Is the Kinect system suitable for evaluation of the hip joint range of motion and as a screening tool for femoroacetabular impingement (FAI)?

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1 Technology and Health Care 23 (2015) DOI /THC IOS Press Is the Kinect system suitable for evaluation of the hip joint range of motion and as a screening tool for femoroacetabular impingement (FAI)? Matthias Lahner a,, Dennis Mußhoff a, Christoph von Schulze Pellengahr a, Roland Willburger a, Marco Hagen b, Andreas Ficklscherer c, Lars Victor von Engelhardt d, Ole Ackermann e, Nina Lahner f and Gregor Vetter a a Department of Orthopaedic Surgery, St. Josef-Hospital, Ruhr-University Bochum, Bochum, Germany b Biomechanics Laboratory, Department of Sport and Movement Sciences, University of Duisburg-Essen, Essen, Germany c Department of Orthopaedic Surgery, Physical Medicine and Rehabilitation, University Hospital of Munich, Campus Großhadern, Munich, Germany d Faculty of Health Sciences, University of Witten/Herdecke, Witten, Germany e Sportklinik Duisburg, Duisburg, Germany f Department of Dermatology, Venereology and Allergology, Ruhr-University Bochum, Bochum, Germany Received 14 October 2014 Accepted 17 October 2014 Abstract. BACKGROUND: In the clinical evaluation of femoroacetabular impingement (FAI), there is a lack of quantitative, reliable and informative assessment methods for the overall functional capability of an individual. OBJECTIVE: We compared clinical and radiological measurements of the hip joint with a new methodology based on the concept of 3-dimensional reachable workspace using Microsoft Kinect. METHODS: We assessed the correlation between the alpha angle of Nötzli on full-length radiographs and the clinical internal rotation. We evaluated the accuracy of joint positions and angles of the hip between the Kinect system and clinical examination including range of motion (ROM). RESULTS: The results of our clinical trial with 24 study participants showed a significant difference between normal internal rotation (> 21 ) and reduced internal rotation ( 21 ) in comparison to the radiological alpha angle of Nötzli (P = 0.026). The acquired reachable Kinect data demonstrated a moderate agreement between the Kinect and clinical examination (correlation coefficients between and 0.375). CONCLUSIONS: The findings suggest that a higher grade alpha angle of Nötzli accompanies reduced clinical internal rotation. The Kinect system provides reliable results of hip ROM. However, further test series must be performed for the application of Kinect in the clinical evaluation of FAI. Keywords: Kinect system, femoroacetabular impingement (FAI), alpha angle of Nötzli, range of motion (ROM), goniometer Corresponding author: Matthias Lahner, Department of Orthopaedic Surgery, St. Josef-Hospital, Ruhr-University Bochum, Gudrunstr. 56, Bochum, Germany. Tel.: ; Fax: ; m.lahner@klinikumbochum.de /15/$35.00 c 2015 IOS Press and the authors. All rights reserved

