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227
Conclusion
Pelvic xation is an important technique utilized for increased rigidity at the lumbo-
sacral junction to lessen the risk of pseudoarthosis in long-construct fusions, or
additional xation in trauma cases. These techniques have been shown to be feasi-
ble and safe in percutaneous fashion. Understanding anatomy provides the knowl-
edge to place screws with minimal risk to vital structures.
References
1. Tsuchiya K, Bridwell KH, Kuklo TR, Lenke LG, Baldus C.Minimum 5-year analysis of L5–
S1 fusion using sacropelvic xation (bilateral S1 and iliac screws) for spinal deformity. Spine.
2006;31(3):303–8.
2. O’Brien JR, Yu WD, Bhatnagar R, Sponseller P, Kebaish KM.An anatomic study of the S2
iliac technique for lumbopelvic screw placement. Spine. 2009;34(12):E439–42.
3. Edwards CC 2nd, Bridwell KH, Patel A, Rinella AS, Berra A, Lenke LG.Long adult deformity
fusions to L5 and the sacrum. A matched cohort analysis. Spine. 2004;29(18):1996–2005.
4. Edwards CC 2nd, Bridwell KH, Patel A, Rinella AS, Jung Kim Y, Berra AB, etal. Thoracolumbar
deformity arthrodesis to L5in adults: the fate of the L5-S1 disc. Spine. 2003;28(18):2122–31.
5. Kim YJ, Bridwell KH, Lenke LG, Cho KJ, Edwards CC 2nd, Rinella AS.Pseudarthrosis in
adult spinal deformity following multisegmental instrumentation and arthrodesis. J Bone Joint
Surg Am. 2006;88(4):721–8.
6. Cunningham BW, Lewis SJ, Long J, et al. Biomechanical evaluation of lumbosacral recon-
struction techniques for spondylolisthesis: an invitro porcine model. Spine. 2002;27:2321–7.
7. Lebwohl NH, Cunningham BW, Dmitriev A, etal. Biomechanical comparison of lumbosacral
xation techniques in a calf spine model. Spine. 2002;27:2312–20.
8. McCord DH, Cunningham BW, Shono Y, etal. Biomechanical analysis of lumbosacral xa-
tion. Spine. 1992;17(suppl 8):S235–43.
9. O’Brien JR, Yu WD, Kaufman BE, Bucklen B, Salloum K, Khalil S, etal. Biomechanical
evaluation of S2 alar-iliac screws: effect of length and quad-cortical purchase as compared to
iliac xation. Spine. 2013;38(20):E1250–5.
10. Nottmeier EW, Pirris SM, Balseiro S, Fenton D.Three-dimensional image-guided placement
of S2 alar screws to adjunct or salvage lumbosacral xation. Spine. 2010;10(7):595–601.
11. Schildhauer TA, McCulloch P, Chapman JR, Mann FA.Anatomic and radiographic consid-
erations for placement of transiliac screws in lumbopelvic xations. J Spinal Disord Tech.
2002;15(3):199–205; discussion 205.
12. O’Brien JR, Matteini L, Yu WD, Kebaish KM.Feasibility of minimally invasive sacropelvic
xation: percutaneous S2 alar iliac xation. Spine. 2010;35(4):460–4.
26 Percutaneous Iliac andS2AI Fixation
229© Springer Nature Switzerland AG 2020
J. R. O’Brien et al. (eds.), The Resident’s Guide to Spine Surgery,
https://doi.org/10.1007/978-3-030-20847-9_27
Chapter 27
Anterior Odontoid Screws: Tips andTricks
DanielKerekes, A.KarimAhmed, CamiloMolina, C.RoryGoodwin,
andDanielM.Sciubba
Background
Anterior screw xation as an approach to odontoid neck fractures was rst described
in a series of papers published throughout the 1980s [8, 10, 23, 40, 49]. Since that
time, favorable fusion outcomes and increasing incidence of surgically treatable odon-
toid fractures have cemented the anterior screw as an essential procedure of the cervi-
cal spine surgeon [13, 60]. The purpose of this chapter is to discuss the anatomy and
epidemiology of odontoid fractures, to review indications and contraindications for
anterior screw xation, and to describe common procedural pitfalls of this surgery.
