Keywords sacroiliac joint - spine - sacrum
Introduction
Spinopelvic fixation remains a challenging subject in spine surgery. Biomechanical
forces, anatomical features and bone quality are some of the reasons why surgeons
continue to explore sacropelvic fixation options for correcting deformities that extend
to the pelvis.[1 ]
[2 ]
Studies have shown high rates of complications in long fixations of the spine crossing
the L5-S1 articulation with exclusive distal anchorage in S1.[3 ]
[4 ] To overcome them, sacropelvic fixations were introduced, allowing the use of significantly
larger implants, increasing the rigidity and stability of the frame necessary for
the effective treatment of complex spinal deformities, among other situations.[2 ]
[3 ]
[5 ] A more recently developed option that has shown some benefits in relation to other
methods, due to the relative ease of insertion and lower complication rates, is the
S2-alar iliac (S2AI) technique. In it, a screw with an S2 entry point is inserted
through the sacroiliac joint and travels through the iliac bone toward the anteroinferior
iliac spine (AIIS).[1 ]
[5 ]
[6 ]
[7 ]
[8 ]
[9 ]
[10 ]
Studies on this technique, despite exposing the trajectory of the implant, do not
detail in an in-depth way how to check the parameters necessary for the insertion
of this screw by means of imaging exams. Given this gap in the literature, the present
study aims to describe how to measure the parameters of the sacropelvic segment necessary
for the safe insertion of the ideal S2AI screw for each patient.
Materials and Methods
The research project was approved by the Research Ethics Committee of the institution
where the study was conducted, process no 7088/2019 (CAAE: 08968219.4.0000.5440).
This is a descriptive study considering computed tomography (CT) of the pelvis. The
clinical data included in the study were the gender and age of the patient. The 100
most recent pelvis CTs from the archive of patients of the institution, of both genders > 18
years old, were consecutively selected. Of these, 66 were excluded because they had
previous surgery in the pelvic or lumbosacral region and/or pathologies affecting
the sacropelvic segment (fracture, tumor, infection, congenital anomalies, ankylosing
spondylitis), totaling n = 34 cases, of which 18 were male and 16 female, with ages ranging from 18 to 86
years old (mean of 52.11 and median of 57.5 years old).
All CTs were downloaded in DICOM format, and the exams were transferred to the Horos
software, version 1 1.7. The images were analyzed in a dedicated window for bone structures
and reconstructed in the software, which allows the evaluator to process and edit
2D image data and reconstruct them in 3D multiplanar models with precision, so that
the determination of the entry point of the screw was done with accuracy, as well
as their plane and the direction. The reslicing tool was used to obtain precise anatomical
alignment.
Considering the entry point described for the insertion of S2AI screws at the midpoint
between the dorsal foramina and S1 and S2, the following sacropelvic morphometric
parameters were measured[3 ]
[11 ]:
01) Screw lengths: greater and lesser distances from the entry point to the AIIS, tangent to the medial
cortex of the ilium, for the longest length, and the lateral cortical of the ilium,
to the shortest length.
02) Screw diameter: minimum width of the iliac wing, which corresponds to the virtual channel of the
S2AI screw trajectory, in which there is no violation of any cortex (without risk
of injury to neurovascular structures).
03) Screw trajectory angles in the axial plane: anteroposterior inclination angles for the insertion of the S2AI screw. Measured
between the screw trajectory lines that correspond to the longest and shortest lengths
and the anteroposterior midline of the sacrum.
04) Angle of screw trajectory in the sagittal plane: craniocaudal inclination angle in relation to the S1 plateau for the insertion of
the S2AI screw.
Results
The CT exam is only available in the axial plane. The first step is to reconstruct
the images in the coronal and sagittal planes ([Fig. 1 ]).
Fig. 1 Computed tomography images in (A ) Sagittal reconstruction, (B ) Axial plane and (C ) Coronal reconstruction.
Initially, sagittal reconstruction is used to angle three-dimensionally, using the
3D multiplanar reformatting function (MPR), the axis of the series of axial sections so that it is close to the
long axis of the sacrum ([Fig. 2 ]) and that it is possible to visualize the S2 vertebra, the entry point and the AIIS
in the same plane ([Fig. 3A ]). The entry point of the S2AI screw is demarcated in the posterior cortex of the
sacrum in the axial plane ([Fig. 3A ]), with the aid of the coronal plane to visualize the midpoint between the dorsal
foramina of S1 and S2 ([Fig. 3B ]).
Fig. 2 Reconstruction performed using the 3D MPR function. Axis angulation (blue) of the
axial section series (purple).
Fig. 3 Images obtained through 3D multiplanar reconstruction after the procedure demonstrated
in [Fig. 2 ]. ((A)) Axial plane image in which the S2 vertebra, the entry point (E) and the AIIS
are seen in the same section. ((B)) Coronal reconstruction in which it is possible
to visualize the interval between the dorsal foramina of S1 and S2 for the precise
demarcation of the entry point of the S2AI screw.
