2.2. Endoscopic Surgical Anatomy of Inferior Articular Process in Endoscopic Fusion

Inferior articular process is a process of a cephalad vertebra of the spinal segment that lies on each side of the neural arch and projects downward and processes vertical convex articular facets which face anterolaterally articulating with the superior articular process of the next caudal vertebra. It is convex dorsally and laterally (Figure 1A) [21]. In the magnified field under endoscope, the superomedial aspect of the inferior articular facet is often medial and deep to the mid-point of the bony arch forms from the ipsilateral spinolaminar junction of the cephalad lamina to the most inferomedial rounded edge of the inferior articular process, which we labelled as Wu’s point in this paper for illustration purposes (Figure 1A,B). This point is at the sloping edge of the medial laminofacet junction medial to which the inferior articular process slopes ventrally caudally and laterally to join the medial superior edge of the superior articular process. The rotatores spinae muscles are associated medial to the edge. Profuse bleeding can occur leading to poor visualization of the endoscopic view when soft tissue dissection occurs medial to the edge of Wu’s point. From Wu’s point, the endoscope moves obliquely upwards and laterally on the dorsal convexity of the inferior articular process to the superolateral edge of the inferior articular facet which articulates the superolateral edge of the superior articular facet, which we have labelled as Kim’s point for illustration purposes. Kim’s point is therefore the confluence of the superolateral edge of the superior articular process and the superolateral edge of inferior articular process (Figure 1A,C). The Kim’s point is often covered with facet capsule. Exposure of Kim’s point as an endoscopic anatomical landmark involves using a flexible curve radiofrequency ablator to dissect the facet capsule on the lateral edge of the facet joint, followed by endoscopic drilling at the lower and lateral edges of the inferior articular facet to expose the underlying lateral edge of the superior articular facet. Once the superior articular facet was found, we continued endoscopic drilling in the cranial direction along the exposed facet joint to the superolateral edge of the facet joint, which would be Kim’s point. The multifidus muscle drapes this bony region, lateral to Kim’s point is the intertransverse membrane and ventral to the intertransverse membrane is associated with radicular artery and exiting nerve root (care is taken during endoscopic soft tissue dissection lateral to Kim’s point, so as not to breach the intertransverse membrane and cause unnecessary bleeding of the radicular artery).

(A): Illustration of the margin of inferior articular process. The endoscopic inferior articular facet resection required is marked by a black line. Wu’s point (blue circle) is at the mid-point of the bony arch which forms from the ipsilateral spinolaminar junction of the cephalad lamina to the most inferomedial rounded edge of the inferior articular process. Kim’s point is the confluence of the superolateral edge of the inferior articular facet and the superolateral edge of the superior articular facet. The amount of recommended inferior articular process resection is shaded in blue while the amount of recommended superior articular facet resection is shaded in red. (B): In uniportal endoscopic posterolateral lumbar interbody fusion Inside-Out Approach, ETLIF (I), an oblique upwards and lateral direction endoscopic drilling is carried out from Wu’s point towards Kim’s point. (C): In uniportal endoscopic posterolateral lumbar interbody fusion Outside-in Approach, ETLIF (O), an oblique medial inferior endoscopic drilling is carried out from Kim’s point to Wu’s point.