# Determination of rotation center and diameter of femoral heads using off-the-shelf augmented reality hardware for navigation

## TL;DR

This study shows that augmented reality hardware can accurately determine the center and size of hip joints during surgery, improving precision in hip replacement procedures.

## Contribution

The study demonstrates the use of off-the-shelf AR hardware for real-time, accurate determination of hip joint parameters during total hip arthroplasty.

## Key findings

- AR-based surface mapping achieved a mean diameter error of less than 1 mm for prosthetic heads and cups.
- Inter-observer reliability for diameter measurements was below 2 mm on average.
- AR proved reliable for determining the center of rotation and diameter of osteoarthritic femoral heads.

## Abstract

In total hip arthroplasty (THA), determining the center of rotation (COR) and diameter of the hip joint (acetabulum and femoral head) is essential to restore patient biomechanics. This study investigates on-the-fly determination of hip COR and size, using off-the-shelf augmented reality (AR) hardware. An AR head-mounted device (HMD) was configured with inside-out infrared tracking enabling the determination of surface coordinates using a handheld stylus. Two investigators examined 10 prosthetic femoral heads and cups, and 10 human femurs. The HMD calculated the diameter and COR through sphere fitting. Results were compared to data obtained from either verified prosthetic geometry or post-hoc CT analysis. Repeated single-observer measurements showed a mean diameter error of 0.63 mm ± 0.48 mm for the prosthetic heads and 0.54 mm ± 0.39 mm for the cups. Inter-observer comparison yielded mean diameter errors of 0.28 mm ± 0.71 mm and 1.82 mm ± 1.42 mm for the heads and cups, respectively. Cadaver testing found a mean COR error of 3.09 mm ± 1.18 mm and a diameter error of 1.10 mm ± 0.90 mm. Intra- and inter-observer reliability averaged below 2 mm. AR-based surface mapping using HMD proved accurate and reliable in determining the diameter of THA components with promise in identifying COR and diameter of osteoarthritic femoral heads.