Keywords spinal curvatures - postural balance - lordosis/classification
Introduction
During the last 2 decades, the study of spinopelvic angles and sagittal balance has
been increasingly relevant in spinal surgery, mainly for the correction of adult deformities.
Reportedly, these measures vary according to age, ethnicity, and biotype in asymptomatic
patients, and according to the etiology of the sagittal imbalance in symptomatic patients.[1 ]
[2 ]
[3 ]
Spinal deformities are clinically and radiologically different in adults and adolescents
due to the association with degenerative processes, dissimilar patterns, and natural
history. Furthermore, spinal imbalance at the sagittal plane may result from fractures
or postoperative complications.[4 ]
Lumbar lordosis severity and pain are inversely correlated; in addition, there is
an association between spinopelvic parameters and lumbar lordosis types with disc
degeneration, facet overload, spondylolisthesis, chronic lumbar pain, disc herniation,
and functional disability.[5 ]
[6 ]
[7 ]
[8 ]
[9 ]
[10 ]
The body uses mechanisms such as increased kyphosis/lordosis of adjacent segments,
trunk hyperextension, pelvic anteversion or retroversion, knee flexion, and ankle
extension to compensate for sagittal imbalance.[11 ]
[12 ] These mechanisms, along with spinal anatomical parameters and sagittal alignment,
must be considered for surgical indication and planning because they affect the postoperative
prognosis.[13 ]
Since this assessment is deemed critical, Roussouly et al.[14 ] proposed a classification system that divides the lumbar spine into four types according
to the lordotic apex and sacral tilt angle. Although this classification has been
used for both research and clinical purposes since its introduction, few studies prove
its validation.
The present study aimed to evaluate the reliability and reproducibility of lumbar
lordosis classification using the Roussouly et al.[14 ] system, and to verify if the inter-rater agreement is affected by the expertise
level.
Materials and Methods
A total of 104 panoramic radiographs of the spine in lateral view of men aged between
18 and 40 years old was used.
These radiographs belonged to a database that had been used in previous studies for
other evaluations. Due to the impossibility of contacting these subjects, who were
previously anonymized, the present study was exempted from an informed consent form
and it was approved by the Research Ethics Committee under protocol number 3.828.093.
All radiographs were obtained with the same equipment. Patients were asked to stand
up, with a straight trunk, upper limbs resting in a support, shoulders at 30° flexion,
slightly flexed elbows, and extended knees. Panoramic, lateral radiographs covered
from the base of the skull to the proximal region of the femur. Low-quality images
that did not allow measurements were excluded from the study.
Using a the Surgimap software, version 2.3.1.5 (Nemaris Inc., New York, NY, USA),
6 evaluators, consisting in 2 spine surgeons with > 5 years of experience (A1 and
A2), 2 residents in Spine Surgery (B1 and B2), and 2 residents in Orthopedics and
Traumatology (C1 and C2), measured the spinopelvic angles and the sagittal vertical
axis ([Figure 1 ]). These data were used to classify the type of lumbar lordosis according to Roussouly
et al.[15 ] ([Figure 2 ]). After 1 month, the measurements were repeated by the same evaluators to assess
intra- and inter-rater agreement.
Fig. 1 Example of spinopelvic angles and vertical sagittal axis measurement using the Surgimap
software.
Fig. 2 Lumbar lordosis classification according to Roussouly et al. Type 1: The sacral tilt (ST) is < 35°, and the apex of lumbar lordosis is located at the
center of the L5 vertebral body. Type 2: The ST is < 35°, and the apex of lumbar lordosis is located at the base of the L4
vertebral body. Type 3: The ST ranges from 35° to 45°. Type 4: The ST is > 45°. (adapted from Roussouly et al.[15 ]).
Statistical Methods
Initially, the results were descriptively analyzed to obtain graphs and frequency
tables to characterize the participants of the research. Categorical variables were
expressed as absolute frequency and percentage values. Graphs assessed the frequency
of variables of interest.
