Ashvin Thambyah

Influence of PLIF Cage Size on Lumbar Spine Stability

Study Design. In vitro biomechanical testing on functional spine units with posterior lumbar interbody fusion cage implants of progressively larger sizes. Objectives. To determine the influence of increasing cage size on the restoration of spine stability after total facetectomy. Summary of Background Data. Bilateral insertion of cages in posterior lumbar interbody fusion commonly involves facetectomy. To restore stability with no additional instrumentation, the cages must provide sufficient distraction of the vertebrae and adequate tension in the anulus. The size of cages is therefore an important consideration in posterior lumbar interbody fusion. Methods. Eight human lumbar functional spine units were obtained and divided into two equal groups; one group underwent bending tests and the other twisting. The functional spinal units were tested intact, after total bilateral facetectomy and with three sets of cages that were progressively larger in size. Results. After facetectomy, the functional spine unit’s stiffness reduced significantly from that of the intact spine in extension (48% of intact), lateral bending (25%), and torsion (39%). With the posterior insertion of small cages into the facetectomized functional spine units, only extension stiffness was restored to the intact level, whereas flexion stiffness reduced significantly (41% of intact). The medium cages restored the lateral bending stiffness of the facetectomized functional spine units; only the large cages managed to restore the torsional stiffness. Flexion stiffness of the facetectomized functional spine units with cages remained significantly less than that of the intact spine, regardless of cage size. Conclusion. In the facetectomized lumbar spine unit, cage size influences lateral bending and torsional stability.

Treatment of chondral lesions in advanced osteochondritis dissecans: a comparative study of the efficacy of chondrocytes, mesenchymal stem cells, periosteal graft, and mosaicplasty (osteochondral autograft) in animal models.

Management of chondral lesions in osteochondritis dissecans remains a challenge. This study investigated the efficacy of periosteal graft, osteochondroidal autograft, autologous chondrocyte and mesenchymal stem cell transplants in the treatment of chondral lesions in animal models. Full-thickness articular cartilage defects were created in the weight-bearing surface of the medial femoral condyle in 20-week-old NZW rabbits. A total of 56 knees were randomly divided into four groups as follows: group 1, transfer of cultured chondrocytes; group 2, transfer of cultured mesenchymal stem cells; group 3, repair by periosteal graft; and group 4, mosaicplasty. All of the contralateral knees served as control. Gross, histologic, and biomechanical examinations at 36 weeks after the operation showed that the cultured chondrocytes and mesenchymal stem cells had comparable enhancing effects on the repair of chondral defects in advanced osteochondritis dissecans, whereas mosaicplasty did well initially and periosteal graft did less favorably.

Micro-anatomical response of cartilage-on-bone to compression: mechanisms of deformation within and beyond the directly loaded matrix

The biomechanical function of articular cartilage relies crucially on its integration with both the subchondral bone and the wider continuum of cartilage beyond the directly loaded contact region. This study was aimed at visualizing, at the microanatomical level, the deformation response of cartilage including that of the non‐directly loaded continuum. Cartilage‐on‐bone samples from bovine patellae were loaded in static compression until a near‐equilibrium deformation was achieved, and then chemically fixed in this deformed state. Full‐depth cartilage–bone sections, incorporating the indentation profile and beyond, were studied in their fully hydrated state using differential interference contrast microscopy. Morphometric measurements of the indented profile were used in combination with a force analysis of the tangential layer to investigate the extent to which the applied force is attenuated in moving away from the directly loaded region. This study provides microscopic evidence of a structure‐related response in the transitional zone of the cartilage matrix. It is manifested as an intense chevron‐type shear discontinuity arising from the constraints provided by both the strain‐limiting articular surface and the osteochondral attachment. The discontinuity persists well into the non‐directly loaded continuum of cartilage and is proposed as a force attenuation mechanism. The structural and biomechanical analyses presented in this study emphasize the important role of the complex microanatomy of cartilage, highlighting the interconnectivity and optimal recruitment of the load‐bearing elements throughout the zonally differentiated cartilage depth.

How healthy discs herniate: a biomechanical and microstructural study investigating the combined effects of compression rate and flexion.

