The objective of this paper is to document design lessons learned in the development of low-cost lower-limb prosthetic devices. Four design principles are introduced based on the authors' experience redesigning a low-cost prosthetic knee mechanism. These design principles are valuable sources to complete the conceptual design of the knee mechanism in the absence of a unique or exact methodology that comprises all the aspects concerning engineering design. The paper defines the motivation to improve life quality by restoring knee functionality in terms of mobility and stability. Subsequent sections explore the state of the art, design constraints, gait analysis, and 3-D modeling using CAD. The model is validated using kinematic and structural simulation in NX 7.5TM covering basic geometric parameters and motion assumptions. The four conceptual design principles are: Hybrid Design Model, Simultaneous Modeling, Multidisciplinary Pulses, and Built-in and Post-design Optimization. Each principle is presented to exemplify its contribution in the knee mechanism redesign. Finally, the design principles are intended to assist the designer with empirical guidelines in the development of prosthetic devices.