Posterior tibial slope is the strongest modifiable risk factor for failure after anterior cruciate ligament reconstruction, and it is measured on a plain lateral radiograph that most patients already have in their file. It is not measured in most units, which is why it usually enters the conversation after the second graft has gone.
The measurement needs a true lateral - the femoral condyles superimposed - and a long enough tibia to draw a reliable anatomical axis. On a short film the axis is estimated over too little bone and the slope comes out unreliable, which is the commonest reason a recorded value cannot be trusted. Ask for a long lateral when you request imaging for a failed reconstruction rather than measuring on whatever is available.
The threshold that selected patients in today's series was 10 degrees. Recording the value at the primary reconstruction costs nothing and means that if the graft does fail, the decision about an osteotomy starts from a number rather than from a new set of images and another delay.
- Record posterior tibial slope at the primary reconstruction, not only after a failure
- Require a true lateral with superimposed condyles and enough tibial shaft for the anatomical axis
- Request a long lateral specifically when imaging a failed reconstruction
- 10 degrees is the threshold that selected patients for combined osteotomy in today's series
- A slope measured on a short or rotated film is not a usable number - say so rather than recording it
Why it matters
The measurement that predicts graft failure is usually taken only after the graft has failed.
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