The Knee 22 (2014) 985–986
Contents lists available at ScienceDirect
The Knee
Editorial
New Tensor/Sensor Technologies
I attended the Current Concepts in Joint Replacement Las Vegas meeting last week, and am always intrigued by efforts to introduce new ideas or concepts. One of the presentations done by a dear friend and eminent American knee surgeon was a live broadcast surgical case demonstrating a new bicruciate knee replacement. The concept is not new, and was popularized years ago by Jean Marie Cloutier and Karl Hamelynck to name a few. The technical requirements entail creating a tibial cut that preserves the tibial eminence and then adjusting the medial and lateral gaps to capture the ‘normal’ kinematics of the knee. These are not new concepts, and the basic surgical technique I witnessed on live video was done with a lot of skill, patience, and pure ‘Kentucky windage’. Cloutier used a two pin rectangle frame that captured the femoral rotation axis and then placed cuts of the femur and tibia to match the geometry of the frame. Hamelynk basically expanded on the LCS tibil cut first technique to make gap spaced cuts of the femur in flexion and extension. The objective was to have gaps spaced carefully at 0 and 90 degrees of flexion. For the practicing surgeon, gap balancing takes on the experience of an art form as teachers of this method have difficulty defining the target outcomes. What, for example, constitutes a well-balanced total knee? I recently reviewed the results of contemporary clinical studies that would suggest that four millimeters of varus or valgus laxity would be a typical amount of laxity tolerated in satisfactory total knees through the range of motion. This is greater than we identify in normal knees and may represent the best we can do with the variety adjustments and ‘abnormalities’ that we create and tolerate with our surgical techniques. I would point out to the readership that the horizon of scientific investigation in this field has moved to new research tools such as sophisticated tensors and computer navigation technologies that will help us create and measure these gaps. So the new game is the world of gap tensors and sensors. The key function of a gap tensor is to measure the force/displacement curve of a ligament at some position of knee flexion. Any tensor has the following features: 1.) a vertical distractor that can have a measured load; 2.) a teter-toter plate or rotation point that allows the gap to widen asymetrically; 3.) a caliper that measures the distance of separation of the joint surfaces. Numerous authors have published on the loads needed to test ligament function and I may summarize that loading could vary from 80 to 180 Newtons with the predominance of opinion choosing from 100 to 150 Newtons. I believe knee ligaments are reasonably stiff structures and our goal with tensing is to find a point where the gap doesn’t stretch any more. This is basically a binary solution; the gap is either ‘loose’ or ‘tight’ and the transition in between is very short perhaps on the order of a millimeter. Fred Picard and Peter Walker have produced elegant studies recently that confirm just that point. [1, 2] We do not need to worry as much about the optimal load for a
http://dx.doi.org/10.1016/j.knee.2014.05.009 0968-0160/© 2014 Published by Elsevier B.V.
given patient once we get the ligaments snug. On the other hand, adjustments of abnormally high ligament tension can be used as a way to balance tight ligaments to an optimal zone, as has been suggested with new digitally instrumented sensors. The mechanical tensor from Kobe University and published by Matsumoto, et.al. is designed to assess soft tissue balancing throughout the range of motion with the extensor mechanism reduced [3]. The tensor has a proximal plate that conforms to the geometry of either a posterior stabilized femoral prosthesis or a cruciate retaining implant. Publications from these authors give us some early idea how the gaps seem to widen after 10° of flexion especially in the lateral joint with cruciate sacrifice and then there is a tightening of the gaps after 90° flexion. Another important tensor design comes from the Osaka University group, and as reported Minoda, et.al. adds the potential of articulating the upper see saw plate with an actual mobile bearing tibial insert. [4] These inserts can then be matched to the femoral component, increasing the precision of the gap assessment. Both the Kobe and Osaka tensors assessed the joint gap as a single distance in the middle of the joint and the asymmetry of the joint gap by measuring the varus/valgus tilt of the upper plate. I have recently presented my experience with a new software protocol that uses ‘bone morphing’ as described by Stindel, et.al. to define the anatomy of the medial and lateral femoral condyle [5]. Used in a standard computer navigation scheme, I am now able to accurately measure the medial and lateral joint gaps at each degree of flexion through the range of motion. The first study looked at eight cadaveric specimens, simply assessing the ‘normal’ gaps measured after a proximal tibial cut as one may do in total knee replacement. The findings were novel, but not terribly surprising. Comparing the group of specimens, each specimen had unique medial and lateral gaps at each degree of flexion, and there were significant differences with slope and treatment of the cruciate ligaments. Had I tested prosthetic dimensions with specific features of joint line position and implant geometry, I imagine each would have created another independent variable. Perhaps the most ‘high tech’ new system is the instrumented tibial trials produced by Orthosensor, Inc. That system uses a variety of technologies including wireless radiofrequency transmitters, accelerometers, and micro-load cells and is integrated with a computer consol. The surgeon can then assess range of motion, contact position on the tibial insert, and importantly the ligament tension loads. In a clinical application, the surgeon does gap balancing after trials are implanted to adjust the contact point on the tibial trial and the amount of pressure caused by the weight of the leg. This optimally is set to about 60 to 70 Newtons. Peter Walker published his laboratory experience with this system using the baseline load applied as the weight of the leg [2]. He found that very small changes in the gaps of one to two millimeters
986
Editorial
caused fairly dramatic increase in the stresses measured by the tibial sensor, allowing the surgeon the potential of adjusting that load with simple releases of the ligaments. In summary, these new methods are attractive as they quantify gap distances and the loads needed to space those gaps. We are learning that 0 and 90 degrees of flexion as we have done for many years work well enough but may not account for all outliers. Recent studies have suggested that important abnormal laxity occurs in mid-flexion and there are a number authors who have identified the tightening that occurs past 90° of flexion [4]. As suggested by Peter Walker, important surgical variables include changes in the posterior tibial slope, distal joint line, femoral component anterior/posterior dimension or offset, tight medial or lateral ligaments that stuff the joint, and bone cuts that cause the same stuffing [2]. Future research will investigate these complex issues in the clinical setting and I just want to appraise our readership of upcoming innovations that we will be assessing. I think we will learn that overall, we have done a pretty good job for our patients. However, as we get more technologically sophisticated, we will wonder just how we did it in the ‘old days’.
