Cranial Cruciate Ligament Pathophysiology in Dogs With Cruciate Disease: A Review
Cruciate disease is a common cause of chronic lameness in dogs. Midsubstance rupture of the cranial cruciate ligament (CCL) arises from progressive pathological failure, often under conditions of normal loading in adult dogs with CCL instability. A high risk of rupture is associated with inflammation of the synovium and adaptive or degenerative changes in the cells and matrix of the CCL. In contrast, CCL rupture in puppies is usually associated with traumatic injury and avulsion of the CCL from its sites of attachment.
Introduction
Cruciate disease is one of the most important orthopedic conditions of the dog and usually leads to rupture of the cranial cruciate ligament (CCL). Midsubstance rupture of the CCL develops from progressive pathological fatigue, often under conditions of normal loading in adult dogs. Cruciate disease is particularly common in large- and giant-breed dogs, such as the Labrador retriever, rottweiler, and Saint Bernard; however, any breed of dog may be affected.1 Cruciate disease is a common and crippling problem in dogs of all ages.1 It leads to progressive osteoarthritis and deterioration of limb function over time, even with surgical treatment.12 The pathological mechanisms for CCL rupture are poorly understood. The purpose of this article is to review the current knowledge of the pathophysiology of this important condition in the dog.
Anatomy and Cell Biology of the CCL
The CCL is a complex structure consisting of extracellular matrix maintained by a diverse population of cells. The CCL attaches proximally to the caudal lateral part of the intercondyloid fossa of the femur and runs diagonally in a cranial, medial, and distal direction across the intercondyloid fossa. It attaches distally in the cranial intercondyloid area of the tibial plateau [Figure 1].3 The caudal cruciate ligament crosses the CCL medially.3 Collagen fibrils within the CCL are organized into bundles or fascicles. As the CCL courses from proximal to distal, the cranial medial fascicles are rotated into an outward spiral, and in flexion these fascicles bend around the caudal lateral fascicles.3 The caudal lateral band forms the bulk of the CCL. The CCL has a core region, which is its major axial tissue component, and an epiligamentous region, composed of the synovial intima and underlying loose connective tissue. The latter tissues cover the CCL except where the CCL wraps around the caudal cruciate ligament.4 The CCL probably consists of a continuum of collagenous fascicles that is functionally complex, with multiple portions that experience differing strains and load distribution during locomotion.5 It is an oversimplification to classify CCL fascicles into the smaller cranial medial band (which is taut in flexion and extension and inserts at the cranial medial aspect of the tibial attachment) and the larger caudal lateral band (which is taut only in extension and inserts at the caudal lateral aspect of the tibial attachment).6
The collagen fibrils within the CCL fascicles are principally composed of type I collagen and a smaller amount of type III collagen.7 The fibrils have a hierarchical organization and are made up of subfibrils and microfibrils.7 Normal CCL collagen fibrils have a recurrent undulating wave or crimped structure [Figures 2A–2D].4 During tensile loading of the ligament’s collagen fibrils, the crimp is lost before the fibril ruptures. Crimping is a distinct structural feature of organized collagen fibrils in dense connective tissue and is an important determinant of tissue biomechanical properties.8 The CCL has a relatively tenuous microvasculature. Blood supply arises predominantly from the periligamentous tissue, as opposed to the proximal and distal attachment sites.910 The innervation of the CCL regulates vasomotor tone and proprioception. Cranial cruciate ligament innervation is thought to be derived from branches of the saphenous, common peroneal, and tibial nerves. The proximal region of the CCL contains the greatest number density of mechanoreceptors.11
The predominant cell type in the CCL is the fibroblast. Three different types of fibroblasts have been described: fusiform or spindle-shaped, ovoid, and spheroid.4 It is presently unclear whether these cells represent differing metabolic states of the same cells or whether they are distinctly different fibroblasts. The cytoplasm of fusiform fibroblasts is intimately attached to the extracellular collagen and follows the crimped waveform of the fibrils.12 Different types of fibroblast occur in different regions of the human anterior cruciate ligament.12 In the zone adjacent to the distal attachment of the anterior cruciate ligament in humans, the cells are spheroid and may be located within lacunae or within and among fascicles.12 They are also more commonly positive for α-smooth muscle actin iso-form, which is a marker of myofibroblasts.12 It is not known whether a similar regional distribution of fibroblasts exists in the dog or whether canine CCL fibroblasts express α-smooth muscle actin.
