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Chapter 14
3D-Printed Patient-Specic Guides:
Planning andInstrumentation
ChristinaK.Hardesty
Planning
Once the decision has been made to use 3D-printed patient-specic guides (PSGs)
for a particular case, identify the patient’s particular anatomic anomalies, deformities, and issues, such as small, dysmorphic, or absent pedicles, congenital vertebrae,
or fusion masses. Planning may be simple for idiopathic cases, but more complex
for congenital cases, revisions with distorted anatomy, or cases needing unusual
implants. Consider if the patient needs instrumentation to the pelvis and which
method is to be used. Note the need to work around existing implants, including
plans to exchange or remove them. Identify any unusual or special trajectories
which may be required. Plan for any signicant osteotomies or anterior column
work. Finally, in revision cases identify any dramatic changes to the anatomy, such
as previous laminectomies, which can alter the anatomy when using the guides, but
may also require more meticulous dissection.
Next, between 3–6weeks prior to surgery, the patient obtains a computed tomography (CT) scan with helical pitch less than 1 and continuous axial acquisition
parameters. Slices must be no more than 1.25mm thick. Using this scan, standard
views and 3D reconstructions can be reviewed for unexpected anatomy such as buttery vertebrae that could not be identied on radiographs, unusual pedicles, or
changes that occur once your patient is supine. Select the levels you would like
guided based on both the radiographic and CT images. Some plans consist of guides
for every level of anticipated instrumentation, while others ask for guides only for
certain levels.
C. K. Hardesty (*)
Orthopaedic Surgery, Rainbow Babies and Children’s Hospitals, Case Western Reserve
University, Cleveland, OH, USA
e-mail: Christina.hardesty@uhhospitals.org
Switzerland AG 2024
S. Garg, C. J. Kleck (eds.), Navigation, Robotics and 3D Printing in Spine
Surgery, https://doi.org/10.1007/978-3-031-68678-8_14
191© The Author(s), under exclusive license to Springer Nature
192
C. K. Hardesty
Provide this information along with any preferences such as screw trajectory,
pedicle wall tolerance, pelvic xation plan, implant choices, etc., to the engineers
who will design the initial plan (Fig.14.1). The engineers use the digital le of the
CT scan to create a 3D composite as well as develop trajectories for the screws,
identify an appropriate screw length and width, and develop a device which will
provide mechanical guidance to cannulate the pedicle or pelvis using a drill bit.
Models of the spine and each level-specic mechanical guide will ultimately be 3D
printed from this plan. Once the engineers return the plan, the surgeon should carefully review it.
When reviewing the plan, verify that agreement with the engineers on anatomic
levels, especially in cases with congenital anomalies such as hemivertebrae, in cases
with abnormal numbers of ribs (11 or 13, for example) and abnormal lumbar vertebrae (4 or 6, for example). Review the screw trajectories to ensure they match preferences, especially in the circumstance of pelvic xation, since there is variability
on length, diameter, and appropriate angle (Fig.14.2) [1]. Review the diameter of
the screw proposed for each pedicle to ensure there will be interference t for the
best pullout strength [2]. Sometimes in-out-in or nontraditional trajectories are the
only option, but the drill bits are smaller than the planned screws, so the breach may
not always be felt after drilling and the screw may dilate a pedicle rather than having
a true breach. Other times, the breach is created during drilling and using a cannulated tap can help prevent screw misplacement. Look at the overall sagittal and
coronal planes for screws that might be out of plane with the adjacent levels and
require the use of polyaxial heads (Fig.14.3). Pay attention to the lengths of the
Fig. 14.1 Surgical plan for a single level
14 3D-Printed Patient-Specic Guides: Planning andInstrumentation
Fig. 14.2 Plan for pelvic xation
193
Fig. 14.3 Overall coronal
alignment and anatomy of
a child with lumbar
myelomeningocele
screw to recognize any that might become anteriorly prominent if overseated. Look
for rib prominence and impedance in cases where severe thoracic rotation is present.
Verify implants that will remain (such as previously placed screws, wires, intrathecal catheters, cages, etc.) versus those which will be removed (such as old growing
rods or failed implants). Identify any level where a unilateral guide would be useful,
such as the most cephalic level. Any changes to the plan can be communicated to the
194
C. K. Hardesty
engineers for revision prior to implementation of the plan via printing of the spine
model and guides.
The engineers will send a nal plan, which can be used during the surgery itself
or reviewed prior to the day. Finally, the model and guides will arrive unsterilized a
few days before the actual surgery so they can be tested for t and feel, the surgical
team can review the anatomy in three dimensions, and any additional preoperative
planning can be undertaken (Fig.14.4).
