S2 screws [Neurosurgery Wiki]

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=====S2 screws=====
Remain an effective technique to supplement [[S1 pedicle screw]]s, especially as a salvage technique for those who cannot have iliac fixation, such as those who previously had substantial posterior iliac crest harvested for bone graft
((Kim JH, Horton W, Hamasaki T, Freedman B, Whitesides TE Jr, Hutton WC. Spinal 
instrumentation for sacral-pelvic fixation: a biomechanical comparison between
constructs ending with either S2 bicortical, bitriangulated screws or iliac
screws. J Spinal Disord Tech. 2010 Dec;23(8):506-12. doi:
10.1097/BSD.0b013e3181c37438. PubMed PMID: 20124912.
)).

====Technique====

Commonly directed  laterally abd superiorly into the [[ala of sacrum]].

Three-dimensional image guidance allows for safe placement of large S2 sacral alar screws that can provide additional biomechanical stability to lumbosacral constructs or serve as an alternate point of sacral fixation when S1 pedicle screws cannot be salvaged or placed in a medial trajectory
((Nottmeier EW, Pirris SM, Balseiro S, Fenton D. Three-dimensional image-guided 
placement of S2 alar screws to adjunct or salvage lumbosacral fixation. Spine J. 
2010 Jul;10(7):595-601. doi: 10.1016/j.spinee.2010.03.023. PubMed PMID: 20434406.)).

Leong et al.
((Leong JC, Lu WW, Zheng Y, Zhu Q, Zhong S. Comparison
of the strengths of lumbosacral fixation achieved with techniques
using one and two triangulated sacral screws. Spine
(Phila Pa 1976). 1998;23(21):2289-94.))
found
that a construct consisting of bicortical S1 pedicle screws
in combination with bicortical S2 alar screws that were inserted
30 degrees anterolaterally was significantly stronger
than constructs using bicortical S1 pedicle screws alone. In
a study by Zindrick et al.,
((Zindrick MR, Wiltse LL, Widell EH, et al. A biomechanical 
study of intrapeduncular screw fixation in the lumbosacral 
spine. Clin Orthop Relat Res. 1986;(203):99-112.))
second sacral pedicle is weakest
location and screws aimed laterally into the [[ala of sacrum]] at 45
degrees resist larger axial pull-put loads than straight anteriorly
into the ala. But, diagonal insertion of S2 screw did
not tested and S2 screw is not embedded firmly than iliac
screw.
To overcome the iliac screw disadvantages, the diagonal
S2 screw for lumbo-sacral fixation was described
here. The diagonal S2 screw has many potential advantages
compared with [[iliac screw fixation]] such as a lower profile, avoidance of extra soft tissue dissection, no competition
with iliac bone graft harvest, and easier salvage if
ever infected. But in patient with severe sagittal imbalance,
authors thought iliac screw fixation is appropriate option
for preventing pseudarthrosis.
Authors used a 6.0-mm-diameter, 50-mm-long
variable angle screw in most of the patients and it was sufficient
for solid lumbo-sacral fixation. More larger screw
is able to provide greater stability, but further biomechanical
testing would have to be accomplished to come to any
firm conclusion regarding the strength of S2 screws. In our
cases, a posterolateral fusion was performed in 13 patients
and there was no interbody fusion case. Interbody fusion
provides mechanical advantages in terms of graft compression
and a large, well vascularized fusion surface. But, only
posterolateral fusion with S2 screw fixation provided high
fusion rate (12/13) and authors think interbody fusion
doesn’t need in our techniques.
Authors suggest that the S1-diagonal S2 screw construct
is an appropriate alternative to the S1-iliac screw
construct and it presents certain clinical advantages. However,
the short- and long-term effects of the diagonal S2
screw have not been completely evaluated. Further data
are needed to fully evaluate the diagonal S2 technique.


The ability to place a diagonal S2 screw requires a thorough
understanding of each individual patient’s anatomy.
Standing anteroposterior and lateral lumbar radiographs
and an anteroposterior pelvic radiograph should be obtained.
The overall alignment of the spine and pelvis in
Placing instrumentation into the ilium has been shown to increase the biomechanical stability and the fusion rates, but it has some
disadvantages. The diagonal S2 screw technique is an attractive surgical procedure for degenerative lumbar deformity. Between
2008 and 2010, we carried out long fusion across the lumbosacral junction in 13 patients with a degenerative lumbar deformity
using the diagonal S2 screws. In 12 of these 13 patients, the lumbosacral fusion was graded as solid fusion with obvious bridging
bone (92%). One patient had a rod dislodge at one S2 screw and breakage of one S1 screw and underwent revision nine months
postoperatively. So, we present alternative method of lumbopelvic fixation for long fusion in degenerative lumbar deformity using
diagonal S2 screw instead of iliac screw.
Keywords: Long fusion, Diagonal S2 screw, Iliac screw, Degenerative lumbar deformity
both the coronal and sagittal plane should be evaluated
carefully. Preoperative computerized tomography (CT)
should be performed to evaluate for absent or small
pedicles and congenital and other bony abnormalities. Depending
on the experience, skill, and comfort of the operating
surgeon, intraoperative fluoroscopy is recommended
but not required
((Park YS, Kim HS, Baek SW, Lee SH. Lumbosacral fixation using the diagonal S2
screw for long fusion in degenerative lumbar deformity: technical note involving 
13 cases. Clin Orthop Surg. 2013 Sep;5(3):225-9. doi: 10.4055/cios.2013.5.3.225. 
Epub 2013 Aug 20. PubMed PMID: 24009909; PubMed Central PMCID: PMC3758993.
)).

====Case reports====
Two patients were treated by the senior author (FAS) who underwent surgery employing S2-AI fixation utilizing an electronic conductivity device (Pediguard cannulated probe, Spineguard, Paris, France). The surgical technique, case illustrations, and radiographic outcomes are discussed.

Stable and accurate fixation was attained in both patients. There were no peri-operative complications related to hardware placement.

To the authors' knowledge, this is the first reported literature combining S2-AI screws with electronic conductivity for immediate intraoperative feedback. This technique has the opportunity to provide surgeons with increased accuracy for placement of S2-AI screws while improving overall radiation safety. This feedback can be particularly helpful when surgeons are learning new techniques such as placement of S2AI screws
((Sandhu FA, McGowan JE, Felbaum DR, Syed HR, Mueller KB. S2-AI screw placement 
with the aide of electronic conductivity device monitoring: a retrospective
analysis. Eur Spine J. 2017 Aug 1. doi: 10.1007/s00586-017-5242-0. [Epub ahead of
print] PubMed PMID: 28766018.
)).