Tibial slope and ACL rerupture: the relative risk holds at every age, the absolute risk does not

Design

Retrospective cohort, single centre, minimum 6-year follow-up

Population

581 patients after primary hamstring-autograft ACL reconstruction

Primary outcome

Graft rerupture

Effect

OR 1.28 per degree of slope (95% CI 1.10 to 1.47); OR 0.91 per year of age (0.87 to 0.95)

A French cohort of 581 patients followed for at least six years after hamstring-autograft ACL reconstruction examined how posterior tibial slope and age combine to predict graft rerupture. There were 38 reruptures.

Each extra degree of slope raised the odds of rerupture by about 28%, and each extra year of age lowered them by about 9%. The relative effect of slope did not change with age. But because younger patients start from a much higher baseline, the absolute risk differs enormously: for a 15° slope, the model predicted a rerupture probability of about 37% at 18 years and about 4% at 45. Predicted risk fell below 10% at about 27 years for a 12° slope and 35 years for a 15° slope.

This gives a practical way to frame slope-reducing osteotomy and lateral extra-articular tenodesis decisions: slope matters most in the young. The estimates come from a model fitted to one centre's patients and need external validation.

• Measure posterior tibial slope on a lateral knee radiograph before every primary ACL reconstruction.
• Weigh slope together with age: a steep slope in a teenager carries far more absolute risk than the same slope at 40.
• Consider lateral extra-articular tenodesis or slope correction discussions for young patients with high slope.
• Use absolute risk figures when counselling patients, not only the relative risk per degree.

Why it matters

Relative risk alone overstates the case for slope correction in older patients and understates it in teenagers.

Don't overread it

The predicted probabilities come from a single-centre model with 38 events and have not been validated externally.

The statistics, in plain English

An odds ratio of 1.28 per degree compounds: five extra degrees roughly multiplies the odds by more than three. The absolute predictions (36.6% at 18, 3.9% at 45) are model estimates, not observed rates in patients of exactly those ages.

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