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432
Fig. 32.5 Thigh thrust
test. Also called
posterior/femoral shear
test because a shearing
pressure is applied to the
sacroiliac joint. Patient
lies supine and examiner
stands on the
contralateral side of
symptomatic joint. The
hip and knee at the
affected side are flexed
to 90°. Examiner puts
the right hand behind the
sacrum to stabilize it and
uses the left hand to
push down on the flexed
knee to exert a posterior
force
Fig. 32.6 Sacral thrust
test. The purpose of this
test is to apply an
anteriorly directed shear
force to the sacroiliac
joint. With the patient
prone, the examiner puts
hands over the sacrum
and applies a downward
force. Hands are
positioned as if doing
cardiac compression
during a
cardiopulmonary
resuscitation
S.C. Yson et al.
intra-articular local anesthetic injection [13].
Relief of the pain by the local anesthetic strongly
points to the hip as the source of pain. Advance
imaging (MRI, MR arthrogram) may also be beneficial in some cases.
Diagnosing symptomatic spine pathology
may be straightforward or difficult. When clear
radiculopathy that correlates with imaging is
relieved by a selective nerve root block or by a
targeted transforaminal epidural steroid injection,
then confidence is high about the spine diagnosis.
Radicular pain can be generated by the sacroiliac
joint, perhaps from cytokine presence near the
traversing lumbosacral plexus [
14]. Facet load-
ing and diagnostic facet blocks can also be helpful. Differentiation of axial discogenic pain is
much more challenging. MRI with Modic end
plate changes can be suggestive. Discography
was more commonly used previously but is currently a source of significant controversy.
32 Sacroiliac Joint Fusion
433
Fig. 32.7 Pelvic torsion test. Popularly known as
Gaenslen’s test. Typically performed with the patient
supine with the leg of symptomatic side dangling on the
side edge of examining table. Patient is requested to hold
the contralateral knee as close to the chest as possible.
Examiner pushes the thigh of symptomatic side down,
If the physical exam, imaging studies and
diagnostic injections are all consistent with sacroiliac joint pathology and rule out other pain
generators, then the presumptive diagnosis is
established. Prior to considering surgery, patients
should have had a reasonable trial of nonsurgical
management. At a minimum this should involve
evaluation and treatment by a skilled physical
therapist with expertise in the sacroiliac joint and
spine. Therapeutic steroid injections and radiofrequency ablations are both commonly used
nonoperative treatment methods. Lastly, addressing non-spinal factors, including medical and
mental health problems, obesity, osteoporosis/
osteopenia, smoking, opioid dependence, secondary gain issues, etc., can never be
overemphasized.
Surgical Technique
When a trial of nonsurgical management has
failed, surgery can be considered. More recently
with the approval of multiple devices, minimally
hyperextending the hip. This maneuver can also be performed with the patient lying on the side with the symptomatic side up. This modification is helpful in patients
who are at higher risk of falling off the table in the supine
dangling position (e.g., obese patients)
invasive techniques have been predominantly
applied. New devices are regularly being introduced and it is not possible to cover all the
nuances of each system. Interested surgeons are
thus advised to reach out to manufacturers to
avail themselves of individual surgical technique
guides and videos specific to each system.
Needless to say, before attempting MIS SIJ
fusion, the surgeon has to thoroughly study the
SIJ anatomy and the system he/she is planning to
use and go through recommended/mandatory
training offered by manufacturers, including performing the procedure on a cadaver or model.
The authors have utilized two different systems – one utilizing triangular plasma-sprayed
titanium rods relying on bony ingrowth to the rod
at both sides of the joint and a screw-based system that allows for joint decortication and bone
graft placement in a circular area around the
screw. While both systems are more commonly
used with C-arm fluoroscopic imaging, the
authors have extensive experience in placing
them using computer navigation with intraoperative 3-D imaging.
434
S.C. Yson et al.
1. C-Arm Fluoroscopy
Both screw- and rod-based systems uti-
lize transgluteal transiliac sacral fixation.
