Abstract:
A method of determining wear of an artificial knee assembly (AKA) includes acquiring a first set of computed tomography (CT) data about the AKA in vivo. A first volumetric file is generated based on the first set of CT data. A first point cloud data set is generated based on the first volumetric AKA file. A first dimensional analysis of the AKA is performed using the first point cloud data set. A second volumetric file is generated based on a second set of acquired CT data before implantation or from a model. A second point cloud data set is generated based on the second volumetric AKA file. A second dimensional analysis is performed using the second point cloud data set. The first dimensional analysis is compared to the second dimensional analysis and a determination is made if they are different from each other.

Description:
TECHNICAL FIELD 
     This disclosure relates to orthopedic implants. In particular, the disclosure relates to systems and methods of measuring wear of artificial knee joint replacements in vivo. 
     BACKGROUND 
     Total joint arthroplasty is an operation involving the replacement of a damaged joint with an artificial joint in order to restore motion to the joint and function to the muscles and ligaments and other soft tissue structures that operate and control the joint. 
     The operation is typically performed on individuals with a painful, disabling arthritic joint that is no longer responsive to conservative treatment regimens. This operation typically entails implantation of two or more artificial joint members into respective natural joint members to replace deteriorated natural articulating surfaces with artificial equivalents. 
     Artificial joint assemblies have been devised for a variety of joints including hips, knees, ankles, shoulders, elbows, fingers, toes and wrists. While artificial joint components are designed to provide stable and permanent attachment to the natural adjacent body tissue(s), at attachment interfaces, over time, the artificial joint can relocate, loosen and/or wear, which can lead to a loss of function, bone deterioration and tissue debris generation. An increase in wear to the articulating surfaces of the artificial joint typically results in reduced function of the artificial joint and, in addition, produces joint debris, which are expelled from the joint area to the surrounding tissues and may cause adverse reactions in these tissues. As wear of a joint progresses and larger amount of particles are expelled to the surrounding tissues, further bone absorption and loosening of the joint implant may occur. Such loosening of a prosthetic joint implant and damage to surrounding tissues is often left undetected in a patient even if regularly checked by a physician. 
     Most modern methods currently employed for determining the extent of wear of an artificial joint rely on X-ray imaging, computer tomography, isotope bone scans, magnetic resonance and the like to image the implanted joint. Known methods are known to have insufficient accuracy or are technically difficult to perform and/or interpret, even by highly skilled professionals. Most modern joint replacement assemblies incorporate metal backed plastic components, metallic components, or ceramic components within metallic shells and the available imaging methods cannot produce sufficient resolution in order to determine artificial joint loosening and/or articulating surface wear. 
     As a result of inefficient detection methods, oftentimes the only indication of early joint loosening is the pain and discomfort suffered by the patient. Bone absorption may progress to a stage necessitating replacement surgery using larger implants, and/or bone grafts to accommodate for the lost bone tissue. The prognosis for success and service life of the implant after such a corrective operation is less predictable and depends, among other factors, on the extent of bone absorption suffered. If performed relatively early on, such corrective surgery has an increased chance of success. Therefore, a method capable of detecting the extent and depth of wear of the articulating surfaces of an artificial joint, or a method that is capable of detecting minute displacement of artificial joint components, is important both to the patient and the treating physician. Also, a more exact understanding of the condition of the components of numerous joints through their lives can facilitate faster improvements to joint design for better performance and longer life. Typical life of an artificial joint now is 15 to 20 years. With such high precision in vivo measurements, joint like may be able to be increased to 25 years and more in a matter of years of developments rather than over decades. 
     It will be appreciated that this background description has been created to aid the reader, and is not to be taken as an indication that any of the indicated problems were themselves appreciated in the art. While the described principles can, in some respects and embodiments, alleviate the problems inherent in other systems, it will be appreciated that the scope of the protected innovation is defined by the attached claims, and not by the ability of any disclosed feature to solve any specific problem noted herein. 
