BACK MUSCLE INJURY AFTER POSTERIOR LUMBAR SPINE SURGERY - TOPOGRAPHICEVALUATION OF INTRAMUSCULAR PRESSURE AND BLOOD-FLOW IN THE PORCINE BACK MUSCLE DURING SURGERY

Study Design. Intramuscular pressure and blood flow of the back muscle s 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 pres sure or blood flow during posterior lumbar surgery and the back muscle injury after surgery. Summary of Background Data. Iatrogenic back mus cle 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 b een published. Methods. The contact pressure between the retractor bla de and muscle tissue was monitored in 10 pigs during posterior surgery of the lumbar spine. On one side, intramuscular pressure at 5, 10, an d 20 mm lateral to the retractor and on the other side blood flow of t he 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 ev aluated 3 hours after surgery. Results. The contact pressure decreased exponentially with time. Intramuscular pressure 5 mm lateral to the r etractor was 114 +/- 31 mm Hg and was significantly higher than at 10 mm and 20 mm. Blood flow markedly decreased during surgery and recover ed incompletely after releasing the retractor at 5 mm and 20 mm latera l 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 w ere exposed to pathophysiologic condition by a retractor during poster ior lumbar spine surgery. External compression by a retractor increase s intramuscular pressure to levels that impede local muscle blood flow . The muscle degeneration after surgery could be explained by direct m echanical damage and by the increased intramuscular pressure of muscle tissue by the retractor.