Bone Plating

Question 1

What are the 2 types of fracture fixation?

Answer 1

1. NON-SURGICAL: external coaptation (splinting)
2. SURGICAL: external skeletal fixator, pins, wires, plates, screws

Question 2

What are the goals of fracture fixation (immobilization)?

Answer 2

• align joints and bones to restore length
• maintain some stress on bone and permit early ambulation to promote healing
• minimize motion at fracture ends
• balance forces that promote bone healing vs. those that promote bone resorption

Question 3

What is Wolff’s Law?

Answer 3

bone remodels based on the forces applied to it

• remodels and thickens in response to increased sustained forces
• resorbs and weakens in response to decreased sustained forces

Question 4

What are 4 pros to internal/external fixation?

Answer 4

1. variety of fixation options to promote stable repair
2. can promote normal muscle/joint function during bone healing
3. typically fewer rechecks than with external coaptation (except with external coaptation)
4. nothing externally to monitor (except with external coaptation

Question 5

What are 3 cons to internal/external fixation?

Answer 5

1. expense to clinic and owner - requires lots of specialized equipment and inventory
2. requires training for appropriate application
3. may require second surgery for explantation due to infection, loose/broken implants, or irritation/rejection

Question 6

What are 3 pros to external coaptation?

Answer 6

1. limited supplies necessary for placement
2. need for highly specialized training is limited
3. avoids prolonged surgical procedure - diaphyseal and transverse fractures heal well

Question 7

What are 5 cons ot externap coaptation?

Answer 7

1. requires frequent rechecks and bandage changes ($$$)
2. limited effective applications - only below elbow and stifle
3. risk of bandage morbidity preventing continued use
4. immobilized joints
5. bandage sores

Question 8

What are some indications for external coaptation?

Answer 8

• fractures below the elbow or stifle
• minimally displaced fractures
• transverse, simple, closed fractures
• non-articular fractures
• fractures expected to heal rapidly - Greenstick fractures

Question 9

What are 2 approaches to internal fixation?

Answer 9

1. open anatomic reduction/reconstruction - primary bone healing with perfect bone reconstruction and rigid fixation
2. biological osteosynthesis - plates placed away from the callus, avoiding disruption of fracture hematoma —> less rigid, secondary healing

Question 10

What fractures require open anatomic reconstruction?

Answer 10

• articular fractures
• transverse, oblique, segmental, and non/minimally comminuted

Question 11

What is the open but do not touch approach to biological osteosynthesis?

Answer 11

fracture is surgically approached and visualized, but the fracture ends are NOT manipulated during the placement of implants

• minimal disturbance of fracture hematoma, periosteum, and blood supply

Question 12

What is minimally invasive osteosynthesis? What is used for guidance?

Answer 12

implants are placed through incisions distant to the fracture —> fracture not approached, closed reduction

intra-operative fluoroscopy - guides implant placement and confirms fracture reduction and alignment

Question 13

What is the goal of biological osteosynthesis?

Answer 13

return limb alignment and length to normal without disruption of fracture

Question 14

What considerations are included when determining implant selection?

Answer 14

• fracture type and location - articular vs. metaphyseal vs. diaphyseal, comminuted vs. simple
• bone affected - forces acting on bone
• patient factors - age, comorbidities, environment, size/weight
• surgeon preference and experience

Question 15

What are examples of primary and secondary implants?

Answer 15

PRIMARY - bone plates, interlocking nails, external skeletal fixators

SECONDARY - Kirschner wires, cerclage wire, interfragmentary screws (provide additional rigidity)

Question 16

What are the 2 most common types of bone plates?

Answer 16

1. STAINLESS STEEL - 316L iron-carbon alloy, very stiff, fails by bending
2. TITANIUM - less stiff, better fatigue resistance, less reactive, expensive —> bendable, so good for growing bones

Question 17

What is a dynamic compression plate (DCP)?

Answer 17

plate that can be used for compression, neutralization, or bridging with screw holes designed to allow screw placement that promotes compression of fracture ends

• flat surface = friction between plate and bone creates stability

Question 18

How do limited contact compression plates (LC-DCPs) compare to DCPs?

