Background This study investigated the relationship between the height of osteotomy and the correction of the kyphotic angle during posterior closing wedge osteotmy with instrumentation and the spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach in thoracolumbar kyphosis, and using this relationship as the basis of the preoperative design.Methods From April 1996 to June 2007, 30 thoracolumbar kyphosis patients with complete medical records and clear X-ray photograms have undergone operation. Of these 30 cases, 16 cases underwent posterior closing wedge osteotmywith instrumentation while the height of the osteotomy and the correction of the angle have been measured; 14 casesunderwent spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posteriorapproach while the height of the osteotomy, the height and the place of the cage and the correction of the angle were also measured. A simple geometrical model was simulated to calculate the relationship between the height of the osteotomy and the correction of the angle and these results are finally compared with the data coming from the actual measuring by the Wilcoxon statistic method.Results The distribution of data from the 16 cases by posterior closing wedge osteotomy with instrumentation was as such: 9 male and 7 female, the mean age was 49.2 years (range 38-70), the kyphosis improved from an average of 30° (range 15°-45°) preoperatively to 4° (range -26°-30°) postoperatively, the kyphosis was corrected on average 2.5° per 1 mm in the height of the osteotomy. The results from the simple geometrical model were that the mean of the correction of the angle per 1 mm was 2.2°. As a result, there was no significant difference (P >0.05) when comparing the measurement collected with the result simulated from the geometric model. The distribution of data from the 14 cases by spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach was as such: 5 male and 9 female, the mean age was 35.3 years old (range 15-57), the kyphosis improved from an average of 64° (range 34°- 95°) preoperatively to 8.7° (range -10°- 22°) postoperatively. The kyphosis was corrected on average of 6.2° per 1 mm in the height of the psteotomy. The results from the simple geometrical model is that the mean of the correction of the angle per 1 mm was 6.6°. There was also no significant difference (P >0.05) when comparing the measurement collected with the result simulated from the geometric model.Conclusions The therapeutic effect is significant for both posterior closing wedge osteotomy with instrumentation and spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach.The posterior closing wedge osteotomy with instrumentation is an easier approach with the mean angle of the correction per 1 mm of 2.5° and the maximum angle of correction of 45°. The spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach is more efficient with the mean angle of correction per 1 mm of 6.2°. It should be reserved for the severe cases of thoracolumbar kyphosis. We can also use the formula to help us constructing preoperative design.