Surgical Approaches to the Lumbar Hidden Zone: Current Strategies and Future Directions☆

The lateral lumbar spinal canal may be subdivided into the subsequently necessitate fusion surgery. Other, more lateral approaches subarticular (lateral recess), the foraminal (pedicle) and the extraforaminal (far lateral) zone. Within these regions lies the “hidden zone”, an area known for its difficult surgical exposure (Fig. 1A) (Macnab, 1971). Common pathologies of this region include foraminal osseous stenosis (narrowing of the foramen through which the nerve root exits the spinal canal) as well as disc herniations. It has been estimated that roughly 10–20% of all disc herniations migrate in a craniolateral direction and may hence be located in the preforaminal and foraminal regions of the “hidden zone”. Due to the local anatomy, these lesions may affect both the traversing (level below) as well as the exiting (same level) nerve root. Patients typically present with neurological symptoms of (poly-)radiculopathy, including pain, weakness and numbness. Commonly, and in contrast to the above-mentioned zones, all types of disc herniations that affect the exiting nerve root at the same level are referred to as “faror extremelateral”, including pre-, intraand extra-foraminal herniations. Whilst a variety of effective techniques for approaching extraforaminal and purely intraforaminal lesions have been developed, there continues to be disagreement with regard to the optimal approach to lesions located in the preand intra-foraminal regions of the hidden zone. In order to understand this discord, it is crucial to comprehend the difficulties and patient-specific concerns associated with the surgical exposure of this region. Anatomically, the medial hidden zone is an area bordered laterally by the pedicle, ventrally by the dorsal part of the vertebral body and covered dorsally by the pars interarticularis of the hemilamina (Fig. 1A). Open surgical exploration of this region via the traditional interlaminar route (Fig. 1B) is therefore only possible after at least partial removal of the ipsilateral hemilamina (extended laminotomy or even hemilaminectomy) and may additionally require partial or complete facetectomy (removal of the facet joint) (Schulz et al., 2014). Extended laminotomy as a means to approach the hidden zone has therefore lost popularity, since the associated removal of biomechanically important bony structures has been suggested to increase the risk of secondary segmental instability (Abumi et al., 1990) andmay