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Accuracy of Balancing at Total Knee Surgery Using an Instrumented Tibial Trial
The Journal of Arthroplasty xxx (2016) 1e5
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Original Article
Accuracy of Balancing at Total Knee Surgery Using an Instrumented Tibial Trial Patrick A. Meere, MD, Svenja M. Schneider, BS, Peter S. Walker, PhD * Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, New York, New York
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2015 Received in revised form 17 February 2016 Accepted 22 February 2016 Available online xxx
Background: Balancing is an important part of a total knee procedure, and in recent years, more emphasis has been given to quantifying the process. Methods: During 101 total knee surgeries, initial bone cuts were made using navigation. Lateral and medial contact forces were determined throughout flexion using an instrumented tibial trial. Balancing was defined as a ratio of the medial and total force, the target being 0.5 (equal lateral and medial forces). Based on the initial values, surgical corrections were selected to achieve balancing. The most common corrections were soft tissue releases (63 incidences), including MCL, posterolateral corner, posteromedial corner, and changing tibial insert thicknesses (34 incidences). Results: After final balancing, the mean ratio was 0.52 ± 0.14, between 0.35 and 0.65 being achieved in 80% of cases. In 84% of cases, only 0-2 corrections were required. The average total force on the condyles was 215 ± 86 N. Conclusion: Our study provides data to surgeons on the results to expect when balancing a knee, which can enhance both accuracy and consistency of the procedure. © 2016 Elsevier Inc. All rights reserved.
Keywords: total knee soft tissue balancing
The main surgical requirements for restoring optimal function in a total knee arthroplasty (TKA) are accurate bone cut alignments and soft tissue balancing throughout flexion [1]. Balancing has been described as equal and rectangular gaps between the resected bone surfaces in both extension and flexion, implying that the lateral and medial soft tissues maintain equal tensions. From an unbalanced state, balancing can be achieved in different ways, including soft tissue releases, making small modifications to bone cuts, and changing component sizes. Central tracking of the femur on the tibia is also a necessary part of balancing, achieved by correct rotation of the components. Balancing has been shown to have advantages such as postoperative patient satisfaction and clinical outcome scores [2-4]. On
No author associated with this paper has disclosed any potential or pertinent conflicts which may be perceived to have impending conflict with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2016.02.050. Initial technical work on this project was carried out by Christopher Bell. The work was supported by the Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, and by an Education Grant from Orthosensor Inc. * Reprint requests: Peter S. Walker, PhD, Laboratory for Knee Implant Design, New York University Hospital for Joint Diseases, 301 East 17th Street, New York, NY 10003. http://dx.doi.org/10.1016/j.arth.2016.02.050 0883-5403/© 2016 Elsevier Inc. All rights reserved.
the other hand, inadequate soft tissue balance has been linked to increased implant wear, loosening, instability, decreased range of motion, and increased pain [2,5-8]. Until now, the most widely used procedure for balancing has involved the use of spacer blocks or distractors, which is dependent on the experience or skill level of the surgeon and does not provide quantitative information. Possibly stimulated by the expanding use of newer methods for achieving accurate bone cut alignments, instrumented distractors and tibial trials have recently been introduced which measure the distraction or contact forces across the knee, allowing for a more quantitative measure of soft tissue balancing at the time of TKA surgery. Once such device is the OrthoSensor Verasense Knee Balancer (OrthoSensor Inc, Sunrise, FL), a wireless load-sensing tibial component that transmits real-time lateral and medial contact forces to a monitor in the operating room [9,10]. A pictorial view of the contact locations allows for assessment of the rotational position of the tibial component. In a recent study using this instrumented tibial trial component, odds ratios analysis demonstrated that balanced joints were 2.5, 1.3, and 1.8 times more likely to achieve a significant improvement in American Knee Society Score, Western Ontario and McMaster Universities Osteoarthritis Index, and activity level, respectively [11]. Increased patient satisfaction was also achieved after balancing using this method [12]. These examples illustrate
2
P.A. Meere et al. / The Journal of Arthroplasty xxx (2016) 1e5
Fig. 1. Ratio of medial to total (medial þ lateral) intercompartmental forces before and after balancing. The standard deviation was considerably reduced after balancing.
