Nigerian Journal of Clinical Practice

ORIGINAL ARTICLE
Year
: 2019  |  Volume : 22  |  Issue : 9  |  Page : 1276--1280

Finite element analysis on the hollow porous design at the proximal end of cementless femoral prosthesis stem


S He1, J Zhu1, J Zhao2,  
1 Department of Orthopaedics, Taixing People's Hospital, Taixing, PR China
2 Department of Orthopaedics, Jinling Clinical Medical College, Nanjing Medical University, Nanjing, Jiangsu, PR China

Correspondence Address:
Dr. S He
Department of Orthopaedics, Taixing People's Hospital, Taixing, Jiangsu
PR China

Abstract

Aims: The present study aims to improve the design of cementless femoral prosthesis stem and achieve better bone ingrowth and long-term stability. Materials and Methods: Four models with different directional hollow holes at the proximal end of femoral prosthesis were designed and finite element analysis was applied to calculate the magnitude of conducting force within the differently angled holes and the stress distribution of the femur and prosthesis. Results: Holes in prostheses make no difference on the stress values of femoral inner walls. The conducting forces in models trepanned on the lateral plane were 6.60N (0° pore) and 8.40N (45° pore) while forces in models trepanned on the anterior-posterior planes were 0.45N (upper 0° pore), 0.48N (lower 0° pore) and 1.57N (upper 45° pore), 1.51N (lower 45° pore), respectively. Conclusion: The position and direction of hollow holes influenced the conducting force in holes but had no influence on stress values of femoral inner walls. Prostheses with one 45° hole trepanned on the lateral plane of proximal prostheses presented best in elevating conducting force.



How to cite this article:
He S, Zhu J, Zhao J. Finite element analysis on the hollow porous design at the proximal end of cementless femoral prosthesis stem.Niger J Clin Pract 2019;22:1276-1280


How to cite this URL:
He S, Zhu J, Zhao J. Finite element analysis on the hollow porous design at the proximal end of cementless femoral prosthesis stem. Niger J Clin Pract [serial online] 2019 [cited 2019 Oct 15 ];22:1276-1280
Available from: http://www.njcponline.com/text.asp?2019/22/9/1276/266154


Full Text



 Introduction



Total hip arthroplasty (THA) is well recognized as an effective treatment to restore hip function and improve patients' quality of life.[1] As more patients undergo THA younger than 55 years of age,[2] how to achieve the long-term stability of femoral prosthesis, prolong its service life span and reduce the rate of revision have been one of research hotspots in the field of joint surgery.[3]

Despite the excellent technique of orthopedic surgeons, the design and material of prostheses make a great contribution to a successful THA.[4],[5],[6] Ingrowth of bone into the prosthesis surface makes major contribution to the biological fixation of femoral bone-stem and decrease in the incidence of mechanical loosening.[7] However, the osseointegration range and intensity on the prosthesis-femur interface for most commercial femoral stems appear to be insufficient, which may result in micromotion and aseptic loosening of the stem and eventually impact on the long-term stability of prostheses.

According to Wolff's law, the growth of bone will change its structure in response to mechanical loads, and mechanical stress is able to induce bone formation.[8],[9],[10] Based on this theory, we speculated that the mechanical stress transferred through the directional inclined holes from femoral wall to the autologous bone accelerate bone ingrowth into the prosthesis-femur interface, which induced the formation of internal bone lock-bolt and strengthened force between prostheses and femur. Better design of hollow holes in femoral stem is able to prolong the stability of prostheses.

With the help of finite element analysis, we established three-dimensional model of prosthesis-femur system, calculated the stress level, distribution of femur and proximal prosthesis and the magnitude of transmitted force in the hole to optimize the porous geometry design at the proximal end of femoral prosthesis. This study aims to provide a theoretical basis and reference for the optimal design of femoral bioprostheses.

 Materials and Methods



Establishment of femoral finite element model

Radiologic images of a normal femurs were first obtained from a 63-year-old male with the weight of 72 kg. 0.9-mm width cuts of computed tomography scans were obtained from 64-slice spiral CT scanner, which scanned from 10 cm above the greater trochanter apex to the knee plane. All the scanning data were imported to Mimics17 system (Materialise, Belgium) for three-dimensional (3D) modeling, followed by developing the volume mesh to materializing the 3D model. Data of materialized 3D model of femurs were saved in IGS format and then imported to the finite element analysis software (UG8.0, Siemens PLM Software, Germany), which was referenced to the L.C.U femoral prostheses produced by Link Corporation.

