surgical parameters
Surgical parameters Group 1 Group 2 P (χ2
)
Duration of surgery (min) 204.44±49.6 205.63±53.7 0.898
Hospitalization (day) 3.22±1.1 3.23±0.8 0.945
Number of surgical levels
1 level 43 36 0.051
2 levels 17 16
3 levels 3 12
Number of each segment
L1‐2 2 1
L2‐3 7 16
L3‐4 25 34
L4‐5 44 52
L5‐S1 8 1
Figure 2: Comparison of segmental ROM (a) and total ROM (b) of dynamic
and rigid rods in preoperative, postoperative 3rd, 6th, 12th month and late
follow-up
a
b
Varol, et al.: Comparison of dynamic and rigid instrumentation
352 Journal of Craniovertebral Junction and Spine / Volume 13 / Issue 3 / July‐September 2022
level. Fusion was detected in 36 cases(57%) and 30 cases(47%)
in groups 1 and 2, respectively (P = 0.247).
Early complications were observed in 17 cases: five (8%) in
group 1 and 12 (19%) in group 2 (P = 0.016). Four cases (6.4%) in
group 1 and eight cases(13%) in group 2 required reoperation.
The complications in the two groups are summarized in Table 3.
The results showed that the preoperative and postoperative
lumbar–leg VAS and ODI values in the rigid group and the
dynamic group were significantly decreased. The preoperative and
postoperative measurements showed no significant difference in
VAS and ODI values between the two groups [Figure 1].
The comparison of the preoperative segmental ROM
values of the rigid group and the dynamic group patients
showed P = 0.31. There was no significant difference in
the preoperative segmental ROM values of the patients in
either group. The preoperative–postoperative comparison
showed P < 0.001. The postoperative segmental ROM was
significantly decreased in both groups. The comparison of
the postoperative segmental ROM in both groups showed
P < 0.001. The segmental ROM values of the patients in the
dynamic group were significantly higher than in the rigid
group, which indicated that it was better maintained in the
former group.
The comparison of preoperative total ROM values in the rigid
group and the dynamic group resulted in P = 0.77. There was
no statistically significant difference in the preoperative total
ROM values of the patients in either group. The preoperative
and postoperative comparisons showed P = 0.001 and
P = 0.007, respectively. The postoperative total ROM was
significantly decreased in both groups. In the last follow‐up,
the comparison of the postoperative total ROM in both groups
showed P = 0.042. The total ROM values in the dynamic
group were significantly higher than in the rigid group, which
indicated that it was better maintained [Figure 2].
The preoperative and postoperative DHI values of the
patients in the rigid and dynamic groups were P = 0.48 and
P = 0.94, respectively. Accordingly, the preoperative and
postoperative DHI changes in the rigid group and dynamic
group were not significant. That is, there was no significant
change in preoperative and postoperative DHI values in either
Table 3: List of early and late complications
Complications Group 1 (%) Group 2 (%)
Early complications 5 (8) 12 (19)
Screw malposition (early period) 1 (1.6) 4 (6.2)
Superficial site infection 2 (3.2) 5 (7.8)
Epidural hematoma 1 (1.6) 0
CSF fistula 1 (1.6) 3 (4.7)
Late complications 27 14
ASD 19 (30) 9 (14)
Pseudoarthrosis 8 (13) 5 (8)
ASD: Adjacent segment disease, CSF: Cerebrospinal fluid
Figure 1: Comparison of low back VAS scores(a), leg VAS scores(b) and ODI
scores (c) of dynamic and rigid rods in preoperative, postoperative 3rd, 6th,
12th month and late follow-up
b
c
a
Varol, et al.: Comparison of dynamic and rigid instrumentation
Journal of Craniovertebral Junction and Spine / Volume 13 / Issue 3 / July‐September 2022 353
group. However, although there was no difference in the
preoperative DHI, it was found to be statistically significantly
higher in the postoperative rigid group.
The comparison of the preoperative and postoperative FH
values in the rigid and dynamic groups showed that the
values were significantly increased postoperatively in both
groups. However, no statistically significant difference was
observed between the preoperative and postoperative
groups [Figure 3].
