Internal Fixation of Spine Updated Flashcards
What are the three main functions of the Spine?
Motion
Weight Transfer
Protection of Neural Elements
In the cervical spine, the facets are oriented in the ____ plane
Axial
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
Frontal
In the lumbar spine, the facets are oriented in the _____ plane
Sagittal
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)
80%
Define decompression
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
Define arthrodesis
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
- Size the plate
- Position and provisionally stabilize the plate
- Create a pathway for the screws
- Insert the screws
- Performing final tightening
What are two methods used to size a plate
Calipers, templates
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?
- Create a pathway
- Insert the screw
- Connect the screw to a rod
- Perform final tightening
What are the 6 steps for an interbody fusion?
- Access the disc
- Remove the disc
- Create an osteogenic environment
- Trial
- Prepare spacer
- Implantation
What is used to scrape disc material from the bone?
Curette
Which allograft type should be thawed before implantation?
Frozen allograft
Which allograft type should be rehydrated in saline prior to implantation?
Freeze dried
What can happen if you don’t thaw or rehydrate an allograft?
Fracture
What are the 3 functions of a screw head?
- Acts as stop to prevent screw from traveling entirely through bone
- Holds or lags a plate securely against bone
- 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?
- Can cut pathway for screw
2. Guides the screw into bone
What are the two main functions of a screw thread?
- Upon rotation, they exert force to insert a screw into bone
- 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
Pullout strength
What are the two main factors that influence screw pullout strength?
- Quality of bone
2. Thread design (more thread = better pullout strength)
The vertical distance between threads
Pitch
What two things does pitch affect?
- Speed of insertion
2. Pullout strength
The distance the screw travels through the bone per 360 degree rotation
Pitch
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
Major diameter
The width of the shaft EXCLUDING the threads
Minor diameter
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
Cortical
The goal for achieving purchase in ________ bone is to maximize the surface area of the screw in contact with the bone
Cancellous
A cortical screw’s minor diameter is slightly ______ than the hole prepared for it
Larger
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 _______
Larger, sharp
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)
Dual core
This screw has two threads traveling down the shaft of the screw simultaneously
Dual lead
A dual lead screw yields twice the insertion speed of a single lead without sacrificing what?
Pullout strength
Partially threaded screws have two main advantages
Ability to lag
Tissue protection
A type of screw used to pull two pieces of bone together
Lag screw
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
Head
What prevents lagging in a plate construct?
Threads at plate to screw interface
A standard screw has a tip that requires ______ a pilot hole for the screw and then ________ the hole before inserting the screw
Drilling, tapping
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
Load bearing
When a construct shields the fusion site from feeling force, this is known as what?
Stress shielding
Allowing the fusion site to feel load but protecting it from damaging forces
Load sharing
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 _____
Bearing
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)
Locking
________ 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
What are the three different thread profiles commonly found in locking caps
V-Thread
Buttress Thread
Square Thread
A V-shape is not ideal for locking caps for what two reasons?
Easy crossthreading
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?
Buttress thread
What are three key considerations when selecting implant materials?
Elasticity
Effect on post-operative imaging
Biocompatibility
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
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
Osseointegration
An alloy of iron with corrosion resistant properties
Stainless steel
SS modulus of elasticity is _______ than titanium
Higher
Stainless steel may create a _____ _____ or _____ effect in MRI images
Signal void, starburst effect
Stainless steel does not osseointegrate
True
Some patients may be allergic to ____ in stainless steel
Nickel
3 types of titanium used
Unalloyed
Commercially Pure
Alloyed with other metals
What is the most common metal used in spinal implants?
CP Titanium
CP Ti creates ____ interference in MRI imaging than SS
Less
Titanium is exceedingly biocompatible, osseointegrates, and allergies are rare
True
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)
True
PEEK stands for…
Polyetheretherketone
PEEK modulus of elasticity falls between that of cortical and cancellous bone
True
PEEK does not osseointegrate and is instead…
Hydrophobic
Cobalt chromium have a ______ modulus of elasticity than that of SS
Higher
