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Eur J Orthop Surg Traumatol (2008) 18:189–195
DOI 10.1007/s00590-007-0284-3
123
ORIGINAL ARTICLE
It is time to reconsider the classiWcation of dens fractures:
an anatomical approach
Demetrios S. Korres · Andreas F. Mavrogenis ·
Petros Gratsias · George P. Lyritis ·
Panayiotis J. Papagelopoulos
Received: 2 May 2007 / Accepted: 10 September 2007 / Published online: 20 December 2007
© Springer-Verlag 2007
Abstract A clinical study was performed to evaluate a
new classiWcation for odontoid process fractures. The exter-
nal and internal anatomy of the axis has been studied. The
Wles of 97 patients with a fracture of the odontoid process
admitted to our institution were reviewed and classiWed
according to the proposed new classiWcation. The method
was tested for reliability and validity. The patients were fol-
lowed for a mean period of 14 years, in order to evaluate
the outcome of these fractures and to correlate it to the frac-
ture type. Intraobserver and interobserver agreement was
excellent with intraclass correlation coeYcients at levels of
0.98 and 0.85, respectively. Four types of odontoid process
fractures are distinguished. Type B fractures are associated
with high risk of pseudarthrosis. Existing classiWcations are
inadequate and misleading since they do not include all
types of odontoid process fractures, nor they consider the
internal anatomy of the odontoid process. ClassiWcation of
the odontoid process fractures has to be reconsidered as
novel-imaging technology has shown new patterns of frac-
tures. The analysis of the imaging data in the present study
justiWes the new classiWcation.
Keywords Anatomy · Axis · Trauma · Dens · Fractures ·
ClassiWcation
Fractures de l’odontoïde: il est temps de revoir leur
classiWcation; une approche anatomique
Résumé Une étude clinique a été entreprise pour évaluer
une nouvelle classiWcation sur les fractures de l’apophyse
odontoïde. L’anatomie et l’architecture interne de l’axis ont
été étudiées sur le plan anatomique. Une série de 95
patients avec fracture de l’apophyse odontoïde admis dans
notre institution a été revue et classée selon la nouvelle
proposition. La méthode a été testée en vue de sa Wabilité et
de sa validité. Les patients ont été suivis sur une période
moyenne de 14 années pour apprécier le pronostic de ces
fractures en fonction de leur type. La concordance intra et
inter observateur a été excellente avec des taux respectifs à
0,98 et 0,85 %. La classiWcation proposée comprend quatre
types de fracture de l’apophyse odontoïde. Le type B est
associé à un risque élevé de pseudarthrose. Les classiWca-
tions existantes sont inadéquates et trompeuses car elles
n’incluent pas tous les types de fractures et ne prennent pas
en considération l’anatomie interne de l’apophyse odon-
toïde. La classiWcation des fractures de l’apophyse odon-
toïde devrait être revue en intégrant l’apport des nouvelles
technologies d’imagerie. L’analyse de ces nouvelles don-
nées justiWe cette nouvelle classiWcation.
Mots clés Anatomie · Axis · Traumatisme ·
Apophyse odontoïde · Fracture · ClassiWcation
Introduction
Fractures of the cervical spine are relatively common [13,
25]. Odontoid process fractures account for approximately
10–14% of cervical spine fractures [4, 7, 18, 21]. Because
of the unique anatomy and architecture of the odontoid
D. S. Korres (&) · A. F. Mavrogenis · P. Gratsias · G. P. Lyritis ·
P. J. Papagelopoulos
First Department of Orthopaedics,
Athens University Medical School,
10 Heyden Str, 10434 Athens, Greece
e-mail: dskorres@med.uoa.gr
G. P. Lyritis
Laboratory for the Research of the Musculoskeletal
System “Th. Garofalidis”, KAT Hospital, Athens, Greece
190 Eur J Orthop Surg Traumatol (2008) 18:189–195
123
process, this anatomical element responds to injuries in a
peculiar manner leading to speciWc pattern of fractures; the
understanding of the mechanism of the fractures and the
biomechanics of the odontoid process is important for treat-
ment and prognosis. The recognition of instability is also
important since it is an absolute indication for surgical
stabilization through an anterior or a posterior approach
[13, 25].
There have been many attempts to classify the odontoid
process fractures, based on radiographic criteria depending
on the level of the fracture and the direction of the line of
the fracture [1, 3, 26, 27]. Evolutions in diagnostic imaging
have demonstrated fracture patterns that have not been
included in previous classiWcations. This study attempted to
classify the odontoid process fractures according to plain
and CT image.
