Topstar 6990 hard floor castors are generally designed for hard floors such as parquet, tile and stone and can be used as replacement castors for office chairs. However, there is no specific guarantee that they will fit the Topstar Ergo Free Plus office chair directly, without further details of the chair's construction and requirements.

To ensure that these castors are compatible, you should check the following points:
- Pin diameter: approx. 10 mm
- Pin length: approx. 25 mm
- Material and construction of the chair

If these specifications match those of your office chair, the castors should fit.