Abstract. Objectives. Young patients receiving metallic bone implants after surgical resection of bone cancer require implants that last into adulthood, and ideally life-long. Porous implants with similar stiffness to bone can promote bone ingrowth and thus beneficial clinical outcomes. A mechanical remodelling stimulus, strain energy density (SED), is thought to be the primary control variable of the process of bone growth into porous implants. The sequential process of bone growth needs to be taken into account to develop an accurate and validated bone remodelling algorithm, which can be employed to improve porous implant design and achieve better clinical outcomes. Methods. A bone remodelling algorithm was developed, incorporating the concept of bone connectivity (sequential growth of bone from existing bone) to make the algorithm more physiologically relevant. The algorithm includes adaptive elastic modulus based on apparent bone density, using a node-based model to simulate local remodelling variations while alleviating numerical checkerboard problems. Strain energy density (SED) incorporating stress and strain effects in all directions was used as the primary stimulus for bone remodelling. The simulations were developed to run in MATLAB interfacing with the commercial FEA software ABAQUS and Python. The algorithm was applied to predict bone ingrowth into a porous implant for comparison against data from a sheep model. Results. The accuracy of the predicted bone remodelling was verified for standard loading cases (bending, torsion) against analytical calculations. Good convergence was achieved. The algorithm predicted good bone remodelling and growth into the investigated porous implant. Using the standard algorithm without connectivity, bone started to remodel at locations unconnected to any bone, which is physiologically implausible. The implementation of bone connectivity ensures the gradual process of bone growth into the implant pores from the sides. The bone connectivity algorithm predicted that the full remodelling required more time (approximately 50% longer), which should be considered when developing post-surgical rehabilitation strategies for patients. Both algorithms with and without bone connectivity implementation converged to same final stiffness (less than 0.01% difference). Almost all nodes reached the same density value, with only a limited number of nodes (less than 1%) in transition areas with a strong density gradient having noticeable differences. Conclusions. An improved bone remodelling algorithm based on strain energy density that modelled the sequential process of bone growth has been developed and tested. For a porous metallic bone implant the same final bone density distribution as for the original adaptive elasticity theory was predicted, with a slower and more fidelic process of growth from existing surrounding bone into the porous implant. Declaration of Interest. (b) declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research reported:I declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of the research project

Numerous papers present in-vivo knee kinematics data following total knee arthroplasty (TKA) from fluoroscopic testing. Comparing data is challenging given the large number of factors that potentially affect the reported kinematics. This paper aims at understanding the effect of following three different factors: implant geometry, performed activity and analysis method. A total of 30 patients who underwent TKA were included in this study. This group was subdivided in three equal groups: each group receiving a different type of posterior stabilized total knee prosthesis. During single-plane fluoroscopic analysis, each patient performed three activities: open chain flexion extension, closed chain squatting and chair-rising. The 2D fluoroscopic data were subsequently converted to 3D implant positions and used to evaluate the tibiofemoral contact points and landmark-based kinematic parameters. Significantly different anteroposterior translations and internal-external rotations were observed between the considered implants. In the lateral compartment, these differences only appeared after post-cam engagement. Comparing the activities, a significant more posterior position was observed for both the medial and lateral compartment in the closed chain activities during mid-flexion. A strong and significant correlation was found between the contact-points and landmarks-based analyses method. However, large individual variations were also observed, yielding a difference of up to 25% in anteroposterior position between both methods. In conclusion, all three evaluated factors significantly affect the obtained tibiofemoral kinematics. The individual implant design significantly affects the anteroposterior tibiofemoral position, internal-external rotation and timing of post-cam engagement. Both kinematics and post-cam engagement additionally depend on the activity investigated, with a more posterior position and associated higher patella lever arm for the closed chain activities. Attention should also be paid to the considered analysis method and associated kinematics definition: analyzing the tibiofemoral contact points potentially yields significantly different results compared to a landmark-based approach

Posterior stabilized (PS) total knee arthroplasty (TKA), wherein mechanical engagement of the femoral cam and tibial post prevents abnormal anterior sliding of the knee, is a proven surgical technique. However, many patients complain about abnormal clicking sensation, and several reports of severe wear and catastrophic failure of the tibial post have been published. In addition to posterior cam-post engagement during flexion, anterior engagement with femoral intercondylar notch can also occur during extension. The goal of this study was to use dynamic simulations to explore sensitivity of tibial post loading to implant design and alignment, across different activities. LifeModeler KneeSIM software was used to calculate tibial post contact forces for four contemporary PS implants (Triathlon PS, Stryker; Journey BCS and Legion PS, Smith & Nephew; LPS Flex, Zimmer Biomet). An average model of the knee, including cartilage and soft tissue insertion locations, created from MRI data of 40 knees was used to mount and align the component. The Triathlon femoral component was mounted with posterior and distal condylar tangency at: a) both medial and lateral condylar cartilage (anatomic alignment), b) at the medial condylar cartilage and perpendicular to mechanical axis (mechanical alignment with medial tangency), and c) at lateral condylar cartilage and perpendicular to mechanical axis (mechanical alignment with lateral tangency). The influence of implant design was assessed via simulations for the other implant systems with the femoral components aligned perpendicular to mechanical axis with lateral tangency. Five different activities were simulated. The anterior contact force was significantly smaller than the posterior contact force, but it varied noticeably with tibial insert slope and implant design. For Triathlon PS, during most activities anatomic alignment of the femoral component resulted in greater anterior contact force compared to mechanical alignment, but absolute magnitude of forces remained small (<100N). Mechanical alignment with medial tangency resulted in greater posterior contact force for deep knee bend and greater anterior force for chair sit activity. For all implants, peak posterior contact forces were greater for activities with greater peak knee flexion. The magnitude of posterior contact forces for the various implants was comparable to other reports in literature. Overall activity type, implant design and slope had greater impact on post loading than alignment method. Tibial insert slope was shown to be important for anterior post loading, but not for posterior post loading. Anatomic alignment could increase post loading with contemporary TKA systems. In the case of the specific design for which effect of alignment was evaluated, the changes in force magnitude with alignment were modest (<200N). Nonetheless, results of this study highlight the importance of evaluating the effect of different alignment approaches on tibial post loading

