Increasing health care costs, limited resources and increased demand makes cost-effective and cost-efficient delivery of Adolescent Idiopathic Scoliosis (AIS) management paramount. Rising implant costs in deformity surgery have prompted justification of high implant density. The objective of this study was to analyse the costs of thoracoscopic scoliosis surgery, comparing initial learning curve costs with those of the established technique and to the costs involved in posterior instrumented fusion from the literature.

189 consecutive cases from April 2000 to July 2011 were assessed with a minimum of 2 years follow-up using a prospective database covering perioperative factors, clinical and radiological outcomes, complications and patient-reported outcomes. The patients were divided into three groups to allow comparison; 1. A learning curve cohort, 2. An intermediate cohort and 3. A third cohort using our established technique. Hospital finance records and implant manufacturer figures were corrected to 2013 costs. A literature review of AIS management costs and implant density in similar curve types was performed.

The mean pre-op Cobb angle was 53°(95%CI 0.4) and was corrected postop to mean 22.9°(CI 0.4). The overall complication rate was 20.6%, primarily in the first cohort, with a rate of 5.6% in the third cohort. The average total costs were $46,732, operating room costs of $10,301 (22.0%) and ICU costs of $4620 (9.8%). The mean number of screws placed was 7.1 (CI 0.04) with a single rod used for each case giving average implant costs of $14,004 (29.9%). Comparison of the three groups revealed higher implant costs as the technique evolved to that in use today, from $13,049 in Group 1 to $14577 in Group 3 (P<0.001). Conversely operating room costs reduced from $10,621 in Group 1 to $7573 (P<0.001) in Group 3. ICU stay was reduced from an average of 1.2 to 0 days. In-patient stay was significantly (P=0.006) lower in Groups 2 and 3 (5.4 days) than Group 1 (5.9 days).

Our thoracoscopic anterior scoliosis correction has evolved to include an increase in levels fused and reduction in complication rate. Implant costs have risen, however, there has been a concurrent decrease in those costs generated by operating room use, ICU and in-patient stay with increasing experience. Literature review of equivalent curve types treated posteriorly shows similar perioperative factors but higher implant density, 69–83% compared to the 50% in this study. Thoracoscopic Scoliosis surgery presents a low density, reliable, efficient and effective option for selected curves.

Introduction: Contrary to the prevailing conviction that lumbar segments affected by lytic spondylolisthesis are unstable, multiple studies have failed to find evidence of increased or abnormal motion at these segments. Affected segments do not exhibit excessive anterior translation: the so-called slip. Previous studies, however, did not use techniques that might reveal abnormalities in the quality of motion, as opposed to its magnitude.

Methods: To determine if features of instability could be detected in the radiographs of patients with spondylolisthesis, a retrospective, cohort study was conducted of the kinematics of the lumbar spine of patients with spondylolisthesis compared with asymptomatic normal subjects. The flexion-extension radiographs of 15 patients with spondylolytic spondylolisthesis were analysed to determine the location of their instantaneous centres of rotation, and their magnitudes of translation and sagittal rotation. Normative data were obtained by applying the same techniques to the radiographs of 20 asymptomatic subjects.

Results: All but one of the 15 patients exhibited at least one segment with abnormal motion. Only one patient had excessive translation at the lytic segment. Four had minor abnormalities affecting either the lytic segment or ones above. Nine patients exhibited major abnormalities. Seven had paradoxical motion at the lytic segment, in which the centre of rotation was located above L5, instead of below, and in which L5 translated backwards, instead of forwards, during flexion. Two patients exhibited axial dropping of L4, instead of horizontal translation, during extension.

Discussion: Not all patients with spondylolisthesis show features of instability. However, a proportion of patients exhibit highly abnormal movements that are consistent with instability. The abnormalities involve movements within normal range but in abnormal directions. Visual inspection of radiographs will not reveal these abnormalities but they can be detected by plotting the instantaneous axes of rotation.

Introduction: Structural changes to the intervertebral disc (IVD) in the form of anular lesions are a feature of IVD degeneration. Degeneration has been related to changes in the mechanical function of the IVD. This study determined the mechanical effect of individual concentric tears, radial tears and rim lesions of the anulus in an in vitro experiment.

Methods: The lumbar spines from five sheep were taken post mortem and divided into three motion segments. The disc body units were tested on a robotic testing facility, using position control, in flexion/extension, lateral bending and axial rotation. Concentric tears, radial tears and rim lesions were experimentally introduced and the motions repeated after the introduction of each lesion. The mechanical response after the lesion creation was compared to the undamaged response to assess the mechanical effect of each lesion.

Results: It was found that an anterior rim lesion reduced the peak moment resisted by the disc in extension, lateral bending and axial rotation. Concentric tears and radial tears did not affect the peak moment resisted, however, radial tears reduced the hysteresis of response in flexion/extension and lateral bending. The neutral zone was not affected by the presence of IVD lesions.

Discussion: These results show that rim lesions reduce the disc’s ability to resist motion. Radial tears change the hysteresis of response indicating an altered stress distribution in the disc. These changes may lead to overloading of the spinal ligaments, muscles and zygapophysial joints, possibly damaging these structures. This suggests a mechanism for a cycle of degeneration that is instigated by small changes in the mechanical integrity of the IVD.

Introduction: A computer model of the L4/5 human intervertebral disc is currently under development. An integral aspect of this model is the material properties assigned to its components. Detailed data on the material properties of the anulus fibrosus ground matrix are not available in the existing literature. To determine these properties, mechanical tests were carried out on specimens of anulus fibrosus harvested from sheep spines. The tests included unconfined uniaxial compression, simple shear and biaxial compression. Data on the strain required to cause permanent damage in the anulus ground matrix and data on the mechanical response of the anulus to repeated loading were obtained.

