Back muscle injury after posterior lumbar spine surgery - Topographic evaluation of intramuscular pressure and blood flow in the porcine back muscle during surgery

Study Design. Intramuscular pressure and blood flow of the back muscles were evaluated topographically during posterior lumbar spine surgery. The topographic damage of the back muscle after surgery was studied. Objective. To investigate the relationship between intramuscular pressure or blood flow during posterior lumbar surgery and the back muscle injury after surgery. Summary of Background Data. Iatrogenic back muscle injury in an animal and human model has been reported previously. Changes of intramuscular pressure and blood flow during surgery might be related to the muscle injury. No previous study on this issue has been published. Methods. The contact pressure between the retractor blade and muscle tissue was monitored in 10 pigs during posterior surgery of the lumbar spine. On one side, intramuscular pressure at 5, 10, and 20 mm lateral to the retractor and on the other side blood flow of the back muscle at 5 and 20 mm during surgery were measured. Histologic changes of the back muscle at 5, 10, and 20 mm to the midline were evaluated 3 hours after surgery. Results. The contact pressure decreased exponentially with time. Intramuscular pressure 5 mm lateral to the retractor was 114 +/- 31 mm Hg and was significantly higher than at 10 mm and 20 mm. Blood flow markedly decreased during surgery and recovered incompletely after releasing the retractor at 5 mm and 20 mm lateral to the retractor. Blood flow at 5 mm was significantly lower than at 20 mm throughout surgery. The muscle damage 3 hours after surgery was more severe near the retractor blade. Conclusions. The back muscles were exposed to pathophysiologic condition by a retractor during posterior lumbar spine surgery. External compression by a retractor increases intramuscular pressure to levels that impede local muscle blood flow. The muscle degeneration after surgery could be explained by direct mechanical damage and by the increased intramuscular pressure of muscle tissue by the retractor.