Effects of transcutaneous CO2 application on fracture healing in a rat fracture model of disuse osteoporosis.

Transcutaneous carbon dioxide (CO2) therapy has been shown to accelerate fracture healing and osteogenesis by enhancing endochondral ossification and angiogenesis. We aimed to investigate the effects of this therapy on fracture healing in a rat hindlimb suspension model of disuse osteoporosis. Eleven-week-old male Sprague-Dawley rats were randomly assigned to three groups: 1) Control group (normal ambulation for 3 weeks, followed by femoral fracture with Kirschner-wire fixation); 2) Hindlimb suspension (HS) group (HS; 3 weeks of HS before fracture); 3) HS + CO2 group (same as HS, followed by transcutaneous CO2 therapy after fracture). HS was induced by tail suspension at a 30° head-down tilt to prevent hindlimb weight-bearing. Transcutaneous CO2 therapy involved applying a CO2-permeable hydrogel and enclosing both hindlimbs in polyethylene bags filled with 100% CO2 for 20 min/day, 5 days/week. The HS + CO2 group showed significantly higher modified Radiographic Union Score for Tibial fractures than did the HS group at weeks 3 and 4, as well as a significantly higher radiographic union rate at week 4. They also showed significantly higher Allen's histological grading scores at week 3. Compared with the HS group, the CO2-treated animals showed significantly higher expression levels of runt-related transcription factor 2 and Osterix at weeks 1 and 2, respectively. These findings suggested that transcutaneous CO2 therapy promoted osteogenic activation and accelerated radiological and histological fracture healing of disuse osteoporosis, supporting its potential as an adjunctive strategy for fragility fractures.