Forward Head Posture and Decreased Lordotic Curvature of the Cervical Spine
Grant Tully, B.S., D.C. 1
1. Private Practice of Chiropractic, Rochester Hills, MI
Abstract Objective: The purpose of this review is to identify and discuss potential pathological conditions secondary to forward head posture and decreased lordotic curve of the cervical spine.
Discussion: Forward head posture and a decreased cervical curve affect the biomechanics of the spine. This can accelerate cervical spine degeneration, contribute to cervical myelopathy, affect proprioception, and contribute to neurodegenerative diseases. There are physiological mechanisms that can explain these phenomenon and this review examines them.
Conclusion: Forward head posture and a decrease in cervical lordosis may play a role in numerous pathologies by altering biomechanics.
Keywords: Forward head posture, cervical spine, lordosis, kyphosis, vertebral subluxation
Forward head posture occurs when the head is anterior to a vertical line through the individual's center of gravity.1 This is problematic because the significant mass of the head is supported by the cervical spine, and significant deviation fromnormal alignment increases cantilever loads and muscular activity.2 The forward head posture is one of the most prevalent conditions in western society and has become more widespread as technology and subsequent dependence has progressed.3 The conditions related to use of technology have been termed by some as Visual Display Terminal Syndrome, and includes forward head posture.4 People spend an average of 2-4 hours per day with their heads tilted reading and texting on smart phones and similar devices. Cumulatively, this is 700-1400 hours a year of excess spine stress. On the upper end of estimates are high school students who may spend 5,000 hours in this poor posture annually.5 This is associated with the progression of forward head posture and subsequent loss of lordosis in the cervical spine.6 A relationship was found between posture and curvature of the cervical spine; a more forward posture of the cervical spine was related to a partly reversed curvature.7 Conservative estimates believe that 66% of the population suffers from the abnormal postural conditions.8
This excessive, abnormal load may have implications in accelerated cervical spine degeneration, cervical spondylotic myelopathy, aberrant proprioceptive input to the central nervous system (CNS), neurodegenerative disorders, and many more related conditions that are not yet documented.1,5,9,10 The purpose of this paper is to explore the role of forward head posture and its potential sequelae.
Accelerated Cervical Spine Degeneration
Degeneration of the cervical spine has long been considered a normal consequence of aging. However, loading, genetics, and local autocrine factors all influence the rate and degree of disk degeneration. Mechanical disturbance of the cervical spine is considered one of the most deteriorative factors for cervical spondylosis.8 Axial loads to the spine are converted to tensile strain on the annular fibers and then transmitted to the vertebral end plates. With continuous loading, creep occurs in the nucleus pulposus. This decreases its effectiveness to dampen forces and degeneration will occur.11 After degeneration of the intervertebral disks, the vertebral bodies add bone to the region adjacent to the IVD to allow the vertebral bodies to more effectively absorb compressive loads.12 This is known as subchondral sclerosis and is an indicator of spondylosis. The body does not haphazardly lay down extra bone. It is a response to strain and follows Wolf's law.
The natural biomechanics of the spine rely on a lordotic curvature in which the posterior columns withstand approximately 65% of the load and the anterior columns support 35%. Thus, the posterior neural arch is responsible for most of the load transmission down the cervical spine.13 As the head shifts forward of a neutral position, it increases strain on the anterior columns. Abnormal posture elicits abnormal stress and strain in many structures including bones, IVD's, facet joints, musculotendinous tissues, and neural elements.14,15 The facets are particularly affected by the forward head posture.16 These stresses can lead to early wear, tear and degeneration.5
Studies in the literature appear to support the notion that abnormal stress promotes cervical degeneration. Jumah et al found that 63.6% of Ghanans who routinely carried bags on their head had spondylotic changes of the cervical spine, while 36% of those who did not had spondylotic changes. These observations indicated that excessive load on the neck is a promoting factor in degeneration. A study by Okada et al revealed that the frequency of progression of age-related changes of the cervical disks during 10 years was significantly higher in the non-lordosis group.15 Harrison et al reported that the vertical load exerted on the vertebral body of the cervical spine was at least ten times stronger at the apex of a kyphosis than that of lordosis.15 Higher rates of degeneration are seen in professions that spend a lot of time in the flexed head position such as dentists, miners and meat carriers.17
While there may be no studies that confirm the cause and effect relationship of forward head posture and loss of lordosis contributing to accelerated degeneration, there is a physiological mechanism to potentially explain it as well as a few studies demonstrating a correlation. As described above, in normal alignment 65% of stress is placed on the posterior elements of the vertebrae while 35% is on the anterior column. As an individual develops forward head posture, it is reasonable to assume that the instantaneous axis of rotation moves from the posterior elements onto the anterior elements. This may lead to accelerated degeneration of the cervical spine.
