Keywords
osseointegration - bone–screw interface - superelasticity - pseudoplasticity
Introduction
Surgical management of adult patients with spinal deformity often involves posterior
instrumentation such as rod–screw constructs. Behavior of this instrumentation when
patients sustain traumatic spinal injury often involves implant failure. This is the
result of the biomechanical properties of the spine and spinal instrumentation as
well as the mechanical load sustained at the time of injury.
Report of Case
An otherwise healthy 24-year-old man underwent posterior spinal surgery for adult
acquired kyphoscoliosis. Surgical reconstruction included instrumentation and fusion
with nickel–titanium pedicle screw–rod construct from T2 to L2 with Smith–Petersen
osteotomies at T7–T8, T8–T9, T9–T10, T10–T11, T11–T12, and T12–L1. His postoperative
course was uneventful and postoperative CT imaging demonstrated correction of scoliosis
from 65 degrees to 20 degrees, with a persistent thoracic kyphosis of 67 degrees.
One year later the patient, an unrestrained driver who was ejected from the vehicle,
sustained an extension–distraction injury with T8 vertebral body fracture seen on
CT imaging ([Fig. 1]). He also sustained a right subtrochanteric femur fracture, left clavicle fracture,
and left fibular fracture. There was no neurologic deficit postinjury.
Fig. 1 Sagittal spine computed tomographic image obtained immediately after the MVA demonstrates
T8 vertebral body fracture involving the superior and inferior endplate.
Postoperative computed tomographic (CT) images with 2-mm cuts to verify screw placement
were compared with CT images with 2-mm cuts obtained after the motor vehicle collision
to evaluate injury to the spinal column. Cobb angles of the thoracic sagittal images
were measured using a digital measuring device and the values were recorded.
Initial postoperative sagittal CT images demonstrate a 67-degree residual thoracic
kyphosis ([Fig. 2]) compared with the post-motor vehicle accident (MVA) sagittal CT images, which reveal
a 54-degree thoracic kyphosis ([Fig. 3]) and a 13-degree improvement in sagittal alignment. The measurement of the anterior
height of T8 vertebral body reveals an increase in height from 10 to 18 mm, an increase
of 80% ([Figs. 4] and [5]).
Fig. 2 Cobb angle measurement (superior endplate T3 to inferior endplate T12) on sagittal
spine computed tomographic image obtained 3 months postsurgery but prior to MVA demonstrates
a residual thoracic kyphosis of 67 degrees.
Fig. 3 Cobb angle measurement (superior endplate T3 to inferior endplate T12) on sagittal
spine computed tomographic image obtained immediately after the MVA demonstrates a
partial correction of thoracic kyphosis to 54 degrees (previously 67 degrees).
Fig. 4 Measurement of the anterior T8 vertebral body pre-MVA.
Fig. 5 Measurement of the anterior T8 vertebral body post-MVA.
Discussion
It is unusual for a patient with long posterior instrumentation of the spine to sustain
a spinal fracture without breakage of the rods.[1]
[2]
[3]
[4] In this particular case, the rods were 6-mm nickel–titanium (Ni–Ti) alloy with two
crosslinks. Although sustaining plastic deformation, the rods maintained their integrity
to the degree that the patient required no subsequent treatment to his spine at 12
months follow-up and has remained neurologically intact.
The biomechanical events of this case can be correlated to the biomechanical properties
of Ni–Ti alloy. There are two characteristics of this alloy that contributed to the
observed clinical event, namely, the mechanical properties of the material as well
as the likelihood of osseointegration of the pedicle screws.[5]
[6] Stress versus strain relationships are shown schematically for implant grade cobalt
and stainless steel (Co and Fe), titanium (Ti–Al–V), and Ni–Ti alloys[7] ([Figs. 6] and [7]).[8]
[9]
[10]
[11] Under the conditions of same size, shape, and crosslinks with similar fixation,
stress versus strain relationships are proportional to load versus deformation.[6]
[12]
Fig. 6 Stress versus strain curve. Titanium alloy, which is capable of bone–screw integration,
can provide greater deformation at the same in vivo load without system (construct)
breakdown.
Fig. 7 Schematics of stress versus strain of metallic implants (EL, elastic limit). The
initial segments of the curves up to the EL show the elastic modulus (linear portion)
is lowest for Ni–Ti and highest for cobalt and stainless steel alloys. Thus, for any
applied elastic stress (load), the strain (deformation) is highest for Ni–Ti. Also,
Ni–Ti, in addition to being superelastic, also exhibits pseudoplasticity, which is
reflected as increased strain (deformation) without significant increase in stress
(load) as represented by the horizontal portion of the curve for Ni–Ti. Co, Fe, and
Ti–Al–V alloys exhibit neither superelasticity nor pseudoplasticity.
Under the conditions where the bone–screw interface is osseous, integrated with mature
bone as with Ni–Ti, and remains integrated during loading, the highest capacity for
strain (deformation) without mechanical failure resides with Ni–Ti. The induced deformation
beyond the elastic limit for Ni–Ti is permanent, and in this case, the rod assumed
a new and permanent contour, maintaining the spine in a corrected position.
When bone–screw integration is combined with the plastic attributes of Ni–Ti, greater
deformation at the same in vivo load without construct breakdown can be predicted.[6] Ni–Ti alloy would appear to demonstrate biomechanical advantages of plastic deformation
and bone integration that rivals stiffer alloys, such as cobalt and stainless steel,
as demonstrated in this case.[13]
[14]
Thus, an unusual extension–distraction injury of the spine is presented, which in
this case, demonstrated improved sagittal alignment after trauma. To our knowledge,
there are no reports of this observed phenomenon in the spine literature.