the potential clinical value of achieving accurate balancing using instrumented devices at surgery. However, even through the use of such technology, a systematic process for achieving balancing has not been clearly defined. For example, the pattern of lateral and medial contact forces during a flexion range needs to be translated into the most suitable surgical correction to move to a more balanced state. In a previous study on knee specimens [10], we explored different tests to obtain the contact force data and concluded that initially a sag test, where the leg was elevated by lifting the heel, could be used to establish tibial rotation and the best thickness of tibial insert. The heel push test, where the knee was flexed by sliding the heel toward the buttocks keeping the leg in a vertical plane, was a convenient way to obtain contact force data through a full flexion range. From the heel push data, appropriate surgical corrections were deduced, and the sensitivity of the corrections to only small dimensional or angular changes was determined. The present study extends the previous work to actual surgical conditions on arthritic knees. The purpose of the study was to address 2 key questions. First, what is the initial state of balancing after bone cuts have been made and the trial components placed, and how accurately can knees be balanced after surgical corrections within the time frame of a normal surgical procedure? Second, what is the effect of each specific surgical correction that is selected based on the state of imbalance and how many corrections are required to achieve acceptable balancing? The results of the study are intended to provide guidelines to the surgeon for the balancing values and the effects of surgical corrections during the balancing procedure and the final balancing results that are likely to be achieved. The expectation is that the clinical outcomes will be more reproducible if a consistency of balancing can be achieved. Methods A method was formulated for obtaining the required data from each surgical case. Screen capture software (Screenflow;
Telestream, Nevada City, CA) was used to simultaneously record the video and audio feed of the surgery from the computer’s camera and the real-time information from the instrumented tibial trial component. The equipment was set up to afford a full view of the leg, from heel to pelvis, so an accurate flexion angle measurement could be computed. Data were collected for 101 TKA cases using a PCL-retaining device (Triathlon; Stryker, Mahwah, NJ). Initial bone cuts were made using computer navigation based on measured resection. The proximal tibia was cut first perpendicular to the long axis. The distal femur was then cut perpendicular to the mechanical axis of the leg (center of hip to center of ankle). Finally, the posterior and anterior femoral cuts were made, rotating the femur appropriate to each case. The femoral trial was inserted, and the instrumented tibial trial was used to measure the magnitudes and location of the contact forces on the lateral and medial sides and the contact point rotations throughout the flexion range. The first step was to determine the correct thickness of the tibial insert using the sag test (foot lifted upward). The knee was then flexed and extended and the tibial rotation was checked and corrected if necessary. The heel push test, in which the leg is pushed in a cephalad direction through the flexion range [10], was then used to determine the initial value of balancing, defined as a ratio of medial to total force at 0 , 30 , 60 , and 90 flexion. This ratio was used to normalize the data to the range 0-1. A balanced knee with equal lateral and medial forces throughout flexion would show a value of 0.5. In our study, this 0.5 value was used as the target based on the traditional guidelines of equal gaps and tensions in flexion and extension. Surgical corrections were then performed based on the initial values relayed by the tibial trial, with the goal of achieving symmetrical medial and lateral loads and avoiding contact points close to the edges. The corrections could be soft tissue releases achieved by “pie crusting”; increasing or decreasing the thickness of the tibial insert; coronal plane modification to the bone cuts on the proximal tibial or distal femur; increasing the tibial posterior slope; and modifying the rotation of the tibial component. A single
Table 1 Changes in Tibiofemoral Force Ratios Before and After the Four Most Common Surgical Corrections. Change Posterolateral corner release Posteromedial capsule release Medial collateral ligament release
Increase in tibial liner thickness
Average Medial-to-Total Force Ratio Before 0.22 ± 0.23 0.70 ± 0.17 0.64 ± 0.14
Average Medial-to-Total Force Ratio After 0.40 ± 0.15 0.59 ± 0.19 0.53 ± 0.16
Total Force Before
Total Force After
141.36 ± 75.35
235.71 ± 102.31
Significance P ¼ .0001 P ¼ .001 P ¼ .001
P ¼ .0001
P.A. Meere et al. / The Journal of Arthroplasty xxx (2016) 1e5
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Fig. 2. The medial-to-total force ratio before and after posterolateral corner release from 0 to 90 flexion.
surgical variable was applied at a time during surgery and changes in force were determined. The analysis focused on the 4 most common surgical corrections, namely, posterolateral corner (PLC) release, posteromedial capsule (PMC) release, medial collateral ligament (MCL) release, and increasing the tibial insert thickness. For the other corrections, there were insufficient cases to achieve statistical significance in the analysis. An MCL release was performed in cases where the medial-tototal force ratio was considerably above 0.50, indicating tight medial soft tissues. Tightness in flexion and extension was differentiated such that the anterior MCL was released in knees that were tight in flexion while the posterior MCL was released in cases that were tight in extension [1]. A PLC release was done, usually involving the arcuate ligament, popliteus tendon, and/or the lateral collateral ligament, if the medial-to-total force ratio was considerably below 0.50 throughout flexion, indicating a tight lateral side. The illiotibial band was released only if the knee was excessively tight only in extension because it is slack and inactive in flexion [1]. A PMC release was done if the medial-to-total force ratio was considerably above 0.50 throughout flexion. It resulted in an average decrease in medial tightness comparable to the change seen after an MCL release. The PMC release was done in cases with high medial force early in flexion while the MCL release was more often used in cases with high medial load at high flexion angles. The tibial insert thickness was increased if the total forces on the medial and lateral condyle were too low. A total force in the range of 120-145 N was aimed for based on previous research [10,13]. The changes in the tibial insert thickness reported on here were necessary because of changes in overall contact forces from soft tissue releases during the procedure. The heel push test was done to evaluate balance at the initial state and after each correction. Real-time medial and lateral contact forces and contact point rotation data were recorded at
0 , 30 , 60 , and 90 flexion ± 5 of the dynamic testing. CAD software was used to measure the angles of flexion from the surgical video. The contact forces were recorded, and in cases of soft tissue releases, the forces were expressed as a ratio of medial to total force to highlight changes in mediolateral balancing. Results After making the bone cuts and inserting the tibial components, the average medial-to-total compartmental force ratio was 0.49 ± 0.27. After final balancing, the ratio was 0.52 ± 0.14 (Fig. 1). The initial data for all knees were scattered between 0.0 (lateral force only) and 1.0 (medial force only). The data after balancing show a clear narrowing of the range of imbalance between the compartmental loads based on the standard deviation values. Of the 101 knees, a final balancing medial-to-total force ratio between 0.35 and 0.65 was achieved in 80% of cases from 0 to 30 ; 79% of cases from 0 to 60 ; and 77% of cases from 0 to 90 . This range was specified on the basis of being feasible to obtain in most of the cases. When the balancing data were separated into initially medially tight and initially laterally tight knees, the change between initial and final balancing was statistically significant with P values of .0034 and < .0001, respectively. An error analysis was also done using 0.5 as the ideal force ratio. The average error for all knees initially was 0.22, and the average error for all surgically balanced knees was 0.10. The difference between these 2 error groups was statistically significant with a P value < .0001. The average total force on the condyles from 0 to 90 was 283.8 ± 157.9 N (63.8 ± 35.5 lbs) initially and 206.0 ± 83.2 N (46.3 ± 18.7 lbs) after final balancing. All of the above values were obtained with both the femoral and tibial trials in place. In 84% of the cases, 0-2 corrections were needed to obtain balancing. One case required 5 corrections, the maximum recorded.
Fig. 3. The medial-to-total force ratio before and after posteromedial capsule release from 0 to 90 flexion.
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Fig. 4. The medial-to-total force ratio before and after medial collateral ligament release from 0 to 90 flexion.
