JP2014108328A - Orthopedic implant strength evaluation method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an orthopedic implant strength evaluation method capable of performing strength evaluation in which a clinical condition is reflected and the actual use environment of an orthopedic implant is assumed.SOLUTION: An orthopedic implant strength evaluation method includes a static load testing process, a load determination process, and a fatigue testing process. In the static load testing process, a load is applied in a constraint state in which a part of a body is considered to which gamma nail as an orthopedic implant is applied, and a displacement amount is measured. In the load determination process, a load-displacement amount curve is created from the measurement value, and an estimator sets a point of inflection, and a first approximate straight line is calculated for the load and the displacement amount from the starting point to the point of inflection, a second approximate line is calculated for the load and the displacement amount from the point of inflection to the end point, and the intersection between the approximate straight lines is calculated as a point of inflection. In the fatigue testing process, a load in the point of inflection is applied, and a fatigue testing is performed.

Description

  The present invention relates to an orthopedic implant strength evaluation method.

  Among orthopedic implants, implants such as hip stems, knee stems, and intramedullary fixation materials are inserted and fixed near the narrowest part of the bone for the purpose of long-term stability. However, the insertion end portion of the implant causes stress shielding, that is, the load transmitted to the bone is reduced depending on the case, so that the bone density is reduced and the bone is contracted to cause loosening of the implant. As countermeasures, there are known a method of changing the shape and material of the stem and a method of treating the surface of the stem in order to prevent stress shielding due to stress concentration in the bone.

  However, at present, an appropriate evaluation method for evaluating the strength of the end portion of the implant such as the distal portion when the shape or material of the stem is changed has not been proposed. As a method for evaluating the mechanical safety of the stem in the orthopedic implant, there are methods defined by standards of International Standardization Organization ISO 7206-4, ASTM F 1264-03, JIS T0313, and the like.

  ASTM F 1264-03 is a regulation for the material strength test of the stem body material, and the strength is evaluated by a four-point bending test as shown in FIG.

  In addition, ISO 7206-4 and JIS T0313 are intended to evaluate the stem proximal portion or the diaphysis portion having a large moment load among the stems, and these methods cannot evaluate the strength of the distal portion of the stem. .

  Normally, in the stem, a large load or moment is applied to the proximal portion of the stem, but the load or moment is distributed and transmitted from the proximal portion of the stem to the distal portion.

  However, each of the above three standards ISO 7206-4, ASTM F 1264-03, and JIS T0313 has the following problems, and can transmit only a smaller load and moment than those that occur in clinical practice, or it has a shape effect. Not evaluated.

  As shown in FIGS. 10 and 11, ISO 7206-4 and JIS T0313 are tests in which loads and moments are less likely to be transmitted than in a clinical state where there is a gap between the stem and bone because the stem S is completely fixed. It is a system and can only transmit small loads and moments to the distal part. In addition, since ASTM F 1264-03 is a normal four-point bending test, it can evaluate the strength of a simple long-shaped stem or the like, but cannot evaluate a complicated-shaped stem or the like.

  Clinically, the moment generated in the orthopedic implant will be described using gamma nail as an example. Gamma nail is applied to fractures such as trochanteric fractures and interosseous fractures. At that time, a moment is generated in the gamma nail due to the influence of muscles and weight, and a load is applied to the distal portion.

  When the proximal femur A is bent as shown in FIG. 12 by the action of the iliopsoas muscle, the gamma nail tries to move with the femur while the distal femur B moves. As a result, a load is generated between the distal portion of the gamma nail and the femur.

  Further, since the gamma nail and the screw are completely fixed to the load due to the body weight, a force F on the lower left in FIG. 13 is transmitted to the gamma nail when the body weight is loaded. The proximal portion to which this force F is loaded serves as a force point and the fracture site serves as a fulcrum, and a moment M due to the force F is generated at the distal portion, so that a load acts on the distal portion.

  Specifically, a load due to muscle is generated in the sagittal plane, and a load due to weight is generated in the frontal plane. Here, the sagittal plane is a plane assumed to be a position where the human body is divided into left and right, and a plane parallel to this plane. The frontal plane is a plane that divides the human body into front and rear and is perpendicular to the sagittal plane. In this way, a moment is generated in the orthopedic implant near the end opposite to the portion where the load is applied due to the muscle and the weight, and the load acts on the distal portion of the gamma nail, for example.

