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practice, it is challenging to objectively determine compromise of these
biomechanical functions. Because it is known that many radiographic and
MRI abnormalities can be found in asymptomatic individuals, it can be
difficult to determine whether imaging evidence of instability or degeneration
correlates with a patient’s symptoms. These challenges must be addressed
because most of the available surgical treatments for spinal disorders alter the
biomechanics of the spine, either in the process of decompression or in the
process of restoring stability. It is helpful for clinicians to understand the
biomechanics of the spine and how implants can alter motion.
The basic biomechanics of the spine have been thoroughly reviewed in
the previous literature.
1,2
In addition to those foundational texts, surgeons
should keep current on recent findings regarding the spine’s normal loadbearing capacity and arc of motion as well as the loading incurred through
typical biomechanical alterations resulting from degenerative changes and
pathologic processes.
Normal Spine Biomechanics
The spine usually supports loads that would cause a vertebral body to
collapse if it did not have sufficient load-bearing capacity or would cause
excessive displacement between vertebrae if the intervertebral motion
restraints were incompetent. A healthy vertebra can support the load-bearing
requirements needed to accomplish the normal and diverse activities of daily
living (ADLs). A vertebral body will fail when the load-bearing capacity of
the vertebra has been reduced (by age-related or pathologic bone loss) or
when applied loads substantially exceed normal loads (as in trauma).
Estimating the risk of fracture or when a previously injured or treated spine
has regained sufficient load-bearing capacity to resume normal ADLs
requires an understanding of both the spine’s normal load-bearing
requirements and its normal load-bearing capacity. The load-bearing capacity
is partially determined by the material properties of the vertebral bone, but
also by several other factors (Figure 1).
Spinal alignment can affect the loads applied to the spine and frequently
has been described in relation to the overall biomechanics of the spine. Spinal
alignment is a complex topic that has been addressed in the recent scientific
literature.
2
Figure 1
Schematic drawing showing a cylindrical bone sample
subjected to a tensile force (F) and the resulting deflection
(ΔL). Dotted lines = the dimensions of the cylinder before force
application, L = the original length. B, The force deflection plot, in
which the applied tensile force and corresponding deflection are used
to determine the structural properties of the cylindrical bone. C, The
stress-strain plot, in which the tensile force is normalized over the
cross-sectional area of the bone sample and the deflection is
normalized over the original length to determine the material
properties of the bone. (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.)
Load-Bearing Requirements
Multiple techniques have been used to estimate the load-bearing requirements
of the spine. These techniques range from simple free-body diagrams to
sophisticated computer models. These techniques for estimating load-bearing
requirements make use of data from load-measuring devices mounted to the
spine, intervertebral disk pressure measurements, measurements of muscle
activity, and measurements of the loads that an individual can apply with his
or her body.
Measurements of the pressures within intervertebral disks are helpful in
appreciating the relative load-bearing requirements of various ADLs because
the forces that result in the measured pressures can be estimated. When a
healthy individual with no degenerative changes or injuries to the spine is
standing, the pressure within the L4-L5 disk is approximately 0.5 MPa.3 That
is nearly the pressure exerted on an object submerged to a depth of
approximately 40 m. The compressive force required to create 0.5 MPa
intradiscal pressure is approximately 530 N. Sitting can increase or decrease
spinal loads by up to 40% relative to those of standing, depending on whether
the individual is supported or unsupported, the amount of thoracic kyphosis,
and the position of the arms. Lying down can reduce pressure on the spine by
20% to 33% compared with standing. While supine, slight flexing of the legs
can further reduce intradiscal pressure. Spikes and variations in pressure
occur with changes in position. Intradiscal pressure measurements also help
explain the gradual swelling of the disk that can occur after 7 hours of sleep,
because pressures at the end of a sleep period are twice as high as at the start
of a sleep period.