Design of multiple directional holes at the proximal end of femoral prosthesis

Four models were established according to different locations and directions of the hollow holes at the proximal end of femoral prosthesis [Table 1] and [Figure 1]. The brief introductions of each model were as follows: (1) Model 1, one 0° hole trepanned on the lateral plane of proximal prostheses; (2) Model 2, one 45° hole trepanned on the lateral plane of proximal prostheses; (3) Model 3, two 0° holes trepanned on the anterior-posterior planes of proximal prostheses; (4) Model 4, two 45° holes trepanned on the anterior-posterior planes of proximal prostheses.{Table 1}{Figure 1}

Assembly of the femur and prosthesis

Four prosthesis models and 3D femoral model were all imported to finite element analysis software (UG8.0), and 3D femoral-prosthesis assembly models were constructed. In order to simplify the model, the rounded corner at the contact between prostheses and the femur was ignored. Assembly models were further imported to Hypermesh10.0 software (Altair, America) to conduct mesh generation of both femur and prostheses.

Finite element calculation and analysis

First, material attributes were defined. The femoral prostheses were made of titanium alloy and the fillers within the pores were cancellous bone [Table 2].{Table 2}

Second, contact mode was defined. Considering different working conditions, the proximal and distal ends of femur-prosthesis interface were assigned with different contact attributes. The surface friction coefficient was 0.5 for the contact between inner wall of femoral medullary cavity and the head face of filler, 0.4 for the contact between inner hole surface and proximal prosthesis and 0.3 for the contact between inner hole surface and distal prosthesis.

Third, force was loaded on models and finite element analysis was analyzed. In order to simplify the calculation, the influence of femoral anteversion angle was ignored and the force model was simplified to apply a force of 750N in the -z direction to the loading point earlier selected on the prosthesis head. All fixed constraints were applied to the distal femur to form the boundary conditions, which meant that the displacement of each node at the distal end was 0 on the X, Y, and Z axes.

ABAQUS 6.14 software (Dassault Systèmes, France) was employed for analysis and processing, and its main tasks were to calculate and analyze the magnitude of conducting force within the differently angled holes at the proximal end of femoral prosthesis as well as to calculate the stress distribution of the femur and prosthesis.

 Results



Holes in prostheses make no difference on the stress values of femoral inner walls

After loaded with a force of 750N, the stress values of femoral inner walls were first analyzed [Figure 2]. The stress values of femoral inner walls corresponding to the treated prostheses interior and lateral walls were 34.1 and 13.1 MPa in Model 1 and 33.3 and 12.3 MPa in the same positions of solid prosthesis, respectively. Similar results were observed in Model 2. No significant difference was observed between solid prosthesis and prosthesis with holes trepanned on the lateral plane [Table 3].{Figure 2}{Table 3}

The stress values corresponding to the treated prostheses anterior and posterior walls were 20.7 MPa (upper hole), 29.8 MPa (lower hole) and 38.5 MPa (upper hole), 41.4 MPa (lower hole) in Model 3 and 21.4 MPa (upper hole), 31.2 MPa (lower hole) and 38.2 MPa (upper hole), 40.0 MPa (lower hole) in the same positions of solid prosthesis, respectively. Similar results were observed in Model 4. No significant difference was also observed between solid prosthesis and prosthesis with holes trepanned on the anterior-posterior planes [Table 3]. Hence, holes in prostheses made no difference on the stress values of femoral inner walls.

Conducting forces significantly increase in the lateral plane of proximal femoral prostheses

Furthermore, the conducting forces in holes were analyzed, which was an important factor of accelerating bone ingrowth into the prosthesis-femur interface. The conducting forces in Model 1 and Model 2 were 6.60N and 8.40N, which indicating that 45° hole was better than 0° hole in elevating conducting forces [Figure 3].{Figure 3}

The conducting forces in Model 3 and Model 4 were 0.45N (upper pore), 0.48N (lower pore) and 1.57N (upper pore), 1.51N (lower pore), respectively. Similar with pores trepanned on the lateral plane, 45° hole was better than 0° hole in elevating conducting forces. Also, no significant difference was observed between upper and lower pores [Figure 3].

