SA MS 2: Fracture Biomechanics And Classification Flashcards
Bones
Subjected to many forces (which are often combined) during normal function
When magnitude of these forces exceeds ultimate strength of the bone, fracture will occur
Forces that cause fractures
Bending, torsion, compression, tension
Same forces must be neutralized when repairing fracture
Geometry of the fracture and the location of the muscle groups attached to the fragment involved will determine which forces predominate in the fractured bone
Bending
Bending force acts to move the ends of the bone out of line with the bone’s long axis
—all fractures have some tendency to bend
Torsion
Tendency for a bone or its pieces to twist around the bone’s long axis
Produces rotation of the fragments
Compression
Acts along the long axis of the bone to move the ends of the bone toward each other
Often causes fragments to override
Shearing
Component of compression
Acts within bone or bone fragments
Essentially tendency of two pieces to slide past each other
Tension
Acts along longitudinal axis of the bone but pulls ends away from each other, producing distraction
- -negative form of compression
- -Causes avulsion fractures of the apophyses like the olecranon and the calcaneus
Shape of the bone
Has some influence on the fracture pattern that will result when excessive force is applied
Fracture pattern more dependent on the orientation of the forces that caused the fracture and the relative strength of the bone in each loading orientation
Compressive forces result in?
OBLIQUE FRACTURES!
Tensile forces result in?
TRANSVERSE FRACTURES
Why do compressive forces result in oblique fractures?
Bone, as as result of osteonal and collagen fiber arrangement, = dramatically weaker in shear than in compression
When loaded in compression, bone fails along lines of highest shear stress rather than compressive stress - lines tend to be at an angle of 30-45 degrees to the direction of the compressive force
Why do tensile forces result in transverse fractures?
Transverse fractures = approx perpendicular to the direction of loading
Fractures resulting from pure tensile forces = rare (most w/ bending)
–influenced by bone shape
–location and shape of physis in immature bone
Bending forces result in what type of fracture?
TRANSVERSE FRACTURES
Why do bending forces result in transverse fractures?
When bone subjected to bending force, convex side = under tension, concave side under compression
–Neutral axis lies somewhere toward the center of the bone
Because bone weaker in tension than compression, fracture begins on the tension surface and propagates toward the compressive surface
Bones loaded in bending and compression
Suffer combo of bending and compressive patterns
Often as the crack propagates from the tension to the compressive surface, it splits and deviates proximally and distally along the shear stress line, creating a butterfly fragment on the compressive side
Torsional forces result in which types of fractures?
SPIRAL FRACTURES!
How do torsional forces cause spiral fractures?
Initiated by the formation of a crack along the long axis of the bone
Crack then spirals - starts at one end of the longitudinal crack, through the bone along the 45 degree shear stress planes, until winds up back at the originating longitudinal crack, completing spiral with longitudinal fracture segment
Loading rate
Influences appearance and severity of fracture patterns
–ie whether a fracture is simple or highly comminuted depends not just on the magnitude of the specific force but also how fast the force is applied
Bone as viscoelastic material
Strength of bone depends on the rate at which it is loaded
- -bone = stronger when loaded rapidly than when loaded slowly
- -if force applied slowly, less of it can be absorbed before the bone deforms and breaks
- -if force applied rapidly, more energy is absorbed before the bone finally breaks but when it does break, it shatters because has absorbed so much energy
Highly comminuted fractures
Product of relatively more energy absorption/faster loading than simple fractures are
Tend to be associated with a lot more concomitant ST damage
How classify fractures
-extent of ST injury
-degree of cortical disruption
-geometry
-location within the bone
-degree and direction of displacement
+/- underlying cause
ST disruption: closed
No wound connecting the bone to the outside world
ST disruption: open
There IS a wound connecting the bone to the outside world
- four types
ST disruption: open Type I
Open fracture
Small laceration (<1cm)
Clean
ST disruption: open Type II
Open fracture
Larger laceration (>1cm)
Mild ST trauma
No flaps/avulsions
ST disruption: open Type IIIa
Open fracture