     SUMMARY 
     In one aspect, the present disclosure describes a method of determining wear of an artificial knee assembly, the method including acquiring a first set of computed tomography data about the artificial knee assembly in vivo. A first volumetric artificial knee assembly file is generated based on the first set of computer tomography data. A first point cloud data set is generated based on the first volumetric artificial knee assembly file. A first dimensional analysis of the artificial knee assembly is performed using the first point cloud data set. A second volumetric artificial knee assembly file is generated based on a second set of acquired computed tomography data about the artificial knee assembly before implantation or acquiring the second volumetric artificial knee assembly file from a model. A second point cloud data set is generated based on the second volumetric artificial knee assembly file. A second dimensional analysis is performed using the second point cloud data set. The first dimensional analysis is compared to the second dimensional analysis. A determination is made if the first dimensional analysis is different from the second dimensional analysis an amount that exceeds a selected tolerance of the artificial knee assembly. 
     In another aspect, a system is disclosed for determining wear of an artificial knee assembly, including a CT x-ray machine configured to scan an artificial knee assembly in vivo and a computer system in communication with the CT x-ray machine. The computer system is configured to acquire a first set of computed tomography data about the artificial knee assembly in vivo, generate a first volumetric artificial knee assembly file based on the first set of computer tomography data, generate a first point cloud data set based on the first volumetric artificial knee assembly file and perform a first dimensional analysis of the artificial knee assembly using the first point cloud data set. The computer system is further configured to at least one of generate a second volumetric artificial knee assembly file based on a second set of acquired computed tomography data about the artificial knee assembly before implantation or acquire the second volumetric artificial knee assembly file from a model, generate a second point cloud data set based on the second volumetric artificial knee assembly file, perform a second dimensional analysis using the second point cloud data set, compare the first dimensional analysis to the second dimensional analysis, and determine if the first dimensional analysis is different from the second dimensional analysis an amount that exceeds a selected tolerance of the artificial knee assembly. 
     Further and alternative aspects and features of the disclosed principles will be appreciated from the following detailed description and the accompanying drawings. As will be appreciated, the principles related to determining wear of artificial joints as disclosed herein are capable of being carried out in other and different embodiments, and capable of being modified in various respects. Accordingly, it is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and do not restrict the scope of the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an expanded view of an embodiment of an artificial joint assembly. 
         FIG. 2  is a front view of the artificial joint assembly of  FIG. 1  in vivo. 
         FIG. 3  is a front view of the artificial joint assembly in a holding fixture. 
         FIG. 4  is an embodiment of a system of measuring the initial, pre-implanted dimensions of an artificial knee joint assembly. 
         FIG. 5  is an embodiment of a system of measuring the dimensions of an artificial knee joint assembly in vivo. 
         FIG. 6  is a flowchart illustrating an embodiment of a method of determining wear of an artificial knee assembly. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exploded view of a conventional artificial knee joint assembly  24  including a femoral component  30 , a tibial tray  32 , and a spacer or insert plate  34  positioned between the femoral component  30  and the tibial tray  32 . The femoral component  30  is a rounded, cup-shaped component that may be made of metallic or ceramic materials. The insert plate  34  includes an upper slide surface  36 , which is shaped and sized to movably receive the femoral component  30 . The slide surface  36  permits articulation of the femoral component  30  on the insert plate  34  while supporting the motion of the femoral component by discouraging misalignment or displacement of the components. For clarity, a patellar component is not shown in this disclosure. 
     The insert plate  34  may be made of any suitable bearing material such as polyethylene. An upper surface or tray part  38  of the tibial tray  32  receives the insert plate  34 . The tibial tray  32  also includes an anchor part  40  extending from the tray part and may be made of metallic or ceramic materials. 
