Introduction: High tibial osteotomy is a recognised method of treatment for malalignment and osteoarthritis in young patients. Today computer aided surgery provides a chance to improve the existing techniques with a traceable planning and a higher degree of accuracy. Intraoperative use of fluoroscopy can be reduced and the results regarding leg axis can be improved.

Method: In our department since two years nearly all patients with malalignment of the lower legs had osteotomies guided with a navigation system. We used the Medivison-Praxim system in five, the Orthopilot prototype software in 12 and the Brain LAB System in 15 patients. The most common operation type was an open wedge osteotomy of the proximal tibia. A single cut osteotomy to correct the torsion and valgus deformity after a distal femur fracture is also possible with the Brain LAB system. Stabilisation was achieved using a plate with head locking screws (Tomofix, Synthes).

The degree of correction was controlled during the operation with the navigation system and compared with pre- and postoperative 2.5D ultrasound measurements to avoid projection errors of long standing x-rays.

Results: In all cases the intraoperative analysis was possible with the navigation systems. In one case, the computer crashed down due to interference of the fluoroscopy machine. No surgical problems were noted due to computer guidance noted. Fluoroscopy was used in all cases to verify the implant position as well as the resection plane after inserting the k-wires for saw blade guidance. The additional time for navigation was about 15 minutes.

The postoperative 2.5D ultrasound leg axis analysis showed a maximum of +/− 2° difference between the pre-, intra- and postoperative measurements.

Discussion: The chance to track the patient’s leg geometry through the complete procedure until bone fixation is the main benefit of computer assistance. The chance of failure during reduction and fixation can also be minimised and potential misalignment can be improved immediately. In addition, like in navigated joint replacement, the result of the surgical treatment can be simulated and judged before any action; values can be influenced showing the consequence right away. The final result regarding the leg axis is determined not only by the computer guidance, but by the primary stability of the implant as well. The chosen Tomofix plate is supposed to provide highest initial stability.

This first results show a promising increase of accuracy while radiation can be reduced. The actual values show that the main goal to increase the intraoperative accuracy in corrective osteotomies can be achieved with computer aided surgery.

Corrective osteotomies are often planned and performed on the basis of normal anatomical proportions. We have evaluated the length and torsion of the segments of the lower limb in normal individuals, to analyse the differences between left and right sides, and to provide tolerance figures for both length and torsion.

We used CT on 355 adult patients and measured length and torsion by the Ulm method. We excluded all patients with evidence of trauma, infection, tumour or any congenital disorder.

The mean length of 511 femora was 46.3 ± 6.4 cm (±2sd) and of 513 tibiae 36.9 ± 5.6 cm; the mean total length of 378 lower limbs was 83.2 ± 11.4 cm with a tibiofemoral ratio of 1 to 1.26 ± 0.1. The 99th percentile level for length difference in 178 paired femora was 1.2 cm, in 171 paired tibiae 1.0 cm and in 60 paired lower limbs 1.4 cm.

In 505 femora the mean internal torsion was 24.1 ± 17.4°, and in 504 tibiae the mean external torsion was 34.9 ± 15.9°. For 352 lower limbs the mean external torsion was 9.8 ± 11.4°. The mean torsion angle of right and left femora in individuals did not differ significantly, but mean tibial torsion showed a significant difference between right (36.46° of external torsion) and left sides (33.07° of external torsion). For the whole legs torsion on the left was 7.5 ± 18.2° and 11.8 ± 18.8°, respectively (p < 0.001). There was a trend to greater internal torsion in femora in association with an increased external torsion in tibiae, but we found no correlation. The 99th percentile value for the difference in 172 paired femora was 13°; in 176 pairs of tibiae it was 14.3° and for 60 paired lower limbs 15.6°.

These results will help to plan corrective osteotomies in the lower limbs, and we have re-evaluated the mathematical limits of differences in length and torsion.