High Tibial Osteotomy (HTO) is an established treatment for unicompartmental osteoarthritis of the knee with malalignment. The classic procedure for correcting varus deformity is the lateral closed wedge osteotomy of the tibia with osteotomy of the fibula. The disadvantages of this technique are well known. Open wedge osteotomy from the medial side eliminates the risk of compartment syndrome and peroneal nerve injuries. A new fixation device (TomoFix) with an adapted surgical technique allows stable fixation of the osteotomy without the need to fill the osteotomy gap with bone grafts. In a prospective study, 92 consecutive cases were treated with this procedure. Bony healing with remodelling of the medial and posterior cortical bone was observed. Full weight-bearing was possible ten weeks after surgery. There were no implant failures. Complications included one delayed union, two revarisations and one deep infection.