Kenneth A. Johnson bone, resulting in delayed or non-union is a vexing clinical problem in all species of animals. Recently, the focus of attention in orthopaedics has been on the treatment of fractures by bone grafts, mesenchymal stem cells, growth factors, biomaterials and improved mechanical environment, either alone or in some combination. These therapies have produced clearly demonstrable benefits in treating delayed and non-union. However there is still much to learn about the complex processes of fracture healing, and why it fails in some patients despite the management by ‘state of the art’ surgery employing, for example, minimally invasive osteosynthesis, locking implants, exogenous growth factor application, and 3D printed biomaterials and stem cells for the reconstruction of critical sized defects. Even so, in a proportion of fracture patients, endogenous factors predispose to impaired fracture healing. The pivotal role of immunity in fracture healing and bone regeneration was one such endogenous factor under discussion at the symposium. The immune system has direct involvement in guiding all stages of fracture healing starting with inflamImmune System and Bone Regeneration

http://dx.doi.org/10.3415/VCOT-15-04-0067 Bone defects and non-union are important clinical problems, complicating the outcome in about 10% of human patients suffering from long-bone fractures. While it would be reasonable to assume that there is a similar risk of such problems complicating fracture management in animals, there is surprisingly little objective data derived from large prospective studies on the incidence of such complications in animals. There are many reasons why it is difficult to conduct such outcome studies in animals. This challenge is further compounded by a host of factors such as differences amongst species, fracture type and location, treatment method, soft tissue injury, wound contamination, and the use of bone grafts, scaffolds, and growth factors as well as numerous other modalities. Anecdotal observations suggest that the interval required for complete bone union is progressively longer, as we move “up” the phylogenetic scale from birds and rodents, through domesticated animals such as the cat, dog, sheep, cow, and horse, and ultimately to humans. It is difficult to determine if these species differences are entirely endogenous, or due to differences in genetics, life span, body weight, metabolic rate or other factors. It is apparent that certain long bones of some species have a predilection for nonunion; for example the distal antebrachium of toy breed dogs, and the tibial diaphysis of the cat and humans. The findings of a recent study led the authors to conclude that use of external skeletal fixation of tibial fractures in the cat strongly predisposed to development of complications and non-union (1). However, aside from the importance of fracture fixation devices, the fracture biological environment also has a powerful influence on outcome, particularly in regards to the interaction of cells, scaffolds and growth factors. Autologous cancellous bone continues to be held up as the gold standard as a potent stimulant of bone regeneration and healing, but the search for alternatives continues. Of the growth factors, exogenously delivered bone morphogenetic protein-2 is now often used clinically but it too has some limitations. Autologous platelet-rich plasma also contains various growth factors that promote osteogenesis. There are many studies evaluating the effect of platelet-rich plasma on bone healing, and the results are somewhat mixed. A study of bone defect healing in rabbit long bone reported in this issue found that platelet-rich plasma promoted defect healing with new bone (2). While these are very encouraging results, we should consider that successful transfer of this treatment methodology to clinical patients requires more objective data on the influence of factors such as species difference, concentration of growth factors in the platelet-rich plasma, as well as the timing and frequency of administration. In the study by Barnes and colleagues, shock wave therapy did not have any further demonstrable positive effects on the healing of tibial osteotomies in clinical canine patients, over and above that of autologous cancellous bone grafting alone (3). It would seem that the ‘jury is still out’ in regards to the value of this treatment modality in fracture healing. As the authors of this study recognized, there are many possible explanations for this lack of effect of shock wave therapy that require elucidation. Imminently, this June with the support of the ISFR (International Society for Fracture Repair) and AO (AOTrauma Germany), there will be an International Symposium on Bone Regeneration in Berlin, with the focus on Crosstalk in bone regeneration, especially in regards to the immune system, cell matrix interactions, and progenitor and stem cells. This will further promote a greater awareness of the value of an expanded understanding of the complex biology in bone regeneration. See the next page for more information on this.

