Infant Cranium 101 – for parents who like to dork out ♥

The infant skull (or cranium) is a fascinating structure.  It goes through rapid growth in the first year of life, yet maintains many unique features that serve special functions while simultaneously allowing room for the expanding brain. As with all childhood development, the more we understand about each stage and its needs, the better we are as parents and practitioners at meeting those needs and creating an environment that helps our kids to grow at their best. 

6 Cool Facts About the Infant Cranium!

1. An infant’s head is about 1/4 the size of their body. (An adult head is 1/8 the size of their body. 
2. Babies have 6 fontanelles. The most prominent and the one that lasts the longest (between 12-24 months) is the anterior fontanelle (the one on the top of the head). Fontanelles are neat because we can monitor hydration status and intra-cranial pressure if they are sunken or bulging. 
3. Sutures and fontanelles are not open “gaps” between the cranial bones. In an infant they are fibrous, membranous soft tissue structures. In an adult, they become interlinked puzzle pieces.
4. Then infant cranium is not very bony. It takes about 30 years for the adult skull to fully form, although sutures gradually fuse and our skull becomes more rigid throughout our entire lifetime.
5. The sphenobasilar synchondrosis is a joint at the base of the skull that remains cartilaginous (not bony) until 5 years of age and does not fully close until 18-25 years old. This is often an important area of focus during craniosacral therapy.
6. The cranial nerves all exit the cranium (hence their name…).  These specialized nerves serve important functions that control things like sucking and tongue movement and digestive secretions, which are all important for infant feeding success!

6 Differences Between Adult and Infant Cranium

1. The ratio between face and back of the skull is different in adults and babies. The back of a baby’s head is relatively large and their face is kind of squished in the front (at about an 8-9 to 1 ratio). This ratio changes over the years until the adult face takes up a considerably large proportion of the skull (now at a 2 to 1 ratio). 
2. If you happen to be familiar with the adult cranium, then it is of note that there are a few key landmarks that are missing in infants. There is no styloid process and no mastoid process.  These develop over time as muscles pull on attachment points.  The lack of a mastoid process makes the facial nerve more vulnerable to trauma, particularly during delivery. 
3. The infant’s ear canal is also very “underdeveloped” compared to the adult and at a much more horizontal plane.  This is one of the reason why kids get ear infections easier than adults; the “plumbing” is less slanted and more prone to congestion.
4. An adult skull has 22 bones. An infant technically has the same number of named bones, but depending on stage of development, many of those bones may function as 2 or more separate pieces.  Some of these separate pieces will eventually ossify together completely to form one bone, whereas others will remain divided by adult sutures. For example, the frontal bone is two pieces separated by the metopic suture in infants. The metopic suture fuses completely to form one solid bone in over 90% of the adult population.
5. The growth of the neurocranium (basically everything but the face) and the growth of the brain are inter-related. They influence each other. The brain and the skull grow together.
6. The base of the infant skull is a harder type of premature-bone than the top of skull. The base has a cartilaginous foundation that solidifies earlier. This is more stable and stiff, which allows for growth but keeps the exiting cranial nerves more well protected. The top (or vault) of the skull goes through intramembranous ossification. This means it kind of skips the cartilaginous phase of bone formation, which is really great for allowing more adaptability to both compressive (think birth) and expansive (think growth) forces; unfortunately, it also means that the cranial vault is a lot more malleable and easy to mould by both outside forces and internal tension patterns (think flat spots and odd shapes).  

The Goal: Not too big, not too little, relatively symmetrical.

Most people are aware of “flat spots” that may develop on the back of an infants head.  Both the size and the shape of an infant’s cranium are important health metrics. 

Size does matter when it comes to the growing cranium. The skull is measured routinely in infants to make sure it is staying “on its curve”, just like weight and length.  We do not want it to be too little (microcephaly) or too big (macrocephaly) as either may be indicators of more serious health concerns.  

Curious as to where your baby’s head is on the curve? Check out the WHO head circumference growth charts. (Keep in mind that there are boy charts and girl chart and that there are specific landmarks on where to measure.)

We also strive for relative symmetry within the cranial structures.  (FYI, anything less than 5 degrees of difference is considered relatively symmetrical. Nobody is perfect.) There are two different conditions which may cause considerable asymmetries in an infants cranium.  The first is very uncommon but very serious. It is called craniosynostosis and it is an early fusion of one or several sutures. The second is very common and may range from mild to severe. This is Non-Synostotic Deformational Plagiocephaly, or more commonly known as “plagio”.  There are different patterns that these head shape aberrations may present (most common are scaphocephaly, brachycephaly, and plagiocephaly).  The important thing to note here is that early identification and early intervention make a very big difference in outcomes. 

As I mentioned above, the base of the skull is a harder type of bone and it solidifies earlier than the top. There is a lot happening here in the first 6 months of life, so it is very important to load this area properly during its growth over that time. Movement asymmetries (like baby turning their head to the right more than to the left), positional preferences (like torticollis), tissue restrictions, and delays in gross motor development all impact the ways that we LOAD these structures.  This is why torticollis and cranial asymmetries and motor delays often go together. It is also the reason why adequate and early tummy time is an important early start for both structural and motor development. 

Do I need to do something about this?

A thorough evaluation of the cranium including measurements is an important part of a complete infant musculoskeletal exam. Because of all of the neat tidbits of information that I shared above, it serves as a crucial health screener. Deviations from normal may be indicators of more serious health concerns that require referral to medical practitioners or an orthotist.  More often, they remain within conservative musculoskeletal management through your paediatric physiotherapist, chiropractor, or occupational therapist. 

The important take-away message here for parents is to address anything that seems out of the ordinary sooner rather than later.  There is some research to support early musculoskeletal assessment done as screeners so that problems are caught and treated sooner.  In our office, both Dr Amanda Stevens (myself) & Dr Anna Marie Gierach are trained and experienced in infant musculoskeletal care including the incredible cranium.   

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The infant cranium is an amazing feat of structure and function commingling and developing simultaneously.  I hope you enjoyed the dork-fest and learned a few new latin words.  😉 

References & Resources

• Ellwood J, Draper-Rodi J, Carnes D. The effectiveness and safety of conservative interventions for positional plagiocephaly and congenital muscular torticollis: a synthesis of systematic reviews and guidance. Chiropractic & manual therapies. 2020 Dec;28(1):1-1.
• Hegazy AA, Hegazy MA. Newborns’ Cranial Vault: Clinical Anatomy and Authors’ Perspective. International Journal of Human Anatomy. 2018 Jul 6;1(2):21.
• Lee K, Chung E, Lee BH. A comparison of outcomes of asymmetry in infants with congenital muscular torticollis according to age upon starting treatment. Journal of physical therapy science. 2017;29(3):543-7.
• Margulies SS, Thibault KL. Infant skull and suture properties: measurements and implications for mechanisms of pediatric brain injury. Journal of biomechanical engineering. 2000 Aug 1;122(4):364-71.
• Martini M, Klausing A, Lüchters G, Heim N, Messing-Jünger M. Head circumference-a useful single parameter for skull volume development in cranial growth analysis?. Head & face medicine. 2018 Dec;14(1):1-8.
• Miller J, Fontana M, Jernlås K, Olofsson H, Verwijst I. Risks and rewards of early musculoskeletal assessment: An evidence-based case report. British Journal of Midwifery. 2013 Oct;21(10):736-43.
• WHO Head Circumference Charts: https://www.who.int/tools/child-growth-standards/standards/head-circumference-for-age