Guide Multi-axial Fatigue of Trabecular Bone with Respect to Normal Walking

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Fatigue Mechanisms

The tenets of diagnosis for subchondral bone disease remain the same as for many other musculoskeletal conditions in the horse—a thorough clinical examination including static and dynamic, and subjective and objective evaluations to localize the source of lameness, diagnostic analgesia, and diagnostic imaging examination. Subchondral bone injury may be identified on standard radiographic projections, but also may be missed depending on the location and time-course of the disease.

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Diseased bone may radiographically have the appearance of areas of decreased opacity surrounded by areas of increased opacity, or may be visible as a distinct fracture. A lack of radiographically observable abnormalities does not rule out the presence of subchondral bone disease and repeat imaging in 10—14 days or use of a different, more advanced imaging modality should be considered. Depending on the severity and chronicity of the injury, nuclear scintigraphy, magnetic resonance imaging MRI , and computed tomography CT can be considered for further evaluation of changes within the subchondral bone.

In severe cases where overlying articular cartilage damage is also present, diagnostic arthroscopy may be considered for further evaluation of the subchondral bone. Volumetric imaging techniques, such as MRI and CT that provide information in three-dimensions have arguably revolutionized our clinical ability to assess subchondral bone and the health of the joint as a whole.

Magnetic resonance imaging has facilitated the identification of diseased subchondral bone.

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These terms all describe lesions within the subchondral bone with high signal intensity on fluid-sensitive sequences Figure 3. Bone marrow lesions have a characteristic decreased signal intensity on T1-weighted images and increased signal intensity of T2-weighted images 41 , Multiple theories and etiologies have been proposed for the formation of bone marrow lesions, and histologic examination of these lesions have identified a wide spectrum of abnormalities More recently, compelling reports have been published citing bone marrow lesions as early indicators of structural deterioration of the joint and may serve as a marker for maladaptive changes within the subchondral bone and articular cartilage 43 — Further research is essential and forthcoming in this field with the continued development of high-field MRI systems that can provide increased detail.

Figure 3. Images of subchondral bone injury as detected using magnetic resonance imaging MRI. There is marked subchondral and trabecular bone sclerosis in the palmar aspect of the third metacarpal condyle white arrows , with the lateral condyle slightly more affected than the medial condyle.

There is a fissure visible within the bone on the palmar-axial aspect of the lateral condyle that also exhibits increased signal D , arrow.

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In addition to identification of bone marrow lesions, increased bone mineral density or sclerosis is frequently identified on MRI in areas of signal changes. These changes are identified as areas of low signal intensity, but this depends on the sequences used for evaluation. Furthermore, diseased subchondral bone is not always sclerotic and although increased bone density may be present, signal changes on MRI are non-specific and may represent an increased volume of lower density bone.

Increased signal intensity on MRI should be interpreted with caution, as this may not truly represent the trabecular thickening inherent to sclerosis 2. Magnetic resonance imaging remains as the only modality capable of being used to identify fluid e. Fine bone detail can be very challenging to identify using MRI, as the appearance of bone and soft tissue can overlap. In cases of confluent tissue—such as in a joint with osseous proliferation with adjacent soft tissue thickening—the signal intensity of MRI will be the same and it can be challenging or impossible to distinguish between the structures.

The volumetric information from reconstructed CT images can illustrate subtle to extensive internal and external osseous remodeling Figure 4. In specific sites, remodeling changes observed on CT have been validated to indicate pathologic change and impending fractures The tissue density observed on CT can be translated into numerical values known as Hounsfield units HU. Information about the specific densities of the bone may provide valuable insight into unique patterns of bone change, and furthermore provides an objective metric for comparison if serial examinations are performed.

Figure 4. Image of the distal metacarpus of a year-old Thoroughbred racehorse obtained post-mortem with marked palmar osteochondral disease obtained using computed tomography CT in the sagittal, transverse and dorsal planes.

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  • Subchondral bone sclerosis and lysis black arrows and extensive articular cartilage loss is visible on these images and was present on gross evaluation. The information afforded through volumetric imaging modalities such as CT and MRI, have increased our knowledge in the behavior of subchondral bone and its response to training and injury. Historically, CT and MRI evaluation in the equine patient has been limited due to gantry size or requirements for general anesthesia. MRI and CT should be considered as complimentary to one another when evaluating the subchondral bone, as each provides unique and valuable information.

    An additional notable volumetric imaging modality that is receiving increasing attention as a developing technology for assessment of subchondral bone is positron emission computed tomography PET. Recent work has demonstrated that PET is able to identify lesions that were not visible using other imaging modalities, and furthermore to distinguish between active and inactive lesions As PET gains further justification, the potential applications for evaluation of subchondral bone injury will likely warrant further discussion.

