Equine subchondral lucencies: Knowledge from the medial femoral condyle.

Equine subchondral lucencies (SCL) have been described since the first availability of suitable radiographic equipment. The initial clinical sign can be lameness, but SCLs are often first found on surveys of juvenile horses and are primarily a radiographic concern for public auctions. When lameness is present, it varies from subtle to obvious and can be intermittent. Some SCLs heal spontaneously, and some remain blemishes, but when the SCL and lameness are persistent, further damage to the joint and limitations to an athletic career are likely. SCLs were initially described in the distal limb followed by the stifle, and the medial femoral condyle (MFC) is now considered the most common location. The aim of this review is to highlight the initial pathology and discuss the clinical and experimental information available on equine SCLs. SCL treatment has evolved from rest alone and has progressed to debridement, grafting, intralesional injection, and most recently, transcondylar screw and absorbable implant placement. Comparison of success rates between techniques is difficult due to variations in follow-up and outcome measures, and no single technique is best for all SCLs. Treatment appears to increase success by 15%-20% over rest alone, but the method chosen depends on many factors. This review emphasizes the need for further work to fully understand SCL formation and all aspects of trabecular bone healing to optimize surgical therapy and improve treatment success.

Compression generated by cortical screws in an artificial bone model of an equine medial femoral condylar cyst.

OBJECTIVE: Determine compression generated by lag and neutral screws over 12 h using two bone analogs. STUDY DESIGN: Experimental study. SAMPLE POPULATION: Bone analogs were made of composite synthetic bone (CSB) or three-dimensional printed polylactic acid (PLA). Analogs had a 2 mm exterior shell with a 10 mm thick internal layer of open-cell material. METHODS: Bone analogs were opposed, making a 4-sided box with open ends. A central channel contained the sensor and the screws passed through it to engage both paired analogs. Four screw/analog conditions were tested: neutral and lag screw with bicortical engagement, neutral and lag screw with unicortical engagement. All screws were tightened to 2 Nm torque and compression values recorded at 0, 0.5, 1, 2, 6, and 12 h (six trials per condition). Medians were compared across groups for statistical significance. RESULTS: There was no difference in median compression between lag and neutral bicortical screws. For PLA, greater median compression was generated by neutral (median 437 N) and lag (median 379 N) bicortical screws compared to neutral unicortical screws (median 208 N, p < .001); lag bicortical screws generated greater median compression than lag unicortical screws (median 265 N, p = .012). For CSB, lag bicortical screws (median 293 N) generated greater median compression than neutral unicortical screws (median 228 N, p = .008). CONCLUSION: Lag and neutral screws generated similar compression. Bicortical screws had higher median compression than unicortical screws in bone analogs. CLINICAL SIGNIFICANCE: Neutral screws generate compression in cancellous bone analogs that can be increased with bicortical bone engagement.

The failure mode of a mechanically loaded equine medial femoral condyle analog with a void and the impact of lag and neutral screw placement.

OBJECTIVE: To determine the failure method of simulated equine medial femoral condyle (MFC) subchondral bone defects under compression and the influence of screw placement on failure resistance. STUDY DESIGN: In vitro study. SAMPLE POPULATION: Composite disks (CD) simulating the moduli of yearling bone in the MFC. METHODS: Four CD conditions were tested, all with a 12.7 mm void (n = 6 per condition): intact (no void), void only, void with a 4.5 mm screw placed in neutral fashion, and void with a 4.5 mm screw placed in lag fashion. Composite disks of each condition were tested under monotonic compression to 6000 N and cyclic compression to 10 000 cycles. Observable failure, load at first observable failure, and displacement at peak 2000 N load were compared among conditions. RESULTS: Specimens failed by cracking at the superior aspect of the void or the screw exit hole. After monotonic loading, cracks were observed 6/6 CD with a void, 6/6 CD with a void/lag screw, and 5/6 CD with a void/neutral screw. After cyclical testing, cracks were noted only on the superior aspect of 6/6 CD with a void and 3/6 CD with a void/lag screw. Displacement at peak load was 0.06 mm (intact), 0.32 mm (void), 0.24 mm (void/lag screw), and 0.11 mm (void/neutral screw). CONCLUSION: Model MFC voids failed by superior cracking that was resisted by lag and neutral screw placement. CLINICAL SIGNIFICANCE: Neutral screws may be an acceptable treatment for subchondral lucencies in the MFC.

