>10° of angular motion between adjacent endplates on lateral flexion / extension radiographs when compared with adjacent proximal & distal levels
On AP view, spinous process & facet malalignment may suggest segmental instability
Indications
Degenerative spondylolisthesis with back pain may represent true segmental instability
Curve progression or lateral listhesis in degenerative lumbar scoliosis may imply relative instability, which may worsen after a posterior decompression
Excessive segmental or junctional kyphosis may be an indicator of segmental instability at that motion segment
Intra-operative structural alterations that may lead to instability include the following:
Excessive removal of facet joints for adequate decompression
>50% resection of each facet joint at same level leads to unacceptable segmental instability
Therefore, when a facetectomy of 50% or more is performed, posterolateral arthrodesis should be strongly considered
Disc excision
Most disc herniations that occur in this group represent extrusions or free fragments of disk at level of foramen
Simple removal of these disk fragments at time of decompressive laminectomy is sufficient
Radical disc excision involves removal of as much of the disc material & endplates as possible
This destabilises anterior column after posterior column has been compromised by decompressive laminectomy & may lead to iatrogenic spondylolisthesis
Therefore, if a radical discectomy is considered necessary, a concomitant posterolateral arthrodesis is often considered
However, isolated discectomy without iatrogenic destabilisation of posterior column does not necessarily mandate fusion
A 2nd or 3rd disc herniation at same motion segment may be considered by some as evidence of instability at that particular interspace, even without radiologic confirmation
Role of lumbar fusion in various degenerative conditions:
Recurrent disc herniation
Occurrence of ³2 episodes of disc herniation at same segment is a relative indication for arthrodesis/fusion
Spinal stenosis
Preoperative instability may be judged on basis of presence of any of following factors, as determined radiologically
Degenerative spondylolisthesis or lateral listhesis
Flexible or progressive degenerative scoliosis or kyphosis
Recurrent spinal stenosis at same segment
Instability after decompression may be considered a potential risk in presence of
Excessive removal of facet joints
Radical disc excision
Removal of pars interarticularis
Pars fracture
Degenerative spondylolisthesis
Posterolateral arthrodesis is indicated for decompressive lumbar surgery in patients who have stenosis as well as preexisting degenerative spondylolisthesis or isthmic spondylolisthesis
Degenerative scoliosis
Relative indications include curve progression & sagittal &/or coronal imbalance with unremitting back pain
Curve flexibility
If >50% curve correction (as measured on supine forced side-bending films) has been achieved, a decompressive laminectomy alone may increase risk of curve progression
Curve progression
Radiculopathy
If patient has scoliosis with predominant radiculopathy within concavity of curve, a decompressive laminectomy with partial facetectomy may not be sufficient to decompress nerve root in concavity
This is because nerve root may be compressed between adjacent pedicles
Use of instrumentation with distraction of adjacent pedicles on concavity & neutralisation or compression along convexity may be necessary to reduce pedicular kinking & unload compression on nerve root
Loss of lumbar lordosis
On standing lateral radiograph, plumb line drawn inferiorly from odontoid should normally pass through posterior 50% of L5 vertebral body
In “flat-back” deformity, plumb line will lie anterior to L5,
Flat-back deformity or relative lumbar kyphosis by itself may cause increasing back pain & can lead to impaired ability to stand upright
Therefore, improving sagittal alignment through segmental instrumentation & fusion +/- osteotomy should be considered at time of decompressive surgery
Fixed lateral listhesis
If motion of hypermobile segment is demonstrated on side-bending films, then instability is suggested
Because decompression of this segment may result in further decompensation of curve & increased lateral listhesis, a concomitant arthrodesis should be considered, which also lowers pseudarthrosis rate
Extent of intra-operative decompression
Magnitude of curve by itself is not an indication for arthrodesis
If none of previous 6 (high-lighted) factors is present, decompression alone is sufficient
When fusion is indicated, it is not necessary to fuse entire length of degenerative curve
Curve will often ascend into lower portion of thoracic spine & down to sacrum
Such long fusions are unnecessary in elderly patients & may, in fact, contribute to significant morbidity
Fusion should end at a disc space that appears to be horizontal with neutral rotation
Fusion should restore sagittal alignment & include decompressed spinal segments.
Degenerative disk disease
Indications for arthrodesis include
Unremitting pain & disability for >1 year
Failure of a trial of aggressive physical conditioning & non-operative treatment lasting >4 months
MRI findings consistent with advanced disc degeneration, preferably at a single level
Normal findings from a psychiatric evaluation
Success rate highly variable (50 to 80%)
Techniques
Posterolateral arthrodesis:
Fusion mass sufficiently close to center of vertebral motion to prevent movement that may stimulate a pain response
Resected spinous processes applied to laminae as local autogenous graft – can be supplemented with allograft and/or synthetic graft
Now used rarely as an isolated procedure because it associated with high rate of pseudarthrosis
Pedicle-screw instrumentation:
Adding posterior instrumentation to posterolateral arthrodesis increases fusion rate (85%)
Better clinical outcomes with regard to pain, function, & neurological recovery than without instrumentation (80%)
Lumbar interbody arthrodesis:
Those who advocate this technique generally consider disc to be primary source of pain
Excision of disc & interbody arthrodesis is thought to remove source of pain & to prevent motion
Bone graft placed closer to centre of vertebral motion, theoretically achieving greater stiffness when fusion has occurred
In addition, intervertebral height may be restored, & a smaller volume of bone graft may be used compared with that required for posterior techniques
Posterior lumbar interbody arthrodesis
85 to 90% good to excellent clinical results
90 to 95% fusion rate
Anterior lumbar interbody arthrodesis
Advantages include direct removal of involved disc & avoidance of iatrogenic trauma associated with posterior paraspinal muscle dissection & partial denervation
Compared with posterior lumbar interbody arthrodesis, allows more complete excision of disc (which is believed to be primary cause of pain)
Concerns related to this technique include injury to great vessels & risk of injury to presacral plexus, potentially resulting in retrograde ejaculation & sterility
70% good to excellent clinical results
50% multiple level & 70% single level fusion rate
Interbody fusion cages:
Metal & carbon-fibre implants filled with nonstructural cancellous bone have been found to provide immediate structural support & a biological substrate to promote fusion
Inserted by anterior or posterior approach
Potential risks during insertion of posterior (PLIF) cages with damage to dura, cord or nerve roots
Difficulties associated with removal if revision surgery required for failed fusion
85% good to excellent clinical results
95% fusion rate
Circumferential arthrodesis:
Treatment of trauma, deformity, failure of a previous operation on lumbar spine, & use as primary procedure for disabling low-back pain
Theoretical advantages of this procedure include elimination of all potential sources of pain in anterior & posterior structures as well as maximization of stability with a resulting increase in rate of fusion
Posterior approach alone or combined anterior & posterior approach
Intervertebral disc prosthesis:
Controversial & investigational
Involves removal of disc to relieve pain, followed by implantation of a prosthesis to simulate stability, mobility, & weight-bearing properties of disc
Complications include migration or dislocation of implant & fracture of metal
88% of prostheses subside to some extent into vertebral end plates
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