An impactor for positioning and holding a surgical or veterinary prosthetic implant while it is being driven into position, said impactor including: (a) an impaction means; (b) an impaction shoe which is configured to receive and support a prosthetic implant in a predetermined position; (c) connection means which connect the impaction means to the impaction shoe and are configured to transmit an impact from the impaction means to the impaction shoe; (d) arms configured to engage an implant supported upon the impaction shoe; (e) said arms being biased into engagement with said implant, but movable against the bias out of engagement with the implant; (f) said connection means being adjustable between a first setting at which the impaction shoe is spring-loaded so as to bias an implant supported upon the impaction shoe into engagement with said arms but at which the impaction shoe can slide relative to said arms, against said spring bias; and a second setting at which both the impaction shoe and the arms are rigidly engaged with an implant supported upon the impaction shoe, to hold the implant in a predetermined position and orientation relative to the impaction means.