CN113081402A

CN113081402A - Femoral stem prosthesis

Info

Publication number: CN113081402A
Application number: CN202110351151.1A
Authority: CN
Inventors: 刘博伦; 郑诚功
Original assignee: Beihang University
Current assignee: Beihang University
Priority date: 2021-03-31
Filing date: 2021-03-31
Publication date: 2021-07-09
Anticipated expiration: 2041-03-31
Also published as: CN113081402B

Abstract

The invention discloses a femoral stem prosthesis, which belongs to the technical field of orthopedic implants and comprises a stem body, wherein a notch is formed in the outer side of the stem body, and an expansion structure is arranged in the notch. The invention can well reduce the stress shielding of the inner side and the outer side of the proximal femur, avoid the bone absorption of the proximal femur and improve the aseptic loosening problem of the femoral stem caused by the stress shielding after total hip replacement. In addition, the auxetic structure is a porous structure, and the pores communicated with the internal structure are beneficial to the transportation of nutrient substances and metabolites, so that the bone can be promoted to grow in, and the medium-term and long-term stability of the prosthesis is further improved.

Description

Femoral stem prosthesis

Technical Field

The invention relates to the technical field of orthopedic implants, in particular to a femoral stem prosthesis.

Background

Total hip replacement is an effective means for treating serious hip joint diseases, and achieves the purposes of relieving pain, recovering function and correcting deformity by implanting an artificial femoral stem and an artificial acetabular cup to replace a diseased or malformed hip joint. Only in 2015, the annual replacement amount of the chinese artificial hip joint has reached 40 thousands, and this figure is also increasing at a rate of 25% -30% per year. The clinical need for high performance hip prostheses is also increasing due to the extended life expectancy of patients, and the need for complex loading conditions and mobility.

The most common failure mode of hip prostheses is aseptic loosening of the femoral stem, and stress shielding is the major mechanical factor for producing aseptic loosening. After the femoral stem is implanted into the femur, the load transmission mode is changed. On the one hand, the load, which is originally transmitted directly through the trabecular bone structure at the proximal end of the femur, becomes transmitted through the shear force of the prosthesis-bone contact interface; on the other hand, the higher bending stiffness of the femoral stem results in reduced bending deformation of the femur. These two factors result in less load, i.e., stress shielding, being experienced by the femur after surgery. Since the remodeling of bone tissue is a dynamic process that varies with load, i.e., higher loads promote bone growth and lower loads promote bone resorption. Stress shielding causes bone resorption of the bone surrounding the prosthesis due to lack of sufficient stress stimulation, resulting in failure of the femoral stem-bone fixation interface, which in turn causes aseptic loosening.

In order to avoid stress shielding, the femoral stem is designed mainly around the idea of reducing rigidity, such as a tapered flat stem, a far-end groove, a far-end cross, a porous structure and the like. The conical flat handle with the rectangular cross section realizes initial stability in a mode of embedding the edge into the cortical bone, and as the medullary cavity does not need to be completely filled and the sectional area of the conical flat handle is smaller than that of the conical round handle fixed by filling, the inertia moment of the cross section is smaller, and further the rigidity of the femoral handle is smaller; the design of the groove or the far-end cross-shaped groove on the surface of the femoral stem essentially reduces the rigidity of the femoral stem through the reduction of the material of the stem body; the elastic modulus of porous structured metals is closer to human bone than dense metals, and related studies have also shown that stress shielding can be reduced to some extent.

Another way to reduce stress shielding is to change the load transfer mode, such as taper design, short handle design, neck design, etc. The femoral stem with the taper curved surface can convert the axial shear load into pressure perpendicular to a fixed interface, so that stress shielding is reduced; different from the mode that the long handle generates press fit fixation at the femoral shaft part, the short handle transmits the load to the area near the small rotor to the maximum extent through full press fit and filling fixation with the metaphysis, and reduces the load transmitted to the far end; the neck collar realizes the transmission of larger longitudinal stress to the inner side of the femur in a mode of directly contacting with the femoral stem.

Although studies have found that the above design can reduce the stress shielding of the proximal medial femur to some extent, the load transferred to the proximal lateral femur is limited as the femoral stem is retracted inwardly under bending loads in the body, since the femoral stem is not integral with the femur. The above design has limited effect on improving stress shielding of the proximal lateral femur.

Disclosure of Invention

The invention aims to provide a femoral stem prosthesis to reduce stress shielding of the proximal end of a femur.

In order to solve the technical problems, the invention provides the following technical scheme:

a femoral stem prosthesis comprises a stem body, wherein a notch is formed in the outer side of the stem body, and an expansion structure is arranged in the notch.

