Atlantoaxial Instability

• Lance Wilson • University of Queensland Veterinary

Medical Centre

Past fellowship examination questionsFELLOWSHIP EXAMINATION JUNE/JULY 2005 SMALL ANIMAL SURGERY PAPER 2 – PRACTICE • Write notes on the following:

– Surgical repair of L7-S1 luxations. – Treatment options and published

prognoses for a dynamic lesion at C5-6 in an 8-year-old Doberman FN (Female Neutered).

– Surgical stabilization of congenital Atlanta-axial subluxation in a miniature poodle.

Fellowship Examination June 2013 Small Animal Surgery Paper 2 1. Answer all parts of this question: a) After a traumatic incident at a park, a two-year-

old Chihuahua presents with neurologic findings suggesting a lesion within the C1–C5 neuroanatomical segment.

i. Detail the specific neurological examination findings you would expect in this patient. (6 marks)

ii. Provide a list of differential diagnoses ranked in order of likelihood.

(3 marks) iii. Describe and justify your plans for diagnostic

investigation and pre-surgical management for this patient. (9 marks)

iv. List two (2) techniques for dorsal stabilisation of the atlantoaxial joint, and two (2) techniques for ventral stabilisation of the atlantoaxial joint.

AAI - overview

• Anatomy • Pathophysiology • Signalment and clinical

findings • Diagnosis and

differential diagnoses • Treatment options • Complications • Prognosis

Atlantoaxial instability (AAI)

• Congenital or developmental malformation or trauma to bones, joint or ligaments of AA region

• Dorsal subluxation of the axis into the vertebral canal

• Direct compressive and concussive effects on the cervical spinal cord

Anatomy - Atlas• First cervical vertebra • Develops from three bony

elements • Two neural arches and body

(Intercentrum 1) • No dorsal spinous process • Large transverse processes

(wings) • Reduced vertebral body • Articulates cranially with

occipital condyles via cotyloid cavities

• Articulates caudally with axis via glenoid cavities

• Ventral process • Transverse and lateral vertebral

foraminae

Anatomy - Axis

• Second cervical vertebra

• Develops from seven bony elements • Centrum of proatlas,

centrum 1 (C1), intercentrum 2, centrum 2, epiphysis and two neural arches

• Dens or odontoid process

• Prominent dorsal spinous process

Anatomy – AA joint

• Synovial joint • No intervertebral disc • Motion largely

rotational with some lateral

• Dens located within fovea of the dens of atlas

• AA joint contiguous with atlanto-occipital joint

Anatomy – AA joint ligaments

Five major ligaments 1.Apical 2. Left alar 3.Right alar 4.Transverse 5.Dorsal atlantoaxial Reber et al Vet Surg 2013

• Concluded alar ligaments to be the most important in preventing dorso-ventral shear

Pathophysiology• Two main presentations Congenital/developmental • Dysplasia, hypoplasia or aplasia of

dens • Abnormal physeal closure of dens in

miniature breeds • deLahunter et al

• Form of avascular necrosis of dens • Leads to failure of normal

ligamentous development • Absent transverse ligament • Incomplete ossification of the atlas

• Parry et al Vet Radiol Ultrasound 2010

• Atypical breeds • 35 x more likely to have AAI

• Block vertebrae Results in supramaximal forces

exerting on ligamentous stabilisers

Pathophysiology

Traumatic • Any breed or age • Severe neck flexion

• Ligamentous or osseous damage

• Often related to underlying congenital defect

End result is dorsal angulation of axis and compression and concussion of spinal cord

Signalment and clinical findingsSignalment • Toy and small breeds

• Yorkshire terrier, Toy and Min Poodle, Chihuahua, Pomeranian, Pekingese

• Congenital AAI typically seen in young dogs (52-70% within first year)

• Traumatic AAI seen at any age Clinical findings • Can be acute or chronic onset • Cervical pain

