Sutures are the fine, irregular lines of junction between articulating cranial bones. The bones of the skull originate through intramembranous or endochondral bone formation. Ossification gradually progresses until only the suture lines with their thin layer of interposing fibrous tissue remain. Fontanels or “soft spots” (membranous areas) in an infant represent areas where ossification of the cranial bones has not yet occurred.
Throughout adult life the sutures gradually undergo closure and are bridged by bone union. The rates of closure are fairly constant in time of occurrence and sequence, though the age ranges are broad. Still, the determination of suture closure is one of the basic methods of assessing skeletal age in adult specimens. There are many regionally defined sutures in the skull, as follows:
- Coronal suture, runs laterally across the top of the skull separating the frontal and parietal bones.
- Sagittal suture, runs longitudinally across the skull from the occipital to the frontal bone and separates the parietal bones from one another.
- Lambdoid suture, is an inverted “V” in shape and separates the occipital bone from the parietals. This suture terminates laterally on each side at the temporal bone.
- Squamosal sutures, roughly semicircular in configuration and separate the parietal bones from the superior portion of the temporal bones. These sutures extend from the sphenoid bone anteriorly to the supra-mastoid crest posteriorly.
- Parieto-mastoid sutures, continuous posteriorly with the squamosal suture, separating the parietal bone from the mastoid region of the temporal bone.
- Occipito-mastoid sutures, separate the occipital bones from the mastoid regions of the temporal bones.
- Spheno-temporal sutures, separate the sphenoid and temporal bones.
- Spheno-occipital suture (also called the basilar suture), separates the sphenoid and occipital bones.
- Spheno-parietal sutures, separates the sphenoid and parietal bones.
- Spheno-frontal sutures, separates the sphenoid and frontal bones.
- Fronto-nasal suture, separates the frontal and nasal bones.
- Internasal suture, separates the two nasal bones from one another.
- Fronto-zygomatic suture, separates the frontal and zygomatic bones.
Non-Metric Traits of the Skull
Apical bone - Accessory bone located at the intersection of the sagittal and lambdoidal sutures (lambda).
Asterionic bone - Accessory bone located at the intersections of the lambdoidal and squamosal sutures.
Auditory exostosis - Bony nodule located in the external auditory meatus. Note if the nodule occludes \(1 / 4\), \(1 / 2\), \(3 / 4\), or the entire meatus.
Bipartite occipital condyles - Division of the occipital condyles in the area of fusion between the basilar aspect of the occipital and the squamous portion of the occipital.
Bregma bone Accessory bone located at the intersection of the coronal and sagittal sutures (bregma).
Condylar canal - Foramen or canal located posterior to the occipital condyles. note if the canal is complete or partial. To measure completeness, use a thin pipe cleaner and gently try to insert it into the canal.
Divided hypoglossal canal - Foramen or canal located at the anterior end of the occipital condyles (beneath the condyles). Note if the canal is divided into two canals; there may also be a partial division. When noting division, observe if the bony spicule is within the canal or on the lateral or medial aspects.
Epiteric bone - Accessory bone located at the intersection of the frontal, parietal, sphenoid, and temporal bones.
Foramen ovale - Located on the inferior aspect of each of the greater wings; they are the only holes (foramina) of the sphenoid that are oval in shape.
Foramen rotundum - Most anterior and medial of the sphenoidal foramina; circular holes in each of the cranial fossae of the greater wings.
Foramen spinosum - Located on the inferior aspect of each of the greater wings, these foramina are the most lateral foramina on the sphenoid and generally the smallest of the circular holes in the sphenoid.
Frontal grooves - Supraorbital, shallow grooves which are tracks for vessels and nerves. May be seen running into supraorbital notch/foramen. May or may not be present.
Inca bone (bipartite, tripartite) - A transverse suture divided the squamous portion of the occipital, creating an accessory bone. The Inca bone is seen at a higher frequency among South American populations. the Inca bone may be singular, bipartite or tripartite. If bipartite an vertical suture will separate the bone into two pieces, if tripartite two vertical sutures will separate the bone into three pieces.
infraorbital foramen - Additional foramen located medially, inferior to the lower margin of the orbit on the maxillary bone. One is always present.
Infraorbital suture - Accessory suture located medially, inferior to the lower margin of the orbit on the maxiallary bone Generally runs between the margin of the orbit and the infraorbital foramen. May be either complete or partial.
Inion spike - Ridge or inferiorly projecting hook of bone in the region of the external occipital protuberance; most often seen in males.