Materials and methods
From 1970 to 2005, 97 patients (13.5% of all cervical spine
fractures and 4.3% of all spinal fractures) were managed to
the authors’ institution for an odontoid process fracture.
These were 59 men and 38 women with a mean age of
46.7 years. A motor vehicle accident was the main cause of
injury in 66 patients (64.3%), a fall from height in 25
patients (26.4%), and other causes in the remaining 5
patients (5.75%). Neurological deWcits were detected in 17
patients (19.5%); however only in 6 of them (6.8%) these
were related to the odontoid process fracture. The fractures
were classiWed initially according to the Anderson and
D’Alonzo classiWcation and later according to the Roy-
Camille and the authors’ new classiWcation.
The proposed herein classiWcation was based on the
study of the “open mouth” dens views. All the X-rays con-
cerning the fractured odontoid process were reviewed by
three independent radiologists who were asked to classiWed
these fractures according to these three classiWcations.
The new classiWcation distinguishes four types of odon-
toid process fractures (types A–D) and recognizes a zone
where practically no fractures are ever noticed (the neutral
zone); this zone is located at the level of the transverse liga-
ment (Fig. 1).
Type A fractures represent avulsion fractures of the apex
of the odontoid process, at the insertion of the alar or apical
ligaments (Fig. 2).
Type B fractures occur at the neck of the odontoid pro-
cess, caudal to the insertion of the transverse ligament, to
the line between the two inner corners of the lateral masses
of the axis (Fig. 3).
Type C fractures occur in the region between the type B
fractures and the anatomical base of the odontoid process.
Type C fractures may be subdivided into Type C1 that are
typical fractures of the base of the odontoid process
(Fig. 4), and Type C2 that are “derooting” fractures with
both ends within the odontoid process but may also extend
to the body of the axis (Fig. 5). The extension to the body
of the axis (type C2) depends not only to the characteristics
of the bone at this region and the degree of accumulation of
trabeculae at certain points but also to the vector of the
applied forces.
Type D fractures are complex (split, vertical, double or
burst) fractures of the odontoid process. The fracture line
may extend through two or more levels (Fig. 6).
Fig. 1 Schematic presentation on the open mouth radiograph of the
four types of odontoid process fractures (types A–C) and the neutral
zone
Fig. 2 Type A fracture of the odontoid process
Eur J Orthop Surg Traumatol (2008) 18:189–195 191
123
The reliability and the validity of this classiWcation were
checked. All the X-rays were re-reviewed by three indepen-
dent reviewers (two spine surgeons and one radiologist)
using interobserver and intraobserver kappa values and
intraclass correlation coeYcients. Forty cases were
reviewed by three orthopaedic residents at their Wnal year
of specialty. Finally, ten cases were repeated in order to tes-
tify intraobserver reliability
Results
According to the Anderson-D’Alonzo classiWcation [3],
two patients had a fracture of the apex of the odontoid, 38
had a type II and 58 a type III fracture. One patient sus-
tained two fractures, one at the apex and the second at the
base of the odontoid process. According to our classiWca-
tion, 25 patients with a type III Anderson-D’Alonzo frac-
ture had a fracture of the body of the axis, rather than a
fracture of the odontoid process.
According to the Roy-Camille classiWcation [27, 28], 45
patients had an anterior oblique fracture, 36 had a horizon-
tal fracture and 10 patients had a posterior oblique fracture.
Six fractures did not meet the criteria to be included in this
classiWcation. According to our classiWcation, 20 of the
45 patients with an anterior oblique fracture, 4 of the 10
patients with a posterior oblique fracture, and 1 of the 6
patients with unclassiWed Roy-Camille fractures of the
odontoid process had rather a fracture of the body of the
axis.
According to the authors’ classiWcation, two patients had
a type A fracture, 30 patients had a type B fracture, 34
patients had a type C fracture (type C1 in 9 patients and
type C2 in 24 patients), and 6 patients had a type D fracture
of the odontoid process. Twenty-Wve patients were not
included in this study since according to the proposed
herein classiWcation they had a fracture of the body of the
axis rather than a fracture of the odontoid process.
