Medical Engineering & Physics 36 (2014) 721–725
Contents lists available at ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Technical note
Smart instrumentation for determination of ligament stiffness and ligament balance in total knee arthroplasty W. Hasenkamp a,b,∗ , J. Villard d , J.R. Delaloye e , A. Arami a,c , A. Bertsch a,b , B.M. Jolles e , K. Aminian a,c , P. Renaud a,b,∗ a
École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland Microsystems Laboratory – LMIS4, Lausanne, Switzerland c Laboratory of Movement Analysis and Measurements – LMAM, Lausanne, Switzerland d Klinikum rechts der Isar, Technische Universität München, Munich, Germany e Centre Hospitalier Universitaire Vaudois and University of Lausanne, Lausanne, Switzerland b
a r t i c l e
i n f o
Article history: Received 28 February 2012 Received in revised form 12 November 2013 Accepted 1 December 2013 Keywords: Load monitoring Strain sensors Total knee arthroplasty Soft-tissue balance Ligament balance Ligament stiffness
a b s t r a c t Ligament balance is an important and subjective task performed during total knee arthroplasty (TKA) procedure. For this reason, it is desirable to develop instruments to quantitatively assess the soft-tissue balance since excessive imbalance can accelerate prosthesis wear and lead to early surgical revision. The instrumented distractor proposed in this study can assist surgeons on performing ligament balance by measuring the distraction gap and applied load. Also the device allows the determination of the ligament stiffness which can contribute a better understanding of the intrinsic mechanical behavior of the knee joint. Instrumentation of the device involved the use of hall-sensors for measuring the distractor displacement and strain gauges to transduce the force. The sensors were calibrated and tested to demonstrate their suitability for surgical use. Results show the distraction gap can be measured reliably with 0.1 mm accuracy and the distractive loads could be assessed with an accuracy in the range of 4 N. These characteristics are consistent with those have been proposed, in this work, for a device that could assist on performing ligament balance while permitting surgeons evaluation based on his experience. Preliminary results from in vitro tests were in accordance with expected stiffness values for medial collateral ligament (MCL) and lateral collateral ligament (LCL). © 2013 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Total knee arthroplasty (TKA) has become a standard procedure for the treatment of degenerative diseases at the knee joint [1]. The main goals of TKA are relief of pain and restoration of motion [2,3]. Currently, there are several prosthesis designs on the market for different indications, each having a set of dedicated surgical instruments used during TKA procedure that depend on surgical specifications and surgeon preference [4]. To maximize the surgical outcome, the TKA procedures rely on the surgical principle of softtissue balancing (or ligament balancing) to manage post-operative knee stability and mobility, and avoid early implant failure [5,6]. Despite all the efforts on developing sets of instruments for TKA, ligament balance is still difficult to measure objectively during the
∗ Corresponding authors at: École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. Tel.: +41 21 69 36752/32596. E-mail addresses: [email protected] (W. Hasenkamp), philippe.renaud@epfl.ch (P. Renaud). URL: http://lmis4.epfl.ch (P. Renaud).
operation, leaving its evaluation much to surgeon experience, feel and opinion [7]. The ligament balancing in TKA has the objective of distributing the tibialfemoral compressive forces symmetrically between the medial and lateral compartments as well as to reestablish an equivalent tibialfemoral gap in both flexion and extension [8]. Inadequate ligament balance can lead to unequal load distribution at the tibial-bearing surface consequently increasing prosthesis wear and resulting in early surgical revision [9,10]. Ideally an instrument that measures simultaneously tibialfemoral forces and flexionextension gaps could assist surgeons to determine the optimal balance for the artificial knee and restore laxity characteristics similar to the natural intact knee [11]. Also a device that allows the determination of the soft-tissue biomechanical properties, such as ligament stiffness, can contribute a better understanding of the intrinsic mechanical behavior of the individual knee joint and benefit the post-operative outcome [12]. To help improve ligament balance a number of techniques and devices have been developed for assessing soft-tissue stiffness (i.e. distractor, tensers, spacer blocks, trial components, electric instruments, and navigation system) [5,11,13–20] but, to the best
1350-4533/$ – see front matter © 2013 IPEM. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.medengphy.2013.12.001
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W. Hasenkamp et al. / Medical Engineering & Physics 36 (2014) 721–725
Fig. 1. Instrumented distractor prototype capable of continuous and real-time measurements of (A) tibial-femoral forces and (B) flexion-extension gaps.
