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[Evaluation of methods for radiographic measurement of the tibial slope. A study of 83 healthy knees].

PURPOSE OF THE STUDY
We compared on lateral X-rays of 83 healthy knees, 6 methods measuring the tibial sagittal slope. Each method determined the tibial slope according to an independent anatomical axis. The goals of the study were to: 1) detect the differences between the 6 methods; 2) determine if any mathematical relation could be observed between the 6 methods; 3) compare accuracy of proximal anatomical axis versus long anatomical axis to evaluate the tibial slope.


MATERIAL
50 healthy patients (83 knees) knees were included in the study. The patients (26 male and 24 females) were 39.3 +/- 15.8 years old.


METHODS
We obtained true lateral X-rays orientated with an image intensifier in order to obtain the exact superposition of the femoral condyles. The measurements were carried out by an observer according to two procedures: 1) by manual goniometric measurements twice; the mean value between both lectures was recorded; 2) with a digitizer (Orthographics TM, Salt Lake City, Ut). We measured the angle between the tangent to the medial tibial plateau and the perpendicular direction to each of the studied anatomical axis: tibial proximal anatomical axis (TPAA); tibial shaft anatomical axis (TSAA); posterior tibial cortex (PTC); fibular proximal anatomical axis (FPAA); fibular shaft axis (FSA); anterior tibial cortex (ATC). The values obtained with the digitizer were used to compare the six methods. The values obtained with the TSAA were considered as reference.


RESULTS
Tibial slope values were different with the 6 methods. ATC gave the higher values and PTC the smaller. The difference could be 5 degrees between two methods measuring the same posterior tibial slope. However different, the values obtained with the 6 methods were strongly correlated (R > 0.85; p = 0.0001). We determined mathematical relationships between the values obtained with the 6 methods according to the regression analysis. The correlation with the values obtained with TSAA (reference values) was stronger for TPAA and TPC (respectively R = 0.92 and R = 0.9). The mean error between manual and digitized measurements was 1.28 degrees, but for the same knee the error could exceed 4 degrees. The highest error was 4.64 degrees with the TSAA, likewise the error frequency was higher with the TSAA (12 knees with an error > 3 degrees versus 7 knees for the other methods (p = 0.01)). In our 33 bilateral knees, after one side measurement, the forecast of the contralateral tibial slope showed an error of 5 degrees or more in 5 patients (15.1 per cent) and an error of 3 degrees or more in 13 patients (39.3 per cent). Among the proximal anatomical axis, only the TPAA and the TPC were not influenced by age, sex, patient height or weight.


CONCLUSION
Values of posterior tibial slope observed with the 6 methods were different but correlated. Among the proximal axis, the TPAA and PTC gave higher reliability. The values obtained with these two methods: 1) were not influenced by morphometric variables, 2) were strongly correlated with the references values obtained with the TSAA, 3) gave low error with manual measurements by comparing with digitized measurements. Forecast of the contralateral tibial slope after one side measurement is unreliable. Satisfactory accuracy could be obtained with a two times manual goniometric measurement, but using a digitizer improves measurement accuracy and is less time consuming.