This paper presents the Smart Knee Rehab, a low- cost system for post-Total Knee Arthroplasty (TKA) rehabil- itation that combines a fully passive mechanical guide with markerless computer vision to measure knee range of motion (ROM) during flexion and extension exercises. The passive rail constrains motion to the sagittal plane to promote safer execution at home, while the camera-based algorithm provides quantitative ROM feedback without wearable sensors. In validation against an iPhone-based inclinometer reference (Measure app on iPhone 13 Pro Max; gyroscope), the system achieved a mean absolute error of 2.38◦ and a root mean square error of 4.09◦.