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.

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.