The inwardly pointing knee angle: A novel computed tomography-based measurement technique and validation study

Abstract

Purpose

Inwardly pointing knee (IPK) is a clinically recognized manifestation of torsional malalignment in patients with patellofemoral pain and instability. However, current imaging protocols primarily quantify isolated components of malalignment, such as femoral anteversion or tibial torsion, without also capturing the net orientation of the knee relative to the direction of gait. The purposes of this study were to describe a functional computed tomography (CT) protocol for patellofemoral assessment and to introduce and validate the measurement of the inwardly pointing knee angle (IPKA).

Methods

A total of 1230 bilateral CT examinations, performed between 2017 and 2025 in patients with anterior knee pain and/or patellofemoral instability, were retrospectively reviewed. Patients were positioned according to their individual foot progression angle (FPA), documented from plantar footprints obtained during walking before image acquisition. CT scans were obtained with the knees in full extension (0°) and at 30° of flexion while the feet remained fixed on the recorded FPA footprint. In addition to conventional torsional and patellofemoral parameters, IPKA, FPA and AVA were measured on these images. Reliability of IPKA was assessed in 60 randomly selected knees measured by three observers on three occasions, using intraclass correlation coefficients (ICCs), the standard error of measurement (SEM) and the smallest detectable change (SDC).

Results

The mean IPKA was 13.97° ± 7.46° at 0° of flexion and 6.52° ± 5.56° at 30° of flexion. Interobserver reliability was excellent at both 0° (ICC 0.962) and 30° (ICC 0.986) of knee flexion and intra-observer agreement was excellent (ICC 0.972–0.978 in full extension and 0.989–0.992 at 30°). The SEM was 1.76° in full extension and 0.88° at 30°, with corresponding SDC of 4.88° and 2.43°.

Conclusions

This functional CT protocol enables assessment of the net axial orientation of the knee relative to the body's line of progression during gait, rather than the assessment of isolated torsional variables alone. IPKA demonstrated high reproducibility and may serve as a useful adjunct in the preoperative evaluation of patients with patellofemoral pain or instability and suspected torsional malalignment. IPKA provides a reproducible, objective measure of the combined torsional effect of the acetabulum, femur, tibia and FPA during gait and, pending validation of normative values and clinically meaningful thresholds, may serve as an adjunct to conventional torsional imaging in the functional assessment of complex patellofemoral disorders.

Level of Evidence

Level III, diagnostic cross-sectional study.