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.
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.