Studies regarding personalized hip endoprosthesis with net structures.

Ghiba, Mihai Ovidiu ; Prejbeanu, Radu ; Rusu, Lucian 等

1. INTRODUCTION

Hip joint replacement is one of the most common surgical procedures performed around the world. Just in the United States 250.000-300.000 total hip arthroplasties (THA) are performed each year, the number of such a surgical procedure considerably growing with each country (Taylor & Wroblewski 2009).

The latest research in the endoprostheses field refers to net structures. The manufacturing of this kind of structures is possible owing to the Rapid Prototyping technology, especially electron beam melting (EBM) (Cremascoli et al., 2007).

A good endoprosthesis stem must be able to assure a stable, safe and long-term fixation. A good osteointegration is capable of offering this kind of fixation between the stem and the bone (Ghiba et al., 2009).

Osseointegration is a process during which the bone tissue grows on the surface of the hip stem endoprosthesis, therefore anchoring it into the bone.

The surface treatments that are being used nowadays have been having good results, but they do not offer the most favorable conditions for osseointegration. Net structures can achieve a better osseointegration than the conventional coated stems (Kusakabe et al., 2003).

This paper exhibits the process of design for one personalized hip stem endoprosthesis with different net structures, placed in the upper part of the stem.

By combining a personalized hip stem endoprosthesis with net structures, it can be achieved the best possible fit and also sufficient contact between stem surface and vital bone tissue. Furthermore, this is very prolific for a good osseointegration.

Because the bone continuously rebuilds its structure, the models were simulated in order to asses which net structure offers the best osseointegration. The process consists of three major steps:

1) Acquisition and processing of the computer tomography (CT) data in order to collect the most important dimensions for the hip stem;

2) Projecting a hip stem with a different design of the net structures;

3) Simulation analyze, in order to evaluate the stresses exerted against the structures and their displacements;

The dimensions collected in step (1) are used in the process of design for the hip stem endoprostheses (2). Step (3) consists of a simulation which helps evaluating the behavior of the structure.

2. RESEARCH METHODOLOGY

2.1 Acquisition and Processing of the CT Data

The CT data were collected using spiral scan method. The patient was a 57 year old male.

The dimensions of the hip stem endoprosthesis were determined with the help of the CT scans. Mimics software was used in order to perform these measurements.

The dimensions used so that to design the personalized hip stem implant, were determined as follows: the CCD (the angle of the neck) (127[degrees]), the length of the neck (65 mm), the depth at which the endoprosthesis stem penetrates the femur (140 mm), the diameters of the medullar canal (10,3 to 23,8 mm), and the diameter of the femoral head (40 mm). These dimensions offer a better geometrical fit, between the endoprosthesis stem and the cortical bone and assure that less bone tissue will be removed.

2.2 Designing a Hip Stem with a Net Structure

One personalized hip stem endoprosthesis with different types of structures was designed by using the dimensions acquired during step (1).

With a view to designing the structure, three parameters were taken into consideration: dimensions, orientation of the layers and the degree of the penetration of the stem.

The position of the structure in the stem was determined by performing several simulations on the hip stem with and without the mesh structure. This is shown in Fig. 1a.

On account of the dimensions that were used for designing these structures, one of them is rare (Fig. 1b) and the other one is dense (Fig. 1c). There are 5x14 layers of the rare structure, each of them with a thickness of 700 [micro]m. They are separated by a distance of 1500 [micro]m. The square holes of the rare structure measure 700x700 [micro]m in dimension. There are 9x23 layers, with a thickness of 400 [micro]m each, construct the dense structure. These layers are placed at a distance of 900 [micro]m between them. The dense structure has square holes with the dimensions of 400x400 [micro]m. The details are being exhibited in Fig. 2a, b.

Taking into consideration the orientation of the layers, two types of structures become obvious: an horizontal one and an oblique one. These structures are also dense and, respectively, rare, respecting the dimensions described above. The obliquity of these structures is demonstrated by the horizontal elements of the vertical layers, which have an inclination of 45[degrees] from the horizontal plane. The details are being exhibited in Fig. 1a, b.

Another parameter that was taken into account is the degree of the penetration of the stem. Depending on this parameter, the surface structures (Fig. 2c) and the cross structures (Fig. 2d) can be observed. The surface structure has a depth of 2000 [micro]m.

