Tanya C. Garcia

Hoof accelerations and ground reaction forces of Thoroughbred racehorses measured on dirt, synthetic, and turf track surfaces

OBJECTIVE To compare hoof acceleration and ground reaction force (GRF) data among dirt, synthetic, and turf surfaces in Thoroughbred racehorses. ANIMALS 3 healthy Thoroughbred racehorses. PROCEDURES Forelimb hoof accelerations and GRFs were measured with an accelerometer and a dynamometric horseshoe during trot and canter on dirt, synthetic, and turf track surfaces at a racecourse. Maxima, minima, temporal components, and a measure of vibration were extracted from the data. Acceleration and GRF variables were compared statistically among surfaces. RESULTS The synthetic surface often had the lowest peak accelerations, mean vibration, and peak GRFs. Peak acceleration during hoof landing was significantly smaller for the synthetic surface (mean + or - SE, 28.5g + or - 2.9g) than for the turf surface (42.9g + or - 3.8g). Hoof vibrations during hoof landing for the synthetic surface were < 70% of those for the dirt and turf surfaces. Peak GRF for the synthetic surface (11.5 + or - 0.4 N/kg) was 83% and 71% of those for the dirt (13.8 + or - 0.3 N/kg) and turf surfaces (16.1 + or - 0.7 N/kg), respectively. CONCLUSIONS AND CLINICAL RELEVANCE The relatively low hoof accelerations, vibrations, and peak GRFs associated with the synthetic surface evaluated in the present study indicated that synthetic surfaces have potential for injury reduction in Thoroughbred racehorses. However, because of the unique material properties and different nature of individual dirt, synthetic, and turf racetrack surfaces, extending the results of this study to encompass all track surfaces should be done with caution.

Quantitative evaluation of perfusion and permeability of peripheral tumors using contrast-enhanced computed tomography.

Rationale and Objectives:Our purpose was to validate contrast-enhanced computed tomography (CECT)-derived quantitative measures of perfusion and permeability against gold standard techniques of fluorescent microspheres and Evans Blue dye, respectively. Materials and Methods:Normal and tumor-bearing (R3230AC) Fischer 344 rats were used. CECT perfusion measurements of normal and tumor tissue were compared with quantitative fluorescent microsphere perfusion measures. CECT permeability measurements from tumors were compared with semiquantitative Evans Blue Dye permeability estimates. CT images were obtained precontrast and an imaging plane was selected. Serial, stationary images were obtained every 2 seconds for 2 minutes after intravenous bolus of iodinated contrast. Permeability and perfusion were measured by applying Patlak analysis to time-density data from normal tissue or tumor and femoral artery. Results:There was good correlation between fluorescent microsphere and CECT measurements of perfusion (r2 = 0.681, P ≪ 0.001) and between Evans Blue Dye and CECT measurements of permeability (r2 = 0.873, P = 0.0007). Conclusions:CECT provides useful, quantifiable measures of perfusion and permeability in peripheral tumors.

Mechanical Comparison of 3.5 mm Broad Dynamic Compression Plate, Broad Limited-Contact Dynamic Compression Plate, and Narrow Locking Compression Plate Systems Using Interfragmentary Gap Models

OBJECTIVES To compare (1) pullout properties between 3.5 mm cortical and locking screws, and (2) mechanical properties and gap displacements between the 3.5 mm broad limited-contact dynamic compression plate (LC-DCP), broad dynamic compression plate (DCP), and narrow locking compression plate (LCP), during axial loading of plate-stabilized diaphyseal fragments with an interfragmentary gap. STUDY DESIGN In vitro mechanical testing of implanted polyurethane foam (PUF) hollow cylinders that simulated compact or osteopenic diaphyseal bone. SAMPLE POPULATION (1) Five cortical and locking screws and (2) 4 PUF-plate constructs for each plate type; using high- and low-density (0.8 and 0.32 g/cm(3)) cylinders. METHODS (1) Screws were completely extracted at 5 mm/min. (2) Plated constructs were axially compressed at 300 N/s for 10 cycles from 5 to 355 N to determine gap displacement during physiologic loading, followed by single cycle increasing load to failure. RESULTS Pullout properties were not different between screw types. All plate constructs had yield loads over 3 times trotting loads. Gap closure occurred with LC-DCP and DCP constructs, but not LCP constructs. LCP construct properties were most similar to LC-DCP and DCP construct properties in the low-density model. CONCLUSION All plate systems sustained physiologic limb loads. Only LCP constructs maintained some gap integrity, although LC-DCP and DCP screws were placed in neutral position. CLINICAL RELEVANCE The LCP system is more likely than LC-DCP and DCP systems, with neutrally positioned screws, to maintain a planned interfragmentary gap, although gap strains range from 0% to 15% across the 2 mm gap during a trot load.

