with hypoplastic first ribs, do the second ribs take their place--joining the manubrium and providing attachment for the scalene muscles?

Question

When someone has hypoplastic first ribs, do the second ribs take their place attaching at the manubrium? What about the scalene muscles? Do they cram together on the shortened first rib, or do they attach at the second rib instead?

Answer 1

Although you asked this question quite awhile ago, I will provide an answer in hopes you or others could benefit from the information.

Hypoplastic First Ribs:

AKA rudimentary rib/s or underdeveloped rib/s. A hypoplastic first rib is a congenital anomaly that may or may not have clinical implications. There will be no distal attachment to the manubrium by the first rib. The second rib will articulate with the manubrium at its typical location despite no first rib articulation. A hypoplastic first rib can be bilateral, unilateral, and vary in its presentation. It can appear shortened to varying degrees and may have a psuedoarticulation or even accessory joint with the lateral-medial second rib.

The first rib has 4 different muscles that attach: scalenus anterior, scalenus medius, subclavius, and intercostal muscles. All of these muscles are accessory breathing muscles (meaning if removed or absent breathing would continue unhindered). The two scalenes that insert also assist in movements of the neck, (because they assist and are not primary movers, they can also be removed or absent without dysfunction). In the case of first rib hypoplasia, scalenus anterior and medius will still insert on the shortened version of the first rib, however the complex anatomical space (involving passage of nerves and blood vessels) will be effected (likely compressed due to reduced space) and lend to much higher risk of Thoracic Outlet Syndrome, TOS.

There are more details that can be discussed regarding this congenital anomaly and its implications for neurogenic and/or vascular TOS. In the case of TOS caused by abnormal anatomy, surgical intervention is typically indicated depending on the patient's presentation.