INTRA-ARTICULAR LOOSE BODIES R E G A R D E D AS O R G A N CULTURES I N V I V U
DR H. J. BARRIE East General and Orthopedic Hospital, Toronto, M4C 3E7, Canada
PLATESLXVIII-LXXI T H O U G Hthe literature on intra-articular loose bodies is so extensive that v. Schmieden (1900), was already apologising for adding to it, very little notice has been taken of the earliest changes in loose bodies after their detachment. Another aspect which is of interest in view of recent work on the behaviour of cartilage in culture (reviewed by Sokoloff, 1974) is that loose bodies can be regarded as organ cultures in vivo. They may be derived from any of the intra-articular structures, and origin from the bearing surface is shown by the presence of hyaline cartilage or cartilage and bone. Osteocytes do not survive detachment, but chondrocytes do, as may the mesenchymal cells of the marrow. This paper describes the sequence of changes which occur in loose bodies containing hyaline cartilage, and underlines the fact that hyaline cartilage is not as inert as has often been depicted. MATERIALS AND METHODS From current material and slides in our files, 100 loose bodies from the knee joint were studied microscopically under direct and polarized illumination. Of these, 35 consisted of detached osteoarthritic spurs or metaplastic synovium, four of fragments of menisci, and 56 of parts of the joint surface identified by the presence of hyaline cartilage. One was not classifiable. The 56 formed the subject of this study, and further sections from these paraffin blocks were stained, where necessary with phosphotungstic acid hematoxylin (P.T.A.H.), periodic acid-Schiff (P.A.S.), Alcian blue, and Gomori’s silver reticulin stain. The ones which were hard had been softened by nitric acid, or by formic acid and sodium citrate or, in the case of recent materia1, by R.D.O. Most of the sections were cut, or had been cut, at right angles to the old face of the cartilage. It was not possible to affix any specific time factor to the changes to be described, because of the well-known lack of correlation between the surgical findings, the length of symptoms, and any history of trauma.
RESULTS In ten cases, the material consisted of single or multiple small irregular flakes of cartilage which were often described by the surgeon as rising to the surface when the knee joint was being explored because of traumatic laceration of a meniscus. It is doubtful if these fragments would have become permanent loose bodies, but their microscopic appearance has relevance to the study. Received 19 July 1911; accepted 29 Nov. 1911 J. PATH.-VOL.
125 (1978)
163
0
164
Dr. H. J. BARRIE
Penetrating deeply into these fragments were numerous spindle-shaped or stellate cells, predominantly isometric. The appearance of some nuclei suggested mitotic activity, but much of the crowding of cells seemed to be due to loss of chondrin. The P.T.A.H. stain showed that the earliest changes showed some of the lines to be described later as " gullying " and illustrated in fig. 2. Changes in loose bodies proper Freshly detached loose bodies differed from the small flakes already described by their much larger size and possession of four surfaces, one of which represented the original surface of the cartilage, two being the sloping lines of fracture and one the base formed either of fractured cartilage or bone. The subsequent changes which transformed this roughly conical fragment into an ovoid body differed in degree according to the parts exposed. Changes at the face The original surface sometimes remained almost unaffected while growth was occurring on the other side, but usually a narrow band of fibroblastic transformation occurred (fig. I), caused by a combination of loss of chondrin, cell de-differentiation and cell division. When change was less superficial, it took the form of a remarkably selective loss of chondrin, resulting in lines of condensed collagenous stroma which radiated from cell to cell and gave the appearance of a gullied landscape (gullying) (fig. 2). This was brought out best by the P.T.A.H. stain which also showed that delicate ribbons of cytoplasm extended from the chondrocytes between the condensed fibres. Towards each pole of the loose body, the transformation of chondrocytes penetrated most deeply, sometimes forming fields of collagen in which islands of normal matrix were isolated like ice floes (fig. 3), sometimes progressing to the more diffuse fibrous involution described in the very small fragments. Changes at the sloping sides Superficial chondrocytes developed increased numbers of P.A.S. positive granules in their cytoplasm and apparently lysed the surrounding stroma, thus liberating themselves (fig. 4). They then became spindle-shaped. Cell proliferation followed, the P.A.S. positive granules disappeared, a collagenous stroma was formed and this underwent metaplasia into fibrocartilage so that the sides became covered and rounded. Changes at the base When the base was formed of cartilage, the changes resembled those at the sides. When bone formed the base, the deep mesenchymal cells of the marrow had died but some of the superficial ones had remained viable and the stroma immediately below the surface usually became transformed into a pale, ghostlike substance which filled the spaces between the osseous trabeculae like glue.
