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There has been a dramatic increase in the use of orthopedic and dental implants worldwide. Studies across anatomic areas have demonstrated that one of the central problems is component loosening.
Implants may loosen due to a lack of strong initial fixation at the bone-implant interface as well as due to non-uniform stress distribution. One manifestation of non-uniform stress distribution is stress shielding. Stress shielding refers to the reduction in bone density (osteopenia) as a result of removal of typical stress from bone by an implant (for instance, the stem of a joint prosthesis). This is because by Wolff's law, bone in a healthy person or animal will remodel in response to the loads it is placed under. After surgery, the bone shares its load with the implant. However, the modulus of elasticity varies between the metal and bone, resulting in changes of how the forces are distributed. Therefore, as the loading on a bone decreases, the bone will become less dense and weaker with resultant bone resorption. The magnitude of stress shielding and the specific locations of bone loss are based on the differences between the properties of the implant and the properties of the underlying bone.
There is extensive literature reporting that stress shielding can result in severe bone resorption across anatomic sites. For example, in shoulder replacement, short stem uncemented humeral components have been reported to have rates of bone resorption of greater than 40 to 70% in short term follow-up. In longer term studies, at a mean of 8 years, one reference reported stress shielding in 47% uncemented stems with partial or complete greater tuberosity resorption in 100% uncemented stems.
Stress shielding also occurs when plates or intramedullary nails are used to repair fractures. While the rigid nature of plates and nails helps to stabilize the fracture and facilitates early mobility, the increased stiffness of the plate or intramedullary nail results in bone loss due to decreased loading of the bone. It has been shown that bone remodeling is extremely sensitive to even small changes in cyclic bone stresses. Changes in cyclic bone stresses of even less than 1% of the ultimate strength can result in measurable changes in bone remodeling after only a few months.
Orthopedic and dental implants function as rigid osseous anchors within bone. The mechanical behavior of implant materials, surface coatings, and shape result in adaptive bone remodeling. The mechanical mismatch between host bone and metallic implants has been a long-lasting concern. For example, the elastic modulus of bone is 10-30 GPa. For two of the most commonly used implant materials, the elastic modulus is approximately 100 GPa for pure titanium and 230 GPa for cobalt-chrome. In this environment, contacted bone is often inappropriately stress shielded, and hence, implants lose supportive bone at the implant-bone region over time.
The current practice in orthopedics has been the use of implants that have been designed without regard to the variability of internal bone architecture. For example, the design and manufacturing of stems for joint arthroplasty has been traditionally driven by ease of and cost of manufacturing. Therefore, humeral stems and stemless implants have been designed to be symmetric and can be used in right or left sides. This minimizes inventory as well as the cost of manufacturing. This long held practice has been forcing the anatomy to fit the implant rather than making the implant match the anatomy and underlying bone architecture.
These concerning findings highlight the need for technology and strategies to address the causes of implant loosening with technology and strategies to optimize the bone-implant interface. These strategies can include improved immediate implant fixation as well as improved stress distribution at the bone-implant interface.
The present disclosure addresses the aforementioned drawbacks by providing systems and methods for implant design and manufacturing to optimize the bone-implant interface. These strategies can improve the immediate fixation as well as the stress distribution at the bone-implant interface. The implant design and methodology may include taking into account the anatomy of a bone of a subject to optimize the bone-implant interface considerations for the subject. The implant design may be asymmetric, and/or may include asymmetric bone-growth promoting coatings applied at locations determined to optimize the bone-implant interface.
In one configuration, a method is provided for manufacturing an orthopedic implant for repairing a part of a bone in a subject. The method includes forming the implant to include at least one material property determined by: i) obtaining an image of the bone from at least one viewing plane; ii) orienting on the image a cross section indicating a maximum width of a feature of the bone from a first border of the bone to an opposite second border of the bone; and iii) determining the at least one material property to optimize the bone-implant interface using the maximum width of the feature of the bone. In one configuration, determining the at least one material property may include to reduce stress shielding using the maximum width of the feature of the bone.
In some configurations, the material property includes at least one of elasticity, surface coating treatment, porosity, thickness, or shape. The material property may create an asymmetric implant. The material property may include the surface coating treatment of the implant, which may correspond to an anatomic location determined by a location of the cross section of the bone. The surface coating treatment may be asymmetric on the surface of the implant.
In some configurations, the method includes determining a quality of the bone using the maximum width of the feature of the bone. Determining the quality of the bone may include determining a score for the quality of the bone in an anatomic location.