2 76 M. Lahner et al. / Kinect system for evaluation of the hip joint 1. Introduction Health care systems will have to face rising economic costs due to arthrosis of the hip joint in the coming years [1]. In this context, the concept of the femoroacetabular impingement (FAI) represents a novel attempt to explain mechanical etiology of hip osteoarthritis [2 4]. A large proportion of the idiopathic hip osteoarthritis can be traced back to FAI [3 5]. In the pathogenesis of FAI, the mechanical concept comprises an abutment of the femoral head-neck junction against the acetabular rim. Two different types of FAI are known, an acetabular type (pincer type) and a femoral type (cam type) [6]. The cam type impingement derives from a prominence of the anterolateral femoral head-neck junction [7]. Causes for cam impingement include aspheric deformity of the femoral head, late closure of the femoral epiphysis, slipped capital femoral epiphysis and Legg-Calvé-Perthes disease [8 10]. Pincer type impingement is due to an overly acetabular cover of the femoral head and is associated with acetabular retroversion, protrusio acetabuli or coxa profunda [11 13]. The first choice of imaging diagnostics for FAI after clinical impingement tests and case history is an anteroposterior (AP) pelvic radiograph and a second lateral or oblique view. To quantify FAI, the alpha angle of Nötzli is described as an adequate parameter to analyse the degree of the disorder of the headneck junction [7,14]. The alpha angle is formed by the mid-axis of the femoral neck and the connecting line between the head of femur and the point first exceeding the radius of the cartilage-covered femoral head [7]. The pathological alpha angle can also be measured in standing AP radiographs [15]. However, the diagnosis of FAI cannot be ensured in all cases because clinical impingement tests must be involved which often depends on the clinician and his experience. Consumer depth-sensing cameras, such as Microsoft Kinect, have gained importance in various medical applications including bearing control evaluation, in-home fall risk analysis, appraisement of the upper extremity and sport analysis [16 21]. The Kinect system is based on a colour camera and an infrared depth sensor for an accurate real-time 3D scene detection with built-in skeletal tracking capabilities [22]. To our best knowledge, there is no study that examines the potential outcome measure of the Kinect system relating to the evaluation of range of motion (ROM) of the hip joint and diagnostics of FAI. The purpose of our study was to investigate the technical aspect and accuracy of Kinect for the evaluation of the hip ROM compared to clinical and radiological findings. 2. Materials and methods 2.1. Clinical and radiological examination This study follows the principles of the Declaration of Helsinki. The ethics committee of the Ruhr- University Bochum approved the study protocol (registration number ). All study participants have voluntarily agreed to the study, are over the age of 18 and have provided written informed consent. For the study group, criterion for exclusion was any kind of previous hip surgery, inflammatory or metabolic rheumatic disease. ROM was measured in 24 hip joints of 24 patients. Flexion, extension, abduction, adduction, internal and external rotation were recorded by D.M. under the supervision of two orthopaedic surgeons (M.L. and G.V.). All participants in the study had a free ROM without a functional deficit. The results were evaluated in degrees.

3 M. Lahner et al. / Kinect system for evaluation of the hip joint 77 Fig. 1. The alpha angle is formed by the mid-axis of the femoral neck and the connecting line between the head of femur and the point first exceeding the radius of the cartilage-covered femoral head (59.20 in this case). The measurement of the alpha angles was described in a previous study of Lahner et al. [23]. The standing anteroposterior (AP) radiograph was carried out with the participant facing the radiographic tube and the patellae pointing anteriorly. According to the study protocol, the radiographs were performed under free exposure to a tube voltage of 81 kv with an exposure value of 25 mas and a focus film distance (FFD) of 180 cm. All radiographs were transferred as DICOM data files from the PACS (Version 3.0, VISUS, Bochum, Germany) into the OrthoPlanner software version (LOCALITE Biomedical Visualization Systems, St. Augustin, Germany). The alpha angle was measured using the method described by Nötzli et al. [24]. The alpha angle is formed by the mid-axis of the femoral neck and the connecting line between the head of the femur and the point first exceeding the radius of the cartilage-covered femoral head (Fig. 1) Experimental setup with Kinect For the evaluation of the reachable workspace, we used Kinect for Windows camera (Microsoft, Redmond, WA) which is capable of capturing colour and depth images with 30 frames per second (fps). The depth acquisition is based on active sensing of infrared pattern projected onto the scene, which provides

78 M. Lahner et al. / Kinect system for evaluation of the hip joint Fig. 2. Assessment of the range of motion (ROM) of the hip joint by using the Kinect system.

4 78 M. Lahner et al. / Kinect system for evaluation of the hip joint Fig. 2. Assessment of the range of motion (ROM) of the hip joint by using the Kinect system. a robust estimation of depth at each pixel with the resolution of pixels and accuracy of mm in the range of 1 4 m [25]. A customized graphical user interface for visualization, control and data collection was integrated into the software. Detection points for the joint position are provided and the actual position of the study participant is described as a vector. The Kinect system connects vectors as a triangle between the examined hip and both knee joints (Figs 2 and 3) Statistical analysis All examination results were recorded on a case report form. The selected patient cohort was grouped in an Excel file (version 2003, Microsoft Corporation, Seattle, WA, USA). The metric variables were verified by the Shapiro-Wilk test for the normal distribution. The correlation was measured between the goniometer-based measurements with those taken using the Kinect system with either Pearson s or Spearman s Rho correlation coefficient. A correlation coefficient of > 0.9 indicates a very high agreement, while a coefficient of < 0.2 states a very low agreement. The values were recorded in IBM SPSS Statistics 22 (PASW 22, SPSS Inc, Chicago, IL, USA). Statistical significance was defined as a P value < 0.05.

deg 62.6 ± 5.66 57.5 ± 4.91 0.026 Alpha angle, difference to 55,deg 7.9± 6.24 2.5 ± 4.91 0.027 Significant at P<0.05. Fig. 3. Screenshot of the appliance of Kinect system.

