THE BIANGULAR BACK REVISITED

THE BIANGULAR BACK REVISITED –-

USE, MISUSE AND CLINICAL POTENTIALS

Simon A. Margolis, CO, ATS/P

ABSTRACT

In 1987, Wengert, Margolis and Kolar described a back design that addressed the problem of head control/positioning and pelvic positioning/alignment – the BiAngular Back (BAB). This paper relies heavily on that work for background and design information. Since then, many attempts have been made to commercialize the design. Most have been less than successful and the concept of the BAB has not been widely used to meet the seated positioning needs of people with disabilities. This paper presents the background behind and indications/contraindications for the BAB. This paper is neither based on "hard" science nor on extensive outcomes studies and research. It is based, rather, on the author’s empirical observations and subjective clinical successes and failures using the BAB design with over 2,500 clients from 1986 to 1993.

BACKGROUND

Head Control/Position

To determine the etiology of most head control problems, we must look lower on the anatomy at the normal curvature, or lack of normal curvature, of the spine and at the position of the pelvis. In most cases, when the curves of the spine are "abnormal", the resulting head position will be "abnormal". To approximate a normal curvature in the spine we must stabilize the pelvis in the desired position; recreate the normal lumbar lordosis; thoracic kyphosis; and cervical lordosis; and thereby balance the head over the pelvis.

Pelvic Position/Stabilization

Often, when seating the client with cerebral palsy, traumatic brain injury and other diagnoses, we are faced with problems of increased hip extensor activity, resulting in "pelvic thrust" or a posterior pelvic tilt. A commonly accepted approach to dealing with this problem is attempting to flex the hip joint past 90 degrees by decreasing the seat-to-back angle of the seated positioning system; or by using a "rolled" or anti-thrust seat design that has the effect of decreasing the seat-to-back angle.

Repeatedly, this concept is often used, or misused, without taking into consideration:

limitations in passive range of motion at the hip joint;

the ability of the seated positioning components to insure appropriate and reproducible pelvic position;

the relationship of the angle of the anatomical hip joint to the mechanical angle between the seat and back of the seated positioning system; and

potential decrease in upper extremity function due to excessive rearward tilt forcing the musculature to function against gravity.

Limitations in PROM at the Hip Joint

In the case of limitations in the passive range of motion at the hip joint, decreasing the seat-to-back angle forces the pelvis into further deformity by increasing posterior tilt. This may have the effect of decreasing the mechanical advantage of the hip extensor muscles at the expense of promoting potentially rigid or even fixed deformities of the spine - often at the expense of head control/position. When the pelvis is forced to tilt posteriorly the entire spine is affected. Normal lordosis is lost, thoracic kyphosis is increased, cervical lordosis is decreased, and the neck and head fall into flexion. If the client has active neck extension, then the typical "stacking" pattern may result.

Appropriate Pelvic Stabilization

If we are not careful to stabilize the pelvis, the body will slide forward, or push forward during an "extensor thrust" and end up with the same result as with insufficient range of motion at the hip joint. Often times in individuals with increased hip extensor activity, a seat belt is not adequate to stabilize the pelvis - even when securely tightened and attached with a correct line of pull, i.e., below the anterior superior iliac spine (ASIS). This is often due to the subjective nature of how tight a securely fastened seat belt should be and/or user or caregiver compliance. In these cases, a Subasis Bar (Margolis, 1985) is more effective in securing the pelvis and preventing forward thrusting at the hips. It is also useful to reduce and/or prevent the progression of rotational deformities at the pelvis.

Mechanical vs. Anatomic Angles

Another important factor to consider in designing a seated positioning system is that even when the mechanical seat-to-back angle is 90 degrees, the anatomic hip angle, even with the pelvis properly secured, may be greater than 90 degrees. In a study by Nwaobi, results showed that "related hip flexion angle and the angle between the backrest and the seat are not equal in most seating positions". In this study, when the seat-to-back angle was 90 degrees or less, the hip flexion angle was always greater than the seat-to-back angle. This was true for both non-disabled subjects and subjects with cerebral palsy.

