HPMC-Products

Both laboratory and clinical examinations confirm that Healon , Viscoat and many other viscoelastics have no inflammatory effects when produced in the proper combinations. Specific tests (such as the owl monkey eye test) carried out prior to the release of a batch of hyaluronic acid confirm this finding in Healon , Healon 5 and Healon GV (see below). For HPMC products, implantation tests... 

HPMC is usually sterilized by autoclave, which is to its advantage, as opposed to other viscoelastics, because it remains unchanged by the autoclave process. HPMC does, nonetheless, have a tendency for microbial contamination, which is why special attention is given to filtering processes post-autoclave to ensure endo- and exotoxin removal from the substance. Quality control is of particular relevance here since HPMC products retain precipitates. More advantages of HPMC are storage at room temperature, availability and low HPMC production-costs. The necessary biotechnical purification-processes are, however, costly. Hydroxypropylmethylcellulose lasts 3 years, at room temperature. 

Similar inconsistencies are seen in critical analyses of the information given for HPMC products (Fig. 30). Visco Shield , for example, is listed as having a far higher molecular weight and supposedly a distinctly lower viscosity at rest than HPMC Ophtal H, although their concentrations are cited as being the same. These variations prove that manufacturer information should be critically studied. 

HPMC products can be stored at room temperature. Recommended cannula sizes for injection are between 20 G (Ocucoat ) and 25 G (HPMC Ophtal L), i.e., wider cannula openings, as the pseudoplastic character of these viscoelastics is far less highlighted than in hyaluronic acid viscoelastics (see above). 

The duration of measurements and the influence exerted by prolonged measuring time on rheologic viscoelastic results was examined within the context of pilot studies. Both the examined hyaluronic acid products as well as the HPMC products evaluated, showed a substantial reduction of viscoelastic mass after a maximum of 15 minutes (Fig. 33,34). This relationship had never before been described in the literature. With prolonged measuring times, viscosity values were... 

Fig. 34. Decreasing weight of a HPMC-product (Ocucoat ) with increasing time...

With HPMC products, viscosity at rest was hardly demonstrable, as expected. The exception was made by HPMC Ophtal H and LA GEL which showed higher viscosity, such as seen in some hyaluronic acid products. In contrast, Adatocel Acri-visc HPMC Ophtal L Coatel PeHa-Visco and Ocucoat all showed rather low zero shear viscosity values (Fig. 36, Table 6). 

The viscoelastic behavioral profile of HPMC Ophthal H corresponded closely with Amvisc Plus, a hyaluronic acid product. Compared to the remaining HPMC products, a slight pseudoplastic property was recognized in this product, similar to hyaluronic acid products. These results support the conclusion that categorizing viscoelastics in substance groups does not always make evident their functional differences. 

Fig. 42. The G7G" (Pa) cross point serves as a characteristic to describe the extent of viscosity and elasticity of a viscoelastic substance and their according frequencies (in alphabetical order n=6). The elas-tical properties of a viscoelastic substance will dominate with increasing frequencies (to a different extent compare with the individual graphs). Due to the division by 2n the unit rad/sec wil be exchanged into Hertz (Hz). In the case of 5 HPMC-products no cross point could be determined for they should exceed (have to be located outside of) the 100 rad/sec value. The rheological characteristics mentioned above are better described by the following example (Figure 43 and 44)...

HPMC products are not suitable to maintain anterior chamber depth with concomitant increased vitreous pressure due to their low elasticity. Higher molecular weight sodium hyaluronate was proven to be superior for this clinical application (Strobel, 1997). 

Similarly, after use in ECCE, many clinical studies revealed insignificant or clinically irrelevant differences in endothelial cell loss and corneal thickness when comparing different viscoelastics. (Alpar, 1985 Alpar et al., 1988 Lane, 1991 Smith 8c Lindstrom, 1991). In a trial led by Pederson (1990), central corneal thickness was statistically significant higher following phacoemulsification and use of HPMC products than Healon . 

Regarding OVD application with intraoperative complications, tissue stability and movement must be differentiated from selective tissue isolation (Fig. 53). Healon 5, Healon GV, Microvisc (Plus), Morcher Oil (Plus), Viscorneal (Plus) (identical to Allervisc (Plus) in content), and other viscoelastic substances are considered suitable to counter a flattened anterior chamber, widen a small pupil, simplify a complicated capsulorhexis, resolve synechiae, or counter iridal and vitreous prolapse (Fig. 54). Endothelial dystrophy (Fuchs Dystrophy, advanced cornea guttata), torn posterior capsule or vitreous prolapse are particularly well suited for Viscoat application (followed by AMO Vitrax , HPMC Ophta H, Visco Shield , and other HPMC products). These viscoelastics are preferred substances in case of zonular dialysis with vitreous body prolapse or sinking nucleus (Fig. 55). 

MscomeaL HioLon Prime, Healon, other HPMC products... 

Hydroxy propyl methyl cellulose (HPMC).35 HPMC is a partly O-methylated and 0-(2-hydroxypropylated) cellulose available in several grades that vary in viscosity and extent of substitution. It is used widely in pharmaceutical formulations, especially in oral products, as a tablet binder, in film coating, and as controlled release matrix. Soluble in cold water, it forms a viscous colloidal solution. For a 2% aqueous solution (20°C), viscosity can range from 2.4 to 120,000 mPa-s. High-viscosity grades can be used to retard the release of water-soluble drugs from a matrix. 

Monolithic non-gas-generating systems are matrix tablets consisting of hydrocolloids that form an external gel layer when hydrated. The internal tablet core remains dry with an overall density lower than that of the gastric fluid. Hydroxypropylmethycellulose (HPMC) is the most commonly used hydrocolloid. This approach has been developed into marketed drug products as the Hydrodynamically Balanced System (HBS) invented by Sheth and Tossounian.93 Gastric retention and flotation times up to 6 hours were achieved. Valrelease (diazepam) and Madopar (levodopa and benserazide) were two marketed products developed using this approach. 

Cellulose ethers have also been used in the ceramic industry (7). Since their appearance in 1959, water-based cellulose ethers have replaced solvent-based adhesives. The adhesives used for ceramic tile are ready-mixed products based on natural or synthetic rubber, polyvinyl acetate, and other resins, and they all contain cellulose ethers of one kind or another (e.g. MC, EC, HPMC, HEMC, HEC). These cellulose ethers reduce water loss, modify the viscosity of the mix, and can provide excellent adhesion for dry, very porous tiles. 

There is a growing interest in using non-animal-derived products for formulation of the capsule shells to address cultural, religious, and dietary requirements. HPMC (e.g., V-caps, Quali-VC, Vegicaps) and pullulan shells (NPCaps) and starch are alternatives. 

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