The bones of the base develop from the cartilaginous chondrocranium. At birth, development is still incomplete.(4) The occipital bone is in four parts, united by intraosseous articular cartilage. The spheroid is in three parts, the temporal in two, the maxilla in two, the frontal frequently in two.

The cranial suture is designed for a very small but vital degree of motion.(5) How much greater is the potential motion of the bones of the developing newborn skull! At this time each part of each of these bones functions virtually as a separate bone, moving in relation to its other parts.

Let us consider the occiput. It is most commonly the presenting part, and therefore the part that may take the brunt of the trauma of labor. The four developmental parts surround the foremen magnum. The base articulates anteriorly with the base of the spheroid. Posterolaterally, it articulates with the lateral masses. The hypoglossal nerve, which innervates the muscles of the tongue, passes out of the skull between the base and the lateral mass, through the intraosseous cartilage in the space that will become the condylar canal. The occipital condyle, which articulates with the atlas, spans the intraosseous cartilage; its anteromedial third is found on the base, the posterolateral two-thirds on the lateral mass.

Immediately anterolateral to this condylar area is the jugular foremen, a space between the condylar part of the occiput and the petrous portion of the temporal. This foremen gives passage not only to the jugular vein but also to cranial nerves IX, X, and XI (glossopharyngeus, vague, and accessorius, respectively). The vagus nerve provides innervation to the gastrointestinal and cardiorespiratory systems.

The supraocciput formed in cartilage fuses with the membranous interparietal bone to form the occipital squama. Compression transmitted through the squama to the condylar part on one side may disturb the function of the vagus and/or hypoglossal nerve, causing vomiting, irregular respiration, and difficulty in sucking. If this compression is transmitted further to the base, the relationship of the base of the occiput to the base of the spheroid may be distorted, causing a lateral strain of the sphenobasilar articulation and a parallelogram deformity of the cranium(5) (Figure 1).

Figure 1. Lateral strain of the sphenobasilar articulation. Viewed from above, the sphenobasilar symphysis has been strained (displaced), with the basisphenoid moving to one side and the basiocciput to the other. Both bones side-bend about parallel vertical axes in the same direction. The lesion is named from the position of the basisphenoid: lateral strain with the spheroid to the right, etc. (From Magoun, H. Osteopathy in the Cranial Field.)

Bilateral condylar compression may cause a buckling type of strain of the cranial base, producing a vertical strain between the occiput and the spheroid at the sphenobasilar articulation. This may be associated not only with vagal dysfunction but also with symptoms of tension, spasticity, opisthotonic spasms, sleeplessness, and excessive crying due to the irritation of the pyramidal tracts on the anterior and lateral aspects of the brain stem in the foremen magnum. This should be considered as a precursor of the spastic type of cerebral palsy.

The spheroid bone is in three parts at birth; the central body bears the lesser wings, with the greater wings (from which the pterygoid process subtends) on either side. The greater wing-pterygoid unit articulates with the body by an intraosseous cartilage. This is situated immediately beneath the cavernous sinus, through which pass cranial nerves III, IV, and VI, innervating the extraocular muscles, and the ophthalmic division of V, which is sensory to the orbit, upper face and scalp. The body of the spheroid articulates with the base of the occiput posteriorly and is therefore distorted by the lateral or vertical strains resulting from condylar compression. Anteriorly the body carries the lesser wings, which enter into the formation of the orbit. The orbit is approximately pyramidal in shape; the apex is at the optic foremen-that is, the root of the lesser wing at the body. Its anatomic integrity is dependent on the relationship of the greater wing to the lesser wing, which is in fact the relationship of the greater wingpterygoid unit to the body.

In the event of a lateral strain at the base due to unilateral condylar compression of the occiput, the orbit will be distorted by rotation of the base of the spheroid carrying the lesser wing anterior on one side and posterior on the other. In the parallelogram head due to lateral compression, the greater wing is compressed medially and carried forward on one side and posterior on the other. In either event, lateral muscle imbalance of the eyes is commonly found in varying degrees ranging from mild esophoria or exophoria to severe strabismus.

The temporal bone is in two parts at the time of birth -the petromastoid portion, developed in cartilage that projects obliquely between the occiput and the greater wing of the spheroid to articulate at its apex with the body of the spheroid, and the squamous portion, developed in membrane the forms the greater part of the lower lateral wall of the skull. The tympanic portion is not yet a bony canal but resembles a horseshoe adherent to the inferior posterior aspect of the squama. These two parts, the squamous and tympanic, unite just before birth. The petromastoid portion contains the auditory and the vestibular apparatus.

The auditory apparatus consists of the bony eustachian tube emerging between the petrous and squamous portions, from which the cartilaginous tube extends to the fossa of Rosenmuller. The eustachian tube is susceptible to distortion, which may impair hearing if lateral stress compresses the squamous portion. Laterally the eustachian tube opens into the middle ear, which, by the ossicular mechanism, transmits the auditory vibrations received from the tympanic membrane to the internal ear. The vestibular apparatus includes the semicircular canals, precisely related to each other and geometrically balanced with those of the opposite side. Distortion of the axis of the petrous portion may disturb this delicate mechanism of equilibrium.

The maxilla develops in two parts-the premaxilla, which will give origin to the incisor teeth, and the body, which carries the canine and all the other upper teeth. Angulation between these two developmental parts of the maxilla gives rise to malalignment and malocclusion in later years.

Thus far our consideration has been directed to certain structural changes that may sometimes be visible and are always palpable following various difficulties of labor. Radiologic techniques have been developed to substantiate many of these palpatory observations and confirm their persistence in childhood problems.(7)