This work was supported by Beijing Hospitals Authority Clinical Medicine Development of Special Funding Support [grant numbers: XMLX202139]. We would like to express our gratitude to Diego Wang for valuable suggestions.

The protocol was approved by the Ethics Committee of Xuanwu Hospital (grant number: 2021-224) and was conducted in accordance with the Declaration of Helsinki. Informed consent was obtained from next of kin to use the cadavers.
1. Installation of angle measuring device
<ol>
	<li>Turn on the switch of the femur and tibia angle measuring device. Open the angle measurement software on the tablet computer, scan the QR codes of the two measuring devices, and click Bluetooth connection.</li>
	<li>Place the two angle measuring instruments on the horizontal table, click the Calibration button to calibrate, and tie them with straps 10 cm above and below the knee to measure the flexion angle of the knee in real time (Figure 1).</li>
</ol>
2. Standardized Oxford UKA osteotomy
<ol>
	<li>Place a cadaver in the supine position with the lower extremities draped in flexion and abduction over the outside of the operating table.</li>
	<li>Open the joint cavity by the medial parapatellar approach with a scalpel. Make a cut 3 cm distal to the joint line along the apex of the medial border of the patella, ending distally at 1 cm medial to the tibial tuberosity. Ensure the incision depth reaches the joint cavity.</li>
	<li>Remove osteophytes of the medial femoral condyle, intercondylar fossa, and anterior tibia using a rongeur.</li>
	<li>Insert different sizes of femoral sizing spoons to hook the posterior femoral condyle, and when the end of the spoon is about 1 mm away from the cartilage surface, the size of the femoral prosthesis corresponding to the spoon is suitable.</li>
	<li>Select a 3 mm G-clamp. Connect the G-clamp, tibial saw guide, and femoral sizing spoon together. Ensure that the guide&#39;s shaft is parallel with the long axis of the tibia in both the coronal and sagittal planes, and the ankle yoke points toward the ipsilateral anterior superior iliac spine.</li>
	<li>Make vertical and horizontal cuts on the tibia. Use the reciprocating saw to make a vertical tibial saw cut. Ensure the cut is just medial to the apex of the medial tibial spine. Advance the saw vertically down until it rests on the surface of the saw guide.</li>
	<li>Remove the shim from the tibial resection guide and insert the slotted 0 shim. Use the oscillating saw blade to excise the plateau. Remove the slotted shim, lever the plateau up with a broad osteotome, and remove it with the knee in extension.</li>
	<li>Make a hole in the distal femoral condyle. Ensure the hole is situated 1 cm anterior to the anterior edge of the intercondylar notch and in line with its medial wall.</li>
	<li>Insert the intramedullary rod in the hole. Connect the femoral drill guide with the intramedullary rod. Perform femoral drilling with the help of the femoral drill guide.</li>
	<li>Install the posterior resection guide and insert it into the drilled hole. Ensure the oscillating saw blade is guided by the underside of the posterior resection guide and perform the posterior femoral condyle osteotomy. Remove the guide and the bone fragment.</li>
	<li>Excise the medial meniscus. Leave a small cuff of the meniscus to protect the MCL. Completely remove the posterior horn.</li>
	<li>Insert a 0 femoral spigot. Attach the spherical mill onto the spigot and perform distal femoral milling.</li>
</ol>
3. Preliminary assessment of the flexion-extension gap
<ol>
	<li>Insert a femoral trial. Assess flexion-extension gap by gap gauge.</li>
	<li>Use angle measurement devices to monitor the flexion angle. Define the appropriate flexion-extension gap balance by inserting the gap gauge into the joint space with slight resistance, and the perceived tension as almost equal when the knee is in the flex position at 20&#176; (extension gap) and 110&#176; (flexion gap). If the gaps are not equal, grind the femur according to the difference value between the flexion and extension gaps until they are equal.</li>
</ol>
4. Application of sensor combination to measure flexion and extension gap pressure
<ol>
	<li>Remove the sensor magnetic induction power switch. Open the pressure measurement software on the tablet computer, scan the sensor QR code, and enter the measurement interface.</li>
	<li>Click the Connect Device button; the sensor will be automatically calibrated after successful connection.</li>
	<li>Select the appropriate thickness cushion block according to the gauge specification. Put the sensor on the metal base and install the cushion block on the sensor (Figure 2).</li>
	<li>Click Start working on the tablet computer. Insert the wireless sensor combination into the medial compartment and fit the metal base to the tibial osteotomy surface (Figure 3).</li>
	<li>Measure the flexion and extension gap pressure at 110&#176; (Figure 4A,B) and 20&#176; (Figure 4C, D) of knee flexion. Calculate the average values separately for three consecutive measurements.</li>
</ol>

