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Soft Tissue Balance Changes Depending on Joint Distraction Force in Total Knee Arthroplasty
The Journal of Arthroplasty 29 (2014) 520–524
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Soft Tissue Balance Changes Depending on Joint Distraction Force in Total Knee Arthroplasty Kanto Nagai, MD a, Hirotsugu Muratsu, MD a, Tomoyuki Matsumoto, MD b, Hidetoshi Miya, MD a, Ryosuke Kuroda, MD b, Masahiro Kurosaka, MD b a b
Department of Orthopaedic Surgery, Steel Memorial Hirohata Hospital, Himeji, Japan Department of Orthopaedic Surgery, Kobe University Graduate School of Medicine, Kobe, Japan
a r t i c l e
i n f o
Article history: Received 31 March 2013 Accepted 21 July 2013 Keywords: soft tissue balance joint distraction force total knee arthroplasty
a b s t r a c t The influence of joint distraction force on intraoperative soft tissue balance was evaluated using Offset RepoTensor® for 78 knees that underwent primary posterior-stabilized total knee arthroplasty. The joint center gap and varus ligament balance were measured between osteotomized surfaces using 20, 40 and 60 lbs of joint distraction force. These values were significantly increased at extension and flexion as the distraction force increased. Furthermore, lateral compartment stiffness was significantly lower than medial compartment stiffness. Thus, larger joint distraction forces led to larger varus ligament balance and joint center gap, because of the difference in soft tissue stiffness between lateral and medial compartments. These findings indicate the importance of the strength of joint distraction force in the assessment of soft tissue balance, especially when using gap-balancing technique. © 2014 Elsevier Inc. All rights reserved.
The acquisition of appropriate soft tissue balancing and accurate alignment is an essential procedure in total knee arthroplasty (TKA) [1–3]. To assess the intraoperative soft tissue balance that reflects postoperative condition after TKA, an offset-type tensor was developed. This tensor enables surgeons to assess soft tissue balance after reduction of the patellofemoral joint (PF) and with the femoral component in place. Initial intraoperative measurements obtained using the tensor [4] demonstrated the importance of PF joint reduction and femoral component placement in measuring soft tissue balance [5,6]. The different patterns between cruciate-retaining (CR) and posterior-stabilized (PS) TKA [6,7] and postoperative soft tissue balance, which reflects intraoperative values [8], have also been reported. However, in the previous study series, the tensor was used with 40 lbs of joint distraction force to assess soft tissue balance in TKA, using the measured resection technique. This distraction force was determined based on a preliminary study that adjusted the thickness of the polyethylene insert. However, in the gap-balancing technique, the tensor is used as a surgical tool for determining the rotational alignment of the femur; therefore, the amount of distraction force is quite important. Intraoperative soft tissue balancing, originally described by Freeman et al [9] and Insall et al [2], is an established method for preparing equal rectangular flexion and extension gaps by releasing
The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.07.025. Reprint requests: Hirotsugu Muratsu, MD, PhD, Department of Orthopaedic Surgery, Steel Memorial Hirohata Hospital, 3-1, Yumesaki-cho, Hirohata-ku, Himeji 671-1122, Japan. 0883-5403/2903-0015$36.00/0 – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.07.025
various soft tissue structures around the knee. However, the lateral tibiofemoral articulation is physiologically lax; as a result, the flexion gap may not be rectangular [10–12]. In this context, Tokuhara et al [13] reported that the tibiofemoral flexion gap in a normal knee was not rectangular and that the lateral joint gap was significantly lax, as assessed by magnetic resonance imaging (MRI). By using the stiffness measurements, Asano et al [14] showed that the soft tissue complex around the knee was elastic and consequently extensible and that the joint gap in TKA depended on the strength of the joint distraction force applied. Based on the above evidence, we hypothesized that a larger joint distraction force leads to a larger varus ligament balance and joint gap, due to the difference in soft tissue stiffness between the lateral and medial compartments. Therefore, the purpose of the present study was to investigate the influence of the joint distraction force on intraoperative soft tissue balance and to evaluate the stiffness of the soft tissue complex of the knee in PS TKA using the measured resection technique. Materials and Methods The subjects were 78 consecutive patients (78 osteoarthritic knees) who underwent primary PS TKA between 2009 and 2012. All knees had varus deformity, and those with valgus deformity and severe bony defects were excluded. The patient population comprised 70 women and 8 men with a mean age of 74.8 ± 5.7 years (± standard deviation; SD). The average preoperative coronal plane alignment in varus was 11.2° ± 4.9° (± SD). Each surgery was performed by the same senior author (H.M.) using cemented PS TKA
K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524
(NexGen LPS Flex, Zimmer, Inc, Warsaw, IN) with a standardized surgical technique. Surgical Procedure Using a tourniquet, we performed a medial parapatellar arthrotomy. Each surgery was carried out using a measured resection technique with a conventional resection block. