Pedicle screws fixation to stabilize lumbar spinal fusion has become the gold standard for posterior stabilization. However their positioning remain difficult due to variation in anatomical shape, dimensions and orientation, which can determine the inefficacy of treatment or severe damages to close neurologic structures. Image guided navigation allows to drastically decrease errors in screw placement but it is used only by few surgeons due to its cost and troubles related to its using, like the need of a localizer in the surgical scenario and the need of a registration procedure. An alternative image guided approach, less expensive and less complex, is the using of patient specific templates similar to the ones used for dental implants or knee prosthesis.

Like proposed by other authors we decided to design the templates using CT scans. (slice thickness of 2.0 mm). Template developing is done, for each vertebra, using a modified version of ITK-SNAP 1.5 segmentation software, which allow to insert cylinders (full or empty) in the segmented images. At first we segment the spine bone and then the surgeon chose screw axes using the same software. We design each template with two hollow cylinders aligned with the axes, to guide the insertion in the pedicle, adding contact points that fit on the vertebra, to obtain a template right positioning. Finally we realize the templates in ABS using rapid prototyping. After same in-vitro tests, using a synthetic spine (by Sawbones), we studied a solution to guarantee template stability with simple positioning and minimizing intervention invasiveness. Preliminary ex-vivo animal testing on porcine specimens has been conducted to evaluate template performance in presence of soft-tissue in place, simulating dissection and vertebra exposure. For verification, the surgeon examined post-operative CT-scans to evaluate Kirschner wires positioning.

During the ex-vivo animal test sessions, template alignment resulted easy thanks to the spinous process contact point. Their insertion required no additional tissue removal respect to the traditional approach. The positioning of contact points on vertebra's lamina and articular processes required just to shift the soft tissue under the cylinders bases. The surgeon in some cases evaluated false stable template positions since not each of the 4 contact points were actually in contact with the bone surface and tried the right position. CT evaluation demonstrate a positive results in 96.5% of the Kirschner wires implanted.

Our approach allows to obtain patient specific templates that does not require the complete removal of soft tissue around vertebra. Guide positioning is facilitated thanks to the using of the spinous processes contact point, while false stable positions can be avoided using four redundant contact points. The templates can be used to guide the drill, the insertion of Kirschner in case of use of cannulated screws or to guide directly the screw. After these preliminary ex-vivo animal tests we obtained the authorization of the Italian Health Ministry to start the human study.

Introduction and Objective. The surgical strategy for acetabular component revision is determined by available host bone stock. Acetabular bone deficiencies vary from cavitary or segmental defects to complete discontinuity. For segmental acetabular defects with more than 50% of the graft supporting the cup it is recommended the application of reinforcement ring or ilioischial antiprotrusio devices. Acetabular reconstruction with the use of the antiprotrusion cage (APC) and allografts represents a reliable procedure to manage severe periprosthetic deficiencies with highly successful long-term outcomes in revision arthroplasty. Objective. We present our experience, results, critical issues and technical innovations aimed at improving survival rates of antiprotrusio cages. Materials and Methods. From 2004 to 2019 we performed 69 revisions of the acetabulum using defrosted morcellized bone graft and the Burch Schneider anti-protrusion cage. The approach was direct lateral in 25 cases, direct anterior in 44. Patients were re-evaluated with standard radiography and clinical examination. Results. Eight patients died from causes not related to surgery, and two patients were not available for follow up. Five patients were reviewed for, respectively, non-osseointegration of the ring, post-traumatic loosening with rupture of the screws preceded by the appearance of supero-medial radiolucency, post-traumatic rupture of the distal flange, post-traumatic rupture of the cemented polyethylene-ceramic insert, and dislocation treated with new dual-mobility insert. Among these cases, the first three did not show macroscopic signs of osseointegration of the ring, and the only areas of stability were represented by the bone-cement contact at the holes in the ring. Although radiographic studies have shown fast remodeling of the bone graft and the implant survival range from 70% to 100% in the 10-year follow up, the actual osseointegration of the ring has yet to be clarified. To improve osseointegration of the currently available APC whose metal surface in contact with the bone is sandblasted, we combined the main features of the APC design long validated by surgical experience with the 3D-Metal Technology for high porosity of the external surface already applied to and validated with the press fit cups. The new APC design is produced with the 3D-Metal technology using Titanium alloy (Ti6Al4V ELI) that Improves fatigue resistance, primary stability and favorable environment for bone graft ingrowth. We preview the results of the first cases with short-term follow up. Conclusions. Acetabular reconstruction with impacted morcellized bone graft and APC is a current and reliable surgical technique that allows the restoration of bone loss with a high survival rate of the implant in the medium to long term. The new 3D Metal Cage is designed to offer high friction for the initial stability. The high porosity of the 3D Metal structure creates a favorable environment for bone growth, thus providing valid secondary fixation reproducing the results achieved with the 3D metal press fit cup

