Clinical Significance of Cervical Vertebral Flexion and Extension Spatial Alignment Changes
From: Spine. 2009 Jan 1;34(1):21-26
Physiologic cervical lordosis maintains basic biomechanical balance of the cervical spine, through intricate interrelation between physiologic lordosis, anteroposterior intervertebral disc edge space height, and sagittal spatial alignment. Cervical vertebral spatial alignment changes induce changes in the sagittal cervical curvature. During dynamic flexion and extension, each cervical vertebral body and intervertebral disc allows for relative spatial displacement, adjusting according to postural changes. At present, there are few measurement reports analyzing the effect of dynamic changes on cervical vertebrae, direct vertebral body alignment, physiologic curvature changes, and its relationships to adjacent vertebrae. The primary goal of orthopedic cervical kyphosis correction is restoration of the physiologic cervical curvature, optimizing normal segmental vertebral spatial structure and alignment. Orthopedic cervical kyphosis correction, especially with severe cervical kyphosis presents high surgical risks and lacks standardized theoretical surgical design guidelines, therefore surgeons are unable to plan correction methods or predict results. This may result from insufficient knowledge and understanding of changes in alignment and curvature of the cervical vertebral column in the sagittal plane. To understand cervical vertebrae and curvature dynamic variation patterns in the sagittal plane and provide a theoretical reference for orthopedic correction of cervical kyphosis, they measured spatial position and alignment of the cervical vertebrae on sagittal plane films of various postures.
In the neutral position on lateral view, the cervical spine is normally lordotic. A majority within the normal population indicates a -21° ∼ -22° physiologic lordosis in the cervical vertebral sequence. This physiologic arch is formed and maintained by mainly anteroposterior discrepancy in height of the intervertebral disc. The height of the anterior edge of the vertebral body is also an important factor of maintaining the physiologic curvature of the cervical vertebra. In cervical vertebral fractures, cervical vertebral body height is flattened thus decreased. Cervical vertebral fracture dislocation is also a common cause of cervical kyphosis. But whether the anterior edge of the cervical vertebra is higher than the posterior edge requires measurement verification. During flexion and extension, cervical vertebral body shape remains unchanged; it is the change in height of the anterior and posterior edges of the intervertebral disc and the coordinative movement of the articular process, which induces consecutive change in cervical physiologic curvature.
This study indicates 87% of cervical curvature apex position located between C4 to C5, a rate lower than reported by Wang Cheng-lin. Differences may be because of different measurement methods used; however, with conclusion conformity. Cervical curve apex mainly lies between the upper boundary of C4 and lower boundary of C5; during flexion and extension, apex distribution remains unaffected or by 1 vertebra at most.
It is concluded that flexion and extension of the cervical vertebrae is centered between C4 and C5, and sagittal vertebral rotation causes a change within the space between the anterior and posterior edges of the intervertebral disc, resulting in physiologic curvature change. Rotation of the cervical upper vertebral terminal end is a direct superposition of the lower vertebral rotation, causing a gradual increase in vertebral rotation amplitude moving upward from C7 toward C2. In addition, all vertebral bodies are displaced during rotating, therefore not affecting apex position during dynamic motion.
The cervical spine’s sagittal profile represents a balance between these anterior (vertebral body, intervertebral disc, anterior longitudinal ligament, etc.) and posterior (lamina, facets, posterior ligament, etc.) elements. A deficiency in either area can lead to sagittal abnormality. Pathologically, posterior cervical structure traction strength is weak, or with increased pressure of anterior structures, induce vertebral spatial alignment change causing a lowered anterior and raised posterior height change in intervertebral discs, and with further development lead to chondronecrosis of the anterior vertebral edge, dissymmetrical vertebral growth, vertebral compression, and wedging, causing cervical kyphosis. Although in the beginning, cervical kyphosis is recoverable, with further exacerbation to severe or rigid kyphosis with vertebral and articular process bone fusion. In cervical kyphosis, there is a pathologic change in vertebral spatial alignment (abnormal vertebral rotation in the sagittal plane) and orthopedic correction surgery is performed with intent to regain spatial alignment of each cervical segment (surgical derotation).
Dynamic cervical curvature changes based on a central apex (C4 or C5) stems from vertebral rotation and displacement in the sagittal plane, revealing cervical spine dynamic variation patterns providing vertebral spatial alignment value suggesting kyphosis correction range beginning at the cervical curve apex and extending cranially and caudally. In addition, inferior terminal lamina movement in the sagittal plane is clinically significant conforming kyphosis correction to pre surgical planning and design; kyphosis correction mainly refers to the vertebral inferior terminal endplate angle indicative of anterior intervertebral space distraction amplitude, whereas pre surgical correction design is based on vertebral body spatial alignment angles.
Vertebral rotation and displacement in the sagittal plane of the central apex (C4 or C5) stimulates dynamic motion changes in physiologic cervical curvature. Our study identified dynamic changes and regularities in reference to the cervical spine, providing reference values to cervical vertebral spatial position and alignment, providing a theoretical reference standard for cervical kyphosis.
