| Summary: | Knot impingement can cause shoulder-joint motion pain after rotator-cuff repair. Previous studies have revealed only subacromial effusion in magnetic resonance imaging (MRI) evaluations of knot impingement. We report two cases of patients with symptomatic knot impingement. In both patients, bursal-side partial-thickness tear of the supraspinatus tendon had been repaired by a single-row technique using one suture anchor and two polyester sutures with a long-chain polyethylene core. Three-dimensional computed tomography and arthroscopy revealed bony erosion at the lateral side of the anterior half of the acromial undersurface in both patients. The size of the erosion was 1.7 cm (anteroposterior direction) × 0.7 cm (mediolateral direction) in one patient and 1.2 cm × 0.5 cm in the other. Arthroscopy showed that suture knots that had been placed at the muscle–tendon junction of the supraspinatus tendon were impinging on the area of bone erosion during shoulder abduction. Although the sutures themselves were of soft material, knot-tying made them stiff and thus led to bone erosion. Surgeons need to be aware of the possibility of subacromial bone erosion caused by suture knots in arthroscopic rotator cuff repair. Keywords: Bone erosion, Computed tomography, Impingement, Rotator cuff repair, Suture knots, Three-dimensional
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