Surgery – Diagnostic testing – Detecting nuclear – electromagnetic – or ultrasonic radiation
Reexamination Certificate
1999-09-20
2002-04-09
Casler, Brian L. (Department: 3737)
Surgery
Diagnostic testing
Detecting nuclear, electromagnetic, or ultrasonic radiation
Reexamination Certificate
active
06370418
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a method for measuring the position of an implant relative to at least one bone in a body, to which bone the implant is connected, which bone has at least one bone marking, and which implant has at least one predetermined identifying mark, according to the following steps:
A. generating radiation and directing said radiation onto said at least one bone marking and said at least one predetermined identifying mark;
B. receiving a radiation image of said at least one bone marking and said at least one predetermined identifying mark;
C. determining the position of the implant with respect to the bone on the basis of the radiation image received in step B.
A method of this kind is in frequent use in hospitals and is described, for example, in K. Søballe, “Migration of hydroxyapatite coated femoral prostheses”, Journal of Bone and Joint Surgery, volume 75-B, No. 5, September 1993, pp. 681-687.
U.S. Pat. No. 5,778,089 discloses a method and equipment for measuring forms and orientations of bones in living beings. This patent gives an analysis of how e.g. the morphology of the vertebrae can be established based on bone density measurements by means of X-ray detection. Use is made of a computer which analyses data received and which uses the data to accurately define the shape and size of the vertebrae under investigation. Moreover, the computer is programmed to use the data to indicate the vertebral condition in medical terms.
As a further example, U.S. Pat. No. 5,577,089 describes measurements with respect to the human femur. Here, anatomically fixed points, like the proximal limit and the medial epicondyle, are detected but they are only used to measure the femur length. Moreover, based on bone density measurements, a femur axis and a femur neck axis are calculated, as well as a femur head centerpoint. The patent discloses that these latter three features may be used to provide “an indication of any possible shifting of the prosthetic joint with respect to the femur” in case an artificial hip joint is implanted. Thus, U.S. Pat. No. 5,577,089 discloses measuring shifting of a prosthetic joint relative to a bone supporting this joint, based on bone density measurements. However, the method proposed is very laborious since it needs the calculation of the intersection of two imaginary lines, and of an imaginary femur head centerpoint, for which many data elements of the femur and the prosthetic joint need be established.
WO-A-96/25086 is directed to providing a solution for the problem that prosthetic devices may be lost over time. To that end, this document discloses a method of evaluating bone density around a radiolucent composite prosthesis. Since the prosthetic device is transparent to x-rays the prosthesis is provided with three radio-opaque reference markers embedded in the prosthesis. By means of a suitable densitometer, the boundaries of the prosthesis and the surrounding bone are identified and stored for later use. A region of interest is defined which is the area of the bone adjacent to the prosthesis. In this region of interest, the density of the bone is measured over time in order to establish any degradation of the bone to which the prosthesis is fixed. From the measurement data, the loosening of the prosthesis from the bone can be established.
The locations of the three radio-opaque markers are also stored in order to compare measurements later in time with prior measurements. To this end, a stored template with three template reference markers is used which are fitted to measured reference markers. To be sure that later measurements can be compared with former measurements, also, in the case of a hip implant, also the location of the lesser trochanter is identified and stored. Thus, both the three markers and the lesser trochanter are used to ensure that subsequent scans of a patient will be properly aligned and may be used for direct comparison with earlier scans.
A method of a first embodiment is used, for example, for determining the position of a hip prosthesis with respect to the femur, to which the hip prosthesis is connected on one side. On the other side, the hip prosthesis is in contact with an acetabular prosthesis, which is attached in the pelvis.
Wear to the hip joint leads to a very painful limitation of the movements which a person is able to carry out. Since the 1970s, hip prostheses have been widely used in orthopaedics to replace a hip joint which has become worn. However, treatment of the arthrosis does not end with the fitting of a hip prosthesis, but rather in practice is the beginning of a long period of careful monitoring of the patient. The monitoring consists both of physical examinations and of the study of regular X-rays.
When the method was introduced, the minimum age of patients to be treated was approximately 70 years. However, nowadays hip prostheses of this kind are also fitted to people of an increasingly young age. Younger people have a higher activity level than older people, with the result that hospitals are confronted to an increasing extent with hip prostheses which become detached from the bone to which they are connected.
FIG. 1
diagrammatically shows an X-ray of a hip prosthesis
1
, which at the top is in contact, by means of a spherical end
2
, with an acetabular prosthesis
19
which is attached to the pelvis
9
, and at the bottom is connected by means of a pin
3
to the femur
6
.
Just below the hip joint, the femur
6
has two marked projections, the outer projection
8
being referred to as the trochanter major and the inner projection
7
as the trochanter minor. The surface of the trochanter major is rough, so as to increase the contact area for the attached gluteus and thigh muscles. The trochanter minor lies on the inside and points 30° towards the rear. Only one muscle is attached to the trochanter minor, and this muscle, when tightened, causes the hip joint to bend and the femur to rotate outwards. Both tubercles are situated at a fixed location. This means that the shape, the location with respect to the leg and the size are not affected by positioning a prosthesis
1
in the femur. The trochanter major, the trochanter minor, as well as the axis of the knee joint (not shown in FIG.
1
), are situated at fixed anatomical positions which form orientation points for the correct positioning of the prosthesis
1
.
If a hip prosthesis
1
becomes detached from the femur
6
, the result is that the prosthesis
1
slowly sinks into the femur
6
, causing damage to the femur
6
. If such an event is only discovered at a late stage, considerable amounts of bone may already have been lost, and this first has to be replaced with donor bone in order to repair the anatomy to a sufficient extent for the same prosthesis
1
to be replaced.
The “moment” at which the mechanical detachment occurs is not precisely known. With standard current X-ray techniques, it is only possible to detect whether a prosthesis is attached or has become detached, or at least whether the prosthesis
1
has moved more than 5 mm. In medical circles, the assumption is that the increase in the speed of migration is the “moment” of detachment. The speed of migration is understood to mean the rate at which the prosthesis
1
moves with respect to the femur
6
.
The above-mentioned article by Søballe describes a standard procedure which can be used to measure the current position of the prosthesis
1
with respect to the femur
6
. This method is known as the X-ray stereophotogrammetry analysis (RSA).
This standard procedure can be used to measure the movement of the prosthesis
1
in the femur
6
to an accuracy of 0.1 mm. In order to be able to make use of this standard procedure, during the hip operation it is necessary to arrange various small tantalum balls, which usually have a diameter of 0.8 mm, at various locations in the bone before fitting the hip prosthesis
1
. Moreover, the prosthesis
1
itself also has to be provided with at least one small tantalum ball, which serves as a referen
Casler Brian L.
Young & Thompson
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