Degenerative scoliosis is a new scoliosis that occurs after skeletal maturation due to degenerative changes in the spine, mostly in middle-aged and elderly people, mainly in the thoracolumbar or lumbar segments, and some patients often need to opt for surgery due to persistent pain (low back pain or radicular pain), neurogenic intermittent claudication or aggravation of the deformity affecting cardiopulmonary function. The main purpose of surgery is to relieve pain, prevent progression of scoliosis, re-establish spinal balance, restore normal function and improve quality of life.
There has been much debate on how to choose a fused spine in surgical treatment. The author provides a review of issues related to the selection of distal fusion vertebrae for the surgical treatment of degenerative scoliosis.
1. Clinical features of degenerative scoliosis
Degenerative scoliosis often occurs on the basis of intervertebral disc degeneration, with a high incidence of spinal stenosis. The parietal vertebrae are most often located in the L3/4 or L2/3 interval, followed by the L1/2 interval, often with rotational subluxation of the L3 and L4 vertebrae and tilt of the L4 and L5 vertebrae.
The pain that occurs in degenerative scoliosis may be caused by degenerative lumbar disc disease and small joint hyperplasia that cause symptoms of spinal stenosis, or it may be caused by a deformity that imbalances the coronal and sagittal planes of the spine resulting in biomechanical changes that fatigue the muscles. Compared to other degenerative lumbar spine diseases, degenerative scoliosis is quite different: while degenerative diseases (lumbar spinal stenosis, lumbar disc herniation, lumbar spondylolisthesis) usually occur best in the L4/5 or L5/S1 segment and often require only intervention in the diseased segment during surgery, degenerative scoliosis, in addition to spinal stenosis, disc herniation, and displacement of the vertebral body (sagittal or coronal), also has three-dimensional rotation of the vertebral body.
Studies have reported that the size of the Cobb angle in the coronal plane of degenerative scoliosis does not significantly correlate with surgical outcome, whereas the restoration of lumbar anterior convexity, correction of lumbar body (L3) tilt and slip in the coronal plane, and reconstruction of force lines in the sagittal plane of the spinal pelvis are the key factors affecting prognosis. Based on the characteristics of degenerative scoliosis, the surgery must not only resolve the spinal stenosis, but also consider the balance of the lumbar spine and even the entire coronal and sagittal planes of the spine, so the fused segments are often longer.
2.Surgical treatment of degenerative scoliosis
There are three main indications for surgical intervention for degenerative scoliosis.
(1) Severe low back pain affecting normal life and ineffective conservative treatment;
(2) Thoracic bending Cobb angle ≥ 50°-60°, lumbar bending Cobb angle ≥ 30°; lumbar coronal and/or sagittal plane deformity progressing more than 5° per year;
(3) Nerve root compression presenting with radicular pain, intermittent claudication of neurogenic origin, and symptoms of cauda equina injury. The goal of surgery is to improve the patient’s function by achieving two basic goals, namely, complete decompression of the damaged nerve and reconstruction of coronal and sagittal balance. Preoperative evaluation and careful consideration of the patient’s potential complications and surgical outcomes are essential.
Birknes et al. concluded that the preoperative period requires a comprehensive consideration of the following aspects.
(1) The deformity itself. For example, the size of the Cobb angle for coronal scoliosis, loss of sagittal convexity, parietal offset and rotation; the overall sagittal balance of the spine, the relationship between the pelvic projection angle (PI) and lumbar lordosis (LL), pelvic tilt (PT), sacral tilt (SS), and reconstruction of the spinal-pelvic balance force line.
(2) Degeneration-related. Schwab et al. concluded that PI can be used to assess the optimal degree of lumbar pronation: LL=PI±9°; pelvic tilt is a compensatory mechanism to maintain normal standing posture, with normal PT<25°; and sagittal balance of spinal deformity is compensated by pelvic translation. The balance of the sagittal plane of spinal deformity is compensated by the translation of the pelvis.
Li et al. found that PI was significantly higher in adults with scoliosis than in the normal population. In addition to thoracic kyphosis and pelvic projection angle, age is also an important factor affecting the sagittal parameters of the spinal pelvis. Silva and Lenke classified degenerative scoliosis into 6 grades, i.e., grades I-VI, based on neurological symptoms, degree of back pain, anterior-posterior and lateral slippage, size of the Cobb angle in the coronal plane, bone formation, anterior lumbar convexity, and overall spinal balance. grade I, simple lumbar decompression; grade II, lumbar decompression + posterior limited fixation; grade III, lumbar decompression + lumbar fusion fixation; grade IV, lumbar decompression + anterior-posterior combined spinal fusion fixation; grade V, extension to
Grade V, extension to thoracic segment fixation and fusion; Grade VI, lumbar decompression + osteotomy orthopedic surgery. Lumbar decompression alone is mainly indicated for.
