Keywords
interventional radiology - fracture - screw - osteoplasty - ablation - pelvis - metastases
Up to 100,000 new patients develop bone metastases each year. The lifetime incidence
is estimated at up to 75% in patients with breast and prostate cancer, 60% in thyroid
cancer, 40% in bladder and lung, and 25% in renal cell carcinoma.[1] The prevalence of metastatic disease involving the bones is increasing, in large
part due to significant recent advances in systemic therapy that have led to more
patients living longer with metastatic disease. As a result, long-standing osseous
involvement can lead to significant complications affecting mobility and quality of
life.
Metastatic involvement of the pelvis can be especially problematic, given the significant
stresses present during weight bearing.[2] A frank protrusio fracture of the femoral head through a weakened acetabulum can
be catastrophic, rendering patients essentially bedbound in severe pain. Surgical
repair for these devastating fractures has traditionally required an extensive Harrington-type
reconstruction to rebuild the acetabulum consisting of tumor curettage, extensive
cementation of the pelvic defect, and placement of an antiprotrusio cage.[3]
[4] The complication rates of these extensive surgeries are high and physical recovery
from such operations can be long, requiring extensive physical therapy to regain mobility.[5] Additionally, essential multimodality, systemic, and radiation therapies are often
delayed or interrupted for weeks while surgical incisions heal.
Pathologic fractures of the sacrum may present as a similar clinical conundrum with
limited therapeutic options for destructive metastases. Neither transsacral screws
nor sacroplasty often provide adequate stabilization alone and true spinopelvic fixation
can bring significant morbidity.
Therefore, patients presenting with pelvic metastases causing instability, especially
when symptomatic, can greatly benefit from minimally invasive stabilization techniques
that provide pain relief, maintain mobility, and essentially remove significant delays
or interruptions in critical systemic therapies.[6]
[7]
[8]
[9]
[10]
[11]
Herein, we describe some of the clinical and technical considerations of our pelvic
“screw and glue” technique developed in close collaboration between our interventional
radiology and orthopaedic oncology services and performed in over 120 patients over
the past decade.
Preprocedural Considerations
Preprocedural Considerations
Patient Considerations
It is essential to consider certain individual patient factors when considering treatment
options. These include, but are not limited to, the severity of pain, inadequacy or
side effects of analgesic medications, degree of mobility limitations, life expectancy,
quality of life, and oligometastatic versus widely metastatic disease.
In general, percutaneous stabilization is advantageous for painful or high fracture
risk lesions in the following settings: poor open surgical candidates, concern for
wound healing complications, extensive disease on presentation requiring prompt initiation
of systemic and/or radiation therapy, and rapidly progressive disease requiring uninterrupted
continuation of systemic and/or radiation therapy. Contraindications to percutaneous
stabilization are generally relative, and even suboptimal stabilization through a
percutaneous approach may be able to achieve acceptable short-term success in palliating
pain and immobility, which may be adequate when considered within a patient's goals
of care.
Open surgical fixation of the acetabulum can be considered in patients with oligometastatic
disease, favorable tumor biology, and good performance status; however, in our practice
it is only rarely performed initially, as we have found that previous percutaneous
stabilization has generally facilitated rather than hindered subsequent surgical reconstruction.
Tumor Type
Tumor type is an important factor when considering whether percutaneous stabilization
is necessary. In general, all tumor types will benefit from percutaneous stabilization
if lesions are large and destructive, especially with associated pain or particularly
aggressive growth. Small and moderate lesions of tumors considered responsive to systemic
and radiation therapy such as breast, prostate, lung, myeloma, and lymphoma can usually
be observed while theses therapies are pursued first, unless there is disability clearly
related to an existing fracture.
Even patients without pain or disability may still carry a significant fracture risk
following initiation of therapy as disease recedes and leaves less structural support.
Although no clear guidelines exist at present, there may be clinical benefit to prophylactic
stabilization in select asymptomatic patients, analogous to prophylactic femoral rodding
in the setting of metastatic disease, to prevent a devastating fracture.
On the other hand, for moderate size tumors more resistant to systemic and radiation
therapy such as renal cell and bladder carcinomas, early stabilization, combined with
ablation when feasible, and often followed with standard radiation therapy, may be
appropriate to prevent skeletal-related complications from progressive disease.
Myeloma is somewhat unique in that an initial favorable response to systemic and radiation
therapy is often achievable but can leave the skeleton at significant risk of fracture
if lytic lesions are moderately large and in weight-bearing portions of the pelvis.