Histopathological Changes in a Ruptured CCL
During progressive disruption of the CCL, distinct tissue repair responses arise in the epiligamentous and core regions of the ligament.4 A bridging scar does not form in the rupture site.4 Distinct phases of tissue repair, including an inflammatory phase, an epiligamentous repair phase, a proliferative phase, and a remodeling phase, occur after rupture of the anterior cruciate ligament in humans.13 Whether similar phases exist in the dog is not known.
Expansion of the volume of the epiligamentous tissue does occur in the dog during a repair phase that lasts many weeks.413 Eventually, synovial tissue covers the ruptured ends of the CCL.413 In a ruptured human anterior cruciate ligament, many cells in the synovial layer express the gene for the contractile isoform α-smooth muscle actin, and therefore differentiate into myofibroblasts.13 Production of actin may play a role in the retraction of the torn ends of the CCL and the lack of bridging scar formation. During the epiligamentous repair phase, a metabolic response to tissue hypoxia is seen in the ruptured canine CCL.14 Cells positive for hypoxia-inducible factor-1α(HIF-1α) are seen in the epiligamentous tissue.14 The nuclear transcription factor HIF-1 is a major regulator of the hypoxia response, and it controls the expression of erythropoietin, as well as the expression of genes that mediate metabolic functions such as glucose transport and angiogenesis.15 Hypoxia-inducible factor plays an important role in cell survival in tissues affected by interruption of blood flow.16 During hypoxia, HIF expression may have different tissue-specific effects, may promote cell survival, or may lead to an up-regulation of apoptosis.1617
Loss of fibroblasts occurs from the core region of a ruptured CCL, whereas in the surrounding epiligamentous region, the number of fibroblasts is similar to an intact CCL.4 Increased numbers of devitalized lactate dehydrogenase-negative fibroblasts are also seen in ruptured CCLs.14 The number of apoptotic cells are similar in dogs with either ruptured or intact CCLs.14 In canine ruptured CCL, the number of typical fibroblasts (i.e., fusiform and ovoid cells) are decreased, and the numbers of cells exhibiting chondroid transformation (i.e., spheroid cells) are increased.418 These cellular changes are associated with extensive disruption of the ligamentous matrix. Decreased birefringence and elongation of the crimping in the remaining collagen fibrils occur [Figures 2A–2D], suggestive of progressive mechanical overload.414 These adaptive or degenerative changes are not closely correlated with age, sex, body weight, or duration of lameness in the dog.414 The cellular and matrix changes that develop in ruptured CCL may result from the effects of remodeling and adaptation to various factors, such as joint inflammation, mechanical loading, ligament microinjury, and ischemia.
