Internal Fixation

Question 1

what are the main advantages to internal fixation?

Answer 1

allows for rigid fixation of fracture fragments until they are healed while also allowing patient to move limbs unencumbered and to bear weight as soon as they are comfortable

Question 2

what is the main disadvantage to internal fixation?

Answer 2

these methods are invasive and require surgical approach to the bone

any damage from surgical approach or manipulation of fragments can potentially compromise vascular supply (and negatively impact healing)

Question 3

fill in the blank: absolute stability is especially important for fractures invading the _____ surface

Answer 3

articular

Question 4

what is a neutralization implant?

Answer 4

= this implant neutralizes all forces being borne by the bone

• this is an implant that calls for anatomic reconstruction = both perfect alignment AND perfect reduction
• the bone supports fixation while at the same time fixation supports the bone

Question 5

anatomic reconstruction is necessary for WHAT TYPE of fractures?

Answer 5

articular fractures – a smooth articular surface is essential in order to minimize future cartilage injury

Question 6

what is a bridging implant?

Answer 6

= this implant bridges the fracture gap – carries all the weight, and resists all the forces applied to the bone

• this implant is for when anatomic reconstruction is not possible (complex fracture)
• this implant must be rigidly fixed to major proximal and distal segments that can bear the entire load of the patient onto the bone
• priority is preserving blood supply

Question 7

what is a buttress implant?

Answer 7

like a bridging implant (supports fracture), but this term describes implants that are near the joints

• short up cortical defects within metaphyseal region
• negates compression & shearing forces
• relatively uncommon fractures in vet setting

Question 8

if the fracture can be reconstructed, and anatomic reconstruction is desired, what are the AO principles of fixation to achieve primary bone healing?

Answer 8

1. fracture reduction to restore anatomic relationships
2. fracture fixation provides absolute/relative stability depending on the ‘personality’ of the fracture, patient, and injury needs
3. preserve blood supply to bone & soft tissues
4. early and safe mobilization of injury and patient as whole

Question 9

what are 6 types of fractures that are suitable for open anatomic reconstruction?

Answer 9

transverse
short oblique
long oblique
segmental
minimally comminuted
articular fractures

Question 10

what’s important to keep in mind when it comes to open anatomic reconstruction and soft tissue?

Answer 10

open anatomic reconstruction needs some degree of soft tissue dissection to manipulate, reduce, and rigidly stabilize fracture segments – there will be iatrogenic damage (to soft tissue, periosteum, fracture hematoma) that can impede healing

Question 11

what are 4 principles of biological osteosynthesis?

Answer 11

1. indirect fracture reduction via limited surgical approaches with minimal to no disturbance of fracture hematoma
2. stabilize fracture using bridging implants over anatomic reconstruction or rigid fixation
3. little reliance on secondary implants
4. limited to no use of bone grafts

Question 12

what is the foundation of biological osteosynthesis?

Answer 12

Reducing iatrogenic trauma to the fracture site using less precise reconstruction and less rigid fixation will encourage the formation of callus with rapid secondary bone healing.

Question 13

what types of implants for fracture repair exist?

Answer 13

primary implants = bone plates, interlocking nails, intramedullary pins, external skeletal fixators

secondary / supplemental implants = Kirschner wires (K wires), cerclage wires, interfragmentary screws

Question 14

orthopedic wire: what is a tension band? what does it do, how is it used and how does it work?

Answer 14

this wire apposes the pull of muscle or ligament on the bone fragment

• often used in tandem with pins that align the fragment and prevent rotation
• tension converted into compression by the band, which may also facilitate healing

Question 15

orthopedic wire: what is cerclage wire? what does it do, how is it used and how does it work?

Answer 15

this wire fully encircles a bone to hold fragments apposed

• Supplementary implant for long oblique fractures of the diaphysis
• Minimum of 2 wires should be used
• Anatomic reconstruction of the ENTIRE bone column is essential – any loose wire and movement will impede healing

Question 16

orthopedic wire: what is hemicerclage wire? what does it do, how is it used and how does it work?

Answer 16

this is used in fractures of diaphysis when it is not possible to place a full cerclage

• this may improve alignment but does NOT add stability
• primary role is to keep alignment while a definitive fixation is applied, absolutely CANNOT be relied on as primary fixation

Question 17

orthopedic wire: what is interfragmentary wire? what does it do, how is it used and how does it work?

Answer 17

this wire is placed between two fragments of bone

• used in long bone fractures but is most often used between fragments of mandible or maxilla
• fracture fragments should make good contact and ideally should interdigitate!

Question 18

what are two main types of pins used as internal fixation and how are they applied?

Answer 18

1. kirschner wires (K wires)
2. steinmann pins

often used within the medullary cavity of long bones to counter any bending forces or skewer fragments to hold location and prevent rotation

Question 19

normograde vs retrograde pin placement: what’s the difference?

