Spinal Surgery- Principles Flashcards
Why do the effects of aspirin take 10 days to reverse?
Aspirin irreversibly blocks COX enzymes.
Plts have no nucleus and can thus not replace the enzymes.
The average life of a plt is 10/7, hence it takes 10/7 for the entire plt population to be replaced.
How quickly can normal haemostasis be demonstrated in the context of antiplatelets?
Normal haemostasis can be demonstrated with as few as 20% normal plts and some studies now suggest the effects of aspirin can wear off after 2-4/7.
Prone positioning for posterior cervical approaches?
Prone
Mayfield
Chin flexed and head lifted to maximise the space between the back of the shoulders and the occiput.
It is important to ensure the chin is not touching the operating table which will often be in a head-up position.
The arms are usually beside the patient.
Prone positioning for lumbar spinal surgery?
Maybe on a frame or a spinal operating table that flexes the patient’s lumbar spine.
Important to get the width of the supports correct to prevent abdominal pressure (which impairs venous return).
Some surgeons use the knee-elbow position though care must be taken to avoid nerve palsies.
Key considerations w.r.t. prone positioning?
Neck not overextended
Axillae not compressed.
No pressure on the eyes
No compression of external male genitalia.
Positioning for anterior cervical spine surgery?
Horseshoe
Rolled towel behind the shoulders to aid neck extension.
Pad the eyes.
Lateral positioning
Used for extreme lateral interbody fusion procedures
True lateral and AP views must be obtained to avoid parallax errors.
The surgical plane is perpendicular to sagittal which allows abdominal contents to fall forwards.
Neural monitoring, especially for the L4/5 level, is recommended to avoid injury to lumbar plexus
Contraindications to extreme lateral interbody fusion approach
Anomalous vascular anatomy
Peritoneal scarring
Spondylolisthesis greater than grade II
What approach to interbody fusion provides better access to L5/S1?
Anterior as the XLIF approach is limited by the iliac crests
Complications of ALIF
Vascular injury
Visceral damage
Retrograde ejaculation
Sympathetic disruption
Def: parallax error
The error/displacement caused in the apparent position of the object due to the viewing angle that is other than the angle that is perpendicular to the object.
Minimised by keeping needles close to the structure being marked and that the XR machine is correctly positioned.
SSEPs
Sensory stimulus over a nerve measured with percutaneous electrodes over the sensory cortex
MEPs
Measured from a muscle after a stimulus is given to the motor cortex
SSEP threshold suggestive of dorsal column insult intra-operatively
10% increase in latency from baseline or a 50% reduction in amplitude.
MEP threshold suggestive of anterior corticospinal tract injury
Any significant change in waveform considered significant SC insult as motor potentials are considered “all or nothing” by many neurophysiologists
Fusion rates for ACDF
Around 90%
What must happen for metalwork to succeed in spinal surgery
Bony fusion.
Bony fusion in traumatic SC surgery vs MSCC
More likely to occur in trauma, therefore in MSCC the construct must be longer.
Factors promoting bony fusion
Decortication of bone surfaces
Adding autograft (iliac crest)
Allogenic cancellous or decorticated bone
Factors inhibiting bone fusion
NSAIDs odds ratio of 3.0 for non-union
Smoking also inhibits
Recombinant bone morphogenic protein 2
Promotes osteoblast differentiation and is effective in promoting spinal fusion.
Safety concerns include include implant displacement, subsidence, infection, urogenital events, retrograde ejaculation, radiculitis, ectopic bone fromation, osteolysis and poorer global outcomes.
Use of internal fixation in spinal surgery?
Correction of deformity:
Kyphosis, scoliosis
Stabilisation:
Fracture, dislocations, malignant and degenerative pathology
General principle of screw-rod fixation systems
Utilise pedicles and lateral masses as fixation points
Allows instrumentation of the sacrum
Parts of the screw
The section that contains the thread
Head
Variation of different types of screws is based on the type of head, the type of interface between the screw head and shaft, whether the shaft is fully or partially threaded and if the shaft is fenestrated.