(1) intermittent claudication due to central canal stenosis with limited stenosis.
(2) Small joint subluxation of no more than 2 mm, with satisfactory sagittal and coronal balance.
(3) No or only mild back pain.
(4) Scoliosis of less than 30° without combined thoracic segmental kyphosis Frazier et al. used simple posterior decompression in 15 patients with degenerative scoliosis with spinal stenosis, and all patients had significant improvement in VAS pain scores after surgery. However, decompression alone may lead to progression of the deformity or worsening of symptoms.
Lumbar decompression + limited posterior fixation is mainly indicated for.
(1) with intermittent claudication or radiating pain in the lower extremities;
(2) Scoliosis less than 30°, subluxation of the tuberosity more than 2 mm, and no bone formation in front of the decompression area;
(3) No/slight back pain, no combined thoracic segment kyphosis. However, the disadvantage of this approach is the high incidence of pseudarthrosis. Grubb et al. reported a postoperative pseudarthrosis rate of approximately 40% in the treatment of degenerative scoliosis using posterior decompression followed by simple posterior fusion.
Lumbar decompression and fusion fixation of the lumbar curvature are primarily indicated for primary low back pain caused by spinal deformities, and therefore patient selection is critical. Patients usually have lumbar scoliosis greater than 45°, small joint subluxation greater than 2 mm, and lack of bone formation in front of the lumbar decompression zone. Lumbar decompression and anterior-posterior fusion fixation: anterior fixation helps to re-establish lumbar lordosis and sagittal balance; in addition, foraminal opening provides indirect decompression; and reduces the incidence of pseudarthrosis.
Lumbar decompression and osteotomy orthopaedic surgery is indicated for patients with degenerative scoliosis combined with thoracic kyphosis or spinal stenosis, especially in the presence of trunk imbalance or coronal imbalance, generally with scoliosis greater than 45° and small joint subluxation greater than 2 mm.
(1) Bending like superior scoliosis flexibility <30% (stiff deformity);
(2) Intermittent claudication or lower extremity radicular pain;
(3) Scoliosis greater than 30°, small joint subluxation more than 2 mm, and no bone formation in front of the decompression area;
(4) Posterior convexity deformity of the lumbar segment.
3.Selecting the range of fixed fusion
The overall principle is to minimize the number of fused segments, not only to preserve the mobility of the lumbar spine, but also to prevent further degeneration of adjacent segments, and Tsai et al. concluded that fusion should not stop at the following segments: vertebrae with incomplete posterior column structure, parietal vertebrae with scoliosis/posterior convexity, and vertebrae with slipped displacement/rotational subluxation. Vertebrae with junctional kyphosis, coronal or sagittal.
Because the parietal, slipped displaced, or rotated displaced vertebra is often the most unstable vertebra in the entire region of scoliosis, stopping at that vertebra can result in excessive stress on the pedicle screw there, which can easily loosen, break, and pull out, leading to failure of internal fixation, especially in patients with severe osteoporosis; in addition, fixation to an unstable vertebra cannot rebuild the stability of the lumbar spine and the entire spine, which can easily lead to loss of compensation of scoliosis and The rapid progression of scoliosis can occur. At the same time, the fusion should not stop at an adjacent segment with an obvious degenerative disc, otherwise postoperative degeneration will be accelerated and another revision surgery will have to be performed.
4. Selection of distal fusion vertebrae
The selection of the distal fusion spine for adult degenerative scoliosis has been controversial, with the current focus being on whether and under what circumstances the L5/S1 segment needs to be preserved for motion.
4.1 Distal fusion vertebrae to L5
In general, fusion should be performed between the neutral and stable vertebrae, and the fusion zone must contain the degenerated and subluxated associated vertebrae as a necessary condition for successful fusion.