Blood supply is just as critical to the viability of normal healthy bone, as it is to the repair and remodelling processes that follow fracture, disease and surgical insertion of orthopaedic implants. The importance of bone blood supply in fracture repair is well accepted by orthopaedic surgeons. The third AO principle of fracture treatment by internal fixation, as set forth more than fifty years ago, was the requirement for ‘atraumatic operative technique preserving the vitality of bone soft tissues and bone’ (1). The importance of this third principle has achieved even greater prominence with the more recent introduction of the concept of minimally invasive plate (percutaneous) osteosynthesis (2). Bone blood supply is not a very popular topic for orthopaedic research and investigation, in comparison to say biomechanical testing of bone and implants. The reasons behind this are several. A major problem is that bone is dense and opaque, so it is difficult to section bone or to directly visualize bone blood vessels with current imaging methods without removal of the mineral. Consequently progress for the discovery of new knowledge in this field has been slow. Murray Brookes wrote an interesting account of the history of research into bone blood supply which can be traced back about 350 years (3). Antonie van Leeuwenhoek, a founding father of microscopy in The Netherlands, described surface veins on the shin bone of a calf, and observed small holes passing to the inside of the bone which he imaged connected to ‘small pipes going longways in the bone’. Later in 1691, Clopton Havers described in his book how a large nutrient artery pieces the shaft of long bones and ramified in bone marrow. He presumed that an oily medullary substance was expelled into a system of straight pores within the bone, these pores becoming known as Haversian canals. It is now well accepted that the nutrient artery penetrating the nutrient foramen, divides into ascending and descending branches of the medullary artery and these are the main source of centrifugal blood flow in the cortical bone of long bones. Some long bones have more than one nutrient foramen; for example about 50% of human femora have two nutrient foramina. The medullary blood vessels arborize and anastomose with both the metaphyseal vessels as well as the periosteal vessels in regions of strong soft tissue attachment. This allows increased compensatory blood flow from the anastomosing supply should one system, such as the medullary supply be knocked out. The macroscopic anatomy of vascular system of bone has been well described, using techniques of vascular injection of barium sulphate, imaging and the Spalteholz method (3–6). In this issue of the Journal, a detailed study using the Spalteholz method to compare the morphology of the medullary blood supply of the tibia of cats and small dogs is reported (7). The underlying purpose of the study was to ask if poor blood supply to the tibia could be a contributing factor to the higher incidence of delayed and non-union of tibial fractures in the cat. Surprisingly, these authors found that the absolute and relative diameters of the main medullary artery in the mid and distal regions of the feline tibia were larger than in small dogs. Sometimes such an unexpected finding that fails to support our assumptions can be more powerful in advancing our knowledge than a confirmatory outcome. It might provoke us to think again. It could be we did the right study, but have asked the wrong question, or vice versa. For example, one problem is that morphological studies of bone blood vessels can provide only limited information about haemodynamics. Apparently the cross-sectional area of a nutrient foramen of a long bone is related to blood flow requirements of the bone. It was interesting to learn that palaeontologists found the foraminal area of ten species of dinosaurs are generally larger than in mammals (8). These investigators concluded that this indicated that dinosaurs had a highly active and aerobic lifestyle! Fortunately bone blood flow can be Kenneth A. Johnson

Purpose: The objective of this study was to determine whether differences in the composition and metabolism of the extracellular matrix (ECM) in canine anterior cruciate ligaments (ACLs) might relate to mechanical properties of the canine knee. Variations in ACL biochemistry and knee mechanics could account for divergent predispositions to ligament rupture.Methods: Eleven knee joints were obtained from both cadaveric Labrador Retrievers (rupture predisposed) and ex-racing Greyhounds (non-rupture predisposed). Anterioposterior laxity and tensile testing determined mechanical properties of the knee joints and ACL samples respectively. The thermal properties of the collagenous matrix were determined by differential scanning calorimetry (DSC) and the biochemical properties by measuring collagen content. collagen cross-links, glycosaminoglycan (GAG) levels, matrix metalloproteinase-2 (MMP-2), tissue inhibitors of metalloproteinase (TIMP).Results: The anterioposterior laxity was significantly greater (p = 0.04) in the Labrador Retriever knee joints. and their ACLs tended to be weaker (p = 0.06). Greater collagen turnover was demonstrated by significantly higher (P = 0.02) concentrations of pro-MMP-2, and lower enthalpy of denaturation (p = 0.05) in Labrador Retriever ACLs.Conclusions: The different metabolism of the collagenous matrix in the ACLs of dogs predisposed to rupture was related to greater knee joint laxity and lower ligament material properties (ultimate tensile stress). This may be suggestive of a link between ligament rupture and eventual knee osteoarthritis in both dogs and humans. (C) 2004 Orthopaedic Research Society. Published by Elsevier Ltd. All rights reserved.

