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Repairing and Restoring Spine Biomechanics
All spine fusion, disk arthroplasty, and spine stabilization devices and related
surgery are intended, at least in part, to influence spine biomechanics.
Fusion
Fusion is sometimes used to provide stability to a spinal level deemed
unstable (unable to maintain normal alignment and intervertebral motion
during ADLs). Fusion also may be used to provide stability to a level deemed
likely to become unstable after decompression surgery. The goal of fusion is
to create a mechanical environment conducive to the formation of bone that
bridges between vertebrae. Because bridging bone only forms where motion
is below a threshold, the primary technical goal is to reduce motion to below
that maximum threshold. Many approaches to creating a fusion environment
have been studied, from uninstrumented fusions to fusions with
instrumentation in multiple regions, including anterior, lateral, posterior, and
posterolateral regions. Pedicle screws are the preferred fixation method to
enhance lumbar fusion and have been extensively studied in most clinically
relevant scenarios.
Although the various approaches to achieving fusion can be modeled and
compared using cadaver studies or computer models,79 surgeons typically
rely on experience and their perception of the need for using instrumentation
to improve the stability of the treated level(s) in determining how much
fixation is adequate for an individual patient. Although posterior lumbar
systems can appear mechanically robust, these systems do not eliminate all
motion between vertebrae. Most studies of fixation hardware in cadaver
spines document some residual motion between vertebrae after
instrumentation.79 Because mineralized bone can form only where there is
minimal motion, the most likely locations for initial bone formation are areas
with the lowest motion. After bone forms in a location, intervertebral motion
is further reduced, thus providing a mechanical environment conducive to the
formation of bridging bone at other locations. It is important that bridging
bone forms a new path for transmission of forces between vertebrae before
the hardware fails from the repetitive stress of providing the load
transmission pathway.
Knowing the location of the densest bone in the spine may be beneficial
when using instrumentation such as pedicle screws. Cortical screws recently
have been clinically used for spinal fixation. These screws are placed in a
pathway intended to mechanically engage with dense bone at multiple points
along the inner pedicle and posterolateral aspects of the vertebral body
13
(Figure 8). Use of a cortical trajectory requires substantially higher torque for
insertion.80 Higher insertional torque is generally associated with a greater
ability to transfer loads from the bone of one vertebra through the hardware
and onto the bone of the adjacent vertebra. However, the shorter screw
trajectory and the decreased length of cortical screws compared with pedicle
screws require careful planning of the entire screw pathway to optimize
biomechanical purchase and approximate or exceed the fixation strength of
traditional trajectories that use larger screws.
81
Optimal cortical screw size has been determined by measuring pullout
strength. The most predictive factor of fixation strength is the percentage of
screw length in the vertebral body. Higher percentages of screw length in the
vertebral body correlate with stronger lumbar fixation. Screw diameter
strongly correlates with pullout strength. It was determined that the ideal
cortical screw has a diameter of 5.5 mm and a length of 35 mm. A major
potential limitation is that most lumbar levels may not safely contain this size
screw without surgical complication or an increased risk of pedicle or pars
interarticularis fracture.
Because cortical screws do not generally traverse anterior and posterior to
the center of rotation of a spinal segment, they have a mechanical
disadvantage compared with longer screws that pass the center of rotation.
For this reason, the use of cortical screws with interbody support is
suggested. A 2013 study suggested that cortical screws without interbody
support are less stiff than traditional pedicle screws only in axial rotation.
79
However, the biomechanical differences between cortical screws and
traditional pedicle screws have yet to be fully evaluated, especially regarding
clinical effect. One study reported higher failure rates of cortical screws in
osteoporotic bone when tested under cyclical loading.
82
Figure 8
Illustrations compare the cortical screw trajectory (green)
with the trajectory of traditional lumbar pedicle screws
(white) for fixation of fusion hardware to the spine. Sagittal (A), axial
(B), and coronal (C) views are shown.
Disk Arthroplasty
Preserving normal motion between vertebrae as opposed to stopping motion
(as with fusion) is intuitively appealing. Substantial research has been
undertaken on various motion-preserving strategies in the spine. The FDA
approved several cervical and lumbar disk arthroplasty devices that are
generally capable of providing motion between vertebrae, although the
motion may not always fully reproduce the quantity and quality of normal
motion. The quantity of motion is typically assessed in clinical trials of disk
arthroplasty based on the magnitude of intervertebral rotation and translation.
