1 Introduction to hyper-kyphosis in spine: an overlooked entity?

Maintenance of an erect posture is one of the fundamental critical tasks of the human spine. Physiologic alignment protects the pathways for the large vessels of the trunk to the lower extremities and provides the scaffolding for pulmonary cardiac and digestive organ function. Aside from these primary life-supporting qualities the spinal column provides a circumferential conduit for the spinal cord and cauda equina. The regional human anatomy with its four sub-regions is shaped to serve level-specific tasks, with cervical and lumbar lordosis balanced with thoracic and lumbar kyphosis. Spinal alignment for an erect human being is a composite multidimensional accomplishment with passive and active components. Passive components maintaining spinal alignment include osseous vertebral elements, ribcage structures and ligaments. Active components include intervertebral discs with their hydrodynamic function and paraspinal musculature. Healthy balance of the active and passive spinal column components is required to avoid premature decay and loss of alignment. Loss of physiologic balance will predictably lead to premature breakdown of the mechanically overloaded aspect of the spine and results in a wide variety of symptoms.

2. Impact of losing alignment

There are regional differences regarding the impact of loss of alignment. In the cervical spine, loss of lordosis will likely result in premature disc degeneration, pain, loss of range of motion and eventually loss of neural canal space with potential for encroachment of neural elements. Throughout its course, the spinal cord is very vulnerable to anterior compression due to location of the anterior spinal artery and the presence of its major motor units in the anterolateral cortico-spinal tracts. For the thoracic spine, loss of physiologic kyphosis may have additional consequences beyond pain and neurologic compromise. From a muscular stabilization perspective, progression of kyphosis will not be counteracted at some point in time by extensor musculature, which due to resultant vector forces will act as flexor muscles. Eventually progressive collapse of the spinal column will eventually lead to an invagination of the trunk with intususception of the abdominal structures into the chest cavity and resultant cardio-pulmonary impairment. Specific numbers for kyphosis angulation relating to impairment of pulmonary vital capacity and cardiac output are hard to come by and are affected by multiple other factors, such as location and cause of kyphosis as well as a number of general health factors. Without question, however, malnourishment and cardiopulmonary compromise are a predictable end-result of structural thoracic deterioration.

Malalignment of one region of the spinal column will not only affect the local organ systems, but also have implications on the alignment of other spinal segments. Hyperkyphosis of the thoracic spine, for instance, will usually be compensated by the bearer’s cervical and/or lumbar spine with a reactive hyperlordosis up to the point of reaching their specific functional limitations. Secondary deterioration and symptoms can be expected as these reactive compensatory zones will succumb to mechanical overload.

3 Current systems for measuring kyphosis

For a structured approach towards kyphosis assessment, measurement of the deformity quantification of physiologic impact, differentiation of etiology and eventually grouping into classification can be used.

As previously implied, specific numeric values for physiologic alignment of the spinal column are hard to come by. Measurements of kyphosis have been described by Cobb and others and are widely used to quantify this deformity [1, 2]. For the cervical and lumbar spine any loss of physiologic lordosis to the point of having a straight spine or less has been widely used as a relatively simple baseline for definition of kyphosis. Sagittal convexity in the thoracic spine of younger populations has been suggested to range from 20–40° [3]. For older adults, the mean kyphosis angle has been described to measure about 48°–50° in women and 44° in men [4]. The angle increases with age, but there are no universally accepted thresholds for defining either kyphosis or “normal” thoracic spine or “normal” changes associated with aging. One longitudinal study reported a mean thoracic angle increase of 3° per decade [5].

There are also more general measures of imbalance, such as the sagittal balance described by Farcy Asher and others to be considered [6–8]. More recently the importance of including the proximal femurs in the structural balance evaluation has become an appealing method of measuring sagittal imbalance and planning corrective osteotomies.

From the standpoint of physiologic impact, assessment of pulmonary parameters such as total lung capacity vital capacity functional residual capacity decreased maximum oxygen uptake, ventilatory dysfunction and obstructive ventilatory dysfunction is possible, but has not been specifically paired with spinal conditions or deformity magnitude. Similarly physical function as determined by impaired physical mobility, difficulties with activities of daily living, decreased bench-press strength, reduced gait speed, increased stair climbing time, less functional reach, more difficulty rising from a chair have all been suggested as simple measures of physical function, but have not been matched with thoracic deformities. Finally there are well documented neurological impairment scales available for documentation of spinal cord injury.

Causes of hyperkyphosis are diverse, and include dysplasia from congenital or developmental causes, fracture, tumor, infection, inflammatory disease, postsurgical factors and acquired nontraumatic deformities from a number of environmental and age related conditions and neuromuscular causes. Kyphosis is also a common component of a variety of scoliotic disorders. In short, the human spine has a great preponderance to sagging into kyphosis as a result of a wide variety of disorders and disturbances. Any management consideration is contingent upon correct identification of the underlying etiology of kyphosis and appreciation of the ramifications of the deformity on the affected patient. For this purpose a classification of kyphosis would be helpful in order to aid in decision-making and research.

Sadly a systematic review of the spine literature revealed only two classification systems. The system introduced by Takemitsu [9] in 1988 differentiated 4 grades of lumbar kyphosis based upon severity of radiographic deformity along with prevalence rates. Jang [10] in 2007 addressed the thoracolumbar spine in his classification system, but limited his system to compensatory regional type curve pattern and a decompensated global curve pattern structural (primary) type deformities. Neither of these systems addresses neurologic or other clinical findings. Surprisingly there have been no attempts at providing a more comprehensive or differentiated classification system to date.