Recovery of Grip Strength Over Time After Complete Resection of <i>KRAS</i>‐Positive Lung Cancer With Hypertrophic Pulmonary Osteoarthropathy: A Case Report

Hypertrophic pulmonary osteoarthropathy is a rare paraneoplastic syndrome affecting < 1% of patients with non-small cell lung cancer, characterized by clubbed fingers, periosteal proliferation, and arthritis. Although symptoms improve after treatment, objective functional recovery has not previously been reported. We present a 52-year-old male heavy smoker with right upper lobe adenocarcinoma and hypertrophic pulmonary osteoarthropathy causing profound grip strength impairment (5/2 right/left kilogram-force (kgf)). Genetic testing identified the KRAS G12C mutation. The patient underwent right upper lobectomy after which his arthralgia resolved immediately and grip strength recovered progressively (16/12 kgf, postoperative day 3; 40.9/32.9 kgf, 3 months), reaching normal adult levels. Periosteal changes initially persisted but resolved by 1 year. This case provides the first objective documentation of functional recovery in a patient with hypertrophic pulmonary osteoarthropathy, suggesting that impaired grip strength may be caused by joint inflammation and edema affecting bone-tendon attachments, rather than by muscle weakness alone. The KRAS G12C mutation may contribute to the development of hypertrophic pulmonary osteoarthropathy through upregulation of vascular endothelial growth factor via the Raf pathway. This case provides valuable insights into the pathophysiology of hypertrophic pulmonary osteoarthropathy and confirms that functional impairment is reversible with appropriate treatment.