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) were both resected. Distal femoral resection was performed perpendicular to the mechanical axis of the femur according to preoperative long-leg radiographs. Femoral posterior condylar resection was performed using the anterior reference technique. The surgical epicondylar axis was preoperatively measured using computed tomography (CT). As Berger et al [15] reported, the surgical epicondylar axis is a line connecting the sulcus of the medical epicondyle and the lateral epicondylar prominence, and the angle between the surgical epicondylar axis and posterior condylar line is defined as the posterior condylar angle. Femoral external rotation was preset at 0°, 3°, 5°, and 7° relative to the posterior condylar axis, which was determined on the basis of preoperative CT, intraoperative Whiteside line, and the trans-epicondylar axis. The mean femoral external rotation was 4.0° ± 1.1° relative to the posterior condylar axis. Proximal tibial resection was then performed with each cut made perpendicular to the mechanical axis in the coronal plane and with 7° of posterior inclination along the sagittal plane. No bony defects were observed along the eroded medial tibial plateau. After each resection, we removed the osteophytes, released the posterior capsule along the femur, and corrected any ligament imbalances in the coronal plane by appropriately releasing the medial soft tissues. The resection and soft tissue release were performed using a spacer block. Intraoperative Measurement with the Offset Repo-Tensor® (OFR Tensor; Zimmer) The OFR tensor consists of three parts: an upper seesaw plate, a lower platform plate with a spike, and an extra-articular main body, as previously described [7–13] (Fig. 1). The offset connection arms from the main body, which connect the two independent plates at the anteromedial corner of the tibia, are passed through the medial parapatellar arthrotomy. The seesaw plate is attached to the offset connection arm of the main body via a single shaft, providing a central pivot in the coronal plane. In addition, the seesaw plate can move in a
521
proximal–distal direction by means of a rack-and-opinion mechanism within the main body. After both tibial and femoral osteotomies, two flat plates are placed at the center of the knee, and the OFR tensor can be firmly fixed to the osteotomized tibia using the spikes and additional pins on the platform plate. The seesaw plate has a post at the proximal center to fit the intercondylar space and the cam of the femoral trial prosthesis used in the PS TKA. This post-and-cam mechanism controls the tibiofemoral position in both the coronal and sagittal planes, reproducing the joint constraint and alignment after the prostheses are implanted. This device is ultimately designed to permit surgeons to measure the joint center gap and ligament balance, both before and after placement of the femoral trial prosthesis, while applying a constant joint distraction force. Joint distraction forces ranging from 20 lbs (9.1 kg) to 60 lbs (27.2 kg) can be exerted between the seesaw and platform plates using a specially made torque driver, which can change the maximum torque value. After sterilization, this torque driver is placed on a rack that contains a rack-and-pinion mechanism along the extra-articular main body. Subsequently, the appropriate torque is applied to generate the required distraction force. In preliminary in vitro experiments we obtained an error for joint distraction within ± 3%. Once appropriate distraction is achieved, attention is focused on two scales that correspond to the OFR tensor: the angle (°, positive value in varus balance) between the seesaw and platform plates and the distance (mm, joint center gap) between the center midpoints of the upper surface of the seesaw plate and the proximal tibial cut. By measuring these angular deviations and distances under a constant joint distraction force, we are able to measure the joint center gap and ligament balance. Intraoperative Measurement A conventional gap measurement was performed between the osteotomized surfaces in a parallel orientation at extension and flexion of the knee. We loaded 20, 40, and 60 lbs of distraction force and measured each joint center gap and varus ligament balance. All measurements were obtained with the PF joint reduced. We loaded this distraction force several times until the joint component gap remained constant. This was done to reduce the error that can result from creep elongation of the surrounding soft tissues. After evaluating soft tissue balance between the osteotomized surfaces, the femoral trial component was placed with the OFR tensor on the surface of the tibial bone cut, and the PF joint was temporarily reduced by applying stitches proximally and distally to the connection arm of the OFR tensor. Joint component gap assessments were carried out at eight knee flexion angles of 0°, 10°, 30°, 45°, 60°, 90°, 120°, and 135° with 20, 40, and 60 lbs of joint distraction force at each angle. During each measurement, the thigh was held and the knee was aligned in the sagittal plane to eliminate the external load on the knee at each angle of knee flexion. After measurements were obtained, a NexGen prosthesis was implanted using cement. Examined Parameters
Fig. 1. The Offset Repo-Tensor® (anteroposterior view). The tensor consists of three parts: an upper seesaw plate, a lower platform plate with a spike and an extra-articular main body. The offset connection arms from the main body connecting two independent plates at the anteromedial corner of the tibia are passed through the medial parapatellar arthrotomy, which permits reduction of the patellofemoral joint, while performing measurements.
The joint center gap (mm) and varus ligament balance (°) between the osteotomized surfaces of the knee were measured at extension and flexion and after the femoral component trial was placed. The joint component gap (mm) and varus angle (°) between the component surfaces were also measured at each flexion angle. Subsequently, the medial and lateral compartment gaps (mm) were calculated at each flexion angle using the joint component gap, varus angle, and the width between the medial and lateral apexes of the femoral component; these apexes represent the contact points of the polyethylene insert, and the width was consistent for each size of implant (Fig. 2). The lateral compartment gap = [component
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K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524 Table 1 Joint Center Gap and Varus Ligament Balance between Osteotomized Surfaces. Joint Distraction Force
Joint center gap (mm) Extension Flexion Varus ligament balance (°) Extension Flexion
20 lbs
40 lbs
60 lbs
23.2 ± 0.2* 22.6 ± 0.3**
26.2 ± 0.3* 25.7 ± 0.3**
28.3 ± 0.2* 27.7 ± 0.3**
4.3 ± 0.3† 1.9 ± 0.4††
6.0 ± 0.4† 3.9 ± 0.4††
6.9 ± 0.4† 4.7 ± 0.5††
Values are shown as mean ± SEM (mm). Statistical difference between each joint distraction force (*, **, †, ††P b 0.01).