LOOSE BODIES
165
On the surface, there was cell proliferation leading to the formation of fibrocartilage. Changes in the centre of the loose bodies As the surfaces became coated with new growth, the centre of the loose bodies underwent complex changes, with varying combinations of cell hyperplasia, dechondrification, and unmasking of collagen preceding the onset of necrosis and calcification. At one end of the spectrum, but only found in three cases, were sharply defined areas containing stellate cells and a myxoid stroma (the Weichselbaum change, fig. 5) (Weichselbaum, 1877). More commonly, there was unmasking of collagen fibres (fig. 6). In places, an apparent increase of cells (fig. 7) was probably the result of loss of glycans as the cells gave no indication of proliferation. Combinations of cell hyperplasia and unmasking of fibre often produced a polka-dot pattern making it impossible to define the boundaries of the original hyaline cartilage because much of the newly formed cartilage had a similar structure. Further blurring of the edges of the hyaline cartilage was produced by slow replacement of normal chondrin by a very pale matrix. The indolence of this change made it impossible to determine whether chondrocytes were altering or replacing the original matrix. Another common change was basophilia round individual chondrones as if each was producing its own tide mark. Confluence of these lines led to a mosaic pattern which was not abolished by decalcifying agents. Secondary remodelling Loose bodies which had reached a haven such as a pre-patellar bursa, often had developed bizarre shapes resembling jacks. This was caused by cells resembling osteoclasts eroding bays which alternated with nodules of active growth (fig. 8). This remodelling was sometimes very marked and the nodules were nearly pinched off, suggesting an explanation for the genesis of sonie multiple loose bodies. The bays contained an oedematous connective tissue often resembling adipose tissue. Sometimes these bays were buried under new growth from their margins and the pattern became exceedingly complex. DISCUSSION
The changes just described show the versatility of hyaline cartilage lying free in a fluid medium and are paralleled by alterations of cartilage in situ as most of them were illustrated by Weichselbaum in his classic paper correlating senile changes in cartilage with osteoarthritis. Fibrous involution of chondrocytes has been described for over 100years in many pathologic conditions and in this study was seen in its most active stage in the small, free-floating fragments. This seemed to be an accentuation of the lesser changes so commonly seen in surgical scrapings from chondromalacia. If very small fragments of detached cartilage are not taken up immediately by synovium, they probably become completely involuted while free-floating. If it were not so, loose bodies would be commoner and more often multiple.
166
Dr. H . J. BARRIE
Various suggestions have been made to explain loss or denaturing of the matrix of cartilage apart from simple leaching of the soluble chondrin. Lysosomes are abundant in chondrocytes (Howell, 1975) and it is possible that they may become activated. Proteases have been identified (Howell). Hyaluronidase has been identified in synovial fluid and may gain access to cartilage following rupture of the surface (Bollet, 1967). Observation of loose bodies suggests that multiple factors may be involved. Initial limitation of the loss to a narrow peripheral band suggested leaching or the action of some factor in the synovial fluid such as hyaluronidase. (In the tiny fragments, the whole flake would be affected.) Where loss of matrix was lacunar, it seemed to be mediated by the chondrocytes themselves (fig. 4). The remarkable appearance of “ gullying ” (fig. 2) again argued strongly for a dominant activity of the chondrocytes, as long cell processes were demonstrable among the condensed collagen fibres. Disruption of the collagen fibres had obviously occurred to allow the calving of the “floes”. The only enzymes said to be capable of splitting collagen at a physiological p1-I are collagenases, and these have been found in pathologic joints (Lane and Weiss, 1975). The action of commercial collagenases on cartilage has been found to be delayed in proportion to the content of polysaccharides left (Hamerman, 1969). The appearance of “ gullying” was well illustrated by Weichselbaum as occurring in senile cartilage but he did not discuss its pathogenesis. The somewhat similar descriptive terms “matrix streaks ” (Lothe, Spycher and Ruttner, 1973) and “ ravines ” (Meachim and Fergie, 1975) refer to quite different changes than those described here as ‘‘gullying”.