Odontoid Anatomy
The odontoid serves as a structural focal point of the C1-C2 atlantoaxial joint. The
anterior surface of the odontoid articulates with the anterior arch of the atlas, and the
posterior surface of the odontoid articulates with the transverse ligament of the
atlas. The apex of the odontoid serves as an attachment site for the apical ligament,
which joins the axis to the skull via attachment at the anterior rim of the foramen
magnum. Paired alar ligaments insert just below the apex on either side of the odon-
toid, providing robust connection to the occipital condyles. The odontoid’s complex
anatomy and intimate association with surrounding structures are explained by
D. Kerekes · A. K. Ahmed · C. Molina · D. M. Sciubba (*)
Department of Neurosurgery, Johns Hopkins University School of Medicine,
Baltimore, MD, USA
C. R. Goodwin
Department of Neurosurgery, Duke University Medical Center, Durham, NC, USA
230
embryologic contributions of the rst and second cervical sclerotomes as well as the
early atlas to odontoid process formation [1].
Classication ofOdontoid Fractures
In 1974, Anderson and D’Alonzo divided odontoid fractures into three classes: type I,
a fracture in the tip of the odontoid process, considered an avulsion fracture of the alar
ligaments; type II, a fracture at the base of the odontoid process; and type III, a fracture
involving both the odontoid process and the body/lateral masses of C2 [3]. Type I
fractures are very rare and relatively stable and are most commonly managed nonop-
eratively [37, 53]. Type II fractures are common and are typically the result of oblique
trauma to the head– “goose egg over the eye” [2, 24]. Type II fractures are typically
unstable and preferred management is surgical [37, 53]. They are the primary indica-
tion for anterior screw xation and the focus of this chapter. Type III fractures are rela-
tively stable and are typically the result of midline trauma to the head– “goose egg on
the forehead” [24]– and are typically managed nonoperatively [37, 53].
Epidemiology
Odontoid fractures represent 10–20% of all acute cervical spine fractures, and type
II is the most common class [53]. The population distribution is bimodal, with peaks
in early adulthood and in the elderly [12, 51]. In patients under 40, these fractures
usually occur in the setting of high-energy trauma, such as a motor vehicle accident,
and anterior displacement of the odontoid process is most common [17, 51]. In
patients over 60, these fractures often present after a fall from a standing height or
other low-energy mechanism, and displacement of the odontoid process is most
commonly posterior [30, 51]. Notably, the incidence of type II fractures has
increased compared to other fractures of the spine in the last two decades, which is
likely a reection of the aging population [60].
Anterior Screw Fixation Versus Other Management
In the population of patients with type II odontoid fractures, the literature is con-
vincing that surgical management offers superior fusion outcomes compared to
nonoperative management: nonunion rates in these fractures after external immobi-
lization alone typically range from 40% to 80% [31, 44, 54, 64], whereas nonunion
rates after surgery are often 25% or much lower [5, 9, 36, 61]. A 2009 meta- analysis
of the literature conrmed overall fusion superiority of operative management com-
pared to nonoperative management, but found that outcomes between those two
cohorts were not statistically different in patients under the age of 45 or in patients
D. Kerekes et al.
231
with anteriorly displaced fractures [50]. While there are some reports of high fusion
rates for these fractures with halo vest management [19, 59], well-documented
complications of halos (cardiac arrest, pneumonia, DVT/PE, pin site infection, pres-
sure sores, respiratory decline, nerve injury, headache), particularly in the elderly,
and the benet of immediate stabilization with operative xation further compel
surgical management in this patient population [7, 39, 45, 51, 62].
