HTO is an abbreviation of 'high tibial osteotomy'.
Realigning the knee joint
In a high tibial osteotomy the upper tibia bone is cut almost through and the bone wedged (usually an opening wedge) and fixed in place with a plate and screws until the bone heals in the new position. This is done to correct the forces going through the joint, frequently in patients with bow-leg (varus) deformity.
High tibial osteotomy in varus knees: indications and limits. Loia MC, Vanni S, Rosso F, Bonasia DE, Bruzzone M, Dettoni F and Rossi R. Joints. 2016 Apr-Jun; 4(2): 98–110.