Flexible growing rods: a biomechanical pilot study of polymer rod constructs in the stability of skeletally immature spines
Bylski-Austrow et al. Scoliosis and Spinal Disorders
Flexible growing rods: a biomechanical pilot study of polymer rod constructs in the stability of skeletally immature spines
Donita I. Bylski-Austrow 0 1
David L. Glos 1
Anne C. Bonifas 1
Max F. Carvalho 1
Matthew C. Coombs 0
Peter F. Sturm 0 1
0 University of Cincinnati , Cincinnati, OH , USA
1 Orthopaedics, Cincinnati Children's Hospital Medical Center , Cincinnati, OH 45229-3039 , USA
Background: Surgical treatments for early onset scoliosis (EOS) correct curvatures and improve respiratory function but involve many complications. A distractible, or 'growing rod,' implant construct that is more flexible than current metal rod systems may sufficiently correct curves in small children and reduce complications due to biomechanical factors. The purpose of this pilot study was to determine ranges of motion (ROM) after implantation of simulated growing rod constructs with a range of clinically relevant structural properties. The hypothesis was that ROM of spines instrumented with polymer rods would be greater than conventional metal rods and lower than noninstrumented controls. Methods: Biomechanical tests were conducted on six thoracic spines from skeletally immature domestic swines (35-40 kg). Paired pedicle screws were used as anchors at proximal and distal levels. Specimens were tested under the following conditions: control, then dual rods of polyetheretherketone (PEEK) (diameter 6.25 mm), titanium (4 mm), and cobalt-chrome alloy (CoCr) (5 mm). Lateral bending (LB) and flexion-extension (FE) moments were applied, and vertebral rotations were measured. Differences were determined by two-tailed t-tests and Bonferroni for four primary comparisons: PEEK vs control and PEEK vs CoCr, in LB and FE (α = 0.05/4). Results: In LB, ROM of spine segments after instrumenting with PEEK rods was lower than the non-instrumented control condition at each instrumented level. ROM was greater with PEEK rods than with Ti and CoCr rods at every instrumented level. Combining treated levels, in LB, ROM for PEEK rods was 35 % of control (p < 0.0001) and 270 % of CoCr rods (p < 0.01). In FE, ROM with PEEK was 27 % of control (p < 0.001) and 180 % of CoCr (p < 0.01). At proximal and distal adjacent non-instrumented levels in FE, mean ROM was lower for PEEK than for either metal. Conclusions: PEEK rods increased flexibility versus metal rods, and decreased flexibility versus non-instrumented controls, both over the entire instrumented segment and at each individual level. Smaller mean increases in ROM at proximal and distal adjacent motion segments occurred with PEEK compared to metal rods, which may help decrease complications, such as junctional kyphosis. Flexible growing rods may eventually help improve treatment options for young patients with severe deformity.
Early onset scoliosis; Growing rods; Spine instrumentation; Biomechanics; Range of motion; PEEK rods; Polymer; Polyetheretherketone; Titanium; Cobalt chrome alloy
Early onset scoliosis (EOS) presents before the age of
10 years  and is associated with high morbidity and
mortality rates compared to adolescent idiopathic
scoliosis (AIS) due to chest wall deformities that
restrict pulmonary development . Fusion of thoracic
spinal deformities at this early age is contraindicated
[3, 4]. Current treatments include serial casting [5–7],
but conservative methods are not always effective.
Surgical treatments include spine distraction and rib
expansion . Distractible ‘growing rods’ (Fig. 1) have
been used for several decades in the attempt to control
both the spinal deformity and to allow for spinal
growth  and have reported to be effective [9–14].
