Posterior tibial slope is not an exotic measurement and it does not need a special study. A true lateral radiograph of the knee with the femoral condyles superimposed will give it: draw the proximal tibial anatomical axis, take the perpendicular, and measure the angle to the medial tibial plateau line. It takes under a minute and the film is usually already in the patient's folder from the day of injury.
The reason to do it routinely rather than selectively is that you cannot tell a steep slope by examining the knee, and the patients in whom it matters most — the adolescents and young adults — are the ones in whom it is least likely to be looked for, because the assumption is that a young athlete's knee is a straightforward reconstruction.
Two practical cautions. The measurement is sensitive to rotation, so a lateral that is not truly lateral will mislead; if the condyles are not superimposed, do not report a number. And a slope measured on a short film is less reliable than one measured on a full-length lateral, so treat a borderline value as a reason to get a better image rather than as a decision.
- Measure PTS on the pre-operative lateral in every ACL reconstruction, not only revisions.
- Reject the measurement if the femoral condyles are not superimposed.
- Record the number in the operation note so a future revision surgeon has it.
- Re-measure on a full-length lateral before making a decision on a borderline value.
- Tell the patient the number and what it means for their re-injury risk.
Why it matters
The patients whose slope matters most are the ones in whom it is least often measured.
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