Introduction. Hip and knee replacements are being performed at increasing rates and currently account for one of the largest procedure expenditures in the Medicare budget. Outcomes of total knee replacement (TKR) depend on surgeon, patient and implant factors. The impact that the specific implants might have on patient-reported outcomes is unknown. The primary purpose of this study was to evaluate the patient-reported functional outcomes and satisfaction after primary total knee arthroplasty in patients with osteoarthritis who underwent TKR using five different brands of posterior-stabilized implants. Specifically, the aim was to evaluate for any difference in patient-reported outcomes based on implant brand used. The hypothesis was that there would be no difference in functional outcome that could be attributed to the implant used in primary TKR. Methods. Using our institution's total joint arthroplasty registry, we identified 4,135 patients who underwent total knee replacement (TKR) using one of the five most common implant brands used at our institution. These included Biomet Vanguard (N=211 patients), Depuy/Johnson&Johnson Sigma (N=221), Exactech OptetrakLogic (N=1,507), Smith & Nephew Genesis II (N=1,414), and Zimmer NexGen (N=779). Only posterior-stabilized primary TKRs in patients with osteoarthritis were included. Patients were evaluated preoperatively using the Knee Osteoarthritis Outcomes Score (KOOS), Lower Extremity Activity Scale (LEAS), and Short Form-12 (SF-12). Demographics including age, body mass index (BMI), Charleston Comorbidity Index (CCI), ASA physical status classification, sex, and smoking status were collected. Postoperatively, 2-year KOOS, LEAS, SF-12, and satisfaction scores were compared between implant groups. Results. At 2-year follow-up, patient-reported outcome scores were available for a total of 4,069 patients. In multiple regression analysis which separately compared each implant group to the aggregate of all others, after accounting for age, BMI, CCI, ASA status, and sex, there were no clinically significant differences in KOOS score changes from baseline to 2-year follow-up between any of the implants. In 2-year satisfaction, >80% of patients in each implant group were satisfied in all domains measured. In the multivariate regression model, patients in the NexGen group had the highest likelihood of being satisfied (OR 1.63, p=0.006) and OptetrakLogic patients had the lowest likelihood of being satisfied (OR 0.60, P<0.001) although these differences were below the minimal clinically important thresholds. Conclusion. TKR provides patient satisfaction and improvement in function regardless of implant type. While some implant systems had higher outcome scores than others, these differences were all below clinically significant thresholds. Whether these differences are a function of surgeon expertise or implant design is unknown. Future research should focus on identifying specific design elements that contribute to improved patient outcomes. Healthcare administrators may find the similarities in clinical outcomes to be a useful consideration when negotiating implant purchasing contracts

Common post-operative problems in shoulder arthroplasty such as glenoid loosening and joint instability can be reduced by improvements in glenoid design shape, material choice and fixation method [1]. Innovation in shoulder replacement is usually carried out by introducing incremental changes to functioning implants [2], possibly overlooking other successful design combinations. We propose an automated framework for parametric analysis of implant design in order to efficiently assess different possible glenoid configurations. Parametric variations of reference geometries of a glenoid implant were automatically generated in SolidWorks. The different implants were aligned and implanted with repeatability using Rhino. The glenoid-bone models were meshed in Abaqus, and boundary conditions and loading applied via a custom-made Python script. Finally, another MATLAB script integrated and automated the different steps, extracted and analysed the results. This study compared the influence of reference shape (keel vs. 2-pegged) and material on the von Mises stresses and tensile and compressive strains of glenoid components with bearing surface thickness and fixation feature width of 3, 4, 5 or 6 mm. A total of 96 different glenoid geometries were implanted into a bone cube (E = 300 MPa, ν = 0.3). Fixed boundary conditions were applied at the distal surface of the cube and a contact force of 1000 N was distributed between the central nodes on the bearing surface. The implants were assigned UHMWPE (E = 1 GPa, ν = 0.46), Vitamin E PE (E = 800 MPa, ν = 0.46), CFR-PEEK (E = 18 GPa, ν = 0.41) or PCU (E = 2 GPa, ν = 0.38) material properties and the bone-implant surface was tied (Figure 1). The von Mises stresses, compressive and tensile strains for the different models were extracted. The influence of design parameters in the mechanical environment of the implant could be assessed. In this particular example, the 95. th. percentile values of the tensile and compressive strains induced by modifications in reference shape could be evaluated for all the different geometries simultaneously in form of radar plots. 2-pegged geometries (green) consistently produced lower tensile and compressive strains than the keeled (blue) configurations (Figure 2). Vitamin E PE and PCU glenoids also produced lower maximum von Mises stresses values than CFR-PEEK and UHMWPE designs (Figure 3). The developed method allows for simple, direct, rapid and repeatable comparison of different design features, material choices or fixation methods by analysing how they influence the mechanical environment of the bone surrounding the implant. Such tool can provide invaluable insight in implant design optimisation by screening through multiple potential design modifications at an early design evaluation stage and highlighting the best performing combinations. Future work will introduce physiological bone geometries and loading, a wider variety of reference geometries and fixation features, and look at bone/interface strength and osteointegration predictions

