Müller AO Classification of fractures

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The Müller AO Classification of fractures is a system for classifying bone fractures initially published in 1987[1] by the AO Foundation as a method of categorizing injuries according to therognosis of the patient’s anatomical and functional outcome. "AO" is an initialism for the German "Arbeitsgemeinschaft für Osteosynthesefragen", the predecessor of the AO Foundation.

It is one of the few complete fracture classification systems to remain in use today after validation.[2]

Comprehensive classification of the long bones[edit]

The English language version of the system[3] allows consistent in detail description of a fracture in defined terminology by creating a 5-element alphanumeric code:

Localisation Morphology
Bone Segment Type Group Subgroup
1/2/3/4 1/2/3/(4) A/B/C 1/2/3 .1/.2/.3


First, each fracture is given to 2 numbers to describe which bone it affects, and where in the bone:

1 2 3 4
Bone Humerus Radius and Ulna Femur Tibia and fibula
Segment Proximal segment Diaphyseal segment Distal segment Malleolar segment (only used with tibia and fibula


Each fracture is next given a letter (A, B or C) to describe the joint involvement of the fracture:

Segment A B C
1 Extra-articular Partial articular Complete articular
2 Simple Wedge Complex
3 Extra-articular Partial articular Complete articular

The exceptions to this step include:

Localisation A B C
11 - Proximal humerus Extra-articular, unifocal Extra-articular, bifocal Articular
31 - Proximal femur Extra-articular, trochanteric Extra-articular, neck Articular, head
44 - Malleoli Infrasydesmotic Transyndesmotic Suprasyndesmotic

Groups & Subgroups[edit]

Finally, the fracture is given 2 further numbers to denote the fracture pattern and geometry.

For segment 2 (diaphyseal) fractures:

Type Group
1 2 3
A - simple Spiral Oblique Transverse
B - wedge Spiral Bending Multifragmentory
C - complex Spiral Segmental Irregular

For segment 1 and 3 (epiphyseal and metaphyseal) fractures:

Type Group
1 2 3
A - extra-articular Simple Wedge Complex
B - partial articular Split Depression Split-depression
C - articular Simple articular, simple metaphyseal Simple articular, complex metaphyseal Complex articular, complex metaphyseal

Subgroups are then used to describe the fractures in terms of displacement (versus apposition, which is the degree to which the parts are in contact with each other), rotation, angulation and shortening.

AO pediatric comprehensive classification of long bone fractures[edit]

A pediatric version of the long-bone classification was published in 2006[4] to further classify fractures of immature bone and so the effects on future growth:

Localisation Morphology
Bone Segment Type Child Severity Exceptions
1/2/3/4 1/2/3 E/M/D 1-9 .1/.2 I-IV

OTA/AO Classification unifying extension[edit]

The Orthopaedic Trauma Association Committee for Coding and Classification initially published their classification system covering the whole skeleton in 1996.[5] In 2006[6] they published a revision, unifying the Muller/AO and OTA systems into a single alphanumeric classification:

Localisation Region/Bone
Bone Segment
1 4 Scapula
5 Clavicle
3 4 Patella
5[7] 1 Cervical spine
2 Thoracic spine
3 Lumbar spine
6 1 Pelvic ring[8]
2 Acetabulum[9]
7[10] 1 Lunate
2 Scaphoid
3 Capitate
4 Hamate
5 Triquetrum and Pisiform
6 Trapezium and Trapezoid
7 Metacarpus
8 Phalanges
9 Multiple fractures
8[11] 1 Talus
2 Calcaneus
3 Navicular
4 Cuboid
5 Cuneiforms
7 Metatarsus
8 Phalanges
9 Multiple fractures
9 1 Craniomidface[12]
2 Mandible[13]


  1. ^ Müller ME, Nazarian S, Koch P (1987). Classification AO des fractures. Tome I. Les os longs. Berlin: Springer-Verlag. 
  2. ^ Audigé L, Bhandari M, Kellam J (2004). "How reliable are reliability studies of fracture classifications? A systematic review of their methodologies". Acta Orthop Scand. 75 (2): 184–94. PMID 15180234. doi:10.1080/00016470412331294445. 
  3. ^ Müller ME, Nazarian S, Koch P, et al. (1990). The Comprehensive Classification of Fractures of Long Bones. New York: Springer-Verlag. 
  4. ^ Slongo T, Audigé L, Schlickewei W, Clavert JM, Hunter J, International Association for Pediatric Traumatology (2006). "Development and validation of the AO pediatric comprehensive classification of long bone fractures by the Pediatric Expert Group of the AO Foundation in collaboration with AO Clinical Investigation and Documentation and the International Association for Pediatric Traumatology". J Pediatr Orthop. 26 (1): 43–9. PMID 16439900. doi:10.1097/01.bpo.0000187989.64021.ml. 
  5. ^ Orthopaedic Trauma Association Committee for Coding and Classification (1996). "Fracture and dislocation compendium". J Orthop Trauma. 10 (Suppl 1:v-ix): 1–154. PMID 8814583. 
  6. ^ Orthopaedic Trauma Association (2007). "Fracture and Dislocation Classification". J Orthop Trauma. 21 (Suppl): S1–S133. PMID 18277234. 
  7. ^ Magerl F, Aebi M, Gertzbein SD, Harms J, Nazarian S (1995). "A comprehensive classification of thoracic and lumbar injuries". Eur Spine J. 3 (4): 184–201. PMID 7866834. doi:10.1007/bf02221591. 
  8. ^ Tile M (2003). Fractures of the Pelvis and Acetabulum. Philadelphia: Williams & Wilkins. 
  9. ^ JUDET R, JUDET J, LETOURNEL E (Dec 1964). "Fractures of the acetabulum: classification and surgical approaches for open reduction". J Bone Joint Surg Am. 46: 1615–46. PMID 14239854. 
  10. ^ Petracić B, Siebert H (Jan 1998). "AO Classification of fractures of the hand bones". Handchir Mikrochir Plast Chir. 30 (1): 40–44. PMID 9541837. 
  11. ^ Zwipp H, Baumgart F, Cronier P, Jorda E, Klaue K, Sands AK, Yung SW (Sep 2004). "Integral classification of injuries (ICI) to the bones, joints, and ligaments— application to injuries of the foot". Injury. 35 (Suppl 2): SB3–9. PMID 15315874. doi:10.1016/j.injury.2004.07.008. 
  12. ^ Buitrago-Téllez CH, Schilli W, Bohnert M, Alt K, Kimmig M (Oct 2002). "A comprehensive classification of craniofacial fractures: postmortem and clinical studies with two- and three-dimensional computed tomography". Injury. 33 (8): 651–68. PMID 12213415. doi:10.1016/s0020-1383(02)00119-5. 
  13. ^ Spiessl B (ed) (1989). AO Classification of Mandibular Fractures. Berlin: Springer-Verlag. 

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