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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_6032_Библиотеки_им_академика_М_И_Перельмана
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measuring the magnitude of rotation or translation between vertebrae. One
explanation may be that the multiple intervertebral motion restraints provide
redundancy in motion control; if one restraint is damaged, the others can
maintain enough control that motion remains within limits. The quality of
motion can be assessed based on the center of rotation and the translation per
degree of rotation (TPDR). Objective measurement of TPDR may prove
particularly valuable because the TPDR reduces dependency on patient
effort.
20,27,28
The center of rotation is the point at which one vertebra appears
to rotate with respect to an adjacent vertebra when comparing two or more
positions of the spine. The center of rotation is dependent on the level
analyzed, and it falls within a relatively narrow range in radiographically
normal spines.
29
Vertebral Morphology
Vertebrae have many morphologic features that help define how the spine
moves and the loads that can be supported. Morphology can vary
considerably between individuals. Characteristics such as the anteriorposterior width of the vertebra relative to the vertebral body height, the shape
of the end plates, the size and orientation of the facet joints, the shape and
spacing of the spinous processes, and variations in the uncovertebral joints of
the cervical spine have been studied, and some evidence exists to support
their clinical relevance. More research is needed to expand the number of
validated clinical guidelines concerning the use of vertebral morphology in
the diagnosis and treatment of spinal disorders.
Figure 5
A, Sagittal-view illustration of a functional spinal unit
demonstrates the resistance of posterior elements (facet
joints, facet capsular ligaments, and interspinous and supraspinous
ligaments [black arrows]) to displacement in flexion and anterior
translation (gray arrows). B, Transverse-view illustration demonstrates
the resistance of the facet joint and its capsular ligaments (black
arrows) to axial rotation (gray arrow). (Reproduced from Wang M, Rao
RD: The biomechanics of the spinal column, in Rao RD, Smuck M,
eds: Orthopaedic Knowledge Update Spine 4. Rosemont, IL,
American Academy of Orthopaedic Surgeons, 2012, pp 19-31.)
Normal facet joints help guide motion between vertebrae and transmit
loads between vertebrae, particularly in extension. The joint surfaces are
covered with smooth cartilage and include a meniscus that helps provide
smooth and cushioned motion between vertebrae, particularly during
extension of the spine. A joint capsule surrounds each joint and helps control
motion. The synovial lining inside the capsule keeps the joint lubricated.30 A
study of the effect of facet joint orientation on disk pressures found that,
when facet joint orientation was closer to the sagittal plane, higher disk
pressure resulted when the motion segment was exposed to a shear load.
31
Intuitively, facet joints that are more coronally oriented would resist anteriorposterior shear forces better than sagittally oriented facet joints (Figure 6).
Transferring stress to the facet joints might protect the disk to an extent, but
at the possible expense of greater propensity toward facet joint degeneration.
Vertebral end plate morphology also varies among individuals and may
be an important factor in selecting the optimal treatment modality. A grading
Figure 6
system for lumbar end plate morphology has been described, along with its
potential significance with respect to outcomes after lumbar disk
arthroplasty.32 A 2014 study reported better clinical outcomes at 2 years after
disk arthroplasty in patients with flat or convex end plates versus hooked or
concave end plates.33 Although the findings may be specific to the design of
the disk arthroplasty device used in the study, the need to consider the
interaction between an implant and the shape of the end plate when planning
surgery is indicated.
T1-weighted magnetic resonance images of sagittally (A)
and coronally (B) oriented L4-L5 facet joints.
Pathomechanics
Trauma
Understanding and caring for patients with traumatic injury to the spine has
been a primary motivation for much of the literature on spine biomechanics
over the past seven to eight decades. Autopsy studies have shown that a wide
range of injuries to spinal structures can occur from trauma. Soft-tissue
injuries to the cervical spine are one of the most common patterns found in
patients with severe blunt trauma.34 Although clinically important, few
diagnostic tests have been validated as sensitive and specific for the
biomechanical consequences of soft-tissue injuries. Careful measurements
obtained from thin-slice CT scans may allow a reliable diagnosis of some
injuries by the documentation of abnormal separation between vertebrae or
between the occiput and the upper cervical spine.
35,36
The frequency of use of
these measurements in actual clinical practice is unknown.