Iliopsoas tendonitis after total hip arthroplasty (THA) can be a considerable cause of pain and patient dissatisfaction. The optimal cup position to avoid iliopsoas tendonitis has not been clearly established. Implant designs have also been developed with an anterior recess to avoid iliopsoas impingement. The purpose of this cadaveric study was to determine the effect of cup position and implant design on iliopsoas impingement. Bilateral THA was performed on three fresh frozen cadavers using oversized (jumbo) offset head center revision acetabular cups with an anterior recess (60, 62 and 66 mm diameter) and tapered wedge primary stems through a posterior approach. A 2mm diameter flexible stainless steel cable was inserted into the psoas tendon sheath between the muscle and the surrounding membrane to identify the location of the psoas muscle radiographically. CT scans of each cadaver were imported in an imaging software. The acetabular shells, cables as well as pelvis were segmented to create separate solid models of each. The offset head center shell was virtually replaced with an equivalent diameter hemispherical shell by overlaying the outer shell surfaces of both designs and keeping the faces of shells parallel. The shortest distance between each shell and cable was measured. To determine the influence of cup inclination and anteversion on psoas impingement, we virtually varied the inclination (30°/40°/50°) and anteversion (10°/20°/30°) angles for both shell designs. The CT analysis revealed that the original orientation (inclination/anteversion) of the shells implanted in 3 cadavers were as follows: Left1: 44.7°/23.3°, Right1: 41.7°/33.8°, Left2: 40/17, Right2: 31.7/23.5, Left3: 33/2908, Right3: 46.7/6.3. For the offset center shells, the shell to cable distance in all the above cases were positive indicating that there was clearance between the shells and psoas. For the hemispherical shells, in 3 out of 6 cases, the distance was negative indicating impingement of psoas. With the virtual implantation of both shell designs at orientations 40°/10°, 40°/20°, 40°/30° we found that greater anteversion helped decrease psoas impingement in both shell designs. When we analyzed the influence of inclination angle on psoas impingement by comparing wire distances for three orientations (30°/20°, 40°/20°, 50°/20°), we found that the effect was less pronounced. Further analysis comparing the offset head center shell to the conventional hemispherical shell revealed that the offset design was favored (greater clearance between the shell and the wire) in 17 out of 18 cases when the effect of anteversion was considered and in 15 out of 18 cases when the effect of inclinations was considered. Our results indicate that psoas impingement is related to both cup position and implant geometry. For an oversized jumbo cup, psoas impingement is reduced by greater anteversion while cup inclination has little effect. An offset head center cup with an anterior recess was effective in reducing psoas impingement in comparison to a conventional hemispherical geometry. In conclusion, adequate anteversion is important to avoid psoas impingement with jumbo acetabular shells and an implant with an anterior recess may further mitigate the risk of psoas impingement

Introduction:. Varus alignment of the knee is common in patients undergoing unicondylar knee replacement. To measure the geometry and morphology of these knees is to know whether a single unicondylar knee implant design is suitable for all patients, i.e. for patients with varus deformity and those without. The aim of this study was to identify any significant differences between normal and varus knees that may influence unicondylar implant design for the latter group. Methodology:. 56 patients (31 varus, 25 normal) were evaluated through CT imaging. Images were segmented to create 3D models and aligned to a tri-spherical plane (centres of spheres fitted to the femoral head and the medial and lateral flexion facets). 30 key co-ordinates were recorded per specimen to define the important axes, angles and shapes (e.g. spheres to define flexion and extension facet surfaces) that describe the femoral condylar geometry using in-house software. The points were then projected in sagittal, coronal and transverse planes. Standardised distance and angular measurements were then carried out between the points and the differences between the morphology of normal and varus knee summarised. For the varus knee group, trends were investigated that could be related to the magnitude of varus deformity. Results:. Several significant differences between normal and varus knees were found, but most of these were small differences unlikely to be clinically significant or have an influence on implant design. However, two strong trends were observed. Firstly, the version of the femoral neck was significantly less for patients with varus knees (mean difference 9°; p < 0.05). The second trend was a significant difference in the sagittal morphology of the medial condyle. The kink angle, the angle formed by the intersection of the circles fitted to the flexion and extension facet surfaces, and their centres (Figure 1) was either absent or small in normal knees (mean 1°). An absent kink angle occurs when the circle defining the flexion facet surface lies within or makes a tangent to the circle defining the extension facet. However, for varus knees, the mean kink angle was 9°, with positive correlation with the angle of varus deformity (Figure 2). Discussion:. Varus knees have a significantly larger kink angle than normal knees, influencing the relative positions of the flexion and extension facet spheres that define the medial condylar geometry, contributing to the commonly observed ‘flattening’ of the medial condyle in the sagittal plane. Varus knees are also associated with significantly less anteversion of the femoral neck. It has been shown that reduced femoral neck anteversion causes increased loading of the medial condyle [1], and our results support this finding. The data generated in this study will feed further biomechanical testing to investigate the influence of kink angle and femoral neck version on the kinematics and load distribution in the varus knee