knowledge of the authors, there are no TKA instrumentation that simultaneously allow a manual loading control by the surgeon while providing tibial-femoral force measurement and flexionextension gap quantification. In fact, previous devices are either automatic controlled, making it impossible for the surgeon to experience a direct force feedback of the measurement, or they are manually controlled and provide rudimentary quantitative information. The objective of the present work was to modify and expand the current capabilities of a standard manual distractor provided by Symbios Orthopédie SA, currently used in orthopaedic surgery, while maintaining the instrument shape and usage. The concept is based on the description of a similar tool used in human lumbar spine surgery [21]. The modified distractor aim to permit the simultaneous measurement of tibial-femoral forces and flexionextension gaps during the TKA procedure, as well as the estimation of ligament stiffness. 2. Materials and methods 2.1. Specification The specifications of the device were defined in a collaboration with surgeons and consist of force measurement independent of the surgeon’s hand position on the distractor and of the position of the applied load in the tip of the tool, displacement measurement ranging between 13 mm and 27 mm with accuracy of 0.1 mm and force assessment up to 500 N with accuracy of 5 N. The first specification provides the practitioner a freedom to operate, while maintaining the measurement reliability. The force and displacement accuracy are also important in order to provide measurements within expected readouts from the knee structure. 2.2. Design The distractor (single plate instrument) to be modified was provided by Symbios Orthopédie SA (Switzerland) and is part of the current instrumentation set provided to the hospitals to perform a TKA. The distractor’s prototype is presented in Fig. 1 and it is capable of continuous, simultaneous and real-time measurements of tibial-femoral forces and flexion-extension gaps. Our method for measuring tibial-femoral forces was to detect mechanical deformations in the distractor, and for measuring the
flexion-extension gap, to track the distance between the tips of the distractor. The detection of mechanical deformations were made by strain gauges. The strain gauge positioning on the distractor was chosen based on finite element analysis (FEA) performed with the current distractor’s shape and material properties. Also, the position of the strain gauges was chosen to minimize the dependency of the force on the variable position of the hands of the surgeon on the distractor handles and on the position of the applied load in the tip of the tool (Fig. 1A). The strain gauges were connected to a Wheatstone bridge in a half-bridge configuration. The half-bridge configuration was used to compensate for overall temperature drifts and it’s resistance is a standard 120 . The Wheatstone bridge was powered with 2.5 V and the output signal amplified using an instrumentation amplifier INA 114 (Texas Instruments). In order to measure displacement an off-the-shelf hall sensor (Asahi Kasei Microdevices Corporation, HW-322B) was positioned on one shaft of the instrument in front of a magnet positioned on another shaft on the opposite side (Fig. 1B). The hall sensor was powered with a transconductance amplifier which provides a constant current of 10 mA. This configuration was chosen to provide a reliable measurement of the hall sensor signal output. The signal output was connected to another instrumentation amplifier INA 114. Both signals from the strain gauges and the hall-sensor were sampled to a computer with a National Instruments data acquisition board (NI-6259). For displaying real-time measurements a LabView (National Instruments) interface was used. 2.3. Procedure The instrumented distractor calibration consisted of a semiautomated procedure. The displacement sensor was calibrated statically, after assembly to the distractor, using a digital caliper fixed to the tip of the distractor while the signals were sampled to the computer. This collected data was used to extract the coefficients based on a third order exponential decay curve fit. The strain sensors were initially calibrated using a load cell by fixing the tip of the distractor, one side to the load cell and the other side to a rigid structure, such that the instrumented arm was free to be operated. Afterwards, in case of drift, necessary corrections can be performed using shunt resistors. The signal conditioner’s gain and span controls were set to obtain a full-scale electrical output signal. Once the distractor is calibrated, it can be used for measurements. During the measurement, the surgeon can see the force and displacement values on the computer screen through the Labview interface and the ligament stiffness can be estimated by the slope of a linear regression fitting the data. 2.4. Experimental measurements To assess the suitability of the device for the purpose of ligament balancing, a control experiment with a spring, simulating the ligaments, was performed. The spring was fixated to the tip of the distractor and a series of dynamic movements of opening and closing the instrument was completed. The measurement protocol consisted of distracting the spring about 10 consecutive times following a procedure suggested by the surgeon to mimic surgical procedures. The rate of loading and unloading was target to be roughly 0.3 Hz while the force and distance were recorded. 2.5. In vitro measurements After approval of the local ethical committee we were able to test the device in vitro, in two cadaveric legs. The cadaveric leg was positioned according the patient would be in a supine position and the knee exposed in flexion. A 15 cm straight vertical incision
W. Hasenkamp et al. / Medical Engineering & Physics 36 (2014) 721–725
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was made to expose the knee joint. The incision is centered proximally over the shaft of the femur extending to the mid-third of the patella and distally just medial to the tibial tubercle. Once the incision into the joint has been made, the soft tissue around the joint is retracted to expose the bone. The bone is prepared by performing a femoral, tibial, and patellar cut and afterwards the ligament stiffness measurement is made. The goal was to proof the device concept and verify if the acquire parameters were in accordance with expected stiffness values for medial collateral ligament (MCL) and lateral collateral ligament (LCL). The measurement protocol consisted of distracting the MCL and LCL 7–10 consecutive times by the surgeon slowly (roughly 0.3 Hz) loading and unloading the distractor while recording the force and distance. This procedure was suggested according surgeon’s normal practice and experience, but it may vary from surgen to surgen, therefore investigate the impact of the measurement on a diversity of surgeon remains a future work.
