Introduction

An understanding of anatomic variability can help guide the surgeon on intervention strategies. Well-functioning thumb metacarpophalangeal joints (MCPJ) are essential for carrying out typical daily activities. However, current options for arthroplasty are limited. This is further hindered by the lack of a precise understanding of the geometric variation present in the population. In this paper, we offer new insight into the major modes of geometric variation in the thumb MCP using Statistical Shape Modelling.

INTRODUCTION

Hip resurfacing offers a more bone conserving solution than total hip replacement (THR) but currently has limited clinical indications related to some poor design concepts and metal ion related issues. Other materials are currently being investigated based on their successful clinical history in THR such as Zirconia Toughened Alumina (ZTA, Biolox Delta, CeramTec, Germany) which has shown low wear rates and good biocompatibility but has previously only been used as a bearing surface in THR. A newly developed direct cementless fixation all-ceramic (ZTA) resurfacing cup offers a new solution for resurfacing however ZTA has a Young's modulus approximately 1.6 times greater than CoCr - such may affect the acetabular bone remodelling. This modelling study investigates whether increased stress shielding may occur when compared to a CoCr resurfacing implant with successful known clinical survivorship.

This paper reports on a proof of concept project funded by the UK National Council for the Replacement, Refinement and Reduction of Animals in Research (NC3Rs), with the aim of developing an in vitro model to recapitulate the human osteoarthritic joint, based on a multiple human cell type co-culture system, for research and drug development in OA. The targets were: (i) the development of a cell culture platform that could produce a mixed stable cell culture of cell types that represent the key components of the human joint: synoviocytes – type I and type II; osteoblasts; osteoclasts; chondrocytes/cartilage or cartilage-like matrix; adipocytes; and immune cells. (ii) demonstration of cell phenotype stability and viability for at least 72 hours. In order to establish the cell culture platform we have developed an eight-channel cell printer, capable of accurately and reliably printing the required cell types to create osteochondral and synovial cell types within a transwell system. Two different sets of cells have been developed and processed using the cell printer: a set based on using an immortalised hTERT MSC line to create osteoblasts, chondrocytes and adipocytes, with commercial cells lines providing the other cell types, and a set obtained from tissue excised during orthopaedic surgery. This gives both a repeatable set of cells with which to undertake mode of action studies, and a bank of cell sets which will be representative of different stages of osteoarthritis. The co-cultures have been immunohistochemically assessed in order to demonstrate maintenance of phenotype.

Mesenchymal stromal cells (MSCs) are widely used in clinical trials for the treatment of many bone defects. Apatite-wollastonite glass ceramic (A-W) is an osteoconductive biomaterial shown to be compatible with MSCs. This is the first study comparing the osteogenic potential of two MSC populations, heterogeneous plastic adherence MSCs (PA-MSCs) and CD271-enriched MSCs (CD271-MSCs), when cultured on A-W 3D scaffold. The paired MSC populations were assessed for their attachment, growth kinetics and ALP activity using confocal or scanning electron microscopy and the quantifications of DNA contents and p-nitrophenyl (pNP) production. While the PA-MSCs and CD271-MSCs had similar expansion and tri-lineage differentiation capacity during standard 2D culture, they showed different proliferation kinetics when seeded on the A-W scaffolds. PA-MSCs displayed a well-spread attachment with more elongated morphology compared to CD271-MSCs, signifying a different level of interaction between the cell populations and the scaffold surface. PA-MSCs also fully integrated into the scaffold surface and showed a stronger propensity for osteogenic differentiation on the A-W scaffold as indicated by higher ALP activity than CD271-MSCs. Furthermore, A-W scaffold seeded uncultured bone marrow mononuclear cells (BM-MNCs) demonstrated a higher proliferation rate and greater ALP activity compared to freshly isolated CD271-enriched BM-MNCs. Our findings suggest that enrichment of CD271-positive population is not beneficial for osteogenesis when the cells are seeded on A-W scaffold. Furthermore, unselected heterogeneous MSCs or BM-MNCs are more promising for A-W scaffold-based bone regeneration, providing novel insight with potential clinical implications in regenerative medicine for bone defects using an innovative tissue engineering approach.