Answer 18

decreased plate to bone contact allows for improved cortical perfusion, reduced stress concentration at screw holes, and contouring

• can also be used for compression, neutralization, or bridging

Question 19

How does the structure of limited contact DCPs compare to DCPs?

Answer 19

contoured underside allows for stress to be more evenly distributed across the plate and less disruption of periosteal vascularity

Question 20

How are locking compression plates used?

Answer 20

combination of holes accommodate conventional screws and locking head screws, acting as an internal fixator eliminated the need for perfect contouring

• compression, neutralization, bridging

Question 21

What is the purpose of using locking plates with locking head screws?

Answer 21

locks in to prevent the need for plate-bone contact for stable repair = more stability with fewer screws

Question 22

When are locking plates most commonly used?

Answer 22

minimally invasive plate osteosynthesis (MIPO)

• can use both locking and cortical screws

Question 23

How do locking plates compare to conventional plates?

Answer 23

greater forces are required to cause pull-out screws making them better for osteoporotic bone, soft bones, and comminuted fractures

Question 24

What are 3 types of locking plates?

Answer 24

1. string of pearls - easier to contour to the curvature of bones
2. fixin plates
3. polyaxial locking plate (PAX)

Question 25

What are string of pearls plates? What screws are used?

Answer 25

locking plate with added stability and limited contact on bone, making it easier to contour in any direction unicortical screws provide almost the same stability as bicortical screws (neutralization, bridging)

Question 26

What are reconstruction plates? How do they compare to DCPs?

Answer 26

plates with deep notching between holes, which allows for contouring in an additional plane - not as strong as DCP - able to be used in areas that require complex plate contouring, like the mandible

Question 27

What are veterinary cuttable plates (VCPs)? How do they compare to DCPs?

Answer 27

versatile plates able to be but to any length - relatively weak compared to DCP - can be stacked to increase stiffness - does not provide compression

Question 28

What is the goal to using screws to repair fractures? What size diameters are used compared to the bone? What provides maximal stability?

Answer 28

achieve as much contact with bone with sufficiently stable implant of minimal size screw diameter should not exceed 40% of bone diameter when used in diaphysis screws should engage the near (cis) and far (trans) cortex of the bone

Question 29

What do the core and outer diameters of screws determine?

Answer 29

CORE = bending strength OUTER = pull out strength

Question 30

When are cancellous and cortical screws used?

Answer 30

CANCELLOUS - metaphyseal and epiphyseal bone of younger animals and soft bone, since they're harder to pull out CORTICAL - dense, cortical bone

Question 31

How do the diameters, thread, and pitch compare in cancellous and cortical screws?

Answer 31

CANCELLOUS - high outer:core diameter ratio, deep thread, large pitch CORTICAL - low outer:core diameter ratio, shallow thread, decreased pitch

Question 32

What are self-tapping screws?

Answer 32

screws with fluted tips, which do not require tapping - pre-cutting the thread pattern into the bone following drilling

Question 33

What are locking screws? How are they placed?

Answer 33

self-tapping, largest core diameter screw that is stronger and cortical screws and contains a threaded head that locked into the threaded locking plates perpendicular through holes in the locking plate (CANNOT USE OTHER PLATES)

Question 34

How do screws work in conventional plates?

Answer 34

- tightening the screw generates friction between the bone and the plate, which is necessary for stability - loading of the limb results in forces shared between the plate and the bone

Question 35

How do screws in locking plates work?

Answer 35

tightened screws lock the screw into the plate, creating a construct that converts shear and bending stress into compressive forces at the bone-screw interface

Question 36

What are lag screws? What happens when they are tightened?

Answer 36

screws placed perpendicular across an oblique or sagittal fracture end with the glide hole drilled into the cis cortex, cause the trans cortex to be drilled into the core pulls the trans cortex closer to the cis cortex, causing compression across the fracture

Question 37

When are lag screws used?

Answer 37

- articular fractures - oblique fractures can be placed through conventional plate holes

Question 38

How are position screws used? How do they compare to lag screws?

