Repetitive manual handling caused 31% of all work related musculoskeletal disorders in 2015, with the back being the site of injury 38% of the time. Despite its high resilience, studies have shown that intervertebral discs can be damaged during repetitive loading at physiological motions, causing cumulative damage and disc herniation. To understand the mechanism of disc injury resulting from repetitive lifting, it is important to measure disc deformations/strains accompanied by MRI imaging to identify disc tissue damage. Therefore, the aim of this study was to examine associations between the magnitude of 3D internal strains, tissue damage and macroscopic evidence of disc injury after simulated repetitive lifting on normal human lumbar discs.

Sixteen cadaver lumbar functional spinal units (FSUs) were subjected to pre-test MRI. Eight FSUs (control) underwent 20,000 cycles or until failure (5 mm displacement) of loading under compression (1.7 MPa – to simulate lifting a 20 kg weight) + flexion (13°) + right axial rotation (2°) using a novel Hexapod Robot. The remaining eight FSUs (experimental) had a grid of tantalum wires inserted, and stereoradiographs were taken to track internal disc displacements at increasing cyclic intervals. Maximum shear strains (MSS) were calculated from the displacements using radiostereometric analysis at cycle 1 and 20,000 cycles (or failure). Post-test MRI was conducted to determine the extent of tissue damage and associated with regions of highest MSS. A repeated measures ANOVA was performed on MSS with a within–subjects factor of cycle number (cycle 1 and failure cycle) and a between subjects-factor of disc region and failure type (p < 0 .05).

Pfirrmann grading revealed mostly normal discs [I (N=2), II (N=13), and III (N=1)]. No significant difference in MSS between control and experimental groups was found for number of cycles to failure (p=0.279). Pre and post-test MRI analysis revealed that 13 specimens were injured after repetitive lifting with either an endplate failure (N=9) or disc bulge (N=4), and two specimens did not fail. Failure strain was significantly greater than cycle 1 in all regions except posterior, left/right posterolateral (p>0.109). Largest MSS at failure was seen in the anterior (60%), and left/right posterolateral regions (64% and 70%, respectively). MSS at failure for the endplate failure group was significantly larger than the no injury group in all regions except right lateral and nucleus (p>0.707). Disc bulge group MSS was significantly larger than the no injury group in the anterior, right anterolateral, and left/right posterolateral regions (p < 0 .027).

Simulated repetitive lifting led to largest shear strains in the anterior, left and right posterolateral regions that corresponded to annular tears or annular protrusion. The no injury group shear strain was less than 50% in all regions, indicating there may be a threshold that could be associated with tissue damage linked with injuries such as disc bulge and endplate failure. There was no evidence of disc herniation in normal discs, agreeing with current clinical knowledge. These results may be indicative of the effects of repetitive manual handling on normal discs of younger patients.

Summary Statement

Repetitive loading of degenerated human intervertebral discs in combined axial compression, flexion and axial rotation, typical of manual handling lifing activities, causes: an increase in intradiscal maximum shear strains, circumferential annular tears and nuclear seperation from the endplate.

Irradiating allograft bone may compromise the mechanical stability of the prosthesis-bone construct, potentially having adverse effects on the outcome of femoral impaction grafting at revision hip replacement. This in vitro study aimed to determine the effect of irradiation of allograft bone used in femoral impaction grafting on initial prosthesis stability.

Morsellised ovine femoral head bone was irradiated at 0 kGy (control), 15 kGy and 60 kGy. For each group, six ovine femurs were implanted with a cemented polished double taper stem following femoral impaction bone grafting. Dynamic hip joint loading was applied to the femoral head using a servo-hydraulic materials testing machine. The primary outcome was stem micromotion. Tri-axial micromotion of the stem relative to the bone at two sites was measured using linear variable differential transformers and non-contact laser motion transducers. Statistical analysis was performed using SPSS.

Compared to the control and 15 kGy groups, specimens in the 60 kGy group demonstrated statistically significant greater micromotion in the axial, antero-posterior and medio-lateral axes. A multi-factorial post-hoc power analysis based on the overall effect of group size indicated a power of 0.7. There was no difference in micromotion between the control and 15 kGy groups. The average micromotion in the axial axes was 63μm in the control and 59μm in the 15 kGy group.

