### Related Literature  
Anthropological and evolutionary studies of the human hand suggest that the fingers are approximately equal in length and arranged in a circular pattern on the palm [12].  
Notably, the center of this circular arc is not at the anatomical midpoint of the hand; rather, it lies at the base of the thumb, where the lines of action of the fingers intersect. This configuration facilitates the movement of all fingers and the thumb towards one another, enhancing opposition and increasing finger interaction [13].  
Grasping studies of the human hand indicate that it is a two-stage process: (i) prehension, where the arm extends toward the object and the fingers, fully stretched, gradually flex and adduct to conform to the shape and size of the object, and (ii) grasping, where the fingers envelop the object [14]. The number of fingers extended and flexed during grasping depends on both the object grasped and the intended task. Critical to note here is that, during the prehension phase, regardless of the object's shape and size, the fingers used for grasping achieve their maximum \"aperture\", defined as the distance between the tips of the fingers and the thumb. The grasping phase primarily involves finger joint flexion with minimal adduction.  
While these insights suggest that a simplified MCP joint design could benefit prosthetic hands, the literature lacks a consensus on optimal finger placement on the palm. Previous analyses of kinematic data from human participants have sought to determine the ideal number of DoF for prosthetic hands, aiming to simplify the design process based on the required manipulations [15]. Kinematic designs for the MCP joint's planar and spherical mechanisms have been proposed in [16] to minimize coordination errors in abduction/adduction (Ab/Ad) motion. Some studies have attempted to simultaneously actuate both F/E and Ab/Ad motions using flexure-based joints and soft fingers [17]. Other designs have positioned the fingers on a circular arc centered at the MCP joint of the middle finger, with finger inclination based on anatomical and surgical data [18]. Tendon-driven soft fingers were designed to produce 3-dimensional motion by authors in [19].  
MCP joint designs commonly reported in the literature include (i) parallel finger configurations without Ab/Ad motion, (ii) four-DoF fingers with active Ab/Ad joints at the expense of simplicity, and (iii) passive Ab/Ad motion via various mechanisms, materials, or control systems, often leading to reduced fingertip force production. Given the need for effective grasping across various object shapes and sizes, incorporating the functionality of Ab/Ad motion into prosthetic hand designs is essential. Therefore, this work seeks to integrate Ab/Ad functionality into a 3-DoF finger design to minimize design complexity. A preliminary design for a complete prosthetic hand is also presented to test and validate the palmar orientation and finger placements. The scope of the current work is limited to finger orientations and locations of the index, middle, ring, and little fingers.  
The remainder of this paper is organized as follows: Section 2 analyzes the MCP joint, quantifying its significance and calculating finger configuration based on the MCP joint. Section 3 provides the results of the grasp validation study. A preliminary prosthetic hand design is also provided to validate the theory proposed in the work experimentally. Finally, a discussion and conclusion are provided to end the paper.