       FIG. 2  shows the artificial knee joint assembly  24  in vivo, with the soft tissue omitted for clarity, and with the femoral component  30  fixed onto a distal portion of a femur  42 . The tibial tray  32  is fixed onto a proximal portion of a tibia  44  via the anchor part  40 . The insert plate  34  is positioned onto the upper surface  38  of the tibial tray  32  to movably receive the femoral component  30  thereon. It will be understood that all configurations and variations of an artificial knee joint assembly  24  are contemplated by the present disclosure. 
     In a conventional artificial knee joint  24 , an insert plate  34  of the tibial component  32  is typically formed from ultra-high molecular weight polyethylene (UHMWPE). The insert plate wears, albeit slightly, since it slides in contact with the femoral component  30  that is made of a metal or ceramics. It has been known that the insert plate  34  becomes thinner through use over a period of time after being implanted in a human body. Therefore, it is a common practice to design the insert plate  34  with a predetermined thickness (normally from about 2 to 5 mm) and taking the wear loss into account. Since the slide surface  36  of the insert plate  34  is normally a concave surface that is recessed from the uppermost extent of the insert plate, the overall thickness of the insert plate must be made larger than the above-mentioned predetermined thickness to ensure sufficient thickness of the sliding surface. 
     The wear of the insert plate  34  can cause various problems for the user and can eventually lead to deterioration of the performance of the joint. At some point, it may become necessary to replace at least the insert plate  34 . The point at which some or all components of the artificial joint assembly  24  should be replaced is before a critical, predetermined wear threshold is exceeded. Timing of replacement may take into account several factors, including but not limited to medical indications, manufacturers recommendations regarding specified wear amounts, and patient symptoms. For purposes of the present disclosure, the point at which one or more component of the artificial joint assembly  24  should be replaced, typically the insert plate  34  may be referred to as a selected tolerance. 
       FIG. 4  illustrates an embodiment of a system  10  for measuring the initial dimensions of an artificial knee joint assembly, i.e., for measuring the assembled artificial joint assembly before implantation into a patient. The system  10  may include an artificial joint assembly holding apparatus  12 , a computed tomography (CT) x-ray machine  14 , and a computer  16 . 
     In general, CT x-ray machine  14  may be configured to obtain data from imaging artificial knee joint assembly  24  disposed on the joint holding apparatus  12 , while computer  16  may be configured to analyze the data acquired from scanning the artificial knee joint assembly  24 . The computer  16  also may be used to store the analysis for comparison with a CT x-ray scan and analysis of the same artificial knee joint assembly  24  after implantation ( FIG. 5 ), i.e., in vivo. 
     Joint holding apparatus  12  may be any type of elements, machines or a system of elements or machines suitable for holding the artificial knee joint assembly  24  for scanning by the CT x-ray machine  14 . Alternatively, the joint holding apparatus  12  is also configured to apply a selected load on the joint assembly  24  while it is being scanned. In alternate embodiments, the load may be zero or a selected, predetermined load, for example, body weight of the patient. 
     Referring to  FIGS. 3 and 4 , joint holding apparatus  12  includes a femoral component fixture  50  and a tibial tray fixture  52  that comprise a joint assembly-holding fixture  51 . The femoral component fixture  50  has a shape and size to receive and attach to the femoral component  30 . The tibial tray fixture  52  has a shape and size to receive and attach to the tibial tray  32 . 
     When the joint holding apparatus  12  is being employed to hold the artificial knee joint assembly  24 , the insert plate  34  is disposed between the femoral component  30  and the tibial tray  32  such that the artificial knee joint assembly  24  is assembled into an anatomically correct and operable state. The femoral component fixture  50  and tibial tray fixture  52  are both mountable to the joint holding apparatus  12  by any suitable structure and method, including the use of fasteners, such as bolts. 
