Adult degenerative scoliosis (ADS) is defined as scoliosis in adults secondary to degenerative changes in the intervertebral discs and articular synovial joints. Degenerative scoliosis mostly occurs in the thoracolumbar and lumbar segments of the spine, and often requires surgical treatment due to degenerative changes in the diseased segments, spinal stenosis, vertebral body slippage, and vertebral joint instability, resulting in symptoms such as severe low back pain, neurogenic pain, and intermittent claudication. Zou Debo, Department of Orthopedics, Thousand Buddha Mountain Hospital, Shandong Province, China 1. Etiology and pathologic features The pathogenesis of degenerative scoliosis is not yet completely clear, and it is generally believed that the main factor in its occurrence and progression is asymmetric degenerative changes in the spinal discs and intervertebral joints, and osteoporosis and vertebral compression fractures may play a relevant role [1]. First, the nucleus pulposus of the intervertebral disc in the diseased segment is dehydrated and the height of the intervertebral space is reduced, which causes local ligamentous laxity and reduced stability, hypermobility of the corresponding vertebral segments, and an increase in the compressive stress on the joints of the articular synchondromes bilaterally. Due to the local instability, the spine can be tilted to one side, and the asymmetric stress on the intervertebral discs and articular synovial joints increases, resulting in narrowing of the intervertebral space on one side, uneven development of degeneration, and bending of the spine to the side. In the early stages of the disease, a portion of degenerative scoliosis still has the possibility of self-correction [2]. With the development of the disease, the vertebral body is tilted, slipped and rotated to the side, the pedicles of the diseased segments are twisted, and the spine bends to the side more than the compensation of the local muscle ligaments in a vicious circle. Muscle spasm on the concave side increases the tensile force on the corresponding side of the spine, while muscle fatigue on the convex side loses its power to counteract spinal curvature, which can ultimately lead to a complete loss of spinal equilibrium [3]. Alternatively, due to localized nerve root involvement in the spine, compensatory scoliosis of the spine occurs to alleviate neurological symptoms. When the primary disease is left untreated for a long period of time, the spine is no longer able to tolerate the compensatory scoliosis, and the asymmetric intervertebral space collapses causing degenerative changes in both the vertebral body and the articular synchondrosis joints, which ultimately leads to degenerative scoliosis deformity as in the above mentioned mechanisms. Since most patients with degenerative scoliosis are middle-aged or older than 45 years old, and most of these patients are combined with severe osteoporosis, and sometimes osteoporotic vertebral compression fracture can be found in the X-ray films of these patients, some scholars believe that the occurrence of degenerative scoliosis may be related to osteoporosis [4]. Other scholars disagreed with this view because degenerative scoliosis can occur in adult populations without osteoporosis and osteochondrosis, and therefore it was not considered to be directly related to the development of degenerative scoliosis. Thus, osteoporosis may be associated with the progression of scoliosis, but is not the primary cause. Degenerative scoliosis in adults mostly involves the thoracolumbar segment and the lumbar spine, and the extent of involvement is usually short, mostly between T11, T12 to L5 and S1 [5], with the parietal vertebrae most often located in the L3/4 or L2/3 interspace, followed by the L1/2 interspace, and there is often a rotational subluxation of the L3 and L4 vertebral bodies and tilting of the L4 and L5 vertebral bodies [6], and in severe cases there is a loss of equilibrium in both the sagittal and coronal planes. Since the disease occurs on the basis of degenerative spinal changes, in addition to the deformity itself, there are usually spinal lesions that require surgical management. These lesions include herniated discs, spinal stenosis, hypertrophy of the ligamentum flavum, vertebral body slippage and instability, coronal and sagittal angulation, vertebral rotation, and spinal imbalance problems. Unlike adolescent idiopathic scoliosis, the scoliosis in these patients is stiff, poorly flexible, disorganized, and difficult to correct; the size of the Cobb’s angle in its coronal plane has no significant correlation with surgical outcome, whereas the recovery of lumbar lordosis influences the outcome of the treatment [7].2 Clinical manifestationsThe clinical manifestations of degenerative scoliosis in adults are mainly low back pain, radicular pain, intermittent claudication and cauda equina syndrome. Since the coronal Cobb’s angle is usually small in these patients, it is rare for patients to present with cosmetic changes. Degenerative scoliosis in adults involves the spine and the peripheral neuromuscular system, with changes in the spine itself such as herniated discs and subluxation hyperplasia of the articular synovial joints, as well as small nerve entrapments and muscle spasticity and fatigue, which may all contribute to low back pain, either alone or in combination. Sagittal imbalance or flat back deformity is more likely to cause low back pain in patients [8], so patients with lumbar curvature are more likely to have low back pain than patients with thoracic curvature. This pain can be confined to the lesion segment or can be diffuse throughout the low back. Defining the source of the pain is extremely important in determining treatment; for example, when the pain is on the convex side of the spine, consider spinal deformity and muscle strain due to muscular imbalance; when the pain is on the concave side, consider degenerative changes of the articular synovial joints, degenerative changes of the intervertebral discs, or instability of the spine. Epidural closure, selective