DESIGN

The BiAngular Back (BAB) design incorporates a two section back. The lower section is attached, or aligned to, the seating surface of the seated positioning orthosis at an angle of approximately 75 to 85 degrees (as allowed by the clients passive range of motion). The lower back section extends superiorly to the level of the posterior superior iliac spines (PSIS). The lower back is rigidly attached to the upper back section at an angle that complements the seat-to-back angle plus 5 degrees. By inclining the upper back posteriorly, we assure that the anterior/posterior midline of the head is set just posterior to the midline of the pelvis.

The BAB as part of a seated positioning system

A pelvic positioning belt or Subasis Bar positioned to apply a posterior/inferior force, inferior to the ASIS, must be used as a counter force to the anteriorly directed force applied to the PSIS. The effect of this design is to reduce hip extensor tone or "thrusting" by putting the hip extensor muscles on stretch and decreasing the mechanical advantage of the hip extensors. From a clinical perspective, the stability afforded by this design may interrupt pathologic hip extensor patterns and allow the client to use back extensor musculature, when available, for increased function and stability.

The BAB design avoids the problem of lost head control/position caused by usual attempts to reduce hip extensor activity by decreasing seat-to-back angle alone. The fact that the upper section of the back is set at an angle to complement the decreased seat-to-back angle, allows the lumbar spine to extend and places the head in a balanced position over the pelvis. The need for anterior head support is virtually eliminated. A posterior head support applying forces primarily in the area of the occipital shelf, in a superior/anterior direction, is extremely effective in maintaining optimal head control/position and proper alignment relative to capital flexion and extension.

Unless the client has sufficient trunk extensors, a force applied posteriorly in the upper thoracic region is also necessary. A properly positioned anterior thoracic vest or shoulder stabilizers will also accomplish this task. The BAB design reduces the amount of posterior directed force that needs to be generated by these components by appropriately positioning the upper torso and interruption of pathologic tone patterns.

We must assure that there is sufficient room for the scapulae and that they are not simply pushed into the padding of the upper back. A relief may need to be cut into the back padding at a level just below the inferior angle of the scapulae and extending superiorly to the top of the back . This scapular relief helps assure that upper extremity function is not compromised by limiting the "slide-glide" movement of the scapula during shoulder range of movement.

The posteriorly directed force required may also be generated through the use of a mobility base that allows posterior orientation in space (OIS). Using an OIS frame should be carefully considered and monitored because tilt angles of greater than 5 – 10 degrees may compromise functional capability

Other Clinical Application

The BAB design is also an effective tool to position clients who exhibit hypotonicity. Realigning the spine with a BAB, securing the pelvis and providing appropriate suboccipital support is the first step. The BAB system is extremely effective for these individuals because the complimentary nature of the upper BAB angle positions the torso so that the midline of the client’s head is positioned just posterior to the midline of the pelvis. This creates an extension moment in the cervical spine. We can then use the occipital support to bring the head into alignment and to control capitol extension.

Client’s who exhibit a reducible yet profound flexor synergy, i.e., hip flexion, tightened abdominal musculature and shoulder protraction, etc., present a unique seated positioning challenge. In many cases of this nature, attempting to tilt these individuals posteriorly only exacerbates the flexion pattern. Using the BAB design, in conjunction with appropriate pelvic stabilization, provides external input to sacrum and has proved clinically to facilitate lumbar extension over the stable pelvis. This provides the same head control/position possibilities described above.

CONCLUSION

Using a BiAngular Back design, appropriate positioning of the pelvis and alignment of the spine is achieved. The normal complimentary curves of the spine are restored and the head is realigned over the pelvis. To work effectively, the BAB must be used in conjunction with an entire seated positioning system – specifically configured to provide effective and reproducible pelvic stabilization

REFERENCES

Wengert, M.E.; Margolis, S.A.; Kolar, K.: A design for the back of seated positioning orthoses that controls pelvic positioning and increases head control. In: The Proceedings of the 10th Annual RESNA Conference, pp. 216-218, June 1987

Nwaobi, O.M.: The relationship between the mechanical and anatomical hip angle in adaptive seating. In: The Proceedings of the Third International Seating Symposium, p. 119, February 1987.

Margolis, S.A., et. al.: The Subasis Bar: an effective approach to pelvic stabilization in seated positioning. In: The Proceedings of the 8th Annual Conference on Rehabilitation Engineering, pp. 45-47, June 1985

Notes: