David Wilson

Introduction: Osteoarthritis (OA) is a debilitating disease caused by abnormal joint mechanics. There is a need for a non-invasive method to identify the specific mechanical changes that lead to OA. Magnetic resonance imaging (MRI) techniques such as T2 and T1ρ have shown sensitivity to mechanical changes in cartilage. However, the response is depth-dependent, and clinical MRI scanners with lower image resolutions cannot detect such specific changes. The magnetization transfer ratio (MTR) is a newer MRI metric that has shown promise as a measure of cartilage strain and stiffness. This research aimed to validate MTR as a robust method to measure loaded deformation and stiffness of knee cartilage.Methods: Three phases of work are described: first, an electropneumatic compressive loading device was developed and validated to apply step compressive forces to cartilage specimens in a high-resolution small-bore 9.4 Tesla MRI scanner. Second, bovine tibiofemoral osteochondral samples were compressed using this device while acquiring MTR and T2 data, and the correlations between cartilage strain and MTR/T2 were analyzed to determine any significant relationships. Finally, instantaneous stiffness maps were acquired across the same specimens via indentation testing. The relationship between local instantaneous modulus and local MTR/T2 were then assessed.Results: The loading device was successfully developed and validated to apply step compressive loads to cartilage samples in a small-bore MRI. Uniform MTR differences were observed through the cartilage thickness with increasing strain, whereas the T2 reaction to strain was regionally dependent. The relationship between MTR and strain was also found to be less specimen dependent (rrm = -0.25 to -0.36) than the relationship between T2 and strain (rrm = -0.28ivto 0.21). When assessing correlations between local MTR/T2 and local instantaneous modulus, neither MRI metrics were found to be strong predictors of cartilage stiffness (rrm = 0.06 to 0.35). However, both measurements showed promise as indicators of mean cartilage stiffness when averaged across the cartilage plate.Conclusion: Our results suggest MTR could be useful in future in vivo cartilage studies in clinical MRI scanners, which may further our understanding of the mechanical changes that occur in cartilage during OA initiation and progression.

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Spinal muscles play an important role in two inter-related clinical problems in the thoracolumbar spine: 1) age-related progressive kyphosis and 2) proximal junctional kyphosis (PJK) following correction surgery. Although these disorders occur largely in the thoracic spine and show symptoms in weight-bearing postures, studies have not investigated thoracic muscles in postures other than supine. Moreover, almost all the image-based thoracolumbar models are developed from muscle data obtained from supine imaging, which questions its credibility. Hence the objectives of this study were to i) analyze the effect of posture on thoracic spinal muscle parameters in different postures, and ii) develop a method for translation of MRI-derived spinal and muscle data into a thoracolumbar biomechanical model. Two regions (T4-T5 and T8-T9) of the thorax of six healthy volunteers were imaged (0.5T MROpen, Paramed, Genoa, Italy) in four postures (supine, standing, sitting, and flexion). Descriptive guidelines were developed to identify and quantify three muscles- trapezius (TZ), erector spine (ES), and transversospinalis (TS) from axial MR images. Intra- and Inter- segmentation repeatability was assessed using ICC(3,1). The effect of spinal level and posture on muscle parameters (cross-sectional area (CSA) and position (radius and angle)) was evaluated using 2-way repeated measures ANOVA (p0.05). A pipeline was developed to estimate subject-specific sagittal spinal geometry in different postures in order to compute muscle line-of-action in those postures. A correction factor for the direction of MRI scan slice and muscle line-of-action was computed and applied to muscle parameters of ES and TS.The intra- and inter-rater repeatability of segmentation were excellent. The muscle size decreased (~40%) for TZ and increased for ES (~10%) caudally. The trapezius CSA increased (~10%) in standing compared to supine due to activation. CSA of all muscles decreased (~20%) during flexion compared to neutral postures, due to passive stiffness. Although the differences in muscle parameters were found to be small (~15%), they play an important role in controlling the model outputs. The correction factor reduced the overall magnitudes of muscle parameters by about 20%. Overall, this study contributes to the growing database of thoracic muscle literature for clinical and biomechanical modelling applications.