After defining the entry point, the safe limits of length of the screw to be inserted
are checked as follows: knowing that the screw is directed to the AIIS,[3 ] lines are drawn from the entry point, tangential to the inner and outer cortical
of the iliac bone wing ([Fig. 4 ]). The line tangent to the internal cortex corresponds to the longest possible length
of the S2AI.
Fig. 4 Checking the lengths of the S2AI screw. From the entry point, lines are drawn tangent
to the cortical of the iliac wing toward the AIIS. The length of the line equals the
length of the screw.
The largest possible diameter of the screw to be inserted is determined by the shortest
distance between the inner and outer faces of the iliac ([Fig. 5 ]). Because the diameters of the available sacroiliac screws are determined by the
measurement of the implant internal diameter, it is extremely important to subtract
half of the diameter of the screw chosen, medially and laterally, so that the thread
does not violate the cortical of the iliac wing. Afterwards, the length measurements
of the implants are adjusted to the new safety limits of the path ([Fig. 6 ]).
Fig. 5 The largest diameter of the S2AI screw corresponds to the shortest (virtual) space
between the inner and outer cortical of the iliac wing.
Fig. 6 Example in which a sacroiliac screw of 7.5 mm in internal diameter was chosen. From
the maximum diameter of 11 mm, 3.75 mm (half the internal diameter of the implant)
was subtracted medially and laterally to avoid the violation of the corticals of the
iliac wing by the screw threads. Afterwards, the length measures were adjusted according
to the safety limit of the new path.
To determine the angles of the trajectory of the skull in the axial plane, the anteroposterior
midline of the sacrum is used, and the angles formed between it and the lines of the
largest and smallest lengths of the screw are measured ([Fig. 7 ]).
Fig. 7 Measure that determines the anteroposterior inclination angle in the axial plane
of the S2AI screw. The anteroposterior midline of the sacrum is used as a parameter
(perpendicular to the horizontal line of the pelvis) in relation to the lines corresponding
to the screw length measurements.
To determine the craniocaudal inclination angle in relation to the S1 plateau ([Fig. 8 ]), the degree of inclination made in the sagittal plane is used ([Fig. 2 ]) to find the image where the entry point and the AIIS are seen in the same plane
([Fig. 3A ]).
Fig. 8 Measure that determines the craniocaudal inclination angle of the S2AI screw: Angle
formed between the S1 plateau and the degree of inclination performed in the sagittal
plane to obtain the plane of the screw's path ([Figure 2 ]).
In the 34 exams evaluated, it was found that the measurement of the longest screw
length ranged from 86.8 to 133.6 mm, with an average of 112.6 mm, and that of the
shortest length, from 73.3 to 117.6 mm, with an average of 105.6 mm. The smallest
diameter of the virtual space between the internal and external iliac boards ranged
from 9.2 to 20.6 mm, with a mean of 11 mm. Regarding the trajectory angle in the axial
plane for the insertion of the longest screw, we observed a mean of 38° (28.1–46.3°)
and a mean variation in relation to the path angle of the shortest screw length of
3.3° (1.2–9°). The trajectory angle in the sagittal plane varied from 4.8 to 10.2°,
with a mean of 8.3°.
Discussion
The fixation of the spine extending to the pelvis is indicated to add anchoring force
to the instrumentation, aiming to reduce the complications related to the L5-S1 fixation,
in cases of pelvic obliquity, high-grade spondylolisthesis (grade 3 or higher), deformity
with rectification of lumbar lordosis that requires corrective osteotomy, osteoporosis
at the lumbosacral junction, spinopelvic trauma as well as other major spinal deformities.[2 ]
[3 ]
[5 ]
[12 ]
[13 ]
[14 ]
Several factors that may have contributed to the high rates of pseudoarthrosis, fracture
of the S1 pedicle and failure of the implants used in sacral fixation associated with
long arthrodesis in the thoracolumbar spine, including the direction and depth of
the screw insertion, are inadequate bone quality in this region and excessive load
resulting from a long lever arm of fusion above the sacrum.[12 ]
[15 ]
[16 ]
[17 ] To circumvent these complications, several sacropelvic fixation techniques have
been described, including transiliac bars, iliac and sacroiliac screws.[1 ]
[2 ] The Galveston technique, described in the 1980s, uses instrumentation with bars
inserted between the faces of the iliac bone and, for this, often requires complex
and three-dimensional folds of the nails.[13 ] The iliac screw technique, in contrast to the transiliac bars, has the advantage
of easy insertion and provides greater resistance to pullout, although it may require
separate fascial and skin incisions.[2 ]
[4 ] In addition, interchangeable connectors that link lumbar arthrodesis and sacropelvic
fixation are often used, and the wide dissection of the soft tissues necessary for
this increases the morbidity of the procedure, and can compromise the integrity and
vascularization of the musculature and skin flaps in that area, making the distal
