The Effect of Anesthesia on Passive Ankle Dorsiflexion

Equinus contractures can commonly be due to contractures of gastrocnemius muscle or combined contractures of the gastrocnemius-soleus Achilles tendon complex. The decision to release part or all of the gastrocnemius-soleus Achilles tendon complex is often assessed intraoperatively while the patient is under anesthesia. It remains unknown whether the administration of general anesthesia affects the measurement of passive ankle dorsiflexion.

The unaffected, nonoperative limb on 46 foot and ankle patients underwent a Silfverskiold test measuring passive ankle dorsiflexion preoperatively and intraoperatively after administration of general anesthesia using an instrumented force-angular displacement goniometer. To determine clinical significance, we surveyed experienced surgeons to estimate the perceived minimally detectable clinical accuracy for measuring passive ankle dorsiflexion.

Forty-six subjects were included with mean age of 42 ± 14.8 years, mean body mass index of 26.2 ± 4.9, and 52% female. The mean change in dorsiflexion values from before anesthesia to after the administration of general anesthesia was 1.9 degrees with 10 lb of pressure with knee extended (E10), 2.3 degrees with 20 lb of pressure with knee extended (E20), 2.8 degrees with 10 lb of pressure with knee flexed (F10), and 2.3 degrees with 20 lb of pressure with knee flexed (F20) (all P < .001). Thirty-three of 45 (73%) surgeons responded to the survey; all thought their minimally detectable clinical accuracy was 5 degrees or greater.

After the administration of general anesthesia, a small but likely not clinically detectable increase in passive ankle dorsiflexion occurs. The common clinical practice of making intraoperative treatment decisions regarding the presence of a gastrocnemius-soleus driven equinus contractures after general anesthesia without use of paralytic agents appears reasonable given the magnitude of the changes identified in this study.