  Such a problem is not limited to the above-mentioned gamma nail, but a moment is also generated at the end opposite to the portion to which the load is applied in the implant such as the hip joint stem and the knee joint stem.

  The evaluation methods defined by the standards as described above do not reproduce the clinical state, and it cannot be said that the strength evaluation of the orthopedic implant is appropriately performed.

  An object of the present invention is to provide an orthopedic implant strength evaluation method capable of performing strength evaluation reflecting the clinical state and assuming the actual use environment of the orthopedic implant.

The present invention is an orthopedic implant strength evaluation method,
The subject of the orthopedic implant that is the object to be examined is in a restrained state that takes into account the body part to which the orthopedic implant is applied, and a load that increases gradually is added to the subject to determine the amount of displacement of the subject relative to the load. A first step to measure,
A load-displacement curve is created from the start point at which the load is applied to the subject in the first step to the end point at which the load is added to the subject, and the inflection in the created load-displacement curve A second step in which the evaluator sets the points;
A third step of calculating a first approximate straight line with respect to the load and displacement from the start point to the inflection point;
A fourth step of calculating a second approximate straight line for the load and displacement amount from the inflection point to the end point;
A fifth step of calculating an intersection of the first approximate line and the second approximate line;
And a sixth step of performing a strength evaluation test of the subject when the load at the intersection calculated in the fifth step is added to the subject.

In the present invention, the first approximate straight line in the third step is calculated so that the load and the displacement amount have a predetermined correlation with respect to linearity,
The second approximate line in the fourth step is calculated so that the load and the displacement amount have a predetermined correlation with respect to linearity.

In the present invention, when the first approximate straight line passes through the inflection point, the inflection point is moved to the larger displacement side, and the third step is executed again using the inflection point after the movement.
When the second approximate line passes through the inflection point, the inflection point is moved to the side where the amount of displacement is small, and the fourth step is executed again using the inflection point after the movement.

  In the present invention, the strength evaluation test is a fatigue test.

  According to the present invention, the orthopedic implant is a gamma nail or artificial joint stem applied to a fracture.

  According to the present invention, in the first step, the subject of the orthopedic implant that is the object to be inspected is subjected to a load that gradually increases in the restraint state in consideration of the body part to which the orthopedic implant is applied. Then, the amount of displacement of the subject with respect to the load is measured. In the second step, a load-displacement amount curve is created from the start point at which load addition to the subject was started in the first step to the end point at which load addition to the subject was completed, and the created load-displacement The evaluator sets the inflection point in the quantity curve.

  In the third step, a first approximate line is calculated for the load and displacement amount from the start point to the inflection point, and in the fourth step, the second approximation line for the load and displacement amount from the inflection point to the end point is calculated. An approximate straight line is calculated, and the intersection of the first approximate straight line and the second approximate straight line is calculated in the fifth step.

  In the sixth step, a strength evaluation test is performed on the subject when the load at the intersection calculated in the fifth step is added to the subject.

  The load applied in the fatigue test is determined based on the measurement value measured in a restraint state in which the body part to which the orthopedic implant is applied is considered, and the strength evaluation test, for example, the fatigue test, is performed using the determined load load as a test condition. Therefore, in the strength evaluation test, the strength evaluation test assuming the actual use environment of the orthopedic implant can be performed.

  Thereby, more accurate evaluation can be performed about the strength evaluation of the orthopedic implant.

  According to the present invention, the first approximate line in the third step is calculated so that the load and the displacement amount have a predetermined correlation with respect to linearity, and the second approximate line in the fourth step is the load and displacement amount. Are calculated so as to have a predetermined correlation with respect to linearity.

  Thus, the first approximate line and the second approximate line are calculated as approximate lines having a certain linearity.

  According to the invention, when the first approximate straight line passes through the inflection point, the inflection point is moved to the side where the displacement is small, and the third step is performed again using the inflection point after the movement, When the second approximate line passes through the inflection point, the inflection point is moved to the side with the larger displacement, and the fourth step is executed again using the inflection point after the movement.

