Objectives. The primary stability of the cementless Oxford Unicompartmental Knee Replacement (OUKR) relies on interference fit (or press fit). Insufficient interference may cause implant loosening, whilst excessive interference could cause bone damage and fracture. The aim of this study was to identify the optimal interference fit by measuring the force required to seat the tibial component of the cementless OUKR (push-in force) and the force required to remove the component (pull-out force). Materials and Methods. Six cementless OUKR tibial components were implanted in 12 new slots prepared on blocks of solid polyurethane foam (20 pounds per cubic foot (PCF), Sawbones, Malmo, Sweden) with a range of interference of 0.1 mm to 1.9 mm using a Dartec materials testing machine HC10 (Zwick Ltd, Herefordshire, United Kingdom) . The experiment was repeated with cellular polyurethane foam (15 PCF), which is a more porous analogue for trabecular bone. Results. The push-in force progressively increased with increasing interference. The pull-out force was related in a non-linear fashion to interference, decreasing with higher interference. Compared with the current nominal interference, a lower interference would reduce the push-in forces by up to 45% (p < 0.001 One way ANOVA) ensuring comparable (or improved) pull-out forces (p > 0.05 Bonferroni post hoc test). With the more porous bone analogue, although the forces were lower, the relationship between interference and push-in and pull-out force were similar. Conclusions. This study suggests that decreasing the interference fit of the tibial component of the cementless OUKR reduces the push-in force and can increase the pull-out force. An optimal interference fit may both improve primary fixation and decrease the risk of fracture. Cite this article: S. Campi, S. J. Mellon, D. Ridley, B. Foulke, C. A. F. Dodd, H. G. Pandit, D. W. Murray. Optimal interference of the tibial component of the cementless Oxford Unicompartmental Knee Replacement. Bone Joint Res 2018;7:226–231. DOI: 10.1302/2046-3758.73.BJR-2017-0193.R1

The clinical success of osteochondral autografts is heavily reliant on their mechanical stability, as grafts which protrude above or subside below the native cartilage can have a negative effect on the tribological properties of the joint [1]. Furthermore, high insertion forces have previously been shown to reduce chondrocyte viability [2]. Commercial grafting kits may include a dilation tool to increase the diameter of the recipient site prior to insertion. The aim of this study was to evaluate the influence of dilation on the primary stability of autografts. Six human cadaveric femurs were studied. For each femur, four 8.5 × 8mm autografts were harvested from the trochlear groove and implanted into the femoral condyles using a Smith & Nephew Osteochondral grafting kit. Two grafts were implanted into dilated recipient sites (n=12) and two were implanted with no dilation (n=12). Insertion force was measured by partially inserting the graft and applying a load at a rate of 1 mm/min, until the graft was flush with the surrounding cartilage. Push-in force was measured by applying the same load, until the graft had subsided 4mm below congruency. Significance was taken as (p<0.05). Average maximum insertion force of dilated grafts was significantly lower (p<0.001) than their non-dilated equivalent [28.2N & 176.7N respectively]. There was no significant difference between average maximum push-in force between the dilated and non-dilated groups [1062.8N & 1204.2N respectively]. This study demonstrated that significantly less force is required to insert dilated autografts, potentially minimising loss of chondrocyte viability. However, once inserted, the force required to displace the grafts below congruency remained similar, indicating a similar degree of graft stability between both groups

Few studies have been reported focusing on developing implant surface nanofiber (NF) coating to prevent infection and enhance osseointegration by local drug release. In this study, coaxial doxycycline (Doxy)-doped polycaprolactone/polyvinyl alcohol (PCL/PVA) NFs were directly deposited on the titanium (Ti) implant surface during electrospinning. The bonding strength of Doxy-doped NF coating on Ti implants was confirmed by a stand single-pass scratch test. The improved implant osseointegration by PCL/PVA NF coatings in vivo was confirmed by scanning electron microscopy, histomorphometry and micro computed tomography at 2, 4 and 8 weeks after implantation. The bone contact surface (%) changes of NF coating group (80%) is significantly higher than that of no NF group (< 5%, p<0.05). Finally, we demonstrated that Doxy-doped NF coating effectively inhibited bacterial infection and enhanced osseointegration in an infected (Staphylococcus aureus) tibia implantation rat model. Doxy released from NF coating inhibited bacterial growth up to 8 weeks in vivo. The maximal push-in force of Doxy-NF coating (38 N) is much higher than that of NF coating group (6.5 N) 8 weeks after implantation (p<0.05), which was further confirmed by quantitative histological analysis and micro computed tomography. These findings indicate that coaxial PCL/PVA NF coating doped with Doxy and/or other drugs have great potential in enhancing implant osseointegration and preventing infection

Bone ingrowth is desired with uncemented hip implants. Infection is clearly undesirable. We have worked on developing a nanofiber coating for implants that would enhance bone formation while inhibiting infection. Few studies have focused on developing an implant surface nanofiber (NF) coating to prevent infection and enhance osseointegration by local drug release. In this study, coaxial doxycycline (Doxy)-doped polycaprolactone/polyvinyl alcohol (PCL/PVA) Nanofibers were directly deposited on the titanium (Ti) implant surface during electrospinning. The interaction of loaded Doxy with both PVA and PCL NFs was characterized by Raman spectroscopy. The bonding strength of Doxy-doped NF coating on Ti implants was confirmed by a stand single-pass scratch test. The improved implant osseointegration by PCL/PVA NF coatings in vivo was confirmed by scanning electron microscopy, histomorphometry and micro computed tomography at 2, 4 and 8 weeks after implantation. The bone contact surface (%) changes of NF coating group (80%) is significantly higher than that of no NF group (< 5%, p<0.05). Finally, we demonstrated that Doxy-doped NF coating effectively inhibited bacterial infection and enhanced osseointegration in an infected (Staphylococcus aureus) tibia implantation rat model. Doxy released from NF coating inhibited bacterial growth up to 8 weeks in vivo. The maximal push-in force of Doxy-NF coating (38 N) is much higher than that of NF coating group (6.5 N) 8 weeks after implantation (p<0.05), which was further confirmed by quantitative histological analysis and micro computed tomography. These findings indicate that coaxial PCL/PVA NF coating doped with Doxy and/or other drugs have great potential in enhancing implant osseointegration and preventing infection