37
Presumably, abnormal separation would be associated with potentially
harmful instability, although instability can be difficult to diagnose directly.
The effect of soft-tissue injuries on the stability of the injured level(s) and on
clinical outcomes is likely to depend on the severity of the injuries and the
efficacy of attempts to stabilize the spine. Appreciating that patients can
survive even severe dissociative injuries, it is important to understand bestavailable diagnostic approaches and stabilization practices.
38,39
Whiplash
A subset of spine trauma is frequently grouped as whiplash injury. Whiplash
injury is common, and the associated healthcare costs are high.40 It is known
from autopsy and laboratory studies that soft-tissue injuries can result from
the accelerations and decelerations that can occur during rear-end vehicle
collisions.41 Injuries to the soft-tissue motion restraints may lead to abnormal
intervertebral motion, and abnormal intervertebral motion can contribute to
symptoms. Although damage to specific anatomic structures has not been
reliably detected with currently available imaging studies in the setting of
whiplash,
41,42
understanding the mechanisms of injury and the types of
injuries that can occur may be helpful in making a diagnosis.
The mechanism of a common whiplash injury has been described.
41
Initially, a posterior translation of the head and neck is followed by extension
of the cervical spine. Rapid forward acceleration of the head then brings the
cervical spine into a flexed position. The actual movements of the head and
neck relative to the body that occur during a motor vehicle collision or other
blunt trauma can be highly variable, so it may be misleading in clinical
practice to assume that a specific pattern of traumatic motion has occurred.
Consistent with highly variable vehicle crash dynamics, a wide range of
injuries has been documented, including damage to the facet joints, joint
capsules, longitudinal ligaments, the intervertebral disk anulus fibrosus, and
possibly, the muscle.41 The relationship between the presence of the initial
injuries and the subsequent development of clinical symptoms, particularly
chronic symptoms, is poorly understood. Diagnostic tests that reliably detect
specific soft-tissue injuries have yet to be fully validated.
A 2016 study identified radiographic disk metrics associated with
whiplash-related symptoms.43 The change in posterior disk height between
flexion and extension divided by the amount of intervertebral rotation
(reported as ± mean SD compared with radiographically normal symptomatic
volunteers) was abnormally elevated in 19% of the patients who reported
whiplash-related symptoms. This metric was found to be substantially higher
when MRI revealed injury to the posterior intervertebral disk and/or posterior
longitudinal ligament. Although this metric is not currently widely used in
routine clinical practice and requires highly accurate measurements to detect
the differences between normal and abnormal motion, it offers promise for
the detection of the biomechanical consequences of soft-tissue injuries after
blunt trauma. The posterior anulus fibrosus in the cervical spine can be thin
compared with the anterior anulus fibrosus. This factor may substantially
influence the propensity for damage to this region from a motor vehicle
collision, and it is an important consideration when assessing magnetic
resonance images for evidence of injuries. The best treatment approach for
specific soft-tissue injuries remains to be determined.
Degeneration and Age-Related Changes
Degenerative changes in the spine are common.44 It is well documented that
degenerative changes can occur in asymptomatic individuals.45 The
development of symptomatic degenerative changes in the spine is determined
by genetic and environmental factors, although the role of genetics is poorly
understood.
46,47
Biomechanical changes associated with minor trauma from
falls, overexertion during sporting and home activities, and repetitive jobrelated spinal loading likely contribute to degenerative changes. The
degenerative changes resulting from the biomechanical effects of the loads
supported by the spine can then alter intervertebral motion. Abnormal motion
resulting from degenerative changes may contribute to the development of
symptoms.
Limited understanding exists regarding how the myriad of observed
degenerative changes affect the biomechanics of the spine and contribute to
symptom development. Instability of the spine is the most commonly
referenced potential biomechanical consequence of degenerative changes. A
checklist for identifying clinical spinal instability has been described and
provides a general framework for conceptually understanding the clinical use
of the term instability, although it has a poorly defined role in routine patient
care.
1,48,49
A well-validated objective diagnostic test for spinal instability is
lacking.
The lack of objective metrics for defining spinal instability may help
explain why a literature review of the effect of degeneration on spinal
stability found only a trend toward increasing stiffness with degeneration.
