Müller AO classification of fractures—
long bones
Joseph Borrelli, US
AOTrauma Principles Course
Learning outcomes
• Explain the rationale and process of the comprehensive
classification of fractures and how it can be used in
clinical decision making
• Not to provide a detailed guide to implementation
• Discussion is limited to bones, segments, types, and
groups, which is what is normally needed for every day
clinical application and communication
—
Bone Segment
Type Group
.
Subgroup
“The basis of all clinical activity, be it:
• assessment and treatment,
• investigation and evaluation,
• learning and teaching,
must be based upon sound data, properly assembled,
clearly expressed, and readily accessible”.
W M Murphy and D Leu
History
• AO group saw need for “sound data” after ORIF became
“acceptable” in order to assess the efficacy/risks
• AO group started documentation of fracture treatment
• Volume of data collected led to the development of a
classification system
• 1960s–70s almost every fracture had its own
classification which led to the need for a universal
system
“A classification is useful only if it considers the severity
of the bone lesion and serves as a basis for treatment
and for evaluation of the results.”
Maurice E Müller, 1988
Comprehensive classification of fractures
Müller AO Classification
• Not only a way to document fractures
• Helps to understand fractures in biomechanical and
biological terms
• Offers competence in: data acquisition, data storage,
and data retrieval
• Provides a framework a surgeon can recognize, identify,
and describe the injury to the bone
Comprehensive classification of fractures
Müller AO Classification
The alpha-numeric notation serves:
• Guide to assessment of fracture whatever depth the
situation requires
• Allows surgeon to record/store observations
• Dependent upon accurate fracture description
Ground rules
• Colors denote the progressive levels of severity
• Describe fracture localization:
- Bones and segments
• Long bones divided into 1 diaphyseal, 2 epiphyseal and 2
metaphyseal segments
• No distinction between epiphysis and metaphysis
• Metaphysis is defined by a square length = widest part of
epiphysis
• “Center” of fracture needs to be determined
The principles of fracture classification
1
2
3
4
Each bone and bone region
is numbered
The principles of fracture classification
• Long bones are each divided into three segments
• Labeled 1, 2, 3 from proximal to distal
1
2
3
The principles of fracture classification
• Generally, proximal and distal segments are defined by a
square whose sides = length of widest part of epiphysis
• Exceptions:
- Proximal femur
- Proximal humerus
- Malleolar segment
The principles of fracture classification
• After documenting the location of the fracture (bone and
segment), the “type” of fracture is determined (A, B, C):
• Q. Is the fracture simple or multifragmentary?
A. If simple = type A, if multifragmentary = type B or C
• Q. If multifragmentary, is there a single wedge shaped
fragment or a more complex fracture pattern?
A. Wedge = type B, or more complex = type C
Which type of fracture?
Types A, B, C:
A
A = simple pattern
B = multifragmentary, wedge
C = multifragmentary, complex
12-
22-
32-
12
22
32
42
B
42C
—
.
The principles of the fracture classification
Q. Which type of fracture?
A. Metaphyseal/epiphyseal types (1 or 3)
Q. Is the fracture extraarticular or intraarticular?
A. If extraarticular = type A
Q. If intraarticular, does it involve a portion of the articular
surface or the entire articular surface?
A. If partial-articular =type B, if complete-articular = type C
The principles of fracture classification
Review of metaphyseal/epiphyseal types
Which types of fracture? metaphyseal or epiphyseal
A
13
Metaphyseal/epiphyseal types
A
Extra-articular fracture
B
Partial articular fracture – part of joint
remains in continuity with diaphysis
C
Complete articular fracture – no part of joint
remains in continuity with diaphysis
33
21
41
23
43
B
C
Fracture types for 11- and 31Proximal humerus 11-
Proximal femur 31-
A = extraarticular, unifocal
A = trochanteric area
B = extraarticular, bifocal
B = neck fracture
C = intraarticular fracture
C = head fracture
A
1
1-
B
3
1C
Malleolar segment 44• A = infrasyndesmotic lateral lesion
• B = transsyndesmotic fibular fracture
• C = suprasyndesmotic fibular fracture
A
B
C
The principles of the fracture classification
• Fractures are coded to the level of the type
• Groups and subgroups are:
- Ascending order of severity
- According to the morphological complexities and
difficulties inherent in their treatment and their
prognosis
—
Bone Segment
.