5 M. Lahner et al. / Kinect system for evaluation of the hip joint 79 Table 1 Comparison of the alpha angles and the internal rotation Variable Internal rotation ( 21 ) Internal rotation (> 21 ) P value Alpha angle, deg 62.6 ± ± Alpha angle, difference to 55,deg 7.9± ± Significant at P<0.05. Fig. 3. Screenshot of the appliance of Kinect system. The application appears mirror inverted. Fig. 4. Comparison of the alpha angles at an internal rotation of 21 and > 21. Fig. 5. Correlation between the Kinect system and goniometer relating to the measured values of flexion. 3. Results We included 8 men and 16 women in our pilot study. The mean age of the study group was 46.8 ± years (range years). At the beginning of the study, we considered an alpha angle > 55 as cut-off value for our trial because an alpha angle > 55 is associated with FAI [14,24]. We found

6 80 M. Lahner et al. / Kinect system for evaluation of the hip joint overall a mean alpha angle of 60.0 ± 5.82 (range ) of 24 participants. Five participants had an alpha angle 55 and 19 participants an alpha angle > 55. The internal rotation was 27.6 ± 5.13 with an alpha angle 55 and 24.2 ± 8.54 with an alpha angle > 55. Regarding the internal rotation, we stated a significant difference between the alpha angle (mean 57.5 ± 4.91 ) and the normal internal rotation (> 21 ) compared to the alpha angle (mean 62.6 ± 5.66 ) and reduced internal rotation ( 21 ), (P = 0.026). Figure 4 shows the comparison of the alpha angles at the point of an internal rotation of 21 and > 21. Relating to the correlation between the Kinect system and the measured ROM (goniometer), we stated a moderate coherence concerning the flexion, extension, abduction, adduction (correlation coefficient 0.375, 0.302, 0.230, 0,352). Figure 5 shows the correlation between the Kinect and goniometer with regard to the measured values of the flexion. 4. Discussion The purpose of our study was to investigate the technical aspect and accuracy of the Kinect system for evaluating the hip joint compared to clinical and radiological findings. We can confirm a correlation between higher grade alpha angles on radiographs and clinical reduced internal rotation. However, we have stated a moderate correlation concerning the assessment of ROM between the Kinect system and the goniometer measured by an orthopaedic surgeon. The results of the study must be interpreted in light of its limitation. The number of study participants was too low relating to the correlation between the ROM and Kinect. At the beginning of our study, we found spikes between the ROM and the Kinect system. Whilst repeating measurements in the course of the survey, the values harmonized which is due to a learning curve. However, the results of Kinect were described as improvable in the literature. Hawi et al. [20] found as well a poor to moderate agreement between the Kinect system and the ROM of the shoulder joint which is suitable to the results of our study. One reason for the moderate correlation of our study could be that the measurements of the Kinect system were performed in a standing position which may lead to a tilting of the pelvis. However, with the findings of our study, the spectrum of applications of Kinect could expand in the direction of measuring the hip ROM and FAI. Nevertheless, our findings showed that the assessment of the hip joint with the Kinect system was amendable. From a technical viewpoint, the Kinect system can be applied everywhere because the system is portable. In contrast to the accelerometer-based methods, the system could be applied at home, so patients can record the movements themselves, e.g. after surgery of the hip (arthroscopy, prosthesis, osteosynthesis). 5. Conclusion The findings suggest that higher grade alpha angle of Nötzli accompanies reduced clinical internal rotation. The Kinect system provides reliable results of hip ROM. However, further test series must be performed for the application of Kinect in the clinical evaluation of FAI. Acknowledgments We would like to thank Dr. Tanja Kottmann for her statistical computation. Moreover, the authors would like to thank Mr. Michael Müller for the design of the figures.