SummarySurgical procedures such as total hip arthroplasty (THA) or femoral head and neck excision may be indicated as a treatment for traumatic coxo-femoral luxation that is complicated by pre-existing joint disease, concurrent fractures or recurrent luxation. The purpose of our study was to evaluate outcome after treatment of traumatic coxo-femoral luxation by THA. Medical records of dogs undergoing cemented THA from 1996 to 2002 were reviewed. Inclusion criteria were coxofemoral luxation resulting from severe external trauma and radiographic follow-up of at least 3 months. Ten dogs (12 THA) met the criteria for inclusion. Complications included THA luxation (n = 1) and a non-displaced peri-prosthetic femoral fracture (n = 1) that healed without further surgery. Median cumulative function scores from client questionnaires after THA were not significantly different in dogs with previously normal (8, range: 7-10), (n = 5) and dysplastic (9, range: 7–16), (n = 5) hips (P = 0.410). Six dogs were available for re-examination and force plate analysis at greater than 6 months post-THA, and none of these dogs had any visible lameness. Peak vertical force (% BWt) showed a trend towards being less in THA (60.8±5.1) than non-operated (68.1±6.1) hindlimbs (P = 0.057), whereas vertical impulse (% BWt x sec) was similar in THA (9.1±1.6) and non-operated (9.8±1.2) hindlimbs (P = 0.286). Our findings indicate that THA can be a successful treatment for traumatic coxo-femoral luxation, irrespective of the dysplastic status of the joint prior to injury.

Reasons for performing study: Recombinant equine growth hormone (reGH) has recently been evaluated for effects on body condition and wound healing. It has the potential to influence articular cartilage via stimulation of IGF-1.Objectives: To investigate effects of administration on synovial joint metabolism.Methods: Six mature horses were given 20 mug/kg bwt reGH daily for 8 weeks by i.m. injection. Three control horses were injected with sterile water. Serum and synovial fluid samples were collected at 6, 8, 11 and 16 weeks for GH and IGF-1 assays. Articular cartilage harvested at week 16 was evaluated by Western analysis using monoclonal antibodies BC-13, BC-4,8-A-4 and CH-3.Results: Concentrations of IGF-1 in serum and synovial fluid were significantly elevated (P<0.05) at 6 and 8 weeks in the reGH group. Glycosaminoglycan concentrations in synovial fluid were significantly less than controls at these time points, suggesting that reGH may modulate proteoglycan metabolism in articular cartilage. In the reGH group, there were not any alterations in synovial fluid content of 3B3(-) epitope or aggrecan metabolite, or in aggrecan or link protein catabolites retained within cartilage, that might be expected with development of osteoarthritis.Conclusions: Intramuscular administration of reGH may be a more efficient means of delivery of IGF-1 to joints for cartilage resurfacing initiatives.Potential relevance: We found no alterations in cartilage metabolism indicative of development of osteoarthritis.

Lag screw fixation using single 4.5 mm cortical bone screws is a recommended technique for repair of mid-sagittal plane fractures of the distal phalanx in adult horses. However, implant infection and technical difficulties in obtaining adequate interfragmentary compression have made this surgical procedure somewhat controversial. We hypothesized that use of larger diameter screws would result in increased axial compression and improved stability of this fracture. Paired distal phalanges from the forelimbs of 10 adult horses were collected at necropsy and divided in half in the midsagittal plane. Using a randomized block study design, four types of bone screws (4.5 mm cortical, 5.5 mm cortical, 6.5 mm cancellous pre-tapped, and 6.5 mm cancellous non-tapped) were inserted to a depth of 15 mm. During screw insertion, the axial force generated under the screw head was measured with a load washer containing a piezoelectric force transducer, while torque of insertion was recorded with a torsional testing machine. The 6.5mm screw inserted after pre-tapping generated significantly greater axial force (2781 N) than the 4.5 mm (1522 N), 5.5 mm (2073 N) or 6.5 mm non-tapped (2295 N) screws. The relationship between maximal applied torque and axial force generated was linear for each screw type. Each unit of torque applied during insertion of cortical screws resulted in a greater increase in axial compression, as compared to cancellous screws. These data suggest that use of larger diameter screws would result in improved interfragmentary compression of distal phalangeal fractures.

