Mechanical coupling between the ribs and sternum in the dog

Area of Science:

• Anatomy
• Physiology
• Biomechanics

Background:

• The rib cage's biomechanical behavior during respiration is complex.
• Understanding the differential movement of ribs relative to the sternum is crucial for respiratory mechanics.
• Previous studies have not fully elucidated the coordinated axial displacements of the sternum and ribs.

Purpose of the Study:

• To quantify the axial displacements of the sternum and specific ribs during various respiratory maneuvers.
• To investigate the influence of the diaphragm and accessory muscles on rib cage kinematics.
• To determine if upper and lower ribs exhibit distinct movement patterns relative to the sternum.

Main Methods:

• Measurements of sternal and rib axial displacements using linear displacement transducers in anesthetized dogs.
• Experiments included passive breathing, quiet breathing, static inspiratory efforts, and accessory muscle stimulation.
• Procedures were conducted before and after bilateral phrenicotomy to isolate diaphragmatic contribution.

Main Results:

• Quiet inspiration showed sternal caudal displacement and seventh rib cranial displacement; the second rib showed variable or caudal displacement.
• During static inspiratory efforts, the second rib moved caudally with the sternum, while the seventh rib moved cranially, especially post-phrenicotomy.
• Accessory muscle stimulation resulted in disproportionately greater caudal displacement of the second rib compared to the seventh rib.

Conclusions:

• The upper ribs are more rigidly coupled to the sternum than the lower ribs.
• Increasing costal cartilage length inferiorly contributes to differential rib-sternum kinematics.
• The upper rib cage acts more cohesively with the sternum compared to the lower rib cage during respiration.