     The joint holding apparatus  12  includes a base  22  to which the tibial tray fixture  52  is attachable and an optional load measuring system  20  upon which the base  22  is mounted. The joint holding apparatus  12  includes a load-applying system  18  to which the femoral component fixture  50  is attachable. The joint holding apparatus  12  can include an Instron® type system or a similar variable load application system. The joint holding apparatus  12  may be rotatable to provide the CT x-ray system  14  with a plurality of positions for scanning necessary for computed tomography methodology. Alternatively, some of the components of the CT x-ray system  14  can be rotated and the joint holding apparatus  12  can be held stationary. 
     The joint holding apparatus  12  is operably associated with a CT x-ray system  14 . The joint holding apparatus  12  is configured to hold the joint assembly  24  so as to be scanned by the CT x-ray system  14 . The CT x-ray system  14  can include any suitable x-ray scanner that is capable of scanning the joint assembly  24  when held by the holding apparatus  12  and obtaining sufficient quality and amounts of data to accurately image and measure the joint assembly. 
     CT x-ray machine  14  may by any type of machine suitable for performing high-resolution, three-dimensional imaging of artificial knee assembly  24 . As illustrated in  FIG. 4 , CT x-ray machine  14  includes a radiation emitter  26  and a radiation detector  28 . In one embodiment, radiation emitter  26  and radiation detector  28  may be fixed relative to artificial knee assembly  24 . In another embodiment, radiation emitter  26  and radiation detector  28  may be configured to rotate, for example, about 360 degrees around artificial knee assembly  24  (i.e., around a vertical, horizontal or selected axis passing through artificial knee assembly  24 ). Radiation emitter  26  may include any suitable type of x-ray tube, such as for example, a 100-500 kV, high-power or micro-focus x-ray tube. Moreover, radiation emitter  26  may include a plurality of radiation tubes, or sources. Radiation detector  28  may be a digital detector configured to detect radiation emitted from radiation emitter  26 , as affected by the geometry of artificial knee assembly  24 . For example, radiation detector  28  may include a 1-10 megapixel digital radiation detecting mechanism. In one embodiment, radiation detector  28  may include an array, or plurality, of digital detectors configured to cooperate with corresponding radiation emitters  26 . CT x-ray machine  14  may include a built-in processor  16  configured to control the operation of radiation emitter  26  and/or radiation detector  28 . CT x-ray machine may also include built-in processing for data storage, component feature extraction, volume reconstruction, rendering/visualization, dimensional analysis, and/or performing comparisons. In one embodiment, CT x-ray machine  14  may further include a built-in display monitor for displaying three-dimensional representations of components. In another embodiment, the artificial joint assembly  24  is positioned closer to the radiation emitter  26  than the radiation detector  28  to create a magnification effect based on the ratio of the distance of the artificial knee assembly divided by the distance of the artificial knee assembly to the radiation detector. 
     The computer  16  may be integral with the CT x-ray system  14  or separate and in communication with the CT x-ray system. Computer  16  may include a single microprocessor or multiple microprocessors that include control mechanisms to operate CT x-ray machine  14 . Numerous commercially available microprocessors may perform the functions of computer  16 . It should be appreciated that computer  16  could readily embody a general machine microprocessor capable of controlling numerous machine functions. Computer  16  may include or be associated with a memory for storing data such as for example, an operating condition, a design limit, and a performance characteristic or specification of CT x-ray machine  14 , and/or model artificial knee assemblies and actual artificial knee assemblies  24 . Various other known circuits may be associated with computer  16 , including power supply circuitry, signal-conditioning circuitry, solenoid driver circuitry, communication circuitry, and other appropriate circuitry. Moreover, because computer  16  may communicate with other components via either wired or wireless transmission, computer  16  may be disposed in a location remote from CT x-ray machine  14 , if desired. Alternatively, as discussed above, computer  16  may be integral with CT x-ray machine  14 . Accordingly, computer  16  may be configured to receive signals from CT x-ray machine  14  including CT data about the geometry of the artificial knee assembly  24 . Computer  16  may be configured to store, analyze and compare the CT data and send reports, images, alerts, and other information based on the raw or analyzed CT data. 
