The short version, if you’re a patient
If your ACL reconstruction has failed — especially more than once — the problem may not be your graft, your surgeon, or how hard you worked in rehab. It may be the shape of your shin bone.
The top of the tibia (shin bone) isn’t flat. It slopes gently backwards, and this angle is called the posterior tibial slope. In most people it’s a modest tilt of around 7–10 degrees. But in some people the slope is much steeper. A steeper slope acts like a ramp: every time you stand, land or pivot, it nudges the shin bone forwards relative to the thigh — exactly the movement your ACL is there to stop.
So if your slope is steep, your new ACL graft is fighting an uphill battle from day one. It’s under more strain with every step, and over time that extra load can stretch or snap it. This is one reason some ACL reconstructions keep failing despite technically excellent surgery.
The fix is a small, precise operation called a slope-reducing osteotomy. We reshape the top of the tibia to flatten that ramp back towards a normal angle, taking the strain off the graft. When it’s done alongside a revision ACL reconstruction in the right patient, it dramatically improves the odds that the second (or third) reconstruction will finally hold. It’s the difference between rebuilding a house and first fixing the ground it stands on.
Not everyone with a failed ACL needs this — it’s reserved for people whose slope is genuinely part of the problem. Working that out needs careful measurement on dedicated X-rays and scans, which is exactly the kind of assessment we do before planning revision surgery.
The detail, for referring clinicians and colleagues
Why slope drives graft failure
Increased posterior tibial slope (PTS) shifts the resting position of the tibia anteriorly and increases anterior tibial translation under axial load. That translation is resisted by the ACL — so a steeper slope means a chronically higher load on the native ligament and, after reconstruction, on the graft.
The biomechanics are now well quantified. In a cadaveric model varying slope in 2° increments, Bernhardson et al. showed that ACL graft force increases linearly with slope — roughly 0.9 N of additional graft force per degree — independent of flexion angle. Flatter slopes unloaded the graft; steeper slopes loaded it (Bernhardson AS, et al. Am J Sports Med 2019;47(2):296–302).
That translates directly into clinical failure. In a systematic review, Liu et al. found that an increased PTS is associated with a significantly higher risk of graft failure after ACL reconstruction (Liu et al. Knee Surg Sports Traumatol Arthrosc 2022). A steep slope is now recognised as one of the most important — and correctable — morphological risk factors for the failed ACL.
Where the threshold sits
The most commonly cited threshold for intervention is a PTS of ≥12°. A 2025 systematic review by Vivekanantha et al. found that the commonest indication for adding a slope-reducing osteotomy to an ACL reconstruction was graft failure in a knee with a posterior tibial slope of 12° or greater (Vivekanantha et al. Arthroscopy 2025). An international expert Delphi consensus published the same year reached broad agreement on measuring PTS on a true lateral radiograph with a long tibial axis, and on considering slope correction in the revision setting once slope is excessive — typically around this 12° mark, and particularly after multiple failed reconstructions (Assessment and Surgical Correction of Posterior Tibial Slope in Revision ACL Surgery: An International Expert Delphi Consensus Statement. J ISAKOS 2025).
The procedure
Slope correction is usually achieved with an anterior closing-wedge osteotomy, most often performed supratuberosity (proximal to the tibial tubercle) to preserve the extensor mechanism. A wedge of bone is removed anteriorly, the slope is reduced towards a physiological target (commonly in the region of 4–6°), and the correction is fixed and allowed to heal — either as a single-stage procedure with the revision reconstruction or as a staged approach.
Does it work?
The outcome data are encouraging and maturing. Series of complex, multiply-revised ACL knees treated with a slope-reducing osteotomy plus revision reconstruction report high patient-reported outcomes and notably low re-rupture rates compared with the historically poor results of repeat revision alone (e.g. Fritsch B, et al. J Exp Orthop 2025). Longer-term follow-up is emerging too: a 20-year retrospective analysis of ACL reconstruction with slope correction from the Lyon group reported durable results (Lambrey, Sonnery-Cottet, Thaunat, Vieira et al. Am J Sports Med 2025).
In practice
The images below are from a case of ours: the pre-operative lateral radiograph used to assess the posterior tibial slope, and the intra-operative image after an anterior closing-wedge slope-reducing osteotomy with plate-and-screw fixation.
The take-home
In the revision ACL patient — and especially the re-revision patient — posterior tibial slope should be measured, not assumed. If it’s ≥12°, correcting the slope isn’t an optional extra; it addresses the mechanical reason the graft keeps failing. Rebuild the ligament, but level the ramp first.
Key references
- Bernhardson AS, Aman ZS, Dornan GJ, et al. Tibial Slope and Its Effect on Force in Anterior Cruciate Ligament Grafts: ACL Force Increases Linearly as Posterior Tibial Slope Increases. Am J Sports Med. 2019;47(2):296–302.
- Liu Z, et al. An increased posterior tibial slope is associated with a higher risk of graft failure following ACL reconstruction: a systematic review. Knee Surg Sports Traumatol Arthrosc. 2022.
- Vivekanantha P, et al. The Commonest Indication for Performing a Slope-Reducing Osteotomy With an ACL Reconstruction Is Graft Failure With a Posterior Tibial Slope of 12° or Greater: A Systematic Review of Indications, Techniques, and Outcomes. Arthroscopy. 2025.
- Assessment and Surgical Correction of Posterior Tibial Slope in Revision Anterior Cruciate Ligament Surgery: An International Expert Delphi Consensus Statement. Journal of ISAKOS. 2025.
- Fritsch B, et al. Slope-reducing high tibial osteotomy and revision ACL reconstruction leads to satisfying clinical results and a low failure rate. Journal of Experimental Orthopaedics. 2025.
- Lambrey PJ, Vincent A, Colas A, Sonnery-Cottet B, Thaunat M, Vieira TD, Fayard JM. Long-term Outcomes of ACL Reconstruction and Posterior Tibial Slope Correction: A 20-Year Retrospective Analysis. Am J Sports Med. 2025.
Independently verified patient reviews