As previously discussed, these parameters can be difficult to interpret without
understanding the protocol used to encourage patient effort in flexing and
extending the spine. Analyzing motion at each individual level as a
proportion of total motion (eg, rotation at C5-C6 as a percentage of C2 to C7
rotation) has been proposed as a better approach. It was found that disk
arthroplasty tends to preserve the proportion of motion at each level, whereas
fusion tends to distribute motion lost at the treated level to the other
levels.
83,84
The long-term consequence of this finding is not yet fully
understood.
The effect of disk arthroplasty on both the quantity and quality of
intervertebral motion has been addressed using both data from clinical trials
and computer models. Computer models are advantageous because important
variables such as the effect of individual parameters can be precisely
controlled. A limitation of computer models is the difficulty of representing
the wide range of vertebral morphologies, soft-tissue properties, and patient
activity levels that can occur. Despite the available knowledge base, the
effect of most motion-preserving devices on the quality of spine motion and
the effect of the quality of motion on long-term clinical outcomes remains
poorly understood.
Using a retrospective analysis of data from an FDA-regulated study of a
lumbar disk arthroplasty device, several variables significantly associated
with clinical outcomes were identified.33 Variables that proved important to
good outcomes included disk height of less than 8 mm and a low level of
lordosis at the treated level. A retrospective analysis of a cervical disk
arthroplasty device reported that a preoperative short disk height or
implanting a disk such that lordosis was increased by greater than 3°
increased the risk of a poor outcome.85 Finite element models showed that
multilevel lumbar disk arthroplasties can substantially increase the motion
occurring at the treated levels, and placing lumbar disks too far anterior or
posterior can lead to abnormal motions and elevated forces across the facet
joints.86 Specific biomechanical consequences of disk arthroplasty can affect
outcomes. Large studies that systematically assess the effects of
biomechanical factors on outcomes are needed to create validated treatment
guidelines.
Summary
Normal spine biomechanics enable a wide range of repetitive motions
without structural failure while simultaneously protecting the neurovascular
elements. Understanding the load-bearing requirements, load-bearing
capacity, and normal motion of the spine can help in counselling patients
about spinal disorders. A large body of peer-reviewed literature provides
evidence for the many and varied injuries and degenerative changes that can
occur in the spine. Nevertheless, validated diagnostic tests are not currently
available that can identify specific causes for spine-related symptoms in
many patients. It is likely that objective validated diagnostic tests could lead
to substantial improvements in clinical outcomes for many patients with
disorders of the spine. A large body of literature describing the
biomechanical effects of many of the available surgical treatment modalities
also is available; this information can aid surgeons in choosing the best
treatment options for their patients. When available, the widespread use of
validated spine biomechanical metrics in research studies will enable
predictive analytics that can account for both clinical and biomechanical
factors that predict the most effective treatment modalities.
Key Study Points
Clinicians must understand the load-bearing requirements, the loadbearing capacity, and normal intervertebral motion to assess whether the
biomechanics of the spine have been compromised.
Assessment of intervertebral motion can be helpful in diagnosing
incompetent intervertebral motion restraints, but only if the diagnostic test
applied sufficient mechanic stress to the spine to allow a reliable
diagnosis. The stability of the spine can be used to select the best treatment
option.
Clinical outcomes for multiple surgical treatment options depend in part
on the interaction of the treatment with the morphology and mechanical
properties of the spine.
Annotated References
1. White AA III, Panjabi MM: Clinical Biomechanics of the Spine, ed 2. Philadelphia, PA,
JB Lippincott, 1990.
2. Oxland TR: Fundamental biomechanics of the spine: What we have learned in the past
25 years and future directions. J Biomech 2016;49(6):817-832.
The understanding of spine biomechanics has advanced substantially since the
publication of some of the original pioneering research. This review summarizes
advancements in knowledge as well as gaps in knowledge that need to be addressed to
better understand strategies for optimizing clinical outcomes.
3. Dreischarf M, Shirazi-Adl A, Arjmand N, Rohlmann A, Schmidt H: Estimation of loads
on human lumbar spine: A review of in vivo and computational model studies. J
Biomech 2016;49(6):833-845.
