Aim of Study: The aim of the study was to investigate the effect of muscle retractors on intramuscular pressure in the posterior spinal muscles during posterior spinal surgery.

Methods: Twenty patients undergoing posterior spinal surgery were recruited into this study and recordings of intramuscular pressure during surgery were performed using a Stryker® compartment pressure monitoring system, prior to insertion of retractors, 5, 30 and 60 minutes into surgery and on removal of retractors. Prior to and following use of the retractors, muscle biopsies were taken from the erector spinae muscle for analysis.

Results: A significant increase in intramuscular pressure (p< 0.001) was observed during surgery, with pressure rising from 7.1±4.1 mmHg pre-operatively to 26.4±16.0 mmHg 30 minutes into the operation. On removal of retractors, this pressure returned to or near to the original value. Analysis of muscle biopsies using calcium-activated ATPase birefringence revealed a reduction in muscle function following prolonged use of self-retaining retractors.

Discussion: This study demonstrates a substantial rise in pressure in the erector spinae muscle during posterior spinal surgery. Following retraction, marked changes were noted in the function of the muscles. This could be an important factor in the generation of operative scar tissue and post-operative dysfunction of the spinal muscles, and therefore, may be a cause of persistent back pain frequently observed in post-operative patients. Currently, this work is being extended to investigate the relationship between loss of muscle function and duration of retraction, and to study the long term implications of loss of muscle function with respect to surgical outcome and chronic back pain.