1. Relative stability: biomechanics, techniques, and fracture healing
Published: July 2013
Steven R Schelkun, US
AOT Basic Principles Course

2. Learning outcomes Define relative stability
Describe the biological behavior of fractured bone and how it is affected by relative stability
Define indication for selection of relative stability according to AO principles
Explain techniques for achieving relative stability
Teaching points:
Consider the spectrum of stability.

3. How stability affects healing
Fixation of fractures alters the biology of fracture healing
Method of bone healing depends on:
Type of fracture (simple or complex)
Type of reduction (anatomical or alignment)
Type of stability achieved (absolute or relative)
Type of implant chosen (providing absolute or relative stability)
We have learned that the fixation of fractures alters the biology of fracture healing.
The method of bone healing will depend on:
Type of fracture
Type of reduction
Type of stability achieved
Type of implant chosen

4. Definition of relative stability
Indicates there is some motion between fracture fragments
Motion must be below the limits of tolerance of healing bone tissue (otherwise healing disruption will occur)
Best methods to produce relative stability include some type of extramedullary or intramedullary splint
Bones that heal by relative stability are characterized by a callus formation
Unlike absolute stability, relative stability indicates that there is some motion between the fracture fragments.
However, the motion that occurs must be below the limits of tolerance of the healing bone tissue, otherwise disruption of the healing process will occur.
The best methods to produce relative stability include some type of extramedullary or intramedullary splint.
And unlike bones that heal by absolute stability, bones that heal by relative stability are characterized by callous formation.

5. Multifragmentary fractures
Tolerate more motion between the fracture fragments
Overall motion is shared by several fracture planes, which reduces tissue strain or fracture and deformation at the fracture gap
Flexible fixation can stimulate callus formation thereby accelerating fracture healing
X-ray courtesy of Steven R Schelkun, MD
It is well-known that multifragmentary fractures will tolerate more motion between the fracture fragments.
The overall motion is shared by several fracture fragments and fracture planes which reduces tissue strain on the individual fracture fragments.
Flexible fixation can stimulate callus formation thereby accelerating fracture healing as long as the interfragmentary motion does not exceed the tolerance level of the healing granulation tissue of early callous formation.

6. Cascade of events in fracture healing
Acute fracture
Hematoma formation
Microvascular ingrowth of granulation tissue
Type of healing depends on:
Type of fracture
Type of reduction
Stability needed
Implant chosen
Cell differentiation to bone
Surgical intervention
Simple fracture
Complex fracture
Illustration courtesy of Steven R. Schelkun, MD
When we look at the cascade of events that occur in the fracture healing process a surgeon may choose to intervene in this process to provide alignment and stability.
The decision that the surgeon must make should depend on the type of fracture, the type of reduction needed, the type of stability needed, and implant chosen.
When presented with a complex fracture, the surgeon’s goal should be to achieve anatomical alignment. Consequently, the type of implant that he chooses should produce relative stability.
If relative stability is achieved, you can expect healing by indirect bone healing.
Anatomical reduction
Anatomical alignment
Absolute stability
Relative stability
Direct bone healing
Indirect bone healing

7. Complex fractures Cannot be reduced anatomically
Without damaging blood supply
Needs anatomical alignment
Best done with indirect reduction techniques
Needs only relative stability
Heals with callus formation
X-ray courtesy of Steven R. Schelkun, MD
Complex fractures cannot be and should not be reduced anatomically for fear of damaging the blood supply to the individual fracture fragments.
These complex fractures need anatomical alignment not anatomical reduction of each fragment.
This anatomical alignment is best done with indirect reduction techniques.
Consequently this type of fracture needs only relative stability and you can anticipate callous formation.

8. Types of stability required
Multifragmentary fractures can tolerate more motion between the many displaced fragments
Require indirect reduction and only relative stability
X-rays courtesy of Steven R. Schelkun, MD
This complex fracture has two major components.
The articular portion needs anatomical reduction and stable fixation with interfragmentary compression.
The complex metaphyseal portion needs anatomical alignment which is done with indirect reduction techniques and an implant that provides relative stability, such as this locking condylar plate.
As seen on the right this fracture heals rapidly with callus formation.

9. Relationship between fracture and stability
The closer the fracture fragments, the more stability is needed to prevent disruption of healing granulation tissue
Anatomically reduced fractures require absolute stability
X-ray courtesy of Steven R. Schelkun, MD
The Perren strain theory indicates that the closer the fracture fragments, the more stability is needed to prevent disruption of the healing granulation tissue between the fragments.
As a corollary to this is that anatomically reduced fractures require absolute stability. Complex fractures however require only anatomical alignment and an implant that produces relative stability.

10. Clinical indications for relative stability
Any nonarticular, multifragmentary fracture
X-rays courtesy of Steven R. Schelkun, MD
The clinical indications for providing relative stability include any non-articular multifragmentary fracture as seen by these three x-rays.

11. Methods to produce relative stability
Traction
Casts
External fixation
Internal fixators (fixed-angle devices)
Intramedullary nailing
Bridge plating
Listed below are the most common methods to produce relative stability:
Traction
Casting
External fixation
Internal fixators such as fixed angle devices
Intramedullary nailing
Bridge plating

12. Examples of methods for relative stability
Traction
Pictures courtesy of Steven R. Schelkun, MD
Traction has been a time honored approach to producing alignment, relative stability, and healing by callous formation.

13. Examples of methods for relative stability
Casting
X-rays courtesy of Steven R Schelkun, MD
Casting is an example of an external splint that produces relative stability and fracture healing by callous formation.

14. Examples of methods for relative stability
External fixation
Pictures courtesy of Steven R Schelkun, MD

15. Examples of methods for relative stability
Internal fixators
X-rays courtesy of Steven R. Schelkun, MD
Internal fixtures such as fixed-angle plates usually function as bridge plates to provide relative stability and healing by callous formation.

16. Examples of methods for relative stability
Intramedullary nails
X-rays courtesy of Steven R. Schelkun, MD
Intramedullary nails are excellent examples of an internal splint that provides relative stability.

17. Examples of methods for relative stability
Bridge plating
X-rays courtesy of Steven R. Schelkun, MD

18. Indirect bone healing with callus
X-rays courtesy of Steven R. Schelkun, MD
The key features of this technique are to produce anatomical length, rotation, alignment, and relative stability with a bridge plate that will allow the small amount of callous formation and accelerated bone healing.

19. Take-home messages
Relative stability indicates that there is a small amount of motion between fracture fragments
Clinical indication for applying implants for relative stability include all nonarticular, multifragmentary fractures
A small amount of interface with motion will stimulate callous formation and accelerate bone healing
Common methods for relative stability include traction, casting, external fixation, internal fixation, bridge plating, and intramedullary nails