     Referring to  FIG. 5 , CT x-ray machine  14  may perform the same operations as in the configuration shown in  FIG. 4 , but of an in vivo artificial knee assembly  24 . Specifically, the system  10  includes a CT x-ray machine  14  with an x-ray emitter  26  and an x-ray detector  28  arranged as in the device shown in  FIG. 4 . Also, a computer  16  is provided in communication with the CT x-ray machine  14  for receiving the data from the CT x-ray machine, performing storage and computational operations and generating outputs from scanning the artificial knee assembly  24  in vivo. It will be understood that the same operations are performed on the artificial knee assembly  24  that were undertaken during the initial or first imaging and analysis process. The CT x-ray machine  14  includes a base  20  that also may include a load cell. In one embodiment of the disclosure, the load on the artificial knee assembly  24  in vivo is made to be the same as during the initial imaging and analysis process. Loading the artificial knee assembly  24  the same in both processes, i) initially and, ii) in vivo, may ensure consistency of results and a reliable comparison. Tests may be performed with the artificial knee assembly  24  pre-implantation and in vivo with zero load or with a selected load applied to the assembly, such as equal to body weight of the patient or a selected suitable load. 
     In operation, CT x-ray machine  14  may acquire computed tomography data about the geometry of a particular artificial knee assembly  24  before implantation (step  100 ) by performing a scan of the artificial knee assembly. Alternatively, a reference data set may be acquired from a computer file or digitized 3D image in a suitable format of a model and/or reference artificial knee assembly, which will form the basis for comparison between the reference artificial knee assembly and the particular artificial knee assembly in vivo. For example, the model may be supplied by the manufacturer and may represent an ideal, unworn artificial knee assembly  24 . 
     To acquire data of an artificial knee assembly  24 , base  20  may intermittently rotate artificial knee assembly by 360 degrees, in half-degree increments, for example, about an axis. CT x-ray machine  14  may use radiation emitter  26  and radiation detector  28  to generate a cross-sectional artificial knee assembly image corresponding to each of the half-degree increments. Accordingly, CT x-ray machine  14  may generate  720  projection images, or “slices,” of artificial knee assembly  24 , each image being a two-dimensional representation of the features of artificial knee assembly  24  across a particular, rotationally-oriented plane (step  102 ). The number of images acquired in order to obtain a sufficiently detailed data set may be specified based upon the resolution of the detector. 
     The projection images are processed by standard CT software (step  104 ) associated with either CT x-ray machine  14  or computer  16  to generate a volumetric artificial knee assembly data file representing the physical characteristics of artificial knee assembly  24  (step  106 ). Specifically, CT x-ray machine  14  may communicate the acquired CT data to computer  16 , to be assembled into a three-dimensional, volumetric artificial knee assembly data file. Alternatively, the volumetric artificial knee assembly data file may be generated by a processor built-in to CT x-ray machine  14 . From the volumetric artificial knee assembly file cross sections of the artificial knee assembly may be obtained in a conversion process from the volumetric artificial knee assembly file (step  108 ). 
     A process to extract the surface of the scanned components is conducted (step  110 ). The surface extraction process may include beam hardening correction and/or localized corrected edge detection, both of which are known correction processes. 
     Either CT x-ray machine  14  or computer  16  may then generate point cloud data from the volumetric artificial knee assembly data file (step  112 ). Specifically, the volumetric artificial knee assembly file may be converted into point cloud data by various statistical and geometrical methods, with each data point in the cloud representing an approximate location of a point on one of the metal or ceramic surfaces of the artificial knee assembly  24 . A point cloud is a set of data points in a coordinate system. In a three-dimensional coordinate system, these points are usually defined by X, Y, and Z coordinates, and often are intended to represent the external surface of an object. The CT x-ray machine  14  measures a large number of points on an object&#39;s surface, and often output a point cloud as a data file. The point cloud represents the set of points that the device has measured. The generation of the point cloud data may be performed by commercially available software, for example Siemens Imageware™, Innometric Polyworks™, 3D Systems Geomagic™, GOM Inspect, Hexagon PC-DMIS Reshaper, or Volume Graphic Studio™. The point cloud data may be exported to a STL file, which is a file format native to a brand of stereolithography CAD software, or a similar mesh handling. 