OBJECTIVE:To test the effects of bone diameter and eccentric loading on fatigue life of 2.7-mm-diameter cortical bone screws used for locking a 6-mm-diameter interlocking nail.SAMPLE POPULATION:Eighteen 2.7-mm-diameter cortical bone screws.PROCEDURE:A simulated bone model with aluminum tubing and a 6-mm-diameter interlocking nail was used to load screws in cyclic 3-point bending. Group 1 included 6 screws that were centrally loaded within 19-mm-diameter aluminum tubing. Group 2 included 6 screws that were centrally loaded within 31.8-mm-diameter aluminum tubing. Group 3 included 6 screws that were eccentrically loaded (5.5 mm from center) within 31.8-mm-diameter aluminum tubing. The number of cycles until screw failure and the mode of failure were recorded.RESULTS:An increase in the diameter of the aluminum tubing from 19 to 31.8 mm resulted in a significant decrease in the number of cycles to failure (mean +/- SD, 761,215 +/- 239,853 to 16,941 +/- 2,829 cycles, respectively). Within 31.8-mm tubing, the number of cycles of failure of eccentrically loaded screws (43,068 +/- 14,073 cycles) was significantly greater than that of centrally loaded screws (16,941 +/- 2,829 cycles).CONCLUSIONS AND CLINICAL RELEVANCE:Within a bone, locking screws are subjected to different loading conditions depending on location (diaphyseal vs metaphyseal). The fatigue life of a locking screw centrally loaded in the metaphyseal region of bone may be shorter than in the diaphysis. Eccentric loading of the locking screw in the metaphysis may help to improve its fatigue life.

OBJECTIVE:To compare synovial fluid biomarkers of cartilage metabolism in joints with naturally acquired or experimentally induced cranial cruciate ligament (CCL) rupture and determine correlations with stage and severity of disease in dogs.ANIMALS:95 dogs with ruptured CCL, 8 dogs with experimentally ruptured CCL, and 24 healthy dogs.PROCEDURES:Synovial fluid was assayed for chondroitin sulfate neo-epitopes 3B3(-) and 7D4 and glycosaminoglycan (GAG) concentration. Results were correlated with demographic data, duration of lameness, radiographic osteoarthritis score, and intra-articular lesions.RESULTS:The 7D4 concentrations and 7D4:GAG in synovial fluid from joints with naturally acquired CCL rupture and experimental CCL transection were similar and significantly greater than values for healthy control joints. The 3B3(-) concentrations in the CCL-deficient groups were not significantly different, although only values in the naturally acquired CCL rupture group were significantly greater than those in the healthy control group. Within the naturally acquired CCL rupture group there was a significant correlation between 3B3(-) and 7D4 concentrations. However, there were no significant correlations between biomarker concentrations and continuous demographic or disease-related variables or differences in biomarker concentrations with different categories of disease.CONCLUSIONS AND CLINICAL RELEVANCE:Synovial fluid biomarker concentrations were significantly increased in joints with secondary osteoarthritis associated with naturally acquired or experimental CCL rupture; however, lack of apparently simple relationships with demographic variables or stage or severity of disease limits their clinical usefulness.

CLINICAL RELEVANCEMeasurement of markers of cartilage pathology in synovial fluid may provide clinical rheumatologists and osteoarthritis (OA) researchers important information for early diagnosis of OA as well as a method for monitoring disease progression and response to treatment. This study demonstrates the value of this approach in an established model of OA (cranial cruciate ligament rupture) at a point distant enough from the original surgical manipulation so as to have little to no effect on the marker concentrations.OBJECTIVEThe objective of this study was to determine whether measurement of markers of cartilage collagen cleavage and proteoglycan turnover in synovial fluid from a canine model could be used to detect cartilage changes following the onset of joint instability during the development of OA.DESIGNA model of joint instability that develops OA was created in 18 mature dogs using monopolar radiofrequency energy (MRFE). MRFE was arthroscopically applied to one cranial cruciate ligament (CCL) while the contralateral CCL was sham treated. The treated CCLs ruptured approximately 8 weeks (55 +/- 1.6 days) after MRFE treatment. Synovial fluid was collected at time zero prior to MRFE treatment, 4 weeks after MRFE treatment, and at 4, 8, and 16 weeks after CCL rupture. Synovial fluid concentrations of the neoepitope COL2-3/4C long (type II collagen cleavage by collagenase) and epitopes 3B3(-) (proteoglycan aggrecan sulfation) and 846 (associated with aggrecan synthesis) were analyzed.RESULTSCompared to sham treated joints, the synovial fluid concentrations of COL2-3/4C long and 3B3(-) were significantly increased 2.2 fold and 2.9 fold, respectively, in joints with MRFE treated CCLs following CCL rupture. Concentrations of the 846 epitope in synovial fluid showed a trend toward an increase, which was not significant, after CCL rupture.CONCLUSIONSConcentrations of the collagenase-cleaved type II collagen neoepitope and 3B3(-) epitope in synovial fluid were significantly increased by 4 weeks and remained elevated for at least 16 weeks after CCL rupture. This suggests that in dogs the COL2-3/4C long neoepitope and 3B3(-) epitope are sensitive markers for changes in joint cartilage turnover in joints that are developing OA.