Impact of Size and Shape of Equine Femoral Subchondral Bone Cysts With a Transcondylar Screw on Predicted Bone Formation Area in a Finite Element Model.

Equine subchondral bone cysts (SBCs) develop most often in the medial femoral condyle (MFC) of yearlings intended for performance. SBCs often cause lameness and can cause secondary injuries to the meniscus and tibial cartilage. A novel surgical technique using a transcondylar lag screw (TLS) across an MFC SBC has shown success in lameness resolution and radiographic healing of MFC SBC. In a previous study using finite element analysis, our lab showed that a TLS stimulated bone formation on the inner surface of the SBC and altered third principal stress vectors to change the direction of surface compression to align with the screw axis. This work extended the previous study, which was limited by the use of only one idealized SBC. Our objective was to test SBCs of several sizes and shapes in a newly developed equine stifle FEM with a TLS to determine how cyst size affects bone formation stimulation. This study found that a transcondylar screw is most effective in stimulating bone formation in cysts of greater height (proximal-distal). The TLS increases stress stimulus in the bone around the cyst to promote bone apposition and directs compression across the cyst. If full penetration of the screw through the cyst is possible, it is recommended that the transcondylar screw be used to treat subchondral bone cysts. For the treatment of smaller cysts that are not accessible by the current screw surgical approach, future work could study the efficacy of a dual-pitch headless screw that may reach smaller cysts.

Intrasinus bolstering of traumatic maxillary sinus fractures by using Foley catheter balloons in two foals.

OBJECTIVE: To describe the repair of unstable facial fractures by using Foley catheter balloons as intrasinus bolsters. STUDY DESIGN: Case report ANIMALS: Two weanling foals with unilateral fractures of the sinus and orbit secondary to kick injuries. Preoperative imaging that included positive contrast dacrocystorhinography and computed tomography confirmed severe comminution of facial fractures and nasolacrimal duct disruption in both foals. METHODS: Small bone fragments were surgically removed, and large fragments were retained even when denuded of periosteum. Repair procedures included nasolacrimal canaliculosinusotomy and suturing fracture fragments together with polydioxanone sutures. After fixation, the fracture fragments could be depressed into the sinus with manual pressure, so two intrasinus Foley catheters were placed to bolster the sinus wall, with the tubing exiting through a frontal sinus trephine. The skin was completely closed over the fractures. Catheters and nasolacrimal stenting were maintained in place during fracture healing. RESULTS: One foal prematurely dislodged catheters and nasolacrimal stent 11 days after fixation. The catheters and stenting were removed as planned 4 weeks after surgery in the second foal. Wound, fracture healing, and overall cosmesis was good in both foals, and epiphora resolved. Surgical site infection, sinusitis, and sequestration did not occur. Both foals became high-level performance horses with acceptable cosmetic outcome and good bilateral nasal airflow. CONCLUSION: Foley catheter balloons supported sinus fracture repair and maintained stability of the surgical reconstruction during convalescence. CLINICAL SIGNIFICANCE: Suture repair of comminuted sinus fractures can be supported by using Foley catheters, which are readily available.

Subchondral lucencies of the proximal tibia in 17 horses.