Type Group
Subgroup
Diaphyseal fractures
To classify the fracture beyond the type continue with the
“binary” concept of reasoning:
• Q. Is the fracture simple or multifragmentary?
A. If simple then type A fracture
• Q. Was the fracture the result of twisting or bending?
A. Twisting mechanisms typically results in a spiral type
fracture = group 1, if bending, then = group 2 or 3
• Q. Bending?
A. Then, is the inclination of the fracture greater or less
than 30º ? group 2 if > 30º (A2) group 3 if < 30º (A3)
12-
42- Diaphyseal groups
32-
—
>30°
>30°
A1
A2
A3
.
<30°
<30°
B1
B2
B3
C1
C2
C3
Diaphyseal fractures
Type B fractures are multifragmentary wedge type fractures
Groups for B type fractures:
B1 = spiral wedge,
B2 = bending wedge
B3 = fragmented wedge
12-
42- Diaphyseal groups
32-
—
>30°
>30°
A1
A2
A3
.
<30°
<30°
B1
B2
B3
C1
C2
C3
Diaphyseal fractures
Type C fractures are multifragmented complex fractures
Groups for C type fractures:
C1 = complex, spiral
C2 = complex, segmental
C3 = complex, irregular
12-
42- Diaphyseal groups
32-
—
>30°
A1 A2
A3
.
<30°
<30°
B1
B2
B3
C1
C2
C3
Metaphyseal/epiphyseal fractures
These are segment 1 and 3 fractures
Remember:
A = extraarticular
B = partial articular
C = complete articular
Metaphyseal/epiphyseal fractures
A1 = metaphyseal simple
A2 =metaphyseal wedge
A3 = metaphyseal complex
A1
A2
A3
Metaphyseal/epiphyseal fractures
B1 = lateral condyle, sagittal
B2 = medial condyle, sagittal
B3 = frontal plane fracture
Metaphyseal/epiphyseal fractures
• C1 = articular and metaphyseal simple
• C2 = articular simple,metaphyseal multifragmentary
• C3 = articular and metaphyseal multifragmentary
C1
C2
C3
Malleolar segment fractures 4444-A = infrasyndesmotic fibular fracture
44-B = transsyndesmotic fibular fracture
44-C = suprasyndesmotic fibular fracture
A
B
44
C
Proximal humeral and femoral fractures
Humerus proximal 1111-A extraarticular unifocal fracture
11-B extraarticular bifocal fracture
A
11-C articular fracture
11 31
B
• Femur proximal 31• 31-A trochanteric area fracture
• 31-B neck fracture
• 31-C head fracture
C
Outcome validation of the AO/OTA
classification system
Objectives:
• To determine whether a greater severity of injury as
documented by the AO/OTA code would correlate with
poor scores of:
- Impairment
- Functional performance
- Self-reported health status
• 200 patients, 3 Level I centers (Seattle, Nashville,
Baltimore): each patient with unilateral and isolated
lower extremity fracture [MF Swiontkowski et al 2000]
Outcome validation of the AO/OTA
classification system
Conclusions:
C-type fractures had a significantly worse functional
performance and impairment compared with B-type
fractures, but B-type fractures were not statistically different
from A-type fractures [MF Swiontkowski et al 2000]
AO Classification
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
A
B
C
Impairment
ROM
Ambulation
Outcome validation of the AO/OTA
classification system
Conclusions:
Further studies validating the AO/OTA fracture
classification system are required with adequate number of
cases for each region of injury to allow separate analysis of
the results [MF Swiontkowski et al 2000]
AO Classification
4.5
4
3.5
3
2.5
A
2
B
1.5
C
1
0.5
0
Impairment
ROM
Ambulation
Classification
• Bone = 3
• Segment = 3
•• Type
Type =
=C
•• Group
Group =
=3
• 33-C3
Classification
• Bone = 3
• Segment = 2
•• Type
Type =
=A
•• Group
Group =
= 3 (bending, < 30°)
• 32-A3
Summary
• AO comprehensive classification of fractures is:
-
Comprehensive
Adaptable
Consistent
Dynamic
• Depends on the surgeon’s ability to accurately
assess the fracture pattern