Second, in the wake of appreciating the neutral spine alignment, let’s examine exactly what transpires in the slouched sitting position. Slouched sitting (Figure 25) is a common flawed posture used by many, not just dental professionals. Starting from the bottom up, the pelvis is usually rotated posteriorly (Figure 26).
Figure 25. Slouched Sitting.
Figure 26. Posterior Rotation.
This posterior rotation affects several structures. It is impossible to maintain lumbar lordosis with a posteriorly rotated pelvis. Therefore, this reversal of the lumbar lordosis triggers abnormal forces to many tissues. Immediately, it causes the lumbar spine to flex causing widening of the lumbar intervertebral discs and strain on the lumbar ligaments. The paraspinal muscles running along side the spine are elongated and many times rendered inactive due to the elongation. The length/tension curve is no longer ideal for these muscles and they become less electrically active on electromyography (EMG). The deep primary stabilizers, the multifidi, function to assist with extension and rotation. However, during over-flexion of the lumbar spine, these primary stabilizers also become close to electrically silent.
Next, distractive and straining forces are placed on the interspinous ligaments, and the posterior longitudinal ligaments of the lumbar spine. Daily repetitive strain to these ligaments over time can result in one experiencing a generalized lower backache that is often felt across both sides of the lumbar spine. Overtime with the muscles and ligaments strained, the next structures to bear the flexion forces are the intervertebral discs. As stated before, these discs serve important functions such as shock absorption and the provision of nutrition to the vertebral bodies above and below the disc via the vertebral endplates.
The discs also occupy space between two vertebrae to keep the intervertebral foramen patent so the nerve roots can pass from the spine cord out to the periphery. Remember, during slouched sitting, the lumbar spine is in flexion causing distractive forces across the disc and thus weakening the outer layer of the disc called the annulus fibrosis. Over time as repetitive and sustained slouched sitting occurs, more weakening of the disc occurs making us vulnerable to further injuring our discs with simple movements performed independent of sitting, such as bending to tie our shoes or putting on our pants.
Once injured, discs do not easily heal and can start to degenerate. During this degenerative process, not only is the annulus fibrosis weaker, but the inner core, the nucleus pulposis, migrates posteriorly further pushing on the weakened walls of the annulus. In many circumstances, the nucleus causes bulging, herniation, or extrusion. The nuclear content full of water for shock absorption now resides outside of the disc never to be useful in the role of providing shock absorption. Further injury and/or degeneration sets off a cascade of events that over time can lead to neural foraminal stenosis (Figure 27), central canal stenosis (Figure 28), degenerative disc disease, degenerative joint disease (facet disease) and nerve entrapment. At this point, many are familiar with the term “sciatica” which can result from any one of these conditions or others.
Figure 27. Neural Foraminal Stenosis.
Figure 28. Central Canal Stenosis.
Besides affecting the lumbar spine, a posteriorly rotated pelvis in slouched sitting effects all the portions of the spine and not just the lumbar region. Slouched sitting additionally causes the thoracic spine to flex, the head then must also move forward to offset the posterior movement of the pelvis.
Video 2. Slouched Sitting.