• Up to 60% congenital • Most traumatic AAI

• Neurologic deficits • C1-C5 neuroanatomical localisation • UMN/GP ataxia • Range from ambulatory tetraparesis

(up to 94%) to tetraplegia (10%) to death from respiratory arrest

Diagnosis and differential diagnoses

Diagnosis • Signalment/CSx

• Young, small breed dog with C1-C5 neurolocalisation and cervical pain

• Plain radiography • Fluoroscopy • Myelography • CT • MRI

Differentials • IVDD

• Traumatic • Degenerative

• Meningomyelitis • Syringohydromyelia • Discospondylitis • Vertebral fracture or

other luxation

Radiography• Unsedated if possible • Lateral view

• detect increase in space between C1 dorsal lamina and C2 dorsal spinous process

McLear et al Vet Radiol Ultrasound 2000 • Atlantoaxial angle <162 deg

predictive of instability • VD view

• Evaluate dens • Severe cases detect

malalignment • Dynamic flexion views obtained

under fluoroscopy with extreme care

• Myelography typically of little benefit

Advanced imagingCT • Modality of choice • Dens morphology • Atlantal morphology • Fractures • AO overlap • Implant positioning MRI • Parenchymal pathology

• Haemorrhage • Oedema • Syringohydromyelia • Myelomalacia

• AA ligaments • Middleton et al Vet Radiol

Ultrasound 2012

Treatment options

Non surgical Surgical • Dorsal techniques • Ventral techniques

Non-surgical treatmentStability achieved via fibrotic ankylosis Consists of • External coaptation with head in extension

• Ventral splint • Minimum 6 weeks cage confinement • Analgesia Indications

• Acute onset • Minimal neurologic deficits • Normal dens radiographically • Young patients • Owner constraints

Good outcome significantly associated with duration of clinical signs (<30 days)

Complications • Continued instability, corneal ulcers, splint

migration, hyperthermia, dermatitis, respiratory compromise, anorexia and otitis externa

Havig et al JAVMA 2005 • Successful outcome in 10/16 (63%)

Surgical treatment

Dorsal techniques • Atlantoaxial wire • Atlantoaxial suture • Dorsal suture • Kishigami AATB • Dorsal cross-Pinning • Nuchal ligament

technique Ventral techniques

• Transarticular lag screws • Transarticular pins +/-

PMMA • Screws and PMMA • Pins and PMMA • Plates and screws

Indications • Chronic or recurrent

disease • Conservative

management failed • Mature vertebrae • All dogs??

Dorsal techniquesAdvantages • Provide decompression and

stabilisation in dorso-ventral plane

• Approach simple • Reduction readily performed • Biomechanically

advantageous Disadvantages • Unable to resist movement in

all directions • Long term stability relies on

fibrous ankylosis • Paucity of bone for fixation • Odontoidectomy not possible

Indications • Small dogs (<2kg) • Failed ventral fixation Contraindications • Dorsal deviation of the dens

Atlantoaxial wire/suture• Loop of 20-24g orthopaedic

wire • Passed under dorsal arch of

C1 and through two tunnels in dorsal spinous process of C2

Denny et al JSAP 1988 and McCarthy et al Compendium 1995 • 14/27 (52%) successful

outcome with atlantoaxial wire

• Suture can also be used Chambers et al JAAHA 1977

and McCarthy et al 1995 • 5/10 (50%) successful

outcome with dorsal suture

Kishigami AATB

• Commercially available implant

• Hooks over cranial aspect of dorsal arch of C1 in epidural space

• Secured to dorsal spinous process of C2 with orthopaedic wire

Pujol et al Vet Surg 2010 • 6/8 (75%) good to

excellent outcome

Dorsal suture• 3-0 nylon double suture • Placed from obliquus capitas cranialis to obliquus capitas

