Abstract
Objectives:
Patients who need torsion measurement of the lower limb often have metal implants hindering e.g. MRI. A new ultra-low-dose (ULD-)CT protocol might be feasible for torsion measurement at cost of relatively low radiation exposure.
Methods:
We retrospectively included all patients with clinically indicated torsion measurement in the period July 2019 to June 2021 and metal implants in the scanning field. The ULD-CT protocol comprised automated tube current time product and automated tube voltage with reference settings of 100kV/20mAs (hip), 80kV/20mAs (knee) and 80kV/10mAs (ankle). Femoral neck anteversion, tibial, intra-articular knee and overall leg torsion measurements were performed by two radiologists independently. Diagnostic confidence regarding the delineation of the relevant cortical bone was rated on a 5-point Likert scale (1 = non-diagnostic, 5 = excellent).
Results:
102 consecutive patients could be included (BMI 27.38 ± 5.85) with 154 metal implants. Median total dose length product of the ULD-CT-torsion measurement was 16.5mGycm [11-39]. Both readers showed high agreement with a maximum torsional difference of 4.1°. Diagnostic confidence was rated best (5/5) in 92.2% (reader 1) and 93.1% (reader 2) with a worst rating of 3/5.
Conclusion:
The new ULD-CT protocol is feasible for torsion measurement of the lower limb – even in patients with metal implants.
Advances in knowledge:
Metal implants are not an obstacle for ULD-CT torsion measurements of the lower limb.
Introduction
Knowledge about the torsional parameters of the lower limb is important for different patient groups: Among those are patients with recurrent patellar dislocations in which torsional malalignment is a significant risk factor and may be surgically corrected. Torsion measurement of the lower limb is therefore a facultative diagnostic tool in patients with patellar dislocations. 1 Second, femoral or tibial shaft fractures might lead to secondary torsional malalignment that may be the cause of pseudarthrosis or deranged biomechanics and functional losing. 2,3 Especially in patients with such diaphyseal shaft fractures the torsional parameters of the lower limb often have to be controlled repeatedly till the femoral or tibial shafts are healed in a correct manner.
There are some approaches to measure the lower limb torsion with 3D radiography 4,5 and it might also be performed using MRI, 6 but due to e.g. availability torsion measurement is usually performed with CT. Furthermore, patients with metal implants might not be suitable for MR imaging. CT, and especially repeated CT scans, cause relevant radiation exposure for the patients, which should be reduced to a necessary minimum due to the ALARA-(“As low as reasonably achievable”-) principle and the known harmful risks of radiation exposure. 7–9
In recent studies, we developed a new ultra-low dose CT (ULD-CT) protocol with the assistance of automated tube voltage and tube current time product modulation 10,11 for patients without metal implants. Since patients who need torsion measurements of the lower limb often have metal implants, the now presented study aims to analyze the feasibility of such a ULD-CT protocol and the radiation exposure in a patient group with metal implants in the lower limb.
To our knowledge, this is the first study that analyzes the feasibility of ULD-CT for torsion measurement of the lower limb in patients with metal implants.
Methods and Materials
Study design and study participants
The study was approved by the institutional review board (application number 930/2019B02). For this retrospective analysis of clinically acquired data, the need for written informed consent was waived. All consecutive patients with clinically indicated CT-torsion measurements of the lower limb at a level-1 trauma center in the study period 1 July 2019 and 30 June 2021 were included. Exclusion criteria were age under 18 and the absence of metal implants in the scanning field.
Following information of the metal implants were captured: type of metal implant, body region in which the metal implant was visible (hip, knee or ankle) and side-of the extremity where the implant was located.
As radiation dose parameters computed tomography dose index (CTDIvol) and dose length product (DLP) were assessed for evaluated body region. Scan length (SL) for each region was calculated by dividing DLP and CTDIvol.
Demographic parameters such as gender, age at time of torsion measurement as well as patients’ body weight and body-mass-index (BMI) were evaluated.
