FR2704746A1 - Method for manufacture of a special femoral stem by total arthroplasty of the hip and stem obtained - Google Patents

Abstract

According to the method: - tomodensitometry of the hip and of the femur is carried out by taking stepped sections of the proximal third of the femur, - the contour of the medullary canal is digitised by computer means, - the digitised sections are downloaded into CAD (computer-aided design) software and, for each section, a set of so-called "spline" curves are produced, - these splines are given their spatial position, which makes it possible to reconstruct the medullary cavity in three dimensions, - reductions are applied as a function of level, - the parameters of the neck of the prosthesis are input, namely the length, the orientation and the location of the neck on the bone segment of the stem, - the data are determined by planning on conventional radiographs, - a tool path between all the sets of curves is calculated.

Description

 Method of manufacturing a special femoral stem for total hip arthroplasty and the stem obtained.

 The invention relates more particularly to the design of special rods, that is to say custom femoral rods, the non-standardized manufacture of which is obtained by unit from the measurements of the patient.

A femoral stem can be divided into two segments:
- a bone segment,
- an articular segment.

 In a special rod, the bone segment is calculated to be adjusted to the measurements of the internal contours of the femur. The joint segment is calculated to restore physiological function to the hip.

Both segments are tailor-made.

 Various methods are known for making custom prostheses.

 Certain procedures allow special prostheses to be made to measure, from radiographs. Now, we know that a conventional radiography studies a three-dimensional body only by its projection on a plane, thus superimposing the different structures.

Such a solution is not precise.

 To overcome these drawbacks, methods have been proposed using computer measurement systems.

 We can cite for example the European patent application 0097001 which teaches a process for manufacturing three-dimensional prostheses to be implanted in the human body. This method provides for obtaining an image of the contour of the prosthesis produced by proceeding with a computer-assisted radiographic analysis.

 Mention may also be made of French patent 2577697 which describes a process for manufacturing a personalized surgical implant adapted to the medullary canal of a receptor bone. For the most part, this process teaches a nuclear magnetic resonance scanning or resonance operation of a series of "n" images distributed along the reporter bone, in parallel transverse planes whose position is identified with respect to an origin. located on an axis perpendicular to said planes. Different images from the series of images are sampled in order to deduce therefrom a series of sampled representations of the contour of the medullary canal situated in the receptor bone. Each of the sampled representations of the contour is smoothed along an ellipse, so as to obtain a simplified shape of the implant adjusted to the medullary canal. The implant is then machined from digital information of this shape by means of a numerically controlled machine tool.

 These different solutions describe a process for designing and manufacturing only the intraosseous segment of the prosthesis, but does not include in the study, the articular segment of the femoral stem.

In particular, no reference is made to the method making it possible to determine the anteversion angle, that is to say the angle situated in the horizontal reference plane and formed by the axis of the femoral neck or of the prosthesis, on the one hand, and the knee axis, on the other.

 The object of the invention is to remedy these drawbacks in a simple, safe, effective and rational manner.

 The problems which the invention proposes to solve are: - on the one hand, to make precise the definition of the bone contour of the stem in its bone segment, - on the other hand, to finalize the principle of tailor-made by a study the neck of the prosthesis.

 To solve these various problems, a method has been designed and developed to produce a special femoral stem by combining computed tomography sections and conventional radiographs.

According to the invention - a tomodensitometry of the hip and the femur is carried out by making stepped sections of the proximal third of the femur, - the contour of the medullary canal is digitally digitized, - it is transferred into CAD software (Computer Aided Drawing) the digitized sections and for each cut, we make a compound of curves called "spline", - we give these splines their position in space, which allows to reconstruct the medullary cavity in three dimensions, - we apply reductions in depending on the level, - the parameters of the neck of the prosthesis are entered, namely the length,
The orientation and location of the neck on the bone segment of the stem, - these data are determined by planning on conventional radiographs, - a tool path is calculated between all the curve compounds.

 Advantageously, for the implementation of this method, a standard protocol and a special protocol for torsion disorders are carried out for the tomodensitometry.

For the standard protocol: - a cut is made which passes through the center of the head of the femur, - under this upper cut, a series of cuts is made, spaced 5 mm apart to the lower base of the implantation of the lesser trochanter , - beyond this last cut1 a series of diaphyseal cuts spaced 15 mm up to a line cutting the femur is made and located 180 mm from the first cut which passes through the center of the head of the femur, - L ' scanner scale is scale 1
For the special protocol: - a cut through the ankle is made at the malleoli, in order to obtain, after superposition on the condyle axis, the external tibial torsion axis, - a plantar cut through the radiological axis of the foot which connects the head of the second metatarsal and the most posterior point of the calcaneus.

With regard to the radiography, the procedure is as follows - an x-ray of the hip and upper femur is taken from the front on a known scale, - an x-ray of the hip and upper femur is taken in profile on the same scale, a teleradiography at scale 1 of the two lower limbs is performed on foot, with compensation for a previously known length inequality or a teleradiography at scale 1 in the supine position,
The invention is set out below in more detail using the appended drawings, in which:
Figure 1 is a view showing the principle of scanner slices for the standard protocol.