The results of this study suggest that a maximum irradiation dose of 15 kGy may not affect initial prosthesis stability following femoral impaction grafting in this model and provide the basis for us to now proceed to in-vivo studies examining the effect of irradiated bone on implant stability over time.

Introduction: Disc degeneration causes structural and biochemical tissue changes resulting in altered stresses that may affect vertebral bone remodelling. We hypothesized that disc degeneration alters vertebral cortical strains and disc mechanics of the motion segment, with and without the presence of zygapophyseal joints.

Methods: Twenty human lumbar functional spinal units (FSUs) were strain gauged on the lateral and anterior vertebral cortices, below the inferior endplate. Each FSU was preloaded overnight (0.2 MPa) in a bath and subjected to dynamic compression (1 MPa), flexion/extension/lateral bending (500N + 5 Nm), and axial rotation (5 Nm), before and after removal of the zygapophyseal joints. After testing, discs were macroscopically assessed and graded (1–4) for degeneration. Stiffness, phase angle (energy absorption) and principal strains were calculated. ANOVAs with the dependent variable of principal strain/stiffness/phase angle versus disc grade were performed for each testing direction.

Results: Assessment of disc degenerative condition revealed six grade 2 discs, eight grade 3, and six grade 4. Age and degeneration were highly correlated (r=0.80, P< 0.0001). The effect of disc grade on stiffness was significant overall in most loading directions, before and after removal of zygapophyseal joints (P< 0.008), apart for axial rotation (P> 0.587). Post-hoc multiple comparisons for all loading directions apart for axial rotation revealed that the stiffness of grade 4 discs was significantly larger than grades 2 and 3 discs in most loading directions.

For phase angle (approximate magnitude 5°), no significant overall effects due to degeneration were found across any loading direction (P> 0.2). ANOVA analyses on maximum/minimum principal strains found no significant effect due to disc grade (P> 0.063). However, a small number of significant effects due to disc grade were found at particular strain gauge locations for the isolated disc in flexion, the intact FSU in extension, and the intact FSU/isolated disc in right lateral bending.

Discussion: This study represents the first of its kind to investigate the effects of disc degeneration on vertebral bone cortical strain and disc mechanical properties. Significant increases in stiffness were found with increasing degeneration in all test directions apart for axial rotation. Changes in disc stiffness were consistent with other studies and may be a result of the structural and biochemical changes within the disc that accompany the degenerative process.

The non-significant small phase angles suggest that the disc behaves more like an elastic solid than a poroelastic material, and that dehydration associated with degeneration does not adversely affect damping. Principal strains were not significantly affected by disc degeneration overall, suggesting that the cortical shell adjacent to the disc-endplate boundary maintains a relatively homeostatic condition, with more dramatic architectural changes probably occurring within the trabecular bone. Applications of this research include providing important validation data for analytical/finite element models of the intact FSU and isolated disc segment, and a better understanding of the magnitudes of cortical strains that need to be maintained in order to avoid damaging vertebral bone stress-shielding effects after treatments for disc degeneration.

The anulus fibrosus of the human lumbar intervertebral disc has a complex, hierarchical structure comprised of collagens, proteoglycans and elastic fibres. Recent histological studies have suggested that the elastic fibre network may play an important functional role. In this study, it was hypothesised that elastic fibres enhance the mechanical integrity of the extracellular matrix transverse to the direction of the collagen fibres.

Using a combination of biochemically verified enzymatic treatments and biomechanical tests, it was demonstrated that degradation of elastic fibres resulted in a significant reduction in both the initial modulus and the ultimate modulus, and a significant increase in the extensibility, of radially oriented anulus fibrosus specimens. Separate treatments and mechanical tests were used to account for any changes attributable to non-specific degradation of glycosaminoglycans. Additionally, histological assessments provided a unique perspective on structural changes in the elastic fibre network in radially oriented specimens subjected to tensile deformations.

The results of this study demonstrate that elastic fibres play an important and unique role in the mechanical properties of the anulus fibrosus, and provide the basis for the development of improved material models to describe intervertebral disc mechanical behaviour.

During certain motions, the disc is at risk of annular injury. Axial compression coupled with various combinations of excessive flexion, lateral bending or axial rotation has been shown to lead to disc injury. However, similar injuries have also been caused by repetitive activity at lower, more physiological ranges of motion. The primary objectives of this study were to determine the regions of largest shear strain experienced by disc tissues in six degrees of freedom (DOF), since shear is considered a likely tissue failure criterion, and to identify the physiological motions that may place the disc at greatest risk of injury.

