Danger of damaging the medial branches of the posterior rami of spinal nerves during a dorsomedian approach to the spine.

Postoperative atrophy of the deep back muscles may be caused by denervation during a dorsomedian approach to the thoracolumbar spine; ensuing instability of the spine with poor clinical results, perhaps due to such muscle loss, has been observed in 11.7% of cases (Sihvonen et al., 1993, Spine 18:575--581). More specifically, this complication may be caused by damaging the medial branches of the posterior rami of the spinal nerves during lateral retraction of the muscles. To investigate the anatomic topography of the medial branches of the posterior rami of the spinal nerves, 18 carbol-formol-fixed specimens were dissected using an operation microscope; also, 3 fresh cadavers were cut in horizontal and vertical planes with a rotary cryotome to confirm the anatomic topography observed in the fixed specimens. In the thoracolumbar spine the medial branch of the posterior ramus of the spinal nerve is subject to ligamentous fixation by the strong fibers of the mammillo-accessory ligament, which extends between the mammillary process and accessory process infero lateral to the superior articular process. When the dorsomedian approach to the thoracolumbar spine is enlarged laterally to the articular processes by retracting the paraspinous muscles, the medial branches of the posterior rami of the spinal nerves are endangered. This may cause postoperative pain as well as dynamic instability beyond the corresponding segments. The results of our anatomic study suggest that the posterior surgical midline approach to the thoracolumbar spine should not be enlarged laterally to the articular processes to prevent injury to the medial branches of the posterior rami of the spinal nerves. Copyright 2002 Wiley-Liss, Inc.