  When the calculated approximate straight line passes through the inflection point, the actual inflection point is larger than the inflection point used for the calculation, in the case of the first approximate line, the displacement amount is larger. In the case, there is a high possibility that it exists on the side where the displacement is small. When the approximate straight line passes through the inflection point, the first and second approximate straight lines can be calculated more accurately by moving the inflection point and calculating the approximate straight line again.

  According to the present invention, a fatigue test can be performed as a strength evaluation test, and the strength of the orthopedic implant can be evaluated based on the result of the fatigue test.

  Further, according to the present invention, it is possible to perform strength evaluation with high accuracy for orthopedic implants such as gamma nails or stems for artificial joints applied to fractures.

It is a front view which shows the outline | summary of the measurement used for implementation of the shaping implant strength evaluation method of one Embodiment of this invention. It is a perspective view which shows the outline | summary of the measurement used for the orthopedic implant intensity | strength evaluation method. It is process drawing which shows the shaping implant intensity | strength evaluation method of this embodiment. It is a graph which shows an example of a load-displacement amount curve. It is process drawing which shows the inflection point calculation method. It is a flowchart which shows the calculation method of a 1st approximate line. It is a schematic diagram which shows the procedure which extracts the combination of data. It is a graph which shows the front part of a load-displacement amount curve. It is a figure which shows the outline of the 4-point bending test prescribed | regulated to ASTM F 1264-03. It is a figure which shows the outline of the evaluation method prescribed | regulated to ISO 7206-4. It is a figure which shows the outline of the evaluation method prescribed | regulated to JIST0313. It is the schematic for demonstrating the moment which generate | occur | produces in a gamma nail when the load by a muscle is added. It is the schematic for demonstrating the moment which generate | occur | produces in a gamma nail when the load by a weight is added.

  FIG. 1 is a front view showing an outline of measurement used for implementing an orthopedic implant strength evaluation method according to an embodiment of the present invention, and FIG. 2 is a perspective view showing an outline of measurement used for an orthopedic implant strength evaluation method. .

  A gamma nail G will be described as an example of an orthopedic implant to be evaluated. The gamma nail G is provided with a slit b at the tip portion on the side to be embedded in the medullary cavity, and further provided with a long hole e for inserting a screw for screwing in the vicinity of the slit.

  In the evaluation method of this embodiment, the gamma nail G is mounted on the mounting table f so that the longitudinal direction thereof is parallel to the horizontal direction.

  The load is applied vertically upward at the force point d at the end opposite to the end where the slit b of the gamma nail G is provided. A support member a is provided at a fracture site, that is, a portion serving as a fulcrum. The support member a is made of, for example, a cylindrical member, and its axis extends in a direction perpendicular to the longitudinal direction of the gamma nail G and is disposed above the gamma nail G. Further, in a state where the gamma nail G is mounted on the mounting table f, the gamma nail G and the support member a are not in contact with each other, and a predetermined interval is provided. As described above, in the clinical state, the bone fracture is not completely fixed but loose. The looseness in the clinical state is reproduced by the distance provided between the gamma nail G and the support member a.

  When a vertically upward load is applied to the force point d, the gamma nail G is lifted by an interval and comes into contact with the support member a. When a load is further applied, a moment M is generated vertically downward with respect to the distal portion c near the slit b. In this way, a vertically downward moment M is generated in the distal portion c of the gamma nail G. However, since the gamma nail G is placed on the mounting table f, the movement of the distal portion c is limited, Stress is also generated in the distal portion c.

  In the conventional evaluation method, since the fixing force by the support member a is large and does not loosen, the stress when a load is applied is generated only in the vicinity of the support member a. On the other hand, as described above, the present invention reproduces the looseness at the fracture site at the interval between the gamma nail G and the support member a in order to reproduce the clinical state more accurately. A relatively large moment M is generated at the opposite end.

FIG. 3 is a process diagram showing the orthopedic implant strength evaluation method of the present embodiment. The orthopedic implant strength evaluation method of the present embodiment (hereinafter simply referred to as “strength evaluation method”)
(Step S1) Static load test process, (Step S2) Fatigue test load load determination process, (Step S3) Fatigue test process.

