The most expensive operating room procedure that year was spinal fusion, with an aggregate cost of $12 billion – that’s 413,200 stays with a spinal fusion performed at a mean cost per stay of $28,900. That also works out to be 7.3% of the total cost of $163.8 billion for the 9.2 million stays with a first-listed surgery in the ICD-9 coding.There were five other musculoskeletal procedures among the 20 most costly surgery-related admissions: knee arthroplasty was second at $11.8 billion, hip replacement was third at $8.3 billion, treatment of hip and femur fracture/dislocation was ninth at $4.3 billion, amputation of lower extremity was 13th at $2.5 billion, and treatment of lower extremity (other than hip or femur) fracture/dislocation was 14th at $2.4 billion. Those six procedures combined were $41.2 billion in hospital costs, which was a quarter of the total for all stays with a first-listed OR procedure, the AHRQ said.