<ol>
	<li>Spitaels, D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Epidemiology+of+knee+osteoarthritis+in+general+practice:+a+registry-based+study.">Epidemiology of knee osteoarthritis in general practice: a registry-based study.</a> BMJ Open. 10 (1), 031734 (2020).</li><li>Heaps, B. M., Blevins, J. L., Chiu, Y. F., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Improving+estimates+of+annual+survival+rates+for+medial+unicompartmental+knee+arthroplasty,+a+meta-analysis.">Improving estimates of annual survival rates for medial unicompartmental knee arthroplasty, a meta-analysis.</a> The Journal of Arthroplasty. 34 (7), 1538-1545 (2019).</li><li>Goodfellow, J. W., O'Connor, J. J., Pandit, H., Dodd, C. A., Murray, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=.">.</a> Unicompartmental Arthroplasty with the Oxford Knee. , (2016).</li><li>Whiteside, L. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Making+your+next+unicompartmental+knee+arthroplasty+last:+three+keys+to+success.">Making your next unicompartmental knee arthroplasty last: three keys to success.</a> The Journal of Arthroplasty. 20, 2-3 (2005).</li><li>Burger, J. A., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Risk+of+revision+for+medial+unicompartmental+knee+arthroplasty+according+to+fixation+and+bearing+type+:+short-+to+mid-term+results+from+the+Dutch+Arthroplasty+Register.">Risk of revision for medial unicompartmental knee arthroplasty according to fixation and bearing type : short- to mid-term results from the Dutch Arthroplasty Register.</a> The Bone &amp; Joint Journal. 103 (7), 1261-1269 (2021).</li><li>Ridgeway, S. R., McAuley, J. P., Ammeen, D. J., Engh, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+effect+of+alignment+of+the+knee+on+the+outcome+of+unicompartmental+knee+replacement.">The effect of alignment of the knee on the outcome of unicompartmental knee replacement.</a> The Journal of Bone and Joint Surgery. British Volume. 84 (3), 351-355 (2002).</li><li>Suzuki, T., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Evaluation+of+spacer+block+technique+using+tensor+device+in+unicompartmental+knee+arthroplasty.">Evaluation of spacer block technique using tensor device in unicompartmental knee arthroplasty.</a> Archives of Orthopaedic and Trauma Surgery. 135 (7), 1011-1016 (2015).</li><li>Takayama, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Joint+gap+assessment+with+a+tensor+is+useful+for+the+selection+of+insert+thickness+in+unicompartmental+knee+arthroplasty.">Joint gap assessment with a tensor is useful for the selection of insert thickness in unicompartmental knee arthroplasty.</a> Clinical Biomechanics. 30 (1), 95-99 (2015).</li><li>Ettinger, M., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+vitro+kinematics+of+fixed+versus+mobile+bearing+in+unicondylar+knee+arthroplasty.">In vitro kinematics of fixed versus mobile bearing in unicondylar knee arthroplasty.</a> Archives of Orthopaedic and Trauma Surgery. 135 (6), 871-877 (2015).</li><li>Matsumoto, T., Muratsu, H., Kubo, S., Kuroda, R., Kurosaka, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intra-operative+joint+gap+kinematics+in+unicompartmental+knee+arthroplasty.">Intra-operative joint gap kinematics in unicompartmental knee arthroplasty.</a> Clinical Biomechanics. 28 (1), 29-33 (2013).</li><li>Newman, J. H., Ackroyd, C. E., Shah, N. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unicompartmental+or+total+knee+replacement?+Five-year+results+of+a+prospective,+randomised+trial+of+102+osteoarthritic+knees+with+unicompartmental+arthritis.">Unicompartmental or total knee replacement? Five-year results of a prospective, randomised trial of 102 osteoarthritic knees with unicompartmental arthritis.</a> The Journal of Bone and Joint Surgery. British Volume. 80 (5), 862-865 (1998).</li><li>Yang, K. Y., Wang, M. C., Yeo, S. J., Lo, N. N. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Minimally+invasive+unicondylar+versus+total+condylar+knee+arthroplasty-early+results+of+a+matched-pair+comparison.">Minimally invasive unicondylar versus total condylar knee arthroplasty-early results of a matched-pair comparison.</a> Singapore Medical Journal. 44 (11), 559-562 (2003).</li><li>Watson, J., Smith, V., Schmidt, D., Navratil, D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Automatic+implantable+cardioverter-defibrillator:+early+experience+at+Wilford+Hall+USAF+Medical+Center.">Automatic implantable cardioverter-defibrillator: early experience at Wilford Hall USAF Medical Center.</a> Southern Medical Journal. 85 (2), 161-163 (1992).</li><li>D'Ambrosi, R., Vaishya, R., Verde, F. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Balancing+in+unicompartmental+knee+arthroplasty:+balancing+in+flexion+or+in+extension.">Balancing in unicompartmental knee arthroplasty: balancing in flexion or in extension.</a> Journal of Clinical Medicine. 11 (22), 6813 (2022).</li><li>Collier, M. B., Eickmann, T. H., Anbari, K. K., Engh, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Lateral+tibiofemoral+compartment+narrowing+after+medial+unicondylar+arthroplasty.">Lateral tibiofemoral compartment narrowing after medial unicondylar arthroplasty.</a> Clinical Orthopaedics and Related Research. 464, 43-52 (2007).</li><li>Collier, M. B., Eickmann, T. H., Sukezaki, F., McAuley, J. P., Engh, G. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Patient,+implant,+and+alignment+factors+associated+with+revision+of+medial+compartment+unicondylar+arthroplasty.">Patient, implant, and alignment factors associated with revision of medial compartment unicondylar arthroplasty.</a> The Journal of Arthroplasty. 21 (6), 108-115 (2006).</li><li>D'Ambrosi, R., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Radiographic+and+clinical+evolution+of+the+Oxford+unicompartmental+knee+arthroplasty.">Radiographic and clinical evolution of the Oxford unicompartmental knee arthroplasty.</a> The Journal of Knee Surgery. 36 (3), 246-253 (2023).</li><li>Koskinen, E., Paavolainen, P., Eskelinen, A., Pulkkinen, P., Remes, V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unicondylar+knee+replacement+for+primary+osteoarthritis:+a+prospective+follow-up+study+of+1,819+patients+from+the+Finnish+Arthroplasty+Register.">Unicondylar knee replacement for primary osteoarthritis: a prospective follow-up study of 1,819 patients from the Finnish Arthroplasty Register.</a> Acta Orthopaedica. 78 (1), 128-135 (2007).</li><li>ten Ham, A. M., Heesterbeek, P. J. C., vander Schaaf, D. B., Jacobs, W. C. H., Wymenga, A. B. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Flexion+and+extension+laxity+after+medial,+mobile-bearing+unicompartmental+knee+arthroplasty:+a+comparison+between+a+spacer-+and+a+tension-guided+technique.">Flexion and extension laxity after medial, mobile-bearing unicompartmental knee arthroplasty: a comparison between a spacer- and a tension-guided technique.</a> Knee Surgery, Sports Traumatology, Arthroscopy. 21 (11), 2447-2452 (2013).</li><li>Clarius, M., Seeger, J. B., Jaeger, S., Mohr, G., Bitsch, R. G. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+importance+of+pulsed+lavage+on+interface+temperature+and+ligament+tension+force+in+cemented+unicompartmental+knee+arthroplasty.">The importance of pulsed lavage on interface temperature and ligament tension force in cemented unicompartmental knee arthroplasty.</a> Clinical Biomechanics. 27 (4), 372-376 (2012).</li><li>Heyse, T. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Balancing+UKA:+overstuffing+leads+to+high+medial+collateral+ligament+strains.">Balancing UKA: overstuffing leads to high medial collateral ligament strains.</a> Knee Surgery, Sports Traumatology, Arthroscopy. 24 (10), 3218-3228 (2016).</li><li>Heyse, T. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Balancing+mobile-bearing+unicondylar+knee+arthroplasty+in+vitro.">Balancing mobile-bearing unicondylar knee arthroplasty in vitro.</a> Knee Surgery, Sports Traumatology, Arthroscopy. 25 (12), 3733-3740 (2017).</li><li>Jaeger, S., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+influence+of+the+femoral+force+application+point+on+tibial+cementing+pressure+in+cemented+UKA:+an+experimental+study.">The influence of the femoral force application point on tibial cementing pressure in cemented UKA: an experimental study.</a> Archives of Orthopaedic and Trauma Surgery. 132 (11), 1589-1594 (2012).</li><li>Peersman, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Kinematics+of+mobile-bearing+unicompartmental+knee+arthroplasty+compared+to+native:+results+from+an+in+vitro+study.">Kinematics of mobile-bearing unicompartmental knee arthroplasty compared to native: results from an in vitro study.</a> Archives of Orthopaedic and Trauma Surgery. 137 (11), 1557-1563 (2017).</li><li>Sun, X., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Sensor+and+machine+learning-based+assessment+of+gap+balancing+in+cadaveric+unicompartmental+knee+arthroplasty+surgical+training.">Sensor and machine learning-based assessment of gap balancing in cadaveric unicompartmental knee arthroplasty surgical training.</a> International Orthopaedics. 45 (11), 2843-2849 (2021).</li></ol>