OBJECTIVE: To describe subchondral lucencies (SCL) in the equine proximal tibia, several treatment options, and clinical outcomes. STUDY DESIGN: Retrospective study. ANIMALS: Seventeen horses with proximal tibial SCL. METHODS: Medical record and radiograph review. Follow-up was obtained via examination and radiography when possible and by telephone and race records when required. The median duration of follow-up was 20 months (range, 0-48). RESULTS: Proximal tibial SCL were associated with lameness in 14 of 17 horses. Subchondral lucencies were primary in 11 horses and secondary to an ipsilateral medial femoral condyle SCL in six horses. One foal with a primary SCL was euthanized because of osteomyelitis. Six horses ≤1 year old with primary SCL were managed with exercise restrictions only; SCL in three horses without lameness decreased in size, whereas three horses with lameness did not improve. One young horse treated with surgical debridement failed to improve and was euthanized. Lameness resolved in three horses with primary tibial SCL treated with screw fixation. Screw fixation of secondary SCL in five horses led to a reduction in SCL size and degree of lameness. CONCLUSION: Primary tibial SCL healed with rest in 3 non-lame young horses with small SCL, but was not successful in lame horses with larger SC. Radiographic size and associated lameness improved or resolved with screw fixation in primary and secondary proximal tibial SCL. CLINICAL SIGNIFICANCE: Primary tibial SCL that did not cause lameness healed with conservative management, but persistent primary and secondary tibial SCL required screw fixation to reduce lameness.

Evaluation of plasma concentration after intravenous and intramuscular penicillin administration over 24 hr in healthy adult horses.

Penicillin is administered intravenously (IV) or intramuscularly (IM) to horses for the prevention and treatment of infections, and both routes have disadvantages. To minimize these shortcomings, a 24-hr hybrid administration protocol (HPP) was developed. Our objective was to determine penicillin plasma concentrations in horses administered via HPP. Venous blood was collected from seven healthy horses administered IV potassium penicillin G at 0 and 6 hr and IM procaine penicillin G at 12 hr. Blood was collected at 2-hr intervals from 0 to 20 hr and at 24 hr. Plasma penicillin concentrations were measured using liquid chromatography and mass spectrometry. Penicillin susceptibility from equine isolates was examined to determine pharmacodynamic targets. The MIC90 of penicillin for 264 isolates of Streptococcus sp. was ≤0.06 µg/ml. For the 24-hr dosing interval, the mean plasma penicillin concentration was >0.07 µg/ml. Five horses (72%) exceeded 0.06 µg/ml for 98% of the dosing interval, and two horses exceeded this value for 52%-65% of the dosing interval. The HPP achieved mean plasma penicillin concentrations in healthy adult horses above 0.07 µg/ml for a 24-hr dosing interval. However, individual variations in plasma concentrations were apparent and deserve future clinical study.

Impact of a void in the equine medial femoral condyle on bone stresses and peak contact pressures in a finite element model.

OBJECTIVE: To predict bone and medial meniscal stresses and contact pressures in an equine stifle with a medial femoral condyle (MFC) intact or with a 2-cm3 subchondral bone void, under varying degrees of internal femoral rotation (IFR). STUDY DESIGN: Finite element model (FEM) of a cadaveric equine stifle loaded to 8000 N. METHODS: The FEM was constructed from computed tomography (CT) of the right, extended stifle of a yearling. The CT image was segmented into relevant anatomic structures and meshed into 4-node tetrahedrons. Bone material properties were assigned according to Hounsfield units, soft tissue properties were estimated from published data, and the model was loaded to 8000 N in 155° extension. RESULTS: The main stresses found in the intact MFC were in compression, with very small areas of shear and tension. Adding a 2-cm3 MFC void increased peak compression stress by 25%, shear by 50%, and tension by 200%. An MFC void also increased tension and shear placed on the medial meniscus by 30%. Under load, IFR of 2.5° and 5° increased MFC peak stresses 8%-21%. CONCLUSION: A 2-cm3 MFC void in an equine stifle FEM increased stress in the bone and meniscus. Internal femoral rotation slightly increased predicted bone stress. CLINICAL SIGNIFICANCE: Increases in bone and meniscal stress predicted in an MFC with a void provide evidence to understand the persistence of voids and mechanism of damage to the medial meniscus.

Stimulation of subchondral bone cyst healing by placement of a transcondylar screw in the equine medial femoral condyle.