In comparing anterior to posterior surgery, the most noticeable advantage of
anterior xation for the patient is the greater postoperative rotational mobility, as no
articial fusion is introduced with screw placement. Up to 83% of patient surgically
treated with anterior xation retain full range of cervical motion [46, 47]. Other
advantages to an anterior approach include a simpler procedure, a less extensive
dissection, fewer critical anatomical structures in the surgical eld, lack of a bone
graft, and lower need for postoperative immobilization [53].
Indications
The only current indication for anterior screw xation is a type II odontoid fracture
or a high type III fracture with a shallow base. The fracture must also be reducible,
as the technique demands the screw be driven into an anatomic odontoid.
Additionally, studies suggest that fusion outcomes are superior for patients who
undergone surgery closer to the time of injury as compared to those who undergo
delayed surgery, which has been dened as anywhere from 1 week to 6 months
between injury and surgery [5, 17]. To this end, some authors consider delay greater
than 3weeks to be a relative contraindication to screw placement [6]. Other risk
factors for nonunion include greater than 4–6mm of fragment displacement and
greater than 10 degrees angular deformity [6, 12, 26, 28].
Contraindications
Contraindications to anterior screw xation include fractures that are irreducible, that
have an oblique fracture plane, that are associated with rupture of the transverse atlan-
tal ligament, or that are associated with signicant cervical oor cervicothoracic kypho-
scoliotic deformity. Additionally, patients are unlikely to experience good outcomes
with anterior odontoid xation if: 1) they have short necks, 2) have a delayed presen-
tation, 3) have a history of osteoporosis, or 4) are greater than 70 years of age.
An irreducible fracture is an absolute contraindication to anterior xation, as it
makes xation with a screw technically infeasible. In subpopulations with anatomi-
cal constraints, it is technically infeasible to achieve the necessary screw trajectory
for proper placement; therefore, barrel chests and short necks are considered rela-
tive contraindications to this procedure. The ability to achieve the proper trajectory
should be determined during preoperative surgical planning and given special con-
sideration, especially in patients with these characteristics.
27 Anterior Odontoid Screws: Tips andTricks
232
A properly placed screw will apply a reduction force on the fracture in a postero-
superior to anteroinferior direction. In light of these physics, anterior oblique frac-
tures can be another relative contraindication to screw xation, as these fractures
have a tendency to displace postoperatively under the screw’s force [4].
The transverse atlantal ligament (TAL), which articulates with the posterior sur-
face of the odontoid, is the structure most responsible for the anterior stability of the
atlantoaxial complex [42]. As such, patients with TAL incompetency will have atlan-
toaxial instability regardless of the integrity of the odontoid process [27, 42].
Therefore, TAL disruption is a contraindication for anterior screw placement. Up to
10% of patients with a type II odontoid fracture have a concomitant TAL rupture, and
these patients are good candidates for surgical xation via a posterior approach [27].
Anterior Fixation intheElderly andOsteoporotic
The elderly and osteoporotic deserve special consideration regarding surgical repair
of a type II odontoid fracture. Histologic analysis shows that bone mass reduction in
those with osteoporosis is particularly pronounced at the base of the odontoid, with
the base having only 36% the bone mass of the body of the odontoid and the axis in
osteoporotic patients. Furthermore, marked reductions in trabecular bone in osteo-
porotic patients mean fractured odontoids are less likely to heal [2]. Despite these
anatomic concerns, many studies have concluded that anterior screw xation is a
reasonable option with acceptable clinical outcome in the elderly patient [9, 13, 15,
32, 52]. On the other hand, however, anterior screw xation in the elderly is also
shown to carry comparatively higher rates of complication and lower rates of fusion
than posterior transarticular xation or C1 lateral mass/C2 transpedicular xation [4,
16, 20, 52, 57]. As such, in a 2010 systematic review of the literature, Harrop etal.
strongly recommend posterior xation in the elderly, on the basis of consensus opin-
ion [33]. Lastly, although nonoperative management in the elderly is shown to offer
signicantly worse fusion, morbidity (mobility, nutrition, sanitation), and mortality
outcomes than surgery [11, 58, 63], some studies demonstrate comparable patient
satisfaction and quality of life outcomes between these two management strategies
[45]; thus, treatment decisions should be made on an individual patient basis.