The authors have nothing to disclose.

Mobile-bearing UKA is an effective treatment for anteromedial KOA. It has the advantages of less trauma, quick recovery, and maintaining normal knee proprioception11,12,13. The key to UKA is flexion-extension balance; that is, making the flexion gap and extension gap as equal as possible on the premise of restoring MCL tension14. The imbalance may lead to bearing dislocation, prosthesis wear, or progression in the lateral compartment15,16,17,18. Balance techniques are usually related to the surgeon&#39;s experience, which affects patient satisfaction and prosthesis survival.
Gap gauge is a widely used UKA gap balance tool now. The surgeon inserts the gap gauge into joint space and feels the gap tension to roughly determine whether the flexion and extension gap is balanced. This approach relies heavily on the surgeon&#39;s sensation and experience, so it is difficult for beginners to achieve a precise balance, which is one of the reasons for the steep learning curve of UKA and the development of prosthetic complications. In addition, this method does not meet the requirements of millimeter level femur grinding in UKA.
Subsequently, tensors were applied to UKA&#39;s gap balance assessment19. Tensors can apply a constant distraction force to joint space to restore the tension of the MCL. By measuring the distracting distance of the joint space, it can accurately measure the flexion and extension gap. However, because the tensor can exert different distraction forces, the distracting distance of the joint space changes when the MCL is not restored to normal tension or the MCL is over-distracted to injury. At present, an appropriate distraction force that can match different bearing thicknesses has not been agreed on7,8,19.
Different from the above two rough measurement tools, the wireless sensor we used is embedded with three integrated pressure probes, which can display the intra-articular pressure during the knee&#39;s full range of motion in real time. The wireless sensor converts the traditional rough feeling of MCL tension into accurate intra-articular pressure, and with the aid of an angle measuring device, surgeons can accurately assess flexion-extension balance. For surgeons, especially beginners, this can effectively assist in a precise osteotomy, shorten the learning curve, and improve the surgical effect.
In order to match different sizes of bearings and femoral prostheses, wireless sensors are also available in various sizes. In the process of use, the most important thing is to select an appropriate cushion block for the sensor according to the osteotomy plan; otherwise, it may lead to wear of the osteotomy surface and damage the flexion-extension gap balance.
Previous studies have reported sensors with fewer embedded pressure probes resulting in lower accuracy or wired transmission that does not meet the aseptic requirements during surgery20,21,22,23,24,25. The wireless sensor we used considers both measurement accuracy and aseptic requirements. In this in vitro study, we found that even an experienced surgeon could not achieve complete equivalence of flexion and extension gaps. To determine a reasonable pressure range for flexion-extension balance, further large-sample, multicenter in vivo studies are required.
However, this wireless sensor also has some limitations. First, the frequent insertion of the sensor combination with its metal base may wear the osteotomy surface of the tibial plateau, causing osteotomy errors, which is contrary to the original goal of precise flexion-extension balance. Secondly, the wireless sensor we used is a disposable device; the battery can only supply power for about 3 h. Currently, our team is improving the technology to enable the sensor to be reused and meet wireless charging requirements. Additionally, we are also designing a new cushion block that can completely simulate mobile bearings to display the contact point trajectory of the tibial and femoral prostheses in real time during knee flexion and extension movements.
The use of the wireless sensor can help to quantify intra-articular pressure and guide osteotomy to achieve accurate flexion-extension balance. This will compensate for the lack of gap balance experience in beginners and reduce the learning difficulty of UKA. The application of wireless sensors reflects the trend toward individualized and intelligent approaches to joint surgery and the technological innovation brought about by close interdisciplinary cooperation.

