Keywords
lordosis - lumbar vertebrae - spinal fusion - young adult
Introduction
The ability to stand erect is the result of a well-balanced articulation of the spinopelvic
complex.[1]
[2] The sagittal balance (SB) describes these morphological and positional characteristics
of the spine and the pelvis in the sagittal plane.[2]
[3] Several parameters of the SB interact with each other in well-studied, predictable
ways.[1]
[4]
[5]
[6] The pelvic incidence (PI), being static and specific to each individual, is considered
a fundamental parameter in determining the shape of the lumbothoracic spine.[7]
[8]
[9] Lower values of PI tend to lead to a reduction of both lumbar lordosis and thoracic
kyphosis (and vice versa). However, the pelvis also allows for positional adjustment.
When the spine is rigid, the only mechanism for correcting sagittal imbalance is rotating
the pelvis in retroversion or anteversion. The two positional pelvic parameters, sacral
slope (SS) and pelvic tilt (PT), have an arithmetical relationship with the PI through
the equation PI = PT + SS.[1]
[6] Therefore, the maximum pelvic retroversion (SS = 0 and PT = PI) is limited by the
value of PI. Several pathologies and anatomical variations may lead to a sagittal
imbalance of the spine while, on the other hand, deregulation of the SB is considered
an important cause of low back pain and a major mechanical factor in degenerative
progression.[1]
[10]
[11]
Schmorl nodes (SNs), first described in 1927, are the herniation of nucleus pulposus
into the subchondral bone of the vertebral endplate.[12] Prevalence rates in the literature vary greatly, ranging from 3.8 to 77%.[13]
[14]
[15]
[16]
[17] They predominate in men and present a high heredity, frequently affecting the lower
thoracic and upper lumbar spines.[12]
[15]
[18]
[19] The pathophysiology of SNs is still uncertain, and they are usually considered incidental
and idiopathic findings in the clinical practice. Classically, this herniation was
believed to occur during the osteochondral ossification.[12]
[17] Several other etiologies have been proposed since, including traumatic, congenital,
developmental, metabolic, and genetic.[14]
[17]
[20]
[21] The formation of SNs has also been associated with smoking and vascular diseases.[22] Recently, Plompt et al.[14] showed a correlation between SNs and vertebral morphology.[14]
[21] They observed that larger and more circular vertebral bodies, with shorter pedicles,
appear to increase the risk of disc herniation into the vertebral endplate.
While little clinical relevance was given to the presence of SNs in the past, they
have been under focus in the recent literature. Several reports and studies have associated
SNs to low back pain, disc degeneration, Modic changes, vacuum disc phenomenon, and
higher risk of vertebral fractures.[16]
[19]
[22]
[23]
[24] To the best of our knowledge, the relationship between SN and SB has not been explored
before. Thus, it seemed imperative to understand the characteristics and correlations
of the SB in young adults with SNs. Therefore, the present study aims to characterize
and compare the spinopelvic SB of a sample of young adults with SNs with a control
group, to highlight possible changes, and to promote scientific investigation on this
subject.
Materials and Methods
A cross-sectional study was performed on 47 young adults. We consecutively included
patients between 18 and 45 years old who required a consultation with an orthopedic
spine surgeon for low back pain and had an available lumbar magnetic resonance imaging
(MRI) and standing full spine radiographs, from 2012 to 2016. Subjects with a history
of spinal degenerative disease, trauma, infection, tumor, previous spinal surgery,
structural congenital deformities, and hip pathology were excluded from the study.
Data was retrospectively acquired from medical records of the patients. Information
on age, height, weight, and body mass index (BMI) was collected. All radiological
data was blindly evaluated. Magnetic resonance imaging exams were evaluated by two
observers, with discrepancies being settled by consensus. Radiograph measurements
were performed twice by one author, with the average value being used. The presence
of SN was assessed on sagittal T2 weighted MRI ([Fig. 1]). Then, the patients were divided into a study group with SN present in the MRI
(n = 21) and a control group (n = 26). Radiographic measurements were performed in lateral standing full-spine radiographs
of both groups, acquired according to the regular protocol. The following spinal and
pelvic parameters were analyzed ([Fig. 1]):
-
- Sagittal vertical axis (SVA), defined as the horizontal offset from the posterosuperior corner S1 to the
vertical line passing through the center of the C7 vertebral body, in millimeters.
-
- Thoracic kyphosis (TK), defined as the angle between the superior endplate of T4 and the inferior endplate
of T12.
-
- Lumbar lordosis (LL), defined as the angle between the superior endplate of L1 and the superior endplate
of S1.
-
- Pelvic incidence (PI), defined as the angle between the perpendicular to the sacral endplate and the
line that connects its midpoint to the femoral head axis.
-
- Pelvic tilt (PT), defined as the angle between the line that connects the midpoint of the sacral
plate to the femoral head axis and the vertical plane.
-
- Sacral slope (SS), defined as the angle between the sacral endplate and the horizontal plane.
Fig. 1 Example of a patient with Schmorl nodes, seen in magnetic resonance imaging (left); measurement of the spinal and pelvic parameters on a lateral standing full-spine
radiograph (right).
The measurement of the SB parameters was performed using Surgimap software (Nemaris
Inc., New York, NY, USA). Statistical analysis was performed using IBM SPSS Statistics
for Windows, version 22.0 (IBM Corp., Armonk, NY, USA). Parametric tests (Student
independent t-tests) were used to compare the parametric scale variables of the two groups. The
Pearson correlation coefficient was used to access the strength of the linear relationship
between parametric scale variables. The level of significance for all statistical
tests was set at p < 0.05. The confidentiality of the data was guaranteed, with it only being accessible
by the main investigators. Ethical approval for the present study was obtained from
the institutional Ethics Committee, and informed consent was not required for the
present study.