In these situations, stabilization with either cement alone or cement-augmented screws
is helpful. Unique to myeloma, a standard cement, with its moderately exothermic reaction,
generally fills the lytic defect completely, either “melting” the soft tumor or displacing
it, and interestingly even in areas of apparent complete cortical dehiscence on imaging
the periosteum often remains intact and a highly effective barrier for cement. As
such, in our experience, myeloma lesions can often be completely treated locally with
cementation alone without significant residual disease, thereby obviating the need
for additional local therapy with radiation with the added benefits of avoiding radiation-induced
osteoporosis of the already weakened pelvis, preserving bone marrow for transplant,
and preserving radiation as a treatment option for recurrent disease.
Structural Assessment
While this discussion focuses on our combined use of orthopaedic screws with cement,
in many instances, pelvic stabilization may be performed with cement alone.[12]
[13] Polymethylmethacrylate (PMMA) is exceptional in resisting compressive forces and
works well on its own for small lesions within the cancellous bone, especially directly
over the acetabular roof where forces are primarily compressive ([Fig. 1]). But PMMA tends to fail with significant rotational, bending, sheer, or distracting
forces, secondarily allowing persistent motion and resulting in inadequate stabilization.
Therefore, in cases where these forces are present, the addition of screws should
be considered to offer a more stable construct by resisting these forces, similar
to rebar in concrete.[9] Specific examples where we have found benefit in adding screws include when there
is a significant fracture already present, there is significant cortical destruction
along the major buttresses of the pelvis (in addition to cancellous destruction) such
as the ischial spine of the posterior column or superior ramus of the anterior column,
there is anatomic concern for early cement leak leading to early osteoplasty termination
and inadequate stabilization, or there is rapidly progressive disease where ongoing
osseous destruction is likely. In practice, screw placement through an area of prior
osteoplasty can be exceedingly difficult and less accurate; so, we have been aggressive
with initial screw placement when, in our judgement, we feel osteoplasty alone is
likely to fail.
Fig. 1 A 48-year-old male with painful lytic myeloma lesion in the acetabulum (a), treated with polymethylmethacrylate osteoplasty alone (b).
Some specific lesion types can make adequate stabilization challenging. In severely
comminuted fractures, it is often challenging to achieve adequate stabilization even
with the addition of screws due to early and extensive cement extravasation. Sufficient
long-term stabilization depends on adequate fracture reduction; however, this is difficult
to achieve percutaneously for significantly displaced fractures. In these instances,
we prefer open surgical techniques if possible and reserve percutaneous stabilization
for pain palliation in poor surgical candidates with limited mobility where structural
integrity is not as essential. Fractures through the acetabular rim can be more difficult
to stabilize percutaneously, and, if significant, may warrant placement of a surgical
cage to prevent progression. Moreover, extensive bone destruction along the screw
paths, especially entry and exit cortices, may prevent adequate screw purchase and
fixation, even if augmented with copious amounts of cement. While total stabilization
is always preferred, it is true that even suboptimal stabilization can often provide
some degree of meaningful short-term benefit, which may be adequate for select patients
without a better surgical option.[6]
Screw Planning
In our experience, there are three main screw corridors around the acetabulum that
are achievable and can be combined to provide a base structure for adequate fixation,
although from time to time additional screw orientations may be advantageous ([Fig. 2]). The first screw is the ischial screw, buttressing the posterior column of the
acetabulum, entering the ischium and travelling superiorly into the posterior ilium/iliac
wing. The second screw is the superior ramus screw, buttressing the anterior column
of the acetabulum. This screw can be placed in either a retrograde fashion entering
near the pubic symphysis and travelling laterally over the acetabular roof to the
lateral iliac cortex, or in the opposite antegrade fashion from a posterior approach.
The third screw is the anteroposterior or AP screw. This screw bridges the anterior
and posterior columns together, beginning in the anterior inferior iliac spine (AIIS)
and traveling just above the sciatic notch terminating in the posterior superior iliac
spine (PSIS). This screw can also be placed in the opposite orientation from a posterior
approach as a PA screw. Notably, we frequently place two parallel screws in this corridor
stacked superiorly and inferiorly for additional support when appropriate.
Fig. 2 (a) Schematic depicting the three main screw corridors around the acetabulum. (b) Ischial screw corridor from retrograde approach. (c) Ramus screw corridor from either antegrade or retrograde approach. (d) AP screw corridor from either antegrade or retrograde approach.
