Hexapod fixators are a specialized type of external fixators, which are used in the treatment of complex bone fractures. A set of metallic rings, which are attached to the bone fragments using wires and which are connected to each other by six length-adjustable struts, so-called distractors, enables a flexible and precise repositioning of fractures in six degrees of freedom. The lengths of the distractors define the configuration of the hexapod fixator and typically have to be iteratively adjusted over a period of several weeks. However, since the distractors are hard to reach for the patients, the length control is error-prone, limits the patient's autonomy and can even require prolonged hospitalization. To overcome these limitations, a new system for automatically measuring the distractor lengths is proposed, which utilizes optical proximity sensors and wirelessly transmits the results to a reader device. The system achieves a measurement accuracy of less than 1 mm over the full measurement range of 105 mm. It can be powered autonomously from a small button cell for several weeks. With dimensions of 24 mm x 10 mm, the electronics module adds only little volume and weight.