The reliability and the validity of this classiWcation were
evaluated. The intraobserver and interobserver agreement
Fig. 3 Type B fracture of the odontoid process
Fig. 4 Type C1 fracture of the odontoid process
Fig. 5 Type C2 fracture of the odontoid process
Fig. 6 Type D fracture of the odontoid process
192 Eur J Orthop Surg Traumatol (2008) 18:189–195
123
values were 0.98 (range 0.96–0.99) and 0.85 (range 0.79–
0.90), respectively; these values were higher compared to
the values required for the evaluation of the validity of the
methods [29]. Also, these values were lower (0.86 and
0.73, respectively) among the group of residents, thus sug-
gesting that some types as the type C1 fractures may be
confusing if the reviewer is not experienced.
Eight patients were treated surgically through a posterior
approach, using a modiWed Gallie technique [8]; these
patients had a type B fracture of the odontoid process.
Eighty-nine patients were treated conservatively either
using skull traction for 6–8 weeks (early period of this
study) or a halo vest application.
The mean follow-up was 14 years (range 3 months to
28 years). Five patients died because of co-morbidities
unrelated to the fracture of the odontoid process. Twelve
patients were lost during the follow-up of this study. All the
patients treated surgically had an excellent or good result
with bony fusion obtained in 3–5 months.
Eleven patients with a mean age of 61.2 years developed
a pseudarthrosis; all of them had a horizontal or type B
fracture of the odontoid process according to the Roy-Cam-
ille and the authors’ classiWcation, respectively. In 7
patients, spine fusion and instrumentation through a poster-
ior approach was done with satisfactory results. Four
patients denied operative treatment and were managed with
a halo vest; three of them showed union of the fracture and
atlantoaxial stability. Eight of the 79 conservatively treated
patients with a mean age of 35 years developed mal-union
of the fracture. Six of them had a type C2 fracture and two
had a type B fracture. At the latest follow-up, one of the six
patients with a type C2 fracture had neurological deWcits
secondary to cervical spine myelopathy.
Discussion
The fractures of the odontoid process represent the mechan-
ical failure of this anatomic unit following the application
of forces [1]. The direction of these forces, the internal
architecture, the mechanical strength of the bone trabecu-
lae, the proportion of the cortical and cancellous bone, the
magnitude of the odontoid process displacement, the vascu-
lar supply of the odontoid process and the age of the
patients are the most important factors in the creation of
speciWc fracture types and the prognosis of these injuries
[11, 19, 24].
In line but not well documented in the literature [1–3],
radiographic and histomorphometric studies outlined the
structural diVerence between the odontoid process and the
body of the axis. Data from these studies may distinguish
the fractures at the base of the odontoid process and the
underlying body of the axis. This was shown in a recent
study using peripheral quantitative computed tomography
(pQCT) in 20 cadaveric specimens of the axis [19]. More-
over, this study showed the diVerence of the internal archi-
tecture of the axis between young and older patients; in
subjects more than 40-year-old a large void of thin trabecu-
lar bone has been identiWed extending from antero-inferior
to supero-posterior to the base of the odontoid process indi-
cated a mechanically weak region that may predispose to
speciWc fracture patterns [19].
A classiWcation should meet certain criteria to be valu-
able and widely accepted. These should include the ease of
understanding, the ability to be easily recalled, the consid-
eration of the internal architecture of the odontoid process,
the understanding of the mechanism of injury and the spe-
ciWc fracture types, the proposal of therapeutic guidelines
according to the speciWc fracture types, and the provision of
useful information regarding the prognosis and the natural
history of the various fracture types.
Odontoid process fractures typically occur at three loca-
tions and have been classiWed according to clinical, radio-
graphic and mechanical criteria [1, 3, 11, 28]. The
classiWcations of De Mourgues et al. (1972), Anderson and
D’Alonzo (1974), and AlthoV (1979) are based on the posi-
tion of the fracture of the odontoid process [1, 3, 21].
In 1972, De Mourgues et al. [21] described two types of
odontoid process fractures; the fractures of the neck and the
fractures of the base of the odontoid process. Most fractures
of the neck are complicated by pseudarthrosis, while the
fractures of the base of the odontoid process are associated
with a 100% union rate.
The Anderson and D’Alonzo classiWcation [3] is the
most widely accepted. This classiWcation is based on the
level of the fracture and includes three types of dens frac-
tures (types I–III). Type I fractures are avulsion fractures of
the apex and are associated with a good prognosis. Type II
fractures are the most common fractures occurring at the
neck of the odontoid process. These fractures are associated
with a 36% incidence of pseudarthrosis. Type III fractures
occur at the base of the odontoid process and are associated
with an excellent prognosis with more than 90% union.