The choice of which screws to place is patient specific and only rarely are all three
corridors reinforced in the same patient. In general, these three primary screws are
placed parallel to specific cortical buttresses that have been eroded or that are
at high risk of destruction with rapidly progressive disease. In select cases, additional
shorter screws can be placed tangential to specific fracture lines to reduce fracture
distraction and further minimize torsional motion.
For severe sacral lesions with extensive bony destruction, severely comminuted fractures,
or a high likelihood of progression, we have often decided that cement alone would
provide inadequate stabilization. In these cases, we preferentially place two parallel
transverse sacroiliac screws, through the S1 and S2 corridors ([Fig. 3]). We avoid placing only one transsacral screw, as this often provides suboptimal
stabilization and can lead to significant rotational forces around the single axis
of the screw that will eventually lead to hardware loosening.
Fig. 3 (a) Paired S1 and S2 screw corridors. (b) Example of inadequate S1 screw corridor addressed with bilateral iliosacral screws
into the S1 body (c).
Of these five basic screws, only the ischial screw, AP screw, and S2 screw corridors
are consistently achievable. The ramus screw and transverse S1 iliosacral screw corridors
are usually achievable; however, some pelvic anatomies, especially in women with open
pelvic anatomy, make these unachievable without passing outside the cortex for part
of the screw path, and preprocedural multiplanar reformats along the proposed screw
corridors can be extremely helpful when planning screw placement.
When necessary, the S1 iliosacral screw can be replaced by bilateral oblique iliosacral
screws meeting in the S1 body, though this provides less stabilization. The ramus
screw can usually be placed in an antegrade orientation at least over the acetabular
roof and partially into the superior ramus, if not all the way to the pubic bone,
whereas a retrograde ramus screw beginning at the pubic bone may not be able to clear
the acetabular roof without violating the joint space.
In the vast majority of cases, we prefer using cannulated, fully threaded screws in
all corridors. This provides maximal stabilization when augmented with adjacent cement
in areas of extensive bony destruction and poor bone quality. Partially threaded screws
(or lag screws) are generally avoided as they dynamically compress the traversed bone
and can exacerbate compromise of fractured neuroforamina. However, these can be useful
when attempting to reduce specific fracture lines or to mitigate sacral nerve irritation
when there is a high probability of neuroforaminal encroachment by providing a smooth
rather than threaded point of contact with the nerve. In our experience, currently
available image guidance systems have made this relatively unlikely. We tend to use
large cannulated screws between 7 and 8 mm but sometimes place smaller 6.5-mm screws
for additional support or through particularly narrow ramus corridors. A screw with
bicortical purchase is one that is positioned within cortex at both of its ends. This
generally provides added stability and should be performed when feasible and safe.
Cannulated screw sets are available from Stryker (Kalamazoo, MI) and DePuy Synthes
(West Chester, PA). While we use both, the Synthes screw set provides the greatest
flexibility. In this set, fully threaded 7.3-mm screws are available up to 180 mm
in length. While Stryker provides fully threaded screws up to 150 mm in 6.5 and 8 mm
diameters, they do provide longer partially threaded screws up to 180 mm.
For acetabular metastases, the Harrington classification system is well known but
may not be applicable when planning minimally invasive stabilization. A more meaningful
assessment of pelvic lesions includes identifying which cortical buttresses are significantly
weakened, the degree of cancellous bone destruction, and any fracture lines extending
into the joint space or neuroforamina that would make aggressive cement deposition
difficult due to the possibility of cement extravasation.
A multidisciplinary approach is essential to offer patients the best long-term plan.
Interventional radiologists should collaborate deliberately with their orthopaedic
surgery colleagues regarding a detailed procedural plan when possible so as to achieve
the greatest structural reinforcement possible and avoid instrumentation that can
make subsequent open surgical revision more difficult if necessary in the future.
We found this collaboration to be professionally rewarding and greatly benefitting
patients by combining the expertise of orthopaedic surgeons in fractures, bone healing,
and structural principles and interventional radiologists in imaging, image guidance,
and an ability to adapt minimally invasive techniques.
Osteoplasty
In general, aggressive osteoplasty is also performed during all screw fixation cases,
with two specific stabilization goals. First, robust cement deposition provides resistance
against compressive forces across large lytic defects, especially in the region of
the acetabular roof where these compressive forces are significant. Second, cement
serves to further stabilize the screws themselves within areas of osseous destruction
and helps minimize screw motion that can lead to poor healing and/or hardware loosening.