flexion, significantly increased with increase in the joint distraction force (Fig. 3A). Furthermore, the varus ligament balance at extension and flexion significantly increased with increase in the joint distraction force (Fig. 3B). The joint component gaps with 20, 40, and 60 lbs are shown in Fig. 4. Similar to the osteotomized gap described above, the joint component gap became significantly larger with an increase in joint distraction force at each flexion angle, and the patterns of the joint component gap throughout the range of motion were similar among the different joint distraction forces. The stiffness of the medial compartment was significantly greater than that of the lateral compartment at each flexion angle (Table 2, Fig. 5). Fig. 2. Illustration showing parameters obtained and calculated. (A) Joint center gap/ joint component gap. (B) Varus ligament balance/varus angle. (C) Lateral compartment gap. (D) Medial compartment gap.
gap] + 0.5 * [width between the medial and lateral apexes of the femoral component that represent the contact points of the polyethylene insert] * sin (varus angle). The medial compartment gap = [component gap] − 0.5 * [width between the medial and lateral apexes of the femoral component that represent the contact points of the polyethylene insert] * sin (varus angle). The medial and lateral compartment stiffness values (N/mm) were calculated using each compartment gap increment from 20 to 60 lbs of joint distraction force at each flexion angle.
Discussion The main findings of the current study are that the joint center gap and varus ligament balance significantly increased with increase in joint gap distraction force. These findings indicate that the lateral compartment gap tends to be larger than the medial compartment gap as the joint gap distraction force increases. Furthermore, we calculated the medial and lateral compartment gaps using the joint
Statistical Analysis All values are expressed as mean ± standard error of the mean (SEM). The results were analyzed using a statistical software package (Statview 5.0, Abacus Concepts Inc, Berkeley, CA, USA). We performed a repeated measure of analysis of variance (ANOVA) to compare the joint center gap and varus ligament balance between the osteotomized surfaces for different distraction forces and carried out a post hoc analysis using Fisher's protected least significance difference (PLSD) test. The joint component gap was also compared between the different joint distraction forces at each flexion angle using ANOVA and post hoc analysis with Fisher's PLSD test. We also used the Student t-test to compare the stiffness between the medial and lateral compartments at each flexion angle. A P-value b 0.05 was considered statistically significant. Results The mean joint center gaps with 20, 40, and 60 lbs were 23.2 ± 0.2 mm, 26.2 ± 0.3 mm, and 28.3 ± 0.2 mm at extension and 22.6 ± 0.3 mm, 25.7 ± 0.3 mm, and 27.7 ± 0.3 mm at flexion, respectively. The mean varus ligament balances with 20, 40, and 60 lbs were 4.3° ± 0.3°, 6.0° ± 0.4°, and 6.9° ± 0.4° during extension and 1.9° ± 0.4°, 3.9° ± 0.4°, and 4.7° ± 0.5° at flexion, respectively (Table 1). The bone was not compressed or deformed with an increase in the distraction force. The joint gaps, both during extension and
Fig. 3. Joint center gap and varus ligament balance at extension and flexion with 20, 40, and 60 lbs (*P b 0.01). (A) Joint center gaps both at extension and flexion were significantly increased as joint distraction force increased. (B) Varus ligament balance both at extension and flexion was also significantly increased as joint distraction force increased.
K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524
Fig. 4. Joint component gap with 20, 40 and 60 lbs joint distraction force. The joint component gap became significantly larger by the increase of joint distraction force at each flexion angle, and the pattern of the joint component gap change from extension to flexion was almost same between different joint distraction forces (*P b 0.01 versus each distraction force).
component gaps, and found that lateral compartment stiffness was significantly lower than medial compartment stiffness at each flexion angle. This difference between the medial and lateral compartment stiffness causes an increase in the varus ligament balance, accompanied by increase in the joint distraction force. Several mechanisms may underlie the difference between the medial and lateral compartment stiffness. First, preoperative osteoarthritic varus knee joints show contracture of the medial compartment and looseness of the lateral compartment. Second, there is physiological laxity in the lateral compartment of a normal knee. Tokuhara et al [13] used an open MRI system to assess the varus and valgus joint laxity of normal living knees during flexion; they found that the lateral flexion gap was significantly larger (by 4.6 mm) than the medial flexion gap. These discrepancies indicate that the tibiofemoral flexion gap in normal knees is not rectangular and that the lateral joint gap is significantly lax. In addition, few studies have assessed the stiffness of the knee joint. Asano et al [14] evaluated the stiffness of the soft tissue complex of osteoarthritic knees treated with PS TKA and showed that the stiffness at extension was significantly greater than that at flexion. In the present study, the medial and lateral compartment stiffness was quantitatively evaluated, and the lateral compartment stiffness was significantly lower than the medial compartment stiffness. Lateral compartment laxity was observed not only at 90° but also at the mid and deep flexion ranges. Two main techniques are used for obtaining the rotational alignment of the femoral component in TKA, namely the measured resection technique and the gap-balancing technique. The best method for obtaining the rotational alignment of the femoral component during flexion remains controversial. Some investigators favor a measured resection technique, in which bony landmarks (femoral epicondyles, posterior femoral condyles, or the anteroposterior axis) are the primary determinants of the femoral component rotation [15]. Others
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Fig. 5. Medial and lateral compartment stiffness at each flexion angle (20–60 lbs). Medial compartment stiffness was significantly larger than lateral compartment stiffness at each flexion angle (*P b 0.01 versus medial).