The authors prefer to position the patient
prone, as for most spine surgeries. However,
patient may also be positioned supine,
depending on surgeon preference. Generally,
three intraoperative views are useful – inlet,
outlet, and lateral. An inlet view is taken
with the AP beam angled cephalad ~30–45°,
corresponds to a true axial view of the
sacrum, and is helpful in assessing for screw
violation through the anterior sacral cortex
or into spinal canal. An outlet view is taken
with the AP beam angled caudad ~30–45°,
corresponds to a true AP view of the sacrum,
and is helpful in assessing whether the
screw/pin has crossed the joint and its relationship with the sacral foramina. The lateral view is taken along the true lateral plane
of the body and is helpful for identifying.
These correspond to an axial view, a true AP
view, and a lateral view of the sacrum,
respectively.
Both systems initially require placement of
a guide pin (Steinmann) on the desired bony
starting point for each screw/rod. This is best
localized on a lateral image (Fig. 32.8). The
two systems that the authors use have different
suggested pin starting points and trajectories;
thus, there is no one perfect starting point.
However, it is important to avoid placing a pin
above the sacral ala, which is usually seen as a
faint oblique line coursing below or sometimes
crossing the S1 endplate; violation of the
sacral alar cortex may result in L5 nerve root
injury. It might initially seem counterintuitive,
but the sacroiliac joint projected on the lateral
image extends far anterior to the anterior sacral
margin; in fact, the true synovial portion of the
joint is its anterior region. Thus, it is certainly
acceptable and even preferable to have a starting point anterior to the anterior sacral cortex.
However, when doing so, the pin should be
directed posteriorly and should be assessed on
an inlet view prior to advancing the pin across
the joint, in order to prevent injury to pelvic
viscera/vessels.
The pin is advanced using a mallet or power
drill. Once in the ilium, inlet and outlet images
can be utilized to adjust or confirm the pin’s
trajectory. Once acceptable pin trajectory is
confirmed on inlet view, pin advancement is
performed while taking regular outlet view
images. This is to ensure that the pins either
stay short of or avoid the sacral foramina.
Driving the pins deeper than the medial foraminal border increases the risk of canal violation
and is probably unnecessary in most cases.
The steps are repeated for each implant (2
or 3, depending on surgeon’s preference).
Drilling, broaching, and implant placement
can be performed over the guide pin (Figs. 32.9
and 32.10). Final inlet, outlet, and lateral
C-arm images are taken to confirm satisfactory placement of all implants prior to wound
closure (Fig. 32.11).
2. Computer Navigation with 3-D Intraoperative
Imaging
At the authors’ institution, SIJ fusion is generally
performed using an intraoperative 3-D imaging system (O-arm) paired with a navigation
system (Stealth) which allows for automated
image registration. The O-arm is likewise utilized for 2-D fluoroscopic imaging, including
the requisite inlet, outlet, and lateral images.
At the beginning of the procedure, a reference
frame or fiducial marker is attached to a fixed
bony landmark, typically the contralateral
PSIS. A 3-D scan is then taken. Navigation is
utilized for identifying skin entry points,
placing the guide pins, and selecting implant
length. Guide pins are inserted through a navigated drill guide. Since the images shown on
the navigation screen are virtual images, these
may not correspond to the actual guide pin
position; thus, it is imperative that pin position still be checked with inlet, outlet, and lateral images prior to drilling/broaching/
implant placement. These latter steps are carried out in similar fashion to a non-navigated
procedure.
32 Sacroiliac Joint Fusion
Fig. 32.8 Lateral view
of the pelvis taken with
the C-arm. Bony starting
point using Steinmann
pin (white arrow) is best
localized using this view
Fig. 32.9 Intraoperative
Ferguson view showing
a broach being driven
over a guide pin
435
Postoperative Care
Patients are advised early on that they should
observe 50% partial weight bearing on the
affected extremity with bilateral axillary crutches
or walker ambulation for 6 weeks postoperative.
No lifting greater than 10 lbs., avoid excessive
bending or twisting activities. Patient is taught by
the physical therapist regarding ambulation and
transfer techniques either preoperatively or
before going home after surgery. Most patients
stay outpatient overnight (23 h stay), although
some go home the same day and some stay longer for pain control issues, particularly those who
are opioid tolerant/dependent. At the 6-week
visit, repeat radiographs (pelvis inlet-outletlateral) are taken; if stable and doing well, patient
is advanced to full weight bearing. Formal postoperative physical therapy may be initiated at this
point, consisting of pelvic stabilization and
transversus abdominis strengthening program,
similar to nonoperative SIJ-focused PT program.