This additive stabilization advantage is likely increased by using fully threaded
instead of partially threaded screws, maximizing the interdigitation between hardware
and cement. And while the benefit of cement is primarily structural, tumor essentially
does not grow through it, and strategic cement deposition can aid to some degree in
preventing disease progression into augmented areas.
Ablation
Ablation can be helpful to provide pain relief as well as local tumor control.[14]
[15]
[16]
[17]
[18]
[19]
When focusing on pain relief, targeting of bone–tumor interfaces, areas of periosteal
involvement, or when involved, the SI joint itself can maximize benefit.[19]
As for tumor control, ablation of large pelvic metastases is almost never intended
to achieve complete tumor ablation, primarily because the adjacent nerves and hip
joint frequently prevent achieving a complete ablation without unacceptable collateral
damage. However, concurrent ablation can serve two primary roles: local tumor control
and cavity creation to facilitate cement fill. The overall benefit for local tumor
control has not been proven; however, targeted tumor control with ablation along the
strategic margins can be helpful in preventing uninhibited disease progression locally
in certain directions leading to hardware failure. Additionally, our experience suggests
that local tumor debulking in certain radiation-resistant malignancies may be beneficial
through some direct disease control, increasing the effectiveness of radiation on
a smaller quantity of remaining disease. We usually do not perform ablation on malignancies
that respond well to radiation and/or systemic therapy such as breast, lymphoma, or
myeloma.
If ablation is to be utilized, the choice of which ablation technology to use is important,
as each has strengths and weaknesses which have been detailed in the literature elsewhere.[20] Specific to pelvic reconstructions, cryoablation can be advantageous when ablation
occurs near critical structures, often nerves, in which case discrete ice ball (ablation
zone) visualization can reduce the risk of nontarget ablation. It is true that ice
is poorly visualized within bone; however, the ice easily extends across cortex into
the adjacent soft tissues, where it can usually be visualized with CT imaging, without
significant asymmetry of the ablation zone. However, cementation immediately following
cryoablation is challenging due to the residual ice and can result in inadequate augmentation.
Several techniques can be used to minimize this including staging the ablation and
cementation procedures a day apart, waiting at least 1 hour between, during which
time screws can be placed, or utilizing a final extended or rapid thaw cycle to directly
melt the ice ball. For this reason, we prefer to use heat-based ablation systems,
both microwave and radiofrequency ablation, when performing ablation away from critical
structures.
Nerve Review
A thorough documented preoperative neurologic exam is required, and all major pelvic
nerves must be identified beforehand on cross-sectional imaging,[21] accounting for any significant displacement of nerves due to bulky soft-tissue disease.
Various techniques have been described to mitigate nerve injury during ablation.[21]
[22]
[23] In general, unilateral injury to the S2 and S3 nerve roots is tolerated in the setting
of normal contralateral innervation, as are injuries to cutaneous sensory nerves.
The inferior gluteal nerve can often be sacrificed if needed resulting in a Trendelenburg
gait that generally is tolerable. Injuries to the L4–S1 nerve roots, femoral nerve,
and sciatic nerve are more significant, as these can significantly impact function
and ambulation, and may necessitate long-term orthotic devices or the use of an assistive
device. In our experience, the most significant postprocedural complications have
all been related to significant nerve injury, manifesting as either severe pain or
weakness, both of which significantly complicate the postprocedural goals of improved
mobility and quality of life.
Technical Procedural Details
Technical Procedural Details
Patient Preparation
All cases are performed under general anesthesia with a ceiling mounted fluoroscopy
unit capable of cone-beam CT. Supine patient positioning is easiest and preferred
unless an ischial screw is needed through the posterior column—in which case patients
are positioned prone with hips slightly flexed using appropriate bolstering and padding,
often incorporating a radiolucent negative pressure beanbag to minimize patient movement.
A Foley catheter is placed to facilitate bladder drainage, especially relevant if
a ramus screw is planned, and a standard surgical preparation of the operative site
including Ioban adhesive drape is performed.
Creation of 3D Objects for Use with Augmented Fluoroscopy
A fluoroscopy unit with advanced imaging features is essential to a successful outcome.
These features include cone-beam CT and needle guidance software, or more generally
augmented fluoroscopy, which consists of displaying in real time three-dimensional
(3D) objects, registered to the patient's cross-sectional data and overlaid onto live
fluoroscopic imaging ([Fig. 4]). These objects include needle guidance lines used for placement of screws, bone
trocars, ablation probes, and cement cannulae, as well as volumetric segmentation
of targeted lesions or areas of bony destruction. These guides are exceedingly helpful
when targeting soft-tissue disease or areas of severe bone loss where there are no
good bony landmarks to reference on fluoroscopy alone and tactile feedback within
the lesion is significantly diminished.
