CONGENITAL RESPIRATORY TRACT DEFECTS IN LAMBS

One hundred and forty-seven newborn lambs with congenital defects of the respiratory tract were examined during a 3-year investigation into the causes of perinatal lamb mortality. Respiratory defects were found in 3.3% of the 4,417 lambs examined and in one-third (36.6%) of the 401 congenitally malformed lambs. All the defects were secondary to other more extensive malformations. The upper respiratory tract was involved 3 times more than the lower tract. Males were involved more than females (4:3). The upper tract defects were harelip, cleft palate, atelorrhinia, arrhinia, proboscis, fused nostrils and choanal atresia. One of the facial clefts was a rare median harelip associated with cleft palate and fused nostrils. The lower tract defects were pulmonary agenesis, hypoplasia and duplication. The 2 most common defects were choanal atresia and secondary pulmonary hypoplasia. It was concluded that primary defects of the respiratory tract of sheep are rare and that secondary defects are relatively common.     

Morphologic and biochemical study of sternal cartilage autografts for resurfacing induced osteochondral defects in horses.

Using biodegradable pins, sternal cartilage autografts were fixed into osteochondral defects of the distal radial carpal bone in ten 2 to 3-year-old horses. The defects measured 1 cm2 at the surface and were 4 mm deep. Control osteochondral defects of contralateral carpi were not grafted. After confinement for 7 weeks, horses were walked 1 hour daily on a walker for an additional 9 weeks. Horses were euthanatized at 16 weeks. Half of the repair tissue was processed for histologic and histochemical (H&E and safranin-O fast green) examinations. The other half was used for the following biochemical analyses: type-I and type-II collagen contents, total glycosaminoglycan content, and galactosamine-to-glucosamine ratio. On histologic examination, the repair tissue in the grafted defects consisted of hyaline-like cartilage. Repair tissue in the nongrafted defects consisted of fibrocartilaginous tissue, with fibrous tissue in surface layers. On biochemical analysis, repair tissue of grafted defects was composed predominantly of type-II collagen; repair tissue of non-grafted defects was composed of type-I collagen. Total glycosaminoglycan content of repair tissue of grafted defects was similar to that of normal articular cartilage. Total glycosaminoglycan content of nongrafted defects was 62% of that of normal articular cartilage (P less than 0.05). Repair tissue of all defects was characterized by galactosamine-to-glucosamine ratio significantly (P less than 0.05) higher than that of normal articular cartilage. These results at 16 weeks after grafting indicate that sternal cartilage may potentially constitute a suitable substitute for articular cartilage in large osteochondral defects of horses.     

Morphologic study of repair of induced osteochondral defects of the distal portion of the radial carpal bone in horses by use of glued periosteal autografts [corrected

The use of periosteal autografts to resurface osteochondral defects was investigated in 10 horses (2 to 3 years old), and the repair tissue was characterized morphologically. Middle carpal joint arthrotomies were made, and osteochondral defects were induced bilaterally on the distal articular surface of each radial carpal bone. Each defect measured approximately 1 cm2 and extended 3 mm into the subchondral bone plate. Residual subchondral bone plate of control and principal defects was perforated by drilling. A sterile fibrin adhesive was made by mixing a fibrinogen component and a thrombin component. A periosteal autograft was harvested from the proximal portion of the tibia and was glued onto the recipient osseous surface, with its cambium facing the joint cavity. Control defects were glued, but not grafted. Horses were walked 1 hour daily on a walker, starting at postoperative week 7 and continuing for 9 weeks. Sixteen weeks after the grafting procedure was done, carpal radiography was performed, after which horses were euthanatized. Quality of repair tissue of control and grafted defects was evaluated and compared grossly, histologically, and histochemically. Using a reticule, the proportions of various repair tissue types filling each defect were quantitated. Seven weeks after the grafting procedure was done, bilateral arthroscopy revealed synovial adhesions and marginal pannus formation in control and grafted defects. None of the autografts was found floating unattached within the respective middle carpal joints. At 16 weeks, the gross appearance of most grafted and nongrafted defects was similar, and repair was dominated by a fibrous pannus. In 4 grafted defects, bone had formed either concentrically within the defect or eccentrically in the fibrous adhesions between the defect and the joint margin. Histologically, all grafted and nongrafted defects were repaired similarly by infiltration of a mixture of fibrous tissue, fibrocartilage, and bone. Fibrous tissue was the predominant tissue in most defects and its mean proportion was 56 and 59% in the grafted and nongrafted defects, respectively. Fibrocartilaginous tissue in the deeper layers approximated 20%, and woven bone at the base of the defect was 20% in all defects. Histochemically, difference in staining for proteoglycans was not observed between grafted and nongrafted defects. Little remaining original periosteal graft tissue was evident at the defect sites. The only distinguishing feature of grafted defects was the presence of islands of bone formation either at the defect site (n = 2 horses), or in somewhat dorsally displaced tissue that was incorporated in fibrous adhesions (n = 2 horses).(ABSTRACT TRUNCATED AT 400 WORDS)     

