As we perform our slew of daily activities — brushing teeth, checking e-mail, sitting in conference calls, helping the kids with homework after dinner — our bodies work to make those activities possible. They breathe, pump blood, regulate temperature, and provide the energy needed to move and think. It’s hard work, so our bodies also look for opportunities to rest and maintain balance, or to conserve energy when possible.
One result of that natural tendency can be the “slouching” we were admonished for as kids, and that seating professionals working with wheelchair users regularly encounter.
What Posterior Pelvic Tilt Looks Like
Wade Lucas, PT, DPT, ATP/SMS, is the Clinical Education Manager for Quantum Rehab.
“Posterior pelvic tilt,” Lucas said, “is the most common detrimental postural tendency that we see, not only in users of mobility equipment, but people in general. When the human body is forced to hold itself up against gravity, it will naturally seek out the most stable, least fatiguing position. When a person is seated, the position of stability is often a posterior pelvic tilt, as it lowers the center of gravity.”
In layman’s terms, posterior pelvic tilt looks like slouching. Sam Hannah, ATP, Executive VP of Symmetric Designs, explained, “The wheelchair user can present as kyphotic, sacral sitting, having rounded shoulders, tight hamstrings and a forward head position. The pelvis will be tipped backward and the torso forward. The wheelchair user’s head will be forward and their eye level looking down. Their PSIS [posterior superior iliac spine] will be lower than their ASIS [anterior superior iliac spine].”
For wheelchair users and non-wheelchair users alike, this position can feel relaxing and comfortable, at least temporarily. So why is posterior pelvic tilt potentially a problem?
“There is frequently a lack of support from the backrest in the lower lumbar area, and there can also be increased pressure in the sacrum due to sitting in a forward position on the seat cushion,” Hannah said. “There is also increased pressure on the back due to less contact for pressure distribution.”
Lucas noted, “A posterior pelvic tilt can facilitate the development of other postural deviations throughout the rest of the body, including an increased thoracic kyphosis and forward head position. More subtly, you also see abduction and external rotation of the legs and increased weight bearing on the lateral aspects of the foot.”
So this posterior pelvic tilt impacts how the rest of the body is positioned in the wheelchair and how well a wheelchair user functions as a result.
Impact on Other Body Functions
Compare optimal seated posture with what happens in posterior pelvic tilt.
“Ideally, the pelvis is positioned in neutral to a slightly anterior tilted position,” Lucas said. “In this position, the ASIS and PSIS will be level and parallel to the seating surface. The ischial tuberosities will be oriented vertically, and there should be little to no contact of the sacrum with the seating surface.
“When a person’s pelvis tilts posteriorly, the ‘tips’ of the ischial tuberosities slide anteriorly and point more horizontally. There will also be a significant increase in surface contact of the sacrum to the seat, thus increasing the chance of pressure injury.”
“Prolonged posterior pelvic tilt can result in skin breakdown, pressure ulcers due to increased pressure and shear forces on the ischial tuberosities and sacrum, nerve pain, digestion problems and respiratory problems,” Hannah said. “A wheelchair user sitting with a posterior pelvic tilt will also likely have a kyphotic posture, which will negatively affect respiration and circulation by increasing pressure on the lungs and heart. The seat depth, cushion well position, and leg channels can all be in a poor position, resulting in increased pressure when the user is sitting forward on the cushion.”
The different position of the pelvis, Lucas added, impacts how other systems work in the body.
“Posterior pelvic tilt by itself is not always that noticeable at first glance,” he said. “However, the position of the pelvis has a strong correlation to the position of other body segments. This correlation leads to the person sitting in a very ‘slouched’ type [of] posture. When the pelvis is not in a well-balanced, neutral position, then it leads to compensatory postural deviations of the lower extremities, trunk, head/neck, and upper extremities. Posterior pelvic tilt tends to facilitate external rotation/abduction of the hips, increased thoracic kyphosis of the trunk, flexion of the lower cervical spine with hyperextension of the upper cervical spine, rounded shoulders and internal rotation of the arms. This posture can impair a number of body functions — most notably, respiration, chewing/swallowing, digestion, bowel and bladder elimination and even circulation. It can also minimize shoulder range of motion, reach and function; increase pain in the neck, trunk and low back; decrease sitting tolerance; and significantly limit a person’s visual field. In addition, prolonged sitting in this position can cause orthopedic deformities that limit joint range of motion and create non-reducible deformities.”
And as previously mentioned, posterior pelvic tilt can significantly raise the risk of pressure injury.
“Posterior pelvic tilt also can be extremely detrimental to an individual’s skin integrity,” Lucas confirmed. “First, as a person begins to shift from a neutral to a posterior tilted position, there are increased shearing forces that occur at the ischial tuberosities, sacrum and coccyx. This shearing over the bony prominences can be more damaging than pressure itself. Once a posterior tilted position is achieved, there will likely be a higher amount of pressure over the ischial tuberosities and the sacrum. When positioned in a neutral pelvic tilt, there is little to no pressure over the sacrum.
