Biomechanic changes in passive properties of hemiplegic ankles with spastic hypertonia

Abstract

Objective: To investigate quantitatively biomechanic changes in the passive properties of hemiplegic spastic ankles.

Design: Evaluation of spastic hypertonia by moving the ankle joint slowly between dorsiflexion and plantarflexion extreme positions under controlled joint torque and position.

Setting: Institutional research center.

Participants: Twenty-four stroke patients with spastic ankles and 32 healthy controls.

Interventions: Not applicable.

Main outcome measures: Passive resistance torque at controlled dorsiflexion and plantarflexion positions, dorsiflexion and plantarflexion range of motion (ROM) at controlled torques, and quasistatic stiffness and energy loss in dorsiflexion and plantarflexion.

Results: Spastic hypertonic ankles showed significant alterations of the passive properties in plantarflexion (P=.041) as well as in dorsiflexion (P=.016) directions. Compared with healthy controls, spastic ankles showed higher resistance torque (9.51+/-4.79Nm vs 6.21+/-3.64Nm, P=.016), higher quasistatic stiffness (.54+/-.19Nm/deg vs .35+/-.20Nm/deg, P=.001) at 10 degrees of dorsiflexion, larger normalized dorsiflexion energy loss (.068+/-.04J/deg vs .04+/-.02J/deg, P=.037), and decreased dorsiflexion ROM at 10Nm of resistance torque (10.77 degrees +/-8.69 degrees vs 20.02 degrees +/-11.67 degrees , P=.014). The resistance torque, ROM, and stiffness of spastic hypertonic ankles in plantarflexion showed similar changes (P<.05) to those in dorsiflexion. The passive ROM, joint stiffness, and resistance torque at controlled positions correlated with each other and also correlated with the Modified Ashworth Scale (P<.01).

Conclusions: Various biomechanic changes in both plantar- and dorsiflexors are associated with spastic hypertonia of chronic stroke patients, and they can be evaluated quantitatively under well-controlled conditions. With simplifications, the various measures in this study can potentially be used to obtain more comprehensive and quantitative evaluations of spastic hypertonia in a clinical setting.