Flashcards in Internal Fixation of Spine Deck (134):
What are the three main functions of the Spine?
Protection of Neural Elements
The vertebra is the _______, a rigid object that pivots
What is the fulcrum of the spine, allowing the vertebra to move?
Disc and facets
What activates the spine, attaching to the vertebra via the spinous process or TP, pulling on the bone to create movement?
What are the spine's restraints, preventing the spine from exceeding its normal range of motion?
To tilt your head down, the muscles pull __ on the spinous processes in the back of your neck
What makes up a functional spinal unit (FSU) or motion segment?
Two adjacent vertebral bodies and the intervening soft tissue
What is the smallest unit of the spine that behaves biomechanically like the entire spine?
In the cervical spine, the facets are oriented in the ____ plane
Being oriented in the axial plane allows the cervical spine to ____, lateral bend, _____, and _______
Rotate, lateral bend, flexion, and extension
In the thoracic spine the facets are oriented in the ____ plane
In the lumbar spine, the facets are oriented in the _____ plane
Since the facets are in the sagittal plane, the lumbar spine can achieve ____ and ____ with some lateral bending but minimal ________
Flexion, extension, rotation
In a healthy spine, __% of compressive loads are transferred through the anterior column (VBs and discs)
The spine is located _______ in the torso
The _______ in the spine give it flexibility to bend and absorb shock
The vertebrae form a ____ _____ around the spinal cord and spinal nerves
During both trauma and normal, physiologic movement, the _______ play a large role in protecting these neural structures
The spinal ligaments connect vertebra to vertebra and prevent extreme motion that could stretch or damage _______
Ligaments are like _____ _____, they only create resistance when they are stretched beyond their resting length and provide no resistance to compression
Without the ligaments to hold the vertebrae in position, even gentle movements could cause damage to the _____
Define spinal instability
Loss of the ability of the spine under physiologic loads to maintain its pattern of displacement leading to increased risk of neurological deficit, deformity, or incapacitating pain
Two goals of spine surgery
Restoration of stability
The removal of any material causing undue pressure on the neural elements
Define direct decompression
Physically removing the tissue that is exerting pressure onto a neural element
Define indirect decompression
Involves increasing the space for the neural structures by increasing or restoring the height of the disc space
Define restoration of stability
The application of stabilizing implants to provide immediate mechanical stability and preserve neurological function, while allowing the patient to have an early return to function
The body's own bony fusion which will provide long-term stability to the spine
What are the 5 steps for plate and screw constructs
1. Size the plate
2. Position and provisionally stabilize the plate
3. Create a pathway for the screws
4. Insert the screws
5. Performing final tightening
What are two methods used to size a plate
What are two key instruments used in positioning and provisionally stabilizing a plate
Plate holding forceps, temporary fixation pins (ideally place in diagonal plate holes)
What are two main techniques for creating a screw pathway when plating?
Drill (requires both a drill and drill guide, a tap may also be necessary to cut path for thread depending on type of screw)
Awl which ensures starting point of screw is in center of the plate hole by creating an opening in cortical bone
When inserting a screw in a plate, what instrument is used?
A screwdriver with or without a holding sleeve depending on type of screw
What order should screw be inserted in a plate?
Alternating pattern to prevent tilting or lift off
What is an instrument that is occasionally used for final tightening a plate?
Torque limiting driver
What are the 4 steps for implanting a screw and rod construct?
1. Create a pathway
2. Insert the screw
3. Connect the screw to a rod
4. Perform final tightening
What are the two most common methods for creating a pathway for a screw in a screw and rod construct?
Drilling (with drill guide)
Probing (after perforation with awl)
When are holding sleeves especially important?
As incision get smaller with encroaching soft tissue
What are the 6 steps for an interbody fusion?
1. Access the disc
2. Remove the disc
3. Create an osteogenic environment
5. Prepare spacer
What is used to make an annulotomy?
What is used to separate disc material from endplate?
What is used to pull out disc tissue?
