Purpose <p>This study aimed to clarify changes in muscle morphology, including normal muscle, intramuscular fat, and low-density muscle after TKA.</p> Methods <p>The retrospective cohort study was conducted with 71 patients with TKA. The cross-sectional area (CSA) of the lower limb muscles was assessed using computed tomography images. The muscle tissues were divided based on Hounsfield Units (HU) into intramuscular fat (-190 to -30 HU), low-density muscle (-29 to 29 HU), and normal-density muscle (30 to 150 HU). These measurements and comparisons were performed before and more than six months after TKA.</p> Results <p>The results revealed fat ratio (%) was significantly decreased in vastus lateralis, gluteus maximus, medius, minimus, and iliopsoas after TKA (2.0 to 1.9, 11.0 to 8.1, 3.5 to 2.7, 10.5 to 8.2, 2.4 to 1.8, respectively). In these muscles, intramuscular fat CSA also decreased, other than iliopsoas. The normal-density muscle CSA (cm²) increased in vastus lateralis, gluteus maximus, medius, minimus, iliopsoas, and gastrocnemius medial head (7.46 to 9.00, 3.74 to 5.47, 3.81 to 3.93, 3.27 to 3.64, 8.83 to 10.20, 5.70 to 6.03, respectively). The low-density muscle CSA increased in gluteus minimus and ankle plantar flexor muscles, except for gastrocnemius.</p> Conclusion <p>Normal-density muscle CSA increased in the vastus lateralis, gluteus muscles, iliopsoas, and gastrocnemius medial head, while intramuscular fat CSA decreased in the vastus lateralis and gluteus muscles. Future studies clarifying the TKA and muscle morphology causality incorporating control groups and functional outcome measures are required to determine the clinical relevance of these morphological changes.</p>