Normal Tibial Plateau Angle Size on Radiography
The tibial plateau angle (TPA) describes the posterior slope of the proximal tibial articular surface relative to the tibial long axis, measured on lateral radiographs. Accurate assessment of this angle is clinically important because it directly influences anteroposterior knee stability, load distribution across the joint, and surgical planning for tibial osteotomies and total knee arthroplasty.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Lateral | 7-22¡ |
Clinical Significance
A normal TPA on lateral radiography falls between 7° and 22°. An increased posterior tibial slope amplifies anterior tibial translation under quadriceps load, predisposing the knee to anterior cruciate ligament (ACL) strain and graft failure following reconstruction. Conversely, a reduced or negative slope may increase posterior tibial translation and stress the posterior cruciate ligament (PCL).
Abnormal TPA values are encountered across a spectrum of conditions and should prompt correlation with clinical examination and advanced imaging when indicated. Key pitfalls include improper patient positioning — even minor internal or external tibial rotation on the lateral projection significantly alters the measured angle — and failure to account for tibial deformity in post-traumatic cases.
- ACL insufficiency / recurrent laxity — elevated TPA is an independent risk factor
- Tibial plateau fracture malunion — may markedly increase posterior slope
- Developmental deformity (e.g., Blount disease) — abnormal slope in skeletally immature patients
- Total knee arthroplasty malalignment — target slope typically 3°–7° during component placement
- High tibial osteotomy planning — TPA guides slope-correcting cuts
Reference: Keats TE, Sistrom C. Atlas of Radiologic Measurement. Mosby Inc. (2001).
Imaging Notes
TPA is measured on a true lateral radiograph of the knee with the patient standing or supine and the tibial shaft parallel to the image receptor. A line is drawn along the proximal tibial cortical axis, and a second line connects the anterior and posterior margins of the medial tibial plateau; the angle between the perpendicular to the tibial axis and the plateau line defines the TPA. Strict lateral positioning is essential — any rotational obliquity superimposes the femoral condyles asymmetrically and introduces measurement error of several degrees.
Weight-bearing lateral views are preferred when evaluating functional slope in the context of ligamentous instability or osteotomy planning, as they reflect the in vivo loading condition. Digital radiography with on-screen angle tools improves reproducibility; inter-observer variability is reduced when anatomic landmarks are defined consistently before measurement.