Letter to the Editor Regarding “Characteristics of Spinal Morphology According to the “Current” and “Theoretical” Roussouly Classification Systems in a Diverse,Asymptomatic Cohort: Multi-Ethnic Alignment Normative Study”

To the Editor,

We read with great interest the study published by Shen et al ¹ that aimed to classify the spinal morphology of a multi-ethnic cohort of 467 asymptomatic volunteers using the “current” and “theoretical” Roussouly classifications. We would like to thank the authors for their interest in the Roussouly classification; nevertheless, it is important to highlight a critical misunderstanding in their methodology.

First, the authors refer to the original Roussouly classification published in 2005 ² as the “current” classification, while they refer to the updated classification published in 2018 ³ as the “theoretical” classification. We believe that the wording “current” and “theoretical” is inappropriate and confusing. The authors could have used clearer wording such as “original” and “updated” (or “old” and “new”).

Second, the authors’ description of the updated (“theoretical”) classification is incorrect. The incorrect definition presented in their article states that:

“The ‘theoretical’ classification uses [pelvic incidence] PI and lumbar lordosis (LL) to separate the alignments into Type 1 (PI <45° and lumbar apex at L5), Type 2 (PI <45° and lumbar apex at or cranial to L4–L5 interspace), Type 3 (45°≤PI ≤60°) and Type 4 (PI >60°). A subset of Type 3 subjects with anteverted pelvis (35° ≤ SS ≤45°; PT <8°) was assigned Type 3AP.” ¹

It is important to clarify that the only difference between the original (“current”) and updated (“theoretical”) classifications is that Type 3 is subdivided into Type 3 and Type 3AP, as stated in the article describing the updated (“theoretical”) classification:

“A previously undescribed subgroup of type 3 (35° < SS <45°), identified as ‘‘anteverted type 3’’ or type 3AP has emerged. […] This new type has showed important characteristics of type 3 (35° < SS <45°, and long LL) despite a low-grade PI which is one of types 1 and 2 characteristics.” ³

Otherwise, the definitions for Types 1, 2, and 4 have not changed between the original (“current”) and updated (“theoretical”) classifications and remain: Type 1 (SS < 35° and lumbar apex at L5), Type 2 (SS < 35° and lumbar apex at or cranial to L4-L5 interspace), and Type 4 (SS ≥ 45°).

Furthermore, both the original (“current”) and updated (“theoretical”) classifications are based on SS, although the updated classification introduced the importance of also considering pelvic incidence (PI) and pelvic tilt (PT).

Lastly, the authors studied a cohort of asymptomatic individuals and found different numbers (and proportions) of patients with Types 1, 2, and 4 when using the original (“current”) compared to when using the updated (“theoretical”) classifications (Table 1). The discrepancies between the number of volunteers that met the criteria for Types 1, 2, and 4 confirm that the authors misunderstood (and therefore misapplied) the updated (“theoretical”) classification, as there should be no differences between the 2 classifications with regards to Types 1, 2, and 4.

Table 1.

Proportion of Patients Reported by Shen et al. (1) of Each Type According to When Using the “Current” (original) and “Theoretical” (updated) Roussouly Classifications.

Roussouly Classification	Type 1 n (%)	Type 2 n (%)	Type 3 n (%)	Type 3AP n (%)	Type 4 n (%)	Total n (%)
“Current (original)”	35 (7.5)	100 (21.4)	222 (47.5)	52 (11.1)	110 (23.6)	467 (100)
“Theoretical (updated)”	27 (5.8)	87 (18.6)	261 (55.9)	54 (11.6)	92 (19.7)	467 (100)

Abbreviations: n, number of volunteers; AP, anteverted.

We therefore would like to emphasise that both the original (“current”) and updated (“theoretical”) classifications are the same, except that the updated (“theoretical”) classification introduces a new spinal shape (Type 3AP).