In this paper we investigate the potential of a smartphone to measure patients' changes in physical activity before and after a surgical pain relief intervention. Providing an objective intervention outcome measure to clinicians could enhance subjective assessments from patient questionnaires and contribute to optimal patient treatment. Thus, we show a proof of concept for our smartphone system providing physical activity from acceleration, barometer and location data to infer meaningful activity features that measure the intervention's outcome. In a case study, we monitored two patients carrying the smartphone 9 days before and another 9 days after a surgical intervention. Results indicate significant activity changes after intervention while the pain level decreased. Particularly physical activity in the home environment increased significantly for both patients where an averaged 98% increase in walking and a more than 150% gain in fast cadence was measured. Questionnaire assessed activity levels showed no meaningful correlations to activity measurements and turned out to be highly subjective.