436
Fig. 32.10 Intraoperative
Ferguson view showing a
titanium rod being driven
over a guide pin
S.C. Yson et al.
Fig. 32.11 Inlet (a), outlet (b), and lateral (c) views of the pelvis showing proper implant placement
Case Example
History
Physical Examination
She walks with an antalgic gait and localizes her
pain at the PSIS (Fortin finger sign). Her pain
This is the case of a 58-year-old housewife
who presented with a 2-year history of rightsided back pain. She was initially managed by
a physiatrist. Her symptoms were initially
attributed to her spine for which L3–L4 facet
was reproduced by the following provocative
maneuvers: FABER, thigh thrust, and Gaenslen’s.
The following exams were negative: sacral thrust,
pelvic gapping, and compression maneuvers.
Motor and sensory examinations are normal.
injections and radiofrequency ablations were
performed and subsequently her right hip for
Imaging
She reported no relief from the aforementioned
procedures. Her Oswestry Disability Index
(ODI) was 64. Her back pain was 8/10 and
right leg pain was 2/10.
Pelvic inlet, outlet, and lateral views showed
mild osteophytic spurring and subchondral scle-
rosis seen on both sides of the sacroiliac joints.
32 Sacroiliac Joint Fusion
Fig. 32.12 Pelvic inlet (a), outlet (b), and lateral (c) views of a 58-year-old woman diagnosed with sacroiliac joint
dysfunction. Mild degenerative changes are seen on both sides of the joint
437
No lesions, fracture, and gross malalignment
were evident (Fig. 32.12).
Management and Treatment
The patient underwent a diagnostic (anesthetic)
injection of the right SIJ which provided complete relief for several hours. She subsequently
received steroid injection which provided significant but temporary relief. She then underwent a
comprehensive physical therapy for 6 months
which reportedly did not provide substantial
improvement. Eventually she underwent minimally invasive SIJ fusion.
Outcome
At 1 and a half year postsurgery, she reports an
ODI of 4 and no back or leg pain.
Technical Pearls
• Evaluate preoperative pelvis inlet-outlet and
lateral x-rays for sacral dysmorphism [15].
Although different terms have been used to
describe anatomic variations along the same
spectrum (e.g., sacropelvic dysmorphism,
lumbosacral transitional segmentation, lumbarized S1, sacralized L5, etc.), the bottomline is that the anatomy in the region is
different from what is considered typical or
normal, which may likely require modifica-
tions to implant starting points/trajectories.
While an anatomic variation does not affect
diagnosis of the patient’s pain generator one
way or another, it may have profound implications on implant placement (Fig. 32.13). The
lateral sacrum or ala can be vacuous bone and
provide limited fixation. The best bones within
the sacrum are the cortices and the subchondral regions. Optimizing fixation in these
regions is best for fixation but also carries risk
of injury to neural, vascular, and visceral
structures.
• Positioning and draping are key. Authors prefer to use a radiolucent, carbon fiber, fourposter table; this allows for optimal
intraoperative imaging. Care must be taken so
that the pads do not preclude access to the surgical site. Prep and drape must be done with
care in order to not drape oneself out of the
necessary entry site. If using O-arm, arm
boards must be positioned close to the table to
allow the O-arm gantry to slide cephalad and
away from the surgical site.
• When using C-arm, the lateral view is critical.
Ensure that the superior margins of the right
and left sacral alae and the right and left sciatic notches are superimposed as much as
possible, thus giving a true lateral image.
Failure to do so may lead to implant
malposition.
• When using navigation, working in the sacrum
could be challenging as the anatomy is very
different as compared to when using navigation to place pedicle screws. There are several
imaging windows available for viewing.
438
Fig. 32.13 Ferguson view of the pelvis showing a dysmorphic sacrum: upsloping sacral ala (broken white lines),
prominent mammillary processes (white arrow), and noncircular S1 foramen (broken black lines)
While each surgeon may develop his/her own
preference, the authors have found it useful to
use three windows simultaneously: (1) a synthetic true AP of the sacrum or outlet view, (2)
axial window, and (3) coronal window. As
with navigated pedicle screw placement, it is
recommended to adjust the instrument trajectory one plane at a time in order to not lose
orientation.
