Abstract
Objective
Navigated computer-assisted total knee arthroplasty (TKA) shows inconclusive mid- to long-term outcome results and is limited by increased costs, surgery-time and an additional learning curve. We introduced a treatment algorithm preserving computer-assisted TKA for patients with adipositas-per-magna, posttraumatic leg-deformities, osteosynthetic material in-situ or reduced preoperative X-ray quality.
Methods
237 primary unilateral TKA were allocated based on the treatment concept described above. A retrospective pre- and postoperative radiological analysis was performed.
Results
222 TKA (93.7%) were within 3° varus/valgus of mechanical-lower-limb axis (mean absolute deviation: 1.8° ± 1.3°).
Conclusion
This algorithm showed an excellent postoperative implantation-accuracy based on an accurate preoperative surgery-planning.
Keywords: TKA, Navigation, Treatment algorithm, Radiological analysis, Computer-assisted TKA
1. Introduction
Computer-assisted navigation systems for total knee arthroplasty (TKA) were introduced mainly to improve implantation accuracy and ligament balancing management. A better clinical and functional outcome with a higher long-term survival rate was expected. Over the past five to ten years, several studies were published comparing navigated computer-assisted TKA with the conventional non-navigated technique.1, 2, 3, 4, 5 Many authors verified an improved implantation accuracy when using the navigation technique.1, 3, 4, 6 This was even confirmed by recent published meta-analyses.7, 8, 9, 10 However, there are also several articles available reporting no improved reconstruction of mechanical lower limb axis when using computer-assisted navigation systems.11, 12, 13 Although a maximum deviation of 3° varus or valgus was recognized as fundamental for long-term survival,14, 15, 16 recent published articles verified that there is no direct influence of exact reconstructed mechanical axis within 3° varus/valgus on TKA long-term survival rate.17, 18
In addition, there are only few trials available comparing navigated with conventional TKA with a postoperative follow-up term of 5 to 10 years or more.5, 19, 20, 21, 22, 23 Kim et al. 22 reported no difference in implant survival rate after a mean follow-up term of 10.8 years. Similar results were reported by Hernandez-Vaquero et al. 21 after a mean postoperative follow-up of 8.3 years. Both authors verified no significant differences in femoro-tibial angle alignment between navigated and conventional TKA group.21, 22 Lützner et al. 20 showed no clinical benefit for computer-assisted technique in terms of functional or patient perceived outcome after 5 years postoperatively. Similar results were published by Harvie et al. 19 . Hoffart et al. 23 revealed a better Knee Society Score for navigated TKA after 5 years. In a randomized controlled trial Cip et al. obtained a significant improved lower limb axis with partially better score results for the navigated cohort.5 However, no benefit on implant survival rate after a minimum postoperative follow-up of 5 years was found.5
Due to obvious limitations of the navigated computer-assisted technique (e.g. additional learning curve,24, 25 additional costs,26 an enhanced surgery time22) and the inconclusive study results published so far, the benefit of navigation systems for TKA is still not clear. Hence, a treatment algorithm was introduced in our institution where the computer-assisted technique was only used for primary TKA in case of adipositas per magna, posttraumatic leg deformities, osteosynthetic material in situ or reduced preoperative radiological X-ray quality due to flexion contracture or rotational disorders. In all other cases, the conventional non-navigated technique was used for primary TKA.
In the current study, the main aim was to evaluate, whether optimal TKA alignment was achieved even when the navigated technique was preserved for the described indications above. In addition, we also compared computer-assisted navigated TKA (navigated: group NAV) with the conventional non-navigated TKA (regular: group REG) to reveal differences in postoperative implantation accuracy.
2. Methods
This study represents a retrospective radiological analysis of all primary TKA implanted consecutively within 6 years (July 2005–June 2011). In order to address influences by any learning curve, all procedures were performed by one single surgeon (AM) who was well-experienced in both conventional and navigated computer-assisted technique. Inclusion criteria for this trial were defined as follows: primary, unilateral TKA; only non-constraint TKA; procedure performed by one single surgeon (AM). Patients with a bilateral or constraint TKA and all revision procedures as well as subjects with missing pre- or postoperative long-leg weight bearing radiographs were excluded for this analysis. Overall, 256 primary non-constraint unilateral TKA were performed within July 2005 to June 2011. 19 patients were excluded due to missing pre- and/or postoperative radiologic data. Hence, we were able to include 237 TKAs (92.6%) for this retrospective data analysis. This study was approved by the institutional review board.
