[.:#{149}.. Erosive Osteoarthritis and Psoriatic Arthritis: A Radiologic. Comparison in the Hand, Wrist, and Foot

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1 i 25 William Mantel1 Karen J. Stuck1 Aaron M. Dwonin2 Robert G. HylIand2 Received May : accepted after revision September Presented at the annual meeting of the Amencan Roentgen Ray Society. Toronto. Ontario. March Department of Radiology, University of Michigan Medical Center, Ann Arbor. Ml Address reprint requests to W. Mantel... Department of Internal Medicine. University of Michigan Medical Center, Ann Arbor, Ml AJR 134: , January o3x/8o/ $00.00 American Roentgen Ray Society Erosive Osteoarthritis and Psoriatic Arthritis: A Radiologic Comparison in the Hand, Wrist, and Foot [.:#{149}.. The medical records and radiographs of the hands, wrists, and feet of 50 patients with psoriatic arthritis and 34 with erosive osteoarthritis were reviewed. Polyarticular arthritis in psoriatics is often unilateral and the joints of a single ray may be selectively affected. Their erosions tend to occur at the bare areas of joints, whereas in erosive osteoarthritis it is the subchondral cortex that is primarily affected. Linear periosteal bone apposition is common in the latter. The distribution of arthritis, sites of bone erosion, and character of periosteal bone apposition form distinctive patterns in these conditions which are important in differential diagnosis. Erosive osteoarthnitis (EOA) and psoniatic arthritis (PA) are commonly confused with each other and with rheumatoid arthritis. The diagnostic problems are complicated, inasmuch as EOA patients may develop supenimposd rheumatoid arthritis [1 1 and patients with psoniasis may exhibit clinical and radiologic features which are said to be indistinguishable from rheumatoid arthritis [2, 3]. Much has been written concerning the radiologic features of rheumatoid arthritis in the hands and feet [4-6], but the features of EOA and PA have not been as clearly defined. This study characterizes the spectrum of nadiologic features in the hand, wrist, and foot in these two conditions and determines those features that are particularly significant in differential diagnosis. Materials and Methods The medical records and radiographs of 93 patients who had been diagnosed as having either erosive osteoarthritis (EOA) or psoriatic arthritis (PA) at the University Hospital were reviewed. Nine were rejected because they were considered in retrospect to have had rheumatoid arthritis. EOA and PA were erroneously diagnosed in five and two cases, respectively, and two had EOA and rheumatoid arthritis. The rest were divided into two groups, 50 with PA and 34 with EOA. All psoniatic patients had typical skin lesions and chronic arthritis. Patients with EOA had inflammatory, destructive changes in the interphalangeal (IP) joints and clinical findings consistent with that diagnosis [7, 8]. Patients with other rheumatic diseases were excluded. There was clinical follow-up of i year or more in 43 PA and 1 8 EOA patients. Table 1 shows the number of available radiographic examinations of each skeletal region in these two groups and the follow-up examinations. In addition, proximal and distal interphalangeal joints of two cadavers were dissected to clarify the relationships of the articular cartilage, joint capsule and collateral ligaments, and intraarticular osseous surfaces.

