Why do you think the radiographer instructs the patient to flex the knee of 20 30 degrees in the lateral knee projection?

Introduction

Osteoarthritis (OA) is a common and disabling condition, with worldwide estimates suggesting that 500 million people are currently affected.1x[1]Hunter, D.J. Osteoarthritis in 2020 and beyond: a Lancet Commission. The Lancet. 2020; 396: 2

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Patient positioning during WLRs is an inducer of HKA measurement errors. The known factors are knee flexion and extension, foot rotation, hip rotation, weight-bearing, and foot positioning.7x[7]Sanfridsson, J., Ryd, L., Svahn, G., Fridén, T., and Jonsson, K. Radiographic measurement of femorotibial rotation in weight-bearing: The influence of flexion and extension in the knee on the extensor mechanism and angles of the lower extremity in a healthy population. Acta Radiol. 2001; 42: 207–217

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There is little evidence on the test-retest reproducibility of obtaining WLRs. Odenbring et al performed a test-retest study of the measured HKA on a very small scale including 8 lower limbs, with a mean difference of 1.3°.19x[19]Odenbring, S., Berggren, A.M., and Peil, L. Roentgenographic Assessment of the Hip-Knee-Ankle Axis in Medial Gonarthrosis. Clin. Orthop. 1993; : 195–196

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Inaccurate HKA measurements caused by the lack of standardization can result in misdiagnosis, inaccurate preoperative planning, and erroneous assessment of achieved surgical corrections. The radiographic technique should be consistent and accurate, with the desired accuracy of 0.45°, reported by Jones et al as the needed wedge accuracy to achieve target corrections in high tibial osteotomy.22x[22]Jones, L.D., Brown, C.P., Jackson, W., Monk, A.P., and Price, A.J. Assessing accuracy requirements in high tibial osteotomy: a theoretical, computer-based model using AP radiographs. Knee Surg. Sports Traumatol. Arthrosc. 2017; 25: 2952–2956

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Therefore, the purpose of the current study is to find the effects of leg rotation, knee flexion and different X-ray beam heights on the measured HKA. These results can be used to improve current guidelines for obtaining WLRs with improved accuracy and reproducibility of the measured HKA.

Material and methods

Radiography system

This study used a Philips DigitalDiagnost v4.0, with a fixed X-ray beam height during acquisition where the source pivoted and aimed towards the upper, middle, and lower parts of the limb. The fixed distance between the detector plate and X-ray beam source was set to 265 cm. The X-ray settings were equal to the protocol for scanning patients with kV set at 81 and varying mAs. The radiographic system contained a laser pointer (at the X-ray beam source) directed to the lead measurement tape indicating the X-ray beam height.

First, a reference radiograph as shown in Fig. 3 was made with the sawbone model set to 5° varus, 0° leg rotation, and 0° knee flexion, with X-ray beam height centred on the joint space. From this reference position, different combinations of leg-rotation (−10°, 5°, 0°, 5°,10°), knee flexion (0°, 5°,15°) and X-ray beam height (0 cm, 5 cm, 10 cm) were applied. Rotation of the leg was described as positive (external) and negative values (internal).

Fig. 3

Whole leg radiographs exported from PACS image viewer, with the sawbones in 5° varus and 0° knee flexion. Left image illustrates the setup in AP (anteroposterior) direction and right illustrates the image in lateral view.

Results

Discussion

This study showed that leg rotation alone (without knee flexion) can lead to clinically relevant measurement errors when exceeding 9°. When 15° knee flexion was combined with 10° leg rotation the HKA measurement error became 2°, thereby grossly exceeding the desired osteotomy accuracy of 0.45° of wedge opening.22x[22]Jones, L.D., Brown, C.P., Jackson, W., Monk, A.P., and Price, A.J. Assessing accuracy requirements in high tibial osteotomy: a theoretical, computer-based model using AP radiographs. Knee Surg. Sports Traumatol. Arthrosc. 2017; 25: 2952–2956

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Significant effects of leg rotation on the measured HKA were expected and our results correspond with previously published literature.8x[8]Brouwer, R.W., Jakma, T.S.C., Brouwer, K.H., and Verhaar, J.A.N. Pitfalls in determining knee alignment: a radographic cadaver study. J. Knee Surg. 2007; 20: 210–215

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Both Radtke et al and Brouwer et al did not report the possible effect of different X-ray beam heights on the HKA measurement error.8x[8]Brouwer, R.W., Jakma, T.S.C., Brouwer, K.H., and Verhaar, J.A.N. Pitfalls in determining knee alignment: a radographic cadaver study. J. Knee Surg. 2007; 20: 210–215

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Preoperative planning of lower limb osteotomy surgery requires insight into the mechanical medial proximal tibial angle (mMPTA) and the mechanical lateral distal femoral angle (mLDFA).5x[5]Paley, D. Normal Lower Limb Alignment and Joint Orientation. in: Princ. Deform. Correct.Springer Berlin Heidelberg, Germany, Berlin; 2002: 1–18