Fig. 4 Examples of augmented fluoroscopy (AF): Needle guidance during placement of Steinman
guide pins (a, b) over which cannulated screws will be placed. (c) Volumetric segmentation of lytic defect (yellow, filled with cement) and joint capsule
(red) on intraoperative cone-beam CT (c) with corresponding volume rendering (d) and orthogonal contour outline of the yellow volume from different fluoroscopic
projections (e, f). 3D objects used to estimate ablation zone (g) and visualize ablation overlapping ablation zones in relation to sciatic nerve (red,
h).
Ideally, all 3D objects to be utilized are created beforehand on a preprocedural cross-sectional
CT or MR dataset that has been loaded into the workstation. This workflow saves intraprocedural
time and allows for more precise object creation when contrast-enhanced datasets are
available as well as preprocedural determination of screw lengths; however, it requires
accurate fusion of preprocedural and intraprocedural imaging.
At the beginning of each case, a cone-beam CT is performed primarily to assess for
interval changes such as fracture or disease progression but also for purposes of
patient registration with preprocedural datasets and 3D objects using 3D/3D registration.
Unfortunately, this process can be tedious and sometimes require manual alignment;
however, this process can be streamlined significantly by first aligning either the
sacral promontory or pubic symphysis in three orthogonal planes, then rotating one
dataset around that point in each projection. In our experience, 2D/3D registration
workflows designed to avoid an initial cone-beam CT are not sufficiently accurate
for screw placement in narrow corridors. Alternatively, for more straightforward cases,
3D objects can be drawn directly on the initial intraprocedural cone-beam CT dataset,
obviating the need for registration with preprocedural imaging.
It is critically important to continually check registration of the 3D objects with
the patient during the procedure. If good registration between datasets is achieved
initially, then any misregistration of 3D objects on live fluoroscopy is due to patient
movement, which can occur with hammering and drilling; however, with a patient under
general anesthesia, this movement really only occurs from side to side (i.e., no cranial–caudal
or anterior–posterior translation). Therefore, a good rule of thumb is that when checking
or correcting overlay registration, place the C-arm at 0 degrees and only adjust the
overlay left to right until the cortices of the overlay are superimposed on the live
fluoroscopic image ([Fig. 5]). Realigning the overlay when the detector is at an oblique angle will give a false
sense of security by appearing correct in that one projection; however, persistent
misregistration will be obvious in any orthogonal view, at which point starting the
registration process over is usually necessary.
Fig. 5 Example of subtle misregistration when using augmented fluoroscopy between the volumetric
3D object overlay and actual fluoroscopic image, best seen by the misalignment of
the bony cortices (a) resulting in inaccurate needle guidance targeting (b), improved after lateral realignment of the augmented fluoroscopy overlay while in
the AP projection (c).
Bone Access
Our most common procedural workflow is one of ablation, if indicated, followed by
complete screw placement, followed by cementation. Every cortical defect is a potential
area of cement extravasation that can lead to early cement termination; therefore,
bone access is planned carefully to avoid unnecessary access holes and used for multiple
purposes (i.e., ablation accesses are repurposed for both screws and cementation).
Initial bone access is often performed with a 10-gauge bone trocar (Stryker). This
size trocar accepts most ablation probes, including larger water-cooled microwave
ablation probes. In general, we have performed bone ablation in a coaxial fashion
using the base trocar to maintain bone access, prevent probe damage, and adjust probe
position, taking care to use purposefully long ablation probe shafts to enable retraction
of base trocars completely outside the ablation zone during energy delivery. Care
should be taken not to damage ablation probes by advancing them through hard bone,
especially when using cryoablation due to its use of highly pressurized gas. Damage
can be minimized by advancing the base trocar fully through any bone before exchanging
the stylet for the ablation probe and then retracting the trocar.
This size trocar also accepts 2.8-mm Steinmann guide pins ([Fig. 6a]), over which the cannulated orthopaedic screws are placed in a coaxial fashion ([Fig. 6b]). The diamond tip stylet facilitates precise bone entry which can sometimes be difficult
to achieve when entering bone with only a Steinmann guide pin, especially when not
tangential to the cortex (e.g., the lateral ilium when placing transsacral screws).