All implants were non-constraint NexGen® Legacy Posterior Stabilized Flex mobile or fixed bearing knees (LPS Flex; Zimmer Inc., IL, USA). All surgeries were performed with a tibia first technique. For the alignment of the tibial tray an extramedullary instrument system was used. An intramedullary technique was used for implantation of the femoral component. Due to our treatment algorithm, navigated computer-assisted TKA was preserved for patients with adipositas per magna (BMI ≥ 40), posttraumatic leg deformities, osteosynthetic material in situ (e.g. femur nail), a reduced preoperative X-ray quality in case of flexion contracture of the knee or rotational disorders.
The CT-free VectorVision® knee navigation system (BrainLAB, Munich, Germany) was used for computer-assisted TKA. In case of a severe valgus deformity (>10°) in conjunction with a contract lateral collateral ligament situation a lateral surgical approach was performed.27 Baseline characteristics at time of primary TKA were obtained which included gender (female, male), age (years), body mass index (BMI, kg/m2), side of implantation (right, left) and the American Association of Anesthesia (ASA) Classification (Table 1).28
Table 1.
Demographic Data.
| Parameter | Overall (n = 237) | Navigated Group (n = 60) |
Conventional Group (n = 177) |
p-value | |
|---|---|---|---|---|---|
| Gender (n) | female | 155 (65.4%) | 46 (76.7%) | 109 (61.6%) | p = 0.041* |
| male | 82 (34.6%) | 14 (23.3%) | 68 (38.4%) | ||
| Side of TKA (n) | right | 124 (52.3%) | 33 (55.0%) | 91 (51.4%) | p = 0.656 |
| left | 113 (47.7%) | 27 (45.0%) | 86 (48.6%) | ||
| ASA Classification (n) | ASA I | 42 (17.7%) | 5 (8.3%) | 37 (20.9%) | p = 0.158 |
| ASA II | 165 (69.6%) | 48 (80.0%) | 117 (66.1%) | ||
| ASA III | 30 (12.7%) | 7 (11.7%) | 23 (13.0%) | ||
| Age (years) | 69.3 ± 8.8 (40.9–87.3) | 67.4 ± 8.6 (46.2–85.3) | 70.0 ± 8.8 (40.9–87.3) | p = 0.055 | |
| Body Mass Index (kg/m2) | 30.6 ± 6.4 (19.7–56.9) | 34.8 ± 8.3 (21.8–56.9) | 29.2 ± 4.8 (19.7–44.3) | p < 0.001* | |
Values are presented as mean ± standard deviation (minimum–maximum).
* = significant different; ASA = American Society of Anesthesiologists; TKA = Total Knee Arthroplasty
For radiological investigation conventional X-rays of TKA were taken in anterior-posterior (including a long leg weight-bearing X-ray), lateral and axial view. Radiographs taken 6 months pre- and at least 3 months postoperatively were included for radiological analysis. If long-leg weight bearing X-rays were missing patients were excluded for this report. Preoperative radiological data is listed in Table 2.
Table 2.
Preoperative Radiologic Data .
| Parameter | Overall (n = 237) | Navigated Group (n = 60) |
Conventional Group (n = 177) | p-value |
|---|---|---|---|---|
| Relative deviation of neutral mechanical limb axis (- val; + = var) | ||||
| mean ± SD | 5.4 ± 6.9 | 6.6 ± 6.7 | 4.9 ± 6.9 | p = 0.103 |
| max. valgus; max. varus | −16.0; 26.0 | −11.0; 24.0 | −16.0; 26.0 | |
| Absolute deviation of neutral mechanical limb axis | ||||
| mean ± SD | 7.3 ± 4.7 | 7.7 ± 5.3 | 7.2 ± 4.4 | p = 0.484 |
| min.; max. | 0.0; 26.0 | 0.0; 24.0 | 0.0; 26.0 | |
| neutral | 6 (2.5%) | 2 (3.3%) | 4 (2.3%) | p = 0.858 |
| varus | 188 (79.3%) | 48 (80.0%) | 140 (79.1%) | |
| valgus | 43 (18.1%) | 10 (16.7%) | 33 (18.6%) | |
We measured the mechanical lower limb axis (mechanical femorotibial angle), lateral distal femoral angle (LDFA) and medial proximal tibial angle (MPTA) in frontal plane.5, 29 In addition the tibial slope in sagittal plane and patella alpha (α) tilt angle in 45° flexion in transversal view was observed.30, 31
A mechanical lower limb axis within 3° varus or valgus deformity was defined as optimum.14, 15, 16 Tibial slope of 7° (within 4°–10°) respectively 83° (86–80°) in relation to mechanical axis of the tibia was considered as reference value. For patella alpha angle a positive angle was declared as normal, 0° or negative angles were considered as subluxation or luxation of the patella.30, 31 Data was also compared within conventional and computer-assisted navigated technique.