The aim of this study was to investigate the effects of implant design and gender on the outcome of Total Knee Arthroplasty (TKA) in patients with osteoarthritis (OA). In this double blind randomised controlled trial, patients with OA received either a standard posterior stabilised implant (n=28) or high flex version of this implant (n=28). Walking speed, knee flexion under anaesthesia (‘drop test’), knee flexion in sitting and during functional activities as measured by electrogoniometry, daily number of steps, Quality of Life (SF36), the function component of the Knee Society Score, pain (Visual Analogue Score) and extensor strength were measured before and one year after TKA. Type of implant did not have a significant effect on any of the outcome measures recorded, while gender showed significant effects both before and after surgery. Before surgery, females had a significantly lower knee range of motion, (both passive and functional), lower Knee Score function component, walking speed and strength. After surgery they had a statistically significant lower range of knee motion during functional activities such as walking up and down a slope. Strength was also still significantly lower but post-operative self-reported function were similar for both genders. There was also no difference between male and female participants regarding Quality of life, objective daily physical activity or pain. The results of this study showed that there is a clinically and statistically significant difference between the function of female and male patients both before and after total knee arthroplasty. Although female patients seem to benefit more from TKA than males, on average they do not achieve the same functional knee motion after surgery. Unlike gender, implant design did not influence the knee motion or function in this group of patients. This has important implications for future research and treatment planning in order to maximise the functional outcome after TKA

Introduction. Iliopsoas tendonitis after total hip arthroplasty (THA) can be a considerable cause of pain and patient dissatisfaction. The optimal cup position to avoid iliopsoas tendonitis has not been clearly established. Implant designs have also been developed with an anterior recess to avoid iliopsoas impingement. The purpose of this cadaveric study was to determine the effect of cup position and implant design on iliopsoas impingement. Materials. Bilateral THA was performed on three fresh frozen cadavers using oversized (jumbo) offset head center revision acetabular cups with an anterior recess (60, 62 and 66 mm diameter) and tapered wedge primary stems through a posterior approach. The relatively large shell sizes were chosen to simulate THA revision cases. At least one fixation screw was used with each shell. A 2mm diameter flexible stainless steel cable was inserted into the psoas tendon sheath between the muscle and the surrounding membrane to identify the location of the psoas muscle radiographically. Following the procedure, CT scans were performed on each cadaver. The CT images were imported in an imaging software for further analysis. The acetabular shells, cables as well as pelvis were segmented to create separate solid models of each. To compare the offset head center shell to a conventional hemispherical shell in the same orientation, the offset head center shell was virtually replaced with an equivalent diameter hemispherical shell by overlaying the outer shell surfaces of both designs and keeping the faces of shells parallel. enabled us to assess the relationship between the conventional shells and the cable. The shortest distance between each shell and cable was measured. To determine the influence of cup inclination and anteversion on psoas impingement, we virtually varied the inclination (30°/40°/50°) and anteversion (10°/20°/30°) angles for both shell designs. Results. The CT analysis revealed that the original orientation (inclination/anteversion) of the shells implanted in 3 cadavers were as follows: Left1: 44.7°/23.3°; Right1: 41.7°/33.8°; Left2: 40.0/17; Right2: 31.7/23.5; Left3: 33.0/2908; Right3: 46.7/6.3. For the offset center shells, the shell to cable distance in all the above cases were positive indicating that there was clearance between the shells and psoas. For the hemispherical shells, in 3 out of 6 cases, the distance was negative indicating impingement of psoas. With the virtual implantation of both shell designs at orientations 40°/10°, 40°/20°, 40°/30° we found that greater anteversion helped decrease psoas impingement in both shell designs. When we analyzed the influence of inclination angle on psoas impingement by comparing wire distances for three orientations (30°/20°, 40°/20°, 50°/20°), we found that the effect was less pronounced. Further analysis comparing the offset head center shell to the conventional hemispherical shell revealed that the offset design was favored (greater clearance between the shell and the wire) in 17 out of 18 cases when the effect of anteversion was considered and in 15 out of 18 cases when the effect of inclinations was considered. Discussion. Our results indicate that psoas impingement is related to both cup position and implant geometry. For an oversized jumbo cup, psoas impingement is reduced by greater anteversion while cup inclination has little effect. An offset head center cup with an anterior recess was effective in reducing psoas impingement in comparison to a conventional hemispherical geometry. In conclusion, adequate anteversion is important to avoid psoas impingement with jumbo acetabular shells and an implant with an anterior recess may further mitigate the risk of psoas impingement

