SPASTICITY AND STRENGTH CHANGES AS A FUNCTION OF SELECTIVE DORSAL RHIZOTOMY

J.R. Engsberg (a,b), K.S. Olree (a), S.A. Ross (a), T.S. Park( a,b)
(a) Motion Analysis Laboratory, St. Louis Children's Hospital
(b) Washington University School of Medicine, St. Louis, MO 63110

Presented at the 20th Annual Meeting of the American Society of Biomechanics
Atlanta, Georgia. October 17-19, 1996

INTRODUCTION

Selective dorsal rhizotomy (SDR) is performed on children with cerebral palsy to minimize or eliminate the influence of spasticity.5 One contraindication and a limitation of SDR surgery is muscle weakness.1 Currently objective measures to quantify spasticity and weakness are not used in the surgical decision making process or in outcome evaluation. This investigation quantified pre- and post-surgery spasticity and maximum active resultant joint torques in the hamstrings of children with cerebral palsy (CP group) undergoing a selective dorsal rhizotomy. Spasticity values were significantly greater than similar values for children with able bodies (AB controls) prior to surgery but not significantly different from AB controls after surgery. Maximum joint torques did not change as a function of surgery and were significantly less than the AB controls.

REVIEW AND THEORY

Recently, objective measures to quantify spasticity using velocity dependent resistance to stretch'4 as its characterization and the maximum active resultant joint torque at the knee were developed.2,3 The rationale for the development of the spasticity measure was due to the paucity of objective measures utilized in its quantification and the desire for the measure to be based upon sound mechanical principals. The chosen characterization permitted an objective measure based on simple mechanical constructs (i.e., angular velocity and a resistive torque through an angular range of motion). A similar process occurred for the maximum active joint torque measure. However, in addition, the maximum active torque protocol had to allow even the weakest child the opportunity to generate torque over a full range of motion. The purpose of this investigation was to quantify changes in spasticity and active joint torques as a function of SDR surgery. It was hypothesized that a significant decrease in both spasticity and active joint torques would occur as a result of the surgery.

PROCEDURES

Ten children (mean 11 years, range 4-16) with spastic diplegic cerebral palsy and undergoing SDR were tested the day prior, and approximately 8 months post-surgery. The AB controls consisted of 6 children (mean 9 years, range 4-17).

Each child sat on a dynamometer and had knee joint extension range of motion limits established. For the spasticity measure, the machine extended the leg passively through a range of motion at speeds of 10, 30, 60, and 90o/s while monitoring the resistive torque from the hamstrings.2 For the active torque measure, the machine flexed the leg through a range of motion at 10o/s while the child performed a maximum contraction of the hamstrings.3 Torque-angle data were processed to partial out effects of gravity and minimize acceleration and machine dynamic responses. Areas within the torque-angle curves were calculated for each speed and child, yielding work values (Fig 1).

Fig 1. Work calculation of single child at a single speed for the spasticity measure.

For the spasticity measure, linear regression was used to derive the slope of the line of best fit for the work-velocity data. For the strength measure, work by the hamstrings was utilized. Paired and unpaired t-tests were used to test for significant differences pre- and post-surgery and between groups, respectively (p<0.05).

RESULTS

Pre-surgery, the hamstring spasticity in the CP group was significantly greater than that of the AB controls (Fig 2). Post-surgery values were significantly less than pre-surgery values and not significantly different than AB controls. Pre-surgery values for maximum flexion work of the hamstrings were not significantly different from post-surgery values. Both pre- and post-surgery work values were significantly less than similar values for the AB controls. The hypothesis that spasticity would be reduced was supported, while the hypothesis that active joint torques would be reduced was not.

Fig 2. Mean values and standard deviations for spasticity and active torques in CP group (i.e., pre- and post-surgery) and AB controls

DISCUSSION

When determining whether an SDR should be performed, pre-surgery spasticity and weakness and post-surgery weakness are important concerns. Previous investigations have not simultaneously quantified spasticity and maximum active joint torque variables pre- and post-SDR. The present investigation indicated a reduction in hamstring spasticity, but not a reduction in active hamstring work. Whether this trend persists at the ankle and hip is presently unknown.

The standard deviation bars in Figure 2 illustrate the large amount of variation existing within the CP group. In 2 children, the pre-surgery spasticity measures were slightly less than the mean for the AB controls. For the active torque measure, 6 children displayed increases from pre- to post-surgery, but 3 children indicated decreases. This large variation seems to indicate the diversity of children that are undergoing the SDR surgery and leads to at least two considerations. The first is whether all of the children undergoing the SDR have the same degree of post-surgery functional outcome. It could be that those children with low spasticity and active torque values prior to surgery do not improve to the same degree as those with greater pre-surgery spasticity and active torques. The second is whether the measures quantified in this investigation could be used to aid in the selection process for children being considered for the SDR surgery. It is possible that children with spasticity and maximum active torques below a certain level are not candidates for the surgery. Current subjective methods of selecting the children do not appear to identify this subgroup. Investigations quantifying both spasticity and maximum active joint torques at the ankle, knee, and hip and functional measures (e.g., gait) could help clarify these issues.

REFERENCES

1. Cahan, L.D. et al. In Park (ed) Neurosurgery: State of the Art Reviews, 4(2) 477-484, 1989.

2. Engsberg, J.R. et al. Arch of Phys and Rehab Med, In Press, 1996.

3. Engsberg, J.R. et al. Dev Med Child Neuro, Supp 73, 42, 1995.

4. Lance, J.W. In Feldman (ed) Spasticity: Disordered Motor Control, 48, 1980.

5. Park, T.S. et al. Neurosurgery, 33(5), 929-934, 1993.