There is a large variability associated with hip stem designs, patient anatomy, bone mechanical property, surgical procedure, loading, etc. Designers and orthopaedists aim at improving the performance of hip stems and reducing their sensitivity to this variability. This study focuses on the primary stability of a cementless short stem across the spectrum of patient morphology using a total of 109 femoral reconstructions, based on segmentation of patient CT scan data. A statistical approach is proposed for assessing the variability in bone shape and density [Blanc, 2012]. For each gender, a thousand new femur geometries were generated using a subset of principal components required to capture 95% of the variance in both female and male training datasets [Bah, 2013]. A computational tool (Figure 1) is then developed that automatically selects and positions the most suitable implant (distal diameter 6–17 mm, low and high offset, 126° and 133° CCD angle) to best match each CT-based 3D femur model (75 males and 34 females), following detailed measurements of key anatomical parameters. Finite Element contact models of reconstructed hips, subjected to physiologically-based boundary constraints and peak loads of walking mode [Speirs, 2007] were simulated using a coefficient of fricition of 0.4 and an interference-fit of 50μm [Abdul-Kadir, 2008]. Results showed that the maximum and average implant micromotions across the subpopulation were 100±7μm and 7±5μm with ranges [15μm, 350μm] and [1μm, 25μm], respectively. The computed percentage of implant area with micromotions greater than reported critical values of 50μm, 100μm and 150μm never exceeded 14%, 8% and 7%, respectively. To explore the possible correlations between anatomy and implant performance, response surface models for micromotion metrics were constructed using the so-called Kriging regression methodology, based on Gaussian processes. A clear nonlinear decreasing trend was revealed between implant average micromotion and the metaphyseal canal flare indexes (MCFI) measured in the medial-lateral (ML), anterio-posterior (AP) and femoral neck-oriented directions but also the average bone density in each Gruen zone. In contrast, no clear influence of the remaining clinically important parameters (neck length and offsets, femoral anteversion and CCD angle, standard canal flares, patient BMI and weight or stem size) to implant average micromotion was found. In conclusion, the present study demonstrates that the primary stability and tolerance of the short stem to variability in patient anatomy were high, suggesting no need for patient stratification. The developed methodology, based on detailed morphological analysis, accurate implant selection and positioning, prediction of implant micromotion and primary stability, is a novel and valuable tool to support implant design and planning of femoral reconstructive surgery.

Objective: To challenge the validity of using biomarker concentrations in synovial fluid for the assessment of joint pathology.

Hypothesis: Synovial fluid biomarker concentrations are influenced by both cartilage and synovial fluid volumes.

Methods: Synovial fluid volumes were determined from the equine metacarpophalangeal (MCP), proximal inter-phalangeal (PIP) and distal interphalangeal (DIP) joints, which have different disease prevalences.

Chondrocyte density was calculated from a defined site in each joint.

Cartilage volume was measured by novel application of Peripheral Quantitative Computed Tomography (pQCT).

Cartilage oligomeric matrix protein (COMP), glycos-aminoglycans (GAG) and total protein (TP) concentrations were measured and then adjusted for cartilage and synovial fluid volume and compared between joints.

Results: Mean synovial fluid volume was significantly greater in the MCP than the distal joints (p< 0.0001) (3.2 ±0.5ml, 0.5 ±0.1ml and 0.6 ±0.1ml respectively). In contrast, the DIP had the greatest cartilage volume compared to the proximal joints (5360 ±667mm3 2640mm3, 1940 ±331mm3 respectively). There was no significant difference in the cartilage cellularity between all joints.

The DIP had higher TP, COMP and GAG concentrations, however, when values were expressed per unit cartilage volume the opposite was found, with the MCP then exhibiting significantly higher concentrations.

Conclusions: These data show the joint with the highest prevalence to osteoarthritis has the lowest biomarker synovial fluid concentrations but the highest biomarker levels per unit cartilage, suggesting a higher release. These results indicate that meaningful interpretation of biomarkers in synovial fluid require consideration of both fluid and cartilage volume.

The skeletal system exhibits functional adaptation. For bone the mechanotransduction mechanisms have been well elucidated; in contrast, the response of tendon to its mechanical environment is much more poorly understood despite tendon disorders being commonly encountered in clinical practice. This study presents a novel approach to developing an isolated tendon system in vivo. This model is used to test the hypothesis that stress-shielding, and subsequent restressing, causes significant biomechanical changes. We propose a control mechanism that governs this process.