Introduction:. Patient specific instrumentation (PSI) generates customized guides from a magnetic resonance imaging based preoperative plan for use in total knee arthroplasty (TKA). PSI software must be able to accommodate differences in implant design. The purpose of the present study was to determine whether any differences in the accuracy of limb alignment, component alignment, component sizing, or bony resection could be identified in patients undergoing PSI TKA with identical PSI software and one of two different implant systems. Methods:. In this case-control study, two different implant systems from the same manufacturer were evaluated in 37 consecutive PSI TKA (Group 1) and 123 consecutive PSI TKA (Group 2) performed by a single surgeon. A third group (Group 3) consisted of 12 consecutive TKA performed with manual instrumentation and the same implant system as Group 1. Identical software was used to generate a preoperative plan from which planned limb alignment, component alignment, component sizes, and bony resection were determined. Intraoperatively, actual component sizes, bony resection, and recut frequency were determined. Long-standing and lateral radiographs were obtained preoperatively and 4-weeks postoperatively to evaluate limb and component alignment. Results:. Groups were similar with regard to age, gender, BMI, and preoperative alignment. No differences in the accuracy of limb alignment, component alignment, component sizing, or PSI-planned versus actual resection were found between Groups 1 and 2. The rate of recuts required was lower in Group 1 than Group 2 for the proximal tibia (3% vs. 35%; p < 0.05). No differences were found in limb alignment, component alignment, or bony resection between the Groups 1 and 3. Group 1 showed less variation than Group 3 in resection depth of the posterior femur (SD 1.4 mm vs. 2.1 mm) and proximal tibia (SD 1.5 mm vs. 2.3 mm). Discussion:. No discernible differences in the accuracy of limb alignment, component alignment, and component sizing were found between Groups 1 and 2. Group 1 required fewer recuts than Group 2 for the proximal tibia. There may be characteristics of implant design, e.g. the slope of the tibial plateau, that may influence the ability of PSI to accurately determine cut thickness. No differences in limb alignment, component alignment, or bony resection were identified between Groups 1 and 3. Group 1 showed less variability in resection depth than Group 3 in the posterior femur and proximal tibia. This study suggests that PSI can be equally accurate for different implant systems. For a given implant system, PSI shows less variation in resection depth when compared to manual instrumentation

Introduction. Survival rates of recent total ankle replacement (TAR) designs are lower than those of other arthroplasty prostheses. Loosening is the primary indication for TAR revisions [NJR, 2014], leading to a complex arthrodesis often involving both the talocrural and subtalar joints. Loosening is often attributed to early implant micromotion, which impedes osseointegration at the bone-implant interface, thereby hampering fixation [Soballe, 1993]. Micromotion of TAR prostheses has been assessed to evaluate the stability of the bone-implant interface by means of biomechanical testing [McInnes et al., 2014]. The aim of this study was to utilise computational modelling to complement the existing data by providing a detailed model of micromotion at the bone-implant interface for a range of popular implant designs, and investigate the effects of implant misalignment during surgery. Methods. The geometry of the tibial and talar components of three TAR designs widely used in Europe (BOX®, Mobility® and SALTO®; NJR, 2014) was reverse-engineered, and models of the tibia and talus were generated from CT data. Virtual implantations were performed and verified by a surgeon specialised in ankle surgery. In addition to the aligned case, misalignment was simulated by positioning the talar components in 5° of dorsi- or plantar-flexion, and the tibial components in ± 5° and 10° varus/valgus and 5° and 10° dorsiflexion; tibial dorsiflexed misalignement was combined with 5° posterior gap to simulate this misalignment case. Finite element models were then developed to explore bone-implant micromotion and loads occurring in the bone in the implant vicinity. Results. Micromotion and bone loads peaked at the end of the stance phase for both the tibial and talar components. The aligned BOX and SALTO demonstrated lower tibial micromotion (with under 30% of bone-implant interface area subjected to micromotion larger than 100µm, as opposed to > 55% for Mobility; Figure 1). Talar micromotion was considerably lower for all designs, and no aligned talar component demonstrated micromotion larger than 100µm. The aligned SALTO showed the largest talar micromotion (Figure 2). Dorsiflexed implantation of all tibial components increased micromotion and bone strains compared to the reference case; interestingly, the SALTO tibial component, which demonstrated the lowest micromotion for the aligned case, also demonstrated the smallest changes in micromotion due to malpositioning (Figure 3). The posterior gap between the tibia and implant further increased bone strains. Dorsi- or plantar-flexed implantation of all talar components considerably increased micromotion and bone loads compared to the reference case (Figure 2), often resulting in micromotion exceeding 100µm. The SALTO talar component demonstrated the smallest changes in micromotion due to malpositioning. Discussion. The aligned Mobility had greater tibial micromotion than the SALTO and BOX, which agrees with higher revision rates reported in registry data (e.g. NZJR, 2014). The increased micromotion associated with dorsi- or plantar-flexion misalignment highlights the importance of aligning the implant correctly, and implies that SALTO can be more “forgiving” for malpositioning than the other TAR designs. Implant design and alignment are therefore important factors that affect the implant fixation and performance of the reconstructed ankle