Intra- and inter-rater agreement of the Roussouly classification were determined using
the Fleiss kappa test (1981); this is a generalization of the kappa test used when
several people evaluate the same sample on a scale with different categories, such
as the Roussouly classification, consisting in types 1 to 4. The kappa agreement coefficient
ranges from + 1 (perfect agreement) to 0 (agreement equal to that expected by chance)
to - 1 (complete disagreement).[16 ]
The kappa coefficient value was classified according to Landis et al.,[17 ] as shown in [Table 1 ].
Table 1
Kappa coefficient
Strength of agreement
< 0.00
Poor
0.00–0.20
Weak
0.21–0.40
Reasonable
0.41–0.60
Moderate
0.61–0.80
Substantial
0.81–1.00
Virtually perfect
All analyzes were performed with the statistical software R version 3.3.1 (R Foundation,
Vienna, Austria), and the level of significance was set at 5%.[18 ]
Results
[Table 2 ] presents frequency distribution of the lordosis type, attributed by each evaluator
at both measurements. Note that type 3 was the most frequently assigned type (> 50%)
by all evaluators, except for B1, who classified most cases as type 4.
Table 2
Measurement
Evaluator
Lumbar lordosis type according to the Roussouly classification
1
2
3
4
Measurement 1
A1
9 (9%)
11 (11%)
59 (57%)
25 (24%)
A2
7 (7%)
12 (12%)
57 (55%)
28 (27%)
B1
14 (13%)
10 (10%)
23 (22%)
57 (55%)
B2
8 (8%)
8 (8%)
57 (55%)
31 (30%)
C1
7 (7%)
8 (8%)
58 (56%)
31 (30%)
C2
7 (7%)
10 (10%)
59 (57%)
28 (27%)
Measurement 2
A1
6 (6%)
9 (9%)
64 (62%)
25 (24%)
A2
10 (10%)
9 (9%)
57 (55%)
28 (27%)
B1
19 (18%)
8 (8%)
30 (29%)
47 (45%)
B2
10 (10%)
5 (5%)
57 (55%)
32 (31%)
C1
3 (3%)
8 (8%)
61 (59%)
32 (31%)
C2
6 (6%)
14 (13%)
62 (60%)
22 (21%)
Intra-rater Agreement
The intra-rater agreement analysis revealed that A2 had the best level of agreement,
with a virtually perfect coefficient (0.827). On the other hand, the lowest level
of agreement was obtained by B1, with a reasonable coefficient (0.307). A1 and B2
showed substantial agreement (0.601 and 0.710, respectively), whereas C1 and C2 presented
moderate agreement (0.580 and 0.557, respectively). The average intra-rater agreement
was 0.597, which was deemed moderate. All values had a p-value < 0.001 ([Figure 3 ] and [Table 3 ]).
Table 3
Evaluator
Fleiss kappa coefficients
95%CI
p-value
A1
0.601
(0.462–0.740)
< 0.001*
A2
0.827
(0.738–0.915)
< 0.001*
B1
0.307
(0.163–0.452)
< 0.001*
B2
0.710
(0.586–0.833)
< 0.001*
C1
0.580
(0.440–0.720)
< 0.001*
C2
0.557
(0.407–0.708)
< 0.001*
Fig. 3 Fleiss kappa coefficient for intra-rater agreement.
Inter-rater Agreement
In the inter-rater agreement analysis, the general coefficient for the first measurement
was 0.43 (moderate); for the second measurement, this value was slightly lower, at
0.369 (reasonable) ([Table 4 ]).