Among the four models, holes trepanned on the lateral plane were better than holes trepanned on the anterior-posterior planes and 45° holes were better than 0° holes in elevating conducting forces. Model 2 was the best design in elevating conducting forces.

 Discussion



The clinical efficacy of THA is closely associated with the initial fixation of prosthesis, surface coating, prosthetic material and joint friction interface.[11] In addition, the geometry design of prostheses also makes an important contribution to prosthetic stability.[7],[12],[13],[14],[15],[16] With the development of surgical techniques and prosthetic materials and manufacture, THA can provide good initial stability and satisfactory medium-term efficacy. However, the changes in stress conduction mode after THA result in a series of problems.[17],[18] First, the declined stress borne of proximal femur causes adaptive osteolysis and reduced support force.[17],[19] Second, increased femoral distal stress increase the risk of thigh pain, periprosthetic fractures and prosthesis fracture.[18],[20] Third, weight-bearing activities generates shear force and torsional stress on the interface between femoral stem prosthesis and the inner wall of femoral medullary cavity, which lead to bone-prosthesis interface micromovement and followed by prosthetic loosening.[21] Among those problems, micromovement of prosthesis is believed to be a main cause for prosthesis aseptic loosening.[22],[23]

How to reduce the micromovement and maintain the long-term stability of femoral stem prostheses is a complicated task that involves multiple disciplines.[24],[25] From the biological aspect, ingrowth of bone into the porous on prosthesis surface and formation of bone-prosthesis integration is a feasible method to strengthen the stability of prostheses. According to Wolff's law, mechanical load is able to change the growth of bone, and bone remodeling goes through an adaptive response to load changes. Also, the maintenance of normal bone density requires repeated stimulation of loads, and the bone trabecular grows along the direction of principal stress.[26]

Noyama et al.[27] designed a femoral prosthesis with different angled grooves on the medial surface of the stem proximal end and found that groove at a 60° angle could achieve the optimum stress and promote the ingrowth of the corresponding bone into prosthesis grooves. Rawal et al.[28] further designed grooves with different directions and sizes and found that the transmitted force magnitude of grooves on prosthesis surface was associated with their size, position and tilted angle: the greater the tilted angle, the higher the conducted stress. In addition, Wei et al.[21] designed hollow porous on the lateral plane of proximal femoral prosthesis, which had a benefit in bone ingrowth from both in vitro and in vivo experiments.

In the present study, we were intended to optimize the design of creating hollow porous on proximal femoral prosthesis. Four models were included, where differently angled (0° and 45°) hollow holes were designed on both anterior-posterior plane and lateral plane of proximal femoral prosthesis stem. With the help of finite element analysis, we found that holes trepanned on the lateral plane were better than holes trepanned on the anterior-posterior planes and 45° holes were better than 0° holes in elevating conducting forces. Also, holes in prostheses make no difference on the stress values of femoral inner walls. Our results indicated that angled pores trepanned on the lateral plane had the largest conducting forces in holes, which was able to increase the stimulation of mechanical loads to the implanted bone tissue filled in the tilted hole and promote bone growth according to Wolff's law.

In the present study, we designed four different models in order to investigate the effect of directions and angles of hollow pores on the conducting forces in holes in a comprehensive aspect, which was a vital factor in promotion of bone growth. In addition, we investigated the effect of different holes on the stress values of femoral inner walls, which was related to adaptive osteolysis of femur. However, we did not included animal experiment and bone ingrowth in femoral prosthesis hole could not been observed, which will added in the following research.

 Conclusion



Prostheses with one 45° hole trepanned on the lateral plane of proximal prostheses presented best in elevating conducting force. Further animal studies were needed to prove the efficiency of the new-designed model on bone ingrowth.

Acknowledgements

The authors would like to thank Dr. Bao Yidong and Mr. Fan Shengbao for the finite element biomechanical study and data collection.

Financial support and sponsorship

This work was supported by the Clinical Science and Technology Project Foundation of Jiangsu Province (BL2012002), the Scientific Research Project of Nanjing Province (201402007) and the Natural Science Foundation of Jiangsu Province (BK20161385).

Conflicts of interest

There are no conflicts of interest.

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