3. Results The inverse of the calibration curve, distance measured by the caliper as a function of the voltage output of the hall-sensor, and the achieved fitting is shown is Fig. 2A. The fitting was performed with a third order exponential decay function (residual sum of squares = 0.0802 mm and adjusted R2 = 0.9998) and the extracted coefficients used for real-time calculation of the displacement. For large displacements (average 25.3 mm) the standard deviation was 1.6 × 10−2 mm and for small displacements (average 15.3 mm) the standard deviation was 2.9 × 10−4 mm. The inverse of the calibration curve, force output given by the load cell as a function of the voltage output of the Wheatstone bridge after amplification, and correspondent fitting is presented in Fig. 2B. The fitting was performed with a linear function (residual sum of squares = 7.808 N and adjusted R2 = 0.9995) and the extracted coefficients used to voltage into force. For small forces (average 36.3 N) and higher forces (average 399.5 N) the standard deviation was 1.13 N and 1.95 N, respectively. The precision of the measurement for both, displacement and forces, are in accordance with desired specification for the distractor. A control experiment using a common spring was performed. The used spring has an outer diameter of 12.5 mm and a wire section of 2.4 × 1.4 mm. The given spring constant was 11.4 N/mm. The measured displacement and forces are presented in Fig. 3 showing the force as a function of distance. As expected, when loading the instrument to distract the spring the force increases linearly with the distance. Also the stiffness of the spring, considering a linear function regression to the data, is in accordance with the spring constant given by the manufacturer. The shift in the curve for unloading is a consequence of the mechanical friction imposed by the distractor construction. In order to prove the device concept in vitro and verify the ligament stiffness we collected data following the previously described in vitro measurement protocol. Fig. 4 shows the recorded signals, force and distance, as a function of time for one of the experiments. Each measurement was repeated 3 times for each compartment (medial and lateral) and took less than 1 min per measurement. Two in vitro measurements are presented in Fig. 5. The graph shows the force as a function of the distance for determining the stiffness of a MCL for the two cadaveric legs. The results from the in vitro experiments to determine the ligament stiffness are summarized in Table 1 together with recently reported MCL and LCL stiffness [12]. They represent the average and standard deviation of three consecutive measurements performed in each compartment (medial and lateral) for the two cadaveric legs.
Fig. 2. (A) Data acquired during the static calibration of the hall-sensor represented by the dots and the third order exponential decay fit to the data represented by the line. The graph presents the distance as a function of the voltage output of the hall-sensor. (B) Data acquired for the calibration of the strain sensors to transduce the force represented by the star points and the correspondent linear regression to the data. The curve presents the force as a function of the output of the Wheatstone bridge.
Fig. 3. Control experiment with a spring attached to the tip of the distractor. The graph shows the force as a function of the distance.
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Fig. 4. Experiment for determining the ligament stiffness which consists of recording the force and distance in a series of consecutive loading and unloading of the distractor. The graph presents the measured forces and distances as a function of the time.
Fig. 5. Results of two in vitro experiment for determining MCL stiffness. The graph presents the force as a function of the distance. The stiffness of the ligament can be estimated by the slope of a linear regression fitting the acquired data.
4. Discussions Nowadays, ligament balancing is a prerequisite for good function and survival in total knee arthroplasty (TKA), but there is no consensus on how to asses ligament balance intra-operatively. [22,23]. Despite several methods using computer navigation systems has been used to quantify ligament balancing via various algorithms, their application during surgery are not common [24–26]. The difficulty in judging ligament balance and excessive Table 1 Summary of the results from the two in vitro experiments to determine the ligament stiffness and previous reported MCL and LCL stiffness [12]. Experiments
Ligament
Stiffness (N/mm)
Cadaveric leg 1
MCL LCL
62.7 ± 1.37 57.6 ± 1.43
Cadaveric leg 2
MCL LCL
47.5 ± 2.29 40.1 ± 1.94
Reported values [12]
MCL LCL
63 ± 14 59 ± 12
tightness or release may lead to knee instability leading to abnormal motion that increase prosthesis wear and can result in early surgical revision [9]. In our study, we sought to provide a simple ligament balance tool to be used during TKA surgery. A commercially available TKA distractor has been instrumented with sensors to indicate distraction forces and displacement. In order to simulate the environment encountered by the surgeons we have tested the developed distractor using spring to simulate the ligaments and characterized the distractor behavior during dynamic loading and unloading. Also the distractor was tested in vitro in two cadaveric legs. The knee is a complex joint, and there are many factors that have been shown to affect the outcome, however the determined ligament stiffness are in accordance with recent studies performed in dissected medial collateral ligament (MCL) and lateral collateral ligament (LCL) [12]. The small discrepancy observed in the second experiment is attributed to the storage conditions of this particular cadaveric leg. However, the values are still close to the lower boundary deviation expected for MCL and LCL ligament stiffness. We believe that the procedures followed during the in vitro tests do not differ significantly from what the surgeon will encounter during TKA. There are several limitations in the current study, i.e. static calibration and shifts imposed by the mechanical friction, but results demonstrated that the distraction gap can be measured reliably and with 0.1 mm accuracy. The distractive loads could be assessed accurately in the range of 4 N, attending the desired surgeon specification for such device. Also, loads as high as 300 N can be measured and are within the linear range of the instrument. Considering ligament balancing as an important factor for good function and survival in total knee arthroplasty, quantify the tibial-femoral force in the medial and lateral ligaments is essential. Since there is no consensus on how to measure ligament balance intra-operatively no absolute gold standard procedure has been developed. Therefore, it would be very helpful to have a simple and standard tool (e.g. distractor) to build a set of numerical values that could provide comparable values of MCL and LCL stiffness to improve soft-tissue balance in total knee replacement. Overall the measurements demonstrate the device suitability for estimating the force and distance and can assist surgeons when performing ligament balance during the TKA. The developed tool provides surgeons with data to assist their intra-operative decisions, since the surgeon can manually set the ligament balance with the help of his expertise and feeling as well as the displayed values. The manual approach is desirable because it gives the surgeon the complete control over the surgery procedures while performing the measurements. We believe the developed distractor can help to improve the implantation technique to increase TKA post-operative results. Also, such tool can be of great help if introduced during training of young surgeons to improve their learning experience by having a quantitative feedback of their manual perception. Before being tested in vivo, we need to establish the measurement protocol and test the robustness and durability of the device. Also the inclusion of a passive telemetry system is being studied which we believe will improve the usage of the device. We are also addressing technical issues of autoclave sterilization, storage and manufacturing costs.
