# Computational Wear Prediction of TKR with Flatback Deformity during Gait
## 1 Introduction  
Flatback syndrome results in muscular pain in the upper back and lower cervical area, knee pain, and inability to stand erect, and is also referred to as \"fixed sagittal imbalance\" [1, 2]. Flatback syndrome is diagnosed as follows. A plumb line is dropped vertically from the center of the C7 body, and the horizontal linear distance between the plumb line and the posterior-superior part of the sacral end plate is measured. If this distance is more than \(\pm 3\) cm, it is judged that the spine has deformed [3, 4]. Among the known symptoms associated with flatback syndrome is difficulty in maintaining an upright posture is due to loss of lumbar lordosis [2]. As a result, the body compensates for the posture in various ways to maintain the correct posture, one of which is the flexion of the knee as shown in Figure 1. However, this mechanism to compensate for posture through knee flexion increases the load on the knee joint and causes the progression to knee arthritis, causing knee pain [5]. Limits in knee extension and knee pain due to continuous knee flexion can be improved by total knee replacement (TKR) [5]. As shown in Figure 2, a knee artificial joint consists of the femoral components and tibial insert. The femoral component is a metal that is applied to the distal ends of the femur, and the tibial insert is a specially made plastic plate that acts as cartilage inserted to reduce friction. The tibial insert is prone to wear as it absorbs shocks and allows the knee joints to move smoothly during lower limb exercise. The wear of the tibial insert has been regarded as the most important cause of complications after TKR, and many studies have been conducted on this [6; 7; 8]. However, due to the loss of lumbar lordosis, flatback patients are predicted to have different wear characteristics from normal gait because the gait mechanism is different from that of people without flatback syndrome, and changes the contact characteristics of the knee artificial joint. Figure 1: Standing posture in ambulatory patients with flatback: **(left)** Patient with flatback who was unable to maintain an upright posture with the knee extended fully; **(right)** Knee and hip flexion as a compensatory mechanism allows for an erect torso, protects the hip extensors and spine, but increases the demand on the quadriceps (reprinted with permission from ref. [2]. Copyright 2002 Wolters Kluwer Health, Inc., Philadelphia, PA, USA). Currently, related research can confirm the study on the wear of the knee artificial joint in the normal gait mechanism [6; 8; 10; 11; 12; 13; 14; 15; 16] or study the study on the compensation mechanism of the spine and knee by collecting and analyzing kinematic data during gait of flatback patients [2; 5; 17; 18; 19; 20; 21]. These researchers have presented the results of their study, such as a comparison of wear volume according to materials used for friction surface of the knee artificial joint, a comparison of wear volume of the knee artificial joint by experiment and computational simulation, a comparison of wear volume of the knee artificial joint by ISO standards, and an analysis of the effects of loss of lumbar lordosis on the knee in flatback patients by comparing the kinematic data of people without flatback syndrome. However, there has been no study to predict the wear on the knee artificial joint considering the gait mechanism of the flatback patients that is different from that of people without flatback syndrome. Therefore, a comparative study was conducted on the wear characteristics of the gait mechanism of flatback patients, which is different from the gait mechanism of people without flatback syndrome, and it is necessary to present this result as a design guideline for knee artificial joint in flatback patients. The purpose of this study is to predict the wear characteristics of the knee artificial joint in normal gait mechanism and flatback gait mechanism.