A grid of wires was inserted into the mid-transverse plane of nine human lumbar discs that were subjected to each of six principal displacements and rotations. Stereo-radiographs were taken in each position and digitised for reconstruction of the 3D position of each grid intersection. Maximum shear strains (MSS) were calculated from relative grid-intersection displacements and normalised by the input displacement or rotation. Physiological MSS were calculated using the maximum reported physiological lumbar segmental motion for each DOF.

The largest MSS were found in the posterior, posterolateral and lateral regions of the disc. For the translation motions, lateral shear and compression produced the largest MSS (approx. 9%/mm). For the rotation motions, lateral bending had significantly larger MSS than all other tests (5.8±1.6 %/°, P< 0.001).

The physiological MSS was greatest for lateral bending, being significantly larger than all other motions (57.8±16.2%, P< 0.001). In addition, physiological MSS for flexion was also significantly larger than for all remaining motions (38.3±3.3%, P< 0.001).

This study has identified lateral bending and flexion as the lumbar segmental motions that may place the disc at greatest risk of injury. The exact failure criterion for intervertebral disc tissue is not known, and MSS was used because it is related to maximum and minimum principal strains, and it was shown that disc tears may be initiated by large interlamellar shear strains that dominate over radial and circumferential annular fibre strains. These results provide improved understanding of disc behaviours under loading and may also be of value validating finite element models.

Defects in annulus fibrosus induced by needle puncture can compromise mechanical integrity of the disc and lead to degeneration in animal models. This study examined the immediate and short-term mechanical and biological response to annulus injury through needle puncture using small and large gauge needles in a bovine organ culture system.

Bovine caudal intervertebral discs were harvested, assigned to one of two needle puncture groups (small = 25G, N=11; large = 14G, N=12) or an unpunctured control group (N=10), and cultured in organ culture for 6 days. After measuring initial heights, diameters, and wet weights, discs were placed in an organ culture chamber and incubated with constantly circulating media in standard culture conditions under a 0.2 MPa static load. Discs underwent a daily dynamic compression loading protocol for five days from 0.2 – 1 MPa at 1 Hz for one hour. Disc structure and function were assessed with measurements of dynamic modulus, creep, height loss, water content, proteoglycan loss to the culture medium, cell viability and histology.

Needle insertion caused a rapid decrease in dynamic modulus and increase in creep during one hour of loading, although no changes were detected in water content, disc height, or proteoglycan lost to the media. Cell viability was maintained except for localised cell death at the needle insertion site. An increase in cell number and possible remodelling response was seen in the insertion site in the nucleus pulposus.

Relatively minor disruption in the disc from needle puncture had immediate and progressive mechanical and biological consequences with important implications for the use of needle puncture in discography, and repair/regeneration techniques. Results also suggest diagnostic techniques sensitive to mechanical changes in the disc may be important for early detection of degenerative changes in response to annulus injury.

Biomechanical properties of the disc provide both flexibility and shock absorption. We hypothesised that frequency-dependent effects in shear and torsion deformations in which intrinsic viscoelasticity (solid phase) predominates would differ from compression and bending, in which fluid flow-mediated poroelasticity is also present.

Disc-vertebra-disc preparations (N=8) from human lumbar spines were subjected to each of three displacements and three rotations (6 degrees of freedom - DOF) at each of four frequencies (0.001, 0.01, 0.1, and 1 Hz) after equilibration overnight under a 0.4 MPa preload in a bath of PBS at 37C with protease inhibitors. The forces and torques were recorded along with the applied translation or rotation. The stiffness (force/displacement or torque/rotation) and the phase angle (between each force and displacement) were calculated for each degree of freedom from recorded data.

The stiffness significantly increased linearly with the log-frequency in most DOF (P< 0.001) apart for lateral bending and flexion/extension (P> 0.055). The increases over the four decades of frequency were 28%, 23% and 25% for antero-posterior (AP) shear, lateral shear and torsion respectively, and were 53%, 33% and 36% for compression, lateral bending and flexion.