4.1.1 Indications for distal fusion to L5 According to Polly et al, if the L5/S1 intervertebral space height is relatively normal and the disc is not degenerated, and if the patient maintains essentially normal lumbar anterior convexity and overall sagittal balance, distal fusion can be considered to stop at L5, preserving the motion of the L5/S1 segment. Bridwell et al. suggested that if the L5/S1 disc is mildly degenerated and located below the iliac interspinous line, the bilateral iliac bones provide some stabilization and protection to the L5/S1 motion and further degeneration of the L5/S1 disc is relatively protected, the distal fusion could stop at L5, but there is no strong evidence to support this view;
Kuklo et al. suggest that if there is significant degeneration of L5/S1 combined with disc calcification, then this segment is likely to be stable and can be fused without fusion to L5/S1. to S1.
4.1.2 Advantages of distal fusion of the spine to L5 The advantages of preserving the L5/S1 segment are.
(i) preserves lumbosacral motion, reduces S1 stresses and sacroiliac joint stresses, and reduces the rate of internal fixation failure;
②Reduced operative time and reduced operative risk;
③Reducing the number of fused segments and reducing the complications related to internal fixation;
④Reducing the incidence of pseudarthrosis;
⑤ No fusion of L5/S1, avoiding 360° fusion and pelvic fixation;
(6) Reduce the impact on hip function and gait, especially when osteoarthritis is present in the hip joint.
4.1.3 Disadvantages of distal fusion to L5 Edwards et al [19] showed that the most common recent complication of distal fusion ending at L5 is failure of internal fixation of the L5 vertebral body due to improper fixation, poor bone quality, and multisegmental joint fusion; compared to other vertebral bodies, the L5 pedicle is shorter, more lax, and angled toward the middle; a short segmental fusion with two pedicle screws In addition, the strength of the screws is related to the angle, and the deeper L5 vertebrae are more stable and less conducive to stress.
In addition, the strength of the screws is related to the angle, and the deeper L5 vertebrae have good stability, which is not conducive to stress dispersion, and the stress is easily concentrated in the L5 pedicle screws; all these factors tend to lead to the weakening of the extraction resistance of the L5 pedicle screws. When L5 internal fixation fails, loss of lumbar anterior convexity, mild kyphosis or lateral kyphosis deformity often occurs at L4/5; especially long-segment fusion often leads to sagittal imbalance; to reduce L5 internal fixation failure, fusion of the anterior column between L4/5 vertebrae can be performed via posterior interbody fusion (PLIF) or transforaminal approach interbody fusion (TLIF) to protect the L5 pedicle screw screws [11, 19-22].
The most common distal complication of distal fusion stopping at L5 is the higher postoperative secondary degeneration of the L5/S1 disc, which is also controversial, especially when the fused segment is long; degeneration can occur even in patients with intact L5 lamina and posterior arch structures and may be associated with a reduction in lumbar lordosis (LL) and a significant sagittal imbalance, with a degeneration rate of 38% to 61%. Because of the stiffness of the lumbosacral region, L5/S1 is often unable to compensate on its own once a long segmental orthopedic fusion is performed above this segment, and the L5/S1 disc and small joints will be subjected to greater stress, leading to accelerated degeneration and corresponding clinical symptoms.
The findings suggest that fusion ends at L5 and that even a normal L5/S1 disc can lead to overload, which can induce secondary degeneration, resulting in kyphosis, loss of anterior column height, disc herniation, spinal stenosis, degenerative slippage, and loss of overall sagittal balance at that segment. Edwards et al. found that a small percentage of patients requiring fusion extension to S1 secondary to L5/S1 had less satisfactory clinical outcomes after reoperation; moreover, fusion to L5 had significantly poorer postoperative sagittal orthopedic results and maintenance time compared with fusion to the sacrum, but fusion to L5 had a significantly lower complication rate than fusion to S1. However, the complication rate of fusion to L5 was significantly lower than that of fusion to S1, and there was no significant difference in the Scoliosis Research Society (SRS)-24 functional scores between the two.
4.2 Distal fusion to S1
The preservation of L5/S1 is dependent on the quality of the L5/S1 disc, and degenerative scoliosis is most often seen in middle-aged and older adults over 40 or 50 years of age, where the L5/S1 disc is usually degenerated. However, if L5/S1 is only mildly degenerated, it is controversial whether and when to fuse S1. Currently, the degree of disc degeneration is assessed clinically by radiographs, MRI and discography, but the consistency of assessment varies widely and further research is needed to determine which method is more useful for treatment decisions.
4.2.1 Indications for distal fusion of the spine to S1 include.
① pelvic tilt due to unbalanced lumbosacral bending;
(ii) Presence of disc degeneration and spinal stenosis or spondylolisthesis at the L5 to S1 segment;
(iii) history of L5/S1 laminectomy with incomplete posterior structures.
