Susannah Clarke

Evaluating a suitable level of model complexity for finite element analysis of the intact acetabulum

To enable large-scale multi-factorial finite element (FE) studies, the FE models used must be as computationally efficient as is feasible, while maintaining a suitable level of definition. The present study seeks to find an optimum level of model complexity for use in such large-scale studies by investigating which model attributes are most influential over the chosen model outputs of principal stress and strain in the intact acetabulum. A multi-factorial sensitivity study was carried out using 128 FE models, representing combinations of the following variables: bone stiffness distribution, imposed muscle loading, boundary condition location, hip joint contact conditions and patients bone anatomy. The relative sensitivity of each input factor was analysed, and it was concluded that the optimum level of model definition must include CT-dependent trabecular bone properties and a sliding interface at the hip joint. It was found that it was not essential to describe the ligamentous sacroiliac and pubic symphysis joints; these could be rigidly fixed in space; and for the normal walking load case, muscle forces may be neglected. It was also concluded that a variety of bone anatomies should be included in a multi-factorial analysis if results are to be inferred for a wider population.

Unicompartmental knee arthroplasties: Robot vs. patient specific instrumentation

BACKGROUND The technical reliability demonstrated by semi active robots in implant placement could render unicompartmental knee arthroplasties (UKAs) more favourable than they are currently. The relatively untested method using patient specific instrumentation (PSI), however, has the potential to match the accuracy produced by robots but without the barriers that have prevented them from being used more widely in clinical practice, namely operative time. Therefore this study took a step towards comparing the accuracy and time taken between the two technologies. METHODS Thirty-six UKAs were carried out on identical knee models, 12 with the Sculptor, 12 with PSI and 12 conventionally under timed conditions. Implant placement in these knees was then judged against that in a pre-operative plan. RESULTS Tibial implant orientations and femoral implant positions and orientations were significantly more accurate in the PSI group with mean errors of 6°, 2 mm and 4° respectively, than the conventional group which had means of 9°, 4 mm and 10°. There was no significant difference between the robot and PSI generally except in tibial implant orientation (mean robotic error 3°) and tibial implant position did not vary significantly across all three groups. It was also found that use of PSI and conventional methods took half the time taken by the robot (p<0.001). CONCLUSIONS With further development, PSI can match and possibly surpass the accuracy of the robot, as it does with the conventional method, and achieve planned surgery in less time. CLINICAL RELEVANCE This work sets the foundation for clinical trials involving PSI.

A hierarchy of computationally derived surgical and patient influences on metal on metal press-fit acetabular cup failure.

The impact of anatomical variation and surgical error on excessive wear and loosening of the acetabular component of large diameter metal-on-metal hip arthroplasties was measured using a multi-factorial analysis through 112 different simulations. Each surgical scenario was subject to eight different daily loading activities using finite element analysis. Excessive wear appears to be predominantly dependent on cup orientation, with inclination error having a higher influence than version error, according to the study findings. Acetabular cup loosening, as inferred from initial implant stability, appears to depend predominantly on factors concerning the area of cup-bone contact, specifically the level of cup seating achieved and the individual patients anatomy. The extent of press fit obtained at time of surgery did not appear to influence either mechanism of failure in this study.

Validation of FE micromotions and strains around a press-fit cup: introducing a new micromotion measuring technique.

Finite element (FE) analysis provides an useful tool with which to analyze the potential performance of implantations in a variety of surgical, patient and design scenarios. To enable the use of FE analysis in the investigation of such implants, models must be experimentally validated. Validation of a pelvic model with an implanted press-fit cup in terms of micromotion and strain is presented here. A new method of micromotion has been introduced to better describe the overall movement of the cup within the pelvis. The method uses a digitizing arm to monitor the relative movement between markers on the cup and the surrounding acetabulum. FE analysis was used to replicate an experimental set up using a synthetic hemi-pelvis with a press-fitted all-metal cup, subject to the maximum loading observed during normal walking. The work presented here has confirmed the ability of FE models to accurately describe the mechanical performance of the press-fitted acetabulum and surrounding bone under typical loading conditions in terms of micromotion and strain distribution, but has demonstrated limitations in its ability to predict numerical micromotion values. A promising digitizing technique for measuring acetabular micromotions has also been introduced.

User Involvement and User Data: A Framework to Help Designers to Select Appropriate Methods

Many methods have been developed and adapted to help designers to understand, empathise with, and quantify users’ situations, through both direct user involvement and more indirect use of user data. These methods vary widely, with different goals and suited for use in different situations. However, designers often find it difficult to select the most appropriate for their needs, often leading to inappropriate method use. We therefore propose a framework to help designers to make informed decisions about methods. The framework identifies the key information needs of designers in making these decisions, based on observations, interviews, card-sorting studies and a literature review. We further discuss how the framework may be populated, giving an example and discussing key issues.