Four immature dogs were presented with ectrodactyly. Two dogs were initially treated with distraction osteogenesis of a hypoplastic ulna and all had surgical correction of the cleft deformity. In each case proximal metacarpal apposition with or without carpal arthrodesis formed the basis of surgical treatment. Follow-up information was available for three cases and all showed improved limb function. Corrective surgery appears to be a viable alternative to amputation for treatment of ectrodacty1y.

OBJECTIVE To evaluate effects of an orally administered mixture of chondroitin sulfate, glucosamine hydrochloride and manganese ascorbate (CS-G-M) on articular cartilage metabolism in dogs with cranial cruciate ligament (CCL) deficient and reconstructed knees, as reflected by concentrations of synovial fluid 3B3, 7D4 and total sulfated glycosaminoglycan (GAG). METHODS Sixteen adult dogs that underwent unilateral CCL transection were randomized into four groups. Thereafter, group I (N=3) had a sham CCL reconstruction, group II (N=3) had CS-G-M and sham CCL reconstruction, group III (N=5) had CCL reconstruction, and group IV (N=5) had CS-G-M and CCL reconstruction. Synovial fluid collected at 0, 1, 3 and 5 months was examined by ELISA for 3B3 and 7D4 epitope, and by DMMB assay for total GAG. RESULTS Synovial fluid from CCL transected knees of CS-G-M treated dogs contained significantly elevated concentrations of 3B3 (P=0.029), 7D4 (P=0.036) and 7D4/GAG (P=0.007) in comparison to controls, in a cross-sectional analysis at 3 months. Furthermore, 7D4 and 7D4/GAG concentrations remained significantly elevated (P=0.012) in CCL transected knees of CS-G-M treated dogs over the 5 month period. However, when epitope concentrations were expressed as a ratio of CCL-transected to contralateral non-operated knee, treatment effect of CS-G-M was no longer significant. Reconstruction of the CCL had no significant effect on synovial fluid epitope. CONCLUSIONS Administration of CS-G-M was associated with altered concentrations of 3B3 and 7D4 epitope in synovial fluid, suggesting that these compounds may act to modulate articular cartilage matrix metabolism in vivo.

OBJECTIVETo quantify geometric, inertial, and histomorphometric properties at the mid-diaphyseal level of left and right metacarpal bones (MCB) of racing Greyhounds.SAMPLE POPULATIONMCB from 7 racing Greyhounds euthanatized for reasons unrelated to MCB abnormalities.PROCEDURESMid-diaphyseal transverse sections of left and right MCB were stained with H&E or microradiographed. Images of stained sections were digitized, and cross-sectional area, cortical area, and maximum and minimum area moments of inertia of each bone were determined. Histomorphometric data (osteonal density, osteonal birefringence, and endosteal new lamellar bone thickness) were collected in 4 quadrants (dorsal, palmar, lateral, medial). Values were compared between limbs and among bones and quadrants.RESULTSCross-sectional area, cortical area, and maximum and minimum moments of inertia of left MCB-IV and -V were significantly greater, compared with contralateral bones. Overall osteonal densities in the dorsal quadrants of left MCB were greater, compared with lateral and medial quadrants. Also, percentage of birefringent osteons was significantly greater in the dorsal quadrant of left MCB-III, -IV, and -V, compared with the palmar quadrant. Thickness of new endosteal lamellar bone was not significantly influenced by limb, bone, or quadrant.CONCLUSIONS AND CLINICAL RELEVANCEIncreased cortical thickness and geometric properties of left MCB-IV and -V of Greyhounds, together with altered turnover and orientation of osteons in the dorsal quadrants of left MCB, are site-specific adaptive responses associated with asymmetric cyclic loading as a result of racing on circular tracks. Site-specific adaptive remodeling may be important in the etiopathogenesis of fatigue fractures in racing Greyhounds.