2 126 MARTEL ET AL. AJR:134. January i980 TABLE 1 : Number of Radiologic Examinations TABLE 2: Clinical Findings PA (n = 50) EOA ( n = 34) PA EOA Site Examined initial initiai Hands Feet Heels Anatomic Considerations Fig. 1 -Frontal view of DIP joint. Bare areas (in black) smaller on distal than on proxma) joint margin. Articular cartilage is thicker near center on distal surface, whereas on proximal surface it is thinner at center. The earliest sites of cortical erosions in rheumatoid anthnitis have been described [4]. These correspond to those parts of the bone within the joint that are not covered by articular cartilage, the so-called bare areas. These can be found between the attachment of the joint capsule and the edge of the articular cartilage. They are larger on the metacarpal head than on the base of the proximal phalanx, and larger on the distal end of the proximal phalanx than on the proximal end of the middle phalanx. An analogous situation exists in a distal interphalangeal joint (DIP), in that the bare areas on the proximal joint surface are larger than on the distal side (fig. 1). The articulan cartilages on the distal sides of both distal and proximal interphalangeal (PIP) joints are thinner at the periphery than at the center, whereas on the proximal sunfaces the cartilages are thicker at the periphery. However, in a metacarpophalangeal joint (MCP) this relation is neversed; the cartilage is thicker over the center of the metacarpal head than at the periphery, whereas on the phalangeal side of the joint it is thicker at the peripheral aspects than in the center. The trapezium (greaten multangular) articulates with four bones: the trapezoid (lesser multangular), scaphoid (navicular), and the first and second metacarpals. The trapezoid Followup Foliowup No. of patients Gender(F/M) 22/28 32/2 Age at onset of arthritis: Less than Greater than Patients with rheumatoid factor/no. of patients tested 2/45 4/22 Clinical follow-up of 1 year or more TABLE 3: Distribution of Arthritis in Hands and Wrists Location Note -n = number of cases with turns Numbers in parentheses are percentages also articulates with four bones: the trapezium, scaphoid, capitate, and second metacarpal. PA (n = 48) EoA (n = 34) Distal interphalangeal, selective 1 3 (27) 2 (6) Proximal interphalangeal, selective 1 (2) 2 (6) Interphalangeal 28 (58) 31 (91) Interphalangeal and metacarpophalangeal 25 (52) 8 (23) Radiocarpal 1 6 (33) 1 (3) Radioulnar 7 (1 5) o Generalized carpal 14 (29) 0 Trapeziometacarpal 3 (6) 1 5 (44) Scaphotrapeziotrapezoidal 2 (4) 1 7 (50) Trapezoid-metacarpal 0 6 (1 7) Trapezoid-capitate 0 2 (6). Ray pattern 1 2 (25) 0 Polyarticular-unilateral pattern 10 (21) 1 (3) Results Clinical Features The pertinent clinical findings are summarized in table 2. The gender ratio of the psoniatics was close to one, and all but two of the patients with erosive osteoarthnitis (EOA) were female. Psoniatic arthritis (PA) tended to occur in the young, whereas EOA had its onset in middle or late life in the great majority, and in the late 30s in only four cases. The latex fixation test for the rheumatoid factor was positive (1 :320 or greater) in two of the 45 psoniatics and four of the 22 EOA patients tested. (This is consistent with the prevalence of the rheumatoid factor in these age groups in our laboratory), There was clinical follow-up of 1 year or more in 43 PA and 1 8 EOA patients. All cases were white, except two with EOA who were black and hispanic, respectively. Other medical conditions coincidentally found in three EOA patients were neurodenmatitis, diffuse interstitial fibrosis, and pseudogout. Gout was suspected in one PA patient with a serum unate of 12.3 mg/dl, but uric acid crystals were not demonstrated in the joint fluid, Another PA patient with a positive rheumatoid factor (1 :640) had chronic hepatitis. The relative onsets of arthritis and psoniasis were documented in 44 cases. Arthritis developed prior to skin lesions in five with intervals of 2, 3, 5, 1 4, and 1 5 years. In 1 1 cases

