A biosynthetic thumb prosthesis

Abstract

In cases of severe damage to the extremities, the function and structure of compromised tissues must be replaced. If biological reconstruction using autologous tissue is not feasible, amputation and replacement with a synthetic prosthesis is often the next best option. Synthetic prostheses are limited, especially in their ability to restore skin sensation. Here we show a biosynthetic prosthesis that combines the versatility of titanium with the rich sensory capabilities of biological skin. The prosthesis recreates opposition pinch by linking motion of the prosthetic joint to that of the residual biological joint, and is enclosed in neurotized skin from the patient's own body. We validated the biosynthetic thumb's mechanical function on the benchtop and in a cadaver, and showed viability of the skin interface in an animal model. These results provide a framework for functional reconstruction of amputated digits using a combination of synthetic materials and biological tissues.

Keywords: Preclinical research; Translational research; Trauma.

Fig. 1. Surgical approach and device design.

a (left to right) Biological thumb with intact native joints and soft tissue. The IP joint is amputated, and native skin covers the residual proximal phalanx. The device is attached to the local bony structures including the residual proximal phalanx and metacarpal. The reverse radial forearm (RRF) and first dorsal metacarpal artery (FDMA) flaps are harvested and rotated about their pedicles to cover the implant. The donor and recipient sites are sutured closed, and the system is allowed to heal. b The implant is made up of four parts: the thumb tip, structural link, driving link, and metacarpal plate. The linkage connects the MCP and IP joints, such that flexion of the biological MCP joint creates flexion of the synthetic IP joint. Static (non-joint) portions of the implant are covered in porous HDPE, and the synthetic IP joint is protected by a flexible cover. A prototype was manufactured in Ti-6Al-4v.

Fig. 2. Benchtop mechanical testing results.

a Experimental setup, in which the prototype thumb is clamped into an adjustable load frame and loaded with the proxy MCP joint locked at different angles. b MCP-IP angle relationships showing the target trajectory, the idealized linkage design, the clinically viable linkage design, and the experimental performance of the physical prototype (Supplementary Table 1). c Deflection of the thumb-tip marker across seven locked proxy MCP angles. d Stiffness of the device as a function of MCP angle. Stiffness represents the amount of perpendicular force required at the thumb tip to produce 1 mm of linear deflection of the thumb tip. e Experimental MCP-IP angle relationship in the presence of different loads. Dots show average measured angle over 1.5 s, and solid lines show first-order linear regression fits. The dashed line represents model predictions for the clinically viable device (identical to the solid blue line from (b)).

Fig. 3. Surgical design, post-op follow-up, and histological analysis from animal study.

a Rodent surgical technique for both experimental (with implant) and control (no implant) groups. b Post-op photos from immediate post-op through tissue collection. c Representative histology. Top two samples are from the same animal (with implant), taken from the flap (top) and the contralateral epigastric region (bottom). Bottom two samples are from a second animal (no implant), again from the flap (top) and the contralateral epigastric region (bottom). d (top) Percent ingrowth versus time at tissue harvest, where each point represents one animal. (bottom) Smallest distance from external surface of epidermis to external surface of smooth muscle layer for the skin flap (with implant) and contralateral skin sample.

Fig. 4. Cadaver testing results.

a Cadaver hand with biological thumb (left), biosynthetic thumb (middle), and biosynthetic thumb with flap coverage (right). b Radiographic images showing the biosynthetic thumb with flap coverage in the rest position (left) and at full flexion (right). c MCP-IP angle relationship for the cadaver hand with biological thumb and with biosynthetic thumb, when actuated only by pulling on the FPL tendon. Solid blue and green lines represent the mean of 10 trials, and shaded regions show +/−1 standard deviation. Predictions from the as-implanted linkage model are also shown for comparison (red). d IP (top) and MCP (bottom) flexion angle as a function of FPL tendon excursion. Zero excursion represents resting tension in the FPL tendon. Solid lines represent the mean of 10 trials, and shaded regions show +/−1 standard deviation.

Fig. 5. Opposition pinch with biosynthetic thumb, after coverage by FDMA and RRF flaps.

a Radiographs showing opposition pinch between the biosynthetic thumb and digits 2–5. b Photos showing opposition pinch between the thumb and digits 2–5 (left to right). c Pinch grasp holding a small spherical object between the biosynthetic thumb and digits 2–5 (left to right).