The intervertebral foramina are oval in shape when viewed laterally, with the longer vertical axis well exceed
ing the anteroposterior measurement. Foramina are less orifices or apertures with sharply defined edges, than ellip
soid spaces with lateral dimensions, and this is especially evident in the cervical region, where the foramen is more of a canal or gutter and commonly exceeds 1 cm in length. 1 1 7 The thoracic foramina have the shortest lateral traverse and of the three spinal regions these are most like
54 COMMON VERTEBRAL JOINT PROBLEMS
simple openings, albeit with the sharpest though still rounded bony margins.
Thoracic movement, and consequently thoracic root disturbance by traction, are regionally the least, otherwise root sleeve fibrosis would probably be noted as pre
dominantly a thoracic pathological change rather than a cervical one.
Since the amount of research into the nature of degenerative change in the thoracic joints is relatively small,902. 1 125 it may well be that fibrotic change at the thoracic foramina occurs fairly frequently, because of the sharper bony edges and the opportunities for chronic irri
tation, although little is written about it.
The lumbar foramina should perhaps be regarded as regions rather than 'portholes', with the lateral recess of the neural canal, noted on a plan view of a lumbar vertebra (see p. 28), an important part of the region; low lumbar roots are not infrequently compressed by disc material in this cubbyhole of the lateral dimensions of the neural c.anal (Fig. 1.32).
Root sleeve
As the ventral and dorsal roots combine to traverse the foraminal opening, they invaginate the dura and arach
noid to carry with them a separate bilaminar sleeve of these two meninges, and a short lateral continuation of the subarachnoid space (with cerebrospinal fluid) which ends at the posterior root ganglion.'" The dural sheath ends more distally after enveloping the ganglion as a fibrous sheath, then continuing as the epineurium of the spinal nerve.
The afferent fibre population of the combined spinal nerve roots exceeds that of efferent fibres by 3 : 1 in the cervical region, 1 .5 : 1 at thoracic segments and 2 : 1 in lumbar roots. 1193
While the ventral and dorsal roots remam separate they can be individually and selectively trespassed upon by degenerative change. }Q2
Frykholm J92 describes the different effects of stimulat
ing, under local anaesthesia for operations on the cervical spine, (a) the dorsal root, when patients immediately ex
perienced a pain with dermatomal distribution, and (b) the ventral root, when they reported pains in muscles which preoperatively had been painful and tender to pressure.
At the foraminal opening, the cross-sectional area occupied by the root bundle (nerve and sheath), is one
third to one-half, with the remaining half occupied by areolar connective and adipose tissue, the spinal artery and its ventral and dorsal branches, many small veins, lym
phatic vessels and filaments of the sinuvertebral nerve (ramus meningeus). 1 192
Invagination ofthe dura and arachnoid to form the 'root sleeve' directly opposite the vertebral foramen occurs at upper cervical segments, with gradually increasing
obli-quity of successively lower segments causing greater dis
crepancies berween rootlet formation and foraminal level Fig. 2. 1 3). Thus the shortest cervical roots are 1 0 mm in length, and the longest sacral rOOt 1 68 mm. This generally results in the rOOt bundle occupying the upper part of the foramen, but at the cervicothoracic junction the roots may descend intradurally for some millimetres belaw the lower margin of the foramen, with consequently an acute upward angulation of the dural sleeve and a close approx
imation of root complex and lower foraminal bony mar
gin. This angulation is seen in more than one-third of cases under 25 years, and between 25 and 40 years the angulation is seen in about three-quarters of cases (Fig.
2. 1 4).
Lack of attachment to foraminal margins for the most part (see p. 62) allows roots to move about and through the foramen, and further, the relative elasticity of roots allows them to sustain a degree of lengthening which is ample provision for the normal ranges and various combi
nations of vertebral movement.
The elasticity nevertheless has a limit, and in the cervi
cal spine, where the relatively free mobility may induce traction sufficient to harm the roots, twO factors give a degree of protection against additional tensile stress, viz:
(a) the attachment of roots C4-C7 to the foraminal gutter by connective tissue of the roots' epineurium (see p. 57), the prevertebral fascia and other slips from musculoten
dinous attachments to transverse processes (the smaller amplitudes of lumbar and thoracic movement do not hazard roots in this way and such attachments are only seen in the cervical region), and (b) the proximally wider part of the less extensible dural funnel is drawn outwards, and plugs the foramen so as to resist further lateral move
ment.
The relationships becween facer-joint and root complex are such that the root is below and in front of the joint in the cervical region, directly in front of it in the thoracic region and in front and above it in the lumbar region.
Foraminal encroachment
While vertebral movement especially changes the vertical dimensions of the foramen, there is adequate room for these variations to occur without compression of the structures traversing it, which are also protected by the surrounding fat and cerebrospinal fluid, bathing the emerging root as it lies within the subarachnoid space of the sleeve. 5)7 A simple reduction in the height of the inter
vertebral disc is not usually sufficient to produce compres
sion of roots and vessels, so long as the fat and fluid remain around the nerve roots, although reduction of the vertical foraminal dimensions can be caused by a retrolisthesis
with-extension at one segment, when the superior articu
lar process of the vertebra below then trespasses upward and effectively reduces available space.
In the lumbar region the vertical diameter of the
APPLIED ANATOMY---(;ENERAL 55
SPINAL NERVES ( shown on this
side only )
CORD
SEGMENTS
B
10 1 0
I I
1 2 1 2
L I L
2 2
3
V E RT E BRAL BODIES4
S I
2
Coccyqeol Coccyx
Fig. 2.13 The level of spinal cord segments i n .elalion lO vertebral levels. The spinal cord ends a l L 1 - L2.
foramen varies between 1 2 and 19 mm, but the transverse diameter may be as little as 7 mm, and the opportunities for foraminal encroachment due to hon'zomal trespass are much greater.
Diminution of the transverse diameter is more likely to embarrass the foraminal contents, and this space-occupy
ing effect can be due to abnormalities of the disc and facet
joints, so often the cause of acquired spinal stenosis. Nerve tissue will tolerate slow compression quite we1l282, )92 and marked trespass may not give rise to much detectable dis
turbance of function, although repeated frictional trauma against encroaching degenerative thickening, and exos
tosis, may be the more likely cause of reactive changes in the nerve and consequent development of signs and symptoms.
It is a curious fact that in both the lower cervical and lower lumbar regions, where spondylotic changes are very frequently responsible for a reduction in foraminal dimen
sions, the foramina should be naturally smaller than in the middle and upper parts of these regions (see also 'Bio
mechanics of spinal cord and meninges', below).
When the transverse dimensions of the lumbar fora
mina appear developmentally reduced, and this is detect
able on plain lateral films in the absence of acquired foraminal encroachment by spondylosis and arthrosis, a narrow neural canal (spinal stenosis) is almost certainly present.
56 COMMON VERTEBRAL JOINT PROBLEMS Dura mater
(open sac) Spmal cord
C7 ca
T1
T2
T3
T4
T5
T7
foot Ventral rootlets Cui edge of pedIcle
Antenor aspect of dura mater
Fig. 2.14 Scheme of cervicothoracic root angulations-anterior aspect (sec leXt). (Afler Nathan H, Feuerstein M 1974 Angulated course of
spinal nerve roots. Journal of Neurosurgery 32: 349.)