Patent Application
20240033095
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 the persistent and significant rate of stress shielding and associated bone loss.
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 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.
This concerning finding highlights the need to address the cause of stress shielding and the resultant need for technology and strategies to address this significant reason for implant failure.
The present disclosure addresses the aforementioned drawbacks by providing systems and methods for implant design and manufacturing to address stress shielding and/or to improve implant fixation to a bone. The implant design and methodology may include taking into account the anatomy and quality of a bone of a subject to address the stress shielding and fixation considerations for the subject. The implant design may be asymmetric, and/or may include asymmetric bone-growth promoting coatings applied at locations determined to mitigate stress shielding.
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 of the bone; iii) quantifying a quality of the bone based on the cross section; and iv) determining the at least one material property of the implant to reduce stress shielding for the bone using the quantified bone quality.
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 at a central region of the device forming an asymmetric implant. The first section and the second section are configured to reduce stress shielding to a region of the bone.
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 of the bone; iii) quantifying a quality of the bone based on the cross section; and iv) determining the at least one material property of the implant to improve implant fixation to the bone using the quantified bone quality.
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 at a central region of the device forming an asymmetric implant. The first section and the second section are configured to improve implant fixation to the bone.
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 address stress shielding. The implant design and methodology may include taking into account the anatomy and quality of a bone of a subject to address stress shielding considerations for the subject. Implants, components, or coatings can be asymmetrically designed to better match the associated anatomy as well as decrease stress shielding, such as by quantifying bone quality 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 and/or coatings that minimize or eliminates stress shielding. In some configurations, a method for implant design to minimize stress shielding 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. Implant designs may include a variety of implant configurations, such as stemmed implants and stemless implants including inlay, onlay, or a hybrid approach.
In a non-limiting example, the methodology facilitates differentially engaging the better quality inferior and anterior humeral bone as well as the peripheral humeral bone for both immediate fixation as well as to prevent stress shielding. Adjusting the thickness of the fins can be done in several ways, including variable fin length, variable fin thickness, variable bone in-growth coating thickness on the surface of the implant, and the like. One will appreciate that fins can be any protrusion from an implant such as arms, blades, wings, and the like, or part of an implant, or a modified geometry of the implant.
Referring to
In accordance with the present disclosure, an implant shape can be adjusted by applying porous ingrowth coating either thinner or thicker in specific regions to address stress shielding. In another configuration, the shape of the fins of an implant may be adjusted in specific regions of the implant to address stress shielding. In another configuration, the fin length of the implant may be adjusted to engage the best quality peripheral bone. These methods can be used alone or in combination. Additional methods that could be used alone or together with adjusting the coating thickness, implant shape, and fin length include: adjusting implant material elasticity or modulus of elasticity, using different types of coating on the same implant, different porosity of the coating on the same implant, different surface coating treatments, different surface coating shapes; and the like.
The shape of the implant has the potential to increase the immediate fixation and pull out strength of the implant. Additionally, when the fins are made of different lengths to better engage the best quality peripheral bone, this also has the potential to further increase the immediate pull out strength. Moreover, the strategies discussed as a part of the methodology can improve long term fixation by decreasing stress shielding.
In some configurations, cortical and cancellous bone quality can be determined in cross-sections at any location or interval down a bone. Determination of bone quality can also be performed in any angular direction from the center of the bone. With such measurements, a true three dimensional quantification of cortical and cancellous bone quality in a 3-dimensional assessment 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 and/or to target the highest identified bone quality in order to address stress shielding.
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 quality of the specific component of the bone (cortical and/or cancellous). 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.
Referring to
Referring to
Referring to
In a non-limiting example, quantifying bone quality includes determining a bone quality score, with higher scores indicating higher bone quality. Lower bone quality may be reflected by a lower score. In a non-limiting example, a scoring range can then be obtained with 1 being low and 10 being high.
In one configuration, a score that quantifies the quality of the bone may be determined and include a determination of the 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 address stress shielding, such as bone in-growth coatings to an implant and the like.
In another configuration, a score may be determined qualitatively using a clinical feedback that assesses the quality of the bone based upon a location in a cross section of the bone and clinical knowledge of the stress shielding 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 address stress shielding, such as through application of bone in-growth coatings to an implant, location of fins on the implant, and the like.
Looking at
Referring to
Referring again to
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.
Looking at
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.
Referring to
Referring to
Referring to
Finite element and clinical research has demonstrated that changes in bone stress in the proximal humerus with stemless implants are most prominent in the superior humeral region. This is consistent with the superior region also having the poorest quality bone. Thinner peripheral fins on stemless implants have been shown to produce less stress changes than thicker peripheral fins. This may be due to these thinner fins being less rigid, resulting in less stress shielding compared with thicker peripheral designs. To minimize stress shielding, an implant may be configured with thinner fins superiorly and then progressively thicker fins in regions with better bone quality. An implant may also be configured to have the fins thinner in the central region where there is poorer quality bone and wider more peripherally to engage the better quality peripheral bone to decrease stress shielding.
