Powers Ratio

Powers Ratio

Since 1979, Powers Ratio measurement has been utilized by radiologists to assess for atlanto-occipital dissociation (skull to first vertebrae instability or dislocation); and described more specifically, horizontal, anterior atlanto-occipital instability.  It is a severe craniocervical instability where the skull separates rom the upper neck. 

To understand what an abnormal power ratio is, it is essential first to know the anatomy of the atlanto-occipital joint, which is related to it.

What is the atlanto-occipital joint? 

It is formed between the first vertebrae of the neck (the atlas) and the base of the skull (the occiput). The following ligaments stabilize this joint:

      1. Cruciform ligament
      2. Alar ligament
      3. Anterior atlanto-occipital membrane
      4. Posterior atlanto-occipital membrane

How is Powers Ratio measured?

Powers Ratio has been used in the evaluation of anterior atlanto-occipital dislocation. It is defined as the ratio of the distance between basion and posterior spinolaminar line of the atlas to the distance between opisthion and anterior arch of the atlas. The formula can be represented as below:

                 Power ratio = AB / CD

Normal Reference Value:  On X-ray, the typical ratio is less than 1. However, on a CT scan, it is less than 0.9.

Abnormal Reference Value:  Anything that comes out greater than one (1) is considered abnormal.

What symptoms can be experienced? 

Atlantocccipital joint disruption, which affects the power ratio, can present with a wide range of symptoms, which can be due to either traction / compression of that part of the spinal cord which passes through the joint, brain stroke due to artery damage, or cranial nerve damage. The latter is responsible for the sensation and movement of all parts of our head, like the eyeball, mouth, tongue, etc.

Compression of cord:  Loss of sensation or paralysis on one or both sides and incontinence due to loss of the anal or bladder sphincter tone.

Cranial nerve damage:  Difficulty swallowing, difficulty speaking, paralysis of the tongue, stiffness of neck movement.

Ischemic stroke:  Numerous signs depending upon the severity .

It should also be noted that injury to the brain stem could also result in respiratory arrest and death.

What can cause an abnormal Powers Ratio? 

Anything that disturbs the atlantooccipital joint can cause an abnormal power ratio. The following are the possible causes listed in order of frequency.

      1. Trauma that includes hyper-extension, hyper-flexion, or rotational injuries of the neck. One example is whiplash-like injuries.
      2. Rheumatoid arthritis affects the atlantooccipital joint.
      3. Down syndrome results in the laxity of ligaments.
      4. Congenital cervical vertebral fusion syndromes.

Diagnostic Imaging

The sensitivity of the Powers Ratio in the detection of anterior atlanto-occipital dissociation on lateral cervical spine radiographs (XRays) has been found to be between 33% and 60%. Reference  This means it can be missed on XRays quite often.

Measurements using CT images were more reliable with regard to intra- and inter-observer agreement, making CT superior to XRay.   Reference   If CT is not accessible, patients can try to find a CBCT at an upper cervical chiropractor clinic or imaging clinic to obtain imaging to rule out CCI.

How is this treated? 

Treatment begins as soon as the injury is suspected. In-line stabilization of the neck is the first step to make sure there is no further injury to the area.

In severe cases, surgical treatment is necessary. These include halo immobilization or occipito-cervical fixation and fusion. The latter is when the back of the head and neck are held together to keep them stable. The fixations can be done with screws too. These are used carefully to avoid damaging blood vessels, and patients are anesthetized to be pain-free..

Research 

Research that is in first place evidence-wise has been conducted, but these are several years older, although still relevant.

 

PowerReferences

Tubbs RS, Hallock JD, Radcliff V, Naftel RP, Mortazavi M, Shoja MM, Loukas M, Cohen-Gadol AA. Ligaments of the craniocervical junction: a review. Journal of Neurosurgery: Spine. 2011 Jun 1;14(6):697-709.

  1. Powers B, Miller MD, Kramer RS, Martinez S, Gehweiler Jr JA. Traumatic anterior atlanto-occipital dislocation. Neurosurgery. 1979 Jan 1;4(1):12-7.
  2. Garrett M, Consiglieri G, Kakarla UK, Chang SW, Dickman CA. Occipitoatlantal dislocation. Neurosurgery. 2010 Mar 1;66(suppl_3):A48-55.
  3. Horn EM, Feiz-Erfan I, Lekovic GP, Dickman CA, Sonntag VK, Theodore N. Survivors of occipitoatlantal dislocation injuries: imaging and clinical correlates. Journal of Neurosurgery: Spine. 2007 Feb 1;6(2):113-20.
  4. Tubbs RS, Hallock JD, Radcliff V, Naftel RP, Mortazavi M, Shoja MM, Loukas M, Cohen-Gadol AA. Ligaments of the craniocervical junction: a review. Journal of Neurosurgery: Spine. 2011 Jun 1;14(6):697-709.
  5. Garrett M, Consiglieri G, Kakarla UK, Chang SW, Dickman CA. Occipitoatlantal dislocation. Neurosurgery. 2010 Mar 1;66(suppl_3):A48-55.
  6. Joaquim AF, Schroeder GD, Vaccaro AR. Traumatic atlanto-occipital dislocation—A comprehensive analysis of all case series found in the spinal trauma literature. International Journal of Spine Surgery. 2021 Aug 1;15(4):724-39.
  7. Formentin C, dos Santos LD, Maeda FL, Tedeschi H, Ghizoni E, Joaquim AF. Traumatic Atlanto-occipital Dislocation in Children Followed by Hydrocephalus–A Case Report and Literature Review. Arquivos Brasileiros de Neurocirurgia: Brazilian Neurosurgery. 2022 Sep 6.
  8. Garvayo M, Belouaer A, Barges-Coll J. Atlanto-occipital dislocation in a child: a challenging diagnosis. Illustrative case. Journal of Neurosurgery: Case Lessons. 2022 Mar 14;3(11).

Ref. Prof. AB May 2023

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