Bionic ankle-foot prosthesis normalizes walking gait for persons with leg amputation

Abstract

Over time, leg prostheses have improved in design, but have been incapable of actively adapting to different walking velocities in a manner comparable to a biological limb. People with a leg amputation using such commercially available passive-elastic prostheses require significantly more metabolic energy to walk at the same velocities, prefer to walk slower and have abnormal biomechanics compared with non-amputees. A bionic prosthesis has been developed that emulates the function of a biological ankle during level-ground walking, specifically providing the net positive work required for a range of walking velocities. We compared metabolic energy costs, preferred velocities and biomechanical patterns of seven people with a unilateral transtibial amputation using the bionic prosthesis and using their own passive-elastic prosthesis to those of seven non-amputees during level-ground walking. Compared with using a passive-elastic prosthesis, using the bionic prosthesis decreased metabolic cost by 8 per cent, increased trailing prosthetic leg mechanical work by 57 per cent and decreased the leading biological leg mechanical work by 10 per cent, on average, across walking velocities of 0.75-1.75 m s(-1) and increased preferred walking velocity by 23 per cent. Using the bionic prosthesis resulted in metabolic energy costs, preferred walking velocities and biomechanical patterns that were not significantly different from people without an amputation.

Figure 1.

Bionic ankle–foot prosthesis. (a) The bionic prosthesis attaches to the socket via a pylon and has a mass of 2 kg, equivalent to the biological foot and partial shank of an 80 kg person [39]. (b) The prosthesis includes an actuator in-series with a carbon-fibre leaf spring, in parallel with a unidirectional leaf spring, and heel and forefoot leaf springs that provide elasticity. (c) A series-elastic actuator performs negative and positive work. The actuator comprises a 200 W DC brushless motor (Maxon EC-Powermax 30) and ball-screw transmission (Nook 14 × 3 mm) in series with a carbon-composite leaf spring. A 0.22 kg Lithium-polymer rechargeable battery provides energy to the motor. The prosthesis is 67% efficient; approximately 30 J of electrical energy produce 20 J of net positive work during the stance period of walking, the typical energy requirement for an 80 kg person walking at 1.75 m s⁻¹ [35]. A charged battery produces 4000–5000 steps, sufficient to walk 4–5 km at 1.75 m s⁻¹ and exceeding the 3060 ± 1890 steps per day typically walked by an active PWA [40].

Figure 2.

Ankle angle, work and torque versus angle across walking velocities. (a) Average ankle angle at toe-off and (b) ankle work per step during the stance phase of walking for PWA using the powered prosthesis (blue diamonds) were within 2 s.d. of the mean data for non-amputees (green dashed lines). Non-amputee toe-off data are extrapolated from Winter [22] and ankle work data are from Palmer [23]. The foot segment is perpendicular to the shank segment at an ankle angle of zero (inset in (a)). Error bars indicate s.e.m. Grey shaded areas represent ±1 s.d. of the non-amputee average. (c) The prosthetic ankle joint torque versus angle for a representative participant with an amputation using the bionic prosthesis illustrates the positive work (grey shaded areas within the traces) provided by the prosthesis. 1, heel strike; 2, foot flat; 3, maximum dorsiflexion; 4, toe-off. (i) 0.75 m s⁻¹; (ii) 1.00 m s⁻¹; (iii) 1.25 m s⁻¹; (iv) 1.50 m s⁻¹; (v) 1.75 m s⁻¹.

Figure 3.

Gross metabolic cost of transport and preferred walking velocity. (a) PWA using the bionic prosthesis (blue diamonds) had nearly the same average metabolic cost of transport (COT) as non-amputees (green squares) (p > 0.50 for 0.75–1.50 m s⁻¹; p = 0.17 for 1.75 m s⁻¹). Cost of transport is the metabolic energy needed to transport unit weight a unit distance, equal to (metabolic demand)/(body weight × distance travelled). PWA using a passive-elastic prosthesis (red circles) had 11–25% greater cost of transport (p < 0.05) at 1.00–1.75 m s⁻¹ compared with non-amputees. (b) Average preferred walking velocities of PWA using the bionic prosthesis (blue bar) and non-amputees (green bar) were equivalent (p = 0.97). PWA using a passive-elastic prosthesis (red bar) preferred to walk significantly slower (p = 0.008). Values within bars indicate averages (s.e.m.). Asterisks (*) indicate significant differences between PWA using the bionic prosthesis compared with using a passive-elastic prosthesis. Error bars indicate s.e.m. Second-order polynomial curve equations in (a) are, non-amputee: COT = 0.303v² – 0.846v + 0.934, r² = 0.49; bionic prothesis: COT = 0.308v² – 0.808v + 0.895, r² = 0.39; and passive-elastic prosthesis: COT = 0.295v² – 0.750v + 0.295, r² = 0.52. COT is calculated in Joules per Newton body weight per metre. v is velocity in metres per second.

Figure 4.

Step-to-step transition work. (a) Average individual leg mechanical step-to-step transition work from PWA using the bionic prosthesis (blue bars) was not different from non-amputees (green bars) except for trailing leg work at 1.75 m s⁻¹. The leg using a prosthesis is always the trailing leg and the biological leg is always the leading leg in PWA. Average ankle work per step attributed to the bionic prosthesis (white stripes overlaid on trailing leg blue bars) comprised approximately 87% of the overall trailing leg work done on the centre of mass across all velocities. At 1.75 m s⁻¹, the ankle work performed by the bionic prosthesis was greater than the trailing leg step-to-step transition work, implying that ankle work was dissipated at the knee and/or hip joints. PWA using a passive-elastic prosthesis (red bars) generated significantly less trailing leg work and experienced greater leading leg collision work compared with using the bionic prosthesis and to non-amputees. The hash sign (#) indicates a significant difference between PWA using the bionic prosthesis and non-amputees, asterisks (*) indicate significant differences between PWA using the bionic prosthesis compared with using a passive-elastic prosthesis, and carets (^) indicate significant differences between non-amputees and PWA using a passive-elastic prosthesis (p < 0.05). Error bars indicate s.e.m. (b) Step-to-step transition work equals the time-integral of mechanical power (Watts with respect seconds). Mechanical power is the dot product of each leg's ground reaction force (F_trail and F_lead) and the instantaneous centre of mass velocity (v_com) [44].