  In psoriatic arthritis, it is important to understand the joint activity
(represented by swelling and pain) and damage processes because both are
related to severe physical disability. This paper aims to provide a
comprehensive investigation in to both processes occurring over time, in
particular their relationship, by specifying a joint multi-state model at the
individual hand joint-level, which also accounts for many of their important
features. As there are multiple hand joints, such an analysis will be based on
the use of clustered multi-state models. Here we consider an observation-level
random effects structure with dynamic covariates and allow for the possibility
that a subpopulation of patients are at minimal risk of damage. Such an
analysis is found to provide further understanding of the activity-damage
relationship beyond that provided by previous analyses. Consideration is also
given to the modelling of mean sojourn times and jump probabilities. In
particular, a novel model parameterization which allows easily interpretable
covariate effects to act on these quantities is proposed.
