Effects of Hydraulic Ankle-Foot Prostheses on Gait in Individuals with Transtibial Limb loss: A Scoping Review

For persons with transtibial limb loss, successful use of a prosthesis relies on a comfortable fitting socket and an ankle-foot mechanism which allows stable and efficient gait. The anatomical ankle performs eccentric and concentric muscle contractions to control plantarflexion and dorsiflexion in the sagittal plane at various times during the stance phase of the gait cycle. This function is described as the three rockers of stance phase: heel rocker, ankle rocker, and forefoot rocker [1]. Additionally, the anatomical ankle actively dorsiflexes for increased toe clearance in swing phase of gait. Passive prosthetic ankle-foot components are unable to replicate the concentric contractions of the anatomical ankle plantarflexors and dorsiflexors, while designs incorporating compression of elastic elements (e.g. bumpers) or deflection of flexible elements (i.e. heel and keel) attempt to simulate some of the eccentric functions of the anatomical ankle.

The majority of passive prosthetic ankle-foot components, such as energy storing and returning feet (ESAR), flexible keel feet (FK), and solid ankle cushion heel feet (SACH), do not incorporate an articulating ankle joint. Any degree of ankle dorsiflexion or plantarflexion is achieved through compression or deflection of a passive heel or keel element, and a recreation torque is produced at the ankle as these materials act like a spring. Earlier designs of single-axis (SA) feet did include a hinge joint, although compression of elastic bumper elements still created a reaction moment about the ankle. These fixed-ankle designs resulted in a single equilibrium point where no reaction moment acts at the ankle of the prosthesis. Limitations of fixed-ankle and SA ankle-foot components for ambulation over level ground have been described, such as reduced knee flexion and prolonged heel only support [2], and gait deviations on sloped surfaces with fixed-ankle components [3]

As ankle-foot component design has improved, with the first commercial release of a passive hydraulic ankle-foot (HAF) prosthesis, the Echelon (Chas. A. Blatchford & Sons Ltd., Basingstoke, UK), in 2009. Since that time, more HAF prostheses have become commercially available, with each system providing different capabilities in range of motion and carbon fiber heel and keel design. Comparatively, hydraulic controlled technology has been available for persons with transfemoral amputation much longer. The first hydraulic knee components were developed in the 1950s with the Mauch knee [4].

HAF prostheses incorporate a hinge joint, dampened by either a linear or rotary hydraulic unit, and a passive foot component. The foot component can be comprised of a FK or ESAR module, but the foot module generally has a much lower build height compared to other fixed-ankle systems without hydraulic technology. The hydraulic joint typically approximates the ankle joint center of rotation and allows plantarflexion and dorsiflexion range of motion through retraction and extension of the hydraulic unit. Hydraulic componentry provides the main functional difference between HAF and fixed-ankle components (such as SACH, FK, SA, and ESAR). As the hydraulic cylinder retracts and extends through a range of motion, torque is absorbed that would otherwise deflect the passive keel elements in the foot component. The hydraulic unit provides a range of equilibrium points where no reaction torque is created about the ankle, which has been described as the ability to automatically re-align and achieve foot-flat [5]. A resistance to plantarflexion and dorsiflexion in hydraulic ankles is created as the hydraulic fluid passes through small hydraulic ports. The resistance to ankle rotation is dependent upon the rate of retraction or extension of the hydraulic cylinder and the settings that control the size of the hydraulic ports. At smaller hydraulic port size settings and greater rates of retraction or extension, the hydraulic ankle rotational stiffness increases and a larger reaction torque about the ankle can be generated.

One literature review of HAF prostheses included articles which were published prior to March of 2016 and focused on gait kinetics, kinematics, spatial, temporal, and pressure parameters [6]. Recently, additional passive HAF prostheses have become commercially available from seven different manufacturers and more recent evidence of benefits of HAF prostheses has been published. There is a need to review existing research literature on passive HAF technology. Therefore, the purpose of this scoping review was to determine if there is sufficient evidence to conduct a formal systematic review or meta-analysis of passive hydraulic ankle-foot prostheses in individuals with transtibial limb loss. This systematic review will not include evidence of microprocessor controlled hydraulic ankle-foot prostheses, as it will focus only on passive hydraulic ankle-foot prostheses.