Technical parameters of the new ULD-CT protocol for torsion measurement of the lower limb [10]
The ULD-CT were acquired using a 128-slice, single source CT scanner (SOMATOM Definition Edge, Siemens Healthineers, Forchheim, Germany) with a pitch of 1.0, rotation time of 0.5 s, collimation of 128 × 0.6 mm, and a scan time of 2.21 s (hip), 2.11 s (knee), and 1.77 s (ankle). An automated tube current time product modulation (CARE Dose4D, Siemens Healthineers, Forchheim, Germany) was used for all regions (hip, knee, ankle). Furthermore, an automated tube voltage selection (CARE kV, Siemens Healthineers, Forchheim, Germany) was additionally used for all regions and was set to optimize tube current time product for the depiction of osseous structures. Reference settings were set as following: hip (100kV, 20mAs), knee (80kV, 20mAs), and ankle (80kV, 10mAs). Furthermore, basic raw-data based iterative image reconstruction (SAFIRE – Sinogram Affirmed Iterative Reconstruction, Siemens Healthineers, Forchheim, Germany) was used at strength 3 out of 5 for all regions. Image reconstruction was performed using a medium sharp kernel (I50f), 3 mm slice thickness and were displayed in a bone window (center/width: 450HU / 1500HU).
All patients were examined feet first in supine position with the feet secured together. The radiographers followed our standard operating procedure (SOP) with special focus on the minimally necessary scan length (SL) for the complete delineation of the relevant anatomical structures, based on prominent anatomical landmarks in the anteroposterior scout view: At the hip, from top of the femoral head to the upper margin of the lesser trochanter; at the knee, from top of the patella to the middle of the fibular head; and at the ankle, from 2 cm above the tibial plafond to the tip of the medial ankle.
Reference as well as actual acquisition parameters (tube current time product and tube voltage) were documented.
Evaluation of the feasibility
Every ULD-CT torsion measurement was analyzed by two radiologists – one senior radiologist with 14 years of experience in musculoskeletal imaging and one radiologist with five years of experience in musculoskeletal imaging. Torsion measurement was performed separately for femoral neck anteversion, tibial torsion, and overall torsion of the lower limb as described previously 11 in a technique modified from Waidelich et al. 11,12 : Femoral neck anteversion was measured as the angle between a line central through the femoral head and central through the greater trochanter (femoral neck) and a second line along the posterior margin of the femoral condyles. Tibial torsion was measured between a line along the posterior margin of the tibial plateau and a line central through the tibial and fibular parts of the ankle joint. Intra-articular knee torsion was measured between the posterior margin of the femoral condyles and the posterior margin of the tibial plateau. Overall torsion of the lower limb was measured between femoral neck and ankle joint (Figures 1–4). The subjective diagnostic confidence was rated on a 5-point Likert scale: 1 = non-diagnostic, 2 = low, 3 = moderate, 4 = good, 5 = excellent. Crucial hereby was the delineation of the cortical bone, respective the edges of the endoprosthesis (Figures 1–4).
Figure 1.
Visualization of the new ultra-low dose CT protocol for torsion measurement with metal implants (hip region). Exemplary scans of a 58-year-old male patient, hip endoprosthesis, BMI 35.0, CTDIvol 1.41 mGy, total DLP 33 mGycm (A),of a 67-year-old male patient, proximal femoral nail, BMI 22.1, CTDIvol 0.59 mGy, total DLP 12 mGycm (B) and a 54-year-old male patient, femoral plate, BMI 31.6, CTDIvol 0.77 mGy, total DLP 15 mGycm (C).
Figure 2.
Visualization of the new ultra-low dose protocol for torsion measurement with metal implants (knee region, femoral side). Exemplary scans of a 61-year-old male patient, distal femoral plate, BMI 29.4, CTDIvol 0.18 mGy, total DLP 16 mGycm (A), of a 39-year-old male patient, femoral nail, BMI 24.2, CTDIvol 0.18 mGy, total DLP 13 mGycm (B) and of a 58-year-old male patient, knee endoprosthesis, BMI 35.0, CTDIvol 0.33 mGy, total DLP 33 mGycm (C).
Figure 3.
Visualization of the new ultra-low dose protocol for torsion measurement with metal implants (knee region, tibial side). Exemplary scans of a 59-year-old male patient, tibial nail, BMI 23.5, CTDIvol 0.18 mGy, total DLP 14 mGycm (A), of a 58-year-old male patient, knee endoprosthesis, BMI 35.0, CTDIvol 0.33 mGy, total DLP 33 mGycm (B) and of a 19-year-old male patient, tibial plate, BMI 19.5, CTDIvol 0.18 mGy, total DLP 15 mGycm (C).