Instrumentation
The model and guides go through a sterilization process and can be sterilized up to
four times. They should be individually wrapped instead of placed in a pan or sterilization tray so that the small feet which t the level-specic anatomy do not break.
There are specic pans that can be used if pans are preferred, but they must be
designed for the guides. Once the room is opened, the guides and model can be
arranged on the sterile tables. Since the screw sizes are already known, they can be
pulled ahead of time and placed in a convenient location.
Surgery begins in typical fashion with exposure down to the spine; however, care
must be taken to avoid any disruption of the cortical surface, since the guides are
matched to the current anatomy. Soft tissues, however, must be completely cleared
Fig. 14.4 Guides and
model prior to sterilization
14 3D-Printed Patient-Specic Guides: Planning andInstrumentation
195
from the lamina and transverse processes so that it does not alter the position of the
guide (Fig.14.5). Meticulous dissection with electrocautery, a rongeur, or a curette
is helpful, as long as the cortical bone surface is not removed. Once the area of
planned instrumentation is exposed, the guides can be used to cannulate the pedicles. There are several workow options, but two general categories are working
cranially or working caudally.
Working Cranially
Beginning at the caudal end of instrumentation, place the guide matched to the
lowest level on the posterior aspect of the spine, matching the feet with the transverse processes and lamina. The guide can be placed on the model rst to conrm
location, t, and any helpful anatomic landmarks before placing on the patient’s
spine (Fig.14.6). To place the guide securely, feel the feet line up with the matching bone. Fit can be veried by twisting the guide (while holding it rmly against
the bone) since the guide may rock superiorly and inferiorly a bit, but will not
rotate when it is fully seated. Next, the guide can be anchored using two small pins
Fig. 14.5 Exposed spine
with adequate soft tissue
clearance
196
Fig. 14.6 Checking the
guide against the model
during the case
C. K. Hardesty
that are placed through the holes found in the center of the guide. This will provide
extra stability during the drilling process, but placement of pins is not required if
the guide can be secured by holding it carefully. Two metal drill sleeves are then
inserted in the guide through the holes overlying the pedicles (Fig.14.7). A larger
sleeve is available for a 3.2-mm drill bit and a small sleeve is available for a 2.0mm drill bit, specically meant for screws 4.0mm or less in diameter. The top of
the drill sleeve is also the set point for measuring depth, using a calibrated drill bit.
As the drill bit is advanced through the guide, the surgeon should be aware that the
starting point could be on a vertical slope, especially if it is on the transverse process. To help prevent skive (slipping of the drill bit along the bone), the drill should
be started at full speed while still in the drill sleeve and not yet touching bone.
Once the outer cortex has been breached, the drill can be carefully advanced until
its calibrated mark reaches the top of the drill sleeve at a length appropriate for that
level. The preoperative plan provides that information and can be referenced during surgery. The drill is removed from the sleeve and a pedicle probe is used to
check the pedicle walls and oor. Rarely is a breech noted with a drill bit, but one
should be addressed if it was unexpected. If the pedicle is sufcient, a marker can
be placed temporarily to hold the location of the pedicle. If bone wax is used, it can
14 3D-Printed Patient-Specic Guides: Planning andInstrumentation
Fig. 14.7 Guide in place
with drill sleeves engaged
197
also limit bleeding from the pedicle. The second pedicle can be cannulated in the
same way using a drill bit on the opposite side. Unlike the rst side, instead of
placing the marker right away, leave the pedicle probe in place and slide the guide
off over that. Two markers, because of their divergence, will not allow the guide to
be lifted off. Once the guide is removed, place a marker in the second pedicle, in
place of the probe. If working with a trainee or a rst assistant, an alternative
workow is to leave the rst drill bit in place, unchuck it from the drill, and then
have the assistant complete the opposite side. Having the rst drill bit still engaged
inside the drill sleeve provides extra stability for the guide. In this workow, the
assistant drills, checks with a probe and places a marker, then the surgeon removes
their drill bit, checks with a probe, lifts off the guide, and places a marker. This
does not add a signicant amount of time. Once both markers are in place, the next
level proximal is addressed and instrumented in the same fashion. This process
continues cranially until all levels are complete. Care must be taken when dissecting out the most proximal level. The interspinous ligament must still be preserved,
but wider dissection is usually required to allow the guide to t. If the level is very
proximal (such as the upper thoracic spine), it is helpful to use unilateral guides.