OBJECTIVE: To predict the bone formation stimulus of a transcondylar screw across an equine subchondral bone cyst (SBC) in an equine medial femoral condyle (MFC). STUDY DESIGN: Finite element modeling (FEM) of an equine MFC with a 2 cm3 SBC under several transcondylar screw conditions. SAMPLE POPULATION: The right stifle of a yearling thoroughbred without stifle disease that had been euthanized for reasons unrelated to this study and donated to the University. METHODS: The FEM was derived from computed tomography of a yearling thoroughbred and analyzed in ABAQUS v6.14. The transcondylar screw was modeled as a 4.5-mm stainless steel cylinder. The region of interest was the centrodistal MFC, and bone stimulus was calculated. The stimulus threshold for bone formation (BFT) was >60 MPa and is presented as the percentage of total bone surface area (BFA) and frontal plane maps. Principal compressive stress vectors were also determined. Tested variables were daily cycles, load, and screw compression and position. RESULTS: At 750 cycles and 900-N load, <3% of the BFA exceeded the BFT. Increases in BFA > BFT occurred proportionally with load, screw compression, and daily cycles (steps). Compressive stress was oriented vertically on the SBC surface without a screw but aligned with the long axis of well-placed lag screws. Screw placement through the void also increased the number and magnitude of compressive vectors. CONCLUSION: This model predicted that a transcondylar lag screw across an MFC SBC increased surface BFA stimulation and reoriented the compression vector. Increasing screw compression, load, and steps per day increased the bone formation stimulus. CLINICAL SIGNIFICANCE: This study provides evidence that supports the use of a lag screw thorough an MFC SBC to promote bone formation.

Treatment of subchondral lucencies in the medial proximal radius with a bone screw in 8 horses.

OBJECTIVE: To describe the results of screw placement through subchondral lucencies (SCL) of the proximal radius in 8 horses. STUDY DESIGN: Retrospective clinical study. ANIMALS: Horses with cubital SCL causing lameness (n=8). METHODS: Medical record review and clinical follow-up. RESULTS: Eight horses with SCL in the proximal radius causing lameness were treated with a screw placed across the lucency. The horses range in age from 1 to 20 years. In 4 of 8 horses, the lameness had been intermittently severe (apparent at the walk). Lameness was isolated to the cubital joint by intra-articular anesthesia in 5 horses and diagnosed radiographically in all 8. All horses had a 4.5 mm cortical bone screw placed from medial to lateral (6 lag, 2 neutral) across the SCL using fluoroscopic or radiographic control. Postoperative care included stall confinement with hand walking for 30-60 days, followed by an additional 30-60 days of pasture turnout. Radiographic SCL healing (reduction in SCL size) was demonstrated at 3-4 months after surgery in all horses, and 7/8 horses (87.5%) were used as intended (4 performance, 3 pasture turn-out) within 6 months. Lameness in the remaining horse improved initially (dressage) but returned. CONCLUSIONS: A screw placed through the SCL of the proximal-medial radius was effective in reducing or resolving lameness associated with the elbow joint in 7/8 horses (88%). Screw placement in the proximal radius should be considered for horses with lameness caused by an SCL when a quick return to exercise is desired or conservative therapy is ineffective.

Ex vivo equine medial tibial plateau contact pressure with an intact medial femoral condyle, with a medial femoral condylar defect, and after placement of a transcondylar screw through the condylar defect.

OBJECTIVE: To determine ex vivo contact data on the equine medial tibial plateau loaded by an intact medial femoral condyle (MFC), by an MFC with an osteochondral defect, and with a screw inserted in lag fashion through the MFC defect. STUDY DESIGN: Ex vivo experiment. ANIMALS: Stifles (n = 6). Horses (n = 4). METHODS: Stifle joints were axially loaded to 1800 N at 155°, 145°, and 130°, under 3 conditions: Intact, MFC with a 15 mm circular osteochondral defect, and with a transcondylar screw inserted in lag fashion through the defect. An electronic pressure sensor (Tekscan®) on the medial tibial plateau recorded contact area, force, peak pressure, and contact maps. Stress load (N/cm(2) ) was calculated for the entire medial plateau and in 3 sub-regions; cranial, caudal, and central. Significance was set at P ≤ .05. RESULTS: Flexion increased force, contact area, and stress load for all conditions. An MFC defect significantly reduced force at both flexion angles and contact area at 145°. The transcondylar screw returned force to intact values at 130° and reduced contact area in extension. Intact MFC contact maps revealed pressure peaks on the central cartilage at all angles and contact pressure and area expansion and caudal movement with flexion. Contact maps with an MFC defect amplified the caudal and abaxial pressure movement during flexion, and the screw did not further change them. CONCLUSIONS: Stifle flexion increases force, contact area, and stress load on the medial tibial plateau and is most pronounced caudally. An MFC defect alters load on the medial tibial plateau, and a transcondylar screw may reverse some of those changes.