In some configurations, a plurality of cross sections are oriented on the image along a longitudinal axis of the bone. Bone quality may be determined for a plurality of features along the longitudinal axis of the bone using the maximum width of each of the plurality of features. In some configurations, the bone is a humerus and a joint that includes the bone is a shoulder.
In one configuration, a device is provided for repairing a part of a bone in a subject. The device includes a first section having a first material property and a second section having a second material property. The first section is connected to the second section forming an asymmetric implant, where the first section and the second section are configured to optimize the bone-implant interface. In one configuration, the first section and the second section are configured to reduce stress shielding.
In some configurations, the first material property includes at least one of elasticity, surface coating treatment, porosity, thickness, or shape. The second material property may include at least one of elasticity, surface coating treatment, porosity, thickness, or shape. In some configurations, at least one of the first material property or the second material property includes a surface coating treatment of the device, that corresponds to a location of uneven stress distribution. The surface coating treatment may be asymmetric on the surface of the device. The location of uneven stress distribution of the bone may correspond to a location of reduced bone thickness. The bone may be a humerus and a joint that includes the bone may be a shoulder.
In one configuration, a device is provided for repairing a part of a bone in a subject. The device includes a first section having a first material property and a second section having a second material property. The first section is connected to the second section. The first material property or the second material property includes a surface coating treatment forming an asymmetric implant and is configured to optimize the bone-implant interface. In one configuration, the first material property or the second material property includes a surface coating treatment forming an asymmetric implant and is configured to reduce stress shielding.
In some configurations of the device, the first material property is a surface coating treatment and the second material property includes at least one of elasticity, porosity, thickness, or shape. The second material property may include a surface coating treatment and the first material property may include at least one of elasticity, porosity, thickness, or shape. The surface coating treatment may correspond to a location of uneven stress distribution of the bone. The location of the uneven stress distribution of the bone may correspond to a location of reduced bone thickness. In some configurations of the device, the bone is a humerus, and a joint that includes the bone is a shoulder.
The foregoing and other aspects and advantages of the present disclosure will appear from the following description. In the description, reference is made to the accompanying drawings that form a part hereof, and in which there is shown by way of illustration a preferred embodiment. This embodiment does not necessarily represent the full scope of the invention, however, and reference is therefore made to the claims and herein for interpreting the scope of the invention. Like reference numerals will be used to refer to like parts from Figure to Figure in the following description.
Systems and methods are provided for implant design and manufacturing to optimize the bone-implant interface. The implant design and methodology may include taking into account the anatomy of a bone of a subject to address bone-implant interface considerations for the subject. Implants or components can be asymmetrically designed to better match the associated anatomy as well as to optimize the bone-implant interface, such as by quantifying bone density and matching material properties of the implant. Information derived from the methodology can be used to guide the design of the implant resulting in an asymmetric design that optimizes the bone-implant interface. In some configurations, a method for implant design to optimize the bone-implant interface includes an asymmetrical shape of the implant; an asymmetric coating applied to the implant, an asymmetric type of coating applied to the implant; an asymmetric modulus of elasticity of the implant material, and the like. Implants may be configured for repairing a part of a bone in a subject, such as by repairing a fracture, or through arthroplasty, and the like.
Previously, the shape, texturing, as well as material properties for implants like the humerus have been symmetric without regard for the relative thickness of the cortical and cancellous bone. The central flaw and deficiency of symmetric implants is that the implants do not match the anatomy. Anatomy, however, is not symmetric. There are dramatic differences in the thickness of cortical and cancellous bone based on the specific location within the bone.
In some configurations, cortical and cancellous thickness can be measured in cross-sections at any interval down a bone. Measurements of thickness can also be performed in any angular direction from the center of the bone. With such measurements, a true three dimensional thickness of cortical and cancellous bone in a 3-dimensional quantifiable manner may be possible for a bone. The shape and material properties of the implant may then be tailored to match the three dimensional architecture of the bone.
In some configurations, the current methodology facilitates the ability to quantify the thickness of bone at specific locations within the bone. The quantified thickness may be of cancellous or cortical bone. In a non-limiting example, images of a bone may be divided into specific levels. At each level, specific zones may be defined. In some configurations, the quality of the bone in each zone may be quantified. This quantification may be performed using a medical imaging system, such as radiography, CT scan, densitometry, dual energy x-ray absorptiometry (DEXA), and the like. In each zone, a user or an automated system may score the material property of the bone by using the thickness of the specific component of the bone (cortical and/or cancellous) and/or bone quality. This score may then guide the material properties in a corresponding specific region of the implant. The methodology allows the implant to better match the underlying bone with more uniform stress distribution by customizing the implant or a coating of the implant at each corresponding location of the bone. This information can be used to customize the implant for an individual patient. This information could also be generated on a population of patients and implants can then be tailored to specific patient populations.