The frequencies of all the surgical corrections used to achieve balancing were as follows:
63 soft tissue releases 34 changes in tibial insert thickness
17 bone cut adjustments 7 rotations of the tibia
The most common surgical corrections were PLC release (22), PMC release (16), MCL release (14), and increase in the tibial liner thickness (30). All had a large enough sample size to determine a significant change with 80% confidence (Table 1). The actual mean magnitudes of the forces and force ratios as a result of these surgical corrections at different flexion angles are shown in Figures 2-5. The elements of the PLC most frequently released by the surgeon were the arcuate ligament and the popliteus tendon. Before releasing the PLC, the average medial-to-total force ratio was 0.22. After releasing the PLC, the average medial-to-total force ratio was 0.40 (Table 1 and Fig. 2). Before releasing the PLC, the average medial-to-total force ratio was 0.70. After releasing the capsule, the average medial-to-total force ratio was 0.59 (Table 1 and Fig. 3), with the greatest change seen in extension. Before an MCL release, the average medial-to-total force ratio was 0.64. After an MCL release, the average medial-to-total force ratio was 0.53 (Table 1 and Fig. 4). The medial force decreased and the medial-to-total force ratio moved toward 0.50 in all cases. In the 22 cases of 2-mm tibial insert thickness increase, the total force on both the medial and lateral condyle increased by an average of 94.3 ± 85.0 N (21.2 ± 19.1 lbs) throughout the range of flexion (0 -90 ; Fig. 5). This equaled an average percent change of 93%. It was observed that the medial-to-total force ratio did not remain constant and soft tissue releases were often necessary as a secondary correction to obtain optimal balancing across the condyle.
Fig. 5. The change in total force (medial and lateral condyle) after a 2-mm increase in the tibial liner thickness.
Discussion In this study, we determined the balancing status of the knee during the stages of a total knee surgical procedure using an instrumented tibial trial. It was shown that after making the initial bone cuts based on measured resection, the balancing covered a wide range. In any series, the range would depend on the arthritic condition and the proportion of initially varus-to-valgus cases. In our case, the initial mean medial-to-lateral force ratio was 0.49, but with a wide range. The use of the instrumented tibial trial allowed for corrections to be made so that the final mean balancing ratio was 0.52 with a greatly reduced spread of values as shown in Figure 1. The choice of which surgical correction to apply was based on the pattern of force readings over the full flexion range. As indicated in the Methods, the selection was based on whether forces increased or decreased with flexion, the asymmetry, and whether the asymmetry changed with flexion. The force readings from the instrumented tibial trials guided those determinations. The sensitivity of the balancing values to the various surgical corrections was consistent with a previous study showing that changes of 2 mm or 2 could correct most imbalanced states [10]. This was related to the condyle forces being due to soft tissue pretensions, with collateral stiffness being in the region of 50 N/ mm [14,15]. The finding that only up to 2 surgical corrections were needed in most of the cases to achieve an acceptable balancing implied that using a quantitative balancing method would not take additional time compared with qualitative methods. The “acceptable level” in our study from 0.35 to 0.65 medial-to-total force ratio was based on what could readily be achieved and what seemed to be subjectively well balanced. The value is consistent with that of a previous study [11] relating balancing values to clinical outcome. It was interesting however that there was no target value for the actual magnitude of the contact forces, given that there was considerable variation between patients in the finally balanced knees. This presumably reflects the variation in ligament stiffness between individuals. Previous studies of intraoperative balancing have found after measured resection there is a significant imbalance between the medial and lateral compartmental loads and a 2-mm increase to substantially increase tibial force and to exaggerate residual imbalance [6]. A 2-mm increase in tibial insert thickness has previously been shown to increase compressive forces by 54%-131%, with magnitudes particularly high in PCL-retaining and mobile-bearing prostheses [16]. The evidence that 2 mm or 2 results in a change in force sufficient to correct most imbalances is consistent with the high magnitude of change we observed following pie-crusting of ligaments [10]. The number of surgical corrections usually applied in this study is supported by a recent single-surgeon study of soft tissue balancing using the
P.A. Meere et al. / The Journal of Arthroplasty xxx (2016) 1e5
flexion-extension gap method, in which 89.9% of cases required 0-2 surgical corrections [17]. There are a number of limitations to this study. First, the ideal target value or zone for balancing has not yet been validated. While we used 0.5 medial-to-lateral force ratio as the target, in normal knees the ratio may be in the region of 0.55, indicating a higher medial force, based on a study of varus-valgus laxities measured on older individuals [18]. However, in contrast, in younger subjects, the ratio was 0.46 [19]. Moreover, laxity may not correspond to the actual contact forces. A recent study of such forces was carried out using Tekscan (Boston, MA) pressure-sensitive film in knee specimens in conditions resembling surgery [20]. The medial force was highest in extension and was constant after 15 flexion. The lateral force was about half of medial in early flexion and fell to small values in higher flexion. In walking, the medial force was measured at around 65% of the total, in patients with instrumented tibial component [21]. Even with reference to such data, the optimal balancing ratio in total knee patients for optimal function is not known. A related point is which flexion region is the most important to achieve accurate balancing, if it is found that it is difficult to obtain equality throughout flexion. Relating this to function, while the stance phase of level walking involves early flexion, the swing phase requires up to about 60 flexion, while other activities can involve higher angles even in the weight-bearing phase. A further limitation of our study is that the posterior cruciate was retained, which makes the prediction of which ligaments