Introduction. Malpositioning of the tibial component is a common error in TKR. In theory, placement of the tibial tray could be improved by optimization of its design to more closely match anatomic features of the proximal tibia with the motion axis of the knee joint. However, the inherent variability of tibial anatomy and the size increments required for a non-custom implant system may lead to minimal benefit, despite the increased cost and size of inventory. This study was undertaken to test the hypotheses: . 1. That correct placement of the tibial component is influenced by the design of the implant. 2. The operative experience of the surgeon influences the likelihood of correct placement of contemporary designs of tibial trays. Materials and Methods. CAD models were generated of all sizes of 7 widely used designs of tibial trays, including symmetric (4) and asymmetric (3) designs. Solid models of 10 tibias were selected from a large anatomic collection and verified to ensure that they encompassed the anatomic range of shapes and sizes of Caucasian tibias. Each computer model was resected perpendicular to the canal axis with a posterior slope of 5 degrees at a depth of 5 mm distal to the medial plateau. Fifteen joint surgeons and fourteen experienced trainees individually determined the ideal size and placement of each tray on each resected tibia, corresponding to a total of 2030 implantations. For each implantation we calculated: (i) the rotational alignment of the tray; (ii) its coverage of the resected bony surface, and (iii) the extent of any overhang of the tray beyond the cortical boundary. Differences in the parameters defining the implantations of the surgeons and trainees were evaluated statistically. Results. On average, the tibial tray was placed in 5.5 ± 3.1° of external rotation. The overall incidence of internal rotation was only 4.8%: 10.5% of trainee cases vs. 0.7% of surgeon cases (p < 0.0001). The incidence of internal rotation varied significantly with implant design, ranging from 1.7% to 6.2%. Bony coverage averaged 76.0 ± 4.5%, and was less than 70% in 8.6% of cases. Tibial coverage also varied significantly between designs (73.2 ± 4.3% to 79.2 ± 3.8%; p < .0001). Clinically significant cortical overhang (>1 mm), primarily in the posterior-lateral region, was present in 12.1% of cases, and varied by design, as expressed by the area of the tray overhanging the cortical boundary (min: 2.3 ± 6.7 mm. 2. ; max: 4.7 ± 7.9 mm. 2. ; p < .0001). The surgeons and the trainees also differed in terms of the incidence of sub-optimal tibial coverage (10.0% vs. 14.4%, p < 0.001), and cortical overhang (7.4% vs. 9.7%, p < 0.001). Discussion. 1. Malrotation, bony coverage and cortical overhang are all strongly influenced by the design of the tibial tray selected and the experience of the surgeon. 2. Compared to trainees, experienced surgeons tend to position tibial trays in more external rotation, and with less concern for reduced bony coverage and cortical overhang than trainees. 3. This study supports the hypothesis that improvements in the outcome and reliability of TKR may be achieved through attention to implant design

Background. Finite element (FE) models have become a standard pre-clinical tool to study biomechanics of spine and are used to simulate and evaluate different strategies in scoliosis treatment: examine their efficacy as well as the effect of different implant design parameters. The goal of this study is to investigate, in a system of rods and laminar wires, the effect of the number of wires and their pre-stress on whole spine stiffness. Methods. A generic FE model was developed to represent a full human spine, including vertebrae, intervertebral discs, ligaments, facet and costovertebral joints, and ribcage. Intervertebral discs were modeled with 3D rebar elements with linear elastic material properties. Vertebrae, ribs, sternum, facet joints, cartilage and endplates were modeled with brick elements, and costal muscles with shell elements with linear elastic properties. Furthermore, ligaments were modeled with truss elements with nonlinear hypo-elastic properties. The spine model was instrumented from T7 to T12 with rods and wires modeled as titanium. Nonlinear contact properties were defined for rib neck-vertebra, transverse processes-rib and facet joint sets. The FE model was loaded in flexion and the whole spine instantaneous stiffness was calculated for different wire pre-stressing levels (0.1 to 2 MPa). Similar analyses were performed with changed numbers of wires and whole spine stiffness was calculated. Results. The results show that with increasing the pre-stress level the whole spine instantaneous stiffness increases by up to 6%. Reducing the number of wires decreases the whole spine stiffness almost linearly by 5%. These changes also alter center of rotation of the spine. The results suggest that pre-stressing and number of wires have an effect on whole spine stiffness. Conclusions. In summary, the develop FE model can be used to simulate different treatment strategies and to improve implant designs used in surgical treatment of scoliosis. Level of evidence. FEA study

Introduction. Many prosthetic design changes have been introduced in attempt to improve outcomes following TKA; however there is no consensus as to whether these changes confer benefits to patients. This study aimed to assess whether patients treated with a modern implant design had an enhanced patient outcome compared to a traditional model in a double blind randomised controlled trial. Methods. 212 consecutive patients were prospectively randomised to receive either a modern (Triathlon) or a traditional (Kinemax) TKA (both Stryker Orthopaedics). 6 surgeons at a single unit performed all procedures in a standardised manner. A single researcher, blinded to implant allocation, performed all assessments. Patients were assessed pre-operatively, and at 6, 26, 52 weeks post-surgery with the Oxford Knee Score (OKS), Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score, goniometry, timed functional assessment, lower limb power (Leg Extensor Power Rig) and pain numerical rating scales (NRS). Change in scores and between group differences were assessed with Two-Way Repeated Measures ANOVAs. Results. Both groups benefited from TKA demonstrating significant improvement in all parameters (p = < 0.001 in all cases). Significant interaction of the implant was observed in 1 year change score of OKS (F = 4.28, p = 0.04), knee flexion (F = 8.42, p = 0.004), lower limb power output (F = 11.33, p = 0.001) and maximal pain report (F = 7.44, p = 0.001), the modern implant superior in each case. Larger change scores were observed in WOMAC and timed functional assessments, but were not statistically significant. Conclusions. In a large prospective double blind randomised controlled trial, patients receiving a modern implant design demonstrated a significantly enhanced patient outcome in terms of knee range of motion, lower limb power output, reduced maximal pain report, and a clinically meaningful reduction in the OKS compared to a traditional design

