FORCE, PRESSURE AND PELVIC ALIGNMENT OF CHILDREN WITH CEREBRAL PALSY USING A SACRAL PAD AND KNEEBLOCK IN AN ADAPTIVE SEATING SYSTEM

FORCE, PRESSURE AND PELVIC ALIGNMENT OF CHILDREN WITH CEREBRAL PALSY USING A SACRAL PAD AND KNEEBLOCK IN AN ADAPTIVE SEATING SYSTEM.

R.L. McDonald, B.App.Sc.(OT), PostGradDip (Biomechanics)

INTRODUCTION

Seating systems that use a sacral pad and kneeblock arrangement are commonly used throughout the United Kingdom, with varying degrees of perceived success. A method for measuring force exerted through a kneeblock and counterpressure through a sacral pad has been developed and found to be reliable with normal children and weights. This paper describes the relationship between force at a kneeblock and pressure behind the sacrum with the first 9 disabled children to take part in the study. In addition to the exploration of force and pressure, a modified version of the Seated Postural Control Measure (SPCM) (Fife et al, 1991) is used to measure postural alignment. The relationship between pelvic parameters (pelvic tilt and pelvic rotation) were analysed in relation to force measurements.

BACKGROUND

The purpose of using a sacral pad and kneeblock arrangement is to apply forces to the pelvis to create a ‘moment’ (turning effect) by the sacral pad, pushing the pelvis into a neutral position, which is balanced and maintained by an opposing counterforce at the kneeblock (Green & Nelham, 1991, Mulcahy et al, 1988). At present, published information about the efficacy of this type of system is of a descriptive nature only (Reid & Rigby, 1996).

A force transduction device was developed, consisting of strain gauges attached to the normal kneeblock. Pressure at the sacral pad was measured using a commercially available skin interface pressure device (The Oxford Pressure Monitor). 7 normal children took part in the pilot project, ranging from 4 to 12 years of age. Data was collected over the space of a day with the children using the special seating system.

Mean force and mean pressure were collected and analysed together in the normal children and consistent relationship was found. Regression analysis was performed and a positive correlation of .777 between Force and Pressure was shown.

SUBJECTS

Having found a linear relationship between force and pressure in normal children, the next stage is to examine this relationship with the main cohort of children. At the time of writing 9 children with cerebral palsy have begun the main trial, and their first visit are the results presented here. 5 of the children have predominantly athetoid/dystonic type of cerebral palsy and 4 of the children have primarily spastic quadriplegia. All are aged between 4 and 13 years.

METHOD

Children are seen at school or in their own homes. Force and pressure measurements are taken at the initial visit together with a seated postural control measure and other descriptive measures. The children sit in their normal seating system, which is modified only by replacement of the child’s own sacral pad with the instrumented sacral pad, and the instrumented kneeblocks (which are adjusted as closesly as possible to the children’s own kneeblocks) replace the children’s normal kneeblock. During the recording period children are asked to sit still, perform a reaching activity and an eating activity.

RESULTS

Data was transformed to make it conform to a normal distribution and a series of analyses were performed. No relationship between force at the kneeblocks and pressure behind the sacrum was shown to be significant. The results were then further analysed by examination of individual cases. We were able to see that there were at least two identifiable conditions - those whose force versus pressure graphs do conform to a linear relationship (n=5) (figure 2 is one example), those whose problems can be easily identified by slipping down in their chair (n=2,figure 1) is one example) and those whom there was no identifiable relationship and whose response to the system may be due to the artefacts of their individual neurological problem.

Figure 1 - Slipping in Chair Figure 2 - Linear Relationship

Finally, force was analysed against the pelvic parameters of pelvic tilt and pelvic rotation. Children without a relationship between force and pressure were excluded from analysis. No relationship was found between force through the kneeblock and degree of pelvic tilt in the chair (figure 3). However, when looking at pelvic rotation, it was shown that as the difference in force between the left and right side of the kneeblocks increases, pelvic rotation decreases. This is significant at a level of p = 0.01 at greater than 10 degrees of pelvic rotation.

Figure 3 - Force and Pelvic Tilt Figure 4 - Force difference and Pelvic Rotation

DISCUSSION

The data does not show an overall relationship between pressure at the sacrum and force through the kneeblocks, nor a relationship between backwards-pelvic tilt and total force through the kneeblock. However it is interesting to look at which children there is a relationship and which there is not and may be valuable in identifying which children will find this type of seating system useful. Furthermore, the relationship between pelvic rotation and difference between left and right force is encouraging. More children have been recruited into the study and are being followed up on a monthly basis for six months. The outcome for the study will be important for influencing the decision making of therapists in providing this form of seating for children with cerebral palsy.

REFERENCES

1. Fife, S.E., Roxborough, L.A., Armstrong, R.W., Harris, S.R., Gregson, J. L.,Field, D. (1991). Development of a clinical measure of postural control for assessment of adaptive seating in children with neuromotor disabilities. Physical Therapy, (12), 981-993.

2. Green E.M. & Nelham, R.L.(1991). Development of sitting ability assessment of children with a motor handicap and prescription of appropriate seating systems. Prosthetics and Orthotics International, (1991), 15, 203—16.

3. Mulcahy, C M. Pountney, T, Nelham, R L, Green, E M, and Billington, G D. Adaptive seating for motor handicap. Problems, a Solution, assessment and prescription. British Journal of Occupational Therapy, (1988) 51(10), 347-352.

4. Reid, D. & Rigby, P. Development of improved anterior pelvic stabilization devices for children with cerebral palsy. Physical and Occupational Therapy in Pediatrics, 1996, 16 (3), 91-96.

Notes: