Simon Mayo can occasionally be heard cracking his knuckles on the show. This is a habit that Mark Kermode finds both annoying and disquieting, to the point where he put an appeal out to persons with relevant qualifications, possibly a doctor of cartilage, to write in and say it was a risky activity.
In the rambly bits of the show on 3rd March 2017 Jason Webb, consultant orthopedic and trauma surgeon from Bristol - BSc(Hons), MBChB(Hons), FRCS(Eng), FRCS(Tr & Orth) and also LTLSTE - emailed in. He said that the clear consensus from all the arthritis councils worldwide was that knuckle cracking is not associated with the development of arthritis. A paper on the subject called "The Effect Of Knuckle Cracking on Hand Function" by Castellanos and Axelrod can be read for further information about this.
Within the same the rambly bits Dr Martin A. Lee, consultant rheumatologist and honorary senior clinical lecturer, Newcastle-Upon-Tyne NHS Foundation Trust and Newcastle University - MB, BCh, BSc(Hons), MRCP(Rheum), PgDipClinEd, FHEA, MAcadMEd - responded with a short and to the point answer of "No". He referred the Good Doctors to the Ig Noble Prize earned by Donald L. Unger who cracked the knuckles on his left hand and left his right knuckles free for 60 years, demonstrating (in a sample size of one) that knuckle cracking does not cause arthritis.
The noise when cracking your knuckles, also known as metacarpophalangeal joints, is caused by nitrogen gas popping in the joint fluid as the finger is moved.
Mark's dislike of Simon's habit is somewhat a case of being nothing if not inconsistent, as Mark himself once greatly upset Justine Green by cracking his knuckles as he prepared to review Observe And Report.