recommend a gap-balancing technique, in which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament at equal tension [16]. In the gap-balancing technique, the rotation of the femoral posterior condyle resection is determined on the basis of the flexion gap. The findings of the present study showed that the external rotational angle of the femoral posterior condyle resection varies depending on the strength of the joint distraction force at the time of flexion gap evaluation. That is, when the strength of the joint distraction force is low, the varus ligament balance is low and therefore the external rotation angle
Table 2 Medial and Lateral Compartment Stiffness at Each Flexion Angle (20–60 lbs). Compartment Stiffness (N/mm), mean ± SEM Flexion 0° 10° 30° 45° 60° 90° 120° 135°
Medial 24.2 24.7 25.2 27.2 27.4 28.8 25.5 25.8
± ± ± ± ± ± ± ±
1.2 1.8 1.0 1.0 1.2 1.5 2.6 2.4
Lateral 20.1 17.2 15.5 17.1 17.4 18.8 18.6 17.0
± ± ± ± ± ± ± ±
1.5* 1.0* 1.0* 0.8* 0.8* 0.8* 1.2* 1.5*
Statistical difference between medial and lateral compartment stiffness (*P b 0.01 versus medial).
Fig. 6. Influence of the strength of joint distraction force on femoral posterior condyle resection in the gap-balancing technique. (A) When joint distraction force is low load. The varus ligament balance is low and therefore the external rotation angle of femoral posterior condylar resection would be small. (B) When joint distraction force is high load. The varus ligament balance is high and therefore the external rotation angle would be large.
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K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524
of the femoral posterior condylar resection is small (Fig. 6A). In contrast, when the strength of the joint distraction force is high, the varus ligament balance is high and therefore the external rotation angle is large (Fig. 6B). Therefore, surgeons should consider this tension property of the soft tissue and avoid implantation of the femoral component in an excessively external rotated position when using the gap-balancing technique. In addition, it is very important to clearly show the amount of joint distraction force applied when surgeons discuss this procedure. Despite the important findings in this study, there are several limitations. These data were obtained in PS TKA, and the results might differ from data obtained in cruciate-retaining TKA. In addition, the joint center gap and varus ligament balance were measured after femoral and tibial bone resection using the measured resection technique, not before the resection of the femoral posterior condyle. The femoral posterior condyle may influence the soft tissue balance. Therefore, before resection of the posterior condyle, soft tissue balance should be assessed using the gap-balancing technique with different joint distraction force loads. In conclusion, we proved the hypothesis that the joint center gap and varus ligament balance significantly increases as the joint gap distraction force increases, at both extension and flexion. Furthermore, we evaluated the medial and lateral compartment stiffness throughout the range of motion. These findings indicate the importance of the strength of the joint distraction force in the assessment of soft tissue balance, especially when using the gapbalancing technique.
Acknowledgments The authors acknowledge Mrs. Janina Tubby for her assistance in preparation of this manuscript.
References 1. Dorr LD, Boiardo RA. Technical considerations in total knee arthroplasty. Clin Orthop Relat Res 1986(205):5. 2. Insall J, Tria AJ, Scott WN. The total condylar knee prosthesis: the first 5 years. Clin Orthop Relat Res 1979(145):68. 3. Insall JN, Binazzi R, Soudry M, et al. Total knee arthroplasty. Clin Orthop Relat Res 1985(192):13. 4. Matsumoto T, Muratsu H, Tsumura N, et al. Joint gap kinematics in posteriorstabilized total knee arthroplasty measured by a new tensor with the navigation system. J Biomech Eng 2006;128(6):867. 5. Muratsu H, Matsumoto T, Kubo S, et al. Femoral component placement changes soft tissue balance in posterior-stabilized total knee arthroplasty. Clin Biomech (Bristol, Avon) 2010;25(9):926. 6. Matsumoto T, Muratsu H, Tsumura N, et al. Soft tissue balance measurement in posterior-stabilized total knee arthroplasty with a navigation system. J Arthroplasty 2009;24(3):358. 7. Matsumoto T, Kuroda R, Kubo S, et al. The intra-operative joint gap in cruciateretaining compared with posterior-stabilised total knee replacement. J Bone Joint Surg Br 2009;91(4):475. 8. Matsumoto T, Muratsu H, Kubo S, et al. Intraoperative soft tissue balance reflects minimum 5-year midterm outcomes in cruciate-retaining and posterior-stabilized total knee arthroplasty. J Arthroplasty 2012;27(9):1723. 9. Freeman MA, Sculco T, Todd RC. Replacement of the severely damaged arthritic knee by the ICLH (Freeman–Swanson) arthroplasty. J Bone Joint Surg Br 1977;59(1):64. 10. Markolf KL, Mensch JS, Amstutz HC. Stiffness and laxity of the knee–the contributions of the supporting structures. A quantitative in vitro study. J Bone Joint Surg Am 1976;58(5):583. 11. Seering WP, Piziali RL, Nagel DA, et al. The function of the primary ligaments of the knee in varus–valgus and axial rotation. J Biomech 1980;13(9):785. 12. Moore TM, Meyers MH, Harvey Jr JP. Collateral ligament laxity of the knee. Longterm comparison between plateau fractures and normal. J Bone Joint Surg Am 1976;58(5):594. 13. Tokuhara Y, Kadoya Y, Nakagawa S, et al. The flexion gap in normal knees. An MRI study. J Bone Joint Surg Br 2004;86(8):1133. 14. Asano H, Muneta T, Hoshino A. Stiffness of soft tissue complex in total knee arthroplasty. Knee Surg Sports Traumatol Arthrosc 2008;16(1):51. 15. Berger RA, Rubash HE, Seel MJ, et al. Determining the rotational alignment of the femoral component in total knee arthroplasty using the epicondylar axis. Clin Orthop Relat Res 1993(286):40. 16. Dennis DA. Measured resection: an outdated technique in total knee arthroplasty. Orthopedics 2008;31(9):940.
