The rib cage stabilizes the human thoracic spine: An in vitro study using stepwise reduction of rib cage structures

Abstract

The stabilizing effect of the rib cage on the human thoracic spine is still not sufficiently analyzed. For a better understanding of this effect as well as the calibration and validation of numerical models of the thoracic spine, experimental biomechanics data is required. This study aimed to determine (1) the stabilizing effect of the single rib cage structures on the human thoracic spine as well as the effect of the rib cage on (2) the flexibility of the single motion segments and (3) coupled motion behavior of the thoracic spine. Six human thoracic spine specimens including the entire rib cage were loaded quasi-statically with pure moments of ± 2 Nm in flexion/extension (FE), lateral bending (LB), and axial rotation (AR) using a custom-built spine tester. Motion analysis was performed using an optical motion tracking system during load application to determine range of motion (ROM) and neutral zone (NZ). Specimens were tested (1) in intact condition, (2) after removal of the intercostal muscles, (3) after median sternotomy, after removal of (4) the anterior rib cage up to the rib stumps, (5) the right sixth to eighth rib head, and (6) all rib heads. Significant (p < 0.05) increases of the ROM were found after dissecting the intercostal muscles (LB: + 22.4%, AR: + 22.6%), the anterior part of the rib cage (FE: + 21.1%, LB: + 10.9%, AR: + 72.5%), and all rib heads (AR: + 5.8%) relative to its previous condition. Compared to the intact condition, ROM and NZ increased significantly after removing the anterior part of the rib cage (FE: + 52.2%, + 45.6%; LB: + 42.0%, + 54.0%; AR: + 94.4%, + 187.8%). Median sternotomy (FE: + 11.9%, AR: + 21.9%) and partial costovertebral release (AR: + 11.7%) significantly increased the ROM relative to its previous condition. Removing the entire rib cage increased both monosegmental and coupled motion ROM, but did not alter the qualitative motion behavior. The rib cage has a strong effect on thoracic spine rigidity, especially in axial rotation by a factor of more than two, and should therefore be considered in clinical scenarios, in vitro, and in silico.

Fig 1. The six specimen conditions for biomechanical testing and motion analysis.

Specimens were tested in intact condition (1), after removing the intercostal muscles (2), after median sternotomy (3), after removing the anterior rib cage up to rib stumps (4), after removing right sixth, seventh, and eighth rib head (5), as well as after removing all rib heads (6).

Fig 2. The experimental test setup.

Pure moments of 2 Nm were applied in flexion/extension, lateral bending, and axial rotation using a custom-built spine tester [18]. Motion analysis was performed using an optical motion tracking system with six cameras.

Fig 3. The process of motion analysis.

Optical markers were transferred into a point cloud. Relative motions were determined by manual labeling.

Fig 4. The ROM of the thoracic spine (T1-12, n = 6) for 2 Nm pure bending moment.

The ROM is depicted as mean with standard deviation for each the intact condition with entire rib cage and the condition without entire rib cage in all loading directions.