Cervical Spondylotic Myelopathy
Forward head posture and decreased cervical lordosis may eventually progress to a cervical kyphosis. Progressive cervical kyphosis has been associated with myelopathy.16 The deformity leads to draping of the spinal cord against the vertebral bodies, thus increasing the longitudinal cord tension because the spinal cord is tethered by the dentate ligaments and the cervical nerve roots.18,19 As the curve progresses, the cord becomes compressed and flattened.20 The intramedullary pressure increases secondary to tethering of the spinal cord.21-23 This compression leads to neuronal loss and demyelination
of the spinal cord.20
There are also implications involving the microvasculature of the cord. Breig and El-Nadi demonstrated that cervical flexion movements produce flattening of the small feeding vessels.24 This occurs because flexion movements, such as a decrease in lordosis, stretch the spinal cord and canal.25
Dentate ligaments transmit stress from the dura mater to spinal cord tissue. When the dura is pulled taut with increasing canal length, tension is transmitted to the cord through the dentate ligaments.26 The small feeder blood vessels on the spinal cord become flattened, thus leading to a loss of blood supply.13 In addition, the veins of the spinal cord operate at low pressure and are easily occluded by compressive forces. The mechanical tension transmitted to the cord from the dentate ligaments can obstruct these veins and cause stasis of blood and ischemia in the portion of the cord drained by these veins.27
In general, flexion movements of any part of the spinal column will induce abnormal stress in the entire cord. Normal posture will minimize these stresses.28 Forward head posture and a decrease in cervical lordosis are flexion type motions. Therefore, it is reasonable to conclude that this abnormal posture can contribute to myelopathy and other spinal cord pathologies similar to the mechanisms discussed. While forward head posture does not necessarily indicate the presence of a kyphosis, it may eventually progress as a sequela. However, many of the mechanisms that contribute to pathology secondary to kyphosis may exist in a lesser degree to a decreased cervical curve and forward head posture.
Aberrant Proprioceptive Input
Adequate posture control requires the sensory and central nervous system for the human body to function appropriately against gravity and environmental forces. Posture control requires visual and vestibular inputs, as well as both proprioceptive and tactile somatosensory inputs, to control posture-regulating muscles in the whole body. Thus, the CNS needs to control multiple systems simultaneously based on corresponding multisensory inputs.29
Posture, such as normal head posture, is under involuntary control. It is largely dependent upon cervical joint mechanoreceptors and afferent input from ligament and musculotendinous sources to maintain optimal body alignment.1,3 The increased strain from forward head posture causes dysafferentation in response to increased strain placed upon various spinal muscles, such as the splenius capitis, trapezius, SCM and levator scapula forcing them to rely on anaerobic metabolism.30 As anaerobic metabolism progresses, metabolites such as substance P, bradykinin and histamine build up and excite chemosensitive pain receptors, causing a barrage of nociceptive afferent input, resulting in dysafferentation.31,32
The role of the cervical spine and position of the head in postural adaptation due to undisturbed proprioception by cervical vertebral facets was confirmed by a number of authors.33,34 Many studies have supported that many of our postural reflexes, such as the vestibulocollic reflex, cervicocollic reflex, pelvo-ocular reflex, vestibulo-ocular reflex, cervico-ocular reflex and cervical somatosensory input, are housed, or occur, within the head and neck region.3 Therefore, it is possible that dysafferentation in the cervical spine secondary to forward head posture may affect the entire spine. Diab discovered that correcting forward head posture improved sagittal, coronal and transverse planes in the scoliotic posture of adolescents.3 Coincidentally, scoliosis occurs more frequently in people with forward head posture relative to the normal population.35-37 A correction of altered head posture, therefore, could be imperative to achieve optimal full spine postural correction, where the rest of the spine orients itself in a top-down fashion.38
The cervical spine appears to have an important role in proprioception. Forward head posture may disrupt this system. It is possible that this can cause reactivity throughout the spine. Because of the complexity of the CNS, the mechanism by which this regulation occurs is still unknown despite researchers' best efforts.29 However, the mechanisms discussed may deserve further investigation.