Postoperative adjacent segment disease
ASD refers to any changes in motion segments above and
below the surgical site, such as disc herniation, spinal
stenosis, proximal junction kyphosis, and so on. Patients
with radiological and clinical findings of ASD and patients
who reoperated for ASD were recorded. ASD was detected
radiologically and clinically in 19 (30%) patients in the rigid
group during the entire follow‐up period. In the dynamic
group, ASD was detected in nine (14%) patients during the
entire follow‐up period. Four (6.3%) of these patients in the
rigid group and five (7.8%) of these patients in the dynamic
group underwent reoperation to treat ASD. The number and
rate of cases related to the levels of ASD are shown in Table 4.
The comparison of the data on the two groups in our study
yielded P = 0.028. According to this result, a statistically
significant difference was found between the two groups in
terms of ASD (P < 0.05). Therefore, according to our results,
the probability of ASD in cases in which the rigid system was
applied was significantly higher (30%) than in the cases in
which the dynamic system was applied (14%).
Postoperative pseudoarthrosis
The lack of substantial bone fusion 6 months following
surgery is referred to as pseudoarthrosis. Cases with
radiological pseudoarthrosis were recorded, which
showed that pseudoarthrosis was detected radiologically
in eight (13%) patients in the rigid group during the entire
follow‐up period. In the dynamic group, pseudoarthrosis was
detected radiologically in five (8%) patients during the entire
follow‐up period [Table 5].
The comparison of pseudoarthrosis in the two groups
showed P = 0.363. Therefore, there was no statistically
significant difference between the two groups in terms of
pseudoarthrosis(P > 0.05). Considering the number of levels,
pseudoarthrosis was the most common in patients in the
rigid group, who had two levels of instrumentation (41%).
However, this result did not provide evidence that an increase
in the number of levels increased the risk of pseudoarthrosis.
DISCUSSION
Lumbar stenosis typically occurs as a result of complex
degenerative pathologies that compress the neural
elements. Facet joint orientation and facet joint tropism
are closely linked to disc degeneration in the lumbar
spine.[5] Modic alterations and lipid infiltration in the
multifidus and erector spinae muscles are also linked
to disc degeneration.[6] The first step in the treatment
is conservative in mild cases, but its benefit is limited
because the symptoms are aggravated by movement. In
advanced cases, the degenerative process exacerbates
neural stenosis. Therefore, surgical methods are frequently
Table 4: The number and rates of cases related to the number
of levels on adjacent segment disease
Number of instrumentation
segments
Group 1
number
of ASD
Group 2
number
of ASD
1 segment 12/43 (28) 5/36 (14)
2 segments 5/17 (29) 2/16 (13)
3 segments 2/3 (67) 2/12 (17)
Total 19/63 (30) 9/64 (14)
Number of cases operated due to ASD 4 (6.3) 5 (7.8)
ASD: Adjacent segment disease
Table 5: Cases with radiological pseudoarthrosis
Number of
instrumentation
segments
Group 1
Number of screw
pseudoarthrosis (%)
Group 2
Number of screw
pseudoarthrosis (%)
1 segment 1/43 (2) 4/36 (11)
2 segments 6/17 (35) 0/16 (0)
3 segments 1/3 (33) 1/12 (8)
Total 8/63 (13) 5/64 (8)
Figure 3: Comparison of disk height index (a) and foraminal height (b) of
dynamic and rigid rods in preoperative, postoperative 3rd, 6th, 12th month
and late follow-up
a
b
Varol, et al.: Comparison of dynamic and rigid instrumentation
354 Journal of Craniovertebral Junction and Spine / Volume 13 / Issue 3 / July‐September 2022
used in treatment. Microsurgery and lumbar stabilization
using rigid and dynamic systems are the basis of surgical
treatment. In our study, we compared dynamic and rigid
systems in terms of clinical, radiological, and surgical
complications. While there was no difference between the
two groups in terms of VAS and ODI scores, statistically
significant differences were found in terms of ROM, fusion
rates, and the development of ADS.