The phase angle (a measure of energy absorption) significantly decreased overall with increasing frequency in all DOF (P< 0.005) apart for lateral bending. During AP and lateral shear, significant decreases in phase angle of 10% were found between 0.001 Hz compared to 0.01 Hz and 0.1 Hz (P< 0.026) with no differences found at 1 Hz. For torsion, the phase angle at 1 Hz was significantly lower by 40% compared to all slower frequencies (P< 0.001). During compression, a large significant drop in phase angle of 25%–35% occurred between 0.001 Hz and all other frequencies (P< 0.016). No significant post-hoc differences were found for flexion-extension (P> 0.057).

The dynamic effects (stiffness increase, and phase angle decrease with frequency) were consistently greater for deformation modes in which fluid flow effects are thought to be greater. Both the solid phase viscoelasticity and the fluid phase poroelasticity of the tissue appear to contribute to the disc stiffness and energy absorption, although these differences become more apparent at 1 Hz compared to the slower frequencies.

Introduction The influence of annular tears on the biomechanical inter-relationship between the disc and vertebral body has a potentially important role in the mechanism of subsequent biological changes in disc and bone. The disc is a complex structure, exhibiting visco-elastic behaviour that is highly dependent on its condition and fluid content. Studies have shown that the stiffness of the disc is altered by its water content in human, ovine and bovine discs. It has also been shown that disc stiffness or modulus can be preserved if the level of water in the disc is kept constant. The importance of maintaining a reproducible state of stress in the disc during sequential testing of the same specimen is crucial to ensuring consistency of results and minimising systematic experimental errors. The aims of this study were to assess the reliability of sequential testing of the same specimen, and to determine whether stiffness, strains and pressure distribution can be restored to pre-testing levels under a uniform hydration loading environment.

Methods Six ovine FSUs with isolated discs were used in this study. Eight, 1-mm strain gauge rosettes were then bonded to the inferior VB of each FSU at lateral and anterior positions and three heights. FSUs were equilibrated for four hours in a saline bath at room temperature in a materials testing machine. A real-time pressure sensor was placed under the VB. FSUs were tested in axial compression at 0.1 Hz to 1 MPa for 5 sinusoidal cycles. Once tested, the FSU was placed under 0.25 MPa preload for one hour in the water bath for re-equilibration and tested again. Pilot studies by this group have shown that one hour is sufficient to return the disc to its original equilibrium state in a bath after testing, with no associated change in stiffness. This sequence was repeated four times to produce a total of five tests on each FSU. Outcome measures were FSU stiffness, axial strain, peak pressure, average pressure and contact area. Data was statistically analysed using intra-class correlation coefficients (ICC), and repeated measures ANOVA or paired t-tests.

Results The ICC for the five repeated stiffness measures was 0.24 (i.e 24% of the variation in the results was due to between-specimen tests with 76% of the variation due to within-specimen tests). Repeated measures ANOVA found no significant effect on stiffness due to repeating the test five times (P = 0.445). The ICC for the eight axial strains ranged from 0.8 to 0.99. There were no significant differences within any of the eight axial strains over the five repeats (P > 0.287). ICCs, and P values (in brackets) from repeated measures ANOVA, were 0.91 (0.179) for peak pressure, 0.85 (0.44) for average pressure and 0.99 (0.077) for contact area.

Discussion The largest systematic variation was seen for stiffness and this may be due to the tissue changes over the 9 hours of testing. Axial strains showed good to excellent agreement over the five repeated tests as did all pressure parameters. We conclude that the method of allowing one hour for re-equilibration in ovine discs produces a reproducible state of stress in the disc and minimises experimental errors.

Introduction The influence of annular tears on the biomechanical inter-relationship between the disc and vertebral body (VB) has a potentially important role in the mechanism of subsequent biological changes in disc and bone. It is postulated that changes in the disc may result in increased or abnormal spinal segment motion, modified load distribution across the spinal joint and altered cancellous bone architecture. There have been no studies investigating the direct effect of disc injury on functional spinal unit (FSU) stiffness and the distribution of pressure immediately adjacent to the disc inferior endplate. The aim of this study was to determine whether minor and severe radial tear injuries to the disc alters FSU stiffness and VB surface pressure distribution.

Methods Six ovine FSUs were used in this study. The posterior elements were removed leaving the isolated disc in each FSU. The inferior VB was transversely cut immediately inferior to the endplate and the neutral axis of bending (NAB) identified and marked. FSUs were equilibrated in a saline bath at room temperature for four hours under a constant preload of approximately 0.25 MPa prior to testing. After equilibration, FSUs were transferred to a saline bath in a materials testing machine (Instron 8511, Instron, High Wycombe, UK) and a real-time pressure sensor (I-Scan 5076, Tekscan Inc., MA, USA) placed under the inferior VB.

