Inhibitory effects of osteoprotegerin on osteoclast formation and function under serum-free conditions

The purpose of this study was to determine whether osteoprotegerin (OPG) could affect osteoclat differentiation and activation under serum-free conditions. Both duck embryo bone marrow cells and RAW264.7 cells were incubated with macrophage colony stimulatory factor (M-CSF) and receptor activator for nuclear factor κB ligand (RANKL) in serum-free medium to promote osteoclastogenesis. During cultivation, 0, 10, 20, 50, and 100 ng/mL OPG were added to various groups of cells. Osteoclast differentiation and activation were monitored via tartrate-resistant acid phosphatase (TRAP) staining, filamentous-actin rings analysis, and a bone resorption assay. Furthermore, the expression osteoclast-related genes, such as TRAP and receptor activator for nuclear factor κB (RANK), that was influenced by OPG in RAW264.7 cells was examined using real-time polymerase chain reaction. In summary, findings from the present study suggested that M-CSF with RANKL can promote osteoclast differentiation and activation, and enhance the expression of TRAP and RANK mRNA in osteoclasts. In contrast, OPG inhibited these activities under serum-free conditions.     

Expression of receptor activator of nuclear factor kappa-B ligand (RANKL) in neoplasms of dogs and cats

BACKGROUND: Receptor activator of nuclear factor kappa-B (RANK), RANK-ligand (RANKL), and the soluble decoy receptor osteoprotegerin (OPG) form a key axis modulating osteoclastogenesis. In health, RANKL-expressing bone stromal cells and osteoblasts activate osteoclasts through RANK ligation, resulting in homeostatic bone resorption. Skeletal tumors of dogs and cats, whether primary or metastatic, may express RANKL and directly induce malignant osteolysis. HYPOTHESIS: Bone malignancies of dogs and cats may express RANKL, thereby contributing to pathologic bone resorption and pain. Furthermore, relative RANKL expression in bone tumors may correlate with radiographic characteristics of bone pathology. ANIMALS: Forty-two dogs and 6 cats with spontaneously-occurring tumors involving bones or soft tissues were evaluated. METHODS: A polyclonal anti-human RANKL antibody was validated for use in canine and feline cells by flow cytometry and immunocytochemistry. Fifty cytologic specimens were collected from bone and soft tissue tumors of 48 tumor-bearing animals and assessed for RANKL expression. In 15 canine osteosarcoma (OSA) samples, relative RANKL expression was correlated with radiographic characteristics of bone pathology. RESULTS: Expression of RANKL by neoplastic cells was identified in 32/44 canine and 5/6 feline tumor samples. In 15 dogs with OSA, relative RANKL expression did not correlate with either radiographic osteolysis or bone mineral density as assessed by dual energy x-ray absorptiometry. CONCLUSIONS AND CLINICAL IMPORTANCE: In dogs and cats, tumors classically involving bone and causing pain, often may express RANKL. Confirming RANKL expression in tumors is a necessary step toward the rational institution of novel therapies targeting malignant osteolysis via RANKL antagonism.     

Involvement of the Ca2+ signaling pathway in osteoprotegerin inhibition of osteoclast differentiation and maturation

The purpose of this study was to determine whether the Ca²⁺ signaling pathway is involved in the ability of osteoprotegerin (OPG) to inhibit osteoclast differentiation and maturation. RAW264.7 cells were incubated with macrophage colony-stimulating factor (M-CSF) + receptor activator of nuclear factor-κB ligand (RANKL) to stimulate osteoclastogenesis and then treated with different concentrations of OPG, an inhibitor of osteoclast differentiation. The intracellular Ca²⁺ concentration [Ca²⁺]_i and phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (CaMKII) in the different treatment groups were measured by flow cytometry and Western blotting, respectively. The results confirmed that M-CSF + RANKL significantly increased [Ca²⁺]_i and CaMKII phosphorylation in osteoclasts (p < 0.01), and that these effects were subsequently decreased by OPG treatment. Exposure to specific inhibitors of the Ca²⁺ signaling pathway revealed that these changes varied between the different OPG treatment groups. Findings from the present study indicated that the Ca²⁺ signaling pathway is involved in both the regulation of osteoclastogenesis as well as inhibition of osteoclast differentiation and activation by OPG.     