44
A diagnostic test for instability can be used with confidence when
documentation exists that the test detects true biomechanical abnormality and
associations between clinical symptoms and abnormal test results and is
useful in selecting the optimum treatment for the patient.
An association between spondylolisthesis and spinal instability is
frequently inferred, although not all spondylolisthesis is unstable.50 There are
multiple confounding issues in determining the relationship of spinal
instability and symptomatic spondylolisthesis.51 During the natural history of
spondylolisthesis, it is generally assumed that an unstable phase (defined as
having abnormal intervertebral motion) will occur, but motion may return to
normal or subnormal levels as the associated degeneration progresses.
Several phenomena associated with degeneration may help in the
stabilization of a spinal level. Severe disk height loss, osteophyte formation,
vertebral end plate sclerosis, and ligament ossification may offer evidence of
restabilization.51 However, instability cannot be definitively ruled out based
only on radiographic signs of restabilization. Objective diagnostic metrics for
instability may help resolve this dilemma. Instability has been defined as the
presence of more than 3 mm of sagittal plane translation between flexion and
extension.
50,52,53
Although this is a promising definition of instability, it is
confounded by sensitivity to variable patient effort, variable radiographic
magnification, and measurement error. A study of asymptomatic volunteers,
which used a device to correct for differences in radiographic magnification,
reported that the upper limit of normal translation was greater than 3 mm at
most levels of the lumbar spine;20 therefore, more than 16% of spinal levels
in the asymptomatic volunteers would be classified as unstable. In clinical
practice, a scaling device is not used to correct for radiographic magnification
error, so the size of a vertebra measured from a radiograph can be 9% to 63%
larger than the actual size of the vertebra.54 In addition, if a patient exerts
little effort when asked to flex and extend his or her spine during
radiography, the results may appear to indicate low translation when
abnormally high translation is actually present (eg, when the patient bends to
tie his or her shoe). For these reasons, the greater–than–3-mm criterion
should be used with caution.
Another approach used to differentiate between static and dynamic
spondylolisthesis is to compare the relative positions between vertebrae when
the patient is supine (assessed with MRI or CT) with those of the patient
when bearing weight (assessed using radiography).
55,56
A 2014 study
reported that almost 40% of patients with spondylolisthesis as observed in
upright standing radiographic examinations had reduction of the
spondylolisthesis in supine imaging studies.
50
Intervertebral Disk
Because intervertebral disks play a key role in controlling intervertebral
motion, degeneration of a disk can alter intervertebral motion. Evidence
indicates an increasing loss of intervertebral motion control during the early
phases of disk degeneration; however, motion may return to normal levels or
may remain at below-normal levels with severe degenerative changes. These
changes were documented in a cadaver model;57 however, in clinical
practice, the changes in motion that occur with degeneration are subtle.
44
Using weight-bearing MRI, a study of 162 patients found an initial increase
in translation in the early stages of intervertebral disk degeneration, followed
by a decrease in translation with severe degeneration.58 Using a cadaver
model, no association was found between disk degeneration and the
magnitude of sagittal plane translation when spines were subjected to shear
loading;59 however, it is unclear whether this was related to the stabilizing
effect of the compressive load applied to the spine.
Compressive loading has been shown to have a stabilizing effect on the
spine, which can be important when assessing instability.60 If the vertebral
end plates are nearly horizontal when imaging is obtained, gravitational
forces may tend to force interlocking and stabilization of the vertebrae. If the
end plates are substantially tilted with respect to a horizontal orientation, a
large component of the gravitational forces will be exerted parallel to the disk
Figure 7
space and can provoke translation between vertebrae that would increase the
likelihood for detecting abnormal translation at an unstable level. A study of
a large number of lumbar spines in patients with a wide range of ages showed
that disk degeneration tended to occur after facet degeneration, and
intervertebral translation initially increased with progressing degeneration
and then decreased in those with advanced degeneration.61 An association
between facet and disk degeneration can be expected because incompetent
motion control in one region will increase the burden on other elements of the
intervertebral motion control system. Each pair of adjacent vertebrae and the
intervertebral disk and ligaments forming the connection between the
vertebrae form a functional spinal unit (FSU). Each FSU can be mechanically
compromised in isolation, and this can in turn affect adjacent FSUs.