Summary Statement. Proximal femoral bony deficits present a surgical and biomechanical challenge to implant longevity in revision hip arthroplasty. This work finds comparable primary stability when a distally fixing tapered fluted stem was compared with a conical design in cadaveric tests. Introduction. Proximal bony deficits complicate revision hip surgery and compromise implant survival. Longer distally fixing stems which bypass such defects are therefore required to achieve stability compatible with bony ingrowth and implant longevity. Aims. It is hypothesised that a tapered stem will provide superior rotational stability to a conical design. This work therefore aims to compare the primary stability and biomechanical properties of a new design of tapered fluted modular femoral stem (Redapt®, Smith & Nephew) with that of a conical fluted stem (Restoration®, Stryker). Materials & Methods. 7 Pairs of cadaveric femora were obtained according to strict inclusion/exclusion criteria. Each underwent dual energy x-ray absorptiometry and calibration plain-film radiographs were taken. Digital templating was performed using TraumaCad (Voyant Health, Brainlab) to determine implant sizing. Both stems are fluted, modular and manufactured from titanium. The control stem (Restoration) featured a straight conical design and the investigation stem (Redapt) a straight tapered design. Implantation was performed by a revision arthroplasty surgeon familiar with both systems. Proximal bone deficiency was reproduced using an extended trochanteric osteotomy with removal of metaphyseal bone before reattaching the osteotomy. Primary stability in the axial, sagittal and coronal planes was assessed using micromotion transducers (HBM, Darmstadt, Germany) and also by Radiostereometric Analysis (RSA). RSA employs simultaneous biplanar radiographs to measure relative movement. Two 1mm tantalum beads were mounted on the prosthesis with the centre of the femoral head taken as the third reference point. Beads were placed proximally in the surrounding bone as rigid body markers. Each bone was potted according to the ISO standard for fatigue testing and cyclically loaded at 1Hz for at least 3 increments (750–350N, 1000–350N, 1500–350N) for 1000 cycles. RSA radiographs were taken at baseline and on completion of each cycle. A strain analysis was concurrently performed using a PhotoStress® (Vishay Precision Group, Raleigh, USA) photoelastic coating on the medial femoral cortex. Each bone was loaded intact and then with the prosthesis in-situ at 500N increments until strain fringes were identified. Once testing was completed, the stems were sectioned at the femoral isthmus and data is presented on the cross-sectional fit and fill observed. Results. Both stem designs showed comparable primary stability with all stems achieving clinically acceptable micromotion (<150 μm) when loaded at body weight. A larger proportion of the control stems remained stable as loading increased to x2-3 body weight. Transducer-recorded migration appeared greatest in the axial plane (y axis) with negligible distal movement in the coronal or sagittal planes. Point motion analysis (RSA) indicated most movement to be in the coronal plane (x-axis) whereas segment motion analysis showed rotation about the long axis of the prosthesis to be largest. Photoelastic strain patterns were transferred more distally in both designs, however substantial stress shielding was also observed. Discussion/Conclusion. Both designs achieved adequate distal fixation and primary stability under representative clinical loading conditions. This work supports the continued use of this novel stem design for revision surgery in the presence of extensive proximal bone loss