to release more difficult, and it could have different effects at different flexion angles. However, in the intact knee, the lengths of the anterolateral and posteromedial bundles of the PCL increased substantially with flexion [22], implying that the tensions would be relatively low in early to mid-flexion. Hence, balancing in this range may be primarily affected by the collateral ligaments. In addition, while the balancing values were obtained during surgery, there is evidence that stress relaxation of the ligaments occurs in the first 30 minutes after surgery and leads to increased ligament laxity [23]. A related limitation is that the knee was not weight-bearing during the heel push test, but it has previously been shown that passive intraoperative force readings correlate with abnormal postoperative kinematics and condylar liftoff [8]. This however brings into question the advantages of accurate balancing in optimizing function, and whether balancing affects the weight-bearing or swing phases of activity the most. In relation to the surgical corrections themselves, because all of the TKA procedures were performed by 1 surgeon, the technique used to select and achieve each surgical correction may be unique to that surgeon. Hence, data should be obtained from other similarly experienced surgeons as well. In a continuation of this study, further data collection and analysis is being carried out to analyze quantifiable data on every surgical correction including other soft tissue releases, bone cuts, rotation of the tibia, and changes in slope. This information can potentially be used to develop a balancing algorithm that can identify necessary corrections based on an initial pattern of imbalance. One approach for this has been described based on the shape of the varus-valgus laxity patterns during flexion [24], verified by studies on knee specimens. A similar method could be used for the contact force patterns, possibly considering also the contact point locations. This will be a useful tool for surgeons to fully use the information provided by the instrumented tibial trial and the
5
effect of the surgical corrections carried out. This approach has the potential to shorten surgery time and avoid corrections which are not applicable, as well as avoid overcorrection. Finally, all of the patients are currently being studied clinically to gain further insights into the relations between balancing values seen at surgery with varus and valgus stability and functional outcomes. References 1. Whiteside LA. Ligament balancing in total knee arthroplasty: an instructional manual. Berlin: Springer; 2004. 2. Matsuda Y, Ishii Y, Noguchi H, et al. Varus-valgus balance and range of movement after total knee arthroplasty. J Bone Joint Surg Br 2005;87-B:804. 3. Unitt L, Sambatakakis A, Johnstone D, et al. Balancer Study Group. Short-term outcome in total knee replacement after soft-tissue release and balancing. J Bone Joint Surg Br 2008;90(2):159. 4. Winemaker MJ. Perfect balance in total knee arthroplasty. J Arthroplasty 2002;17(1):2. 5. Del Gaizo DJ, Della Valle CJ. Instability in primary total knee arthroplasty. Orthopedics 2011;34(9):e519. 6. D'Lima DD, Patil S, Steklov N, et al. An ABJS Best Paper: dynamic intraoperative ligament balancing for total knee arthroplasty. Clin Orthop Relat Res 2007;463: 208. 7. Lombardi Jr AV, Berend KR, Adams JB. Why knee replacements fail in 2013; patient, surgeon, or implant? Bone Joint J Br 2014;96-B(11 Suppl A):101. 8. Wasielewski RC, Galat DD, Komistek RD. Correlation of compartment pressure data from an intraoperative sensing device with postoperative fluoroscopic kinematic results in TKA patients. J Biomech 2005;38:333. 9. Gustke KA. Use of smart trials for soft-tissue balancing in total knee replacement surgery. J Bone Joint Surg Br 2012;94(11SupplA):147. 10. Walker PS, Meere PA, Bell CP. Effects of surgical variables in balancing of total knee replacements using an instrumented tibial trial. Knee 2014;21(1):156. 11. Gustke KA, Golladay GJ, Roche MW, et al. A new method for defining balance: promising short-term clinical outcomes of sensor-guided TKA. J Arthroplasty 2014;29:955. 12. Gustke KA, Golladay GJ, Roche MW, et al. Increased satisfaction after total knee replacement using sensor-guided technology. Bone Joint J 2014;96-B(10):1333. 13. Zalzal P, Papini M, Petruccelli D, et al. An in vivo biomechanical analysis of the soft-tissue envelope of osteoarthritic knees. J Arthroplasty 2004;19(2):217. 14. Robinson JR, Bull AMJ, Amis AA. Structural properties of the medial collateral ligament complex of the human knee. J Biomech 2005;38(5):1067. 15. Wilson WT, Deakin AH, Payne AP, et al. Comparative analysis of the structural properties of the collateral ligaments of the human knee. J Orthop Sports Phys Ther 2012;42(4):345. 16. Schirm AC, Jeffcote BO, Nicholls RL, et al. Sensitivity of knee soft-tissues to surgical technique in total knee arthroplasty. Knee 2011;18:180. 17. Peters CL, Jimenez C, Erickson J, et al. Lessons learned from selective soft-tissue release for gap balancing in primary total knee arthroplasty: an analysis of 1216 consecutive total knee arthroplasties. J Bone Joint Surg Am 2012;95(152):1. 18. Heesterbeek PJC, Verdonshot N, Wymenga AB. In vivo knee laxity in flexion and extension: a radiographic study in 30 older healthy subjects. Knee 2008;15(1): 45. 19. Schultz SJ, Shimokochi Y, Nguyen A-D, et al. Measurement of varus-valgus and internal-external rotational knee laxities in vivo e part 2: relationship with anterior-posterior and general joint laxity in males and females. J Orthop Res 2007;25:989. 20. Verstraete MA, Meere PA, Salvadore G, et al. Insights from the native knee in load balancing during TKA surgery. Submitted to 22nd Congress of the European Society of Biomechanics. July 10-13, 2016, Lyons, France. 21. Halder A, Kutzner I, Graichen F, et al. Influence of limb alignment on mediolateral loading in total knee replacement: in vivo measurements in five patients. J Bone Joint Surg Am 2012;94:1023. 22. Wang JH, Kato Y, Ingham SJ, et al. Effects of knee flexion angle and loading condition on the end-to-end distance of the posterior cruciate ligament: a comparison of the roles of the anterolateral and posterolateral bundles. Am J Sports Med 2014;42(12):2973. 23. Bellemans J, D'Hooghe P, Vandenneucker H, et al. Soft tissue balance in total knee arthroplasty; does stress relaxation occur perioperatively. Clin Orthop Relat Res 2006;452:49. 24. Schwartkopf R, Hadley S, Abbasi M, et al. Computer-assisted surgery patterns of ligamentous deformity of the knee: a clinical and cadaveric study. J Knee Surg 2013;26(4):233.
