Video 1. Component 1 - Base of Support.
The base of support refers to the placement of the feet. It is common to see narrow base of support with the ankles crossed, the legs stretched out in front with either the ankles uncrossed or crossed, the feet tucked under the chair or the feet just too close together. This narrow base of support interferes with balance and can limit the range of motion the trunk can move through during the performance of procedures.
Ideally, when leaning forward, a person’s center of mass should fall within the base of support. If the center of mass falls outside the base of support, stability and balance are compromised. To illustrate this concept, sit on the edge of your chair with your feet tucked underneath. Try leaning forward. If you lean too far forward, you’ll feel as though you will fall forward. It is at this point that your center of mass is outside your base of support. With a narrow base of support, muscular tension to passive tissue forces increase to sustain your alignment when the center of mass is outside the base of support and thus ultimately create potential for injury.
For protective and stable positioning, the feet should be flat on the floor at least shoulder width apart and in front of the hips. The knees should be slightly lower than the hips. Now, if you sit to the edge of your chair with your feet positioned in a wide base of support, you can lean as far forward as needed without the feeling of falling forward, i.e., your center of mass stays within your base of support.
Video 2. Component 2 - The Pelvis.
The next component of a stable and protective posture is the pelvis. The pelvis and hips should be level on the stool and not oblique. You can see: Obliqueness or unlevelness can trigger deviations in the spine, setting off additional compensations in the shoulder, neck and head regions. These deviations and compensations create potential for fatigue and pain, so monitoring the position of your pelvis is important.
Video 3. Component 3 - The Lumbar Spine.
To position the lumbar spine correctly, raise your tailbone up. This assists with restoring the lordotic curve and placing the spine in a more structurally stable position. The tailbone is part of the sacrum. In a slouched sitting position, you are actually sitting on your sacrum, yielding the turn sacral sitting. Sacral sitting does not pose any dangers to the sacrum itself unless you are insensate in this region. However, sacral sitting directly causes lumbar spine flexion and therefore can impact the pain-sensitive tissues of the lumbar spine. In turn, lumbar flexion triggers compensations and deviations in the thoracic spine, the cervical spine and the shoulder regions. These repetitive deviations can lead to fatigue, musculoskeletal pain and dysfunction. If not corrected, they may potentially lead to disability. Remember, raise your tailbone up.
Hip hinging, with a wide base of support, pelvis and hips level, and the tailbone up. There will still be a need to lean forward to improve direct and indirect vision. As previously shown, rounding the low back and creating further flexion in the thoracic spine is less than ideal. When leaning forward, employ hip hinging versus rounding the low back. Hip hinging preserves the lordotic curve in the low back while the motion occurs at the hips. This strategy minimizes stress to the lumbar ligaments, muscles, and the posterior aspect of the intravertebral discs.
Video 4. Component 4 - Trunk.
The fourth component of the six components of posture.
Too much trunk flexion is the most common fault in the sagittal plane and can be a result of poor lumbar spine position. Trunk flexion cannot occur if the lumbar spine is correctly position. Whether flexion occurs in the lumbar or thoracic spine, it will influence the other and affect the positions of the pelvis, shoulders, and the head and neck.
Hip hinging should be substituted for trunk flexion. During trunk flexion, stress to the spine’s passive tissue forces occurs making the muscles, ligaments, discs, and nerves in the sacral, lumbar, thoracic and cervical regions vulnerable. When the trunk is flexed, the weight of the body is shifted backward. In order for the body to stay balanced, the shoulders and the head and neck move forward, yielding the typical forward head and protracted shoulder position.
We will discuss the consequences of the forward head in more detail later on. If you’ve mastered controlling the pelvis and use hip hinging, excessive trunk flexion should not occur.
Lateral flexion, or side bending, is another common trunk deviation. This misalignment has the potential to cause a weakness of the hips and can alter both the shoulder and neck positions. The consequences of side bending are usually felt in the neck and the shoulders, as they must be positioned to counter-balance the trunk deviation.
Video 5. Component 5 - Shoulders.
The fifth component in the six components of posture.
The shoulders are another source of chronic pain for dental professionals. Pain in the shoulders can originate from the shoulder joint itself or subsist as a source of referred pain from the cervical spine. In seeking medical intervention, an examination of both regions should be performed so the intervention addresses the origin of the pain. As with the trunk, minimizing excessive and prolonged positioning outside of 20 degrees in each plane is recommended to avoid pain and injury to the shoulders.
