37/38: Internal Fixation - Frush Flashcards
4 AO principles
- Anatomical reduction of fx fragments
- stable internal fixation designed to fulfill local biomechanical demands
- Preservation of blood supply to bone fragments and soft tissue by means of atraumatic surgical technique
- Early active pain-free mobilization of m and jt adjacent to fx, to prevent cast dz
review indirect v. direct osseous repair of bone healing
Indirect osseous repair
- Inflammation (1-7 days)
- Soft Callous (~3 weeks)
- Hard Callous (3-4 months)
- Remodeling (months-years)
Direct osseous repair
- Bypasses callous formation
- “cutting cones” form at areas of direct contact
- Gap healing
- Deposition of lamellar bone at 90 degrees to fracture
absolute vs. relative stability - what tools used?
Absolute = no motion at fx site
- use compression plates of screws
- ideal for articular fx
- needs less than 2% strain
Relative stability = varying defrees of motion
- use IM nailing, ex fix, locking plates
- holds fx fragments in place but will likely heal with callous
- needs 2-20% strain
what is strain? how do you decrease strain?
Strain = Deformation of a material when a given force is applied
- Relative change in fx gap divided by fx gap
- Strain decreased by increased gap length and decreased motion
Review Strain Model Movie ***
add notes here
key characteristics of ideal implant material (metallurgy)
- biocompatibility
- strength
- resistance to degradation and erosion
- ease of integration
- minimal adverse effects of imaging
titanium v. stainless steel
Stainless steel
- Corrosion products (Ni, Cr, Mo)
- Pain
- Inflammation
- Allergic rx (nickle allergy very common)
Titanium
- Possible foreign body rx –> osteolysis
- Not as hard as stainless steel
least dense surgically impantable metal
titanium
- used for pt with nickel allergy
material of choice for joint implants
cobalt-chromium
screw anatomy
what part of screw broken here?
runout
cortical vs. cancellous surgical screws
(image of cortical)
Cortical
- Smaller pitch for grasping cortical bone
- 1.25 mm thread pitch
- Fully threaded
Cancellous
- Larger pitch, 1.75 mm
- Thin core, deep threads
- Different tip
- Fully or partially threaded
(picture of cancellous)
what screw do you use for compression?
Lag screw by screw (overdrill, underdrill, coutnersink, measure, tap) or by design
glide hole vs. overdrill
synonymous with each other
- larger drill that usually matches screw outer diameter
smaller drill that matches screw core diameter
glide hole, thread hole and underdrill
instruments needed for lag technique
steps of insertion for lag screw by techinique
(video in lecture)
- glide hole (larger hole)
- thread hole (smaller hole)
- countersink
- depth gauge
- tap
- insertion
why is a guide hole necessary?
no guide hole near fragment –> screw distracts fracture instead of compressing it
steps of insertion lag by design
- thread hole (small hole only*)
- countersink
- depth gauge
- tap
- insertion
- compression
what depth is appropriate for by technique and by design?
By technique: Need to insert 1mm past far cortex to increase “pullout” strength
By design: Do not want to pierce far cortex because they are cancellous screws
charnley’s fx classification
stable = transverse
unstable = oblique, spiral, comminuted
potentially stable = short oblique
what angle do you place an interfragmentary screw?
- perpendicular to bone: max resistance to shear
- perpendicular to fragemnt: max inter-frag compression
self-tapping v. non-self tapping screw types
Self Tapping
- Cut own thread path
- Fluted tip
- Decrease # of steps of screw insertion
- Increased torque required for insertion
- Weakened pull-out strength at fluted areas (17-30 less)
- Not recommended for interfrag. fixation (AO/ASIF)
Non-Self Tapping
- Blunt tip (no flutes)
- Require thread holes (cortical bone)
- Less axial load and torque required
- Ideal for interfrag. compression
Cannulated Screw
- Reduced thread to core ratio
- Decreased pull-out strength
- Simple application
- Wide variety of sizes
good in areas where screw head promience may be problemativ
herbet screw
- two sets of threads
- decrease pull-out and compressive forces
- ex: STJ arthrodesis
what size K wire would you use?
don’t want to use wire larger than 1/3 of the diameter of the bone - could cause fx
same as k-wire but is larger and only comes in smooth variety
steinman pins
- stabilize larger osteotomies or fusion
cerclage =
encircling of a part with a ring or loop
- thin mallelable stainless steel wire
uses k-wire with cerlage wire
tension band wiring
- 1-2 K wires placed across fx in parallel fashion and monofilament wire placed in figure 8 pattern to exert force opposite of fx fragment
- used in fx with stable soft tissue attachment
do staples help with compression?
not traditional
- some newer ones yes (“memory staples”)
4 functions you can achieve with traditional plate
- compression (eccentric drilling, prebending, tension device, tension band)
- neutralization (helps protect inter-frag scres, neutralizes forces to prevent rotational forces)
- buttress (stabilizes fx, anchored to main stable fragment, not necessarily to fragment it is supporting, usually metaphyseal or epiphyseal)
- bridge (plate fixated in two main fragments only, used to hold bone out to length due to comminution of surgery with graft placement)
is total bone plate contact a good thing?
no - can create osteonecrosis under plate
- use a limited contact dynamic compression plate to limit trauma
rules of stabilization
Screw Fixation: The length of the fracture should be at least twice the diameter of the bone involved
Plate Stability: Metatarsal plates need 4 cortices, 2 screws on both sides of fracture; Ankle plates need 6-8 cortices, 3-4 screws
Vassals Rule: Reduce the primary fracture and secondary fractures spontaneously reduce
Two Screws are Better than One: 2 points of fixation to resist rotatory forces
which is better - two small screws or one large screw
two small screws
- resists rotatory force
- 2nd screw perpendicular to cortec: anchor
- 1st screw perpendicualr to fracture line : compression
- if you can only insert one screw - insert is half-way
does a locking plate need to be bicortical?
no - unicortical
- traditional plates need bicortical screws
what should you do pre-op to prevent infection with hardware?
- generally if using hardware, use preop abx to decrease or prevent
- 2 g Ancef IV
- 1 g Vanc IV or 600 mg Clindamycin IV if allergic to penicillin
- leave hardware in if stable with post op infection, take out if unstable
