Philip Riches

The internal mechanics of the intervertebral disc under cyclic loading

The mechanics of the intervertebral disc (IVD) under cyclic loading are investigated via a one-dimensional poroelastic model and experiment. The poroelastic model, based on that of Biot (J. Appl. Phys. 12 (1941) 155; J. Appl. Mech. 23 (1956) 91), includes a power-law relation between porosity and permeability, and a linear relation between the osmotic potential and solidity. The model was fitted to experimental data of the unconfined IVD undergoing 5 cyclic loads of 20 min compression by an applied stress of 1MPa, followed by 40 min expansion. To obtain a good agreement between experiment and theory, the initial elastic deformation of the IVD, possibly associated with the bulging of the IVD into the vertebral bodies or laterally, was removed from the experimental data. Many combinations of the permeability-porosity relationship with the initial osmotic potential (pi(i)) were investigated, and the best-fit parameters for the aggregate modulus (H(A)) and initial permeability (k(i)) were determined. The values of H(A) and k(i) were compared to literature values, and agreed well especially in the context of the adopted high-stress testing regime, and the strain related permeability in the model.

A comparison of sprinting kinematics on two types of treadmill and over-ground

Conventionally motorized treadmills elicit different sprinting kinematics to the over‐ground condition. Treadmills powered by a torque motor have been used to assess sprinting power; yet, the kinematics of sprinting on the torque treadmill are unknown. This study compares the sprinting kinematics, during the constant velocity phase, between a conventional treadmill, a torque treadmill and the over‐ground condition to assess the suitability of each treadmill for sprinting studies and training. After familiarization, 13 recreationally active males performed multiple sprints at various experimental settings on each surface. Ninety sprints, which attained mean velocities over 7.0 m/s, had their lower‐body sagittal plane joint angles during ground contact captured at 250 Hz. These data were low‐pass filtered at 10 Hz, and compared with respect to surface, subject and velocity using an ANCOVA statistical model. Sprinting on the conventional treadmill elicited a longer ground contact time, a longer braking phase, a more extended knee at foot strike and a faster extending hip than the torque treadmill and over‐ground (all P<0.05). The torque treadmill obtained an equivalent sprinting technique to the over‐ground condition, with the exception of a less extended hip at toe‐off, suggesting that it is more appropriate for laboratory sprinting analyses and training than the conventional treadmill.

Determination of the strain-dependent hydraulic permeability of the compressed bovine nucleus pulposus

The hydraulic permeability, k, of the nucleus pulposus (NP) is crucial, both in withstanding compressive stress and for convective transport of nutrients within the disc. Permeability has previously been determined using biphasic mathematical models, but has not been found by direct permeation experiments, which is the objective of this study. Bovine coccygeal nucleus samples (n=64), phi10mm and thickness 683+/-49microm (mean+/-S.D.) were compressed axially to one of lambda=1.0, 0.9, 0.8, 0.7, 0.6, 0.5, 0.4 or 0.3, where lambda is the stretch ratio. Ringers solution was permeated through the sample, with an o-ring ensuring axial flow. During stress equilibrium, k was determined and fitted to four permeability-strain equations. Permeability decreased exponentially with compression, and was best described by Values of k were comparable to those arising from mathematical models, lending confidence to permeability being determined from such models.

Microhardness anisotropy of lamellar bone

The Knoop microhardness test has been utilised to observe in-plane microhardness anisotropy of rat tibiae. The elongated rhombohedral geometry of the Knoop indenter enables the Knoop microhardness (HK) to be calculated for a given indenter orientation. Two indenter orientations were used: the major axis of the indenter was aligned along the length of, and across the mid-sagittal section. The statistical analysis demonstrated that the variation in HK was primarily due to the orientation of the Knoop indenter (p < 0.001). HK was consistently greater when the indenter was aligned with the major diagonal radial on the mid-sagittal section.

Non-invasive computer-assisted measurement of knee alignment.

The quantification of knee alignment is a routine part of orthopaedic practice and is important for monitoring disease progression, planning interventional strategies, and follow-up of patients. Currently available technologies such as radiographic measurements have a number of drawbacks. The aim of this study was to validate a potentially improved technique for measuring knee alignment under different conditions. An image-free navigation system was adapted for non-invasive use through the development of external infrared tracker mountings. Stability was assessed by comparing the variance (F-test) of repeated mechanical femoro-tibial (MFT) angle measurements for a volunteer and a leg model. MFT angles were then measured supine, standing and with varus-valgus stress in asymptomatic volunteers who each underwent two separate registrations and repeated measurements for each condition. The mean difference and 95% limits of agreement were used to assess intra-registration and inter-registration repeatability. For multiple registrations the range of measurements for the external mountings was 1° larger than for the rigid model with statistically similar variance (p = 0.34). Thirty volunteers were assessed (19 males, 11 females) with a mean age of 41 years (range: 20–65) and a mean BMI of 26 (range: 19–34). For intra-registration repeatability, consecutive coronal alignment readings agreed to almost ±1°, with up to ±0.5° loss of repeatability for coronal alignment measured before and after stress maneuvers, and a ±0.2° loss following stance trials. Sagittal alignment measurements were less repeatable overall by an approximate factor of two. Inter-registration agreement limits for coronal and sagittal supine MFT angles were ±1.6° and ±2.3°, respectively. Varus and valgus stress measurements agreed to within ±1.3° and ±1.1°, respectively. Agreement limits for standing MFT angles were ±2.9° (coronal) and ±5.0° (sagittal), which may have reflected a variation in stance between measurements. The system provided repeatable, real-time measurements of coronal and sagittal knee alignment under a number of dynamic, real-time conditions, offering a potential alternative to radiographs.