While maintaining the preload, FSUs were loaded in axial compression at 0.1 Hz through the NAB to 1 MPa in a saline bath for 5 sinusoidal cycles. Once tested, a radial tear was introduced via scalpel injury into the left postero-lateral region of the annulus and tested after one hour of re-equilibration. A final, more severe injury, in the form of removal of a 5 mm x 2 mm window of annulus at the same location was performed and tested after re-equilibration.

Outcome measures were FSU stiffness, peak pressure, average pressure, contact area, and centroid of force location. Data was statistically analysed using repeated measures ANOVA or paired t-tests.

Results No significant differences in stiffness was found as a result of disc injury (P = 0.857), nor for peak and average pressure, contact area and centroid location (P > 0.179).

Discussion These results may not be surprising given that the disc has been shown to be remarkably resilient under axial compression, even with a severe annular or nuclear injury. Further insight will be revealed when other modes of loading are performed in both ovine and human discs for the main study planned to be undertaken in the near future.

Introduction Dynamically identifying the distribution of pressure between any two given surfaces such as articulating joints is of fundamental importance in understanding their interaction. The purpose of this laboratory study was to assess the potential of a dynamic pressure measurement system, Tekscan. ( I-Scan 5076, Tekscan Inc., MA, USA) via a study which observed the changes in the load profile through the vertebral body of harvested ovine lumbar functional spinal units (FSU’s) with a created defect in the intervertebral disc.

The system was used to determine pressure distributions in isolated vertebral bodies inferior to the disc, during axial compression of normal and injured discs of an ovine functional spinal unit.

Methods Four ovine lumbar segments L1-L3 were harvested The superior vertebral body (VB) remained complete, whilst the inferior VB was sectioned 2mm from the endplate and the surface smoothed using emery paper in order to achieve maximum contact area. The neutral axis of bending for each specimen was identified and marked. In accordance with the manufacturer guidelines, the sensor was conditioned and calibrated between 20-200N of load. Testing was carried out in a materials testing machine (Instron 8511, Instron, High Wycombe, UK), where 200N of axial load was applied through the FSU and a snapshot of the instantaneous pressure distribution was taken. A 12 x 2 mm gap defect was created in the right ventro-lateral (2 specimens) and the right lateral (2 specimens) aspect of the IVD. The specimens were returned to the Instron and 200N of load was applied axially through the NAB. A recorded image of the pressure footprint was taken.

Results Comparing the recorded colour-coded images together with their centroids of force of the pre- and post-injury pressure distributions of the vertebral bodies, it was clearly evident that there was a major shift of the load through the IVD. As predicted and as seen in the pressure footprint, the pressure shifted in the opposing direction of the injury in order to maintain a balanced system. A pressure reading validation was also carried out with the use of the Instron, where the experimental pressure of the sensor was within 3% of the NATA calibrated load cell.

Discussion The system was used to sample pressure in real time and display it as a 3D colour-coded map, allowing for visualisation of normal pressure distributions. The associated software has numerous aids and functions, allowing real-time visualisation of the dynamic forces and the balance of forces across two interacting surfaces, making the system an invaluable analytical tool.

The Tekscan system will be used to observe the effect of disc injury on the pressure distribution of the adjacent vertebral body. The relationship between the pressure distribution across the vertebral body and bone architecture will also be studied

This study illustrated that this system is a valid tool for qualitatively and quantitatively assessing dynamic pressure distributions.

Introduction Vertebral deformity, disc disorganisation, and change to vertebral bone architecture are morphological features that are associated with degeneration of the spine and with back pain. Observations from our earlier studies found that the BV/TV is always a maximum in the inferior third of the vertebral body (VB), and minimum in the central third. Animal model studies have reported that the strain in loaded vertebra is a minimum in the central third of the vertebra. There have been no studies investigating the direct affect of VB removal on functional spinal unit (FSU) stiffness, strain magnitude and the distribution of pressure immediately adjacent to the sectioned VB. There were a number of aims for this study. The first aim was to determine whether the strain varies between supero-inferior locations on the VB. The second aim was to determine if strain symmetry was present across the normal VB. The third aim was to determine whether transverse sectioning of the VB alters the stiffness, strains and pressure distributions of the functional spinal unit (FSU) and VB.