Surgical correction of concomitant cranioventral abdominal wall, caudal sternal, diaphragmatic, and pericardial defects in young dogs

Seven puppies with concomitant congenital cranioventral abdominal wall, caudal sternal, diaphragmatic, and pericardial defects were treated surgically when they were between 10 and 12 weeks old. Three pups had ventricular septal defects that were not corrected. Diaphragmatic herniorrhaphy without extension of the diaphragmatic defect was performed in 6 pups. In one pup, paracostal extension of the diaphragmatic defect was necessary to decrease tension on the diaphragmatic closure. All pups were healthy at 6-month follow-up examinations, but 2 of 3 pups with ventricular septal defects had moderate generalized cardiomegaly evident on thoracic radiography. Early surgical correction of the congenital defects in these pups was usually simple because there were few or no thoracic adhesions, the dogs were small, the defects were small in 6 of 7 dogs, and the costal arch was pliable in each dog.     

Biocompatibility of three-dimensional chondrocyte grafts in large tibial defects of rabbits

OBJECTIVE: To evaluate biocompatibility and effects of implantation of 3-dimensional chondrocyte-agarose autografts in tibial defects in rabbits and to compare in vitro and in vivo chondrocyte-agarose constructs with respect to cell viability, differentiation, and matrix production. ANIMALS: 24 adult New Zealand White rabbits. PROCEDURE: Three-dimensional constructs with (grafted group) or without (control group) autogenous chondrocytes were implanted into tibial defects of rabbits and cultured in vitro. During an 8-week period, defects were evaluated radiographically, grossly, histologically, biochemically, and immunohistochemically. In vitro constructs were evaluated histologically, biochemically, and immunohistochemically. RESULTS: Tibial defects had significantly higher radiographic densitometry values at 4 and 6 weeks after implantation in grafted group rabbits, compared with control group rabbits. Number of observed centers of endochondral ossification was significantly greater in defects of grafted group rabbits, compared with control group rabbits. On day 14, glycosaminoglycan concentration was significantly higher in tibial defects of grafted group rabbits, compared to defects of control group rabbits or in vitro constructs. At weeks 2, 4, and 8, glycosaminoglycan concentrations were significantly lower in the in vitro control constructs, compared with other groups. Collagen type I was present in bone and bony callous in defects of grafted and control group rabbits. Collagen type II was identified in cartilaginous tissues of grafted and control group rabbits. Collagen type X was associated with hypertrophic chondrocytes. Only type II collagen was found in the in vitro chondrocyte constructs. CONCLUSIONS AND CLINICAL RELEVANCE: Chondrocyte-agarose grafts are biocompatible in large tibial defects and appear to provide a cell source for augmenting endochondral ossification.     

Biochemical study of repair of induced osteochondral defects of the distal portion of the radial carpal bone in horses by use of periosteal autografts

Periosteal autografts were used for repair of large osteochondral defects in 10 horses aged 2 to 3 years old. In each horse, osteochondral defects measuring 1.0 x 1.0 cm2 were induced bilaterally on the distal articular surface of each radial carpal bone. Control and experimental defects were drilled. Periosteum was harvested from the proximal portion of the tibia and was glued into the principal defects, using a fibrin adhesive. Control defects were glued, but were not grafted. Sixteen weeks after the grafting procedure, the quality of the repair tissue of control and grafted defects was assessed biochemically. Total collagen content and the proportion of type-II collagen were determined. Galactosamine and glucosamine contents also were determined. From these measurements, contents of chondroitin and keratan sulfate and total glycosaminoglycan, and galactosamine-to-glucosamine ratio were calculated. All biochemical variables were compared with those of normal equine articular cartilage taken from the same site in another group of clinically normal horses. Total collagen content was determined on the basis of 4-hydroxyproline content, using a colorimetric method. The proportions of collagen types I and II in the repair tissue were assessed by electrophoresis of their cyanogen bromide-cleaved peptides on sodium dodecyl sulfate slab gels. Peptide ratios were computed and compared with those of standard mixtures of type-I and type-II collagens. Galactosamine and glucosamine contents were determined by use of ion chromatography. In general, the biochemical composition of repair tissue of grafted and nongrafted defects was similar, but clearly differed from that of normal articular cartilage.(ABSTRACT TRUNCATED AT 250 WORDS)     

