Acute Spinal Ligament Trauma, Inferring Spinal Stability

ASNR, 2015David B. Hackney

Michael GrofMary Tenenbaum

Ron Alkalay

Conflicts of Interest/Disclosures

• No conflicts

• No financial interests

• No unapproved or off label applications

ThemesGoals of Imaging

Ligament structure and anatomy

Findings in ligamentous injury CT MR

Sensitivity of imaging w/o MR

Sensitivity and incremental value of MR

Potential risks

? Optimal approach-may not be possible

Imaging Goal- Assess Stability

• Stability is determined by the combination of fractures and ligamentous injuries

• Fractures of anterior and middle column structures relatively are straightforward to evaluate

• Signs of ligamentous injury can be ambiguous in all three columns

• Ligamentous integrity is particularly challenging in the Posterior Ligamentous Complex (PLC)

Predicting Mechanical StabilityPlain Radiographs

Plain radiographs- highly sensitive, delayed instability was exceedingly rare before CT

Their reported sensitivity has dropped in the era of cross sectional imaging

Achieving high sensitivity requires exquisite technique, frequently with multiple repeat exposures

This reduces their advantage for radiation dose and can make them slower than CT

CT has replaced plain radiographs

CT is extremely sensitive

Predicting Mechanical StabilityCT

CT is extremely sensitive

Radiation dose is falling

Acquisition time on the order of 10 seconds

Repeat studies are rarely needed, C7-T1 routinely covered well

Can often provide useful inferences about ligamentous injury

CT has replaced plain radiographs

Predicting Mechanical StabilityMR

Much less sensitive than CT for fractures

STIR/ fat suppressed T2 images may suggest fractures not seen on CT

Far better for soft tissue injuries, canal stenosis, cord compression

Only imaging method that can display ligaments

Much slower than CT

Both CT and MR indicated in severe spine injury

Ligament

StructureAnatomy

Behavior under tension

Ligament Structure

Collagen

Primary building unit is the Tropocollagen molecule.

Typically displays a wave-like configuration when not under tension

This wave pattern in structure is know as crimp

Crimp is crucial to the behavior of ligaments under load

Structure-Function relationship: recruitment mechanisms

A ligament will contain fibers with various levels of crimp. Some have tighter, some more relaxed wave patterns.

Increasing levels of tensile deformation result in recruitment of additional load-bearing fibrils.

This can be modeled as springs are stretched to

their limit as the stiffness of the tissue increases.

Each fibril contributes a small part to the overall stiffness.

As a fibril becomes un-crimped, its stiffness increases, increasing the overall ligament stiffness

Mechanical response

A typical stress-strain curve for ligaments and tendons, when neglecting the effect of the tissue viscoelastic behavior, can be observed to exhibit

three distinct regions;

A toe-in or initial

non-linear region.

Followed by a

linear region.

Finally, yield and

failure regions.

Structure-Function relationship: recruitment mechanisms

The behavior of ligaments under tension depends on the loading rate.

When stretched slowly, fibers lengthen by straightening their crimp.

Individual fibrils can reach the limit of elongation and rupture while more tightly crimped fibers remain intact.

This process can be repeated as the ligament gradually fails under RELATIVELY low load.

Structure-Function relationship:

Initial toe-in region

Strain, e, (L/L0)

Toe region

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The initial non-linear region corresponds to "un-crimping" of the crimp in the collagen fibrils. The crimp pattern offers reduced resistance to being

stretched. In effect only the ground substance is offering resistance, this part of the stress strain curve shows a relatively low stiffness.

Reducing crimp

Structure-Function relationship:

linear responseAs the collagen fibrils starts to become “un-crimped”, the collagen fibril themselves are being stretched giving rise to a stiffer material.Hence the near linear response

Linear region

e

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*Straightened fibers

Loss of crimp

Structure-Function relationship:

Initiation of failureAs individual fibrils within the ligament or tendon begin to fail, damage accumulates, the stiffness is reduced and the ligament/tendons begins to fail. The overall behavior of ligaments and tendons

depends on the individual crimp structure and failure of the collagen fibrils.

FailureFiber rupture

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Low vs High Rate LoadsSlow stretch injuries produce ligament

ruptures with the adjacent bone intact.Under high rate loading, the fibrils act

together to resist lengthening. This results in a stronger ligament.

