The physical examination of the shoulder frequently relies on specific maneuvers to assess the integrity of the rotator cuff muscles. The Empty Can Test and the Full Can Test are two common methods used by clinicians to evaluate the strength and health of the supraspinatus muscle. These resisted movements are designed to isolate and challenge the muscle, helping to screen for pathology like tendon tears or inflammation that may be causing pain or weakness.
Understanding the Supraspinatus Muscle
The muscle targeted by these tests is the supraspinatus, a relatively small but significant muscle located in the upper back. It originates from the supraspinous fossa, a shallow groove on the scapula (shoulder blade). The muscle’s tendon travels laterally, passing underneath a bony arch before attaching to the greater tubercle of the humerus (upper arm bone).
The supraspinatus is one of the four muscles that make up the rotator cuff, a group responsible for stabilizing the shoulder joint. Its primary action is to initiate shoulder abduction, the act of lifting the arm out to the side. Specifically, the supraspinatus performs the first 15 degrees of abduction before the larger deltoid muscle takes over the main effort.
The Classic Empty Can Test Procedure
The original version of this evaluation is known as the Empty Can Test, or the Jobe Test. To perform this test, the patient stands or sits while raising their arm to approximately 90 degrees of abduction. The arm is also positioned about 30 degrees forward, aligning it with the plane of the scapula.
The distinguishing feature of this classic test is the rotation of the arm, where the patient’s thumb points downward, simulating the action of emptying a beverage can. With the arm held in this specific position, the examiner applies downward pressure on the forearm or wrist. The patient is instructed to resist this force while the clinician observes for pain or weakness.
Why the Full Can Test is Often Preferred
The Full Can Test was later proposed as an alternative because the original Empty Can position can sometimes lead to misleading results. The internal rotation required in the Empty Can position causes the supraspinatus tendon to compress against the overlying bone, a phenomenon known as impingement. This mechanical compression can provoke pain even if the tendon is only inflamed, potentially leading to a false-positive result.
The Full Can Test modifies the arm rotation to address this issue while still challenging the supraspinatus muscle. The arm remains abducted to 90 degrees and forward-flexed 30 degrees, but the hand is externally rotated so the thumb points upward, like holding a full can. This external rotation moves the greater tubercle away from the coracoacromial arch, which reduces impingement forces on the tendon. By minimizing mechanical irritation, the Full Can Test is often less painful and provides a more accurate assessment of muscle strength.
What a Positive Test Result Means
A positive result for either the Empty Can or Full Can Test is defined by a patient experiencing significant pain or demonstrating noticeable weakness when resisting the downward pressure. Pain suggests irritation or inflammation of the supraspinatus tendon, referred to as tendinopathy. If the arm cannot resist the pressure or shows asymmetric weakness, this strongly suggests a more serious issue, such as a tear in the supraspinatus muscle or tendon.
A positive outcome also raises the possibility of damage to the suprascapular nerve, which supplies the supraspinatus muscle. These physical tests are indicators of pathology, not definitive diagnoses. To confirm the presence and extent of a tendon tear, a healthcare provider typically requires further diagnostic imaging, such as an MRI or ultrasound.