Knee osteoarthritis (KOA) is a global burden1, for which the stepwise treatment strategy is currently adopted. For end-stage unicompartmental KOA, unicompartmental knee arthroplasty (UKA) is an effective choice, with a 10-year survival rate of over 90%2. Medial UKA only replaces the severely worn medial compartment and preserves the natural lateral compartment, medial collateral ligament (MCL), and cruciate ligament3. The principle is to make the flexion gap and extension gap approximately the same by tibial osteotomy and femoral grinding, and to restore MCL tension after implantation of the prosthesis and bearing4. Compared with total knee arthroplasty, UKA has greater surgical difficulty and technical requirements. The main source is the proper balance of ligaments throughout the full range of motion of the knee3.
Traditionally, after preliminary osteotomy, the surgeon inserts a gap gauge in the joint space and indirectly determines whether the flexion and extension gaps are equal by feeling the tension of the MCL. However, the definition and sensation of balance are hardly the same, even for experienced surgeons. For beginners, it is more difficult to grasp the requirement of balance. The imbalance of the flexion-extension gap can lead to a series of complications5,6, resulting in an increased revision rate.
With the advancement of technology, some researchers have tried to apply tensors to UKA7,8. However, these researches are all on the fixed-bearing UKA, and the tensor may damage the MCL when used.
The emergence of sensors not only meets the demand for displaying the pressure in the knee joint gap, but various sensors often have less risk of MCL damage due to their small size9,10. In addition, the sensors currently used are all wired transmission, which may interfere with the aseptic operation and is not convenient enough to use.
In order to accurately measure the flexion-extension gap balance parameters, we developed a wireless sensor combination for UKA, which consists of a metal base, a wireless sensor with three pressure probes on the front, medial, and lateral sides, and a cushion block. The sensor combination measures and displays the pressure in the joint space in real time to help surgeons accurately assess whether the balance target has been achieved.

<table><tbody><tr><td>angle measuring device</td><td>AIQIAO(SHANGHAI) MEDICAL TECHNOLOGY CO., LTD.</td><td>20203010141</td><td>angle measuring device of femur,angle measuring device of tibia</td></tr><tr><td>Oxford Partial Knee System</td><td>Biomet UK LTD.</td><td>20173130347</td><td>Oxford UKA</td></tr><tr><td>Wireless sensor combination</td><td>AIQIAO(SHANGHAI) MEDICAL TECHNOLOGY CO., LTD.</td><td>20212010325</td><td>a metal base,&amp;nbsp; a wireless sensor with three pressure probes, and a cushion block</td></tr></tbody></table>

in vitro application of a wireless sensor in flexion-extension gap balance of unicompartmental knee arthroplasty

Unicompartmental knee arthroplasty (UKA) is an effective treatment for end-stage anteromedial osteoarthritis (AMOA). The key to UKA is the flexion-extension gap balance, which is closely related to postoperative complications such as bearing dislocation, bearing wear, and arthritis progression. The traditional gap balance assessment is performed by indirectly sensing the tension of the medial collateral ligament by a gap gauge. It relies on the surgeon's feel and experience, which is imprecise and difficult for beginners. To accurately assess the flexion-extension gap balance of UKA, we developed a wireless sensor combination consisting of a metal base, a pressure sensor, and a cushion block. After osteotomy, the insertion of a wireless sensor combination allows the real-time measurement of intra-articular pressure. It accurately quantifies the flexion-extension gap balance parameters to guide further femur grinding and tibia osteotomy, to improve the accuracy of gap balance. We conducted an in vitro experiment with the wireless sensor combination. the results showed that there was a difference of 11.3 N after applying the traditional method of flexion-extension gap balance performed by an experienced expert.

