Summary

Our statistical shape analysis showed that size is the primary geometrical variation factor in the medial meniscus. Shape variations are primarily focused in the posterior horn, suggesting that these variations could influence cartilage contact pressures.

The various disorders of the patellofemoral joint, from pain syndrome to maltracking and arthritis, form a significant subset of knee disorders (Callaghan and Selfe 2007). Several studies have shown significant geography and gender based variation in incidence rates of these disorders and of osteoarthritis in general (Woolf 2003). A number of previous studies have examined patellar shape in this context, focusing primarily on the use of 2D measurements of bony geometry to classify patellar shapes and identify high-risk groups (Baumgartl 1964; Ficat 1970).

Recent developments in imaging and statistical analysis have enabled a more sophisticated approach, characterised by statistical shape models which account for three dimensional shape differences (Bryan 2008). Incorporating soft tissue data into these analyses, however, has been a challenge due to factors including the necessity of multi-modality images, absence of repeatable landmarks, and complexity of the surfaces involved. We present here a novel method which has potential to significantly improve analysis of soft tissue geometry in joints. It is built using Arthron, a UCD-developed biomechanics analysis software package.

The shape modelling process consists of three phases: pre-processing, consistent surface parameterization, and statistical shape analysis. The pre-processing phase consists of several mesh processing operations that prepare the input surfaces for shape modelling. Consistent surface parameterizations are implemented using the minimum description length (MDL) correspondence method (Davies 2002) [Fig. 1]. The statistical shape analysis phase involves the reporting and visualization of geometric variation at the input surface. An algorithm was developed to measure the cartilage thickness at each node on the patellar surface mesh. The initial step in this process was to calculate surface normal vectors at each point. These vectors were then projected through the cartilage surface model in order to calculate the thickness [Fig. 2]. The Matlab software was used to aggregate all cartilage thickness values in a given subgroup and after being normalised for the average patellar centroid size for the subgroup, these thicknesses were visualised on the average shape.

Pilot study data consisted of 19 Caucasian (7 female, 12 male) and 13 Japanese (7 female, 6 male) subjects. These data originated from studies performed by DePuy Orthopaedics Inc. Initial results show ethnicity effects in cartilage thickness to be more significant than gender effects [Fig. 3]. After correcting for patellar centroid size, male subjects display 9% greater average thickness than female subjects, while Caucasian subjects display 17% greater average thickness than Asian subjects. Areas of statistically significant differences (t < 0.05) were found to coincide with expected areas of patellofemoral contact through the flexion cycle, showing the potential for the thickness differential to impact upon patellar kinematics. Principal component analysis of the thickness distributions gives more detailed information about modes of variation.

With further development, this method has potential to enable sophisticated analysis of localised variation in soft tissue geometry, thereby improving understanding of the impact of joint geometry on disease formation.

The management of the dysplastic hip represents a clinical and a technical challenge to the paediatric orthopaedic surgeon. There is a great deal of variation in the degree and direction of acetabular dysplasia. Preoperative planning in the dysplastic hip is still largely based on plain radiographs. However, these plain films are a 2D projection of a 3D structure and measurement is prone to inaccuracy as a result. Hip arthrography is used in an attempt to analyse the 3D morphology of the hip. However, this still employs a 2D projection of a 3D structure and in addition has the risk of general anaesthesia and infection. Geometrical analysis based on multiplanar imaging with CT scans has been shown to reduce analysis variability. We present a system for morphological analysis and preoperative of the paediatric hip using this model. Our system can be used to determine the most appropriate osteotomy based on morphology. This system should increase the accuracy of preoperative planning and reduce the need for arthrography.

Purpose

Locking plates are widely used in clinical practice for the surgical treatment of complex proximal humerus fractures, especially in osteoporotic bone. The aim of this study is to assess the biomechanical influence of the infero-medial locking screws on maintaining reduction of the fragments in a proximal humerus fracture.

Hypothesis

Athletes significantly alter their lumbar spinal motion when performing squat lifting at heavy weights. This altered motion effects a change in pressure in the posterior annulus of lumbar discs.

Hypothesis: Athletes significantly alter their lumbar spinal motion when performing squat lifting at heavy weights. This altered motion effects a change in pressure in the posterior annulus of lumbar discs.

