It has been a long held belief that the amount of partial thickness damage to the rotator cuff correlates with the severity of symptoms and can be used to direct treatment. However, assessment of partial thickness rotator cuff tear (PT-RCT) size and thickness is difficult and imprecise. Despite this, several authors have recommended different treatment regimens for PT-RCTs based on the percentage of thickness of the tendon torn. The current method of measurement arbitrarily compares the amount of exposed bone in the rotator cuff footprint, to historic anatomic data to determine the percentage of tendon involvement. Considering the variability of footprint anatomy, this can lead to imprecise and improper decision making. The purpose of this study was to compare the current method of measurement to the use of an intra-articular depth gauge in the measurement of articular surface PT-RCTs of the supraspinatus tendon. Cadaveric Study: In eight fresh frozen cadaveric shoulders, PT-RCTs of various amounts were created in the supraspinatus tendon. Each tear was then evaluated arthroscopically and the amount of footprint exposed, the total tendon thickness and percentage of tendon thickness torn was measured by comparison to a shaver of known size and using an intra-articular depth gauge. Shoulders were then dissected and the true anatomic measures determined. Clinical Study: Over a 1 year period, patients with articular surface PT-RCTs of the supraspinatus tendon were evaluated arthroscopically using the intra-articular depth gauge and the amount of footprint exposed, the total tendon thickness and percentage of tendon thickness torn was measured. In the cadaveric study, the mean true amount of footprint exposed was 7mm (range: 5-10mm), the mean true total tendon thickness was 13mm (range:10-22mm), and the mean true percentage of tendon thickness torn was 54% (range: 33-70%). The intra-articular depth gauge was more accurate than comparison to/estimation to a known size. In particular, total tendon thickness and percentage of tendon thickness torn was more accurately determined using the intra-articular depth gauge. In the clinical study, 22 patients with PT-RCTs of the supraspinatus tendon were identified. Using the intra-articular depth gauge, the mean footprint exposed was 10mm (range:0-18mm), the mean total tendon thickness was 14mm (range: 10-19 mm) and the mean percentage of tendon thickness torn was 65% (range:0-95%). In 4 cases the treatment decision was altered by the measurement made by the intra-articular depth gauge (i.e. 1 patient repaired instead of debrided; 3 patients debrided instead of repaired.) Weber has recommended that PT-RCTs involving > 50% of the tendon thickness should be repaired. Despite his recommendations, which have become a standard of treatment, measuring tear thickness remains difficult. The intra-articular depth gauge provides a method of direct measurement of the amount of footprint exposed, total tendon thickness and percentage of thickness torn for accurate and precise documentation and decision making. In our clinical study use of the intra-articular depth gauge altered treatment 18% of the time. However, previous treatment recommendations should be scrutinized since these are based on comparisons/estimates and are likely inaccurate.