Arthroscopic impedance probe to detect cartilage degeneration

Details Arthroscopic impedance probe to detect cartilage degeneration Arthroscopic impedance probe to detect cartilage degeneration

2001-02-02

2004-05-11

Van, Quang T. (Department: 3742)

Surgery

Diagnostic testing

Measuring electrical impedance or conductance of body portion

C600S587000

Reexamination Certificate

active

06735468

ABSTRACT:

BACKGROUND OF THE INVENTION
The invention relates to the field of non-destructive arthroscopic diagnostic probes, and in particular to non-destructive arthroscopic diagnostic probes for detecting degeneration of articular cartilage utilizing impedance measurements.
Articular Cartilage
The function of organs in the human body are a direct consequence of their inherent structure. The function of an organ as a whole is more than the sum total of its individual constituents. Articular cartilage (AC) is a rich and illustrative example. An understanding of the composition and physical properties of AC are essential to diagnose a disease with any given device to aid in patient care. AC is a dynamic, living tissue that responds to stimuli in its environment (i.e. external loading, fluid flow, electric fields), and the cells of cartilage (chondrocytes) are able to maintain its intricate extracellular matrix (ECM). The scientific data collected over the past 25 years for normal cartilage, supports a hypothesis that a feedback between mechanical stimulation and chondrocytes must exist to maintain cartilage homeostasis.
By gross visualization, during knee arthroscopy or an open joint procedure, normal AC appears as a homogeneous shiny white substance covering the ends of articulating bones. It is a thin layer from 1 mm to 6 mm depending on the joint and particular surface location. In the presence of synovial fluid, AC provides a very low friction surface that has a coefficient of friction that is less than that of ice on ice. A closer inspection at the light microscopic level reveals AC as a very complex ECM of macromolecules with chondrocytes embedded within. Cartilage is a unique organ as it is aneural, alymphatic and avascular. Nutrient exchange to the chondrocytes proceeds by diffusion from synovial fluid at the articular surface and from the subchondral bone below. The lack of a blood supply severely limits AC's ability to be repaired following injury. The absence of innervation means that pain is transduced from the surrounding bone through unshielded force, or from the joint capsule in response to an inflammatory stimuli response.
The complex structure of AC acts as a loadbearing, shock absorbing, and wear resistant material to protect joint surfaces. In addition to a low friction surface, AC has a high compressive strength critical to pain free joint function. Compressive loads are distributed over a larger area, while also acting as a damping element during high impact loading (i.e. jumping). Cartilage is also strong in tensile loading when subject to shear stresses due to the sliding nature of joint function (i.e. knee joint or intervertebral disk). Lubrication by the synovial fluid also reduces shear stresses and helps protect the cartilage from trauma. These macroscopic mechanical properties are a direct consequence of the composition.
Articular cartilage is mostly water (60-80% of total weight) and ECM that comprises the bulk of the dry weight. The primary structural components of articular cartilage (AC) ECM are produced and maintained by the chondrocytes enmeshed within it. Tissue mechanical properties depend on the organization and structure of macromolecules present in the ECM. The ECM is made up of mainly collagen type II fibrils (along with small amounts of types IX and XI collagen), charged proteoglycans (PGs), and cells. Collagen and PGs form the framework for cartilage that resists applied mechanical forces. The collagen forms a dense cross linked network with PGs embedded within. Proteoglycans are macromolecules that contain polyanionic sulfated glycosaminoglycan (sGAG) chains. The negative fixed charge density of sGAGs is approximately 5.3 mEq/gm dry weight in normal human femoral head cartilage. A slight excess of mobile positive ions within the tissue preserves electroneutrality. At the macroscopic level a Donnan osmotic swelling force develops, caused by the electrostatic charge repulsion between the fixed anionic groups that draws water into the ECM, expanding the collagen network.
The chondrocytes are responsible for PG turnover (synthesis and degradation). The most abundant PG is aggrecan, which has an extended protein core with up to 150 chondroitin sulfate and keratan sulfate chains attached in a “bottle brush” structure providing a high concentration of anions. When first synthesized, aggrecan is mobile, but quickly binds to immobile hyaluranon, stabilized by a link protein, creating the high density of fixed COO
31
and SO
3
−
groups at physiologic pH.
Many other soluble factors play an important role in the maintenance process by participating as mediators of turnover and production of ECM, including ions, growth factors, hormones, cytokines, proteinases (e.g. matrix metalloproteinases) and their inhibitors. Numerous factors are required to maintain homeostasis. They can be produced by the chondrocytes themselves or synthesized elsewhere and transported into the ECM. These factors affect the chondrocytes through cell surface receptors and their transport through the ECM can be prohibited, resulting in pathology.
At the ends of articulating joints, the AC is 3-4 mm thick, with areas on the patella as high as 6-8 mm. Microscopically mature AC has 3 zones based on the shape of the chondrocytes and distribution of the type II collagen. The tangential layer has flat chondrocytes, tangential collagen fibril orientation and a sparse PG content. The intermediate layer is the thickest, with round chondrocytes, oriented in vertical columns. Finally, the basal layer has round chondrocytes and contains the tidemark that separates the uncalcified (nourished by the synovial fluid) and calcified cartilage (that gets fed by the episphyseal vessels). It has been reported that no age changes after maturation are discernible based on histology, including no loss of AC.
Collagen makes up the majority of the dry weight (approximately 50%) of AC, as it is also the most common structural protein in the body. In cartilage the most abundant form of collagen (>90%) is type II that acts as the structural meshwork of the ECM with its associated extensive intermolecular crosslinking via trivalent hydroxylysyl pyridinoline residues. The name “collagen” is a generic term for structural molecules that are rich in glycine, proline and hydroxyproline. Striated fibrils, type I, II, and III have three polypeptide chains wound in a triple helical configuration.
Type II collagen is composed of three left handed tightly interwoven alpha chains (&agr;(II)
1
), 300 nm long and 1.5 nm in diameter., each with a repeating amino acid sequence of GlyPro(Hydroxyproline). It is the triple helical structure that enables collagen type II to have a high tensile strength. Hydroxylysine (some as hydroxylysyl pyridinoline crosslinks) helps type II collagen link together the ECM network.
Small amounts of collagen type IX help connect the various matrix elements together while type XI (approximately 3%) regulates the caliber of the fiber. In addition, collagen types VI and X are also present (<1%). Collagen type VI has a cross linking behavior and an increased amount has been reported in OA models, while collagen type X is associated with growth plate cartilage in the hypertrophic zone, and in the calcified layer of mature cartilage.
Individual aggrecan monomers are attached to a GAG core (hyaluronate), stabilized by link protein, with the number depending on the functional nature of the cartilage. The common PGs that can be found in cartilage include aggrecan, decorin, fibromodulin, and biglycan and make up about 35% of dry weight of AC. Aggrecan molecules form large aggregates (approximately 200 MDa) in cartilage, forming a hydrogel-like structure that is, in turn, immersed within the collagen type II fibers.
Aggrecan is the major PG in AC (around 90%) and is composed of two types of sulfated GAG (sGAG): chondroitin-6-sulfate, chondroitin-4-sulfate (approximately 20,000 MW) and keratan sulfate (approximately 5,000 MW). The amount of sGAG attached to each PG varies depending on the functi

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