Contents lists available at ScienceDirect
The Journal of Arthroplasty journal homepage: www.arthroplastyjournal.org
Soft Tissue Balance Changes Depending on Joint Distraction Force in Total Knee Arthroplasty Kanto Nagai, MD a, Hirotsugu Muratsu, MD a, Tomoyuki Matsumoto, MD b, Hidetoshi Miya, MD a, Ryosuke Kuroda, MD b, Masahiro Kurosaka, MD b a b
Department of Orthopaedic Surgery, Steel Memorial Hirohata Hospital, Himeji, Japan Department of Orthopaedic Surgery, Kobe University Graduate School of Medicine, Kobe, Japan
a r t i c l e
i n f o
Article history: Received 31 March 2013 Accepted 21 July 2013 Keywords: soft tissue balance joint distraction force total knee arthroplasty
a b s t r a c t The influence of joint distraction force on intraoperative soft tissue balance was evaluated using Offset RepoTensor® for 78 knees that underwent primary posterior-stabilized total knee arthroplasty. The joint center gap and varus ligament balance were measured between osteotomized surfaces using 20, 40 and 60 lbs of joint distraction force. These values were significantly increased at extension and flexion as the distraction force increased. Furthermore, lateral compartment stiffness was significantly lower than medial compartment stiffness. Thus, larger joint distraction forces led to larger varus ligament balance and joint center gap, because of the difference in soft tissue stiffness between lateral and medial compartments. These findings indicate the importance of the strength of joint distraction force in the assessment of soft tissue balance, especially when using gap-balancing technique. © 2014 Elsevier Inc. All rights reserved.
The acquisition of appropriate soft tissue balancing and accurate alignment is an essential procedure in total knee arthroplasty (TKA) [1–3]. To assess the intraoperative soft tissue balance that reflects postoperative condition after TKA, an offset-type tensor was developed. This tensor enables surgeons to assess soft tissue balance after reduction of the patellofemoral joint (PF) and with the femoral component in place. Initial intraoperative measurements obtained using the tensor [4] demonstrated the importance of PF joint reduction and femoral component placement in measuring soft tissue balance [5,6]. The different patterns between cruciate-retaining (CR) and posterior-stabilized (PS) TKA [6,7] and postoperative soft tissue balance, which reflects intraoperative values [8], have also been reported. However, in the previous study series, the tensor was used with 40 lbs of joint distraction force to assess soft tissue balance in TKA, using the measured resection technique. This distraction force was determined based on a preliminary study that adjusted the thickness of the polyethylene insert. However, in the gap-balancing technique, the tensor is used as a surgical tool for determining the rotational alignment of the femur; therefore, the amount of distraction force is quite important. Intraoperative soft tissue balancing, originally described by Freeman et al [9] and Insall et al [2], is an established method for preparing equal rectangular flexion and extension gaps by releasing
The Conflict of Interest statement associated with this article can be found at http:// dx.doi.org/10.1016/j.arth.2013.07.025. Reprint requests: Hirotsugu Muratsu, MD, PhD, Department of Orthopaedic Surgery, Steel Memorial Hirohata Hospital, 3-1, Yumesaki-cho, Hirohata-ku, Himeji 671-1122, Japan. 0883-5403/2903-0015$36.00/0 – see front matter © 2014 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.arth.2013.07.025
various soft tissue structures around the knee. However, the lateral tibiofemoral articulation is physiologically lax; as a result, the flexion gap may not be rectangular [10–12]. In this context, Tokuhara et al [13] reported that the tibiofemoral flexion gap in a normal knee was not rectangular and that the lateral joint gap was significantly lax, as assessed by magnetic resonance imaging (MRI). By using the stiffness measurements, Asano et al [14] showed that the soft tissue complex around the knee was elastic and consequently extensible and that the joint gap in TKA depended on the strength of the joint distraction force applied. Based on the above evidence, we hypothesized that a larger joint distraction force leads to a larger varus ligament balance and joint gap, due to the difference in soft tissue stiffness between the lateral and medial compartments. Therefore, the purpose of the present study was to investigate the influence of the joint distraction force on intraoperative soft tissue balance and to evaluate the stiffness of the soft tissue complex of the knee in PS TKA using the measured resection technique. Materials and Methods The subjects were 78 consecutive patients (78 osteoarthritic knees) who underwent primary PS TKA between 2009 and 2012. All knees had varus deformity, and those with valgus deformity and severe bony defects were excluded. The patient population comprised 70 women and 8 men with a mean age of 74.8 ± 5.7 years (± standard deviation; SD). The average preoperative coronal plane alignment in varus was 11.2° ± 4.9° (± SD). Each surgery was performed by the same senior author (H.M.) using cemented PS TKA
K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524
