A 42-year-old female office worker presents with 6 weeks of right shoulder pain exacerbated by overhead reaching and sleeping on the right side. She has pain with forward flexion and internal rotation. Neer's test and Hawkins-Kennedy test are positive. X-ray shows no fracture or dislocation. MRI reveals a partial-thickness tear of the rotator cuff involving the articular surface of the supraspinatus. Which anatomical feature makes articular-sided (bursal-sided) tears clinically significant?