After all the guides have been used, facetectomies can be performed in typical
fashion. Markers can then be exchanged for screws and deformity correction can
proceed.
198
C. K. Hardesty
Working Caudally
One of the reasons markers need to be used when working cranially is that the
guide relies on the anatomy of the level below the one you are instrumenting, so it
cannot be altered by a facetectomy or a screw. If pedicle instrumentation goes from
cranial to caudal, then the anatomy below each level is preserved and the workow
can be altered. In these cases, the standard workow is to apply the guide to the
bone (see above section for a description of this), place the drill sleeves, drill each
pedicle, check the pedicle with a probe, and then place markers. The guide should
still be removed over a marker and a probe since the pathways are divergent. At
this point, however, screws can be placed immediately. If two probes were used,
markers could be avoided altogether. Facetectomies can also be performed at this
level, but not at the level below, since this would disrupt the anatomy for the guide.
Once all the screws were placed, deformity correction would proceed in standard
fashion.
Pelvic Fixation
For S1 and sacro-alar-iliac (S2AI) screws, a common guide is made since there is
usually no motion present between levels of the sacrum. Seating the guide is done
in similar fashion, nding the t of the feet by gently twisting with downward pressure. When placing S2AI screws, the markers must be short and only one drill sleeve
can be used at a time because the two paths cross one another. For S1 screws, drilling just shy of the anterior cortex then popping through with a blunt tool may be
preferred to drilling bicortically.
Use ofthe3D Model
The model itself is a very helpful tool for the surgery. It allows a surgeon to see the
deformity completely in all dimensions, identify previous unknown anomalies,
serve as a fail-safe for pedicle location if guided access fails, and provides an anatomically matched feel for how each guide should t. The model can help plan
osteotomies, can clearly show previous surgical sites and how the anatomy has been
altered, and can identify the margins of bone in tumor, bone loss, or myelomeningocele cases. Comparison of the model to the actual spine can help conrm levels
rather than requiring intraoperative uoroscopy.
14 3D-Printed Patient-Specic Guides: Planning andInstrumentation
199
Conclusion
The preoperative planning process is an essential step when using 3D-printed
PSGs. Understanding the anatomy and being able to safely execute screw placement are two benets of the analysis that is done prior to surgery [3]. While there
is a small learning curve initially, this process becomes familiar very quickly and
the preoperative effort becomes minimal. Once in the operating room, the workow
is simple to understand and reliable.
References
1. Jain A, Brooks JT, Kebaish KM, Sponseller PD.Sacral alar iliac xation for spine deformity.
JBJS Essent Surg Tech. 2016;6(1):e10.
2. White KK, Oka R, Mahar AT, Lowry A, Garn SR.Pullout strength of thoracic pedicle screw
instrumentation: comparison of the transpedicular and extrapedicular techniques. Spine.
2006;31(12):E355–8.
3. Jasty N, Folkman M, Pujabi N, Hardesty CK.Patient matched comparison of spinal deformity
surgery using 3D printed guides. In: Proceedings from the safety in spine Surgery summit;
April 15, 2023; NewYork, NY.Abstract; 2023. p.4.
Chapter 15
3D-Printed Patient-Specic Guides:
Summary ofClinical Results
DevonLeFever, AiyushBansal, andRajivSethi
Clinical Results of3D-Printed Guides inSpine Surgery
The clinical interest in utilizing 3D-printed guides in the treatment of spinal pathologies has been on the rise. Increasing academic efforts have taken place to understand the safety and accuracy of this adaptation of 3D printing technology. To date
a search of 3D-printed pedicle screw guides generates over 100 associated articles.
The results of using 3D-printed guides in spine surgery have been promising, with
overall positive results.
These outcomes can be summarized in the following sections:
• Improved Surgical Precision.
• Enhanced Patient Safety and Improved Clinical Outcomes.
• Cost-Effectiveness.
• Limitations and Challenges.
D. LeFever (*) · A. Bansal
Department of Neurosurgery, Center for Neuroscience and Spine, Virginia Mason Medical
Center, Seattle, WA, USA
R. Sethi
Department of Neurosurgery, Center for Neuroscience and Spine, Virginia Mason Medical
Center, Seattle, WA, USA
School of Medicine, University of Washington, Seattle, WA, USA
Switzerland AG 2024
S. Garg, C. J. Kleck (eds.), Navigation, Robotics and 3D Printing in Spine
Surgery, https://doi.org/10.1007/978-3-031-68678-8_15
201© The Author(s), under exclusive license to Springer Nature