Principles and Equipment

Question 1

What degree of lonitudinal and transverse angulation can be achieved with 4.5 and 5.5mm cortical screws through plates?

Answer 1

DCP - 4.5 mm screws through 4.5mm plate allows 20° (says 25 in Aeur) of longitudinal angulation & 7° in the lateral direction. 5.5mm screws through a 4.5 mm DCP hole can be angled only 10° longitudinally & 3–4° laterally

LC-DCP - cut-outs on underside allow for increased angulation; upto 40° of angulation along the axial plane of the plate w 4.5mm screws. 7° of transverse screw angulation

LCP - DCU hole also allows 40° of angulation along the axial plane & 7º transverse w 4.5mm screws in 4.5/5.0 plate.

5.5mm screw through 4.5/5.0 LCP can be angled 25º axially & 5º laterally

Question 2

Types of pins used for transfixation pin casting, pin size and drill hole size

Answer 2

TYPES - Smooth or threaded pins; can be positive profile or negative profile (not recommended as threads cut into the pin)

POSITIVE PROFILE CENTRALLY THREADED PINS are best; better resistance to pullout

Size - typically 4-6.3mm pins used. Pin holes dhould be approx 20% dorsopalmar bone diameter; >20% assoc w incr risk of catastrophic failure through pin holes dt decr torsional stability

6.3mm IMEX pins typically used in adults. 4.8 mm in foals (5/16”)

DRILL HOLE - should be 0.1mm smaller vs the pin ie 6.2mm for the 6.3mm pin (=0.1mm radial preload - incr strength & stability)

NB pins are named according to shaft not outside thread diameter as for cortex screws; 6.3mm pin has 8mm outer thread diameter.

Question 3

Outer diameter, thread hole diameter, core diameter, head diameter & pitch of the 3.5 cortex screw

Answer 3

Outer diatmeter: 3.5mm

Thread hole: 2.5mm

Core diameter: 2.4mm

screw head: 6mm

Pitch 1.25

Question 4

Outer diameter, thread hole diameter, core diameter, head diameter & pitch of the 4.5mm cortex screw

Answer 4

Outer = 4.5

Thread hole = 3.2mm

Core diameter = 3.1mm

Head = 8mm

Pitch = 1.75

Question 5

Outer diameter, thread hole diameter, core diameter, head diameter & pitch of the 5.5 cortex screw

Answer 5

Outer = 5.5

Thread hole = 4.0

Core = 4.0 Head = 8mm (same as 4.5)

Pitch = 2

Question 6

List some advantages of LC-DCPs over DCPs

Answer 6

1. Improved vascularity to the bone (potentially)
2. Increased screw angulation (40deg for 4.5 vs 20 in DCPs)
3. No middle of the plate - compression can be placed in either direction along the plate
4. Same amont of metal at all points on the plate so no stress risers. Also makes them easier to bend in the press