AN INNOVATIVE APPROACH FOR MANAGEMENT OF ATLANTOAXIAL INSTABILITY WITH

MYELOMALACIC CHANGES EXTENDING TO SUBAXIAL CERVICAL CORD WITH BIOMECHANICAL

ANALYSIS OF THE METHOD

Subhayan Mandal1,Sumit Deb2,P.K.Ganguly3,Dibyendu Ray4, Anup Sadhu5, Atreyee dutta , Sanjib Samadder

1Consultant Neurosurgeon, KPC Medical College & Hospital

2Prof & H.O.D, Department of Neurosurgery, National Medical College & Hospital

3Prof, Department of Neurology, KPC Medical College & Hospital

4Consultant Neurosurgeon, AMRI Hospital, Salt Lake

5Consultant Radiologist, EKO CT Scan Private Ltd.

Address for correspondence:

Dr. Subhayan Mandal , MS ,MCh (Neurosurgery),FAAGAI

Consultant neurosurgeon, KPC Medical College & Hospital, Kolkata

E mail: subhayanmandal@gmail.com

ABSTRACT:

Post traumatic atlantoaxial instability with myelomalacic changes at cervico vertebral junction (CVJ) extending to C2 level in a young lady of 26

years not only pose complex problems regarding the extent of decompression, fixation issues but also require consideration of maintenance of

mobility so that she can enjoy her livelihood with a stable but mobile atlantoaxial junction. Regarding the complexity of organisation of

neurovascular structures at the aforesaid level demands not only extreme accuracy in operative ergonomics to the part of the neurosurgeon

but also lack of standardization of the operative technique to this unique situation demands innovative ideas to fulfil the goal. Here we address

the above situation by performing wide decompression extending from foramen magnum to the upper part of lamina of C3 followed by our

innovative technique of fixation of atlantoaxial segment to reach the operative goal of adequate decompression, stabilization with

maintenance of fair degree of mobility of the atlantoaxial segment in this young lady. We also provide the biomechanical explanation of the

logistics of our operative strategy and science behind our construct in terms of simple laws of physics namely; Newton’s third law, Hook’s law

and law of shear modulus. We suggest that a randomized controlled trial may be conducted to evaluate the proposed surgical technique.

KEYWORDS:

Atlantoaxial instability, innovative, myelomalacic, biomechanical, ergonomics,axial load, equation, CV junction

INTRODUCTION:

Atlantoaxial joint is considered to be the most intricate and delicate area of human body in view of its structural complexity and because of its

close intimacy with the major neurovascular structures. Atlantoaxial segment of the craniovertebral junction (CVJ) is endowed with extreme

mobility whereas the occipitoaxial segment is responsible more for the stability issues of the CVJ. As per the knowledge to date it is the

atlantoaxial segment which is the faulty one in most of the cases of CVJ instability issues. A number of techniques are there to address

atlantoaxial instability but no one stands out to the best making the scientific field open to treat cases of atlantoaxial instability as per merits

of the cases, and also leave open the field for innovation in technique of decompression and fixation of atlantoaxial joint.

CASE REPORT:

26 yrs. aged Lady Mrs. UC presented to the clinic with history of pain in the neck associated with restriction of neck movement, weakness in all

four limbs, difficulty in deglutition, few episodes of neuralgic pain along the lower division of left trigeminal nerve, episodic radiating pain

along the distribution of left C5 to C8 dermatomes turning occasionally to be dysthetic, a band like sensation around the chest, with urgency,

frequency of micturition and urge incontinence along with broad based gait and difficulty in negotiating through narrow passages and

difficulty in going sideways. Patient stated about a history of trivial neck trauma attributed to minor fall dated one and half a month back after

which she remembered the gradual progressive appearances of the above symptoms.

On examination Mrs. UC was found to be alert, conscious, cooperative and oriented. Powers of her upper limbs were 4-/5 [Medical Research

Council (MRC) Scale] bilaterally (B/L), hand grip powers were 4-/5 (B/L) with lower limbs power being 4/5(B/L). All the deep tendon reflexes

(DTR) were 3+(hyperactive reflex). There was absent abdominal reflex and plantar reflexes were extensor in both sides. Rotation of neck was

< 150, flexion and extension were < 100, lateral bending in both sides were restricted to < 100 as measured by protractor and all the

movements of the neck were associated with intense pain. All the modalities of sensations were decreased below C5 dermatome.

