Introduction

We have used CT-based navigation system for THA from 2004 (Fig, 1). The purpose is to set acetabular socket in optimal position. We have used two different matching methods in these navigation THA surgeries. The old one is Land-mark matching method (L-method), using conventional paired point matching procedure during surgery. The new one is CT-based fluoroscopy-matching system (F-method), that is new technology of image matching procedure before starting surgery (Fig. 2). We compared the accuracy of socket setting angle with these two systems and discuss the usefulness of navigation system.

A new type of knee prosthesis capable of making deep knee flexion has been long awaited for Asian and Muslim people. Our research group has developed such a prosthesis and designated it as CFK (Complete Flexion Knee). In order to assess the performance of CFK, we have set up various kinds of simulation/experimental projects, such as a cadaveric study, a mathematical model analysis, a photoelastic analysis and FEM analysis.

For carrying out the above-mentioned projects, we faced the most fundamental problem; the information about the muscles’ forces and the forces acting on the joints is limited to that for ambulatory activities but not for squatting or sedentary sitting.

The objective of this study is to introduce the force acting on the knee joint and the lower limbs’ muscle forces at deep knee flexion. A 2D mathematical model was used. The model was composed with three segments: upper leg, lower leg, and foot. The muscle groups incorporated into our model were gluteal muscles, quadriceps including rectus femoris and the vasti, hamstrings, and calf muscles including gastrocnemius and soleus. And thigh-calf contact was assumed to take place at 130° of knee flexion. Three equations were introduced from the moment equilibrium condition about each joint. Since the number of unknowns was six, being surplus to the number of equations, several muscles were grouped into one basing upon the EMG data.

Double leg ascending motions from deep squatting with heel rising were studied for 10 healthy male subjects age of 24±2 years, height of 172±5.8 cm, weight of 66.5±8.7 Kg. The data of ground reaction force and angle of each joint during the motion were collected using a force plate and video recording system respectively. The length of each segment for each subject was directly measured. The mass of each segment and center of gravity was determined by referring to the literature.

The results demonstrated that both the normal force acting on the knee joint and the quadriceps force became maximum when knee flexion angle became 130°(the angle at which the thigh-calf contact diminished), then decreased according as the knees extended. Both of their maximum value were proportional to the subject’s body weight and about seven times larger than that. Therefore it was justified that the joint force and quadriceps force were normalized by dividing them by the body weight. Ascending speeds did not affect the values of joint and quadriceps forces unless the motion was jumping.