X-ray and MRI analysis of the percutaneous transforaminal endoscopic approach

Objective To radiographically analyze the relationships of the nerve root and the safe trajectory in lumbar using percutaneous transforaminal endoscopic discectomy (PTED) approach. Methods A retrospective study from January 2011 to January 2015 in Tianjin Hospital was carried out. One hundred MRIs of transverse views and 100 X-rays were used for image analysis, those with previous spinal surgery, trauma or gross deformity were omitted from the study. The MRI study was done bilaterally from L2/3 to L5/S1. On transverse MRI, the foraminal width (G), the nerve root-disc distance (H), nerve root-facet distance (I) and the target angle (J°) were analyzed at the superior (s) and inferior (i) margin of the disc. The X-ray study was done from L4/5 to L5/S1 in two planes AP (a) and Lateral (b). The crest height (Q) and target angle (R°) were measured on both planes. Results On transverse MRIs, the foraminal width was larger at superior margin of the disc. The smallest Gi was measured at the inferior margin of L4/5 [(2.72±1.40) mm] and the largest Gs was measured at the superior margin of L2/3 [(5.16±1.53) mm]. The nerve root-disc distance decreased caudally and was measured closer at the inferior margin of the disc. The nerve root lay dorsal to the disc at L2/3 and at L3/4, whereas the nerve root lay ventral to the disc at L4/5 and L5/S1. The nerve root-facet distance was measured shortest at the superior margin than at the inferior margin of the disc. The shortest Is of superior margin was at L4/5 [(-2.55±4.94) mm] and the longest Ii of inferior margin at L5/S1 [(-8.23±11.11) mm]. The target angles (Js, Ji) decreased caudally. The target angles at L2/3 and L3/4 were wider at their superior margin than their inferior margin. In contrast, the angles at the lower lumbar levels, L4/5 and L5/S1, were smaller at their superior margin than those at their inferior margin. On X-rays, L4/5 lied almost at the same level of the iliac crest whereas L5/S1 lay below. The target angle (Ra°, Rb°) at L5/S1 was much steeper than that at L4/5. Conclusions At the lower lumbar levels (L4/5, L5/S1) the foraminal width decreases, the nerve root lies more ventrally to the disc and the trajectory angle is too narrow or steep. During transforaminal percutaneous endoscopic lumbar discectomy these features put the exiting nerve root at risks of iatrogenic injury. On that account, for reasons aforementioned, it is highly advised to enlarge the foramen to achieve adequate access to a safer passage of endoscopic instruments and thus to minimize the risks of nerve injury. Key words: Intervertebral disc; Magnetic resonance imaging; Radiography; Diskectomy; percutaneous; Endoscopes; Surgical approach