“In addition to the increased pressure risk on the buttocks caused by a posterior pelvic tilt, the resultant hip external rotation/abduction increases the pressure-related injury risk at the bony prominences in the lower extremities. These at-risk areas include greater trochanters, lateral femoral condyles, fibular head and lateral malleoli. The resultant increased thoracic kyphosis also creates more prominent spinous processes, which increases the risk for pressure points at the apex of the curve.”
And as Hannah pointed out, “Posterior pelvic tilt also affects the center of gravity of the wheelchair, which changes the level of effort required to propel the wheelchair.”
Why Posterior Pelvic Tilt Occurs
Posterior pelvic tilt can start simply, as the result of the body trying to find a more comfortable position while sitting.
“Prolonged sitting time in the wheelchair reduces hip flexor muscle and back muscle strength,” Hannah said, “which leads to a curvature of the spine. This can lead to a shift in the pelvis where the user’s weight is more focused on the backrest, requiring less engagement of their abdominals, glutes and hamstrings. The less those muscles are engaged, the more there is increased likelihood of posterior pelvic tilt.”
“Posterior pelvic tilt is a position of stability, so the body tends to seek out this position,” Lucas explained. “First of all, it allows the person’s center of gravity to shift posterior to their base of support, which increases stability and decreases the workload on postural muscles. Secondly, the shift towards a posterior tilt causes an increase in surface area contact with the seating support surface, thus increasing stability.
“The person’s physical characteristics can play a role in posterior pelvic tilt development,” he added. “These physical characteristics include hypotonicity/impaired core strength, extensor hypertonicity, tight hamstrings, decreased hip flexion range of motion, and non-reducible thoracic kyphosis.”
Unfortunately, seating can also contribute to posterior pelvic tilt, Lucas said: “Other potential causes include the improperly set-up or insufficient support in the setup of the seating system. For example, if the seat depth is too long, then the pressure on the popliteal fossa [can cause] the person to slide forward and collapse into a posterior pelvic tilt as they seek support from the back of the chair. Too much seat depth can also prevent the person from getting their hips all the way back to the back of the chair, causing the pelvis to rock posteriorly to meet the backrest. Legrest length setup is key to pelvic positioning, as well. If the legrest length is too long, then the person must shift the lower extremities forward to use them. This causes the lower half of the pelvis to slide forward and the top half of the pelvis to rock posteriorly.
“When the legrests are too short, again there is a lack of support/surface contact area on the posterior thigh, and the body will seek out a more stable positioning. Short legrests may also force the person into more hip flexion than they have available, thus forcing the person’s pelvis posteriorly to accommodate for this. Backrest size and placement is also key. The backrest must be placed and adjusted to provide posterior support to the PSIS and lumbar spine. If there is inadequate support in this area, then a posterior pelvic tilt is very likely to occur.”
Seating & Positioning Interventions
Fortunately, the properly specified and fitted seating system can have a big impact on posterior pelvic tilt, Hannah said: “There are a number of factors that affect pelvic positioning within a wheelchair. It is important to choose a seat cushion that has an appropriate contour to support the posterior pelvis, including an appropriately positioned and shaped ischial well with proper depth. The seat dump angle will also affect posture and pelvic pressure and positioning. A pelvic positioning belt can also assist with holding the pelvis in a neutral position. A backrest with increased lumbar support can also assist with supporting a neutral position.”
“Another consideration,” Lucas said, “is the use of tilt (whether it is manual or power tilt) for decreasing posterior pelvic tilt. Allowing the person to tilt back 5° to 15° can decrease the negative effects of gravity on an individual’s posture. Gravity will help keep the pelvis, trunk, and head back against the seating supports in a well-supported position. This tends to be a preferred position of stability and comfort while still allowing the person to complete safe, functional activities.
“This position can also decrease pain and increase sitting tolerance throughout the day. For an individual with a non-reducible thoracic kyphosis, this position can increase the person’s visual field and reduce the need to go into a posterior pelvic tilt to achieve a forward line of sight.”
Shear is always a concern during repositioning, so Lucas said, “When considering the use of a recline system, care should be taken in order to limit friction and shear on the buttocks and back of the client. Using recline by itself may rotate the pelvis posteriorly, and the buttocks/back can slide on the backrest and seat cushion. Tilt should be used prior to using recline, which will utilize gravity to keep the pelvis back and in a neutral position. When the person comes back up to an upright position, they should recline up prior to tilting back up.
“These steps are vital in maintaining pelvic position and skin integrity when using power positioning systems.”
And in helping to ensure posterior pelvic tilt is optimally managed.