(Step S1) Static load test process First, in the static load test process, using the measurement environment shown in FIGS. 1 and 2, the orthopedic implant, in this embodiment, the gamma nail G, gradually increases with respect to a predetermined force point d. A load to be applied is added as a load, and a displacement amount of the gamma nail G with respect to the applied load is measured. The load application to the gamma nail G and the measurement of the displacement amount of the gamma nail G are performed by a universal material testing machine.

  The universal material testing machine measures the magnitude of the load applied to the gamma nail G and the amount of displacement when the load is applied on a one-to-one basis. A displacement amount is measured for each change, and a load-displacement amount curve from a start point at which load addition is started to an end point at which load addition is completed can be created.

  The static load test process ends when a load-displacement curve is created by a universal material testing machine, and the created load-displacement curve is passed to the load determination process of the fatigue test which is the next process.

  If the measurement shown in FIG. 1 and FIG. 2 is performed, the testing machine to be used is not limited to the universal material testing machine, and any measuring machine and test can be used as long as a load-displacement curve can be created. It may be a machine.

(Step S2) Load determination process of fatigue test The inflection point in the load-displacement curve created in the static load test process is obtained, and the load at the obtained inflection point is used as the load in the fatigue test process of the next process. decide.

  FIG. 4 is a graph showing an example of the load-displacement amount curve C. As shown in FIG. The horizontal axis indicates the amount of displacement, and the vertical axis indicates the load. In the load-displacement curve C, the displacement increases as the load applied to the gamma nail G increases. However, the slope changes between the front part and the rear part of the curve, and the inflection point is Exists.

  Here, it was found from the FEM analysis (finite element analysis) performed in advance that the stress generated in the distal portion c of the gamma nail G does not change when the load applied to the force point d exceeds a certain value. The inflection point of the load-displacement curve C represents the load applied to the force point d when the stress generated at the distal portion c does not change, that is, when the maximum stress is generated at the distal portion of the orthopedic implant. ing.

Therefore, if the inflection point of the load-displacement amount curve C is obtained, the load at the obtained inflection point can be determined as a load to be applied when performing the fatigue test.
Hereinafter, the procedure for obtaining the inflection point will be described in detail.

  FIG. 5 is a process diagram showing an inflection point calculation method. In the inflection point calculation method of the present embodiment, first, in step A1, a person (evaluator) who intends to perform the test visually observes the provisional inflection on the load-displacement amount curve created in the static load test process. Specify a point (Point D). Since this inflection point D needs to determine the data range used when calculating the approximate straight line in the following steps, it is necessary to tentatively designate one point as the inflection point. .

  The inflection point is designated by displaying a load-displacement amount curve on a display or the like, and the evaluator designates one point on the curve that will be the inflection point D. When the temporary inflection point D is designated by the evaluator, the process proceeds to step A2.

  In step A2, a first approximate straight line is calculated using data between the start point at which the load is started and the specified inflection point D in the load-displacement amount curve.

  Below, the case where 100 or more data is used is demonstrated as an example. FIG. 6 is a flowchart showing a method for calculating the first approximate straight line. In step B1, all approximate straight lines that can be calculated using data such that the number of data is 100 or more are calculated. The data whose number of data is 100 or more is data obtained by extracting a combination of data whose number of data used for calculating the approximate straight line is 100 or more.

  FIG. 7 is a schematic diagram showing a procedure for extracting a combination of data. In the example shown here, it is assumed that the total number of data from the start point to the inflection point D is 102. The total data is divided into 98 data obtained by removing two data from both ends, two data from the end on the start point side, and two data from the end on the inflection point D side (FIG. 7A )). When 98 data (central data) is essential data, and a combination of data having the number of data of 100 or more is extracted, a combination a (data number 100) of the central data and the two data on the start point side, the central data A combination b (data number 100) of one data on the start point side and one data on the inflection point side, a combination c (data number 100) of the central data and the two data on the inflection point side, Combination d (data number 101) of the central data, the two data on the starting point side, and the one data on the inflection point side, the central data, one data on the starting point side, and 2 on the inflection point side There are six combinations, ie, a combination e (data number 101) and central data, two data on the start point side, and two data on the inflection point side (data number 102).