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Original Article
Accuracy of Balancing at Total Knee Surgery Using an Instrumented Tibial Trial Patrick A. Meere, MD, Svenja M. Schneider, BS, Peter S. Walker, PhD * Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, New York, New York
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 June 2015 Received in revised form 17 February 2016 Accepted 22 February 2016 Available online xxx
Background: Balancing is an important part of a total knee procedure, and in recent years, more emphasis has been given to quantifying the process. Methods: During 101 total knee surgeries, initial bone cuts were made using navigation. Lateral and medial contact forces were determined throughout flexion using an instrumented tibial trial. Balancing was defined as a ratio of the medial and total force, the target being 0.5 (equal lateral and medial forces). Based on the initial values, surgical corrections were selected to achieve balancing. The most common corrections were soft tissue releases (63 incidences), including MCL, posterolateral corner, posteromedial corner, and changing tibial insert thicknesses (34 incidences). Results: After final balancing, the mean ratio was 0.52 ± 0.14, between 0.35 and 0.65 being achieved in 80% of cases. In 84% of cases, only 0-2 corrections were required. The average total force on the condyles was 215 ± 86 N. Conclusion: Our study provides data to surgeons on the results to expect when balancing a knee, which can enhance both accuracy and consistency of the procedure. © 2016 Elsevier Inc. All rights reserved.
Keywords: total knee soft tissue balancing
The main surgical requirements for restoring optimal function in a total knee arthroplasty (TKA) are accurate bone cut alignments and soft tissue balancing throughout flexion [1]. Balancing has been described as equal and rectangular gaps between the resected bone surfaces in both extension and flexion, implying that the lateral and medial soft tissues maintain equal tensions. From an unbalanced state, balancing can be achieved in different ways, including soft tissue releases, making small modifications to bone cuts, and changing component sizes. Central tracking of the femur on the tibia is also a necessary part of balancing, achieved by correct rotation of the components. Balancing has been shown to have advantages such as postoperative patient satisfaction and clinical outcome scores [2-4]. On
No author associated with this paper has disclosed any potential or pertinent conflicts which may be perceived to have impending conflict with this work. For full disclosure statements refer to http://dx.doi.org/10.1016/j.arth.2016.02.050. Initial technical work on this project was carried out by Christopher Bell. The work was supported by the Department of Orthopaedic Surgery, New York University Hospital for Joint Diseases, and by an Education Grant from Orthosensor Inc. * Reprint requests: Peter S. Walker, PhD, Laboratory for Knee Implant Design, New York University Hospital for Joint Diseases, 301 East 17th Street, New York, NY 10003. http://dx.doi.org/10.1016/j.arth.2016.02.050 0883-5403/© 2016 Elsevier Inc. All rights reserved.
the other hand, inadequate soft tissue balance has been linked to increased implant wear, loosening, instability, decreased range of motion, and increased pain [2,5-8]. Until now, the most widely used procedure for balancing has involved the use of spacer blocks or distractors, which is dependent on the experience or skill level of the surgeon and does not provide quantitative information. Possibly stimulated by the expanding use of newer methods for achieving accurate bone cut alignments, instrumented distractors and tibial trials have recently been introduced which measure the distraction or contact forces across the knee, allowing for a more quantitative measure of soft tissue balancing at the time of TKA surgery. Once such device is the OrthoSensor Verasense Knee Balancer (OrthoSensor Inc, Sunrise, FL), a wireless load-sensing tibial component that transmits real-time lateral and medial contact forces to a monitor in the operating room [9,10]. A pictorial view of the contact locations allows for assessment of the rotational position of the tibial component. In a recent study using this instrumented tibial trial component, odds ratios analysis demonstrated that balanced joints were 2.5, 1.3, and 1.8 times more likely to achieve a significant improvement in American Knee Society Score, Western Ontario and McMaster Universities Osteoarthritis Index, and activity level, respectively [11]. Increased patient satisfaction was also achieved after balancing using this method [12]. These examples illustrate
2
P.A. Meere et al. / The Journal of Arthroplasty xxx (2016) 1e5
Fig. 1. Ratio of medial to total (medial þ lateral) intercompartmental forces before and after balancing. The standard deviation was considerably reduced after balancing.