Fig. 6 (a) Steinmann guide pins with drill (white arrow) and screw (black arrow) tips for use
with cannulated screw placement. (b) Coaxial configuration of 2.8 mm guide pins through both a 10-gauge bone trocar (white
arrow) and cannulated, fully threaded screw (black arrow). (c) 10-gauge diamond tip bone trocar, power drill, battery pack (black), and collet
chuck shown with guide pin. Separate chucks (not shown) are used when performing power
screw driving or drilling. (d) Common tools include (top to bottom) a hand screwdriver, guide pin screw measuring
device, drill bit used for overdrilling sclerotic bone, and Steinmann guide pins.
Screw Placement
Our workflow for placing screws generally starts with bone access using a 10-g trocar
followed by placement of a 2.8-mm Steinmann guide pin through this trocar and along
the entire planned screw path using a power drill ([Fig. 6c]) and needle guidance. An oscillating, back-and-forth rotation of the guide pin can
be advantageous during advancement through cancellous bone to minimize inadvertently
exiting the bone cortex; however, a full forward drill rotation will be necessary
when traversing intact cortex such as the sacroiliac joint. Once guide pins have been
placed along all screw paths, a cone-beam CT is performed. Each guide pin is critically
evaluated in bull's-eye and tangential multiplanar reformats to ensure proper position.
If malpositioning is present, the offending guide pins are repositioned, accepting
that previous guide pin tracks can make repositioning challenging. Once the guide
pins are in appropriate position, measurements to determine needed screw lengths are
performed, either by making direct measurements on cone-beam CT reformats or utilizing
mechanical measuring devices that fit over the guide pins. The bone trocar can then
be removed, and the cannulated screw placed over the guide pin, similar to the Seldinger
technique, and advanced using either a power or hand screwdriver. Rarely, analogous
to creating a pilot hole, overdrilling the planned screw path with an appropriately
sized drill bit may be necessary prior to screw insertion in especially sclerotic
lesions ([Fig. 6d]). Screws should be advanced such that the screw head is flush with the entering
cortex, confirmed with an appropriate tangential fluoroscopic projection, without
breaking the cortex. Washers are considered beneath the screw head in situations where
there is extensive bony destruction or poor bone quality at the bone entry site.
Given the complexity of these procedures, operative times and the corresponding radiation
doses can be high, and we routinely reduce fluoroscopic frame rates from the default
of 15 frames per second to 2 to 4 frames per second along with aggressive radiation
beam collimation without significant degradation of image quality.
Maximizing Needle Guidance
Software-based augmented fluoroscopic needle guidance can be extremely helpful in
placing trocars, probes, and screws with a high degree of accuracy by automatically
moving the C-arm to calculated positions, thus providing a bull's-eye or line-of-sight
view as well as orthogonal views to needle paths created on cross-sectional datasets.
However, some nuances of these systems can make accurate placement challenging.
When available, bull's-eye views are highly favored for initial trocar placement,
as this allows for easy identification of the optimal skin entry site. However, the
C-arm often cannot achieve a bull's-eye perspective for all trajectories, forcing
the operator to rely on the calculated orthogonal views. This is most apparent for
ischial access in the prone position. However, these orthogonal projections are often
at oblique angles and not necessarily orthogonal to each other, making it extremely
difficult to efficiently adjust the bone entry site and needle angulation, by even
the most spatially adept operators.
An easy workaround for this limitation is to ignore the calculated projections for
the needle pathway and instead force the C-arm into an AP projection. With the needle
pathway still projected on live fluoroscopy, advance the needle along the path, taking
care to optimize the horizontal displacement and horizontal angulation, until the
needle tip is positioned at the bone entry site. Angulation of the C-arm in either
tangential oblique projection (right anterior oblique or left anterior oblique for
an ischial needle or craniocaudal for a transsacral needle) will likely display some
degree of needle positioning error. While a full orthogonal projection will maximize
the appearance of any malpositioning, it is not necessary for efficient readjustment
so long as the needle position and angulation are adjusted only in the vertical plane
(orthogonal to initial imaging). Restricting needle adjustments to only occur in the
horizontal and vertical planes, even if needle paths are oblique, greatly facilitates
accurate placement of almost any needle using augmented fluoroscopic needle guidance.
Maximizing Cement Fill
Adequate cementation is the most difficult aspect of percutaneous stabilization for
large metastatic lesions but essential for maximal stabilization and long-term success.