Methods Six ovine FSUs with isolated discs were used in this study. Eight, 1-mm strain gauge rosettes were then bonded to the inferior VB of each FSU at lateral and anterior positions and three heights. FSUs were equilibrated in a saline bath at room temperature in a materials testing machine. A real-time pressure sensor was placed under the VB. FSUs were tested in axial compression at 0.1 Hz to 1 MPa for 5 sinusoidal cycles. The inferior VB was then sectioned transversely at 1/3 of its height and placed under preload for one hour for re-equilibration and re-tested. This procedure was repeated at 2/3 of VB height and immediately adjacent to the endplate. Outcome measures were FSU stiffness, axial strain, peak pressure and average pressure. Data was statistically analysed using repeated measures ANOVA or paired t-tests.

Results The results of the first aim found no significant difference in strains within the right lateral or left lateral (P > 0.134) columns of strain gauges. However, for the anterior column of strain gauges, the superior strain was 30% higher than the inferior strain (P = 0.047). The results of the second aim found no significant differences between laterally opposing strain gauges (P > 0.139). For the third aim, transverse sectioning of the VB over three levels produced no significant differences for FSU stiffness (P = 0.275), strains for any strain gauge (P > 0.087), or peak and average pressures (P > 0.076).

Discussion This complex pilot study has shown that overall, axial cortical strain in a normal, ovine FSU did not vary with VB supero-inferior location laterally, but did vary anteriorly. Strains were symmetrical between laterally opposing VB locations at each of three levels, and was not affected by transverse sectioning of the VB at three levels. The finding that anterior column strains differ, may relate to changes in load distribution governed by VB surface second moment of area differences (laterally compared to anteroposteriorly), and the absence of a disc inferiorly. Further insight will be revealed when other modes of loading are performed in both ovine and human discs for the main study planned to be undertaken in the near future.

The effect of screw geometry on the pullout strength of Anterior Cruciate Ligament [ACL] reconstruction is well documented. Most research has looked at the effect of screw length and diameter, however other factors such as the degree of taper may also be important. Tapered screws should in theory be associated with increased pullout strength. This has not been demonstrated either clinically or in vitro before. The aim of this study was to compare the pullout strength of ACL reconstruction with a parallel against a tapered screw.

A parallel and tapered screw were manufactured which were identical in all other respects. Sixty superficial digital flexors from the hind legs of sheep were harvested. The tendons were paired and combined to form a quadruple tendon reconstruction of approximately 7mm diameter as measured with graft sizer. An ACL reconstruction was performed on the proximal tibia of 30 bovine knees, which had been harvested in right and left knee pairs, using the quadruple tendon. Fifteen reconstructions were fixed using tapered screws and fifteen with non-tapered screws. The insertion torque of both tapered and non tapered screws were recorded using an instrumented torque screwdriver. The reconstructions were mounted in an Instron materials testing machine with an x-ray bearing system to eliminate horizontal forces, to ensure that the forces were all directed along the line of the tibial tunnel. The maximum pullout strengths were recorded in each case. Five knee pairs were subjected to bone densitometry scanning to ensure that any difference in pull out strength was not due to changes in bone density between right and left knee pairs.

Results indicated that there was no difference between right and left knee pairs [p = 0.58] and that tapered screws were associated with significantly higher pull-out strengths [p=0.007] and insertion torques [p = 0.001].

We sought to identify the tensile properties of the medial patellofemoral ligament (MPFL), and determine whether its repair was sufficient as a means of restoring stability after acute lateral patella dislocation. We also sought to establish whether there was a correlation between the tensile properties of the anterior cruciate ligament (ACL) and the MPFL.

16 hind limbs of Merino Wethers were obtained and stored fresh frozen. The specimens were thawed overnight, dissected out and then placed in a water bath at 37 degrees centigrade for 30 minutes prior to testing. All testing was carried out in the water bath to approximate a more physiological environment. For each specimen the ACL was first tested to failure on an Instron 8511. The MPFL was then tested to failure, then repaired and retested to failure. Finally a reconstruction was carried out, using a flexor tendon, which was again tested to failure.

