8 I proceed then to record briefly my views in respect of the issue estoppel pleaded by the respondent. His original application for compensation was made under the Workers' Compensation Act, s 18. He claimed that he had suffered a disability within the meaning of that Act and that his employer, the appellant, was liable to pay compensation in accordance with Sch 1 of the Act. He put his case that he had suffered a disability on alternate bases. He failed to establish a personal injury by accident arising out of or in the course of the employment under par (a) of the definition of "disability" in s 5(1) of the Act, but he succeeded in establishing, and the review officer found, that he suffered from a "disease" as defined in s 5(1), a physical ailment, disorder, defect or morbid condition in the form of the degenerative condition of his lower spine, involving a liability to disc protrusion. That was found to be a pre-existing disease aggravated or accelerated to a significant degree by his employment with the appellant as a truck driver. That made it a disability within the meaning of par (d) of the definition of that term.