Marginal tubercle - A protuberance in the region of the masseteric muscle attachment on the inferior margin of the zygomas.
Mastoid foramen - Single or multiple foramina located within the occipitomastoid suture, or near the suture on either the temporal or occipital. Number and location should always be noted during observation.
Maxillary torus - Bulging protuberance located on the lingual margins of the alveoli near the maxillary molars. Generally, maxillary tori are associated with culture groups that use their teeth as tools.
Metopic suture - Divides the frontal bone, located at midline. Generally closes by eight years of age. If present, it should be scored as complete or partial. (Mann and Hunt 2005)
Os Japonicum - Additional suture dividing each zygomatic into two pieces.
Ossicles or wormian bones - Aberrant growth patterns may be manifested by sutural complexities represented by small islands of bone.
Pacchionian pits - Pits with sharply defined margins located on the frontal and parietals, vary in size from small to large. (Mann and Hunt 2005)
Palatine torus - Bulging protuberance located on the along the lingual aspect of the palatine suture. Generally, a palatine torus is associated with culture groups that use their teeth as tools.
Parietal foramen - Single foramen located on the posterior aspect of the parietal along the saggital suture near obelion. Each parietal may display a foramen, although the foramen may be either absent or within the suture itself. Very large or misshapen parietal foramen are sometimes observed, and should be recorded.
Parietal notch bone - Extrasultural bone located in the squamosal suture; anterior to asterion. May be unilateral, bilateral, or absent.
Pharyngeal tubercle/fossa - Round depression or smooth projection located in the center of the basilar aspect of the occipital, on the extocranial surface.
Pterygo-alar bridge/spur - Bridge or spicule of bone originating either on the lateral pterygoid lamina or on the lateral aspect of the foramen ovale.
Pterygo-spinous bridge/spur - Bridge or spicule of bone origination either on the lateral pterygoid lamina or on the medial aspect of the foramen ovale.
Supraorbital foramen - Complete foramina located along the superior margins of the orbits (anterior frontal). Completeness can be measured by passing a thin pipe cleaner through the foramina. Number should be observed for each orbit.
Supraorbital notch - Notches located along the superior margins of the orbits (anterior frontal). Number should be observed for each orbit.
Trochlear spine - Small, sharply-curved bony projection within the orbit; located on the medial aspect of the frontal within the orbit. May also be represented by a notch without the bony projection. Trait may be unilateral, bilateral, or absent (trait is rare). (Mann and Hunt 2005)
Tympanic dehiscence - Hole perforating the tympanic plate or the temporal bone; located on the inferior aspect of the external auditory meatus. Size should be observed if present. May be bilateral, although presence of the trait is rare. (Mann and Hunt 2005)
Zygomatico-facial foramen - Single or multiple foramen located ont he convex surface of the zygomatics, inferior to the orbital margin. This trait may also be absent; number and size (large or small) should be noted during observation.
Mandibular torus - Bulging protuberance located on the lingual margins of the alveoli near the mandibular molars. Generally, mandibular tori are associated with culture groups that use their teeth as tools
Mental Foramen - Foramina located on each side of the labial aspect of the mandible inferior to the secon premolar. Usually singular, but may be multiple.
Mylohyoid bridge - Bony bridge crossing the mylohyoid sulcus; may be partial or complete.
Articulations of the Cranial Bones with One Another
Vomer, Ethmoid, Frontal, Occipital, Parietal, temporal, Zygomatic, and Palatine
Parietal, Temporal, Sphenoid, and Atlas
Sphenoid, Parietal, Ethmoid, Lacrimal, Nasal, Zygomatic, and Maxilla
Occipital, Frontal, Temporal, Sphenoid, and Parietal
Fronatl, Ethmoid, Inferior nasal concha, and Maxilla
Frontal, Ethmoid, Zygomatic, Inferior nasal concha, Vomer, Lacrimal, Maxilla, Nasal, Palatine, and Mandible
Temporal and Maxilla
Frontal, Sphenoid, Maxilla, and Temporal
Occipital, Sphenoid, Parietal, Zygomatic, and Mandible
Sphenoid, Ethmoid, Palatines, and Maxilla
Sphenoid, Frontal, Maxilla, Palatines, Vomer, Inferior nasal concha, and Lacrimal
Inferior Nasal Concha
Ethmoid, Lacrimal, Maxilla, and Palatine
Nasal, Maxilla, and Frontal
Sphenoid, Ethmoid, Maxilla, Inferior nasal concha, Vomer, and Platine