T2-weighted magnetic resonance images show fluid in a
facet joint. If possible, it is helpful to confirm that the fluid is
visible in both the sagittal (A) and axial (B) images.
Facet Joints
The degenerative changes that can occur in the facet joints can be
subjectively assessed using several grading systems.62 It is unclear, however,
how degenerative changes in facet joints affect spinal biomechanics and the
relative contribution of degenerative changes in causing symptoms.
Phenomenologically, it has been shown that patients with lumbar
spondylolisthesis tend to have more sagittally oriented facet joints.
63
Spondylolisthesis is more likely in patients with L4-L5 facet joints oriented
greater than 58° from the coronal plane.64 This may be a consideration when
assessing the potential value of a treatment such as uninstrumented
decompression that requires inherent spinal stability. Recently, it has been
reported that the orientation of the lumbar facet joints may depend, in part, on
ethnicity.65 This finding has not yet resulted in validated clinical guidelines
for using facet joint orientation as a diagnostic tool or in treatment planning.
The presence of fluid in the facet joints observed with MRI is among the
most accepted indicators of lumbar instability (Figure 7). Multiple studies
have reported an association between the presence of fluid in the facet joints
and dynamic spondylolisthesis.
55,66,67
In addition, it appears that the likelihood
of dynamic instability correlates with the amount of fluid; however, it should
be noted that gas can sometimes be seen on CT images of the facet joints.
Gas in the facet joints also is an accepted indicator of instability.68 Gas
appears black on a magnetic resonance image. The MRI fluid sign may be
more sensitive (true in disease) than specific (false in health). Determining
the actual sensitivity and specificity of any test for spinal instability is
challenging because a true validated preferred test is required but has not yet
been established. When assessing facet joints for a fluid sign, it is important
to appreciate that lordosis is reduced in a supine patient, and the cranial-most
aspect of the facet joints can open while the caudal-most aspect is closed. The
accumulation of joint fluid may be seen in the resulting gap at the cranialmost aspect of the joint even at a stable level, so it is important to appreciate
the amount of possible fluid at a stable level.
It has been shown that the amount of sagittal plane intervertebral
translation that occurs (normalized to the amount of intervertebral rotation to
control for variability in patient effort) is substantially higher when the facet
fluid sign is present.20 Because the TPDR is normally small,20 TPDR may be
a more reliable metric for determining instability, although only if sufficient
patient effort has been exerted in flexion and extension to provoke an
abnormal TPDR if present.
In the cervical spine, the morphology of the facet joints such as
asymmetric hypertrophy is associated with pathomechanical changes such as
degenerative spondylolisthesis.69 Asymmetric hypertrophy is defined as a
facet joint on one side having a much larger joint surface area than the facet
joint on the opposite side. Spondylolisthesis in the cervical spine is more
commonly observed in upright standing radiographs than in supine magnetic
resonance images.69 A large difference in the orientations of the left and right
facet joints is associated with the presence of spondylolisthesis in the cervical
spine.70 Spondylolisthesis was found in 20% of symptomatic patients in one
study.
71
Vertebral Morphology
A study evaluating the relationship between MRI-based measurements of
morphologic parameters for the lumbar spine reported little association with
symptoms.72 The thickness and cross-sectional area of the ligamentum
flavum were among the few parameters that had significant association with
clinical symptoms, but even those parameters explained less than 25% of the
variability in Oswestry Disability Index scores. Imaging-based morphologic
measurements have not been found to correlate with functional status.
73
Iatrogenic Instability
Multiple studies have documented that resection of tissue to achieve a
decompression objective can result in loss of stability or spondylolisthesis.
74-
77
Increasing instability has been reported with increasingly greater resection
of ligaments and resection of the lamina and facets.78 Postoperative
instability may be seen in 5.5% of patients following laminectomy or
minimally invasive decompression without fusion.75 In 40 patients with grade
1 spondylolisthesis who were treated with decompression using laminectomy
only (no fusion), postoperative instability was more likely in patients with the
following preoperative findings: intervertebral translation at the level of the
spondylolisthesis greater than 1.25 mm, disk height greater than 6.5 mm, and
facet joint angle greater than 50°.74 Determining the true incidence of
iatrogenic instability after uninstrumented decompression requires a validated
test for instability, an evaluation of preoperative stability, and details of the
decompressive procedure.
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