4.2.2 Advantages of distal fusion to S1 Kuklo et al. noted that fusion to the sacrum has the advantage of protecting the L5 pedicle screw, allowing decompression of the L5 nerve root, preventing spinal slippage, and eliminating secondary degeneration of L5/S1 Cho et al. suggested that fusion to S1 should be performed in patients with sagittal imbalance or reduced lumbar anterior convexity, even if there is only mild degeneration of the L5/S1 disc.
4.2.3 Disadvantages of distal fusion of the spine to S1 Aebi [5] found by a matched cohort
The analysis found a higher rate of complications (including pseudarthrosis, sacroiliac arthritis and sacral fractures) with long segmental fusion distal to the sacrum; increased surgical exposure, prolonged duration and increased complications associated with distal fusion to the sacrum compared to fusion ending at L5; possible sacroiliac and hip degeneration and gait changes, especially if the patient has pre-existing hip osteoarthritis; higher risk of sacral screw dislodgement Emami et al. found a positive correlation between the occurrence of pseudarthrosis and internal fixation failure, which was closely related to the fixation method. They found a 36% rate of prosthesis in patients using the Luque Galveston technique compared to 14% with sacral and iliac screws.
In a study by Weistroffer et al. of 50 cases of long-segment fixation to the sacrum, they found a 24% rate of pseudoarthrosis. In contrast, Bridwell et al [24] recommended that distal fusion in adults with degenerative scoliosis should be extended to S1, despite the increased incidence of various complications, and noted the need for fixation of the ilium and anterior column fusion.Tsuchiya et al noted that although screw placement through the sacroiliac joint may lead to secondary degeneration of the sacroiliac joint, it is not sufficient to cause serious clinical problems because the screws do not damage the cartilage of the sacroiliac joint, and the sacroiliac joint is a very wide joint and placement of only 2 screws would not really fix the joint leading to the development of sacroiliac arthritis.
There is a lack of credible comparative studies on the idea that fusion to the sacrum may alter the patient’s gait. In terms of the reasons for the high incidence of L5/S1 pseudarthrosis, some current studies suggest that biomechanically, L5/S1 is in the stiff pelvic and fusion segment between the 2 main lever arms, which are the translational site of lumbar spine motion and stress concentration. At the same time, the sacrum is not an independent unit, it is closely connected to the pelvis, and simply performing two-point fixation of S1 is inadequate and unsound for the sacro-pelvic complex.
In order to reduce the incidence of pseudoarthrosis, in addition to the application of double cortical sacral screws and strict treatment of the bone graft bed, it is currently advocated to apply cage support and bone morphogenetic protein (BMP) for 360° fusion between the vertebral bodies of the L5/S1 anterior column, and to add bilateral iliac fixation or S2 sacral screw fixation, which can not only improve the lumbosacral fusion rate, but also increase biological stability, improve lumbar pronation, and increase In addition to improving the rate of lumbosacral fusion, it can also increase the biological stability, improve the anterior lumbar convexity, increase the height of the intervertebral space and intervertebral foramen, and reduce the foraminal stenosis. Because of the weak subcutaneous tissue in the iliac region, the iliac screw is more prominent, especially in thin patients, and can sometimes cause strong discomfort.
These patients may be considered for removal of the iliac screws after obtaining a strong fusion 2 years after surgery. Sagittal plane imbalance and increased pelvic PI are high risk factors for postoperative imbalance; sagittal plane imbalance leads to distal complications, including pseudoarthrosis and failure of internal fixation at the lumbosacral junction.
5.Summary
In conclusion, the final fixation fusion range needs to be determined after the range of neural decompression and the range of deformity correction are determined. In most cases, the decompression range is within the range of deformity correction, and the decision to expand the fixation range is based on the degeneration of the disc in the cephalocaudal junction area, the stability of the intervertebral space, and the presence or absence of compression fractures in the junctional vertebrae. In patients with lumbar curvature, distal fusion fixation ending at L5 requires less operative time than extension to the sacrum and/or pelvis, but leaves a potentially painful segment and may lead to sagittal imbalance later in life.
It is generally accepted that distal fusion should end at S1 when there is severe degeneration of the L5/S1 disc, L5/S1 slippage, a history of L5/S1 laminectomy, L5 is within the range of scoliosis, and L5/S1 canal decompression is required. The preservation of L5/S1 will continue to be the focus of debate and research for a long time.