Study Design:

3-D motion analysis of lumbar spinal motion in athletes, during squat weight lifting.

Methods: 48 athletes performed 6 lifts at 40% maximum, 4 lifts at 60% max and 2 lifts at 80% max. 3-D motion analysis system, measured lumbar spine motion. Exercise performed as a ‘free’ squat and repeated with a weight lifting support belt.

4 cadaveric sheep spinal motion segments mounted in purpose built jig, replicating angulation seen in the in vivo motion study. These samples were then fixed to a tension/compression loading frame, replicating the forces seen in the in vivo study. Pressure measurement was achieved using a Flexiforce single element force sensor strip, positioned at the posterior annulus.

Posterior annulus pressure was measured during axial compression and on compression with the specimen fixed at 3° of extension.

Results:

Significant decrease (p< 0.05) in flexion in all groups when lifting at 40% max was compared with lifting at 60% and 80% of max. Flexion from calibrated zero point ranged from 24.7° (40% group), to 6.8° (80% group). A progressively significant increase (p< 0.05) seen in extension in groups studied when lifting at 40% max was compared with lifting at 60% and 80% max lift. Extension from a calibrated zero point ranged from − 1.5° (40% group), to − 20.3° (80% group). No statistically significant difference found between motion seen when performing the exercise as a ‘free’ squat or when lifting using a support belt in any group studied.

Comparing axially loaded specimens with specimens loaded in extension, there was an average increase in pressure of 36.4% in the posterior annulus, when the spine was loaded in 3° of extension at a pressure equivalent to the 80% lift in the in vivo motion study, in comparison to axial loading.

Conclusions: Squat weight lifting at heavier weights, causes athletes to lift at a progressively greater degree of extension. The use of a weight lifting support belt does not significantly alter spinal motion during lifting. The increased extension at heavier weights results in a stress concentration in the posterior annulus of lumbar discs.

Introduction: Complex foot and ankle surgery requires accurate preoperative planning. In the foot corrective osteotomies and lengthening procedures are challenging and can be associated with a range of complications. The aim of planning is to correct only the deformity and prevent extensive surgery on adjacent rays while maintaining biomechanical integrity. Knowledge of foot and ankle morphometry is vital to preoperative planning model. A 3D coordinate system is required for accurate morphological analysis. To conduct meaningful comparison between different subjects the coordinate system between subjects must be constant. Coordinate systems have been defined for other joints, however, there has been no previous description of a coordinate system for 3D analysis in the foot and ankle. We present a coordinate system for 3D data analysis in the foot and ankle and apply this to morphological analysis in the forefoot for preoperative planning.

Methods: CT images of ten anatomically normal feet were segmented in Materialise’s Interactive Medical Image Control System 10.1 (MIMICS) a general purpose segmentation program for grey value images. These files were then imported to a shape analysis program for biomechanics, Arthron. A coordinate frame was defined in a 3 × 3 identity matrix using the inter-malleolar axis and a fibular diaphyseal centroidal axis in the construction. Centroidal vectors were defined in the metatarsals enabling 3D analysis of the forefoot. Correlation of metatarsal length, inter-metatarsal angles, inter-malleolar distance and stature was carried out using Pearson product moment correlation coefficient, r.

Results: The morphology of the forefoot was examined in relation to the medial and lateral columns. The length of the metatarsals had a strong mathematical correlation within each column and between the two columns (0.525 – 0.965). There was also a strong correlation in the length between the lesser metatarsals (0.76 – 0.97) The 3rd metatarsal at the column junction correlated well (−0.583) with the inter-metatarsal angles. There was also a strong correlation between the individual’s stature and the metatarsal length and the inter-malleolar distance (0.60 – 0.89). Specifically in relation to the first metatarsal length there was a good correlation with inter-malleolar (0.75) and suture (0.65).

Discussion: We applied our analysis of the forefoot to the well recognised complication of Hallux Valgus surgery, first metatarsal shortening. This may lead to or exacerbate transfer metatarsalgia due to redistribution of force in the forefoot. Osteotomies of the lesser rays aim to relieve symptoms and re-establish the relationship between metatarsal lengths. However, this does not restore the relationship between metatarsal length, stature and inter-malleolar distance which we have shown to be important. Hurst et al proposed distraction osteogenesis of the first metatarsal to re-establishing length. This would return the normal mathematical relationship, which we have described and seems the most probable way to restore normal foot biomechanics and therefore relieve pain.