This in vitro study was performed on a 60-year-old female cadaver. With the S-size femoral prosthesis and 3 mm bearing the target, after performing femoral grinding and tibial osteotomy, the surgeon used the gap gauge to assess flexion-extension gap tension preliminarily and believed that balance was achieved.
After the femoral trial was installed, the wireless sensor was inserted into the medial joint space, and the intra-articular pressure was measured three times at 110&#176; (flexion gap) and 20&#176; (extension gap) of flexion. The flexion-extension gap pressure was 49.9 N-44.8 N, 47.1 N-25.9 N, and 42.0 N-34.2 N (Table 1). The pressure values for the flexion gap were quite consistent, while the pressure values for the extension gap were quite different. The average pressure in the flexion and extension gaps were 46.3 N and 35.0 N, respectively, with a mean difference of 11.3 N. Postoperative radiographs showed appropriate prosthesis positioning (Figure 5).
<table border="1" fo:keep-together.within-page="1" fo:keep-with-next.within-page="always">
	<tbody>
		<tr>
			<td rowspan="2">Measurement times</td>
			<td colspan="2">Intra-articular pressure (N)</td>
		</tr>
		<tr>
			<td>Flexion 110&#176; (flexion gap)</td>
			<td>Flexion 20&#176; (extension gap)</td>
		</tr>
		<tr>
			<td>1</td>
			<td>49.9</td>
			<td>44.8</td>
		</tr>
		<tr>
			<td>2</td>
			<td>47.1</td>
			<td>25.9</td>
		</tr>
		<tr>
			<td>3</td>
			<td>42.0</td>
			<td>34.2</td>
		</tr>
		<tr>
			<td>Mean</td>
			<td>46.3</td>
			<td>35.0</td>
		</tr>
	</tbody>
</table>
Table 1: Intra-articular pressure measured by the sensor.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/64993/64993fig01.jpg" /> 
Figure 1: The angle measurement devices. (A) The angle measurement devices were installed 10 cm above and below the center of the knee. (B) The measurement software can display the knee flexion angle in real time. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64993/64993fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/64993/64993fig02.jpg" /> 
Figure 2: The structure of the wireless sensor combination. The wireless sensor combination consists of (A) a metal base, (B) a wireless sensor with three pressure probes (yellow arrows), (C) and a cushion block. (D) The combination formed after nested assembly. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64993/64993fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/64993/64993fig03.jpg" /> 
Figure 3: Application of sensor combination. After the osteotomy and installation of the femoral trial, the wireless sensor combination is inserted in the medial compartment for measurement. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64993/64993fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 4" class="xfigimg" src="/files/ftp_upload/64993/64993fig04.jpg" /> 
Figure 4: Position to measure pressure. (A) The flexion gap pressure was measured at 110&#176; of knee flexion; (B) the flexion gap pressure was 49.9 N. (C) The extension gap pressure was measured at 20&#176; of knee flexion; (D) the extension gap pressure was 44.8 N. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64993/64993fig04large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 5" class="xfigimg" src="/files/ftp_upload/64993/64993fig05.jpg" /> 
Figure 5: Postoperative imaging. Postoperative antero-posterior radiograph showed good tibial component positioning and coverage. A postoperative lateral radiograph showed good positioning and flexion angle of the femoral component. Abbreviations: AP = antero-posterior; LT = lateral). <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64993/64993fig05large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty at JoVE.com

In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty

This protocol presents a cadaveric study of a wireless sensor used in medial unicompartmental knee arthroplasty. The protocol includes the installation of an angle measuring device, standardized Oxford unicompartmental knee arthroplasty osteotomy, preliminary assessment of flexion-extension balance, and application of the sensor to measure flexion-extension gap pressure.

In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty

Unicompartmental knee arthroplasty (UKA) is an effective treatment for end-stage anteromedial osteoarthritis (AMOA). The key to UKA is the ...

in-vitro-application-wireless-sensor-flexion-extension-gap-balance

Universidad San Sebastian

Research

JoVE Journal

Medicine

466 Views.  Xuanwu Hospital Capital Medical University. This protocol presents a cadaveric study of a wireless sensor used in medial unicompartmental knee arthroplasty. The protocol includes the installation of an angle measuring device, standardized Oxford unicompartmental knee arthroplasty osteotomy, preliminary assessment of flexion-extension balance, and application of the sensor to measure flexion-extension gap pressure.

Video: In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty

Bioelectric Analyses of an Osseointegrated Intelligent Implant Design System for Amputees

The projected number of American amputees is expected to rise to 3.6 million by 2050.  Many of these individuals depend on artificial limbs to perform routine activities, but prosthetic suspensions using traditional socket technology can prove to be cumbersome and uncomfortable for a person with limb loss.  Moreover, for those with high proximal amputations, limited residual limb length may prevent exoprosthesis attachment all together. Osseointegrated implant technology is a novel operative procedure which allows firm skeletal attachment between the host bone and an implant. Preliminary results in European amputees with osseointegrated implants have shown improved clinical outcomes by allowing direct transfer of loads to the bone-implant interface.  Despite the apparent advantages of osseointegration over socket technology, the current rehabilitation procedures require long periods of restrictive load bearing prior which may be reduced with expedited skeletal attachment via electrical stimulation.  The goal of the osseointegrated intelligent implant design (OIID) system is to make the implant part of an electrical system to accelerate skeletal attachment and help prevent periprosthetic infection. To determine optimal electrode size and placement, we initiated proof of concept with computational modeling of the electric fields and current densities that arise during electrical stimulation of amputee residual limbs.  In order to provide insure patient safety, subjects with retrospective computed tomography scans were selected and three dimensional reconstructions were created using customized software programs to ensure anatomical accuracy (Seg3D and SCIRun) in an IRB and HIPAA approved study.  These software packages supported the development of patient specific models and allowed for interactive manipulation of electrode position and size. Preliminary results indicate that electric fields and current densities can be generated at the implant interface to achieve the homogenous electric field distributions   required to induce osteoblast migration, enhance skeletal fixation and may help prevent periprosthetic infections.  Based on the electrode configurations experimented with in the model, an external two band configuration will be advocated in the future.