On the ultrasonic properties of tendon

The strong dependence of tendon echogenicity on insonation angle is explored by analyzing echo spectra. Combining echo spectra with high-resolution images from several modalities reveals that fluid spaces surrounding fascicles and bundles are likely sources of ultrasonic scatter. Mathematical models of tendon structure are proposed to explain how the anisotropic microstructure of tendon gives rise to angle-dependent echogenicity. Echo spectra from spontaneously damaged equine tendon samples were compared with normal equine tendon and found to exhibit a dramatic decrease in anisotropic properties that appears to be related to the spatial organization and type of collagen generated during repair. Variation in echo spectra with insonation angle is a robust indicator of mechanical damage.

Comparison of macrostructural and microstructural bone features in Thoroughbred racehorses with and without midbody fracture of the proximal sesamoid bone

OBJECTIVE To compare macrostructural and microstructural features of proximal sesamoid bones (PSBs) from horses with and without PSB midbody fracture to gain insight into the pathogenesis of PSB fracture. SAMPLE POPULATION PSBs from 16 Thoroughbred racehorses (8 with and 8 without a PSB midbody fracture). PROCEDURES Parasagittal sections of fractured and contralateral intact PSBs from horses with a PSB fracture and an intact PSB from age- and sex-matched control horses without a PSB fracture were evaluated for visual, radiographic, microradiographic, histologic, and his-tomorphometric differences in bone porosity, vascular channels, heme pigment, trabecular anisotropy, and pathological findings. RESULTS Fractured PSBs and their contralateral intact PSBs had more compacted trabecular bone than did control PSBs. Focal repair or remodeling was evident in the palmar aspect of many fractured and contralateral intact PSBs. Fracture coincided with microstructural features and propagated from the flexor to the articular surface. CONCLUSIONS AND CLINICAL RELEVANCE Fractured PSBs had adapted to high loading but had focal evidence of excessive remodeling and porosity that likely predisposed the horses to complete fracture and catastrophic injury. Detection of focal injury before complete fracture provides an opportunity for prevention of catastrophic injury. Development of diagnostic imaging methods to assess porosity of PSBs may help to identify at-risk horses and allow for modifications of training and racing schedules to reduce the incidence of PSB fracture in Thoroughbred racehorses.

Intervertebral Biomechanics of Locking Compression Plate Monocortical Fixation of the Canine Cervical Spine

OBJECTIVE To evaluate the use of a locking compression plate (LCP) with monocortical screw purchase for stabilization of the canine cervical spine. STUDY DESIGN Experimental study. ANIMALS Cadaveric canine cervical spine specimens (n = 7). METHODS Flexion and extension bending moments were applied to canine cadaveric specimens (C3-C6) in 4-point bending, before and after creation of a ventral slot at C4-C5, and after fixation with a 5 hole, 3.5 mm LCP with monocortical screw placement. Screw placement and penetration into the vertebral canal were determined by radiography. Range of motion, stiffness, and energy for passive physiologic loads were determined for the C3-C4, C4-C5, and C5-C6 vertebral motion units (VMU). Monotonic failure properties were determined for cervical extension. Effects of treatments on biomechanical variables were assessed using repeated measures analysis of variance and least square means (P ≤ .05). RESULTS The ventral slot procedure increased range of motion at the treated VMU. Plate fixation decreased range of motion, increased stiffness, and decreased energy at the treated VMU. No changes were observed at adjacent VMUs. None of the screws penetrated the vertebral canal. Mean (± SD) yield bending moment of plate stabilized, slotted spines was 15.6 ± 4.6 N m. CONCLUSION LCP fixation with monocortical screws stabilized the canine cervical spine.

Biomechanical Evaluation of Screw-In Femoral Implant in Cementless Total Hip System

OBJECTIVE To compare (1) proximal femoral axial strains, (2) femoral head deflection, and (3) failure mechanical properties, between Helica head and neck prosthesis implanted femora and normal femora. STUDY DESIGN In vitro study. SAMPLE POPULATION Cadaveric canine femora (n = 5 pair). METHODS Femoral bone strains and head displacement during in vitro simulation of midstance of the gallop were evaluated using cadaveric femurs cyclically loaded in vitro. Strains and displacements were compared within femurs, before and after, prosthesis implantation; and throughout cycling to seek evidence of movement with cyclic loading. Subsequently, implanted femurs and contralateral, intact femurs were loaded to failure to compare failure mechanical properties and modes of failure. RESULTS Proximal femoral axial strains were significantly different between intact and implanted femora on all 4 cortical surfaces (P < .05). Compressive strains were lower in the implanted femur on all cortical surfaces, except on the caudal surface which was higher. No difference was noted for femoral head angle under an axial load corresponding to gallop (P > .05). Vertical head displacement was ∼0.1 mm greater for implanted femora than intact femora (P < .05). Yield and failure loads and yield energy of implanted femora were 39-54% lower than those for intact femora (P < .05). Mode of failure for both the intact and implanted femora did not appear to be different. CONCLUSION Helica femoral prosthesis alters strain distribution in the proximal aspect of the femur and exhibits initial micromotion. Failure load in axial compression of the Helica-implanted femur is less than that of the normal femur, but greater than that expected in vivo.