High rate injuries produce avulsions of the bone, with the ligament remaining intact.

Although mechanically different, these injuries are functionally equivalent, since stability requires integrity of both the bone and ligament.

Since they are soft tissue injuries, the low rate ligamentous tears are harder to diagnose without MR.

Age and Ligament Degeneration

o Aging produces reductions in elasticity

o Loss of tensile strength

o Implies higher risk of tears for older, more degenerated ligaments

o It is difficult to apply this insight clinically since we know age, but never know the applied forces

Ligamentous Degenerationo No trauma,

deteriorated ISL

Anatomy Review

Anterior Longitudinal Ligament- ALL

Posterior Longitudinal Ligament- - PLL

Intervertebral Disk Annulus Fibrosus

Facet Joint Capsule

Ligamentum Flavum- LF

Interspinous Ligaments- ISL

Supraspinous Ligament-SSL

Nuchal ligament- SSL in cervical spine. Does not attach to cervical spinous processes above C7

Anatomy review

Ligamentous Injury Without Fracture

Often can be inferred from CT or plain radiographs- increased width of joints

Often associated with acute neurological deficits. Patients will undergo MR anyway

Is there a need to obtain MR with normal CT?

CT in Ligamentous Injury• Interpretation appears easy with only three

findings

• Avulsion

• Wide spaces

• Swelling (usually only visible prevertebral)

• Avulsion • Wide space Prevertebral swelling

How Reliable is CT Alone?

Raza, et al, Injury, 2013

Meta-analysis of reports of CT accuracy for detecting ligamentous injury

If you had a CT, but no MR, how would you perform for detecting ligamentous injury?

No one uses the same methods, so all such summaries are limited by the need to collapse studies with different approaches

Summary Stats Meta Analysis CT Alone

Raza, et al, Injury 2013

Summary Stats Meta Analysis CT Alone

These stats suggest that CT is extremely reliable for detecting ligamentous injury

They would imply little point in bothering with MR

However, looking more closely, look at confidence intervals, rather than point estimates.

The confidence intervals include some alarmingly low values!

Reliability of CT Alone

Raza, et al, Injury 2013

Summary Stats Meta Analysis CT Alone

We might be happy with sensitivity in the mid 90’s and negative predictive value over 99%.

We are definitely not happy if the confidence intervals include sensitivities under 50%

These studies are plagued by small sample size, which leads to wide intervals

In spite of years of interest, the real answer to CT reliability remains unknown

It is clear that MR is more sensitive

Interpreting MR Findings

• More complicated since we can see the ligaments

• Includes the inferences one can derive from CT, but also attempts to interpret imaging of the ligaments themselves

• If the ligament appears normal, it presumed to be normal, we believe MR has high sensitivity for functionally significant tears

• (The sensitivity vs. histology is known to be low)

• If the ligaments appears to show a through and through tear we assume complete loss of integrity. We believe there is a low functional false positive rate

Interpreting MR Findings

• If the ligament shows high signal without a through and through tear, this is considered an equivocal indeterminate interpretation, often summarized as “edema”

• From the review of ligament structure, high signal might indicate partial disruption of collagen fibers, suggesting some degree of weakening

• How weak such a ligament might be is completely unknown

• There may be simple “edema” with no fiber disruption or weakening at all

• The structural significance of damage to an individual ligament depends on the other injuries at that level

MR of through and through ligamentous tearsALL and PLL completely disrupted

IVD injury, Annulus ?PLL, ? LF, ?ISL

Definite middle column injury.Attenuated LF, high signal in ISL.Implies PLC Injury. Unclear whether PLC disruption is complete.Unclear whether complete disruption is required to destabilize the spine.

Isolated prevertebral fluid.No ALL injury detected.Probably stable if an isolated finding.Usually treated with collar and follow up.

Prevertebral fluid, no ligamentous defect

Equivocal Findings

“Edema” pattern, high signal in the ISL indeterminate

(There is an IVD injury with annulus disruption and cord contusion)

• Widened facet joint, increased fluid.

• This is usually all we see in joint capsule tears.

• Rarely resolve capsule itself.

• Wide joints and fluid are common in degenerative disease without trauma

• Increased signal in adjacent muscles. Abnormal, but unknown implications for stability.

• Often seen in the absence of evidence of instability.

