Surgery – Truss – Pad
Patent
1994-04-12
1996-02-27
Hindenburg, Max
Surgery
Truss
Pad
A61B 510
Patent
active
054940451
DESCRIPTION:
BRIEF SUMMARY
The present invention concerns a stiffness measuring device and a measuring procedure for assessing the stiffness of rigid tissue.
In particular, the invention deals with a stiffness measuring device for use in assessing the stiffness of articular cartilage, and in the disclosure following below articular cartilage is exclusively considered as tissue that is being measured. However, the stiffness measuring device of the invention is not restricted to be used only in measuring articular cartilage stiffness: it can be applied in like manner in assessing the stiffness of any other equivalent tissues.
Articular cartilage is differentiated connective tissue containing no blood vessels, lymphatic vessels or nerves. Articular cartilage is stiffest among the soft tissues of the body, yet clearly softer than bone. The articular cartilage differs in thickness in different joints, from a few micrometers up to several mm. The cartilage thickness may also be different in different parts of the articular surfaces. The two main tasks of the articular cartilage are: to reduce the load on the underlying bone, and to act towards low friction in the joint.
Chondromalacia is a degenerative disease of the articular cartilage, its causative agent being as yet unknown. It is however highly common, in that as many as one-third of the adult population have been estimated to have changes in their patellar chondral surface. Nearly all symptomatic patients present softened patellar cartilage although the cartilage may appear normal macroscopically. Softening is in fact the first observable change in the chondral surface caused by chondromalacia. At a later stage the chondral surface becomes uneven and frayed. Diagnosis of chondromalacia in its initial stage is important because the changes are irreversible once fraying has taken place.
At present, mechanical characteristics of articular cartilage are mostly measured using material testing apparatus. Such bulky and heavy apparatus is not transportable; it is therefore only applicable in laboratory conditions for studies on specimens.
The simplest method employed in operating theatre conditions for assessment of articular cartilage stiffness is pressing down on the chondral surface with a metallic instrument in connection with surgery. The results of such measurements are however highly subjective and variable.
So far only two measuring instruments have been proposed for peroperative use. One is the cartilage elastometer, its use requiring opening of the knee joint, whereby its use is cumbersome. The other is a measuring instrument which can be used arthroscopically, i.e., in connection with optical viewing of the joint. In this instrument a pressure transducer is employed which has a pressure-sensitive membrane on one end. The transducer is mounted in a curved steel tube in such a way that the pressure-sensitive end of the transducer remains slightly inside the end of the tube. On the end of the tube is attached a sphere made of elastic material, its surface abutting tightly on the transducer membrane and thereby transmitting a pressure signal. The other side of the sphere is located outside the metal tube.
In the measurement the end of the instrument is pressed against the cartilage until the metallic end of the socket precludes further pressing. The elastic sphere on the tip of the measuring instrument is flattened against the cartilage; the higher the stiffness of the cartilage, the greater the flattening of the sphere. At the same time, it transmits to the pressure transducer the force produced by the cartilage. The relationship between cartilage stiffness and tension reading is however not linear. In principle this pressure transducer enables forces produced by the cartilage to be measured, but mathematical modelling of this measurement is difficult because the deformation caused in the cartilage by the transducer is not known. It is also unclear what effect the cartilage thickness has on the result of measurement.
The object of the invention is to eliminate the drawbacks mention
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Jurvelin Jukka
Kiviranta Ilkka
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