Answer:
Grade of metamorphism
Explanation:
Gneiss forms at a higher grade of metamophic slate. Slate forms at low metamorphic conditions just about a few temperature and pressure. 
From the prolith materials exposed to metamorphic conditions, transformations begins as metamorphic changes begins to take place. First, a sediment is transformed into slate, from slate to phyllite, from phyllite to schist, schist to gneiss, gneiss to migmatite and migmatite goes back to melt. These transformation of sediments is controlled by the nature of the prolith materials and the prevailing metamorphic conditions. 
 
        
                    
             
        
        
        
Crystals, good insulators, hard and brittle,conduct electricity, high melting and boiling points
        
                    
             
        
        
        
Answer:
Arthroplasty
Explanation:
The restoration of function of a joint is called Arthroplasty. The bone functions are restored by maintaining the  bones through resurfacing. Some times artificial joints are also used instead of resurfacing the existing bones. 
Arthroplasty is a method to cure Osteoarthritis, or degenerative joint disease where joint cartilages reduces with age. 
Also when medical treatments no longer make any impact on the joints of bone pain then Arthroplasty is done.