Low Significance FindingsReview of MR findings in whiplash patients

and in controls

Many findings seen in trauma were also common in controls

Muscle strain, hemorrhage, fluid and nuchal ligament edema are often seen in normal subjects

Nuchal ligament tears and facet joint fractures and contusions are likely consequences of acute trauma

Odds Ratios Whiplash vs. Control

• Anderson, et al Radiology 2012

PLC Challenges• The PLC is composed of multiple ligaments with

complex anatomy

• The components vary in strength, elasticity and contribution to overall stability

• There are no validated guidelines for which combinations imply unstable spine

• Interobserver reproducibility is poor for ligamentous injury overall, particularly for PLC

• Facet capsules- difficult to image, interobserver terrible

Denis Column Concept

Posterior

Middle

Anterior

Denis Simple, Too Simple

• Simple and easy to apply

• Reproducible

• Familiar

• All injuries to a column treated as functionally equivalent (bone, lig, compression, distraction)

• No detail on nature or orientation of fractures or ligamentous tears

• Grossly oversimplifies the PLC

Subaxial Cervical Spine Injury Classification System

(SLIC)• A carefully developed attempt to classify cervical

spine injuries and guide therapy. Three components

• Morphology, including compression, burst, facet perch or dislocation

• Discoligamentous complex, including interspinous widening, interspace widening, facet perch or dislocation

• Neurologic status

• Should be a better alternative to Denis. A study of reproducibility was disappointing, see next

SLIC Reproducibility

• Van Middendorp, et al The Spine Journal 2013

Inter-rater agreement values of SLIC

MeasureNo. of categories Scale type Kappa

Most affected level 5 Nominal 0.76

Morphology 5 Ordinal 0.29 Disk Lig Complex 3 Ordinal 0.46 Neurology 5 Ordinal 0.7

SLIC Interobserver kappa

Good performance for identifying the most affected level, should be trivial

Unimpressive for determining the presence of disco ligamentous injury

Poor agreement for describing morphology of the injuries

SLIC so far has the best reproducibility of the candidates to replace Denis

Imaging Component of Classification Systems

The limited reproducibility of spine injury classification systems is less of a problem than it may appear.

More of an issue for surgical decision making, treatment planning, outcomes prediction.

Imaging requirements are the same.

All require identification of fractures, avulsions, wide joint capsules, subluxations, and ligament injury

These are only part of the classification schemes

Much of the challenge is translating the score to a prediction of stability

Do We Need MR to Assess Possible Ligamentous Injury?

Many studies of this question

Answers range from a resounding “NO” to resounding “YES”

Examples of results implying MR is not needed and some that indicate it is

Routine MR May be Unnecessary to Clear C Spine

1462 closed head injury and GCS of 14 or less

585 underwent helical CT (many cleared clinically)

100 spine or spinal cord injury

85 fracture or dislocation

15 had spinal cord injury without radiographic abnormality.

No patient had an unstable injury detected on MR that was missed on CT

Many studies with same conclusionsSchuster, et al, Arch Surg, 200520

Routine MR for obtunded trauma patients with normal CT

203 with normal CT, GCS<15, no definite neurologic deficits, but unreliable exam

184 normal MR

18 abnormal MR

2 cleared clinically, 2 operated, 14 discharged in collars

Many studies with same conclusionsMenaker, J Trauma, 2008

MR May be Necessary to Clear C Spine

Why the Discrepancies?

Vague or unstated criteria for “ligamentous injury”

If you cannot agree on presence of tears, unlikely to agree on functional significance of multi-column injuries

Treating this as a binary choice, probably wrong

Failure to distinguish anatomic from functional outcomes

What Question Are We Asking?

• Not “Did CT identify every ligament that appeared abnormal on MR”. Clearly “NO”

• Instead, we ask “Did CT identify abnormalities suggesting spinal instability in every patient in whom MR suggested an unstable ligamentous injury” Often “YES”

• In other words, how reliable are CT-based inferences about spinal stability in the setting of trauma?

Is This a Normal CT? Unrestrained Driver

Motor Vehicle Accident,

Alert at scene

Amnestic to event GCS 14

Multiple severe injuries

Evolution of Prevertebral Swelling

Initial CT.

No swelling.

Does this person have a stable spine?

Does the patient need an MR?

Is This a Normal CT?