What is used to scrape disc material from the bone?
What is used to remove bony osteophytes?
What is used to create osteogenic environment (cartilage remove from endplates and blood flowing to region)?
A rasp or high speed burr
Which allograft type should be thawed before implantation?
Which allograft type should be rehydrated in saline prior to implantation?
What can happen if you don't thaw or rehydrate an allograft?
What are the 3 functions of a screw head?
1. Acts as stop to prevent screw from traveling entirely through bone
2. Holds or lags a plate securely against bone
3. Contains drive mechanism that engages with screwdriver
What is the function of the screw shaft?
When inserted into bone, threads prevent the screw from pulling out
What are the two functions of the screw tip?
1. Can cut pathway for screw
2. Guides the screw into bone
What are the two main functions of a screw thread?
1. Upon rotation, they exert force to insert a screw into bone
2. They increase the force required to remove a screw from the bone
The measurement of the force that is required to pull a screw from the material in which it is embedded
What are the two main factors that influence screw pullout strength?
1. Quality of bone
2. Thread design (more thread = better pullout strength)
The vertical distance between threads
What two things does pitch affect?
1. Speed of insertion
2. Pullout strength
The distance the screw travels through the bone per 360 degree rotation
Screws with a ______ pitch have more thread revolutions than the same length screw with a _______ pitch
Smaller pitch has more threads than greater pitch
The width of the shaft including the threads
The width of the shaft EXCLUDING the threads
Describe the thread design on a cortical screw
The threads on a cortical screw are short and flat to exert forces outward and increase compressive force between the screw and the bone
The goal for achieving purchase in _______ bone is to create compressive forces between the screw and bone
The goal for achieving purchase in ________ bone is to maximize the surface area of the screw in contact with the bone
A cortical screw's minor diameter is slightly ______ than the hole prepared for it
A cancellous screw's minor diameter is the _____ diameter as the minor diameter of the screw
Same size - a large core would simply push the cancellous bone away rather than creating compressive force
The threads on a cancellous screw are _______ and _______
These screws incorporate both cortical and cancellous screw attributes in the same screw (constant major diameter while minor transitions from wider at head to narrower at tip)
This screw has two threads traveling down the shaft of the screw simultaneously
A dual lead screw yields twice the insertion speed of a single lead without sacrificing what?
Partially threaded screws have two main advantages
Ability to lag
A type of screw used to pull two pieces of bone together
In a lag screw, the unthreaded portion is near the _____. As the threaded portion engages the distant fragment and advances, the threads of the screw pull the distant fragment toward the near fragment and the gap descreases
What prevents lagging in a plate construct?
Threads at plate to screw interface
If a screw is not fully inserted into bone, ______ threads can be a problem since close contact between soft tissue and ______ threads can irritate or damage soft tissues
A standard screw has a tip that requires ______ a pilot hole for the screw and then ________ the hole before inserting the screw
A self-tapping screw has a tip that requires ______ or _______ to insert the screw but not tapping
Drilling or probing
What are the two different self-tapping screw designs?
One design uses a flute cut into the tip of the screw to begin cutting the thread pattern
In another design the thread continues to the tip of the screw as the core tapers allowing the threads to bite in and begin to cut without a flute
A self-drilling screw has a tip that can be inserted directly into the bone without ____ or _____
Drilling or tapping
A self-drilling screw will always follow what?
The path of least resistance
Self-drilling screws have what type of tip?
A sharp, pointy tip
If trajectory or screw angle is important, what must be prepared for a self-drilling screw?
A hole using an awl
If a construct is too rigid, or ____ _____ it will bear all of the load and shield the fusion site from feeling force
When a construct shields the fusion site from feeling force, this is known as what?
Allowing the fusion site to feel load but protecting it from damaging forces
Define modulus of elasticity
A measure of the stiffness of a material or how much the material flexes when a force is applied
What is the ideal material for a spinal implant?