For statistical analysis SPSS software (Version 17, IL, USA) was used. To assume normal distributed data the Kolmogorov-Smirnoff test was performed. If data was normal distributed Levene‘s test followed by an independent t-test was used. Skewed and categorical data was compared using Mann-Whitney U test. Fisher‘s exact test or Chi-square test was performed comparing nominal data. Intra- and interobserver reliability was calculated using the Intraclass Correlation Coefficient (ICC, two-way mixed model).
3. Results
Overall 222 (93.7%) of 237 TKA were within an optimal mechanical lower limb axis with a maximum deviation of 3° varus or valgus. Considering a maximum deviation of 4° varus/valgus, 227 (95.8%) of 237 TKA were within this range. The absolute deviation from neutral mechanical lower limb axis was 1.8° ± 1.3° (0–8°). Accordingly, the relative deviation was 0.7° ± 2.1° with a maximum valgus deviation of 5.0° and a maximum varus deviation of 8.0°.
Mean postoperative LDFA was 90.7° ± 1.5° (86–96°) with an absolute deviation of 1.2° ± 1.1° (0–6°) from 90° LDFA. 230 (97.0%) of 237 TKA were within 3° varus/valgus deformity and 235 (99.2%) within 4° varus/valgus deviation. For MPTA a mean postoperative value of 89.9° ± 1.7° (81–96°) was obtained with a mean absolute deviation from 90° MPTA of 1.2° ± 1.2° (0–9°). 230 (97.0%) of 237 TKA were within 3° varus/valgus inaccuracy and 234 (98.7%) were within 4° varus/valgus deviation.
Postoperative tibial slope evaluation revealed a mean value of 6.3° ± 2.0° (2–12°) with an absolute deviation of 1.7° ± 1.4° (0–5°). For patella alpha angle a mean value of 10.0° ± 3.9° (1–22°) was found. (Table 3)
Table 3.
Postoperative Radiologic Data
| Parameter | Overall Group (n = 237) | Navigated Group (n = 60) |
Conventional Group (n = 177) |
p-Values |
|---|---|---|---|---|
| Postoperative mechanical lower limb axis | ||||
| within 3° varus/valgus | 222 (93.7%) | 57 (95.0%) | 165 (93.2%) | p = 0.766 |
| > 3° varus/valgus | 15 (6.3%) | 3 (5.0%) | 12 (6.8%) | |
| within 4° varus/valgus | 227 (95.8%) | 58 (96.7%) | 169 (95.5%) | p > 0.999 |
| > 4° varus/valgus | 10 (4.2%) | 2 (3.3%) | 8 (4.5%) | |
| neutral | 32 (13.5%) | 8 (13.3%) | 24 (13.6%) | p = 0.948 |
| varus | 130 (54.9%) | 32 (53.3%) | 98 (55.4%) | |
| valgus | 43 (31.6%) | 20 (33.3%) | 55 (31.1%) | |
| Relative deviation of neutral mechanical lower limb axis (- val; + = var;) |
0.7 ± 2.1 (−5 to 8) | 0.4 ± 2.2 (−5 to 8) | 0.8 ± 2.0 (−5 to 7) | p = 0.157 |
| Absolute deviation of neutral mechanical lower limb axis |
1.8 ± 1.3 (0–8) | 1.8 ± 1.4 (0–8) | 1.8 ± 1.3 (0–7) | p = 0.950 |
| Lateral Distal Femoral Angle (LDFA) | 90.7 ± 1.5 (86–96) | 90.4 ± 1.6 (87–95) | 90.8 ± 1.5 (86–96) | p = 0.053 |
| within 3° varus/valgus | 230 (97.0%) | 58 (96.7%) | 172 (97.2%) | p > 0.999 |
