Biomechanics of the Thorax: A Clinical Model of in Vivo Function”
The Journal of Manual and Manipulative Therapy, 1983; Vol 1 No 1 (13-21)
Restrictions to motion of the thoracic spine are common. Due to how often people are sitting throughout their day the thoracic spine becomes kyphotic and compressed. Thoracic spine dysfunction is commonly sited as a problem in the health field and addressed in some capacity. It is common practice to work on gross movement patterns of extension and rotation to restore thoracic mobility. These changes can often be short lived with repetitive poor posture and global movement patterns not addressing local dysfunctions. Specifically practitioners have a poor understanding of the biomechanics of the thorax. Without a working model a clinician is limited to unreliable signs and symptoms for direction of treatment planning. Getting into detailed biomechanics of the thoracic spine can be overwhelming and the easy way out is to treat grossly.
Diane Lee wrote a great article in the Journal of Manipulative Therapy in 1983 addressing thorax biomechanics. The purpose of this article was to provide a model of in vivo biomechanics of the thorax which has been used clinically to form a basis of assessment and treatment of mechanical dysfunction of both the spinal and costal joints. First it is important to functional unit of the thoracic spine. It involves a superior vertebra, inferior vertebra, a facet joint, a demi facet joint for the head of the rib on the superior and inferior vertebrae, and the rib itself. Each functional unit has 6 degrees of freedom: 3 degrees of translation (Ant/Post, Medial/Lateral, superior/inferior) and 3 degrees of rotation (coronal rotation-flexion/extension, sagittal trunk side bend, axial rotation-left and right trunk rotation).
Thoracic flexion involves a coupling of several motions. It involves a sagittal plane rotation, anterior translation of the vertebrae, and anterior rotation of the rib. The rib anterior rotation is associated with a superior glide at the costovertebral joint. If the thoracic spine is limited in flexion (which is less common) one must assess and treat rib rotation, PA mobility of the thoracic spine, and rotation of the spinal unit.
Thoracic extension involves a coupling of sagittal plane posterior rotation, posterior translation of the vertebrae, and posterior rotation of the rib. As a clinician, if the rib is stuck into internal torsion (anterior rotation) it will present deeper to the table in prone. This dysfunction would be limited in external torsion (posterior rotation) and limit thoracic extension. If you did repetitive thoracic extension on a foam roll or with press ups you would never address the rib dysfunction and not be able to restore full thoracic extension. When palpating ribs assess for elevation/depression of the rib as well as external torsion (sharp corner to the rib) or internal torsion (dull rib). This will help dictate how the rib is influencing the thoracic spine.
With lateral bending or side bending there is a lateral rotation to the direction of the side bend of the vertebrae (ipsilateral side of vertebrae goes inferior) coupled with a contralateral rotation of the motion segment (Y axis) could with a spinal unit translation to the ipsilateral side of the side bend (X axis). In other words, side bend occurs with opposite rotation of the spine and a gross translation towards the side bend. When assessing side bending, assess how well the contralateral ribs gap, ipsilateral ribs approximate, the vertebrae glides towards the direction of bending, and contralateral rotation of the segment to determine what is the cause of the movement dysfunction.
The article goes into much more details about thoracic rotation and the difference in movement between the upper and lower thoracic spine. The purpose of this article review was to bring to light the complexity of the thoracic and that often persistent deficits in mobility are related to the interplay between rib, facet, demi-facet, vertebra, and soft tissue.