Study Design. Microstructural investigation of compression-induced disruption of the flexed lumbar disc. Objective. To provide a microstructural analysis of the mechanisms of annular wall failure in healthy discs subjected to flexion and an elevated rate of compression. Summary of Background Data. At the level of the motion segment failure of the disc in compression has been extensively studied. However, at the microstructural level the exact mechanisms of disc failure are still poorly understood, especially in relation to loading posture and rate. Methods. Seventy-two healthy mature ovine lumbar motion segments were compressed to failure in either a neutral posture or in high physiological flexion (10°) at a displacement rate of either 2 mm/min (low) or 40 mm/min (high). Testing at the high rate was terminated at stages ranging from initial wall tearing through to facet fracture so as to capture the evolution of failure up to full herniation. The damaged discs were then analyzed microstructurally. Results. Approximately, 50% of the motion segments compressed in flexion at the high rate experienced annulus or annulus-endplate junction failure, the remainder failed via endplate fracture with no detectable wall damage. The average load to induce disc failure in flexion was 18% lower (P < 0.05) than that required to induce endplate fracture. Microstructural analysis indicated that wall rupture occurred first in the posterior mid-then-outer annulus. Conclusion. Disc wall failure in healthy motion segments requires both flexion and an elevated rate of compression. Damage is initiated in the mid-then-outer annular fibers, this a likely consequence of the higher strain burden in these same fibers arising from endplate curvature. Given the similarity in geometry between ovine and human endplates, it is proposed that comparable mechanisms of damage initiation and herniation occur in human lumbar discs. Level of Evidence: N/A

New insights into the role of the superficial tangential zone in influencing the microstructural response of articular cartilage to compression

OBJECTIVE The purpose of this study was to characterize the microstructural response of healthy cartilage in a perturbed physical environment to compressive loading with a novel channel indentation device. Manipulation of the cartilage physical environment was achieved through (1) removal of the superficial tangential zone (STZ) and (2) varying the saline bathing solution concentration. DESIGN Cartilage-on-bone blocks were subjected to creep loading under a nominal stress of 4.5 MPa via an indenter consisting of two rectangular platens separated by a narrow channel relief space to create a specific region where cartilage would not be directly loaded. Each sample was fixed in its near-equilibrium deformed state, after which the cartilage microstructure was examined using differential interference contrast (DIC) optical microscopy and scanning electron microscopy (SEM). The cartilage bulge in the channel relief space was studied in detail. RESULTS STZ removal altered the indentation response at the macro- and microstructural levels. Specifically, the strain in the directly compressed regions was reduced (P=0.012) and the bulge height in the channel relief space was greater (P<0.0001) in the STZ-removed compared with the surface-intact samples. The bulge height in the STZ-removed group was always less than the preloaded cartilage thickness. There was intense shear in the non-directly-loaded regions of intact-cartilage but not in STZ-removed cartilage. Bathing solution concentration influenced only the STZ-removed group, where lower concentrations produced significantly abrupt transitions in matrix continuity between the directly compressed and adjacent non-directly-loaded cartilage (P=0.012). CONCLUSIONS This study showed that while the surface layer was important in distributing loads away from directly-loaded regions, so were other factors such as the matrix fibrillar interconnectivity, swelling potential, and tissue anisotropy.

On new bone formation in the pre-osteoarthritic joint

OBJECTIVE This study investigated the structural alterations in the osteochondral junction, traversing the intact-to-lesion regions, with the aim of elucidating the way in which the pre-osteoarthritic (pre-OA) state progresses to fully developed osteoarthritis (OA). METHOD Thirty bovine patellae showing varying degrees of degeneration, with lesions located in the distal-lateral quarter, were used for this study. Cartilage-on-bone blocks were cut along the lateral facet to include both the lesion site in the distal end and the intact site in the proximal end. The blocks were formalin-fixed, mildly decalcified and microtomed to obtain 30 microm - thick osteochondral slices. Using differential interference contrast optics, the tissue microstructure was captured at high resolution in its fully hydrated state. RESULTS There were structural changes in the osteochondral junction beneath the still-intact articular cartilage adjacent to the lesion site. The changes observed in traversing from the intact to the lesion site exhibited characteristics that were strikingly similar to those associated with primary bone formation. The evidence suggests that disruption of the cartilage continuum by a lesion has wider mechanobiological consequences at the osteochondral junction. CONCLUSION The progression of OA appears to involve new bone formation adjacent to lesion sites. We hypothesise that the new bone spicules that appear in regions beneath intact cartilage adjacent to lesion sites provide a snapshot of the elusive pre-OA state.

