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Classification · Radial head fracture

AO/OTA Classification of Radial Head Fractures in 3D

The seven codes of the AO/OTA 2018 classification of the proximal end of the radius (bone 2R1: A extra-articular, B partial articular, C complete articular), on the radial head fracture 3D model, seen in lateral view at 90° and AP view as on elbow radiographs. With how to read the code, its link with the Mason classification, reliability data, grading pitfalls and free infographics for teaching and publication.

Designed by an orthopedic surgeon

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Infographic: six AO/OTA 2018 codes of the radial head on a 3D elbow, from the simple neck fracture (2R1A2) to the whole head in several fragments (2R1C3), with the lateral view at 90° of flexion and the AP view under each code.
Free to reuse under CC BY 4.0. Download in high definition.

The codes

Types A, B and C, in 3D

Each image is a rendering of the EMET Education radial head fracture model at that code, left elbow seen in lateral view at 90° of flexion, the head beneath the capitellum and the anterior fragment above it, with a narrow field to approach the projection of a lateral radiograph; the infographic adds the AP view. The model draws six of the seven codes: 2R1A1 (avulsion of the bicipital tuberosity) is described without an image. The displacements shown are typical cases, adjustable in the model: the code itself does not say whether the fracture is displaced, that is the role of the modifiers. Under each code, the closest type of the Mason classification according to the model.

  1. 2R1A1Bicipital tuberosity avulsion

    Definition
    Type A, group 1: avulsion of the bicipital tuberosity of the radius, where the biceps tendon inserts. Biceps tendon avulsions usually occur without a fracture; weakness of supination suggests one (AO Surgery Reference).
    Mason, indicative
    Outside Mason, which describes only the head.
    In the 3D model
    Not drawn: the model shows neither the bicipital tuberosity nor the distal biceps tendon.
  2. Left elbow in 3D, lateral view at 90° of flexion: simple fracture of the radial neck, the head tilted on the neck, code 2R1A2.

    2R1A2Neck, simple

    Definition
    Type A, group 2: simple extra-articular fracture of the radial neck, one single line (AO Surgery Reference). When displaced and unstable, the AO Surgery Reference advises looking for injury to the medial collateral ligament (valgus mechanism), to both collateral ligaments (elbow dislocation) or to the interosseous ligament of the forearm (Essex-Lopresti injury).
    Mason, indicative
    Type II, by convention: Mason described only the head, reviews place a displaced or angulated neck in type II (van der Windt et al.).
    In the 3D model
    The head tilts 35° outward, translated 2 mm: one simple line through the neck, with no other fragment.
    Animate 2R1A2 in 3D
  3. Left elbow in 3D, lateral view at 90° of flexion: fracture of the radial neck in several fragments, code 2R1A3.

    2R1A3Neck, multifragmentary

    Definition
    Type A, group 3: extra-articular multifragmentary fracture of the neck. It is uncommon and difficult to repair (AO Surgery Reference).
    Mason, indicative
    Type II, by convention, like the simple neck (EMET Education's reading: Mason's definition applies only to the head).
    In the 3D model
    The ring of the neck in three fragments, the head angulated 25° with a 2 mm gap.
    Animate 2R1A3 in 3D
  4. Left elbow in 3D, lateral view at 90° of flexion: an anterior fragment of the radial head, displaced and tilted, code 2R1B1.

    2R1B1Partial articular, simple

    Definition
    Type B, group 1: simple partial articular fracture: one fragment of the articular surface of the head breaks off, the rest of the head stays attached to the neck (AO Surgery Reference). The fracture is called stable when no ligament injury of the elbow or forearm goes with it; it is unstable with an elbow dislocation or in an Essex-Lopresti injury, and a wide gap between fragments suggests an associated ligament or bone injury.
    Mason, indicative
    Type I with modifier [1] (undisplaced); type II with [2] (displaced: at least 2 mm and 30% of the head according to the threshold attributed to Broberg and Morrey).
    In the 3D model
    Anterolateral fragment of 135° (about 34% of the head), displaced 3 mm, tilted 20° and depressed 1 mm.
    Animate 2R1B1 in 3D
  5. Left elbow in 3D, lateral view at 90° of flexion: partial fracture of the radial head in several fragments, code 2R1B3.