• Some systems come with a pin placement
guide that allows identification of subsequent
bony starting points on the outer iliac cortex
after the first pin had been placed. Although
its use is optional, this may help ensure that
implants at minimum do not hit each other and
promote separation. Emerging biomechanical
data suggests that greater implant separation
and being in a nonlinear pattern appear to
achieve greater initial stability.
S.C. Yson et al.
aborted. With the use of advanced intraoperative
imaging, this is less likely to be a problem.
Typical problems include entry into the sacral
neural canal, rarely to the sacral spinal canal, and
anterior or posterior cortical perforation. With
passage of instruments over the guide pins, inadvertent pin advancement may occur; this may be
avoided/mitigated by switching to a blunt guide
pin. Likewise, guide pins may inadvertently
guide pin held by an assistant to gently push the
guide pin while the drill or broach is being backed
out helps avoid this problem.
The use of local anesthetic in the surgical field
helps to lessen the postoperative pain. Enhanced
recovery after surgery (ERAS) strategy for spine
surgery population is an emerging concept. The
authors have no experience on it but the concept
is promising. This strategy typically uses preemptive multi-pharmaceutical strategy to minimize pain [
16].
Surgical Outcomes
Multiple prospective studies have shown that
minimally invasive SIJ fusion is a viable treatment option for SIJ pain [11, 17, 18]. Compared
to nonoperative treatment, SIJ fusion has been
demonstrated to reduce pain and improve quality
of life [11, 17]. Long-term retrospective studies
seem to suggest that favorable outcomes are
maintained for up to 5 years [19]. Of note, most
of these outcome studies mainly refer to transiliac fixation devices, whether triangular titanium
rods or hollow anchorage screws [20, 21].
Complications and Strategies for Avoidance
Implant malposition is a key complication to be
avoided. This requires appropriate preoperative
anatomic analysis and adequate intraoperative
imaging and image interpretation. Large patients
or low-resolution imaging equipment are typical
causes. If it is not possible to adequately discern
the anatomic landmarks, the case should be
Conclusion
In summary, the diagnosis of sacroiliac joint pain
cannot not be easily distinguished from pain
coming from other sources based on history or
imaging alone. No single physical examination
test has been shown to be pathognomonic for sacroiliac pain. Performing a composite of tests adds
to the validity of results (e.g., more positive tests
lead to a higher likelihood of pain coming from
32 Sacroiliac Joint Fusion
439
the sacroiliac joint). Fluoroscopic- or CT-guided
intra-articular injection is currently the accepted
reference standard for confirming the diagnosis
of a painful sacroiliac joint. Once diagnosis is
confirmed, a trial of nonoperative management
should be done prior to considering surgery.
Minimally invasive fusion procedures are now
available, making surgery less morbid. As with
other elective surgical procedures, careful planning is essential to avoiding intra- and postoperative complications.
References
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regimen of pain provocation tests as an aid to reduce
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validity of individual provocation tests and composites of tests. Man Ther. 2005;10(3):207–18.
11. Polly DW, et al. Two-year outcomes from a randomized controlled trial of minimally invasive sacroiliac
joint fusion vs non-surgical Management for Sacroiliac
Joint Dysfunction. Int J Spine Surg. 2016;10:28.
12. Polly D, et al. Does level of response to SI joint block
predict response to SI joint fusion? Int J Spine Surg.
2016;10:4.
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Biomechanical Principles of Spine Stabilization
Alvin Y. Chan, Jeffrey P. Mullin, Emily Bennett,
Karin Swartz, and Edward C. Benzel
33
Introduction
An understanding of biomechanical principles is
crucial to making appropriate decisions with
respect to proper spine stabilization. Surgeons
have more methods than ever at their disposal to
stabilize the spine, and each option has its own
specific nuances, complications, and advantages;
thus, understanding the fundamental biomechanical principles that lie at the core of each intervention is crucial to matching the patient’s
specific requirements to the most suitable construct. This chapter lays the foundation on which
proper spine stabilization is established and
hopefully encourages the reader to consider the
pertinent biomechanical principles to optimize
patient outcome.
