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

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.

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

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.

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.

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

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.

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.

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.

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.

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

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.

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.

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

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.

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.

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

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.

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