The primary purpose of this study was to investigate the role the ligamentum teres has in providing hip stability using a biomechanical model. The second purpose was to review arthroscopic findings in those with a complete ligamentum teres rupture and question them regarding instability to determine how clinical findings related to the biomechanical model. A string model was created to examine ligamentum teres excursion during various hip positions. A retrospective review of 350 consecutive surgical patients identified 20 subjects with a complete ligamentum teres rupture that was not repaired at the time of surgery. The model found the ligamentum teres to have the greatest excursion when the hip was externally rotated in flexion (ER/FLEX) and internally rotated in extension (IR/EXT). During operative assessment, it was noted that all 20 subjects had laxity during dynamic impingement testing when their hip was in a position of ER/FLEX. Nine (45%) of the 20 subjects with ligamentum teres rupture were available for follow-up (mean 31 months post-op). Five out of these 9 subjects noted instability: 5 of 9 with squatting (ER/FLEX) and 4 of 9 with crossing one leg behind of the other (IR/EXT). These 5 subjects had osseous risk factors that compromised hip stability including inferior acetabular insufficiency. The ligamentum teres may contribute to hip stability when the hip is in ER/FLEX and IR/EXT. Individuals with osseous risk factors for instability, including inferior acetabular insufficiency, may have instability with squatting (ER/FLEX) and crossing one leg behind of the other (IR/EXT). IV.