Figure 4.
Visualization of the new ultra-low dose protocol for torsion measurement in patients with metal implants (ankle region). Exemplary scans of a 50-year-old male patient, external fixator and tibial plate, BMI 28.1, CTDIvol 0.18 mGy, total DLP 19 mGycm (A), of a 58-year-old female patient, external fixator and fibular plate, BMI 30.8, CTDIvol 0.18 mGy, total DLP 19 mGycm (B) and of a 59-year-old male patient, tibial nail and tibial plate, BMI 23.5, CTDIvol 0.18 mGy, total DLP 14 mGycm (C).
Statistical analysis
Statistical analysis was performed using the software package JMP (Version 16.2.0, SAS Institute, Cary, NC, USA). Shapiro-Wilk-W-test was performed for continuous variables to check for normality. Normally distributed variables are reported as arithmetic mean and standard deviation. Non-normally distributed variables are reported as median with total range. For the correlation of ordinal and continuous variables, the chi-squared test in the whole model test of a logistic regression was performed. Correlations of two ordinal variables were analyzed by contingency analysis and a likelihood ratio. The inter-reader agreement was analyzed using an intraclass correlation coefficient (ICC), values were considered as poor reliable (<0.5), moderate reliable (0.5–0.75), good reliable (0.75–0.9), or excellent (>0.9). 13
Results
The study population included in total 202 eligible patients with 221 torsion measurements. 114 patients with 119 CT torsion measurements were excluded due to age under 18 and absence of metal implants (Figure 5).
Figure 5.
Flow Chart of eligible CT torsion measurements (n = 221) in the study period July 2019 till June 2021, excluded patients due to exclusion criteria age <18 and absence of metal implants. Final sample size of n = 102 CT torsion measurements in n = 88 patients.
The patients of the final sample (n = 102 ULD-CT torsion measurements in 88 patients) had a median age of 46.36 [±17.08] years. Participants were more often male (n = 76 male, n = 26 female). The body weight and BMI could be assessed in n = 84 participants (82.35%) with 27.379 [±5.85] kg/m2 and 88.0 [±22.60] kg.The median BMI of males and females was comparable 26.0 kg/m2 with a total range of 19.5–54.1 for males and 18–38.5 for females.
The different types of metal implants in our study population are listed in Table 1 36 of them were implanted on the left side-and 54 on the right side. Only 12 patients had implants on both sides, whereas 36 patients had more than one metal implant (e.g., femur and tibia on the same side).
Table 1.
Types of metal implants in the study population
| Type of metal implant | Localization | n |
|---|---|---|
| External Fixators | Femur | 2 |
| Tibia | 6 | |
|
Internal fixation devices
- Plates |
Symphysis | 1 |
| Femur | 30 | |
| Tibia | 13 | |
| Fibula | 9 | |
| Calcaneus | 1 | |
|
Internal fixation devices
- Nails |
Femur/proximal femoral nail/dynamic hip screw/cling plate | 48 |
| Tibia | 10 | |
| Internal fixation devices - others | Single screws, graft fixation devices | 25 |
| Endoprosthesis | Hip | 4 |
| Knee | 5 | |
| Total | 154 |
Automated tube voltage modulation lead to n = 24 scans of the hip region with 120 kV and n = 78 scans of the hip region with 100 kV, while there was a significant correlation with BMI (p < 0.0001) and with body weight (p < 0.0001). All patients with more than 120 kg body weight (n = 7) were examined with a tube voltage of 120 kV (median DLP 26 mGycm [19-39]) and none of the patients with less than 70 kg body weight (n = 15; median DLP 15 mGycm [total range 11–17 mGycm]). There was no significant correlation of actual tube voltage with the presence of metal implants at hip level (Likelihood ratio p = 0.57).