Variants of CCL Rupture
Traumatic Injury to the CCL
Rupture of the CCL associated with traumatic injury usually involves a multiple ligamentous injury of the stifle and leads to joint luxation.19 Injury to both the CCL and the caudal cruciate ligament, as well as the medial collateral ligament, is common.19 Isolated traumatic injury to the CCL is most often seen in puppies and is typically associated with avulsion of the ligament at the tibial attachment site distally.20 Such injuries are usually unilateral.20 In contrast, most pathological ligament ruptures that occur in more mature dogs are midsubstance tears, and bilateral cruciate disease is often seen in adult dogs.21
Early Cruciate Disease and Mild CCL Disruption
Dogs with early cruciate disease may have little or no palpable instability of the stifle with only mild disruption of the CCL, and they are often presented with lameness, effusion of the stifle joint, and mild osteoarthritis.22 Radiography can distinguish dogs with early cruciate disease from cases of late or end-stage cruciate disease. Mild osteoarthritis is present radiographically in dogs with early disease, whereas in dogs with late disease, osteoarthritis is severe or end-stage. Dogs with end-stage cruciate disease and complete disruption of the CCL may have little palpable stifle instability because of periarticular fibrosis. Because most of the CCL fascicles must be biomechanically disrupted for joint instability to be detected clinically, only dogs with a palpably stable stifle should be diagnosed as having a partial cruciate rupture.3
Risk Factors for CCL Rupture
The risk of CCL rupture is higher in certain breeds, such as the rottweiler, Labrador and Chesapeake Bay retriever, Newfoundland, Akita, Neapolitan mastiff, Saint Bernard, and Staffordshire bull terrier.2324 Dog phenotype may have a significant effect on the structural properties of the CCL, as the CCL of certain breeds (e.g., rottweiler) appears more vulnerable to mechanical overload.25 Although anatomical differences in the shape of the proximal tibia have been documented in dogs with cruciate disease, many dogs with a steep tibial plateau angle do not develop cruciate disease, and the functional tibial plateau angle is parallel to the ground in most dogs.2627 Neutering also increases the risk for cruciate disease, particularly in female dogs, although the cause of this effect is not understood.2324 Larger dogs weighing >22 kg are at greater risk of cruciate disease and tend to develop CCL rupture at younger ages.2324 It remains unclear whether obesity is also a risk factor for the disease independent of the dog’s size.
The prevalence of CCL rupture increases with age and reaches a peak incidence at 7 to 10 years.23 Cranial cruciate ligament structural properties are affected by age. Tensile stiffness decreases with age, particularly in dogs weighing >15 kg.28 Weakening of the CCL with age is associated with adaptive or degenerative changes within the tissues of the ligament, and it includes loss of fibroblasts, metaplasia of surviving fibroblasts to chondrocytes, and disruption of collagen fibril architecture.42829 Histopathological changes of CCL degeneration are more prominent in larger dogs weighing >15 kg, and the onset of the degenerative changes occurs at an earlier age in these large dogs.28
The above data suggests that specific factors may influence the loads the CCL is exposed to and may alter adaptive or degenerative changes to the point that the CCL is more vulnerable to disruption and progressive rupture.
Collagen Remodeling and Degradation in the CCL
Ligaments adapt to joint growth during development, and they remodel in response to joint stresses after maturity. Compared to tendons, ligaments have more numerous cells, a higher deoxyribonucleic acid (DNA) content, a larger amount of reducible collagen cross-linkages, and more type III collagen, suggesting that ligaments are metabolically more active and have a greater adaptive potential than tendons.6 Normal resorption of matrix collagen occurs by fibroblast phagocytosis and intracellular digestion with lysosomal cathepsins, whereas inflammatory remodeling of collagen is thought to be mediated by matrix metalloproteinase (MMP) enzymes.30 After experimental transection of the CCL, 34% of the mass of the transected ligament is resorbed within 10 days.31 The loss of collagen mass and resorption of collagen after anterior cruciate ligament rupture are not associated with expression of MMP enzymes that are able to degrade native triple helical collagens, such as MMP-1 and MMP-13 (i.e., collagenases) and MMP-2 and MMP-9 (i.e., gelatinases).32