3 AJR: 1 34, January 1980 EROSIVE OSTEOARTHRITIS/PSORIATIC ARTHRITIS 127 skin and joint disease began simultaneously, whereas in 28 cases the psoniasis antedated the arthritis by 2-36 years (mean, 12). The inflammatory component of the disease in the fingers was conspicuous in the great majority of the EOA cases. In eight there was also a history or objective evidence of arthritis in the forefoot other than in the first ray. This involved the metatarsophalangeal joints in five cases and the interphalangeal joints in three. Radiologic Features Distribution. Table 3 shows the distribution of affected joints of the hands and wrists. In cases in which the same Fig. 2.-Psoriatic arthritis. First, fourth, and fifth DIP joints are affected. Peniosteal bone apposition has partially obscured bare area erosions in these joints and has caused widening of affected bones. Fig. 3. -Psoriatic arthritis. Ray pattern. Fingers are spared except for joints of thumb. Soft-tissue swelling essentially absent. Tapered erosion of proximal phalanx of thumb and minimal peniosteal bone apposition of radial stybid. Generalized loss of articulan cartilage of carpal and radiocarpal joints. Erosions of tniquetrum, trapezium, and navicular. Patient also exhibited polyarticular unilateral pattern: other hand and wrist were completely spared. skeletal region was examined on different dates, only one examination was used to determine the distribution of anthnitis. Uniform cartilage-space narrowing in the IP or MCP joints, without accompanying bone or soft-tissue changes, was not taken as a sign of joint involvement because this is common in the elderly and does not necessarily indicate arthritis [9]. There were radiographs of the hands and wrists in 48 patients with psoriatic arthritis (PA) and 35 patients with erosive osteoarthritis (EOA). Selective involvement of the DIP joints was more typical of PA than EOA. Selective involvement of PIP joints was uncommon, whereas IP joint involvement was frequent in both diseases. Combined MCP and IP arthritis was more prevalent in PA than EOA. Furthermore, MCP arthritis was not associated with bone erosions in EOA, but such erosions were common in PA. Radiocarpal, radioulnar, and diffuse carpal joint involvement were common in PA but rare in EOA. There was one EOA patient with uniform narrowing of the radiocarpal cartilage space, but this patient proved to have chondrocalcinois and pseudogout. Selective arthritis of the trapeziometacarpal and scaphotrapeziotnapezoidal joints was typical of EOA. The latter was associated with arthritis of the trapezoidmetacarpal and trapezoid-capitate joints in six and two EOA patients, respectively. Although predilection for the DIP joints (fig. 2) of the hands was the most common pattern of finger involvement in PA, two other patterns were also conspicuous. The tendency for the joints of a single ray to be severely affected with sparing of other joints ( nay pattern) (figs. 3 and 4) was noted in 1 2 cases of psoniatic arthritis (PA). The thumbs were involved in four. A single ray was affected in nine cases and two rays were affected in three. In only two Fig. 4.-Psoniatic arthritis. Involvement of joints of middle finger illustrative of nay pattern. Minimal soft-tissue swelling and osteoporosis of this digit.

4 128 MARTEL ET AL. AJR:134, January 1980 performed. Other finger joints were affected I year later. B, 1 0 years later. Several IP joints now affected. Widening of bones due to peniosteal bone apposition, involvement of scaphotrapeziotrapezoidal joints and surgical fusion of previously affected PIP joint on right. 6 7 instances was this associated with diffuse soft-tissue swelling of the affected ray. A second pattern (1 0 cases) was arthritis in several joints of one hand, with the opposite hand completely spared polyarticular unilateral pattern) (figs. 2 and 3). The night hand was affected in three cases and the left in seven. Although the ray and polyarticulan unilateral patterns often persisted for months or years, follow-up radiographs in eight cases disclosed progression to other joints so that these initial patterns were subsequently not recognizable. One EOA patient had a polyarticular unilateral pattern with four IP joints affected. In two EOA patients the disease Fig. 6.-Psoniatic arthritis. Selective arthritis of IP joints of first and fifth toes. Severe destruction of subchondral bone. particularly in great toe, associated with widening of intenosseous space and erosion of terminal tuft. Relative absence of osteoporosis. Fig. 7. -Erosive osteoarthnitis. Diffuse subchondrat erosion in P joint of left fifth toe, associated with soft-tissue swelling. Similar lesion was present in right fifth toe and fingers were severely affected. was monanticular, with the PIP joint of a middle finger involved in both, It persisted as monarticular disease for 6 months in one patient and 2 years in the other, with other IP joints subsequently affected in both (fig. 5), Table 4 shows the distribution of arthritis in the feet. Radiographs were available in 40 PA and 1 7 EOA patients. The arthritis was common in PA and often severe. There was a predilection for the IP joint of the great toe, and severe destruction was common in this joint. In three cases of PA (psoniatic arthritis), severe intenphalangeal joint destruction was symmetrical and peculiarly limited to the first and fifth digits (fig. 6). Arthritis was less common in EOA

