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.
Curves of the spine
Cervical lordosis
Thoracic kyphosis
Lumbar lordosis
Sacral kyphosis
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
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.
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 is the tectorial ligament?
Cranial extension of the PLL
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
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.
