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Posterior muscle-splitting approaches are the most commonly performed surgery for spinal fusion in those diagnosed with adolescent idiopathic scoliosis (AIS). Although short-term postsurgical changes have been documented regarding the posterior paraspinal musculature, there is a paucity of studies regarding the long-term effects.Sixty-three consecutive patients with AIS were offered participation via mail invitation. Magnetic resonance imaging (MRI) studies were reviewed to analyze axial images at each level distal to the fusion mass. Symmetry, muscle quality, and fatty infiltration were graded according to the Goutallier classification of the iliocostalis, longissimus, and spinalis muscles.Twenty patients were enrolled in this study with a mean age of 26 yr and a mean follow-up of 12 yr. These patients had a preoperative mean major curve of −55 degrees and a postoperative mean major curve of −25 degrees. The mean fusion length performed was approximately 11 levels. In total 49 levels were graded for change. Thirteen patients (65%) had no qualitative abnormalities across 34 levels (69.4%), and seven patients (35%) had abnormalities. Of those with abnormalities, five had unilateral greater than grade 2 right-sided changes and two had grade-4 changes in the spinalis muscle.The open posterior muscle splitting approach for spinal fusion demonstrates very little long-term qualitative fatty degeneration distal to the fusion mass on MRI. Future studies should focus on what patient-specific and surgery-specific factors may predispose someone to long-term damage to the paraspinal muscles after spinal fusion.