Background

Lateral lumbar spine statistical shape models (SSM) have been used previously to describe associations with osteoarthritis and back pain. However, associations with factors such as osteoporosis, menopause and parity have not been explored.

Purpose and Background

Both overall spine shape and the size and shape of individual vertebrae undergo rapid growth and development during early childhood. Motor development milestones such as age of walking influence spine development, with delayed ambulation linked with spinal conditions including spondylolysis. However, it is unclear whether associations between motor development and spine morphology persist into older age. Therefore, these associations were examined using data from the MRC National Survey of Health and Development, a large nationally-representative British cohort, followed up since birth in 1946.

An increased prevalence of osteoarthritis (OA) in post-menopausal women has led to the suggestion that hormonal factors may play a role in the pathogenesis. This study aims to examine if undergoing a hysterectomy, both with retention and removal of ovaries, predisposes women to OA and secondly if the development is influenced by hormone replacement therapy (HRT).

Statistical shape modelling (SSM) is a method of image analysis allowing for detection of subtle shape variation described by landmark points. Through the generation of linearly independent modes of variation, each image can be described in terms of numerical scores. 149 radiographs from female participants of the Osteoarthritis Initiative (OAI) were examined to compare hip morphology in those who had undergone hysterectomies compared to controls.

No differences were observed in BMI, age, height or weight between groups. ANOVA and Games-Howell post-hoc analysis showed that modes 3 and 5 were statistically significant. Lower mode 3 scores were associated with hysterectomy (p=0.019), with narrowing of the femoral neck and increased acetabular coverage. Lower mode 5 scores were associated with hysterectomy and oophorectomy (p=0.049), displaying reduced coverage of the femoral head, superolateral migration of the femoral head and larger greater trochanter. No associations were observed between HRT use and OA.

The subtle morphologic features of hip OA present in only hysterectomised women suggests undergoing a hysterectomy may be a predisposing factor and a clinical consideration. The use of HRT was not observed to influence the development of OA and thus cannot be suggested as a protective measure.

Purpose

To investigate associations between sagittal thoracolumbar spine shape with sex and measures of adiposity throughout adulthood.

Purpose and Background:

Healthy adults with a curvy (lordotic) lumbar spine were shown to lift a load from the floor by stooping, while straight (flat) spines squatted. Since skin-surface motion capture often misrepresents internal curvature this study calculated internal lumbar curvature during lifting in the same cohort and compared lumbosacral motion.

Background and Aim

Spinal stability is associated with low back pain and affects the spines ability to support loads. Stability can be achieved if the applied force follows the curvature of the spine, passing close to the vertebral centroids. Previously we showed that calculated muscle forces required for stability in an idealised model increased with increasing and more evenly distributed lumbar curvatures. The purpose of this study was to calculate the muscle forces required for stability in standing in a group of healthy adults.

Purpose and Background

Low birth weight is related to decreased lumbar spine vertebral canal size and bone mineral content later in life, suggesting that antenatal factors affect spine development. The purpose of this study was to explore associations between antenatal factors and lumbar spine morphology in childhood.

In uncemented total hip arthroplasty (THA), the optimal femoral component should allow both maximum cortical contact with proximal load transfer and accurate restoration of individual joint biomechanics. This is often compromised due to a high variability in proximal femoral anatomy. The aim of this on-going study is to assess the variation in proximal femoral canal shape and its association with geometric and anthropometric parameters in primary hip OA.

In a retrospective cohort study, AP-pelvis radiographs of 98 consecutive patients (42 males, 56 females, mean age 61 (range:45-74) years, BMI 27.4 (range:20.3-44.6) kg/m2) who underwent THA for primary hip OA were reviewed. All radiographs were calibrated and femoral offset (FO) and neck-shaft-angle (NSA) were measured using a validated custom programme. Point-based active shape modelling (ASM) was performed to assess the shape of the inner cortex of the proximal femoral meta- and diaphysis. Independent shape modes were identified using principal component analysis (PCA). Hierarchical cluster analysis of the shape modes was performed to identify natural groupings of patients. Differences in geometric measures of the proximal femur (FO, NSA) and demographic parameters (age, height, weight, BMI) between the clusters were evaluated using Kruskal-Wallis one-way-ANOVA or Chi-square tests, as appropriate.