AlthoV [1], based on an experimental anatomical study
described four types of odontoid process fractures (types A–
D) in relation to the anatomy of the odontoid process.
According to AlthoV these were (a) fractures above the neck
of the dens that are associated with a pseudarthrosis rate of
approximately 64%, (b) fractures through the neck that are
associated with a pseudarthrosis rate of approximately 55%,
(c) fractures through one of the articular processes that are
associated with a pseudarthrosis rate of approximately 50%,
and (d) fractures through the body of the axis that are associ-
ated with a pseudarthrosis rate of less than 13%.
The Roy-Camille et al. classiWcation [27, 28] distin-
guishes three types of odontoid process fractures (anterior
Eur J Orthop Surg Traumatol (2008) 18:189–195 193
123
oblique, posterior oblique and horizontal fractures) based
on the direction of the line of the fracture. The horizontal
and the posterior oblique fractures are the most unstable
patterns leading to high rate of pseudarthrosis. The classiW-
cation proposed by Roy-Camille et al. is generally consid-
ered a more realistic approach to odontoid process fractures
since it provides information not only for the mechanism of
the fracture but also for the stability and the prognosis of
the speciWc fracture type. We have been using this classiW-
cation at our institution since 1987.
Evolutions in diagnostic imaging have been useful in
demonstrating other types of odontoid process fractures
such as the vertical type [6, 9, 16, 17, 20]. At present, plain
radiographs, except “open mouth” views of the odontoid
process, are not considered the best way to identify a frac-
tured odontoid process. In our series, the initial evaluation
and classiWcation of the odontoid process fractures was
based on the “open mouth” plain radiographs of the odon-
toid process. Computed tomography scan with image
reconstruction is considered the gold standard for a proper
diagnosis since it may also document the presence of con-
comitant fractures [11, 23]. Also, as in the present series
computed tomography scan has been useful to evaluated
complex (type D) fractures of the odontoid process.
In the present classiWcation, we distinguish four types of
the odontoid process. Type A fractures are located at the
apex of the odontoid process. These are avulsion type frac-
tures due to excessive rotation forces applied through the
ligaments attached to this area (Fig. 2). Type B are fracture
of the neck located between the lower edge of the trans-
verse ligament and the line connecting the medial corners
of the upper articular facets of the axis. The fracture is due
to forces in Xexion applied through the transverse ligament
or to Xexion in lateral bending that occasionally may be
associated to a fracture of the lateral articular mass (Fig. 3).
Type C fractures are located between the previously men-
tioned line and the base of the odontoid process (type C1).
These fractures are secondary to forces in Xexion or exten-
sion and/or lateral bending of the neck (Fig. 4). The exten-
sion to the body of the axis (type C2; Fig. 5) depends not
only to the characteristics of the bone and the degree of
accumulation of trabeculae (the presence of void) at this
region but also to the vector of the applied forces. Type D
or complex fractures involve more than one level of the
odontoid process (Fig. 6). The mechanism of these frac-
tures is multifactorial.
It is important to understand the diVerence between
types B and C fractures. Type B fractures are located higher
in the neck of the odontoid, while type C fractures are
almost at the level of the base of the neck in contact to the
upper part of the body of the axis. In the former, the bone is
thick mainly cortical, while in the later the bone is thin
mainly trabecular, with an altered internal architecture due
to fewer number of trabeculae and the presence of a void
resulting in a mechanically weak area [19].
Type D or complex fractures are uncommon; modern
imaging modalities such as multi-slice and reconstruction
computed tomography scans are useful for the diagnosis
[6, 9, 12, 14, 15, 20]. These fractures are more frequent in the
elderly. The mechanism of injury is unclear. It is speculated
that type D fractures result from the combination of two or
more diVerent forces acting simultaneously or subsequently
on a speciWc area with minor resistance [17, 20]. The pres-
ence of arthrosis has also been suggested to contribute to
the creation of this type of fractures [16, 18].
In our series no fracture was seen in the area in front of
the transverse ligament. This area is called “the neutral
zone”. It consists of thick cortical bone that has been
embryogenetically reinforced by the continuous compres-
sive forces this area is exerted by the transverse ligament.