Summary Statement. Computational models are the primary tools for efficient design-phase exploration of knee replacement concepts before in vitro testing. To improve design-phase efficiency, a subject-specific computational platform was developed that allows designers to assess devices in realistic conditions by directly integrating subject-specific experimental data in these models. Introduction. Early in the design-phase of new implant design, numerous in vitro tests would be desirable to assess the influence of design parameters or component alignment on the performance of the device. However, cadaveric testing of knee replacement devices is a costly and time-consuming procedure, requiring manufacture of parts, preparation of cadaveric specimens, and personnel to carry of the experiments. Validated computational models are ideally suited for pre-clinical, high-volume design evaluation. Initial development of these models requires substantial time and expertise; once developed, however, computational simulations may be applied for comparative evaluation of devices in an extremely efficient manner [Baldwin et al. 2012]. Still, computational models are complementary of experimental testing and for this reason, computational models tuned with subject-specific experimental data, e.g. soft tissue parameters, could bring even more efficiency in the design phase. The objective of the current study was to develop a platform of tools that easily allows for subject-specific knee simulations. The system integrates with commercially available medical imaging and finite element software to allow for direct, efficient comparison of designs and surgical alignment under a host of different boundary conditions. Patients & Methods. MRI image was acquired, and 3D bone models were generated using the Mimics Innovation Suite® (Materialise NV, Leuven, Belgium). The two models (1) tibiofemoral (TF) joint laxity including ligamentous constraint and (2) whole joint (TF and patellofemoral (PF)) mechanics during dynamic activities of daily living (e.g. gait, squat, chair-rise), developed in Abaqus/Explicit (SIMULIA, Providence, RI), were then be adapted with integrated subject-specific attachment sites. Results. The suite of tools provides a platform for baseline evaluation of design factors, comparison of new implant designs with predicate devices, and assessment of robustness to surgical alignment. This platform is currently capable of taking into account subject-specific factors in order to provide realistic results in relation with experimental data. Implant material properties, ligament properties and initial conditions can be varied, and results compared, to evaluate the influence of a host of design and surgical factors on implant performance. The interface allows users without complex finite element expertise to setup, analyze and compare devices and interpret results. Discussion/Conclusion. A platform which allows implant designers to evaluate their design ideas in realistic conditions integrating subject-specific parameters and to compare with predicate devices has the potential to substantially decrease the development time for new devices. Designers can perform iterative modification to their devices to focus on an optimal design solution prior to in vitro testing, reducing the number of pre-clinical cadaveric experiments that may be required, and ultimately improving TKR mechanics in the patient population

Introduction. The results of the original mobile bearing Oxford unicompartmental knee replacement (UKR) in the lateral compartment have been disappointing because of high dislocation rates (11%). This original implant used a flat bearing articulation on the tibial tray. To address the issue of dislocation a new implant (domed tibia with biconcave bearing to increase entrapment) was introduced with a modified surgical technique. The aim of this study was to compare the risk of dislocation between a domed and flat lateral UKR. Methods. Separate geometric computer models of an Oxford mobile bearing lateral UKR were generated for the two types of articulation between the tibial component and the meniscal bearing: Flat-on-flat (flat) and Concave-on-convex (domed). Each type of mobile bearing was used to investigate three distinct dislocation modes observed clinically: lateral to medial dislocation, with the bearing resting on the tray wall (L-M-Wall); medial to lateral dislocation, out of the joint space (M-L); anterior to posterior dislocation, out of the joint space (A-P). A size C tray and a medium femoral component and bearing were used in all models. The femoral component, tibial tray and bearing were first aligned in a neutral position. For each dislocation the tibial tray was restrained in all degrees of freedom. The femoral component was restrained from moving in the anterior-posterior directions and in the medial-lateral directions. The femoral component was also restrained from rotating about the anterior-posterior, medial-lateral and superior-inferior directions. This meant that the femoral component was only able to move in the superior-inferior direction. Different bearing sizes were inserted into the model and the effect that moving the femoral component medially and laterally had on the amount of distraction required to cause bearing dislocation was investigated. Results. The average femur distraction to allow bearing dislocation in the A-P, M-L and L-M-wall directions was 1.62 mm (27%), 0.51 mm (26%) and 1.2 mm (24%) greater respectively for the domed bearing. There was a 3% increase in femoral distraction required to cause L-M-Wall dislocation, per increment of bearing thickness for both the domed and lateral bearings. There was on average a 7% increase in femoral distraction required to cause L-M-Wall dislocation per mm increment of medial femoral component movement. Discussion. Dislocation over the tray wall is a particular clinical problem and using a domed bearing can lead to an increased required femoral distraction of between 25% and 37%. This may be significant during everyday activities and demonstrates that the new domed design should reduce the incidence of bearing dislocation by increasing the amount of entrapment. Increasing the thickness of the bearing has a small effect on the distraction required to allow bearing dislocation. Lateral placement of the femoral component markedly reduced the femoral distraction required for bearing dislocation over the tray wall. Medial placement of the femoral component is advisable so long as impingement with the tray wall is avoided

Our aim was to assess the intra- and inter-observer reliability in the establishment of the anterior pelvic plane used in imageless computer-assisted navigation. From this we determined the subsequent effects on version and inclination of the acetabular component.

A cadaver model was developed with a specifically-designed rod which held the component tracker at a fixed orientation to the pelvis, leaving the anterior pelvic plane as the only variable. Eight surgeons determined the anterior pelvic plane by palpating and registering the bony landmarks as reference points. The exact anterior pelvic plane was then established by using anatomically-placed bone screws as reference points.

The difference between the surgeons was found to be highly significant (p < 0.001). The variation was significantly larger for anteversion (sd 9.6°) than for inclination (sd 6.3°). The present method for registering pelvic landmarks shows significant inaccuracy, which highlights the need for improved methods of registration before this technique is considered to be safe.