An interactive sculpting tool is being widely used to segment a 3-D object on a volume rendered image for improving the intuitiveness. However, it is very hard to segment only an outer part of a 3-D object, since the conventional method cannot handle the depth of removal. In this paper, we present an effective method to determine the depth of removal, by using the proposed spring-rod model and the voxel-opacity. To determine the depth of removal, the 2-D array of rigid rods is constructed after a 2-D closed loop is defined on a volume-rendered image by a user. Each rigid rod is located at a digitized position inside the user-drawn closed loop and its direction is coincident with that of projecting rays. And every rod has a frictionless ball, which is interconnected with its neighboring balls through ideal springs. In addition, we assume that an external force defined by the corresponding voxel-opacity value is exerted on each ball along the direction of the projected ray. Using this spring-rod system model, we can determine final positions of balls, which represent the depths of removal. Then, the outer part can be properly removed. The proposed method is applied to various medical image data and is evaluated to provide robust results with easy user-interaction.