In the entire cohort, mean FO was 39.0 mm, mean NSA was 131 degrees. PCA identified 10 independent shape modes accounting for over 90% of variation in proximal femoral canal shape within the dataset. Cluster Analysis revealed 6 shape clusters for which all 10 shape modes demonstrated a significantly different distribution (p-range:0.000-0.015). We observed significant differences in age (p=0.032), FO (p<0.001) and NSA (p<0.001) between the clusters. No significant differences with regard to gender or BMI were seen.

Our preliminary analysis has identified 6 different patterns of proximal femoral canal shape which are associated with significant differences in femoral offset, neck-shaft-angle and age at time of surgery. We are currently evaluating the entire dataset of 345 patients which will allow a comprehensive classification of variation in proximal femoral shape and joint geometry. The present data may optimise preoperative planning and improve future implant design in THA.

Surgical reconstruction of the injured Anterior Cruciate Ligament (ACL) is an effective solution to knee instability, but not all grafts incorporate well. The biological environment in the knee that controls graft integration is not well understood, and this study aims to fill that gap as the first step towards a translational approach to optimise outcomes.

Over two stages, tissue samples and knee fluid samples were harvested from patients undergoing ACL reconstruction. These samples were cultured and stored to allow batch analysis for a variety of cytokines, growth factors and collagenases.

Stage 1 (n=14) identified the presence of specific pro-inflammatory cytokines, growth factors and latent collagenase. Information gathered allowed a more targeted approach to be used in stage 2 (n=18). Stage 2 data from tissue cultures suggest that collagenase activity peaks later than 6 hours post-op. The relationships between collagenase activity and levels of TNF-alpha, IL-1beta and bFGF are of potential interest, and the profiles of patients will be compared with longer term follow-up data to determine any effects on outcomes.

Further detailed assessment of the biology of ACL graft incorporation is required, but these preliminary data have clarified some of the details worthy of further study.

Introduction: Tendon ruptures are increasingly common, repair can be difficult and healing poorly understood. Tissue engineering approaches often require expansion of cell numbers to populate a construct, and maintenance of cell phenotoype is essential for tissue regeneration.

Methods: In this study we characterise the phenotype of human Achilles tenocytes and assess how this is affected by passaging. Tenocytes, isolated from tendon samples from 6 patients receiving surgery for rupture of the Achilles tendon, were passaged 8 times. Proliferation rates and cell morphology were recorded at passages 1, 4 and 8. Total collagen, the ratio of collagen types I and III and decorin were used as indicators of matrix formation and expression of the integrin ‘alpha’1 subunit as a marker of cell-matrix interactions.

Results: With increasing passage number, cells became more rounded, were more widely spaced at confluence and confluent cell density declined from 18700 /cm2 to 16100 /cm2 (P=0.009). No change to total cell layer collagen was observed but the ratio of type III to type I collagen increased from 0.60 at passage 1 to 0.89 at passage 8 (P< 0.001). Decorin expression significantly decreased with passage number, from 22.9 ± 3.1 ng/ng DNA at passage 1, to 9.1 ± 1.8 ng/ngDNA at passage 8 (P< 0.001). Integrin expression did not change.

Conclusion: We conclude that the phenotype of tenocytes in culture rapidly drifts with progressive passage.

Introduction: Aseptic loosening of the acetabular cup is the commonest cause for revision surgery of total hip replacements (THR). Whereas a sound bone cement micro-interlock is believed to contribute most to the stability, most surgeons prefer to enhance their fixation by adding a macro-interlock. Drilling of additional keyholes creates cement pegs intended to resist rotational forces at the acetabular bone-cement interface. Only a few attempts have been made to investigate the effect of number, configuration and shape of these keyholes. Following the limited experiments by Oh (1983) on beechwood blocks, Mburu (1999) systematically optimised diameter, depth and number of keyholes using the same beechwood model. He subsequently developed a novel drill, bell-shaped in cross-section, aiming to minimise the stress concentration observed at the base of cement pegs.

