Measuring Posterior Tibial Slope: Which Radiographic Measurement Method Is Most Reliable?

“Modified 20% anatomical axis method demonstrates the highest reliability for posterior tibial slope measurement on radiographs”

Dr Shradha A. Kulkarni, Dr Takaaki Hiranaka and Dr David Parker. Journal of ISAKOS, July 2026

Posterior tibial slope (PTS) is an important measured radiographic parameter in the planning of ACL reconstruction (ACLR), total knee arthroplasty (TKA) and high tibial osteotomy (HTO). Yet despite its clinical importance, there is no clear consensus within the Orthopaedic community on the most reproducible and precise method for measuring it.

This study, published in the Journal of ISAKOS in July 2026, compared four commonly used radiographic measurement methods to determine which method provides the most reliable, precise and reproducible measurement of PTS.

The Aim:

The study aimed to identify the most reliable and reproducible technique for measuring PTS, while also determining whether measurements differed between long and short lateral radiographs and whether they differed if the axis of measurement was changed (i.e. between anatomical and mechanical axis methods).

The protocol

The retrospective study analysed 30 patients who had undergone medial opening wedge high tibial osteotomy (MOWHTO) for varus deformity. Preoperative and six-month postoperative radiographs were assessed using four PTS measurement techniques:

• Modified 20% anatomical axis (AA) method by Hiranaka et al.

• Mechanical axis (MA) estimation method by Sonnery-Cottet et al.

• Two-circle AA method by Ni et al.

• 5-15 cm AA method using short lateral knee radiographs by Dejour et al.

Two fellowship-trained orthopaedic surgeons independently performed the measurements twice, more than one week apart for pre-operative and post-operative radiographs. Intraobserver and interobserver reliability were assessed using intraclass correlation coefficients (ICC) and 95% confidence intervals. Cohen’s D was used to ascertain the effect size.

What did the research show?

The modified 20% anatomical axis method consistently demonstrated the highest overall reproducibility and precision of the four techniques. The other techniques also demonstrated high ICC values indicating good precision and reproducibility.

Interobserver reliability was consistently strongest for the modified 20% AA method, while all four techniques demonstrated satisfactory reliability for clinical use. The study also found negligible differences in PTS measurements when anatomical axis methods were used to measure PTS on long versus short lateral radiographs.

However, the choice of reference axis did make a measurable difference. PTS measurements using anatomical axis methods were, on average, approximately 2° higher than measurements using the mechanical axis.

What does this mean for surgeons and patient care?

The findings have practical implications wherever PTS measurements influence surgical decision-making. A difference of 2° may appear small, but it can become clinically important when measurements are used against thresholds for ACLR failure risk or when planning procedures designed to modify tibial slope.

The study therefore supports consistency in the method used to measure PTS and suitable adjustment in the threshold according to the axis used for measuring PTS. The modified 20% AA technique provides a highly reproducible approach, while familiarity with a chosen measurement technique remains important for achieving reliable results.

For surgeons, this is particularly relevant when comparing measurements between patients, studies or treatment pathways. A numerical difference in PTS may reflect on the measurement method rather than a true anatomical difference.

The take-home message...

Measuring PTS is not simply a matter of obtaining an angle on an X-ray. The reference axis and measurement technique can influence the result. Consistent methodology is essential when PTS measurements are being used to perform surgical planning and influence clinical decision-making.