Tibial slope and ACL rerupture: age sets the absolute risk

Design

Retrospective cohort, penalised logistic regression, minimum 6-year follow-up

Population

581 patients with primary hamstring-autograft ACL reconstruction

Primary outcome

ACL graft rerupture

Effect

OR 1.28 per degree of slope (95% CI 1.10 to 1.47); predicted rerupture with 15° slope 36.6% at age 18 vs 3.9% at 45

A French cohort of 581 patients with primary hamstring ACL reconstruction, followed for at least six years, modelled how posterior tibial slope and age combine to predict graft rerupture. There were 38 reruptures.

Each degree of slope was associated with 28% higher odds of rerupture (OR 1.28, 95% CI 1.10 to 1.47), and each year of age with 9% lower odds. The relative effect of slope did not change with age. The absolute risk did: with a 15-degree slope, the predicted rerupture risk was 36.6% at 18 and 3.9% at 45. Predicted risk fell below 10% at about 27 years for a 12-degree slope and about 35 for a 15-degree slope.

The practical point is that a steep slope matters most in the young. A teenager with a 15-degree slope is in a very different position from a 40-year-old with the same measurement.

These are model-based estimates from a single country's cohort with relatively few events. They help frame counselling and the case for slope-reducing osteotomy or lateral extra-articular tenodesis, not set a fixed threshold.

• Measure posterior tibial slope on lateral radiographs before primary ACL reconstruction
• Interpret a raised slope in the light of age; risk is highest in teenagers and young adults
• Discuss augmentation such as lateral extra-articular tenodesis with young patients who have a steep slope
• Use absolute risk, not odds ratios, when counselling patients

Why it matters

It turns a familiar relative risk into numbers a patient can understand.

Don't overread it

The age thresholds are model estimates from 38 reruptures, not validated cut-offs for osteotomy.

The statistics, in plain English

An odds ratio of 1.28 per degree applies at all ages, but because young patients start from a much higher baseline risk, the same slope adds far more absolute risk at 18 than at 45. The absolute figures are model predictions, not observed rates in each age group.

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