Conclusion: We have presented a means defining a coordinate system for 3D data analyses in the foot and ankle. We have shown this coordinate system to be effective in the morphometrical analysis of the forefoot. This analysis shows the importance of a proportional metatarsal length within the forefoot but also in relation to stature and the inter-malleolar distance in preservation of a normal biomechanical environment. This coordinate system can now be used for meaningful comparison of anthropological and morphological data between multiple subjects.

Background: Fixation of complex fractures of the proximal humerus is challenging. Fixed angle plates have been shown to give good results in younger patients with good bone quality, however failure of screws to maintain fixation in older patients remains a problem [1]. It has been shown that the bone quality within the humeral head sharply declines with age leaving a large bone void [2]. We propose that filling this bone void with a synthetic bone graft will improve screw purchase, and reduce the likelihood of construct failure.

Aims: We aim to use CT based finite element analysis to examine the effect of augmenting plate fixation with synthetic bone graft in the presence of a poor bone stock.

Methods: A computer tomography (CT) scan of an intact cadaveric fresh frozen humerus from a 78 year old male was obtained. The CT Hounsfield units were calibrated using a water phantom. Both the external contour and internal structure of the hummers were accurately defined using the threshold method. Using proprietary software [Simpleware Simpleware Ltd., Exeter], Boolean subtraction was employed to simulate an anatomically reduced four-part proximal humerus fracture with a representative bone void within the humeral head. A digital representation of a fixed angle proximal humerus plate was created and located so as to fix the fracture. The geometry of the plate fixed four-part fracture was then used to create a hexa-hederal dominant finite element mesh with over six hundred thousand elements created. Linear elastic properties were assigned to each element within the mesh representing bone using established relationships based on local Hounsfield number in the original CT scan [3]. The model was imported into the finite element preprocessor [Abaqus CAE, Simulia Inc, USA]. Contact interactions between the bone fragments, implant and bone graft substitute were defined. A pressure load was applied to the articular fragment to simulate maximum physiological joint reaction forces on the proximal humerus [4,5]. Simulations were run on the facilities at the Irish Center for High End Computing (ICHEC) The effect of adding synthetic bone graft to fixation with a fixed angle device was studied.

Results: In all models the peak pressures were along the lateral cortex and at the implant bone interface. This agrees with common clinical modes of failure being lateral collapse, valgus impaction and cutout of screws into the glenoid fossa. Finite element models where the simulated bone void was filled with bone graft substitute showed 60% reduction in the bearing pressures at the implant/bone interface.

Conclusions: Our results suggest that augmenting plate based fixation of complex proximal humerus fractures with synthetic bone graft results in a more robust construct. It a clinical setting this is likely to result a lower incidence of failure of fixation and subsequent revision surgery.

The cervical spine exhibits the greatest range of motion amongst the spinal segments due to its tri-planar components of movement. As a result, measurement of movements has proved difficult. A variety of methods have been used in an attempt to measure these movements but none have provided satisfactory triplanar data.

This paper uses the Zebris ultrasonic 3-D motion analysis system to measure flexion, extension, range of lateral bending and range of axial rotation in five similar male and five similar female subjects with no history of neck injuries. The subjects were tested unrestrained and in soft and hard collars, as well as in Philadelphia, Miami J and Minerva orthoses.

Results show that the Minerva is the most stable construct for restriction of movement in all planes in both groups (p< 0.001 vs. all groups (p=0.01 vs. Philadelphia in female extension), ANOVA). In the male group, the standard hard collar provides the second best resistance to flexion, lateral bending and axial rotation. The female group showed no one orthosis in second place overall. Looking at these results allows ranking of the measured orthoses in order of their three-dimensional stability. Furthermore, they validate the Zebris as a reliable and safe method of measurement of the complex movements of the cervical spine with low intersubject variability.

In conclusion, this paper, for the first time presents reproducible data incorporating the composite triplanar movements of the cervical spine thus allowing comparative analysis of the three-dimensional construct stability of the studied orthoses. In addition, these results validate the use of the Zebris system for measurement of cervical spine motion.

