At present, several different fixators are in clinical use in the full range of thoracic and lumbar spine disorders. Nevertheless, an effort must be made to continue to improve pedicle fixation devices so that more safety, strength, adaptability and user-friendliness can be obtained. Fixation systems are subject to high loads, especially in major spinal injuries with marked loss of stability. Implant failure, producing poor clinical results, is a series problem. For safer and easier instrumentation a new fixator called the modular spine fixator (MSF) has been designed. The MSF consists of only a few basic components. Its main advantages are the smooth construction and small size, the modularity and easy handling, the high mechanical stability and optimal fatigue properties. In contrast to other fixators, the pedicle screws of the MSF act as compression screws. In high-load static and fatigue testing the MSF revealed no component loosening, material yielding or breaking. More than other systems, the MSF is suitable for short one-level instrumentation, thus avoiding overbridging uninjured motion segments. Between 1991 and 1993 60 instrumentations in the treatment of unstable thoracic and lumbar injuries were performed with the MSF. Single-level instrumentations were done in 34, two-level instrumentations in 24, three- and four-level instrumentations in 2 cases, all combined with posterior allogenic bone grafting. Since the beginning of 1993 transpedicular anterior bone grafting has been performed in addition, using autogenic bone material. In this series no implant fatigue failure has been noted. There were three infections requiring removal of the posterior bone graft in two and graft in fixator removal in one.(ABSTRACT TRUNCATED AT 250 WORDS)