References [1] Wilson WT, Deakin AH, Wearing SC, Payne AP, Clarke JV, Picard F. Computer-assisted measurments of coronal knee joint laxity in vitro are related to low-stress behavior reather than the structural properties of the collateral ligaments. Comput Aided Surg 2013 [Epub ahead of print, PMID: 23697384]. [2] Walker PS, Meere P, Bell CP. Effects of surgical variables in balancing total knee using instrumented tibial trial. Knee 2014;21:156–61. [3] Matsumoto T, Kuroda R, Kubo S, Muratsu H, Muzuno K, Kurosaka M. The intraoperative joint gap in cruciate-retaining compared with posterior-stabilized total knee replacement. J Bone Joint Surg Br 2009;91:475–80. [4] Minoda Y, Iwaki H, Ikebucchi M, Hoshida T, Nakamura H. Intraoperative assessment of midflexion laxity in total knee prosthesis. Knee 2014;21:810–4. [5] Stindel E, Briard JL, Merloz P, Plaweski S, Dubrana F, Lefevre C, et al. Bonemorphing: 3D morphological data for total knee arthroplasty. Comput Aided Surg 2002;7(3): 156–68.
Jim Stiehl Editor-In-Chief 24 October 2014
Contents lists available at ScienceDirect
The Knee
Editorial
New Tensor/Sensor Technologies
I attended the Current Concepts in Joint Replacement Las Vegas meeting last week, and am always intrigued by efforts to introduce new ideas or concepts. One of the presentations done by a dear friend and eminent American knee surgeon was a live broadcast surgical case demonstrating a new bicruciate knee replacement. The concept is not new, and was popularized years ago by Jean Marie Cloutier and Karl Hamelynck to name a few. The technical requirements entail creating a tibial cut that preserves the tibial eminence and then adjusting the medial and lateral gaps to capture the ‘normal’ kinematics of the knee. These are not new concepts, and the basic surgical technique I witnessed on live video was done with a lot of skill, patience, and pure ‘Kentucky windage’. Cloutier used a two pin rectangle frame that captured the femoral rotation axis and then placed cuts of the femur and tibia to match the geometry of the frame. Hamelynk basically expanded on the LCS tibil cut first technique to make gap spaced cuts of the femur in flexion and extension. The objective was to have gaps spaced carefully at 0 and 90 degrees of flexion. For the practicing surgeon, gap balancing takes on the experience of an art form as teachers of this method have difficulty defining the target outcomes. What, for example, constitutes a well-balanced total knee? I recently reviewed the results of contemporary clinical studies that would suggest that four millimeters of varus or valgus laxity would be a typical amount of laxity tolerated in satisfactory total knees through the range of motion. This is greater than we identify in normal knees and may represent the best we can do with the variety adjustments and ‘abnormalities’ that we create and tolerate with our surgical techniques. I would point out to the readership that the horizon of scientific investigation in this field has moved to new research tools such as sophisticated tensors and computer navigation technologies that will help us create and measure these gaps. So the new game is the world of gap tensors and sensors. The key function of a gap tensor is to measure the force/displacement curve of a ligament at some position of knee flexion. Any tensor has the following features: 1.) a vertical distractor that can have a measured load; 2.) a teter-toter plate or rotation point that allows the gap to widen asymetrically; 3.) a caliper that measures the distance of separation of the joint surfaces. Numerous authors have published on the loads needed to test ligament function and I may summarize that loading could vary from 80 to 180 Newtons with the predominance of opinion choosing from 100 to 150 Newtons. I believe knee ligaments are reasonably stiff structures and our goal with tensing is to find a point where the gap doesn’t stretch any more. This is basically a binary solution; the gap is either ‘loose’ or ‘tight’ and the transition in between is very short perhaps on the order of a millimeter. Fred Picard and Peter Walker have produced elegant studies recently that confirm just that point. [1, 2] We do not need to worry as much about the optimal load for a
http://dx.doi.org/10.1016/j.knee.2014.05.009 0968-0160/© 2014 Published by Elsevier B.V.