Preliminary investigation of the treatment of equine medial femoral condylar subchondral cystic lesions with a transcondylar screw.

OBJECTIVE: To determine if medial femoral condylar (MFC) subchondral cystic lesions (SCL) causing lameness will demonstrate radiographic evidence of healing and lameness reduction after placement of a transcondylar screw in lag fashion. STUDY DESIGN: Retrospective case series. ANIMALS: Horses (n = 20) with lameness attributed to a MFC SCL. METHODS: A 4.5 mm screw was inserted in lag fashion across MFC SCL in horses with hindlimb lameness. Post-operative radiography and lameness examinations were performed at 30-60 day intervals after surgery for 120 days, and SCL radiographic area and lameness were graded. Treatment was considered successful if lameness was eliminated and the radiographic area of the SCL on a caudocranial projection decreased ≥ 50% by 120 days. RESULTS: Twenty-six limbs were treated. Nine horses (11 limbs) had autologous adjunctive biologics placed into the SCL. Lameness was reduced by 1-2 grades by 60 days after surgery in 18 horses and was eliminated in 15 horses by 120 days, at which time the SCL area had decreased ≥ 50% and work had resumed without lameness (mean follow-up, 12 months). Biologic therapies had no impact on treatment success. Treatment was less successful in horses >3 years of age. CONCLUSIONS: A MFC transcondylar screw decreases the area of a MFC SCL on craniocaudal radiographs and eliminates lameness in ∼ 75% of horses by 120 days. The simplicity and lack of specialized equipment required make this technique a useful option for the treatment of equine SCL causing lameness.

Improvement in Bilateral Carpal Valgus Deviation in 9 Foals After Unilateral Distolateral Radial Periosteal Transection and Elevation.

OBJECTIVES: To evaluate the effect of periosteal transection and elevation in foals with naturally occurring carpal valgus deviation. STUDY DESIGN: Prospective clinical study. ANIMALS: Foals (n = 9) with bilateral carpal valgus. METHODS: Foals with bilateral carpal valgus had distolateral radial periosteal transection and elevation (PE) including distal ulnar transection on 1 limb. Foals were stall confined until the skin incisions healed, and then allowed free exercise in a small paddock or round pen. Dorsopalmar carpal radiographs were obtained at 14 day intervals to determine the carpal valgus angle (CVA) until it was <5°. RESULTS: All limbs had a reduction in CVA and there was no significant difference in total carpal valgus angular correction or the rate at which the correction occurred between the surgical and control limb. CONCLUSIONS: Unilateral distolateral radial PE had no effect on carpal angulation in 9 foals with naturally occurring bilateral carpal valgus deviation when performed between 19 and 43 days.

Comminuted fracture of the accessory carpal bone removed via an arthroscopic-assisted arthrotomy.

A 16-year-old American paint horse gelding was presented for evaluation of a left forelimb lameness grade III/V. Radiographs and computed tomography revealed a comminuted fracture of the accessory carpal bone involving the entire articulation with the distal radius and the proximal aspect of the articulation with the ulnar carpal bone. Multiple fragments were present in the palmar pouch of the antebrachiocarpal joint. An arthroscopic-assisted open approach was necessary to remove all fractured fragments. Subsequently the horse was re-admitted for lameness and was treated successfully with antibiotics and long-term supportive bandaging.