Introduction. The design and manufacture of patient specific implants at Hospital for Special Surgery (HSS) was started in the fall of 1976. The first implant designed and manufactured was an extra large total knee. This effort expanded to include all arthroplasty devices including hips, knees, shoulders and elbows along with fracture fixation devices. In the 1980s, the hospital was designing and manufacturing over 100 custom implants per year. This reduced significantly in the 1990s due to the introduction of modular total knee replacements. In 1996, HSS ceased manufacture due to rising costs and a greater regulatory burden. However, implants are still designed at HSS with manufacturing outsourced to commercial companies. Since 1976, the hospital has designed over 2500 implants. Patient Population. Currently, we design implants for ∼30 cases per year, hips, knees, and upper extremity devices (mainly elbow). We've seen an increase in acetabular revision cases over the last few years and now design about 10 revision acetabular components each year. Regulatory Challenges. Patient specific implants can be provided under a variety of regulatory pathways. Some have received 510(k) clearance as the manufacturer has provided a matrix of sizes; if the device fits within the matrix, the device can be supplied commercially. For devices that fall outside of the 510(k) pathway, the process is more complex. The 2012 FDA Safety and Innovation Act attempts to clarify the ambiguous nature of the prior statute in which the number of custom implants allowed of a specific type was unclear, setting the limit to 5 units per year of a particular device type. However, such a low number has led to rationing by the device manufacturers and even cessation of manufacturing of custom implants by one of the major companies. Another option available for obtaining patient specific implants is the Compassionate Use process. This process has been used, for example, when a patient needs a replacement of a part for a device that is not approved for use in the United States. The surgeon can request an exemption from FDA to use the part. To obtain this exemption, the surgeon must provide to FDA a justification letter explaining why using this device is in the best interests of the patient, a letter from a second surgeon who agrees with this conclusion, a letter from the implant company agreeing to manufacture the device, IRB approval from the institution where the device will be implanted, and consent from the patient. Though gathering this information can be time consuming, once the application is submitted, FDA has rapidly responded in providing exemptions (in less than two weeks). Conclusion. Even with the increase in modular components and the variety of devices in the commercial market, the need still exists for patient specific implants at HSS. The regulatory environment creates additional challenges to obtaining these devices. In spite of these challenges, we are able to provide the devices required for our patients

Introduction. Many uncemented femoral implant designs have had successful outcomes in total hip arthroplasty (THA). Different uncemented stem designs achieve initial and long term stability through shape, size, coating and fit. There is increasing emphasis on bone preservation, particularly in younger and more active patients. The desire to optimize load transfer has led to the development of short stems that seek to achieve fixation in the proximal femur. Short stems designed to achieve stability by engaging the metaphysis or the proximal femoral necks are currently in clinical use. The purpose of this study was to examine the extent to which five stems designed to achieve proximal fixation contact the bone in the proximal femur. Using three-dimensional CT models of 30 femurs, we assessed the fit, fill and contact of each of the five different implants. Methods. Using three-dimensional computerized templating software designed to navigate robotic surgery, pre-operative CT scans of 30 patients were analyzed. Each of five femoral implant designs (TRILOCK, ARC, ABGII, CITATION, ACCOLADE) was then optimized for size and fit based on manufacturer technique guide and design rationale. The proximal femoral metaphysis was divided into four zones in the axial plane. Five contact points were determined on the frontal plane using anatomical landmarks. Each zone was assessed for cortical contact and fill of the bone-implant interface. We graded contact from 1 to 5, with 5 being 100% contact. Results. In the 150 different templates analyzed significant variability existed in contact areas of the proximal femur depending on implant design and femoral morphology. High femoral neck resection design (ARC) had the greatest contact area in the most proximal zones (Figure 1). The ABG II and Trilock stems had comparable contact in the antero-medial zones, while the ABG II had greater fill in the sagittal plane (Figures 2 & 3). The Trilock was the only stem that consistently achieved lateral cortical contact at the distal landmarks. All stems showed a pattern of mostly posteromedial contact proximally and mostly anteromedial distally. Discussion. To our knowledge, this is the first study to examine the contact points of metaphyseal engaging stems in the proximal femur. By directly comparing implant contact points in the same femur we found significant variability in the extent of fit, fill and contact of the metaphysis. These differences in proximal femoral contact are like to have implications for fixation in bone of varying quality and for long term proximal bone remodeling

Purpose: Finite element analysis can be used to assess the behaviour of loaded structures. We used this method to evaluate the influence of glenoid implant design on the behaviour of an osteoarthritic scapula. Material and methods: A 76-year-old female patient scheduled for a shoulder prosthesis underwent preoperative computed tomography of the osteoarthritic shoulder. Two polyethylene implants were evaluated: one with a triangular stem and the same prosthesis with three studs. 3D reconstruction of the glenoid cavity with the implants was then obtained and processed with the finite elements method. Three loadings were applied to the model: centred loading to reproduce the case of an ideally stable prosthesis with a normal tendinomuscular environment and excentred loading to simulate a deficient rotator cuff or prosthesis instability. Results: With centred loading, stress remained low, to the order of 7 MPa, at the stem-glenoid cavity interface. Excentered loading produced peak stress on the borders of the glenoid implants, directly under the loading zone and at the tip of the stem, at the bone-cement interface, reaching 20 MPa. The implant tended to bend in the anteroposterior direction producing strong shear forces on the posterior part of the glenoid cavity. These forces caused micromovement at the cement-bone interface. There was no significant difference between the stem and stud implants. Discussion: Eccentric loading of the glenoid implant appears to have a negative effect on long-term survival, the stress reaching levels greater than the values of cement fatigue fracture. Peak stress was situated on the posterior border of the cement layer due to the small space available between the implant the cortical bone in the posterior part of the osteoarthritic scapula. In this situation, the tip of the stem or the studs tend to come into contact with the posterior cortical of the scapula. When inserting a total shoulder prosthesis, it appears to be more important to keep in mind the geometry and the mechanical properties of the scapula than the implant design

