# Prosthesis Embodiment in Lower Extremity Limb Loss: A Narrative Review
## 4 Results
### Bidirectional Devices and Modular Design  
Historically, prosthetic devices have been unidirectional devices, focusing primarily on mechanical functional output to replicate the patient's active range of motion of the joints. These traditional devices have not been designed to transmit sensory feedback from the external environment back to the patient. However, the sense of ownership is a product of the integration of visual and tactile sensory channels [12; 37; 38; 39]. For example, the rubber hand illusion is diminished in cases where visual and tactile stimulation are asynchronous, demonstrating that the congruence of multisensory inputs is vital to the illusion [12]. Therefore, for a lower limb prosthesis to engage one's sense of ownership, the sensory feedback from that system must be strategically designed and tuned.  
Advanced neuroprosthetic technology includes bidirectionality and can communicate sensory feedback to the patient, which is fundamental to enhancing the functionality and user experience of lower limb prostheses, including mobility and balance. Figure 1 summarizes a system diagram for a bidirectional mechanoneural interface (MI) proposed by [PERSON] and [PERSON] in [40]. Their proposed design is modular and identifies the two modules for sensory feedback: a proprioceptive mechanoneural module interface (PMI) and cutaneous mechanoneural interface (CMI). Example PMIs are discussed in Sections 4.3 and 4.4, and an example of CMIs is discussed in Section 4.5.  
The idea of a modular system began in an attempt to represent external forces experienced while walking, such as gravity, in a proprioceptive framework [41]. This resulted in unnatural sensations that could not be characterized solely within a proprioceptive context, motivating the inclusion of cutaneous mechanoneural interfaces in their design to provide natural sensations and improve biomechanical fidelity. However, it should be noted that the intersection between cutaneous sensation and proprioception can be gray. For example, [PERSON] et al. demonstrated that proprioception can be simulated by cutaneous stimulation [42] by using surface skin electrodes to transmit information about the angle of the knee joint to the wearer. [PERSON] et al. were able to generate both perception modalities using the C-FINE devices described in Section 4.5 [43]: Table 2 shows the variety of sensations that can be generated with such devices, some of which are proprioceptive.  
Figure 1: System diagram for a bidirectional mechanoneural interface (MI) proposed by [PERSON] and [PERSON] in [40]. The MI framework comprises two parts: a proprioceptive mechanoneural interface and a cutaneous mechanoneural interface. The proprioceptive mechanoneural interface provides bidirectional control of the prosthesis through a biophysical controller, whereas the cutaneous mechanoneural interface relays topical sensory feedback afferents to the central nervous system.  
Prosthesis embodiment is a multifaceted and holistic concept. A modular prosthesis that can integrate both proprioceptive and cutaneous feedback creates a more natural and intuitive sensory experience and reinforces the sense of ownership of the limb. This design is a first step towards that. In addition, studies have not yet been conducted to evaluate a combination of interventions discussed in the review, and could be a direction of future research.