Editor's note: Dr. Upledger has asked guest author Lisa Johnson Zee to share her thoughts on cranial bone movement in this month's column.

In anatomy and physiology, I learned that cranial bones fuse in early adulthood or childhood.¹ Gray's Anatomy supports the theory that the sutures grow together, creating a solid mass of bone called the calvarium. The fused skull functions as a helmet in which volume or pressure changes in blood, cerebrospinal fluid (CSF) or brain tissue cause corresponding pressure changes in other systems to prevent an increase of intracranial pressure.

However, there is a sizable body of literature that documents a small, rhythmic movement of the cranial bones. The bulk of these studies come from the cranial osteopathy medical field. The following is a synopsis of some of these studies.

Tettambel used force transducers to measure movement between the frontal bone and bilateral mastoid processes of the temporal bone in 30 subjects.² She recorded three rhythms including the cardiac and respiratory rhythms. She hypothesized that the third pulse, which averaged eight cycles per minute, was the craniosacral rhythm.

Frymann studied the rhythmic changes in the circumference of the head using a U-shaped frame with a differential transducer.³ Changes in the diameter of the skull were measured by the displacement of metal rods. This study is unique because it measured movement in live human subjects. Frymann found a pulsating rhythm between six and eight cycles per minute separate from cardiac and respiratory rates. The amount of displacement was measured between 10 and 30 microns.

Another study by Adams, et al., looked at parietal bone mobility in cats.⁴ These researchers fastened strain gauges to feline parietal bones to measure movement when injections of artificial CSF were given. The bones moved significantly, varying from 17 to 70 microns. External lateral head compression caused a measurable widening of the sagittal suture with an inward rotation of the parietal bones.

Researchers at the University of Michigan College of Osteopathic Medicine have looked at cranial bone mobility in adult primates.⁵ Michael and Retzlaff used a direct screw attachment on the right parietal bone and measured movement with a pressure transducer. They also measured blood pressure, heart rate and respiration rate. The parietal bones moved spontaneously in two distinct rhythms, one corresponding to the respiration rate and a second, slower rate of five to seven cycles per minute.

These four studies indicate cranial bones may show a slow, steady, cyclical movement. A relatively new theory for Western medical science, it represents a dramatic shift. Bringing controversial ideas into the status quo of scientific thought is not easy, but the body of literature supporting cranial movement is growing. Although inconclusive, it deserves to be approached with an open mind.

In CranioSacral Therapy (CST), the rhythm of CSF can be palpated at all parts of the body due to the passive action of fascial connective tissue. The rhythm occurs in two distinct phases: flexion (outward movement) and extension (inward movement). In physical therapy terms, flexion is a decreasing measurement of degrees in the angle of the joint. The sphenobasilar joint is where the posterior sphenoid articulates with a ridge on the occipital bone.

When Dr. William Garner Sutherland, the "father of osteopathy," palpated the movement of these bones, he noticed this joint does indeed flex or reduce angle size on the inferior side. The flexion of this angle is accompanied by subtle outward movement in the body, which Sutherland called flexion. Therefore, in CST, the cranium, along with the rest of the body, is in flexion when it widens and in extension when it narrows.

Anatomy of Suture Closure

To discover more about cranial bone motion, let's examine the nature of cranial sutures. If the sutures remain flexible throughout adulthood, some degree of motion is possible when driven by pressure changes in the craniosacral system. If the tissues fuse and become immobile, rhythmic motion is unlikely.

Several studies have examined the nature of the cranial sutures. Retzlaff, et al.,used light and scanning microscopy to examine tissue samples of adult primate sutures.⁶ They found connective tissue, blood vessels and nerve fibers present in the sutural space. They described a five-layered pattern of fibers and cells containing collagenous bundles. Tissue was reported to be arranged in a wavy pattern. The researchers hypothesized the purpose of the tissue might be to control the elongation of the collagen bundles. They reported no evidence of fusion in the adult primate sutures.

In a separate study, Upledger and Retzlaff examined the sagittal suture in primate skulls.⁷ They found not only connective tissue, but also a vascular network and neuronal plexuses and receptors in sutural tissue. In one specimen, they were able to trace a single dendrite through the dural membrane into the brain, terminating in the third ventricle containing CSF. Further study of this neural tract may bring answers to how the homeostatic feedback mechanism in the brain's CSF hydraulic system functions.

In the 1920s, Todd and Lyon published two articles examining a timeline of sutural closure in the male human skull.⁸ These researchers hypothesized that cranial sutures fuse at some point in the human lifetime. They started with 427 specimens, but rejected 81 due to abnormal suture closure or "delayed union." Furthermore, some of the skulls were termed lapsed union, which meant failure of the suture to close due to a concentration of bone along the edge of the articulatory surface. For reasons unclear, they counted these skulls as fused, which biased results toward earlier suture closure. The data they found is as follows:

• Sagittal suture closed at 31 years.
• Coronal suture closed at 38 years.
• Lambdoidal sutures closed at 47 years.
• Masto-occipital closed at 70-80 years.
• Masto-parietal closed at 70-80 years.
• Spheno-temporal rarely closed.

The authors concluded that the sutures tend to close along this timeline. However, there is a high degree of variability reported. This study also was conducted some 80 years ago. Standards of protocol in scientific research have changed.

Researchers have studied one suture in-depth using different human specimens. Kokich examined one suture in the facial area - the frontozygomatic suture.⁹ Of his 61 specimens, he found none demonstrated closure until after age 80, and some weren't completely fused even after age 90. He noted that bony interdigitations formed along the suture with advancing age, but did not affect the patency of sutural movement. Kokich, like Retzlaff and Upledger, found clear evidence of collagen fibers within the suture. He stated that frontozygomatic suture remains a functioning "articulation" until late in life.

A conclusive statement about whether and when sutural fusion occurs cannot be made from existing research.¹⁰ Clearly the subject remains open for debate. Having palpated the craniosacral rhythm with my own hands, I believe cranial sutures maintain flexibility that might best be called articulation. This flexibility allows the bones to move passively as they are driven by the craniosacral system.

References

1. Zemlin W. Speech and Hearing Science: Anatomy and Physiology. Englewood Cliffs, NJ: Prentice-Hall, 1988.
2. Tettembel, et al. Recording of cranial rhythm impulse. Journal of the American Osteopathic Association, 1978;78:149.
3. Frymann V. A study of the rhythmic motions of the living cranium. Journal of the American Osteopathic Association, 1971;70(9):928-45.
4. Adams T, Heisey R, Smith M, Briner B. Parietal bone mobility in the anesthetized cat. Journal of the American Osteopathic Association, 1992;92:599-622.
5. Michal D, Relzlaff E. A preliminary study of cranial bone movement in the squirrel monkey. Journal of the American Osteopathic Association, 1975;74:866-69.
6. Retzlaff E, Michael D, Roppel R, Mitchell F. The structure of cranial bone sutures. Journal of the American Osteopathic Association, 1976;75:106-7.
7. Upledger J. CranioSacral Therap. I. Study Guide, Palm Beach Gardens, FL.: The Upledger Institute, 1987.
8. Todd T, Lyon D. Endocranial suture closure. American Journal of Physical Anthropology, 1924;7:325-84.
9. Kovich V. Age changes in the human frontozygomatic suture from 20 to 95 years. American Journal of Orthodontics, 1976;69:411-30.
10. Rogers J, Witt P. The controversy of cranial bone motion. Journal of Orthopedic Sports Physical Therapy, 1997;26(2):95-103.