Arthroscopic Subchondral Bone Plate Microfracture Technique Augments Healing of Large Chondral Defects in the Radial Carpal Bone and Medial Femoral Condyle of Horses

OBJECTIVE: To evaluate the effect of arthroscopic subchondral bone microfracture on healing of large chondral defects in horses. STUDY DESIGN: Short- (4 months) and long-term (12 months) in vivo experimental chondral defect model. ANIMALS: 10 horses, aged 2 to 5 years. METHODS: Each horse had a 1 cm2 full-thickness chondral defect created in both radial carpal bones and both medial femoral condyles. One carpus and one femoral condyle of each horse had the subchondral bone plate under the defect perforated using an orthopedic awl. All horses were exercised, five horses were evaluated after 4 months and five horses after 12 months. Gross, histologic, and histomorphometric examination of defect sites and repair tissues was performed, as was collagen typing of the repair tissue. RESULTS: On gross observation a greater volume of repair tissue filled treated defects (74%) compared with control defects (45%). Histomorphometry confirmed more repair tissue filling treated defects, but no difference in the relative amounts of different tissue types was observed. There was an increased percentage of type II collagen in treated defects compared with control defects and evidence of earlier bone remodeling as documented by changes in porosity. CONCLUSIONS: In full-thickness chondral defects in exercised horses, treatment with subchondral bone microfracture increased the tissue volume in the defects and the percentage of type II collagen in the tissue filling the defects when compared to nontreated defects. CLINICAL RELEVANCE: No negative effects of the microfracture technique were observed and some of the beneficial effects are the basis for recommending its use in patients cases with exposed subchondral bone.     

Postoperative integrity of veterinary surgical gloves

A multicenter, prospective study was performed to document the incidence of defective gloves postoperatively in veterinary surgery and to correlate defects with a variety of influencing factors. Gloves were collected after surgical procedures performed by the small animal clinical services at two veterinary teaching hospitals and one institution's student surgery laboratories. Gloves were evaluated for defects using electrical resistance testing. The overall incidence of glove defects was 23.3%. Significantly more defects occurred in gloves used for nonsoft-tissue procedures and in gloves worn on the nondominant hand. Eighty-four percent of all defects occurred in procedures lasting >60 minutes. No differences were detected in the brands of gloves used nor among surgeons of different experience levels. The individuals performing the surgery were not able to accurately predict the presence of a defect in their gloves. Surgeons should remain alert for possible glove defects and consider measures such as changing gloves every 60 minutes or double-gloving to minimize potential complications.     

Study of how frequently surgeons' gloves are perforated during operations

Surgical gloves were collected after 231 first opinion and referral surgical procedures performed at the University of Liverpool Small Animal Hospital. They were tested for defects by the water load test and the site of any defects was recorded; 10.2 per cent of the gloves had defects, and at least one glove became defective during 51 (22.1 per cent) of the surgical procedures. Significantly more defects occurred during orthopaedic procedures and the likelihood of a defect occurring increased with the duration of the surgical procedure. The majority (78 per cent) of the defects occurred in the gloves worn on the non-dominant hand and 64 per cent of them were in the index finger. The surgeon or assistant was aware of a glove being punctured in only 3.8 per cent of cases.     

Abaxial implantation of the middle piece in spermatozoa and spermatids in related sterile boars

Spermatozoa and developing spermatids showing neck region abnormalities have been studied in material from 2 genetically related boars. In both boars the defects were abaxial implantation of the tails and lack of substance in the neck region. In many spermatozoa, a too wide space between the capitulum and the basal plate was more pronounced in epididymal spermatozoa compaired to testicular material. This implies that the defect aggravated, and might be connected with the migration of the cytoplasmic droplet in the epididymis. Since the defects were observed in spermatids, it its concluded that the defects were heriditary.This conclusion was further supported by the observation of similar defects in 6 other related boars, examined by light microscopy only.     