Activities that can substantially increase disk pressure include forward
bending (pressure up to 3.6 times greater than that of standing, depending on
how much the individual flexes forward), and lifting (as much as seven times
greater pressure than standing, with stooped lifting creating pressure 35%
greater than that of squat lifting). In a 2014 study, vertebral body
replacements used in the management of L1-L3 trauma were modified to
enable in vivo measurements of spinal loading.4 Ten activities that resulted in
particularly high spinal loads were identified (Figure 2). Maximum loads
were found with lifting a weight from the ground and elevating a weight with
arms extended straight out from the body. Simple activities such as tying a
shoe can result in high spinal loads. These data suggest that, in the setting of
lumbar corpectomy with vertebral body replacement, the effect of bracing is
unpredictable. Although only the portion of the load passing between the
vertebral bodies was measured (some load was supported by the posterior
elements), the load data are helpful in understanding differences in loading
between activities in the setting of anterior column reconstruction.
Figure 2
Bar chart shows spinal loading measured using in vivo
instrumented vertebral body replacements in five patients
(P1 to P5). The 10 activities with the highest measured loads are
shown along with the variability in loading between patients and
activities. These data can be used to educate patients on activities to
avoid when greater spinal protection is needed (such as after spine
surgery). (Adapted with permission from Rohlmann A, Pohl D, Bender
A, et al: Activities of everyday life with high spinal loads. PLoS ONE
2014;9[5]:e98510.)
Good data are available to support recommendations to patients for
activities that can place the greatest mechanical stress on the lumbar spine.
3,4
In vivo measurements of lumbar spinal loads also may help in the decision to
recommend an orthosis, because the data show that, although loads may be
reduced in some patients, an orthosis can increase spinal loads in other
patients.5 Measurements of disk pressure in the cervical spine have been
restricted to cadaver specimens and are not as helpful for understanding
clinical load-bearing requirements.
Load-Bearing Capacity
The load-bearing capacity of vertebrae has been documented using ex vivo
Figure 3
tests of isolated vertebrae subjected to controlled loading conditions.
Whereas long bones have a safety factor because load-bearing capacity can
substantially exceed load-bearing requirements,6 documented failure loads
for an intact vertebra have been in the range of the load-bearing requirements
previously described.
7,8
This may partly be the result of the advanced age of
the cadaver spines tested. A study on vertebral fracture risk reported that the
estimated mean compression strength of vertebrae in postmenopausal women
with no compressive fracture is 5,746 ± 1,873 N.9 The factor of risk for a
vertebral fracture has been defined as the load-bearing requirement divided
by the load-bearing capacity.10 The factor of risk for young spines varied
between 0.07 for upright standing and 0.61 for lifting a 10-kg weight. For
older spines, the factor of risk varied from 0.12 to 0.99, suggesting that in
certain individuals with compromised vertebral bone quality, even loads
encountered with normal ADLs could cause insufficiency failure.
Three-dimensional CT reconstructions of the spine of a
patient with lumbar stenosis before fusion surgery. The left
half of the spine has been electronically removed to allow easier
interpretation of the images. A, Image shows all tissue with a density
of 180 Hounsfield units (HU) or higher. B, Image shows all tissue with
a density of 480 HU and higher. C, Image obtained at a 780 HU
threshold shows that the densest bone is found in the region of the
pedicles, pars interarticularis, and lamina, and that posterior elements
play an important role in spinal load bearing. If insertion of screws into
the strongest bone is required, this imaging technique is helpful in
locating the densest bone.
It is important to appreciate that repetitive loading can increase the risk of
fracture.11 Because structural redundancy exists in normal vertebrae, failure
can occur in a proportion of trabeculae within the vertebra without overall
failure.12 In osteoporotic vertebrae, however, structural redundancy is
diminished because of the internal pattern of bone loss.12 This is important
because, unlike a long-bone fracture that can effectively prevent weight
bearing, a compression fracture in the spine may result in some collapse of
vertebral body height but does not prevent further weight bearing.
The posterior elements are important to the load-bearing capacity of the
spine. The densest bone typically forms in regions where the highest loads
are supported. For example, the densest bone is in the calcar region of the
proximal femur, which is the location of convergence of trabeculae patterns
that begin in the femoral head. The apparent brightness of bone on CT
imaging is proportional to the density of the bone. Most software for viewing
CT images allows tissue with low-signal intensity to be eliminated by
adjusting how tissues of various densities appear on the display (Figure 3).