Cell proliferation There is no doubt that the chondrocytes gave rise to the proliferating cells which were such a marked feature of the fractured surfaces. There is an interesting relationship between cartilage cells and stroma in tissue culture in vitro. Chondrocytes cultured in mono-layer proliferate in stellate form, but in organ culture they show little proliferation and can form matrix (Chacko, et al., 1969; Sokoloff, 1974). So it was with the loose bodies. Peripheral cells proliferated, but it appeared that they had to free themselves from the inhibiting effect of stroma before this could occur. As de-differentiated cells, they were able to branch out anew into collagen production and then synthesis of matrix. The so-called “ Weichselbaum change ” calls for some comment. This is a sharply defined area of myxoid stroma and stellate fibroblasts. Weichselbaum described it as part of an absorptive process at the borders of ageing cartilage and he postulated that the joint surface could expand or contract according to functional requirements. The usual absorption, or fibrous involution seen at the borders of cartilage is much less florid than that pictured by Weichselbaum. It can be seen from his other illustrations that his sections were much thicker than the ones in use today and this exaggerated their cellularity. In our material, the Weichselbaum change was found when remodelling
PLATE LXVIII
BARKIE
LOOSEBODIES
FIG. 1 .-Fibrous
involution and cell proliferation localised to surface of loose body. PAS. x 480.
FIG.2.--Gullying of matrix. PTAH. x 360.
PLATELXlX
BARRIE LOOSEBODIES
FIG.3.-Break-up
FIG.4.-Lacunar
of matrix into floes. PTAH.
X
144.
lysis of matrix at sides of loose body.
PLATELXX
BAnRIri LOOSEBODIES
FIG.5.-"
FIG. 6.-Unmasking
Weichselbauni
"
change in new-formed cartilage. PTAH. x 180.
of collagen fibres (Zerfaserung) in deep part of loose body. PAS. ~ 4 8 0 .
PLATELXXI
BARRIE
LOOSEBODIES
FIG.7.-Apparent
FIG.8.-Osteoclastic
loss of glycans without cell proliferation. PAS.
remodelling at periphery. Haemolysin and eosin.
x 190.
x 120.
LOOSE BODIES
167
was occurring and one origin for it may be explained as follows. When rigid or semi-rigid structures such as bone, cartilage, or calcified cartilage are eroded, the cavity formed cannot collapse, and, in the absence of any other cell proliferation, is filled by fibroblasts and myxoid stroma. When artificially promoted cavities are prevented from collapse, the same thing occurs and the myxoid tissue is ultimately replaced by fat (Pochin, 1949). In loose bodies, the tissue resembling fat (Strangeways, 1920) is a sequel to osteoclastic absorption at the surface. The deeper Weichselbaum change probably results from a process of giant lacunar absorption. The presence of osteoclastic erosion shows that it is the chondrocytes which control their own density according to functional requirements. Signs of remodelling were most marked in the loose bodies which were described by the surgeon as being sequestered in synovial pouches. It is probable that, as long as the loose bodies were mobile and subject to pressure and friction, the predominant feature was growth of fibrocartilage and the maintenance of bearing surfaces. When sequestered, there was differentiation into osteoclasts and erosion of the surfaces. This transformation has parallels in fixed tissues. Kalayjian and Cooper (1972) found that chondrocytes trapped in the epiphysis could take the appearance of osteoclasts. Sledge (1968) found that exposure of the upper surface of the epiphyseal plate to high concentrations of oxygen led to the production of a marrow cavity complete with osteoclasts even though no vascularisation had occurred. In the deep part of the cartilage, the patchy loss of chondrin, the “zerfaserung” and the development of a basophil mosaic occurred after the loose bodies had developed a shell of new growth. The degenerative changes were not surprising as chondrocytes only remain viable for a distance of 3 mm from the surface (Bywaters, 1937). In their staining reactions, the mosaic lines resembled the tide mark. Fawns and Landells (1953) regarded this as marking the cartilage most recently calcified by diffusion of calcium ions from the bone. The