As a ways of avoiding issues with conventional socket suspended prostheses, a new type of fixation for limb amputees has been developed and is gradually gaining popularity around the world. Rather than tightly clamping the prosthesis around the skin of the residual limb, a titanium fixture is implanted into the bone and a transcutaneous pin is used to anchor the prosthesis directly to the skeleton. This method, called osseointegration, has been common practice for dental implants for decades but since it is relatively new for limb amputations some challenges still remain. One of these challenges concerns below-the elbow amputations. As there are two bones in the forearm, two titanium implants and transcutaneous pins are present. When performing pronation and supination of the forearm, these two pins make a complicated motion that has proven to be difficult to preserve in a prosthetic attachment device while maintaining stability and reliability. In this thesis an attachment device is designed and validated that preserves natural forearm rotation for below-the-elbow amputees making use of osseointegrated prostheses. Tests are performed to examine how well the motion is preserved, how the ability to perform the motion affects the performance of everyday tasks, and how the device distributes loading over the two implants. The developed device was tested by a single patient. Nearly the full range of pronation and supination was preserved, performance improved in a number of tasks requiring forearm rotation, and it was shown that it can be predicted which implant receives the majority of the loads when carrying weight so an as natural distribution as possible can be simulated. A number of challenges remain present for existing patients. These include issues with satisfactory prosthetic control through surface electrodes, carrying of large weights, and adjustability to various patients. Future developments will hopefully tackle some of these challenges, further enabling the device developed in this thesis to contribute to the development of the next generation of prosthetic limbs