RESEARCH - Joint symmetry in early and late RA and PsA: comparison with a mathematical model

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Arthritis Rheum. 2000 Apr;43(4):865-71.

Joint symmetry in early and late rheumatoid and psoriatic arthritis:

comparison with a mathematical model.

Helliwell PS, Hetthen J, Sokoll K, Green M, Marchesoni A, Lubrano E, Veale

D, Emery P.

Rheumatology and Rehabilitation Research Unit, University of Leeds, UK.

OBJECTIVE: To establish a mathematical model to predict the probability of

symmetry of joint involvement as a function of the number of joints involved

and to compare expected with actual probabilities in psoriatic arthritis

(PsA) and rheumatoid arthritis (RA) and in early and late disease. METHODS:

Random involvement of joints was assumed, and the binomial theorem was used

to give the frequency distribution of involved joints as a function of each

joint count. Ten joint pairs were included: shoulder, elbow, wrist,

metacarpophalangeal joints, proximal interphalangeal (PIP) joints of the

hands, hip, knee, ankle, metatarsophalangeal joints, and PIP joints of the

feet. Observed probabilities were obtained from subjects with early

(duration < or =12 months) and late PsA and RA. RESULTS: The number of

subjects in each of the disease subgroups was as follows: early PsA n = 33,

late PsA n = 77, early RA n = 61, late RA n = 93. Observed probabilities of

symmetry exceeded predicted probabilities for all disease subgroups. The

median number of involved joints in each group was as follows: early PsA 4,

late PsA 8, early RA 8, late RA 15 (chi2 = 95.3, 3 degrees of freedom, P =

0.0001, by Kruskal-Wallis test). After correcting for the discrepancy in the

number of involved joints, no difference in joint symmetry was found between

the groups (chi2 = 1.77, P = 0.62 by Friedman two-way analysis of variance).

Similar results were obtained when individual hand and foot joints were

analyzed separately.

CONCLUSION: The pattern of joint involvement is often used to distinguish

between rheumatoid and psoriatic arthritis. This study confirms that

symmetry is largely a function of the total number of joints involved and

that, in terms of joint pattern, differences between these disorders are

more quantitative than qualitative. Both disorders have high absolute values

of symmetry, particularly in the joints of the wrist and hand.

PMID: 10765932

http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve & db=pubmed & dopt=Abstra\

ct & list_uids=10765932 & itool=iconabstr & query_hl=29 & itool=pubmed_DocSum

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