A custom-built external fixator was used to functionally isolate the ovine patellar tendon(PT). In group 1 animals(n=5) the right PT was stress-shielded for 6 weeks. This was achieved by drawing the patella towards the tibial tubercle, thus slackening the PT. In group 2 (n=5) the PT was stress-shielded for 6 weeks. The external fixator was then removed and the PT physiologically loaded for a further 6 weeks. In each case, the PT subsequently underwent tensile testing and measurement of length(L) and cross-sectional area(CSA). The untreated left PTs acted as controls (n=10).

6 weeks of stress-shielding significantly decreased material and structural properties of tendon compared to controls (elastic modulus(E) 76.2%, ultimate tensile strength(UTS) 69.3%, stiffness(S) 79.2%, ultimate load(UL) 68.5%, strain energy(SE) 60.7%; p< 0.05). Ultimate strain(US), L and CSA were not significantly changed. 6 weeks of subsequent functional loading (Group 2) caused some improvement in material properties, but greater recovery in structural properties (E 79.8%, UTS 91.8%, S 96.7%, UL 92.7%, SE 96.5%). CSA was significantly greater than Group 1 tendons at 114% of control value.

Previous models of tendon remodelling have relied on either joint immobilization or direct surgical procedures. This model allows close control of the tendon’s mechanical environment whilst allowing normal joint movement and avoiding surgical insult to the tendon itself. The hypothesis that stress-shielding, and subsequent restressing, causes significant biomechanical changes has been upheld. We propose that the biomechanical changes observed are governed by a strain homeostasis feedback mechanism.

Introduction: The ‘gold standard’ currently used to assess bone quality is bone mineral density (BMD) measured by Dual Energy X-ray Absorptiometry (DEXA). However BMD accounts for no more than 60 – 70% of bone strength. X-rays are affected primarily by the mineral phase of bone; the organic phase remains essentially invisible. Yet it is known that the material strength and toughness of bone is critically dependent on its organic phase. A Raman spectroscopic technique was used that permitted visualisation of both phases of bone deep to unbroken skin by successfully removing spectral information from the overlying tissues.

Hypothesis: Spectral features of both the mineral and organic phases of bone from different murine genotypes can be measured objectively through the unbroken skin using time-resolved Raman spectroscopy.

Methods: We used an 800 nm probe laser (1 kHz, 1 ps pulses, focussed to 1 mm diameter) with a synchronised 4 ps optical Kerr gate that had a variable picosecond delay that effectively shuttered out photons from the overlying tissues. We measured bone spectra at a point 2mm above the carpus from two mouse genotypes: wildtype and oim/oim (matched for age, sex and weight) at a typical depth 1.1mm. We then repeated the measurements once the overlying tissues had been carefully removed to expose the bones directly. Oim/oim mice produce only homotrimeric collagen I, (á1(I)3), associated with this change in collagen is a poor mineralisation of the bone tissue, making it an ideal model for a this study.

Results: We recorded the main spectral features in both phases of bone and showed that the ratios of spectral bands from the two phases were similar within each genotype, whether measured through the skin or directly from exposed bone. However, there was a significant difference in the same ratios between genotypes associated with a reduced mineralisation in the oim/oim mice; a significant difference that was apparent both directly from bone and through skin. The band associated with CH2 wag of collagen (organic phase) showed a frequency shift between the genotypes.

Discussion: Measurements of the spectra and their analysis were similar whether made directly on bone or transcutaneously. We were able to detect changes in mineralisation between genotypes and, unlike measurements of BMD, we showed also changes in collagen. Since the material strength of bone is critically dependent on collagen, this indicates an appreciable advantage of this technique over DEXA.

Conclusions: This novel technique allowed objective transcutaneous spectral measurements of bone tissue and was able to distinguish between normal and unhealthy bone tissue. With a laser focussed to 1 mm diameter that was readily moveable, these measurements were specific to that site (2 mm proximal to the carpus). After further optimisation, this technology is likely to improve fracture risk assessments in comparison to the use of DEXA alone, opening opportunities for screening in anticipation of the predicted increase in fragility fractures.

We examined the effect of periosteal devascularisation upon the early healing of osteotomies of sheep tibiae held in an instrumented external fixation system with an axial stiffness of 240 N/mm. At 14 days, cortical blood flow measured by the microsphere technique was 19.3 ml/min/100g in the well-vascularised osteotomies, but only 1.7 ml/min/100g in the devascularised osteotomies, despite an increase in medullary flow (p less than 0.0005). Delay in healing of the devascularised osteotomies was suggested by an in vivo monitoring system and confirmed by post-mortem mechanical testing. We suggest that the osteogenic stimulus of dynamic external fixation is dependent on the early restoration of cortical blood flow in devascularised fractures.