Complex foot and ankle surgery and reconstruction require accurate preoperative planning. In the foot procedures are challenging and can be associated with a range of complications. The aim of planning is to correct only the deformity and prevent extensive surgery. Knowledge of foot and ankle morphometry is vital. For comparison between different subjects the coordinate system must be constant. To the authors knowledge there has been no previous description of a coordinate system for the foot and ankle.

CT images of ten anatomically normal feet were segmented in a general purpose segmentation program for grey value images and imported to a shape analysis program for biomechanics. A coordinate frame was defined in a 3 × 3 identity matrix using the inter-malleolar axis and a fibular diaphyseal centroidal axis in the construction. Centroidal vectors were defined in the metatarsals. Correlation of metatarsal length, inter-metatarsal angles, inter-malleolar distance and height was carried out.

The forefoot was examined in relation to the medial and lateral columns. Metatarsal length had a significant correlation within each column and between the two columns notably in the 3rd (0.525 – 0.965) metatarsal at the columns junction. The 3rd metatarsals also correlated significantly (−0.583) with the inter-metatarsal angles. There was a weak correlation between the 1st 3rd and the 3rd 5th inter-metatarsal angles directly however, each had a large correlation with the 1st 5th inter-metatarsal angle (0.734 – 0.950). There was also a large correlation between the individual’s stature and the metatarsal length and the inter-malleolar distance.

We have presented a means defining a coordinate system for three dimensional analyses in the foot and ankle. This coordinate system can be used for meaningful comparison of data between multiple subjects. We have shown that this coordinate system to be effective in practice in the morphometrical analysis of the normal forefoot.

Graft choices for revision anterior cruciate ligament (ACL) reconstruction and complex ligament reconstructions of the knee are controversial. The aim of our study was to analyze the biomechanical effect of harvesting bone plugs from both the distal and proximal poles of the patella, to simulate a simultaneous harvesting of a Bone – Patellar Tendon – Bone and Quadriceps Tendon – Bone grafts, in a transverse stress environment.

Sixty Bovine Patellae were analysed. They were divided into 4 groups – based on the residual bone bridge (percentage of total length of patella) remaining after bone plug resection. 0 – 10%, 11 – 20%, 21 – 30% and > 30%. All patellae were tested in a modified 4 – point bending environment, to a maximum load of 10,000N, in a customized designed jig. This method simulates the axial bending stress on the patella during knee flexion. All dimensions of the patellae were recorded including Depth of patella at bone resection and wall thickness adjacent to plug resection site.

All patellae with a 0% bone bridge fractured (Ultimate Tensile Strength/UTS) at a mean Tensile Force of 5863N (Range 3140 – 8730N). There was a subgroup of incomplete fractures – extra-articular fractures – which fractured at 6542N (Range 5085 – 9180N). The remaining specimens failed to fracture. Comparing the UTS and the patellar dimensions, using Weibull’s Statistical Analysis we demonstrated that less than 60% bone plug resection carried a very low probability of fracture.

This study demonstrates the safe criteria for bone – tendon graft harvesting from both the proximal and distal poles of the patella. With regards to a normal human patella, a 40% bone – bridge is approximately a 20mm bone – bridge. We conclude that the simultaneous harvesting of Bone – Patellar Tendon – Bone and Quadriceps Tendon – bone grafts from a patella has no significant increase in the fracture risk of the patella.

Hallux rigidus was first described in 1887. Many aetiological factors have been postulated, but none has been supported by scientific evidence. We have examined the static and dynamic imbalances in the first metatarsophalangeal joint which we postulated could be the cause of this condition. We performed a finite-element analysis study on a male subject and calculated a mathematical model of the joint when subjected to both normal and abnormal physiological loads.

The results gave statistically significant evidence for an increase in tension of the plantar fascia as the cause of abnormal stress on the articular cartilage rather than mismatch of the articular surfaces or subclinical muscle contractures. Our study indicated a clinical potential cause of hallux rigidus and challenged the many aetiological theories. It could influence the choice of surgical procedure for the treatment of early grades of hallux rigidus.

Introduction: The foot is a very complex structure acting as the platform for all gait patterns. At present, little is known about the exact biomechanics of the foot due to the difficulties in modeling all of the components of the foot accurately. This has made it virtually impossible to develop a complete understanding of the aetiology of many diseases of the foot including hallux rigidus. We hypothesize that sagittal plane incongruency of the rotation of the 1^st Metatarsophalangeal Joint (MTPJ), or an increase in the tension of the intrinsic plantar flexors is responsible for the development of hallux rigidus.