Methods: This study compares the novel drill against a conventional drill using the same beechwood model and the previously optimised number, diameter and depth of keyholes, the shape of the drills being the only difference. The tests were performed on twenty beechwood blocks, half of the blocks allocated to the novel drill and half to the conventional drill. Since the three keyholes were not equidistant, it was also tested whether the direction of torque had an influence on the results. This was done by applying torque clockwise in half of the specimens and anticlockwise in the other half.

Results: The results showed that the static torque to failure was superior in the novel drill (mean: 163 Nm) compared to the conventional drill (mean: 127 Nm), but this was not statistically significant (p= 0.12) due to the wide variation of results. There was no difference for the direction of torque (p= 0.8) and the type of drill and the direction of torque did not show any interaction (p= 0.5).

Conclusions: Results suggest that there is an improvement with the novel drill in static torque to failure. Further testing appears therefore to be worthwhile. Results also suggest that the difference or improvement achieved with the novel drill is less pronounced than it is for number, depth and position of keyholes (Mburu, 1999). However, more variability than expected was encountered. Despite recognition as a possible model for keyhole testing, beechwood blocks testing three keyholes at the same time may not be appropriate when the load to failure for cement pegs is approximating the load to failure for beechwood as encountered in our experiment. Simplification of the system like testing single pegs should be considered.

Introduction: Since the mid-1800’s it has been believed that the human femur functions in a similar way to a crane in which the distal end is fixed and body weight is applied to the femoral head (Meyer, 1867, Williams, 1995). This results in tension in the lateral femoral shaft and in the so-called ‘principal tensile system’ of trabeculae while, compression is found in the medial shaft and in the ‘principal compressive system’. Most studies have concentrated on the shaft to find ways of avoiding these tensile stresses and recognised that the inclusion of muscle forces is essential in any realistic modelling. The state of stress in the proximal femur has not been satisfactorily resolved, though a minority view is that muscle forces put all of the trabeculae into compression (Strange, 1965). Our hypothesis is that the majority of the proximal femur is in compression and that the so-called ‘principal tensile system’ functions as an arch, transferring compressive stresses to the diaphysis.

Methods: To begin to test this, we have developed a 2D finite element (FE) model of the femur. The distal end was constrained and a force of half body weight, representing two-legged stance and negligible muscle forces, was applied to a representation of the acetabulum. The material properties used were 17 GPa for cortical bone, and 100–400 MPa for cancellous bone, with a higher modulus assigned to areas of greater apparent density. The model was meshed, using eight-node quadrilateral elements, and solved using ANSYS 8.0 software (ANSYS, Inc., USA). Recognising that the joint capsule is a substantial structure, ligamentous forces were included by spring-like link elements. Contact elements were used between the femoral head and acetabulum.

Results: In the absence of the capsular ligaments, stresses in the proximal femur were similar to those predicted by the crane model, i.e. corresponding to the traditional description of tensile and compressive trabeculae. The inclusion of ligamentous forces resulted in compressive stresses being generated over most of the proximal femur. When the denser trabecular systems were given a higher modulus the stresses become focused along the arch of the horizontal trabeculae.

Discussion: This study shows that inclusion of ligamentous forces results in compressive stresses being generated in the proximal femur and transmitted through the arch-like structure of trabeculae. It is notable that the capsular ligaments are thick and strong and are aligned with the femoral neck. Also, though ignored in this study, some of the major muscle groups have a significant component lying in the same direction. These result in a considerable force pressing the femoral head into the acetabulum and compressive stresses in most of the head and neck. This makes best use of the mechanical properties of bone, which functions better in compression than tension (Cowin, 2001), and avoids tensile forces in the diaphysis.

Synthetic graft expanders have recently been developed for use in impaction grafting revision hip arthroplasty, but their true role has yet to be determined.

