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Musculoskeletal > External skeletal fixation > Flashcards

External skeletal fixation Flashcards

1
Q

What types of pin design can be used with an ESF?

A

Smooth, positive profile, negative profile, negative profile with a tapered thread run-out (Duraface)

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2
Q

How do positive profile pins compare biomechanically to negative profile pins with a tapered thread run out?

A

Tapered run out prevents stress riser while maximizing external shaft diameter. Shown to have increased stiffness, strength and fatigue life compared to positive profile pins

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3
Q

What are the different commercially available ESF connecting clamps?

A

IMEX SK, Securos Titan, and Securos U-clamps

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4
Q

What is the effect of using a larger connecting bar on the pins?

A

A larger connecting bar results in a stiffer construct. This decreases the load/stress on individual pins (more evenly distributes load) helping to protect the pin/bone interface

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5
Q

Are titanium or carbon fiber connecting bars stronger?

A

Titanium are twice as strong as carbon fiber of comparable size

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6
Q

Describe the different configurations of ESF

A
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7
Q

How do you increase the strength/stiffness of an ESF frame?

A

Use a more complex frame design, augmentation techniques (interconnecting bars, combined frames [combine with IM pin or interlocking nail - may or may not be ‘tied-in’])

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8
Q

What are the two types of interconnecting bar configurations?

A

Articulations - don’t cross the fracture gap.
Diagonals - do cross the fracture gap.
Diagonals result in greater increases in stability as compared to articulations.

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9
Q

What are the general recommendations of acrylic connecting bar stiffness and size?

A

Should be 2-2.5 times the diameter of bone. In order to reach similar stiffness should be 3-4 times the diameter of a comparable stainless steel connecting bar.

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10
Q

What limits the size of an acrylic connecting bar?

A

Once diameters exceed 25mm vaporization can occur. This is a sequelae of excessive heat within the column that can result in vacuum and potential voids. This reduces the density and stiffness of the construct.

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11
Q

What are the biomechanical differences of epoxy resin and methylmethacrylate acrylic connecting bars for ESF application?

A

Epoxy resin is 4 x stiffer but methylM absorbs 6 x the energy prior to failure.

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12
Q

What can be used to increase the stability of the acrylic bar/pin interface in an ESG system?

A

Knurled pins increase the pin-epoxy resin interface strength by 40%. Epoxy resin creates a bond with smooth pins that is almost 4x as strong as with methylmethacrylate

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13
Q

To prevent thermal injury during curing how far should an acrylic bar be positioned from the patient?

A

1cm

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14
Q

Describe the recommendations for wire size and tension based on ESF ring components

A
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15
Q

Describe the components of a ring ESF

A
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16
Q

What is the equivalent pin strength of a 1.6mm wire tensioned in a ring ESF?

A

Equivalent to a 4mm pin in strength

17
Q

In ring ESFs stabilized with fixation wires only, what is the primary determinant of construct stiffness?

A

The ring size

18
Q

How do you minimize translation of bone along fixation wires when using a ring ESF?

A
  1. Place the fixation wires as close to 90 degrees to one another as possible (the lower the angle the less resistance to translation).
  2. Use opposing Olive wires
19
Q

How can bone segments be moved in relation to one another using a ring ESF?

A

Can be translated, angled or transported axially

20
Q

What rate of bone movement per day facilitates distraction osteogenesis?

A

1mm per day

21
Q

As it relates to distraction osteogenesis what is the period of latency?

A

The period after the initial osteotomy and the initiation of distraction. This is usually 3-5 days and allows the fracture hematoma to form

22
Q

How do tensioned fine wires in a ring ESF respond to loading?

A

Non-linear stiffness. Initial low stiffness with easy deflection, followed by an exponential increase in stiffness and then linear portion. Linear portion usually reached within 1mm of travel. This allows for axial micromotion.

23
Q

How can the axial micromotion of a ring ESF be reduced?

A

Increased tensioning of wires, larger wires.

24
Q

What are the biologic disadvantages of using rigid transarticular ESF?

A

If used for greater than 4 weeks can result in arthrosis secondary to joint adhesions, soft tissue contracture, and degenerative articular changes

25
Q

Describe the safe soft tissue corridors for ESF placement

A
26
Q

How many ESF pins should be placed per segment?

A

4 per segment. Additional pins will increase stiffness but not enough to offset morbidity

27
Q

How far from the fracture and joint should ESF pins be placed?

A

3/4 the bone diameter from joints, 1/2 bone diameter from fracture

28
Q

How does the stiffness of the pin relate to its exposed (extracortical) length?

A

Inversely proportional to extracortical length to the 3rd power.

29
Q

How far should clamps be placed from the skin surface?

A

About 1cm - trade-off between reducing working length of fixation pins and allowing post-op care of tracts and for swelling

30
Q

By placing pins on alternating sides of a connecting bar what divergent planar angle can be achieved?

A

Up to 35 degrees (acts like a type 1b fixator)

31
Q

What is pin feed rate?

A

The rate at which the fixation pin advances into bone, and is determined by thread pitch and rotational speed. It is important to apply appropriate axial pressure to prevent interference with feed rate that may lead to cortical stripping.

32
Q

Describe strategies to preserve the pin-bone interface

A
33
Q

What are some disassembly options for an ESF if the surgeon wishes to pursue dynamization?

A
34
Q

What is reverse dynamization?

A

The process by which frame components are added during healing to increase frame stiffness and improve fracture stability

35
Q

How far should pins be placed from the physis in immature animals to avoid inadvertent disruption?

A

1cm or 3 pin diameters

36
Q

What is the critical size of a bone defect where non-union is likely to occur?

A

1.5 times the bone diameter in the feline tibia (likely to depend on animal species, age, breed and bone location).

37
Q

What are the most common complications associated with ESF application?

A

Soft tissue injury, pin tract infection, construct complications

Musculoskeletal (11 decks)

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