caudalis bilaterally and tied near occiput Sánchez-Masian et al VCOT 2014

• 10/15 (66%) had successful outcome with another 2 successfully revised

• Recommended for dogs <1.5kg

Dorsal cross-pinning• Paired 0.045 inch k-wires • Dorsal spinous process of

C2 into the wings of C1

• Dorsal bone burred and bone graft placed

• Wires cut, bent and incorporated into PMMA

Jeffery ND JSAP 1996 • Single case report

successfully treated

Ventral techniques

Advantages • Potential for AA

arthrodesis • Increased availability of

bone stock • Odontoidectomy

possible Disadvantages • Reduction more difficult • Approach more difficult • Increased potential for

iatrogenic trauma • Bulk of implants

Indications • Mature dogs >2kg • Atlantoaxial fractures • Dorsal dens deviations • Failed dorsal fixation Contraindications • Small, immature dogs

Approach for ventral AA techniquesTwo options • Routine ventral midline

approach to AA joint • Modified ventral midline/

parasagittal Shores et al vet Surg 2007

• Incision between right sternothyroid and sternocephalic muscles

• Gives improved exposure of AA joint and protection of regional structures

• AA joint exposed, cartilage debrided and ACBG placed to encourage arthrodesis

AAI reduction ventral techniques

Pike et al Vet Surg 2012 Forterre et al VCOT 2012

• Small gelpi retractors placed from the intercondyloid notch/incisure cranially and a small slot in the cranial endplate of C3/4 caudally.

• Second pair placed orthogonal to retract the longus colli muscles

Platt et al Vet Surg 2004 • Screw in caudal ventral

C2 attached to wire and used to lever up C3

Transarticular lag screws/pins• Bilateral implants directed

across AA joint Reves et al Vet Surg 2013

• 40 deg medial to lateral and 20 deg ventral to dorsal

• Mean corridor length 7mm (4.5mm-8mm) and width 3-5mm

• Aim for medial aspect of alar notch

• Technical issues include correct angulation, erroneous implant placement and fracture of the cranial axis

Denny et al JSAP 1988 • 90% success rate with lag

screws without grafting

Screws and PMMA• Cortical bone screws

1.5mm-2.0mm • Placed in ventral C1 and C2

and encased in PMMA • Ensure PMMA is not bulky Platt et al Vet Surg 2004

• Transarticular pins • 2-3 screws in atlas and 2 in

axis • 16/19 (84%) considered to

have a successful outcome Sanders et al JAAHA 2004

• 2-3 screws in atlas and 4 in axis

• Joined by small pins secured by wire

• 11/12 (92%) successful

Pins and PMMA

• 0.035-0.062 inch pins inserted into C1, C2 and transarticular

• Cut 10mm from bone and entombed in PMMA

Aikawa et al Vet Surg 2013 • 46/49 (94%) improved

neurologically

Schultz et al vet Surg 1997 • 8/9 (89%) considered

successful

Plates and screws

!• 11 cases in total

• Successful outcome in 8/11 (73%)

Dickomeit et al VCOT 2011 • Locking butterfly plates

resulted in excellent outcomes in 3/3 dogs

Surgical complications• Seen in 9 – 70% of reports Neurologic deterioration

• Surgical manipulation or erroneous implant placement Respiratory system compromise

• Ventral techniques • Surgical retraction or implant related compression • Can result in laryngeal paresis, tracheal obstruction or necrosis • Aspiration pneumonia

Implant failure • Migration or breakage of implant • May result in destabilisation

Atlas or axis fracture • Thin or soft bone predisposes

Ongoing pain or neurological signs • Inadequate decompression or fixation failure • Failure of arthrodesis

Postoperative carePost-op imaging

• Radiographs • CT

Monitor neurological and respiratory systems Analgesia

• Opiods • Nsaids, CCSs?

Physiotherapy External coaptation? Strict cage confinement

• 6-8 weeks

Exercise modification Avoid neck collars/games etc.