MRI cervical spine with CV junction (Fig-1) showed mildly displaced old communited type I &II odontoid fracture associated with non displaced

old communited fracture of anterior arch of C1. There was also evidence of myelomalacic changes of cervical cord behind C2 along with

radiologically obvious mild CV Junction stenosis (diameter 9mm).

Fig:1-MRI cervical spine with CV junction revealing-

1. Old communited fracture with mild displacement in tip ( ) and also at the base of odontoid ( ) process (DENS) (Type I &II

old odontoid fractures).

2. Nondisplaced old communited fracture also noted in anterior arch of Atlas( ).

3. Mild soft tissue noted in the prevertebral space of C1 vertebra-sequel of previous contusion.

4. Hyperintense foci noted in body of C2 vertebra/sequel of previous contusion.

5.Intensity noted in upper cervical cord with contracted upper cervical cord myelomalacic changes /sequel of previous traumatic

cervical myelopathy ( ).

6 .Intact adjacent Tectorial membrane.

7. Mild stenosis noted at CV junction (9mm).

8. Minimal Disc desiccation noted at C2C3, C3C4, C5C6, C6C7 levels.

3D CT CV Junction (Fig-2) revealed disruption of atlantoaxial alignment, atlantoaxial instability without any evidence of platybasia

From the above clinico-radiological findings the diagnosis was made as follows-

1. Post traumatic atlantoaxial instability

2. Compressive myelopathy at CV junction due to stenosis with contribution from post traumatic myelomacic changes up to C2 level

OPERATIVE GOALS AND TECHNIQUE:

Goals of operative technique were therefore

1. To achieve adequate decompression of CV junction and thereby to restore a platform for optimal functioning of relevant neurovascular

structures concerned

2. Near normal physiological restoration of alignment

3. Stabilization of CV junction of which here the atalantoaxial component is in fault; in accordance to the rule that in most of the cases of

CV junctional instability it is the atalantoaxial segment which remains usually responsible

4. Motion preservation at atlantoaxial segment

To accomplish the aforesaid goals operative technique we used was an innovative one and comprised of the following-

➢ In a standard fashion through posterior approach we explore the CV junction bilaterally, under C ARM (ALLENGERSTM) guidance we

reached to C1 –C2 facet joint spaces bilaterally by dissecting the posterior capsule of the joint to negotiate a SPACER(GESCOTM) to

stabilize the joint followed by driving lateral mass screws at C1 & C2 lateral masses bilaterally and with an addition of lateral mass

screw bilaterally also at C3 to shift centre of pressure through which axial load with transmit along the construct. Moreover both bio-

ergonomically as the number of fixation point of the said construct had increased by incorporation of C3 the construct, it was ought to

confer superior rigidity thereby satisfying our goal 3.

➢ Decompression was performed by cutting inferior margin of foramen magnum for a length of 2 cm, C1 posterior arch and continuing

the extent of decompression up to upper aspect of C3 lamina thereby providing adequate decompression of vital neurovascular

structures passing beneath CV junction. Thus we achieved the goal 1 efficiently.

➢ When connecting the heads of the polyaxial screws by titanium rods during tightening because of the leverage action, alignment of the

said anteriorly dislocated posterior arch of C1 got corrected as evidenced through CARM fluoroscopy. Now imparting compression

Fig: 2- 3D CT CV Junction revealing–

1. Atlantoaxial alignment seen disrupted due to separated odontoid process of atlas suggesting instability( ).

2. There is minor anterior displacement of posterior arch of C1 vertebra( ).

3. No atalantoaxial assimilation/platybasia is seen.

force between C1 C2 polyaxial screw segment had made the construct rigid over the spacers those were in driven bilaterally in the

facet joint spaces as said earlier thus fulfilling our goals of 2 and 3.

➢ Lastly as we used polyaxial screws and as per the properties of the polyaxiality of the screw we succeed to preserve motion at

atlantoaxial segment and thereby able to achieve the goal 4. In addition translational movement in either of the anterior or posterior

directions were being prevented by insertion spacer at C1 C2 Facet joint interfaces bilaterally and by the lateral mass screw rod

construct.