OPG/RANKL/RANK系统的生理功能

骨是一个代谢活跃的器官,在整个生命周期中都在进行不断的更新与重建。在骨形态发生和重建的过程中主要受到细胞因子和激素的调节,同时骨代谢也受到OPG/RANKL/RANK系统的调控。论文主要对骨骼生理结构与骨组织细胞之间的关系,骨代谢和骨的形成过程,OPG/RANKL/RANK系统的生理功能,OPG/RANKL/RANK系统的影响因子以及骨代谢紊乱所引起的疾病几个方面作一综述。     

Effects of RANKL, osteoprotegerin, calcium and phosphorus on survival and activation of Muscovy duck osteoclasts in vitro

The aim of this study was to determine whether receptor activator of nuclear factor NF-κB ligand (RANKL), osteoprotegerin (OPG) and a calcium:phosphorus (Ca:P) ratio of 2:1 could affect survival and activation of Muscovy duck osteoclasts (OCs). Bone marrow cells were obtained from 5-day-old Muscovy ducks and cultured with (Group A) No added factors, (B) 30 ng/mL soluble RANKL (sRANKL), (C) 30 ng/mL sRANKL and 10 ng/mL OPG, (D) 10 ng/mL OPG, (E) 50 ng/mL OPG, (F) 100 ng/mL OPG and (G) 30 ng/mL sRANKL, 6 mmol/L Ca and 3 mmol/L P.sRANKL promoted the survival of OCs on day 2, whereas the number of OCs decreased with addition of OPG in a dose-dependent manner. OPG and Ca:P (2:1) both inhibited OC survival induced by RANKL. RANKL stimulated bone resorption by OCs, whereas OPG, but not Ca:P (2:1), inhibited the activity of OCs induced by RANKL. RANKL promotes the survival and activation of OCs from Muscovy ducks, whereas OPG and, to a lesser extent, Ca:P (2:1) reduce the life span and inhibited the activation of OCs induced by RANKL.     

Regulation of matrix metalloproteinase-9 protein expression by 1α,25-(OH)2D3 during osteoclast differentiation

To investigate 1α,25-(OH)₂D₃ regulation of matrix metalloproteinase-9 (MMP-9) protein expression during osteoclast formation and differentiation, receptor activator of nuclear factor κB ligand (RANKL) and macrophage colony-stimulating factor (M-CSF) were administered to induce the differentiation of RAW264.7 cells into osteoclasts. The cells were incubated with different concentrations of 1α,25-(OH)₂D₃ during culturing, and cell proliferation was measured using the methylthiazol tetrazolium method. Osteoclast formation was confirmed using tartrate-resistant acid phosphatase (TRAP) staining and assessing bone lacunar resorption. MMP-9 protein expression levels were measured with Western blotting. We showed that 1α,25-(OH)₂D₃ inhibited RAW264.7 cell proliferation induced by RANKL and M-CSF, increased the numbers of TRAP-positive osteoclasts and their nuclei, enhanced osteoclast bone resorption, and promoted MMP-9 protein expression in a concentration-dependent manner. These findings indicate that 1α,25-(OH)₂D₃ administered at a physiological relevant concentration promoted osteoclast formation and could regulate osteoclast bone metabolism by increasing MMP-9 protein expression during osteoclast differentiation.     

