Thoracic AOMSI

In health care objective evidence of an injury is vital during litigation.  A lot of spinal injuries do not always have a way to shown objectively.  Some of the easiest objective evidence we have for spine is imaging.  MRI, CT and x-rays can be vital in showing objective injuries.  My podcast has spent a lot of time discussing disc pathology seen on an MRI and ligament damage seen on x-rays. 

I have spent a lot of time with my podcasts covering ligament damage and how this can be seen on x-rays.  We talked about the AOMSI studies for the cervical spine in the second podcast and the AOMSI study for the lumbar spine in episode 19.  Today I want to spend time covering the AOMSI study for the thoracic spine. 

This study is A Biomechanical study of the Ligamentous Stability of the Thoracic Spine in Man published by Panjabi, Hausfeld and white in Scandinavian orthopedic journal in 1981.  This study is very similar to the cervical and lumbar study.  They wanted to quote “establish the thresholds of  thoracic spine stability” end quote.  Once again it appears the study was done to assess when thoracic spine fusions could be justifiably done and the surgeons could get paid.  They said that once the information from this study is done quote “Clinical judgment can be made to decide if the patient is to have minor treatment such as a brace or perhaps major treatment in the form of surgery such as spinal fusion” end quote.   

This study was once again similar to the cervical and lumbar studies by using cadaver motion units.  They took 2 vertebra free of disease.  In this case they kept around 3 cm of ribs to not destroy the transverse process.  These vertebra were prepared and frozen to later assess what it takes to destroy the thoracic spine ligaments and create instability in the thoracic spine.  

As seen in other studies the thoracic spine is very stable given the protection from ribs.  This make instability in the thoracic spine very difficult.  This made their study difficult to assess failure.  Instead of pulling the vertebra apart to assess for damage they loaded each level with 43% of the body weight and then cut the ligaments.  This is a large change from the other 2 studies.  They cut ligaments to see what the spine looked like prior to failure and at failure.  They found this failure point to be 1.9mm translation and 5 degrees change in angular rotation.  They assumed a 30% magnification with x-rays and moved the 1.9mm translation to 2.5mm translation.   

They admitted that this study may have flaws with comparison to patient on x-rays.  They reported that with translation or horizontal displacement only 5% of stable spines will be misclassified as unstable spines, but up to 35% of injured spines may be misclassified as stable.  They reported that with change in angular motion none of the intact stable thoracic spines will be misclassified as unstable, but that 71% of injured spines will be misclassified as stable. 

It is important to note that only 0-5% of uninjured or stable spines will be misclassified as unstable.  However, using this method of assessing stability 35-71% of injured spines will be classified as stable uninjured.   They reported that there is an increased stability contribution by the thorax making the thoracic spine more stable than the cervical and lumbar spine. 

Real World

I am asked in depositions how common an AOMSI injury is.  I answer that I am not sure how common this injury is.  They ask me to guess how common this type of injury is in my practice.  I really don’t have an idea of how common this injury is.  I typically answer with ligament damage in trauma patients is very common.  It does seem less common for these trauma patients to rise to the level of AOMSI.  I believe the attorney in the depositions is trying to show that this is a very rare injury and therefore unlikely to happen. 

The AMA Guides to the Evaluation of Permanent Impairment 5^th edition on  page 379 says quote “When routine x-rays are normal and severe trauma is absent, motion segment alteration is rare; thus, flexion and extension x-rays are indicated only when the physician suspects motion segment alteration from history or findings on routine x-rays” end quote. 

I believe the attorneys in depositions have asked me how common this injury is to see if I agree with the AMA Guides findings of this injury being rare.  This is what I want to talk about today for the real world example.  I have neve had a thoracic AOMSI patient.  The thoracic spine is very stable with the ribs and I have never seen an unstable thoracic spine.  I would also say that I have only seen a dozen or so lumbar AOMSI cases in my career.  I do see a lot of cervical AOMSI cases.  Keep in mind that with me doing a lot of IMEs and personal injury care I do see a lot of trauma patients.  I do not see a lot of AOMSI cases in patients without trauma. 

The AMA Guides use the term severe trauma.  I find this very interesting.  I often wonder what they mean by this term.  I have a few thoughts on MVCs and their ability to cause severe traumas.  Vehicles have become a very common item in society.  I believe most people use vehicles multiple times a day.  These can be a person vehicle or public transportation.  According to a google search vehicles have increased  on average around 1,000 lbs since 1980.  When I was a child the freeway speeds were around 55mph.  Now the common is 70 mph and up to 80 mph. 

It is common for a lot of drivers to exceed these speed limits.  We are very used to very heavy vehicles with the average weight around 4-5,000 lbs. that go a very high speed.  This is typically not scary to us since we experience and likely participate in this multiple times a day.  If we take a step back and analyze this situation it can be shocking the potential forces involved in a vehicle.

The equation for force is mass times acceleration.  It is clear that the mass is very large in vehicles.  It is also clear that the acceleration of a vehicle could be very high.  In most cases a MVC will cause a negative acceleration of stopping.  I think we loose sight of how serious injuries can be in a MVC.  A simple rear end collision can lead to very high forces given the large mass and sudden acceleration of the vehicle. 

I’ve often thought about this in a different way.  If I was sitting on a chair and a large NFL player ran as fast as he could and collided with the back of my chair how injured would I be?  The largest NFL player ever weight 410 lbs.  This is 10 times smaller that most vehicles.  The fastest NFL players run around 22 mph.  I’m sure the 410 lbs NFL player was not running around 22 mph, but lets say this was the case.  This would be so much lower force than most MVC due to the low mass and low speed.  We all know we would likely be really injured in this scenario. 

Of course there are differences.  The mass of the chair would be substantially less than the mass of a vehicle.  The safety features in a chair would be much lower than a vehicle.  However, we would likely be really hurt with this type of collision.  How much more potential are there in MVC?  

It is my opinion that even low speed collisions have the potential for severe injuries or no injuries.  To me it is not as important to think about the speeds and mechanism of injury as it is to assess the patients individually to see what injuries they have.  What type of picture does each patient present is what matters to me.  I know vehicle have a lot of force and capable of causing all types of injuries.  

I know I’ve spent a lot of time talking about ligament damage and injuries like AOMSI.  This is not the most common injury I see, but it is a significant one that is demonstrable.  We owe it to the patient’s in MVC to not miss a diagnosis like this.  This type of injury, although not common is significant and has a life long impact and deserves fair value in a settlement and future care.