Identifying the bone with the highest density is helpful so that screws can be
engaged in the densest bone when using the cortical bone trajectory technique
for lumbar fusion
13,14
Intervertebral Motion
In addition to maintaining the required load-bearing capacity of the vertebrae,
the spine also must simultaneously facilitate a wide range of intervertebral
motions while protecting the neurovascular elements passing through and
near the spine. These difficult requirements are achieved in part by a complex
system of soft-tissue structures, including the intervertebral disks and
intervertebral ligaments, which control relative motion between vertebrae.
The other essential elements in achieving diverse and controlled motions are
the muscles and a nervous system that can sense the position of vertebrae and
control the activity of the muscles. The intervertebral disk, intervertebral
ligaments, and facet joints must work together to maintain intervertebral
motion within normal limits.
The biomechanics of the intervertebral disk have been extensively
studied, although much of this knowledge has yet to be effectively
assimilated into validated diagnostic and treatment algorithms for routine
clinical use. Resources are available that describe the fundamental structure
of the normal intervertebral disk, including the anulus fibrosus and nucleus
pulposus; the deformation of these structures during ADLs; and the role of
soft-tissue structures in the diagnosis and treatment of spinal disorders.
15
In clinical practice, overall motion through the spine can be measured
using goniometers and electronic devices (including smart phones). Injuries
or pathologic or degenerative changes to the disks or intervertebral ligaments
may best be appreciated through precise measurements of the relative motion
between vertebrae. In the sagittal plane, relative motion between vertebrae is
commonly assessed from flexion-extension radiographs. The potential
advantages of using upright flexion-extension MRI to assess abnormal
intervertebral motion has been described but has not yet become a routine
clinical tool.16 Regardless of the imaging modality used, if the normal limits
of translation are known, abnormal motion can be detected as translation
outside of the normal limits.
Intervertebral motion in the sagittal plane has been extensively studied,
although it is important to appreciate that the intervertebral motion that
occurs in the coronal and axial planes during twisting or bending to the side
may be important. Intervertebral motion also is coupled because, as the spine
bends laterally, the vertebrae also will flex or extend and twist in the axial
plane.17 Coupled motion is less pronounced in sagittal plane flexionextension because only small amounts of twisting or lateral bending occur
between vertebrae with sagittal plane flexion-extension; therefore,
intervertebral motion can be largely assessed in a single two-dimensional
plane. One reason that sagittal plane flexion-extension radiography has been
the primary clinical imaging modality for assessing instability is that only
two-dimensional imaging is required. Axial rotation or lateral bending are
best studied using three-dimensional imaging of the spine in multiple
positions,18 or using geometric detail from three-dimensional imaging
combined with kinematic information from two-dimensional imaging;
19
however, these methods are not clinically practical in most situations. Even
with two-dimensional imaging, variability in image acquisition protocols can
confound diagnostic utility.
Substantial variability exists in the peer-reviewed literature for what is
considered normal intervertebral motion.20 Intervertebral motion classified as
normal based on data from one study may be considered abnormal motion
using similar data from a different study. This variability may be attributed in
part to differences in the populations studied, but may largely result from
variability in the instructions given to tested individuals as they are asked to
perform spinal flexion and extension, or may result from the specific
methodology used to produce the motion measurements.