We performed a series of experiments to investigate the properties of one such porous hydroxyapatite material (IG-Pore, ApaTech Ltd). IG-Pore was mixed with fresh-frozen human allograft chips and impacted into composite femoral models with a similar biomechanical profile to human bone (Sawbones Europe). Exeter hip prostheses (Stryker Howmedica Ltd) were implanted with cement and each model was axially loaded for 18000 cycles at physiological levels using an Instron servohydraulic materials testing machine. Four test groups with 0%, 50%, 70% and 90% IG-Pore were used, and there were eight femora in each group.

Pre- and post-loading radiostereometric analysis was performed to characterise migration of the prosthesis. Total subsidence was measured and was separated into that occurring at the prosthesis-cement and cement-femur interfaces. Cyclical compression and expansion of the graft-containing models was measured using the Instron.

Median values (interquartile range) for total subsidence were 0.43 mm (0.28 to 0.55) for the pure allograft group, 0.31 mm (0.20 to 0.55) for the 50% IG-Pore group, 0.23 mm (0.07 to 0.34) for the 70% allograft group and 0.13 mm (0.06 to 0.18) for the 90% IG-Pore group. These differences were statistically significant (p=0.034, Kruskal-Wallis). Subsidence at the prosthesis-cement interface was also lower for IG-Pore containing models (p=0.019, Kruskal-Wallis), although there was no significant difference at the cement-femur interface. Specimens with a higher proportion of IG-Pore showed smaller cyclical movements on loading (p=0.005, ANOVA).

Higher proportions of IG-Pore do appear to reduce subsidence in a mechanical model of impaction grafting. A randomised clinical trial using RSA to compare a 50% IG-Pore/allograft mix with pure allograft is in progress to investigate the use of this material as a bone graft expander in the clinical setting.

Impaction grafting procedures have found a widespread role in revision hip arthroplasty. Synthetic graft expanders have recently been introduced, but the optimal ratio of expander to allograft is unknown.

We performed a series of in vitro experiments to investigate the optimal ratio for one commercially available porous hydroxyapatite material (IG-Pore, ApaT-ech Ltd). IG-Pore was mixed with fresh frozen human allograft chips from osteoarthritic femoral heads and with blood. Graft was impacted into fibre-glass femoral models (Sawbones Europe) with a similar biomechanical profile to human bone, and Exeter hip prostheses (Stryker Howmedica Ltd) were cemented in place. Each model was loaded using an Instron servohydraulic materials testing machine for 18000 cycles. The magnitude and frequency of the loading cycle was based on physiologically measured values. Four test groups with 0%, 50%, 70% and 90% IG-Pore were used, with eight femora in each group.

Tantalum marker beads were attached to the prosthesis, the femoral model and the cement mantle, and radio-stereometric analysis (RSA) was performed pre- and post- loading to determine migration and rotation of the prosthesis in each axis. Pre-loading films were repeated in sixteen cases for precision measurements, and twelve specimens had delayed post-loading films performed to measure any re-expansion of the unloaded graft.

The primary end-point was RSA-measured subsidence of the prosthesis, defined as vertical movement of the tip. Median subsidence was 0.43mm, 0.31mm, 0.24mm and 0.13mm in the 0%, 50%, 70% and 90% IG-Pore groups respectively (P=0.034, Kruskal-Wallis test). The precision, given as the median absolute difference, was 0.0065mm.

All specimens showed a cyclical compression and expansion throughout the loading cycle. Specimens with a higher proportion of IG-Pore tended to be more resistant to this and the mean values for cyclical movement were 1.76 0.27mm, 1.65 0.21mm, 1.57 0.18 mm and 1.45 0.14mm for the 0%, 50%, 70% and 90% IG-Pore groups.

Higher proportions of IG-Pore appear to reduce subsidence in impaction grafting. Other issues such as the handling qualities of the graft and the biological effect of synthetic materials also need to be considered, however. A randomised clinical trial using RSA to evaluate a 50% IG-Pore/allograft mix in comparison to pure allograft is ongoing in our institution, and we hope that this will answer some of these questions definitively.