Objective-To evaluate effects of intra-articular and extracapsular reconstruction of the cranial cruciate ligament (CCL) on metabolism of articular cartilage as reflected by concentrations of chondroitin sulfate epitopes 3B3 and 7D4 in synovial fluid.Animals-13 adult dogs.Procedure-Each dog underwent unilateral CCL transection (CCLT). One month after CCLT, sham CCL reconstruction (3 dogs), intra-articular CCL reconstruction (5), or extracapsular CCL reconstruction (5) was performed. Synovial fluid was collected by direct arthrocentesis from CCLT and contralateral stifle joints immediately before (time 0) and 1, 3, and 5 months after CCLT. Fluid was examined for concentrations of 3B3 and 7D4 epitopes and total sulfated glycosaminoglycan (GAG) content.Results-Concentrations of 3B3, 7D4, and GAG, 3B3:GAG, or 7D4:GAG in CCLT joints did not differ significantly among treatment groups nor in the ratios of these variables in CCLT joints to contralateral joints at 3 months. In a longitudinal analysis, concentrations of 3B3 and 7D4, 3B3:GAG, and 7D4:GAG in CCLT joints in all groups changed significantly with time, but we did not detect time X group interactions.Conclusions and Clinical Relevance-Transection of CCL resulted in significant perturbation in articular cartilage metabolism as reflected by alterations in concentrations of 3B3 and 7D4 in synovial fluid. These changes over time were not significantly influenced by method of CCL reconstruction. We did not find evidence that surgical stabilization of CCL-deficient joints by intra-articular or extracapsular techniques had any effect on preventing alterations in composition of synovial fluid that have been associated with secondary osteoarthritis.

OBJECTIVE Walking exercise alleviates some symptoms, such as pain, in patients with mild to moderate knee osteoarthritis (OA). However, a major concern is that weightbearing exercise on osteoarthritic joints may exacerbate articular cartilage degradation. Loading of proteoglycan depleted articular cartilage in vitro increased expression of the chondroitin sulphate epitope 3B3, suggesting that loading may influence metabolism of osteoarthritic cartilage. This study aimed at evaluating the effects of walking exercise on articular cartilage metabolism in patients with knee OA, as reflected by changes in concentrations of synovial fluid markers. METHODS Thirty elderly patients with knee OA (Kellgren-Lawrence grades II to IV) were randomly allocated to control (n = 15) and 12 week exercise (n = 15) groups. Synovial fluid obtained from 21 of the patients at time zero and after 12 weeks was examined by enzyme linked immunosorbent assay (ELISA) for the chondroitin sulphate epitopes 3B3 and 7D4, and by a dye binding assay with 1,9-dimethylmethylene blue for total sulphated glycosaminoglycan (GAG) concentrations. The 3B3/GAG and 7D4/GAG ratios were calculated. RESULTS No significant changes in concentrations of 3B3, 7D4, GAG, 3B3/GAG, or 7D4/GAG between time zero and 12 weeks were found in either group. However, there were significant declines in 3B3 (p=0.001), GAG (p=0.007), and the 3B3/GAG ratio (p=0.049) with aging. CONCLUSION Twelve weeks of walking exercise had no demonstrable adverse effects on articular cartilage metabolism, as reflected by the concentrations of synovial fluid GAG or the chondroitin sulphate epitopes 3B3 and 7D4.