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In one configuration, a score that quantifies the thickness of the bone and/or the quality of the bone may be determined based upon a location in a cross section of the bone. Different radial locations in the same cross section of the bone may have different scores and therefore would have differing amounts of intervention to optimize the bone-implant interface, such as bone in-growth coatings to an implant and the like. In a non-limiting example, a score for the thickness of a cortical and/or cancellous bone at a radial location of a cross section of a bone may be determined by using the centerline of the bone as the origin of the radial coordinates. The bone may vary in thickness or bone quality at different radial locations in the cross section and thereby different scores may be determined for different radial locations of the cross section of bone.
In another configuration, a score may be determined qualitatively using a clinical feedback that assesses the thickness of the bone and/or the quality of the bone based upon a location in a cross section of the bone and clinical knowledge of the stress distribution effects to be expected. Different radial locations in the same cross section of the bone may have different scores and therefore would have differing amounts of intervention to optimize the bone-implant interface, such as through application of bone in-growth coatings to an implant and the like.
Significant variability exists in thickness of cortical and cancellous bone based on the specific location within the bone. This understanding may be applied to the manufacture of implants in accordance with the present disclosure. An implant may be configured to better match the variability that exists in the thickness of the cortical and cancellous bone, as well as quality of the bone, facilitating more even stress transfer and optimizing the bone-implant interface.
Optimization may also include increasing the immediate torque-out strength and optimal loading of an implant, as described in U.S. Patent Application Publication No. 2019/0105169, which is incorporated by reference herein in its entirety for all purposes. A testing protocol for quantifying resistance to torque-out failure can be found in U.S. 2019/0105169. Increasing the immediate torque-out strength may include increasing the torque-out strength of the implant immediately following implantation and prior to any significant resulting bone in-growth. The torque-out strength may be increased above a threshold value, where the threshold value is the minimum value for a symmetrical, non-optimized implant to remain fixed upon implantation. After implantation, optimization may also include optimizing longer term and more even stress distribution between the bone and the implant.
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In some embodiments, the plurality of lateral lines 58a to 58j can be placed at equidistant intervals distally from intersection point 41a, or from a proximal aspect line, such as proximal greater tuberosity line 44, or from the bone cut line 46. In some embodiments, the equidistant interval can be in a range from 0.1 to 50 millimeters. In a non-limiting embodiment, the equidistant interval can be 25 millimeters. As such, example measurements can be made at 25 millimeters, 50 millimeters, 75 millimeters, 100 millimeters, 125 millimeters, 150 millimeters, 175 millimeters, and 200 or more millimeters distal to the intersection point 41a, or from a proximal aspect line, such as proximal greater tuberosity line 44, or from the bone cut line 46. One can add more lines to provide for determining the contour of the bone with higher resolution.
In a non-limiting example embodiment, a first reference distance can be measured for a first line 71b extending perpendicularly from a first point 62b of the plurality of first intersection points 62a to 62h to a first point 64b of the plurality of second intersection points 64a to 64h. A second reference distance can be measured of a second line 71e extending perpendicularly from a second point 62e of the plurality of first intersection points 62a to 62h to a second point 64e of the plurality of second intersection points 64a to 64h. A third reference distance can be measured of a third line 71h extending perpendicularly from a third point 62h of the plurality of first intersection points 62a to 62h to a third point 64h of the plurality of second intersection points 64a to 64h.
The anatomic shape of the first border 48 can be extrapolated based on the first reference distance of the first line 71b, the second reference distance of the second line 71e, and the third reference distance of the third line 71h. A first curvature of the anatomic shape can be extrapolated between the first point 62b of the plurality of first intersection points 62a to 62h and the second point 62e of the plurality of first intersection points 62a to 62h based on the first reference distance and the second reference distance. A second curvature of the anatomic shape can be extrapolated between the second point 62e of the plurality of first intersection points 62a to 62h and the third point 62h of the plurality of first intersection points 62a to 62h based on the second reference distance and the third reference distance.
In another version of the method of the disclosure, the anatomic shape of the first border 48 and the second border 50 together can be extrapolated based on a fourth reference distance of the lateral line 58b, a fifth reference distance of the lateral line 58e, and a sixth reference distance of the lateral line 58h. A first curvature of the anatomic shape can be extrapolated between the first point 62b of the plurality of first intersection points 62a to 62j and the second point 62e of the plurality of first intersection points 62a to 62j based on the fourth reference distance and the fifth reference distance. A second curvature of the anatomic shape can be extrapolated between the second point 62e of the plurality of first intersection points 62a to 62j and the third point 62h of the plurality of first intersection points 62a to 62j based on the fifth reference distance and the sixth reference distance.
