Ch. 8 - Fracture Fixation

Question 1

What is a bone fracture?

Answer 1

A bone fracture is a structural failure of the primary load-carrying apparatus of the body.

Question 2

What is the difference between a traumatic and a pathologic/fragility fracture?

Answer 2

Traumatic - results from high-force impact or high stress
Pathologic/Fragility - results from a medical condition (e.g. osteoporosis, bone cancer, etc.) and can occur under physiological load

Question 3

What happens during the first stage of fracture healing?

Answer 3

The first stage is Hematoma Formation. This is an inflammatory phase when torn blood vessels haemorrhage and become clotted. A hematoma forms at the fracture site and it becomes swollen and painful.

Question 4

What occurs during the first and second stages of the Repair Phase of fracture healing?

Answer 4

1st stage of Repair Phase - Fibrocartilaginous Callus Formation:
- granulation tissue (soft callus) forms from the hematoma a few days after fracture
- it is poorly organized fibrous tissue
- internal callus has cartilaginous characteristics
- external callus has more bone-like characteristics
2nd stage of Repair Phase - Bony Callus Formation:
- new trabecular bone appears in the fibrocartilaginous callus
- hard callus replaces the original soft callus to bridge the fracture gap
- callus increases in structural stiffness

Question 5

What happens during the last phase of fracture healing?

Answer 5

The last phase is bone remodelling:

• compact bone is laid down to reconstruct shaft walls
• excess material on the bone shaft exterior and in the medullary canal is removed (stiffness is maintained but strength increased, sometimes stiffness and strength exceed that of adjacent bone)

Question 6

List the 6 stages of fracture healing.

Answer 6

1. Inflammatory phase
2. Callus formation
3. Callus proliferation
4. Bony bridging
5. Callus consolidation
6. Remodelling

Question 7

Describe 2 different approaches to fracture treatment.

Answer 7

1. Functional Bracing
   Introduce function (weight bearing and motion) to enhance natural healing process. Aims to maintain a proper balance btw callus stimulation and excessive fragment motion
2. Rigid Fixation
   Put fracture fracture sites in close contact using plates. Healing is reduced to normal bone remodelling without callus formation. Healing can be very rapid but there is a risk of infection due to surgery and a risk of stress shielding. Precise fragment positioning is needed.

Question 8

What is the benefit of external fixation?

Answer 8

It allows for adjustment of stiffness during the healing process. Side bar stiffness can be adjusted to control amount of loading of the fracture callus.

Question 9

How do stiffness and strength progress over the 4 stages of fracture healing?

Answer 9

Stage 1 - low stiffnes + low strength
Stage 2 - high stiffness + low strength
Stage 3 - high stiffness + medium strength
Stage 4 - high stiffness + high strength

Question 10

What types of load is a long bone usually subjected to?

Answer 10

• Axial loading
• Bending
• Torsion

Question 11

What assumptions would we make to model the mechanics of an intramedullary rod?

Answer 11

1. Bone and rod are concentric cylinders

2. Rod fills intramedullary rod

Question 12

How would you treat/model loads located at the fracture site differently than those away from the fracture site?

Answer 12

At fracture site - rod alone may carry the load (Beam Theory)
Away from fracture site - rod and bone can act as a unit (Composite Beam Theory)

Question 13

How may torsional coupling between an intramedullary rod and bone be achieved and why would this be needed?

Answer 13

Torsional couping may be desired to reduce fragment motion.

• Locking screws at the ends
• Longitudinal flutes

Question 14

How does distraction osteogenesis work?

Answer 14

• distracts 1 mm per day
• physical therapy 2hr per day to allow muscles, tendons and soft tissue to adapt to the new bone size
• frame allows weight bearing

Question 15

What assumptions would we make to model the mechanics of an external fixation device?

Answer 15

1. Stiff side bar
2. Bone is stiff with respect to pins
3. Deformation due to deflection in the pins

Question 16

What does bone implant anchorage depend on?

Answer 16

1. Bone implant interface
   - size
   - location
   - mechanical properties
2. Peri-implant bone
   - architecture
   - material properties
3. Implant
   - geometry
   - stiffness
   - loading

Question 17

Fixation of fractures occurs nowadays through fixating plates. Additionally to the conventional design, a new system known as “locking screws fixator” is becoming more popular. Name 2 advantages of such system

Answer 17

1. Tissue doesn’t need to be compressed against plate to be stable
2. Failure mechanism is much more sturdy. The conventional system relies on shear strength on the threads-bone interface and failure mechanism is known as “sequential pullout”. On the other hand, locking screws systems rely more on normal strength and overall tissue strength: the failure mechanism is the “en bloc” rupture of the tissue, which is much more difficult to achieve.

Question 18

Give a disadvantage of the “locking screw fixator” system.

Answer 18

Lack of possibility of drilling screw in other directions other than perpendicular to the bone. It has been shown that having screws in different directions from one another decreases the risk of failure

Question 19

What is a Dynamic Hip Screw?

Answer 19

A type of orthopaedic implant designed for fixation of certain types of hip fractures which allows controlled dynamic sliding of the femoral head component along the construct.

Question 20

What is the idea being the mechanism of a Dynamic Hip Screw?

Answer 20

The idea behind the dynamic compression is that the femoral head component is allowed to move along one plane; since bone responds to dynamic stresses, the native femur may undergo primary healing: cells join along boundaries, resulting in a robust joint requiring no remodeling.

Question 21

Explain the Tension Band Principle to prevent fracture of the femur.

Answer 21

If a device (here a plate) is applied to the tension side
of the bone, so that opening cannot occur, the load
becomes a compressive one over the whole width of
the bone. The lateral plate functions as “tension
band”. Tension is absorbed by the plate. The
compression cortex becomes stable.