Proximal tibial anatomical axis and anterior tibial cortex-based measurements of posterior tibial slope on lateral radiographs differ least from actual posterior tibial slope-A biomechanical study.

PURPOSE

To compare different measurement techniques of the posterior tibial slope (PTS) on lateral radiographs with the actual in situ PTS and evaluate the effect of tibial malrotation and image section length.

METHODS

Actual PTS was measured on eight fresh-frozen tibiae using a portable 6-axis measuring arm with an accuracy of ±0.01°. True lateral radiographs were taken in the neutral position and after applying 10/20/30° internal/external rotation (IR/ER) and 5/10/15° varus/valgus rotation. The PTS was measured radiographically using five different reference axes: anterior tibial cortex (ATC), anatomical tibial axis, proximal tibial anatomical axis (PTAA), posterior tibial cortex (PTC) and fibular shaft axis (FSA).

RESULTS

The ATC and PTAA methods showed the lowest deviation from the actual PTS, while the PTC method showed the highest difference of 5.5 ± 1.5° (medial) and 7.1 ± 1.8° (lateral) among all tested methods ( < .001). The PTAA technique showed a 1.9 ± 1.4° (medial) and 2.9 ± 1.8° (lateral) difference from the actual slope (n.s.). ER caused the PTS to increase 0.7 ± 2.0° (10° ER, n.s.) to 3.4 ± 2.1° (30° ER,  < .05), whereas IR caused the PTS to decrease 1.6 ± 1.3° (n.s) to 4.1 ± 1.7° ( < .05) when comparing to the PTAA method for the neutral position. Varus and valgus rotation showed the highest deviation from the neutral rotation at 15° valgus (3.1 ± 2.1°, n.s.).

CONCLUSION

Tibial slope measurements have a high degree of variability between different measurement methods, while the ATC and PTAA methods showed the least deviation from the actual PTS measured in this in vitro model. Malrotation resulted in a severe distortion of the PTS values, which may alter preoperative planning and intraoperative results. Therefore, radiographic PTS measurements may be contrasted with more objective, reproducible and reliable measuring methods.

LEVEL OF EVIDENCE

There is no level of evidence as this study was an experimental laboratory study.