DLP at hip level was significantly higher (median 17 mGycm vs 11 mGycm), if actual tube voltage was 120 kV compared to 100 kV (p < 0.0001). Median DLP at knee and ankle level were almost equally low (3 vs 2 mGycm). Total DLP of the ULD-CT for torsion measurement of the lower limb could be performed at a median of 16.5 mGycm. SL at knee level showed a relatively wide total range of 10.0–42.86 cm (for more information see Table 2).
Table 2.
Radiation dose parameters of ULD-CT for torsion measurement in patients with metal implants
| hip | knee | ankle | total | ||
|---|---|---|---|---|---|
| Voltage (kV) | 120 | 100 | 80 | 80 | |
| n | 24 | 78 | 102 | 102 | 102 |
| Presence of metal implant in this region | 16 (66.67%) | 47 (60.26%) | 64 (62.75%) | 30 (29.41%) | |
| reference current (mAs) | 12 | 20 | 20 | 10 | |
| actual current (mAs) | 17 [14-33] | 18 13–28 | 9 [9-29] | 10 9,10 | |
| CTDIvol (mGy) | 1.18 [0.94–2.26] | 0.73 [0.52–1.25] | 0.18 [0.18–0.54] | 0.18 [0.18–0.19] | |
| DLP (mGycm) | 17.5 [13-32] | 11 7–22 | 3 2–9 | 2 1–3 | 16.5 [11-39] |
| SL (cm) | 14.75 [11.06–19.86] | 14.20 [12.31–20] | 16.67 [10–42.86] | 11.11 [5.56–16.67] | 42.05 [28.70–67.40] |
ULD-CT = ultra low dose computed tomography, CTDIvol = computed tomography dose index, DLP = dose length product, SL = scan length; variables were non-normally distributed, results are reported as median with total range.
Femoral neck anteversion, tibial torsion, intra-articular knee torsion and overall torsion measurements are shown in Table 3. Maximum differences between the readers ranged from 2.4° to 4.1° (tibial torsion left leg). The ICC ranged from 0.986 to 0.999 (Table 3). Diagnostic confidence was rated 5/5 (excellent) on the 5-point Likert scale in 92.2% (n = 94; reader 1) and 93,1% (n = 95; reader 2) of the ULD-CT. Worst rating of both readers was 3/5 (moderate; n = 3; 2.9%; both readers). The ratings of “moderate image quality” regarding the delineation of the cortical bone were on examinations of patients with comminuted fractures and multiple osteosynthesis materials.
Table 3.
Torsion measurements of two readers with inter-reader agreement
| Reader 1 (°) | Reader 2 (°) | Δ (total range, °) | ICC | |
|---|---|---|---|---|
| Femoral neck anteversion right | 23.5 [±12.9] | 23.6 [±12.9] | 0–3.3 | 0.999 [0.998–0.999] |
| Femoral neck anteversion left | 22.8 [±11.8] | 22.9 [±11.8] | 0–3.5 | 0.999 [0.998–0.999] |
| Tibial torsion right | 41.3 [±11.2] | 41.2 [±11.0] | 0–2.4 | 0.999 [0.998–0.999] |
| Tibial torsion left | 37.3 [±9.4] | 37.3 [±9.3] | 0–4.1 | 0.999 [0.996–0.998] |
| Intra-articular knee torsion right | 1.3 [±5.7] | 1.5 [±6.0] | 0–3.6 | 0.989 [0.984–0.993] |
| Intra-articular knee torsion left | 2.0 [±4.6] | 2.2 [±4.6] | 0–3.6 | 0.985 [0.978–0.990] |
| Overall external torsion right | 19.1 [±14.4] | 19.3 [±14.3] | 0–3.2 | 0.999 [0.999–0.999] |
| Overall external torsion left | 17.8 [±11.6] | 17.8 [±11.6] | 0–3.4 | 0.999 [0.999–0.999] |
Discussion
In this study, we aimed to analyze the feasibility of a ULD-CT protocol for torsion measurement of the lower limb in case of metal implants. With best-rated diagnostic confidence for the delineation of cortical bone in the vast majority of evaluated patients and a worst rating of 3/5 on a Likert scale while reaching an excellent agreement of torsion measurements by the two readers the study shows distinct results. A maximum difference of the readers of 4.1° (tibial torsion left) might be considered acceptable, since intra-individual torsional differences of 4.3 [±2.3]° at the femoral bone and 6.1 [±4.5]° are considered normal and torsional differences of up to greater 15° (tibial torsion) are considered pathological. 12
The radiation exposure with a median DLP of 16.5 mGycm and a total range of 11–39 mGycm was quite similar to reported results of the same ULD-CT protocol in patients without metal implants of 15 mGycm [total range 11–26] with a calculated effective radiation exposure of 0.17 mSv [total range 0.08–0.80], mentioning that this is less than a single standard anteroposterior radiograph of the pelvis (according to www.xrayrisk.com). 10
Meanwhile SL at the knee region showed a surprisingly great range. This may be due to a few well-meaning radiographers misconceived the ULD-CT protocol and extended SL to cover a relevant region with implants in total in order to omit a simultaneously enrolled high-quality CT of this region.