The Anderson and D’Alonzo classiWcation [3] oVers a
simple and topographic approach to odontoid process frac-
tures. However, it does not contribute to the thorough
understanding of the mechanism of the fracture, nor it
incorporates any biomechanical characteristics or charac-
teristics of the internal architecture of the odontoid process.
In addition, the many attempts made for its improvement
[10, 12] or replacement [1, 21, 28], the existence of alterna-
tive classiWcations based on the direction of the fracture
line [27, 28], the heterogenity of the reported pseudarthro-
sis rates at type II fractures and the presence of various
unclassiWed fracture types such as some vertical or oblique
fractures [6, 9, 15] may suggest the inadequacy of the
aforementioned classiWcation schemes. The classiWcation
proposed by Anderson and D’Alonzo is misleading and
contributes to confusion regarding fracture location [5, 14].
The Anderson and D’Alonzo type III fracture is not a frac-
ture of the odontoid process, but rather a horizontal rostral
fracture through the upper aspect of the body of the axis
[5]. Further adding of subtypes such as type IIA [12], type
IIB [10], and type IIIA [14] perpetuate the confusion.
Once recognized in the literature that there are fractures
not corresponding to the already existing classiWcations [6,
17] it was clear that a more appropriate classiWcation would
be obligatory. The proposed herein classiWcation is based
on the structural, anatomical and biomechanical properties
of the odontoid process. This classiWcation is considered
more realistic since it is simple, it includes the whole spec-
trum of fractures, it refers only to one anatomical structure
(the odontoid process), it correlates to the biomechanical
characteristics of the axis, it indicates the prognosis of the
diVerent fracture types, and it suggests the adequate treat-
ment.
A classiWcation should also facilitate the evaluation and
management of the fractures [10]. Although the study
presented herein is a retrospective study with therapeutic
194 Eur J Orthop Surg Traumatol (2008) 18:189–195
123
indications based on previous studies [27, 28], adequate
management for odontoid process fractures has been
addressed; this has been consistent with the literature [3–5,
10–13, 21, 27, 28]. Indications for surgical treatment are
based on the speciWc fracture pattern, the internal anatomy
of the odontoid process, and the severity of the injury. Type
B fractures are at risk for pseudarthrosis and early surgical
management is recommended. Surgical treatment is also
recommended for all types of odontoid process fractures if
neurological symptoms or instability is documented. Type
C fractures should be treated conservatively because of the
void area that may be associated with high rates of implant
loosening [19]. Fracture types A and D can be treated con-
servatively provided that adequate immobilization can be
obtained.
The proposed herein classiWcation also reXects the prog-
nosis of the fractures. The relationship of the fracture type
with pseudarthrosis and malunion has been documented in
the present series. Type A fracture are associated with an
excellent outcome and high union rate. Type B fractures are
associated with the highest pseudarthrosis rate since they
occur at a region of predominant cortical bone. Type C
fractures are frequently associated with an unacceptable
anterior angular deformity. Type C1 fractures, although
unstable, are associated with a more favorable prognosis
due to the vicinity with the cancellous bone of the body of
the axis. The extension to the body of the axis (type C2)
depends not only on the characteristics of the bone and the
degree of accumulation of trabeculae at this region but also
to the vector of the applied forces. Type D fractures are
usually observed in the elderly and are associated with a
favorable prognosis.
Type C2 fractures are associated with high malunion
rates. In these series, although this complication had been
expected to be observed in older patients because of the
thin trabeculae and the biggest void area, it was more
often observed in younger patients. The volume of the
void at this region could be the reason for the kyphotic
deformity these fractures may develop particularly in this
age group. This may also be secondary to inappropriate
immobilization or increased physical activity in younger
patients [19].
Conclusion
Currently available classiWcation schemes for odontoid pro-
cess fractures are inadequate and misleading since they do
not consider the internal anatomy, nor they include the
whole spectrum of fractures of the odontoid process. Frac-
ture types involving the structurally weak area of bone at
the base of the odontoid process or the underlying body of
the axis should be considered as a separate entity.
In order to classify the diVerent fractures types of the
odontoid process we believe that it is necessary to accept a
new anatomical classiWcation of odontoid process fractures.
The rationale of the new classiWcation is to understand the
mechanism and to improve the interpretation of the diVer-
ent odontoid process fracture types, to propose therapeutic
guidelines and to evaluate the prognosis of the speciWc frac-
ture types. The analysis of the imaging data in the present
study justiWes the new classiWcation.
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