Article: How Much Fixation is Enough

Rigid fixation: internal fixation that is stable enough to prevent micromotion between bony fragments under normal function

Functionally Stable Fixation: internal fixation that is not “rigid” but satisfies the goals of maintaining fragment alignment to permit healing during active use of the bone

Load-Bearing

• Devices that are of sufficient strength and rigidity that it can bear the entire loads applied to the mandible during function.
• Injuries that require load-bearing fixation are comminuted fractures of the mandible, those fractures where there is very little bony interference because of atrophy, or those injuries that have resulted in a los of a portion of the mandible.
• Plates >2.0mm in diameter (most commonly 2.4, 2.4, 2.7mm)

Load-Sharing

• Insufficient stability to bear all functional loads applied
• Requires solid bony fragments on each side of the fracture
• 2.0 miniplates
• Lag screw technique is load sharing

• Compression plates have the ability to compress the fractured bony margins, helping to bring them closer together, and imparting additional stability by increasing the frictional interlocking between them.
• Safest to use in fractures where there is minimal obliquity and sound bony buttresses on each side of the fracture that can be compressed by the plate.
• Compression plates can also cause widening of the mandible if not properly overbent.

• Locking plates function as internal fixators, achieving stability by locking the screw to the plate.
• Locking plates are advantageous when the plate is not intimately in contact with the underlying bone.

• To apply the lag screw technique, two sound bony cortices are required because this technique shares the loads with the bone.
• Lag Screw refers to a partially threaded screw
• Lag Screw Technique: Drill a Gliding Hole in the proximal segment by using a drill bit with the same diameter as the threads of the screw. Drill a Pilot Hole in the distal segment using a drill bit with the same diameter of the screw. The distal segment will only engage. By tightening the screw, a tensile force is created within the screw that compresses the bony cortices together, tightly reducing the fracture.
• Countersinking: To spread the contact area of the screw head and reduce the stress on the cortex, the gliding hole may be countersunk. This also results in the head of the screw being less prominent, which may be desirable in subcutaneous bone.
• Screw should protrude 1-2mm through the opposite cortex
• At least two screws are necessary to provide stable fixation
• One should always place the lag screw in a direction that is perpendicular to the line of fracture to prevent overriding and displacement during tightening of the screws.

Titanium:

• No allergies
• Non-magnetic

Stainless steel

• Most allergic reactions to nickel; then to chromium
• Should not be used in contact with titanium-based or cobalt-based implants due to risk of galvanic corrosion)