Study Design. Intervertebral foraminal anatomy of L2-S1 was investigat
ed by the anatomic dissection of 96 foraminal levels in 12 human cadav
eric spines. Objectives. The goal of this study was to determine the d
imensions of the ''safe zone'' and a safe point of insertion in percut
aneous intradiscal procedures and the largest safe working cannula dia
meter. Summary of Background Data. Working cannulas are used in percut
aneous lumbar procedures. The dimensions of the safe zone of insertion
and determination of the optimal cannula size based on these measurem
ents and point of insertion are unclear from the literature. Methods.
Safe zone dimensions from F2-L3 to L5-S1 were determined as were two c
annula diameters, C1 and C2. C1 was the maximal cannula size that can
be placed within the safe zone. C2 is the maximal size allowable when
the point of insertion lies in the midline of the pedicle. Results. Th
e average triangular safe zone was 18.9 mm wide and 12.3 mm high; the
hypotenuse measured 23.0 mm. The maximum C1 diameter ranged 5.0-10.0 m
m. The corresponding safe point of insertion lay along the medial one
third of the pedicle. The maximum C2 diameter ranged 4.0-8.9 mm. The c
orresponding safe point of insertion lay in the midline of the pedicle
. Conclusions. Either a 7.5-mm cannula placed in line with the medial
one third of the pedicle or a 6.3-mm cannula located in the midline of
the pedicle appears safe.