# Artificial-Hand Technology--Current State of Knowledge in Designing and Forecasting Changes
## 3 Construction
### Joint Types  
Finger classification is one of the basic and superficial types of categorisation. For closer determination of prehension ability, joint analysis has to be conducted. First of all, the nomenclature must be clear. The concept of degrees of freedom is commonly used in mechanical engineering and robotics to describe mechanism mobility [91]. This term states the number of independent input parameters required to explicitly determine the state of the whole system. For example, a kinematic chain with two revolute joints, each driven by single actuator, has 2 DOFs. The same system but coupled, so one actuator controls both links, is perceived as having 1 DOF. The necessity of this explanation comes from the arbitrary usage of \"degrees of freedom\". Authors exploit it to show mechanical construction (only the number and type of joints) as well as hand dexterity (including actuation and transmission) [92; 93; 94]. In compliance with the above definition, the first application is not correct in some cases and could lead to misunderstandings. In this regard, the term \"degree of motion\" (DOM) is introduced in this chapter to describe the mechanics of robotic hands with no interconnections. The concept \"degree of freedom\" is used in the context of the whole hand in the next chapter where actuating systems are considered. In addition, some research groups  
Figure 3: Timeline of number of fingers in robotic hands. distinguish both aspects by using DOF and active DOF, where the latter means joints with independent actuation [89; 95]. There are several kinds of joints that are applied to artificial hands and they are summarized in Table 1. Most frequently, a simple hinge is used for an axial motion like flexion and extension of PIP and DIP links [96; 97; 98; 99]. The gimbal is a combination of two or three hinges that rotate around a geometric center point, and is often employed in MCP joints where abduction is preferred [92; 100]. Another type, the pivot ball, has three degrees of motion, which is arguably a good solution for thumb CMC application; however, in MCP, one of the motions must be prevented from torsional activity. This joint was used in [101; 102] as an MCP connection, but neither author mentioned the phenomenon of third axis rotation. Alternative designs are ellipsoidal or saddle joints that are inspired by the human skeletal system. Both [93; 100; 103] devices were based on biocorrect bone models, thereby replicating a natural-link strategy. Contrary to previously described constructions, these were not equipped with a mechanism that keeps parts of the structure together. Hence, an elastic ligament was provided to stabilize bones and constrain motions, as well as to hold synovial fluid that reduces friction between two articulated surfaces. There is also a relatively new group of elastic devices that have pneumatic actuation. They are made of very stretchable silicone (EcoFlex) that, through inner chambers, can bend in a specified direction when pressurised air is applied. [PERSON] et al. in [104] presented a single-modal hand where even joints could not be identified as only sections of five fingers, and inner and outer palm were mentioned. In addition, they were controlled by four supplying channels. Despite that, the hand not only passed the Kapandji test, but was also able to perform 31 out of the 33 Feix grasps. On the other hand, [PERSON] et al. [105] proved that properly projected fingers and multichannel actuators (described in the next section) lead to humanlike construction. This hand had 2 DOM in MCP and CMC joints, and 1 in PIP, DIP, and IP, so there was no difference from typical rigid devices. Robotic hands where nothing except joints is made of flexible components like rubber or spring belong to elastic devices [106; 107; 108]. Such a structure is cheap and easy to apply, but it does not protect from side or torsional impact that may cause damage to the actuation and transmission system.