This review paper provides a resource for information about the loads that must be
supported by the spine during ADLs. This information may be helpful when counseling
patients on recommended activity restrictions.
4. Rohlmann A, Pohl D, Bender A, et al: Activities of everyday life with high spinal loads.
PLoS One 2014;9(5):e98510.
This large database of loads measured by instrumented vertebral body replacements can
serve as a reference when helping patients understand how to avoid activities with high
spinal loads.
5. Rohlmann A, Zander T, Graichen F, Bergmann G: Effect of an orthosis on the loads
acting on a vertebral body replacement. Clin Biomech (Bristol, Avon) 2013;28(5):490-
494.
Based on loads measured using an instrumented lumbar vertebral body replacement in
patients, an orthosis was found to reduce loads in some patients and for some activities,
whereas loads for other patients and other activities were increased. These results
suggest caution when recommending use of an orthosis to restrict loading of the spine.
6. Biewener AA: Safety factors in bone strength. Calcif Tissue Int 1993;53(1
suppl 1):S68-S74.
7. Windhagen HJ, Hipp JA, Silva MJ, Lipson SJ, Hayes WC: Predicting failure of thoracic
vertebrae with simulated and actual metastatic defects. Clin Orthop Relat Res
1997;344:313-319.
8. Zhao F-D, Pollintine P, Hole BD, Adams MA, Dolan P: Vertebral fractures usually
affect the cranial endplate because it is thinner and supported by less-dense trabecular
bone. Bone 2009;44(2):372-379.
9. Melton LJ III, Riggs BL, Keaveny TM, et al: Structural determinants of vertebral
fracture risk. J Bone Miner Res 2007;22(12):1885-1892.
factor of risk for vertebral fracture: A population-based study using QCT. J Bone Miner
Res 2006;21(9):1475-1482.
musculoskeletal disorder risk: A systematic literature review. Hum Factors
2013;55(1):108-124.
This literature review aids in the appreciation of the magnitude of applied loads and the
number of loading repetition interactions in determining the risk of musculoskeletal
disorders.
J Bone Miner Res 2012;27(10):2152-2158.
Vertebrae can continue to provide load-bearing support after some trabeculae have
failed because of structural redundancy. The combination of bone reduction within
vertebrae and the loss of structural redundancy results in a much more serious vertebral
fracture than can be explained by low bone volume fraction alone.
screw techniques. Oper Tech Orthop 2015;25(3):187-193.
Cortical screws used in fusion hardware fixation take advantage of the dense cortical
bone in the region of the pars interarticularis. The surgical exposure can be less
extensive than with traditional pedicle screws. Knowledge of the biomechanical and
clinical support for fixation using cortical screws is presented.
pedicle screws. Spine J 2009;9(5):366-373.
Medical Publishers, 2009.
lumbar segmental motion at levels adjacent to disc herniation. Eur Spine J
2016;25(1):222-229.
No effect of disk herniation on intervertebral motion at the adjacent levels was observed
in a clinical study in which intervertebral motion was measured using magnetic
resonance images obtained during the performance of lumbar flexion and extension.
validation of spinal analytical models. J Biomech 2016;49(5):780-785.
The authors of this study provide comprehensive reference data for intervertebral
motion in the lumbar spine in a form usable for computer model validation.
In vivo three-dimensional analysis. Spine (Phila Pa 1976) 2006;31(2):155-160.
kinematics in the healthy young adult cervical spine during dynamic functional loading.
J Biomech 2015;48(7):1286-1293.
A sophisticated approach was used to measure three-dimensional intervertebral motion
in young volunteers with no history of spine problems.
during seated flexion-extension radiographs of 658 asymptomatic nondegenerated
levels. J Neurosurg Spine 2015;23(6):731-738.
Reference data describing intervertebral motion in the lumbar spine of asymptomatic
volunteers are presented. Data from prior publications are tabulated and support the
need for standardization of lumbar flexion-extension studies.
relation to lumbosacral stability: Investigation of moderate and maximum motion. Spine
(Phila Pa 1976) 2000;25(6):732-737.
Radiol 1944;25:593-609.
intervertebral disc mechanics: A new definition of the neutral zone. BMC Musculoskelet
Disord 2011;12(1):38.
A mathematical definition is proposed for the neutral zone, which is a fundamental
aspect of spine movement. Justification for the use of a flexion-extension study in the
reliable diagnosis of instability is presented.
