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

The lateral lumbar spinal canal may be subdivided into the 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 “far- or extreme-lateral”, including pre-, intra- and 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 pre- and intra-foraminal regions of the hidden zone. Fig. 1 Lumbar anatomy and surgical approaches to the hidden zone. A) Overview of applied neurosurgical anatomy of the lumbar spine, depicting a representative craniolateral disc herniation affecting both the exiting L2 and the passing L3 nerve root. Besides ... 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) and may subsequently necessitate fusion surgery. Other, more lateral approaches have been suggested; however, these require specific anatomical knowledge, and offer inferior access to more medial spinal pathologies of the hidden zone. In 1998, Di Lorenzo et al. (1998) proposed a less invasive direct procedure by utilizing a translaminar approach (TLA) through a fenestration of the pars interarticularis, thus circumventing facetectomy or hemilaminectomy in many cases (Fig. 1C). The increasing availability of high-definition imaging modalities (MRI, CT) has contributed to the growing popularity of the TLA, since identifying the exact location and extent of the spinal lesion is crucial for surgical planning to limit unnecessary biomechanical damage and prevent intraoperative conversion to conventional approaches. In recent years, several studies have demonstrated the feasibility, safety and efficacy of this technique to successfully treat disc herniations affecting the foraminal and preforaminal regions. Endoscopic approaches to the hidden zone have been suggested, including endoscopic transforaminal (Fig. 1D) or translaminar techniques (Schulz et al., 2014; Dezawa et al., 2012). However, whilst the endoscopic TLA might offer an incremental improvement with regard to trauma, transforaminal endoscopic procedures are not recommended for the more medial foraminal lesions of the hidden zone due to imposed spatial restrictions, especially in the lower lumbar levels. Consequently, endoscopic transforaminal approaches to these pathologies have been associated with increased operating times as well as higher complication and revision rates (Schulz et al., 2014; Lee et al., 2007). Nevertheless, even though the TLA seems to be the method of choice to approach craniolateral disc herniations, some authors have argued that this technique also has its limitations. Due to segment-dependent changes of vertebral anatomy, Di Lorenzo's approach must be located very laterally in the more upper lumbar levels in order to reach the medial hidden zone. Disruption of the lateral hemilamina (pars interarticularis), however, has been linked to an increased risk of stress fracture and instability (Ivanov et al., 2007). This becomes more relevant as the relative risk of cranial disc sequestration increases significantly in higher lumbar levels and cranial sequestration is strongly correlated with increased age (Daghighi et al., 2014). Since older patients are also more likely to suffer from osteoporosis and degenerative spinal disorders such as facet joint hypertrophy, which may manifest segmental instability, less invasive medial approaches to the hidden zone are warranted. Recently, Reinshagen et al. (2015) suggested approaching craniolateral disc herniations via a crossover translaminar approach (cTLA), which utilizes a fenestration of the contralateral hemilamina at the base of the spinous process to reach the hidden zone (Fig. 1E). Besides avoiding disruption of the lateral half of the hemilamina, this facet-sparing technique might additionally offer advantages when treating recurrent patients who previously underwent extended laminotomy, as approaching the recurrent pathology from the contralateral side avoids additional ipsilateral bone resection. A minimally invasive technique, similar to that reported by Reinshagen et al., has been proposed by Alimi et al. (2014)). Although not a translaminar approach, Alimi's technique also features a crossover route to the foraminal region and demonstrated good results for treating foraminal stenosis in a series of 32 patients. The main limitation of both TLA and cTLA techniques is their restricted access to the intervertebral disc space, especially at lower lumbar levels. Although cranial disc herniations mostly appear as completely sequestered fragments, preoperative imaging and meticulous surgery planning is crucial in order to minimize reversion to conventional approaches. In the future, combining the TLA or cTLA with preoperative simulation software as well as intraoperative neuronavigation might prove helpful in further minimizing surgical tissue trauma when treating these challenging pathologies. In conclusion, access to the hidden zone remains surgically challenging. However, with an increasing number of reliable techniques the surgeon can now decide which procedure is the most appropriate for a patient's individual pathology. Furthermore, even though common sense implies that less bone disruption increases spinal stability, data on TLA and cTLA approaches still need to be supported by a large prospective randomized trial to assess the preservation of spinal stability and patient outcomes compared to conventional approaches.