Treatment goals in EOS include minimizing spinal
deformity over the life of the patient, the extent of any
Fig. 1 Radiographs of a patient with early onset scoliosis,
preoperative (top) and after implantation of growing rod construct
(bottom). (Left) Posterior-anterior view. (Right) Sagittal view
final spinal fusion, complications, procedures,
hospitalizations, and burden for the family; and maximizing
thoracic function including motion of the chest and
Surgical treatments typically require multiple surgeries
and involve many complications, including infection,
instrumentation failures, corrosion, joint fusion, and
changes to adjacent motion segments [13, 15].
Complications of growing rod treatment for EOS were reported
. In a study of 141 patients, the investigators
concluded that management of EOS is prolonged regardless
of treatment modality, and so complications are frequent
and expected. Complications may be reduced by
delaying initial implantation when possible, using dual rods,
and limiting the number of lengthening procedures.
Early changes of the thoracic geometry after
implantation of a growing rod were shown to have a corrective
effect on chest wall geometry . Constructs that
lengthen magnetically reduce the number of surgeries
, but the instrumentation is stiff, the elongating
section cannot be contoured, and MRI is contraindicated
. Construct mechanical properties, therefore, affect
both treatment efficacy and some of the complications.
Early biomechanical studies have been reported .
In one study, distraction of long non-segmental spinal
constructs was shown to result in load-sharing across
multiple levels, rather than a local concentration of
distractive effects, during a simulated distraction
maneuver . A foundation composed of four pedicle
screws implanted in two adjacent vertebral bodies
provided a stronger construct in pullout tests compared to
laminar hooks , and cross-links were not shown to
enhance fixation. Spine versus rib anchors have also
been assessed in biomechanical tests . A pediatric
cadaveric study  reported differences in distraction
failure forces due to anchor points on ribs, laminae, or
pedicles. The effect of distraction force  and timing
 were explored using computer models.
Development of a “smart” growing rod system has been
The immature porcine spine has been reported to
provide a reasonably similar growth rate and anatomical
dimensions to the EOS patient population . An in
vivo study in swine showed an increase in vertebral body
height in distracted segments compared to
nondistracted control segments . In a clinical case series,
growing rod treatment performed with lengthening
procedures every 6 months was reported to stimulate
growth in vertebrae within the instrumented levels .
Length gains, however, tend to decrease with time.
These “diminishing returns” have been attributed to
auto-fusion of the spine from prolonged immobilization
by a rigid device . The force required to distract the
spine doubled by the fifth lengthening in a study on
EOS patients in which distraction forces were measured
during lengthening procedures .
Some of the complications stem from mechanical
factors. Rod fractures, a relatively common complication,
may be related to the significant increase in distraction
force over time. In a retrospective review of a multicenter
database, rod fracture occurred in 15 % of patients .
The high stiffness of conventional metal rods creates
compliance mismatches between spine and instrumentation,
stress concentrations, and motion redistribution, factors
which likely contribute to rod breakage, screw pull-out,
auto-fusion, and junctional kyphosis [15, 32, 33]. Using a
computational model, a more flexible non-fusion
correction system for AIS which used non-locking polyaxial
pedicle screws and mobile connectors was reported to
reduce intervertebral rotation less than more rigid implants
. Growing rods with a telescopic sleeve component
have been designed to reduce constraints to axial rotation,
with the expectation that growth would be allowed while
maintaining the axial flexibility of the spine for improved
capacity for final correction .
