Children with partial hand differences are those with congenital limb differences of the hand or amputation of the bones distal to the wrist joint. In the United States, approximately 1,500 babies are born each year with a partial hand anomaly.1 These anomalies can occur with ipsilateral shortening of the forearm and underdeveloped vestigial digits.2 Digit amputation is more commonly treated with a prosthetic device since the child has not matured with a limb difference.3 The prosthetics profession refers to both of these types of differences as minor, but they can be severely disabling, especially if the amputation involves the thumb or multiple digits. In fact, patients with partial hand amputation report a higher level of disability than those with other major unilateral upper-limb amputation.4 While the number of individuals with partial hand amputations is ten times greater than all other categories of upper-limb amputation combined, the state of available technology for this underserved patient population is relatively poor.5 Today, there are rapid manufacturing technologies like plastic and metal 3D printers, which can scale prosthetic componentry to appropriate sizes for many pediatric patients.6 However, a commercially available robust prosthetic digit for pediatric partial hand difference is still lacking. In collaboration with the VA Biomechatronics Development Laboratory and with funding from the U.S. Department of Veterans Affairs Rehabilitation Research and Development Service, Point Designs designed a scalable miniature prosthetic finger spanning 55–75mm lengths to serve children with limb differences who are between three and 15 years of age (Figure 1).7
An Innovative Design
The design of the miniature digit was based upon our prior Point Digit work, which has been commercialized by Point Designs (www.pointdesignsllc.com).8 The miniaturization of the digit affects the function of the mechanical systems within it. Multiple mechanisms within the patent-pending Point Digit were scaled down, including (1) the ratcheting mechanism, (2) the bump release mechanism, and (3) the push-button release mechanism. These changes were accomplished while maintaining reliability and function.
We sought to reduce the weight of the miniaturized Point Digit by 50 percent to a weight of 25g to serve children with body weights below 90 lb. To achieve this, we manufacture the finger using titanium and create hollow geometries within the components using direct metal laser sintering techniques. This new design for the pediatric population is referred to as the Point Mini.
A battery of verification tests was performed to ensure that all design specifications of the Point Mini had been met. In all tests, the digit met or exceeded the specification (Table 1). In some cases, the digit exceeded the specification by several factors. The digit met the mass and length requirements of a five-year-old child. In the most demanding test, the static load test at the distal tip of the finger, the static strength of the Point Mini at the distal fingertip exceeded the specification by 18 times (279 lb.). No digits exhibited failures during cyclic loading tests, and all digits had no visible signs of wear, damage, or loss of function. These results verify that miniaturization of the digit did not cause a decrease in the mechanical performance of the mechanism. In fact, the Point Mini meets all the same mechanical specifications as the larger Point Digit, providing equal strength and durability to the pediatric population.
A preliminary case study using the existing Point Digit technology was conducted with an 11-year-old patient. The subject presented with a congenital limb absence of digits two through four and an intact thumb and little finger. The 80mm Point Digit devices were provided for the patient’s index, middle, and ring fingers since the Point Miniwas not yet available. Without a prosthesis, the patient was able to achieve opposition between the existing thumb and little finger but lacked the support and grip strength necessary to participate in his favorite sports. A prosthesis was fabricated with digits positioned to oppose the intact thumb and form stable grasps. The subject is an active boy whose goals were to play basketball and baseball and practice archery. The robust ratcheting system provided by the Point Digittechnology enabled him to improve function during activities of daily living and participate in his favorite sports. Application of the Point Mini will provide similar access and ability to even physically smaller and younger children in the future.
Jacob L. Segil research Healthcare ScientistRocky Mountain Regional VA Medical Center
Co-founder, Point Designs
References are available at www.oandp.org/page/ATcurrent.