ACL Reconstruction and the “Delta Angle”

https://pubmed.ncbi.nlm.nih.gov/40690738/

Evidence base

Ayuob A, Raj S, Jessup J, Searle HKC, Ahmed I, Thompson P, Shah F, Metcalfe A, Smith N. Introducing “Delta Angle” of Posterior Tibial Slope and Its Impact on ACL Reconstruction Failures: A Regression Analysis of 1174 Patients. Am J Sports Med. 2025 Aug;53(10):2387-2396. doi: 10.1177/03635465251355213. Epub 2025 Jul 21. PMID: 40690738.

This study was conducted by our team at University Hospitals Coventry and Warwickshire to better understand how the tibial slope, and difference on either side of the knee affect outcomes after ACL reconstruction

Key points

  • A new measurement called the delta angle helps predict ACL reconstruction failure
  • The delta angle reflects the difference between the inner and outer slopes of the tibia
  • For every 1 degree increase in delta angle, the risk of ACL failure increases by around 70 percent
  • A delta angle of around 6 degrees increases the risk of failure more than fivefold
  • This may help identify patients who need additional procedures to protect the ACL graft

What is the delta angle?

The tibia (shin bone) is not flat at the top. It has a slope that influences how forces pass through the knee.

Traditionally, surgeons have looked at the overall slope. This study shows that the difference between the inner (medial) and outer (lateral) slope is more important.

This difference is called the delta angle.

  • The medial slope is measured on the inner side of the knee
  • The lateral slope is measured on the outer side
  • The delta angle is the difference between the two

Why is this important?

ACL reconstruction can fail for a number of reasons. One of the key factors is the shape of the bone.

A steeper slope increases forward movement of the tibia. When there is also a difference between the inner and outer slopes, this creates additional rotational forces.

These combined forces increase strain on the ACL graft and can increase the risk of failure.

What did the study involve?

We analysed 1174 patients who had undergone ACL reconstruction.

Patients were grouped into:

  • successful primary ACL reconstruction
  • revision ACL reconstruction (failed graft)
  • contralateral ACL injuries

We measured:

  • medial slope
  • lateral slope
  • delta angle

We then assessed how these related to the risk of graft failure.

What were the results?

Differences in bone shape

Patients with failed ACL reconstructions had:

  • Lower medial slope, around 3.6 degrees compared to 5.2 degrees
  • Higher lateral slope, around 9.7 degrees compared to 8.3 degrees
  • A significantly higher delta angle, around 6.2 degrees compared to 3.1 degrees

This shows that it is not just the slope itself, but the imbalance between the two sides that matters.

Risk of ACL failure

The delta angle was a strong predictor of failure:

  • Each 1 degree increase increased the risk of revision surgery by around 70 percent
  • A delta angle of 6 degrees increased the risk by approximately 5 times compared to a neutral knee.

This is one of the strongest anatomical risk factors identified for ACL failure.

What does this mean for you?

This study helps explain why some ACL reconstructions fail despite good surgery and rehabilitation.

In particular:

  • Bone shape plays a major role in outcomes
  • Patients with a high delta angle may be at higher risk
  • Additional procedures, such as lateral extra-articular tenodesis or slope-correcting osteotomy, may be considered in selected cases

Key considerations

  • Not all patients need additional surgery
  • The delta angle is one of several factors considered when planning treatment
  • This measurement may help personalise surgery and reduce the risk of failure