The effort that patients exert when asked to perform flexion and extension
spinal movements also can result in large differences in apparent
intervertebral motion.21 A patient must apply enough stress to the spine
during flexion and extension testing to allow a diagnosis of possible
abnormal motion. An analogous situation involves the parameters that must
be met when the anterior drawer test is used in the diagnosis of a torn anterior
cruciate ligament (ACL). When the anterior drawer test is performed, it is
appreciated that substantial force must be applied to the lower leg to assure
that the examiner can detect that the ACL is being tensioned (if it is intact)
and thereby confirm that the ACL can restrict translation of the tibia with
respect to the femur. Similarly, if the goal is to determine if the disk and
intervertebral ligaments can restrict intervertebral motion to within normal
limits, then adequate force must be applied to the spine (by the patient during
flexion and extension) to apply stress to the intervertebral motion restraints
(if intact) and thereby assure a reliable diagnosis of any incompetent
intervertebral motion restraints.22 It is important for the clinician to assure
that flexion-extension tests apply adequate stress to the spine to detect any
incompetency of the intervertebral motion restraints. If insufficient stress is
applied to the spine, the patient’s radiation exposure during flexion-extension
radiography is unjustified because spinal instability may not be detected.
Criteria to determine whether flexion-extension spinal radiography is
properly performed have been primarily based on studies of cadaver spines
and the definitions of neutral and lax zones.
The concept of a neutral zone has been used to describe the small range
of motion where vertebrae can move relatively freely with application of only
small loads23 (Figure 4). Within the neutral zone, the intervertebral motion
restraints are not stressed to the point at which they substantially restrict
intervertebral motion. Analogous to the previously mentioned anterior drawer
test for an ACL injury in which a small amount of translation between the
tibia and femur can occur before the ACL is tensioned, the healthy spine
allows a small amount of intervertebral movement before the ligaments and
Figure 4
disks begin to restrict motion (Figure 5). At the borders of the neutral zone in
the spine, the disks, ligaments, and facet joints will begin to become
mechanically stressed such that increasingly larger forces are required to
achieve more intervertebral motion. The border of the neutral zone has been
referred to as the lax zone.24 Past the lax zone, there is a zone where the
relationship between applied loads and displacements is approximately linear.
A spine flexion-extension test should ideally load the spine sufficiently such
that intervertebral motion is past the neutral and lax zones and into the linear
elastic zone of the load versus deformation curve (Figure 4). The size of the
combined neutral and lax zones can be measured in the laboratory by
preparing the spine so that controlled loads can be applied while measuring
resultant displacements.
24
A typical load-displacement plot of a functional spinal unit
when loaded along one of its six degrees of freedom. The
neutral zone (NZ) represents the amount of displacement occurring
near the neutral position of the spine, the elastic zone (EZ) represents
the amount of displacement occurring in the physiologically loaded
region, and the range of motion (ROM) represents the sum of the NZ
and the EZ, as measured in millimeters (for translation) or degrees (for
rotation). (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.)
Although application of controlled loads to the spine is not practical in
patients (so the size of the neutral zone cannot be directly measured and used
diagnostically), laboratory measurements of the neutral zone can help
determine how much intervertebral motion is needed to assure that the spine
is stressed sufficiently to detect any incompetent intervertebral motion
restraints. In the lumbar spine, laboratory data support that at least 5° of
sagittal plane intervertebral rotation is required for the motion of the
intervertebral level to be outside of the neutral and lax zones and into the
linear elastic zone of the load-deformation curve24 (Figure 4). In an
asymptomatic population of volunteers who performed flexion and extension
movements from a seated position, less than 5% of lumbar levels had less
than 5° of intervertebral motion at any level of the spine, and the average
intervertebral rotation was more than 11° at every level.20 In the cervical
spine, at least 4° of intervertebral rotation may be required to assure
intervertebral motion occurs outside the neutral and lax zones.
The relative role of each individual intervertebral ligament and each
component of an intervertebral disk in controlling intervertebral motion is
complex. In the normal spine, the intervertebral disks, the intervertebral
ligaments (anterior and posterior longitudinal ligaments, ligamentum flavum,
interspinous ligaments, and supraspinous ligaments), and the facet joints
(including the facet capsules) collectively contribute to the control of
intervertebral motion.25 Because this motion control system is complex,
computer models are currently the most promising tool for understanding the
role of each component.26 These computer models may help in determining
optimal diagnostic and treatment methods based on specific patterns of
traumatic and degenerative compromises to the intervertebral motion
restraints.
It is difficult to detect damage to intervertebral motion restraints by
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