What are the two types of interface between the screw head and shaft?
Monoaxial
Polyaxial
Monoaxial screws
Do not allow movement between the screw head and the shaft
Use of monoaxial screws
For deformity correction
Polyaxial screws
Joint with a spherical head enclosed in a housing which allows the screw head to move in relation to the shaft
Allow the surgeon some flexibility in connecting the screws to rods.
Fail at the head-shaft interface rather than along the shaft or rod
Subdivisions of polyaxial screws
Uniplanar- allow movement in the cranial or caudal plane only
Multiplanar- allow movement in multiple planes
Subdivisions of multiplanar polyaxial screws
Biased- allow angulation up to 55 degrees in one direction
Non-biased- allow up to 30 degrees of angulation in each direction
Types of shaft design
Fully threaded
Smooth shank screws
Features of smooth shank screws
10mm unthreaded segment and are used in the C1 lateral mass where the smooth shank is designed to prevent the thread irritating the C2 nerve root.
Lag screws
Unthreaded segment and a distal half that is threaded
Used in odontoid peg fixation where the thread sits in the peg and the shaft in the C2 vertebra
Tightening the screw compresses the fracture thus improving bony fusion.
Lag screw sits flush
Features of fenestrated screws
A hole down the middle which enables K-wire insertion to guide screw placement or the insertion of cement down the screw
Def: pitch of a screw
Distance travelled by the screw in one 360 degree turn
Fine pitched screw travels a short distance, used in cortical bone and generally has a higher pull out strength
Coarse screw travels further with each turn, requires less force to insert and is used in cancellous bone.
Rescue screws
Can be used where a screw has a poor hold.
Usually has a wider diameter and coarser pitch.
Types of screw tips
Rounded- need tapping to start the screw off
Self-tapping- has cutting flutes at the tip but still requires a pilot hole
Self-drilling- sharp and not requiring pilot hole.
Types of plates
Locking or non-locking/dynamic plates
Locked plates
The screw is held in the plate so that the angle of the screw to the plate is fixed
Dynamic plates
Allow some movement as the fracture settles
Forces resisted by screw/plate or screw/rod vs cages
Scfew/plate- distraction
Cage- compression
Advantages of minimal access surgery
Less pain
Less analgesia requirements
Shorter duration of stay
Disadvantages of minimally invasive spine
May be more difficult
Risk of incorrect port placement
More XR exposure
Equipment is often extensive
Prevention of infection in spinal surgery
Razer rather clippers for hair removal
Laminar flow theatres (no RCTs)
Gent scrub
Double gloving
Prophylactic antibiotics.
VTE and spinal surgery
Early mobilsation, good hydration, compression stockings.
LMWH
Incidence of post-op compressive epidural haematoma
0-1%
Component of the vertebral column
24 mobile vertebrae
C7
T12
L5
Fused vertebrae
S4-5 + C4
What limits movement of mobile vertebrae?
Intervertebral discs
Paired posterior synovial facet joints
Intervertebral ligaments
Paraspinal muscles
Diurnal variation in length of vertebral column
2cn
Age-related loss of vertebral column length
Related to dehydration fo the inrevertebral dsiscs
Primary roles of the vertebral column
Protect the spinal cord
Provide stability and mobility
To control the transmittance of movement from the upper and lower extremities
Curves of the spine
Cervical lordosis
Thoracic kyphosis
Lumbar lordosis
Sacral kyphosis
Which part of the vertebral column is responsible for the majority of the load transfer fo the spine
Anterior column (vertebral bodies and intervertebral discs)
Typical articulation between two prevertebral vertebrae
Two posterior intervertebral facet joints between the inferior and superior articulating facets and the intervertebral disc.