Vast ST laceration or flaps or high energy trauma
ST available for wound coverage
ST disruption: open Type IIIb
Open fracture
Extensive ST injury loss
Bone exposure present
Periosteum stripped away from bone
ST disruption: open Type IIIc
Open fracture
Arterial supply to the distal limb damaged
+/- arterial repair required for limb to salvage
Extent of Cortical Disruption
- Greenstick
- Fissure
- Complete
- Depressed
Extent of Cortical Disruption: greenstick
Bending fracture
Cortex doesn’t break all the way through but the bone deforms relative to its longitudinal axis
–usually a fracture of young animals
Extent of Cortical Disruption: Fissure
Crack
Only involves one side of he bone (one cortex) and it’s usually longitudinally oriented
Extent of Cortical Disruption: Complete
Both cortices disrupted
Fracture goes all the way through the bone resulting in 2 separate fragments
Extent of Cortical Disruption: depressed
When a fracture segment is displaced into the cavity it once formed a wall of - sinuses, skulls
Geometry of fracture lines: transverse
Fracture perpendicular to the long axis of the bone
Rotation of fragments = big problem with this one
Geometry of fracture lines: oblique
Fracture occurs at an angle greater than 30 degrees to the long axis of the bone
–can be short or long
–long oblique fracture = one in which the length of the fracture line equals or exceeds twice the diameter of the bone at this point
Compression: major issue with this fracture type (fragments shear past each other)
Geometry of fracture lines: spiral
Fracture line curves around the diaphysis
Acts a lot like an oblique fracture
Geometry of fracture lines: comminuted
More than 2 pieces Fractures have zero inherent stability Types: --butterfly --highly comminuted --segmental --multiple
Comminuted: butterfly
Aka wedge –> wedge shaped chunk of bone broken off but the proximal and distal fragments still touch each other at some point
Highly comminuted
Bunch of pieces
Ex of a complex fracture
Pieces should share a common fracture line
Comminuted: segmental
Section interposed between the most proximal and the most distal fragments so they can’t touch
Pieces that do not share a common fracture line
Comminuted: multiple
Segmental fracture in which the interposed segment is in more than one piece
Ex of a complex fracture
Location within the bone: articular
Fracture involving joint surface
Location within the bone: physeal
Fracture involving the growth plate
6 types
Location within the bone: physeal SH Type I
Fractures run through physis
Location within the bone: physeal SH Type II
Fractures run through physis and portion of metaphysis
Location within the bone: physeal SH Type III
Fractures run through physis and epiphysis
Generally IA
Location within the bone: physeal SH Type IV
IA, run through epiphysis, across the physis, and through the metaphysis
Location within the bone: physeal SH Type V
Crushing of physis which is not visible radiographically but is evident several weeks later when physeal growth closes
Location within the bone: physeal SH Type VI
Partial physeal closure resulting from damage to a portion of the physis
Location within the bone: metaphyseal
Fracture involving the metaphysis
Location within the bone: diaphyseal
Usually will specify whether fracture in the proximal, middle or distal third of the diaphysis
Location within the bone: condylar, supracondylar, subtrochanteric
If there’s a part of the bone with a cool anatomic name, you correctly remember it, and the fracture involves it or is near to it, then show off a little
Displacement: non-displaced
Bone fractured but pieces flying close in formation in their proper alignment
Suggests there is still an intact periosteal sleeve holding them together
Displaced
Pieces not in usual alignment
Displacement described by referring to the distal fragment’s location relative to the proximal fragment
–Ex: if the distal piece is caudal and lateral to the proximal fragment, then you say the fracture has a CD-L displacement
Causes
- Traumatic
- Pathologic
- Fatigue
- Iatrogenic
Cause: Traumatic
Force may either be direct (blow from outside) or indirect (w/ bone fixed or planted in a certain position, such as when an animal’s own muscles break the bone)
Cause Pathologic
Something wrong with the bone that weakens it to the point that otherwise normal, ordinary forces placed upon it will cause it to fracture
- -osteopenia in humans
- -neoplasia in animals
Cause: Fatigue
Caused by repetition of forces that wouldn’t be sufficient too break the bone in a single application - racing greyhounds
Iatrogenic
Create these by using screws that are a little too large when fix another fracture
Why care about fracture classification and forces?