There is a need to develop alternative prosthesis attachment due to limb loss attributed to vascular occlusive diseases and trauma. The goal of the work is to introduce an osseointegrated intelligent implant design system to increase skeletal fixation and reduce periprosthetic infection rates for patients needing osseointegrated technology.

The projected number of American amputees is expected to rise to 3.6 million by 2050.  Many of these individuals depend on artificial limbs to perform ...

Biology

A Mini-Invasive Internal Fixation Technique for Studying Immobilization-Induced Knee Flexion Contracture in Rats

Joint contracture, resulting from a prolonged joint immobilization, is a common complication in orthopedics. Currently, utilizing an internal fixation to restrict knee joint mobility is a widely accepted model to generate experimental contracture. However, implanting application will inevitably cause surgical trauma to the animals. Aiming to develop a less invasive approach, we combined a muscle-gap separation modus with a previously reported mini-incision skill during the surgical procedure: Two mini skin incisions were made on the lateral thigh and leg, followed by performing muscle-gap separation to expose the bone surface. The rat knee joint was gradually immobilized by a preconstructed internal fixation at approximately 135&#176; knee flexion without interfering essential nerves or blood vessels. As expected, this simple technique permits rapid postoperative rehabilitation in animals. The correct position of the internal fixation was confirmed by an x-ray or micro-CT scanning analysis. The range of motion was significantly restricted in the immobilized knee joint than that observed in the contralateral knee joint demonstrating the effectiveness of this model. Besides, histological analysis revealed the development of fibrous deposition and adhesion in the posterior-superior knee joint capsule over time. Thus, this mini-invasive model may be suitable for mimicking the development of immobilized knee joint contracture.

Here, we present a protocol to describe a minimally invasive technique for knee joint immobilization in a rat model. This reproducible protocol, basing on muscle-gap separation modus and the mini-incision skill, is suitable for studying the underlying molecular mechanism of acquired joint contracture.

Joint contracture, resulting from a prolonged joint immobilization, is a common complication in orthopedics. Currently, utilizing an internal fixation to ...

Developmental Biology

Combined In vivo Optical and &#181;CT Imaging to Monitor Infection, Inflammation, and Bone Anatomy in an Orthopaedic Implant Infection in Mice

Multimodality imaging has emerged as a common technological approach used in both preclinical and clinical research. Advanced techniques that combine in vivo optical and &#956;CT imaging allow the visualization of biological phenomena in an anatomical context. These imaging modalities may be especially useful to study conditions that impact bone. In particular, orthopaedic implant infections are an important problem in clinical orthopaedic surgery. These infections are difficult to treat because bacterial biofilms form on the foreign surgically implanted materials, leading to persistent inflammation, osteomyelitis and eventual osteolysis of the bone surrounding the implant, which ultimately results in implant loosening and failure. Here, a mouse model of an infected orthopaedic prosthetic implant was used that involved the surgical placement of a Kirschner-wire implant into an intramedullary canal in the femur in such a way that the end of the implant extended into the knee joint. In this model, LysEGFP mice, a mouse strain that has EGFP-fluorescent neutrophils, were employed in conjunction with a bioluminescent Staphylococcus aureus strain, which naturally emits light. The bacteria were inoculated into the knee joints of the mice prior to closing the surgical site. In vivo bioluminescent and fluorescent imaging was used to quantify the bacterial burden and neutrophil inflammatory response, respectively. In addition, &#956;CT imaging was performed on the same mice so that the 3D location of the bioluminescent and fluorescent optical signals could be co-registered with the anatomical &#956;CT images. To quantify the changes in the bone over time, the outer bone volume of the distal femurs were measured at specific time points using a semi-automated contour based segmentation process. Taken together, the combination of in vivo bioluminescent/fluorescent imaging with &#956;CT imaging may be especially useful for the noninvasive monitoring of the infection, inflammatory response and anatomical changes in bone over time.

Combined In vivo Optical and  &#181;CT Imaging to Monitor Infection, Inflammation, and Bone Anatomy in an Orthopaedic Implant Infection in Mice

Combined optical and &#956;CT imaging in a mouse model of orthopaedic implant infection, utilizing a bioluminescent engineered strain of Staphylococcus aureus, provided the capability to noninvasively and longitudinally monitor the dynamics of the bacterial infection, as well as the corresponding inflammatory response and anatomical changes in the bone.

Multimodality imaging has emerged as a common technological approach used in both preclinical and clinical research. Advanced techniques that combine in ...

Novel Diagnostics in Revision Arthroplasty: Implant Sonication and Multiplex Polymerase Chain Reaction

In orthopedic patients, foreign body-associated infections, especially periprosthetic joint infections (PJIs), are a devastating complication of arthroplasty. Infection requires complex treatment, may result in long hospitalization and causes considerable costs. Multiple surgical revisions can be necessary in these patients, with a loss in function as well as in quality of life.
The routine preoperative diagnostics include blood examination for C-reactive protein (CRP) and other biomarkers, as well as joint aspirate analysis for cell count, differentiation, and culture. Intraoperative specimens for histology and microbiology are also standard procedure. The microbiological examination of removed implants with sonication, in combination with the implementation of molecular biology techniques in microbiology, represent two novel techniques currently employed to enhance the differential diagnostics of PJI.
We present here the step-wise procedure of analyzing joint aspirate and sonication fluid, using a cartridge-based multiplex polymerase chain reaction (PCR) system. Results were matched against conventional cultures and consensus criteria for PJI. Conventional microbiological cultures from tissue biopsies, joint aspirate and sonication fluid showed a sensitivity of 66.7%, 66.7%, and 88.9%, respectively, and a specificity of 82.3%, 54.6%, and 61.5%, respectively. The PCR diagnostic of the sonication fluid and the joint fluid showed a sensitivity of 50.0% and 55.6%, respectively, and both a specificity of 100.0%. Both PCR diagnostics combined had a sensitivity of 66.7% and a specificity of 100.0%. The multiplex PCR therefore presents a rapid diagnostic tool with moderate sensitivity but high specificity in diagnosing PJI.