Answer 38

screw is placed across a fracture line to hold fragments in place without providing compression weaker repair compared to lag screws - able to be placed through conventional plate holes

Question 39

Where on the bone are plates placed?

Answer 39

on the side of most tension

Question 40

How are conventional plates placed? What is required for stable repair?

Answer 40

precisely contoured to match the normal shape of the bone surface with maximal contact to prevent distraction of fracture ends during screw placement screw purchase of at least 6 cortices above and below fracture ---> 3 screws

Question 41

How are locking plates placed? What is required for stable repair?

Answer 41

minimal to no contouring required screw purchase of at least 4 cortices above and below fracture ---> 2 screws

Question 42

What affects a plate's function?

Answer 42

how it is placed on the bone in relation to the fracture - compression - neutralization - buttress - bridging

Question 43

When are compression plates most commonly placed? What does this allow?

Answer 43

transverse or nearly transverse fractures where compression is applied by the spherical gliding principle compression of bone at the fracture site

Question 44

What is compression mode? How are screws loaded?

Answer 44

plate applied to achieve compression across the fracture - commonly used for transverse fractures because it does not cause shearing eccentrically loaded to apply compression by the spherical gliding principle

Question 45

What carries most of the load in compression mode? What does this allow?

Answer 45

mostly the bone promotes primary bone healing

Question 46

What is neutralization mode? How does the plate act?

Answer 46

plates used in addition to primarily placed lag or positional screws protects against shearing, bending, and rotational forces that would otherwise damage the interfragmentary repair achieved by the screws

Question 47

DCP plate in neutralization mode:

Answer 47

spiral convoluted fracture with wire for stabilization

Question 48

What is bridging mode? What bears the load?

Answer 48

plate spans fractured area which cannot be anatomically reconstructed - comminuted area is bridged by the plate plate - no load sharing at fracture with screw holes left empty ---> micro-motion promotes secondary bone healing

Question 49

What plate/screw combination is used in bridging mode? When is it most commonly used?

Answer 49

longer plate with fewer screws biological osteosynthesis, MIPO

Question 50

When is buttress mode used? What screws are used?

Answer 50

metaphyseal fractures to prevent collapse of the adjacent articular surface lag screws ---> most holes should be filled

Question 51

What is subject to load in buttress mode?

Answer 51

plate ---> axial forces do not help with fracture compression and do not promote load sharing by the bone (supports cortex and resists displacement)

Question 52

What is required for stable repair with locking plates? How do they compare to cortical screws?

Answer 52

screw purchase of at least 4 cortices above and below fracture ---> 2 screws - have increased core diameter = increased bending resistance - converts shear stress in axial loading to compression forces at screw-bone interface

Question 53

What are the 3 principles of locking plate placement?

Answer 53

1. 2 screw minimum per segment (4 cortices) 2. 3rd screw at each section increases torsional stability 3. addition of more than 3 screws has little advantage

Question 54

How does the size of the fracture gap alter locking plate placement?

Answer 54

small (1 mm) = screws omitted from 1-2 holes adjacent to fracture larger (> 6 mm) = screws places as close to fracture site as possible, which decreases working length

Question 55

What is an interlocking nail? When is it used?

Answer 55

internal fixation that combines benefits of an intramedullary rod and a plate ---> has a removable jig to guide placement of screws - diaphyseal comminuted fracture biological osteosynthesis - NOT used for fractures of the radius

Question 56

Interlocking nail:

Answer 56

Question 57

What are the major risks of internal fixation?

Answer 57

- implant failure: loosening, breakage, migration - osteomyelitis - impingement of nerves common in femoral IM pins - osteopenia secondary to implates that are too strong - delayed, nonunion, malunion

Question 58

What are the most common causes of implant loosening with DCP/LC-DCP and locking plates?

Answer 58

axial load exceeding friction causing sequential pull out axial load exceeding compression force causing implants to fail all together

Question 59

What is the most common cause of implant bending/breaking?

Answer 59

axial forces cause cycling of the implant depending on material and working length - more holes left open = decreased strength of the plate