[The thoraco-lumbar junction. Orientation of zygapophyses, mamillary process and vertebral rotation]

The orientation of the zygapophyseal joints at the thoraco-lumbar level is an important factor involved in the axial rotation of the human spine. The functional role of the Mamillary Processes (MP) is unknown. This study was carried out on 55 adult dried spines. The aims were 1) to record the interzygapophyseal joint angles. 2) to determine morphological and functional angle types. 3) to correlate the various types and the corresponding MP lengths. 4) to propose hypotheses about the MP role. There were 2 distinct morphotic types founded on 2 different populations of angles clearly visible on histograms at the T12 level and statistically detectable at the T11 and L1 levels. T11-T12-L1 realized an homogeneous anatomical unit with 2 separate groups depending on the zygapophyseal joint orientations. A functional classification was also possible depending on the location of the geometric axial center of rotation. The "thoracic" type which center was on vertebral body. 95% of T10 vertebrae and 88% of T11 vertebrae belonged to the "thoracic" type whereas 97% of L1 ans 98% of L2 belonged to the lumbar type. At the T12 level, we found 35% of "thoracic" type and 65% of "lumbar" type. Long MP were found on "lumbar" type vertebrae with little interzygapophyseal angles (p < 0.001). "Anatomical" types were not related to different MP lengths. On the contrary, the "functional" classification showed shorter MP on "thoracic" type vertebrae than on "lumbar" vertebrae (1.9 +/- 2.9 mm vs 4.5 +/- 3 mm, p < 0.03). A large interzygapophyseal angle is known to enhance the axial rotation of the thoraco-lumbar junction. We infer that MP also play a role in axial rotation.(ABSTRACT TRUNCATED AT 250 WORDS)