**1. Introduction**

The spine connects the pelvis and the head with 24 vertebral bodies that can move against each other in three directions of movement. It stabilizes the torso and enables verticalization. The posture and movements of the spine are individually varied and highly characteristic of each person [1]. Visual inspection and posture analyses are important aspects of the basic examination of patients affected by spinal disorders [2]. Many musculoskeletal examiners have reported that visual estimations are one of their most commonly used assessment tools when analyzing spinal posture in clinical practice [3]. Although these visual assessments are simple and quick to perform, their results are relatively subjective, and their interrater reliability is statistically poor [3,4]. This becomes problematic when the results contribute to the clinical decision making process or are used in follow-up examinations to assess the progress and outcomes of the initiated therapies [5]. In order to address this problem, the collection of data regarding spinal posture should be objective and standardized using valid, reliable and reproducible measurement approaches. It is crucial for the assessments to be non-invasive for the patient and quick and easy to conduct

**Citation:** Huthwelker, J.; Konradi, J.; Wolf, C.; Westphal, R.; Schmidtmann, I.; Drees, P.; Betz, U. Reference Values for 3D Spinal Posture Based on Videorasterstereographic Analyses of Healthy Adults. *Bioengineering* **2022**, *9*, 809. https://doi.org/10.3390/ bioengineering9120809

Academic Editors: Christina Zong-Hao Ma, Zhengrong Li, Chen He and Aurélien Courvoisier

Received: 14 November 2022 Accepted: 12 December 2022 Published: 15 December 2022

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in daily clinical routines. Videorasterstereography (VRS) seems to be a corresponding method that is increasingly becoming established in clinical practice [6–8].

The VRS system is based on a horizontal light line pattern projected onto the patient's unclothed back and creates a virtual plaster cast of the individual back surface within only a few seconds [7]. In addition to information about the surface topographic curvature picture, the system is able to precisely estimate the position of every vertebral body (from C7 to L4) and the pelvis in a virtually constructed three-dimensional model of the human vertebral column [7,9–12]. VRS has evolved since its initial development in the 1980s and has been described in various publications [6,7,13]. The system has been proven to be valid and highly reliable compared to the clinical gold standard (X-ray imaging) [8,14–17].

In order to implement VRS for spinal posture analysis as a routine assessment in clinical practice, it is essential to have systematic reference data available for comparison with the potential pathological findings. Unfortunately, the current datasets are only conditionally able to fulfill these requirements, as they have several limitations.

Thus far, there are reference data for the global spine parameters of children [18], young adults [19–21] and young and middle-aged adults [22,23]. Either relatively heterogeneous study cohorts with very small numbers of participants have been analyzed without any further subgroup specifications [22,23], or subgroup-analyses have focused only on the potential differences between female and male participants in young, relatively homogenous study cohorts [19–21]. Possible changes in physiologic spinal posture according to sex and/or age over the adult life span have not yet been investigated. This knowledge, however, is essential for the consultation of reference data in clinical practice, in which not only young but also older patients are examined using VRS measurement devices.

In order to close this gap in our knowledge, the first aim of the current study was to provide practitioners and researchers with an additional set of VRS reference data that, firstly, included a preferably high number of healthy participants. Secondly, structured subgroup analyses were used depict possible physiologic changes in the spinal posture parameters according to sex and age over an adult life span of 18 to 70 years.

The second aim of this study was to provide the respective reference data for specific spine parameters: the isolated position of each vertebral body from C7 to L4 in all three dimensions of movement. These data are currently missing from the literature. As of up to a few years ago, only global spine parameters such as the thoracic kyphosis and lumbar lordosis angles were exportable from the DICAM 3 software. Meanwhile, the three-dimensional position of each vertebral body can be analyzed using an additional export interface.

In contrast to the work previously published by our own research group, describing a subgroup analysis of 100 asymptomatic females based on the dataset included here [24], this project involved a more differentiated analysis providing reference data for three different age cohorts (18–30 years, 31–50 years and 51–70 years) and for both sexes, respectively.

#### **2. Materials and Methods**

The data analyzed in this work were part of a prospective, explorative, cross-sectional and monocentric study assessing the three-dimensional spinal posture and movement behavior of healthy participants in the upright standing position and at four different walking speeds (2 km/h, 3 km/h, 4 km/h and 5 km/h). Ethical approval was obtained from the responsible ethics committee of the Rhineland-Palatinate Medical Association, and the study is registered with the World Health Organization (WHO) (INT: DRKS00010834). Based on a statistical sample size calculation, 201 healthy participants (sex ratio of 2/3 females to 1/3 males, aged 18–70 years) who gave their informed consent prior to participation were included in three different age cohorts (young (18–30 years), middle (31–50 years) and old (51–70 years)).