Cranial cruciate ligament rupture and remodeling in older dogs are associated with localization of cathepsin K and tartrate-resistant acid phosphatase within CCL tissue.29 In dogs with CCL rupture, the presence of cells containing cathepsin K and tartrate-resistant acid phosphatase is especially prominent in the epiligamentous tissue surrounding remodeling fascicles, in which fibroblasts have undergone chondroid transformation.29 Type I collagen is the major biological substrate for the cysteine protease cathepsin K, and the collagenolytic activity of cathepsin K is dependent upon the formation of a complex with chondroitin sulphate.3334 The role of collagenases (MMP-1 and MMP-13) and gelatinases (MMP-2 and MMP-9) in the mechanism of CCL rupture has not been studied in detail. Although proinflammatory mediators stimulate expression of protease activity in canine CCL explants, the specific enzymes responsible for this degradative activity have not been determined.35
Cruciate disease in dogs is often associated with infiltration of leukocytes into the synovial membrane of the stifle joint and the development of inflammatory changes in the synovial fluid.36–39 Release of collagenolytic proteases, such as cathepsins and MMPs, from the synovium into the stifle synovial fluid can significantly degrade the structural properties of the CCL and increase the likelihood of a mid-substance rupture rather than a CCL avulsion at one of its attachments.40 Inflammatory changes within the tissues of the stifle joint and increased expression of collagenolytic proteases (such as cathepsins, tartrate-resistant acid phosphatase, and MMPs) may contribute to progressive degradation of CCL collagen and progressive rupture of the ligament [Figure 3]. Cathepsin K has only been recently discovered and is thought to have a role in joint destruction in humans with rheumatoid arthritis and osteoarthritis.4142
The type of the leukocytes that infiltrate the CCL and the stifle synovium in dogs with cruciate disease is not known, although the cells most likely arise from the macrophage/monocyte lineage. The specific factors that lead to their recruitment and differentiation in various tissues of the stifle joint are speculative, although an autoimmune mechanism through exposure to CCL type I collagen from cyclic loading and microinjury, and to type II collagen from articular cartilage damaged by joint instability and protease degradation, has been suggested.4344
Conclusion
A fundamental understanding of the pathways that lead to degradation of CCL type I collagen and the weakening of CCL structural properties is key to the development of new approaches for the early diagnosis and treatment of this important disease. Development of a specific diagnostic test for dogs with early cruciate disease and a palpably stable stifle will facilitate selection of medical or surgical treatments that will preserve the structural properties of the CCL before excessive degradation of collagen occurs.
Acknowledgments
The authors thank Zhengling Hao and other members of the Comparative Orthopedic Research Laboratory group, without whom their work on cruciate disease would not have been possible.
Citation: Journal of the American Animal Hospital Association 40, 5; 10.5326/0400385
Citation: Journal of the American Animal Hospital Association 40, 5; 10.5326/0400385
Citation: Journal of the American Animal Hospital Association 40, 5; 10.5326/0400385
Photographic view of the canine stifle joint illustrating the cranial medial and caudal lateral bands of the cranial cruciate ligament (CCL). The probe is placed on the caudal lateral band, which comprises the bulk of the CCL. The twist in the fascicles of the cranial medial band is clearly visible (arrow). (L=lateral; M=medial)
Longitudinal sections of an intact CCL from a 1.5-year-old female beagle (A, B) and a ruptured CCL from a 4-year-old castrated male Chesapeake Bay retriever (C, D) viewed using bright light (A, C) and circularly polarized light (B, D) microscopy (bar=100 μm). (A, B) In this intact CCL from a young dog, birefringence of the extracellular matrix collagen and the crimped structure are clearly visible in polarized light as banded bright fibrils (arrows) (B). (C, D) The disorganized regions of the extracellular matrix in the ruptured CCL lack birefringence and crimping (D) and have a marked loss of fibroblasts (C). Surviving fibroblasts are predominantly spheroid (arrows) (C).
Schematic diagram of the current understanding of canine cruciate disease. Rupture of the CCL occurs progressively over time in the majority of affected dogs. The specific reasons for this progressive failure are not understood. Progressive degradation of collagen within the ligament matrix is likely to be a key step that precedes rupture of the ligament. Green boxes describe normal physiological events. Red boxes describe pathological events associated with cruciate disease. (MMPs – matrix metalloproteinases; TRAP – tartrate-resistant acid phosphatase.)
Contributor Notes
