Ann Rheum Dis (1983), 42, Supplement p 54

Intra-articular apatite crystal depositionH. RALPH SCHUMACHER,' P. VERGHESE CHERIAN,' ANTONIO J. REGINATO,1THOMAS BARDIN,2 SUSAN ROTHFUSS1

From the 'University ofPennsylvania School ofMedicine, Philadelphia, USA and 2Clinique Rheumatologique, HospitalLariboisiere, Paris, France

Introduction

The role of apatite crystals in calcificperiarthritis is well recognised.1-5 Formany years, however, intra-articularapatites were not thought to play anypart in joint disease. They wereidentified in only a small percentage ofmenisci examined grossly and by x-raydiffraction by McCarty et al. in 1966.6In 1975-7 we7' and Dieppe et al.'using electron microscopy andelemental analysis on joint fluidsfound apatite crystals in varioussituations including acute otherwiseunexplained arthritis. We review ourfindings since that time.

Methods for crystal identification

Techniques used to identify apatitecrystals in synovial fluids have variedin different series and may affect theimportance of the findings. Sensitivity

and specificity of most technicnot been fully defined. Regimicroscopy may allow visualilarger clumps of apatite crglossy, slightly irregular glob2-10,um in diameter (Fig. 1few of such aggregalbirefringent.' Stains for calcphosphorus such as Von Koalizarin red stains can hellidentify these globules.' Nexamined the sensitivity of thred stain and find that it cc0- 005 ,ug of synthetic apatitesynovial fluid.' The techniqinot totally specific and is usedscreening test.Scanning electron mic

(SEM) can also identify clunsame shape1" and allowsprobe elemental analysis, whinot only whether calciiphosphorus are present Iwhether their approximate r

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ques have close to the 1 67:1 expected with wellular light crystallised apatite.isation of Transmission electron microscopyrystals as allows identification of the tiny thinules from 50-250A diameter needles within the). Only a clumps that are typical of apatite.' Thistes are technique may be used on smallcium and amounts of fluid and also on either thinissa's and sections or synovial fluid dried ontop further formvar coated grids.11 Used on synovialWe have fluid the technique detects downie alizarin to at least 0-002 mg/ml of apatite andan detect avoids possible artefacts of chemicalin 1 ml of fixation. Examination of thin sectionsue is still also allows demonstration of whetheronly as a and by which cells crystal clumps are

phagocytised. This may obviously be:roscopy important for the phlogisticips of the significance of the apatite. Elementselectron may also be analysed as with SEM.

ich shows Electron diffraction patterns can alsoum and be made for comparison with knownbut also standards, but the problems and valueratios are of this have not yet been worked out.

When large amounts of suspectedapatite are present x-ray diffraction isgenerally accepted as the definitivemethod of analysis, although smallamounts of certain crystals may bemissed when mixed with otherpredominant crystals. A semi-quantitative screening techniquefor apatite using 14C disodiumetidronate binding is also beingstudied.12 Intracellular crystals mightbe missed with this method. Infrared

,04| 1t spectrophotometry has been used tosupplement findings with othertechniques1" and with the use ofFournier transform may allow

^.$ detection of small amounts of crystalsmixed in with other predominant ones.

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Fig. 1 Lightmicroscopical viewof irregularclumpsofapatite crystals (arrows) insynovial fluid. Note erythrocytes (E) and neutrophils (N) for size comparison.

Apatite in joints

Apatite has been described in knees,shoulders, hips, wrists, firstcarpometacarpal joints, meta-carpophalangeal, proximal anddistal interphalangeal joints, and

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metatarsophalangeal joints includingthe 1st.5 89 13 We have studied a totalof 104 patients in whom apatite wasfound by transmission electronmicroscopy and find a similardistribution of joints containing thesecrystals. Cases reported have generallybeen from studies of specific diseasesor selected by referral so the incidenceof identifiable apatite in the generalpopulation remains unclear.Women seem to be affected more

often than men,5 and we know of noreports in children except inassociation with collagen disease. Ourstudy included 58 women and 46 men;there was a high incidence of elderlypatients because we included 44patients from a study of osteoarthritis,the youngest patient was 36 years old.

CLINICAL PRESENTATIONS

Otherwise unexplained acuteinflammatory arthritis with jointeffusions containing apatite has beenreported by Fam et al., Dieppe et al.,and our group at knees, wrists, hips,proximal interphalangeal joints,metacarpophalangeal joints, and firstmetatarsophalangeal joints' 8 9 andmay well occasionally occur at anyjoints where crystals can deposit.Some patients have had multipleattacks and have also occasionally hadcalcific periarthritis. Attacks mayresolve without treatment; someprogress to chronic erosive arthritis."3Synovial fluid leucocyte counts inclinically acutely inflamed joints haveranged from 1 8-84 x 109/P5 9