Materials & Methods: Ground reaction forces and kinematic data from gait analysis together with anthropometric data from MRI scans of a 24 y.o. female were used to create a Mimics model of the articulation of a normal 1st MTPJ during a gait cycle. The centre of rotation was calculated by triangulating the articular surfaces. Finite element analysis was performed on the model and on similar models with the hypothesized;

joint incongruency,

Results: The results demonstrated a significant increase in the peak stresses, contact areas and stress distributions between the incongruent models compared to the congruent models.

Discussion: To the best of our knowledge this is the most accurate FE model of the 1st MTPJ calculated. Hallux Rigidus is a very common forefoot disorder, with multiple etiologies and treatments advocated. This model demonstrates that an increased tension in the plantar flexors results in a reduced ROM with increased contact stresses on the joint surface.

Conclusion: While it is known Hallux Rigidus has a multi-factorial etiology, the authors feel the above study demonstrates an important inherent etiology.

Many pedicle screw instrumentation systems are currently available to the spine surgeon. Each system has its unique characteristics. It is important for the surgeon to understand the differences in these pedicle screw systems¹

Following the introduction of a new spinal instrumentation set to our clinical practice we encountered two cases of pedicle screw breakage. We thus decided to investigate the mechanism of this screw failure (screw A) in these particular cases and to compare the biomechanical properties, through independent analysis, of a variety of pedicle screws from different manufacturers.

Samples of the broken pedicle screws were retrieved at surgery. Surface analysis of the fracture area using the electron microscope, demonstrated features consistent with fatigue fracture.

Pedicle screws of comparable size from a variety of manufacturers were gathered for independent analysis. Shadowgraph analysis was performed of each screw allowing multiple measurements to be taken of the screw’s geometry. Using this data stress concentration factors were determined demonstrating screw A to have larger values than all the other screws ranging from 2 – 3.6 times the nominal stress. The smaller teeth of screw A, spaced further apart than in the other screws, means that the large proportion of the load which would be carried by the threads is distributed over a smaller area resulting in higher stresses in the threads. The sharp corner at the root of the thread, acting as a stress concentrator, would become the focal point of these high stresses, and magnify them by 2 to 3.6 times.

These increased stresses most likely account for an increased susceptibility to fatigue fracture seen in screw A.

In conclusion it is important to be careful with the introduction and use of new pedicle screw materials and designs, that all the standard biomechanical testing has been performed to a satisfactory standard.

Knowing the physical characteristics of the available pedicle screw instrumentation systems may allow the choice of pedicle screw best suited for a given clinical situation.

Slipped upper femoral epiphysis (SUFE) is a condition, which affects the immature hip joint. Many theories have been postulated as to its underlying aetiology however; its exact cause is, as yet unknown. The final common pathway appears to be failure of the of the growth plate to resist shearing forces, giving rise to displacement of the femoral head. We hypothesized that the lubricating ability of the synovial fluid in hip joints of children with SUFE was defective, thereby allowing increasing shear forces on the physis to occur, so that when the joint is loaded it will fail at its weakest point, namely the capital epiphysis.

Aim: The aim of this study was to establish a biomechanical porcine cadaveric model to study SUFE and use this model to determine how defective joint lubrication may increase the probability of SUFE.

Methods: Using immature porcine femurs and a custom-made rig, a torsional load was applied about an axis perpendicular to the growth plate in order to cause the femur to fail along the growth plate. The Hounsefield testing machine applied a tensile load and recorded the associated elongation. From these figures the torque applied to the femoral head and the associated angular rotation could be computed. Using Weibel analysis we were able to determine the probability of SUFE occurring for a range of synovial fluid coefficients of friction, a range of joint reaction forces and neck-shaft angles.

Results: We found that it is possible to induce a SUFE in an immature porcine loaded hip joint and that there was an increased probability of slip with increased coefficient of friction of synovial fluid, increased percentage body weight and increased neck-shaft angle.

Conclusion: Defective lubrication may be a key underlying aetiological factor responsible for SUFE.