    2R1B3Partial articular, fragmentary

    Definition
    Type B, group 3: fragmentary partial articular fracture: the detached portion of the articular surface is itself in several pieces, the rest of the head staying attached to the neck (AO Surgery Reference). The same stability conditions as for 2R1B1 apply.
    Mason, indicative
    Type II (Hotchkiss's reading: reconstructable); Mason reserved type III for the whole head being comminuted (van der Windt et al.). Modifier [1] or [2] says whether it is displaced.
    In the 3D model
    A 135° sector in 3 fragments, displaced 3 mm, tilted 15° and depressed 1.5 mm.
    Animate 2R1B3 in 3D
  6. Left elbow in 3D, lateral view at 90° of flexion: the radial head split and detached from the neck, code 2R1C1.

    2R1C1Complete articular, simple

    Definition
    Type C, group 1: simple complete articular fracture: the whole articular surface of the head is detached from the neck, the fracture remaining simple, without fragmentation (AO Surgery Reference). It is stable without an associated ligament injury, unstable with an elbow dislocation or in an Essex-Lopresti injury; a gap between fragments suggests an associated ligament or bone injury.
    Mason, indicative
    Type III, the model's reading: the whole head is involved, but Mason does not describe this case separately (Hotchkiss: depending on whether the head is reconstructable).
    In the 3D model
    Fractured neck (3 mm gap, 10° angulation, 1 mm translation) and head split in two: the model's reading of "simple" (two lines).
    Animate 2R1C1 in 3D
  7. Left elbow in 3D, lateral view at 90° of flexion: the radial head shattered into several fragments, code 2R1C3.

    2R1C3Complete articular, multifragmentary

    Definition
    Type C, group 3: multifragmentary complete articular fracture: the whole head is detached from the neck and fragmented (AO Surgery Reference). Beyond 3 articular fragments, fixation becomes unreliable (13 of 14 unsatisfactory results: Ring et al.).
    Mason, indicative
    Type III: comminuted fracture of the whole head. Modifier [1] or [2] says whether it is displaced.
    In the 3D model
    The neck and four fragments of the head (3 to 5 if chosen), displaced 4 mm, tilted 20° and depressed 2 mm.
    Animate 2R1C3 in 3D

Schematic rendering, for teaching: the model illustrates each code on a single elbow, it does not reproduce a radiograph. Definitions reworded from the 2018 compendium (Meinberg et al.) and the AO Surgery Reference; their figures and pictograms are not reproduced. The bar shows the type: one segment A, two B, three C. The correspondences with Mason are the model's, for guidance.

Reading the code

From Müller (1990) to the 2018 compendium

The classification comes from Müller et al.'s classification of long-bone fractures, adopted by the Orthopaedic Trauma Association, whose 2018 compendium is the reference version. For the proximal end of the radius, it comes down to seven codes that stop at the group: there is no subgroup, and the numbers 2 of types B and C do not exist. The two reliability studies of the radial head (2009) predate this version: their results do not transfer as such to the 2018 code.

The code reads from left to right:

  • 2R1: bone 2, the forearm; the letter R, the radius (U for the ulna: an injury to both bones is coded twice); its segment 1, the proximal end. End segments are delimited by a square, the two forearm bones being taken as a unit;
  • the type: A, extra-articular fracture (the bicipital tuberosity or the neck); B, partial articular (part of the surface of the head stays attached to the neck); C, complete articular (the whole surface of the head is detached from the neck);
  • the group: A1 avulsion of the bicipital tuberosity, A2 simple neck, A3 multifragmentary neck; B1 simple partial articular, B3 fragmentary partial articular; C1 simple complete articular, C3 multifragmentary complete articular;
  • universal modifiers, as numbers in square brackets, shared by all bones: [1] undisplaced, [2] displaced, [3] impaction (articular or metaphyseal), [4] no impaction, [5] dislocation, with its direction ([5a] to [5f]) (Meinberg et al.). Thus 2R1B1[2]: simple partial articular fracture, displaced; 2R1B1[2,5b] with a posterior dislocation. See also the proximal humerus page, which goes into how to read the code.