Methods: Intervertebral discs were isolated from the lumbar spines of recently sacrificed sheep. These discs were sectioned into test specimens ensuring there were no continuous collagen fibres bearing load. The edge dimensions of the cubic specimens were 3 ± 0.2mm. To ascertain the strain to initiate tissue damage, the specimens underwent successive loadings, which were carried out 1 hour apart to allow recovery. The maximum strain in each test was increased incrementally by 5% until a reduction in stiffness was observed in the following test. Separate tests were carried out to quantify and characterise the response of the anulus ground matrix in the three modes of loading and to strains greater than that which initiates damage.

Results: The strains at which permanent tissue damage occurred were between 20 and 27% in uniaxial compression and between 25 and 35% in simple shear. Testing the specimen beyond these strains showed an obvious reduction in stiffness. The biaxial compression tests showed similar changes but did not result in such pronounced losses in stiffness. The material characteristics were reproducible up to 20% strain. Following deformation to higher strains the altered mechanics were also shown to be reproducible indicating that the matrix had been deranged but not failed.

Discussion: Average physiological strains in the L4/5 intervertebral disc are in the order of 10–50% based on maximum deformations observed in vivo. The current results demonstrate that this strain will cause some permanent damage to the anulus ground matrix, however, the matrix will still be capable of carrying stress upon repeated loading. Thompson et. al.¹ found that people over the age of 35 all exhibited signs of disc degeneration. We hypothesise that the regenerative ability of the anulus ceases to function effectively as we age and the continual damage caused to the anulus tissue by daily activities may lead to the degenerative changes seen in the anulus.

Knowledge of the material characteristics up to 20% strain and following exposure to higher strains will enable a more realistic model of the intervertebral disc and the effects of degeneration to be studied.

Introduction: The complexity of the spine has made a complete understanding of its mechanical function difficult. As a consequence, biomechanical models have been used to describe the behaviour of the spine and its various components. A comprehensive mathematical model of the muscles of the lumbar spine and trunk is presented to enable computation of the forces and moments experienced by the lumbar intervertebral joints during physiological activities.

Methods: The model includes the nine major muscles crossing the region and concentrates on improving the estimated line of action for the muscles. The muscles are considered to consist of numerous fascicles, each with its own force producing potential based on size and line of action. The model respects the physical constraints imposed by the skeletal structure by ensuring that muscles maintain their anatomical position in various spinal postures. Validation was performed by comparing model predictions of maximum moments to published data from maximum isometric exertions in male volunteers. To highlight the potential novel uses of the model, three examples of muscle injury caused by surgical procedures were investigated; posterior lumbar surgery, impairment of abdominal muscles from anterior surgery and removal of the psoas major unilaterally during total hip replacement.

Results: The validation indicated that the model predicted forces similar to those measured in normal volunteers. The biomechanical changes resulting from the muscle injuries during the surgical procedures share several common features: decreased spinal compression and production of asymmetric moments during symmetric tasks.

Discussion: The results suggest that interference with muscles crossing or attaching to the lumbar spine can have a significant impact on its function.

We studied the effect of the surface finish of the stem on the transfer of load in the proximal femur in a sheep model of cemented hip arthroplasty. Strain-gauge analysis and corresponding finite-element (FE) analysis were performed to assess the effect of friction and creep at the cement-stem interface.

No difference was seen between the matt and polished stems. FE analysis showed that the effects of cement creep and friction at the stem-cement interface on femoral strain were small compared with the effect of inserting a cemented stem.

Our aim was to determine whether in vitro studies would detect differences in the cellular response to wear particles of two titanium alloys commonly used in the manufacture of joint replacement prostheses. Particles were of the order of 1 μm in diameter representative of those found adjacent to failed prostheses.

Exposure of human monocytes to titanium 6-aluminium 4- vanadium (TiAlV) at concentrations of 4 x 10⁷ particles/ml produced a mean prostaglandin E₂ release of 2627.6 pM; this was significantly higher than the 317.4 pM induced by titanium 6-aluminium 7-niobium alloy (TiAlNb) particles (p = 0.006). Commercially-pure titanium particles induced a release of 347.8 pM. In addition, TiAlV stimulated significantly more release of the other cell mediators, interleukin-1, tumour necrosis factor and interleukin-6. At lower concentrations of particles there was less mediator release and less obvious differences between materials. None of the materials caused significant toxicity.

The levels of inflammatory mediators released by phagocytic cells in response to wear particles may influence the amount of periprosthetic bone loss. Our findings have shown that in vitro studies can detect differences in cellular response induced by particles of similar titanium alloys in common clinical use, although in vivo studies have shown little difference. While in vitro studies should not be used as the only form of assessment, they must be considered when assessing the relative biocompatibility of different implant materials.

In the assessment of fracture healing by monitoring stiffness with vibrational analysis or instrumented external fixators, it has been assumed that there is a workable correlation between stiffness and strength. We used four-point bending tests to study time-related changes in stiffness and strength in healing tibial fractures in sheep. We aimed to test the validity of the measurement of stiffness to assess fracture strength.

At each duration of healing examined, we found marked variations in stiffness and strength. Stiffness was shown to be load-dependent: measurements at higher loads reflected ultimate strength more accurately. There was a biphasic relationship between stiffness and strength: at first there was a strong correlation regardless of loading conditions, but in the second phase, which included the period of ‘clinical healing’, stiffness and strength were not significantly correlated.

We conclude that the monitoring of stiffness is useful primarily in assessing progress towards union but is inherently limited as an assessment of strength at the time of clinical union. Any interpretation of stiffness must take into account the load conditions.