Polymer foams have been used extensively in the testing and development of orthopaedic devices and for verification of computational models. Their use is often preferred over cadaver and animal models due to being relatively inexpensive and their consistent material properties. Successful validation of such models requires accurate material/mechanical data. The assumed range of compressive moduli, provided in the sawbones technical sheet, is 16 MPa to 1.15 GPa depending on the density of foam. In this investigation, we apply two non-contact measurement techniques (digital volume correlation (DVC) and optical surface extensometry) to assess the validity of these reported values. It is thought that such non-contact methods remove mechanical extensometer errors (slippage, misalignment) and restrict the effect of test-machine end-artifacts (friction, non-uniform loading, platen flexibility). This is because measurement is taken directly from the sample, and hence material property assessment should be more accurate. Use of DVC is advantageous as full field strain measurement is possible, however test time and cost is significantly higher than extensometry. Hence, the study also sought to assess the viability of optical extensometry for characterising porous materials.

Testing was conducted on five 20 mm cubic samples of 0.32g/cc (20 pcf) solid rigid polyurethane foam (SAWBONES^TM). The strain behaviour was characterised by incremental loading via an in situ loading rig. Loading was performed in 0.1 mm increments for 8 load steps with scans between loading steps. Full field strain measurement was performed on one sample by micro focus tomography (muvis centre, Southampton) and subsequent DVC (DaVis, Lavision). Calculation of Young's modulus and Poisson's ratio was then preformed through use of the virtual fields method. These results were subsequently corroborated by use of optical extensometry (MatchID). To account for heterogeneities, axial strain measurements were averaged from six points on the front and rear surfaces. A computationally derived correction factor was then applied to account for through volume strain variations. In each test compressive displacement was applied to 900N (∼2MPa) to remain within the linear elastic region.

Significant variability of individual strain measurements were observed from extensometry measurements on the same sample, indicating non-uniform loading did occur in all samples. However by averaging across multiple points linear loading profiles were identified. For all non-contact methods the calculated elastic moduli were found to range between 331–428 MPa whilst the approximated modulus based on cross head displacement was ∼210 MPa. The optical-extensometry gave a considerably higher modulus (p = 0.047) than the DVC results as only surface measurements were made. However, following computational based correction values converged within 6% of one another. Both the DVC and point-tracking results (p = 0.001) indicated substantially higher compressive modulus (137%) than the manufacturer provided properties.

This study demonstrates that methods of measuring displacement data on of cellular foams must be carefully considered, as artefacts can lead to significant errors of up to 137%, and such errors may falsely influence the design and validation of tested devices.

Polymer foams have been extensively used in the testing and development of orthopaedic devices and computational models. Often these foams are used in preference to cadaver and animal models due to being relatively inexpensive and their consistent material properties. Successful validation of such models requires accurate material/mechanical data. The assumed range of compressive moduli, provided in the sawbones technical sheet, is 16 MPa to 1.15 GPa depending on the density of foam. In this investigation, we apply two non-contact measurement techniques (digital volume correlation (DVC) and optical surface extensometry/point-tracking) to assess the validity of these reported values. It is thought that such non-contact methods remove mechanical extensometer errors (slippage, misalignment) and are less sensitive to test-machine end-artifacts (friction, non-uniform loading, platen flexibility). This is because measurement is taken directly from the sample, and hence material property assessment should be more accurate. Use of DVC is advantageous as full field strain measurement is possible, however test time and cost is significantly higher than extensometry. Hence, the study also sought to assess the viability of optical extensometry for characterising porous materials.