The implant shape can be adjusted by applying porous ingrowth coating either thinner or thicker in specific regions. Keeping the base implant the same and adjusting only the coating thickness may have advantages in regard to inventory and accelerated regulatory approval.
In a non-limiting example, thinner fins may be used in regions of poorer bone quality, and thicker fins may be used in regions of better bone quality. The thickness of the fin may be proportional to the bone quality. In another non-limiting example, an inverse configuration may be provided where the thickness of the fin may be inversely proportional to the bone quality, such that thinner fins may be used in regions of better bone quality and thicker fins used in regions of poorer bone quality.
Referring to
Referring again to
Referring to
Referring to
Referring to
Referring to
Referring to
Referring to
Non-limiting example stemless proximal humeral implants have been depicted, but 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 stress shielding 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 reduction of stress shielding 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 decrease stress shielding.
Referring to
Referring to
Referring to
To account for the heterogenous nature of the spatially distributed bone mineral density (BMD), an approach was adopted with BMD in the superior region of the cancellous region set at 0.2 g/cc and the inferior region set at 0.35 g/cc. The mapping between BMD and the elastic modulus used in the simulations was made using experimental testing.
Previous efforts we identified for evaluating stress shielding of implants during design phases undertook computerized methods to analytically predict the likelihood of this behavior. They hypothesized that bone stress in a reconstructed state that was closer to the stress in the intact bone would be less likely to remodel or resorb (from Razfar et al. p. 1080: “For the purpose of this investigation, it is proposed that any changes in bone stress from the intact state are a consequence of joint reconstruction and accordingly should be minimized, as stress changes could lead to a cascade effect whereby stress shielding is initiated”). They go on to state that by “cascade”, some bone would remodel (“strengthen”) due to the presences of higher stress after reconstruction and some would resorb (“weaken”) due to absence of prior stress. Bone that remodels would further propagate the problem over time in the resorbed bone on top of the initial shielding provided by implant. (Basically a “double bogey” for the newly shielded tissue.)
Taking these concepts into account, the modeling results were evaluated by comparing ratio of the average stress (weighted by element volume) in a volume slice of cancellous bone of the implant model to that same slice of bone in the intact model. This method is practical in an engineering sense in that it normalizes the average cancellous bone stress fairly against that of the intact bone. This allows the practitioner to discern the degree of stress change the bone will likely experience when reconstructed with a particular implant, providing a “single metric” means of comparison of implants.
The results from the stemless shoulder designs are presented in the bar chart of
A testing protocol was created to quantify resistance to torque-out failure. Each model was implanted in a rigid polyurethane foam with density of 5 PCF (Pacific Research Laboratories, Inc., Vashon Island, Washington), per the guidelines specified by ASTM F-1839-08 “Standard Specification for Rigid Polyurethane Foam for Use as a Standard Material for Testing Orthopaedic Devices and Instruments”.
A cannulated technique was utilized. A guidewire was placed in the foam bone followed by drilling of a center hole. A preparation tool 1 millimeter in width smaller than the model was used to create channels. This allowed for a 1 millimeter interference fit when the model was placed in the foam. Each model was embedded to a depth such that the medial surface of the model was flush with the surface of the foam.
Each embedded model was rigidly mounted to a 6-axis load cell (ATI Mini 58) via a bolt screwing into the central channel of the model such that the inferiorly located fin of the model pointed downward parallel to gravity. A torque replicating that of a shoulder-elevating motion was applied about the anterior-posterior axis until failure. Failure was defined as the model no longer being fixed within the block due to the foam fracturing around the model or the model sliding free of the foam. During this torque-out process, 3-axis force and 3-axis torque values were recorded at 100 Hz. 6 trials were conducted for each model implant and a summary of the results are show in Table 2 below.
Referring to
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.
The methods in accordance with the present disclosure may be used to result in a population based average of bone quality for an off the shelf solution. A specific number of left and right specific implants with sizes based on a distribution of anatomy can be manufactured. This could include optimized fin length based on the methodology. In some configurations, a semi-custom implant may be provided, where implant options exist for differing patient populations such as a younger patient with good quality bone compared to an older patient with poorer quality bone.
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 minimize stress shielding for that individual patient.
The development of personalized implants may be based on the individual anatomy of a patient with optimized fin length as well as fin designs based on their specific bone quality. A variety of implant designs may be used to decrease stress shielding and improve immediate fixation thereby facilitating the design of smaller implant. Implant designs can be optimized from joint arthroplasty (hip, knee, shoulder, elbow, ankle, spine, etc.), trauma implants, as well as dental implants.
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. 63/120,930 filed on Dec. 3, 2020 and entitled “Systems and Methods to Target Bone Quality 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 |
|---|---|---|---|
| PCT/US2021/040156 | 7/1/2021 | WO |
| Number | Date | Country | |
|---|---|---|---|
| 63120930 | Dec 2020 | US |