The code describes the line and the number of fragments, not how far it is displaced or the neighbouring injuries: a simple fracture of the head may go with a coronoid fracture or a dislocation, which do not appear in the letters and numbers of the code (see the Mason classification and the Mayo suffixes).

Mason and AO/OTA

From the Mason type to the AO/OTA code

The two systems do not overlap: Mason mostly describes displacement and fragment size, the AO/OTA code the line and the number of fragments, with displacement as a modifier. A single code therefore covers several Mason types (2R1B1 is type I or II depending on displacement), and a single type several codes. The table gives the model's correspondence, on the Mason classification page.

The model's and the Mason page's correspondence, for guidance: this is not a table from the compendium or the AO Surgery Reference. References 1, 2 and 13.
Mason typeAO/OTA 2018 codeNote
I, undisplaced2R1B1[1]Fissure or marginal fracture with no displacement: simple partial articular fracture, with modifier [1] (Meinberg et al.). A displaced fragment of less than 2 mm or less than 30% of the head remains type I since Broberg and Morrey (attributed threshold).
II, displaced fragment2R1B1[2]The same code, with modifier [2]: displacement makes the type, not the line.
II, several fragments2R1B3Fragmented sector, head continuous with the neck: read as type II as in Hotchkiss; Mason reserved III for the whole head being comminuted.
II, neck2R1A2 (2R1A3 if multifragmentary)Type II by convention (van der Windt et al.): the neck is extra-articular in the AO/OTA, therefore type A.
III, head detached from the neck and split2R1C1The model's reading: the whole head is involved, Mason does not describe this case separately.
III, comminuted2R1C3The whole head in several fragments: this is Mason's definition.
IV, with elbow dislocationthe head's code, with [5]Dislocation becomes a modifier, with its direction ([5a] to [5f]); Johnston's type IV (1962) has no code of its own. The head's code remains that of its fracture.

Reliability

How reproducible is the code?

Poorly, on radiographs, in the two studies we found: both date from 2009 and concern a version earlier than the 2018 one; we found no reliability study of the 2018 code for the radial head. In both, the Mason (or Hotchkiss) classification agrees better than the AO; in Sheps et al., dropping the last digit of the code improves agreement.

Reliability of the AO code of the radial head (references 4 and 5). Both studies concern a version earlier than 2018, which their abstract does not specify; the numbers of radiographs are not all given in the abstracts; coefficients are not directly comparable.
StudyMeasureValue
Matsunaga et al., 2009AO/ASIF, 4 observers, 3 sessions: inter- then intraobserver kappa, rated poor by the authors (Mason: 0.43–0.56 · 0.58)0.25–0.48 · 0.48
Sheps et al., 2009Full AO code, 5 observers, 43 patients: interobserver kappa, "fair" (Hotchkiss: 0.59, "moderate"), mean raw agreement 37.7%0.26
Sheps et al., 2009AO code without its last digit: interobserver kappa, "moderate" (Hotchkiss with types II and III combined: 0.76, "substantial"), mean raw agreement 67.4%0.46

In these series, the full AO code therefore agrees less well than Mason or Hotchkiss (Matsunaga et al., Sheps et al.), and the simplification, which amounts to keeping only the type and the group, only reaches the low end of the "moderate" range. In practice: the type to communicate, the full code for research, with written thresholds, and the Mason classification or Hotchkiss's to decide.