nerve root block, articular synapse block, discography, and some other diagnostic tests are decisive references for determining the source of pain and the choice of surgical methods [9]. Lower extremity radicular pain is a manifestation of nerve root compression and ischemia caused by spinal stenosis, which usually occurs at the apex of the lateral convexity with typical neurological localization.Liu et al [10] found that nerve root compression in L3 and L4 was mainly caused by stenosis of the foramen magnum or its outlet on the concave side, and nerve root involvement in L5 and S1 was considered to be caused by the stenosis of the convex lateral saphenous fossa. It was shown that the former had a significantly greater effect on the size of Cobb’s angle than the latter. In the presence of lateral displacement of the vertebral body, the likelihood of nerve root compression was higher in L3 and L4 than in L5 and S1.Unlike the nerve root compression in L5 and S1, which is common in general degenerative diseases (e.g., spinal stenosis), the sites of nerve root compression secondary to degenerative scoliosis in adults tend to be wider and higher because the apex of adult scoliosis is often located in the L3, 4, or L2, 3 interspaces, and the apex is often characterized by rotational deformity and lateral slippage, and the there is a correlation between its distribution and the anatomical features of the scoliotic deformity [11]. In addition, neurogenic intermittent claudication is a major reason for consultation in patients with degenerative scoliosis, and as with the above reasons, the location of spinal stenosis in these patients is often at the site of the most severe scoliosis, and the involved nerve roots tend to be more extensive.3 Assessment of the conditionThe choice of treatment for adult patients with degenerative scoliosis relies on the assessment of the detailed clinical manifestations and imaging data. The former includes age, general condition, history of scoliosis, vital organ function, and a thorough orthopedic examination, such as the source and nature of pain, neurologic localization, identification of the etiology of intermittent claudication, softness of the scoliosis, and general balance of the trunk. It is important to take a detailed history of the deformity, treatment and medication, and to assess the psychological impact of the disease on the patient. A detailed understanding of the patient’s expectations of the procedure is a key factor in meeting patient satisfaction. Imaging is crucial in the evaluation of adult degenerative scoliosis. It usually includes evaluation of X-rays, CT, MRI and various contrast images with the aim of evaluating the degree of degeneration, identifying the source of pain and nerve compression, and understanding balance in the coronal and sagittal planes.X-rays entail the taking of full-length orthopantomograms of the spine in the standing position, left and right lateral flexion films in the prone position (coronal plane), and hyperextension and hyperflexion films (sagittal plane). Measurements of the following indices are required in the full-length orthopantomograms: the extent of lateral convexity in the coronal plane, Cobb’s angle, upper and lower endplate inclination, parietal rotation and slippage, stabilizing vertebrae, maximal lateral displacement, and coronal loss of compensation (the distance between the sacral midline and the C7 plumb line); the relationship of the segments in the sagittal plane; and the angle of the lumbar lordosis. Measurement of the above indexes can fully understand the sagittal plane, coronal plane and the overall balance, according to which the degenerative scoliosis can be typed, which is important for the treatment strategy, the selection of specific surgical methods and the prediction of the treatment effect [12-13].MRI can be used to assess whether the central spinal canal and the lateral fossa are stenotic and whether the degeneration of individual discs is present, and exclude the vertebral body and the vertebral canal from occupying lesions.CT, Discography, neurogenography and myelography are used to observe intervertebral foraminal stenosis and nerve root compression, which helps to identify the source of pain [14] and is an important basis for whether to perform surgery to correct scoliosis and the extent of decompression of the spinal canal.4 Surgical treatment is different from the orthopedic treatment plan for adolescent idiopathic scoliosis, and the treatment of degenerative scoliosis in adults should focus on the alleviation or elimination of symptoms, and does not advocate perfect orthopedic surgery. The main goal is to reduce or eliminate symptoms and not to advocate perfect orthopedic surgery. Simple lumbar scoliosis should be treated conservatively if the angle is small, there is no severe spinal stenosis, vertebral body slippage or instability, and the spine is basically balanced in the sagittal and coronal positions. Unless cauda equina syndrome occurs and emergency surgical decompression is required to save neurological function, most patients should also be treated conservatively for a period of time, often due to comorbidities such as hypertension, diabetes, or respiratory disease. Traditional conservative treatments include bed rest, brace protection, massage and physiotherapy, lumbar and dorsal muscle exercise, pain medication, and intravertebral canal closure, etc. Although scholars have confirmed that braces and massages do not have long-term efficacy, the above measures often result in varying degrees of relief of patients’ clinical symptoms [15]. For patients who have been ineffective in conservative treatment, or whose symptoms recur after treatment and affect their daily life, surgery should be performed when the systemic condition permits. It is now generally recognized that the indications for surgery in this disease are intractable and recurrent low back pain and radiating pain in the lower limbs, significant nerve compression symptoms and progressive aggravation. Surgery should also be considered when there is significant segmental