Seating Shapes

When recommending a seating system, one should consider the shape of the seating system and how the shape of the cushion or back will impact the individual's positioning. The spine and pelvis can be supported by more than five different shapes and, if not fitted correctly, will create postural instability. The pelvis and lower extremities can be supported by more than four different shapes that will affect stability. It is important to consider these shapes when accommodating or correcting orthopedic asymmetries. The seating shape may also determine the method of pressure distribution. Lastly, the shape may affect the overall comfort of the individual sitting in the seating system.

The following resource provides a visual for the most common shapes and contours of a seating system, and what the outcomes may be if they are not properly fitted.

Pelvis & Spine Seating Shapes

Assessment Goals

  • Posterior pelvic stability
  • Posterior-lateral pelvic stability
  • Lumbar support
  • Posterior thoracic stability
  • Lateral thoracic stability
Posterior sacral support not present

Posterior Sacral Support Not Present

  • Pelvis will collapse into a posterior pelvic tilt
  • Flattening of the lumbar spine
  • Increase in thoracic spine kyphosis
  • Hips sliding forward
Posterior-lateral sacral support not present

Posterior-Lateral Sacral Support Not Present

  • Pelvis and spine may become asymmetrical
  • Pelvis may collapse into a posterior tilt and rotated position
  • Flattening of the lumbar spine
  • Hips may slide forward
  • Pelvis may shift laterally
  • Pelvis may become oblique, spine may become laterally flexed

Posterior Thoracic Support (Shape)

Posterior thoracic support (shape) - too little

Too Little

  • May cause inadequate accommodation of thoracic spine, creating forward or lateral collapse of trunk
  • May cause poor head position/control
  • In absence of correct shape, may push pelvis and/or trunk forward
Posterior thoracic support (shape) - too much

Too Much

  • May inhibit function
  • May encourage a collapsed trunk posture
  • This can be common with bariatric patients

Lateral Thoracic Support

Lateral thoracic support - Three point control

Vertical Placement Range

  • Location must support the ribcage
  • Provide three points of control
Lateral thoracic support - too low/too shallow

Too Low/Too Shallow

  • Thoracic spine may not be supported
  • May lead to collapse of trunk and poor trunk control
  • May cause skin irritation

Too Deep

  • May interfere with upper extremity function and/or cause injury

Thoracic Support (Height)

Thoracic support (height) - too low

Too Low

  • Thoracic and/or lumbar spine may not be supported
  • May lead to collapse of trunk and poor trunk control
Thoracic support (height) - too high

Too High

  • Upper extremity function may be compromised
  • May cause instability or discomfort
  • May cause sliding away from backrest
  • May cause increased pressure on scapula and thoracic area

Lumbar Support (Shape)

Lumbar support (shape) - too little

Too Little

  • In the absence of posterior pelvic support contour, the lumbar area may collapse
  • May cause posterior pelvic tilt
  • May cause sliding of pelvis forward
Lumbar support (shape) - too much

Too Much

  • Pelvis may rotate posteriorly or anteriorly
  • Trunk can fall forward
  • Extensor muscles may compensate for leaning forward and inhibit function

Pelvis & Lower Extremity Seating Shapes

Assessment Goals

  • Assess hip flexion Range of Motion (ROM)
  • Assess hamstring length
  • Provide lateral stability
  • Provide anterior stability
  • Maximize surface contact area

Pelvic Contour Width

Consider protecting the trochanters via offloading or immersion/envelopment.

Pelvic contour width - too wide

Too Wide

Trochanters not supported may cause:

  • Lateral instability and/or pelvic obliquity
  • Ischial Tuberosities (ITs) can bottom out
  • This may be common in pediatric patients
Pelvic contour width - too narrow

Too Narrow

  • This can be common with clients who are bariatric
  • Creates a pelvic obliquity
  • Increases pressure on greater trochanters

Pelvic Contour Length

Buttocks should be supported while loading femurs for stability. Ischial Tuberosities (ITs) need to be protected during activity.

Pelvic contour length - too long

Too Long

  • ITs can slide forward into posterior pelvic tilt which can lead to additional loading on coccyx
  • Possible inadequate femoral loading
  • May cause increased shear force at ITs
Pelvic contour length - too short

Too Short

  • Results in insufficient space for IT movement for functional activity
  • ITs may press into anterior shelf of cushion, causing potential skin integrity issues

Pelvic Contour Depth

The buttocks should be supported while maintaining optimal hip angle. Correct height depends on difference in height between ischials and posterior aspect of femur.

Pelvic contour depth - too deep

Too Deep

  • May cause interference with hip angle
  • May increase or decrease hip flexion angle, depending on hip ROM and amount of support at posterior pelvis
  • The pelvis may not be optimally loaded, which can lead to additional loading at coccyx
Pelvic contour depth - too shallow

Too Shallow

  • Femurs will not be loaded
  • May not prevent sliding
  • May not provide optimal pressure reduction at the ischials

Femoral Support Length

Femoral loading stabilizes the pelvis, positions the lower extremities, and redistributes pressure.

Femoral support length - too long

Too Long

  • Pulls the hips forward in the seat, which may cause sliding
  • Inhibits function
  • Increases pressure behind knees
Femoral support length - too short

Too Short

  • Not enough surface contact area for loading
  • Ischials may have increased pressure
  • Lower extremities may not be optimally positioned
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References

  1. Ágústsson, A., Sveinsson, Þ., & Rodby-Bousquet, E. (2017). The effect of asymmetrical limited hip flexion on seating posture, scoliosis and windswept hip distortion. Research in Developmental Disabilities,71, 18-23. doi:10.1016/j.ridd.2017.09.019
  2. Rodby-Bousquet, E., Ágústsson, A., Jónsdóttir, G., Czuba, T., Johansson, A., & Hägglund, G. (2012). Interrater reliability and construct validity of the Posture and Postural Ability Scale in adults with cerebral palsy in supine, prone, sitting and standing positions. Clinical Rehabilitation,28(1), 82-90. doi:10.1177/0269215512465423
  3. Waugh, K. and Crane, B. (2013). A clinical application guide to standardized wheelchair seating measures of the body and seating support surfaces (rev. Ed.). Denver, CO. University of Colorado Denver. Available from: http://www.ucdenver.edu/academics/AssistiveTechnologyPartners/resources/WheelchairSeating/Pages/WheelchairGuideForm.aspx

Date: 2019-07-26


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