Furthermore, the notch is a rectangular notch, the height of the notch is 20-40mm, and the depth of the notch is 6-15 mm.

Further, the distance between the upper edge of the notch and the upper edge of the handle body is 3-8 mm.

Further, the length of the handle body is 90-120 mm.

Further, on the coronal plane, the stem body is wedge-shaped, wherein:

the inner side of the near end of the handle body is a curved surface, the radius of the curved surface is 100-120mm, the inner side of the far end of the handle body is an inclined surface, and the included angle between the inclined surface and the vertical direction is 2-5 degrees;

the outer side surface of the handle body is a curved surface, and the radius of the curved surface is 520 mm and 550 mm.

Further, on a sagittal plane, the upper part of the handle body is rectangular; the lower part of the handle body is wedge-shaped, and the included angle between the front plane and the crown plane of the lower part of the handle body and the included angle between the rear plane and the crown plane of the handle body are 2-3 degrees.

Furthermore, the Poisson ratio of the auxetic structure is-0.68 to-0.25.

Furthermore, the auxetic structure comprises a plurality of concave hexagonal auxetic structure unit cells, the height of a vertical rod on the outer side wall of each auxetic structure unit cell is 2-6mm, the length of an inclined rod on the outer side wall is 0.3-0.8mm, the concave angle between the vertical rod and the inclined rod on the outer side wall is 60-80 degrees in an initial state, and the rod thickness of the vertical rod is 0.2-0.6 mm.

Furthermore, the handle body and the auxetic structure are integrally printed and formed by adopting an additive manufacturing technology.

Furthermore, the handle body and the auxetic structure are both made of titanium alloy or cobalt-chromium-molybdenum alloy.

The invention has the following beneficial effects:

according to the femoral stem prosthesis, the notch is formed in the outer side of the stem body, and the tensioning structure is arranged in the notch, so that on one hand, the integral rigidity of the femoral stem is reduced due to the existence of the notch and the use of the tensioning structure, the femoral stem is more prone to bending inwards under bending load, larger load can be transferred to the inner side of the proximal end of the femur, and bone absorption caused by stress shielding of an inner bone is avoided; on the other hand, the tensile expansion structure with the negative Poisson ratio characteristic is designed on the tension side (near end outside) of the femoral stem, so that the femoral stem can expand outwards when being stressed in a body, the load can be transmitted to the near end outside femur (compared with the prior art, the larger load can be transmitted), and the bone absorption of the outside bone due to stress shielding can be avoided. Therefore, the invention can better reduce the stress shielding of the inner side and the outer side of the proximal femur, avoid the bone absorption of the proximal femur and improve the aseptic loosening problem of the femoral stem caused by the stress shielding after the total hip replacement. In addition, the auxetic structure is a porous structure, and the pores communicated with the internal structure are beneficial to the transportation of nutrient substances and metabolites, so that the bone can be promoted to grow in, and the medium-term and long-term stability of the prosthesis is further improved.

Drawings

Fig. 1 is a schematic view of the overall structure of a femoral stem prosthesis of the present invention;

FIG. 2 is a schematic view of a split structure of the femoral stem prosthesis shown in FIG. 1;

FIG. 3 is a schematic structural view of the femoral stem prosthesis shown in FIG. 1, wherein (a) is a front view, (b) is a side view, and (c) is a top view;

FIG. 4 is a schematic perspective view of a unit cell of the auxetic structure of FIG. 1;

FIG. 5 is a schematic diagram of the structure of the auxetic structure cell of FIG. 4, wherein (a) is a front view, (b) is a side view, and (c) is a top view;

FIG. 6 is a schematic structural view of the auxetic structure of FIG. 1, wherein (a) is a front view, (b) is a side view, (c) is a top view, and (d) is a three-dimensional view;

FIG. 7 is a schematic diagram of a finite element analysis process of a femoral stem prosthesis of the present invention, wherein (a) is a conventional solid metal femoral stem, (b) is an auxetic femoral stem of the present invention, (c) is a three-dimensional reconstruction and surgical simulation of a femoral model, (d) is mesh segmentation and material property assignment, and (e) is loading and boundary condition setting;

fig. 8 is a stress cloud of a femur, a solid femoral stem, and an auxetic femoral stem, wherein (a) is a femoral model, (b) a conventional solid metal femoral stem of the prior art is implanted, and (c) an auxetic femoral stem of the present invention is implanted;

fig. 9 is a graph comparing stress values of a femur, a conventional solid femoral stem implanted in the prior art, and a femoral stem implanted in an auxetic structure of the present invention.

