Studies on stress and strain state of a hip joint endoprosthesis.

Sticlaru, Carmen ; Davidescu, Arjana ; Crainic, Nicolae 等

1. INTRODUCTION

Arthroplasty is the reconstruction or reshaping of a damaged or diseased joint. This elective surgery most often involves joint replacement, the implantation of an artificial joint (prosthesis). In addition to osteoarthritis, arthroplasty can be a treatment for conditions including hip fractures, other source of acute trauma and rheumatoid arthritis.

Arthroplasty may be used to:

* Replace all or part of a joint with a prosthesis

* Resurface a joint with the patient's own tissue

* Reshape the bone and cartilage that make up the joint

The purpose of this procedure is to relieve pain, to restore range of motion and to improve walking ability, thus leading to the improvement of muscle strength. Indications for arthroplasty include: osteoarthritis (OA), rheumatoid arthritis (RA), avascular necrosis (AVN) or osteonecrosis (ON), congenital dislocation of the hip joint (CDH), acetabular dysplasia (shallow hip socket), frozen shoulder, loose shoulder, traumatized and misaligned joint, joint stiffness.

The people may benefit from hip replacement surgery if: hip pain limits everyday activities such as walking or bending; hip pain continues while resting, either day or night; stiffness in a hip limits the ability to move or lift your leg; the patients have little pain relief from anti-inflammatory drugs or glucosamine sulfate; the patient's harmful or unpleasant side effects from your hip medications; other treatments such as physical therapy or the use of a gait aid such as a cane do not relieve hip pain.

The hip joint supports most of the upper body weight. As a person ages the bone become thinner and more brittle, increasing the risk for injury. The hip joints are connecting the torso to the legs and support the upper body weight (fig. 1 a). The bones of the pelvis, the pubis, the ischium and the illium form a ball-socket joint together with the head of the femur, the log thigh bone (fig. 1 b).

A total hip prosthesis is composed of two components: the femoral (thighbone) component and the cup component (fig. 2).

During the procedure, the joint is fully exposed and the damaged bone and cartilage are cut away or reshaped. A plastic cup is placed in the enlarged hip socket (fig. 1c). Then, the top of the femur is removed and a metal ball is inserted into the top of the femur (fig. 1c). Also a metal stem is also inserted into the femur to add stability to the prosthesis (fig. 1 d). The joint is tested before the incision is closed. The whole stem component is there only to keep the relatively small ball component fixed to the skeleton. The stem component is big, it engages about one third of the whole thighbone. It is ballast. When the total hip fails, the one third of the thighbone skeleton round this big ballast suffers, it is damaged or destroyed. In the healthy hip joint the femoral head is continually in close and stabile contact with the socket during all movements. during total hip replacement a portion of these supporting structures (muscles, ligaments, capsule) is cut (divided) for easier access to the hip joint. Even if the surgeon tries to restore muscle and soft tissue balance by suturing together the cut ligaments, muscles, and joint capsule after the total hip replacement, there is usually some imbalance of soft tissues left. The total hip prosthesis must be anchored securely within the skeleton for good function.

There are two methods how to secure the fixation of a total hip prosthesis to the skeleton:

1. The cemented total hip--the surgeon uses bone cement for fixation of the prosthesis to the skeleton

2. The cement less total hip--the surgeon impacts the total hip directly into the bed prepared in the skeleton.

When prostheses are used, they may be made of polyethylene, metal, ceramics or silicone. The most common design is metal-on-polyethylene, although metal-on-metal designs have become more popular in recent years.

2. THE SOLID MODEL FOR THE FEMUR AND HIP JOINT PROSTHESIS

The solid model for the femur was obtained using computer tomography images. These images were imported in MIMICs and then exported in ProEngineer to make the assembly with the endoprosthesis. The assembly was created using cut operations to obtain the form for the femoral head. This form is obtained by the medical doctor during the surgery (fig. 3). In fig. 3 are presented 3 cases: a. correct position for the femur; b. the position for leg hitting ground; c. leg swinging free.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

3. THE FINITE ELEMENT MODEL DEVELOPED IN ANSYS

The steps for solving the finite element analysis are:

--import the CAD model from proEngineer (Sticlaru, 2007);

--assigning the materials for the femur and endoprosthesis elements (stainless steel for the stem, ball and exterior cup, polyethylene for interior cup, for the femur (Weinans H., 1992), bone properties (Gahr R.H., 1999));

--creating the mesh for the model (fig. 4) (a. the femur, b. the stem, c. the cup, d. interior cup);

--creating the environment for the assembly;

--run analyze--for different body position (fig. 3 a, b, c).

The environment for the model looks like that presented in fig. 5 (a. the forces of the hip in single leg stance b. x-ray image of a normal hip showing the compression trabeculae oriented parallel to the resultant compressive load on the femoral head (Bibb 2006) c. the environment for the solid model). The simulations were performed with the distal end of the femur rigidly constrained. The load for the assembly was obtained by studying the influence of different positions for the human body (Davidescu, 2007). In normal walking the hip is subjected to wide swings of compressive loading from one-third of body weight in the double support phase of gait to 4 times body weight during the single leg support phase.

In normal walking the hip is subjected to wide swings of compressive loading from one-third of body weight in the double support phase of gait to 4 times body weight during the single leg support phase.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

4. RESULTS AND CONCLUSIONS

From the obtained results there can be depicted some aspects:

* total deformations are presented in fig. 6; it can be observed that the distribution is asymmetrical distributed and the major values appear in the cup;

* directional deformation along the femoral axis is resented in fig. 7--the deformation of the bone is different and the values are greater for the ball at rim of the cup;

* strain state for the cup is presented in fig 8--it is an assymetrical distribution, with greater values for swinging up the leg (c);

* stress state for the stem is presented in fig. 9--the distribution for the stress state is simmilar, but the values are very different--the greater values are for the swinging up the leg position;

[FIGURE 6 OMITTED]

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

[FIGURE 9 OMITTED]

The presented aspects depicted from FEM analyze for a hip joint endoprosthesis are very useful for the orthopedist, because that can be taken better decisions for the patients who need total hip arthroplasty. Using the parameterized capabilities of the proEngineer and the link between ProE and Ansys, many models can be developed easy starting from this one. This study is useful for obtaining personalized hip joint endoprosthesis for patients that have skeleton deformation. For this kind of patients to realize a total hip arthroplasty is difficult. In these cases after a computer tomography, the model of the bone is imported in proE--the stem and the cup are designed using the bone form, than in Ansys and it can be decided if the endoprosthesis is a good one.

5. REFERRING

Bibb R. (2006) Medical modelling--The application of advanced design and development techniques in medicine Woodhead Publishing Limited and CRC Press LLC [C] 2006, Woodhead Publishing Limited;

Davidescu A., Sticlaru C. (2007) Studies by Finite Element Method of some Devices for Treatment of Intertrochanteric Fractures, The 12th IFToMM World Congress, Besancon (France), vol 1, pg.112-117, www.iftomm.org;

Gahr R.H., Leung K.S., Rosenwasser M.P., Roth W.(1999) The Gamma Locking Nail, Einhorn-Press Verlag GmbH Reinbek, ISBN 3- 88756-808-7.

Sticlaru C., Davidescu A. (2007), Comparative Study of Fixation Devices for Intertrochanteric Fractures The 12th IFToMM World Congress, Besancon (France), vol 1, pg.118-123, www.iftomm.org

Weinans H., Huiskes R., Grootenboer H. (1992)--Effects of Material Oroperties of Femoral Hip Components on Bone Remodeling, Orthopaedic Research Society.