Cranial and Craniosacral
SUMMARY
The human cranium is composed of multiple bony plates connected by fibrous sutures that enable it to squeeze through the birth canal and then rebound. Later these sutures fill in with bone, but that "ossification" of the sutures does not completely immobilize them. Afterwards, they participate in an almost imperceptible pumping system by which the cranium expands and contracts very slightly in synchrony with breathing in order to circulate cerebrospinal fluid (CSF). This cranial circulation process is likely to require a smooth accommodation of cranial expansion and contraction by the cranial bones, and the field of cranial osteopathy long ago developed a sophisticated system to identify restrictions to the movement of the cranial bones and methods for using manual manipulation to release them. The movement of CSF also extends into the spinal column, where CSF flow is weaker and where the effectiveness of manipulation may rely more on restoring ranges of motion between the spinal segments than on removing restrictions to CSF circulation.
BACKGROUND
The cranium was considered a rigid shell with a genetically determined shape until high precision ultrasound in the late 20th century showed that it is actually a dynamic system that responds to many different types of stimuli. It changes shape depending on body posture and gravity; and it responds to pressures from coughing and muscle tonus. Inside the cranium, the brain is bathed in cerebrospinal fluid; which, like all tissues, needs circulation to remove waste products. In other parts of the body, waste products are flushed out by the venous and lymphatic drainage that gets stimulated by functional movements; because veins have one-way valves, and exercise operates them like a pump to help drain the capillary beds. However, the cranium is too rigid to employ movement to assist in the drainage of waste products; therefore, during sleep, the body employs the negative pressure produced in the chest during each inbreath to drain the waste products from cerebrospinal fluid (CSF). Each inbreath sucks about a half a drop of CSF out of the cranium, accompanied by contraction of the bony shell, which serves to clean out by-products from the neural metabolism and move them into circulation during sleep much like functional forces serve to clean out by-products from the rest of the body. In fact, this CSF drainage system works so much like a lymphatic system for the glial cells of the brain that it is widely known as the “glymphatic” system. After each CSF draining inhalation, during the outbreath, arterial pulsation provides the fast movement of small volumes of CSF that refills the cranium and allows the bony shell to rebound and expand.
CRANIAL OSTEOPATHY
The complex movements of the individual cranial bones during this expansion and contraction process were elegantly described by cranial osteopaths long before anyone understood that they were caused by breathing. Sutherland believed that cranial respiration was a primary process, - beginning in the brain and then moving through all the other tissues. Magoun believed that it was caused by spontaneous oscillations of the brain and spinal cord. Upledger believed that it was caused by a stretch reflex in the cranial sutures, which responded to each incident of cranial expansion due to production of CSF by reducing CSF production to maintain intracranial pressure equilibrium. These cranial osteopaths described in great detail the movements that each cranial bone should undergo in this cranial respiration process, and they prescribed a series of low force manual manipulations that could be used to restore movements that were restricted. Even if they misunderstood the source of the rhythm, their work to identify blockages to that rhythm and techniques for restoring it with light manual pressure may turn out to be extremely valuable as we develop better understanding of the importance of CSF drainage for prevention of neurodegenerative diseases like Alzheimers. In addition, combining osteopathic cranial work with an understanding of cranial hydrodynamics will lead to new tedcchniques, such as using an inversion or a trampoline to expand and contract the cranium.
Although we do not yet have technology sensitive enough to measure the movements of individual cranial bones, it's certainly reasonable to think that movement restrictions at the sutures connecting them could impair glymphatic drainage during sleep. We know that completely stopping the movement at a cranial suture is detrimental. In a condition called craniosynostosis, an early head injury or birth trauma causes a cranial suture to fuse, which impairs growth in all the surrounding cranial bones, alters head shape, and causes a number of symptoms. A partial or limited craniosynostosis could cause a less dramatic but still significant health impairment by restricting CSF drainage from portions of the cranium. The brain consistently turns out to be more sensitive than we realize, and it could be affected in many ways that we do not yet understand by even minute changes in the pressure of its housing.
One common source of partial or limited craniosynostosis between the two maxillary bones could be the rigidity of many upper nightguards and oral appliances. To prevent our oral appliances from restricting the natural range of movement between the two maxillary bones, we employ minimal acrylic crossing the midline, as shown below left. We also offer appliances that have no rigid materials crossing the midline but instead have the two sides connected only by loops of flexible braided stainless steel wire, as shown below right.
THE ROLE OF THE BITE IN POSITIONING THE CRANIAL BONES
A fact that has not been recognized or employed in cranial and craniosacral work is that the positions of the facial bones are largely controlled by bite forces. In a study illustated below, bite forces applied to a cranium coated with stress sensitive paint were distributed throughout the front of the cranium, and they are much stronger than the forces used to make cranial adjustments. In the future, the contours of the bite table may be altered to reposition the facial bones. For example, steeply interdigitated bites or deep overbites can produce strains between facial bones by locking together the upper and lower jawbones (partial craniosynostosis of the maxillo-mandibular suture).
To understand how bite forces are applied to the cranium, the bite needs to be seen as a cranial suture connecting the upper and lower jawbones. Cranial sutures provide adaptability to accommodate irregularities in growth of individual bones, and such adaptability is needed in the bite, because the upper and lower jawbones grow by very different mechanisms, at slightly different rates, and in slightly different directions. However, in many modern human bites, the unworn teeth have become so steeply interdigitated that they cannot accommodate the natural discrepancies between the normal growth patterns in the upper and lower jawbones.The elongation of the mandible cannot push its corpus (the area containing the lower teeth) further forward, because that area of the mandible is locked to an upper jaw that grows by expanding rather than translating; and the upper jaw cannot expand, because it is locked to a mandible which grows by translating rather than expanding. Relieving this partial synostosis between the jawbones requires adjusting the bite.
CRANIOSACRAL TREATMENT - is a relatively recent extension of cranial work to include the spinal canal, which is enclosed in the same CSF reservoir as the cranium. The hydrodynamics of CSF in the spinal canal are better known than those in the cranium, because they are more easily imaged and monitored. Blockages to the normal CSF flow there due to Chiari malformation, spina bifida, a cyst, spinal cord tethering, space occupying lesions, trauma, or infection impair the flow immediately above and below the blockage. Distal to the blockage, CSF flow resumes with epidural and cord pulsation. In minor blockages, the flow reflects local turbulence and eddies. In major blockages, the flow may be stopped completely. Removing the blockage surgically restores the flow and eliminates the symptoms.
Craniosacral manipulation could function in the spinal canal like cranial manipulation in the cranium to free up blockages to CSF circulation, but CSF flow in the spine is slower and less consequential than CSF movement in the cranium, and I believe that much of the clinical success from craniosacral treatment is produced by other benefits of manual manipulation of the spinal column. The cranium functions as the superior end of the spine, therefore holding the occiput in one hand and the sacrum in the other when a patient lies on a table enables a therapist to feel the mobility of the whole spinal column and manipulate it in various ways that can enhance its range of motion which improves intervertebral joint circulation and may also provide other benefits that we don't yet understand.