portion of the incision more prone to dehiscence of the surgical wound.[2 ]
Retrospective studies comparing short-term complications between the iliac screw and
S2AI screw techniques show that S2AI fixation was associated with a reduction in the
incidence of implant breakage, of surgical site infection and surgical wound dehiscence,
in addition to having less need for surgical revision and lower rate of posterior
pelvic pain in the postoperative period.[9 ]
[18 ] The entry point of the traditional iliac screw is in the posterosuperior iliac spine,
and its insertion requires considerable dissection of soft parts, including fascia
and paravertebral musculature.[2 ] The S2AI screw is an anatomically viable pelvic fixation technique, with an entry
point at the level of the second sacral vertebra, that is, lower, medial and anterior,
thus avoiding the prominence of the instrumentation, which can lead to pain at the
implant site and increased risk of surgical wound dehiscence.[3 ]
[9 ]
[19 ]
[20 ] Another advantage of the S2AI technique is that it eliminates the need for modular
connectors, used in traditional fixation in the iliac screw technique, to join the
sacropelvic fixation to the posterior spine arthrodesis. This is due to the fact that
the S2AI entry point is in line with the entry point of the S1 pedicle screw, which
can also reduce a potential cause of implant-related failure.[1 ]
[2 ]
[3 ]
[5 ] In addition, studies suggest that the minimally invasive percutaneous fixation of
the S2AI screw, as well as the insertion guided by stereotaxic or robotic navigation
are viable, safe and accurate options for the precise insertion of the screw.[4 ]
[21 ]
[22 ]
[23 ]
[24 ]
On the other hand, one of the potential disadvantages of this technique, according
to studies on cadavers, is the fact that 60% of S2AI screws can violate the sacroiliac
joint cartilage without effectively achieving arthrodesis.[2 ] However, the clinical significance of this is still unknown.[13 ]
[18 ]
[19 ]
[20 ]
Among the risks of the S2AI technique, there are violations of the vertebral canal,
injuries to visceral structures, such as intestines and urogenital organs, and neurovascular
structures, mainly internal iliac arteries and veins and the lumbosacral plexus.[3 ]
[25 ] Detailed knowledge of the anatomy and trajectory characteristics of the screw is
necessary to minimize these possible damages.
Regarding the biomechanical properties of S2AI, it is known that the lumbosacral transition
resists shear forces and is, evidently, one of the most important regions of the spine
in terms of mobility and load support.[3 ] The current literature shows that there is no significant difference in biomechanical
properties related to rigidity and failure of instrumentation between the techniques
of S2AI and conventional iliac screws.[1 ]
[5 ]
[8 ]
[9 ] O'Brien et al. showed that the 65 mm S2AI screw was biomechanically equivalent to
the 90 mm iliac screw and the 80 mm S2AI screw.[1 ] Although this result seems contradictory, the iliac fixation is performed in a spongy
bed, and the sacroiliac fixation has cortical penetration in the joint, which can
offer additional strength despite the shorter length.[1 ]
In relation to the sacral pedicle screws of S1 and S2, the greater length of the S2AI,
as well as the acquisition of multiple corticals, due to the penetration of the sacroiliac
joint, makes this instrumentation method biomechanically more stable.[3 ] Burns et al. did not find, in their study, significant differences for torsional
stiffness in extension, flexion and lateral inclination between S2AI constructions
and iliac screw. There were also no significant differences between S2AI and iliac
screws for insertion torque or pullout resistance.[5 ]
An Asian population study[3 ] shows that screws between 85 and 120 mm in length are potentially favorable in this
group with no difference between genders. In the present article, a mean of similar
length, 112.6 mm, was observed, and that the 7.5 mm diameter sacroiliac screws were
viable, which is in accordance with Wu et al.,[9 ] who showed, in a literature review, that the diameter of the S2AI screws varied
from 6.5 to 8.5 mm. The anteroposterior tilt angle found by Elder et al.[19 ] varied from 30 to 45°, and the craniocaudal inclination angle, from 20 to 45°. This
last finding differs from the result found in the sample of Brazilian individuals
in the present assessment.
Most studies evaluated the trajectory of spinopelvic implants by means of imaging
tests or in cadavers without any injury or deformity.[3 ]
[10 ]
[26 ] As spinopelvic fixation is often performed for the purpose of correcting major deformities,
cases in which pelvic asymmetry is occasionally present, we believe that individualization
of surgical planning is essential to avoid complications and, consequently, for the
acquisition of good results. The present study, therefore, is important for the surgeon
to know how to assess in detail the sacropelvic parameters necessary for the safe
insertion of the S2AI screw for each case.
Conclusion
It was possible to adequately assess, through multiplanar CT reconstructions, the
sacropelvic parameters necessary for the safe insertion of the ideal S2AI screw for
each patient.