5 AJR:134, January 1980 EROSIVE OSTEOARTHRITIS/PSORIATIC ARTHRITIS 129 TABLE 4: Distribution of Arthritis in Feet Location PA (n - 40) EOA (n - 17) 28 (70) Interphalangeal, great toe 3(18) Metatarsophalangeal, great toe 30 (75) 9 (53) Interphalangeal, other 23 (57) S (29) Metatarsophalangeal, other 23 (57) 0 Tarsal 1 0 (25) 0 Note -n = number of cases with films umbers in parentheses are percentages TABLE 5: Distribution of Calcaneal Lesions TypeandLocation. PA (n = 31) EOA (n = 13) Erosions: Posterosupenior 8 (26) 0 Posteroinferior 2 (6) 0 Bone proliferation: Posterosuperior 9 (29) 0 Posteroinferior 21 (68) 5 (38) Note-n - number of cases with films Numbers in parentheses are percentages Fig. 8.-Psoriatic arthritis. Extensive calcaneal erosion posterosupeniorly, associated with diffuse, irregular periosteat bone apposition. Other heel was similarly affected. Fig. 9.-Erosive osteoarthnitis in left hand. A, Arthritis in fingers, 1 year dunation. Uniform loss of articular cartilage and subchondral erosions of IP joints resulting in gull wings deformity of distal surface of DIP of index finger. IP soft-tissue swellings and uniform loss of thickness of anticulan cartilages of MCP joints. B. 3 years later. Further subchondnal erosion of IP joints. Extensive erosions of thumb with widening of interosseous space, and lack of change in MCP joints. Right hand was similarly affected. and, although the joints of the first digit were most often affected, subchondnal bone destruction was absent in these joints. However, in five cases of EOA (erosive osteoanthnitis), the arthritis of the IP joints, other than in the first digit, was associated with extensive subchondnal bone erosion similar to that observed in the fingers (fig. 7). One psoniatic patient had the polyanticulan unilateral pattern in both hands and feet with involvement on the left side. There were lateral radiognaphs of the heels of 31 PA and 1 2 EOA patients (table 5). Calcaneal erosions were common in PA, particularly posterosupeniorly, but were absent in EOA. Calcaneal bone appostion was common in PA, panticularly infeniorly, and was typically diffuse and poorly defined (fig. 8). In five cases of EOA it appeared as a localized, well marginated spur posteroinfeniorly. Articular Cartilage. Joint involvement in both conditions was typically associated with uniform narrowing of the cartilage space. However, in EOA some of the joints, particularly those not severely affected, exhibited nonuniform loss of cartilage consistent with common osteoarthnitis. Widening of the interosseous space was frequent in PA in the IP joint of the great toe and was always associated with extensive bone destruction (fig.6). Such widening was occasionally present in EOA, especially in the IP joint of the thumb, but was much less marked and was always associated with severe bone destruction (fig. 96). Bone Erosion. The sites of intnaarticular bone erosion differed significantly in these conditions (table 6). Erosions were confined for the most pant to the bane areas of the interphalangeal (figs. 2 and 1 06), metacarpophalangeal, wrist, and metatarsophalangeal joints in 22 PA patients, but erosions did not occur at these sites in patients with EOA. However, erosions occurred exclusively in the subchondnal cortex in seven PA and 32 EOA patients (fig. 1 1 ). They involved the bare areas as well as the subchondral cortex in 1 8 PA cases. Erosion of the bare area at the radial aspect of the base of the proximal phalanx of the index finger was