5. Conclusion Extending the capabilities of current available surgical instruments is a versatile and easy way to improve surgical outcome without changing surgical procedures or impose to surgeons new surgical techniques. In contrast with the existing ligament balance tools the developed instrumented distractor allows a manual
W. Hasenkamp et al. / Medical Engineering & Physics 36 (2014) 721–725
loading control by the surgeon while providing both measurements of tibial-femoral forces and flexion-extension gaps. The determination of ligament stiffness was also achieved with good reliability. The quantification of such parameters can possibly contribute to validate the way surgeons implant prosthesis at the same time that it can provide useful in vivo information for a better understanding of the complex knee joint. Acknowledgements We would like to thank Symbios and Dr. V. Leclercq for supporting this work and also Dr. K. Boulos for their valuable advice and surgical expertise. Competing interests: All authors confirm that there are no financial and personal relationships with other people or organizations that could inappropriately influence this work. Funding: This work has been supported by the Swiss federal program funding, Nano-tera, through the SImOS (Smart Implants for Orthopaedics Surgery) project - SNF20NAN1-123630. Ethical approval: Ethical approval was given by the cantonal ethics committee for research on human being in Lausanne, Switzerland, chaired by Prof. Dr. Roger Darioli under the protocol 441/11. References [1] Schirm AC, Jeffcote BO, Nicholls RL, Jakob H, Kuster MS. Sensitivity of knee soft-tissues to surgical technique in total knee arthroplasty. The Knee 2011;18(3):180–4. [2] Basic orthopaedic biomechanics and mechano-biology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004. [3] Jolles BM, Grzesiak A, Eudier A, Dejnabadi H, Voracek C, Pichonnaz C, Aminian K, Martin E. A randomised controlled clinical trial and gait analysis of fixed- and mobile-bearing total knee replacements with a five-year follow-up. Journal of Bone and Joint Surgery, British Volume 2012;94(5):648–55. [4] Ritter Ma, Faris PM, Keating EM. Posterior cruciate ligament balancing during total knee arthroplasty. Journal of Arthroplasty 1988;3(4):323–6. [5] Matsumoto T, Muratsu H, Tsumura N, Mizuno K, Kuroda R, Yoshiya S, Kurosaka M. Joint gap kinematics in posterior-stabilized total knee arthroplasty measured by a new tensor with the navigation system. Journal of Biomechanical Engineering 2006;128(6):867–71. [6] Crottet D, Maeder T, Fritschy D, Bleuler H, Nolte L-p, Pappas IP, Modeling A. Device designed to improve the ligament balancing procedure in TKA. Evolution 2005;52(9):1609–11. [7] Attfield SF, Warren-Forward M, Wilton T, Sambatakakis A. Measurement of soft tissue imbalance in total knee arthroplasty using electronic instrumentation. Medical Engineering & Physics 1994;16(6):501–5. [8] Almouahed S, Gouriou M, Hamitouche C, Stindel E, Roux C. Design and evaluation of instrumented smart knee implant. IEEE Transactions on Bio-medical Engineering 2011;58(4):971–82. [9] Werner FW, Ayers DC, Maletsky LP, Rullkoetter PJ. The effect of valgus/varus malalignment on load distribution in total knee replacements. Journal of Biomechanics 2005;38(2):349–55.
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[10] Arami A, Simoncini M, Atasoy O, Hasenkamp W, Ali S, Bertsch A, Meurville E, Tanner S, Dejnabadi H, Leclercq V, Renaud P, Dehollain C, Farine P-A, Jolles BM, Aminian K, Ryser P. Instrumented prosthesis for knee implants monitoring. In: 2011 IEEE international conference on automation science and engineering. 2011. p. 828–35. [11] Marmignon C, Leimnei A, Cinquin P. Robotized distraction device for knee replacement surgery. International Congress Series 2004;1268:638–43. [12] Wilson WT, Deakin AH, Payne AP, Picard F, Wearing SC. Comparative analysis of the structural properties of the collateral ligaments of the human knee. Journal of Orthopaedic and Sports Physical Therapy 2012;42(4):345–351. [13] In Y, Kim S-J, Kim J-M, Woo Y-K, Choi N-Y, Kang J-W. Agreements between different methods of gap balance estimation in cruciate-retaining total knee arthroplasty. Knee Surgery, Sports Traumatology, Arthroscopy: Official Journal of the ESSKA 2009;17(1):60–4. [14] Takahashi T, Wada Y, Yamamoto H. Soft-tissue balancing with pressure distribution during total knee arthroplasty. Journal of Bone and Joint Surgery, British Volume 1997;79(2):235–9. [15] Skrinskas T, Viskontas D, Ferreira L. Application of an intra-operative load measuring system for knee replacement surgery. In: Medical image computing and computer-assisted intervention - MICCAI 2003. 