True synovial cysts of the lumbar spine: an epiphenomenon of instability of the functional spine unit?

Dear Editor, We read with great interest the paper of Landi et al. titled: “Microsurgical excision without fusion as a safe option for resection of synovial cyst of the lumbar spine: long-term follow-up in mono-institutional experience.” published in Neurosurg Rev, 2012 Apr; 35(2):245–53 [16]. In this report, the authors present their series of 18 patients with synovial cysts who have been operated from 1995 to 2007. After excluding three patients who presented spinal instability at the preoperative assessment, the surgical procedure performed in all other 15 patients was restricted to surgical resection of the synovial cyst without fusion. According to their results, no patient presented signs of instability at the 2year follow-up as evaluated by dynamic x-rays. With basis on such data, the authors defend that, once excluded preoperative instability, the majority of the patients with lumbar synovial cysts can be optimally treated with surgical resection of the synovial cyst and partial arthrectomy (usually restricted to 1/3 of the involved facet) without fusion. Although these are the general lines upon which we base our surgical decision in the daily clinical practice, some special remarks about the present study are opportune. Although the authors found a very low incidence of preoperative instability (around 20 % of the patients with synovial cysts), other series have found much higher rates (up to 65 % in some series) [15] despite the fact that, similarly to the study of Landi et al., usually less than 10 % of the patients received upfront fusion [4, 22], In fact, two previous series reported the presence of at least some degree of spondylolisthesis in plain x-rays in up to 55 % of the patients [18, 20]. Even if such patients may occasionally present no motion in flexion/extension preoperative x-rays, they present a very high risk of developing symptomatic instability after a surgical procedure for resection of the synovial cyst which involves a partial facetectomy and, therefore, also a high likelihood of requiring fusion at the long-term; the reason for which they should be closely followed. Regarding the nomenclature of synovial cysts arising from (or adjacent to) the lumbar zygapophyseal joint, the literature presents a broad range of variation. The first report of such lesion dates from 1880 by von Gruker [12]. Vosschulte and Borger were the first to report associated nerve root compression secondary to a cyst adjacent to the facet articulations in 1950 [28]. Historically, the term juxta-facet cyst was first used by Kao et al. to describe cysts located adjacent to the facets joints or arising from the ligamentum flavum [14]. As the authors mention, from a morphological standpoint, such cysts can be basically subdivided in “synovial”, “ganglion”, “ligamentum flavum,” and “posterior longitudinal ligament cysts” (Fig. 1). True synovial cysts have a synovial lining membrane which communicates with the facet joint. Ganglion cysts, in opposition, have no synovial lining and develop from mucinous degeneration of the periarticular tissue and usually contain proteinaceous fluid. Ligamentum flavum and posterior longitudinal ligament cysts usually contain clear or xanthochromic fluid and also present no real communication with the facet joint [22]. Although some authors have previously defended that such anatomopathological differentiation does not bear any clinical significance, as one cyst type could evolve into the other and, ultimately, all of them would receive the same treatment, we truly believe that it is possible, in a significant proportion of the cases, to subclassify such entities after a T. A. Mattei (*) Department of Neurosurgery, University of Illinois College of Medicine at Peoria, 530 NE Glen Oak, #7430, Peoria, IL 61637, USA e-mail: tobias.a.mattei@osfhealthcare.org