Post operative period was uneventful; drain removed on 2nd postoperative days and she was able to ambulate by self with minimal

assistance within 18 hours of operation. A 3D CT scan was done on 3rd post-operative day and she was discharged home on 8th post-

operative days. At the time of discharge she was stable in vital parameters. Her powers of all the four limbs including hand grip powers

were 4/5(MRC scale), DTRs were 3+ with extensor plantar reflexes B/L. Postoperatively she could rotate her head by 70 0 B/L, flexion &

extension were 600 for each and she was able to carry out lateral bending to 300 in either side with minimal pain attributed to post

operative wound (Fig-3) and all the movements of her neck were videographed.

A-LEFT LATERAL BENDING:

ANTERIOR AND POSTERIOR

VIEWS

B- RIGHT LATERL BENDING:

ANTERIOR AND POSTERIOR

VIEWS

C-FLEXION: ANTERIOR AND

POSTERIOR VIEWS

D-EXTENSION: ANTERIOR

AND POSTERIOR VIEWS

E- ROTATIONAL

MOVEMENT

Fig: 3- Analysis of movements of

the neck through real time

images

Her 3D CT scan with reconstruction with report was as follows (Fig-4)-

Fig: 4- Post-operative CT Scan and 3D CT reconstruction of bony and instrumental orientation:

1. Restoration of atlantoaxial alignment with correction of anterior displacement of posterior arch

of C1

2. No atlantoaxial instability was noted in comparison to previous 3D CT CV junction

3. Implants related artifacts are noted bilaterally.

• Spacer- indicated in red arrows ( )

• Poly axial screws in yellow arrows ( )

• Anterior arch of atlas in sky blue arrow ( )

• Odontoid process in light green arrow ( )

• Foramen magnum(FM) and FM view in black arrow ( )

DISCUSSION:

Innovation is the driving force of progress of science, technology and portrays superior understanding of ergonomics in surgical field to meet

the unmet targets. Variations in the neurosurgical approaches to address atlantoaxial instability suggest progressive better scientific

appreciation of the three dimensional structural complexity of the region, coupling of acculturation of superior technologies and

ergonoinformatics to solve the diverse intricate issues of atlantoaxial instability.

For a quite long time CV junction remained relatively invincible unexplored part of the human body because of its structural delicacy, intimate

relationship with vital neurovascular structures and because of its complex three dimensional orientations. Approach to this region not only

demands a superior cognition of intricate 3D anatomy of the region and skill of neurosurgeon but also a supreme knowledge armoured instant

decision making process. Moreover though a success of operation on CV junction is often an ecstatic event, failure is often gauzed with

gruesome lethality.

Treatment of atlantoaxial instability(AAI) has been traversed a long path from the days of ‘external immobilization only’ to wire fixation by

Osgood and Mixter(1) in 1910,Gallie(2), Brooke Jenkins followed by Sontag modification of Gallie’s technique and lastly Halifax(4,5) in succession

but were fallen into disfavour because of poor biomechanical stability issues, high incidence of non-union, chances of posterior rachischisis(3)

and lastly due to potential chances of cord compression in a ‘clothline’ fashion(4). Moreover requirement of an intact posterior arch had made

these endeavours inapplicable to provide adequate decompression at atlantoaxial level of cervical cord (6,7) if needed. All these techniques

required an external immobilization post-operatively and therefore issues of immediate postoperative mobility were ignored at a large. In the

later era of screw fixation method Magerl’s technique (8) of transarticular screw fixation, followed by in close succession Goel’s Technique (9) of

fixation by plates connecting C1 lateral mass & C2 pedicle screws and lastly Harm’s technique (10, 11) of incorporation of rod instead of plate in

Goel’s technique in order to provide superior mobility after construct were come into practice. Magerl’s technique was long been considered

as gold standard technique but requirement of supreme operative skill, failure to provide solution to issues of decompression and in around

20% of the specimen due to the anatomical infidelity of the position of foramen transversarium endangering ipsilateral vertebral artery (11)

made this technique less popular to young neurosurgeons.

Although either Goel or Harm’s techniques were good in term of providing optimal decompression at atlantoaxial segment with good

biomechanical stability following fixation, presence of narrow C2 pars preclude safe placement of screw in terms of injuring vertebral artery. In

our method as we had placed the screw through lateral mass injuring vertebral artery was not possible as the direction of screw was away

from outer border of foramen transversarium (Fig-5). Lastly chance of injuring the cord during placement of C2 pedicular screw in case of a

narrow pedicle was of real thereat whereas in our technique as the direction of screw was away from the cervical canal therefore it was not

possible (Fig-5).