Radiology
Imaging plays a pivotal role in the evaluation and surgical management of type II
odontoid fractures. Multiple modalities are often utilized in order to acquire a com-
plete understanding of the character of the injury. Plain radiographs are inexpensive
and widely accessible and are frequently the rst line investigation into cervical
pain. Cervical spine plain lms with odontoid and lateral exion-extension views
can be used to initially assess mobility of the fractured odontoid fragment, but have
been shown to have poor sensitivity for cervical fractures (detecting as little as 39%)
[65]. Separation distance greater than 3mm between the anterior C-1 ring and the
D. Kerekes et al.
233
odontoid may indicate transverse ligament disruption [22]. Follow-up exion and
extension radiographs are commonly used to evaluate postoperative fusion and sta-
bility [56].
Computerized tomography (CT) is essential for comprehensive assessment of the
odontoid and can be used to subclassify the fracture and determine extent of bony
involvement and severity of displacement. CT images are also used for surgical plan-
ning, particularly to determine the appropriate screw length for the procedure and to
assess the relative density of the cortical shell of C2, if anterior xation is being consid-
ered. Magnetic resonance imaging (MRI) may be a helpful adjunct to determine extent
of soft tissue involvement, including the direct evaluation of the integrity of the trans-
verse atlantal ligament and the possibility of a spinal cord injury [48]. CT and MRI play
additional important roles in diagnosing congenital conditions of the odontoid and
potential underlying pathologies, such as rheumatoid arthritis or infection [34].
Procedure
The patient is placed in a supine position. The patient’s mouth is propped open with
a cork, bite block, roll of gauze, or other radiolucent object to allow for adequate
plain radiograph evaluation of the fracture. To put the cervical spine into extension,
a blanket or pad is placed beneath the patient’s interscapular region, except in cases
of a severely retrolisthesed fragment concerning for potential basilar artery injury.
Once the patient’s spine is in appropriate alignment, the surgeon may wish to secure
the patient’s head to the bed (Halter traction, radiolucent Mayeld clamp, or
Gardner-Wells tongs with tape) to prevent incidental movement during the opera-
tion. At this point, lateral and anteroposterior (AP) images should be taken to ensure
that an appropriate view of the cervical spine and odontoid can be attained for pur-
poses of the procedure and also to demonstrate reduction of the fracture. (Historically,
biplanar uoroscopy with two C-arms has been the modality of choice; however,
recent studies have described advantages to screw placement that are conferred by
the use of the O-arm [66] and neuro- navigation [35, 38].) Flexion or extension of
the neck may be judiciously applied as necessary to ensure proper reduction. The
patient is prepped and draped in the usual sterile fashion.
After sterilization, imaging is used to visually approximate the desired screw
trajectory (the surgeon may wish to use a radiodense tool such as a probe or
Kirschner wire to aid in this). A transverse skin incision is made according to the
planned trajectory, typically at the level of the C5/C6 disc space, starting medially
and continuing laterally. Exposure progresses in the same fashion as is used for
anterior cervical discectomy and fusion: division of the platysma, blunt dissection
of the plane medial to the sternocleidomastoid, lateralization of the carotid sheath,
and mobilization medially of the trachea and esophagus. Upon exposure of the pre-
vertebral space, the longus colli muscles are elevated and retracted. Blunt preverte-
bral dissection is used to create a tunnel toward C2-C3. Upon reaching the C2-C3
disc space, the level is conrmed by imaging.
