Purpose: To show that retrovertebral extension of neoplastic and infectious disease proceeds in a predictable manner, with the anatomic superstructure determining the shape of the advancing process.
Methods: We examined 58 patients who had neoplastic (n = 44) and infectious (n = 14) processes that caused canal compromise. In total, 140 levels were examined by means of MR imaging only (48 patients), CT only (1 patient), CT plus MR imaging (3 patients), and MR imaging plus CT myelography (6 patients).
Results: At 136 levels, the retrovertebral disease process took the shape of a smoothly marginated, bilobulated mass that was broad-based against the posterior surface of the vertebra. Various degrees of mass effect were observed: symmetric on 108 levels and asymmetric on 28 levels; on 4 levels, expansion of the anterior epidural space was unilobar, reflecting unilateral canal invasion. With advanced stages of canal invasion, this bilobulated pattern was maintained but there was a tendency toward detachment of the midline septum (20 levels).
Conclusion: The observed patterns are determined by the particular anatomy of the anterior epidural space. The shape of the mass is imposed by the posterior longitudinal ligament and by the attached lateral membranes, with the central tether produced by the attached midline septum. The two compartments expand independently.