37/38: Internal Fixation - Frush

Question 1

4 AO principles

Answer 1

1. Anatomical reduction of fx fragments
2. stable internal fixation designed to fulfill local biomechanical demands
3. Preservation of blood supply to bone fragments and soft tissue by means of atraumatic surgical technique
4. Early active pain-free mobilization of m and jt adjacent to fx, to prevent cast dz

Question 2

review indirect v. direct osseous repair of bone healing

Answer 2

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

Question 3

absolute vs. relative stability - what tools used?

Answer 3

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

Question 4

what is strain? how do you decrease strain?

Answer 4

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

Question 5

Review Strain Model Movie ***

Answer 5

add notes here

Question 6

key characteristics of ideal implant material (metallurgy)

Answer 6

• biocompatibility
• strength
• resistance to degradation and erosion
• ease of integration
• minimal adverse effects of imaging

Question 7

titanium v. stainless steel

Answer 7

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

Question 8

least dense surgically impantable metal

Answer 8

titanium

• used for pt with nickel allergy

Question 9

material of choice for joint implants

Answer 9

cobalt-chromium

Question 10

screw anatomy

Answer 10

Question 11

what part of screw broken here?

Answer 11

runout

Question 12

cortical vs. cancellous surgical screws

(image of cortical)

Answer 12

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)

Question 13

what screw do you use for compression?

Answer 13

Lag screw by screw (overdrill, underdrill, coutnersink, measure, tap) or by design

Question 14

glide hole vs. overdrill

Answer 14

synonymous with each other

• larger drill that usually matches screw outer diameter

Question 15

smaller drill that matches screw core diameter

Answer 15

glide hole, thread hole and underdrill

Question 16

instruments needed for lag technique

Answer 16

Question 17

steps of insertion for lag screw by techinique

(video in lecture)

Answer 17

1. glide hole (larger hole)
2. thread hole (smaller hole)
3. countersink
4. depth gauge
5. tap
6. insertion

Question 18

why is a guide hole necessary?

Answer 18

no guide hole near fragment –> screw distracts fracture instead of compressing it

Question 19

steps of insertion lag by design

Answer 19

1. thread hole (small hole only*)
2. countersink
3. depth gauge
4. tap
5. insertion
6. compression

Question 20

what depth is appropriate for by technique and by design?

Answer 20

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

Question 21

charnley’s fx classification

Answer 21

stable = transverse

unstable = oblique, spiral, comminuted

potentially stable = short oblique

Question 22

what angle do you place an interfragmentary screw?

Answer 22

• perpendicular to bone: max resistance to shear
• perpendicular to fragemnt: max inter-frag compression

Question 23

self-tapping v. non-self tapping screw types

Answer 23

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

Question 24

good in areas where screw head promience may be problemativ

Answer 24

herbet screw

• two sets of threads
• decrease pull-out and compressive forces
• ex: STJ arthrodesis

Question 25

what size K wire would you use?

Answer 25

don’t want to use wire larger than 1/3 of the diameter of the bone - could cause fx

Question 26

same as k-wire but is larger and only comes in smooth variety

Answer 26

steinman pins

• stabilize larger osteotomies or fusion

Question 27

cerclage =

Answer 27

encircling of a part with a ring or loop

• thin mallelable stainless steel wire

Question 28

uses k-wire with cerlage wire

Answer 28

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

Question 29

do staples help with compression?

Answer 29

not traditional

• some newer ones yes (“memory staples”)

Question 30

4 functions you can achieve with traditional plate

Answer 30

• 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)

Question 31

is total bone plate contact a good thing?

Answer 31

no - can create osteonecrosis under plate

• use a limited contact dynamic compression plate to limit trauma

Question 32

rules of stabilization

Answer 32

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

Question 33

which is better - two small screws or one large screw

Answer 33

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

Question 34

does a locking plate need to be bicortical?

Answer 34

no - unicortical

• traditional plates need bicortical screws

Question 35

what should you do pre-op to prevent infection with hardware?

Answer 35

• 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