Answer 19

normograde = pin started outside bone and advanced INTO medullary canal– necessary for closed placement

retrograde = advancing pin from fracture site out of the bone; fracture is reduced, and pin is advanced into opposing fragment

Question 20

crossed pin vs. rush pin

Answer 20

crossed-pin = using small diameter pins starting from sides of joint, pins penetrate cortex opposite

rush pin = do not penetrate cortex opposite and generate a “spring effect”; they are driven at angle that they glance off the opposite cortex and continue up medullary canal

Question 21

what are interlocking nails? what forces do they resist? when are they used?

Answer 21

what? IM rod that locks prox & dist to fix cortical bone to major bone segment

forces? resist bending, rotation, compression, tension

used? for treatment of closed, communited diaphyseal fractures (except radius); also for angular deformity and some metaphyseal fractures

Question 22

what are the advantages of using a nail compared to a plate?

Answer 22

+ nails are placed near neutral bone axis while plates are placed eccentrically on surface – plates fatigue at lower loads especially with unstable fractures

+ interlocking nail has more resistance to bending compared to similar sized bone plate

+ because nail is placed in intramedullary location, this eliminates the risk of screw pull-out failure, unlike plates

Question 23

what are the disadvantages of interlocking nails?

Answer 23

mostly biomechanical disadvantages:

• the breaking of screw, bolt or nail at the level of the hole is possible – must select your device carefully
• there is some rotational instability compared to plate-rod constructs
• limitation of what fracture types you can repair

Question 24

what are the two types of screws based on their shaft threads?

Answer 24

cortical screws – small pitch, less depth
cancellous screws – large pitch, more depth

• pitch = longitudinal distance between threads

Question 25

what are the two types of screws based on their screw heads?

Answer 25

locking screws = physically engage the plate and produce a change in the force transfer between bone-screw-plate

standard / smooth head screw = smooth on external surface

Question 26

what screws can be used to achieve a “lag effect”?

Answer 26

• partially threaded cancellous screws (as long as threads engage far cortex)
• shaft screw (stronger in bending)

screws used in lag fashion aren’t enough to resist bending forces on the weight-bearing bone, so always apply a plate!!

Question 27

what are cannulated screws used for?

Answer 27

use is for after closed reduction or where exposure of fragments is difficult

= screw has axial hole in shaft, and k wire is used to maintain reduction

Question 28

what are self-tapping screws designed for?

Answer 28

there speed up the insertion process – typically screw needs to have a hole drilled and tapped; without tapping, you can crack bone

self-tapping screws have cutting flute on tip of the screw so they cut their own threads as they are inserted – these must be advanced to pass through far cortex to achieve full holding power

Question 29

the (greater/lesser) the radius of the screw, the (greater/lesser) its area moment of inertia and the (greater/lesser) its strength and stiffness

Answer 29

greater
greater
greater

• larger core diameter the better for resisting bending

Question 30

the optimal orientation of a lag screw is (angled/perpendicular) to the fracture plane

Answer 30

perpendicular

Question 31

what is a position/fixation screw?

Answer 31

this is used to hold fragments in a specific location, placed with threads engaging BOTH near/far fragments

this is less stable than compressive fixation, and necessary if compression would collapse fragments (ie. articular fractures)

Question 32

what are bone plates for?

Answer 32

these produce excellent stability of the fracture site; resisting tension, compression, and rotational forces placed on a limb

if bone and plate don’t work together mechanically (like in bridging) the plate is more susceptible to bending

Question 33

what are non-locking plates?

Answer 33

these are lagged to the bone, held in place by friction; versatile & reliable

ex. Dynamic Compression Plate (DCP), Limited Contact Dynamic Compression Plate (LC-DCP), cuttable plates

Question 34

what are locking plates?

Answer 34

these are guided by threads, designed for biological osteosynthesis = avoids tight plate-bone contact, thus limiting damage to periosteum and preserves extraosseous blood supply

ex. locking compression plate (LCP), string of pearls (SOP), advanced locking plate system (ALPS), fixing, polyaxial locking plate system (PAX)

Question 35

what factors must be considered when applying a plate?

Answer 35

• type and size of plate
• patient weight
• dimensions of specific bone
• application (plates must be contoured to the surface where they are applied)
• expected loads (best to plate on the surface of the bone that is loaded in tension)

Question 36

What force contributes to the strength and stiffness of a pin or nail?

Answer 36

the area moment of inertia

= greater the radius of the cylinder, stiffer it is

Question 37

what are the five functional applications of plates?

Answer 37

• dynamic compression – simple transverse or short oblique fractures
• neutralization – long oblique, spiral, butterfly fractures
• bridging – withstands all loads placed on fracture and prevents collapse of segments
• buttress plating – a plate that negates compression and shear forces within metaphyseal region
• elastic plate osteosynthesis – long, thin plates that span the length of bone with fewest screws, for young animals

Question 38

what is plate-rod? how to place it?

Answer 38

In some complex fractures, the strength of a repair can be improved by using a plate and intramedullary pin together!

plate attached to major prox and dist fragment = prevents axial collapse and rotation

should take up between 35-40% of medullary cavity diameter

rod may need screws, minimum 2 (4 cortices engaged) but ideally 3 (6 cortices) recommended for compression and neutralization