2003. p. 246–53. [16] Viskontas DG, Skrinskas TV, Johnson JA, King GJ, Winemaker MJ, Chess DG. Computer-assisted gap equalization in total knee arthroplasty. The Journal of Arthroplasty 2007;22(3):334–42. [17] Bellemans J, Ries M, Victor J. Total knee arthroplasty: a guide to get better performance. Heidelberg: Springer-Verlag; 2005. [18] Rupp S, Kohn D. Das besondere Instrument Der so genannte Knee Balancer. Operative Orthopädie und Traumatologie 2000;60(Heft 3):256–60. [19] Swank M, Romanowski JR, Korbee LL, Bignozzi S. Ligament balancing in computer-assisted total knee arthroplasty: improved clinical results with a spring-loaded tensioning device. Proceedings of the Institution of Mechanical Engineers Part H: Journal of Engineering in Medicine 2007;221(7): 755–61. [20] Marmignon C, Leimnei A, Lavallée S, Cinquin P. Automated hydraulic tensor for total knee arthroplasty. The International Journal of Medical Robotics Computer Assisted Surgery: MRCAS 2005;1(4):51–7. [21] Ambrosetti-Giudici S, Pfenniger A, Krenn MH, Piotrowski WP, Ferguson SJ, Burger J. Surgical instrumentation for the in vivo determination of human lumbar spinal segment stiffness and viscoelasticity. Medical Engineering & Physics 2009;31(9):1063–8. [22] Aunan E, Kibsgard T, Clarke-Jenssen J, Rohrl SM. A new method to measure ligament balancing in total knee arthroplasty: laxity measurements in 100 knees. Archives of Orthopaedic and Trauma Surgery 2012;132(8):1173–81. [23] Rousseau M-A, Lazennec J-Y, Catonne Y. Early mechanical failure in total knee arthroplasty. International Orthopaedics 2008;32(1):53–6. [24] Nizard R. Computer assisted surgery for total knee arthroplasty. Acta Orthopaedica Belgica 2002;68(3):215–30. [25] Picard F, Deakin AH, Clarke IV, Dillon JM, Kinninmonth AW. A quantitative method of effective soft tissue management for varus knees in total knee replacement surgery using navigational techniques. Proceedings of the Institution of Mechanical Engineers Part H: Journal of Engineering in Medicine 2007;221(7):763–72. [26] Hakki S, Coleman S, Saleh K, Bilotta VJ, Hakki A. Navigational predictors in determining the necessity for collateral ligament release in total knee replacement. Journal of Bone and Joint Surgery, British volume 2009;91(9): 1178–82.
Contents lists available at ScienceDirect
Medical Engineering & Physics journal homepage: www.elsevier.com/locate/medengphy
Technical note
Smart instrumentation for determination of ligament stiffness and ligament balance in total knee arthroplasty W. Hasenkamp a,b,∗ , J. Villard d , J.R. Delaloye e , A. Arami a,c , A. Bertsch a,b , B.M. Jolles e , K. Aminian a,c , P. Renaud a,b,∗ a
École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland Microsystems Laboratory – LMIS4, Lausanne, Switzerland c Laboratory of Movement Analysis and Measurements – LMAM, Lausanne, Switzerland d Klinikum rechts der Isar, Technische Universität München, Munich, Germany e Centre Hospitalier Universitaire Vaudois and University of Lausanne, Lausanne, Switzerland b
a r t i c l e
i n f o
Article history: Received 28 February 2012 Received in revised form 12 November 2013 Accepted 1 December 2013 Keywords: Load monitoring Strain sensors Total knee arthroplasty Soft-tissue balance Ligament balance Ligament stiffness
a b s t r a c t Ligament balance is an important and subjective task performed during total knee arthroplasty (TKA) procedure. For this reason, it is desirable to develop instruments to quantitatively assess the soft-tissue balance since excessive imbalance can accelerate prosthesis wear and lead to early surgical revision. The instrumented distractor proposed in this study can assist surgeons on performing ligament balance by measuring the distraction gap and applied load. Also the device allows the determination of the ligament stiffness which can contribute a better understanding of the intrinsic mechanical behavior of the knee joint. Instrumentation of the device involved the use of hall-sensors for measuring the distractor displacement and strain gauges to transduce the force. The sensors were calibrated and tested to demonstrate their suitability for surgical use. Results show the distraction gap can be measured reliably with 0.1 mm accuracy and the distractive loads could be assessed with an accuracy in the range of 4 N. These characteristics are consistent with those have been proposed, in this work, for a device that could assist on performing ligament balance while permitting surgeons evaluation based on his experience. Preliminary results from in vitro tests were in accordance with expected stiffness values for medial collateral ligament (MCL) and lateral collateral ligament (LCL). © 2013 IPEM. Published by Elsevier Ltd. All rights reserved.
1. Introduction Total knee arthroplasty (TKA) has become a standard procedure for the treatment of degenerative diseases at the knee joint [1]. The main goals of TKA are relief of pain and restoration of motion [2,3]. Currently, there are several prosthesis designs on the market for different indications, each having a set of dedicated surgical instruments used during TKA procedure that depend on surgical specifications and surgeon preference [4]. To maximize the surgical outcome, the TKA procedures rely on the surgical principle of softtissue balancing (or ligament balancing) to manage post-operative knee stability and mobility, and avoid early implant failure [5,6]. Despite all the efforts on developing sets of instruments for TKA, ligament balance is still difficult to measure objectively during the
∗ Corresponding authors at: École Polytechnique Fédérale de Lausanne, Lausanne, Switzerland. Tel.: +41 21 69 36752/32596. E-mail addresses: [email protected] (W. Hasenkamp), philippe.renaud@epfl.ch (P. Renaud). URL: http://lmis4.epfl.ch (P. Renaud).
