There is evidence that articular malunions resulting in considerable incongruities or instabilities lead to posttraumatic arthritis. However, evidence connecting impact injury to arthrosis is not entirely convincing. Several studies using various animal models have shown that articular impacts can lead to changes consistent with early arthritis, but none of these experiments has generated changes consistent with advanced clinical disease. In one study, Thompson et al29 impacted dog patella-femoral joints in vivo. The joints of these animals showed changes consistent with very early arthritis at 6 months, but when dogs undergoing similar impacts were evaluated at 1 year, the cartilage had stabilized.14
There has been substantial interest in the role of triggered chondrocyte death, or apoptosis, in osteoarthritis (OA).1,2,8,12 Studies of human osteoarthritic cartilage have shown a direct correlation between disease severity and the degree of chondrocyte apoptosis.2,8,12 Although it is not known whether chondrocyte apoptosis plays a primary or secondary role in the pathogenesis of OA, the finding is clearly important. Chondrocytes are the only cells available to maintain and repair the matrix, and it is the matrix that ultimately provides cartilage with its unique biomechanical properties. It has been known for some time that impact injury can impair cell viability.25 More recently, impact injury has been associated with chondrocyte apoptosis.4 Impact injury has been shown to cause apoptosis in humans and animals.3–6,14,21,22 These observations raise the possibility that chondrocyte apoptosis caused by impact injury could initiate a pathogenesis similar to that seen in OA.
The long-term effects of impact injury on articular cartilage are not known. Nevertheless, the observation that impact can cause chondrocyte apoptosis offers the best evidence for potential long-term problems. Therefore, in this paper we will examine apoptosis and impact injury.
Apoptosis and Cartilage Degeneration
Apoptosis describes a particular type of cell death best identified by a series of morphologic changes within dying cells. Kerr et al11 coined the term apoptosis in 1972 to describe a mechanism of controlled cell deletion. Kerr and his coworkers noted a pattern of cell death that occurred without inflammation in numerous normal and abnormal tissues. Most importantly, based on their observations, they suggested that apoptosis was an “active, inherently programed phenomenon” that could be initiated or inhibited by various environmental stimuli.11 They chose the term apoptosis, which is used in Greek to describe the falling of petals from a flower or leaves from a tree, to emphasize their belief that apoptosis was an essential process in the homeostasis of animal cell populations. Although the essential elements of apoptosis were identified in 1914 by the German anatomist Ludwig Gräper, his work remained largely unrecognized until Kerr’s group revisited the idea.11 In 1984, Wyllie et al33 recognized that the pattern of chromatin breakup that had been noted in the breakdown of irradiated lymphocytes could be used to identify apoptotic cells. With this identification of a marker for apoptosis, the research in this area grew.15 Finally, there was a proliferation of papers after the identification of several genes that govern apoptosis therefore generating the hope of intervening to prevent it.9
The morphologic features of apoptosis are similar across different cell types and species.9 First, there is cell shrinkage and dense chromatin condensation. The cell then breaks into membrane-bound, ultrastructurally well-preserved, fragments. These fragments, known as apoptotic bodies, are then either shed from epithelial lined surfaces or are taken up by other cells where they undergo autolysis (Fig 1).11 Apoptosis can be identified histomorphologically by the presence of apoptotic bodies, but this approach is laborious, and only reflects a late stage of the process. Several techniques have been developed to identify apoptotic cells at earlier stages. However, each technique has its advantages and disadvantages.1 One technique commonly used to look for apoptotic cells in cartilage is TdT-mediated dUTP nick end-labeling (TUNEL) staining. The TUNEL technique identifies DNA strand breaks that occur early in the apoptotic process. Unfortunately, the technique is prone to technical errors and if not carefully applied, can overestimate the number of apoptotic cells.1
Although the interest in apoptosis is relatively recent, there has been a long-standing assumption that cell death is a central feature in the pathogenesis of OA. As described by Sokoloff,26 essays by Axhausen and Pommer26 from 1911–1913, note the propensity for experimental animals to display cell death in articular cartilage spawned a generation of research into the primacy of cartilage necrosis in OA. There was particular interest in the changes in cell density that occurred with age and OA.18,19,30 Decreases in cell density were noted to accompany fibrillation, raising the possibility that cell loss could be responsible for OA.19,30 Meachim and Roy20 described the degradation of human articular chondrocytes into “cytomembrane remnants” well before the term apoptosis was coined. Moreover, investigators recognized the importance of chondrocyte degradation even before the mechanisms were understood.
Chondrocyte apoptosis has been identified at high rates in osteoarthritic and traumatically injured cartilage and most likely plays some role in the pathogenesis of joint degeneration. Blanco et al2 found a mean of 6% apoptotic cells in sections of osteoarthritic cartilage, whereas no apoptotic cells were identified in normal cartilage. Others have made similar observations, and rates of apoptosis as high as 22% have been reported.8 We found an apoptosis rate of 35% in cartilage samples harvested from patients with articular fractures compared with a 1% rate in normal cartilage harvested for mosaic plasty.22 Aigner and Kim1 suggested that these high apoptotic rates may be an artifact of TUNEL staining, but they do not discount the importance of identifying apoptotic cells. They reported that cell degradation occurs rapidly once apoptosis begins; therefore, an apoptosis rate as high as 12% would result in rapid cartilage degradation inconsistent with the clinical course of OA. However, Aigner and Kim1 also emphasized that the mere presence of apoptosis confirms that it plays some role in the pathogenesis of OA.