6 130 MARTEL ET AL, AJR:134, January 1980 TABLE 6: Significant Radiologic Features Features PA (n = 50) EOA (n = 34)..Bare area erosions 22 (44) 0 Terminal tuft erosions, hands 8 (1 6) 0 Terminal tuft erosions, great toe 1 5 (30) 0 Irregular periosteal bone apposition 35 (70) 0 Cortical thickening 22 (44) 25 (73) Feet more severely affected than hands 6 (1 7) 0 Phalangeal fusion 6 (1 2) 5 (15) Severe bone destruction without regional osteoporosis 5 (10) 0 Subluxations, hands 9 (1 8) 0 Note -Numbers in parentheses are percentages. Based on 35 PA patients who had films of hands and feet on same date A B C frequent; this area appeared to be somewhat selectively affected in seven cases (fig. 1 1 ). Three cases (two PA and one EOA) had discrete, cystlike, 3-5-mm-diam erosions of the subchondnal bone. These erosions had thin sclerotic margins in the latter. The feet were much more severely affected than the hands in six PA cases (table 6). Smooth erosion of the metadiaphyseal regions of one or more bones of hands or feet, appearing as a tapered pencillike deformity, was observed in 1 5 PA cases (fig. 3). This was not observed in EOA. Ten of these patients exhibited very severe joint destruction, so-called arthritis mutilans (fig. 1 2). This occurred in the metatansophalangeal joints in six cases and the thumb joints in four. Fig Psoniatic arthritis. Progressive erosion in DIP joint. A, Soft-tissue swelling, no bone erosion. B. 3 years later. Typical bare area erosions. C. 6 years after B. Diffuse subchondral destruction with widening of intenosseous space and soft-tissue swelling. Fig Psoniatic arthritis. A, Left. B, Right. Bare area erosions at radial aspect of base of proximal phalanx of both index fingers (large arrows) associated with selective osteoporosis in these fingers. Subtle intraarticulan erosions of right DIP and PIP joints (small arrows) adjacent to focal thickening.

7 AJR:134, January 1980 EROSIVE OSTEOARTHRITIS/PSORIATIC ARTHRITIS 131 Erosions occurred in PA in the terminal tufts of one or more distal phalanges of the hands (fig. 13) in eight cases and in the great toes in 1 5. Diffuse surface erosion of the distal phalanx of the great toe was noted in three of these cases, and in one this was associated with sclerosis of the distal phalanx with only minimal changes in the adjacent joint (fig. 14). Swelling of the acral soft tissues of the hand or great toe was observed in 1 0 cases, usually in association with tuft erosion (fig. 1 3B). Extensive bone destruction in PA was usually associated with osteoporosis which was often severe. However, in five cases with severe bone destruction regional osteoporosis was conspicuously absent (fig. 6). Periosteal Bone Apposition. Peniosteal bone apposition was common in both groups of patients (table 6). This was poorly defined and irregular (figs. 8 and 1 4) in 35 PA patients, but it was always linear and smooth in EOA (figs. SB and 9). There was smooth cortical thickening in 22 PA patients and 25 EOA patients (figs. 2 and 1 5). In many patients in both groups, the bone diameter, usually phalanx 1! I. 4 : Fig 1 2 -Psoniatic arthritis. Arthritis mutilans with bony ankylosis of IP joints and osteoporosis. Other foot was similarly affected. Fig Two patients with psoniatic arthritis. A. Resorption of terminal tufts of distal phalanges associated with mmimal thickening of acral soft tissues. Arthnitis of DIP joints. particularly of middle and ring fingers. Other hand was similarly affected B. Pronounced acral softtissue swelling associated with resonplion of terminal tufts, especially of thumb and index finger Other hand was similar)y affected. Minimal involvement of DIP joints. except fifth which is fused. Fig Psoniatic arthritis. Softtissue swelling and diffuse surface erosion of distal phalanx associated with sclerosis and irregular peniosteal bone apposition. Adjacent joint minimally affected. Fig. 15.-Psoniatic arthritis. Focal areas of cortical thickening secondary to peniosteal bone apposition and residual erosions at bane areas. Typical configuration of erosions in DIP of index finger causing mouse ears appearance at distal margin. A B

8 132 MARTEL ET AL, AJR:134, January 1980 Fig Erosive osteoarthnitis in night wrist. A, Severe erosion of trapeziometacanpal joint associated with linear peniosteat bone apposition and sclerosis. Similar severe erosive changes were present in IP joints and left hand and wrist were affected likewise. B, 6 years later. Anticulan surfaces have remodelled with bone formation at sites of erosions. Sclerosis has regressed. First metacarpal remains wide. or metacarpal, was widened, apparently due to peniosteal bone apposition and concomitant endosteal resorption (fig. 9). Peniosteal bone apposition in PA was common in the radial styloid (nine cases), whereas linear bone apposition in EOA was frequent in the first metacarpal (1 0 cases) and was always associated with arthritis of the trapeziometacarpal joint. In one case of severe arthritis of the latter, sclerosis of the affected bones regressed after 6 years (fig. 16). Joint Deformities. Flexion deformities and medial or lateral deviations occurred both in PA and EOA (figs. 