operation, leaving its evaluation much to surgeon experience, feel and opinion [7]. The ligament balancing in TKA has the objective of distributing the tibialfemoral compressive forces symmetrically between the medial and lateral compartments as well as to reestablish an equivalent tibialfemoral gap in both flexion and extension [8]. Inadequate ligament balance can lead to unequal load distribution at the tibial-bearing surface consequently increasing prosthesis wear and resulting in early surgical revision [9,10]. Ideally an instrument that measures simultaneously tibialfemoral forces and flexionextension gaps could assist surgeons to determine the optimal balance for the artificial knee and restore laxity characteristics similar to the natural intact knee [11]. Also a device that allows the determination of the soft-tissue biomechanical properties, such as ligament stiffness, can contribute a better understanding of the intrinsic mechanical behavior of the individual knee joint and benefit the post-operative outcome [12]. To help improve ligament balance a number of techniques and devices have been developed for assessing soft-tissue stiffness (i.e. distractor, tensers, spacer blocks, trial components, electric instruments, and navigation system) [5,11,13–20] but, to the best
1350-4533/$ – see front matter © 2013 IPEM. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.medengphy.2013.12.001
722
W. Hasenkamp et al. / Medical Engineering & Physics 36 (2014) 721–725
Fig. 1. Instrumented distractor prototype capable of continuous and real-time measurements of (A) tibial-femoral forces and (B) flexion-extension gaps.
knowledge of the authors, there are no TKA instrumentation that simultaneously allow a manual loading control by the surgeon while providing tibial-femoral force measurement and flexionextension gap quantification. In fact, previous devices are either automatic controlled, making it impossible for the surgeon to experience a direct force feedback of the measurement, or they are manually controlled and provide rudimentary quantitative information. The objective of the present work was to modify and expand the current capabilities of a standard manual distractor provided by Symbios Orthopédie SA, currently used in orthopaedic surgery, while maintaining the instrument shape and usage. The concept is based on the description of a similar tool used in human lumbar spine surgery [21]. The modified distractor aim to permit the simultaneous measurement of tibial-femoral forces and flexionextension gaps during the TKA procedure, as well as the estimation of ligament stiffness. 2. Materials and methods 2.1. Specification The specifications of the device were defined in a collaboration with surgeons and consist of force measurement independent of the surgeon’s hand position on the distractor and of the position of the applied load in the tip of the tool, displacement measurement ranging between 13 mm and 27 mm with accuracy of 0.1 mm and force assessment up to 500 N with accuracy of 5 N. The first specification provides the practitioner a freedom to operate, while maintaining the measurement reliability. The force and displacement accuracy are also important in order to provide measurements within expected readouts from the knee structure. 2.2. Design The distractor (single plate instrument) to be modified was provided by Symbios Orthopédie SA (Switzerland) and is part of the current instrumentation set provided to the hospitals to perform a TKA. The distractor’s prototype is presented in Fig. 1 and it is capable of continuous, simultaneous and real-time measurements of tibial-femoral forces and flexion-extension gaps. Our method for measuring tibial-femoral forces was to detect mechanical deformations in the distractor, and for measuring the
flexion-extension gap, to track the distance between the tips of the distractor. The detection of mechanical deformations were made by strain gauges. The strain gauge positioning on the distractor was chosen based on finite element analysis (FEA) performed with the current distractor’s shape and material properties. Also, the position of the strain gauges was chosen to minimize the dependency of the force on the variable position of the hands of the surgeon on the distractor handles and on the position of the applied load in the tip of the tool (Fig. 1A). The strain gauges were connected to a Wheatstone bridge in a half-bridge configuration. The half-bridge configuration was used to compensate for overall temperature drifts and it’s resistance is a standard 120 . The Wheatstone bridge was powered with 2.5 V and the output signal amplified using an instrumentation amplifier INA 114 (Texas Instruments). In order to measure displacement an off-the-shelf hall sensor (Asahi Kasei Microdevices Corporation, HW-322B) was positioned on one shaft of the instrument in front of a magnet positioned on another shaft on the opposite side (Fig. 1B). The hall sensor was powered with a transconductance amplifier which provides a constant current of 10 mA. This configuration was chosen to provide a reliable measurement of the hall sensor signal output. The signal output was connected to another instrumentation amplifier INA 114. Both signals from the strain gauges and the hall-sensor were sampled to a computer with a National Instruments data acquisition board (NI-6259). For displaying real-time measurements a LabView (National Instruments) interface was used. 2.3. Procedure The instrumented distractor calibration consisted of a semiautomated procedure. The displacement sensor was calibrated statically, after assembly to the distractor, using a digital caliper fixed to the tip of the distractor while the signals were sampled to the computer. This collected data was used to extract the coefficients based on a third order exponential decay curve fit. The strain sensors were initially calibrated using a load cell by fixing the tip of the distractor, one side to the load cell and the other side to a rigid structure, such that the instrumented arm was free to be operated. Afterwards, in case of drift, necessary corrections can be performed using shunt resistors. The signal conditioner’s gain and span controls were set to obtain a full-scale electrical output signal. Once the distractor is calibrated, it can be used for measurements. During the measurement, the surgeon can see the force and displacement values on the computer screen through the Labview interface and the ligament stiffness can be estimated by the slope of a linear regression fitting the data. 2.4. Experimental measurements To assess the suitability of the device for the purpose of ligament balancing, a control experiment with a spring, simulating the ligaments, was performed. The spring was fixated to the tip of the distractor and a series of dynamic movements of opening and closing the instrument was completed. The measurement protocol consisted of distracting the spring about 10 consecutive times following a procedure suggested by the surgeon to mimic surgical procedures. The rate of loading and unloading was target to be roughly 0.3 Hz while the force and distance were recorded. 2.5. In vitro measurements After approval of the local ethical committee we were able to test the device in vitro, in two cadaveric legs. The cadaveric leg was positioned according the patient would be in a supine position and the knee exposed in flexion. A 15 cm straight vertical incision
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was made to expose the knee joint. The incision is centered proximally over the shaft of the femur extending to the mid-third of the patella and distally just medial to the tibial tubercle. Once the incision into the joint has been made, the soft tissue around the joint is retracted to expose the bone. The bone is prepared by performing a femoral, tibial, and patellar cut and afterwards the ligament stiffness measurement is made. The goal was to proof the device concept and verify if the acquire parameters were in accordance with expected stiffness values for medial collateral ligament (MCL) and lateral collateral ligament (LCL). The measurement protocol consisted of distracting the MCL and LCL 7–10 consecutive times by the surgeon slowly (roughly 0.3 Hz) loading and unloading the distractor while recording the force and distance. This procedure was suggested according surgeon’s normal practice and experience, but it may vary from surgen to surgen, therefore investigate the impact of the measurement on a diversity of surgeon remains a future work.