Type and location of fracture has very strong influence on how easily it will heal, how likely it is that nasty complications like IFX, quadriceps tie-down and OA will appear and what kind of repair is appropriate (or possible) to perform
–Goal: choose form of fracture fixation that effectively neutralizes forces that will be acting on your fracture once it has been reduced
Casts and splints
Good for controlling bending
Help control rotation if fit well
Wires
Help control tension (while producing compression themselves) but terrible at controlling all the other forces - should almost never be used by themselves
Intramedullary pins
Control bending
Three repair mechanisms that control all of the forces?
- Bone plates
- Interlocking nails
- External fixators
Bone plates, external fixators, interlocking nails
Pretty good at controlling all of the forces providing you’ve chosen hardware that’s strong enough and in proper placement and configuration
External Coapation: def
Immobilization of a body part with externally applied support
Ex: casts, splints, braces, bandages
External Coapation: Forces neutralized
Bending forces - depends on how rigid a form of compaction you’ve chosen
Torsion - fair, depending on how form-fitting coapation is
DOES NOT CONTROL COMPRESSION AND TENSION
External Coapation: +
Minimal disruption of blood supply (assuming applied correctly)
Minimal effect on physeal growth
Initial application often cheaper than sx repair
–frequent bandage changes and rechecked may make total cost comparable to sx
External Coapation: -
Poor control over compressive and distractive forces
Less rigid stabilization than internal or external fixation
Alignment and reduction may be difficult to achieve in a closed fashion - generally only obtainable with incomplete or transverse fractures
Joints above and below must be immobilized
–jt immobilization can lead to jt stiffening and arthritic changes
Internal Fixation
Fracture repair by means of stabilization apparatus that is somehow directly attached to the bone --Can provide more rigid stabilization than external coaptation does Ex: 1. IM pins 2. Cross pins 3. Cerclage wire 4. Tension bandage wiring 5. Locked IM nail 6. Plates and screws 7. External skeletal fixator
Intramedullary (IM) Pins
Rods that stabilize broken bones by passing longitudinally within medullary canal, providing support very close to the neutral axis of the bone
Intramedullary (IM) Pins: forces neutralized
BENDING ONLY
Intramedullary (IM) Pins: +
Availability:
–cheap, readily available, don’t req fancy equip to place
Axial alignment:
–fracture fragments skewered along linear rod, good axial alignment easy to achieve
Bending control
Intramedullary Pins: -
Only control bending
Anatomic repair req’d
Potential injury to surrounding structures from pin placement/migration
Inadequate for repair of certain bones - flat bones, radius*
Cross Pins
Similar to IM pins
Placed so that they cross the two cortices as well as the fx line
Cross Pins: forces neutralized
Mod effective for bending
Better than IM pins for torsion, compression
When appropriate?