    Multi-axial Fatigue of Trabecular Bone with Respect to Normal Walking |

    With increasing accessibility to these volumetric imaging modalities and evolving understanding of the role of subchondral bone in joint disease, it is likely that assessment of subchondral bone will become an integral part of diagnostic imaging for veterinary patients affected by orthopedic conditions. The causes of pain associated with diseased subchondral bone, including bone resorption, still remain poorly understood. Proposed contributors to pain may include instability, increased intraosseous pressure, hypertension, and impingement of sensitive structures such as the periosteum, ligaments or joint capsule.

    The two consensus goals for treatment of subchondral bone disease include: 1 restoring function and 2 preventing progression of disease to failure through pain relief and restoration of normal bone architecture 2. It is challenging to create a specific treatment regimen since subchondral bone disease encompasses a large spectrum of abnormalities.

    The plethora of methods proposed to treat subchondral bone disease reflects the variety of challenges associated with managing this condition. If pain or lameness are recognized early in the maladaptive response process, exercise modifications including altering the intensity, duration or type e. A notable critique of these alternative training methods is that they utilize instability, or alterations in the distribution of weightbearing forces across the articular surfaces Instability has been shown to prevent vessel ingrowth, and despite the fact that subchondral bone disease is not necessarily a primary vascular condition, avascular areas are commonly present within diseased subchondral bone 2.

    Bisphosphonates have also been used commonly to treat subchondral bone disease, but their potential effectiveness has yet to be fully elucidated. Proponents of bisphosphonate therapy argue that inhibition of osteoclastic activity benefits those cases undergoing active degradation of the subchondral bone, and additional analgesic and anti-inflammatory effects have also been suggested 2 , 50 , Despite these potential benefits and positive reports in the human medical literature, there has been no definitive consensus in the treatment of subchondral bone disease using bisphosphonates in veterinary species.

    Bisphosphonates are also not licensed for use in juvenile horses, which likely represent the largest population of cases of subchondral bone injury through the racehorse industry. The frequency and intensity that subchondral bone disease is observed would justify this as an economic issue, however there is a paucity of data on the incidence and specific demographics of this disease. The inherent difficulties in identifying subchondral bone disease prior to the occurrence of more severe sequelae makes epidemiologic studies very challenging. At the current time, the best prevention of subchondral bone disease is focused on reducing risk.

    Despite inconsistent agreement in a specific definition for clinical signs of subchondral bone disease, it is well-agreed that cumulative exercise is associated with an increased risk of subchondral bone disease 2. Exercise and training regimes should be tailored to each specific horse, with specific attention to the clinical condition and response of the horse.

    Published work has recognized that exposure to exercise at the end of growth, but before skeletal maturity is beneficial for subchondral bone development, and this must be balanced with the adaptive response of each animal. Further work is essential in order to understand those cases that may be at an increased risk for development of subchondral bone disease through the use of multi-modality imaging or potentially biomarker panels.

    Despite the voids in the current knowledge about subchondral bone disease, exercise modulation will likely remain a central tenet of disease management. Substantial insight has been gained about the biomechanical influences of the joint on the subchondral bone, with the relationship between subchondral bone injury and articular cartilage loss and the development of degenerative joint disease only beginning to be elucidated.

    Continued investigation of the adaptive and maladaptive changes within the subchondral bone by researchers and clinicians alike will continue to yield valuable information about the behavior of this unique component of the joint. Further discovery of the delicate balance of factors that maintain the integrity of the subchondral bone and homeostasis within the joint will surely enhance and direct our understanding of subchondral bone disease in across both veterinary and human patients.

    Both HS and CK contributed equally to the preparation, developing, writing and editing of the review article submitted. The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Gomoll AH, Farr J, editors.

    The osteochondral unit. In: Cartilage Restoration. Google Scholar. Science in brief: report on the Havemeyer Foundation workshop on subchondral bone problems in the equine athlete. Equine Vet J. Joint Disease in the Horse. Louis, MO: Elsevier Inc The structure of the human subchondral plate. J Bone Joint Surg Br.

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    Response of joints to impact loading. Arthritis Rheum. The response of joints to impact loading — II In vivo behavior of subchondral bone. J Biomech. Response of joints to impact loading — III: Relationship between trabecular microfractures and cartilage degeneration. Influence of exercise and joint topography on depth-related spatial distribution of proteoglycan and collagen content in immature equine articular cartilage.

    Mankin H, Radin E. Structure and function of joints. In: McCarthy D, editors. Equine Surgery. Louis, MO PubMed Abstract Google Scholar. The role of subchondral bone in joint disease: a review.

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    • Role of subchondral bone in the initiation and progression of cartilage damage. Clin Orthop Relat Res. Calculation of joint reaction forces in the equine distal forelimb during walking and trotting. Frost HM. Redefining Wolff's Law: the bone modeling problem. Anat Rec. Coupling the activities of bone formation and resorption: a multitude of signals within the basic multicellular unit. Bonekey Rep.