3. Results The inverse of the calibration curve, distance measured by the caliper as a function of the voltage output of the hall-sensor, and the achieved fitting is shown is Fig. 2A. The fitting was performed with a third order exponential decay function (residual sum of squares = 0.0802 mm and adjusted R2 = 0.9998) and the extracted coefficients used for real-time calculation of the displacement. For large displacements (average 25.3 mm) the standard deviation was 1.6 × 10−2 mm and for small displacements (average 15.3 mm) the standard deviation was 2.9 × 10−4 mm. The inverse of the calibration curve, force output given by the load cell as a function of the voltage output of the Wheatstone bridge after amplification, and correspondent fitting is presented in Fig. 2B. The fitting was performed with a linear function (residual sum of squares = 7.808 N and adjusted R2 = 0.9995) and the extracted coefficients used to voltage into force. For small forces (average 36.3 N) and higher forces (average 399.5 N) the standard deviation was 1.13 N and 1.95 N, respectively. The precision of the measurement for both, displacement and forces, are in accordance with desired specification for the distractor. A control experiment using a common spring was performed. The used spring has an outer diameter of 12.5 mm and a wire section of 2.4 × 1.4 mm. The given spring constant was 11.4 N/mm. The measured displacement and forces are presented in Fig. 3 showing the force as a function of distance. As expected, when loading the instrument to distract the spring the force increases linearly with the distance. Also the stiffness of the spring, considering a linear function regression to the data, is in accordance with the spring constant given by the manufacturer. The shift in the curve for unloading is a consequence of the mechanical friction imposed by the distractor construction. In order to prove the device concept in vitro and verify the ligament stiffness we collected data following the previously described in vitro measurement protocol. Fig. 4 shows the recorded signals, force and distance, as a function of time for one of the experiments. Each measurement was repeated 3 times for each compartment (medial and lateral) and took less than 1 min per measurement. Two in vitro measurements are presented in Fig. 5. The graph shows the force as a function of the distance for determining the stiffness of a MCL for the two cadaveric legs. The results from the in vitro experiments to determine the ligament stiffness are summarized in Table 1 together with recently reported MCL and LCL stiffness [12]. They represent the average and standard deviation of three consecutive measurements performed in each compartment (medial and lateral) for the two cadaveric legs.
Fig. 2. (A) Data acquired during the static calibration of the hall-sensor represented by the dots and the third order exponential decay fit to the data represented by the line. The graph presents the distance as a function of the voltage output of the hall-sensor. (B) Data acquired for the calibration of the strain sensors to transduce the force represented by the star points and the correspondent linear regression to the data. The curve presents the force as a function of the output of the Wheatstone bridge.
Fig. 3. Control experiment with a spring attached to the tip of the distractor. The graph shows the force as a function of the distance.
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Fig. 4. Experiment for determining the ligament stiffness which consists of recording the force and distance in a series of consecutive loading and unloading of the distractor. The graph presents the measured forces and distances as a function of the time.
Fig. 5. Results of two in vitro experiment for determining MCL stiffness. The graph presents the force as a function of the distance. The stiffness of the ligament can be estimated by the slope of a linear regression fitting the acquired data.