Growing rods with greater flexibility might result in a
sufficiently straight and more flexible spine with fewer
surgical complications. The polymer polyetheretherketone
(PEEK) has a lower modulus than traditional rod materials,
which might allow for greater range of motion (ROM) than
standard metal cobalt-chrome alloy (CoCr) or titanium
(Ti) rods. The bending stiffness of PEEK is about 10 % of a
titanium rod of the same diameter . Rods made of
PEEK have been previously reported for use in adult, short
segment, lumbar spine surgery. In cadaveric tests, short
PEEK rods provided comparable stability to titanium rods
of equivalent diameter [37, 38]. PEEK rods have been
shown to affect disc pressure in levels adjacent to spinal
instrumentation . To the investigators’ knowledge, no
prior biomechanical study was performed on PEEK rods of
the length of the thoracic spine. A preliminary report
suggested that PEEK rods of dimensions suitable for EOS
patients might provide sufficient stability to correct a curve
and withstand physiological loads, at least in very small
children . No previous report has presented effects of
rods of different material properties on the motion of each
intervertebral joint, in particular, motion at the adjacent
Therefore, the purpose of this study was to determine
changes to the biomechanical properties of skeletally
immature spines after implantation of simulated growing
rod constructs with a range of clinically relevant structural
properties. The primary hypotheses were that ROM of
spines instrumented with PEEK rods are 1) lower than
non-instrumented controls, 2) greater than metal rods, and
3) closer to controls than to metal rod constructs. Further,
adjacent segment motion was expected to be lower with
polymer rods compared to conventional systems.
In vitro biomechanical tests were conducted on six
porcine thoracic spines harvested from skeletally
immature Yorkshire cross pigs (10–14 weeks of age weeks of
age, body mass 35–40 kg). The spines were obtained
after death from animals that had been previously
utilized for other studies that had not involved the spine
(approved by IACUC, University of Cincinnati). Spines
were sectioned to include vertebrae T1-T13 (domestic
pigs have 14 to 15 thoracic vertebrae), then were frozen
at -20 °C until testing. To prepare test specimens,
muscle was carefully removed to preserve ligaments,
joint structures, transverse processes, and rib
articulations. Paired pedicle screws (polyaxial, 5.0 X 35 mm, Ti;
DePuy Spine, Raynham MA) were inserted into T3 and
T4 for the proximal anchors, and into T10 and T11 for
the distal anchors. A non-instrumented intervertebral
joint remained above and below the upper and lower
instrumented vertebrae. Pedicle screws were inserted
freehand. The entry point was prepared using an awl at
the junction of a line between the transverse process and
lateral border of the pars. The pedicle canal was created
using a pedicle probe. The pedicle wall integrity was
verified using a ball-tip probe before inserting each
The specimens were carefully aligned in neutral
orientation while potting the specimen into end blocks in
fiberglass-reinforced resin (Bondo, St. Paul, MN) to
facilitate reproducible positioning into the loading device.
For flexion-extension testing, specimens were placed in
the system with the caudal and cranial end-blocks level
with the base. Testing was performed at room
temperature and specimens were kept moist using
physiological saline solution.
Each specimen was tested before and after
instrumentation using a repeated measures experimental design. The
order of testing was: 1) before rod insertion (Control),
followed by 2) PEEK rods (6.25 mm diameter, n = 6,
Quadrant Plastics, Fort Wayne IN) (Fig. 2), 3) titanium
rods (n = 6, 4 mm diameter, Synthes, Paoli PA), and 4)
cobalt-chrome-molybdenum alloy rods (CoCr) (n = 4,
5.5 mm diameter, DePuy Spine, Raynham MA) (Fig. 2,
radiograph with CoCr rods). The rods were approximately
200 mm long, and straight in both coronal and sagittal
planes for this pilot study, as PEEK cannot be contoured
at room temperature.
Tests were conducted in lateral bending (LB) followed
by flexion-extension (FE). Moments of ±5 Nm were
applied using a materials test system (Instron 4465;
Instron, Norwood, MA) with control and data
acquisition software (TestWorks 4; MTS, Eden Prairie, MN)
and a custom cable-floating pulley fixture (Fig. 3). The
system allowed for continuous cycling from full flexion
to full extension, or left to right lateral bending, as
Fig. 2 Spine test specimen with dual titanium rod construct. Left:
Coronal view (Reproduced from Reference 40, Fig. 1: Bylski-Austrow
DI, Glos DL, Bonifas AC, Carvalho MF, Coombs MT, Sturm PF. Flexible
growing rods: A pilot study to determine if polymer rod constructs
may provide stability to skeletally immature spines. Scoliosis 2015,
10(Suppl 1):O73.) Right: Sagittal view. The rods were anchored using
two pairs of pedicle screws at the proximal end, and a second set of
two pairs at the distal end. The discs adjacent to the instrumented
region at each end were not instrumented
previously described . Specimen motion was largely
in the plane. However, coupled motions were allowed
and the rotational axes were not prescribed. Loads were
measured using the load cell (5 kN) of the test system.