 Figure 2 is a view showing the principle of scanner slices for the special protocol.

 Figure 3 is a view showing an embodiment of a prosthesis obtained according to the method of the invention.

 Figures 4, 5, 6 and 7 show the principle of measuring the anteversion of the cervix.

 According to the invention, a computed tomography and an X-ray of the hip and femur is carried out, by carrying out determined cuts at the level of the femur, as will be indicated in the following description.

The scanner sheets are cut into several shots, 6 in particular. The pictures are digitized by a device such as those sold by HEWLETT PACKARD under the reference SCANJET
MORE. The cuts are computerized using software such as that marketed by HEWLETT PACKARD under the reference SCANGAL.

 These provisions make it possible to trace the interior contours of the femur using drawing software of the type known under the reference MICROGRAFX.

 According to the invention, to perform the computed tomography, a standard protocol and a special protocol for torsion disorders are defined. For this, the positioning of the femur is fundamental. The femur must be parallel to the plane of the table, seen in profile, and parallel to the edge of the table, seen from the front, being wedged there and fixed in this position.

 As shown in Figure 1, for the standard protocol a cut is made (C) which passes through the center of the head (F1) of the femur (F).

Under this cut (C), a series of cuts (C1) are spaced 5 mm apart. These cuts (C1) are made up to the lower base of the implantation of the lesser trochanter, symbolized by the line (XX ').

 Beyond this last cut, diaphyseal cuts (C2) are made spaced 15 mm apart. These cuts (C2) are made up to a line (YY ') cutting the femur and located 180 mm from the first cut (C).

 Finally, a cut (C3) is made which passes through the femoral condyles at their widest portion in order to measure the anteversion, that is to say the angle between the femoral neck and the knee. All cuts are approximately 1 mm thick.

 With regard to the special protocol for torsion disorder (Figure 2), this allows the ideal direction of the prosthetic neck to be calculated in the horizontal plane.

 To define this protocol, a cut is made (C4) passing through the ankle at the level of the malleoli. This cut (C4) superimposed on the axis of the condyles, gives the angle of external tibial torsion. A plantar section is made passing through the radiological axis of the foot which connects the head of the second metatarsal and the most posterior point of the calcaneus. As a result, at the end of the calculation, the axis of the neck of the prosthesis is perfectly perpendicular to the axis of the foot. The measurement of the external tibial torsion makes it possible to detect the errors in particular those of measurement of the axis of the foot.

 In the event that the ideal axis of the cervix passes behind the bicondylar line, the anteversion of the prosthetic cervix is reduced to + 2 ".

For conventional radiographs, the procedure is as follows:
An X-ray of the hip and the femur, seen from the front, is carried out on a known scale close to 1 / 15th, so that 100 cm real gives 115 cm on the X-ray. A second X-ray of the hip and the femur considered in profile is then carried out, always on a scale of 111 Seme. If the radiology table is not calibrated to this scale, it is necessary to place in the same plane as the femur considered, a radiographed reference whose actual length is known and cited.

 Teleradiography can be done standing or lying down. For standing X-ray, the patient is placed with feet joined parallel, the knees extended. The pelvis and the last lumbar vertebrae must appear on the radiograph. In the event that there is an inequality in length, a second telemetry must be carried out by balancing the pelvis by means of a wedge placed on the shortest member. The thickness of this wedge is noted on the picture and should allow the complete extension of the knees. This teleradiography must be carried out at scale 1. For the teleradiography lying down the patient is placed with feet joined parallel with the knees extended. The pelvis and the last lumbar vertebrae must appear on the radiograph. Teleradiography is performed at scale 1.

Given this process and to allow computer-aided fabrication of the prosthesis, the scanned cuts as defined are recovered in software known under the reference
MASTERCAM. The different compounds of curves (splines) are created in each scanner section and are differentiated from the digitized scanner images by smoothing and reduction. This computer-aided manufacturing is then carried out by a specific function which calculates a tool path between all the curve compounds. The tool path thus determined gives the final shape of the machined femoral stem.

The prosthetic neck is designed, at this stage, by manual planning. It is possible to vary the anteversion, the length,
The inclination, and the lateralization of the cervix.

 The anteversion is the angle of the horizontal plane formed by the axis of the cervix and the axis of the knee (posterior bicondylar line). Anteversion is the sum of two angulations, namely helitorsion and flexion.

The helitorion is the angle (HT) formed by the posterior bicondylar axis, on the one hand, and the horizontal axis of the most proximal section of the rod, on the other hand (Figure 4). Flexion is the angle of the sagittal plane formed by the axis of the femoral stem in this plane on the one hand, and the axis of the prosthetic neck in this same plane on the other hand (Figure 5). When the flexion angle passes behind the sagittal axis of the femoral stem, it is considered negative and called the deflection angle (DF).