A feline assay using osteoclasts generated in vitro from peripheral blood for screening anti-resorptive agents

Musculo-skeletal diseases are a major cause of pain and suffering in cats and several conditions involve increased bone resorption by osteoclasts. However, little is known about the biology of these cells in the cat. In this study we established a method to generate feline osteoclasts from blood mononuclear cells stimulated by macrophage colony stimulating factor (M-CSF) and receptor activator of NF-kappaB ligand (RANKL). Cultured osteoclasts are multinucleated, express tartrate resistant acid phosphatase (TRAP), form F-actin rings and resorb bone. They express alpha(v)beta3 vitronectin receptor and osteoclast enzymes, cathepsin K and MMP9; the myeloid antigen, CD18, and the megakaryocyte/platelet integrin, CD41, are absent. This phenotype is typical of osteoclasts from other species. Three resorption inhibitors were examined for activity against feline osteoclasts. Calcitonin, bisphosphonate and RGD integrin inhibitory peptide all reduced bone resorption at doses similar to those efficacious in rabbit or human. We conclude that blood-derived osteoclast cultures are a suitable in vitro system for assessing the ability of drugs to inhibit bone resorption in domestic cats.     

Effects of dietary calcium to available phosphorus ratios on bone metabolism and osteoclast activity of the OPG /RANK/RANKL signalling pathway in piglets

Hydroxyapatite, a mineral form of calcium (Ca) and phosphorus (P) that gives bones their rigidity, is the major and essential component of bones and teeth in the human and animal body. A suitable ratio of Ca and P is vital for bone growth. The aim of this study was to explore the effects of dietary calcium to available phosphorus ratios (Ca/AP) on bone metabolism and osteoclast activity of the osteoprotegerin (OPG)/receptor activator of nuclear factor kappa B ligand (RANKL) signalling pathway in piglets. At days 15 and 29, the piglets were assessed for growth performance, blood indicators, cytokines and the OPG/RANK/RANKL signalling pathway. Our results showed that piglets fed a dietary Ca/AP ratio of 2:1 increases growth performance and regulates blood indicators and cytokines (parathyroid hormone (PTH), calcitonin (CT), vitamin D₃ (VD₃)_, insulin‐like growth factor‐1 (IGF‐1), transforming growth factor‐β (TGF‐β), interleukin‐1 (IL‐1), interleukin‐6 (IL‐6), carboxyterminal propeptide of type I procollagen (PICP), tartrate‐resistant acid phosphatase (TRACP), alkaline phosphatase (ALP) and osteocalcin (OCN) content). We also demonstrated that this ratio affects hormone secretion and further bone metabolism through the OPG/RANK/RANKL signalling pathway of osteoclasts. These results indicate that a suitable dietary Ca/AP ratio is vital for bone growth and reduce the incidence of bone diseases such as osteoporosis, providing a practical basis for the raising of piglets.     

Effect of computed tomography display window and image plane on diagnostic certainty for characteristics of dysplastic elbow joints in dogs

OBJECTIVE: To test the effects of computed tomography (CT) image plane and window settings on diagnostic certainty for CT characteristics associated with dysplastic elbow joints (elbow joint dysplasia) in dogs and to provide optimal display guidelines for these CT characteristics. SAMPLE POPULATION: CT images of 50 dysplastic elbow joints from 49 lame dogs and 10 elbow joints from 5 sound dogs. PROCEDURES: CT image data were obtained in transverse, sagittal, and dorsal planes. Each plane was examined by use of 3 Hounsfield unit (HU) window settings. Two veterinary radiologists independently evaluated sets of CT images for evidence of 7 CT characteristics. Effect of elbow joint status, image plane, and window settings on diagnostic certainty for these CT characteristics was tested by use of a visual analogue scale. RESULTS: Diagnostic certainty for abnormalities of the medial coronoid process (MCP) and radial incisure was highest in the transverse plane, subchondral defects or sclerosis of the trochlea humeri was highest in the dorsal plane, and joint incongruity was highest in the sagittal plane. Certainty for hypoattenuating subchondral defects or fissures was highest at 2,500 or 3,500 HUs, whereas certainty for subchondral sclerosis was highest at 1,500 HUs and lowest at 3,500 HUs. CONCLUSIONS AND CLINICAL RELEVANCE: Diagnostic certainty for CT characteristics of elbow joint dysplasia in dogs was affected by image display variables. Diagnostic certainty for altered subchondral bone density was primarily influenced by window settings, whereas structural MCP abnormalities and joint incongruity were influenced most by image plane.     