Detailed Description

In order to make the technical problems, technical solutions and advantages of the present invention more apparent, the following detailed description is given with reference to the accompanying drawings and specific embodiments.

The invention provides a femoral stem prosthesis, which comprises a stem body 1, wherein a notch 10 is arranged on the outer side of the stem body 1, and an expansion structure 2 is arranged in the notch 10, as shown in figures 1-6.

It should be noted that the positional reference herein is based on the human body after the femoral stem prosthesis is implanted into the human body, the inner side refers to the side of the stem body 1 where the femoral neck 11 and the ball joint 12 are located (i.e., the right side in the embodiment shown in fig. 1-2), and the outer side refers to the side of the stem body 1 away from the femoral neck 11 and the ball joint 12 (i.e., the left side in the embodiment shown in fig. 1-2).

The auxetic structure material belongs to the category of Metamaterials (Metamaterials), and generates negative Poisson ratio characteristics which are not possessed by natural materials through the deformation of the structure of the auxetic structure material, namely, the integral structure respectively expands/contracts in the radial direction under the action of axial tension/compression load, and the negative Poisson ratio characteristics of the integral structure are derived from the rotation and the bending of the inner structure when the integral structure is stressed. Auxetic structures offer significant advantages over traditional materials in shear resistance, fracture resistance, permeability variability, and energy absorption. With the rapid development of additive manufacturing technology, it has become possible to prepare auxetic structures with complex structures and small volumes.

Preferably, the notch 10 is a rectangular notch sized according to the size of the femoral stem, and the height H1 of the notch 10 may be 20-40mm, such as 24-33mm, and the depth L1 may be 6-15mm, such as 8-12 mm. Also, the distance H2 between the upper edge of the notch 10 and the upper edge of the shank 1 may be 3-8mm, for example 5 mm.

As shown in FIG. 3, in order to reduce the load transmission to the distal end and increase the load transmission to the proximal end of the femur, the stem body 1 is preferably designed as a short stem, and the length L2 of the stem body 1 may be 90-120mm considering the height of the patient. In order to enhance the axial stability of the femoral stem prosthesis, the stem body 1 is wedge-shaped in the coronal plane. For the convenience of processing, the inner side of the proximal end of the handle body 1 (i.e. the curved surface 1) is a curved surface with a radius of 100 mm and 120mm, for example 118mm, the inner side of the distal end of the handle body 1 is an inclined surface (i.e. the plane 1), and the included angle between the inclined surface and the vertical direction is 2-5 degrees, so as to play a role of transition; the outer surface (i.e. the curved surface 2) of the handle body 1 is a curved surface with a radius of 520 mm and 550mm, such as 541 mm.

In the sagittal plane, the upper part of the shank 1 is preferably rectangular (see plane 2 and plane 3 shown in fig. 3 (b)). In order to adapt to the change of the shape of the medullary cavity and reduce the stress stimulation of the far end of the femoral stem to the cortical bone, the lower part of the stem body 1 is wedge-shaped, and the included angle between the front plane (i.e. the plane 4) and the back plane (i.e. the plane 5) of the lower part of the stem body 1 and the coronal plane is 2-3 degrees, for example 2 degrees.

As shown in fig. 1-3, to reduce femoral splitting during implantation, the edge of the stem body 1 is preferably designed as a transition fillet, and the chamfer radius can be flexibly selected, for example, 2 mm.

The above-described design of the femoral stem prosthesis in fig. 1-3 can improve the installation firmness of the prosthesis in the femur and further reduce the stress shielding of the proximal end of the femur.

During the research process, the inventor finds that the Poisson ratio of the auxetic structure 2 is preferably-0.68 to-0.25, namely the ratio of the strain in the vertical load direction to the strain in the load direction when the auxetic structure is under tension is-0.68 to-0.25. The size of the load and the mechanical transmission rule between the femoral stem and the femur of the tension-expansion structure are comprehensively considered, the mechanical distribution rule of the postoperative proximal femur under the Poisson's ratio condition is closer to the normal human body proximal femur stress condition, and the femoral stem prosthesis can achieve the best effect of reducing stress shielding.