Introduction. In total hip arthroplasty, press-fit anchorage is one of the most common fixation methods for acetabular cups and mostly ensures sufficient primary stability. Nevertheless, implants may fail due to aseptic loosening over time, especially when the surrounding bone is affected by stress-shielding. The use of acetabular cups made of isoelastic materials might help to avoid stress-shielding and osteolysis. The aim of the present numerical study was to determine whether a modular acetabular cup with a shell made of polyetheretherketone (PEEK) may be an alternative to conventional titanium shells (Ti6Al4V). For this purpose, a 3D finite element analysis was performed, in which the implantation of modular acetabular cups into an artificial bone stock using shells made of either PEEK or Ti6Al4V, was simulated with respect to stresses and deformations within the implants. Methods. The implantation of a modular cup, consisting of a shell made of PEEK or Ti6Al4V and an insert made of either ceramic or polyethylene (PE), into a bone cavity made of polyurethane foam (20 pcf), was analysed by 3D finite element simulation. A two-point clamping cavity was chosen to represent a worst-case situation in terms of shell deformation. Five materials were considered; with Ti6Al4V and ceramic being defined as linear elastic and PE and PEEK as plastic materials. The artificial bone stock was simulated as a crushable foam. Contacts were generated between the cavity and shell (μ = 0.5) and between the shell and insert (μ = 0.16). In total, the FE models consisted of 45,282 linear hexahedron elements and the implantation process was simulated in four steps: 1. Displacement driven insertion of the cup; 2. Relief of the cup; 3. Displacement driven placement of the insert; 4. Load driven insertion of the insert (maximum push-in force of 500 N). The FE model was evaluated with respect to the radial deformations of the shell and insert as well as the principal stresses in case of the ceramic inserts. The model was experimentally validated via comparison of nominal strains of the titanium shells. Results. The maximum radial deformation of the shell made of PEEK was 581 μm (insertion) and 470 μm (relief) and therefore multiple times higher compared to the Ti6Al4V shell (42 μm and 21 μm). As a result, larger deformations occurred at the PE and ceramic inserts in combination with the PEEK shell. Partially, the deformations were above an usual clearance of 100 μm. When the ceramic insert was combined with the shell made of PEEK, maximum principal stresses in the ceramic insert amounted to 30 MPa and were clearly lower than approved bending strength of the ceramic material (948 MPa). Conclusion. The examined acetabular shell made of PEEK was intensively deformed during insertion compared to the geometrically identical Ti6Al4V shell and is therefore not suitable for modular acetabular cups. In future studies it should be clarified to what extent acetabular cups with shells made of carbon fiber reinforced PEEK materials with higher stiffness lead to reduced deformations during the insertion procedure

Introduction: Adequate congruency and primary stability are vital for good long-term results after mosaicplasty. The strength of press-fit stability of the grafts depends upon the length and diameter of the graft, extent of dilation and bone quality. The aim of our study was to quantify the effect of graft diameter and dilation length on the primary stability of single osteochondral grafts against compression and compare the stability of single and multiple osteochondral grafts in an in vitro biomechanical animal model. Methods: In the single graft series one osteochondral graft was transplanted from the trochlea of porcine femurs to the weight-bearing area of the lateral femoral condyle, while in the multiple graft series three grafts were transplanted in a row or in circular fashion in the same position. We used the MosaicPlasty instruments (Acufex, Smith & Nephew Inc. MA, USA). The specimen was installed on a testing machine (Computer controlled ZWICK FR005TH type tensile machine, Zwick GmbH Ulm, Germany) and the graft was first pushed in level with the surrounding cartilage surface, then it was pushed 3 mm deeper. The push-in forces were measured and the compression curve was registered. Results: In the case of single 4.5-mm grafts, the mean level push-in force was 43.5 N, pushing 3 mm deeper needed a mean of 92.5 N (n=13). In the case of single 6.5-mm grafts, level push-in needed a mean of 76.2 N, while for pushing 3 mm deeper a mean of 122.2 N force had to be used (n=14). The length of the drill-hole and the dilation were both 20 mm in each setting. When using 20 mm long drill-holes and 15 mm dilation length, the values above were found to be 36.6 N and 122.5 N in the case of 4.5-mm grafts (n=12). In case of multiple grafting level push-in needed a mean force of 31.8 N in the row series, while pushing 3 mm deeper needed a mean of 52.17 N (n=7). In the circle series level push-in needed a mean of 30.44 N, while for pushing 3 mm deeper a mean of 54.33 N force had to be used (n=9). Conclusions: These results suggest that grafts of greater diameter are more stable in absolute values and the stability may be increased by shorter dilation length, while level push-in forces do not increase significantly. Multiple grafts may not be as stable as single grafts after transplantation and transplantation in a row or in circular fashion does not influence stability