Digital radiographic preoperative planning and postoperative monitoring of total hip replacements The, Bertram

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Publication date: 2006 Link to publication in University of Groningen/UMCG research database

Citation for published version (APA): The, B. (2006). Digital radiographic preoperative planning and postoperative monitoring of total hip replacements: techniques, validation and implementation0 s.n.

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General Discussion

Chapter 8 - General discussion

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Chapter 8 - General discussion

General discussion The aim of this thesis was to create a scientific foundation for studying the possibilities of preoperative planning and postoperative measurements on total hip replacements. A common theme in all our studies was the possibility to implement the results in daily practice of orthopedic surgeons (in both specialized and nonspecialized centres). Therefore, the measurements were performed on a standard PC and commonly available radiographs. We have made the choice to perform all studies using digital radiographs (either digitized or direct) in the light of the current trend towards digitalization of radiological departments throughout the medical community. A survey in the Netherlands estimated the percentage of hospitals using a PACS at 37% in 2003 1, while a survey in 2005 even estimated that 80% of the hospitals in the USA with more than 500 beds were using a PACS system.2 Using digital equipment has given us the opportunity to investigate more possibilities of preoperative planning and postoperative measurements, which are unpractical or unfeasible without digital tools. Most currently available software packages, including the ones we have used, (HyperOrtho, OrthoView, Q Bone Planner, MediCAD) are still in development or early versions, and few have been scientifically assessed. For this reason orthopedic software development should at least partly be science-driven. Vice-versa, scientific research in this field should be directed in such a way that the final result is implementable in practical solutions, such as improvement of software applications. Using digital environments that were still in an early phase of development has resulted in less desirable research conditions. At the start of the study the University Medical Centre Groningen did not have acces to direct digital radiographs, which forced us to work with digitized x-ray films. Theoretically, a loss of image quality can be expected, and we have tried to minimize this effect by using the highest resolution possible without overloading the capacity of the available workstations. A drawback of working with experimental software was the weak user interface, thus leading to a low level of user-friendliness. This has had implications on the practical execution of at least one study (Chapter 4). Fortunately, we were able to replicate the study in the final phase of this research project with most of the software problems solved (Chapter 5). An intuitive endpoint in studies concerning optimizing preplanning and development of predictive measurements on THR is survival of the implant. Because of obvious time limitations of the studies in this particular project, we could not study survival rates. We have thus used early endpoints such as correct preoperative prediction of implanted component sizes, direct postoperative evaluations (leg length discrepancies, positioning of implants), and focussed on optimization of measurement of wear rates which have a close relation to failure of THRs. 3-7 We have dedicated two studies to preoperative planning of THRs (Chapters 4 and 5). Although the first study showed comparable results for digital and analogue planning,

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it was deemed possible that a source of bias had influenced the digital plannings in a negative way. A second, randomized clinical trial on the same topic showed trends in favour of digital planning (and in any case no adverse effects). The difference in results with preoperative planning of TKRs was striking: Accuracy of digital planning of TKR was superior in comparison with analogue planning. The fact that THR planning was less successful in predicting the component sizes might be explained by several factors. A first possibility is a difference in prosthetic determinants. The differences in size between components was greater for TKRs than for THRs, which made the distinction between well or ill-fitting components more clear for TKRs. Although this determinant is capable of affecting the measured accuracy of preoperative planning, it does not cause the decrease in accuracy itself. It merely potentiates or weakens true causes of inaccuracy: using small differences in sizes will enable the detection of small problems with accuracy. So, this determinant is of no concern to research aimed at increasing accuracy of preoperative planning. It should merely be kept in mind, when comparing figures of accuracy concerning different types of prostheses. A second possibility is a difference in functional anatomic determinants. More specifically, the femoral anteversion of the femur is an extra factor to be taken into account in THRs. It is attempted to be neutralized in the standardized pelvic radiographs, but might still account for some of the difference in accuracy between preoperative planning of TKRs and THRs. Since it is not clear if this is really a relevant source of errors, future studies should be aimed at quantifying its role. The third possibility is a difference in radiographic determinants. Correct positioning of the calibration object is less of a problem in knee radiographs than pelvic radiographs. This was considered a potentially important factor and led us to direct our research toward a fundamental issue: magnification factor of radiographic projections of the hip joint for preoperative planning. Following common practice had seemed to be a reasonable starting point, since other scientific papers 8;9, expert opinion 10, and logical reasoning seemed to form a foundation for it. This assumption proved to be false, and we decided to explore other possibilities to enhance the accuracy of correcting for the magnification factor (Chapter 2 and 3). We have focused on enhancing wear measurement methods only of all-polyethylene cups since the all-polyethylene tool was the only available wear measurement tool during the project. This has led to a scientific challenge of a different kind than enhancement of measurements on metal backed cups: The problem to be solved with metal backed cups is detection of the metal shell and reproducible conversion into a reference for wear measurements. This is an issue of reliability and can for the most part be solved by software adjustments as others have shown. 11 With all-polyethylene cups, the problem is validity: projection differences potentially result in a distortion of the relationship between reference markers used for wear measurements. A mathematical solution for this problem was developed and proved to be successful (Chapter 6 and 7). It has resulted in a two-dimensional wear measurement method

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which is no longer as sensitive to differences in two-dimensional projection of the hip joint as conventional measurements. The basic idea which was the foundation for our approach will hopefully be valuable for constructing solutions for other measurements.