? Wide posterior disk space C2-3

? Wide interspinous space

No prevertebral widening

Called back

MR Two Days LaterNow there is

prevertebral fluid.ALL,PLL, LF appear

intact.Posterior annular

tearISL “edema”.Treated with a collar,

no surgery.Was this a CT false

negative?25

How to Characterize the Imaging Results?

• Was this a CT false negative?• Was this a CT true positive, but a radiologic miss?• Was this an MR false positive?• Was this an overall imaging false positive?• What is the gold standard:• Totality of the imaging findings? • The clinical outcome?• Direct inspection of the ligaments at surgery?

(unknown if no surgery)

MVA

Sup facet fx, perched (PLC). Ant and middle column fx. Distracted contralateral facet (PLC)

Joint Capsule Integrity, MR

We know the capsules are destroyed. Minimal increased fluid or surrounding tissue edema. In absence of anatomic findings, these would be false negatives

• PLL appears intact!• So what?• ALL, LF probably

damaged, perhaps disrupted

• ISL “edema”• So what?• MR shows many

things not seen on CT. But CT provides all the clinically relevant information about stability.

• This is a three-column injury by CT alone!

PLC Hierarchy

• Some ligaments more important than others

• ISL/SSL complete tears probably imply loss of PLC integrity, regardless of LF, facets

• Isolated unilateral facet capsule probably stable

• Multiple components with incomplete injury, edema, unknown significance

Risk-Benefit Radiologist's ViewMinimize risk of deterioration due to treatable

but undetected mechanical instability

Seek the highest sensitivity, accept high functional false +

Assume low risk of spine MR

Accept uncertain significance of many ligament findings. Identify them, let others worry about mechanical significance

Equivocal findings often handled with bracing and reevaluation

We assume this management is benign30

Risk of MR and Late Collar Removal

Simulation: literature-reported risk estimates

Severely brain injured patients with no apparent spinal deficits and normal CT

Early removal (w/o MR) risk: quadriplegia

Transport and MRI risks: hypoxia, hypotension, ↑ICP

Late removal risks:↑ICP, delirium

Risk of missed unstable C spine 2.5%

Outcome: Quality-Adjusted Life Months

Dunham, et al, Crit Care, 2008, J Neurotrauma 2011

Early Collar Removal vs. MR and Late Removal-Severe Brain Injury

High risks associated with late collar removal

High risks associated with transport to and performance of MR

Complications associated with late collar removal and MRI predicted to produce worse losses of functional survivorship than quadriplegia

Conclusion: It would be safer to accept the CT result and remove the collar than to wait and do MR

Dunham, et al, Crit Care, 2008, J Neurotrauma, 2011

Practical Implications• CT is highly sensitive

• There is risk of missing ligamentous injury with CT alone

• This risk is real, unknown, but likely very low

• There is a risk of delaying collar removal

• There is a risk of obtaining MR

Ligaments-integral to spinal stability

Deteriorate with age and degeneration

Lose strength and elasticity

Can fail partially or completely

MR can probably distinguish partial from complete tears

Do not know how strong each ligament needs to be

Ligaments nearly invisible on CT

Generally visible on MR

CT- Highly accurate for unstable injury

MR- Probably somewhat more sensitive for ligamentous injury, how much, unclear

Unknown functional significance of “edema” pattern

• Surgeons are aware of false positives. Often manage ambiguous results by immobilizing and reevaluating

• Count on radiology to avoid false negatives

• In spite of simulation results, many consider it far worse to miss an unstable injury than to keep a patient in a collar, EVEN IF THIS IS BETTER CARE. Difficult to justify letting a patient become quadriplegic after imaging is complete

• Implications: Do a lot of imaging

• Much of this imaging will be unnecessary

• We don’t know which imaging studies are unindicated in a given patient

• Generous use of imaging may or may not be best for patients

• Patients should be the only stakeholders, but they are not

• Keep (defense) lawyers happy

• Make hospital administrators unhappy

Optimal Strategy?

Low threshold for obtaining spine CT

Tight interpretation of CT for evidence of ligamentous/facet injury. Wide interspaces, spinous process distance and angle

( false +)

MR in symptomatic, abnl neuro exam, suspected abnl CT

obtunded with suspicious mechanism 35

Goals and Outcome

• Minimize (eliminate?) missed unstable injuries

• Accept this is an expensive goal• Carefully report all evidence of

ligamentous injury on CT and MR• Hope this will identify all unstable

injuries• Know this will identify many equivocal

findings•