This creates three points of articulation between each vertebra
The trend in width and depth of vertebra
They increase from cranial to caudal, corresponds with increased compressive strength due to the greater axial loads experienced in the more caudal segments.
Microscopic arrangement of the vertebral body
Outer cortical layer
Inner cancellous bone with the microarchitecture adapted so that the trabeculae are orientated along the lines of application of load to provide resistance to dynamic loading.
How is the C1 vertebra atypical?
No vertebral body
Anterior arch with odontoid peg directly behind it
Flattened posterior arch with a small midline posterior tubercle
The articular processes are medially orientated to permit both flexion/extension and rotational movements
Features of C2
Odontoid peg- a superior bony extension of the vertebral body
Has the largest spinous process in the spine.
Subaxial vertebrae?
C3-7
Uniform morphology
In this region, the uncinate processes on the superior anterolateral aspects of the vertebral bodies articulate with a corresponding articulation of the vertebral body above. This uncovertebral joints aids rotation in the cervical spine
Features of thoracic vertebrae
Uniform morphology with a long inferiorly angulated spinous process and vertically orientated facet joints
There are costovertebral joints from T1 to T12 that provide articulation between the vertebra and corresponding rib.
There are also rib articulations on the transverse processes from T1 to T10.
In this region, the ribs, sternum and intercostal structures provide extra stability.
Orientation of lumbar facet joints
Sagittal plane
Which cervical vertebra does not have a bifid spinous process?
C7
Structure of intervertebral disc
Found between the inferior and superior endplates of the vertebrae above and below.
Has a central nucleolus pulposus with a surrounding annulus fibrosis.
Acts to transmit load, allow movement, provide stability
Composition of the nucleus of the intervertebral disc
Comprised of mostly water (80-90%)
Type 2 collagen and multiple hydrophilic proteoglycans.
The interaction between the annulus fibrosis and these proteoglycans leads to a unique hydrostatic structure.
When the disc is loaded in compression water seeps out and the height of the disc is reduced, but the turgidity of the nucleus allows the transmission of the force to the obliquely orientated fibres in the multiple lamellae of the annulus.
Which fibres connect the annulus fibrosis to the cortical bone of the vertebral body?
Sharpey’s fibres
What happens to intervertebral discs with age?
The ability to resist axial compression.
The proteoglycan composition of the nucleus changes with a decrease in water retention.
There is a loss of disc height
Disc bulbing is often seen.
Asymmetric loading of the disc in a pathological curve of the spine leads to migration of the nucleus pulposus to the convex side causing disc bulging (not herniation).
Posterior elements of the spinal cord
Laminae
Spinous processes
Bilateral facet joints
Zygapophyseal joints=
Facet joints
Features of spinal facet joints
Synovial plane joints between the superior and inferior articular processes of the lumbar vertebrae
In the cervical cord, they are orientated coronally allowing flexion-extension, lateral bending and rotation.
In the lumbar spine, they are orientated sagittally, resisting rotational movement but allowing flexion/extension.
Why is there a lower incidence of anterior subluxation at L5/S1?
The facet joints have a more coronal orientation.
Degenerative spondylolisthesis is thus more common at L4/5.
Where is the spinal canal most narrow?
T7
What structures can contribute to stenosis of the spinal canal?
Bulging of the annulus due to dehydration
Overgrowth of facet joints due to OA
Folding in of the ligamentum flavum due to disc height
Causing the classical “Trefoil” appearance of the spinal canal.
What connects the anterior and posterior elements of the vertebral canal?
Pedicles
Features of the pedicles
Signficant variation in pedicle size and orientation seen through the spine.
Smallest at T4
Ligaments that provide stability to the spine
Anterior and posterior longitudinal ligaments
Interspinous ligaments
Ligamentum flavum
Intertransverse ligaments
Capsular ligaments
Movement resisted by interspinous lkigaments
Flexion
Movement resisted by anterior longitudinal ligament?