(NexGen LPS Flex, Zimmer, Inc, Warsaw, IN) with a standardized surgical technique. Surgical Procedure Using a tourniquet, we performed a medial parapatellar arthrotomy. Each surgery was carried out using a measured resection technique with a conventional resection block. The anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) were both resected. Distal femoral resection was performed perpendicular to the mechanical axis of the femur according to preoperative long-leg radiographs. Femoral posterior condylar resection was performed using the anterior reference technique. The surgical epicondylar axis was preoperatively measured using computed tomography (CT). As Berger et al [15] reported, the surgical epicondylar axis is a line connecting the sulcus of the medical epicondyle and the lateral epicondylar prominence, and the angle between the surgical epicondylar axis and posterior condylar line is defined as the posterior condylar angle. Femoral external rotation was preset at 0°, 3°, 5°, and 7° relative to the posterior condylar axis, which was determined on the basis of preoperative CT, intraoperative Whiteside line, and the trans-epicondylar axis. The mean femoral external rotation was 4.0° ± 1.1° relative to the posterior condylar axis. Proximal tibial resection was then performed with each cut made perpendicular to the mechanical axis in the coronal plane and with 7° of posterior inclination along the sagittal plane. No bony defects were observed along the eroded medial tibial plateau. After each resection, we removed the osteophytes, released the posterior capsule along the femur, and corrected any ligament imbalances in the coronal plane by appropriately releasing the medial soft tissues. The resection and soft tissue release were performed using a spacer block. Intraoperative Measurement with the Offset Repo-Tensor® (OFR Tensor; Zimmer) The OFR tensor consists of three parts: an upper seesaw plate, a lower platform plate with a spike, and an extra-articular main body, as previously described [7–13] (Fig. 1). The offset connection arms from the main body, which connect the two independent plates at the anteromedial corner of the tibia, are passed through the medial parapatellar arthrotomy. The seesaw plate is attached to the offset connection arm of the main body via a single shaft, providing a central pivot in the coronal plane. In addition, the seesaw plate can move in a
521
proximal–distal direction by means of a rack-and-opinion mechanism within the main body. After both tibial and femoral osteotomies, two flat plates are placed at the center of the knee, and the OFR tensor can be firmly fixed to the osteotomized tibia using the spikes and additional pins on the platform plate. The seesaw plate has a post at the proximal center to fit the intercondylar space and the cam of the femoral trial prosthesis used in the PS TKA. This post-and-cam mechanism controls the tibiofemoral position in both the coronal and sagittal planes, reproducing the joint constraint and alignment after the prostheses are implanted. This device is ultimately designed to permit surgeons to measure the joint center gap and ligament balance, both before and after placement of the femoral trial prosthesis, while applying a constant joint distraction force. Joint distraction forces ranging from 20 lbs (9.1 kg) to 60 lbs (27.2 kg) can be exerted between the seesaw and platform plates using a specially made torque driver, which can change the maximum torque value. After sterilization, this torque driver is placed on a rack that contains a rack-and-pinion mechanism along the extra-articular main body. Subsequently, the appropriate torque is applied to generate the required distraction force. In preliminary in vitro experiments we obtained an error for joint distraction within ± 3%. Once appropriate distraction is achieved, attention is focused on two scales that correspond to the OFR tensor: the angle (°, positive value in varus balance) between the seesaw and platform plates and the distance (mm, joint center gap) between the center midpoints of the upper surface of the seesaw plate and the proximal tibial cut. By measuring these angular deviations and distances under a constant joint distraction force, we are able to measure the joint center gap and ligament balance. Intraoperative Measurement A conventional gap measurement was performed between the osteotomized surfaces in a parallel orientation at extension and flexion of the knee. We loaded 20, 40, and 60 lbs of distraction force and measured each joint center gap and varus ligament balance. All measurements were obtained with the PF joint reduced. We loaded this distraction force several times until the joint component gap remained constant. This was done to reduce the error that can result from creep elongation of the surrounding soft tissues. After evaluating soft tissue balance between the osteotomized surfaces, the femoral trial component was placed with the OFR tensor on the surface of the tibial bone cut, and the PF joint was temporarily reduced by applying stitches proximally and distally to the connection arm of the OFR tensor. Joint component gap assessments were carried out at eight knee flexion angles of 0°, 10°, 30°, 45°, 60°, 90°, 120°, and 135° with 20, 40, and 60 lbs of joint distraction force at each angle. During each measurement, the thigh was held and the knee was aligned in the sagittal plane to eliminate the external load on the knee at each angle of knee flexion. After measurements were obtained, a NexGen prosthesis was implanted using cement. Examined Parameters
Fig. 1. The Offset Repo-Tensor® (anteroposterior view). The tensor consists of three parts: an upper seesaw plate, a lower platform plate with a spike and an extra-articular main body. The offset connection arms from the main body connecting two independent plates at the anteromedial corner of the tibia are passed through the medial parapatellar arthrotomy, which permits reduction of the patellofemoral joint, while performing measurements.