Testing was conducted on five 20 mm cubic samples of 0.32g/cc (20 pcf) solid rigid polyurethane foam (SAWBONESTM). The strain behaviour was characterised by incremental loading via an in situ loading rig. Loading was performed in 0.1 mm increments for 8 load steps with scans between loading steps. Full field strain measurement was performed on one sample by micro focus tomography (muvis centre, Southampton) and subsequent DVC (DaVis, Lavision). Average strains in each direction were then calculated to enable modulus and Poisson's ratio calculation. These results were subsequently corroborated by use of optical point-tracking (MatchID). To account for heterogeneities, axial strain measurements were averaged from six points on the front and rear surfaces (fig.2). In each test compressive displacement was applied to 900N (∼2MPa) to remain within the linear elastic region.

Significant variability of individual strain measurements were observed from point couples on the same sample, indicating non-uniform loading did occur in all samples. However, by averaging across multiple points, linear loading profiles were ascertained (fig.2). For all non-contact methods the calculated elastic moduli were found to range between 331–428 MPa whilst the approximated modulus based on cross head displacement was ∼210 MPa, similar to the manufacturer's quoted value (220MPa). The point-tracking gave a significantly higher modulus (p = 0.047) than the DVC results as only surface measurements were made. It is thought that a correction factor may be ascertained from the finite element method to correct this. Both the DVC and point-tracking results (p = 0.001) indicated a substantially higher compressive modulus than the manufacturer provided properties.

This study demonstrates that methods of measuring displacement data on cellular foams must be carefully considered, as artefacts can lead to errors of up to 70% compared to optical and x-ray based techniques.

Introduction

Periprosthetic bone remodelling after Total Knee Arthroplasty (TKA) may be attributed to local changes in the mechanical strain field of the bone as a result of the stiffness mismatch between high modulus metallic implant materials and the supporting bone. This can lead to significant loss of periprosthetic bone density, which may promote implant loosening, and complicate revision surgery. A novel polyetheretherketone (PEEK) implant with a modulus similar to bone has the potential to reduce stress shielding whilst eliminating metal ion release. Numerical modelling can estimate the remodelling stimulus but rigorous validation is required for use as a predictive tool. In this study, a finite element (FE) model investigating the local biomechanical changes with different TKA materials was verified experimentally using Digital Image Correlation (DIC). DIC is increasingly used in biomechanics for strain measurement on complex, heterogeneous anisotropic material structures.

Introduction

Implant-cement debonding at the knee has been reported previously [1]. The strength of the mechanical interlock of bone cement on to an implant surface can be associated with both bone cement and implant related factors. In addition to implant surface profile, sub-optimal mixing temperatures and waiting times prior to cement application may weaken the strength of the interlock.

Introduction

Fatigue and wear at the head/stem modular junction of large diameter total hip replacements can be exacerbated as a result of the increase in frictional torque. In vivo, a “toggling,” anterior-posterior (A-P) movement of the head taper on the trunnion may facilitate corrosion in the presence of physiological fluids, leading to increased metal ion release. Clinically, metal ion release has been linked to the formation of pseudo tumours and tissue necrosis [1].

Introduction:

Novel biomaterials may offer alternatives to metal arthroplasty bearings. To employ these materials in thin, bone conserving implants would require direct fixation to bone, using Titanium/HA coatings. Standard tests are used to evaluate the adhesion strength of coatings to metal substrates [1], versus FDA pass criteria [2]. In tensile adhesion testing, a disc is coated and uniform, uniaxial tension is exerted upon the coating-substrate interface; the strength is calculated from the failure load and surface area. Rapid failure occurs when the peak interface stress exceeds the adhesion strength, as local failure will propagate into an increasing tensile stress field.

Ceramics and reinforced polymers (e.g. carbon-fibre-reinforced PEEK), have considerably different stiffness (E) and Poisson's Ratio (ν) from the coating and implant metals. We hypothesised that this substrate-coating stiffness mismatch would produce stress concentrations at the interface edge, well in excess of the uniform stress experienced with coatings on similar stiffness metals.