The auxetic structure 2 may adopt various structural forms in the prior art, the shape of which is adapted to the shape of the femoral stem prosthesis, for the convenience of implementation, referring to fig. 4-5, the auxetic structure 2 may include a plurality of auxetic structure unit cells of concave hexagonal structure, and the size parameter of each auxetic structure unit cell may be as follows:

the height H3 of the vertical rods on the outer side walls may be 2-6mm, for example 4 mm;

the length L3 of the diagonal bars on the outer side walls may be 0.3-0.8mm, for example 0.5 mm;

the internal concave angle theta between the vertical rod and the diagonal rod on the outer side wall in the initial state can be 60-80 degrees, for example 70 degrees;

the vertical rod may have a rod thickness t of 0.2-0.6mm, for example 0.4 mm.

In the embodiment shown in the figures, considering the size of the stem body 1 and the size of the gap 10, the auxetic structure 2 may specifically be stacked with a total of 380 auxetic structure units of 6, 6 and 15 respectively in the medial-lateral, anterior-posterior and superior-inferior directions of the femoral stem prosthesis. The auxetic structure 2 can expand outwards when being pulled apart, and further can generate mechanical stimulation to bones contacted with the auxetic structure.

In order to avoid local stress concentration and ensure the smooth implantation of the femoral stem prosthesis, the expansion structure part can be trimmed according to the size of the stem body surface and the edge transition fillet so as to ensure that the expansion structure part and the stem body are in transition and smooth, as shown in fig. 6.

The installation of the auxetic structure 2 in the notch 10 of the handle 1 can be performed by various conventional techniques, such as bonding, welding, etc., however, considering that the auxetic structure 2 is small in size and porous inside, and needs to maintain a good connection with the handle 1, it is preferable to integrally print and form the auxetic structure 2 and the handle 1 by using an additive manufacturing technique (such as a selective laser melting technique, etc.) for the convenience of manufacturing.

The material of the auxetic structure 2 is preferably a titanium alloy (e.g. Ti6Al4V) or a cobalt chromium molybdenum alloy (CoCrMo) in view of the requirement for adequate deformation, adequate strength and good biocompatibility of the auxetic structure part. For strength reasons, the shank 1 is preferably made entirely of solid, dense metal, preferably also a titanium alloy or a cobalt-chromium-molybdenum alloy.

The following provides relevant experimental data to illustrate the performance of the femoral stem prosthesis of the present invention.

Finite element analysis of the invention for improving stress shielding effect by the expansion bone handle

1. Method of producing a composite material

And reconstructing a right femur three-dimensional model according to CT scanning data of a healthy male lower limb. And (4) simulating and cutting the femoral head according to clinical practical operation, determining the implantation position of the femoral stem by referring to the anatomical structure of the femur, and completing the simulated implantation of the femoral stem prosthesis (as shown in (c) in fig. 7). The size of the femoral stem handle body with the tension and expansion structure adopted in the experiment is as follows: the length of the handle is 110mm, the height of the notch is 30mm, the depth is 10.6mm, and the distance between the notch and the upper edge of the handle body is 5 mm. In order to research the influence of the auxetic structures with different Poisson ratios on the stress distribution of the postoperative femur, the auxetic femoral stems 1, 2 and 3 are arranged, and the corresponding Poisson ratios are-0.25, -0.42 and-0.68 respectively. Two control groups are respectively provided, namely a normal femur and a conventional solid femoral stem with the outline consistent with that of the femoral stem with the auxetic structure. The shapes of the solid femoral stem and the auxetic structure femoral stem are shown in fig. 7 (a) and (b).

And (3) carrying out grid division on the thighbone by adopting a tetrahedron 1-order unit, wherein the side length of the unit is 2 mm. And meshing the femoral stem handle body by adopting a tetrahedron 1-order unit. The cell size was 1 mm. The auxetic structure was gridded using hexahedral cells, with a cell size of 0.2mm (as shown in fig. 7 (d)). The material parameters of the femur were assigned according to the gray value of the CT image of the femur, which was a linear relationship with the density of the bone tissue, and according to the method of Charalampos et al, the density of the highest gray value (1835 HU in this study) was set to 1.75g/cm³The gray value (100HU) of low-density cancellous bone is collected in the greater trochanter region, and the density is 0.05g/cm³. And according to the principle that one straight line is determined from two points, a linear relation formula of density and gray value is obtained. Then according to the formula (E is 3790 rho) between elastic modulus (E) and density (rho)³) Setting of the femoral elastic modulus is completed (as shown in fig. 7 (e)). The poisson's ratio of the femur is 0.3. The elastic modulus of the prosthesis is 110GPa, and the Poisson ratio is 0.3. All materials are assumed to be homogeneous and isotropic.