development of mosaic lines in loose bodies when chondrocytes are degenerating makes it more likely that calcium ions can diffuse from the synovial fluid, and that one of the normal functions of the chondrocyte is to prevent their union with chondroitin sulphate. The relationship between metachromasia and calcification in the peri-cellular zone is discussed by Rubin and Howard (1950). Loose bodies derived from menisci show very little growth, probably because the cells in them are locked in more securely than in hyaline cartilage and thus the initial freeing does not take place. In some species of skate, loose bodies formed of fibrocartilage are a normal component of a complex mandibular joint (Davies, 1948). The ability of hyaline cartilage to transform itself, when exposed to unusual stimuli, must have important applications to a wide range of pathologic conditions, including osteoarthritis. The replacement of hyaline cartilage by pannus in rheumatoid arthritis is usually ascribed to growth of fibrous tissue over the surface, but fibrous change precedes the advance of the capillary net as Hamerman, Barland and Janis (1969) have already pointed out. In their
168
Dr. H . J. BARRZE
paper, the illustration of rheumatoid cartilage (fig. 11, p. 294) might well have been used for this paper on loose bodies. The question of the response of hyaline cartilage to direct trauma is too complex to discuss here, but loose bodies are witness to the fact that when the appropriate stimuli are present, chondrocytes can free themselves from chondrin and show great versatility. SUMMARY
A study of the changes which occur in the intra-articular loose bodies which contain hyaline cartilage, showed the chondrocytes to be very versatile. To effect the transformation of a fractured portion of the articular surface from an angular to a rounded body, peripheral chondrocytes lyse the surrounding glycans and revert to fibroblasts before proliferating and forming fibrocartilage. The central chondrocytes show a wide variety of changes which may mask the original structure of the hyaline cartilage. Remodelling can occur by the formation of osteoclasts. Analogies are drawn between the behaviour of chondrocytes in culture and in some pathologic processes. I wish to thank Mrs C. Hughes and staff for preparing the histological material.
REFERENCES BOLLET,A. J. 1967. Connective tissue polysaccharide metabolism and the pathogenesis of osteoarthritis. A h . Int. Med., 13,33-60. BYWATERS, E. G. L. 1937. The metabolism of joint tissues. J. Path. Bact., 44,247-268. CHACKO,S . , ABBOTT,J., HOLTZER,S., AND HOLTZER,E. 1969. The loss of phenotypic traits by differentiated cells: VT. Behaviour of the progeny of a single chondrocyte. J. Exp. Med., 130, 417-442. DAVIES, D. V. 1948. The synovialjoints of the skate (raia). J . Anat., 82,9-20. FAWNS,H. T., AND LANDELLS, J. W. 1953. Histochemical studies of rheumatic conditions: I. Observations on the fine structures of the matrix of normal bone and cartilage. Ann. Rheum. Dis., 12, 105-113. HAMERMAN, D. 1969. Cartilage changes in the rheumatoid joint. Clin. Orthop. and Rel. Res., 64, 91-97. HAMERMAN, D., BARLAND, P., AND JANIS,R. 1969. The structure and chemistry of the synovial membrane in health and disease. Zn The biological basis of medicine, edited by E. Bittar and R. Bittar, Academic Press, London, vol. 1, p.269. HOWELL, D. S. 1975. Degradative enzymes in osteoarthritic human articular cartilage. Arthr. and Rheum., 18,167-177. KALAYJIAN, D. B., AND COOPER,R. R. 1972. Osteogenesis of the epiphysis: a li&t and electron microscopic study. Clin. Orthop. and Rel. Res., 85, 242-256. LACK,C. H. 1959. Chondrolysis in Arthritis. J. of Bone and Joint Surg., 41B, 384-387. LANE,J. M., AND WEISS,C. 1975. Review of articular cartilage collagen research. Arthr. and Rheum., 18, 553-562. LOTHE,K., SPYCHER, M. A., AND RUTTNER,J. R. 1973. “Matrix Streaks”; A peculiar pattern in the cartilage of the femoral head of ageing human subjects. J. of Bone and Joint Surg., 55B, 581-587. MEACHIM,G.,AND FERGIE, I. A. 1975. Morphological patterns of articular cartilage fibrillation. J. Path., 115, 231-240. POCHIN,E. E. 1949. Local deposition of adipose tissue, experimentally induced. Clin. S C ~ 78, . , 97-107.