Robust cement deposition superior and medial to the acetabular roof is critical when
there has been bony destruction in this high-stress area. Cement deposition at screw/screw
interfaces and screw/bone interfaces can greatly enhance rotational and torsional
stability and reduce postoperative motion and hardware loosening, which is why we
prefer to use fully threaded screws whenever possible. In extreme cases, screws can
be “potted” into cement alone when there is no good bone present for purchase. Additionally,
adequate cementation at bone/tumor interfaces such that cement extends from the lytic
cavity, interdigitating into normal bone, is highly preferred to simply filling the
central portion of a lytic defect. In our experience, while not aesthetically pleasing,
cement leakage in noncritical tissue planes is preferable to inadequate cementation.
For a small subset of straightforward cases, particularly single screw ramus fractures
or very small lesions, distal cementation can technically be performed through a partially
advanced screw, which can then be “potted” into the distal cement before it sets.
We have found an 11-g bone trocar advanced through the cannulated Stryker screws to
be ideal for this approach, as they are perfectly sized to prevent retrograde cement
migration around the delivery cannula inside the cannulated screw. However, for larger
or complex lesions, cement deposition through partially advanced screws usually leads
to suboptimal results due to cement's finite working time, including inadequate cement
deposition and incomplete advancement of the screw into the setting cement.
In these cases, it is almost universally preferable to place all screws completely
before cementing through independent bone access trocars in a coaxial fashion. Generally,
these cement trocars are placed parallel to our ischial and/or AP screws. A curved
nitinol vertebroplasty needle can facilitate cementation over the acetabular roof
and allow for multiple repositionings from the same access site as long as cement
is kept flowing slowly. Cement is then injected under intermittent fluoroscopy, taking
care to view complex geometries such as the acetabulum from all angles. At the conclusion
of every procedure, a cone-beam CT is performed to assess the adequacy of cement deposition,
and when necessary, additional trocars are placed to facilitate further cement deposition.
Sometimes, significant destruction or dehiscence of the cortex can obscure the intended
margins of cementation on fluoroscopy alone. In such cases, if available, one can
create or segment a 3D volumetric object encompassing the lytic defect or intended
fill zone. In many fluoroscopy systems, the outline of this object can then be overlaid
onto live fluoroscopy, providing real-time feedback as to the adequacy of cement fill
([Fig. 7]).
Fig. 7 3D volumetric object outline (white lines) of lytic defect can be helpful to confirm
in real-time adequate cement deposition when lesion borders are difficult to visualize
fluoroscopically.
In select cases, achieving maximal cement deposition safely can be uniquely challenging.
This is especially true when the anatomy of interest is oriented such that it is superimposed
in an AP fluoroscopic projection. Despite steep angulation of the fluoroscopy unit,
the complex geometry of the pelvis can be challenging to interpret quickly. In these
cases, most notably involving the posterior acetabulum and sacrum, intermittent cross-sectional
imaging with a true CT scanner can provide increased confidence and prevent early
cessation of cement deposition; however, the significant streak artifact associated
with indwelling orthopaedic screws significantly degrades image quality, and in our
practice, even with a combined fluoroscopy–CT imaging suite, CT imaging is reserved
for only the most challenging cases.
Care must be taken to prevent cannulae from becoming stuck within hardening cement.
This is most likely to occur during prolonged, high-risk cement deposition when using
smaller gauge cement delivery needles such as a curved nitinol needle. In the event
a needle becomes stuck within cement, most will come free with persistent traction,
sometimes over several minutes; however, avoidance of this situation is advisable.
For large base cannulae, a simple 360-degree rotation periodically with the stylet
in place is very effective even if buried deep within cement for long periods of time.
Complication Management
Significant nerve injuries are usually the result of ablation and can result in significant
functional limitation and pain. These symptoms may improve if the nerve injury is
incomplete; however, this can still take several months if it occurs at all. The potential
for recovery of nerve function is multifactorial; however, it is likely higher with
cold-based ablation technologies than with heat-based ablation, as the neuronal architecture
is generally preserved. Acute high-dose steroids may be considered if nerve compromise
is felt due to extrinsic compression from postablation edema.
Functional motor deficits may require temporary or permanent orthoses, such as an
ankle–foot orthosis in the case of sciatic nerve injury, extensive physical and occupational
therapy rehabilitation, and utilization of ambulatory-assist devices to maximize function.
Neuropathic analgesics should be considered for persistent pain in addition to traditional
analgesics.
Cement will occasionally extravasate outside the bone, despite appropriate diligence.
Small volumes of cement inside the hip joint are usually inconsequential but should
be treated with full range of motion manipulation of the hip while on table to smooth
out any cement within the joint before setting fully. In cases of a significant intra-articular
extravasation, refractory hip pain and joint degeneration can result and arthroscopy
may be considered for cement removal if total hip reconstruction is prohibitive.