The joint center gap (mm) and varus ligament balance (°) between the osteotomized surfaces of the knee were measured at extension and flexion and after the femoral component trial was placed. The joint component gap (mm) and varus angle (°) between the component surfaces were also measured at each flexion angle. Subsequently, the medial and lateral compartment gaps (mm) were calculated at each flexion angle using the joint component gap, varus angle, and the width between the medial and lateral apexes of the femoral component; these apexes represent the contact points of the polyethylene insert, and the width was consistent for each size of implant (Fig. 2). The lateral compartment gap = [component
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K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524 Table 1 Joint Center Gap and Varus Ligament Balance between Osteotomized Surfaces. Joint Distraction Force
Joint center gap (mm) Extension Flexion Varus ligament balance (°) Extension Flexion
20 lbs
40 lbs
60 lbs
23.2 ± 0.2* 22.6 ± 0.3**
26.2 ± 0.3* 25.7 ± 0.3**
28.3 ± 0.2* 27.7 ± 0.3**
4.3 ± 0.3† 1.9 ± 0.4††
6.0 ± 0.4† 3.9 ± 0.4††
6.9 ± 0.4† 4.7 ± 0.5††
Values are shown as mean ± SEM (mm). Statistical difference between each joint distraction force (*, **, †, ††P b 0.01).
flexion, significantly increased with increase in the joint distraction force (Fig. 3A). Furthermore, the varus ligament balance at extension and flexion significantly increased with increase in the joint distraction force (Fig. 3B). The joint component gaps with 20, 40, and 60 lbs are shown in Fig. 4. Similar to the osteotomized gap described above, the joint component gap became significantly larger with an increase in joint distraction force at each flexion angle, and the patterns of the joint component gap throughout the range of motion were similar among the different joint distraction forces. The stiffness of the medial compartment was significantly greater than that of the lateral compartment at each flexion angle (Table 2, Fig. 5). Fig. 2. Illustration showing parameters obtained and calculated. (A) Joint center gap/ joint component gap. (B) Varus ligament balance/varus angle. (C) Lateral compartment gap. (D) Medial compartment gap.
gap] + 0.5 * [width between the medial and lateral apexes of the femoral component that represent the contact points of the polyethylene insert] * sin (varus angle). The medial compartment gap = [component gap] − 0.5 * [width between the medial and lateral apexes of the femoral component that represent the contact points of the polyethylene insert] * sin (varus angle). The medial and lateral compartment stiffness values (N/mm) were calculated using each compartment gap increment from 20 to 60 lbs of joint distraction force at each flexion angle.
Discussion The main findings of the current study are that the joint center gap and varus ligament balance significantly increased with increase in joint gap distraction force. These findings indicate that the lateral compartment gap tends to be larger than the medial compartment gap as the joint gap distraction force increases. Furthermore, we calculated the medial and lateral compartment gaps using the joint
Statistical Analysis All values are expressed as mean ± standard error of the mean (SEM). The results were analyzed using a statistical software package (Statview 5.0, Abacus Concepts Inc, Berkeley, CA, USA). We performed a repeated measure of analysis of variance (ANOVA) to compare the joint center gap and varus ligament balance between the osteotomized surfaces for different distraction forces and carried out a post hoc analysis using Fisher's protected least significance difference (PLSD) test. The joint component gap was also compared between the different joint distraction forces at each flexion angle using ANOVA and post hoc analysis with Fisher's PLSD test. We also used the Student t-test to compare the stiffness between the medial and lateral compartments at each flexion angle. A P-value b 0.05 was considered statistically significant. Results The mean joint center gaps with 20, 40, and 60 lbs were 23.2 ± 0.2 mm, 26.2 ± 0.3 mm, and 28.3 ± 0.2 mm at extension and 22.6 ± 0.3 mm, 25.7 ± 0.3 mm, and 27.7 ± 0.3 mm at flexion, respectively. The mean varus ligament balances with 20, 40, and 60 lbs were 4.3° ± 0.3°, 6.0° ± 0.4°, and 6.9° ± 0.4° during extension and 1.9° ± 0.4°, 3.9° ± 0.4°, and 4.7° ± 0.5° at flexion, respectively (Table 1). The bone was not compressed or deformed with an increase in the distraction force. The joint gaps, both during extension and
Fig. 3. Joint center gap and varus ligament balance at extension and flexion with 20, 40, and 60 lbs (*P b 0.01). (A) Joint center gaps both at extension and flexion were significantly increased as joint distraction force increased. (B) Varus ligament balance both at extension and flexion was also significantly increased as joint distraction force increased.
