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+ Polarised light microscopy remains the only practical way of identifying these particles in the clinical setting. + Polarised light microscopy is a test with major problems in both its sensitivity and specificity for the identification of these crystals. + The sensitivity problem is in part attributable to the problems caused by low concentrations of crystals or the presence of crystals of very small size, a negative report could always be a false negative therefore, and crystal presence cannot be regarded as an “on-oV” phenomenon. + The specificity problem can be improved by the use of good equipment, good laboratory practice, specific training and quality control exercises—initiatives to develop these improvements need supporting. PAUL DIEPPE ANGELA SWAN MRC Health Services Research Collaboration, University of Bristol, Canynge Hall, Whiteladies Road, Bristol BS8 2PR 1 Dieppe PA, Calvert P. Crystals and joint diseases. London: Chapman and Hall, 1983. 2 Eisenberg JM, Schumacher HR, Davidson PK, Kaufman A L. Usefulness of synovial fluid analysis in the evaluation of joint eVusions; use of threshold analysis and likelihood ratios to assess a diagnostic test. Arch Intern Med 1984;144:715–19. 3 RadcliVe K, Pattrick M, Doherty M. Complications resulting from misdiagnosing pseudogout as sepsis. BMJ 1986;293:440–1.

4 Garrod AB. A treatise on gout and rheumatic arthritis. London: Longmans, Green, 1876. 5 McGill N, Dieppe PA, Bowden M, Gardiner DJ, Hall M. Identification of pathological mineral deposits by Raman microscopy. Lancet 1991;337: 77–8 6 Gathercole L, Swan A, Price G, Dieppe P. Nanometre scale surface features of arthropathic microcrystals and their relation to protein adsorption. A study by scanning probe microscopy and wide angle X-ray diVraction. J Mater Sci-Materials in Medicine 1996;7:511–16. 7 Hasselbacher P. Variation in synovial fluid analyses by hospital laboratories. Arthritis Rheum 1987;30:637–42. 8 Schumacher HR, Sieck MS, Rothfuss C, Clayburne GM, Baumgarten DF, Mochan BS. Reproducibility of synovial fluid analyses; a study among four laboratories. Arthritis Rheum 1986;29:770–4. 9 McGill NW, York H. Reproducibility of synovial fluid examination for crystals. Aust NZ J Med 1991;34:710–13. 10 Gordon C, Swan A, Dieppe PA. Detection of crystals in synovial fluids by light microscopy; sensitivity and reliability. Ann Rheum Dis 1989;48:737– 42. 11 Swan AJ, Chapman B, Heap P, Seward H, Dieppe P. Submicroscopic crystals in osteoarthritic synovial fluids. Ann Rheum Dis 1994;53:467–70 12 Bjelle A, Crocker P, Willoughby D. Ultramicrocrystals in pyrophosphate arthropathy. Crystal identification and case report. Acta Med Scand 1980; 207:89–92. 13 The identification of crystals in synovial fluid. Dieppe P, Swan A. University of Bristol, 1998 (Practical Hankbook available on request). 14 Swan AJ, Heywood BR, Dieppe PA. Extraction of calcium containing crystals from synovial fluids and articular cartilage. J Rheumotol 1992;19: 1763–73. 15 Honig S, Gorevic P, HoVstein S, Weissman G. Crystal deposition disease. Diagnosis by electron microscopy. Am J Med 1977;63:161–4. 16 Dieppe P, Pascal E, Swan A. The identification of crystals in synovial fluids; the EULAR quality control initiative. Rheumatology in Europe 1997;26: 74–5.

Unusual and memorable Series editor: Gary D Wright A 39 year old man was referred with increasing swelling of the right hallux, and a similar, smaller swelling on the left side. This was progressive over the preceding 12 months, and was painful on weight bearing. There was marked, diffuse soft tissue swelling, tenderness, slight restriction in joint range and associated nail changes (fig 1). Plain radiograph (fig 2) suggested bone proliferation, and computed tomography was requested (fig 3). These show florid bony excresences arising from the terminal phalanges of both halluces, thickened cortices, normal medullary cavities, and extensive soft tissue swelling.

Figure 1

Figure 2

Figure 3

Periostitis and bone proliferation may be associated with soft tissue swelling and nail changes on the same digit, and can be an early feature of psoriatic arthritis (and other spondyloarthropathies) before significant joint involvement occurs. Periosteal bone may entirely cloak the cortical surface (particularly of the terminal phalanges of the toes), and when associated with trabecular thickening can cause the entire phalanx to appear radiodense, the so called “ivory phalanx”.1 1 Resnick D, Broderick RW. Bony proliferation of terminal phalanges in psoriasis. The “ivory phalanx”. J Can Assoc Radiol 1977;28:187. Contributors: D O’GRADAIGH, C BLUNDELL, T MARSHALL, K GAFFNEY, P CHAPMAN. Norfolk and Norwich Hospital, Norwich.