7 M. Lahner et al. / Kinect system for evaluation of the hip joint 81 Funding No external funding was received for this study. The Kinect software has been made freely available for download. Conflict of interest There is no conflict of interest. References [1] Pivec R, Johnson AJ, Mears SC, Mont MA. Hip arthroplasty. Lancet. 2012; 380(9855): [2] Ganz R, Leunig M, Leunig-Ganz K, Harris WH. The etiology of osteoarthritis of the hip: An integrated mechanical concept. Clin Orthop Relat Res. 2008; 466(2): [3] Leunig M, Beaulé PE, Ganz R. The concept of femoroacetabular impingement: current status and future perspectives. Clin Orthop Relat Res. 2009; 467(3): [4] Leunig M, Ganz R. Femoroacetabular impingement. A common cause of hip complaints leading to arthrosis. Unfallchirurg. 2005; 108(1): 9-10, [5] Iman S, Khanduja V. Current concepts in the diagnosis and management of femoroacetabular impingement. Int Orthop. 2011; 35(10): [6] Ganz R, Parvizi J, Beck M, Leunig M, Nötzli H, Siebenrock KA. Femoro-acetabular impingement: A cause for osteoarthritis of the hip. Clin Orthop Relat Res. 2003; 417: [7] Nötzli HP, Wyss TF, Stoecklin CH, Schmid MR, Treiber K, Hodler J (2002) The contour of the femoral head-neck junction as a predictor for the risk of anterior impingement. J Bone Joint Surg Br. 2002; 84(4): [8] Siebenrock KA, Wahab KH, Werlen S, Kalhor M, Leunig M, Ganz R. Abnormal extension of the femoral head epiphysis as a cause of cam impingement. Clin Orthop Relat Res. 2004; 418: [9] Leunig M, Casillas MM, Hamlet M, Hersche O, Nötzli H, Slongo T, Ganz R. Slipped capital femoral epiphysis: early mechanical damage to the acetabular cartilage by a prominent femoral methaphysis. Acta Orthop Scand. 2000; 71: [10] Snow SW, Keret D, Scarangella S, Bowen JR. Anterior impingement of the femoral head: A late phenomenon of Legg- Calvé-Perthes disease. J Pediatr Orthop. 1993; 13: [11] Beck M, Leunig M, Parvizi J, Boutier V, Wyss D, Ganz R. Anterior femoro-acetabular impingement: Part II. Midterm results of surgical treatment. Clin Orthop Relat Res. 2004; 418: [12] Kim WY, Hutchinson CE, Andrew JG, Allend PD. The relationship between acetabular retroversion and osteoarthritis of the hip. J Bone Joint Surg Br. 2006; 88(6): [13] Corten K, Ganz R, Chosa E, Leunig M. Bone apposition of the acetabular rim in deep hips: A distinct finding of global pincer impingement. J Bone Joint Surg Am. 2011; 93 Suppl. 2: [14] Kassarjian A, Yoon LS, Belzile E, Connoly SA, Millis MB, Palmer WE. Triad of MR arthrographic findings in patients with cam-type femoroacetabular impingement. Radiology. 2005; 236(2): [15] Clohisy JC, Nunley RM, Otto RJ, Schoenecker PL. The frog-leg lateral radiograph accurately visualized hip cam impingement abnormalities. 2007; 462: [16] Clark RA, Pua YH, Fortin K, Ritchie C, Webster KE, Denehy L, Bryant AL. Validity of the Microsoft Kinect for assessment of postural control. Gait and Posture. 2012; 36: [17] Stone E, Skubic M. Evaluation of an inexpensive depth camera for in-home gait assessment. Journal of Ambient Intellegence and Smart Environments. 2011; 3(4): [18] Lowes L, Alfano L, Yetter B, Worthen- Chaudhari L, Hinchmann W, Savage J, Samona P, Flanigan KM, Mendell JR. Proof of concept of the ability of the Kinect to quantify upper extremity function in dystrophinopathy. PLoS Currents. 2013; 14(5). [19] Kurillo G, Chen A, Bajcsy R, Han JJ. Evaluation of upper extremity reachable workspace using Kinect camera. Technol Health Care. 2013; 21(6): [20] Hawi N, Liodakis E, Musolli D, Suero EM, Stuebig T, Claassen L, Kleiner C, Krettek C, Ahlers, Citak M. Range of motion assessment of the shoulder and elbow joints using a motion sensing input device: A pilot study. Technol Health Care. 2014; 22(2):

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