Prognosis• Fair to good if clinical signs

mild-mod and guarded if severe Conservative • Influenced by duration of

clinical disease (>30 days negative prognostic indicator)

• 50-63% success rate • Lack of long term follow up Surgery • Overall success rates 47-94% • Positively influenced by

• Age < 24 months • Duration of CSx < 10 months • Lower neurologic grade

• Perioperative mortality rates 0-30% with most recent reports <10%

Dorsal techniques • 50-75% success rate (average

61%) Ventral techniques • 47-94% success rate • Recent reports average 90%

Important review articles/references• Beaver DP, Ellison GW, Lewis DD, et al: Risk factors

affecting the outcome of surgery for atlantoaxial subluxation in dogs: 46 cases (1978–1998). J Am Vet Med Assoc 216:1104, 2000.

• Havig ME, Cornell KK, Hawthorne JC, et al: Evaluation of nonsurgical treatment of atlantoaxial subluxation in dogs: 19 cases (1992–2001). J Am Vet Med Assoc 227:257, 2005

• Cerda-Gonzalez S, Dewey CW. Congenital diseases of the craniocervical junction in the dog. Veterinary Clinics of North America. Small Animal Practice 40, 121–41, 2010

• Dewey DW, Marino DJ, Loughin CA: Craniocervical junction abnormalies in dogs. NZVJ 61: 202, 2013.

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References• Evans HE.: Miller’s anatomy of the dog. 3rd edition. Philadelphia: W.B. Saunders; 1993. • Havig ME, Cornell KK, Hawthorne JC, et al: Evaluation of nonsurgical treatment of

atlantoaxial subluxation in dogs: 19 cases (1992–2001). J Am Vet Med Assoc 227:257, 2005

• Beaver DP, Ellison GW, Lewis DD, et al: Risk factors affecting the outcome of surgery for atlantoaxial subluxation in dogs: 46 cases (1978–1998). J Am Vet Med Assoc 216:1104, 2000.

• McCarthy RJ, Lewis DD, Hosgood G: Atlantoaxial subluxation in dogs. Compend Contin Educ Pract Vet 17:215, 1995.

• Denny HR, Gibbs C, Waterman A: Atlantoaxial subluxation in the dog: a review of thirty cases and an evaluation of treatment by lag screw fixation. J Small Anim Pract 26:37, 1988.

• Sanders SG, Bagley RS, Silver GM, et al: Outcomes and complications associated with ventral screws, pins, and polymethyl methacrylate for atlantoaxial instability in 12 dogs. J Am Anim Hosp Assoc 40:204, 2004.

• Platt SR, Chambers JN, Cross A: A modified ventral fixation for surgical management of atlantoaxial subluxation in 19 dogs. Vet Surg 33:349, 2004.

• Aikawa T, Shibata M, Fujita H: Modified Ventral Stabilization Using Positively Threaded Profile Pins and Polymethylmethacrylate for Atlantoaxial Instability in 49 Dogs. Vet Surg 42;683, 2013

• Lorinson D, Bright RM, Thomas WB, et al: Atlantoaxial subluxation in dogs: the results of conservative and surgical therapy, Canine Pract 23(3):16–18, 1998.

• Gilmore DR: Nonsurgical management of four cases of atlantoaxial subluxation in the dog. J Am Anim Hosp Assoc 20:93, 1984.

• Jeffery ND: Dorsal cross pinning of the atlantoaxial joint: new surgical technique for atlantoaxial subluxation. J Small Anim Pract 37:26, 1996.

• LeCouteur RA, McKeown D, Johnson J, et al: Stabilization of atlantoaxial subluxation in the dog, using the nuchal ligament. J Am Vet Med Assoc 177:1011, 1980.

