The Intricacies Of Our Thoracic Spine

Vertebral body of thoracic spine

Since my accident, I've been researching and reading so I know how to best treat the thoracic spine when it's someone else who's hurt their thoracic spine and ribcage. So this week's post is to share what I've been reading up on the anatomy, structure and function of our thoracic spine.  Also by writing it here, I can always send this link to patients who want to know more. 

Cancellous bone

The vertebral body is not a solid block of bone. In fact, it is just a shell of dense cortical bone (outer surface of bone) surrounding cancellous bone (pictured above). This shell is not strong enough for lengthwise compression and it collapses like a cardboard box with too heavy loads (see picture below).

It is reinforced by vertical struts between the top and bottom surfaces. A strut acts like a solid, but narrow block of bone. As long as it is kept straight, it can sustain immense longitudinal loads.

Vertical struts straining under load (b)

However, struts tend to bend or bow when longitudinal forces are too strong, although a box with vertical struts is still stronger than an empty box.

Stronger with horizontal cross-beams

When cross-beams that are connected are introduced to the struts, the strength of that box is further enhanced. So, when a load is applied, this cross-beams hold the struts in place to prevent them from deforming and preventing the box from collapsing. 

Our vertebral bodies follow this internal architecture described above. The struts and cross-beams are formed by thin rods of bone called vertical (VT) and transverse trabeculae (TT). This trabeculae provides weight bearing strength and resilience to the vertebral body.

Any load applied to the vertebral body is first borne by the vertical trabeculae. When the load is too much, the horizontal trabeculae picks up the slack. Hence the load is sustained by a combination of vertical pressure and transverse tension in the trabeculae.

The advantage of this design (when it is not a solid block of bone) is that a strong but lightweight load bearing structure is constructed with minimal use of bone.

Another benefit is that the space between the trabeculae is used as channels for the blood supply and venous drainage for the vertebral body. Under some conditions, it allows for haemopoiesis (making new red blood cells) and this helps with transmitting load and absorbing force.

So how did my T4 fracture when it is supposed to be strong and resilient? Well, it wasn't just a simple fall, I was rear ended by a motorbike.

References

Bogduk N and Twomey LT (1987). Clinical Anatomy Of The Lumbar Spine. Longman Group UK.

Oliver J and Middleditch A (1991). Functional Anatomy Of The Spine. Butterworth-Heinemann Ltd.