Representative pre-clinical analysis is essential to ensure that novel prosthesis concepts offer an improvement over the state-of-the-art. Proposed designs must, fundamentally, be assessed against cyclic loads representing common daily activities [Bergmann 2001] to ensure that they will withstand conceivable in-vivo loading conditions. Fatigue assessment involves:

–

cyclic mechanical testing, representing worst-case peak loads encountered in-vivo, typically for 10 million cycles, or

Cyclic stresses from gait loading are super-imposed upon residual assembly stresses. In thick walled devices, the residual component is small in comparison to the cyclic component, but in thin section, bone preserving devices, residual assembly stresses may be a multiple of the cyclic stresses, so a different approach to fatigue assessment is required.

Modular devices provide intraoperative flexibility with minimal inventories. Components are assembled in surgery with taper interfaces, but resulting residual stresses are variable due to differing assembly forces and potential misalignment or interface contamination. Incorrect assembly can lead to incomplete seating and dissociation [Langdown 2007], or fracture due to excessive press-fit stress or point loading [Hamilton 2010]. Pre-assembly in clean conditions, with reproducible force and alignment, gives close control of assembly stresses. Clinical results indicate that this is only a concern with thick sectioned devices in a small percentage of cases [Hamilton 2010], but it may be critical for thin walled devices.

A pre-clinical analysis method is proposed for this new scenario, with a case study example: a thin modular cup featuring a ceramic bearing insert and a Ti-6Al-4V shell (Fig. 1). The design was assessed using FE predictions, and manufacturing variability from tolerances, surface finish effects and residual stresses was assessed, in addition to loading variability, to ensure physical testing is performed at worst case:

–

assembly loads were applied, predicting assembly residual stress, verified by strain gauging, and a range of service loads were superimposed.

The predicted worst-case stress conditions were analysed against three ‘constant life’ limits [Gerber, 1874, Goodman 1899, Soderberg 1930], a common aerospace approach, giving predicted safety factors. Finally, equivalent fatigue tests were conducted on ten prototype implants.

Taking a worst-case size (thinnest-walled 48 mm inner/58 mm outer), under assembly loading the peak tensile stress in the titanium shell was 274 MPa (Fig. 2). With 5kN superimposed jogging loading, at an extreme 75° inclination, 29 MPa additional tensile stress was predicted. This gave mean fatigue stress of 288.5 MPa and stress amplitude of 14.5 MPa (R=0.9). Against the most conservative infinite life limit (Soderberg), the predicted safety factor was 2.40 for machined material, and 2.03 for forged material, or if a stress-concentrating surface scratch occurs during manufacturing or implantation (Fig. 3). All cups survived 10,000,000 fatigue cycles.

This study employed computational modelling and physical testing to verify the strength of a joint prosthesis concept, under worst case static and fatigue loading conditions. The analysis technique represents an improvement in the state of the art where testing standards refer to conventional prostheses; similar methods could be applied to a wide range of novel prosthesis designs.

Developmental dysplasia of the hip is a condition in which the acetabulum provides insufficient coverage of the femoral head in the hip joint. This configuration gives poor biomechanical load distribution, with increased stress at the superior aspect of the joint surfaces, and can often lead to degenerative arthritis. Morphologically, the poor coverage may be due to an acetabulum that is too shallow or oriented in valgus.

The dysplastic deformity can be treated surgically with a group of similar procedures, often labeled periacetabular osteotomies or rotational acetabular osteotomies. Each involves separating the acetabulum from the pelvis and fixating the fragment back to the pelvis in an orientation with increased coverage of the femoral head. This redistributes the biomechanical loads relative to acetabulum.

Bone remodeling at the level of trabeculae is an accepted concept under research; however, it is unclear whether the hip undergoes gross morphology changes in response to changes in biomechanical loading. An understanding of the degree to which this remodeling occurs (if at all) may have an impact on surgical planning.