the potential clinical value of achieving accurate balancing using instrumented devices at surgery. However, even through the use of such technology, a systematic process for achieving balancing has not been clearly defined. For example, the pattern of lateral and medial contact forces during a flexion range needs to be translated into the most suitable surgical correction to move to a more balanced state. In a previous study on knee specimens [10], we explored different tests to obtain the contact force data and concluded that initially a sag test, where the leg was elevated by lifting the heel, could be used to establish tibial rotation and the best thickness of tibial insert. The heel push test, where the knee was flexed by sliding the heel toward the buttocks keeping the leg in a vertical plane, was a convenient way to obtain contact force data through a full flexion range. From the heel push data, appropriate surgical corrections were deduced, and the sensitivity of the corrections to only small dimensional or angular changes was determined. The present study extends the previous work to actual surgical conditions on arthritic knees. The purpose of the study was to address 2 key questions. First, what is the initial state of balancing after bone cuts have been made and the trial components placed, and how accurately can knees be balanced after surgical corrections within the time frame of a normal surgical procedure? Second, what is the effect of each specific surgical correction that is selected based on the state of imbalance and how many corrections are required to achieve acceptable balancing? The results of the study are intended to provide guidelines to the surgeon for the balancing values and the effects of surgical corrections during the balancing procedure and the final balancing results that are likely to be achieved. The expectation is that the clinical outcomes will be more reproducible if a consistency of balancing can be achieved. Methods A method was formulated for obtaining the required data from each surgical case. Screen capture software (Screenflow;
Telestream, Nevada City, CA) was used to simultaneously record the video and audio feed of the surgery from the computer’s camera and the real-time information from the instrumented tibial trial component. The equipment was set up to afford a full view of the leg, from heel to pelvis, so an accurate flexion angle measurement could be computed. Data were collected for 101 TKA cases using a PCL-retaining device (Triathlon; Stryker, Mahwah, NJ). Initial bone cuts were made using computer navigation based on measured resection. The proximal tibia was cut first perpendicular to the long axis. The distal femur was then cut perpendicular to the mechanical axis of the leg (center of hip to center of ankle). Finally, the posterior and anterior femoral cuts were made, rotating the femur appropriate to each case. The femoral trial was inserted, and the instrumented tibial trial was used to measure the magnitudes and location of the contact forces on the lateral and medial sides and the contact point rotations throughout the flexion range. The first step was to determine the correct thickness of the tibial insert using the sag test (foot lifted upward). The knee was then flexed and extended and the tibial rotation was checked and corrected if necessary. The heel push test, in which the leg is pushed in a cephalad direction through the flexion range [10], was then used to determine the initial value of balancing, defined as a ratio of medial to total force at 0 , 30 , 60 , and 90 flexion. This ratio was used to normalize the data to the range 0-1. A balanced knee with equal lateral and medial forces throughout flexion would show a value of 0.5. In our study, this 0.5 value was used as the target based on the traditional guidelines of equal gaps and tensions in flexion and extension. Surgical corrections were then performed based on the initial values relayed by the tibial trial, with the goal of achieving symmetrical medial and lateral loads and avoiding contact points close to the edges. The corrections could be soft tissue releases achieved by “pie crusting”; increasing or decreasing the thickness of the tibial insert; coronal plane modification to the bone cuts on the proximal tibial or distal femur; increasing the tibial posterior slope; and modifying the rotation of the tibial component. A single
Table 1 Changes in Tibiofemoral Force Ratios Before and After the Four Most Common Surgical Corrections. Change Posterolateral corner release Posteromedial capsule release Medial collateral ligament release
Increase in tibial liner thickness
Average Medial-to-Total Force Ratio Before 0.22 ± 0.23 0.70 ± 0.17 0.64 ± 0.14
Average Medial-to-Total Force Ratio After 0.40 ± 0.15 0.59 ± 0.19 0.53 ± 0.16
Total Force Before
Total Force After
141.36 ± 75.35
235.71 ± 102.31
Significance P ¼ .0001 P ¼ .001 P ¼ .001
P ¼ .0001
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Fig. 2. The medial-to-total force ratio before and after posterolateral corner release from 0 to 90 flexion.
surgical variable was applied at a time during surgery and changes in force were determined. The analysis focused on the 4 most common surgical corrections, namely, posterolateral corner (PLC) release, posteromedial capsule (PMC) release, medial collateral ligament (MCL) release, and increasing the tibial insert thickness. For the other corrections, there were insufficient cases to achieve statistical significance in the analysis. An MCL release was performed in cases where the medial-tototal force ratio was considerably above 0.50, indicating tight medial soft tissues. Tightness in flexion and extension was differentiated such that the anterior MCL was released in knees that were tight in flexion while the posterior MCL was released in cases that were tight in extension [1]. A PLC release was done, usually involving the arcuate ligament, popliteus tendon, and/or the lateral collateral ligament, if the medial-to-total force ratio was considerably below 0.50 throughout flexion, indicating a tight lateral side. The illiotibial band was released only if the knee was excessively tight only in extension because it is slack and inactive in flexion [1]. A PMC release was done if the medial-to-total force ratio was considerably above 0.50 throughout flexion. It resulted in an average decrease in medial tightness comparable to the change seen after an MCL release. The PMC release was done in cases with high medial force early in flexion while the MCL release was more often used in cases with high medial load at high flexion angles. The tibial insert thickness was increased if the total forces on the medial and lateral condyle were too low. A total force in the range of 120-145 N was aimed for based on previous research [10,13]. The changes in the tibial insert thickness reported on here were necessary because of changes in overall contact forces from soft tissue releases during the procedure. The heel push test was done to evaluate balance at the initial state and after each correction. Real-time medial and lateral contact forces and contact point rotation data were recorded at
0 , 30 , 60 , and 90 flexion ± 5 of the dynamic testing. CAD software was used to measure the angles of flexion from the surgical video. The contact forces were recorded, and in cases of soft tissue releases, the forces were expressed as a ratio of medial to total force to highlight changes in mediolateral balancing. Results After making the bone cuts and inserting the tibial components, the average medial-to-total compartmental force ratio was 0.49 ± 0.27. After final balancing, the ratio was 0.52 ± 0.14 (Fig. 1). The initial data for all knees were scattered between 0.0 (lateral force only) and 1.0 (medial force only). The data after balancing show a clear narrowing of the range of imbalance between the compartmental loads based on the standard deviation values. Of the 101 knees, a final balancing medial-to-total force ratio between 0.35 and 0.65 was achieved in 80% of cases from 0 to 30 ; 79% of cases from 0 to 60 ; and 77% of cases from 0 to 90 . This range was specified on the basis of being feasible to obtain in most of the cases. When the balancing data were separated into initially medially tight and initially laterally tight knees, the change between initial and final balancing was statistically significant with P values of .0034 and < .0001, respectively. An error analysis was also done using 0.5 as the ideal force ratio. The average error for all knees initially was 0.22, and the average error for all surgically balanced knees was 0.10. The difference between these 2 error groups was statistically significant with a P value < .0001. The average total force on the condyles from 0 to 90 was 283.8 ± 157.9 N (63.8 ± 35.5 lbs) initially and 206.0 ± 83.2 N (46.3 ± 18.7 lbs) after final balancing. All of the above values were obtained with both the femoral and tibial trials in place. In 84% of the cases, 0-2 corrections were needed to obtain balancing. One case required 5 corrections, the maximum recorded.
Fig. 3. The medial-to-total force ratio before and after posteromedial capsule release from 0 to 90 flexion.
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Fig. 4. The medial-to-total force ratio before and after medial collateral ligament release from 0 to 90 flexion.