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A longitudinal bone axis 254 can be oriented on the image 240. The longitudinal bone axis 254 may extend from intersection point 41a from
A plurality of lateral lines 258a, 258b, 258c, 258d, 258e, 258f, 258g, 258h, 258i, 258j can be oriented on the image 240 at different distances from the proximal end 243, or from point 41a. Each of the plurality of lateral lines 258a to 258j can extend perpendicularly from one of a plurality of first intersection points 262a, 262b, 262c, 262d, 262e, 262f, 262g, 262h, 262i, 262j on the first border 248 of the bone 42 to one of a plurality of second intersection points 264a, 264b, 264c, 264d, 264e, 264f, 264g, 264h, 264i, 264j intersecting the longitudinal bone axis 254 at one of a plurality of second intersection points 264a to 264j. Each of the plurality of lateral lines 258a to 258j can further extend perpendicularly from one of a plurality of second intersection points 264a to 264j on the longitudinal bone axis 254 to one of a plurality of third intersection points 268a, 268b, 268c, 268d, 268e, 268f, 268g, 268h, 268i, 268j on the second border 250 of the bone 42. The anatomic shape of the bone 42 can be extrapolated based on the plurality of first intersection points 262a to 262j, and the plurality of second intersection points 264a to 264j. Specifically, the anatomic shape of the first border 248 of the bone 42 can be extrapolated from the plurality of first intersection points 262a to 262j and the plurality of second intersection points 264a to 264j. The anatomic shape of the second border 250 of the bone 42 can be extrapolated from the plurality of second intersection points 264a to 264j and the plurality of third intersection points 268a to 268j.
In some embodiments, the plurality of lateral lines 258a to 258j can be placed at equidistant intervals distally from the proximal end 243. In some embodiments, the equidistant interval can be in a range from 0.1 to 50 millimeters. In a non-limiting embodiment, the equidistant interval can be 25 millimeters. As such, example measurements can be made at 25 millimeters, 50 millimeters, 75 millimeters, 100 millimeters, 125 millimeters, 150 millimeters, 175 millimeters, and 200 or more millimeters distal to the proximal end 243. One can add more lines to provide for determining the contour of the bone with higher resolution.
In a non-limiting example embodiment, a first reference distance can be measured for a first line 271b extending perpendicularly from a first point 262b of the plurality of first intersection points 262a to 262h to a first point 264b of the plurality of second intersection points 264a to 264h. A second reference distance can be measured of a second line 271e extending perpendicularly from a second point 262e of the plurality of first intersection points 262a to 262h to a second point 264e of the plurality of second intersection points 264a to 264h. A third reference distance can be measured of a third line 271h extending perpendicularly from a third point 262h of the plurality of first intersection points 262a to 262h to a third point 264h of the plurality of second intersection points 264a to 264h.
The anatomic shape of the first border 248 can be extrapolated based on the first reference distance of the first line 271b, the second reference distance of the second line 271e, and the third reference distance of the third line 271h. A first curvature of the anatomic shape can be extrapolated between the first point 262b of the plurality of first intersection points 262a to 262h and the second point 262e of the plurality of first intersection points 262a to 262h based on the first reference distance and the second reference distance. A second curvature of the anatomic shape can be extrapolated between the second point 262e of the plurality of first intersection points 262a to 262h and the third point 262h of the plurality of first intersection points 262a to 262h based on the second reference distance and the third reference distance.
In some configurations, the anatomic shape of the first border 248 and the second border 250 together can be extrapolated based on a fourth reference distance of the lateral line 258b, a fifth reference distance of the lateral line 258e, and a sixth reference distance of the lateral line 258h. A first curvature of the anatomic shape can be extrapolated between the first point 262b of the plurality of first intersection points 262a to 262j and the second point 262e of the plurality of first intersection points 262a to 262j based on the fourth reference distance and the fifth reference distance. A second curvature of the anatomic shape can be extrapolated between the second point 262e of the plurality of first intersection points 262a to 262j and the third point 262h of the plurality of first intersection points 262a to 262j based on the fifth reference distance and the sixth reference distance.
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Implants may take on any form including, but not limited to, joint arthroplasty, trauma, dental implants, and the like. Non-limiting examples include stems for joint arthroplasty, stemless implants for joint arthroplasty, trauma implants including intramedullary nails and plates, sports medicine implants such as anchors and screws, as well as dental implants.
