Several surgical techniques and devices have been developed to help patients born with limb deformities, limb length inequalities, and extreme short stature. People with such ailments often experience pain, dysfunction, and joint degeneration. The primary method of treating such deformities is an osteotomy followed by callus distraction [1]. Commonplace lengthening devices are external fixators and intramedullary devices, but each has its drawbacks.Traditional external fixators, such as the Ilizarov device and Taylor spatial frame, are cumbersome, painful, and produce large residual scars [2]. Due to pin tract infection rates of 10–20%, lengthening with these methods requires careful surveillance [3]. Intramedullary lengthening devices can cause severe complications such as intramedullary infection [4]. Surgeons have recently experienced success with a motorized, intramedullary nail (Fitbone), but pediatric use of this device can be limited due to interference with open growth plates [5]. The investigators have designed an extramedullary device that retains the attractive qualities of an intramedullary nail, without the risk of deep infection or damage to growth plates. Additionally, the device can be equipped with a six-axis force-torque sensor capable of measuring forces and moments in real time.The device is designed to function much like an external fixator, but without external hardware or tissue penetration with bone pins. Powered by a single stepper motor and lead screw, the device is capable of 85 mm of extension or compression, which is consistent with current limb lengthening techniques [6]. Shown in Fig. 1, the device rests outside of the medullary canal of the bone, and therefore, can be used in children without concern for injury to the growth plates. Additionally, the device will not leave scars from the pin sites because the device is placed inside the skin of a patient. The device is also equipped with a six-axis force/torque sensor capable of measuring the full set of reaction forces and moments in real-time.The motor and gear set chosen for prototype design were the 12 mm diameter Faulhaber ADM1220 two-phase stepper motor with 6 V winding and 1024:1 gear reduction drive (MicroMo Solutions, Clearwater, FL) rated for 0.450 Nm maximum intermittent torque output at 20 steps per revolution per phase. In two-phase operation, 40,960 input steps are needed for a lead screw translation of one pitch. The method by which the motor will be powered has not been selected, but existing medical devices rely on induction through the patient's skin to provide power. Wireless power or the use of a battery could be adapted for this application.A ¼ in. −20 stainless steel ACME threaded lead screw and bronze ACME threaded nut were selected for the device (Nook Industries, Inc., Cleveland, OH). The lead screw-nut assembly has a rated dynamic load capacity of 1387 N and a static load capacity of 4448 N. The efficiency of the lead screw-nut assembly is 0.36, which provides a theoretical axial thrust force of 801 N from the drive assembly.Extension of the device is supported by two cylindrical telescopic cases. Contour features on the inner surface of the device's outer casing support the torsional loads applied to the device from the actuator as well as the soft tissue forces during lengthening. The telescopic cases of the device and the bone attachment plates will be fabricated from 316 L medical grade stainless steel. This material was chosen for its high strength as well as its biocompatible properties. The device is to be fixed to the bone using self-tapping, locking head screws. Thus, the device will not directly contact the bone and periosteal blood supply is preserved [7].To measure the real time forces and torques acting on the device, an optional six-axis force-torque sensor was included in the design. The force torque sensors can be located at both ends of the device between the mating surfaces of the bone plate and device end. To minimize the size of the sensor, a monolithic cylindrical design was chosen. Strain gauges on the surface of the sensor independently measure the six moment and force components of the load applied to the device.Design requirements were based on a literature review and surgeon interviews. The device must be capable of 85 mm lengthening at a rate of 1.0 mm per day [8,9]. Since the device is intended for pediatric applications, a small device envelope must be maintained. An approximate device envelope was developed at 200 mm in length and 20 mm in diameter. By maintaining this envelope the device can be slid under the patient's skin and fixed to the patient's bone using two incisions.Based on reported values from pediatric studies, the maximum force required to lengthen a child's femur was estimated at just over 500 N [10–12]. Therefore, maximum theoretical distraction force of the device, 801 N, was calculated to be 1.6 times the force required to lengthen a child's femur.A step resolution of 0.25 mm is commonly recommended for callus distraction [13]. The current theoretical precision of the device is 31 nm per step. However, the actual output accuracy depends largely on the lead screw lash and system compliance. It was determined that the device's actual step resolution was sufficiently small.Since the device is extramedullary (located external to the bone) it will be subject to a high bending moment. Analysis has indicated that the device's telescopic design will lend to challenges with a large internal friction force. To mitigate this force, the addition of low friction sliding surfaces and rolling elements between the two cases were investigated.A prototype device was fabricated to evaluate the device's range of motion and assumed extension rate control. The prototype was made from Accura 60 stereolithography (SLA) resin using a Viper Si2 SLA system (3D Systems, Rock Hill, SC) with a 0.0025 mm resolution. SLA was chosen to reduce fabrication time and cost. Bench top testing revealed that the prototype design would be able to extend or compress the bone callus 85 mm at a rate of 1 mm per day. However, the SLA resin was not able to perform under realistic loading, so an evaluation of the device's strength was not achieved.A prototype, implantable extramedullary device was successfully designed and evaluated using information from current literature and surgeon interviews. The design of the device addresses major drawbacks of current limb lengthening techniques. It avoids issues with pin tract infections and scarring by being implanted underneath the patient's skin. In addition, it has promise as a pediatric device because it will not interfere with open growth plates.The device provided an interesting design challenge due to high internal friction forces between the two telescopic cases. Dynamic friction analysis indicated that including rolling elements or a low friction sliding surface between the cases would effectively reduce the internal friction force experienced during lengthening.Complications during distraction osteogenesis are often a result of the poorly understood response of both hard and soft tissues to lengthening. Several studies suggest that issues such as premature consolidation and nonunion of the bone callus could be avoided using force-feedback during lengthening [10,14–16]. Including a force-torque sensor in the design provides the opportunity for force-feedback control of the device. An instrumented device also has potential for use as a clinical research tool. The proposed device could provide valuable data about callus distraction forces in both children and adults.Limb length inequalities and limb deformities are serious medical conditions that are often overlooked by researchers and orthopedic device manufacturers because of the small size of the patient population. The device presented offers an alternative treatment option that addresses the needs of patients of all ages.This material is based upon work supported by the National Science Foundation Graduate Research Fellowship under Grant No. DGE-1256259.