4. Discussions Nowadays, ligament balancing is a prerequisite for good function and survival in total knee arthroplasty (TKA), but there is no consensus on how to asses ligament balance intra-operatively. [22,23]. Despite several methods using computer navigation systems has been used to quantify ligament balancing via various algorithms, their application during surgery are not common [24–26]. The difficulty in judging ligament balance and excessive Table 1 Summary of the results from the two in vitro experiments to determine the ligament stiffness and previous reported MCL and LCL stiffness [12]. Experiments
Ligament
Stiffness (N/mm)
Cadaveric leg 1
MCL LCL
62.7 ± 1.37 57.6 ± 1.43
Cadaveric leg 2
MCL LCL
47.5 ± 2.29 40.1 ± 1.94
Reported values [12]
MCL LCL
63 ± 14 59 ± 12
tightness or release may lead to knee instability leading to abnormal motion that increase prosthesis wear and can result in early surgical revision [9]. In our study, we sought to provide a simple ligament balance tool to be used during TKA surgery. A commercially available TKA distractor has been instrumented with sensors to indicate distraction forces and displacement. In order to simulate the environment encountered by the surgeons we have tested the developed distractor using spring to simulate the ligaments and characterized the distractor behavior during dynamic loading and unloading. Also the distractor was tested in vitro in two cadaveric legs. The knee is a complex joint, and there are many factors that have been shown to affect the outcome, however the determined ligament stiffness are in accordance with recent studies performed in dissected medial collateral ligament (MCL) and lateral collateral ligament (LCL) [12]. The small discrepancy observed in the second experiment is attributed to the storage conditions of this particular cadaveric leg. However, the values are still close to the lower boundary deviation expected for MCL and LCL ligament stiffness. We believe that the procedures followed during the in vitro tests do not differ significantly from what the surgeon will encounter during TKA. There are several limitations in the current study, i.e. static calibration and shifts imposed by the mechanical friction, but results demonstrated that the distraction gap can be measured reliably and with 0.1 mm accuracy. The distractive loads could be assessed accurately in the range of 4 N, attending the desired surgeon specification for such device. Also, loads as high as 300 N can be measured and are within the linear range of the instrument. Considering ligament balancing as an important factor for good function and survival in total knee arthroplasty, quantify the tibial-femoral force in the medial and lateral ligaments is essential. Since there is no consensus on how to measure ligament balance intra-operatively no absolute gold standard procedure has been developed. Therefore, it would be very helpful to have a simple and standard tool (e.g. distractor) to build a set of numerical values that could provide comparable values of MCL and LCL stiffness to improve soft-tissue balance in total knee replacement. Overall the measurements demonstrate the device suitability for estimating the force and distance and can assist surgeons when performing ligament balance during the TKA. The developed tool provides surgeons with data to assist their intra-operative decisions, since the surgeon can manually set the ligament balance with the help of his expertise and feeling as well as the displayed values. The manual approach is desirable because it gives the surgeon the complete control over the surgery procedures while performing the measurements. We believe the developed distractor can help to improve the implantation technique to increase TKA post-operative results. Also, such tool can be of great help if introduced during training of young surgeons to improve their learning experience by having a quantitative feedback of their manual perception. Before being tested in vivo, we need to establish the measurement protocol and test the robustness and durability of the device. Also the inclusion of a passive telemetry system is being studied which we believe will improve the usage of the device. We are also addressing technical issues of autoclave sterilization, storage and manufacturing costs.
5. Conclusion Extending the capabilities of current available surgical instruments is a versatile and easy way to improve surgical outcome without changing surgical procedures or impose to surgeons new surgical techniques. In contrast with the existing ligament balance tools the developed instrumented distractor allows a manual
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loading control by the surgeon while providing both measurements of tibial-femoral forces and flexion-extension gaps. The determination of ligament stiffness was also achieved with good reliability. The quantification of such parameters can possibly contribute to validate the way surgeons implant prosthesis at the same time that it can provide useful in vivo information for a better understanding of the complex knee joint. Acknowledgements We would like to thank Symbios and Dr. V. Leclercq for supporting this work and also Dr. K. Boulos for their valuable advice and surgical expertise. Competing interests: All authors confirm that there are no financial and personal relationships with other people or organizations that could inappropriately influence this work. Funding: This work has been supported by the Swiss federal program funding, Nano-tera, through the SImOS (Smart Implants for Orthopaedics Surgery) project - SNF20NAN1-123630. Ethical approval: Ethical approval was given by the cantonal ethics committee for research on human being in Lausanne, Switzerland, chaired by Prof. Dr. Roger Darioli under the protocol 441/11. References [1] Schirm AC, Jeffcote BO, Nicholls RL, Jakob H, Kuster MS. Sensitivity of knee soft-tissues to surgical technique in total knee arthroplasty. The Knee 2011;18(3):180–4. [2] Basic orthopaedic biomechanics and mechano-biology. 3rd ed. Philadelphia, PA: Lippincott Williams & Wilkins; 2004. [3] Jolles BM, Grzesiak A, Eudier A, Dejnabadi H, Voracek C, Pichonnaz C, Aminian K, Martin E. A randomised controlled clinical trial and gait analysis of fixed- and mobile-bearing total knee replacements with a five-year follow-up. Journal of Bone and Joint Surgery, British Volume 2012;94(5):648–55. [4] Ritter Ma, Faris PM, Keating EM. Posterior cruciate ligament balancing during total knee arthroplasty. Journal of Arthroplasty 1988;3(4):323–6. [5] Matsumoto T, Muratsu H, Tsumura N, Mizuno K, Kuroda R, Yoshiya S, Kurosaka M. Joint gap kinematics in posterior-stabilized total knee arthroplasty measured by a new tensor with the navigation system. Journal of Biomechanical Engineering 2006;128(6):867–71. [6] Crottet D, Maeder T, Fritschy D, Bleuler H, Nolte L-p, Pappas IP, Modeling A. Device designed to improve the ligament balancing procedure in TKA. Evolution 2005;52(9):1609–11. [7] Attfield SF, Warren-Forward M, Wilton T, Sambatakakis A. Measurement of soft tissue imbalance in total knee arthroplasty using electronic instrumentation. Medical Engineering & Physics 1994;16(6):501–5. [8] Almouahed S, Gouriou M, Hamitouche C, Stindel E, Roux C. Design and evaluation of instrumented smart knee implant. IEEE Transactions on Bio-medical Engineering 2011;58(4):971–82. [9] Werner FW, Ayers DC, Maletsky LP, Rullkoetter PJ. The effect of valgus/varus malalignment on load distribution in total knee replacements. Journal of Biomechanics 2005;38(2):349–55.
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