ASD is a potential long‐term complication of spinal fusion. This
condition includes several symptoms, such as disc degeneration,
facet joint changes, and spinal stenosis. The reported incidence
of symptomatic ADS has been defined as 5%–20% with varying
follow‐up times and different techniques. The etiology of
ADS has not yet been fully defined. Two theories have been
developed to explain this mechanism.[7] The first theory is
focused on mechanical causes, such as the increased load
exposure of the adjacent segment under stress and increased
intradiscal pressure.[8] Cadaver studies have shown that the load
on the instrumented segments after fusion was transferred to
the adjacent segment, which increased the intradiscal pressure
on the adjacent segment.[9,10] Moreover, the displacement of
the rotation center in flexion and the formation of relative
hypermobility comply with this theory.[11] The second theory
emphasizes the natural progression of age‐related degeneration
without the involvement of a mechanism.[12]
Patient age and sex are risk factors for ASD. Aota et al. found
that the risk increased in patients over 55 years.[13] Decreased
proteoglycan and water content in elderly patients results in
disc degeneration and causes the transfer of axial loading to
the facet joint. Previous findings showed that ASD developed as
a result of joint instability.[10] Previous reviews of the literature
on ASD found that being over 55 years old is a major risk
factor.[7,4] In Guigui et al., the risk factors for ASD were defined
as patient age, female gender, and use of a rigid instrument.[14]
In our study, while 26 of 28 patients with ASD were female,
only two were male. The general female gender ratio, which
was 78% in our study, was 93% in cases with ASD. Similarly,
while the mean age in our study was 56.44, the mean age of
patients who developed ASD was 60.32 years. In accordance
with the literature, our results indicated that age and female
gender were risk factors for ASD. However, no significant
result was found to support that smoking, diabetes mellitus,
and hypertension were risk factors for ASD.
Ghiselli et al., in their case series of 123 patients, found that
this rate was higher in patients who underwent long‐segment
fusion and lower in patients who underwent shorter fusion
based on an average follow‐up of 6 to 7 years.[15] Nagata
et al. found that the longer the instrumented segment, the
shorter the amount of time required for ASD development
and the higher the risk of ASD development.[16] Shono et al.
reported that more rigid and longer‐segment instrumentation
increased the risk of ASD.[17] In Miyakoshi et al., the results
of single‐segment instrumentation were more positive than
those of previous studies in the literature.[18]
In our study, we found that adjacent segment degeneration
developed in 21.5% of patients with single‐segment
instrumentation, 21.2% of patients with two segments, and
26.7% of patients with three segments, according to the fusion
levels. Based on these results, it could not be concluded that
the number of instrumentation levels is a risk factor for ASD.
In Park et al.’s review of 56 studies, the incidence of symptomatic
ASD was defined as 5.2%–18.5% with varying follow‐up times
and different techniques.[7] Nakashima et al. conducted a
retrospective study of 101 patients who were followed up for at
least 10 years after fusion. Their findings showed that 80 cases
had worsening lumbar spinal stenosis at the adjacent level
and 87 cases had increased disc degeneration in the adjacent
segment.[19] However, there have been fewer studies on ASD
requiring revision surgery. Aiki et al. reported 7.7%[20] in their
2‐year follow‐up, and Gillet reported ASD requiring reoperation
in 20% of patients in their minimum of 5‐year follow‐up.[21] In
Guigui et al., although 49% of ASD was observed radiologically,
8% became symptomatic and were reoperated.[14]
Kim et al. reported that fixation in the dynamic system,
whether single or multilevel, caused less hypermobility in
the adjacent segment and significantly reduced the risk
of ASD.[22] Another study showed that the more rigid the
instrumentation type used, the shorter the time required
for patients to develop ASD.[23] Yang and Jiang’s comparative
study showed that the Dynesys dynamic system caused less
ROM in the adjacent joint compared with the rigid system,
and it preserved the disc structure in the adjacent segment,
thus reducing ASD rates.[24] Thoracic kyphosis and pelvic tilt
were found to be important indicators of overall rigidity
and reference the ability of the spine to compensate for the
sagittal plane deformity after spinal fusion.[25]
In our study, degeneration was found in the radiological
adjacent segment in nine (14%) of 64 cases in which the
dynamic system was used and in 19 (30%) of 63 cases in which
the rigid system was used, according to instrumentation
type. ASD became symptomatic in nine (7%) of all cases, and
revision surgery was performed. Radiological and surgical
ASD rates have been reported widely in the literature, which
is consistent with the literature in our study.
Varol, et al.: Comparison of dynamic and rigid instrumentation
Journal of Craniovertebral Junction and Spine / Volume 13 / Issue 3 / July‐September 2022 355
In our study, we used PEEK rods as a dynamic system.