     For example, each of the femoral component  30  and the tibial component  32  may be represented by approximately at least 100,000 data points defining a geometry of each component and a space therebetween that is occupied by the insert plate  34 , thereby defining the critical geometry of the insert plate. Even in the event that a selected type of CT x-ray machine  14  only obtains each data point to 6 or 7 um, the analysis of thousands of neighboring data points may be used to improve the imaging resolution to the micron level. For example, statistical averages and probabilities may be used to optimize an approximated location for a data point. Accordingly, the geometry of each of the femoral component  30  and the tibial component  32 , and even the insert plate  34  of the artificial knee assembly  24 , may be determined to a resolution as low as 3-5 um. 
     If the point cloud data is noisy, the noise may be removed according to any well-known process such as the use of a best fit process, wherein the data is compared to known geometric configurations, such as planes, spheres, cones, cylinders, and so on. Alternatively, or in addition, the data may be filtered with a robust filter (step  114 ) based on, for example, spline filtering. 
     There are two options for analyzing the wear or change of dimensions of a subject in vivo artificial knee assembly. The subject in vivo artificial knee assembly  24  may be compared to a reference standard artificial knee assembly (step  116 ) or alternatively, the subject in vivo artificial knee assembly may be compared to the same artificial knee assembly scanned before implantation (step  118 ). The subject in vivo artificial knee assembly  24  may be scanned, analyzed and compared to a reference or the pre-implanted assembly at selected time intervals so as to monitor the wear of components of the assembly over time. 
     The subject in vivo artificial knee assembly  24  is subjected to location and alignment procedures to perform a 3D comparison with the reference or the pre-implanted assembly (step  120 ). Differences between the reference or the pre-implanted assembly to the subject in vivo artificial knee assembly are quantified (step  122 ). The quantified differences are used to determine if the subject in vivo artificial knee assembly  24  exceeds a selected wear tolerance. If it is found that the wear tolerance has been exceeded, a decision can be made to replace the worn component. 
     The above-described steps of: generating the volumetric artificial knee assembly file, generating the point cloud data, and performing dimensional analysis may be performed within CT x-ray machine  14  or by computer  16 . Alternatively, each step may be performed on a separate computer  16 , separate processor, and/or a separate software package, in a so-called “parallel processing” or “pipe-lining” process. Because the processing steps may be divided across distinct computers, processors, and/or software suites, the processing of data in these steps may be expedited to a pace that may make possible the real time imaging of artificial knee assembly  24 . Full processing from imaging to 3D construction results can be accomplished in 10 minutes or less. Geometric dimensional analysis can then be performed after this under 10 minute data acquisition step. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is applicable to monitoring wear in an artificial joint, such as an artificial knee assembly in vivo. The subject in vivo knee assembly can be scanned and the assembly analyzed to acquire a point cloud data set that is comparable to a reference artificial knee assembly or the actual implanted knee assembly scanned before implantation. Wear of the knee assembly can be monitored over time and determined to a highly accurate degree. 
     It will be appreciated that the foregoing description provides examples of the disclosed system and method. However, it is contemplated that other implementations of the disclosure may differ in detail from the foregoing examples. All references to the disclosure or examples thereof are intended to reference the particular example being discussed at that point and are not intended to imply any limitation as to the scope of the disclosure more generally. All language of distinction and disparagement with respect to certain features is intended to indicate a lack of preference for those features, but not to exclude such from the scope of the disclosure entirely unless otherwise indicated. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. 
     Accordingly, this disclosure includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the disclosure unless otherwise indicated herein or otherwise clearly contradicted by context.