LOOSE BODIES
169
RUBIN,P. S., AND HOWARD, J. E. 1950. Histochernical studies on the role of acid rnucopolysaccharides in calcifiability and calcification. In Metabolic Interrelations: Transactions of the Second Conference, p. 155, edited by E. C. Reifenstein, Jr., New York: Josiah Macy, Jr., Foundation. SCHMIEDEN, V. 1900. Ein Beitrag zur Lehre von den Gelenkmaiisen. Arch. f. Klinische Chirurgie, 62, 542-579. SLEDGE,C. B. 1968. Biochemical events in the epiphyseal plate and their physiological control. Clin. Orthop. and Rel. Res., 61, 37-47. SOKOLOFF, L. 1974. Cell biopsy and the repair of articular cartilage. J. Rheumatol., 1, 9-16. STRANGEWAYS, T. S. P. 1920. Observations on the nutrition of articular cartilage. Brit. Med. J., 1, 661-663. WEICHSELBAUM, A. 1877. Die senilen Veranderungen der Gelenke u. deren Zusamrnenhang mit der Arthritis deformans. Akad. d. Wissenschaften. Sitzungsberichte der Mathematish-NaturwissenschaftlichenClasse, 75-76, Abtheilung, 3, 193-243.
DR H. J. BARRIE East General and Orthopedic Hospital, Toronto, M4C 3E7, Canada
PLATESLXVIII-LXXI T H O U G Hthe literature on intra-articular loose bodies is so extensive that v. Schmieden (1900), was already apologising for adding to it, very little notice has been taken of the earliest changes in loose bodies after their detachment. Another aspect which is of interest in view of recent work on the behaviour of cartilage in culture (reviewed by Sokoloff, 1974) is that loose bodies can be regarded as organ cultures in vivo. They may be derived from any of the intra-articular structures, and origin from the bearing surface is shown by the presence of hyaline cartilage or cartilage and bone. Osteocytes do not survive detachment, but chondrocytes do, as may the mesenchymal cells of the marrow. This paper describes the sequence of changes which occur in loose bodies containing hyaline cartilage, and underlines the fact that hyaline cartilage is not as inert as has often been depicted. MATERIALS AND METHODS From current material and slides in our files, 100 loose bodies from the knee joint were studied microscopically under direct and polarized illumination. Of these, 35 consisted of detached osteoarthritic spurs or metaplastic synovium, four of fragments of menisci, and 56 of parts of the joint surface identified by the presence of hyaline cartilage. One was not classifiable. The 56 formed the subject of this study, and further sections from these paraffin blocks were stained, where necessary with phosphotungstic acid hematoxylin (P.T.A.H.), periodic acid-Schiff (P.A.S.), Alcian blue, and Gomori’s silver reticulin stain. The ones which were hard had been softened by nitric acid, or by formic acid and sodium citrate or, in the case of recent materia1, by R.D.O. Most of the sections were cut, or had been cut, at right angles to the old face of the cartilage. It was not possible to affix any specific time factor to the changes to be described, because of the well-known lack of correlation between the surgical findings, the length of symptoms, and any history of trauma.
RESULTS In ten cases, the material consisted of single or multiple small irregular flakes of cartilage which were often described by the surgeon as rising to the surface when the knee joint was being explored because of traumatic laceration of a meniscus. It is doubtful if these fragments would have become permanent loose bodies, but their microscopic appearance has relevance to the study. Received 19 July 1911; accepted 29 Nov. 1911 J. PATH.-VOL.
