implant failure of posterior lower spinal instrumentation · pdf fileimplant failure of...
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Implant Failure of Posterior Lower Spinal Instrumentation
(A clinical & surgical Challenge)
B YMohamed Mohi El-Din, MD
The Department of Neurosurgery, Faculty of Medicine,
Cairo University
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The purpose of this study • To highlight the topic of implant
failure • To study the causes of failure and its
prevention• To enhance the radiological
differences between successful and failed cases (markers of failure) indicating the sources of error and assessing the implant status
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Over the last two decades
Pedicle screws in spinal stabilization
has dramatically increased
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Despite its advantages
It is associated with risks of screw/rod breakage
and/or screw/rod disconnection
Whichcan necessitate
additional surgical procedures to achieve
spinal fusion
ALL THESE ARE
RELATIVE ?!
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Radiological Failure analysis1. Manifestations of failure : loose
screws, loose rods, rod-screw disconnection, rod fracture, and/or screw fracture;
2. The effect of implant failure; 3. The point of instrumentation failure; 4. Screw axis, screw length & diameter,
rod length & diameter, and rod contouring;
5. The radiographs of the failed cases were interpreted in comparison to the successful cases
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We found that
Most of the failures (90%) occurred within 6 months after surgery, with small number (10%) in the following 6 months (usually
coinciding with some form of trauma), with no reported cases after 1 year postoperative
? F USI O N ?
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We found that
All forms of implant failure were more common in patients with multilevel
fusions
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We found that
The commonest form fracture screw (34%),
followed byrod fracture (24%),
rod loosening (22%), then the least form,loose screw (16%)
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Our data showed
The screw fracture (most common failure), most of them in cases of traumatic instability with anterior
column defects
Defects more than 50% collapse of the anterior vertebral body height was more frequent in the failure group
trauma cases
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Our data showed
The most common in mechanical failure
clinically is at the lower screw
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Our data showed
The loose screw (least form of failure), most of them in degenerative instability &
rest in iatrogenic instability
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Our data showed
• Autograft ( laminectomy augmented by bone strips from the iliac crest) was the routine procedure in 90% of cases of the control group, with 10% of cases having anterior interbody fusion
• Posterolateral fusion alone, using only the autograft obtained from laminectomy, allografts, and/or synthetic bone was the rule in the majority (80%) of cases of the failure group, with 20% of the cases having no bony fusion what so ever
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Another important finding in this study
The addition of supplemental fixation sites
was a cr iterion of many cases of the control group:
• use of a metal crosslink, • addition of two more screws at the
pathology level or extra level, especially in junctional zones,
• or anterior interbody fusion with cages
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What is going on ?
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Failures are not unavoidable complications or ongoing
degeneration
• In a properly performed fusion
• In the right patient • For the right diagnosis• By the right surgeon !!!!?
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Failure After Spinal Fusions..
WHY ??The 3 Ws
1. The wrong patient2. The wrong diagnosis3. The wrong surgeon
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The wrong patient ?
• A patient chosen for fixation with an underlying pathology that will not benefit from the procedure
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" The Patient Comes First "William James Mayo, M.D. (1861-1911)
Consider all the time that you are at a
time…The Patient
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The wrong diagnosis ?
• Misdiagnosis• Inadequate
preoperative studies
• Poor choice of surgical technique
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The wrong surgeon ?F D A F inal Rule
should be performed only by experienced spinal surgeons with
specific training
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Hidden agendas
play an important role in influencing medical care
in general
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Objective of lumbar spine stabilization
is to stop the motion
at a painful segment in the spine.
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The ultimate success of a spinal fusion rests on
whether the fusion heals solidly in the desired
position with NO
PSEUDOARTHROSIS
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Pseudarthrosis(failure of successful spinal
fusion)
Implant FailureThe instrumentation (pedicle screws and
rods) loosen and move from their original positions or even break
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Fusion (Arthrodesis ) = bone graft Stabilization = instrument (implants)
Instrumentation is used to assist in
obtaining a solid fusion
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BIOMECHANICS OF CONSTRUCT FAILURE
• Failure of the Implant• Failure of the Implant–
Bone Interface• Failure of Bone–Bone
Interface
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Failure of the Implant
The point of failure (At points of maximum
stress)
1. usually at the screw–plate/rod junction
2. between the tip of the screw and the plate/rod
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Moreover,
Using a spinal fixation device
may induce stress shieldingin the vertebral
body
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Screw fractures
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Similar to screws
Plates/rods break at the point at which maximum stress is
applied
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screw
screw
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Rod Fractures
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Important to know
REGARDING:
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The maximum stresses in the rod & screws
• The rod diameter is the most important factor
• The maximum stress values decreases as the diameters increases
• The axial compressive force increases as the rod diameter increases.
• The stress levels in the distal screw are always higher than those in the proximal screw
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Both anterior and posterior surgery will be required?
whenthe objectives of surgery cannot be achieved with a single approach• complete burst fractures • anterior defect • gross residual anterior
compression
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In instability at the lumbosacral junction
The bending moments on the implant may
result in implant failure.
In this situation interbody anterior column structural
support should be considered.
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Posterior pedicle screw fixation systems
require two level stabilization for single level burst injuries,
but single level stabilization may be adequate for fracture dislocations
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Anterior column reconstruction
1. at the thoracolumbar junction after a burst fracture & at the sacroiliac junction.
2. The absence of satisfactory anterior stabilizing devices
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Also important
Long rigid multisegmental fixation constructs tend to load the more caudal screws far more than the
rostral screws THUS
,the use of a shorter length implant, may decrease the
risk of caudal construct failure
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Failure of the Implant–Bone Interface
The surgeon should attempt to distribute loads such that no single portion of the implant or spine carries an excessive
portion of the loadBy
1. improving the integrity of the existing implant–bone interfaces,
2. providing additional implant–bone interfaces, improving the integrity of the bone, and
3. by normalizing the geometry
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Screw pullout
pullout is mainly a function of the volume of bone between the threads
Pullout resistanceis proportional not only to the volume of
bone between screw threads but also to the triangular area defined by the screw
ThusTriangulation of pedicle screws provides
additional resistance to pullout
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Screw Loosening
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Rod Placement
It is important to secure the rod with the rod holder while tightening the locking nut
to prevent unnecessary torque on the adjoining bone/screw
interfaces
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Rod Loosening
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To Conclude
The spine is a dynamic structure and The optimal treatment for complex
spinal conditions remains a challenge