Fracture comminutive de l'os du carpe accessoire enlevé à l'aide d'une arthrotomie assistée par arthroscopie. Un cheval American Paint Horse âgé de 16 ans a été présenté pour l'évaluation d'une boiterie de la jambe avant gauche de grade III/V. Les radiographies et la tomodensitométrie ont révélé une fracture comminutive de l'os du carpe accessoire touchant toute l'articulation avec le radius distal et l'aspect proximal de l'articulation avec l'os du carpe cubital. Des fragments multiples étaient présents dans la poche palmaire de l'articulation antébrachio-carpienne. Une approche ouverte assistée par arthroscopie a été nécessaire pour retirer tous les fragments fracturés. Le cheval a ensuite été réadmis pour boiterie et a été traité avec succès à l'aide d'antibiotiques et de pansements de soutien à long terme.(Traduit par Isabelle Vallières).

An ex vivo model to evaluate the effect of cyclical adductory forces on maintenance of arytenoid abduction after prosthetic laryngoplasty performed with and without mechanical arytenoid abduction.

OBJECTIVES: To (1) develop a model of cyclical adduction force on an abducted left arytenoid cartilage that mimics swallowing or coughing; (2) determine if arytenoid abduction by a clamp before knot tying will improve the maintenance of abduction under cyclical adduction testing. STUDY DESIGN: Experimental. SAMPLE POPULATION: Cadaveric equine larynges (n = 14). METHODS: Left laryngoplasty performed using a single suture of #5 Ethibond with (n = 7) and without (n = 7) abducting the arytenoid with a clamp before knot tying. Each laryngoplasty was loaded cyclically from 2 to 26 N at 0.5 Hz for 5000 cycles in a servohydraulic test frame. Arytenoid displacement data were collected at 1 Hz intervals and median percent loss of abduction compared between groups. Significance was set at P < .05. RESULTS: Median left arytenoid abduction distance was 16.9 mm (range, 9.8-19.8 mm). One larynx in each group failed at <1000 cycles. Loss of abduction was determined by progressive displacement of the testing actuator and confirmed by measurement. There was no difference in loss of abduction between clamped and non-clamped larynges after 5000 cycles. This model of cyclical adduction resulted in arytenoid displacements similar to those seen in the 1st week postoperatively. CONCLUSIONS: Ex vivo cyclical adductory forces produced a significant loss of laryngoplasty abduction. The use of a clamp to abduct the arytenoid cartilage before knot tying did not reduce the loss of abduction.

Radiographic analysis in Thoroughbreds reveals morphological changes in healthy maturing stifle joints and possible association between subchondral lesions and femoral condyle width.

OBJECTIVE: Assess femorotibial features in foals with and without medial femoral condyle (MFC) subchondral radiolucencies (SR+ and SR-). METHODS: 3 independent, sequential radiographic studies were performed. Study 1 retrospectively measured femorotibial morphological parameters in repository radiographs (SR- and SR+). Study 2 qualitatively compared drawings of intercondylar notch shape in postmortem radiographs (SR-). Study 3 prospectively measured femorotibial parameters in 1-month-old foals (SR-). In studies 1 and 3, 13 morphologic parameters were measured. Limb directional asymmetry was assessed in 2 age groups (< 7 or ≥ 7 months). RESULTS: Study 1 (SR- group; n = 183 radiographs) showed increased femoral measurements with maturation, except the distal femoral intercondylar notch width (FINwal), which decreased. In contrast, in SR+ stifles (53 radiographs), 3 femoral parameters (MFC width [MFCwpf], MFC height, or FINwal) showed no changes. Tibial plateau width alone increased with maturation in both groups. Interobserver reliability was good to excellent. Study 2 (n = 53 radiographs) confirmed a distal FINw decrease in SR- foals. In study 1, left SR- stifles in greater than or equal to 7-month-old fillies had significantly larger femoral bicondylar width and FINw, while right SR+ stifles in fillies greater than or equal to 7 months had a significantly larger MFCw. In study 3 of 1-month-old foals (n = 94 SR- radiographs), the MFCw, femoral condyle bicondylar width, and lateral femoral condyle height were all greater on the left, whereas the intercondylar intereminence space width was larger on the right. CLINICAL RELEVANCE: In SR+ stifles, the distal femur exhibited divergent maturation, indicating a wider MFC in the right stifle in older foals. As SR lesions are more common on the right, this suggests a potential association with MFC morphology.