Neighbouring classifications

Mason, Broberg and Morrey, Hotchkiss, Mayo

For the radial head, the AO/OTA is mostly the code of registries and studies; the neighbouring classifications, older, steer decisions or describe the associated injuries. Their principles, from the reviews and the original papers:

References 8 to 12. Only the reliability comparison comes from studies; the relations of principle are the page's.
ClassificationPrincipleRelation to the AO/OTA
Mason, 1954 and Johnston, 1962I: undisplaced fissure or marginal fracture; II: displaced marginal sector; III: whole head comminuted; IV (Johnston): head fracture with elbow dislocationDescribes displacement and dislocation, which the AO/OTA leaves to modifiers. See the Mason page.
Broberg and Morrey, 1987Type II: at least 2 mm of displacement and at least 30% of the articular surface; takes up Johnston's type IVThreshold attributed to these authors by the reviews: it gives a numerical meaning to modifier [2] for the head.
Hotchkiss, 1997I: undisplaced or minimally displaced (under 2 mm), no block; II: displaced (often over 2 mm) or angulated neck, reconstructable; III: comminuted, not reconstructable, excision or replacementReasons in terms of management. Interobserver agreement "moderate" (κ 0.59) against "fair" for the AO (0.26) in Sheps et al.
Mayo suffixes (van Riet and Morrey, 2008)The Mason type, followed by letters for associated injuries: c coronoid, o olecranon, l LCL, m MCL, d distal radioulnar jointAssociated injuries are not in the head's AO/OTA code: the model adds them to its reading.

Use

In research and in clinical practice

In research

The AO/OTA code is the language of trauma registries and databases: it fits on one line, sorts and groups. In a prospective series of 201 radial head or neck fractures, increasing complexity according to the AO-OTA (version not specified in the abstract), radiographic comminution and age, and the choice to operate, were independent predictors of a poorer outcome (Duckworth et al.). The 2018 code is also used to label radiographs for artificial intelligence: across 5,367 elbow examinations and 54 fracture types, the weighted mean area under the curve was 0.80 for the proximal radius (Pettersson et al.).

In clinical practice

The type, A, B or C, sums up the essentials: neck, partial articular fracture (a fragment stays attached to the neck) or fracture of the whole head. The code alone tells neither whether the fracture is displaced nor whether it is stable. The AO Surgery Reference points out that a displaced, unstable neck fracture calls for a search for collateral or interosseous ligament injury, and that an articular fracture is unstable with an elbow dislocation or in an Essex-Lopresti injury (AO Surgery Reference).

It is the Mason classification (or Hotchkiss's), more than the AO/OTA code, that steers treatment: fragment displaced at least 2 mm and 30% of the head, number of articular fragments (beyond three, fixation is unreliable: Ring et al.), mechanical block, associated injuries. The AO/OTA code is recorded alongside, with displacement modifier [1] or [2], for the report and for research.

Grading pitfalls

Five pitfalls when coding a radial head fracture

  • Displacement is not in the code. A simple articular fragment is a 2R1B1, displaced or not: record [1] or [2], and state the threshold used (2 mm and 30% of the head, attributed to Broberg and Morrey). Without a modifier, the code does not say whether the fracture is operated.
  • The neck is an A, even if the head is tilted. A line through the neck alone is extra-articular: 2R1A2 if simple, 2R1A3 if fragmentary. The head is B or C only if its articular surface is broken.
  • B or C? The question is whether any part of the articular surface stays attached to the neck. On radiographs, agreement between readers is poor (Sheps et al.): write down the criterion used and, when in doubt about the number of fragments, review the cross-sectional imaging.
  • Simple or fragmentary? An articular fragment in several pieces is a B3, even if the head stays continuous with the neck: it is a Mason type II in Hotchkiss's scheme. Do not write 2R1B2 or 2R1C2: these numbers do not exist for the radial head.
  • What is not in the code. The coronoid, the lateral collateral ligament, the distal radioulnar joint and the dislocation: the dislocation goes in modifier [5], the rest in the description (Mayo suffixes). A displaced, unstable neck fracture should prompt a search for an Essex-Lopresti injury (AO Surgery Reference).

Self-test

Which code?

The same elbow seen from three-quarters, at the six codes the model draws, in random order. Find the code of each one; the answer comes with the definition.

Elbow in 3D seen from three-quarters, at an unknown AO/OTA code of the radial head, for the self-test.

Which AO/OTA code is this?