Displacements of vertebrae and mounting blocks were
recorded using high definition video (Nikon D7000, with
Tokina At-X Pro Macro 100 F2.8 D lens; Nikon, Tokyo,
Japan). Five cycles were applied at a frequency of
0.10 Hz for FE or 0.05 Hz for LB, using a sinusoidal
waveform. The fourth cycle was analyzed.
Vertebral orientation at each level was determined
from a triplet LED array which was rigidly pinned to
each vertebra. Sampling frequency was 24 Hz, as was
the video frame rate. Tests were performed with room
lights off to allow for ease of distinguishing markers
from background. Rotations were calculated using a
customized program (Mathworks, MATLAB R2011b,
MathWorks, Natick, MA) . Range of motion,
determined from the moment-rotation curve for each motion
segment, was defined as the maximum side-to-side
rotation. Range of motion over the entire treated region
was determined by adding the ROM at each
instrumented level (T3-T4 to T10-T11).
Statistical differences between treatments in ROM
over the instrumented segments were determined by
two-tailed paired t-tests and Bonferroni correction. Four
primary comparisons were used: PEEK vs control and
PEEK vs CoCr, in LB and FE (α = 0.05/4 = 0.0125).
For non-instrumented control spines, ROM in LB (Fig. 4)
and in FE (Fig. 5) gradually decreased from proximal to
mid-thoracic segments, then increased from mid- to
lower thoracic levels. Control values for mean ROM in
LB ranged from 6° at T7-T8 to 16° at T2-T3, the
proximal adjacent segment. In FE, ROM ranged from 5° at
mid-thoracic to 10° at the proximal adjacent level. For
all three instrumented conditions, the smallest ROM
values, less than 1° in both LB and FE, were at
midconstruct, and all three showed large differences in
ROM across the proximal and distal junctions, 8° to 15°,
compared to the differences of 1° to 3° in the control
In lateral bending, ROM after each treatment,
including PEEK rods, was lower than non-instrumented
control at every instrumented level (Fig. 4). Range of
motion was greater with PEEK rods than for Ti or CoCr
rods at every instrumented level. Conversely, at the
proximal and distal non-instrumented segments of the
instrumented specimens, ROM was greater for every
instrumented condition compared to the control
condition, and the order was reversed. That is, at both
proximal and distal non-instrumented levels, mean ROM
was lowest for control, then PEEK, Ti, and CoCr. The
largest difference in ROM between adjacent levels, 15°,
was between the upper instrumented vertebra and first
proximal adjacent level with Co-Cr rods.
In flexion-extension, ROM after each treatment,
including PEEK rods, was lower than non-instrumented
control at every instrumented level (Fig. 5). Range of
motion was usually greater with PEEK rods than Ti or
CoCr rods at individual levels, but variability was greater
in FE than in LB. Mean ROM at proximal and distal
non-instrumented levels was at least slightly lower for
PEEK than for Ti and CoCr. At the distal adjacent
segment, but not the proximal adjacent, the pattern of
mean ROM was reversed compared to instrumented
levels, as with LB. The largest difference in ROM
between adjacent levels, 11.5°, was between the lowest
instrumented vertebra and first distal adjacent level with
The ROM over all of the instrumented segments in
lateral bending (Fig. 6) for each condition were:
Control 67.9° (±7.4°), PEEK 23.9° (±3.3°), Ti 13.1° (±3.3°),
CoCr 10.1° (±3.8°). Differences between Control and
PEEK (p < 0.0001) and PEEK and CoCr (p < 0.002)
were both significant. Over the instrumented levels,
ROM for spines with PEEK rods was 35 % of
noninstrumented controls, and 2.7 times greater than
Fig. 3 Spine test specimen with PEEK rod construct mounted for a
flexion-extension test. Bottom: At each vertebra, a marker array with
3 white LEDs was inserted into the anterior aspect for video motion
analysis (Reproduced from Reference 40: Bylski-Austrow DI, Glos DL,
Bonifas AC, Carvalho MF, Coombs MT, Sturm PF. Flexible growing rods:
A pilot study to determine if polymer rod constructs may provide
stability to skeletally immature spines. Scoliosis 2015, 10(Suppl 1):O73.)
