Abstract

Purpose of study and background

Spinal muscle area (SMA) is often employed to assess muscle functionality and is important for understanding the risk individuals may have of developing back pain or the risk of postural instability and falls.. However, handgrip strength (HGS) has also been utilized as a measure of general muscle capacity. This study aimed to examine the relationship between SMA and HGS to assess whether the latter could be used as an accurate indicator of the former.

Purpose of study

This study aims to establish the micro-structure of the vertebral endplate and its interface with the adjacent bone and disc in fresh, unstained tissue so that the structure can be related to normal and pathological function.

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.

The objective of this study was to assess the reliability and appropriateness of statistical shape modelling for capturing variation in thoracic vertebral anatomy for future use in assessing scoliotic vertebral morphology.

Magnetic resonance (MR) images of the thoracic vertebrae were acquired from 20 healthy adults (12 female, 8 male) using a 1.5 T MR scanner (Intera, Philips). A T1 weighted spin-echo sequence (repetition time = 294 ms, echo time = 8 ms, number of signal averages = 3) was used. A set of slices (number = 27, thickness = 1.9 mm, gap = 1.63 mm, pixel size = 0.5 mm) were acquired for each vertebrae, parallel to the mid-transverse plane of the vertebral body. Repeated imaging, including participant repositioning, was performed for T4, T8 and T12 to assess reliability. Landmark points were placed on the images to define anatomical features consisting of the vertebral body and foramen, pedicles, transverse and spinous processes, inferior and superior facets. A statistical shape model was created using software tools developed in MATLAB (R2013a, The MathWorks Inc.). The model was used to determine the mean vertebral shape and ‘modes of variation’ describing patterns in vertebral shape. Analysis of variance was used to test for differences between vertebral levels and subjects and reliability was assessed by determining the within-subject standard deviation from the repeated measurements.

The first three modes of variation, shown below (green = mean, red and blue = ±2 standard deviations about the mean), accounted for 70% of the variation in thoracic vertebral shape (Mode 1 = 44%, Mode 2 = 19%, Mode 3 = 4%). Visual inspection indicated that these modes described variation in anatomical features such as the aspect ratio of the vertebral bodies, width and orientation of the pedicles, and position and orientation of the processes and facet points. Variation in shape along the thoracic spine, characterised by these modes of variation, was consistent with that reported in the literature. Significant differences (p< 0.05) between vertebral levels and between some subjects were found. The reliability of the method was good with low relative error (Mode 1 = 5%, Mode 2 = 8%, Mode 3 = 19%).

Statistical shape modelling provides a reliable method for characterizing many anatomical features of the thoracic vertebrae in a compact number of variables. This is useful for robustly assessing morphological differences between scoliotic and non-scoliotic vertebrae and in assessing entry points and trajectories for pedicle screws.

The shape of the vertebral bodies from L1 to L4 was assessed from lateral dual-energy x-ray absorptiometry (DXA) images using an active shape model. The output from the model was compared to measurements of areal bone mineral density in L1 to L4 (aBMD) using a stepwise linear regression model. A significant relationship was found between aBMD and vertebral shape that suggests that the method may be useful for correcting artefacts such as osteophytes.

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.

Back pain may be related to abnormal segmental movement and suggested treatment is segmental fusion. Recent techniques using cages can achieve fusion rates of over 90% but the clinical results are no better. We hypothesise that the cages integrate fully to adjacent vertebrae taking all the load, producing abnormal stress patterns in the vertebrae producing pain.

In this study a simple FE model of a disc and its adjacent vertebral bodies was developed using ANSYS software. The dimensions of the model were based on those of a human lumbar disc. The normal disc was modelled as having nucleus with fluid properties (bulk modulus 1720 MPa). To model the degenerate disc, the material properties of the nucleus were changed to be the same as the annulus (Young’s modulus, E=5 Mpa; Poisson’s ratio, n=0.49). To model fusion of the disc, the nucleus was replaced with a simple representation of a one of three of the commonly used cages. In all the models the material properties of the cancellous bone (E = 100 MPa; n = 0.3) and the cortical bone (E=12000MPa;n=0.3) remained the same. The model was loaded axially with 1.5 kN.

The vertical and horizontal stress patterns around a loaded degenerate disc showed areas of increased loading in the endplate and cancellous bone. The inclusion of cages in the model showed high concentration of tensile and compressive stresses at the point of contact with the cages and in the cancellous bone of the vertebral bodies. The stress pattern showed more similarity to that of degenerate disc, than a normal one.

Fusion cages alter the pattern of stress distribution in the adjacent vertebral bodies from that of the normal disc. The excellent fusion rates of the cages are not mirrored by improvement in clinical results. It supports the concept that abnormal load transfer may be a more significant cause of back pain than abnormal movement.

Introduction: Despite a very high fusion rate (90%) achievable by present techniques, the clinical success rate for curing back pain is in the range of 50%. We hypothesise that disc degeneration gives rise to abnormal stress patterns in the bone. Although the cages integrate fully, load is taken by the cage producing abnormal stress patterns in the vertebrae. Unless a near normal stress pattern in the vertebrae is established, pain may continue.

Method: A simple finite element model of a disc and its adjacent vertebral bodies was developed using ANSYSS software. The dimensions of the model were based on the human lumbar disc. The normal disc was modelled as a fluid with a bulk modulus of 1720 MPa. The degenerate disc was modelled as having the same material properties for the nucleus and the annulus. Fusion of the disc was modelled by replacing the nucleus with commonly used cages. In all the models, the material properties of the cancellous bone (E=100 MPa; v=0.3) and the cortical bone (E=12000 MPa; v=0.3) remained the same. The model was loaded axially with 1.5 kN.

Results: The vertical and horizontal stress patterns around a loaded degenerate disc showed areas of increased loading in the endplate and the cancellous bone confirming the authors’ previous work using load transducers. The introduction of the cages in the model changed the stress distribution – they caused an increase in the compressive stresses in the cancellous bone, and a high concentration of tensile and compressive stresses at the point of contact with the cages.

Conclusion: This study has shown that fusion cages alter the pattern of stress distribution in the adjacent vertebral bodies similar to that of a degenerate disc. It supports the concept that abnormal weight transfer is a more significant cause of back pain as compared to abnormal mobility.