Which clinical finding best distinguishes a supraspinatus tear from an infraspinatus tear in a patient with rotator cuff pathology?

Explanation

## Distinguishing Supraspinatus from Infraspinatus Tears **Key Point:** The drop arm test is the most specific clinical discriminator for supraspinatus tears, whereas external rotation lag signs are pathognomonic for infraspinatus involvement. ### Supraspinatus Tear Hallmark **High-Yield:** The **drop arm test** is positive when the patient cannot maintain passive abduction at 90° or drops the arm suddenly when released. This reflects loss of the supraspinatus's primary role in initiating and sustaining abduction. - Inability to initiate abduction (first 15°) = supraspinatus paralysis - Inability to maintain abduction once achieved = supraspinatus weakness - This is the most **specific** finding for isolated supraspinatus pathology ### Infraspinatus Tear Hallmark **Clinical Pearl:** The **infraspinatus lag sign** (also called external rotation lag sign) is elicited by: 1. Passively externally rotating the shoulder to 90° with elbow flexed at 90° 2. Asking the patient to maintain this position 3. Observing involuntary internal rotation (lag) when the examiner releases support This demonstrates weakness of external rotation, the primary function of infraspinatus. ### Comparison Table | Feature | Supraspinatus Tear | Infraspinatus Tear | | --- | --- | --- | | **Primary motor loss** | Abduction (0–15°) | External rotation | | **Pathognomonic test** | Drop arm test | Infraspinatus lag sign | | **Muscle atrophy** | Supraspinous fossa | Infraspinous fossa | | **Weakness pattern** | Initiating abduction | Resisting external rotation | | **Lag sign** | Absent | Present (external rotation lag) | ### Why Drop Arm Test Discriminates Best The drop arm test is: - **Specific** to supraspinatus function (abduction initiation) - **Easily reproducible** at the bedside - **Positive in >70% of full-thickness supraspinatus tears** - **Negative in infraspinatus-only tears** (external rotation is preserved in abduction) **Mnemonic:** **SUPRA = Start (initiate) ABduction** → Drop arm test [cite:Netter's Atlas of Human Anatomy Ch Shoulder Joint; Gray's Anatomy 42e Ch Rotator Cuff] ### Clinical Correlation **Clinical Pearl:** A patient with a pure supraspinatus tear will have: - Positive drop arm test - Normal external rotation strength (infraspinatus intact) - Atrophy of the supraspinous fossa (not infraspinous) Conversely, an infraspinatus tear alone will have: - Negative drop arm test (abduction preserved) - Positive infraspinatus lag sign - Atrophy of the infraspinous fossa