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Introduction: Osteoarthritis (OA) is the most prevalent joint disease in Canada, affecting millions of people. OA begins with softening of cartilage and is characterized by progressive loss of the tissue resulting in joint impairment. Because cartilage’s primary function is mechanical, and because OA disrupts cartilage’s mechanical function, there is a substantial need for a non-invasive method to assess cartilage mechanics. Contrast-enhanced computed tomography (CECT) using charged contrast agents is an imaging method developed to quantify Glycosaminoglycan (GAG) content of cartilage.Since GAG is a key determinant of cartilage compressive stiffness, CECT measurements may be correlated with cartilage stiffness. The objective of this study was to determine whether CECT using a novel cationic contrast agent (CA4+) is correlated with cartilage stiffness in intact human joint surfaces.Methods: Six human femoral condyle compartments with intact healthy cartilage (ICRS grade 0 or 1) were used. Cartilage stiffness was measured across the surface in a Mach-1 testing system(Biomomentum, Montreal) using an indentation test. The samples were then immersed in CA4+ solution for 48 hours and then scanned at 41μm resolution in a hr-pQCT scanner (Xtreme CT,Scanco, Zurich). The averages of CECT attenuations at the sites of the indentation tests were computed for both superficial cartilage (600μm depth) and for the full thickness of cartilage.Correlations between stiffness and CECT attenuation were assessed with scatter plots and Pearson’s correlation coefficient.Results: A significant and positive correlation was observed between stiffness data and mean CECT attenuations in superficial cartilage across all samples, with correlation coefficients ranging fromr=0.4 to 0.72, and p0.01. When data from all locations were pooled (n=221), the correlation coefficient was r=0.55 and a regression line fitted to the data predicted stiffness from CECT measurements with an error of 20% of the stiffness range. However, correlations between stiffness and CECT attenuations in full-depth cartilage were substantially lower and not significant in half of the tested specimens. CECT identified regions of reduced cartilage stiffness and the expected depth dependent changes in GAG concentration.Conclusion: CECT of superficial cartilage using CA4+ provides a surrogate measure of compressive cartilage stiffness in intact joint surfaces.

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Cam femoroacetabular impingement (FAI) is a mechanical process thought to cause of hip osteoarthritis (OA). In cam FAI, it is thought that a ‘cam deformity’ on the femoral head-neck junction intrudes into the intra-articular joint space, inducing elevated mechanical force on acetabular cartilage. However, few experimental studies have measured contact mechanics in FAI. Open MRI in functional positions has potential to directly and non-invasively assess cam FAI, but MRI measures have not been related to mechanics. This thesis asked, in cadaver hips positioned in a simulated anterior impingement posture: (1) Does open MRI show intrusion of a cam deformity into the intra-articular joint space? (2) Is a cam deformity associated with elevated acetabular contact force? (3) Are MRI measures of cam FAI related to acetabular contact force?Cadaver hips (cam, n=9; controls, n=3) were positioned in a simulated anterior impingement posture, then imaged using open MRI with multi-planar reformatting. The β-angle was measured at 72 locations about the circumference of the femoral neck, and a binary ‘MRI cam-intrusion sign’ was defined (positive if βmin0°). Hips were then instrumented with a piezoresistive sensor before conducting six repeated impingement trials, measuring acetabular contact force (F), centroid location, and distribution. A binary ‘contact-force sign’ (positive when F>20N) defined elevated contact force.Minimum β-angle ranged from 1.4° to -28.5° in cams versus 4.6° to -0.2° in controls. Cam hips were significantly more likely than controls to have a positive MRI cam-intrusion sign (p=0.0182, Fisher’s exact test) and positive contact-force sign (p=0.0083). There was a significant relationship between the MRI cam-intrusion sign and contact-force sign (p=0.033).This thesis established that open MRI measures of cam FAI relate to contact mechanics, indicating that open MRI has significant potential to investigate the biomechanics of cam impingement. Open MRI can be used to establish treatment guidelines and understand why some hips develop OA and some do not.