Strong attachment to the vertebral bodies but not the intervertebral discs.
Rists extnesion
Attachments of the ALL
Skull base to sacrum
Posterior longitudinal ligament
Clivus to coccyx
Provides limited resistance to flexion.
Made up of two layers
Attached to the intervertebral discs.
Constrains herniation of the nucleus pulposus
Extent of the ligamentum flavum?
C2 to S1
Close to the axis of rotation.
Why is the ligamentum flavum yellow?
Due to its high elastin content
What is the significance of the relatively high elastin content of the ligamentum flavum?
Prevents the ligament buckling in extension and narrowing the spinal canal diameter
What is the tectorial ligament?
Cranial extension of the PLL
Where is the ligamentum flavum deficient?
In the midline
What are the capsular ligaments?
Capsule ligament enclosing the spinal facet joints,
What are the components of the cruciate ligament?
It consists of the transverse ligament of the atlas, along with additional fibres above and below.These fibres are also known as “longitudinal bands
What is the importance of the transverse ligament of atlas?
Significant role in prevent C1/2 subluxation
Apical ligament of axis?
Runs from the tip of the odontoid peg to the skull base
Alar ligament
Run from the sides of the odontoid peg to the skull base
Ligamentous structures stabilising the cranio-cervical junction?
Cruciate ligament (transverse ligament and superior and inferior longitudinal bands)
Apical ligament
Alar ligaments
Accessory part of tectrorial membrane
Atlanto-occipital membrane
Causes of congenital variation in spinal anatomy
Failure of segmentation or formation.
Scoliosis results from what?
Three dimensional deformity.
Klippel-Feil Syndrome
Triad
Low posterior hairline
Short “webbed” neck
Limited cervical range of movement
Associated renal/congenital cardiac disease/ brainstem abnormalities/ cerbical scoliosis/ Sprengel’s deformity
Sprengel’s deformity
Sprengel’s deformity (also known as high scapula or congenital high scapula) is a rare congenital skeletal abnormality where a person has one shoulder blade that sits higher on the back than the other.
Pathophysiology of Klippel-Feil syndrome
Failure of formation or segementation of the cervical somites at 3-8/40.
Categorisation of function othe spine
Static (protection of SC)
Dynamic (provision of stability and mobility, to control transmittance of movement to the upper and lower extremities)
Advice to patients with Klippel-Feil
Aim for conservative Mx unless spinal instability or neurological deterioration
Avoid collision sports
Frequently develop degenerative cervical conditions
Normal flexion/extension at C0/C1
15 degrees
Normal flexion/extension at C1/2
10 degrees
Normal flexion extension in the subaxial C-spine
7 degrees in the upper
20 in the lower.
Flexion-extension in lumbar spine
Increases caudally up to 20 degrees at L5/S1
What prevents flexion in the thoracic spine?
Splinting by the ribs
Rotation in the ccervical spine
45 degrees at C1/2
Between 7 and 10 degrees at each level.
No rotation at Co
At which level of the spine is there no lateral flexion
C1/2
What is the functional spinal unit?
Two adjacent vertebrae with the intervening disc and associated ligaments.
It is the smallest anatomical spinal segment that can be considered to exhibit the biomechanical characteristics of the whole spine.
Hooke’s law
For small displacements, the size of deformation is proportional to the force applied.
What is the zone of plastic deformation
The point beyond the elastic limit at which a material totally recovers when a stress is applied.
Coupled movements in the spine
Often coupled so that movements along one axis leads to a movement along another.
This is mostly due to orientation of the facet joints e..g in the cervical spine where lateral bending results in rotation of the spinal processes away from the concavity of the curve.
Def: Clinical stability of the spine
Described by White and Panjabi
`the ability of the spine under physiological loads to limit patterns of displacement so as not to damage or irritate the spinal cord or nerve roots and in addition, to prevent incapacitating deformity or pain due to structural changes.
