Measuring the Right Angle

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January/February 2005

By Kell G Waugh, MA, PT

Figure 1: Wheelchair support surface angles.

The seating angles of a wheelchair’s body support system can significantly affect the sitting posture of a patient, and, consequently, their overall function, health, skin safety, and comfort. Therefore, proper specification of seating angles is a critical component of a wheelchair seating assessment, especially for those individuals with poor sitting balance or posture, abnormal muscle tone, or joint contractures.

Although research has shown that the orientation of the seat and back support can affect trunk posture, muscle hypertonus, and respiratory and upper extremity function,^1-5 evidenced-based guidelines for prescribing the primary relative seat support angles are lacking. Therefore, these primary seating angles—seat to back support angle, seat to leg support angle, and leg support to foot support angle—should be individually determined by an experienced clinician (PT or OT) as part of the standard wheelchair seating assessment. While it is equally critical to specify the orientation of the seat and back support with respect to gravity, this article will focus on the primary clinical variables to consider when determining the three relative seat support angles.

SUPPORT SURFACE ANGLES
The following support surface angles describe the angular orientation of the three primary wheelchair support surfaces relative to each other in the sagittal plane (Figure 1):

Seat to back support angle
Seat to leg support angle
Leg support to foot support angle

Note that the seat to leg support angle is dependent on the location of the front edge of the seat support relative to the back edge of the foot support.

BODY SEGMENT ANGLES
We can use the terminology of body segment angles to describe a person’s sitting posture (actual or desired) when at rest. The following body segment angles describe the relative angle between adjacent body segments in the sagittal plan (Figure 2):

Trunk to thigh angle
Pelvic to thigh angle
Thigh to leg angle
Leg to foot angle

Figure 2: Body segment angles.

Body segment angles describe a static position, not a range of motion; however, a person’s joint range of motion will affect the body segment angles that he or she can comfortably maintain in the sitting position. The pelvic to thigh angle reflects the position of true hip flexion, and when this differs from the overall trunk to thigh angle, one can assume the spine is either flexed or extended from the neutral position. Note that if a seated person’s body segments are parallel to the wheelchair’s support surfaces, then the body segment angles will be the same as the support surface angles, provided the seat depth matches the depth of the thigh (Figure 3).

Figure 3: Body segment angles and support surface angles.

However, it is very important to differentiate between the angular measures of the seating device and of the seated person’s body, as they may or may not be the same.

A wheelchair’s primary support surfaces should support the person in the desired resting posture, but allow functional movement away from, and back to, that resting posture. In order to prescribe the desired wheelchair support surface angles, the therapist and wheelchair user need to first determine the desired resting posture in the sagittal plane, which can be described by using body segment angle terminology.

One of the most important variables to consider when determining these angles is joint range of motion. During the mat evaluation portion of a wheelchair seating assessment, joint range of motion is assessed to determine end comfortable range for seating at the hips, knees and ankles while maintaining desired pelvic/spinal alignment. These numbers are used to determine the preliminary desired body segment angles for sitting, which in turn help to determine the corresponding support surface angles (see Table 1).

Following is a review of the primary variables to consider during assessment to determine each of the three support surface angles. Remember that the individual must be able to function and be comfortable at the prescribed angles.

SEAT TO BACK SUPPORT ANGLE
Your general objectives for the seat to back support angle are (1) to support the person in their maximum comfortable hip flexion that still allows maintenance of that person’s optimal and desired pelvic/spinal alignment, and (2) to position the trunk for balance and functional movement. The following individual-specific variables should be assessed and considered when prescribing the wheelchair seat to back support angle:

Hip flexion range of motion
Abnormal muscle tone
Balance and trunk control
Functional activities

HIP FLEXION RANGE OF MOTION
In most individuals, as the hips are flexed, tension in the hip extensors will at some point cause posterior rotation of the pelvis with progressive flexion of the spine.⁶ Sacral sitting with a kyphotic spine can have numerous negative consequences for long-term sitters, and it is therefore generally accepted that this posture should be prevented if possible.^7-12 The point in hip flexion range when the pelvis begins to rotate varies with each individual, and for many wheelchair sitters, it occurs before reaching 90°of hip flexion.