The idea that biomechanical pathology can contribute to neurodegenerative disorders appears to be growing and centers around abnormal cerebrospinal fluid hydrodynamics. Altered CSF flow has been implicated in neurodegenerative brain diseases including multiple sclerosis, Parkinson's disease and dementia, etc.39 This is problematic because in addition to being a shock absorber for the brain, CSF has a role in circulating nutrients and chemicals filtered from the blood and removing waste from the brain and other nervous system tissues. Abnormal flow of CSF may be affected by many factors. As it relates to forward head posture and loss of cervical lordosis, it is known that abnormal tension on the cord can be caused by a loss of cervical lordosis. Tension on the cord can cause a caudal migration of the cerebellar tonsils as well as increase intramedullary pressure on the cord.21-23,40
Abnormal CSF flow can occur as a result of the cerebellar tonsils obstructing the foramen magnum, preventing CSF from properly circulating through, and around the brain. In addition, disturbance of spinal alignment can also impose abnormal tension on the vasculature, causing stretching or compression resulting in impaired arterial and/or venous flow.41 In the supine position, the main drainage route for the cerebral venous blood outflow is the internal jugular vein. In the erect position, the internal jugular vein collapses and the cerebral venous blood outflow occurs through a complex venous network that ultimately drains through the vertebral plexuses.42 The veins of the vertebral plexus, dural sinuses, and emissary veins have no valves and therefore are dependent on hydrostatic pressure gradients.43 If pressure downstream is elevated secondary to an increase in intramedullary pressure on the spinal cord and blood vessels, or by cerebellar tonsillar ectopia, it stands to reason CSF hydrodynamics may be altered.
Obstruction and back pressure in drainage systems of the brain have been implicated in the literature as a possible contributor to neurodegenerative disorders previously.44-46 Given the unexplained tendency for multiple sclerosis to be periventricular in its distribution, the question arises as to whether abnormal CSF hydrodynamic effects play a role in the genesis of MS lesions.47,48 The resulting obstruction of CSF from the brain impairs the outflow of CSF from the lateral ventricles. This may increase ventricular CSF pressure that in turn leads to "leakage" of CSF and its content of polypeptides and antigenic proteins into surrounding brain parenchyme.49 It is possible, therefore, that improper biomechanics found with forward head posture and resulting loss of cervical lordosis can alter the normal hydrodynamics of CSF and contribute to neurodegenerative disease.
The purpose of this paper was not to prove a cause and effect relationship between forward head posture, loss of cervical lordosis and potential damaging sequelae. It was merely to implicate a biomechanical role in the progression of certain conditions. Often, the emphasis is placed on pharmaceuticals and pharmokinetics to address disease. The body is more complex than merely the sum of its chemical processes. They do not occur in a vacuum. Biomechanics influence the body as well and warrant further investigation in some pathology. As evidenced previously, it already appears to have a role in accelerated cervical degeneration, cervical spondylotic myelopathy, aberrant proprioceptive input to the CNS, and neurodegenerative disorders.
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