Effect of hyperbaric oxygen treatment on incorporation of an autogenous cancellous bone graft in a nonunion diaphyseal ulnar defect in cats

OBJECTIVE: To determine whether hyperbaric oxygen treatment (HBOT) would affect incorporation of an autogenous cancellous bone graft in diaphyseal ulnar defects in cats. ANIMALS: 12 mature cats. PROCEDURE: Bilateral nonunion diaphyseal ulnar defects were created in each cat. An autogenous cancellous bone graft was implanted in 1 ulnar defect in each cat, with the contralateral ulnar defect serving as a nongrafted specimen. Six cats were treated by use of hyperbaric oxygen at 2 atmospheres absolute for 90 minutes once daily for 14 days, and 6 cats were not treated (control group). Bone labeling was performed, using fluorochrome markers. Cats were euthanatized 5 weeks after implanting, and barium sulfate was infused to evaluate vascularization of grafts. Ulnas were evaluated by use of radiography, microangiography, histologic examination, and histomorphometric examination. RESULTS: Radiographic scores did not differ between treatment groups. Microangiographic appearance of grafted defects was similar between groups, with all having adequate vascularization. Differences were not observed between treated and nontreated groups in the overall histologic appearance of decalcified samples of tissue in grafted defects. Mean distance between fluorescent labels was significantly greater in cats given HBOT than in nontreated cats. Median percentage of bone formation in grafted defects was significantly greater in cats given HBOT. CONCLUSIONS: Hyperbaric oxygen treatment increased the distance between fluorescent labels and percentage of bone formation when incorporating autogenous cancellous bone grafts in induced nonunion diaphyseal ulnar defects in cats, but HBOT did not affect revascularization, radiographic appearance, or qualitative histologic appearance of the grafts.     

The effect of compacted cancellous bone grafting on the healing of subchondral bone defects of the medial femoral condyle in horses

Objective— To compare the quality of second-intention healing and that of compacting sternally harvested cancellous bone into subchondral bone defects of the medial femoral condyle in horses.
Study Design— A controlled experiment using a surgical technique that minimizes soft tissue trauma, customized for consistency among horses.
Animals or Sample Population— Ten horses, aged 2 to 5 years, free of hindlimb lameness and with radiographically normal stifles.
Methods— After a 12.7-mm-diameter × 19-mm-deep defect was created into randomly selected medial femoral condyles, bone and cartilage healing was evaluated over a 6-month period in control horses (  n = 5  ) and horses receiving a compacted cancellous bone graft (  n = 5  ). Healing was evaluated using lameness assessment, radiographic and microradiographic interpretation, arthroscopic appearance, percent bone fill, proteoglycan content, and histology.
Results— Six months after surgery, there was no significant difference between grafted and ungrafted defects with respect to lameness, radiographic score, or percent bone fill. Histologically, grafted defects were characterized by the presence of dead graft and secondary cyst formation in four defects. Ungrafted defects filled with fibrous tissue and no cyst formation were identified.
Conclusions— Grafted defects do not heal better than ungrafted defects, and lameness was not affected by surgical technique.
Clinical Significance— Cartilage healing is similar in grafted and ungrafted defects in the equine medial femoral condyle at 6 months, suggesting that surgical debridement alone of cystic structures remains the treatment of choice.     

Congenital Cardiac Defects in Neonatal Foals: 18 Cases (1992–2007)

Background: Literature available regarding congenital cardiac defects in foals is limited to reports of individual cases or small case series.
Objective: To describe the clinical, echocardiographic, and necropsy findings and breed predilection of congenital cardiac defects in neonatal foals.
Animals: Eighteen foals <15 days of age with 1 or more congenital cardiac defects.
Methods: Medical records of foals diagnosed with congenital cardiac defects at the William R. Pritchard Veterinary Medical Teaching Hospital were reviewed. Data collected included history, signalment, clinical signs, laboratory data, diagnostic and necropsy results, and outcome.
Results: Arabian foals represented 39% of cases with congenital cardiac defects and were significantly ( P = .004) overrepresented (OR = 4.7 [CI: 1.8–12.4]) compared with the general hospital population. Ventricular septal defect (VSD) (14/18), tetralogy of Fallot (5/18), and tricuspid valve atresia (4/18) were the most common defects identified. A ≥3/6 heart murmur (14/14) accompanied by tachycardia (14/17), tachypnea (17/17), and cyanosis of mucous membranes (7/16) were the most common clinical signs. Concurrent congenital defects were common (9/18). Two foals, both with VSD, survived for ≥ 8 years after diagnosis and 1 was a successful performance horse.
Conclusions and Clinical Relevance: Arabian horses appear to have a predisposition for cardiac defects. The presence of a loud murmur (≥ 3/6), cyanotic membranes, and tachycardia or tachypnea in a neonatal foal should warrant thorough evaluation of the heart for congenital defects. Foals with cardiac defects should be closely evaluated for concurrent congenital defects in other body systems.     