Results:

There was no correlation between ACL and MPFL strength (p=0.677). Statistical analysis showed that the intact MPFL was significantly stronger than the repaired MPFL (P=0.001) but no different to the reconstructed MPFL (P=0.224), with no difference between repaired and reconstructed (P=0.174). A Power analysis showed that there was not adequate power to detect a significant difference between the last two pairs, and that we would have needed over 35 specimens to show a difference.

This study does not support carrying out a repair of the MPFL following an acute lateral patella dislocation, as it does not restore its tensile properties. It further suggests that a reconstruction may better restore the tensile properties of this ligament.

Introduction: Accelerated rehabilitation programs following ACL reconstruction require adequate fixation strength. Staple fixation of grafts outside the tibial tunnel has been shown to have fixation strength comparable to interference screws. The use of bioresorbable polymer implants has potentially significant advantages especially if revision is required. The purpose of this study was to evaluate a new bioresorbable fixation staple using an ovine model.

Materials and Methods: Forty-eight mature sheep underwent unilateral cranial cruciate ligament (CrCl) reconstruction. The reconstruction comprised a loop of superficial digital flexor tendon (autograft) joined to a prosthetic ligament (LK-15). Femoral fixation was by endobutton. Tibial fixation of the LK-15 was with either a new Poly-L-lactic acid (PLLA) staple (Zimmer Japan/Gunze Ltd.) or a Cobalt-chrome (CoCr) staple. Biomechanical and histological responses were evaluated at 0, 6, 12 and 24 weeks.

Results: At all times post-reconstruction there were no significant differences between staple types for construct strength or stiffness (p> 0.05). The staple was not the site of reconstruction failure, and there were no adverse tissue reactions, for either staple type. Fibrous tissue was more often found at the interface of the CoCr staple.

Conclusions: The PLLA staple performed biomechanically as well as the metal staple for tibial fixation of cruciate ligament reconstructions. There were no significant observable adverse histological responses over the time intervals examined.

Purpose: A variety of second generation femoral interlocking intramedullary nails, in which the proximal lag screw is engaged in the femoral head, are now available for the treatment of complex comminuted pertrochanteric femoral fractures. Jamming of the lag screw results in a rigid device which is more likely to cut-out of the femoral head. The aim of this study was to determine the sliding characteristics and jamming potential of the lag screws of five different devices used to treat these fractures.

Method: The devices examined include; the single lag screw devices: the DHS, the Gamma nail and the Intramedullary hip screw (IMHS), and the double lag screw devices: the Russell-Taylor Reconstruction nail (RTN) and the Austofix Hip nail. The devices were mounted in a servo-hydraulic testing apparatus and examined by two different techniques. The first set-up looked at lag screw motion with respect to loads applied which were representative of the single limb stance phase of gait (SLSPOG). The second set-up which, was first described by Kyle in 1980, looked at the forces required to initiate sliding.

Results: For the first set up (SLSPOG), all single lag screw devices demonstrated sliding across the normal physiological range of applied load. The Russell Taylor Reconstruction nails demonstrated conflicting results with the lag screws of two nails sliding and one nail jamming. All the Austofix nails jammed at the higher angles of the normal physiological range (1590, 1640).

Using the Kyle set-up, the forces required to initiate sliding were found to be lowest with the Synthes DHS (42.33±5.77N), Zimmer CHS (52.67±26.56N), and the IMHS (45.33±10.97N). These were closely followed by the Gamma nail (79.33±8.39N) and the Richards Classic hip screw (82.00±16.37N). The highest forces were for the RTN (98.00±18.52N) and the Austofix hip nail (283.00±70.62N). These results were significantly different. (p< 0.001, ANOVA)

Conclusion: The results demonstrate that double lag screw implants require greater loads to initiate sliding and have a greater potential for jamming. Whilst all single lag screw nails slide, barrel length does alter the forces required to initiate sliding. Further testing using a lubricant is currently being undertaken.

Purpose: The aim of this study was to examine the intervertebral disc (IVD) biomechanics in a sheep model with concentric tears.

Methods: Fifty two adult merino wethers were randomly allocated into two groups with circumferential tears introduced by injection with saline (group 1) or needle stick with no saline (group 2). They were then sacrificed at 0, 1, 3, 6, 12 and 18 months for biomechanical testing. An additional ten sheep were used as an unoperated control at time 0 (Group 0). Biomechanical tests on each functional spinal unit (FSU) and IVD were performed.