Computed tomography versus arthroscopy for detection of canine elbow dysplasia lesions

Objective— To describe associations between computed tomography (CT) and arthroscopy in dogs with elbow dysplasia lesions.
Study Design— Retrospective clinical study.
Sample Population— Canine elbows (n=101) investigated by CT and arthroscopy.
Methods— CT scans were reviewed for 10 predetermined CT signs and graded for osteophyte size. Surgical reports were reviewed for specific disease features and cartilage erosion grades. Associations between variables were investigated with multivariate logistic regression and correlation between osteophyte size and cartilage erosion with Spearman's rank order correlation.
Results— Medial coronoid process (MCP) fragment on CT was significantly associated with the arthroscopic identification of a displaced MCP fragment, cartilage erosion affecting the MCP, and cartilage erosion affecting the humeral condyle. Irregular radial incisure of the ulna on CT was significantly associated with the arthroscopic identification of cartilage erosion affecting the MCP. Osteophytes on CT were significantly associated with an abnormal arthroscopic examination. There was a moderately significant correlation between CT osteophyte grade and cartilage erosion grades for areas of the medial joint compartment (r_s=0.44–0.48).
Conclusion— Some CT signs are significantly associated with arthroscopic features of elbow dysplasia lesions in dogs; however, other CT signs were not associated with arthroscopic findings, and CT and arthroscopy can provide contradictory information. Osteophyte size is moderately correlated with cartilage erosion of the medial joint compartment.
Clinical Relevance— CT can provide valuable information for the investigation of dogs with elbow dysplasia, but the absence of CT signs (or the absence of arthroscopic abnormalities) does not rule out elbow lesions.     

Osteoclast progenitors from cats with and without tooth resorption respond differently to 1,25-dihydroxyvitamin D and interleukin-6

Both vitamin D and inflammatory cytokines can stimulate osteoclast formation and activity. We studied the effect of 1,25-dihydroxycholecalciferol (1,25(OH)(2)D), and interleukin-6 (IL-6), on the formation and activity of feline osteoclasts, using peripheral blood mononuclear cells (PBMCs) from cats with and without tooth resorption (TR(+) and TR(-)) as a source of osteoclast precursors. The formation of osteoclast-like cells (defined as multinucleated, tartrate-resistant acid phosphatase-positive cells) was assessed at 7 and 14 days. In the presence of M-CSF and RANKL, with and without IL-6, more osteoclasts were formed from TR(-) PBMCs than from TR(+) PBMCs on plastic. More osteoclasts were formed from TR(+) PBMCs on bone slices in the presence of M-CSF/RANKL with 1,25(OH)(2)D. This opposite effect may be due to a higher expression of the vitamin D receptor (VDR) in TR(+) osteoclasts and precursors on bone. Formation of resorption pits was analyzed and confirmed with scanning electron microscopy. In conclusion, we propose that TR(+) PBMCs when cultured on bone are sensitive to 1,25(OH)(2)D, whereas the differentiation of TR(-) PMBCs on bone seem more sensitive to IL-6, suggesting that osteoclast precursors from cats with and without tooth resorption respond differently to osteoclast stimulating factors.     