Results
A total of 47 patients were included in the study, with 72.3% females (n = 34). The mean age was 28.2 years old, and the mean body mass index (BMI) was 22.9 kg/m2.
No significant differences between groups were found regarding gender, age or BMI
([Table 1]).
Table 1
|
SN group
(n = 21)
|
Control group
(n = 26)
|
p-value
|
|
Age (years old)[*]
|
28.1 (6.7)
|
28.3 (7.7)
|
0.935
|
|
BMI (kg/m2)[*]
|
21.8 (7.1)
|
23.8 (3.7)
|
0.332
|
|
Gender (females)
|
66.7% (n = 14)
|
76.9% (n = 20)
|
0.435
|
Regarding the measured SB parameters: the average angle of LL was 54.5° in the SN
group and 64.3° in the control (p < 0.001); the average SS was 36.2° for the SN group and 41.4° for the control (p = 0.016) ([Table 2]). No significant differences between groups were observed for SVA, TK, PI or PT
([Fig. 2]).
Fig. 2 Representation of sagittal balance parameters in the control versus SN groups. Lumbar
lordosis and sacral slope were significantly different between groups. SN – Schmorl
nodes, SVA – sagittal vertical axis, TK – thoracic kyphosis, LL – lumbar lordosis,
PI – pelvic incidence, PT – pelvic tilt, SS – sacral slope.
Table 2
|
Measure
|
SN group
|
Control group
|
p-value
|
|
SVA
|
- 33.1° (28.3)
|
- 40.3° (24.1)
|
0.363
|
|
TK
|
25.3° (11.3)
|
32.6° (6.1)
|
0.096
|
|
LL
|
54.5° (8.2)
|
64.3° (8.2)
|
< 0.001
|
|
PI
|
46.5° (12.5)
|
51.4° (9.1)
|
0.143
|
|
PT
|
11.9° (7.1)
|
10.0° (5.5)
|
0.335
|
|
SS
|
36.2° (7.7)
|
41.4° (6.1)
|
0.016
|
In both the control and SN groups, significant and moderate to strong positive correlations
were found between the LL-SS (r = 0.707 and r = 0.540, respectively), PI-PT (r = 0.744 and r = 0.812, respectively) and PI-SS (r = 0.812 and r = 0.672, respectively). In the SN group, significant positive correlations between
SVA-TK (r = 0.756), SVA-LL (r = 0.769), TK-LL (r = 0.896), and LL-PI angles (r = 0.380) were also observed.
Discussion
In the present study, the LL and the SS were significantly lower in patients with
SN. This finding reveals that these patients have a flatter spine and a more vertical
pelvis. These characteristics show a trend toward the sagittal profile described in
patients with low back pain, disc herniation, and degeneration – a straighter spine
with a decrease in both lumbar lordosis and sacral slope (flat back).[1]
[6]
[11]
[25] Although the SB changes found were minor (9.8° in LL and 5.2° in SS), these can
be early changes that might have a greater clinical impact later.
In both groups, positive strong correlations were found for PI-PT and PI-SS, agreeing
with the equation PI = PT + SS.[1]
[6] Additionally, an LL-SS correlation was also present in both groups, a finding well
described in the literature.[6] Some parameters were found to be significantly correlated only in the SN group:
SVA-TK, SVA-LL, TK-LL, and LL-PI. The strong associations between these parameters
indicate that the sagittal profile identified is indeed characteristic to the whole
SN group. Nonetheless, no significant difference was found in the SVA between the
two groups, suggesting that all patients were able to maintain a well-balanced spine
through compensatory mechanisms.
With our study design, no assumptions on the causality between SB and SN are possible.
Although SNs may have a multifactorial etiology, they are assumed to develop early
in life, during the osteochondral ossification, and are often present in subjects
with otherwise healthy and well-balanced spines.[13]
[17]
[19] Barrey et al.[11] showed that younger patients with disc lesions had lower values of PI, presuming
that the SB influences the development of disc herniation or degeneration. Since no
significant differences in the PI were found between the two groups in our sample,
it may imply that SNs affect the SB and not vice versa.
Several studies report that there is an association between SN and disc pathology.
Although there must be shared etiologies regarding disc disease, the resulting horizontalization
of intervertebral discs increases mechanical loading and disc pressure, possibly leading
to an increased risk of early disc degeneration.[1]
[6]
[11] Hence, we believe the SB changes observed in the SN group may partially explain
the link between SNs and disc pathology.
The main limitation of the present study is its small sample size (due to the strict
inclusion and exclusion criteria). Despite the absence of statistically significant
gender differences between groups, the increased prevalence of female patients in
the control group may be a bias regarding the sagittal balance measurements. Also,
the influence of the number or localization of the SNs was not evaluated. Furthermore,
our sample was composed by symptomatic patients (which could introduce a bias), and
as the background patient data collected was limited, other potential factors associated
with disc disease (such as smoking, heavy labor or bone fragility) were not considered.
Conclusions
The study of SB is an important tool to understand the mechanical behavior of the
spine and the pathophysiology of several spine diseases. In the present study, patients
with SN presented a particular SB profile, characterized by a decreased lumbar lordosis
and sacral slope when compared with a control group. Our results show that SNs may
be relevant clinical findings, which may signal patients under risk of having SB variations
associated with earlier disc degeneration and, therefore, should trigger a SB assessment.
Thus, further studies are necessary to fully understand the relationships between
SNs and SB, as well as their role in other spine diseases.