Therefore the advantages we obtained were as follows-

• Chances of injuring either the vertebral artery or the exposure of C2 screw in the spinal canal were least in comparison to either

Goel’s or Harm’s techniques.

• Difficulty in connecting the rod between C1 lateral mass screw to C3 lateral mass screw through C2 screw was simplified by inserting

C2 lateral mass screw instead of pedicle screw as in the later case difficulty would occur later in connecting rod due to different

orientation of head of C2 pedicular screw. In that case we had to bend the connecting rod to connect all the screw heads and

therefore the bending force would affect the rigidity and longevity of the construct.

• Addition of a polyaxial screw at C3 lateral mass provides extra length for transmitting the force vector dividing nearly equally over

three vertebrae thereby minimises stress on C2 vertebra and thus biomechanically confers less chance of screw pull out from C2

vertebra.

• Inserting spacer at C1 C2 facet joints bilaterally had made the joint rigid and locked thereby preventing anterior slippage of C1 once

reduced during operation at the time of tightening the screws over rod under fluoroscopic guidance.

• Finally due to incorporation of polyaxial screw and rod system instead of screw plate system (Goel’s Technique) to create the

construct gave us biomechanical advantages providing more generous degree of motion which was one of our goal.

Fig-5: Comparison of directions of lateral mass ( ) and

transpedicular screw ( ). Foramen transversarium ( ) is shown in

relation to the direction of screw pathways.

BIOMECHANICAL CONSIDERATION OF THE TECHNIQUE AND THE CONSTRUCT -

From biomechanical point of view if we consider the free body diagram of our construct in relation to the bony arrangement around the

atlantoaxial junction we can construct the free body diagram as the following –

Here:

F = Force acting through the centre of C1 vertebra on standing ( equals to the weight of the skull responsible for imparting axial load in

absence of any other axial loading forces)

M1, M2, M3 = Masses of C1, C2, C3 vertebrae

M1g, M2g = Corresponding weights of C1, C2 vertebrae

Θ = Angle conformed by force F at centre of C1 with the horizontal plane

FS = Frictional force offered by spacer at C1 C2 facet joint

FP = Pull out force acting along the shaft of each screw

Fr = Friction force offered by the titanium screw bone interface resisting screw pull out

Y= Young’s Modulus of titanium metal

S= Shear Modulus of titanium metal

From the above free body diagram we can derive the following –

• F cosΘ is the horizontal force component responsible for anterior translation of atlas (C1) and bio mechanistically also be the

responsible force that can cause fracture of any arches of C1 with or without fracture odontoid process if an enormous axial load is

being imparted through the vault due to traumatic or non-traumatic reasons. In our case we had found that the lady had non displaced

fracture of anterior arch of atlas along with fracture of odontoid process thereby explaining our assumption. In our construct we had

given a spacer to oppose this component of force by inserting a space r at C1 C2 which provide frictional force FS in the opposite

direction of F cos Θ and the state of equilibrium thus conforms the following equation-

F cosΘ = FS .........................................................................(1)

• The vertical component of the force ; ie, F sinΘ as going alonf the cervical cord is responsible for trauma in addition to F cos Θ causes

myelomalacic changes of the cord by compressing it in both vertical and sagittal (horizontal) directions as we observed in our case.

• In each segment downward from the level of C1 as the weight of the previous vertebra is added to the magnitude of axial load F sinΘ ;

therefore magnitude of axial loads will be

I. At C2 level –

F sinΘ + M1g ............................................................................(2)

II. At C3 level-

Θ

Fr

Fr

Fr

Fp

Fp

Fp

FF SSiinn ᶱɵ

FF CCooss ᶱɵ

M2

M3

M1 FS

F

Fig-6: Force vector diagram of the construct

F sinΘ + M1g+M2g ............................................................................(3)

If we think the average force acting at the centre of free body of the construct (at centre of C2 body) ,then the value of the force will be

Favg = (F sinΘ+ F sinΘ + M1g+ F sinΘ + M1g+M2g)

3 ............................................................................(4)

ie, Favg = F sinΘ + 2𝑀1𝑔

3 +

𝑀2𝑔

3 ............................................................................(5)

