Pathology of ossification of the ligamentum flavum

Abstract

Introduction Ossification of the ligamentum fl avum (OLF) is a pathological condition that causes myelopathy, radiculopathy, or both [1,2]. Reports have shown that it is relatively common in the Japanese population compared to that in American or European populations [3,4]. The etiology of hypertrophy and Ossification of the ligamentum fl avum is still not fully understood, but an association with Ossification of the posterior longitudinal ligament (OPLL), or diffuse idiopathic skeletal hyperostosis, has been found. OLF can be diagnosed on lateral radiographs, manifesting as Ossification of the spinal foramen. When comparing the narrowing of the spinal canal as seen by computed tomography (CT) or magnetic resonance imaging (MRI), the CT scan may provide information superior to that of MRI because it shows precisely the areas where there is protruding Ossification from the posterior to the anterior aspect of the spinal canal [3-6]. Historically, OLF was fi rst observed on lateral radiographs and reported by Polgar in 1920 [7]. In 1938, Anzai [1] described the fi rst case with neurological symptoms and identifi ed OLF in a specimen removed during the operation. Oppenheimer [8] also observed OLF on plain radiographs in diffuse idiopathic skeletal hyperostosis and ankylosing spondylitis. He speculated that such Ossification might be responsible for a radicular neuropathy. In 1960 Yamaguchi et al. [2] reported an operative case with severe myelopathy; Koizumi [9], Yanagi [10], and Nagashima [11] subsequently reported similar cases. Most cases of OLF occur in the thoracic spine, especially the lower third of the thoracic or the thoracolumbar spine; OLF rarely occurs in the cervical spine [12,13]. Because thoracic spinal canal stenosis resulting in thoracic myelopathy or radiculopathy has been noted recently [14,15], OLF is now recognized as a clinical entity causing thoracic myelopathy manifesting as OPLL and spondylosis [12,16-19]. When OLF was considered a contributing factor in patients with herniated thoracic discs, the surgical results were poorer than those in patients without OLF [20]. However, outside Japan, unlike OPLL in the cervical spine, thoracic myelopathy secondary to OLF is sometimes overlooked or misdiagnosed as degenerative overgrowth by the posterior spinal element consisting of the superior articular processes [21-24]. This error results from a lack of knowledge about this pathological condition. OLF has been recognized as a composite lesion because the combination of Ossification of the spinal ligaments with hyperostotic changes is frequently encountered [10,12,25,26]. Small degrees of OLF may be considered a degenerative change, as its incidence in radiographic studies of the spinal columns of aged persons has ranged from 4.5% [27] to 25.0% [13]. It has been suggested that the mechanism of hypertrophy, overgrowth, and progression of Ossification of the ligaments plays an important role in the pathological process of myelopathy. This chapter describes the pathology of OLF by comparing autopsy specimens with typical specimens obtained during surgery. It also discusses the pathogenesis of OLF and the recent literature about the contribution of growth factors. © 2006 Springer.