Neutrophils have ranged from 27%YO to90% with a predominance in acutecases. Crystals may be seen to bephagocytised by synovial fluidneutrophils and mononuclear cells(Fig. 2).8Most recent studies have also

identified apatite crystals in some jointeffusions without any dramaticevidence of inflammation.8 9 18 14 15Many such joints have hadosteoarthritis, but the real incidence ofapatite deposition in osteoarthritis andhow often it occurs early in still grosslynormal cartilage is not yet known. Wefound apatite in 44 of 100osteoarthritic effusions afterexamining just one grid on each bytransmission EM.'5 Dieppe et al.found apatite in 9 out of 34 unselectedosteoarthritic fluids with scanningelectron microscopy (SEM).6 Using

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Fig. 2 A clump oftiny crystals proved to be apatite by elemental analysis lie in a

vacuole ofa synovial fluid mononuclear cell. Electron micrograph x20 000(original magnification).

TEM Ali found apatite more often inosteoarthritic than other cartilages butdid not describe incidence.'7 Even inosteoarthritis with 'non-inflammatory'joint effusions apatite has beenphagocytised by synovial lining cellsthat might release proteases andcollagenase and be a major factor inprogressive joint damage."3

RADIOGRAPHIC FINDINGSJoints with acute or subacuteapatite-associated arthritis may showcalcification in the synovial space (Fig.3).' The linear cartilage or meniscalcalcification seen with CPPD is notexpected. Some x-ray films show nocalcification despite laterdemonstration of large amounts ofapatite. In fact, only 21 of our 104patients had obviousx-ray evidence ofcalcification. Patients may also haveperiarticular calcifications at thesymptomatic site or elsewhere.Erosive arthritis may develop'" '8 andchanges of osteoarthritis may bepresent.'4

In patients with clinicalosteoarthritis the severity ofradiographic changes of osteoarthritistends to correlate with the incidence ofapatite deposition.

Apatite associated with other diseases

Apatite crystals have been found in ahigh percentage of specimens ofsynovial fluid or cartilage frompatients known also to have CPPDdeposition. 420-22 Either or bothcrystals may be phagocytosed; in ourstill limited experience CPPD seemsmore often to be intracellular.Other diseases that have been

associated with clinically significantapatite arthritis include scleroderma,dermatomyositis, and systemic lupuserythematosus" ; renal failure treatedby haemodialysis;l 19 and possibly highdose vitamin D treatment.26 Apatitealso occurs in other joint diseases,presumably as a secondarycomplication. We found apatite in 8out of 32 effusions from joints with

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Fig. 3 X-ray films ofhand, showing apatite deposits best in the joint space ofthefifth metacarpophalangeal joint and the capsule of the secondmetacarpophalangeal joint. This patient had a history of recurrent acute arthritisthat evolved into erosive arthritis.

rheumatoid arthritis; the crystalscorrelated with the severity ofsecondary osteoarthritis.27 In certaincases acute dramatic inflammation in asingle joint may be induced by crystals

rather than by the underlyingrheumatoid arthritis. Table 1 showsdiagnoses other than osteoarthritis inour 104 patients with apatitedeposition.

Table 1 Associated diseases other than osteoarthritis in 104 patients withintra-articular apatite crystals

Diseases No ofpatients

Dermatomyositis, scleroderma 6Renal failure on dialysis 10Oxalate deposition 1Hypothyroidism 3Haemochromatosis 3Osteochondromatosis 1Acromegaly 2CPPD deposition 22Gout 5Rheumatoid or psoriatic arthritis 10Temporal arteritis with aseptic necrosis I

Characteristics of crystals

It is impressive that there istremendous morphological variationamong apparent apatites we havefound in joints. Crystals may be almostpunctate, (Fig. 2), rods or thicker'boat-like' structures, (Fig. 4) or longneedles (Fig. 5); they may be denselyor loosely packed, and larger crystalsmay have the suggestion of an internalstructure at high magnification.Crystals may be rare or profuseenough to make the fluid milky. Rareclumps may obviously have differentclinical significance than massiveamounts. Crystals are frequentlyphagocytosed (Fig. 6) but seem mostoften to be ingested by mononuclearcells rather than polymorphonuclearneutrophils. Elemental analysis alsoshows some variation with someapparent apatite having Ca:P ratioswell below the expected 1 67:1,suggesting the presence of someamorphous or poorly crystallised salts.Substitution of chloride and carbonatein apatite also varies. As yet we knowof no good correlation of elementalanalysis findings with crystalmorphology.

Apatite was most often detected insynovial fluid because of itsaccessibility and from knees ratherthan other joints for the same reason.Arms, including finger joints, wrists,elbows, and shoulders seemedinvolved more often than the legs inpatients with collagen-vascular diseaseor those undergoing renal dialysis.Apatite was seen in surgical specimensof cartilage either as round clumpsnear to chondrocytes (but not clearlyin matrix vesicles) or as irregularmasses on the surface or in theinterstitium. Apatite in synovium wasfrequently phagocytosed or enfoldedby synovial cell processes.

