No, an interstitial tear is NOT a full-thickness tear
An interstitial tear (also called intrasubstance or intratendinous tear) in the supraspinatus tendon is definitionally a partial-thickness tear, not a full-thickness tear, regardless of its location in the critical zone or its severity grade. This is a fundamental anatomical and pathological distinction.
Understanding the Classification
The supraspinatus tendon consists of multiple layers of intertwining fiber bundles 1. Tears are classified based on which layers are disrupted:
- Full-thickness tears: Complete disruption through all layers from the bursal (superficial) surface to the articular (deep) surface
- Partial-thickness tears: Incomplete disruption involving only some layers, which includes:
- Articular-side tears (deep surface)
- Bursal-side tears (superficial surface)
- Interstitial/intratendinous tears (within the substance of the tendon, not communicating with either surface)
1 explicitly describes this classification system, noting that partial-thickness tears include "lesions of the superficial, deep and central surface or tendon delamination."
Clinical Implications
The distinction matters significantly for treatment decisions. The 2020 AAOS guidelines provide strong evidence supporting either conversion to full-thickness or in-situ repair for high-grade partial-thickness tears 2, indicating these are treated as distinct entities from full-thickness tears requiring different surgical considerations.
Key Diagnostic Features
An interstitial tear will NOT demonstrate:
- Communication between the subacromial bursa and glenohumeral joint
- The "bubble sign" on arthroscopy (air bubbles flowing from high-pressure subacromial space into low-pressure joint) 3
- Contrast extravasation through the entire tendon thickness on arthrography
The "Critical Zone" Caveat
The term "critical zone" refers to the watershed area of relatively poor vascularity approximately 1 cm proximal to the supraspinatus insertion—this is a location descriptor, not a tear classification. Most tears (79-89%) occur in the anterior half of the supraspinatus 4, but location does not determine whether a tear is partial or full-thickness.
Common Pitfall
Do not confuse "moderate-grade" or "high-grade" partial-thickness tears with full-thickness tears. Grade refers to the percentage of tendon thickness involved (typically >50% for high-grade 5), but even a 90% partial tear remains partial-thickness if one surface remains intact. Studies show that only 16% of high-grade partial tears progress to full-thickness over one year 5, confirming these are distinct pathological entities with different natural histories.