Chondrocyte apoptosis is important in the pathogenesis of joint degeneration regardless of whether it initiates the degeneration. Articular cartilage consists of a sparse population of chondrocytes and a large extracellular matrix (ECM). Chondrocytes comprise approximately 2% of the articular cartilage volume, but they are essential in mediating the synthesis, degradation, and turnover of the macromolecules that compose the biomechanically functional extracellular matrix.24 Cartilage degeneration results from failure of the ECM, and chondrocytes are responsible for maintaining this matrix. The loss of functioning chondrocytes to repair and maintain the matrix ultimately could weaken it, making it more susceptible to damage. However, damage to the matrix in fact, could cause the apoptosis.1,28 Still, the result would be the same: there would be less cells to maintain and repair the matrix.
Mechanical Injury and Apoptosis
It is difficult to determine whether articular impact causes apoptosis directly, by damaging chondrocytes, or secondarily, by damaging the matrix. The health of the matrix is dependent on the chondrocytes, and the chondrocytes are dependent on the matrix (Fig 2). Regardless, a vicious cycle could be established resulting in progressive cell death and matrix degradation. Numerous studies3–6,14,21,23 have shown that impact injury can result in chondrocyte apoptosis.
Numerous studies have shown a high rate of apoptosis after articular impacts.3–6,14,21,22 In one study, Borrelli et al3 impacted the rabbit femoral condyles with 60-MPa and 80-MPa stresses. The apoptosis rates for these impacts were 11% and 3%, respectively, whereas the rate in control specimens averaged 1%.3 Others have reported similar findings with apoptosis rates ranging from 15–40% depending on the magnitude and nature of the impact.4–6,14,21 Apoptotic cells mostly have been observed in the superficial tangential zone. With increasing impact magnitude, the depth at which apoptotic cells are located increases, but still dead cells primarily are located in the upper zones of the cartilage. Apoptotic cells are present regardless of visible damage to the cartilage structure.6,21 In cases where cracks form, apoptotic cells are seen along the edges of the cracks.3,6,14,21
It is difficult to draw conclusions regarding the relationship of impact magnitude and apoptosis. The rate of apoptosis increases as the impact stress increases.3,6,21 However, the degree of cartilage injury from a given impact is also greatly dependent on the loading rate, the shape of the impactor, and the loading conditions for the experiment. Additionally, for in vivo experiments, it is difficult to actually quantify the magnitude of the impacts.32 One very important factor seems to be whether the specimen with impacted cartilage is constrained or unconstrained. An unconstrained specimen is one that is able to deform radially under load. For example, a piece of cartilage laying on a flat surface would be considered unconstrained. A constrained specimen is prevented from expanding radially; a piece of cartilage loaded in a custom well would be unable to expand laterally. Cartilage impacted in vivo is constrained by the cartilage’s attachment to the bone. Unconstrained specimens consistently show greater damage and higher rates of apoptosis than constrained specimens.21
Milentijevic et al21 impacted bovine cartilage explants in confined compression. When the cartilage was loaded to 10 MPa, cell death was observed in the upper 2% of the specimen.21 When the impact was increased to 60 MPa, cell death was observed in the upper 6% of the specimen.21 None of the specimens showed gross damage. In contrast, another single-impact study using unconfined compression showed that cell death occurred throughout the specimens at 25 MPa.25 Tissue damage at these stresses was severe.25 The findings of Milentijevic et al are consistent with the findings of Borelli et al3 that in vivo impacts of 60 MPa did not cause visible structural damage, although they did result in apoptosis.
The experiments of Milentijevic et al21 suggest that chondrocytes are more likely to be damaged when impacts result in deformation of the matrix. Impacts that simply elevate the pressures experienced by the chondrocytes seem to be less damaging.6,21 When a specimen is impacted in an unconfined configuration, it can deform in all directions.25 Confined specimens can only deform in the axial direction, and then only as water leaks out.21 The decreases structural damage and lower rates of apoptosis seen in confined specimens and particularly in the lower layers of confined specimens suggests that matrix deformation is detrimental to chondrocytes.14,21,25
It is interesting to speculate how impact-induced apoptosis relates to clinical observations. If we assume that impact-induced apoptosis initiates a pathogenesis similar to that of OA, we would expect changes to occur slowly over a long time. This may explain why attempts to create animal models of impact-induced arthritis have succeeded in reproducing only very early disease.7,14,29 It may be necessary to allow the animals to live for an extended time to see the late changes we expect. In his large series of acetabular fractures, Letournel and Judet13 observed that joint degeneration occurred at two times. When the reduction was inadequate joint degeneration occurred within the first couple of years. Numerous patients had arthritis develop despite anatomic reduction, however the degeneration developed over several years. The initiating factor in the late development could have been apoptosis resulting from the initial impact. Martin and Buckwalter16 suggested that, as chondrocytes age, their metabolic activity degrades and they are less able to support the ECM. They offer this as an explanation for the observation that intraarticular fractures lead to a higher rate of posttraumatic arthritis in the elderly than in the young.10,13,17,27,31 This phenomenon also would make cartilage in the elderly more susceptible to impact-induced apoptosis.
The long-term effects of articular impact injury remain unknown. However, evidence showing the potential importance of chondrocyte apoptosis in OA and the observation that apoptosis results from articular impacts suggests a possible link between impact injury and late joint degeneration. Most exciting is the possibility that interventions can be devised to reduce or prevent apoptosis and therefore prevent posttraumatic arthritis.5
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