Other variables that potentially influence image quality and radiation exposure are body weight and BMI, especially when automated tube current time product and automated tube voltage modulations are used. Our study population showed a mean BMI of 27.379 kg/m2, which corresponds to overweight according to WHO classification 14 and is comparable to the average BMI in Europe and in high-income English-speaking countries. 15 This indicates that our study population is representative in terms of body habitus. However, even the images of 7 patients with a body weight above 120 kg could be successfully evaluated with a median DLP of only 26 mGycm.
There are different ways to reduce radiation dose for the patient in CT imaging. 16 One way is to adapt scan parameters and to accept increased image noise when it does not bother the validity of the scan. That can easily be done at every scanner without sophisticated (and often expensive) software or hardware. Other starting points that are often complementary in regard to radiation exposure reduction are the use of tin filters for spectral shaping, 17–19 the use of iterative image reconstruction 20,21 and - in the future probably more and more important - the use of artificial intelligence e.g. deep learning based algorithms, for example de-noising tools. 22,23
ULD-CT has been and will be an important topic in musculoskeletal radiology. 24 It potentially delivers relevant diagnostic information at ultra-low radiation exposure compared to that of conventional CT, but superior to conventional radiography. A recently published study on phantoms and cadavers showed adequate value of tin-filtered ULD-CT compared to conventional CT regarding osseous anatomy and pathologies of the pelvis. 25 Other groups described the diagnostic performance of ULD-CT in the field of fracture detection with mixed results: Murphy et al 26 compared the diagnostic performance of ULD-CT with that of conventional radiography and that of conventional CT in the field of the peripheral extremities (wrist/hand and ankle/foot) at n = 64 patients with 34 fractures. Radiation exposure was reduced by the factor seven in ULD-CT compared to conventional CT, thereby it showed lower sensitivity and specificity compared to conventional CT and higher sensitivity and specificity compared to conventional radiography. Hamard et al 27 found in n = 69 patients with 36 fractures of the spine, the pelvic ring and the hip a better sensitivity for ULD-CT compared to conventional radiography at comparable radiation exposure. Similar results are published by Alagic et al 28 with a study collective of n = 203 extremities (wrist and ankle) with 109 fractures at comparable radiation exposure.
To our knowledge, there is no other study so far that analyzed the feasibility of ULD-CT for torsion measurement of the lower limbs in patients with metal implants. The presented results show that metal implants are not necessarily an obstacle for ULD-CT especially in the investigated setting of torsion measurement of the lower extremities. Further studies are warranted to extend this finding to other clinical questions.
The retrospective monocentric setting of the study is clearly a limitation. However, we are convinced that the study population is large enough to draw valid conclusions that withstand scrutiny. The validation of the feasibility is limited due to the subjective character of the evaluation and the missing correlation with high-dose CT. Although the presented ULD-CT protocol already found its way into clinical routine of all torsion measurements of the lower limb at the regional level-1 trauma center as an imaging tool for surgical planning. Future clinical applications in different trauma centers with varying CT technology could prove broad applicability.
The presented ULD-CT protocol is able to serve for torsion measurement of the lower limb at cost of relatively low radiation exposure – even in patients with metal implants.
Contributor Information
Gabriel Keller, Email: gabriel.keller@med.uni-tuebingen.de.
Leonard Grünwald, Email: lgruenwald@bgu-tuebingen.de.
Fabian Springer, Email: fabian.springer@med.uni-tuebingen.de.
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