BONE SCINTIGRAPHY FOR THE DIAGNOSIS OF AN ABNORMAL MEDIAL CORONOID PROCESS IN DOGS

Few reports have been published regarding the use of scintigraphy in the diagnosis of elbow joint lameness in dogs. Some authors have speculated about the potential use of bone scintigraphy and its suspected high sensitivity for the early diagnosis of abnormalities of the medial coronoid process (MCP) in dogs. Scintigraphy is used routinely in our institution in dogs presented for thoracic limb lameness and/or suspected of abnormalities of the MCP when radiographic findings were equivocal. Radiographic, scintigraphic, and surgical findings of the elbow joints of 17 dogs with elbow joint lameness were compared with radiographic, scintigraphic, and necropsy findings of the elbow joints of 12 clinically healthy Labrador Retrievers. Quantitative evaluation of scintigraphic images was performed to determine relative radiopharmaceutical uptake in the region of the MCP. Maximum relative uptake of the coronoid process in the normal dogs was taken as a threshold value to classify elbows as positive or negative for an abnormal MCP after all 24 elbows of the 12 healthy dogs were confirmed as being normal at necropsy. All 17 elbows from lame dogs were positive on scintigraphy and confirmed as having chondromalacia, a fissure, or fragmentation of the MCP. Based on our results, bone scintigraphy may be a valuable diagnostic tool for the diagnosis of abnormalities of the MCP in dogs, and particularly in older dogs where clinical and radiographic changes may be ambiguous.     

Urine N-telopeptide excretion in dogs with appendicular osteosarcoma

Canine appendicular osteosarcoma (OSA) is a commonly diagnosed cancer that is capable of inducing pathologic bone remodeling. Investigating surrogate indices of bone metabolism may contribute to the diagnostic and therapeutic management of bone malignancies in companion animals. This study evaluated the excretion of N-terminal telopeptide (NTx), a marker of bone resorption that is detected in urine. Sixty-three dogs with appendicular OSA were compared with 29 age-matched healthy dogs. Dogs with appendicular OSA had significantly higher baseline urine NTx excretion than healthy controls (P < .0001). In 17 dogs with OSA treated with either amputation or standardized palliative therapies, significant reductions in urine NTx excretion were observed, suggesting that excessive bone resorption in dogs with OSA may be linked with focal skeletal osteolysis or its consequences. To identify any relationship between indicators of pathologic bone turnover, baseline urine NTx excretion was correlated with serum bone alkaline phosphatase (bALP) or radiographic tumor lengths at diagnosis. No significant correlations were identified between baseline urine NTx excretion and either bALP or tumor length. The findings from this study suggest that high urinary NTx excretion may support the diagnosis of focal skeletal osteolysis in dogs, and reductions in urine NTx excretion after treatment may reflect elimination or minimization of pathologic bone resorption.     

Clinical and Radiographic Evaluation of Intertrochanteric Osteotomy in Dogs: A Retrospective Study of 18 Dogs

Objective- To determine whether intertrochanteric osteotomy (ITO) can prevent the progression of degenerative joint disease (DJD) in dysplastic hip joints.
Study Design- The results of ITO were assessed retrospectively by using owner questionnaires, physical examination, and radiographic evaluation.
Animals- Eighteen client-owned dogs (29 coxofemoral joints were evaluated).
Methods- Lameness was scored according to a grading system. A scoring system was also developed to assess radiographically evident osteoarthritis on a ventrodorsal projection of the coxofemoral joints in extension.
Results- Twenty-nine ITO were performed in 18 dogs with varying degrees of hip dysplasia. The dogs were lame on 19 of 29 rear limbs on physical examination before surgery. In 22 of the 29 hip joints, palpation caused signs of pain. The median age at the first and second surgical procedure was 14.5 months and 18 months, respectively. Follow-up evaluation was performed on average at 9.77 (FU I), 22.52 (FU II), and 47.50 months (FU in) after surgery. Based on the owners' evaluation, there was a tendency toward improvement after surgery. The results of the physical examination at the preoperative examination and at the three follow-up examinations did not differ significantly. The follow-up radiographic scores showed significantly worse DJD than the preoperative scores.
Conclusions- ITO does not prevent progression of DJD in the dysplastic hip.
Clinical Relevance- Knowledge of the long-term effects of ITO is essential for surgeons trying to achieve improvement in dogs with hip dysplasia.     

