Poster 208: Changes in Posterior Tibial Slope Following "L" and "Inverse L" High Tibial Osteotomy: A Biomechanical Study

OBJECTIVES: High tibial osteotomy (HTO) is a well-established procedure for addressing varus deformity of the knee or offloading the medial compartment in cases of medial compartment osteoarthritis. Altering the weightbearing axis of the tibia through HTO may result in unintended changes to the posterior tibial slope (PTS) and thus the sagittal stability of the knee. In 2003, Lobenhoffer described a biplanar “L” shaped HTO with an ascending cut in the coronal plane, leaving the tibial tubercle on the distal side of the osteotomy. Since then, multiple techniques have been described, including an “Inverse L”, leaving the tibial tubercle on the proximal end of the osteotomy. We present a modified “flex” version to the plate fixation used in the “L” and “Inverse L” technique with the goal of minimizing changes in the PTS during HTO. To quantify the effect of osteotomy technique on PTS, “L” (n=5), “Inverse L” (n=5), “L with flex” (n=6), and “Inverse L with flex” (n=6) techniques were performed on cadaveric specimens following medial opening wedge HTO (Figure1). Pre- and post-osteotomy fluoroscopic images were taken and PTS measured to determine whether one technique was more effective at minimizing PTS change after medial HTO. METHODS: The tibia from 22 fresh frozen cadaveric male specimen (11 pairs, mean age 46 + 14) were removed of all soft-tissue. Native anteroposterior and lateral radiographs were taken. Three independent observers measured the PTS angle using the circle method. Two 2.4mm Kirschner wires were placed through the medial cortex of the tibia at the metaphyseal-diaphyseal junction aimed towards the tip of the fibular head as a guide for the sagittal cut performed in the posterior four-fifths of the tibia. The oblique osteotomy stopped 10-15mm medial to the lateral tibial cortex and the joint line to avoid fracture. The osteotomy site was then medially opened 10mm. One specimen from each pair was randomized to receive the “L” shaped ascending HTO with the contralateral limb receiving the “Inverse L” shaped descending HTO. Vertical cuts on the coronal plane extending 3-4cm (proximally for the “L” and distally for “Inverse L”) were completed. 6 specimens from each group were subsequently fixed proximally with an anteriorly angled plate (15 degrees) which was forced posteriorly and fixed to the shaft (Figure 2). This plate “flex” technique was applied to purposefully correct PTS. All osteotomies were fixed using a standard locking AO Tomofix plate with 3 proximal screws and 4 distal screws. Pre- and post-HTO proximal tibia plateau orientations were collected using eight OptiTrack motion capture cameras as an adjunct to tibial slope measures completed through radiographic analysis. Three rigid body marker triads were used: distally along the anterior midline of tibia, medial superior articular surface, and the fixture frame. Post-HTO, the independent observers again measured PTS with the circle method. Change in tibial slope data was averaged among radiograph reviewers and compared to motion capture data. A one-way ANOVA statistical analysis was completed. RESULTS: PTS as measured by reviewers before and after HTO using the “circle method” and the motion capture analysis of the change in orientation of the tibial plateau are reported in Table 1. For all specimens, the posterior tibial slope decreased by 1.77 degrees (+/- 3.2). There was no statistically significant difference in change of tibial slope amongst all techniques, measured by lateral radiographs or motion capture analysis. CONCLUSIONS: There was no statistical difference in the change in posterior tibial slope across treatment types. Reviewer calculations of tibial slope using the circle technique was not different from measures observed through high resolution motion capture. No difference in the change in PTS may allow surgeons to select the technique that they feel most comfortable using on their patient with less concern of causing changes in the PTS and sagittal instability during an HTO. Additionally, this may allow for versatility in the event that patient anatomical characteristics interfere or impede the use of a specific technique. Future studies incorporating pre- and post-CT data to quantify hinge fracture and further characterize slope are underway.