Pharmacokinetic-pharmacodynamic cutoff values for benzylpenicillin in horses to support the establishment of clinical breakpoints for benzylpenicillin antimicrobial susceptibility testing in horses.

Introduction: The aim of this international project was to establish a species-specific Clinical Breakpoint for interpretation of Antimicrobial Susceptibility Testing of benzylpenicillin (BP) in horses. Methods: A population pharmacokinetic model of BP disposition was developed to compute PK/PD cutoff values of BP for different formulations that are commonly used in equine medicine around the world (France, Sweden, USA and Japan). Investigated substances were potassium BP, sodium BP, procaine BP, a combination of procaine BP and benzathine BP and penethamate, a prodrug of BP. Data were collected from 40 horses that provided 63 rich profiles of BP corresponding to a total of 1022 individual BP plasma concentrations. Results: A 3-compartment disposition model was selected. For each of these formulations, the PK/PD cutoff was estimated for different dosage regimens using Monte Carlo simulations. The fAUC/MIC or fT>MIC were calculated with a free BP fraction set at 0.4. For fAUC/MIC, a target value of 72 h (for a 72h treatment) was considered. For fT>MIC, efficacy was assumed when free plasma concentrations were above the explored MIC (0.0625-2 mg/L) for 30 or 40 % of the dosing interval. For continuous infusion, a fT>MIC of 90 % was considered. It was shown that a PK/PD cutoff of 0.25 mg/L can be achieved in 90 % of horses with routine regimen (typically 22,000 IU/kg or 12.4 mg/kg per day) with IM procaine BP once a day (France, Japan, Sweden but not USA1) and with IM sodium BP at 14.07 mg/kg, twice a day or IV sodium BP infusion of 12.4 mg/kg per day. In contrast, penethamate and the combination of procaine BP and benzathine BP were unable to achieve this PK/PD cutoff not even an MIC of 0.125 mg/L. Discussion: The PK/PD cutoff of 0.25 mg/L is one dilution lower than the clinical breakpoint released by the CLSI (0.5 mg/ L). From our simulations, the CLSI clinical breakpoint can be achieved with IM procaine BP twice a day at 22,000 IU i.e. 12.4 mg/kg.

Osteomyelitis of the patella in eight foals.

OBJECTIVE: To describe the characteristics of osteomyelitis lesions of the patella and to report short- and long-term outcome after treatment in 8 foals. STUDY DESIGN: Retrospective case series. ANIMALS: Foals (n = 8). METHODS: Medical records (2003-2007) and radiographs of foals that had osteomyelitis of the patella were reviewed. Inclusion criteria included clinical, radiographic, and surgical findings consistent with osteomyelitis of the patella, and a long-term follow-up of >15 months. Information acquired included signalment, hematologic and serum biochemical profile results, clinical and radiographic signs, surgical technique and perioperative treatment. Follow-up radiographs were evaluated and outcome was determined from veterinary examination, race records, and telephone questionnaire. RESULTS: Six foals survived long term (15 months-4 years); all had intralesional and systemic antimicrobial therapy, along with synovial lavage and antimicrobial medication. All were sound and achieved either yearling sales (n=3), show hunter or racing (2). Two foals died in the short term from renal failure and suppurative peritonitis secondary to cecal perforation, 1 remaining lame with suppurative osteonecrosis confirmed at necropsy. This foal was not administered intralesional antimicrobial therapy. CONCLUSIONS: Prompt medical and surgical therapy for osteomyelitis of the patella can result in a good prognosis for soundness and a potential athletic career. Concurrent septicemia or other systemic perinatal disease can result in prolonged therapy and delayed recovery.

Microstructural features of subchondral radiolucent lesions in the medial femoral condyle of juvenile Thoroughbreds: A microcomputed tomography and histological analysis.