The frequencies of all the surgical corrections used to achieve balancing were as follows:
63 soft tissue releases 34 changes in tibial insert thickness
17 bone cut adjustments 7 rotations of the tibia
The most common surgical corrections were PLC release (22), PMC release (16), MCL release (14), and increase in the tibial liner thickness (30). All had a large enough sample size to determine a significant change with 80% confidence (Table 1). The actual mean magnitudes of the forces and force ratios as a result of these surgical corrections at different flexion angles are shown in Figures 2-5. The elements of the PLC most frequently released by the surgeon were the arcuate ligament and the popliteus tendon. Before releasing the PLC, the average medial-to-total force ratio was 0.22. After releasing the PLC, the average medial-to-total force ratio was 0.40 (Table 1 and Fig. 2). Before releasing the PLC, the average medial-to-total force ratio was 0.70. After releasing the capsule, the average medial-to-total force ratio was 0.59 (Table 1 and Fig. 3), with the greatest change seen in extension. Before an MCL release, the average medial-to-total force ratio was 0.64. After an MCL release, the average medial-to-total force ratio was 0.53 (Table 1 and Fig. 4). The medial force decreased and the medial-to-total force ratio moved toward 0.50 in all cases. In the 22 cases of 2-mm tibial insert thickness increase, the total force on both the medial and lateral condyle increased by an average of 94.3 ± 85.0 N (21.2 ± 19.1 lbs) throughout the range of flexion (0 -90 ; Fig. 5). This equaled an average percent change of 93%. It was observed that the medial-to-total force ratio did not remain constant and soft tissue releases were often necessary as a secondary correction to obtain optimal balancing across the condyle.
Fig. 5. The change in total force (medial and lateral condyle) after a 2-mm increase in the tibial liner thickness.
Discussion In this study, we determined the balancing status of the knee during the stages of a total knee surgical procedure using an instrumented tibial trial. It was shown that after making the initial bone cuts based on measured resection, the balancing covered a wide range. In any series, the range would depend on the arthritic condition and the proportion of initially varus-to-valgus cases. In our case, the initial mean medial-to-lateral force ratio was 0.49, but with a wide range. The use of the instrumented tibial trial allowed for corrections to be made so that the final mean balancing ratio was 0.52 with a greatly reduced spread of values as shown in Figure 1. The choice of which surgical correction to apply was based on the pattern of force readings over the full flexion range. As indicated in the Methods, the selection was based on whether forces increased or decreased with flexion, the asymmetry, and whether the asymmetry changed with flexion. The force readings from the instrumented tibial trials guided those determinations. The sensitivity of the balancing values to the various surgical corrections was consistent with a previous study showing that changes of 2 mm or 2 could correct most imbalanced states [10]. This was related to the condyle forces being due to soft tissue pretensions, with collateral stiffness being in the region of 50 N/ mm [14,15]. The finding that only up to 2 surgical corrections were needed in most of the cases to achieve an acceptable balancing implied that using a quantitative balancing method would not take additional time compared with qualitative methods. The “acceptable level” in our study from 0.35 to 0.65 medial-to-total force ratio was based on what could readily be achieved and what seemed to be subjectively well balanced. The value is consistent with that of a previous study [11] relating balancing values to clinical outcome. It was interesting however that there was no target value for the actual magnitude of the contact forces, given that there was considerable variation between patients in the finally balanced knees. This presumably reflects the variation in ligament stiffness between individuals. Previous studies of intraoperative balancing have found after measured resection there is a significant imbalance between the medial and lateral compartmental loads and a 2-mm increase to substantially increase tibial force and to exaggerate residual imbalance [6]. A 2-mm increase in tibial insert thickness has previously been shown to increase compressive forces by 54%-131%, with magnitudes particularly high in PCL-retaining and mobile-bearing prostheses [16]. The evidence that 2 mm or 2 results in a change in force sufficient to correct most imbalances is consistent with the high magnitude of change we observed following pie-crusting of ligaments [10]. The number of surgical corrections usually applied in this study is supported by a recent single-surgeon study of soft tissue balancing using the
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flexion-extension gap method, in which 89.9% of cases required 0-2 surgical corrections [17]. There are a number of limitations to this study. First, the ideal target value or zone for balancing has not yet been validated. While we used 0.5 medial-to-lateral force ratio as the target, in normal knees the ratio may be in the region of 0.55, indicating a higher medial force, based on a study of varus-valgus laxities measured on older individuals [18]. However, in contrast, in younger subjects, the ratio was 0.46 [19]. Moreover, laxity may not correspond to the actual contact forces. A recent study of such forces was carried out using Tekscan (Boston, MA) pressure-sensitive film in knee specimens in conditions resembling surgery [20]. The medial force was highest in extension and was constant after 15 flexion. The lateral force was about half of medial in early flexion and fell to small values in higher flexion. In walking, the medial force was measured at around 65% of the total, in patients with instrumented tibial component [21]. Even with reference to such data, the optimal balancing ratio in total knee patients for optimal function is not known. A related point is which flexion region is the most important to achieve accurate balancing, if it is found that it is difficult to obtain equality throughout flexion. Relating this to function, while the stance phase of level walking involves early flexion, the swing phase requires up to about 60 flexion, while other activities can involve higher angles even in the weight-bearing phase. A further limitation of our study is that the posterior cruciate was retained, which makes the prediction of which ligaments to release more difficult, and