Top: A floating pulley system was used to convert linear displacement
of crosshead to rotation to apply moment to the cranial end of
spines with CoCr rods. For flexion-extension (Fig. 6),
the total ROM of each motion segment within the
instrumented segment for each test group was:
Control 51.3° (±14.7°), PEEK 13.9° (±4.8°), Ti 10.2° (±4.4°),
CoCr 8.6° (±4.3°). Differences between control and
PEEK (p < 0.0005) and PEEK and CoCr (p < 0.005)
were both significant. Over the instrumented levels,
ROM for spines with PEEK rods was 27 % of
noninstrumented control, and 1.8 times greater than
spines with CoCr rods.
At the proximal and distal adjacent discs in LB and FE,
ROM was always greater for CoCr than for PEEK. The
mean difference in ROM between PEEK and CoCr was 0.9°
(± 0.5°). Peak-to-peak moment (ΔM) for each group,
Control, PEEK, Ti, and CoCr, respectively, were, in LB: 10.7 Nm
(± 0.28 Nm), 10.7 Nm (± 0.26 Nm), 10.9 Nm (± 0.56 Nm),
10.7 Nm (±0.37); and in FE: 10.8 Nm (0.39 ± Nm), 10.9 Nm
(± 0.40 Nm), 10.8 Nm (± 0.32 Nm), 10.6 Nm (±0.25). No
differences were found in applied moments between groups
(p > 0.25), and the target maximum moment was met in all
cases of each condition in both loading directions.
The structural properties of the rods were shown to
significantly affect the biomechanical properties of the
spine in a simulated growing rod construct. Range of
motion of spines instrumented with PEEK rods was
closer to that of metal rods than to that of the control,
non-instrumented condition. Range of motion with
PEEK rods was 27 to 35 % of control. By contrast, ROM
with PEEK rods was 1.8 to 2.7 times greater than with
Co-Cr rods. Therefore, results supported the first two
hypotheses, as the mean ROM with PEEK rods was
between the control condition and the metal rods. The
third hypothesis, which was based on the very high
flexibility of single, isolated, PEEK rods, was not supported.
The polymeric growing rod constructs when implanted
as dual rods did, in fact, very significantly decrease
thoracic spine motion compared to the control
condition. Further, smaller increases in mean ROM of
adjacent discs compared to control usually, but not
always, occurred with PEEK compared to the metal rods,
specifically at the distal end in FE and at both proximal
and distal ends in LB.
Fig. 4 Range of motion (ROM) in lateral bending at each motion segment for each rod type. Control non-instrumented condition, and with
dual rods of polyetheretherketone (PEEK), Titanium, and Cobalt-Chrome alloy (CoCr) are shown. The instrumented levels spanned T3-T4
through T10-T11. Ceph-T3 indicates the proximal adjacent level, and T11-REF the distal adjacent level
The magnitude of the ROM at adjacent discs, and the
differences between the ROM between the first
instrumented motion segment and the adjacent disc, may be
expected to affect the risk of junctional kyphosis .