The contact property between the femoral stem and the femur is frictional contact, and the friction coefficient is 0.1. And simulating the mechanical response of the femur after joint replacement according to the limit load state of the hip joint in a normal gait cycle by adopting finite deformation static analysis. The load size was 1600N (about 2 times body weight), the loading position was femoral head center, and the loading direction was vertically downward. In addition, the distal femur is constrained and fixed when a load is applied. And finally, observing and recording the stress distribution condition of the thighbone after the solid femoral stem is implanted and the femoral stem with the tension expansion structure is implanted, comparing the stress distribution condition and evaluating the influence of the femoral stem with the tension expansion structure on stress shielding.

2. Results

Femur, conventionThe stress cloud of the solid femoral stem and the auxetic stem of the present invention are shown in fig. 8. The Von-Mises stresses of all cells located in each Gruen partition of the femur were extracted separately and the mean value was calculated (as shown in FIG. 9). According to the formula

Femoral stress changes were quantified after implantation of different femoral stems, with positive values indicating increased stress (as shown in table 1). And according to the formula

To quantify the improvement of stress shielding by the auxetic stem relative to the solid stem, positive values indicate an improvement in stress shielding (as shown in table 2).

The results show that implantation of a conventional solid femoral stem results in stress shielding of the femur in regions Gruen1, 2, 5, 6, 7, with

regions

1, 2 on the lateral side of the proximal femur and 7 on the medial side of the proximal femur being more severe. After the femoral stem with the expansion structure is implanted, the stress shielding on Gruen1 areas and 7 areas is obviously reduced, and taking the expansion femoral stem 2 as an example, the stress shielding on the Gruen1 areas, 2 areas and 7 areas of the femur can be respectively reduced by 46.25%, 28.10% and 35.63% compared with a solid metal stem. In addition, with the increasing of the absolute value of Poisson ratio of the auxetic femoral stem (namely | -0.25| < | -0.42| < | -0.68|), the stress of the femurs in the 1 region, the 2 region and the 7 region is increased, which shows that the expansion effect of the auxetic structure is more obvious, and the expansion structure can generate larger load transmission to the medial proximal femur and the lateral proximal femur. But the stress distribution level of the femur implanted with the auxetic stem 2 is closest to that of a normal femur.

TABLE 1 postoperative stress Change in Gruen zones (%)

TABLE 2 improvement of stress shielding (%)

3. Conclusion

The femoral stem with the stretching structure can respectively transmit more loads to the proximal outer side and the proximal inner side of the femur under the action of in-vivo loads, so that stress shielding of the femur at the position can be avoided; the larger the absolute value of Poisson's ratio of the auxetic structure is, the more beneficial the larger load is transferred to the proximal femur; the stress level of the implanted auxetic stem 2 (Poisson ratio: -0.42) femur is closest to the normal femur.

While the foregoing is directed to the preferred embodiment of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the appended claims.

Claims

1. A femoral stem prosthesis comprises a stem body and is characterized in that a notch is formed in the outer side of the stem body, and an expansion structure is arranged in the notch.

2. The femoral stem prosthesis of claim 1, wherein the notch is a rectangular notch having a height of 20-40mm and a depth of 6-15 mm.

3. The femoral stem prosthesis of claim 2, wherein the distance from the upper edge of the notch to the upper edge of the stem body is 3-8 mm.

4. The femoral stem prosthesis of claim 1, wherein the stem body has a length of 90-120 mm.

5. The femoral stem prosthesis of claim 1, wherein the stem body is wedge-shaped in a coronal plane, wherein:

6. The femoral stem prosthesis of claim 5, wherein the upper portion of the stem body is rectangular in the sagittal plane; the lower part of the handle body is wedge-shaped, and the included angle between the front plane and the crown plane of the lower part of the handle body and the included angle between the rear plane and the crown plane of the handle body are 2-3 degrees.

7. The femoral stem prosthesis of claim 1, wherein the Poisson's ratio of the auxetic structure is between-0.68 and-0.25.

8. The femoral stem prosthesis according to claim 1, wherein the auxetic structure comprises a plurality of concave hexagonal auxetic structure unit cells, the height of the vertical rod on the outer side wall of each auxetic structure unit cell is 2-6mm, the length of the inclined rod on the outer side wall is 0.3-0.8mm, the internal concave angle between the vertical rod and the inclined rod on the outer side wall in the initial state is 60-80 degrees, and the rod thickness of the vertical rod is 0.2-0.6 mm.

9. The femoral stem prosthesis of any one of claims 1-8, wherein the stem body and auxetic structure are integrally printed using additive manufacturing techniques.

10. The femoral stem prosthesis of claim 9, wherein the stem body and the auxetic structure are both titanium alloy or cobalt chromium molybdenum alloy.