Mechanisms involved in apatitedeposition disease

The ability of apatite to induce acuteinflammation in joints has been shownby intra-articular injection into theknees of dogs using the same model asthat used for urate and CPPD crystals.8Apatite has also producedinflammation at other sites-forexample, the pleural space andsubcutaneous tissue'°-afterexperimental injection. Apatite is not

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I

Fig. 4 Rod and 'boat-like' apatite crystals. Some foamy internal str

seen at high magnification. Electron micrograph x125 000 (originmagnification).

Fig. 5 Long needle-like apatite crystals from joint fluid stained w

conjugated antibody to IgG demonstrating binding of this immuno

labelled by ferritin particles (arrows) to clump. Electron micrograj

(original magnification).

always associated with detectableinflammation (the same is true of urateand CPPD28 29) and the reasons for thisrequire study.

In TEM sections finely granularmaterial is generally associated withthe synovial fluid crystals (Fig. 6)8 and

with most clumpsembedded in synoviumcartilage. Preliminaelectron microscopicallaboratory show thglobulins are at least oi

of this granular materia

possibility that other materials, includ-ing fibronectin, osteonectin enzymes,and complement, are present shouldalso be considered. As previously sug-gested with urate crystals,' substancesadhering to crystals could encourageor inhibit inflammation. Effects couldeven vary depending on other cellularand humoral aspects of the joint.Complement may be activated by apa-tite as well as by other crystals.3'

Apatite almost certainly appears injoints by different mehanisms. In somecases (as with CPPD) it is thought to bea result of degenerative changes incartilage and to deposit first incartilage.'7 In other examples,especially when associated withmigratory calcific periarthritis at othersites, it probably deposits outsidecartilage due to the still unknownsystemic factors that seem to causemost such calcific periarthritis.Phosphate excess or other features ofrenal failure and haemodialysis seemto contribute to some cases.'9 Serumand synovial fluid calcium and

ructur canbe phosphate concentrations have beenal normal in most cases. Occasionally

apatite is seen to have risen fromfragments of bone released intoseverely destroyed joints.

Apatite deposition in articularcartilage occurs spontaneously inaging rabbits or after administration ofvitamin D; synovial calcification withapatite can be induced byintra-articular calciphylaxis.3' Inneither of these established types ofexperimental apatite calcification hasacute inflammation occurred. Longterm studies are still needed todetermine if osteoarthritis evolves or ifvarious modifications can produceattacks of acute arthritis.

Treatment

Acute inflammation associated withapatite deposition generally seems to

ith ferritin respond to treatment with)globulin indomethacin, other non-steroidalph x80 000 agents, or colchicine.' Aspirin has

been ineffective in some cases.Intra-articular depot corticosteroidshave also appeared to be helpful.' We

of crystals have no personal experience withor superficial measures to prevent or decreasery immuno- apatite deposition. Disodiumstudies in our etidronate has been reported toat immuno- decrease calcinosis but with chronicne component use may also effect bone(Fig. 5). The mineralisation.32 In patients with

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Fig. 6 Apatite crystals embedded iet electron dense material in a vacuole of a

synovial fluid mononuclear cell. Electron micrograph x32 000 (originaltnagnification).

abnormal calcium or phosphorusmetabolism as, for example, occurs in

many of those with renal failure, bettercontrol of phosphorus concentrationsmay decrease apatite deposits. Somecalcinosis in or around joints has alsobeen reported to resolve

spontaneously. In one patient of ours

with a scleroderma-like disease apatitedeposits in finger joints resolved whenjoints gradually stitfened withprogressive skin disease .2

Indomethacin was thought to decreasepara-articular ectopic ossification inthree cases after total hiparthroplasty.33 Neither this nor otherpossible ways of decreasing calcinosishave been adequately andsystematically evaluated.

Other calcium-containing crystals

Other calcium-containing crystalshave been described in intra-articularsites. Brushite (CaHPO4 2HlO) hasbeen reported in small amounts ofuncertain clinical importance and

usually in association with CPPD."4Whitlockite and octacalciumphosphate may also occur in diseasedjoints along with apatite. {'

Recently, we have found calciumoxalate crystals in synovial effusions in

three patients undergoing haemo-dialysis for chronic renal failure.37 Inone patient x-ray films showed fingercalcifications similar to those seen withapatite and knee chondrocalcinosislike that seen with CPPD deposition.Detailed analyses at necropsy revealedmassive oxalate deposits and no

CPPD. The list of crystals identifiablein joints is almost certainly not yetcomplete. We and others have seenother unidentified birefringent or

electron dense material that is stillunder evaluation.

Supported in part by grants from theVeterans Administration. NationalInstitutes of Health (1-R21-AG 02815-01),McCabe Foundation, and Barsumian Fund.We thank Miss Gilda Clayburne, Miss

Marie Sieck, and Mrs Susan Rothfuss forexcellent technical help on the work from

our laboratory and appreciate the carefultyping of Miss Mary Ellen Maguire.

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