L5 radiculopathy caused by L5 nerve root entrapment by an L5-S1 anterior osteophyte

Discussion

There have been only a few case reports of L5 radiculopathy caused by anomalous bony articulations between L5 and S1. Although this patient had an L5 radiculopathy, it is different from the “far-out syndrome” traditionally described by Wiltse et al.⁸ He did not fit into any of the categories described in their series that included elderly patients with degenerative scoliosis and a younger group of patients with isthmic spondylolisthesis with at least 20% slip who showed symptoms of an L5 radiculopathy caused by entrapment of the L5 nerve root between the transverse process of L5 and the sacral ala. This patient had entrapment of the nerve root by a large anterior osteophyte protruding anterolaterally from the L5-S1 disc space.

Matsumoto et al⁶ reported on a series of 28 patients with extraforaminal entrapment of the L5 nerve root at the lumbosacral junction. CT imaging showed the presence of marked osteophytes at the posterolateral margin of the L5-S1 vertebral bodies. By use of a microscopic or microendoscopic posterior approach, they were all decompressed with partial resection of the sacral ala, L5 transverse process, and L5-S1 facet joint along the L5 root. The authors did not make any attempt to resect the osteophyte to avoid damage to retroperitoneal vascular structures, nor was any patient initially fused. All patients in their series had immediate pain relief; however, 4 underwent revision with posterior interbody fusion for recurrent radicular pain due to intraforaminal stenosis in 3 patients and inadequate extraforaminal decompression in the other. In contrast, in our case report, an anterior retroperitoneal approach was used to directly visualize the osteophyte, which was resected, and anterior lumbar interbody fusion was performed concurrently.

Abe et al⁹ first reported on the use of an anterior approach in a single patient to decompress the L5 nerve root compressed by an anomalous alar transverse process. In addition to diagnostic injections and traditional imaging, they used selective radiculography to confirm the diagnosis and believed that this was the most useful diagnostic test to diagnose this condition. Selective radiculography was not used in our case, because there was no evidence of traditional spinal canal or foraminal pathology present on imaging. However, it could have been used to gain further confirmation and accurate localization of the entrapment. Furthermore, the use of diagnostic injections should absolutely be performed diagnostically and exhausted therapeutically, before proceeding with any type of surgical intervention.

The use of advanced imaging was critical to the diagnosis of our patient's condition. Moon et al¹⁰ found that 77% of patients in their retrospective review of preoperative CT scans in 27 patients showed an asymmetric enlargement of the anterior primary division of the L5 nerve root that was significantly associated with foraminal or extraforaminal compression of the root at the lumbosacral junction. In our case, even more valuable than the CT scan was its extrapolation to create a 3D model. This was helpful to the surgical team's understanding and visualization of the patient's anatomy and pathology. Intraoperatively, the surgeons repeatedly referred to the model. The resected osteophyte closely resembled what was expected. In such cases, use of the 3D model is recommended because most surgeons will be unfamiliar with this rather rare pathoanatomy.

Osteophytes and other anomalous bony abnormalities account for some cases of entrapment, but thickened lumbosacral ligaments (LSL) can also contribute to root compression. In a cadaveric study of 42 specimens, large osteophytes on the inferior border of L5 and tightness of the LSL were found to contribute to entrapment of the L5 nerve root.¹¹ Olsewski et al¹² identified an 11% rate of LSL compression of the L5 nerve root in 102 specimens. Although a thickened LSL may contribute to compression in cadavers, it may not always be symptomatic in vivo. Extraforaminal disc herniations may also play a role in entrapment of the L5 nerve root at the lumbosacral junction. O'Toole et al¹³ reported a case of microendoscopic discectomy performed at the lumbosacral junction with successful relief of the patient's symptoms.

In our case, the decision was made to perform a concurrent arthrodesis based on the patient's history of increased pain with transition to sitting or standing. It was thought that this would increase the chances of achieving a successful outcome by eliminating movement across the joint as a potential source of compression. In other case reports, this has not been routinely performed concurrently, but there is not an abundance of evidence to support or negate its use.

In conclusion, extraforaminal entrapment of the L5 nerve root has many different causes and must be considered in patients presenting with an L5 radiculopathy. In this case, a large anterior L5-S1 osteophyte was removed safely using the anterior retroperitoneal approach, which yielded a good clinical result.