• Just at the point of breakage of the construct, Favg should be sufficient enough to cause a unitary translation at the neck of the screw, so

that ∆𝑙 > 0 ;then by the law of shear modulus we can state-

S = Favg

𝐴 x

𝐿

∆𝑙 ................................................................(6)

[

𝑤ℎ𝑒𝑟𝑒 𝐴 = 𝑠𝑢𝑟𝑓𝑎𝑐𝑒 𝑎𝑟𝑒𝑎

𝐿 = 𝑙𝑒𝑛𝑔𝑡ℎ 𝑜𝑓 𝑡ℎ𝑒 𝑠ℎ𝑎𝑓𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑐𝑟𝑒𝑤

∆𝑙 = 𝑑𝑖𝑠𝑝𝑙𝑎𝑐𝑒𝑚𝑒𝑛𝑡 𝑜𝑓 𝑙𝑜𝑤𝑒𝑟 𝑒𝑛𝑑 𝑜𝑓 𝑠𝑐𝑟𝑒𝑤 𝑏𝑒𝑦𝑜𝑛𝑑 𝑡ℎ𝑒 𝑝𝑜𝑖𝑛𝑡 𝑜𝑓 𝑒𝑙𝑎𝑠𝑡𝑖𝑐𝑖𝑡𝑦 ]

Because of the huge value of shear modulus of titanium metal (approximately 40 GPa=4 x 1010 Newton /m2 = 4.07 x 105 kg/cm2), ∆𝑙

approaches to near zero.

In reference to the diagram-

• FP is the pull out force that tries to pull out the screw off the bone acting along the shaft of the screw opposed by the frictional force Fr.

When the system is in equilibrium it could be stated that

FP = Fr ..............................................................(7)

Here Fr can well be assumed that it depends on the quality of the bone under consideration as an osteoporotic bone will confer lesser

frictional force than a good quality bone ,therefore chance of pull out of the screw will be more for osteoporotic bone that a normal bone as in

that case FP > Fr. It is also quite obvious that more the length of screw shaft is in driven in the bone, more will be the magnitude of frictional

force as there is more of the lateral surface of the screw is present to interact with the bone to produce frictional force.

An interesting phenomenon occurs when the patient rotate the head suddenly over the point or on the fulcrum of rotation, where an

enormous amount of rotational force or torque is generated due to muscular pull that tries to break the construct at any particular level and

because of very short duration of presence of the force it acts before the frictional force opposes the rotational force causing breakage of

screw at the neck. The same thing also applies when the patient suddenly flex the head by the action of neck flexors of both sides. In both the

situations as per the Hook’s law we can state that force Frt when just crosses elastic limit of titanium metal then -

Y = Frt

A x

∆𝑙

𝐿 ...........................................................................(8)

[Where Frt = Muscular force ]

Favg

∆𝑙 > 0

ɩ

Frt

Rotation

Pivot point or Fulcrum of rotation

We know that Young’s modulus of titanium is around 100 GPa = 1011 Newton /m2 = 1.019 x 106 kg/cm2 , therefore a massive value of torque is

needed to break the construct at the neck.

Lastly, as use polyaxial screw we know because of the polyaxiality of the lateral mass screw it is possible to rotate or lateral bend the neck

instead of having a rigid construct with very less chance of breakage as enormous magnitude of the force is required to break the screw at

neck as evidenced from the above equation which is a real nightmare for the surgeon that obviates the professional to advice the patient a

rigid immobilization as have already been discussed in relation to various atlantoaxial fixation methods.

CONCLUSION:

In medical field biomechanics provides neurosurgeon to describe ergonomics of a new operative technique scientifically, however to proof

superiority of an operative technique requires randomised control trial to find out the superior result of the technique over the existing

technique and moreover requires a substantial long time follow up of the case to confirm the better surgical outcome. In recent days finite

element analysis of a model is also used to accurately describe the biomechanics of a construct.

Our innovative method of atlantoaxial segment decompression and fixation has been applied to a young patient to compensate the need of

the patient vis a vis to achieve the goal to give stability amidst mobility so that she can continue her living with a mobile but stable atlantoaxial

joint. It is fair enough to commit that we do not have sufficient data in terms long-term outcome data to prove superiority of our technique

over others as described earlier and therefore we recommend usage of our technique in more cases and to assess outcomes over longer

period of follow up before strongly commenting on the superiority or inferiority of our innovative technique of atlantoaxial segment

decompression and fixation.

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