Download and cite

Free infographics, for teaching and publication

Use them in a course, a presentation, a social media post or a scientific article, under the Creative Commons Attribution 4.0 license: free reuse, including commercial, provided EMET Education is credited with a link to this page. The license covers EMET Education's images, not the classification itself.

Portrait infographic of six AO/OTA 2018 radial head codes, one row per code.
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Cite this figure

Ready to paste. For a journal figure, add the original reference (Meinberg et al., 2018).

VancouverEMET Education. AO/OTA classification of radial head fractures: 3D visualization [Internet]. 2026 [cited]. Available from: https://www.emet-education.com/en/classifications/ao-ota-radial-head/. CC BY 4.0.
APAEMET Education. (2026). AO/OTA classification of radial head fractures: 3D visualization [Infographic]. https://www.emet-education.com/en/classifications/ao-ota-radial-head/ (CC BY 4.0)
HTML<a href="https://www.emet-education.com/en/classifications/ao-ota-radial-head/"><img src="https://www.emet-education.com/en/classifications/ao-ota-radial-head/img/infographic.webp" alt="AO/OTA classification of radial head fractures in 3D" width="960"></a><br>Figure: <a href="https://www.emet-education.com/en/classifications/ao-ota-radial-head/">EMET Education</a>, CC BY 4.0

Questions

Frequently asked questions

What does "2R1B1" mean in the code?

The 2 is the forearm, the letter R the radius (U: the ulna), the 1 the proximal segment. B is the type (partial articular fracture: a fragment of the head stays attached to the neck) and 1 the group (simple). There is no subgroup for the radial head. Modifiers, in square brackets ([1] undisplaced, [2] displaced, [5] dislocation), come after.

Which AO/OTA codes for Mason I, II, III and IV?

There is no official table; the page's correspondence is the model's, for guidance. Type I: 2R1B1[1]. Type II: 2R1B1[2] for one fragment, 2R1B3 for several fragments, 2R1A2 or 2R1A3 for the neck (by convention). Type III: 2R1C3 for the comminuted head, 2R1C1 for the detached, split head (the model's reading). Type IV: the head's code, with the dislocation modifier [5].

Why is a neck fracture "extra-articular"?

Because the compendium places it in type A: the AO Surgery Reference calls simple neck fractures 2R1A2 and multifragmentary neck fractures 2R1A3, under the heading "radius, extraarticular". Type B or C supposes that the articular surface of the head is itself broken.

AO/OTA or Mason: which one to use for the radial head?

To decide, Mason (or Hotchkiss): in the two 2009 studies we found, the AO code agrees less well (κ 0.26 against 0.59 for Hotchkiss in Sheps; 0.25–0.48 against 0.43–0.56 for Mason in Matsunaga). For a registry or a study, the AO/OTA code, with its displacement modifier. The two are recorded together.

Does the AO/OTA code say whether the fracture is displaced, stable or needs surgery?

No. Displacement is a modifier ([1] or [2]); stability is judged on association with an elbow dislocation, an Essex-Lopresti injury or a ligament injury, which the AO Surgery Reference mentions without including them in the code. The indication for surgery rests on displacement (2 mm and 30% of the head, threshold attributed to Broberg and Morrey), the number of fragments, mechanical block and associated injuries.

Can I reuse these images in a course or an article?

Yes. The images and infographics on this page are under the Creative Commons Attribution 4.0 license: free reuse, including commercial, with credit to EMET Education and a link to this page. The "Cite this figure" box gives ready-to-paste citations.

Do the images reproduce the AO drawings? How were they made?

No: no figure or pictogram from the compendium is reproduced, and the definitions are reworded. They are renderings of the EMET Education radial head fracture model, set to each code: the radius, humerus and ulna, taken from CT, are cut along the lines of the code and displaced so that they never cross each other. It is a schematic model for teaching; it does not reproduce a radiograph. EMET Education is not affiliated with the AO Foundation or the Orthopaedic Trauma Association.

Sources

Author and references

Page written by Dr. Mikaël Chelli, orthopedic surgeon. Last reviewed November 4, 2026.