Estimates of genetic parameters for hip and elbow dysplasia in Finnish Rottweilers

Data from 2,764 Rottweiler dogs born from 1987 to 1996 were analyzed with a Restricted Maximum Likelihood procedure using a mixed linear animal model to obtain variance component estimates for hip and elbow dysplasia. The data included 2,764 hip dysplasia and 2,278 elbow dysplasia records. Hip joints were scored as normal (0), borderline (1), slight (2), moderate (3), and severe (4, 4.5, and 5) hip dysplasia. Elbow joints were graded normal or borderline (0), slight (1), moderate (2), and severe (3) elbow dysplasia. The mean for the hip scores was 1.07 and for the elbow scores .60. Environmental effects influencing hip dysplasia were age, birth year, birth year x season interaction, and experience of the veterinarian responsible for x-raying the dog. For elbow dysplasia, statistically significant effects were age, birth year, sex of the dog, and panelist responsible for each screening. Estimates of heritability for hip and elbow dysplasia were .58 +/- .04 and .31 +/- .04, respectively, with a genetic correlation of .37 +/- .08 between the traits. Genetic improvement of almost one genetic standard deviation was observed in both traits during the 10 yr covered by the data.     

COMPUTED TOMOGRAPHY OF CANINE ELBOW JOINTS AFFECTED BY PRIMARY AND CONCOMITANT FLEXOR ENTHESOPATHY

Flexor enthesopathy is an important differential diagnosis for elbow lameness in dogs. The disorder can be a primary cause of elbow lameness or concomitant with other elbow pathology. Since treatment differs for primary and concomitant forms of flexor enthesopathy, a noninvasive method for distinguishing between them is needed. In the current prospective study, computed tomographic (CT) examination was performed before and after IV injection of contrast in 17 dogs with primary flexor enthesopathy, 24 dogs with concomitant flexor enthesopathy, 13 dogs with elbow dysplasia, and seven normal dogs. Dogs were assigned to groups based on results of clinical examination and at least three other imaging modalities. Computed tomographic lesions consistent with flexor enthesopathy were found in all clinically affected joints with primary flexor enthesopathy and in 29 of the 30 clinically affected joints with concomitant flexor enthesopathy. Those lesions were not found in sound elbows or joints affected by elbow dysplasia. Flexor lesions detected in dogs with primary flexor enthesopathy were not significantly different from those detected in dogs with the concomitant form. Findings indicated that CT can be applied to detect flexor enthesopathy, but a distinction between the primary and concomitant forms was not always possible. Authors recommend the use of multiple diagnostic techniques for treatment planning in affected dogs.     

Efficacy of an oral hyaluronate and collagen supplement as a preventive treatment of elbow dysplasia

One hundred and five Labrador dogs were randomly divided into two groups to determine the number of animals that develop elbow dysplasia when treated with an oral supplement compared to untreated ones. Efficacy of the oral treatment was also evaluated once illness was diagnosed. The supplement (Hyaloral) contained hyaluronic acid, hydrolysed collagen, glucosamine, chondroitin sulphate, and gamma oryzanol. Clinical evaluation of the elbow joints was completed at months 3, 6, 12, and 20 by orthopaedic evaluations, radiography, serologic and blood analysis, and veterinarian evaluation of dysplasia symptoms. All side effects were recorded. In the control group, 33.3% of the dogs developed radiographic evidence of elbow dysplasia compared to 18.5% in the treated group. Symptoms of dysplasia at 12 months differed between the treated (12.5%) and control (61.5%) animals, and were significantly different at 20 months (p < 0.05). Differences in lameness along with movement and swelling of the elbows between groups were observed after 12 months. The treated group had improved significantly by the last visit (p < 0.05). No adverse side effects were reported. In conclusion, oral treatment with Hyaloral may have a potential cumulative action that provides protection against dysplasia and significantly improves symptoms of elbow dysplasia.     