  An approximate straight line is calculated for all the extracted data of these six combinations. The number of data is 100 or more is an example, and depending on the number of data of the created load-displacement curve, the number may be less than 100, and the number of data may be 300 or more, 500 or more, or more. May be.

  As a method for calculating an approximate straight line from a plurality of data, a known method may be used, and for example, it can be calculated by a least square method.

  Returning to the flowchart of FIG. 6, in step B2, the determination coefficient limit value is set to 0.9999. The determination coefficient is a coefficient representing the linearity of the approximate line. If the approximate line is calculated by the least square method, the determination coefficient is calculated by the square of the correlation coefficient.

  In step B3, all approximate straight lines whose determination coefficients are 0.9999 or more, which is the determination coefficient limit value determined in step B2, are extracted. What is necessary is just to compare the determination coefficient of each approximate line with 0.9999 which is a limit value, and extract the approximate line with a determination coefficient of 0.9999 or more.

  In step B4, it is determined whether or not an approximate straight line corresponding to a condition having a determination coefficient of 0.9999 or more has been extracted. If it has been extracted, the process proceeds to step B5. If it has not been extracted, the process proceeds to step B6.

  If no approximate straight line has been extracted, the subsequent processing cannot be executed. Therefore, in step B6, the determination coefficient limit value is reduced by 0,0001 to 0,9998, and the process returns to step B3. Extract the approximate line again.

  In Step B5, an approximate straight line that maximizes the number of data used for calculation among the extracted approximate straight lines is temporarily set as a temporary first approximate straight line. In step B7, it is determined whether or not the approximate straight line extracted in step B5 uses the data of the inflection point D as data for calculation. If the data of the inflection point D is used, the process proceeds to step B8, and if not, the process proceeds to step B9. The fact that the data of the inflection point D was used means that the tentatively determined first approximate line passes through the inflection point D. In this case, the point to be the inflection point is further displaced. This indicates that there is a high possibility that the amount is on the larger side. Therefore, in step B8, the inflection point D is shifted by 60 data to the side where the displacement is larger to obtain a new inflection point D, and the process returns to step B1 to calculate the first approximate line again. Note that the number of data to be shifted is set to 60 is merely an example, and the number of data to be shifted may be appropriately determined according to the total number of data.

  In Step B9, it is confirmed that the calculated first approximate straight line does not use the data of the inflection point D for the calculation, and the inflection point D shifted in Step B10 is the first temporarily specified. Returning to the inflection point D, the calculation of the first approximate straight line is completed.

  Returning to the process diagram of FIG. 5, in Step A3, a second approximate straight line is calculated using data from the inflection point D to the end point at which the load is terminated in the load-displacement amount curve. As the method of calculating the approximate line, the second approximate line can be calculated by the same method as the flowchart shown in FIG.

  When the calculation of the first approximate straight line is completed, the inflection point has returned to the first inflection point D temporarily specified in step B10 in FIG. 6, so that the second approximate straight line is also changed from step B1 in FIG. The processing can be started and calculated in the same manner as the first approximate line. In the case of the second approximate straight line, the process may be terminated when it is confirmed in step B9 that the inflection point D is not used.

  In step A4, the inflection point D is obtained by calculating the intersection of the first approximate line and the second approximate line calculated in step A2 and step A3, respectively.

  As shown in the graph of FIG. 4, the first approximate straight line L1 is calculated from the data of the front part of the load-displacement amount curve C, and the second approximate straight line L2 is calculated from the data of the rear part of the load-displacement amount curve C. Thus, the inflection point D can be obtained as the intersection of these two approximate lines.

  Assuming that the obtained inflection point D has coordinates D (X, Y), the load at the inflection point D is Y on the ordinate, and this Y value becomes the measurement target of the load-displacement amount curve C. It is a load value to be applied when evaluating the strength of the gamma nail G.

  As described above, in the load determination process of the fatigue test, it is possible to determine the load that is one of the test conditions when performing the fatigue test in the next fatigue test process.

(Step S3) Fatigue Test Process In the fatigue test process, the orthopedic implant fatigue test is performed using the load determined in the load determination process of the fatigue test in step S2. The fatigue test can be performed using a general fatigue tester.