instability, subluxation, sagittal and coronal imbalance, excessive or progressive worsening of scoliosis, and a combination of severe retrovertebral deformity that affects quality of life. Age and mild medical conditions are not contraindications to surgical treatment [16].Kluba compared the effects of conservative and surgical treatments and concluded that surgery improved the patient’s ability to walk and quality of life, and reduced the amount of pain medication taken [17]. In surgical treatment, the source of symptoms should be determined according to the patient’s clinical manifestations and imaging examinations, and in addition to solving the problems of spinal stenosis, vertebral body slippage, and intervertebral instability, the sagittal and coronal balance of the entire spine should also be considered, so that the deformity can be corrected as much as possible, and the normal spinal alignment can be restored, especially to rebuild the physiological anteversion of lumbar vertebrae. Many scholars have demonstrated that good sagittal balance is more important than correction of scoliosis deformity and is significantly associated with good postoperative outcomes [18-20]. During surgical treatment, the focus should also be on correcting the lateral displacement of the vertebrae [21]. The choice of access for surgical treatment relies on the pain-causing segment, the flexibility of the bend, the inclination of the distal vertebrae, and the extent of the bend. After surgical decompression, the deformity can be corrected using internal fixation devices and fused with implants depending on the degree of deformity. Commonly used surgical procedures include: simple spinal canal decompression; multisegmental open window or butterfly decompression; spinal canal decompression and posterior fusion with internal fixation; spinal canal decompression with anterior and posterior fusion and posterior internal fixation. Among them, posterior fusion internal fixation can be divided into short-segment fusion fixation and long-segment fusion fixation [22]. Some authors have also used Smith-Peterson osteotomy + pedicle screw internal fixation to reestablish the balance between the coronal and sagittal planes of the spine after spinal canal decompression [23]. Regardless of the surgical approach, obtaining a solid fusion is critical to achieving the desired outcome. There are still no accepted principles for fusion and fixation of adult degenerative scoliosis. For the selection of proximal fusion vertebrae, some scholars believe that the thoracolumbar segment (T11-L2) is the turning point of spinal mechanics, and that fusion stopping at T11-L2 will concentrate the stress in the cephalad adjacent segments, so proximal fusion should be extended to T10 or above, which will result in better spinal stability, surgical results and longer maintenance of spinal function [24]. However, Shufflebarger concluded that there is no credible data to suggest that proximal fusion to T10 and above improves long-term outcomes [25].Bridwell [26] stated that:Not all scoliosis requires fusion, and the extent of fusion for posterior surgery should be based on a judgment of the major and minor scoliosis.Cobb’s angle, Cobb’s angle, parietal displacement, and parietal rotation measurements are decisive in determining the primary and secondary scoliosis. In addition, fusion should be performed between the neutral and stabilized vertebrae, and the fusion zone must include the degenerated and subluxated vertebrae involved, which is necessary for successful fusion. It is unwise to terminate the fusion in a rotationally subluxated vertebra, or in a sagittally curved parietal vertebra.Cho[22] concluded by comparing long-segment and short-segment fusion for the treatment of degenerative scoliosis that short-segment fusion is sufficient for patients with a small Cobb’s angle and a well-balanced spine; whereas, for patients with large Cobb’ s angle and good spinal balance, short-segment fusion is sufficient; whereas, for patients with large Cobb’s s angle and rotational subluxation, long-segment fusion should be chosen to avoid adjacent segment disease. The choice of distal fusion vertebrae is also highly controversial. If the L5/S1 intervertebral space height is relatively normal and there is no degenerative disc change, and at the same time, the patient maintains a basically normal lumbar anterior convexity angle and overall sagittal balance, distal fusion stopping at L5 can be considered, preserving the kinematic function of the L5/S1 segments [27]. When there is significant degeneration and combined disc calcification in L5/S1, it indicates that this segment is stable and fusion to S1 is not required [28]. Fusion to the sacrum is only required when there is a slipped L5/S1 vertebra or previous laminectomy, L5/S1 spinal stenosis requiring decompression, severe degeneration, or significant tilt of the L5 with respect to the sacrum (>15 degrees) [26].According to Cho, if there is a decrease in the lumbar lordosis or a sagittal imbalance, then the L5/S1 needs to be fused to the S1, even if the lesion is very mild [29 ]. Fusion to the sacrum requires a more involved surgical procedure and has more postoperative complications. For these reasons, some authors propose to avoid fusion to the sacrum whenever possible. However, fusion stopping at L5 is associated with 61% of adjacent segmental disease, or concomitant sagittal plane changes.5 Conclusion Degenerative scoliosis in adults usually presents with low back pain, neurogenic symptoms, and intermittent claudication requiring surgical treatment. However, the age of the patient, medical comorbidities, scoliosis stiffness, degenerative changes and severe osteoporosis make surgical treatment difficult. Decompression and reestablishment of spinal sagittal and coronal balance, especially in the lumbar lordosis, should precede orthopedics per se. Although there is a typology of degenerative scoliosis in adults, there are still no recognized principles to guide the fixation of a fused curved spine for surgical treatment other than decompression and bone grafting.