S A Sampath: Relationship between mechanical axis of the lower limb and measurements taken from anteroposterior radiographs for patients with osteoarthritis of the knee

Abstract

INTRODUCTION

For patients undergoing total knee replacement, it has been shown that the degree of preoperative malalignment determines the complexity and duration of the procedure1 and the choice of implant. Despite this, long leg alignment views are rarely carried out prior to surgery in the UK. Sclerosis of subchondral bone is a well-recognized radiographic and surgical feature of osteoarthritic knees and initial research2 indicated associations between measurements of the sclerotic area taken from standard anteroposterior radiographs of the knee and the angle of the load line, measured by the Orthopilot navigation system during surgery. The current study extended that initial research by additionally measuring the mechanical axis of the lower limb from weight-bearing, long leg radiographs in order to establish whether measurements taken from anteroposterior radiographs may be able to offer valuable information on leg alignment prior to surgery.

Keywords: ImageJ, Knee, Mechanical axis, Anteroposterior Radiographs, Subchondral bone sclerosis, Osteoarthritis

1. METHODS

Anonymised radiographs from patients with osteoarthritis were analyzed using ImageJ. Each patient had both a standard anteroposterior radiograph of the knee and a long leg view taken while weight bearing. In each case, the mechanical axis of the lower limb was measured by calculating the angle formed by the femoral and tibial axes, as described by Goker and Block3 . The area of increased bone density was manually outlined using the polygon selection tool and a number of measurements and shape descriptors of this area recorded.

2. RESULTS

Inter-tester analysis indicated excellent reliability (ICC = 0.99) for the mechanical axis measurement and moderate to excellent (ICC between 0.61 and 0.99) for the anteroposterior measurements.

Preliminary results (based on 43 patients) showed that, for the lateral segment, there was a significant correlation between the mechanical axis of the lower limb and the outlined area (r= -0.41), length of the minor axis (r = -0.51), minimum Feret’s diameter (r = -0.44) and the roundness (r = -0.44). Multiple linear regression, based on these four predictor variables, explained 27% of the variation in the mechanical axis. For the medial segment, there was a significant correlation between the mechanical axis of the lower limb and Feret’s diameter (r = 0.30).

3. DISCUSSION

The preliminary results indicate significant relationships between measurements easily obtained from anteroposterior radiographs and the mechanical axis of the lower limb. This suggests that simple analysis of standard anteroposterior radiographs using ImageJ may provide useful information regarding knee alignment prior to surgery.

Keywords

Knee, Mechanical axis, Anteroposterior Radiographs, Subchondral bone sclerosis, Osteoarthritis

Administrative data

Presenting author: S A Sampath
Organisation: The Bluespot Knee Clinic, 32 Orchard Road, Lytham, Lancashire , United Kingdom

co-authors: Sandra Lewis, Manchester Metropolitan University, Crewe, United Kingdom

Islay McEwan, Manchester Metropolotan University , Crewe, United Kingdom

Matteo Fosco, Rizzoli Orthopaedic Institute, University of Bologna,Via Pupilli 1,40136 Bologna, Italy

Domenico Tigani,Orthopaedic Surgery, Santa Maria alle Scotte Hospital , viale Bracci 1, 53100, Siena, Italy