• Kishigami M: Application of an atlantoaxial retractor for atlantoaxial subluxation in the cat and dog. J Am Anim Hosp Assoc 20:413, 1982

• Cerda-Gonzalez S, Dewey CW. Congenital diseases of the craniocervical junction in the dog. Veterinary Clinics of North America. Small Animal Practice 40, 121–41, 2010

• Pike F, Kumar M, and Boudrieau R: Reduction and Fixation of Cranial Cervical Fracture/Luxations using Screws and Polymethylmethacrylate (PMMA) Cement: A Distraction Technique Applied to the Base of the Skull in Thirteen Dogs. Vet Surg 41: 235, 2012.

• Revés N, Stahl C, Stoffel M et al: CT Scan Based Determination of Optimal Bone Corridor for

• Atlantoaxial Ventral Screw Fixation in Miniature Breed Dogs. Vet Surg 42: 819, 2013.

• Stead AC, Anderson AA, Coughlan A: Bone plating to stabilize atlantoaxial subluxation in four dogs. J Small Anim Pract 34:462, 1993.

• Dickomeit M, Alves L, Peckarkova M, et al: Use of a 1.5 mm butterfly locking plate for stabilization of atlantoaxial pathology in three toy breed dogs. VCOT 24:246, 2011.

• McLear RC, Saunders HM: Atlantoaxial mobility in the dog. Vet Radiol Ultrasound 41:558, 2000

• Dewey CW, Berg JM, Stefanacci JD, et al. Caudal occipital malformation syndrome in dogs. Compend Contin Educ Pract Vet 2004;26(11):886–95.

• deLaHunta and Glass. Veterinary neuroanatomy and clinical neurology. Saunders 2009.

• Tobias and Johnson. Veterinary surgery small animal. Elsevier 2012. • Lorenz, Coates, Kent. Handbook of veterinary neurology. Elsevier, 2011. • Watson AG, de Lahunta A: Atlantoaxial subluxation and absence of transverse

ligament of the atlas in a dog. J Am Vet Med Assoc 195:235, 1989. • Thomas WB, Sorjonen DC, Simpson ST: Surgical management of atlantoaxial

subluxation in 23 dogs. Vet Surg 20:409, 1991. • Schulz KS, Waldron DR, Fahie M: Application of ventral pins and

polymethylmethacrylate for the management of atlantoaxial instability: results in nine dogs. Vet Surg 26:317, 1997.

• Chambers JN, Betts CW, Oliver JE: The use of nonmetallic suture material for stabilisation of atlantoaxial subluxation. J Am Anim Hosp Assoc 13:602, 1977.

• Pujol E, Bouvy B, Omana M, et al: Use of the Kishigami Atlantoaxial Tension Band in eight toy breed dogs with atlantoaxial subluxation. Vet Surg 39:35, 2010.

• Middleton, Hillman, Trichel et al: Magnetic resonance imaging of the occipitoatlantoaxial region in the dog.. Vet Rdiol Ultrasound 53; 545, 2012

• Shores A, Tepper LC: A modified ventral approach to the atlantoaxial junction in the dog. Vet Surg 36:765, 2007

• Dewey DW, Marino DJ, Loughin CA: Craniocervical junction abnormalies in dogs. NZVJ 61: 202, 2013.

• Forterre F, Vizcaino-Reves N, Stahl C, et al: An indirect reduction technique for ventral stabilization of atlantoaxial instability in minature breed dogs. Vet Comp Orthop Traumatol 25: 332, 2012.

• Sánchez-Masian D, Luján-Feliu-Pascua A, Font C et al: Dorsal stabilization of atlantoaxial subluxation using non-absorbable sutures in toy breed dogs. Vet Comp Orthop Traumatol 27: 62, 2014.

• Parry AT, Upjohn MM, Schlegl K, et al: Computed tomography variations in morphology of the canine atlas in dogs with and without atlantoaxial subluxation. Vet Radiol Ultrasound 51: 596, 2010.

• Reber K, Bürki A, Vizcaino Reves N et al: Biomechanical Evaluation of the Stabilizing Function of the Atlantoaxial Ligaments Under Shear Loading: A Canine Cadaveric Study. Vet Surg 42: 918, 2013.

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