However, all rod types, when anchored with two pairs of
pedicle screws at each end, created relatively large
changes in motion across the junctions. Range of motion
of the adjacent disc was always greater for CoCr than for
PEEK. Whether a magnitude of difference of 1° is
clinically significant, however, is not yet known.
Limitations of this study include in vitro tests on
physiologically normal quadruped spines. The use of
porcine spines, without ribs, for in vitro studies using
pedicle screws and transverse process hook anchors
has been reviewed [19, 32]. The lack of a rib cage
certainly decreased the stiffness of the thoracic spine.
For the specific aim of this study, to determine if
polymer rods might provide increased stiffness to the
spine in a construct, the use of the isolated spine was
simpler and conservative. That is, because the PEEK
rods clearly provided increased support to a thoracic
spine without the rib cage, it would also do so for
the stiffer structure of a spine plus the intact rib
cage. However, no model can mimic very well the
Fig. 5 Range of motion (ROM) in flexion-extension at each motion segment for each rod type. Control non-instrumented condition, and with
dual rods of polyetheretherketone (PEEK), Titanium, and Cobalt-Chrome alloy (CoCr) are shown. The instrumented levels spanned T3-T4 through
T10-T11. Ceph-T3 indicates the proximal adjacent level, and T11-REF the distal adjacent level
Fig. 6 Range of motion (ROM) of entire instrumented region in
lateral bending (left) and flexion-extension (right). Control
noninstrumented condition, and with dual rods of polyetheretherketone
(PEEK), Titanium, and Cobalt-Chrome alloy (CoCr) are shown.
*** p < 0.000125, ** p < 0.00125, * p < 0.0125 (α = 0.05/4 =
0.0125). (Reproduced from Reference 40, Bylski-Austrow DI, Glos
DL, Bonifas AC, Carvalho MF, Coombs MT, Sturm PF. Flexible
growing rods: A pilot study to determine if polymer rod
constructs may provide stability to skeletally immature spines.
Scoliosis 2015, 10(Suppl 1):O73.)
severe deformities of the spine and thorax of a young
child with EOS.
Other limitations include that specimen numbers were
relatively small, especially for the Co-Cr condition.
Physiological loads of body weight, activity, and curve
correction in the relevant patient populations are not yet
well defined. The two rods were each intact, and did not
contain any distraction mechanism. Further tests are
needed in buckling and torsion, and strength and fatigue
properties are essential. Total ROM was defined as the
sum of individual motion segment ROMs, which is not
necessarily the same ROM if it were determined by
directly measuring the ROM within the instrumented
levels, due to differences in timing of motion along the
spine caused by loading method and specimen
viscoelasticity. The boundary or constraint conditions used, as
well as the loading method, may affect ROM patterns to
some extent. The inability to plastically deform PEEK to
contour the initial rod configuration in situ to better
approximate a desirable sagittal profile is a limitation of
this material. Magnetically controlled growing rods that
are remotely lengthened using an actuator without
additional surgeries are a recent advance in the field. Other
implant structural factors that may be considered are
composite structures that include a partial PEEK rod as
part of a magnetically controlled construct, tapered rod
diameter, and novel connector designs.
Specific results will depend on material and geometric
properties. Rod length was the same for all conditions, but
material and diameter both changed. This set of
conditions was chosen to reflect a range of structural properties.