Differential diagnostics in painful arthroplasty is crucial for treatment success. Joint aspiration is routinely performed preoperatively. Multiplex polymerase chain reaction of the joint aspirate and the sonication fluid, a feasible tool for quick pathogen detection from these samples is described.

In orthopedic patients, foreign body-associated infections, especially periprosthetic joint infections (PJIs), are a devastating complication of ...

Using a Knee Arthrometer to Evaluate Tissue-specific Contributions to Knee Flexion Contracture in the Rat

Normal knee range of motion (ROM) is critical to well-being and allows one to perform basic activities such as walking, climbing stairs and sitting. Lost ROM is called a joint contracture and results in increased morbidity. Due to the difficulty of reversing established knee contractures, early detection is important, and hence, knowing risk factors for their development is essential. The rat represents a good model with which the effect of an intervention can be studied due to the similarity of rat knee anatomy to that of humans, the rat's ability to tolerate long durations of knee immobilization in flexion, and because mechanical data can be correlated with histologic and biochemical analysis of knee tissue.
Using an automated arthrometer, we demonstrate a validated, precise, reproducible, user-independent method of measuring the extension ROM of the rat knee joint at specific torques. This arthrometer can be used to determine the effects of interventions on knee joint ROM in the rat.

The goal of the protocol is to measure the extension range of motion of the rat knee. The effects of various diseases that increase the stiffness of the knee joint and the effectiveness of treatments can be quantified.

Normal knee range of motion (ROM) is critical to well-being and allows one to perform basic activities such as walking, climbing stairs and sitting. Lost ...

A Probing Device for Quantitatively Measuring the Mechanical Properties of Soft Tissues during Arthroscopy

Probing in arthroscopic surgery is performed by pulling or pushing the soft tissue, which provides feedback for understanding the condition of the soft tissue. However, the output is only qualitative based on the &#8220;surgeon&#8217;s feeling&#8221;. Herein is described a probing device developed to address this issue by measuring the resistance of soft tissues quantitatively with a tri-axial force sensor. Under both conditions (i.e., pull- and push-probing certain tissues mimicking the acetabular labrum and cartilage), this probing device is found to be useful for measuring some mechanical properties in joints during arthroscopy.

Probing during arthroscopy surgery is normally done to assess the condition of the soft tissue, but this approach has always been subjective and qualitative. This report describes a probing device that can measure the resistance of the soft tissue quantitatively with a tri-axial force sensor during arthroscopy.

Probing in arthroscopic surgery is performed by pulling or pushing the soft tissue, which provides feedback for understanding the condition of the soft ...

In Vivo Quantification of Hip Arthrokinematics during Dynamic Weight-bearing Activities using Dual Fluoroscopy

Several hip pathologies have been attributed to abnormal morphology with an underlying assumption of aberrant biomechanics. However, structure-function relationships at the joint level remain challenging to quantify due to difficulties in accurately measuring dynamic joint motion. The soft tissue artifact errors inherent in optical skin marker motion capture are exacerbated by the depth of the hip joint within the body and the large mass of soft tissue surrounding the joint. Thus, the complex relationship between bone shape and hip joint kinematics is more difficult to study accurately than in other joints. Herein, a protocol incorporating computed tomography (CT) arthrography, three-dimensional (3D) reconstruction of volumetric images, dual fluoroscopy, and optical motion capture to accurately measure the dynamic motion of the hip joint is presented. The technical and clinical studies that have applied dual fluoroscopy to study form-function relationships of the hip using this protocol are summarized, and the specific steps and future considerations for data acquisition, processing, and analysis are described.

In Vivo Quantification of Hip Arthrokinematics during Dynamic Weight-bearing Activities using Dual Fluoroscopy

Dual fluoroscopy accurately captures in vivo dynamic motion of human joints, which can be visualized relative to reconstructed anatomy (e.g., arthrokinematics). Herein, a detailed protocol to quantify hip arthrokinematics during weight-bearing activities of daily living is presented, including the integration of dual fluoroscopy with traditional skin marker motion capture.

Several hip pathologies have been attributed to abnormal morphology with an underlying assumption of aberrant biomechanics. However, structure-function ...

Sensor Assessment of Gap Balance in Mobile-Bearing Unicompartmental Knee Arthroplasty

The most important procedure of mobile-bearing unicompartmental knee arthroplasty (UKA) is to balance the knee flexion and extension gap. Conventionally, the balance was determined by the subjective assessment of plugging out the feeling gauge. Since it mainly depended on the surgeons' experience, the accuracy was always in doubt. In the past 10 years, pressure sensors have been introduced to guide the gap balance in total knee arthroplasty (TKA). However, the sensor technique was introduced to UKA very recently. Herein is our sensor assessment of the gap balance in 20 cases UKA by one experienced surgeon. The sensor was a custom-designed force sensor matrix according to the shape of the tibial trial of mobile-bearing UKA. The postoperative clinical outcomes and radiographic results were recorded for future comparison. We aim to use this method to assess more than 200 cases of UKA by various surgeons to ultimately standardize the gap-balance result.