Table 4
Evaluator
Fleiss kappa coefficients
95%CI
p-value
Measurement 1
0.430
(0.344–0.516)
< 0.001
Measurement 2
0.369
(0.288–0.451)
< 0.001
Among evaluators with the same level of expertise, there was a statistically significant
agreement (p < 0.001) between all groups, and spine surgeons with > 5 years of experience presented
the highest level of inter-rater agreement, ranging from substantial at the first
measurement (0.619) to moderate at the second measurement (0.439). Spine surgery and
orthopedics and traumatology residents showed reasonable levels of inter-rater agreement
within their classes ([Table 5 ]).
Table 5
Measurement
Evaluator
A1
A2
B1
B2
C1
C2
Measurement 1
A1
1.000
A2
0.619
1.000
B1
0.192
0.261
1.000
B2
0.488
0.412
0.236
1.000
C1
0.565
0.583
0.218
0.434
1.000
C2
0.597
0.584
0.196
0.516
0.496
1.000
Measurement 2
A1
1.000
A2
0.439
1.000
B1
0.222
0.138
1.000
B2
0.483
0.458
0.283
1.000
C1
0.515
0.539
0.168
0.449
1.000
C2
0.440
0.404
0.166
0.496
0.325
1.000
Discussion
In addition to dictating treatment or providing prognosis, an adequate classification
must be reproducible for professionals with different expertise levels. The Roussouly
classification for lumbar lordosis was introduced as a tool to analyze the sagittal
alignment of the spine while considering pelvic orientation, characterizing an individual
biotype.[14 ]
Roussouly types 1 and 2 have lower sacral tilt (ST) (<35o ) and lower angular lordosis, increasing the load at the anterior spine, with a potentially
higher association with disc degeneration[3 ]
[6 ]
[7 ] and chronic low lumbar pain.[9 ] Type 3 is the most frequent type in asymptomatic populations, even among different
ethnicities and age groups.[1 ]
[2 ] Type 4 has the highest amount of ST (> 45o ) and lumbar lordosis, and it is more related to spondylolisthesis and facet overload.[8 ]
In our study, type 3 lordosis was the most commonly found by the evaluators, which
is consistent with previous studies in the asymptomatic population.[1 ]
[2 ] Roussouly considered this type of lordosis as more physiological.[14 ]
The classification requires the measurement of spinopelvic angles, which can be performed
manually using a goniometer on panoramic, lateral radiographs of the spine including
the pelvis and femoral heads; however, the Surgimap software has been validated to
facilitate measurement.[19 ]
Even though the Roussouly classification describes objective criteria depending mainly
on measurable, well-defined references, a variation of a single degree in ST may change
the type of lordosis. As a result, patients with borderline cutoff values (∼ 35o or 45o ) can receive different ratings from different observers or at different measurements
from the same observer. In addition, the definition of the lordotic apex may be doubtful,
allowing for divergences between types 1 and 2. Thus, the hypothesis that the greater
or lesser presence of spines with these characteristics may affect classification
reproducibility is valid.
The present study revealed the good reproducibility of the Roussouly classification,
since both the intra- and inter-rater agreements were at least reasonable (> 0.20)
according to Fleiss kappa coefficients. The intra-rater agreement ranged from reasonable
to virtually perfect, whereas the inter-rater agreement ranged from reasonable to
moderate.
Evaluator B1 stood out with the lowest intra- and inter-rater agreement; in addition,
he was the only one to find a higher prevalence of type 4 lordosis. These differences
may be explained by some divergence in the interpretation of the classification, technical
measurement errors, or be inherent to the fact that measurements with close values
can be classified as different types.
Experience seems to affect the reproducibility of the classification, since the most
experienced evaluators (A1 and A2) showed greater intra- and inter-rater agreement.
This finding may be explained by the fact that spinal surgeons have greater familiarity
with these measurements and understanding of the spinopelvic angles than residents
in training.
Conclusion
The Roussouly classification demonstrated good reliability and reproducibility. Intra-
and inter-rater agreements were at least reasonable, ranging from substantial to virtually
perfect in some situations. Experts with a higher level of experience showed greater
intra- and inter-rater agreement.