The extremely low bending stiffness of single isolated
PEEK rods suggested that not even these relatively large
diameter rods would form a construct much less flexible
than the intact isolated thoracic spine. In a study of
comparative mechanical properties of commonly used spinal
rods , the stiffness of PEEK rods was only 4 % that of
Ti rods of the same diameter, whereas carbon fiber
reinforced PEEK was close to titanium. That study also
reported that the effect of mechanical property differences
increased with decreasing rod diameter. Therefore, the
present investigators did not expect a priori that even a
relatively large diameter PEEK rod would substantially
decrease ROM compared to control. The larger diameter
PEEK rod, clinically relevant Co-Cr rod, and the small
diameter titanium rod used in the present study provided
a range of properties, whereas the primary comparisons
were between the Co-Cr and PEEK. Differences among
the moduli of the materials was the primary factor
affecting bending stiffness differences, whereas the diameter
was a secondary factor, over a range of test conditions
which spanned, and slightly exceeded, the physiologically
Potential advantages of PEEK for implants include
high biocompatibility, fatigue resistance, and lower
modulus than titanium. Lower stiffness imparts greater
load sharing with the anterior column, reduced stress at
the bone-to-screw interface, and reduced beam
scattering artifact in MRI and CT [37, 42–45]. Titanium
induces significant artifacts on CT or MRI which
constrain post-operative assessment of adjacent
structures, whereas PEEK, without addition of compounding
material, is radiolucent, neither distorting nor visible in
MRIs . A biomechanical study of a lumbar fusion
construct concluded that segments instrumented with PEEK
rods more closely mimicked intact physiologic loading in
the subadjacent level than titanium . PEEK was
reported to be relatively inert biologically with no evidence
of inflammatory reaction to wear debris .
Possible adverse effects of PEEK rods in temporary,
long-rod, non-fusion constructs may include lower
deformity correction, loss of initial correction, higher
infection rate, and elastic, high deformation, failure
mode. Potential disadvantages of PEEK rods might be
discerned, in part, from those reported for different
but related uses. Clinical outcome studies on
commercially available flexible fusion-promoting systems
have shown higher failure rates with early
reoperations compared to traditional metal fusion-promoting
constructs [37, 49, 50]. In a retrieval analysis of
explanted PEEK rods used for lumbar fusion in
which 11 of 12 PEEK rod systems were employed for
fusion at one level, and motion preservation at the
adjacent level, no cases of PEEK rod fracture or pedicle
screw fracture were noted. Permanent indentations by the
set screws and pedicle screws were the most prevalent
observations on the surface of explanted PEEK rods [51, 52].
Further studies quantifying wear debris and biological
effects in a typical application are required.
Polyetheretherketone may have increased bacterial activity on its
surface compared to titanium [51, 53, 54]. In fatigue tests,
PEEK was shown to be notch-sensitive . Cyclic
deformation was predominately elastic in the lifetime
range . Those authors concluded that the clinical
significance was the potential for gross failure of PEEK
implant devices without any substantial period of
detectable difference in structural behavior. Therefore, for any
application of PEEK to growing rods, design-related stress
concentrations would require careful consideration.
The results of this pilot study did not rule out PEEK
as a possible rod material for growing rods or, perhaps
more likely, as a component of a composite structure.
To the investigators’ knowledge, this is the first study to
test polymers for applications in EOS. Plans for the next
phase of the study include a set of components that
comprise a full, clinically relevant, construct.
In a biomechanical pilot study, simulated growing rod
constructs using polymer rods provided greater stability
compared to controls, greater flexibility compared with
cobalt-chrome, and a more gradual motion and stiffness
transition across junctions than conventional rods. This is
a first feasibility study. A number of other design changes
are possible and many additional preclinical tests would
be necessary prior to translation of this concept. However,
results showed that polymers may become a part of better
treatment options for EOS. Maintenance and retention of
greater spine flexibility would likely allow for fewer
complications and higher satisfaction for patients, parents,
DBA oversaw and participated in the design of the study and methods,
performed the statistical analysis, and drafted the manuscript. DLG drafted
test methods, designed and fabricated the continuous loading fixture, and
oversaw testing. ACB performed the biomechanical tests and carried out the
analysis. MFC performed the surgical instrumentation of the specimens,
participated in test development, and provided surgical expertise and
judgment. MTC helped perform the tests, adapted the program for data
analysis, and oversaw the reduction of the data. PFS was responsible for the
overall concept and clinical relevance, participated in study design, and
reviewed the manuscript. All authors read and approved the final
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