Sitting with the hips at a 90-degree angle or less can cause various microtrauma, including trigger points in the iliopsoas muscle, leading to low back pain.
Click here to enlarge imageThe nature of dentistry makes intermittent forward leaning virtually unavoidable. This combination of thighs parallel to the floor with forward leaning causes the pelvis to roll backward and flatten the low-back curve. Research shows that this flattening of the lumbar curve has detrimental effects on both the spinal musculature and discs. Muscular activity in the lower back increases, which can cause ischemia. Pressure within the disc also increases, which can lead to premature disc degeneration. Research supports positioning the hips higher than the knees, which allows for increased hip angle with lower associated low-back muscle activity and disc pressure.
Chairs with a tilting seat feature open the hip angle to about 110 degrees, (saddle-style stools open to about 135 degrees) which maintains the low-back curve, decreases disc pressure, enables closer positioning to the patient, and helps reduce low-back pain.
Maintaining this essential low-back curve also places the spinal curves above it in a more balanced posture, minimizing strain on the other spinal discs, muscles, and ligaments. While I have discussed the reasons for tilting or saddle-style stools extensively in past articles, there is an even more compelling reason for those 90-degree sitters who suffer from low back pain.
Trigger points in the iliopsoas (hip flexor) muscles can develop as a result of sitting in a 90 degree (or less) hip angle, referring pain to the low back.
As a dental professional, you have probably already experienced trigger points, whether you realize it or not. A trigger point is a group of muscle fibers that are in a constant state of contraction. It feels like a hard knot, and when pressed upon may be painful or may refer pain to a distant part of the body.
The involved muscle tends to become tight, short, and weak. Trigger points may be active (painful) or latent (causing stiffness and restricting range of motion). Because they are caused by prolonged muscle contraction, postural asymmetry and mental stress (among other factors), it is easy to see why trigger points are so common among dental professionals. They are often the cause for “mysterious” low back pain syndromes that elude Western medicine practitioners.
The iliopsoas (pronounced ilio-so´-az) muscle is especially vulnerable to developing painful trigger points due to the seating biomechanics inherent to delivering dental care. The muscles function primarily to flex the hip and are constantly activated while the practitioner sits actively (not resting on a backrest), which is a majority of the time for most operators.
Iliopsoas trigger points are especially aggravated by prolonged sitting with the hips acutely flexed 90 degrees or more (i.e., on a non-tilting seat pan). Although the muscle is located on the front of the body, trigger point pain from this muscle is usually referred to the low back (Fig. 2) and sometimes the upper buttocks and interior thigh. Pain is common when standing and walking, and relieved when lying down, sitting, or flexing the hip. Trigger points in this muscle can cause improper posture, which aggravates other muscles and leads to trigger point development in them.
Identifying tightness in the hip flexor muscle requires a special test called the Thomas Test, which is performed by a physical therapist or similar health-care practitioner. There is also palpable tightness and pain in the muscle. The quadratus lumborum muscle should be assessed for involvement, as these two muscles are often “partners in crime.”