How critical are the tibiofemoral joint reaction forces during frequent squatting in Asian populations

This study examines tibiofemoral joint moments and forces when performing a squat. The relevance of studying such an activity is to understand better the mechanical factors involved in the higher incidence of tibiofemoral osteoarthritis in Asian populations where squatting is a common daily activity. In this study, motion analysis data of walking versus squatting were compared, specifically looking at net external knee flexion moments, ground reaction forces and tibiofemoral contact forces. It was found that while squatting resulted in more than 2.5 times larger peak external moments compared with walking, tibiofemoral contact forces were not significantly different. This was due to reduced ground reaction forces recorded for the squatting phase compared to the larger dynamic effects of deceleration at heel strike during walking. The most significant finding of this study was that in squatting, there was a reversal in the tibiofemoral shear reaction force from posterior-directed to anterior-directed, occurring under full compressive load and within a fraction of a second. It is believed that repeated squatting results in many such reversals in shear reactions that may ultimately have significant implications to the long term mechanical function and structural integrity of the joint cartilage.

Microstructural changes in cartilage and bone related to repetitive overloading in an equine athlete model.

The palmar aspect of the third metacarpal (MC3) condyle of equine athletes is known to be subjected to repetitive overloading that can lead to the accumulation of joint tissue damage, degeneration, and stress fractures, some of which result in catastrophic failure. However, there is still a need to understand at a detailed microstructural level how this damage progresses in the context of the wider joint tissue complex, i.e. the articular surface, the hyaline and calcified cartilage, and the subchondral bone. MC3 bones from non-fractured joints were obtained from the right forelimbs of 16 Thoroughbred racehorses varying in age between 3 and 8 years, with documented histories of active race training. Detailed microstructural analysis of two clinically important sites, the parasagittal grooves and the mid-condylar regions, identified extensive levels of microdamage in the calcified cartilage and subchondral bone concealed beneath outwardly intact hyaline cartilage. The study shows a progression in microdamage severity, commencing with mild hard-tissue microcracking in younger animals and escalating to severe subchondral bone collapse and lesion formation in the hyaline cartilage with increasing age and thus athletic activity. The presence of a clearly distinguishable fibrous tissue layer at the articular surface immediately above sites of severe subchondral collapse suggested a limited reparative response in the hyaline cartilage.

Micromechanics of annulus–end plate integration in the intervertebral disc

BACKGROUND CONTEXT The intervertebral disc plays a major functional role in the spinal column, providing jointed flexibility and force transmission. The end plate acts as an important structural transition between the hard vertebral tissues and the compliant disc tissues and is therefore a region of potentially high stress concentration. The effectiveness of anchorage of the tough annulus fibers in the end plate will have a major influence on the overall strength of the motion segment. Failure of the end plate region is known to be associated with disc herniation. PURPOSE The aim of this study was to investigate the mechanism of anchorage of the annular fibers in the end plate. STUDY DESIGN A microstructural analysis of the annulus-end plate region was carried out using motion segments obtained from the lumbar spines of mature ovine animals. METHODS Motion segments were fixed and then decalcified. Samples incorporating the posterior annulus-end plate were then removed and cryosectioned along the plane of one of the lamellar fiber directions to obtain oblique interlamellar sections. These sections were imaged in their fully hydrated state using differential interference contrast optical microscopy. RESULTS The annular fiber bundles on entering the end plate are shown to subdivide into subbundles to form a three-dimensional multileaf morphology with each leaf separated by cartilaginous end plate matrix. This branched morphology increases the interface area between bundle and matrix in proportion to the number of subbundles formed. CONCLUSIONS Given both the limited thickness of the end plate and the intrinsic strength of the interface bond between bundle and end plate matrix, the branched morphology is consistent with a mechanism of optimal shear stress transfer wherein a greater strength of annular fiber anchorage can be achieved over a relatively short insertion distance.

On the Design of Learning Outcomes for the Undergraduate Engineer's Final Year Project.

The course for the final year project for engineering students, because of its strongly research-based, open-ended format, tends to not have well defined learning outcomes, which are also not aligned with any accepted pedagogical philosophy or learning technology. To address this problem, the revised Blooms taxonomy table of Anderson and Krathwohl (2001) is utilised, as suggested previously by Lee and Lai (2007), to design new learning outcomes for the final year project course in engineering education. Based on the expectations of the engineering graduate, and integrating these graduate expectations into the six cognitive processes and four knowledge dimensions of the taxonomy table, 24 learning outcomes have been designed. It is proposed that these 24 learning outcomes be utilised as a suitable working template to inspire more critical evaluation of what is expected to be learnt by engineering students undertaking final year research or capstone projects.