given patient once we get the ligaments snug. On the other hand, adjustments of abnormally high ligament tension can be used as a way to balance tight ligaments to an optimal zone, as has been suggested with new digitally instrumented sensors. The mechanical tensor from Kobe University and published by Matsumoto, et.al. is designed to assess soft tissue balancing throughout the range of motion with the extensor mechanism reduced [3]. The tensor has a proximal plate that conforms to the geometry of either a posterior stabilized femoral prosthesis or a cruciate retaining implant. Publications from these authors give us some early idea how the gaps seem to widen after 10° of flexion especially in the lateral joint with cruciate sacrifice and then there is a tightening of the gaps after 90° flexion. Another important tensor design comes from the Osaka University group, and as reported Minoda, et.al. adds the potential of articulating the upper see saw plate with an actual mobile bearing tibial insert. [4] These inserts can then be matched to the femoral component, increasing the precision of the gap assessment. Both the Kobe and Osaka tensors assessed the joint gap as a single distance in the middle of the joint and the asymmetry of the joint gap by measuring the varus/valgus tilt of the upper plate. I have recently presented my experience with a new software protocol that uses ‘bone morphing’ as described by Stindel, et.al. to define the anatomy of the medial and lateral femoral condyle [5]. Used in a standard computer navigation scheme, I am now able to accurately measure the medial and lateral joint gaps at each degree of flexion through the range of motion. The first study looked at eight cadaveric specimens, simply assessing the ‘normal’ gaps measured after a proximal tibial cut as one may do in total knee replacement. The findings were novel, but not terribly surprising. Comparing the group of specimens, each specimen had unique medial and lateral gaps at each degree of flexion, and there were significant differences with slope and treatment of the cruciate ligaments. Had I tested prosthetic dimensions with specific features of joint line position and implant geometry, I imagine each would have created another independent variable. Perhaps the most ‘high tech’ new system is the instrumented tibial trials produced by Orthosensor, Inc. That system uses a variety of technologies including wireless radiofrequency transmitters, accelerometers, and micro-load cells and is integrated with a computer consol. The surgeon can then assess range of motion, contact position on the tibial insert, and importantly the ligament tension loads. In a clinical application, the surgeon does gap balancing after trials are implanted to adjust the contact point on the tibial trial and the amount of pressure caused by the weight of the leg. This optimally is set to about 60 to 70 Newtons. Peter Walker published his laboratory experience with this system using the baseline load applied as the weight of the leg [2]. He found that very small changes in the gaps of one to two millimeters
986
Editorial
caused fairly dramatic increase in the stresses measured by the tibial sensor, allowing the surgeon the potential of adjusting that load with simple releases of the ligaments. In summary, these new methods are attractive as they quantify gap distances and the loads needed to space those gaps. We are learning that 0 and 90 degrees of flexion as we have done for many years work well enough but may not account for all outliers. Recent studies have suggested that important abnormal laxity occurs in mid-flexion and there are a number authors who have identified the tightening that occurs past 90° of flexion [4]. As suggested by Peter Walker, important surgical variables include changes in the posterior tibial slope, distal joint line, femoral component anterior/posterior dimension or offset, tight medial or lateral ligaments that stuff the joint, and bone cuts that cause the same stuffing [2]. Future research will investigate these complex issues in the clinical setting and I just want to appraise our readership of upcoming innovations that we will be assessing. I think we will learn that overall, we have done a pretty good job for our patients. However, as we get more technologically sophisticated, we will wonder just how we did it in the ‘old days’.
References [1] Wilson WT, Deakin AH, Wearing SC, Payne AP, Clarke JV, Picard F. Computer-assisted measurments of coronal knee joint laxity in vitro are related to low-stress behavior reather than the structural properties of the collateral ligaments. Comput Aided Surg 2013 [Epub ahead of print, PMID: 23697384]. [2] Walker PS, Meere P, Bell CP. Effects of surgical variables in balancing total knee using instrumented tibial trial. Knee 2014;21:156–61. [3] Matsumoto T, Kuroda R, Kubo S, Muratsu H, Muzuno K, Kurosaka M. The intraoperative joint gap in cruciate-retaining compared with posterior-stabilized total knee replacement. J Bone Joint Surg Br 2009;91:475–80. [4] Minoda Y, Iwaki H, Ikebucchi M, Hoshida T, Nakamura H. Intraoperative assessment of midflexion laxity in total knee prosthesis. Knee 2014;21:810–4. [5] Stindel E, Briard JL, Merloz P, Plaweski S, Dubrana F, Lefevre C, et al. Bonemorphing: 3D morphological data for total knee arthroplasty. Comput Aided Surg 2002;7(3): 156–68.
Jim Stiehl Editor-In-Chief 24 October 2014