Purpose: The use of a bioabsorbable suture anchor across a joint as a means of internal stabilization has not previously been described. This study assesses the iatrogenic damage caused by such a procedure in the normal immature porcine hip.

Materials and Methods: Six twelve week old pigs underwent unilateral transarticular suture anchorage of the hip using a Panalok® RC Quick Anchor® Plus with Panacryl® suture. (Mitek® Products Johnson and Johnson). Anteroposterior pelvic radiographs were taken pre-operatively and six weeks post-operatively. Acetabular index, diameter of the femoral head ossific nucleus of both hips on both occasions were measured and compared. Pigs were sacrificed six weeks post-operatively. Specimens were analysed macroscopically for femoral head diameter, acetabular dimensions, and presence of gross chondrolysis. Histological analysis was performed to assess the presence of articular chondrolysis, and proximal femoral physeal arrest.

Results: In four out of six hips the rate of change of the acetabular index slowed as compared to the unoperated side though none worsened. The diameter of the femoral ossific nucleus on the operated side continued to increase in size at a similar rate as the unoperated side, despite the surgical procedure according to radiographic comparison. Similar findings were made in the macroscopic analysis of the hip geometry. Gross and histological analysis of the articular cartilage show only local areas of chondrolysis related to the drill holes, and in one hip where a second hole was drilled, cartilage regeneration was noted. Metaphyseal growth at the proximal femoral physis was unaffected by the procedure.

Conclusions: The use of a trans-articular suture anchor across the hip appears to cause marginal retardation of acetabular development in the normal hip. The procedure does not appear to affect proximal femoral physeal or epiphyseal growth and the presence of a bioabsorbable suture within the joint did not result in chondrolysis.

The aim of this study is to evaluate the effectiveness of the application of vibration, during the femoral cementation, as a cementing technique.

It has been demonstrated that when vibration of a constant frequency was utilised, flow of low viscosity cement increased with vibration of increasing amplitude up to a particular acceleration. Above this acceleration there was little additional benefit. It has also been shown that when constant amplitude was used the flow increase was uniform over a wide frequency range, eventually falling off over a particular frequency. These results prove that the flow of orthopaedic bone cement is significantly affected by mechanical vibration of the receiving structure. It is our hypothesis that vibration promotes the ingress of bone cement into cancellous bone.

The effect of mechanical vibration in the frequency range 0–500 Hz on the cadaveric human femur has been assessed in the past. It was found that when the bone was fixed at both ends, its resonant frequency was markedly affected by end loading and damping. If the conditions of the experiment were designed to simulate the condition of the femur when prepared for a total hip replacement, it was found that the bone did not resonate but behaved in a mass-like mode. The significance of this observation is that in the event of vibration being applied to enhance the penetration of orthopaedic bone cement, the movement induced in the bone will be proportional to the force applied regardless of frequency.

Introduction: The magnitude of the initial chondral injury and the residual articular step-off are amongst prognostic factors implicated in outcome following intra-articular fractures. The alignment of an intra-articular fracture line may be an as yet unrecognised prognostic variable.

Hypothesis: That fractures in the coronal plane of the medial femoral condyle result in worse outcomes than those in the sagittal plane.

Aim: To compare the effect of displaced intra-articular osteotomies (ie simulating fractures fixed in an incongruent position) of the medial femoral condyle – in one group performed in the sagittal plane, in the other in the coronal plane.

Materials and Methods: The study was conducted in two arms: in vitro and in vivo.

In vitro study: A pneumo-electric rig was designed and built. Ten freshly harvested porcine knee joints underwent osteotomy (test specimens: 5 sagittal, 5 coronal). 5 control specimens underwent no osteotomy. Specimens were mounted on the rig and subjected to cyclical flexion and extension under load (40,000 cycles over 11 hours). Transarticular pressure measurements were performed before and after testing. Surface roughness was measured following testing using laser interferometry.

In vivo study: Three groups (A to C), each comprising 15 New Zealand white rabbits were utilised. Rabbits from each group were consigned to a control (5), coronal osteotomy (5) or sagittal osteotomy (5) group. Rabbits in group A were sacrificed at 3 weeks (early outcome), group B at 10 weeks (immediate) and group C at 20 weeks (long term). The knee was then harvested en bloc and prepared for light microscopy. A further 10 specimens underwent electron microscopy of the medial meniscus.