The end of hip flexion range of motion for seating can be defined as that point just before the pelvis starts to rotate posteriorly, or before discomfort, soft tissue, or joint restrictions cause increased resistance to movement.

The minimum thigh to trunk angle that can be comfortably tolerated while maintaining the desired pelvic/spinal alignment should be determined by an experienced therapist as part of the initial seating assessment. If you decrease a person’s trunk to thigh angle past this determined minimum by closing up the seat to back support angle, the pelvic to thigh angle will not change (as there is no more true hip flexion range) and a kyphotic sitting posture will result if there is spinal flexion mobility.

ABNORMAL MUSCLE TONE
To help decrease extensor hypertonus and posturing, the seat to back support angle should support the individual in their maximum comfortable hip flexion that still allows maintenance of a neutral to mild lumbar lordosis. Maintaining a position of maximum hip flexion is thought to inhibit hip extensor spasticity by placing the spastic muscles on prolonged stretch,^3,13 and a neutral to mild lordosis can facilitate normal spinal extension, postural stability, and upper extremity function. “Wedging” the seat surface upward to achieve a seat to back support angle of less than 90° is a common practice intended to increase the angle of hip flexion and thereby reduce extensor tone and thrusting.

While this sometimes successfully reduces hypertonus, it can frequently result in either thoracolumbar flexion with poor head posture and decreased upper extremity function, or discomfort with a triggering of increased hypertonus. This may occur because many people with extensor spasticity have a limitation in hip flexion range of motion. While the person may appear to be sitting in greater hip flexion on a wedged seat due to a decrease in their trunk to thigh angle, closer inspection reveals that their pelvis has rotated posteriorly¹⁴ with a resulting increase in their pelvic to thigh angle (which equals less hip flexion). In order to decrease hypertonus while also maintaining spinal alignment, it is important to support the pelvis posteriorly at its minimum comfortable pelvic to thigh angle by properly angling (or shaping) the lower back support surface. This may require a seat to back support angle greater than 90°, but you may actually achieve a greater degree of hip flexion by maintaining control of the pelvis posteriorly.

RISK OF INSTABILITY AND SLIDING
The more you open up a seat to back support angle past 90°, the greater the tendency to slide forward on the seat, especially when the person has low muscle tone and a heavy torso. Decreasing the thigh to trunk angle to at least 105° to 100° either by closing up the seat to back support angle or by contouring the seat or back support can help prevent sliding.

A posterior inclination of the seat surface, with or without changes in the seat to back support angle, can also help prevent sliding and increase feelings of postural stability; however, pay attention to limitations in hip flexion when determining the final desired body segment angles.

BALANCE AND TRUNK CONTROL
Changes in the seat to back support angle will affect the position of the trunk and head with respect to the vertical. Therefore, the effect of orientation in space on an individual’s head and trunk control, abnormal muscle tone, and pressure distribution across the buttocks needs to be assessed and considered carefully in the context of their therapeutic and functional goals.

Kyphosis deformities, fixed posterior rib humps, or a forward head posture can shift a person’s center of gravity anterior to their hip joint causing forward instability, and a more open trunk to thigh angle may be required for balance. Conversely, a more upright trunk position achieved by either decreasing the trunk to thigh angle or slightly tilting the seat and back support anteriorly may facilitate active spinal extension and improved posture in an individual with low muscle tone.⁴

FUNCTIONAL ACTIVITIES
Because of its effect on orientation in space, changes in the seat to back support angle can affect a variety of functional issues, such as upper extremity reach, access to rear wheels for self-propulsion, visual regard of the environment, independent transfers, and physiological functions such as swallowing and breathing. For example, sitting with a trunk to thigh angle that is too open may make it difficult for an individual to use their arms to access a computer. Conversely, sitting with a trunk to thigh angle that is too acute may prevent the rib cage expansion necessary for optimal respiration.