125 (1978)
163
0
164
Dr. H. J. BARRIE
Penetrating deeply into these fragments were numerous spindle-shaped or stellate cells, predominantly isometric. The appearance of some nuclei suggested mitotic activity, but much of the crowding of cells seemed to be due to loss of chondrin. The P.T.A.H. stain showed that the earliest changes showed some of the lines to be described later as " gullying " and illustrated in fig. 2. Changes in loose bodies proper Freshly detached loose bodies differed from the small flakes already described by their much larger size and possession of four surfaces, one of which represented the original surface of the cartilage, two being the sloping lines of fracture and one the base formed either of fractured cartilage or bone. The subsequent changes which transformed this roughly conical fragment into an ovoid body differed in degree according to the parts exposed. Changes at the face The original surface sometimes remained almost unaffected while growth was occurring on the other side, but usually a narrow band of fibroblastic transformation occurred (fig. I), caused by a combination of loss of chondrin, cell de-differentiation and cell division. When change was less superficial, it took the form of a remarkably selective loss of chondrin, resulting in lines of condensed collagenous stroma which radiated from cell to cell and gave the appearance of a gullied landscape (gullying) (fig. 2). This was brought out best by the P.T.A.H. stain which also showed that delicate ribbons of cytoplasm extended from the chondrocytes between the condensed fibres. Towards each pole of the loose body, the transformation of chondrocytes penetrated most deeply, sometimes forming fields of collagen in which islands of normal matrix were isolated like ice floes (fig. 3), sometimes progressing to the more diffuse fibrous involution described in the very small fragments. Changes at the sloping sides Superficial chondrocytes developed increased numbers of P.A.S. positive granules in their cytoplasm and apparently lysed the surrounding stroma, thus liberating themselves (fig. 4). They then became spindle-shaped. Cell proliferation followed, the P.A.S. positive granules disappeared, a collagenous stroma was formed and this underwent metaplasia into fibrocartilage so that the sides became covered and rounded. Changes at the base When the base was formed of cartilage, the changes resembled those at the sides. When bone formed the base, the deep mesenchymal cells of the marrow had died but some of the superficial ones had remained viable and the stroma immediately below the surface usually became transformed into a pale, ghostlike substance which filled the spaces between the osseous trabeculae like glue.
LOOSE BODIES
165
On the surface, there was cell proliferation leading to the formation of fibrocartilage. Changes in the centre of the loose bodies As the surfaces became coated with new growth, the centre of the loose bodies underwent complex changes, with varying combinations of cell hyperplasia, dechondrification, and unmasking of collagen preceding the onset of necrosis and calcification. At one end of the spectrum, but only found in three cases, were sharply defined areas containing stellate cells and a myxoid stroma (the Weichselbaum change, fig. 5) (Weichselbaum, 1877). More commonly, there was unmasking of collagen fibres (fig. 6). In places, an apparent increase of cells (fig. 7) was probably the result of loss of glycans as the cells gave no indication of proliferation. Combinations of cell hyperplasia and unmasking of fibre often produced a polka-dot pattern making it impossible to define the boundaries of the original hyaline cartilage because much of the newly formed cartilage had a similar structure. Further blurring of the edges of the hyaline cartilage was produced by slow replacement of normal chondrin by a very pale matrix. The indolence of this change made it impossible to determine whether chondrocytes were altering or replacing the original matrix. Another common change was basophilia round individual chondrones as if each was producing its own tide mark. Confluence of these lines led to a mosaic pattern which was not abolished by decalcifying agents. Secondary remodelling Loose bodies which had reached a haven such as a pre-patellar bursa, often had developed bizarre shapes resembling jacks. This was caused by cells resembling osteoclasts eroding bays which alternated with nodules of active growth (fig. 8). This remodelling was sometimes very marked and the nodules were nearly pinched off, suggesting an explanation for the genesis of sonie multiple loose bodies. The bays contained an oedematous connective tissue often resembling adipose tissue. Sometimes these bays were buried under new growth from their margins and the pattern became exceedingly complex. DISCUSSION
The changes just described show the versatility of hyaline cartilage lying free in a fluid medium and are paralleled by alterations of cartilage in situ as most of them were illustrated by Weichselbaum in his classic paper correlating senile changes in cartilage with osteoarthritis. Fibrous involution of chondrocytes has been described for over 100years in many pathologic conditions and in this study was seen in its most active stage in the small, free-floating fragments. This seemed to be an accentuation of the lesser changes so commonly seen in surgical scrapings from chondromalacia. If very small fragments of detached cartilage are not taken up immediately by synovium, they probably become completely involuted while free-floating. If it were not so, loose bodies would be commoner and more often multiple.