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Cooke and Sheehy proposed a WLR protocol with the purpose of eliminating leg rotation, at the same time accounting for torsional deformity of the tibia.14x[14]Cooke, T.D.V and Sled, E.A. Optimizing limb position for measuring knee anatomical axis alignment from standing knee radiographs. J. Rheumatol. 2009; 36: 472–477

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WLR acquisition guidelines should focus on eliminating leg rotation to minimize the possible effect of knee flexion and account for the mean tibial rotation. It should deliver reproducible radiographs while being quick and easy to perform by X-ray technicians with fixed positioning for the feet and leg rotation. First, there needs to be a consensus about which anatomical landmark is easy to define and useable for knee joint rotation assessments. A viable landmark to define proximal tibial rotation on CT scans is the Akagi line, which can represent the AP alignment of the knee-joint.28x[28]Saffarini, M., Nover, L., Tandogan, R., Becker, R., Moser, L.B., Hirschmann, M.T., and Indelli, P.F. The original Akagi line is the most reliable: a systematic review of landmarks for rotational alignment of the tibial component in TKA. Knee Surg. Sports Traumatol. Arthrosc. 2018; 27: 1018–1027

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This report proposes a set of guidelines for reproducible WLR acquisition, based on the findings of this study (control leg rotation and knee flexion, with no effects of different X-ray beam heights) and what is already described in the literature. Patients should be positioned in maximum knee extension, which is more straightforward to apply by X-ray technicians compared to a certain knee flexion angle. The feet are pointed outwards with 10° of rotation and with 10 cm between the centre of their heels. The angle of 10° is situated between the longitudinal axes of the foot.24x[24]Turgut, A., Göktürk, E., Seber, S., Köse, N., Hazer, B., and Günal, I. Rotational profile of the lower extremity and foot progression angle: computerized tomographic examination of 50 male adults. Arch. Orthop. Trauma Surg. 2002; 120: 255–258Google ScholarSee all References,28x[28]Saffarini, M., Nover, L., Tandogan, R., Becker, R., Moser, L.B., Hirschmann, M.T., and Indelli, P.F. The original Akagi line is the most reliable: a systematic review of landmarks for rotational alignment of the tibial component in TKA. Knee Surg. Sports Traumatol. Arthrosc. 2018; 27: 1018–1027

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Aligning the feet at 10° to define the AP alignment does not account for individual variances in tibiofibular torsion and femoral anteversion. However, the standard deviation of tibiofibular torsion and femoral anteversion within the population of both angles is below 9°, which means that approximately 68% of the patients show rotational variances below 9°23x[23]Bråten, M., Terjesen, T., and Rossvoll, I. Femoral anteversion in normal adults: Ultrasound measurements in 50 men and 50 women. Acta Orthop. 1992; 63: 29–32

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In cases with a suspicion of a large lower limb torsional deformity (tibia and/or femur), 3D imaging techniques in the work-up is advisable. Indications for rotational errors on an AP knee radiograph are no (or too much) tibia-fibular overlap and femoral condyle asymmetry.35x[35]Maderbacher, G., Schaumburger, J., Baier, C., Zeman, F., Springorum, H.-R., Dornia, C., Grifka, J., and Keshmiri, A. Predicting knee rotation by the projection overlap of the proximal fibula and tibia in long-leg radiographs. Knee Surg. Sports Traumatol. Arthrosc. 2014; 22: 2982–2988

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Our study has many limitations. First, our model could not represent the knee joint kinematics in terms of soft tissue compression and tension. During knee flexion, knee joint kinematics are complex due to muscle contractions and ligament tensions. Our sawbone model did not incorporate these parameters and the influence of weight-bearing. However, this study tried to mimic the knee-joint articulation by positioning the hinge points of the Ilizarov frame slightly above the knee-joint (on Blumensaat line).36x[36]in: M. Çakmak, C. Şen, L. Eralp, H.I. Balci, M. Civan (Eds.) Basic Techniques for Extremity Reconstruction. Springer International Publishing, Cham; 2018

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Conclusion

This study showed that leg rotation alone (without knee flexion) can lead to clinically relevant measurement errors when exceeding 9°. When there is 15° of knee flexion and 10° leg rotation the error becomes approximately 2°. Varying X-ray beam heights within a range of 10 cm does not affect the accuracy. Based on these findings, we propose guidelines for system setup and patient positioning during a WLR that is easy to apply and aims at minimizing errors when measuring the HKA.

Funding

This study received no funding.

Disclaimer

None

Declaration of competing interest

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Ethical approval & informed consent

As the study was performed using sawbones, ethical approval and informed consent was not needed.

Author contributions

WPG and HCN conducted this study, after developing the research question together with RJHC and NvE. KHS helped with the design of the study. WP and HC analysed the data and performed the statistics. HW and RJBS helped with the interpretation of the data. All authors contributed to the writing of the manuscript.