Fatigue fracture of the cuboidal bones of the foot, especially the navicular tarsal bone, is common in athletes and dancers. The racing greyhound is a naturally occurring animal model of this injury because both microcracking and complete fracture occur in the right central (navicular) tarsal bone (CTB). The right limb is on the outside when racing in a counter-clockwise direction on circular tracks, and is subjected to asymmetric cyclic compressive loading. We wished to study in more detail adaptive modeling in the right CTB in racing greyhounds. We hypothesized that cyclic asymmetric loading of a cuboidal bone induced by racing on a circular track would induce site-specific bone adaptation. We also hypothesized that such an adaptive response would be attenuated in greyhounds that were retired from racing and no longer subjected to cyclic asymmetric loading. Central tarsal bones from racing greyhounds (racing group, n = 6) and retired greyhounds being used for breeding (nonracing group, n = 4) were examined using quantitative computed tomography (CT). Bone mineral density (BMD) was determined in a 3-mm diameter region-of-interest (ROI) in six contiguous 1-mm-thick sagittal CT slices of each CTB. Bones were subsequently examined histomorphometrically and percentage bone area (B.Ar./T.Ar., %) was determined in 10 ROI from dorsal to plantar in a transverse plane, mid-way between the proximal and distal articular surfaces. The BMD of the right CTB was greater than the left in all greyhounds (p < 0.001). In comparing ipsilateral limbs between groups, BMD of the racing group was greater than the nonracing group for each side (p < 0.005). In sagittal plane histologic sections, bone in the dorsal region of the right CTB had undergone adaptive modeling, through thickening and compaction of trabeculae. B.Ar./T.Ar., % in the right CTB of the racing group was greater than in the contralateral CTB (p < 0.001), and the ipsilateral CTB of the nonracing group (p < 0.001). In the nonracing group, B.Ar./T.Ar., % in the right CTB was not significantly different from left CTB (p > 0.8; power = 80% at Δ = 48%). It was concluded that greyhounds racing on circular tracks develop site-specific bone adaptation with compaction of trabecular bone and increase in BMD in the right CTB in particular, the most common site for fatigue fracture. Our data also suggested that partial reversal of this adaptive process occurred in retired, nonracing greyhounds, after cessation of asymmetric cyclic loading at racing speed. Racing greyhounds provide a model in which to study fatigue fracture and adaptation of cuboidal foot bones subjected to cyclic loading.

Magnetic resonance imaging was conducted on previously frozen left carpi from six normal dogs using a 1.5 Tesla magnet in combination with a transmit/receive wrist coil. Three-millimeter thick T1-weighted spin-echo images and 1-mm thick T2*-weighted gradient-recalled 3-D images were obtained in dorsal and sagittal planes. Carpi were embedded, sectioned, and stained. Anatomic structures on the histologic sections were correlated with the MR images. All of the carpal ligaments plus the radioulnar articular disc and the palmar fibrocartilage were identified on MR images. The accessorio-quartile ligament, which had not been well described previously in dogs, was also identified. It originated on the accessory carpal bone and inserted on the fourth carpal bone. The T2*-weighted gradient echo imaging technique provided better images than T1-weighted technique, largely because thinner slices were possible (1 mm vs. 3 mm), resulting in less volume averaging of thin ligaments with surrounding structures. Although MRI is currently the imaging modality of choice to identify ligamentous injury in humans, further studies are needed to determine if abnormalities can be detected in canine carpal ligaments using MRI.

Greyhound central tarsal bone (CTB) from animals with (n = 11) and without CTB fatigue fracture (n = 15) was examined histologically for the presence, numerical density, and morphology of in vivo microdamage. Complete fracture of the right CTB is a common occurrence during dog racing, because this is the outside limb when running counterclockwise on a circular or oval track. The CTB consisted of both remodeled cortical bone and inner trabecular bone. Thickening and coalescence of trabeculae were observed, particularly dorsally and medially, causing reduction or elimination of the marrow void spaces. A band of tightly packed transverse osteons was also observed adjacent to the concave proximal joint surface. Typical linear microcracks were most often seen in remodeled cortical and trabecular bone and were often observed adjacent to vascular channels. In contrast, ultra-microcracking, represented by diffuse staining with basic fuchsin, was consistently observed in the plantar process around the attachment site for the plantar ligament complex. Dog status (fractured or intact) and side (left or right) both had a significant effect on microcrack density and microcrack surface density (p < 0.05). Microcrack density and microcrack surface density were increased in the right (fractured) CTB from greyhounds with CTB fracture. There was also a trend for side to have a significant effect on microcrack length, with microcrack lengths being higher in the right CTB of both intact and fractured dogs. These data support the general hypothesis that fatigue fracture occurs because of ongoing cyclic stresses after induction of reparative remodeling. Development of methods for biomechanical testing of small cuboidal bones should allow investigation of relationships between accumulation of loading cycles and bone weakening because of microdamage.