  The load determined in the load determination step of the fatigue test is repeatedly applied to the orthopedic implant a predetermined number of times, and the number of times the load is applied until it is broken is used as the result of the fatigue test. If it is not destroyed up to a preset number of times, the orthopedic implant is accepted, and if it is destroyed within the set number of times, the orthopedic implant is rejected.

  As described above, according to the present invention, the load applied in the fatigue test is determined based on the measurement value measured in the restraint state in which the body part to which the orthopedic implant is applied is considered in the load determination process. In the fatigue test process, the determined load load is used as a test condition. Therefore, even in the fatigue test, a fatigue test in which the actual use environment of the orthopedic implant is assumed can be performed. With such a fatigue test, a more accurate evaluation can be performed for the strength evaluation of the orthopedic implant.

  The present invention is an orthopedic implant having a long shape and a portion where a load is applied directly (a point of action) and a portion where a moment is generated (a point of action) across a position serving as a fulcrum in an embedded state in a bone Is particularly suitable for the strength evaluation of orthopedic implants that are at remote positions and are processed at the point of action such as slits or through holes to reduce the strength.

  In the above description, a gamma nail is an object to be examined as an example of an orthopedic implant. It may be an inspection object.

(Example)
Using the gamma nail G having the shape shown in FIGS. 1 and 2, the static load test in step S1 was performed, a load-displacement amount curve was created, and an inflection point was obtained. The static load test was performed under the conditions shown in Table 1.

  Next, a load-displacement amount curve was created based on the measured value obtained by the static load test. FIG. 8 is a graph showing the front part of the load-displacement amount curve. The vertical axis represents the load load (N), and the horizontal axis represents the displacement (mm). In addition, the graph shown in FIG. 8 has shown the front part from the starting point to the inflection point D designated by the evaluator.

  Based on such a load-displacement curve, two approximate straight lines in step S2 are created, the inflection point D is obtained from the intersection, and the vertical coordinate value of the inflection point D is set as the maximum load load value. . In this example, the static load test was performed a plurality of times, and the inflection point was obtained from the load-displacement amount curve for each, and the load load value was determined. The results are shown in Table 2.

  Finally, a fatigue test for gamma nail G was performed. It should be noted that the maximum value among a plurality of obtained load values, that is, 420 (N) in this example, was set as the load under the test conditions of the fatigue test condition. Table 3 includes other test conditions.

  When the fatigue test was performed under the conditions shown in Table 3, the gamma nail G was not destroyed even when the maximum number of repetitions was reached, and it was found that the test reflected the clinical state had sufficient strength. .

Claims (5)

An orthopedic implant strength evaluation method comprising:
The subject of the orthopedic implant that is the object to be examined is in a restrained state that takes into account the body part to which the orthopedic implant is applied, and a load that increases gradually is added to the subject to determine the amount of displacement of the subject relative to the load. A first step to measure,
A load-displacement curve is created from the start point at which the load is applied to the subject in the first step to the end point at which the load is added to the subject, and the inflection in the created load-displacement curve A second step in which the evaluator sets the points;
A third step of calculating a first approximate straight line with respect to the load and displacement from the start point to the inflection point;
A fourth step of calculating a second approximate straight line for the load and displacement amount from the inflection point to the end point;
A fifth step of calculating an intersection of the first approximate line and the second approximate line;
A sixth step of performing a strength evaluation test of the subject when the load at the intersection calculated in the fifth step is added to the subject. The first approximate straight line in the third step is calculated so that the load and the displacement amount have a predetermined correlation with respect to linearity,
The method according to claim 1, wherein the second approximate line in the fourth step is calculated so that the load and the displacement amount have a predetermined correlation with respect to linearity. When the first approximate straight line passes through the inflection point, the inflection point is moved to the side with the larger displacement, and the third step is executed again using the inflection point after the movement,
2. When the second approximate line passes through the inflection point, the inflection point is moved to the side where the amount of displacement is small, and the fourth step is executed again using the inflection point after the movement. Or the orthopedic implant strength evaluation method of 2 or 2.   The method for evaluating the strength of an orthopedic implant according to any one of claims 1 to 3, wherein the strength evaluation test is a fatigue test.   The orthopedic implant strength evaluation method according to any one of claims 1 to 4, wherein the orthopedic implant is a gamma nail or an artificial joint stem applied to a fracture.

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