Results:

In vitro study: A significant difference in loading patterns was noted between the sagittal, coronal and control groups. Specimens from the sagittal group sustained significantly more wear on the apposing medial tibial articular surface (p=0.04), with the meniscus having a protective effect on the underlying articular surface.

In vivo study: Light microscopy confirmed degenerative changes in the apposing tibial articular cartilage, being more marked in sagittal specimens. On the femoral side of the knee, the healing response of the femoral osteotomy was significantly better in sagittal test specimens than coronal (p< 0.05).

Conclusion: In contrast to the hypothesis, sagittal femoral step-offs gave rise to more tibial wear. This can be explained by the short duration of exposure of the coronal incongruity to the apposing joint during the flexion extension cycle. The sagittal step-off was constantly exposed, giving rise to persistently elevated tibial joint loading pressures opposite the high side of the step-off.

In contrast, the coronal femoral osteotomies had a worse healing response. The alignment of the fracture line perpendicular to the plane of motion of the joint exposes the repair tissue within it to increased shear and tensile stresses. This may play a negative role in the repair of these coronal defects when compared to sagittal osteotomies, which are relatively protected from the high transarticular pressures and showed a greater tendency to remodel their articular surface.

Percutaneous Radiofrequency Ablation (RFA) has become the method of choice in the treatment of a wide spectrum of disorders. It was introduced for the treatment of Trigeminal Neuralgia and has since been used both extensively and successfully in the treatment of this disorder. Over the past two decades it has been advocated in the treatment of hepatic metastases, lung tumours and cerebral tumours. In 1992 Rosenthal et al reported using this procedure for the treatment of Osteoid Osteomas with good outcome. Further case series have supported this modality of treatment. However, the biomechanical effects of RFA on cortical bone have not been reported to date.

The study comprised of 16 large white land-raised male pigs. All were between 70–80kg in weight at the time of treatment. RFA was performed on the femur, tibia and humerus of each animal 24 hours, 1 week and 4 weeks before euthanisation. RFA was carried out via a percutaneous technique under fluoroscopic guidance. The fibula was not treated in each case and used as an intrinsic control to account for inter-group variability. The Modulus of Rigidity, Maximum Torsional Strength of all bones were determined and compared.

There were three pathological fractures, all occurring in the hemerii and all occurring at 4 weeks post treatment. The Modulus of Rigidity and Maximum Torsional strength were significantly reduced at 24 hours and 1 week when compared with the control. However in the 4 weeks group the biomechanical strength of cortical bone was not significantly different and had almost returned to normal which is contradictory to the clinical setting. There was no significant difference at 24 hours and 1 week.

RFA has become well established as the method of choice for the treatment of Osteoid Osteomas, however the biomechanical consequences of this procedure have not been reported to date. The torsional strength of RF ablated cortical bone is severely attenuated after 1 week, 40% reduction in torsional strength when compared with the control group. This study demonstrated that RFA of cortical bone is an effective treatment for cortical lesions however the biomechanical weakness promotes the need for weight-bearing restrictions when managing these patients postoperatively.

Aim: Prosthetic loosening has emerged as a most serious long-term complication after Joint Arthroplasty and the most common cause for revision. Arthroplasty is performed either under a general anaesthesia or a spinal/ epidural or a combination of the two. During general anaesthesia Sevoßurane is used for induction and maintenance. We investigated the effect of Sevoßurane on bone cement in an in vitro setting. Materials & Methods:. 40 beads of roughly the same size were prepared from 2 mixes in a sterile condition in vacuum. 20 of these beads were scanned initially under an electron microscope at 2 levels of magniþcation. The surface images of all the cement beads were analysed. Equal numbers of scanned and unscanned beads were separated into 2 groups of 20 each. They were immersed into 2 jars of normal saline. One was connected to the anaesthetic apparatus and exposed to Sevoßurane at a concentration of 2.5%. The other group (control) was exposed to oxygen. This was performed for 2 hours in an orthopaedic theatre. All the beads were then scanned. Results & Conclusions: The post Sevoßurane exposure images revealed a large number of pits of irregular dimensions on the surface. There were no changes on the surface of control beads. This suggests that in clinical concentrations Sevoßurane can affect the surface of bone cement and its mechanical properties. This can in turn affect the bone cement interface and be a potential cause of prosthetic loosening.