| > 3° varus/valgus | 7 (3.0%) | 2 (3.3%) | 5 (2.8%) | |
| within 4° varus/valgus | 235 (99.2%) | 59 (98.3%) | 176 (99.4%) | p = 0.443 |
| > 4° varus/valgus | 2 (0.8%) | 1 (1.7%) | 1 (0.6%) | |
| neutral | 65 (27.4%) | 18 (30.0%) | 47 (26.6%) | p = 0.425 |
| varus | 127 (53.6%) | 28 (46.7%) | 99 (55.9%) | |
| valgus | 45 (19.0%) | 14 (23.3%) | 31 (17.5%) | |
| Relative deviation of 90° LDFA (− = val; + = var) | 0.7 ± 1.5 (−4 to 6) | 0.4 ± 1.6 (−3 to 5) | 0.8 ± 1.5 (−4 to 6) | p = 0.053 |
| Absolute deviation of 90° LDFA | 1.2 ± 1.1 (0–6) | 1.2 ± 1.1 (0–5) | 1.3 ± 1.1 (0–6) | p = 0.637 |
| Medial Proximal Tibial Angle (MPTA) | 89.9 ± 1.7 (81–96) | 90.0 ± 1.6 (87–96) | 89.9 ± 1.8 (81–94) | p = 0.737 |
| within 3° varus/valgus | 230 (97.0%) | 59 (98.3%) | 171 (96.6%) | p = 0.682 |
| > 3° varus/valgus | 7 (3.0%) | 1 (1.7%) | 6 (3.4%) | |
| within 4° varus/valgus | 234 (98.7%) | 59 (98.3%) | 175 (98.9%) | p > 0.999 |
| >4° varus/valgus | 3 (1.3%) | 1 (1.7%) | 2 (1.1%) | |
| neutral | 67 (28.3%) | 19 (31.7%) | 48 (27.1%) | p = 0.795 |
| varus | 95 (40.1%) | 23 (38.3%) | 72 (40.7%) | |
| valgus | 75 (31.6%) | 18 (30.0%) | 57 (32.2%) | |
| Relative deviation of 90° MPTA (− = var; + = val) | −0.1 ± 1.7 (−9 to 6) | 0.0 ± 1.6 (−3 to 6) | −0.1 ± 1.8 (−9 to 4) | p = 0.737 |
| Absolute deviation of 90° MPTA | 1.2 ± 1.2 (0–9) | 1.1 ± 1.1 (0–6) | 1.3 ± 1.2 (0–9) | p = 0.400 |
| Tibial slope | 6.3 ± 2.0 (2–12) | 6.1 ± 2.2 (2–12) | 6.3 ± 2.0 (2–12) | p = 0.406 |
| Absolute deviation to 7° tibial slope | 1.7 ± 1.4 (0–5) | 1.8 ± 1.5 (0–5) | 1.6 ± 1.3 (0–5) | p = 0.473 |
| Patella alpha angle | 10.0 ± 3.9 (1–22) | 10.4 ± 4.0 (1–18) | 9.9 ± 3.9 (1–22) | p = 0.408 |
Values are presented as mean ± standard deviation (minimum–maximum).
val = valgus; var = varus.
Postoperatively there were no significant differences within navigated and conventional group. There were no statistical differences regarding absolute (1.8° ± 1.4° in group NAV versus 1.8° ± 1.3° in group REG, p = 0.950) or relative deviation (0.4° ± 2.2° versus 0.8° ± 2.0° in group REG, p = 0.157) from postoperative neutral mechanical lower limb axis. Same number of patients were within a maximal deviation of 3° varus or valgus from neutral mechanical lower limb axis (p = 0.766) respectively within a maximum deviation of 4° varus/valgus (p > 0.999). (Fig. 1)
Fig. 1.
Postoperative Deviation from Neutral Mechanical Lower Limb Axis.
Mean postoperative LDFA was 90.4° ± 1.6° in navigated versus 90.8° ± 1.5° in conventional group (p = 0.053). There were also no statistical differences within mean absolute deviation (1.2° ± 1.1° versus 1.3° ± 1.1°, p = 0.637) from 90° LDFA. Same number of patients were within a maximum deviation of 3° respectively 4° varus/valgus inaccuracy (p > 0.443). (Fig. 2)
Fig. 2.
