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Background: We report changes in the natural history of hip instability with nusinersen treatment among patients with spinal muscular atrophy (SMA) type II after onset of weakness, historically wheelchair-bound but now potentially ambulatory in the era of disease-modifying therapy., Methods: Patients with genetically confirmed diagnoses of SMA type II who received intrathecal nusinersen from January 1, 2018, to June 30, 2022, were screened for inclusion. Patients with less than 6 months of follow-up, or prior hip surgeries were excluded. Primary clinical outcome measures included scores from Hammersmith motor functional scale expanded (HMFSE), revised upper limb module (RULM), 6-minute walk test (6MWT), and ambulatory status. Radiographic outcomes, including Reimer migration index, the presence of scoliosis, and pelvic obliquity, were also assessed. Secondary outcomes involved comparisons with a historical cohort of SMA type II patients treated at our institution who never received nusinersen., Results: Twenty hips from 5 boys and 5 girls were included in the analysis, with a mean follow-up of 3 years and 8 months. The median age at time of nusinersen initiation was 6.8 years old, ranging between 2.5 and 10.3 years. All patients developed lower limb motor weakness before nusinersen initiation. After treatment with nusinersen, 1 previously stable hip (5%) developed subluxation, 15 hips (75%) remain subluxated, 3 hips (15%) remain dislocated, and 1 hip (5%) remained stable, with a statistically significant difference between the pretreatment and posttreatment groups ( P <0.01). Six patients (60%) were ambulatory at latest follow-up. Six patients (60%) had improved ambulatory ability; 2 had static ambulatory ability (20%); and 2 had deterioration in their walking ability. The median HFMSE score improved from 18.5 (range 0 to 46) to 22 (range 0 to 49) ( P =0.813), whereas the median RULM score improved from 17 (range 2 to 28) to 21.5 (range 5 to 37), which was statistically significant ( P =0.007)., Conclusions: Hip instability persists despite treatment with nusinersen among patients with SMA type II who received nusinersen after onset of lower limb weakness., Level of Evidence: Therapeutic Level IV., Competing Interests: The authors declare no conflicts of interest., (Copyright © 2024 The Author(s). Published by Wolters Kluwer Health, Inc.)