Preoperative motor strength and time to surgery are the most important predictors of improvement in foot drop due to degenerative lumbar disease.

Objective: Palsy of dorsiflexion, or foot drop, may be due to degenerative lumbar disease and amenable to posterior spinal decompression. The objective of this study is to measure prognostic factors of and time to foot drop improvement after posterior lumbar decompression.

Methods: We retrospectively reviewed 71 patients undergoing first-time, posterior lumbar decompression for foot drop due to degenerative spinal disease. Patient sex, age, comorbidities (Charlson Comorbidity Index), preoperative anterior tibialis strength (manual muscle testing, MMT), and duration of foot drop were ascertained from clinical notes. Prognostic factors affecting foot drop improvement were calculated with a discrete time proportional hazards model, in which follow-up times and outcome measures were binned into six time intervals: 1 week, 6 weeks, 3 months, 6 months, 1 year, and ≥ 1 year.

Results: Of the 71 patients, the mean age was 54.6 ± 16.0 years, and 66.2% (n=47) were males. The mean Charlson Comorbidity Index was 2.42. During a mean follow-up of 30.4 months, dorsiflexion function improved postoperatively in 73.2% (n=52) of patients. The median time to surgery from onset of foot drop was within 6 weeks, and the median preoperative MMT strength of patients with foot drop improvement was 3. Following a discrete-time proportional hazards model, duration of anterior tibialis palsy (HR=0.67, P=0.004) and preoperative muscle strength (HR=1.10, P=0.010) were significant predictors of foot drop improvement. Following an adjusted Kaplan-Meier analysis, the median time to foot drop improvement was within 6 weeks of surgical intervention.

Conclusions: Preoperative muscle strength and palsy duration were statistically significant predictors of foot drop improvement. Furthermore, the median time to improvement was 6 weeks.