In this retrospective study, computed tomography (CT) scans of 13 patients (2 male, 11 female, 40 ± 9 years of age) undergoing unilateral periacetabular osteotomies were examined; scans were taken both pre-operatively and at least a year post-operatively with an in-plane resolution of 0.55 mm and a slice thickness of 1.25 mm. Scans were segmented to produce triangulated meshes for the proximal femurs and the pelvis. These scans were manually processed to isolate the articular portions of the femoral heads and acetabulums, respectively; the fovea, acetabular fossa, any osteophytes and any segmentation artifacts were excluded.

Post-operative meshes were registered to their pre-operative counterparts for both the femoral head and the acetabulum, for both the operative and non-operative hips, using the iterative closest point (ICP) algorithm to 20 iterations. To account for differences in defining the edges of the articular surfaces in the manual isolation, metrics were only calculated using points that were within 0.3 mm of a normal from the opposing mesh. With the resulting matched data, nearest neighbour distances were calculated to form the remodeling metrics. Select spurious datapoints were removed manually.

For the operative femoral heads, the registered post-operative points were 0.24±0.53 mm outside of the pre-operative points. The maximum deviation was on average 1.94 mm with worst-case of 2.99 mm; the minimum deviation was −0.62 mm with worst-case of −2.06 mm. Positive numbers indicate the post-operative points are ‘outside’ of the pre-operative points – that is, farther from the head centre. The non-operative femoral heads have similar deviation values, 0.21±0.46 mm outside, with maximum and minimum deviation averaging to 1.24 mm and −0.74 mm respectively, with worst cases of 2.99mm and −1.80mm.

For the operative acetabulums, the post-operative deviations were −0.08±0.43mm. The maximum and minimum deviations averaged to 0.62mm and −0.82mm, with worst cases of 2.14mm and −1.51mm across the set. Again, the non-operative acetabulums were very similar; post-operative deviations were −0.02±0.43mm, maximum and minimum deviations averaged to 1.24mm and −0.65mm, with worst cases of 1.97mm and −2.00mm.

These quantitative measurements were reflected in manual examination of the meshes; generally speaking, there were small deviations with no overarching patterns across the anatomy.

All metrics were very similar across the same anatomy (that is, femoral head or acetabulum) regardless of whether the hip operative or non-operative. Femurs tended to ‘grow’ slightly post-operatively, but by less than a half voxel in size. Given that the CT voxels are large compared to the measured deviations, it is possible the results may be sensitive to the manual segmentations used as source data.

Manual examination of the deviations indicated a few potential trends. Seven operative and eleven non-operative acetabulums had a small patch of positive deviation (1mm to 1.5mm) in the anterosuperior aspect. This can be seen in the plot as the yellow-red area near the top right of the leftmost rendering. Other high-deviation areas included the superior aspect of the acetabulum (both positive and negative) and the superior aspect of the femoral head (generally positive).

The edges of the mesh were often a source of high deviation. This is likely an artifact of over-inclusion the manual isolation of the articular surfaces, as joint surfaces become non-articular as they move away from the joint interface.

Overall, the superior and anterosuperior aspects of the acetabulum and the superior aspect of the femoral head showed some indication of systemic changes; further study may clarify whether these data represent consistent anatomical changes. However, as the magnitude of the deviations between pre- and post-operative scans are on or below the order of the CT voxel size, we conclude that (in the absence of other strongly compelling evidence) periacetabular osteotomies for adults should be planned without the expectation of gross remodeling of the articular surfaces.

INTRODUCTION

Resurfacing prostheses are implanted by impaction onto the prepared femoral head. Ceramic resurfacings can be proposed as an alternative to metal implants, combining bone conservation with mitigation of sensitivity reaction risks. With low wall-thickness required for bone conservation, their strength must be verified. This study aimed to assess a ceramic resurfacing prosthesis' strength under surgical loads using a computational model, tuned and verified with physical tests.