Biomechanical evaluation of acetabular cup implantation in cementless total hip arthroplasty.

OBJECTIVE To report biomechanical properties of the Biologic Fixation System (BFX) acetabular cup impacted into a normal canine pelvis and to compare the effect of implant positioned to and beyond the medial acetabular wall. STUDY DESIGN In vitro cadaveric study. ANIMALS Hemipelves of mature, large-breed dogs (n=6). METHODS For each dog, 1 hemipelvis was reamed to the depth of the acetabular wall (group A) and 1 was reamed an additional 6 mm after penetration of the medial cortex of the acetabulum (group B). The hemipelves were implanted with acetabular cups and loaded in compression through a matching femoral prosthetic component until failure. Specimen stiffness, and failure displacement, load, and energy were determined from load and displacement data and results between groups compared with a paired t-test. RESULTS Mean failure load was greater in group A (3812 ± 391 N) than group B (2924 ± 316 N; P<.014). No other differences (P>.05) were observed between groups. Bone fracture (n=5) and cup displacement (1) occurred in group A whereas in group B there were 3 fractures and 3 cup displacements. CONCLUSIONS Although medial placement of the BFX cup affected compressive failure loads, failure loads for both groups exceeded normal physiologic loads. CLINICAL RELEVANCE Medial positioning of the acetabular cup does not appear to compromise acetabular implant-pelvic stability under normal physiologic loads. Because arthroplasty candidates often have abnormal acetabular architecture, mechanical properties of the cup placed in acetabula without a dorsal rim should be investigated.

Biomechanical comparison of mono- and bicortical screws in an experimentally induced gap fracture

OBJECTIVES To compare the bending and torsional mechanical properties of mono- and bicortical locking screws in a canine cadaveric tibial gap ostectomy bridged by a locking compression plate (LCP). METHODS A 10-hole 3.5 mm LCP was applied medially to the tibia with a gap ostectomy using locking screws in the two proximal and distal plate holes. One tibia of each pair was randomly assigned monocortical screws and the other bicortical screws. Constructs were tested non-destructively in mediolateral and caudocranial four-point bending and torsion, and then to failure in four-point bending. Stiffness, yield and failure variables were compared between screw lengths and load conditions using analysis of variance. RESULTS Caudocranial and mediolateral four-point bending stiffnesses were not different between screw constructs. Torsional stiffness was greater and neutral zone smaller for bicortical constructs. Constructs were stiffer and stronger in caudocranial bending than in mediolateral bending. In caudocranial bending, bicortical constructs failed by bone fracture and monocortical constructs by screw loosening. CONCLUSION Bicortical constructs were stiffer than monocortical constructs in torsion but not bending. Bicortical screw constructs failed by bone fracture under the applied loads whereas monocortical screw constructs failed at the bone-screw interface. CLINICAL RELEVANCE Bicortical screw placement may be a safer clinical alternative than monocortical screw placement for minimally invasive percutaneous osteosynthesis LCP-plated canine tibiae with comminuted diaphyseal fractures.

How do metacarpophalangeal joint extension, collateromotion and axial rotation influence dorsal surface strains of the equine proximal phalanx at different loads in vitro?

The biomechanical circumstances that promote sagittal fracture of the equine proximal phalanx (P1) are poorly understood. In order to improve our understanding of equine metacarpophalangeal joint (MCPJ) biomechanics and potential aetiologies of sagittal P1 fractures, the study objectives were to quantify P1 bone strains, collateromotion and axial rotation during MCPJ extension under controlled loading circumstances. Unilateral limbs from six cadavers were instrumented with bone reference markers for measurement of P1 movement relative to third metacarpal bone positions during axial limb loading to 10,500N. Bone reference markers recorded by video were digitized and the movement analyzed during MCPJ extension. Concurrently, dorsoproximal P1 surface strains were measured with one uniaxial and one rosette strain gauge. Strain gauge data was reduced to determine principal and shear strain magnitude and direction. External axial rotation and collateromotion increased with increasing MCPJ extension. Maximum principal strain increased linearly as load increased from 2000 to 10,500N. Minimum principal and maximum shear strains had curvilinear relationships with limb loading, with negligible strain magnitude until approximately 6000N load, after which strain increased rapidly. The direction of P1 minimum principal strain shifted approximately 30-40° as load increased from 5400N to 10,000N, moving from proximolateral-distomedial to a nearly proximodistal direction. At near maximal MCPJ extension, with concurrent axial rotation and collateromotion, a rapid increase in dorsoproximal P1 bone strain and a change in principal strain direction occurred. The alterations in principal strain magnitude and direction associated with maximal MCPJ extension may support a biomechanical theory for sagittal P1 fracture occurrence in horses.