BACKGROUND: The aetiology of equine medial femoral condyle (MFC) subchondral bone radiolucencies (SR) is unknown. OBJECTIVES: Characterise the microstructural structural features of MFC SR in juvenile Thoroughbreds with microcomputed tomography (µCT) and histology. STUDY DESIGN: Cross-sectional post-mortem study. METHODS: Distal femurs were collected at post-mortem. Conventional tomodensitometry was employed to scout for MFCs with and without SR lesions (SR+ and SR-, respectively). Group 1 were CT MFC SR+ and Group 2 age-matched SR- controls. Both underwent µCT and histological analysis. Group 3 CT MFC SR- foals, <6 months, were selected to search for chondronecrosis. Histological sections, processed from the lesion (Group 1) and a corresponding site in Groups 2 and 3, were assessed for chondronecrosis, fibrin, fibroplasia and osteochondral separation. Group 3 sections were surveyed for chondronecrosis alone. RESULTS: A total of 178 femurs from 89 Thoroughbreds were harvested. Of these horses 19.1% (95% CI: 10.9%-27.3%) were CT MFC SR+ (17/23; 7.46 ± 4.36 months) and met the inclusion criteria for Group 1. Group 2 included 30 CT MFC SR- specimens (5.00 ± 2.73 months) and Group 3 had 44 CT MFC SR- s (2.68 ± 1.74 months). SR were located axially in foals <7 months of age, and centrally thereafter. All SRs had areas of thickened cartilage on histology and separation at the osteochondral junction containing fibrin (acute event) and fibroplasia (chronicity) in 73.9% (17/23; 95% CI: 56%-91.9%). In Group 1 specimens, chondronecrosis was present in 82.6% (19/23; 95% CI: 67.1%-98.1%) but four MFC SR+ had no evidence of chondronecrosis. Chondronecrosis was not detected in the Group 3 foal MFCs. MAIN LIMITATIONS: No longitudinal follow-up. CONCLUSIONS: The absence of chondronecrosis, pathognomic of osteochondrosis, in four MFC SR+s and in all of the CT MFC SR- foals suggests that osteochondrosis is not the cause, or the only cause, of these lesions and favours trauma as an alternate aetiological hypothesis.

Osteoclast density is not increased in bone adjacent to radiolucencies (cysts) in juvenile equine medial femoral condyles.

BACKGROUND: There is a knowledge gap about how equine MFC subchondral radiolucencies (SR) arise and evolve. Osteoclasts are believed to have a role but have not been studied in situ. OBJECTIVES: To measure and compare osteoclast density and the percentage of chondroclasts in healthy and MFC SR specimens from juvenile Thoroughbreds. STUDY DESIGN: Cadaveric study. METHODS: Medial femoral condyles (MFC) from a tissue bank of equine stifles were studied. Inclusion criteria were MFCs (≤8 months old) with a computed tomography SR lesion and histological focal failure of endochondral ossification (L group). Contralateral, lesion-free, MFCs were a control group (CC). Osteochondral slabs were cut through the lesion (L), a healthy site immediately caudal to the lesion, (internal control; IC) and the contralateral, site-matched controls (CC). Histological sections were immunostained with Cathepsin K for osteoclast counting. Osteoclasts in contact with the growth cartilage (chondroclasts) were also counted. The sections were segmented into regions of interest (ROI) at different depths in the subchondral bone: ROI1 (0-1 mm), ROI2 (1-3 mm) and ROI3 (3-6 mm). Osteoclasts were counted and the bone area was measured in each ROI to calculate their density. Chondroclasts were counted in ROI1 . RESULTS: Sections were studied from L and IC (n = 6) and CC sites (n = 5). Osteoclast density was significantly higher in ROI1 when compared with ROI3 in all groups. Although higher osteoclast density was measured in ROI1 in the L group, no significant differences were detected when compared with control ROIs. The proportion of chondroclasts in ROI1 was lower in the L sections when compared with controls but no significant differences were detected. MAIN LIMITATIONS: Limited sample size. CONCLUSIONS: Osteoclasts are important actors in MFC subchondral bone development, digesting both growth cartilage (chondroclasts) and bone, but the pathophysiology of early MFC SRs cannot be explained solely by an increased osteoclast presence in the subchondral bone.