Although resurfacing hip replacement (RHR) is associated with a more demanding patient cohort, it has achieved survivorship approaching that of total hip replacement. Occasional failures from femoral neck fracture, or migration and loosening of the femoral head prosthesis have been observed, the causes of which are multifactorial, but predominately biomechanical in nature. Current surgical technique recommends valgus implant orientation and reduction of the femoral offset, reducing joint contact force and the femoral neck fracture risk. Radiographic changes including femoral neck narrowing and ‘pedestal lines’ around the implant stem are present in well performing hips, but more common in failing joints indicating that loosening may involve remodelling. The importance of prosthesis positioning on the biomechanics of the resurfaced joint was investigated using finite element analysis (FEA).

Seven FE models were generated from a CT scan of a male patient: the femur in its intact state, and the resurfaced femur with either a 50mm or 52mm prosthesis head in

neutral orientation,

The fracture risk during trauma was investigated for stumbling and a sideways fall onto the greater trochanter, by calculating the volume of yielding bone. Remodelling was quantified for normal gait, as the percentage volume of head and neck bone with over 75% post-operative change in strain energy density for an older patient, and 50% for a younger patient.

Resurfacing with the smaller, 50mm prosthesis reduced the femoral offset by 3.0mm, 4.3mm and 5.1mm in varus, neutral and valgus orientations. When the 52mm head was used, the natural joint centre could be recreated rrespective of orientation, without notching the femoral neck. The 50mm head reduced the volume of yielding femoral neck bone relative to the intact femur in a linear correlation with femoral offset. When the natural femoral offset was recreated with the 52mm prosthesis, the predicted neck fracture load in stumbling was decreased by 9% and 20% in neutral and varus orientations, but remained in line with the intact bone when implanted with valgus orientation. This agrees with clinical experience and justifies currently recommended techniques. In oblique falling, the neck fracture load was again improved slightly when the femoral offset was reduced, and never fell below 97% of the natural case for the larger implant in all orientations.

Predicted patterns of remodelling stimulus were consistent with radiographic clinical evidence. Stress shielding increased slightly from varus to valgus orientation, but was restricted to the superior femoral head in the older patient. Bone densification around the stem was predicted, indicating load transfer. Stress shielding only extended into the femoral neck in the young patient and where the femoral offset was reduced with the 50mm prosthesis. The increase in remodelling correlated with valgus orientation, or reduced femoral offset. The trend would become more marked if this were to reduce the joint contact force, but there was no such correlation for the 52mm prosthesis, when the natural femoral offset was recreated. Only in extreme cases would remodelling alone be sufficient to cause visible femoral neck narrowing, i.e. patients with a high metabolism and considerably reduced femoral offset, implying that other factors including damage from surgery or impingement, inflammatory response or retinacular blood supply interruption may also be involved in femoral neck adaptation.

The results of this FEA biomechanical study justify current surgical techniques, indicating improved femoral neck fracture strength in stumbling with valgus position. Fracture risk under oblique falling was less sensitive to resurfacing. Furthermore, the results imply that reduced femoral offset could be linked to narrowing of the femoral neck; however the effects of positioning alone on bone remodelling may be insufficient to account for this. The study suggests that surgical technique should attempt to recreate the natural head centre, but still aim primarily for valgus positioning of the prosthesis, to reduce the femoral neck fracture risk.

Introduction: Double bundle anterior cruciate ligament (DBACL) reconstruction has been shown to improve objective measurements of rotational stability. However, subjective improvement in patient outcome has yet to be shown. The aim of this study was to investigate whether double bundle ACL reconstruction could provide satisfactory subjective and objective outcome in a consecutive patient series at a minimum of two years post operatively.

Methods: From May 2006 a consecutive group of patients underwent double bundle ACL reconstruction using doubled semitendinosus (anteromedial bundle) and gracilis (posterolateral bundle) tendons fixed with interference screws in both femur and tibia. Pre and post operative subjective and objective IKDC 2000 knee scores were collected including pivot shift and KT-1000 arthrometer assessments. A comparison was made to a cohort of single bundle ACL reconstructions performed by the same surgeon whose data was collected previously.

