Cranial Syndesmoses
Syndesmosis is a type of joint where bones are connected by fibrous connective tissue – ligaments and membranes. In the current anatomical classification, the following ligaments are considered syndesmoses of the skull (syndesmoses cranii):
- ‣ Stylohyoid ligament
- ‣ Pterygospinous ligament
- ‣ Sphenomandibular ligament
- ‣ Stylomandibular ligament
- ‣ Auricular ligaments
In addition to the mentioned ligaments, there are several other syndesmoses of the skull, among which the pterygoalar ligament1 should be mentionned. This ligament is located in close proximity to the pterygospinous and is found in 50-60% of people2,3. Due to the close location and variable anatomy, clear differentiation of these ligaments can be challenging. Perhaps this is why the term "pterygoalar ligament" has not yet been introduced into the official anatomical nomenclature. Nevertheless, considering the clinical significance of these syndesmoses, the ligamentum pterygoalare has been included in the Anatomical Standard model and is discussed below.
It should be noted that ligaments attached to the (occipital bone are not classified as syndesmoses of the skull, as the occipital bone is functionally considered part of the vertebral column (C0). Therefore, the corresponding syndesmoses are classified as ligaments and membranes of the spine.
Clicking on the image to hide all the skull bones except the temporal bones, which are directly linked with the stylohyoid ligaments.
As the name suggests, the stylohyoid ligament connects the styloid process of the temporal bone (processus styloideus) with the lesser horn of the hyoid bone (os hyoideum). All the aforementioned anatomical structures develop from the cartilage of the second pharyngeal (visceral) arch during embryogenesis1. After birth, the cartilaginous structure transforms into the stylohyoid ligament, with ossification beginning at its ends, gradually elongating the styloid process on one side and forming part of the hyoid bone on the other. This ossification continues throughout life, and in extreme cases, can form a solid bony structure connecting the skull to the hyoid bone2. In some cases, elongation of the styloid process can cause compression of nearby blood vessels and nerves, leading to the so-called Eagle syndrome (named after the physician who first described the syndrome)3.
The list of terms
| Lig. stylohyoideum | – | Stylohyoid ligament |
| Processus styloideus | – | Styloid process |
| Cornu minus ossis hyoidei | – | Lesser horn of the hyoid bone |
Click on the image to hide all the skull bones except the sphenoid bone, which is directly linked with the mentioned ligaments.
The location of these ligaments relative to the foramen ovale has significant clinical importance. The mandibular nerve passes through the foramen ovale and, branching out, comes into contact with the pterygospinous and pterygoalar ligament, passing either between the ligaments or through openings formed by the ligaments and skull1. Both of these ligaments have a tendency to ossify, forming solid bony bars in 2–6% of cases2,3. This can lead to gradual compression of the branches of the trigeminal nerve, resulting in disorders such as paralysis of the masticatory muscles, as well as loss of taste and numbness in the tongue and cheek4,5. Additionally, ossified ligaments near the foramen ovale can complicate needle access to the mandibular nerve during the treatment of trigeminal neuralgia3.
The list of tems
| Lig. pterygospinale | – | Pterygospinous ligament |
| Lamina lateralis processus pterygoidei | – | Lateral plate of the pterygoid process |
| Spina ossis sphenoidei | – | Spine of sphenoid bone |
| Lig. pterygoalare | ≈ | Pterygoalar ligament |
| Lamina lateralis processus pterygoidei | – | Lateral plate of the pterygoid process |
| Ala major ossis sphenoidei | – | Greater wing of the sphenoid bone |
Click on the image to hide all the skull bones except the temporal and sphenoid bones, which are directly linked to the stylomandibular and sphenomandibular ligaments.
The stylomandibular and sphenomandibular ligaments limit the range of motion of the temporomandibular joint1 and can be damaged by forceful movements or trauma to the lower jaw. The chronic pain caused by this damage is known as Ernest syndrome2,3.
The list of terms
| Lig. sphenomandibulare | – | Sphenomandibular ligament |
| Spina ossis sphenoidei | – | Spine of sphenoid bone |
| Ramus mandibulae | – | Ramus of mandible |
| Lingula mandibulae | – | Lingula of mandible |
| Lig. stylomandibulare | – | Stylomandiobular ligament |
| Processus styloideus (ossis temporalis) | – | Styloid process of the temporal bone |
| Angulus mandibulae | – | Angle of mandible |
Click on the image to move the lower jaw from the central occlusion to the position of maximal depression
Both ligaments – the stylomandibular and sphenomandibular – limit the protrusion and displacement of the mandibular condyles1 i.e. are functionally associated with movements of the lower jaw. For this reason, they are often referred as the "extracapsular ligaments" of the temporomandibular joint2.
The stylomandibular and sphenomandibular ligaments separate the pterygoid muscles, allowing them to move independently and supporting their function. Another role of these ligaments is to prevent neuromuscular conflicts. For example, the sphenomandibular ligament, by pulling the medial pterygoid muscle away from the mandible, prevents the compression of the inferior alveolar nerve as it passes toward the mandibular foramen1,2.
Last update: 22.10.2024