 The natural heliosion is restored by the shape of the rod which follows the internal contours of the femur.

 Relative to the horizontal plane, the angle of flexion (F +) or deflection (DF-) is the artifice of calculations which makes it possible to correct the anteversion induced by the sole helitorion of the subject (Figure 6).

 The advantage of this method is to obtain equivalent internal and external rotations of the lower limb of the patient. Indeed, in the hypothesis where the anteversion is excessive with a very strong helitorion, for example in a congenital disease of the high congenital luxation type, this angle of excessive anteversion will reproduce the gait feet inside the subject with a standard prosthesis whose neck is in line with the proximal portion of the stem.

 In this same scenario, the method according to the invention makes it possible to rotate the neck back by subtracting the angle of deflection and to balance the rotations. Ideally and apart from certain exceptional cases, the final axis of the neck of the prosthesis can be placed perpendicular to the axis of the foot after the latter has been superimposed on the CT scan of the condyles.

 In the opposite situation of a malformation with natural retroversion of the femoral neck, the anteversion is insufficient and the gait of the subject is made feet apart. The possibility of increasing the angle of anteversion by adding a substantial flexion angle to the subject's helitorion, here again, allows the rotations to be balanced (Figure 7).

The calculation of the length of the inclination and the location of the prosthetic neck can be done as follows:
The cut of the diseased femoral neck (cervicotomy), for hip joint replacement, is an oblique cut in the frontal reference plane and sometimes oblique in the sagittal plane. A computer reconstruction of the different cervicotomies is carried out using the standard protocol CT slices. This reconstruction makes it possible to determine the best cutting height and the best cutting angle. The selection criteria are:
- conservation of the femoral skeleton as much as possible,
- maximum penetration orifice for the most filling prosthesis possible.

 When this level of cervicotomy is chosen, a transparency of the proximal femur simulates it.

 If an extension is necessary, it is then reproduced on a transparent uniting pelvis and femur.

 The diaphyseal lateralization (outside) or medialization (inside) is then calculated relative to the median axis of the pelvis according to the usual stability criteria.

 After positioning the prosthetic cup and determining the ideal center of rotation, the inclination and lateralization of the cervix automatically result from this preparation.

 The length and lateralization parameters are checked on the scanner by taking two reference points at the start of the study, one on the femur, the other on the pelvis.

 The femoral stem is machined by a numerically controlled machine whose programs come from the computer where the prosthesis is designed under the conditions indicated above. The stem is covered in whole or in part with a coating of calcium hydroxyapatite or simply sanded with corundum.

 The advantages are apparent from the description.

Claims (1)

CLAIMS the orientation and location of the neck on the bone segment of the stem, - these data are determined by planning on conventional radiographs, - a tool path is calculated between all the curve compounds, -2- Method according to the Claim 1, characterized in that a standard protocol and a special protocol for torsion disorders are carried out for the tomodensitometry. -1- Method for manufacturing a special femoral stem for total hip replacement, characterized in that: - a CT scan of the hip and femur is performed by performing staged sections of the proximal third of the femur, - we digitize by computer the contour of the medullary canal - we transfer the digitized sections into CAD software (Computer Aided Drawing) and for each cut, we make a compound of curves called "splines", - we give these splines their position in space , which makes it possible to reconstitute the medullary cavity in three dimensions, - reductions are applied depending on the level, - the parameters of the neck of the prosthesis are introduced, namely the length, -3- Method according to claim 2, characterized in that for the standard protocol - a cut is made which passes through the center of the head of the femur, - under this upper cut, a series of cuts spaced by a distance is made determined (dl) to the lower base of the implantation of the lesser trochanter, - beyond this last cut, a series of diaphyseal cuts spaced by a determined distance (d2) to a line cutting the femur is made and located at a determined distance (d3) from the first cut which passes through the center of the head of the femur, - a cut is made which passes through the condyles at their widest portion, in order to measure the anteversion. -4- The method of claim 3, characterized in that the distances (dl) are 5mm, (d2) 15 mm and (d3) 180 mm, the thickness of the cuts being 1 mm.  -5- The method of claim 1, characterized in that for the special protocol - a cut is made through the ankle at the malleoli, in order to obtain, after superposition on the axis of the condyles5 the axis of tibial torsion external, - a plantar section is made passing through the radiological axis of the foot which connects the head of the second metatarsal and the most posterior point of the calcaneus. -6- The method of claim 1, characterized in that the radiography comprises the following steps: - an x-ray of the hip and the upper femur is performed in front of a known scale, - an x-ray of the hip and of the femur is carried out upper profile on the same scale, - we carry out a foot teleradiography on scale 1 of the two lower limbs, with compensation for a previously known length inequality or a teleradiography on scale 1 in the supine position, -7- Femoral stem obtained by implementing the method according to any one of Claims 1 to 6. -8- Rod according to claim 7, characterized in that it is covered over all or part of its height, with a calcium hydroxyapatite coating.