56 and 9). However, subluxations occurred in nine PA patients but none with EOA (table 6). Bone Fusion. Interphalangeal fusion occurred in six PA and five EOA patients (table 6). More than one joint was affected in two patients in each group (fig. 1 7), In all cases the fused joints were in the hands, except for one psoniatic in whom they occurred in the foot (fig. 1 2). Discussion Some of the nadiologic features of psoniatic arthritis (PA) cited here have been previously described. These include predilection for the distal interphalangeal (DIP) joints of the hands, interphalangeal joint of the great toe and heels, peniosteal bone apposition, intenphalangeal fusion, and nesorption of the tufts of distal phalanges of hands and feet [2, 3, ]. Features previously recognized in erosive osteoarthnitis (EOA) include severe interphalangeal joint (IP) destruction with relative sparing of the metacanpophalangeal (MCP) joints [9] and frequent involvement of the trapeziometacarpal joint [13]. Several important aspects of both diseases have been insufficiently emphasized on not previously recognized. The Fig Erosive osteoanthnitis. Fusion of PIP joints of middle three digits. Arthritis of scaphotnapeziotnapezoidal joint and narrowing of cantilages of MCP joints without surface erosions., ray and polyarticular unilateral patterns of arthritis in the hands in PA and the bare-area type of erosions, panticulanly in the DIP joints, are examples. Diagnostically significant features of EOA, not previously emphasized, include the configuration of the eroded subchondnal cortex of the IP joints and linear peniosteal bone apposition, Selective anticular disease of the wrist is not necessarily restricted to the trapeziometacarpal joint and destructive interphalangeal arthritis of the small toes may represent the same pathologic process that occurs in the fingers. Funthenmore, analysis of the radiographic features of these two diseases in parallel have permitted us to draw attention to characteristics that have special significance in differential diagnosis. PA resembles rheumatoid arthritis in that the earliest bone erosions tend to occur at the bane areas. However, in PA they are often less sharply demarcated, probably due to obscuration by concomitant bone apposition, a feature usually lacking in rheumatoid arthritis [5]. Furthermore, in PA the erosions not only occur in the PIP, MCP, and wrist joints, but in DIP joints as well [1 41 The erosions on the distal surface of the DIP joints have a characteristic appearance which suggests mouse ears. By contrast, the erosions of the IP joints in EOA are largely a consequence of destruction of the articulan cartilage. The resultant deformity of the distal subchondnal cortex suggests, gull wings because the most marked bone erosion is peripheral, whereas on the proximal side of the joint, the erosion is usually most marked near the center of the bone (figs ). It is felt that these configurations reflect the fact tnat the articulan cartilage on the distal side ofthese joints is normally thinnest at the periphery, whereas on the proximal side it is thinnest centrally. Such gull wings erosions occurred in

9 AJR:134. January 1980 EROSIVE OSTEOARTHRITIS/PSORIATIC ARTHRITIS 133 Fig. 18.-Psoniatic arthritis. Typical. bane area erosions of DIP joint of ring finger giving mouse ears appearance (see fig. 20). Associated periosteal bone apposition giving frayed appearance adjacent to mouse ears erosions and causing widening of involved bones. Fig Erosive osteoanthnitis. Typical subchondnal erosions of DIP joints of third and fifth digits causing gull wings configuration of bones of distal phalanges (see fig. 20). Erosions of proximal surfaces of these joints are more centrally placed. particularly in third digit. Fig. 20.-DIP joint. A, Bare area erosions of psoriatic arthritis C mouse ears ). B, Subchondral erosions of erosive osteoarthritis C gull wings ). the lp joints in 24 EOA cases. However, these erosions are nonspecific, and may be observed in the late stages of PA (psoniatic arthritis) in the IP joints or rheumatoid arthritis in the PIP joints, following complete destruction of the articular cartilage. An important feature distinguishing the advanced stages of rheumatoid arthritis and EOA is the absence of bone erosion in the MCP joints in the latter [9]. Although degenenative changes occasionally occur in these joints in EOA, they are not characterized by surface erosions. However, the erosions in these joints in rheumatoid arthritis are often conspicuous, particularly