Although they are not marketed as a dynamic stabilization
device, PEEK rods have a softer profile than all other metal
systems and therefore create a softer structure in the
posterior lumbar spine. Compared with other dynamic
systems, PEEK rod systems can reduce screw loosening by
allowing the self‐movement of the screw.[26] Because the PEEK
modulus of elasticity is similar to bone, using this polymer
as part of a pedicle screw–rod structure offers sufficient
rigidity for fusion to occur, but it will not be exposed to
the rigid stresses created by a titanium structure.[27,28]
Biomechanical studies have shown that PEEK rods provide
greater durability, strength, and general biomechanical
profiles compared with metallic rod systems.[29] PEEK rods
reduce the ROM of an unstable spinal segment with no
significant difference in stability compared with titanium
rods. The potential advantages of using PEEK rod systems
for the spine are as follows: shares the load on the anterior
column, which facilitates interbody fusion, reduces the stress
between the bone and screw surface, reduces the rate of
screw mobilization, and reduces the incidence of adjacent
level disease in the long term.[30]
A potential disadvantage associated with the PEEK rod
is the theoretical risk of pseudoarthrosis due to reduced
hardness and rod breakage. Moreover, PEEK rods are difficult
to follow in radiological imaging due to their radioactive
properties. Inappropriate placement of spinal implants may
complicate the perception of clinical results, and rod breaks
may not be identified in postoperative imaging. However,
radiopaque markers can be added to these rods to provide
a radiographic evaluation of the position of the PEEK rods.[31]
Some studies have reported good or excellent results with
low complication rates in PEEK rod systems.[3,30,27,32] For rigid
stabilization, to decrease pseudoarthrosis four‐rod technique
was recommended.
Whether a dynamic stabilization system maintains disc height
is still controversial. Huang et al. analyzed 38 patients treated
with the PEEK rod system and found that DHI increased
slightly but gradually decreased below preoperative levels.[33]
Their findings suggested that a pedicle‐based dynamic system
could not restore disc height. Kumar et al. compared disc
lengths between dynamic and fusion levels and found
that they decreased after surgery, but this change was not
statistically significant at the 2‐year follow‐up.[34]
In our study, based on our findings from the literature
review, our evaluations of the DHI and FH parameters
showed that the DHI was significantly higher in the rigid
group, but a decrease was observed in both groups during
the postoperative follow‐up period. There was no difference
between the preoperative and postoperative groups in either
group. Regarding FH, the postoperative increase in the rigid
group decreased to preoperative values in the following
months, and a positive significant difference was observed
between the preoperative and postoperative values in the
dynamic group.
Wang et al. compared the K‐Rod dynamic system and fusion in
a 2‐year follow‐up of 98 patients. DHI and FH were increased
in both groups compared with preoperative values, but
there was no difference between the two groups. Similarly,
although VAS and ODI values were significantly decreased
in both groups compared with preoperative values, no
statistically significant difference was observed between the
two groups. Regarding segmental ROM and total ROM,
the dynamic group was found to be significantly mobile in
the fusion group. These results indicated that the dynamic
system resulted in less restriction on physiological lumbar
movements.[35]
Ogrenci et al. observed that in 172 patients who underwent
PEEK rod instrumentation during an average 2‐year follow‐up,
fusion rates were similar to those in which titanium rods
were used according to the literature, but that ASD and other
long‐term complications were fewer than those in which
titanium rods were used, and physiological lumbar movement
was better maintained.[1] Ozer et al. also argued that in
71 patients who were operated on using various dynamic
systems, less ASD was shown in follow‐ups of at least 2 years
compared with rigid systems; moreover, lumbar lordosis and
disc height were maintained at a reasonable level.[2]
CONCLUSION
In early experiences using PEEK rod systems, physiological
spine movement, increasing fusion rates, minimal
complications, reduction in adjacent segment degeneration,
and biomechanical compatibility were demonstrated.
Although further long‐term studies are needed, and the cost
of PEEK systems is likely to be a barrier, these results indicate
the benefits of the use of PEEK rods in spinal surgery.
The clinical results of dynamic systems applied under
appropriate conditions and with appropriate indications
have shown similar efficacious results. Moreover, it has been
observed that their advantages outweigh those of standard
rigid systems with regard to long‐term complications and
physiological parameters. However, it is too early to make a
final judgment regarding their usage, and longer follow‐ups
and larger case series studies are needed.
Varol, et al.: Comparison of dynamic and rigid instrumentation
356 Journal of Craniovertebral Junction and Spine / Volume 13 / Issue 3 / July‐September 2022
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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