Results: The effect of procedure overall was significant for torsion (P< 0.022), axial compression (P< 0.014), extension (P< 0.001) and left lateral bending (P< 0.004) for both the FSU and IVD. In almost every case, both groups 1 and 2 were significantly stiffer than group 0 but no different to each other. The effect of time overall was significant for flexion (P< 0.0028) and right lateral bending (P< 0.022) for both the FSU and IVD. In torsion, twisting to the left was significant for the intact FSU (P=0.008) and twisting to the right for the isolated IVD (P=0.009).

Discussion: The results of this study show that any intervention in the disc alters the biomechanics compared to an unoperated control group. To our knowledge this has not been shown before and these findings may have relevance to any intervention into the disc in the patient.

Introduction: The purpose of this study was to evaluate the micromotion of a femoral prosthesis relative to the femur in a revision hip replacement model.

Methods: A series of Ovine hip hemiarthroplasties were mechanically tested to detect micromotion of the femoral prosthesis relative to the femur 12 weeks following implantation. A mechanical testing device utilising muscle simulation of the major groups around the femur was designed. A 3D targeting system was developed using non-contact LASER transducers on the implant referenced to a second target on the overlying femur. Movement of this second target was measured with three LVDT’s (linear variable differential transformers).

Results: The system error was quantified in each femur to a resolution of the order of 15 microns. The mean micromotion, in 3D at two points assuming rigid body mechanics, was less than 50 microns for clinically stable implants. One stem was determined to be clinically loose and had a corresponding mean micromotion of 150 microns.

Conclusion: The method enabled measurement of 3Dmicromotion of a femoral prosthesis within the femur, during a laboratory approximation of normal physiological load cycles. The micromotion values corresponded to clinical outcomes, in a manner consistent with other reports in the literature. This system can be modified to allow targeting of different implants within a variety of bone types.

Aim: To determine the intra operative biomechanical properties of a semitendinosus graft used in ACL reconstruction.

Introduction: ACL reconstruction has become a commonly performed operation with 1,139 of these procedures being performed in South Australia in 1997 (SA Health Commission)

The majority of the scientific literature is based on data obtained from elderly cadaveric material. Little is known about the biomechanical properties of the soft tissue grafts currently used prior to implantation. The correct preconditioning and intraoperative tensioning of the soft tissue grafts has also not been investigated.

The initial graft biomechanical properties are important. Inadequate tension will lead to continuing instability whilst excessive tension may cause accelerated joint arthrosis. The tension in the graft may decrease by 30% if it has not been cyclically pretensioned.

Methods: A machine has been designed that will allow the intraoperative biomechanical testing of soft tissue grafts immediately prior to their implantation into the patient during ACL reconstruction. Data will be available on creep, stress relaxation, and tensile testing.

This device will also allow the accurate preconditioning of the graft, providing objective data that can then be compared to the subsequent clinical progress of the patient.

All testing will be accomplished during the time it takes to prepare the tunnels for insertion of the graft, and as such will not prolong unnecessarily the operative time.

Procedure: Once the graft has been prepared prior to fixation, it will be placed between two clamps. One is fixed to a load cell whilst the other is coupled to a linear actuator. The linear actuator will be driven by a computer controlled stepper motor under close-loop control. Custom software will cyclically load the autograft between two definable load points. A linear variable differential transformer (LVDT) will be used to monitor displacement of the autograft and load will be monitored with a load cell of capacity 125Kg.

This set-up will be immersed in a saline water bath maintained at body temperature during testing. The load cell will be hermetically sealed, with clamps and water bath being autoclavable. With the facilities for draping, the test area will remain sterile. The auto graft clamps will be designed to allow fixation of various graft materials (eg semitendinosus, gracilis, bone-patella tendon-bone) and adjustable for graft lengths. The water bath will house a thermocouple, heating mat and controller to maintain the saline temperature to within 1°C.

The testing system will be mounted on a stainless steel trolley for mobility in the operating room with an underlying shelf to house the associated electronics and a retractable side draw for storage of the laptop computer.

The autograft will be preconditioned between two known loads for 20 cycles recording load and displacement simultaneously on a laptop computer. Once preconditioned, the autograft will then be used for the ACL reconstruction in the standard way.

Summary: Objective data on preconditioning of ACL grafts, has never before been available intra-operatively. We outline the experimental set-up which has been designed and is undergoing testing prior to its use in a prospective study.