Future research 1 The question remains if preplanning to obtain an optimal fit of the prosthetic components as we have done so far, should not be replaced by planning with the primary aim to biomechanically optimize the geometrical relations of the hip joint for favourable dynamic loads (i.e. seeking for a combination of lever arms in the artificial hip joint that lead to low hip joint contact forces). The choice to first explore “optimal fit” planning had two reasons: The first reason is that the current way of preoperative planning mostly resembles “optimal fit” planning. Sizing of the prosthetic components is primarily determined by the shape of the acetabulum and proximal femoral medullary canal, which puts optimization of the dynamic biomechanical relations – such as constructing the most favourable lever arms – of the hip joint to second place. This seems to be a wise choice since analogue preoperative planning lacked the accuracy to justify more rigorous primarily biomechanically based reconstructions. After all, most orthopedic surgeons who engage in preoperative planning value it mostly because they regard it as an essential pre-theatre preparation of surgery to prevent intra-operative surprises, and not because of exact prediction of outcome. The second reasons is that – even if accuracy of preoperative planning is substantially increased – the choice for “optimal fit” planning makes sense. Seeking to optimize the bone-implant interface by choosing the best fitting implant with the least destruction of bony structures is thought to be desirable, and correlates with good long-term follow-up results.12;13 On the other hand, it can be reasoned that strength and quality of the interface are just one factor in a multifactorial mechanism which determines the success or failure of an implant. Favourable biomechanics of the artificial joint might be just as important or even more important. It could reduce the forces acting upon the interfaces and reduce the velocity of wearing out of the cup. Despite the attractiveness of this approach, it remains to be clarified what is actually favourable and what is unfavourable? Should the orthopedic surgeon try to restore the patients’ own anatomy, or perhaps try to copy the geometric relations of the contralateral side 10;14, or is the fact that the patient already has developed arthrosis of the hip an indication that his own anatomy should not serve as a reference? Can the orthopedic surgeon use a set of rules of thumb, such as medialization of the cup, preventing cranialisation of the centre of rotation, and restoration of the femoral offset 15-21, or should he be using individualized modelling techniques to define the optimal balance of several parameters in each patient?22-24 And if so, how can widely implementable two-dimensional biomechanical plans be extracted from the complex three-dimensional reality? With this approach the

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preoperative planning seems to evolve more and more into an exact science, although the ability of the orthopedic surgeon to translate this exact science into exact results remains an uncertain factor. Surgery with the aid of navigation systems aim to solve this 25;26, but before we know the answer to the questions posed above, how do we know in which direction we should navigate?

Future research 2. While the accuracy of preoperative planning increases, the question arises if they could be used for the purpose of stock control in order to reduce the financial burden for hospitals and society of the growing stocks.27;28 The main purpose would then be to accurately predict the type of implant (for example lateralized versus non-lateralized stems) and component sizes which are needed. Cost reduction through this route is dependent on several factors. On the level of the manufacturer it depends on the percentage of clients which participate in standard preplanning of components. On the level of the hospital it depends on the type of agreement on implant supply between manufacturer and clinic and indirect costs such as inventory space and insurance policies, as well as the price reduction which the manufacturer is willing to offer for participation in a preplanning programme. In conclusion, the study question that is to be answered is: how much cost reduction at the various levels can be achieved by implementation of different stock control strategies?

Future research 3. It would be valuable to enhance the reliability and validity of other measurements than wear of all-polyethylene cups. A necessary next step would be to explore the possibilities of wear measurements on metal backed cups. In these cups the validity of the measurements is not as much an issue as in all-polyethylene cups, but the reliability of the measurements certainly is. Automatic edge detection algorithms might prove to be valuable in enhancing the reliability of the results.29 Other measurements which deserve the attention are cup migration, stem subsidence and osteolysis measurements, since they also are related to long term success of the THR.30-38 Of these, the migration and subsidence measurements might be approached using similar models as have been used in our studies for enhancing validity of wear measurements, since the main threat to validity is again posed by radiographic two-dimensional projection differences. Enabling digital osteolysis measurements using conventional radiographs is probably the most challenging of all, but may be a solution to the low reliability of human visual assessments.39 First of all, only part of the osteolysis can be visualized on a twodimensional projection.40 Secondly, differences in intensity of radiation and radiation absorbing tissues would probably force us to implement a new kind of calibration

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tool. With the extension of reliable and valid measurements it might be possible to construct more accurate prediction and diagnostic models for survival or failure of an implant, which is of interest to both the scientific community, and to the clinician involved in individual patient care.

Conclusions 1. We can correct for the magnification factor of pelvic x-rays with good accuracy (Chapters 2 and 3). We have reduced the width of range of errors from 16% to 6% using the most recent study results, although we think there still is room for improvement. 2. Digital preoperative planning compared slightly favourable to analogue planning (Chapters 4 and 5). The differences were only small, but it should be kept in mind that these results have been obtained using the first calibration protocol, which had a 95% margin of error of 6% to 12%. 3. Two-dimensional wear measurements on all-polyethylene cups can mathematically be enhanced (Chapters 6 and 7). Using the algorithm which was developed during this study alters conventional linear wear measurements in such a way that they are no longer greatly affected by projectional differences of the THR at reference and follow-up radiographs.

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Reference List 1. 2. 3. 4. 5. 6.

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