in the metacarpal head, and are frequently present when the interosseous space is nanrowed. Small notchlike deformities at the bases of the proximal phalanges, were occasionally observed in EOA. These should not be confused with bane area erosions. The anticular cartilages are usually of normal thickness; there are no erosions in the adjacent metacarpal; and the cortex within the notch is intact. In one case the notches in the fifth digit were bilateral and symmetrical and did not change over 5 years. Dihlmann [1 5] called attention to cortical irregularities in the bases of the proximal phalanges in asymptomatic individuals and cautioned against confusing these with dinically significant erosions [15]. Peniosteal bone apposition is well known in PA [1 0, 16], but is not a widely recognized feature of EOA. It is often irregular and exuberant in PA, whereas in EOA it is always linear and usually minimal to moderate in degree. In EOA it occurs typically near affected joints. After the inflammatory phase of psoniatic arthritis subsides, fluffy peniosteal bone apposition becomes more compact and linear. Hence, smooth cortical thickening may result in both conditions; if endosteal resorption supenvenes, bone widening results. Linear bone apposition is extremely common in EOA in the first metacarpal, associated with arthritis of the trapeziometacarpal joint. The frequency with which the joints of a single ray were selectively affected in the hands in PA was unexpected. The. sausage digit phenomenon emphasizes diffuse, marked, soft-tissue swelling of an entire digit due to tenosynovitis and arthritis of the joints of that digit [1 7]. However, the significant diagnostic feature from a radiologic viewpoint is the selective involvement of the joints of a particular ray. The soft-tissue swelling may be inconspicuous or absent, as in the majority of our cases, and more than a single ray may be so affected. It is important for us to be aware of the diagnostic significance of the nay pattern without soft-tissue swelling. It is conceivable that earlier radiography in some of these patients would have shown sausage digits but there is no evidence to conclude that the ray phenomenon is necessanily associated with massive, diffuse, soft-tissue swelling in the early stages.

10 134 MARTEL ET AL. AJR134. January 1980 The frequent asymmetry of psoniatic arthritis has been repeatedly noted [2, 3]. However, asymmetry in the arthritis of the hands and wrists occasionally occurs in senopositive rheumatoid arthritis. It is therefore important to emphasize that in PA the arthritis of the hands and wrists may not only be asymmetrical but unilateral as well. Furthermore, such patients may exhibit the ray pattern simultaneously; this was true in three cases. In four patients with the unilateral pattern, there was selective involvement of the DIP joints. These patterns may be relatively specific for PA, inasmuch as they have not been described in other rheumatic diseases. One EOA patient had arthritis of four DIP joints of one hand with sparing of the contralateral side. This may be a coincidence, but it suggests that the polyarticular unilatenal pattern may not be specific for PA. This patient had no skin lesions on other radiologic stigmata of PA, but there was no clinical or radiologic followup. In a recent review of the radiologic findings in 56 cases of Reiter s disease [1 8], these patterns of arthritis in the hands were not encountered. Nevertheless, the specificity of these patterns deserves further evaluation. There is considerable overlap of nadiologic and clinical manifestations in PA and Reiten s disease, but there are significant differences [18-21 ]. Typically, the feet are more severely affected than the hands in the latter [1 8] and severe destruction in the metatansophalangeal (MTP)joints is common [1 8]. It is significant that these two features may occasionally be encountered in PA. Another feature that has been emphasized both in Reiten s disease and PA is the lack of osteoporosis in regions of severe bone destruction. Although we observed this in several cases, it was more common for osteoporosis to be present. Tnapeziometacarpal arthritis [22] is a common feature of EOA, but other trapezial and trapezoidal joints may be affected. It is usually difficult to visualize these joints with routine projections. Involvement may be limited largely to the scaphotnapeziotrapezoidal joint [23]. Although trapezoid-metacarpal and trapezoid-capitate arthritis occasionally occurs, it was minimal