The problem….. 

B-rep modeling (boundary representation) is presently the most efficient tool for the representation of 3D CAD objects (solids or surfaces). There exists today a consensus around the STEP standard for the exchange of topological entities (solids, shells, faces,…) and geometric entities (circles, cones, Bezier, Nurbs, B-spline, cylinder,…), but there is no accepted standard to check if a definition is valid or not. The different modelers, and even the different functions of the same modeler, have different tolerance values regarding "defective definition". For instance, a badly defined face with an open trimming loop will be displayed as curves and won't be shown in solid mode. 

the solution….. 

3D DOCTOR is the most productive tool in the market that allows the automatic correction of some of the flaws you may encounter. This will enable a greater number of functions and modelers to accept the validity of the definition of objects without changing their geometry. 

and more….. 

3D DOCTOR offers also some operations to complete or modify the definition e.g. fill up holes or delete some trimming curves.