A C2-C3 anterior discectomy (removing one-third to one-half the disc) is per-
formed to expose the anterior inferior endplate of C2. If one screw will be placed, an
27 Anterior Odontoid Screws: Tips andTricks
234
appropriate starting point in the midline of the base of C2 is identied and conrmed
by lateral and AP imaging. If two screws will be placed, an appropriate location
3–4mm off the midline is identied and conrmed. A small pilot hole is drilled at the
identied location(s). If a cannulated system is being utilized, a Kirschner wire
(K-wire) is directed into the pilot hole and advanced systematically under lateral and
AP imaging guidance across the fracture to the tip of the odontoid until the distal cor-
tex is penetrated. Depending on the surgeon preference, a lag screw or a fully threaded
screw under lag technique is now placed such that the tip of the screw is in the distal
cortex of the odontoid apex and the base of the screw is ush against C2in the C2-C3
disc space. Hyperextension or exion of the patient’s neck may be required to keep the
fracture reduced in anatomic position. Screws should be selected based on the length
from odontoid tip to base of C2, as determined by preoperative CT or length of K-wire
used. They are typically 4mm in diameter. If a cannulated system is not being utilized,
these steps are carried out without the guidance of a K-wire. Percutaneous approaches
to anterior screw xation have also been described and are an alternative option [11].
Proper nal position of the screw is conrmed with lateral and AP imaging.
Hemostasis is carefully obtained, and the wound is irrigated. The platysma is re-
approximated with interrupted sutures as needed, and the skin incision is closed.
One Screw or Two?
In some of the earliest reports of anterior screw xation, investigators describe the
placement of two screws (Fig.27.1) [7]. Since then, many studies have shown one
screw to be biomechanically and clinically equivalent to two screws for the
Fig. 27.1 Postoperative
radiograph of a properly
placed anterior odontoid
screw
D. Kerekes et al.
235
purposes of this procedure [21, 25, 36, 43, 55, 61]. If the anatomy of the fracture or
concerns for stability compel consideration of the use of two screws, it should be
noted that cadaveric studies have suggested an odontoid external transverse diame-
ter of at least 9.2mm is necessary to allow for ideal spacing [13].
Common Pitfalls
Common pitfalls for anterior screw xation are frequently associated with poor pre-
operative planning and include screw trajectory, screw insertion site, anterior oblique
fracture, screw placement, use of imaging, and Kirschner wire implementation.
Guiding a screw along an ideal trajectory for proper placement within the odon-
toid process can be a nuanced and challenging task. Barrel chests and short necks
are common obstacles to executing an appropriate trajectory. Severe cervicotho-
racic kyphosis can also impair proper drill bit positioning. Frequent references to
intraoperative imaging early on are crucial to mastering the 3D anatomy that is at
the center of this meticulous procedure.
The proper site of insertion for an odontoid screw on the inferior endplate of C2
can be awkward to reach and relatively inaccessible compared to the anterior face of
C2. Therefore, a common mistake in this procedure is to penetrate C2 too anteriorly,
leaving the head of the screw anterior, rather than inferior, to the body of C2. This is
problematic for several reasons. First, anterior protrusion of the screw has the poten-
tial to irritate the esophagus and/or trachea postoperatively. Moreover, the cortical
bone of the anterior surface of C2 is much thinner than the cortex lining the interver-
tebral disc space [2]. Screws entering into C2 anteriorly are thus far more likely to
lose purchase and pull out over time, cause fragment malalignment, and prolong
healing time [42]. Improper insertion can be combated by performing an anterior
C2-C3 discectomy, which exposes the ideal entry point at the inferior surface of C2.
Anterior oblique fractures can be particularly troublesome for the surgeon per-
forming anterior xation, as the force applied by the screw is nearly parallel to the
direction of the fracture. As a result, nonunion rates are higher in the subpopulation
of type II odontoid fractures running anteroinferior to posterosuperior [5]. Patients
with this classication of fracture should be considered for posterior xation or
nonoperative management. Alternatively, the surgeon may apply a contoured one-
third tubular plate to the fracture to prevent translation of the oblique fragment [29].