it could have different effects at different flexion angles. However, in the intact knee, the lengths of the anterolateral and posteromedial bundles of the PCL increased substantially with flexion [22], implying that the tensions would be relatively low in early to mid-flexion. Hence, balancing in this range may be primarily affected by the collateral ligaments. In addition, while the balancing values were obtained during surgery, there is evidence that stress relaxation of the ligaments occurs in the first 30 minutes after surgery and leads to increased ligament laxity [23]. A related limitation is that the knee was not weight-bearing during the heel push test, but it has previously been shown that passive intraoperative force readings correlate with abnormal postoperative kinematics and condylar liftoff [8]. This however brings into question the advantages of accurate balancing in optimizing function, and whether balancing affects the weight-bearing or swing phases of activity the most. In relation to the surgical corrections themselves, because all of the TKA procedures were performed by 1 surgeon, the technique used to select and achieve each surgical correction may be unique to that surgeon. Hence, data should be obtained from other similarly experienced surgeons as well. In a continuation of this study, further data collection and analysis is being carried out to analyze quantifiable data on every surgical correction including other soft tissue releases, bone cuts, rotation of the tibia, and changes in slope. This information can potentially be used to develop a balancing algorithm that can identify necessary corrections based on an initial pattern of imbalance. One approach for this has been described based on the shape of the varus-valgus laxity patterns during flexion [24], verified by studies on knee specimens. A similar method could be used for the contact force patterns, possibly considering also the contact point locations. This will be a useful tool for surgeons to fully use the information provided by the instrumented tibial trial and the
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effect of the surgical corrections carried out. This approach has the potential to shorten surgery time and avoid corrections which are not applicable, as well as avoid overcorrection. Finally, all of the patients are currently being studied clinically to gain further insights into the relations between balancing values seen at surgery with varus and valgus stability and functional outcomes. References 1. Whiteside LA. Ligament balancing in total knee arthroplasty: an instructional manual. Berlin: Springer; 2004. 2. Matsuda Y, Ishii Y, Noguchi H, et al. Varus-valgus balance and range of movement after total knee arthroplasty. J Bone Joint Surg Br 2005;87-B:804. 3. Unitt L, Sambatakakis A, Johnstone D, et al. Balancer Study Group. Short-term outcome in total knee replacement after soft-tissue release and balancing. J Bone Joint Surg Br 2008;90(2):159. 4. Winemaker MJ. Perfect balance in total knee arthroplasty. J Arthroplasty 2002;17(1):2. 5. Del Gaizo DJ, Della Valle CJ. Instability in primary total knee arthroplasty. Orthopedics 2011;34(9):e519. 6. D'Lima DD, Patil S, Steklov N, et al. An ABJS Best Paper: dynamic intraoperative ligament balancing for total knee arthroplasty. Clin Orthop Relat Res 2007;463: 208. 7. Lombardi Jr AV, Berend KR, Adams JB. Why knee replacements fail in 2013; patient, surgeon, or implant? Bone Joint J Br 2014;96-B(11 Suppl A):101. 8. Wasielewski RC, Galat DD, Komistek RD. Correlation of compartment pressure data from an intraoperative sensing device with postoperative fluoroscopic kinematic results in TKA patients. J Biomech 2005;38:333. 9. Gustke KA. Use of smart trials for soft-tissue balancing in total knee replacement surgery. J Bone Joint Surg Br 2012;94(11SupplA):147. 10. Walker PS, Meere PA, Bell CP. Effects of surgical variables in balancing of total knee replacements using an instrumented tibial trial. Knee 2014;21(1):156. 11. Gustke KA, Golladay GJ, Roche MW, et al. A new method for defining balance: promising short-term clinical outcomes of sensor-guided TKA. J Arthroplasty 2014;29:955. 12. Gustke KA, Golladay GJ, Roche MW, et al. Increased satisfaction after total knee replacement using sensor-guided technology. Bone Joint J 2014;96-B(10):1333. 13. Zalzal P, Papini M, Petruccelli D, et al. An in vivo biomechanical analysis of the soft-tissue envelope of osteoarthritic knees. J Arthroplasty 2004;19(2):217. 14. Robinson JR, Bull AMJ, Amis AA. Structural properties of the medial collateral ligament complex of the human knee. J Biomech 2005;38(5):1067. 15. Wilson WT, Deakin AH, Payne AP, et al. Comparative analysis of the structural properties of the collateral ligaments of the human knee. J Orthop Sports Phys Ther 2012;42(4):345. 16. Schirm AC, Jeffcote BO, Nicholls RL, et al. Sensitivity of knee soft-tissues to surgical technique in total knee arthroplasty. Knee 2011;18:180. 17. Peters CL, Jimenez C, Erickson J, et al. Lessons learned from selective soft-tissue release for gap balancing in primary total knee arthroplasty: an analysis of 1216 consecutive total knee arthroplasties. J Bone Joint Surg Am 2012;95(152):1. 18. Heesterbeek PJC, Verdonshot N, Wymenga AB. In vivo knee laxity in flexion and extension: a radiographic study in 30 older healthy subjects. Knee 2008;15(1): 45. 19. Schultz SJ, Shimokochi Y, Nguyen A-D, et al. Measurement of varus-valgus and internal-external rotational knee laxities in vivo e part 2: relationship with anterior-posterior and general joint laxity in males and females. J Orthop Res 2007;25:989. 20. Verstraete MA, Meere PA, Salvadore G, et al. Insights from the native knee in load balancing during TKA surgery. Submitted to 22nd Congress of the European Society of Biomechanics. July 10-13, 2016, Lyons, France. 21. Halder A, Kutzner I, Graichen F, et al. Influence of limb alignment on mediolateral loading in total knee replacement: in vivo measurements in five patients. J Bone Joint Surg Am 2012;94:1023. 22. Wang JH, Kato Y, Ingham SJ, et al. Effects of knee flexion angle and loading condition on the end-to-end distance of the posterior cruciate ligament: a comparison of the roles of the anterolateral and posterolateral bundles. Am J Sports Med 2014;42(12):2973. 23. Bellemans J, D'Hooghe P, Vandenneucker H, et al. Soft tissue balance in total knee arthroplasty; does stress relaxation occur perioperatively. Clin Orthop Relat Res 2006;452:49. 24. Schwartkopf R, Hadley S, Abbasi M, et al. Computer-assisted surgery patterns of ligamentous deformity of the knee: a clinical and cadaveric study. J Knee Surg 2013;26(4):233.