166
Dr. H . J. BARRIE
Various suggestions have been made to explain loss or denaturing of the matrix of cartilage apart from simple leaching of the soluble chondrin. Lysosomes are abundant in chondrocytes (Howell, 1975) and it is possible that they may become activated. Proteases have been identified (Howell). Hyaluronidase has been identified in synovial fluid and may gain access to cartilage following rupture of the surface (Bollet, 1967). Observation of loose bodies suggests that multiple factors may be involved. Initial limitation of the loss to a narrow peripheral band suggested leaching or the action of some factor in the synovial fluid such as hyaluronidase. (In the tiny fragments, the whole flake would be affected.) Where loss of matrix was lacunar, it seemed to be mediated by the chondrocytes themselves (fig. 4). The remarkable appearance of “ gullying ” (fig. 2) again argued strongly for a dominant activity of the chondrocytes, as long cell processes were demonstrable among the condensed collagen fibres. Disruption of the collagen fibres had obviously occurred to allow the calving of the “floes”. The only enzymes said to be capable of splitting collagen at a physiological p1-I are collagenases, and these have been found in pathologic joints (Lane and Weiss, 1975). The action of commercial collagenases on cartilage has been found to be delayed in proportion to the content of polysaccharides left (Hamerman, 1969). The appearance of “ gullying” was well illustrated by Weichselbaum as occurring in senile cartilage but he did not discuss its pathogenesis. The somewhat similar descriptive terms “matrix streaks ” (Lothe, Spycher and Ruttner, 1973) and “ ravines ” (Meachim and Fergie, 1975) refer to quite different changes than those described here as ‘‘gullying”.
Cell proliferation There is no doubt that the chondrocytes gave rise to the proliferating cells which were such a marked feature of the fractured surfaces. There is an interesting relationship between cartilage cells and stroma in tissue culture in vitro. Chondrocytes cultured in mono-layer proliferate in stellate form, but in organ culture they show little proliferation and can form matrix (Chacko, et al., 1969; Sokoloff, 1974). So it was with the loose bodies. Peripheral cells proliferated, but it appeared that they had to free themselves from the inhibiting effect of stroma before this could occur. As de-differentiated cells, they were able to branch out anew into collagen production and then synthesis of matrix. The so-called “ Weichselbaum change ” calls for some comment. This is a sharply defined area of myxoid stroma and stellate fibroblasts. Weichselbaum described it as part of an absorptive process at the borders of ageing cartilage and he postulated that the joint surface could expand or contract according to functional requirements. The usual absorption, or fibrous involution seen at the borders of cartilage is much less florid than that pictured by Weichselbaum. It can be seen from his other illustrations that his sections were much thicker than the ones in use today and this exaggerated their cellularity. In our material, the Weichselbaum change was found when remodelling
PLATE LXVIII
BARKIE
LOOSEBODIES
FIG. 1 .-Fibrous
involution and cell proliferation localised to surface of loose body. PAS. x 480.
FIG.2.--Gullying of matrix. PTAH. x 360.
PLATELXlX
BARRIE LOOSEBODIES
FIG.3.-Break-up
FIG.4.-Lacunar
of matrix into floes. PTAH.
X
144.
lysis of matrix at sides of loose body.
PLATELXX
BAnRIri LOOSEBODIES
FIG.5.-"
FIG. 6.-Unmasking
Weichselbauni
"
change in new-formed cartilage. PTAH. x 180.
of collagen fibres (Zerfaserung) in deep part of loose body. PAS. ~ 4 8 0 .
