Internal Fixation

Question 1

define internal fixation and list the different types of internal fixation devices used in vet med

Answer 1

internal fixation: fracture repair using implants that are placed beneath the skin, including screws, plates, wires, pins, and interlocking nails

Question 2

compare and contrast cancellous screws versus cortical screws (not LO)

Answer 2

cancellous screws: wider pitch, larger thread height (more prominent threads to engage softer bone better), smaller core diameter (weaker/bendier), have increased strength in cancellous/trabecular bone

cortical screws: smaller pitch and thread height, larger core diameter, have increased strength in cortical bone (but in SA are almost always used, even in cancellous bone)

Question 3

what are the 3 functions of screws? (not LO)

Answer 3

1. hold plates to bone
2. hold fracture segments together
3. serve as an object to which something can be attached

Question 4

what are the 2 ways screws can be applied to span a fracture? (not LO)

Answer 4

1. positional: application maintains the position that the bones are in relative to one another
2. lag: the screw draws the two bones closer to one another and therefore compresses the fracture line; provides interfragmentary compression between bone fragments; accomplished by
   -positioning the screw perpendicular to the fracture line and
   -placing the screw so that it only engages the trans-cortex (where the bottom of the screw is) and not the cis-cortex (where the screw enters)

Question 5

describe the 2 types of bone plates (not LO)

Answer 5

1. locking: both the screw heads and the plate holes have threads and the screw head locks into the plate at a fixed 90 degree angle; do not reply on bone-plate contact for stability!!; better for curvy bones because you don’t have to contour the plate perfectly to the bone
2. non-locking: neither the screw heads nor the plate holes have threads and the screw compresses the plate to the bone; the friction between the plate and the bone provides stability so you must contour the plate very accurately to the bone; an advantage is that you can angle screws at not just 90 degree angles

Question 6

describe the different ways in which fixation methods are applied

Answer 6

1. compression: plate is applied to generate compression between bone fragments; this requires a plate with a ramped hole design; as the eccentrically placed screw is tightened, the head moves down the ramp towards the fracture gap and the bone segment engaged by the screw moves with it until the fracture segments meet, and then further tightening of the screw will result in compression of the fragments (the goal is primary bone healing and no callus formation)
   -provides load sharing between the bone and the implant, low stress on implants so lower risk of implant failure
   -good for transverse fractures
2. neutralization:
   -fracture is reconstructed/compressed with other implants (other screws), but the function of the bone plate is just to support the other implants, not to compress itself
   -used in situations where heavy compression may displace fracture lines
3. bridging:
   -fracture is not reconstructed/compressed and the bone plate gets the full load (great stress on implants)
   -usually performed out of necessity (comminuted fractures)
   -usually combined with secondary implants, like an intramedullary pin

Question 7

describe orthopedic wiring and give 4 wiring techniques (not an LO)

Answer 7

description: stainless steel wire with little bending strength but a tensile strength that is related to wire diameter

1. cerclage: to use this, the fracture must be anatomically reconstructable and be a long oblique or spiral fracture
2. tension band: used in avulsion fractures; converts distractive forces into compressive forces
3. interfragmentary: drill holes on either side of a fracture and wrap wire to pull together, rarely used
4. interdental: use teeth as anchors to form a brace around the mandible/maxilla until the fracture heals

Question 8

what are the 3 types of intramedullary fixation? (not LO)

Answer 8

1. cross pins: smooth pin
   -ideal for physeal (growth plate) fractures or metaphyseal fractions in young animals that are going to heal quickly
   -does not sufficiently increase rotational strength for diaphyseal fractures, so don’t use for those
2. intramedullary pin: smooth pin
   -counteracts bending extremely well
   -need 3 points of fixation (2 at the metaphysis and 1 at the fracture site)
   -the larger the diameter, the stronger the pin
3. interlocking nail: an intramedullary rod with holes; place bolts or screws through the holes and both cortices of the bone, resists bending like an IM pin but ALSO resists rotation, tension, and compression

Question 9

describe what forces an intramedullary pin counteracts and why it should rarely be used alone

Answer 9

only counteracts bending forces!! does not counteract compression, tension, rotation, or shearing, all of which happen during load bearing, so an IM pin should rarely be used alone (combine with cerclage wire, bone plate, or ex fix) unless using in the metacarpals/metatarsals

Question 10

Explain why an intramedullary pin and/or interlocking nail should NEVER be placed in the radius

Answer 10

there is no extra-articular entry or exit without interfering with the elbow or radiocarpal joint!! so DONT place an IM pin in the radius!!

Question 11

list the potential complications that can occur following internal fixation

Answer 11

1. implant failure
2. infection
3. delayed union, malunion, or nonunion
4. premature physeal closure in young animals
5. osteoarthritis