The recommendations from the Arcam AB User's Manual were taken into account in what concerns the dimensions of the structure.

A hip stem with this kind of structure can be very effectively manufactured by the Arcam EBM S12 machine (Christensen et al., 2007).

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

2.3 The Simulation

The biomaterial that is most used for manufacturing implants is Ti6Al4V. This material can be used with the Arcam EBM S12 machine for the stem fabrication.

For the FEM (Finite Element Modeling) static simulation process it was used Ti6Al4V.

The fix points of the stems were defined on the lateral sides of the structure and in the inferior part of the stems. These points were chosen because the bone tissue grows through the structure hole, and, in that region, fixes the stem into it.

A force of 1630 N was applied against the head of the endoprostheses. The force was applied against the midline of the head of the endoprostheses, coming from an inferior-posterior-lateral direction, with an angle of 10[degrees] in the frontal plane and 10[degrees] in the sagittal plane, as specified in ISO7206-4 (Taylor & Wroblewski 2009).

In order to obtain accurate results, a high quality mesh was generated by using a maximum edge length of 2 mm for all the structures.

The results are presented as plots of the equivalent stress (Fig. 3a, b) and total deformation (Fig. 3c, d).

3. RESULTS

It was concluded that the denser the structure is, the lower the equivalent stresses and total deformations are. There are no major deformations between the surface and the cross structures, but it seems that, by comparison, the surface structures have more decreased values.

It resulted that the hip stem with horizontal-dense structures has the lowest equivalent stress and total deformations.

Considering the orientation of the structures it can be observed that the oblique-dense structures have the highest equivalent stress and total deformations values.

A more rapid fixation of the stem will occur if the surrounding bone tissue is stimulated a little. If the forces that stimulate the surrounding bone tissue are too high the fixation may not appear because the grown tissue might break.

[FIGURE 3 OMITTED]

Based on the simulation results it was decided that the hip stem with a cross-horizontal-dense structure is better for the patient, because the equivalent stresses (70 MPa) and total deformations (0.0011 mm) at the structure level and also the equivalent stresses (136 MPa) and total deformations (0.222 mm), at the stem level, are all acceptable.

This kind of structure offers better osseointegration. Another advantage of this structure is that it only needs a small quantity of material for its making, but, in spite of this, one of the disadvantages is that it takes too long to manufacture.

4. CONCLUSIONS

The personalized hip stem endoprosthesis with net structures offer a better osseointegration than the usual personalized hip stem endoprosthesis and than the conventional coated ones.

Solutions regarding the design of the net structures that can be built using an Arcam EBM Rapid Prototyping machine are also presented.

Future simulation tests will be conducted on structures that can be generated by using specific software that is available on the market, with different penetration degrees and different positions, in order to determine the best structure type.

The next step is to try to design and simulate new structures that are denser than the ones presented

The Arcam EBM S12 machine will be used to build future stems, which will be mechanically tested, in order to validate the results of these simulations.

5. REFERENCES

Christensen, A.; Lippincott, A.L.; Kircher, R. (2007) "Qualification of Electron Beam Melted (EBM) Ti6Al4VELI for Orthopaedic Implant Applications", Available from: http://www.medicalmodeling.com, Accessed: 200903-12

Cremascoli, P.; Lindhe, U.; Ohldin, P., (2007) "New orthopaedic implants produced with Rapid Manufacturing improve people's quality of life", Available from: http://www.arcam.com, Accessed: 2009-05-15

Ghiba, M., O.; Rusu, L. et al., (2009) "Design process of custom-made femoral stem prosthesis", Annals of the Oradea University, pp. 658-663, ISSN 1583-0691, Romania, 05.2009,Editura Universitatii din Oradea, Oradea

Kusakabe, H.; Sakamaki, T., et al., (2003), " Osseointegration of a hydroxyapatite-coated multilayered mesh stem", Available from: http://www.sciencedirect.com, Accessed: 2009-05-17

Taylor, B., M.; Wroblewski, M., (2009) "Effect of Hip Stem Taper on Cement Stresses", Available from: http://www.orthosupersite.com, Accessed: 2009-04-120