From time to time, augmented screw construct failure does occur. One reason for this
is failure to achieve adequate stabilization initially, in which case ongoing motion
within the fixation construct will lead to the cycle of screw loosening, cement fractures,
and progressive instability. For this reason, adequate cement deposition from the
beginning is critical. However, the long-term structural integrity of even the most
stable constructs depends in large part on a patient's capacity to form new bone across
fracture lines and areas of destruction, as all constructs will eventually fail in
the setting of either absent osseous healing or progressive disease leading to further
skeletal erosion. In cases of failure, having maintained a close collaboration with
orthopaedic surgery colleagues throughout the patient's care will facilitate a multidisciplinary
discussion of appropriate treatment options going forward, as major surgical fixation
or additional osteoplasty may be required ([Fig. 8]).
Fig. 8 A 63-year-old man with initial presentation of progressive left hip pain. Pelvic
radiograph (a) and CT (b) showing bilateral destructive acetabular lesions (white arrows). (c) Status post bilateral acetabular percutaneous ablation and “screw and glue” fixation
procedures. Each side was performed 7 days apart. Patient was ambulating with a walker
on postoperative day 1. Adjuvant systemic and radiation therapy were both initiated
within the week after stabilization. Unfortunately, despite initial pain relief, he
developed bilateral hip pain 6 months after minimally invasive stabilization due to
fragmentation of his acetabular rim and rapid joint degeneration bilaterally. He underwent
sequential total hip arthroplasty with an end result similar to a Harrington type
procedure (d), with good postoperative pain relief. The previous screws and cement were described
at the time of surgery as facilitating rather than hindering surgical repair.
Care should also be taken to minimize injury to the femoral head when performing ablation
near the joint space. Microwave and cryoablation energy will easily transmit across
the joint space and can lead to femoral head necrosis and collapse, which can be difficult
to treat without surgical reconstruction. Radiofrequency ablation may be less injurious
if the acetabular cortex is intact to provide some insulation of heat; however, this
is not definitive. We have not tried more complicated adjunct techniques such as joint
irrigation, as a complete ablation is usually not safely achievable and adjuvant therapy
is already planned.
Postoperative Management
For the vast majority of pelvic augmented screw fixations, patients are weight bearing
as tolerated after a couple of hour's bedrest. Patients typically receive nonsteroidal
anti-inflammatory agents and steroids at the time of the procedure to facilitate postoperative
pain control, although most patients will have significant pain relief just from stabilization.
Physical therapy evaluation occurs the following morning primarily to optimize the
use of necessary assist devices prior to discharge. Patients without complicating
issues are often discharged on the first postoperative day.
The value of engaged multidisciplinary management in the postoperative period cannot
be overstated. Nearly all patients will benefit from immediate adjuvant therapy as
residual disease is almost always present. Patients can receive radiation therapy
and systemic therapy generally within a couple of days after their procedure, as significant
complications related to wound healing are extraordinarily low due to the minimal
tissue disruption and small skin incisions. However, this is a significant deviation
from current postoperative algorithms and the expeditious treatment of these patients
often requires proactive education of the treating specialist preoperatively.
Long-term, periodic clinical follow-up is essential to identify any hardware loosening
or local tumor progression and allow for early less invasive treatment before more
significant complications occur. We typically see our patients in follow-up at 2 weeks,
3 months, 6 months, and yearly with radiographic imaging.
Additionally, the optimization of bone health and regeneration in the cancer patient
is essential to long-term success especially as patients live longer with their disease.
Engaging an oncologist or endocrinologist to aggressively prescribe metabolic medications
such as zoledronic acid and denosumab when appropriate can significantly impact fracture
healing and prevent construct failure.
Conclusions
Minimally invasive augmented screw fixation of the pelvis can provide significant
clinical benefit with acceptable risk and minimal interruption to concurrent therapies
and should be offered within a multidisciplinary approach to appropriately selected
patients with unstable pelvic metastatic disease.
Interventional radiologists, with their unique, minimally invasive image-guided experience,
are particularly well suited to perform these procedures; however, integrated preoperative
planning between interventional radiology and orthopaedic surgery can synergize two
complimentary and essential skill sets that are necessary to achieve optimal stabilization
and the best outcomes for these patients within a long-term care plan.
Those interventional radiologists who become experts in these techniques will find
a deeply rewarding practice and be well positioned to provide significant value to
patients and referring clinicians alike.