Measurement techniques and parameter comparisons. J Spinal Disord 1998;11(5):416-
429.
Cervical spine soft tissue responses and biomechanical modeling. Clin Biomech
(Bristol, Avon) 2001;16(1):1-27.
ligamentous lumbosacral spine in flexed and extended postures: Finite element study. J
Biomech 2016;49(6):974-982.
Information is provided to aid in understanding the interaction of intervertebral disks,
vertebral morphology, and intervertebral ligaments in the roles of simultaneously
providing load-bearing capacity and stability.
spine in vivo. Spine (Phila Pa 1976) 1990;15(12):1300-1306.
segmental motion in the lower cervical spine in women with chronic whiplashassociated disorders, grades I-II: A case-control study using a new measurement
protocol. Spine (Phila Pa 1976) 2003;28(19):2215-2221.
Bertagnoli R, McAfee PC, An HS, eds: Motion Preservation Surgery of the Spine. New
York, NY, Elsevier Health, 2008, pp 85-96.
mechanotransduction in normal, injury and degenerative conditions. J Biomech Eng
2011;133(7):071010.
Despite substantial research, a better understanding of the biomechanics of facet joints
and their role in patient symptoms is needed. Existing knowledge of facet joint
biomechanics and pathomechanics is presented.
orientation and the facet tropism in the lumbar spine. Spine J 2013;13(10):1301-1308.
Using computer models, the authors documented little effect of lumbar facet orientation
on intervertebral disk stresses resulting from bending moments; however, shear forces
resulted in higher disk pressure with more sagittally oriented facet joints or facet
tropism.
influence outcomes in lumbar total disc arthroplasty? Part II. Clinical and radiographic
results as evaluated utilizing the Vertebral Endplate Yue-Bertagnoli (VEYBR)
Classification. SAS J 2008;2(2):101-106.
arthroplasty. Eur Spine J 2014;23(10):2127-2135.
Several variables can be measured from preoperative and postoperative radiographs and
MRI studies that can explain variability in clinical outcomes. This variability may help
in the formulation of strategies for optimizing clinical outcomes after lumbar disk
arthroplasty.
dissociation and death as a result of blunt trauma. Spine J 2010;10(12):1128-1132.
assessment of the upper cervical anatomy: What is normal? Spine J 2010;10(3):219-
229.
injuries of the occipitocervical complex. Spine (Phila Pa 1976) 2011;36(9):709-714.
Careful thin-slice CT examination measurements of the anatomic spacing between the
occiput and the first and second cervical vertebrae are required to make a reliable
diagnosis of injuries in this region. Reference data required for interpretation of
measurements are presented.
Common in blunt trauma fatalities and better detected with objective computed
tomography-based measurements. Spine J 2010;10(8):704-707.
dislocation in a series of 17 consecutive survivors during an 8-year period. J Neurosurg
Spine 2006;4(6):429-440.
Traumatic atlantooccipital dislocation: Comprehensive assessment of mortality,
neurologic improvement, and patient-reported outcomes at a Level 1 trauma center over
15 years. Spine J 2015;15(11):2385-2395.
Failure to make a diagnosis of atlantooccipital dissociation is a predictor of mortality in
patients with blunt trauma. Additional support for the need for careful CT-based
measurements to reliably diagnose occipitocervical injuries is presented.
neck pain in the general population: Results of the Bone and Joint Decade 2000-2010
Task Force on Neck Pain and Its Associated Disorders. J Manipulative Physiol Ther
2009;32(2 suppl):S46-S60.
role of tissue damage in whiplash-associated disorders: Discussion paper 1. Spine
(Phila Pa 1976) 2011;36(25suppl):S309-S315.
This review presents the most current knowledge regarding the biomechanics of
whiplash injury.
accidents: A systematic review. Spine (Phila Pa 1976) 2002;27(17):1934-1941,
discussion 1940.
metric identifies disc abnormalities in whiplash patients. Spine J 2016;16(10):S302.
Using carefully assessed high-resolution MRI examinations as the preferred method, a
simple metric that can be objectively measured from cervical flexion-extension
radiographs was found to be substantially elevated in the presence of MRI
abnormalities at the posterior anulus fibrosus and/or posterior longitudinal ligament.
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