Slope is equally dangerous at every age — but only young knees carry the consequence

Design

Retrospective cohort, level of evidence 3, Firth penalised logistic regression with bootstrap validation

Population

581 primary ACL reconstructions with hamstring autograft, minimum six-year follow-up; 431 isolated, 150 with lateral extra-articular tenodesis

Primary outcome

Graft rerupture, modelled jointly against posterior tibial slope and age

Effect

OR 1.28 per degree of slope (95% CI 1.10-1.47) and 0.91 per year of age (0.87-0.95); at 15 degrees, predicted rerupture 36.6% at age 18 versus 3.9% at age 45

Posterior tibial slope and young age are both established predictors of ACL graft rerupture. What has been missing is whether they interact, and what the two together mean in absolute terms for the patient in front of you. This retrospective cohort of 581 primary hamstring-autograft reconstructions, followed a minimum of six years, models both jointly. Four hundred and thirty-one were isolated reconstructions and 150 had a lateral extra-articular tenodesis; there were 38 reruptures.

Per degree of slope, the odds ratio for rerupture was 1.28 (95% CI 1.10 to 1.47). Per year of age, 0.91 (0.87 to 0.95) — protective. The interaction term was not significant (P = 0.685 in the pooled cohort), so each degree of slope multiplies risk by about the same amount whether the patient is 18 or 45. The absolute picture is entirely different. At a slope of 15 degrees, predicted rerupture probability was 36.6% at age 18 and 3.9% at age 45. Predicted risk fell below 10% at roughly 27 years for a 12-degree slope and roughly 35 years for a 15-degree slope, with thresholds checked by 1,000-iteration bootstrap.

The change this makes is in consenting and in deciding who gets more than a standard reconstruction. A 15-degree slope in a 40-year-old recreational player is a number to note; the same slope in a 17-year-old pivoting athlete is a one-in-three chance of losing the graft, and that is the conversation in which lateral extra-articular tenodesis, slope-correcting osteotomy or a frank discussion about returning to that sport belongs. Use the age-and-slope pair to decide, not slope alone — and give the patient the absolute number, because 36.6% and 3.9% mean something to them in a way that an odds ratio never will.

• Pair slope with age before deciding on augmentation — neither number decides alone.
• Quote absolute rerupture risk to the patient, not relative risk.
• Treat a steep slope in a teenager as a trigger to discuss LET or slope-correcting osteotomy.
• Do not escalate surgery for a steep slope in a patient in their forties on this evidence.
• Document the slope, the age and the risk discussed — this is a consent conversation as much as a surgical one.

Why it matters

It tells you which patient with a steep slope actually needs more than a standard reconstruction.

Don't overread it

A retrospective level 3 cohort with 38 events cannot show that correcting the slope lowers the risk.

The statistics, in plain English

The absent interaction is the point that is easiest to misread. It does not mean slope matters less in older patients — each degree multiplies the odds by 1.28 at any age. It means the multiplication starts from a much lower baseline, so the same relative effect produces a small absolute risk in an older knee and a large one in a young knee. That is why absolute risk, not the odds ratio, is what belongs in the consent conversation. The caution is the event count: 38 reruptures across 581 patients is a thin basis for predictions as precise as 36.6%, and a level 3 retrospective cohort cannot exclude that the surgeons chose augmentation for the knees they already judged high-risk. Read the thresholds as a way of framing the decision, not a cut-off.

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