The objective of the study was to verify a positive effect of an additional oblique cancellous screw on the primary rotational stability of complete and incomplete high tibial closed-wedge osteotomies (8°) in ovine tibiae. Of 51 specimen 48 were employed for final results. The osteotomy site was stabilized with L-shaped plates (Allopro, Sulzer Orthopedics GmbH, Freiburg, Germany). The specimen were subdivided in 4 groups: complete (1/2) and incomplete (4/5 of the mediolateral tibial diameter in height of the horizontal sawing-jig) (3/4) osteotomies each with (1/3)) and without (2/4) an additional oblique cancellous screw. Constant axial load of 200 Newton and rotational velocity of 0,496°/sec. was applied during testing. 8 ovine specimen were tested without osteotomy or rigid fixation as a control group (5). Statistical significance (p< 0,05) was determined via the nonparametric Mann-Whitney U-test. The results were charted with SPSS (version 11.0). Correlation between objective measurement parameters and primary rotational stability of the specimen was displayed according to Pearson. The primary rotational stabilty in group 1 (intact medial cortical bone, incomplete osteotomy with additional oblique cancellous screw) was significantly higher than in groups 2, 3 and 4. In this group the resulting torsional moments in the initial part of the charted graphs were even higher than in the control group. Group 2 (incomplete osteotomy without a oblique cancellous screw) showed a significantly higher primary rotational stability compared to the groups with complete osteotomy (group 3/4). Between the groups with complete osteotomy (3/4) no significant differences in rotational stability occured. No significant correlation could be found between the objective measurement parameters of the specimen (length, weight, maximal width of the tibial plateau) and the primary rotational stability of the rigidly fixated ovine tibiae. This biomechanical in-vitro assessment showed that an intact medial cortical bone bridge has a statistically significant impact on the primary rotational stability of lateral closed-wedge osteotomies in proximal tibiae. An oblique cancellous screw through the osteotomy gap has an additional effect concerning rotational stability. In case of complete osteotomy of the proximal tibiae or due to inadequate operative technique the stabilizing effect of the medial cortical bone bridge gets lost. This results in a deterioration of rotational stability at the osteotomy site and in a sufficicantly rigid fixation is no longer guaranteed. In this case an additionally inserted oblique cancellous bone screw leads to higher resistance against rotational forces. A rigid osteosynthetic stabilization of corrective osteotomies in proximal tibiae seems a condition precedent to obtain the desired correction angle.