The alar ligament is a crucial component of the upper cervical spine, playing a pivotal role in maintaining the stability of the craniocervical junction—the complex area where the skull meets the neck. Due to its specific function in controlling head movement, this paired ligament has a lesser-known alternative name that highlights its role: the "check ligament of the odontoid".
Anatomy of the alar ligament
Located in a critical area, the alar ligaments are a pair of short, strong fibrous cords that secure the head to the neck. Each ligament originates from the upper, outer side of the dens (the tooth-like projection of the C2 vertebra) and extends upward and outward to attach to the inner side of the occipital condyles at the base of the skull. They are typically described as tough, rounded cords.
The stabilizing function of the 'check ligament'
The "check ligament of the odontoid" name directly refers to its function as a limiter of motion. The alar ligaments prevent excessive rotation of the head. When the head turns, the opposite-side alar ligament tightens, acting as a brake to prevent over-rotation and maintain the correct alignment of the dens with the C1 vertebra. This function is essential for protecting the spinal cord from damage during extreme movements. They also limit side-bending of the head.
Clinical relevance and ligament injury
Given their role in preventing excessive motion, the alar ligaments are susceptible to injury during trauma such as whiplash or contact sports. An alar ligament injury can lead to craniocervical instability, characterized by excessive movement at the head-neck junction.
Common causes of alar ligament injury include:
- Motor vehicle accidents.
- Direct head and neck trauma.
- Conditions causing ligament laxity, like Ehlers-Danlos Syndrome.
- Aggressive cervical spine adjustments.
Symptoms can vary but often include chronic headaches, neck pain at the base of the skull that worsens with movement, dizziness, and a feeling of a "heavy head". Severe instability may irritate nerves and vessels, causing neurological symptoms. Diagnosis typically involves specialized imaging like dynamic MRI and physical examination.
Alar ligament vs. transverse ligament
Both alar and transverse ligaments are crucial for craniocervical junction stability but differ in function. The transverse ligament is a major part of the cruciform ligament complex and is considered the strongest craniocervical ligament, preventing forward displacement of C1 on C2. The alar ligaments also provide significant strength but primarily limit rotation and lateral flexion.
| Feature | Alar Ligament | Transverse Ligament |
|---|---|---|
| Function | Limits excessive axial rotation and contralateral lateral flexion of the head. | Prevents anterior displacement of the atlas (C1) relative to the axis (C2) during neck flexion. |
| Attachment | Connects the dens (C2) to the occipital condyles (skull). | Extends horizontally between the medial sides of the lateral masses of the atlas (C1). |
| Component of | Accessory ligaments of the atlantoaxial joint. | Part of the cruciform (cruciate) ligament complex. |
| Strength | Very strong, but historically considered to fail under less force than the transverse ligament in mechanical tests. | Considered the thickest and strongest of the craniocervical ligaments. |
Conclusion
Understanding what the alar ligament is also known as, the "check ligament of the odontoid," reveals its critical function. These strong cords limit head rotation, protecting the spinal cord. Injury to the alar ligament, often from trauma, can lead to craniocervical instability with chronic pain and neurological symptoms. Recognizing its role and location is vital for understanding spinal health. For more detailed anatomical information, authoritative resources are available.