Postoperative Deviation from Neutral Lateral Distal Femoral Angle (LDFA).
Similar results were found for postoperative MPTA in frontal plane. Mean MPTA was 90.0° ± 1.6° versus 89.9° ± 1.8° in group REG (p = 0.737). Mean absolute deviation of 90° MPTA was 1.1° ± 1.1° versus 1.3° ± 1.2° in group REG (p = 0.400). Same number of patients were within 3° respectively 4° varus/valgus MPTA deformity (p > 0.682). (Fig. 3)
Fig. 3.
Postoperative Deviation from Neutral Medial Proximal Tibial Angle (MPTA).
No statistical differences were found for both tibial slope and patella alpha angle. Tibial slope showed a mean value of 6.1° ± 2.2° in group NAV versus 6.3° ± 2.0° in group REG (p = 0.406). Mean absolute deviation from 7° tibial slope was 1.8° ± 1.5° versus 1.6° ± 1.3° in group REG. (p = 0.473). Patella alpha angle showed a mean value of 10.4° ± 4.0° in group NAV versus 9.9° ± 3.9° in group REG (p = 0.408). (Table 3)
Intra- and interobserver reliability was measured with Intraclass Correlation Coefficient (ICC; two-way mixed model) and showed a high correlation of 0.9493 and 0.8770, respectively.
4. Discussion
Computer-assisted navigated total knee arthroplasty (TKA) was shown to result in higher implantation accuracy compared to the conventional TKA technique.1, 3, 4, 6, 7, 8, 9, 10 This was in contrast to the findings of several other studies.11, 12, 13 However, there is still no consensus, whether an exact reconstructed mechanical lower limb axis is essential to end up in a higher long-term survival rate with an improved clinical outcome.14, 15, 16, 17, 18 To the best of our knowledge, there are only few recent published articles available reporting on clinical outcome data and survival rate after a postoperative follow-up rate of 5–10 years or more.5, 19, 20, 21, 22 Kim et al. 22 and Hernandez-Vaquero et al. 21 could not find any differences when comparing conventional with navigated TKA after 10.8 respectively 8.3 years postoperatively. Lützner et al. verified no differences in terms of functional or patient perceived outcome after 5 years postoperatively.20 Due to inconclusive study results and obvious limitations of the navigated implantation technique (increased surgery time22, relevant costs26, additional learning curve24, 25) we introduced a treatment algorithm for navigated computer-assisted TKA in our institution. To the best of our knowledge, this is one of the first studies published so far reporting on radiological outcome data where navigated TKA was preserved for special indications as described above.
This trial was restricted by several limitations. First, this report is limited due to the retrospective study design. However, for our main study question a high follow-up rate of 92.6% was achieved. Second, because we performed a retrospective radiological analysis, no clinical data or survival rate was collected. Nevertheless, the results of the current trial based on its study design are unique and have not been published before. Hence, we think that our retrospective radiological outcome data is an important addendum to the current available literature about navigated TKA. Both conventional and computer-assisted implantation techniques have a learning curve that was accounted for in the current trial.24, 25 All procedures were performed by one orthopedic surgeon (AM) who was well experienced with both the conventional and navigated technique. Confalonieri et al. 25 described that the break-even point after which no difference is observed within a beginner and an expert surgeon using computer-assisted TKA occurs after a learning curve of at least 16 cases. However, our results and the described concept for computer-assisted TKA may not be conferrable for surgeons, who are still within the learning curve of both the conventional or navigated technique.
There may be institutions where primary TKA is only performed under support of the navigated technique. Nicholson et al. 32 already described a re-learning curve for returning to conventional total knee arthroplasty after performance of a series of computer-assisted TKA. We think that the computer-assisted TKA is no substitution of conventional surgical technique for TKA but an additional tool for primary total knee replacement. There may occur situations, where navigated TKA may fail during surgery and conversion to the conventional implantation technique is necessary. Hence, all surgeons offering computer-assisted TKA should be familiar with the conventional technique and vice versa. We see the navigated TKA as an important adjuvant especially for patients where reconstruction of an exact lower-limb axis with conventional extra- or intramedullary devices may be difficult due to severe obesity, preoperative bone deformity, osteosynthetic implants in situ or reduced preoperative radiological X-ray quality in case of flexion contractures or rotational disorders. For those patients, computer-assisted technique is a helpful tool. Introducing this treatment algorithm, we end up in a perfect postoperative implant alignment, as our data has shown. Including all navigated and conventional TKA we had an excellent implantation accuracy of 93.7% within ±3° respectively 95.8% within 4° varus or valgus deviation from neutral mechanical lower limb axis. The mean absolute deviation from neutral lower limb axis was 1.8°.