in this series and did not occur as an isolated carpal joint abnormality. All of these joints may be affected in common osteoarthnitis with or without involvement of the fingers. Absence of arthritis in these joints does not exclude EOA. However, in many cases the severity of the erosions in these joints seemed to parallel the degree of interphalangeal bone destruction. Involvement of the radiocarpal joint is characteristic of the arthropathy associated with calcium pyrophosphate deposition disease [24]. Its occurrence in one patient with erosive osteoarthnitis probably represents coexistence of two separate diseases. The arthritic process in EOA occasionally begins in a single interphalangeal joint. In such cases the diagnosis may be difficult, particularly if the inflammatory process is severe and the disease remains monarticular for a significant period. One of our patients underwent surgical exploration because of the presumptive diagnosis of infectious arthritis. There was no histologic on bacteriologic evidence of an infectious process, and histologic examination of biopsy material showed erosion of articular cartilage with focal areas of nonspecific granulation tissue. This patient subsequently developed similar arthritis of other lp joints (fig. 5), Malalignment of finger joints may be observed in both PA and EOA. In EOA these are usually restricted to mild flexion deformity and medial or lateral deviation, but in PA such malalignments may be severe and subluxation also occurs. Osteoarthnitis of the first metatarsophalangeal joint is well known in EOA. Valgus deformity, osteophyte formation, and bone remodeling are typical, but irregular destruction of the subchondral bone is not a feature. To our knowledge, interphalangeal arthritis of the small toes with irregular subchondral bone erosion has not been described in EOA. Well manginated calcaneal spurs, localized to the posterior attachment of the plantan aponeurosis, are commonly the result of mechanical stress. Although they were found in five cases of EOA, we do not feel that this is necessarily a feature of this condition, The significant observation with regard to the finding is that diffuse, poorly defined calcaneal bone apposition, so typical of PA was not observed in any of the EOA cases. Interphalangeal fusion of the fingers is commonly considened to be diagnostic of PA. Our findings indicate that this may be more common in EOA. Such fusion also occurred in the feet in PA (fig. 1 2), but not in EOA. (Fusion of the IP joints of the hands also occurs in adult rheumatoid arthritis, but it is uncommon [4].) In assembling these 84 cases for study, three possible sources of error were recognized. Patients with psoniatic arthritis in whom skin lesions had not yet appeared could have been mistakenly placed in the erosive osteoanthnitis (EOA) group. It is significant that the arthritis preceded the skin disease in 10% of our PA cases. Second, individuals with osteoarthritis could coincidentally have psoniatic skin lesions. This was suggested by the five PA patients who had carpal arthritis limited to the trapeziometacanpal on scaphotrapeziotrapezoidal joints. Third, some of these patients, particularly those with the rheumatoid factor, could have had coincidental rheumatoid arthritis. If such errors did occur, they were few and do not seem to have significantly altered the results. This is supported by the radiologic differences between these two groups of patients and the contrast between these features on the whole and those of rheumatoid arthritis [4-6]. In reviewing the medical records in these patients, it was of interest to note how often other rheumatic diseases were seriously considered in the differential diagnosis by the referring clinician. This was particularly true in those patients with psoniatic arthritis (PA) in whom the arthritis preceded the skin lesions and in erosive osteoanthnitis (EOA) patients in whom the inflammatory changes were particularly severe and protracted. Seronegative rheumatoid arthritis was commonly initially suggested as the diagnosis. Recognition of the nuances in the radiologic manifestations of these diseases will lessen the likelihood of such misdiagnoses. REFERENCES 1. Ehrlich GE. Inflammatory osteoarthnitis. II. The superimposition of rheumatoid arthritis. J Chronic Dis 1972;25: Wright V. Psoriatic arthritis. In: Scott TJ, ed. Copemans textbook of the rheumatic diseases, 5th ed, chap 21. Edinburgh: Churchill, Livingstone. 1978:

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