* \<b\>Background\</b\>: Tremor is a common symptom in people with Parkinson's disease (PD), affecting 75% of patients. While PD tremor usually occurs at rest, 46-93% of patients also have action tremor. Action tremor often does not respond to dopaminergic medication, and stereotactic surgery is invasive and unsuitable for many patients. By interfering with daily activities, action tremor can reduce independence and quality of life, necessitating new treatments.
* \<b\>STIL Orthosis\</b\>: The STIL Orthosis, a CE-certified device, aims to reduce tremor by dampening wrist flexion-extension and forearm pronation-supination. It features dampeners that limit high-frequency movements, such as tremor, while maintaining voluntary movements. In patients with essential tremor (n=24), the STIL Orthosis reduced action tremor severity during daily activities by 65-87% compared to baseline. This suggests that it could be a promising treatment for PD action tremors.
* \<b\>Design\</b\>: In this study, 25 PD patients will be recruited to test whether the STIL Orthosis can be used to treat PD-related action tremor. Action tremor will be measured under three conditions: while wearing the STIL Orthosis (intervention), while wearing an orthosis without dampening (sham), and while wearing no orthosis (baseline). Severity of action tremor in all conditions will be assessed using the TETRAS scale.
* \<b\>Primary outcome\</b\>: The primary outcome of this study is action tremor severity, measured using the \<b\>TETRAS scale\</b\>. The main study endpoint is the \<b\>difference in TETRAS scores between two conditions (intervention vs. sham and intervention vs. baseline)\</b\>.
* \<b\>Secondary outcomes\</b\>: As secondary outcomes, we will measure tremor severity using the MDS-UPDRS part III scale, focusing on items related to postural, kinetic, and rest tremor (3.15, 3.16, 3.17). We will also measure tremor power using surface electromyography (sEMG) and accelerometry. Furthermore, we will assess perceived change in tremor using the PGC-I, user satisfaction on relevant items from the D-QUEST, and product safety using an adverse events report form.
* \<b\>Sample size calculation\</b\>: The sample size calculation is based on a pilot study with 8 Parkinson's disease patients experiencing action tremor in the forearm, which showed an effect size of 1.24 1.86 for MDS-UPDRS part 3 (tremor items 3.15, 3.16, and 3.17) and 2.21 for TETRAS. Given that patients only benefit from a device that has a large effect on their tremor, an effect size \<b\>Cohen's d=0.8\</b\> was chosen, which is also common practice in the medical literature. In total, two main comparisons will be done on the primary outcome (TETRAS score): intervention vs. sham, intervention vs. baseline. To adjust for multiple statistical tests, a Bonferroni correction of \<b\>alpha = 0.05/2 = 0.025\</b\> is used. To achieve \<b\>80% power in a two-sided paired t-test\</b\>, a minimum of \<b\>18 patients\</b\> needs to be included. In order to compensate for possible drop-outs, 25 patients will be included for this clinical investigation.
* \<b\>Statistical analysis\</b\>: All analyses will follow the intention-to-treat principle. Paired-samples t-test will be used to test for intervention-related effects on action tremor severity. In the event that tremor severity scores are not normally distributed, Wilcoxon signed rank tests will be used instead.