Kim et al. 22 reported a lower amount of patients to be within 3° varus/valgus of lower limb alignment in both navigated (89%) and conventional (87%) group. Hoffart et al. 23 published 86.5% in the conventional group and 87.6% in the navigated cohort to be within 3° varus/valgus of lower limb alignment. The encouraging radiological results of the current study may confirm our treatment concept that navigated computer-assisted TKA can be favored especially for those patients as described before. In all other cases optimal component alignment can also be achieved with the conventional non-navigated technique.
Encouraging results were also found for both LDFA and MPTA. All evaluations showed at least 97.0% to be within 3° or 4° varus/valgus deviation from 90° LDFA or MPTA. The absolute mean deviation of 90° LDFA or MPTA was 1.2°. These results were superior in comparison to both conventional and navigated cohort of Kim et al. 22. In contradiction Hernandez-Vaquero et al. 21 published an improved mean deviation of 0.69° for the femoral and 0.96° for the tibial component in coronal plane. In the current analysis we revealed a mean absolute deviation of 1.7° from 7° tibial slope. However, due to different definitions of an optimal tibial slope comparison of literature is difficult.
The comparison of conventional with navigated TKA in the current analysis revealed no differences for all parameters. Excellent implantation accuracy was achieved in both study cohorts in both frontal and sagittal plane. In both groups mean absolute deviation of neutral mechanical lower limb axis was 1.8°. At least 95.8% were within 3° or 4° varus or valgus deviation. Similar results were found for MPTA, LDFA and tibial slope. However, in both groups there were preoperative lower limb deformities with a deviation of at least 24° varus or valgus. Nevertheless, postoperative data showed no statistical differences in frontal plane within both groups. This may confirm, that even for strong valgus or varus deviations an excellent postoperative alignment can be achieved with the conventional implantation technique as good as with the navigated technique. In a previously published randomized controlled trial Cip et al. compared 100 navigated with 100 conventional TKA.5 A significant better reconstruction of the mechanical lower limb axis was found for the computer-assisted navigated TKA group.5 This was in contradiction to the current study results. The single surgeon study design, the treatment concept that navigated TKA was preserved for special indications and the different amount of navigated respectively conventional TKA in the current analysis may account for differences in study results. However, we performed an inter- and intraobserver reliability analysis that showed an excellent match with 0.9493 and 0.8770, respectively. This data may underline the accuracy of our postoperative radiological outcome measurements.
To summarize, we are reporting on data based on a treatment concept for TKA where the navigated computer-assisted technique was only used in patients with adipositas per magna, posttraumatic leg deformities, osteosynthetic material in situ or reduced preoperative radiological X-ray quality due to flexion contracture or rotational disorders. The encouraging current study results underline that this algorithm is an excellent treatment option for orthopedic surgeons in order to address obvious limitations of the navigated computer-assisted implantation technique. With conventional implantation technique, excellent TKA implantation results are possible based on an accurate preoperative surgery planning. Further clinical and survival rate analyses are necessary to confirm these radiological study results.
Conflict of interests & funding
On behalf of all authors, the corresponding author states that there is no conflict of interests.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.
Compliance with ethical standards
Ethical approval: All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. Due to the retrospective study design, no informed consent was necessary.
Footnotes
This work was performed at Department of Orthopedic Surgery, Academic Teaching Hospital Feldkirch, Austria and approved by the institutional review board. All authors have participated in research. The article is not submitted elsewhere. It is a new manuscript submission. On behalf of all authors, the corresponding author states that there is no conflict of interests.
Contributor Information
J. Cip, Email: johannes.cip@aon.at.
M. Widemschek, Email: mark.widemschek@vlkh.net.
C. Bach, Email: christian.bach@vlkh.net.
P. Ruckenstuhl, Email: paul.ruckenstuhl@gmx.at.
T. Benesch, Email: Thomas.benesch@univie.ac.at.
K. Studer, Email: kathrin.studer@kispisg.ch.
A. Martin, Email: arno.martin@vlkh.net.
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