Here, we present a protocol to measure the gap contact force and gap balance in mobile-bearing unicompartmental knee arthroplasty (UKA). Along with the clinical and radiographic data, we hope to determine the normal range of the contact force and set up the threshold of the gap balance.

The most important procedure of mobile-bearing unicompartmental knee arthroplasty (UKA) is to balance the knee flexion and extension gap. Conventionally, ...

FSN Therapy for Knee Osteoarthritis Pain Management

Fu's Subcutaneous Needling for Knee Osteoarthritis Pain

Fu's subcutaneous needling (FSN) is a new acupuncture and dry needling technique based on traditional Chinese medicine. It rapidly produces long-lasting effects in soft tissue injuries, particularly in painful musculoskeletal conditions, by providing stimulation primarily in the subcutaneous area. Osteoarthritis (OA) is the most common joint disease in adults worldwide and is often accompanied by a painful syndrome of structural changes in the peripheral joints of the knee. However, the etiology of OA pain is not fully understood, though myofascial trigger points (MTrPs) are commonly found in the lower limb muscles (so-called "tightened muscles") of patients with knee OA. 
FSN has been used in many fields for the treatment of acute pain problems and can relieve muscle contraction from MTrPs, thereby improving the local circulation. This study recruited patients with pain from knee OA into an FSN group or a transcutaneous electrical nerve stimulation (TENS) group with three treatment sessions and a follow-up over the course of 2 weeks. The results showed that FSN was effective in treating soft tissue pain around the knee with OA. This study aimed to establish and visualize three key technical indicators during FSN therapy, including the FSN needle insertion point and layer; the frequency and duration of the swaying movement; and the manipulation of the reperfusion approach. These findings have great potential for future applications in myofascial pain treatment, especially for pain management. Following this protocol could enhance FSN skills.

Author Spotlight: Fu's Subcutaneous Needling for Knee Osteoarthritis Pain  

We present a protocol for using Fu's subcutaneous needling for knee osteoarthritis pain, which combines swaying movement and reperfusion approach techniques. This protocol has great potential for future applications in myofascial pain treatment and could enhance Fu's subcutaneous needling (FSN) manipulation skills.

Fu's subcutaneous needling (FSN) is a new acupuncture and dry needling technique based on traditional Chinese medicine. It rapidly produces long-lasting ...

Impact of Non-Invasive ACL Injury and Surgery on Protease Activity in Mice

Non-Invasive Compression-Induced Anterior Cruciate Ligament (ACL) Injury and In Vivo Imaging of Protease Activity in Mice

Traumatic joint injuries such as anterior cruciate ligament (ACL) rupture or meniscus tears commonly lead to post-traumatic osteoarthritis (PTOA) within 10-20 years following injury. Understanding the early biological processes initiated by joint injuries (e.g., inflammation, matrix metalloproteinases (MMPs), cathepsin proteases, bone resorption) is crucial for understanding the etiology of PTOA. However, there are few options for in vivo measurement of these biological processes, and the early biological responses may be confounded if invasive surgical techniques or injections are used to initiate OA. In our studies of PTOA, we have used commercially available near-infrared protease activatable probes combined with fluorescence reflectance imaging (FRI) to quantify protease activity in vivo following non-invasive compression-induced ACL injury in mice. This non-invasive ACL injury method closely recapitulates clinically relevant injury conditions and is completely aseptic since it does not involve disrupting the skin or the joint capsule. The combination of these injury and imaging methods allows us to study the time course of protease activity at multiple time points following a traumatic joint injury.

Author Spotlight: Investigating Early Events and Long-Term Effects of ACL Injuries for Osteoarthritis Progression

Non-invasive ACL injury is a reliable and clinically relevant method for initiating post-traumatic osteoarthritis (PTOA) in mice. This injury method also allows for in vivo quantification of protease activity in the joint at early time points post-injury using protease-activatable near infrared probes and fluorescence reflectance imaging.

Traumatic joint injuries such as anterior cruciate ligament (ACL) rupture or meniscus tears commonly lead to post-traumatic osteoarthritis (PTOA) within ...

In Vitro Application of a Wireless Sensor in Flexion-Extension Gap Balance of Unicompartmental Knee Arthroplasty

Summary

Explore More Videos

Chapters in this video

Bioelectric Analyses of an Osseointegrated Intelligent Implant Design System for Amputees

A Mini-Invasive Internal Fixation Technique for Studying Immobilization-Induced Knee Flexion Contracture in Rats

Combined In vivo Optical and µCT Imaging to Monitor Infection, Inflammation, and Bone Anatomy in an Orthopaedic Implant Infection in Mice

Novel Diagnostics in Revision Arthroplasty: Implant Sonication and Multiplex Polymerase Chain Reaction

Using a Knee Arthrometer to Evaluate Tissue-specific Contributions to Knee Flexion Contracture in the Rat

A Probing Device for Quantitatively Measuring the Mechanical Properties of Soft Tissues during Arthroscopy

In Vivo Quantification of Hip Arthrokinematics during Dynamic Weight-bearing Activities using Dual Fluoroscopy

Sensor Assessment of Gap Balance in Mobile-Bearing Unicompartmental Knee Arthroplasty

Fu's Subcutaneous Needling for Knee Osteoarthritis Pain

Non-Invasive Compression-Induced Anterior Cruciate Ligament (ACL) Injury and In Vivo Imaging of Protease Activity in Mice