Osteotomies: Advanced and Complex Techniques

We started performing precise surgery based upon CT plans in the last century – the first embodiment of this approach was a robotic assistant built for total knee replacement, the “Acrobot” [1]. Abundant evidence now exists to confirm that assistive technologies enable surgeons to achieve their preoperative goals [2]. The concept of planned surgery is therefore not novel. Patient-matched instruments share several key elements with the robotic platform, and these formed the basis of this current project. The essential elements include image segmentation, planning, and registration. We applied the know-how of these dimensions to design and build patient-matched guides for a range of tasks using biocompatible polymer 3D printers. Having established a workflow for arthroplasty, the adaptation of the same principles to osteotomy was a short step, requiring software to be developed to deliver semiautomated useful information regarding limb segment alignment and the shapes of bones.

3D printing and unicompartmental knee arthroplasty

In suitable patients, unicompartmental knee arthroplasty (UKA) offers a number of advantages compared with total knee arthroplasty. However, the procedure is technically demanding, with a small tolerance for error. Assistive technology has the potential to improve the accuracy of implant positioning. This review paper describes the concept of detailed UKA planning in 3D, and the 3D printing technology that enables a plan to be delivered intraoperatively using patient-specific instrumentation (PSI). The varying guide designs that enable accurate registration are discussed and described. The system accuracy is reported. Future studies need to ascertain whether accuracy for low-volume surgeons can be delivered in the operating theatre using PSI, and reflected in improved patient reported outcome measures, and lower revision rates. Cite this article: EFORT Open Rev 2018;3 DOI: 10.1302/2058-5241.3.180001

3D printing and high tibial osteotomy

High tibial osteotomy (HTO) is a relatively conservative surgical option in the management of medial knee pain. Thus far, the outcomes have been variable, and apparently worse than the arthroplasty alternatives when judged using conventional metrics, owing in large part to uncertainty around the extent of the correction planned and achieved. This review paper introduces the concept of detailed 3D planning of the procedure, and describes the 3D printing technology that enables the plan to be performed. The different ways that the osteotomy can be undertaken, and the varying guide designs that enable accurate registration are discussed and described. The system accuracy is reported. In keeping with other assistive technologies, 3D printing enables the surgeon to achieve a preoperative plan with a degree of accuracy that is not possible using conventional instruments. With the advent of low dose CT, it has been possible to confirm that the procedure has been undertaken accurately too. HTO is the ‘ultimate’ personal intervention: the amount of correction needed for optimal offloading is not yet completely understood. For the athletic person with early medial joint line overload who still runs and enjoys life, HTO using 3D printing is an attractive option. The clinical effectiveness remains unproven. Cite this article: EFORT Open Rev 2018;3 DOI: 10.1302/2058-5241.3.170075.

Do patient-specific instruments (PSI) for UKA allow non-expert surgeons to achieve the same saw cut accuracy as expert surgeons?

IntroductionHigh-volume unicompartmental knee arthroplasty (UKA) surgeons have lower revision rates, in part due to improved intra-operative component alignment. This study set out to determine whether PSI might allow non-expert surgeons to achieve the same level of accuracy as expert surgeons.Materials and methodsThirty-four surgical trainees with no prior experience of UKA, and four high-volume UKA surgeons were asked to perform the tibial saw cuts for a medial UKA in a sawbone model using both conventional and patient-specific instrumentation (PSI) with the aim of achieving a specified pre-operative plan. Half the participants in each group started with conventional instrumentation, and half with PSI. CT scans of the 76 cut sawbones were then segmented and reliably orientated in space, before saw cut position in the sagittal, coronal and axial planes was measured, and compared to the pre-operative plan.ResultsThe compound error (absolute error in the coronal, sagittal and axial planes combined) for experts using conventional instruments was significantly less than that of the trainees (11.6°±4.0° v 7.7° ±2.3º, p = 0.029). PSI improved trainee accuracy to the same level as experts using conventional instruments (compound error 5.5° ±3.4º v 7.7° ±2.3º, p = 0.396) and patient-specific instruments (compound error 5.5° ±3.4º v 7.3° ±4.1º, p = 0.3). PSI did not improve the accuracy of high-volume surgeons (p = 0.3).ConclusionsIn a sawbone model, PSI allowed inexperienced surgeons to achieve more accurate saw cuts, equivalent to expert surgeons, and thus has the potential to reduce revision rates. The next test will be to determine whether these results can be replicated in a clinical trial.

Novel curved surface preparation technique for knee resurfacing

Conventional tools are incapable of preparing the curved articular surface geometry required during cartilage repair procedures. A novel curved surface preparation technique was proposed and tested to provide an accurate low-cost solution. Three shapes of samples, with flat, 30 mm radius and 60 mm radius surfaces, were manufactured from foam bone substitute for testing. Registering guides and cutting guides were designed and 3-D printed to fit onto the foam samples. A rotational cutting tool with an adapter was used to prepare the surfaces following the guidance slots in the cutting guides. The accuracies of the positions and shapes of the prepared cavities were measured using a digital calliper, and the surface depth accuracy was measured using a 3-D scanner. The mean shape and position errors were both approximately ± 0.5 mm and the mean surface depth error ranged from 0 to 0.3 mm, range - 0.3 to + 0.45 mm 95% CI. This study showed that the technique was able to prepare a curved surface accurately; with some modification it can be used to prepare the knee surface for cartilage repair.