SEAT TO LEG SUPPORT ANGLE
The general objectives for the seat to leg support angle are (1) to support the lower legs so that the knees are in a comfortable degree of knee flexion with sufficient slack in the hamstring muscles to allow maintenance of the person’s optimal pelvic/spinal alignment, and (2) to achieve adequate weight bearing through the foot supports. The following individual-specific variables should be assessed and considered when prescribing the wheelchair seat to leg support angle:

Knee extension range of motion with hips flexed
Abnormal muscle tone or movement patterns
Functional mobility/maneuverability

KNEE EXTENSION RANGE OF MOTION WITH HIPS FLEXED
Many wheelchair sitters—children as well as older adults—have tightness in their hamstring muscles. If shortened or spastic hamstring muscles are overstretched at the knee as a result of foot supports being positioned too far forward, the tension will pull the ischials anteriorly causing a posterior pelvic tilt, thoracolumbar flexion, and sliding.^14,15 In addition to hamstring tightness, long-term wheelchair users may also have fixed knee flexion contractures that limit knee extension range of motion in sitting. Therefore, many individuals will require a seat to leg support angle of less than 120°, some even less than 90°, in order to prevent discomfort and sliding. The maximum thigh to leg angle that can be comfortably tolerated while still maintaining desired pelvic/spinal alignment should be determined by an experienced therapist as part of the initial seating assessment.

ABNORMAL MUSCLE TONE OR MOVEMENT PATTERNS
Positioning the knee in increased flexion by maintaining a thigh to leg angle of 90° or less can help to reduce abnormal extensor tone and thrusting of the lower extremities. This is often critical to maintaining postural stability in individuals with severe extensor tone.

FUNCTIONAL MOBILITY NEEDS/MANEUVERABILITY
A wheelchair’s seat to leg support angle affects how far the foot supports project forward, and thus impacts the overall length of the wheelchair and its turning radius. Therefore, an individual who needs to maneuver their wheelchair in tight spaces may require a “tighter” seat to leg support angle.

LEG TO FOOT SUPPORT ANGLE
The general objective in prescribing the leg support to foot support angle is to support the foot at a comfortable angle of dorsiflexion while also maximizing weight bearing through the foot. A 90° leg support to foot support angle is usually best for helping to support the weight of the legs and stabilize the pelvis and thighs, provided there is sufficient ankle flexibility. The following individual-specific variables should be assessed and considered when prescribing the wheelchair leg support to foot support angle:

Ankle range of motion
Abnormal muscle tone or movement

ANKLE RANGE OF MOTION AND DEFORMITIES
A person must have at least zero degrees of dorsiflexion in order to comfortably sit with a leg to foot angle of 90°. If this cannot be achieved passively, then the foot must be supported in a position of plantar flexion by angling the footplate; otherwise, there will be decreased contact and support to the foot, which may increase feelings of instability. The optimal leg to foot angle should be determined by an experienced therapist as part of the initial seating assessment.

ABNORMAL MUSCLE TONE AND MOVEMENT PATTERNS
Maintaining a small degree of dorsiflexion by decreasing the leg support to foot support angle can help control extensor tone and posturing provided range of motion is adequate. However, the objective of inhibiting extensor tone distally is often better achieved through the use of ankle foot orthoses.

JUST THE BEGINNING
The support surface angles reviewed here are just a few of the many features of a wheeled mobility device that the therapist must discuss and determine with the wheelchair user, and communicate effectively to the rehabilitation technology supplier (RTS), for successful technology prescription. Observing and measuring a seated person’s body segment angles is a valuable clinical tool that facilitates effective problem solving during the assessment and prescription process.

Kelly G. Waugh, MA, PT, is an assistive technology specialist working in the Wheelchair Seating and Mobility Clinical Services division of Assistive Technology Partners, University of Colorado Health Sciences Center, Denver.

REFERENCES

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Hobson DA. Contributions of posture and deformity to the body-seat interface variables of a person with spinal cord injury. Proceedings of the Fifth International Seating Symposium. Memphis, Tenn: University of Tennessee, Memphis Rehabilitation Engineering Program; 1989:153-171.
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January/February 2005