  1. Meinberg EG, Agel J, Roberts CS, Karam MD, Kellam JF. Fracture and Dislocation Classification Compendium—2018. J Orthop Trauma. 2018;32 Suppl 1:S1–S170. doi:10.1097/BOT.0000000000001063
  2. AO Foundation. AO Surgery Reference, orthopedic trauma, adult trauma, proximal forearm: radius, extraarticular (avulsion of bicipital tuberosity 2R1A1, simple 2R1A2, multifragmentary 2R1A3), partial articular (2R1B1, 2R1B3) and complete articular (2R1C1, 2R1C3), definitions. surgeryreference.aofoundation.org. Consulté le 4 novembre 2026.
  3. Müller ME, Nazarian S, Koch P, Schatzker J. The Comprehensive Classification of Fractures of Long Bones. Berlin: Springer; 1990.
  4. Matsunaga FT, Tamaoki MJ, Cordeiro EF, et al. Are classifications of proximal radius fractures reproducible? BMC Musculoskelet Disord. 2009;10:120. doi:10.1186/1471-2474-10-120
  5. Sheps DM, Kiefer KR, Boorman RS, et al. The interobserver reliability of classification systems for radial head fractures: the Hotchkiss modification of the Mason classification and the AO classification systems. Can J Surg. 2009;52(4):277–82. PMID 19680511
  6. Pettersson A, Axenhus M, Stukan T, Ljungberg O, Nåsell H, Razavian AS, Gordon M. Use of artificial intelligence for classification of fractures around the elbow in adults according to the 2018 AO/OTA classification system. BMC Musculoskelet Disord. 2025;26(1):848. doi:10.1186/s12891-025-09161-2
  7. Duckworth AD, Watson BS, Will EM, et al. Radial head and neck fractures: functional results and predictors of outcome. J Trauma. 2011;71(3):643–8. doi:10.1097/TA.0b013e3181f8fa5f
  8. Mason ML. Some observations on fractures of the head of the radius with a review of one hundred cases. Br J Surg. 1954;42(172):123–32. doi:10.1002/bjs.18004217203
  9. Johnston GW. A follow-up of one hundred cases of fracture of the head of the radius with a review of the literature. Ulster Med J. 1962;31(1):51–6. PMID 14452145
  10. Broberg MA, Morrey BF. Results of treatment of fracture-dislocations of the elbow. Clin Orthop Relat Res. 1987;(216):109–19. doi:10.1097/00003086-198703000-00017
  11. Hotchkiss RN. Displaced fractures of the radial head: internal fixation or excision? J Am Acad Orthop Surg. 1997;5(1):1–10. doi:10.5435/00124635-199701000-00001
  12. van Riet RP, Morrey BF. Documentation of associated injuries occurring with radial head fracture. Clin Orthop Relat Res. 2008;466(1):130–4. doi:10.1007/s11999-007-0064-8
  13. van der Windt AE, Langenberg LC, Colaris JW, Eygendaal D. Which radial head fractures are best treated surgically? EFORT Open Rev. 2024;9(5):413–21. doi:10.1530/EOR-24-0035
  14. Ring D, Quintero J, Jupiter JB. Open reduction and internal fixation of fractures of the radial head. J Bone Joint Surg Am. 2002;84(10):1811–5. doi:10.2106/00004623-200210000-00011

3D model

The 3D model and related classifications

The images on this page come from an interactive model, the radial head fracture 3D model. Choose the AO/OTA 2R1 code (or the Mason type), set the fragment size, displacement, tilt and number of fragments from the imaging, add the coronoid, the dislocation, the Osborne-Cotterill lesion or the LCL tear, then play the fixation (headless screws, plate) or the radial head replacement.

ElbowRadial head fractureMason and AO/OTA, coronoid, dislocation, Osborne-Cotterill lesion, LCL, headless screws, plate and replacement. ClassificationsMason classification of the radial headTypes I to IV, the classification that steers treatment, in 3D. ClassificationsAll 3D classificationsArranged by joint.

Explain the fracture in 3D

The radial head fracture model opens in your browser, at the code you choose, with no sign-up.

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