Results: The group included 29 patients (26 male, mean age 30; range 18–47). A significant improvement in subjective IKDC 2000 score between preop (54.8) and last follow up (76.3) was shown (p = 0.00); 96% of subjects were IKDC grade A or B; 85% of subjects had a negative pivot shift on last review with 15% eliciting a pivot glide. There were no differences in subjective or objective outcomes between the double bundle and single bundle groups.

Discussion: This data compares favourably with other published series of double bundle ACL reconstruction. Although subjective improvement in functional outcome is seen, early results do not show an improvement over single bundle reconstruction. It remains to be shown if this technique will confer an overall improvement in long term outcome via the more anatomical reconstruction.

The purpose of this study was to describe our experience of the Calaxo Osteoconductive interference screw (Smith & Nephew) when used for both femoral and tibial graft fixation in Double Bundle ACL reconstruction.

Since May 2006, all patients with an ACL deficient knee were reconstructed using the Double Bundle technique. All were followed prospectively and outcome data collected. Evidence of fixation failure was established subjectively by clinical examination (Lachman, Anterior Draw, Pivot Shift) and objectively via KT-1000 arthrometer. Following ethical approval, post-operative CT scans (immediate and 1 year) were performed on our first 10 patients allowing assessment of tunnel dimensions/fill.

Thirty two patients (29 male, 3 female) with a mean age of 30 (range 18-46) were included. At last follow-up, no evidence of graft/fixation failure was found; KT-1000 mean side-side difference 1.4mm (range −3 to +6). All patients had a positive pivot shift preoperatively which was abolished postoperatively. One patient had a postoperative infection with no other complications reported. Radiologically the screws did not show complete resorption but areas of new bone were identified.

We have shown satisfactory results with use of the Calaxo screw when used in Double Bundle Reconstruction. We have not had any cases of the adverse local soft tissue reaction, which has led to this screw being withdrawn from clinical use. Even when using a total of four screws in each knee. A previous study published by Seibold (2007) has shown tunnel widening and communication when suspensory fixation is used in Double Bundle reconstruction. This has the potential risk of leading to fracture between the tunnels. This has not been seen with the Calaxo screw which may be a result of the biological action of the screw which should ultimately lead to a reduction in these risks.

Introduction: The purpose of this study was to describe our experience of the Calaxo Osteoconductive interference screw (Smith & Nephew) when used for both femoral and tibial graft fixation in Double Bundle ACL reconstruction.

Methods: Since May 2006, all patients with an ACL deficient knee were reconstructed using the Double Bundle technique. All were followed prospectively and outcome data collected.

Evidence of fixation failure was established subjectively by clinical examination (Lachman, Anterior Draw, Pivot Shift) and objectively via KT-1000 arthrometer.

Following ethical approval, post-operative CT scans (immediate and 1 year) were performed on our first 10 patients allowing assessment of tunnel dimensions/fill.

Results: Thirty two patients (29 male, 3 female) with a mean age of 30 (range 18–46) were included. At last follow-up, no evidence of graft/fixation failure was found; KT-1000 mean side-side difference 1.4mm (range −3 to +6). All patients had a positive pivot shift preoperatively which was abolished postoperatively. One patient had a postoperative infection with no other complications reported. Radiologically the screws did not show complete resorption but areas of new bone were identified.

Discussion: We have shown satisfactory results with use of the Calaxo screw when used in Double Bundle Reconstruction. We have not had any cases of the adverse local soft tissue reaction, which has led to this screw being withdrawn from clinical use. Even when using a total of four screws in each knee.

A previous study published by Seibold (2007) has shown tunnel widening and communication when suspensory fixation is used in Double Bundle reconstruction. This has the potential risk of leading to fracture between the tunnels.

This has not been seen with the Calaxo screw which may be a result of the biological action of the screw which should ultimately lead to a reduction in these risks.