Range of Motion and Neutral Zone of All Human Spinal Motion Segments: A Data Collection of 30 Years of In Vitro Experiments Performed Under Standardized Testing Conditions

Background

Spinal flexibility can vary among spinal sections and single motion segments. The purpose of this work was to provide a comprehensive overview of the range of motion (RoM) and neutral zone (NZ) values of all spinal levels collected during 30 years of in vitro experiments under standardized testing conditions.

Methods

RoM and NZ data obtained from in vitro testing of intact human mono- and polysegmental specimens with pure moments of 2.5 Nm for the cervical, 5 Nm for the thoracic, and 7.5 Nm for the lumbar spine were collated from the internal database of the authors' institution. Descriptive statistics were performed with median values and median absolute deviations. Outliers were defined as values beyond twofold standard deviation and excluded from evaluation. Normal distribution was verified using the Shapiro–Wilk test.

Results

RoM and NZ data of N = 1139 functional spinal units were collected with sample sizes ranging from n = 6 to n = 224 per segmental level. The cervical spine was very flexible in flexion/extension and moderately flexible in lateral bending, while in axial rotation, the motion segment C1-C2 was as flexible as the subaxial cervical spine combined. The thoracic spine was the least flexible section in flexion/extension but allowed moderate lateral bending and axial rotation. The rib cage had a strong effect on thoracic spinal flexibility, particularly in axial rotation and at the mid-thoracic spinal levels. The lumbar spine exhibited moderate flexibility in flexion/extension and lateral bending but showed the lowest RoM and NZ in axial rotation.

Conclusions

This unique summary of RoM and NZ data, acquired under the same loading conditions in the same spine tester, provides a detailed insight into overall spinal flexibility and will serve as a valid dataset for the validation of in vitro studies and numerical models of the single motion segments of the spine.