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Introduction: Medial opening-wedge high tibial osteotomy (HTO) is a surgical procedure intended to shift load from the medial to the lateral compartment of the knee. The 10-year success rates of HTO are variable. One factor affecting success may be how effectively the procedure corrects alignment in the coronal plane (wedge angle) and sagittal plane (slope angle). The objective of this study was to determine the effect of changing tibial slope for a range of tibial wedge angles in medial opening wedge HTO on knee joint contact pressure location and kinematics during continuous loaded flexion/extension.Methods: The accuracy and repeatability of Novel pliance capacitive pressure sensors were measured under relevant compressive forces using a materials testing system (Instron ElectroPuls E10000). Seven male cadaveric knee specimens (mean age 62 (14)) cycled through simulated squatting. Tibiofemoral and patellofemoral kinematics were measured using an infrared (Optotrak Certus) motion tracking system. Contact pressure was measured using capacitive pressure sensors (Novel Pliance). This assessment was repeated for seven clinically relevant combinations of wedge and slope. Results: The capacitive pressure sensors had a maximum error of 13 ± 2.1% when applying force across the entire sensor. Significant differences (p0.05) were found between all tibiofemoral and patellofemoral kinematic parameters and contact pressure parameters measured for all wedge and slope combinations. An increase in slope from neutral to 5° resulted in 7mm of anterior translation of the centre of pressure on the medial compartment for a 15° wedge (p0.000). Increasing the wedge angle resulted in anterior translation of the tibia relative the femur (p0.000). Increasing the wedge angle to 7.5° resulted in a reduction in the proportion of load in the medial compartment by 17% (p0.001).Conclusion: Knee kinematics and contact pressure are altered as a result of changes to the wedge and slope of the osteotomy. There is a large focus on the effect of the coronal plane correction in HTO, but the sagittal plane correction also plays an important role in contact pressure at the tibiofemoral joint.

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Excessive or abnormal joint loading that leads to cartilage degeneration has been associated with hip osteoarthritis (OA). Before preventative measures for OA can be designed, such as physiotherapy techniques, braces, or surgical interventions, the connection between load-bearing and cartilage degeneration needs to be validated experimentally. As a first step towards such a validation, a method of measuring the load distribution across the hip joint is needed; ideally that can be used in vivo and can detect changes in the load distribution during an applied load. The objective of this study was to assess the accuracy of using biplanar radiography combined with CT imaging to estimate hip cartilage strain across the joint as an indication of the load distribution. Estimating cartilage strain using biplanar radiography and CT imaging is a multi-device multi-step measurement protocol that has error associated with each step. While biplanar radiography systems are commonly assessed on their ability to measure radio-opaque bead locations, to the author’s knowledge no studies have quantified errors in the additional steps of estimating cartilage strain. The present study used a phantom hip joint to quantify the errors in measuring bone displacement with biplanar radiography, segmenting 3D joint surfaces from a CT image, and measuring the relative proximity of joint surfaces in the biplanar radiography coordinate frame. The quantified errors were much lower than ex vivo hip cartilage deformation results in the literature, which demonstrated the potential for using this technique to estimate cartilage strain in the hip.As a proof of concept, cartilage strain was estimated in the ex vivo hip joint during a compressive load. Two hemi-pelvis/proximal femur specimens, with radio-opaque beads inserted in each bone, were loaded in compression in a materials testing machine, with biplanar radiographs acquired throughout. A small amount of cartilage deformation (0.1mm) was detected across the hip joint; however, due to the low load applied the deformation results were not comparable to the literature. The largest cartilage strains were identified in the anterior and superior regions, which was consistent with the literature. Future studies using higher loads are needed to further assess the capabilities of our system.

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Cam-type femoroacetabular impingement is a painful disorder common in young adults, caused by decreased concavity of the femoral head-neck. It is associated with hip osteoarthritis, though the exact mechanism of joint damage is not fully understood. Gait analysis has shown that cam deformities cause changes to coupled motions in vivo, though it is unclear whether these changes are compensatory or due to direct bony contact. The objective of this study was to determine how cam deformities and surgical resection affect patterns of hip rotation, translation of the center of rotation, and force required to flex and abduct the hip.We assessed the relationship between deformity and coupled motions, translations of center of femoral rotation, and force required to create active unconstrained flexion and abduction ex vivo. Three deformities were simulated on each of six hemi-pelvis/proximal femur specimens. Four muscles were simulated by cables drawn from the distal tendon to the location of proximal attachment. Motion was created by actively shortening one of these cables while statically loading the others. Markers on the femur and pelvis were tracked, allowing for calculation of joint rotations and translations. A load cell on the active cable allowed for measurement of the applied force.We found that deformity resulted in increased external rotation, adduction and translation during flexion and increased internal rotation, extension and decreased translation during abduction. We also found that when a more severe deformity was present, more force was required to create both flexion and abduction to the same angle. Further, we found that resection resulted in increased internal rotation and translation during flexion and decreased internal rotation during abduction. Less force was required to create flexion and abduction following resection. Changes to motion patterns occur as a result of changed contact loads between the femoral head and acetabulum, resulting in loading of regions of articular cartilage which may not be optimized for these loads and may, therefore, begin a degenerative cascade leading to osteoarthritis. As coupled motions were observed within ranges of flexion and abduction required for daily living, it is recommended that resection be performed in an attempt to slow the progression of osteoarthritis by limiting contact between the femoral head-neck and acetabulum.

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