Most freq used for repairing physeal or very distal/prox physeal fractures
Cerclage Wire
Added to other types of repair to:
- -prevent propagation of fissure
- -reconstruct fragments to aid in fx reduction
- -hold fx in reduction while definitive repair applied
Cerlage wire - forces neutralized
Help control torsion/compression when main form of repair is an IM pin
do NOT provide much bending control
Tension Band Wiring
Involves placing 2 small, parallel pins perpendicular to fx line
Figure of 8 wire then passed through hole distal to fx, around the protruding ends of pin, and tightened
Tension Band Wiring - Forces neutralized
Counteracts tensile forces
Converts them to compressive forces at a transverse fx site to favor direct bone union
Locked IM Nail
Special rod placed within medullary canal –> fixed with screws that pass from outside the bone, through holes in the rod, and back out thru cortex of bone
Locked IM Nail - Forces Neutralized
Very strong countering bending forces
Screws = quite strong against compression, tension, torsion
–prevent nail from migration
Locked IM Nail +
Fixing comminuted fx w/o req anatomic reconstruct
Moderately less disruptive of blood supply
Not usually removed
Locked IM Nail -
Used almost exclusively at femur, humerus, tibia
Cannot be applied to radis
Plates and Screws
Bone plates = screwed directly onto the fx’d bone so that the screws transfer the forces of WB from the bone to the plate by increasing friction btw the 2
- -better the friction btw plate and bone, stronger plate/bone/screw construct is
- -Most plates need to be closely contoured to fit the bone
Locking Plates
When screws not only lock into the bone but also the plate
Little internal external fixators
Very strong
Don’t need to be closely contoured to the bone –> can be less invasive
Plates, Screws Forces Neutralized
ALL!
Screws alone control torsion and compression or tension
Very strong fx repair = IM pin + plate
Screws, Plates +
Anatomic reconstruct not necessary
Plate sits directly against bone surface –> entirely buried, can be used in areas w/ lots of overlying muscle
Very little intervention by o - usually do not req removal
Screws, Plates -
Req most bone exposure –> most disruptive to the blood supply
Screw placement dictated by location of preformed holes in the plate = difficult or impossible to capture very short prox or distal fragments w/ minimum # of screws necessary for stability
If IFX - plate must be removed once bone is healed.
External Skeletal Fixator
Splints a bone by passing fixation pins from outside of the body and through both cortices of the bone in a more or less transverse fashion
–pins then attached to at least 1 external connecting bar
Can be combined w/ IM pin –> aids in alignment of fragments, controls bending
External Skeletal Fixator Forces Neutralized
ALL!
Main Goal for Tx of Any Fx
Early return of patient to full fxn
Fx Fixation Should
- Restore limb alignment
2. Stabilize fracture
What should you consider when planning fx repair?
- Age of P
- Wgt
- Concurrent injuries
- Overall health of animal
- expected activity level/intended use of the animal
- Ability of owner to provide post-op care
Reconstruction
Permits load sharing btw implants and the bone
Protects implants from fatigue and early failure
Biologic Environment
Young animals w/ active periosteum and metaphyseal fx - quick to heal in most situations
Comminuted high energy fractures may have impaired vascularity –> longer healing times anticipated
Geriatric or debilitated animals/animals with substantial ST injury will experience prolonged healing times = stable implants for longer period of time
Indications for Anatomic Reconstruction
- Articular fx
- Simple fx
- Fx w/ 2 or 3 lrg segments
Indications for Major Segment Alignment
Severely comminuted fx: tx w/ plates, plate-rod combos, locked IM nails, or external fixators
Open Reduction
Allows for bone grafting and anatomical reconstruction of articular and comminuted fx
- -prolongs sx time
- -impairs blood supply
Closed Reduction
Utilizes indirect reduction
–alignment of fragments by distraction of bone ends
Preserves blood supply/biology of fx
–Comes at cost of fx alignment
Best for minimally displaced or incomplete fx or for comminuted fx tx’d w/ external fixation
Indications for Open Reduction
- Articular fx
- Simple displaced fx
- Communities fx tx’d by major segment alignment and cancellous bone grafting
Indications for Closed Reduction
- Non-displaced or incomplete fx
2. Comminuted fx tx’d w/ external fixation
Fx Planning Checklist
Decide on appropriate fixation based on fx and patient assessment Plan fx reduction Plan fx fixation Select sx approach(es) Check implant inventory Perform dx Critically eval post-op RADS