While alumina ceramic-ceramic THA has been performed in the US for more than 12 years, the phenomenon of frequent, clinically reproducible squeaking is relatively new. The current study investigates the influence of implant design on the incidence of squeaking. We reviewed implant information on 1275 consecutive revision THAs performed from 10/2002 through 10/2007 to identify any patients who had complained of squeaking or grinding. We also identified, 2778 consecutive primary ceramicceramic THA. Of these, we reviewed the clinical records of 1,039 patients (37%) to date. Any patient complaint of squeaking or grinding at the time of an office visit or by phone interview was recorded. Hips were divided into group 1: flush mounted ceramic liner; group 2a: recessed ceramic liner mated with a stem made of TiAlV and using a 12/14 neck taper; and group 2b: recessed ceramic liner mated with a stem made of a beta titanium alloy comprised of 12% molybdenum, 6% Zirconium, and 2% Iron and using a neck taper smaller than a 12/14 taper. Of the revision THAs, 5 hips (0.4%) were in patients who had complained of squeaking or grinding. All 5 hips had a recessed, metal-backed ceramic liner and evidence of metallosis. In primary THAs, Group 2b had statistically significantly (p=0.04) more squeaking (7.6%) than group 2a (3.2%) which had statistically significantly (p=0.002) more squeaking than group 1 (0.6%). Squeaking following ceramic-ceramic THA is associated with use of a recessed metal-backed ceramic liner in combination with a femoral component made of a betatitanium alloy and using a relatively small head-neck taper. Since all revised hips in our study had metallosis, it is possible that metal debris is adversely affecting the bearing and that the elevated metal rim combined with a small head neck taper and the beta-titanium alloy contribute to this problem. Use of bearings with a flush-mounted ceramic liner mated with femoral components made of TiAlV and using a 12/14 taper appears to be prudent

Abstract. Objectives. Currently, total hip replacement surgery is an effective treatment for osteoarthritis, where the damaged hip joint is replaced with an artificial joint. Stress shielding is a mechanical phenomenon that refers to the reduction of bone density as a result of altered stresses acting on the host bone. Due to solid metallic nature and high stiffness of the current orthopaedic prostheses, surrounding bones undergo too much bone resorption secondary to stress shielding. With the use of 3D printing technology such as selective laser melting (SLM), it is now possible to produce porous graded microstructure hip stems to mimics the surrounding bone tissue properties. Method. In this study we have compared the physical and mechanical properties of two triply periodic minimal surface (TPMS) lattice structure namely gyroid and diamond TPMS. Based on initial investigations, it was decided to design, and 3D print the gyroid and diamond scaffolds having pore size of 800 and 1100 um respectively. Scaffold of each type of structure were manufactured and were tested mechanically in compression (n=8), tension (n=5) and bending (n=1). Results. Upon FEA validation of the scaffold in Abaqus, the desired scaffold for hip implant application was evaluated to have a young's modules of 12.15 GPa, yield strength of 242 MPa and porosity of 55%. Topology and lattice optimization were performed using nTopology to design an optimised graded porous hip implant based on stress shielding reduction. It was understood that the designed optimised hip implant can reduce the stress shielding effect by more than 65% when compared to the conventional generic implant. Conclusions. The designed hip implant presented in this work shows clinical promise in reducing bone loss while having enhanced osseointegration with the surrounding cortical bones. Hence, this will help reduce the risk of periprosthetic fracture and the probability of revision surgery

High flexion designs are intended to provide a greater range of knee flexion and possibly improve flexion in stiff knees. This study assessed the effects of two implant designs. A posterior stabilised high flexion mobile bearing (MB) design vs a cruciate retaining standard fixed bearing (FB) design. The aim of this study was to assess whether implant design has an effect on the functional outcome one year after total knee arthroplasty (TKA). Methods. Ninety patients with knee osteoarthritis on the waiting list for unilateral TKA were recruited and randomly allocated to either the MB or FB group. Patients were assessed between one and four weeks before, and one year after TKA. Primary outcome was knee flexion during high flexion activities of daily living such as stair ascending and descending and squatting as measured using gait analysis. Knee flexion in long sitting using a manual goniometer and the WOMAC were also recorded. Two sample t-tests were used to investigate statistical differences between the two groups pre- and postoperatively. Results. Average age was 69 years. Thirty-three received the MB design and 39 the FB design. Age, gender balance and pre-operative flexion (112 and 113 degrees in the FB and MB groups respectively) were the same in both groups. There were no statistically significant differences in post-operative knee flexion during functional activities. Knee flexion in sitting and the stiffness and function components of the WOMAC were also similar between the two groups (p>0.05). However, post-operatively the WOMAC pain component was slightly higher in the MB group (4.2 vs 2.4 points, p<0.05). Conclusion. In our patient group with a mean pre-operative flexion of 112.7 degrees, the high flexion mobile bearing design did not improve knee flexion during high flexion functional activities of daily living