Response of induced bone defects in horses to collagen matrix containing the human parathyroid hormone gene

OBJECTIVE: To determine whether human parathyroid hormone (hPTH) gene in collagen matrix could safely promote bone formation in diaphyseal or subchondral bones of horses. ANIMALS: 8 clinically normal adult horses. PROCEDURE: Amount, rate, and quality of bone healing for 13 weeks were determined by use of radiography, quantitative computed tomography, and histomorphometric analysis. Diaphyseal cortex and subchondral bone defects of metacarpi were filled with hPTH(1-34) gene-activated matrix (GAM) or remained untreated. Joints were assessed on the basis of circumference, synovial fluid analysis, pain on flexion, lameness, and gross and histologic examination. RESULTS: Bone volume index was greater for cortical defects treated with hPTH(1-34) GAM, compared with untreated defects. Bone production in cortical defects treated with hPTH(1-34) GAM positively correlated with native bone formation in untreated defects. In contrast, less bone was detected in hPTH(1-34) GAM-treated subchondral bone defects, compared with untreated defects, and histology confirmed poorer healing and residual collagen sponge. CONCLUSIONS AND CLINICAL RELEVANCE: Use of hPTH(1-34) GAM induced greater total bone, specifically periosteal bone, after 13 weeks of healing in cortical defects of horses. The hPTH(1-34) GAM impeded healing of subchondral bone but was biocompatible with joint tissues. Promotion of periosteal bone formation may be beneficial for healing of cortical fractures in horses, but the delay in onset of bone formation may negate benefits. The hPTH(1-34) GAM used in this study should not be placed in articular subchondral bone defects, but contact with articular surfaces is unlikely to cause short-term adverse effects.     

Ventricular septal defects in the horse

Clinical, echocardiographic, and right-side cardiac catheterization data were collected in 6 horses with ventricular septal defects. The defects were confirmed by necropsy in 5 horses. On echocardiography, the cardiac dimensions were normal in 3 horses and enlarged in 2 others. A step-up in partial oxygen pressure between right atrium and right ventricle suggested a left-to-right shunt in 3 of the 4 horses catheterized. In 1 foal, a small defect without PO2 step-up was documented by angiocardiography. The oxymetry and pressure data were compared with previous cases from the reviewed literature, and the importance of pressure measurements to rule out concomitant defects was emphasized.     

Repair of sinocutaneous fistulae in two horses

Chronic facial defects extending into the sinuses of two mares were repaired using periosteal flaps. Partial osseous bridging was established over the defects following repair. Previous attempts to close the defects with skin alone had failed. A periosteal flap is a simple surgical technique that provides the foundations for successful repair, despite the chronicity of some fistulae.     

Prevalence and types of birth defects in Ontario swine determined by mail survey.

Preweaning mortality in piglets constitutes a major loss to the swine industry. Congenital defects account for a small but significant proportion of these losses. To implement appropriate strategies to reduce such losses, it is necessary to identify the specific causes and their relative importance. Consequently, a mail survey of swine production in Ontario was carried out to determine the prevalence and types of birth defects. Statistical comparisons of the prevalence of overall defects were made between accurate and estimate records, breeds (cross vs. purebred), size of operation (number of sows) and geographic location. The mean litter size of 11 pigs born per sow was not significantly different for those with accurate versus estimate records, but the difference in the prevalence of defective pigs (live and dead) was significant (accurate 3.1% vs. estimate 4.1%). Splayleg (spraddleleg) was the most common defect. The next four defects for both groups were belly rupture, other rupture, ridglings and other, but not in the same ranking. Purebred and small farm operations (< 25 sows) had a significantly higher prevalence of birth defects for estimated data only. Geographic location had no effect. Further work is required to determine whether recording prevalence of birth defects in Ontario swine will provide a useful monitor of environmental stress. The study provides a baseline for the prevalence and type of defects in Ontario swine.     

