Personalised 3D knee compliance from clinically viable knee laxity measurements: A proof of concept ex vivo experiment.

Personalised information of knee mechanics is increasingly used for guiding knee reconstruction surgery. We explored use of uniaxial knee laxity tests mimicking Lachman and Pivot-shift tests for quantifying 3D knee compliance in healthy and injured knees. Two healthy knee specimens (males, 60 and 88 years of age) were tested. Six-degree-of-freedom tibiofemoral displacements were applied to each specimen at 5 intermediate angles between 0° and 90° knee flexion. The force response was recorded. Six-degree-of-freedom and uniaxial tests were repeated after sequential resection of the anterior cruciate, posterior cruciate and lateral collateral ligament. 3D knee compliance (C6DOF) was calculated using the six-degrees-of-freedom measurements for both the healthy and ligament-deficient knees and validated using a leave-one-out cross-validation. 3D knee compliance (CCT) was also calculated using uniaxial measurements for Lachman and Pivot-shift tests both conjointly and separately. C6DOF and CCT matrices were compared component-by-component and using principal axes decomposition. Bland-Altman plots, median and 40-60th percentile range were used as measurements of bias and dispersion. The error on tibiofemoral displacements predicted using C6DOF was < 9.6% for every loading direction and after release of each ligament. Overall, there was good agreement between C6DOF and CCT components for both the component-by-component and principal component comparison. The dispersion of principal components (compliance coefficients, positions and pitches) based on both uniaxial tests was lower than that based on single uniaxial tests. Uniaxial tests may provide personalised information of 3D knee compliance.

The equivalence of multi-axis spine systems: Recommended stiffness limits using a standardized testing protocol.

The complexity of multi-axis spine testing often makes it challenging to compare results from different studies. The aim of this work was to develop and implement a standardized testing protocol across three six-axis spine systems, compare them, and provide stiffness and phase angle limits against which other test systems can be compared. Standardized synthetic lumbar specimens (n=5), comprising three springs embedded in polymer at each end, were tested on each system using pure moments in flexion-extension, lateral bending, and axial rotation. Tests were performed using sine and triangle waves with an amplitude of 8Nm, a frequency of 0.1Hz, and with axial preloads of 0 and 500N. The stiffness, phase angle, and R2 value of the moment against rotation in the principal axis were calculated at the center of each specimen. The tracking error was adopted asa measure of each test system to minimize non-principal loads, defined as the root mean squared difference between actual and target loads. All three test systems demonstrated similar stiffnesses, with small (<14%) but significant differences in 4 of 12 tests. More variability was observed in the phase angle between the principal axis moment and rotation, with significant differences in 10 of 12 tests. Stiffness and phase angle limits were calculated based on the 95% confidence intervals from all three systems. These recommendations can be used with the standard specimen and testing protocol by other research institutions to ensure equivalence of different spine systems, increasing the ability to compare in vitro spine studies.

Effect of degeneration on the six degree of freedom mechanical properties of human lumbar spine segments.

While the effects of disc degeneration on compression and rotation motions have been studied, there is no data for shear loading. Clinical research has shown that those with low back pain (a potential consequence of degeneration) experience a 75% greater lateral shear force than those without it. Therefore, the aim was to compare the effect of degeneration on spine segment stiffness and phase angle in each of six degree of freedom (6DOF) loading directions. Fourteen intact functional spinal units (FSU) were dissected from human lumbar spines (mean (SD) age 76.2 (11) years, Thompson grades 3 (N = 5, mild), 4 (N = 6, moderate), 5 (N = 3, severe)). Each FSU was tested in ±6DOFs while subjected to a physiological preload, hydration, and temperature (37°C) conditions in a hexapod robot. A one-way ANOVA between degenerated groups was performed on stiffness and phase angle for each DOF. Significant differences in stiffness were found between mild and moderate degenerative groups in lateral shear (p = 0.001), and axial rotation (p = 0.001), where moderate degeneration had decreased stiffness. For phase angle, significant differences were seen in anterior shear (p = 0.017), and axial rotation (p = 0.026), where phase angle for mild degeneration was less than moderate. Trends of stiffness and phase angle changes between degenerative groups were similar within each DOF. Clinically, the identification of the DOFs that are most affected by degeneration could be used in rehabilitation to improve supplemental stabilization of core muscle groups. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1399-1409, 2016.

Effect of potting technique on the measurement of six degree-of-freedom viscoelastic properties of human lumbar spine segments.