A.Y. Chan, BS • K. Swartz, MD
Department of Neurosurgery, Froedtert Hospital
and the Medical College of Wisconsin,
8701 W Watertown Plank Rd., Milwaukee,
WI 53226, USA
J.P. Mullin, MD • E. Bennett, MD
E.C. Benzel, MD (*)
Department of Neurosurgery, Cleveland Clinic,
Neurological Institute, 9500 Euclid Avenue,
Cleveland, OH 44195, USA
e-mail:
benzele@ccf.org
Basic Principles of Spine Biomechanics
Biomechanically Relevant Spinal Anatomy
The main structural element of the spine is the
vertebral body (VB) which provides the main
resistance against axial loading. The following
terms should be defined: (1) the “width” of the
VB is measured from the right-left direction, (2)
the “depth” is measured in anteroposterior planes,
and (3) the “height” is measured craniocaudally.
The VB is generally cylindrical in shape, where
the depth and width measurements are typically
greater than the height. The VB has a rim of cortical bone, an interior of cancellous bone, and is
flanked craniocaudally by two end plates.
Furthermore, the width and depth of the VB
increase as you move caudally down the spine,
leading to a larger cross-sectional area to accommodate for the increased axial loading at the base
of the spine. An exception to this generalization
is the L5 VB, which tends to be narrower in depth
than the L4 VB (Fig. 33.1).
Two adjacent vertebral bodies combine with
the intervening intervertebral disc and adjoining
ligaments to compose a functional spinal unit
(FSU) or motion segment. The intervertebral disc
serves as a “shock absorber” and a primary stabilizing structure of the FSU [1]. Although the disc
© Springer International Publishing AG 2017
L.T. Holly, P.A. Anderson (eds.), Essentials of Spinal Stabilization,
DOI 10.1007/978-3-319-59713-3_33
441
442
Fig. 33.1 Vertebral
body diameter versus
spinal level. The width
(solid line) and depth
(dashed line) of the
vertebral bodies are
depicted separately
(Fig. 1.1 in
Biomechanics of spine
stabilization, Benzel E,
ed. Printed with
permission from Thieme
Medical Publishing)
A.Y. Chan et al.
is vaguely similar in outline to the VB in depth
and width, the composition is vastly different. It
consists of the nucleus pulposus (proteoglycans
suspended in a loose collagenous network)
located posterocentrally and is surrounded by the
annulus fibrosus (a fibrocartilaginous ring).
Similar to the VB, the intervertebral discs
increase in cross-sectional area in the caudal
direction, allowing the lower region of the spine
(e.g., lumbar) to sustain higher axial loading [
2].
Moreover, the type of loading influences how
the disc responds. For example, concentric axial
loading creates an equally distributed force
within the disc, while an eccentric axial load will
bulge the annulus fibrosis on the ipsilateral side
and displace the nucleus pulposus to the contralateral side. The sharply angulated fibers of the
annulus fibrosus provides the disc’s main resistance to shearing and rotational forces which
allows for increased force during a broad range
of activities. For example, during normal walking, the compressive axial loading on the discs in
the lumbar region can be up to 2.5 times the body
weight. When lifting 14–27 kg objects, the axial
load can increase further to nearly ten times the
body weight [3, 4]. Increasing activity requires
the discs to undergo significant and repetitive
forces without failure.
Between motion segments of the spine, the
facet joint is the main load-bearer and stabilizer.
The orientation of the facet joints differs depending on the spinal level (e.g., cervical, thoracic,
lumbar), and these differences allow for contrasting degrees of motion and resistance among
them. Generally, the pattern of flexibility
decreases in the cranial to caudal direction.
Specifically, the facet joint articulations in the
cervical spine lie in the coronal plane which
allows for high degrees of motion in flexion,
extension, and rotation, whereas the lumbar facet
joint articulations lie in the sagittal plane preserving flexion and extension but allowing for less
rotation than in the cervical spine. The thoracic
facet joint articulations lie in between the coronal
and sagittal planes and therefore provide an
“intermediate” range of motion (Fig. 33.2). The
rib cage also stabilizes the thoracic region by acting as a barrel attached to the spine. Stress, extension, and ventrally directed forces “load” the
facets, while flexion and dorsally directed forces
“unload” the facets. Facet joints take on additional load-bearing responsibilities when other