Accuracy of a freehand sculpting tool for unicondylar knee replacement

Unicondylar knee replacement is technically challenging and malalignment of the implant components is one of the factors that results in high failure rates. Surgical robotics with navigation is emerging as a potential solution to improve the accuracy of implant placement.

Standardising the clinical assessment of coronal knee laxity

Clinical laxity tests are used for assessing knee ligament injuries and for soft tissue balancing in total knee arthroplasty. This study reports the development and validation of a quantitative technique of assessing collateral knee laxity through accurate measurement of potential variables during routine clinical examination. The hypothesis was that standardisation of a clinical stress test would result in a repeatable range of laxity measurements. Non-invasive infrared tracking technology with kinematic registration of joint centres gave real-time measurement of both coronal and sagittal mechanical tibiofemoral alignment. Knee flexion, moment arm and magnitude of the applied force were all measured and standardised. Three clinicians then performed six knee laxity examinations on a single volunteer using a target moment of 18 Nm. Standardised laxity measurements had small standard deviations (within 1.1°) for each clinician and similar mean values between clinicians, with the valgus laxity assessment (mean of 3°) being slightly more consistent than varus (means of 4° or 5°). The manual technique of coronal knee laxity assessment was successfully quantified and standardised, leading to a narrow range of measurements (within the accuracy of the measurement system). Minimising the subjective variables of clinical examination could improve current knowledge of soft tissue knee behaviour.

Is the coactivation of biceps femoris during isometric knee extension affected by adiposity in healthy young humans

This study aimed to verify if the level of biceps femoris antagonist activity measured during isometric knee extension was affected by the individual degree of adiposity in 14 young healthy subjects of both genders aged between 18 and 24. Surface EMG signals were recorded from the biceps femoris muscle of the dominant leg during isometric knee extension at three levels of voluntary contraction: maximum (MVC), 80% MVC and 200 N, respectively. In addition, whole-body percentage of fat, volume of the thigh and skinfold thickness below the electrodes were achieved. Biceps femoris coactivation values were: 28.5 +/- 17.9%, 30.9 +/- 17.7% and 25.3 +/- 17.5% for MVC, 80% MVC and 200 N trials, respectively (NS). Neither the whole-body percentage of fat nor the skinfold thickness influenced percentage coactivation, irrespective of the intensity of contraction. However, an increase in the whole-body percentage of fat showed a tendency to augment the biceps femoris coactivation (P(I)=0.079; P(II)=0.575). No differences in coactivation were observed between genders. In addition, the duration of contraction did not affect the level of coactivation.

Knoop microhardness anisotropy of the ovine radius

The Knoop indenter has been used to characterise fully the Knoop microhardness (H(K)) anisotropy of compact bone. 2120 indentations were performed on mature ovine radii and a linear relationship was found between H(K) and the angle between the major diagonal of the indenter and the lamella boundaries (p<<0.001). H(K) increased significantly with ash fraction (p<0.001), but decreased with atmospheric vapour pressure (p<0.05). A significant interaction was found between ash fraction and atmospheric vapour pressure (p<0.01). H(K) significantly varied with indentation position along the diaphysis and around the cortex (both p<<0.001), however radial variation in H(K) was not statistically significant. The variation of ash fraction showed similar trends. These data show that H(K) varies similarly to Vickers microhardness, but in addition, can provide clear information on the anisotropy of Haversian bone without the need for excising many different indentation planes. A large number of indentations are required to obtain low type I and type II errors in the statistical analysis.

3D bioactive composite scaffolds for bone tissue engineering

Bone is the second most commonly transplanted tissue worldwide, with over four million operations using bone grafts or bone substitute materials annually to treat bone defects. However, significant limitations affect current treatment options and clinical demand for bone grafts continues to rise due to conditions such as trauma, cancer, infection and arthritis. Developing bioactive three-dimensional (3D) scaffolds to support bone regeneration has therefore become a key area of focus within bone tissue engineering (BTE). A variety of materials and manufacturing methods including 3D printing have been used to create novel alternatives to traditional bone grafts. However, individual groups of materials including polymers, ceramics and hydrogels have been unable to fully replicate the properties of bone when used alone. Favourable material properties can be combined and bioactivity improved when groups of materials are used together in composite 3D scaffolds. This review will therefore consider the ideal properties of bioactive composite 3D scaffolds and examine recent use of polymers, hydrogels, metals, ceramics and bio-glasses in BTE. Scaffold fabrication methodology, mechanical performance, biocompatibility, bioactivity, and potential clinical translations will be discussed.