The absence of an intervertebral disc between the first cervical vertebra (C1) and the second cervical vertebra (C2) is primarily due to the unique structure and function of these two vertebrae, which are known as the atlas and axis, respectively.
The atlas (C1) is responsible for supporting the skull and allows for the nodding motion of the head, while the axis (C2) has a projection called the odontoid process or dens that acts as a pivot point. This specialized articulation allows for a greater range of motion in the neck, particularly rotation.
Having an intervertebral disc between C1 and C2 would hinder this range of motion due to the way discs function in the spine. Discs serve as shock absorbers and provide cushioning between vertebrae, but in the case of C1 and C2, their anatomical design promotes stability and allows for the unique rotational movement without the additional complexity of a disc. The absence of a disc here also reduces potential friction and improves the efficiency of movement in this highly mobile region of the spine.
In summary, the lack of an intervertebral disc between C1 and C2 is a result of their specific roles and the need for a high degree of flexibility in the cervical region of the spine, which is achieved through their unique bony structure instead of a disc.