The material properties of the implant, such as a plate, intramedullary nail, and the like, can be tailored to the specific anatomic site. The thickness of the cortices varies significantly based on the specific anatomic location where a plate may be placed. An implant designed with these factors taken into account can result in more physiologic loads to the underlying bone and has the potential to minimize uneven stress distribution and improve fracture healing. Moreover, the material properties of an implant, such as an intramedullary nail may be optimized for patients with different quality bone in specific locations, such as the difference between young patients compared to older patients. This has the potential for wide implementation since the distribution of many fractures constitutes a clear bimodal population. Femur fractures occur most frequently in either high velocity accidents in young males or simple falls in elderly women.
Implant elasticity and the long-term bone integrity associated with adaptive bone remodeling are strongly related. Altering implant material properties can alter elastic properties. Adjusting titanium porosity results in elastic properties closer to those of bone than to solid titanium surfaces. Functionally graded material (FGM), where the mechanical properties can be tailored to vary with position within the material, may be used to adjust the modulus of elasticity within an implant.
In some configurations, the modulus of elasticity of the implant may be modified by adjusting the coating of the implant in regards to type, porosity, thickness, and the like with respect to location on the implant. Modification of elasticity may also include using different coatings in different areas of the implant. Modification of elasticity may also include altering the shape of the implant, such as by matching a fluted, oval, spherical, or other shape to the anatomy of the bone. Modification of elasticity may also include adjusting the modulus of elasticity of the implant material in a zone specific manner.
Surface roughness treatments such as polishing, sandblasting, plasma-spraying, or porous beading may have an impact on the distribution of stresses at the bone-implant interface. In some configurations, the use of surface roughness methods may result in improved stress distribution in peri-implant tissues. In FEA analysis, porous-surfaced implants were shown to distribute stress in a more uniform pattern around the implant compared with smooth surfaced implants. However, these coatings have traditionally been applied in a symmetric manner. Moreover, in these FEA analysis, bone has been simplified and modeled to be symmetric. In accordance with the present disclosure, surface roughness methods may be applied or utilized asymmetrically on an implant. Imaging or modelling the asymmetry in bone architecture may be used to apply asymmetric coatings to an implant to better match the implant to the anatomy and improve stress distribution.
In some configurations, the method utilizes the bone properties measurements of a wide population of patients. Implants may then be manufactured in mass to fit an average size or configuration rather than being patient specific. In some configurations, different implant size and configuration options may be created based on differing populations. In non-limiting examples, an implant may include characteristics optimized for a younger patient with thicker cortices and good quality bone, and another implant may be optimized for an older patient with bone that has thinner cortices and poorer quality bone. A true centerline measurement generated from the population averages may be used to define the true center of the canal and can be used to design implants that match the shape of the canal and result in more uniform stress distribution.
Individualized implants may be designed using additive manufacturing and 3D printing to customize the implant for an individual patient in accordance with the present disclosure. The material properties of the implant including coatings, shape, and material properties may be adjusted to create in an implant designed to optimize the bone-implant interface for that individual patient.
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One skilled in the art would appreciate that the implant design and methodology may include taking into account the anatomy of a bone of a subject to address stress shielding considerations for the subject. Stress shielding may develop over time in a subject with an implant, and optimizing the bone-implant interface may address stress shielding considerations as well, such as by preventing bone resorption in a subject over time. As described above, implants or components can be asymmetrically designed to better match the associated anatomy as well as to address stress shielding when the bone-implant interface is optimized, such as by quantifying bone density and matching material properties of the implant. Information derived from the methodology can be used to guide the design of the implant resulting in an asymmetric design that optimizes the bone-implant interface. In some configurations for addressing stress shielding, a method for implant design to optimize the bone-implant interface includes an asymmetrical shape of the implant; an asymmetric coating applied to the implant, an asymmetric type of coating applied to the implant; an asymmetric modulus of elasticity of the implant material, and the like.
The present disclosure has described one or more preferred embodiments, and it should be appreciated that many equivalents, alternatives, variations, and modifications, aside from those expressly stated, are possible and within the scope of the invention.
This application claims the benefit of U.S. Provisional Patent Application Ser. No. 62/975,458 filed on Feb. 12, 2020 and entitled “Systems and Methods to Reduce Stress Shielding in Implants,” which is incorporated herein by reference as if set forth in its entirety for all purposes.
Filing Document | Filing Date | Country | Kind |
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PCT/US2021/017817 | 2/12/2021 | WO |
Number | Date | Country | |
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62975458 | Feb 2020 | US |