In the sagittal plane, the shoulders should not be slumped forward, but attempt to hold the shoulders down and back using the muscles between the shoulder blades. Shoulder flexion not exceeding 20 degrees will allow the operator to use the intrascapular muscles for support. When the shoulders are flexed greater than 20 degrees forward, greater activation of the upper trapezius muscles can occur, making the intrascapular muscles difficult to activate or engage, resulting in upper trapezius or neck pain.
The combination of the arms too far in front and a forward head can lead to problems associated with the brachial plexus, which is a group of nerves that exit the spinal column, pierce the scalenes, or the muscles on the side of the neck, and dive deep under the chest muscles, and finally branch into the arm, forearm, wrist and hand. Gliding restrictions between the nerves and their associated interfacing tissues are common in the presence of muscle tightness, and consequently manifest as an unpleasant sensation.
For example, the tightness in the neck, the shoulder and not the least the forearm, wrist or hand, can project a number of symptoms to include but not limited to burning, pulling, numbness and tingling.
In the frontal plane, the shoulders should be level with the trunk and not elevated up toward the ears. The shoulders should not be abducted out to the side greater than 20 degrees. Abduction greater than 20 degrees further engages the upper trapezius muscles.
Video 6. Component 6 - Head and Neck.
The sixth component of the six components of posture.
With the neck region being one of the main areas of complaint, it is crucial to understand proper positioning and alignment in all three planes in motion. These planes include the sagittal plane, which we see flexion and extension, the transverse plane for rotation, and the frontal plane for lateral flexion.
First, in the sagittal plane, the ears should be aligned with the AC joint of the shoulder. Most often, the ear is significantly forward of this landmark, and hence the term forward head. In proper alignment the ears should line up closely with the AC joint of the shoulder.
While the term forward head seems relatively benign, upon examination of the altered cervical curve, we can begin to understand the consequences of prolonged abnormal positioning. In the forward head position, the upper cervical spine is in excessive extension and the lower cervical spine is in relative flexion, potentiating the ‘hump’ effect. In the upper cervical spine, upper cervical extension causes joint compression and shortening of the surrounding muscles. Over time, repetitive and/or static joint compression can lead to degenerative changes of the joints. Similarly, the shortening of the upper cervical muscles cannot only further cause joint compression and chronic pain from the prolonged static contraction of the surrounding muscles, but muscle shortening can constrict the neurovascular structures supplying the head and neck, resulting in headaches.
The consequences of lower cervical flexion include stress to the passive tissues, with over-activity in the paracervicals and the upper trapezius muscles. Over time there is a risk for disc pathology with or without nerve involvement and chronic pain from repetitive and/or prolonged static contraction of the surrounding muscles.
While the expectation is not that the operator limit their movement into rotation, flexion, or lateral flexion, it is the intention that the operator limits his or her movement to safe ranges of motion, to 20 degrees in each plane. Any repetitive and/or static position, whether in structurally stable alignment or not, can lead to pain and/or injury.
Knowledge of the principles of proper alignment is the key to minimizing stress to pain-sensitive tissue over time.
Dental professionals who lack the understanding of proper body mechanics do not have the knowledge to actively participate in protecting themselves from musculoskeletal disorders. While many accept discomfort as part of the profession, early intervention upon realization of symptoms may assist with preventing serious or permanent injury.
Dental professionals at risk for musculoskeletal disorders may experience the following symptoms:
Although these symptoms may not necessarily lead to a musculoskeletal disorder, it is important to recognize them and seek intervention if they persist. Acknowledging that fatigue in any position, whether correct or incorrect, is a precursor to discomfort that can lead to symptoms that may cause pain/injury is important. When fatigued, clinicians should alter their position, move to another quadrant, or take a short break. Awareness of proper body mechanics is key to making informed decisions about sitting and moving to minimize pain and injury.
Table 2 summarizes the protective positions and ranges for each of the six body regions discussed in the videos and can be used as a decision-making guide for posture and positioning.1 Constructed upon the foundational principles underlying alignment, the parameters set in the table aim to minimize load and stress to pain sensitive tissues when sitting. Sitting properly is not intended to be a static event, but fluid movement in the most energy efficient ranges and patterns. In turn, the conservation of energy will minimize fatigue, and in the long run, pain and/or pathology.
Table 2. Summary of Protective Positions and Ranges.