The effectiveness of selective thoracic fusion for treating adolescent idiopathic scoliosis: a systematic review protocol

Abstract

Review question/objective: The objective of this review is to assess the effectiveness of selective thoracic fusion as a form of treatment in adolescent idiopathic scoliosis (AIS). This will be compared with all other forms of operative management for major structural thoracic curves. Background: Scoliosis is defined as a lateral curvature of the spine of at least 10 degrees, as measured by the Cobb angle. It can be categorized into three broad categories - neuromuscular, congenital and idiopathic. Of these categories, idiopathic is by far the most common, and is a diagnosis of exclusion. Idiopathic scoliosis can then be further broken down into categories based on age of onset. Of these, AIS (children presenting at 10 years of age or older) accounts for 80-85% of cases.1 Scoliosis curves have a proven complex deformity, consisting of a three-dimensional deformity involving the coronal, sagittal and rotational planes. Each curve (of which there may be many in one patient) can be described with an apex (the vertebra with the greatest lateral distance from the centre of the spine) and the two vertebrae at the end of the curve (named the end vertebrae). The Cobb angle, measured by the intersection of parallel lines from the endplates of the superior and inferior end vertebrae, is the standard way of quantifying the magnitude of scoliosis curves. Major or primary curves are the largest abnormal curves as classified by the Cobb angle. These curves are almost always structural. In addition, secondary or tertiary curves are described as structural if the Cobb angle cannot be reduced to under 25 degrees, on side bending radiographs.2 Due to the permanent nature of physiological and morphological change of the vertebral bodies and ligaments, structural curves will usually progress as the patient matures, usually at 1 degree per year after maturity.3 Non-structural curves usually do not progress as the patient matures; instead they are hypothesized to be a product of the body's instinctive nature to provide truncal balance. For many years spinal surgeons have been debating whether a more rigid and straighter spine or a mobile and less straight spine provides better outcomes.4 The treatment for AIS can include both an operative and non-operative approach. However when the Cobb angle is above 400, the likelihood of curve progression is high and surgical treatment is warranted.5 Although technology has advanced, the primary goals for operative management have remained constant. The primary goals of surgical treatment in AIS should be to optimize coronal and sagittal correction and avoid further curve progression. This involves not only correction of the major primary curve but also any minor (secondary) curves, while maintaining adequate thoracic kyphosis and lumbar lordosis. Ideally, a balance should be struck between fusing the lowest number of mobile segments and properly correcting the existing deformity. This is where selective spinal fusion has a role to play. The premise of selective thoracic fusion is that after fixation of the primary thoracic curve, there is spontaneous coronal correction of the unfused lumbar curve.6 Thus the thoracic curve can be exclusively fused to allow for a more mobile lumbar spine.4,6 This has been described in studies since the 1950s.7,8 However since then, results have varied greatly in the extent of spontaneous lumbar correction. Studies have shown that the degree of spontaneous correction of the lumbar spine is somewhat close to the correction of the thoracic curve; however the extent of optimal correction that can be achieved is uncertain.9-11 The alternative to selective thoracic infusion involves complete fusion of both the primary thoracic and secondary lumbar curve in a consecutive series. This can be done via either an anterior or a posterior approach. Complete fusion gives better correction of both curves. It also diminishes the risk of coronal decompensation, adding on phenomenon, junctional kyphosis and eventual revision surgery.4 However this needs to be calculated against the risk of sagittal decompensation, increased risk of lumbar degeneration and chronic back pain, all of which seem to be more prevalent in patients with fusion of both curves.12 Another goal of surgical intervention is the need to avoid complications. Examples of complications of selective spinal fusion include: junctional kyphosis, coronal imbalance, adding-on and revision surgery.9,13,14 Junctional kyphosis is described as kyphosis of over 10 degrees more than pre-operative measurements. This is measured by the angle between the inferior end plate of the highest instrumented vertebrae and the superior end plate of the vertebra two levels higher. Coronal imbalance is when the distance between the C7 plumb line and the central sacral vertical line is greater than 2 centimeters. The adding-on phenomenon is described as progression or extension of the primary curve after fusion.10 In 2001, Lenke et al 2 reported a classification for AIS that has been able to identify those patients who may benefit from a selective spinal fusion (1C, 2C, 5C). A three-tiered approach is used with the Lenke classification system involving curve type, lumbar modifier and sagittal modifier. Firstly the curves of the spinal column (proximal thoracic, main thoracic and thoracolumbar/lumbar) are classified as structural or non-structural before a lumbar modifier (A, B, C) based on the distance from the central sacral vertical line and the lumbar apical vertebra is applied. Further classification is then undertaken measuring the kyphosis of the thoracic curve T5-T12 (-, N, +). Lenke proposed that a selective thoracic fusion could be undertaken when the primary curve is structural and the compensatory lumbar curve is non-structural and that additionally certain radiological criteria were met such as the Cobb angle magnitude, apical vertebral translation and rotation. These are all objective markers that can be accurately measured on plain radiographs, with good inter-and intra-observer reliability.2 However all surgeons do not routinely accept these treatment guidelines. It has been reported that only approximately 49-67% of experienced surgeons are performing a selective thoracic fusion in Lenke 1C curves.15,16 This may be due to the fear of complications (of which the rates are relatively unknown) and well as misunderstanding of how much correction can be achieved by the un-fused compensatory lumbar curve. A search of PubMed, the Cochrane Library, PROSPERO and the JBI Databases of Systematic Reviews and Implementation Reports found no current systematic review assessing the effectiveness of selective thoracic fusion compared to other approaches. As such, the aim of this review is to evaluate and critically appraise available evidence on selective thoracic fusion in order to provide a suitable estimate of the radiological and functional outcomes of this type of surgical intervention as well as the approximate complication rate in order to give patients correct information prior to their providing their informed consent.