One in which the modulus of elasticity is close to that of bone so that the construct approximates a natural state for the body
The stiffer a material is, the more likely it is to be load _____
What are the two ways that a screw remains locked to a plate?
Locking screw rigidly connected to plate
Blocking screw prevented from backing out of plate by secondary blocking mechanism
A ________ screw is rigidly connected to a plate and the two implants interlock to prevent any movement at the screw-to-plate interface (maximizes pullout strength)
What are the two common locking screw designs?
Conical locking screw
A locking screw that is cone shaped and threaded (the plate hole is similarly threaded)
Conical locking screw
How does a conical locking screw work?
As the threads on the screw and plate engage, they pull the screw deeper into the plate and the conical shape of the screwhead creates a "wedge effect" increasing friction between the screw and plate
How does an expansionhead locking screw work?
The larger expansionhead screw is inserted through the plate and into the bone - the smaller locking screw is then inserted into the expansionhead screw. The locking screw expands the head of the expansionhead screw so that it interlocks with the plate and creates a rigid construct.
How does a blocking screw work?
A blocking screw is prevented from backing out of the plate by a secondary blocking mechanism however the blocking mechanism may not prevent motion at the plate to screw interface
How does a blocking clip work?
A clip embedded inside a plate expands and snaps into a groove in the screwhead when the screw is inserted through the plate. This prevents the screw from backing out of the plate.
________ and _______ screws provide greater resistance to pullout
Convergent and divergent
Why do convergent and divergent screws provide greater resistance to pullout?
All of the bone tissue between the screws and plate must be overcome
When screws are not locked to the plate, screw orientation does not have as much impact on pullout strength because the screws can ________
What are the three different thread profiles commonly found in locking caps
A V-shape is not ideal for locking caps for what two reasons?
Tend to exert force both outward in addition to downward
What does a buttress thread prevent?
The right triangle threads prevent the thread from exerting outward force that could splay the screwhead
What does a square thread provide more resistance to?
Cross-threading - they lack the sharp points that can cut into the opposing implant's metal and are more likely to follow machined path
What is more likely to exert outward force and cause splay, square or buttress thread?
What are three key considerations when selecting implant materials?
Effect on post-operative imaging
Why are metal implants not ideal for CT/X-Ray?
Can't assess fusion
Why are metal implants not ideal for MRI?
MRI uses powerful magnets that can cause movement or heating in metal implants
MR Safe implants are defined as implants that....
Pose no hazard
MR Condition implants are defined as implants that...
Pose no hazard under certain conditions
MR Unsafe implants are defined as implants that...
Pose a hazard
Why is it never acceptable to leave instrument or instrument fragment in patient?
May not be MR Safe or Conditional
What is an imaging concern with MRI?
Artifact causing signal loss (black spots) or image distortion around the implant making it difficult to asses the region surrounding for fusion or pathology
When bone grows in and around an implant providing additional fixation
An alloy of iron with corrosion resistant properties
SS modulus of elasticity is _______ than titanium
Implant grade _____ L stainless steel
Stainless steel may create a _____ _____ or _____ effect in MRI images
Signal void, starburst effect
Stainless steel does not osseointegrate
Stainless steel has been implanted in humans for over _ decades
Some patients may be allergic to ____ in stainless steel
3 types of titanium used
Alloyed with other metals
The modulus of elasticity of CP Ti is _____ than SS
What is the most common metal used in spinal implants?
CP Ti creates ____ interference in MRI imaging than SS
Titanium is exceedingly biocompatible, osseointegrates, and allergies are rare
Titanium alloys are similar to CP TI in terms of elasticity, imaging, biocomp but may have greater tensile strength (max amount of strain before breaking)
PEEK stands for...
PEEK modulus of elasticity falls between that of cortical and cancellous bone
PEEK does not osseointegrate and is instead...
Cobalt chromium have a ______ modulus of elasticity than that of SS
Ultra-high molecular weight polyethylene (UHMW) is a ______
PMMA stands for...
PMMA is a ____
PMMA modulus of elasticity is _____