The bone biologic effects of zoledronate in healthy dogs and dogs with malignant osteolysis

BACKGROUND: Malignant osteolysis is a process whereby cancer cells in concert with osteoclasts erode bone matrix. Aminobisphosphonates (NBPs) such as zoledronate induce osteoclast apoptosis and thereby decrease malignant skeletal destruction, severity of bone pain, and frequency of pathologic fracture. HYPOTHESIS: IV-administered zoledronate will reduce homeostatic bone turnover in healthy dogs and pathologic bone resorption in dogs diagnosed with primary and secondary bone tumors. ANIMALS: Six healthy dogs and 20 dogs with naturally occurring primary or metastatic bone tumors were administered zoledronate IV. METHODS: Prospective study: In all dogs, healthy (n = 6) and with malignant osteolysis (n = 20), the bone biologic effects of zoledronate were evaluated by quantifying changes in serum C-telopeptide (CTx) or urine N-telopeptide (NTx) concentrations or both. In dogs with osteosarcoma (OSA) (n = 10), serial changes in tumor relative bone mineral density (rBMD) assessed by dual-energy x-ray absorptiometry were used to characterize zoledronate's antiresorptive effects within the immediate tumor microenvironment. Additionally, the biochemical tolerability of zoledronate was assessed in 9 dogs receiving multiple (> or =2) consecutive treatments. RESULTS: All dogs had significant reductions in serum CTx or urine NTx concentrations or both after zoledronate administration. In a subset of dogs with appendicular OSA, reduced urine NTx concentrations and increased primary tumor rBMD coincided with improved limb usage as reported by pet owners in dogs treated with zoledronate and concurrent oral analgesics. Multiple zoledronate infusions were not associated with biochemical evidence of toxicosis. CONCLUSIONS AND CLINICAL IMPORTANCE: In dogs with skeletal neoplasms, IV-administered zoledronate exerts bone biologic effects, appears safe, and can provide pain relief.     

MAGNETIC RESONANCE IMAGING OF PRIMARY AND CONCOMITANT FLEXOR ENTHESOPATHY IN THE CANINE ELBOW

Flexor enthesopathy is a recently recognized elbow disorder in dogs and considered to be an important differential diagnosis for elbow lameness. Primary and concomitant forms of the disease have been previously described and treatments differ for the two forms. The goal of this prospective study was to compare magnetic resonance imaging (MRI) findings for dogs with primary flexor enthesopathy (n = 17), concomitant flexor enthesopathy (n = 23), elbow dysplasia alone (n = 13), and normal elbows (n = 7). Each elbow joint underwent MRI using the same low‐field scanner. Sequences included transverse and sagittal T1‐weighted (before and after IV contrast), transverse and sagittal T2‐weighted, and dorsal STIR. For each elbow, MRI lesions were recorded based on a consensus of two observers unaware of group status. Magnetic resonance imaging lesions involving flexor tendons were found in 100% of clinically affected joints with primary flexor enthesopathy and 96% of clinically affected joints with concomitant flexor enthesopathy. Thickened flexor muscles were the most common lesions, followed by hyperintense tendon signal and contrast enhancement. Irregular, thickened medial humeral epicondyle, edema, and calcified body lesions were less frequently observed. Magnetic resonance imaging characteristics of flexor enthesopathy were not found in normal joints or those affected by elbow dysplasia alone. No significant differences in frequencies and details of individual MRI characteristics were found between primary and concomitant flexor enthesopathy groups. Findings indicated that MRI is a sensitive technique for detection of flexor enthesopathy lesions in dogs, however, MRI characteristics do not allow differentiation of primary versus concomitant forms of the disease.