Polymethyl methacrylate (PMMA) and Wood's Metal are fixation media for biomechanical testing; however, the effect of each potting medium on the measured six degree-of-freedom (DOF) mechanical properties of human lumbar intervertebral discs is unknown. The first aim of this study was to compare the measured 6DOF elastic and viscoelastic properties of the disc when embedded in PMMA compared to repotting in Wood's Metal. The second aim was to compare the surface temperature of the disc when potted with PMMA and Wood's Metal. Six human lumbar functional spinal units (FSUs) were first potted in PMMA, and subjected to overnight preload in a saline bath at 37 °C followed by five haversine loading cycles at 0.1 Hz in each of 6DOF loading directions (compression, left/right lateral bending, flexion, extension, left/right axial rotation, anterior/posterior, and lateral shear). Each specimen was then repotted in Wood's Metal and subjected to a 2-h re-equilibrating preload followed by repeating the same 6DOF tests. Outcome measures of stiffness and phase angle were calculated from the final loading cycle in each DOF and were expressed as normalized percentages relative to PMMA (100%). Disc surface temperatures (anterior, left/right lateral) were measured during potting. Paired t-tests (with alpha adjusted for multiple DOF) were conducted to compare the differences in each outcome parameter between PMMA and Wood's Metal. No significant differences in stiffness or phase angle were found between PMMA and Wood's Metal. On average, the largest trending differences were found in the shear DOFs for both stiffness (approximately 35% greater for Wood's Metal compared to PMMA) and phase angle (approximately 15% greater for Wood's Metal). A significant difference in disc temperature was found at the anterior surface after potting with Wood's Metal compared to PMMA, which did not exceed 26 °C. Wood's Metal is linear elastic, stiffer than PMMA and may reduce measurement artifact of potting medium, particularly in the shear directions. Furthermore, it is easier to remove than PMMA, reuseable, and cost effective.

A novel method to replicate the kinematics of the carpus using a six degree-of-freedom robot.

Understanding the kinematics of the carpus is essential to the understanding and treatment of wrist pathologies. However, many of the previous techniques presented are limited by non-functional motion or the interpolation of points from static images at different postures. We present a method that has the capability of replicating the kinematics of the wrist during activities of daily living using a unique mechanical testing system. To quantify the kinematics of the carpal bones, we used bone pin-mounted markers and optical motion capture methods. In this paper, we present a hammering motion as an example of an activity of daily living. However, the method can be applied to a wide variety of movements. Our method showed good accuracy (1.0-2.6°) of in vivo movement reproduction in our ex vivo model. Most carpal motion during wrist flexion-extension occurs at the radiocarpal level while in ulnar deviation the motion is more equally shared between radiocarpal and midcarpal joints, and in radial deviation the motion happens mainly at the midcarpal joint. For all rotations, there was more rotation of the midcarpal row relative to the lunate than relative to the scaphoid or triquetrum. For the functional motion studied (hammering), there was more midcarpal motion in wrist extension compared to pure wrist extension while radioulnar deviation patterns were similar to those observed in pure wrist radioulnar deviation. Finally, it was found that for the amplitudes studied the amount of carpal rotations was proportional to global wrist rotations.

Assessment of the initial viscoelastic properties of a critical segmental long bone defect reconstructed with impaction bone grafting and intramedullary nailing.

INTRODUCTION: This study compared the initial viscoelastic properties of a segmental tibial defect stabilized with intramedullary nailing and impaction bone grafting to that of a transverse fracture stabilized with intramedullary nailing. MATERIALS AND METHODS: Seven sheep tibiae were tested in compression (1000N), bending and torsion (6Nm) in a six degree-of-freedom hexapod robot. Tests were repeated across three groups: intact tibia (Intact), transverse fracture stabilized by intramedullary nailing (Fracture), and segmental defect stabilized with a nail and impaction bone grafting (Defect). Repeated measures ANOVA on the effect of group on stiffness/phase angle were conducted for each loading direction. RESULTS: The Intact group was significantly stiffer than the Fracture and Defect groups in bending and torsion (p<0.022 for both loading directions), and was marginal for the Defect group in compression (p=0.052). No significant differences were found between the Fracture and Defect groups (p>0.246 for all loading directions) for stiffness/phase angle. In compression and bending, phase angles were significantly greater for the Fracture and Defect groups compared to Intact (p<0.025), with no significant differences between groups in torsion (p=0.13). Sensitivity analyses conducted between the Fracture and Defect group differences found that they were not of clinical significance. CONCLUSION: The initial properties of a segmental defect stabilized with intramedullary nailing and impaction bone grafting was not clinically significantly different to that of a transverse fracture stabilized with intramedullary nailing.