In addition to screw trajectory, proper nal screw positioning is a concern of mil-
limeters and is a constant challenge. Likely in light of the vital anatomy just distal
to the odontoid, a common error is to stop the screw before xation into the strong
apical cortical bone is achieved. If purchase in the cortical bone of the tip is not
attained, the screw is more likely to pull out or fail, and fracture compression will
not be maximized [5, 15, 42]. If patient anatomy presents safety concerns for strong
distal cortical xation, some studies recommend the use of the fully threaded
variable- pitch screw [41] or cannulated cancellous lag screw [18].
Other difculties with screw positioning arise in the context of the lag technique. If
a lag screw is being utilized, it is important that the threaded section of the screw does
27 Anterior Odontoid Screws: Tips andTricks
236
not span the fracture, or else a lag effect across the fracture is not realized, and compres-
sion will not be attained. Likewise, if a lag technique is being utilized, it is important
that the proximal fragment of the odontoid is “overdrilled” up to the point of fracture,
or else a lag effect will not be realized. Precise determination of proper screw length by
preoperative CT or K-wire estimation can aid in achieving robust screw placement.
Another complication of anterior xation is found in the use of biplanar uoros-
copy. Although the utilization of two C-arms allows for efcient imaging of both the
lateral and AP views, a limitation of this set up is that only one view is visualized at a
time. As such, it is tempting for the surgical team to follow screw trajectory in only
one view– most commonly the more intuitive lateral view. However, without appro-
priate attention paid to the AP view, it is possible for a screw to seemingly track the
odontoid well and still end up in soft tissue. In the cases using biplanar uoroscopy,
frequent consultation of both views is imperative for successful screw placement. The
real-time feedback of neuro-navigation is essential to avoid a trajectory error [35].
The last pitfall comes with the use of a K-wire. Proper use of a K-wire requires
application of force upon the cannulated screw parallel to the direction of the K-wire.
If restrictions of the environment limit ability to apply this force suitably and the
screw is driven at an angle to the wire, a shear force may be applied to the K-wire,
resulting in the K-wire being driven forward into the brainstem, or the tip of the
K-wire breaking off to remain in the odontoid [42]. Thus, due diligence must be paid
to the maintenance of force in the appropriate direction when cannulated drill bits
and screws are used in conjunction with a K-wire. As an extra precaution, the end of
the K-wire may be held with a needle holder to prevent incidental advancement.
Outcomes
Fusion rates for anterior screw xation range from 73% to 96% in fractures less than
6months old [5, 9, 13, 36, 61]. Morbidity for this surgery is generally considered to be
low [12], with complication rates ranging from 8% to 25% [4, 8, 14]. Major complica-
tions include dysphagia, need for a feeding tube, hardware failure, and cervical instabil-
ity. A 2015 systematic review found that anterior xation offers a long- and short-term
survival advantage for patients older than 60 compared to nonoperative management
and that this advantage was not different from that offered by posterior xation [58].
Qualitatively, a well-placed anterior screw offers instant return of cervical stability, and
most patients have near-immediate improvement of neck pain with preserved mobility.
Conclusion
Anterior screw xation is an effective and valuable procedure in many adult patients
with an odontoid fracture. As the population ages and the incidence of type II frac-
tures rises, demand for this procedure can be expected to increase. Although
D. Kerekes et al.
237
technical expertise is required for success, an anterior screw properly placed offers
great quality of life improvement to patients with these unstable fractures and pro-
vides several advantages over posterior xation. Looking forward, there is anticipa-
tion of further improvement in outcomes as neuro-navigation modalities become
more widely applied and studied in this setting.
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27 Anterior Odontoid Screws: Tips andTricks