first 0 to 15 degrees of this motion before the larger deltoid muscle takes over. The supraspinatus also works continuously with the other rotator cuff muscles to keep the head of the humerus centered within the shoulder socket.
Performing the Empty Can and Full Can Tests
Both the Empty Can Test and the Full Can Test begin with the patient standing or sitting. The affected arm is raised in the scapular plane (approximately 30 degrees forward from the side of the body) and elevated to about 90 degrees of abduction, or level with the shoulder. This position aligns the supraspinatus muscle fibers for optimal contraction and assessment.
To perform the Empty Can Test, the patient fully internally rotates the arm so the thumb points toward the floor, resembling the motion of pouring liquid out of a can. The clinician then applies a steady downward force just above the patient’s elbow or wrist, and the patient resists this pressure.
The Full Can Test uses the same arm elevation and scapular plane position as the empty can maneuver. The difference is that the patient externally rotates the arm so the thumb points upwards, as if holding a full can of liquid. The clinician applies a downward force to the arm, and the patient resists. This change in rotation helps move the supraspinatus tendon away from the bony arch of the shoulder, reducing the likelihood of impingement pain.
Interpreting Positive Test Findings
A positive result for either the Empty Can or Full Can Test is defined by two findings: pain or weakness. Pain, without significant weakness, often suggests irritation or inflammation of the supraspinatus tendon (tendinopathy) or subacromial impingement.
Weakness, which is compared to the unaffected side, is a more serious finding and can indicate a significant structural issue. This weakness suggests the muscle is unable to generate force effectively, potentially due to a partial or full-thickness tear of the supraspinatus tendon. Weakness can also indicate a problem with the suprascapular nerve, which supplies the muscle. When both pain and weakness are present, the likelihood of a tear increases.
Clinical Utility and Diagnostic Accuracy
The Full Can Test is often preferred by clinicians over the original Empty Can Test in modern clinical practice. Although both positions result in similar electrical activity in the supraspinatus muscle, the empty can position is more provocative and likely to cause pain. The internal rotation in the Empty Can Test increases compression of the supraspinatus tendon against the acromion bone, often leading to pain even without a tear.
Conversely, the external rotation in the Full Can Test reduces tendon compression, making the test less painful while still effectively isolating the supraspinatus muscle. Research shows that when muscle weakness indicates a tear, the Full Can Test has a slightly higher accuracy (around 75%) compared to the Empty Can Test (about 70%). While these manual tests are valuable for guiding a physician’s initial assessment, a definitive diagnosis requires confirmation through advanced imaging, such as magnetic resonance imaging (MRI).