PLATELXXI
BARRIE
LOOSEBODIES
FIG.7.-Apparent
FIG.8.-Osteoclastic
loss of glycans without cell proliferation. PAS.
remodelling at periphery. Haemolysin and eosin.
x 190.
x 120.
LOOSE BODIES
167
was occurring and one origin for it may be explained as follows. When rigid or semi-rigid structures such as bone, cartilage, or calcified cartilage are eroded, the cavity formed cannot collapse, and, in the absence of any other cell proliferation, is filled by fibroblasts and myxoid stroma. When artificially promoted cavities are prevented from collapse, the same thing occurs and the myxoid tissue is ultimately replaced by fat (Pochin, 1949). In loose bodies, the tissue resembling fat (Strangeways, 1920) is a sequel to osteoclastic absorption at the surface. The deeper Weichselbaum change probably results from a process of giant lacunar absorption. The presence of osteoclastic erosion shows that it is the chondrocytes which control their own density according to functional requirements. Signs of remodelling were most marked in the loose bodies which were described by the surgeon as being sequestered in synovial pouches. It is probable that, as long as the loose bodies were mobile and subject to pressure and friction, the predominant feature was growth of fibrocartilage and the maintenance of bearing surfaces. When sequestered, there was differentiation into osteoclasts and erosion of the surfaces. This transformation has parallels in fixed tissues. Kalayjian and Cooper (1972) found that chondrocytes trapped in the epiphysis could take the appearance of osteoclasts. Sledge (1968) found that exposure of the upper surface of the epiphyseal plate to high concentrations of oxygen led to the production of a marrow cavity complete with osteoclasts even though no vascularisation had occurred. In the deep part of the cartilage, the patchy loss of chondrin, the “zerfaserung” and the development of a basophil mosaic occurred after the loose bodies had developed a shell of new growth. The degenerative changes were not surprising as chondrocytes only remain viable for a distance of 3 mm from the surface (Bywaters, 1937). In their staining reactions, the mosaic lines resembled the tide mark. Fawns and Landells (1953) regarded this as marking the cartilage most recently calcified by diffusion of calcium ions from the bone. The development of mosaic lines in loose bodies when chondrocytes are degenerating makes it more likely that calcium ions can diffuse from the synovial fluid, and that one of the normal functions of the chondrocyte is to prevent their union with chondroitin sulphate. The relationship between metachromasia and calcification in the peri-cellular zone is discussed by Rubin and Howard (1950). Loose bodies derived from menisci show very little growth, probably because the cells in them are locked in more securely than in hyaline cartilage and thus the initial freeing does not take place. In some species of skate, loose bodies formed of fibrocartilage are a normal component of a complex mandibular joint (Davies, 1948). The ability of hyaline cartilage to transform itself, when exposed to unusual stimuli, must have important applications to a wide range of pathologic conditions, including osteoarthritis. The replacement of hyaline cartilage by pannus in rheumatoid arthritis is usually ascribed to growth of fibrous tissue over the surface, but fibrous change precedes the advance of the capillary net as Hamerman, Barland and Janis (1969) have already pointed out. In their
168
Dr. H . J. BARRZE
paper, the illustration of rheumatoid cartilage (fig. 11, p. 294) might well have been used for this paper on loose bodies. The question of the response of hyaline cartilage to direct trauma is too complex to discuss here, but loose bodies are witness to the fact that when the appropriate stimuli are present, chondrocytes can free themselves from chondrin and show great versatility. SUMMARY
A study of the changes which occur in the intra-articular loose bodies which contain hyaline cartilage, showed the chondrocytes to be very versatile. To effect the transformation of a fractured portion of the articular surface from an angular to a rounded body, peripheral chondrocytes lyse the surrounding glycans and revert to fibroblasts before proliferating and forming fibrocartilage. The central chondrocytes show a wide variety of changes which may mask the original structure of the hyaline cartilage. Remodelling can occur by the formation of osteoclasts. Analogies are drawn between the behaviour of chondrocytes in culture and in some pathologic processes. I wish to thank Mrs C. Hughes and staff for preparing the histological material.
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