PERINATAL LAMB MORTALITY IN WESTERN AUSTRALIA 7. Congenital Defects

Four hundred and one malformed lambs were examined during a 3-year investigation into the causes of perinatal lamb mortality. Frequence of involvement of the various body systems was: musculo-skeletal 55.4%, digestive 12.7%, cardiovascular 9.7%, uro-genital 7.1%, nervous 6.0%, special senses 3.5%, integument 3.2%, and endocrine 1.5%. A third of the defects (32.9%) involved more than one body system. There was a sex difference of 3 males: 2 females. The common defects in the lambs autopsied in order of frequence were: agnathia, atresia ani, cardiac defects, arthrogryposis, micrognathia, scrotal bifurcation, embryonic duplications, hernias, meningocele, and perosomus elumbis. Agnathia, atresia ani and persistent umbilical haemorrhage appeared to be hereditary. There was no relationship between congenital defects and the time of lambing and phytoestrogens.     

Retrospective review of congenital heart disease in 976 dogs

Background: Knowledge of epidemiology is important for recognition of cardiovascular malformations. Objective: Review the incidence of congenital heart defects in dogs in Italy and assess breed and sex predispositions. Animals: Nine hundred and seventy‐six dogs diagnosed with congenital heart disease (CHD) of 4,480 dogs presented to Clinica Veterinaria Gran Sasso for cardiovascular examination from 1997 to 2010. Methods: A retrospective analysis of medical records regarding signalment, history, clinical examination, radiography, electrocardiography, echocardiography, angiography, and postmortem examination was performed. Breed and sex predisposition were assessed with the odds ratio test. Results: CHD was observed in 21.7% of cases. A total of 1,132 defects were observed with single defects in 832 cases (85%), 2 concurrent defects in 132 cases (14%), and 3 concurrent defects in 12 cases (1%). The most common defects were pulmonic stenosis (PS; 32.1%), subaortic stenosis (SAS; 21.3%), and patent ductus arteriosus (20.9%), followed by ventricular septal defect (VSD; 7.5%), valvular aortic stenosis (AS; 5.7%), and tricuspid dysplasia (3.1%). SAS, PS, and VSD frequently were associated with other defects. Several breed and sex predispositions were identified. Conclusions and Clinical Relevance: The results of this study are in accordance with previous studies, with slight differences. The breed and sex predilections identified may be of value for the diagnosis and screening of CHD in dogs. Additionally, the relatively high percentage of concurrent heart defects emphasizes the importance of accurate and complete examinations for identification. Because these data are from a cardiology referral center, a bias may exist.     

Effects of phenylbutazone on bone activity and formation in horses

OBJECTIVE: To determine the effects of phenylbutazone (PBZ) on bone activity and bone formation in horses. ANIMALS: 12 healthy 1- to 2-year-old horses. PROCEDURES: Biopsy was performed to obtain unicortical bone specimens from 1 tibia on day 0 and from the contralateral tibia on day 14. Fluorochromic markers were administered IV 2 days prior to and on days 0, 10, 15, and 25 after biopsy was performed. Six horses received PBZ (4.4 mg/kg of body weight, PO, q 12 h) and 6 horses were used as controls. All horses were euthanatized on day 30 and tissues from biopsy sites, with adjacent cortical bone, were collected. Osteonal density and activity, mineral apposition rate (MAR), and percentage of mineralized tissue filling the biopsy-induced defects in cortical bone were assessed. Serum samples from all horses were analyzed for bone-specific alkaline phosphatase activity and concentration of PBZ. RESULTS: MAR was significantly decreased in horses treated with PBZ. Regional acceleratory phenomenon was observed in cortical bone in both groups but was significantly decreased in horses treated with PBZ. Osteonal activity was similar at all time points in all horses. In control horses, percentage of mineralized tissue filling the cortical defects was significantly greater in defects present for 30 days, compared with defects present for 14 days. Differences in percentage of mineralized tissue were not detected in horses treated with PBZ. CONCLUSIONS AND CLINICAL RELEVANCE: PBZ decreased MAR in cortical bone and appeared to decrease healing rate of cortical defects in horses.