K. Nagai et al. / The Journal of Arthroplasty 29 (2014) 520–524
Fig. 4. Joint component gap with 20, 40 and 60 lbs joint distraction force. The joint component gap became significantly larger by the increase of joint distraction force at each flexion angle, and the pattern of the joint component gap change from extension to flexion was almost same between different joint distraction forces (*P b 0.01 versus each distraction force).
component gaps, and found that lateral compartment stiffness was significantly lower than medial compartment stiffness at each flexion angle. This difference between the medial and lateral compartment stiffness causes an increase in the varus ligament balance, accompanied by increase in the joint distraction force. Several mechanisms may underlie the difference between the medial and lateral compartment stiffness. First, preoperative osteoarthritic varus knee joints show contracture of the medial compartment and looseness of the lateral compartment. Second, there is physiological laxity in the lateral compartment of a normal knee. Tokuhara et al [13] used an open MRI system to assess the varus and valgus joint laxity of normal living knees during flexion; they found that the lateral flexion gap was significantly larger (by 4.6 mm) than the medial flexion gap. These discrepancies indicate that the tibiofemoral flexion gap in normal knees is not rectangular and that the lateral joint gap is significantly lax. In addition, few studies have assessed the stiffness of the knee joint. Asano et al [14] evaluated the stiffness of the soft tissue complex of osteoarthritic knees treated with PS TKA and showed that the stiffness at extension was significantly greater than that at flexion. In the present study, the medial and lateral compartment stiffness was quantitatively evaluated, and the lateral compartment stiffness was significantly lower than the medial compartment stiffness. Lateral compartment laxity was observed not only at 90° but also at the mid and deep flexion ranges. Two main techniques are used for obtaining the rotational alignment of the femoral component in TKA, namely the measured resection technique and the gap-balancing technique. The best method for obtaining the rotational alignment of the femoral component during flexion remains controversial. Some investigators favor a measured resection technique, in which bony landmarks (femoral epicondyles, posterior femoral condyles, or the anteroposterior axis) are the primary determinants of the femoral component rotation [15]. Others
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Fig. 5. Medial and lateral compartment stiffness at each flexion angle (20–60 lbs). Medial compartment stiffness was significantly larger than lateral compartment stiffness at each flexion angle (*P b 0.01 versus medial).
recommend a gap-balancing technique, in which the femoral component is positioned parallel to the resected proximal tibia with each collateral ligament at equal tension [16]. In the gap-balancing technique, the rotation of the femoral posterior condyle resection is determined on the basis of the flexion gap. The findings of the present study showed that the external rotational angle of the femoral posterior condyle resection varies depending on the strength of the joint distraction force at the time of flexion gap evaluation. That is, when the strength of the joint distraction force is low, the varus ligament balance is low and therefore the external rotation angle
Table 2 Medial and Lateral Compartment Stiffness at Each Flexion Angle (20–60 lbs). Compartment Stiffness (N/mm), mean ± SEM Flexion 0° 10° 30° 45° 60° 90° 120° 135°
Medial 24.2 24.7 25.2 27.2 27.4 28.8 25.5 25.8
± ± ± ± ± ± ± ±
1.2 1.8 1.0 1.0 1.2 1.5 2.6 2.4
Lateral 20.1 17.2 15.5 17.1 17.4 18.8 18.6 17.0
± ± ± ± ± ± ± ±
1.5* 1.0* 1.0* 0.8* 0.8* 0.8* 1.2* 1.5*
Statistical difference between medial and lateral compartment stiffness (*P b 0.01 versus medial).
Fig. 6. Influence of the strength of joint distraction force on femoral posterior condyle resection in the gap-balancing technique. (A) When joint distraction force is low load. The varus ligament balance is low and therefore the external rotation angle of femoral posterior condylar resection would be small. (B) When joint distraction force is high load. The varus ligament balance is high and therefore the external rotation angle would be large.
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of the femoral posterior condylar resection is small (Fig. 6A). In contrast, when the strength of the joint distraction force is high, the varus ligament balance is high and therefore the external rotation angle is large (Fig. 6B). Therefore, surgeons should consider this tension property of the soft tissue and avoid implantation of the femoral component in an excessively external rotated position when using the gap-balancing technique. In addition, it is very important to clearly show the amount of joint distraction force applied when surgeons discuss this procedure. Despite the important findings in this study, there are several limitations. These data were obtained in PS TKA, and the results might differ from data obtained in cruciate-retaining TKA. In addition, the joint center gap and varus ligament balance were measured after femoral and tibial bone resection using the measured resection technique, not before the resection of the femoral posterior condyle. The femoral posterior condyle may influence the soft tissue balance. Therefore, before resection of the posterior condyle, soft tissue balance should be assessed using the gap-balancing technique with different joint distraction force loads. In conclusion, we proved the hypothesis that the joint center gap and varus ligament balance significantly increases as the joint gap distraction force increases, at both extension and flexion. Furthermore, we evaluated the medial and lateral compartment stiffness throughout the range of motion. These findings indicate the importance of the strength of the joint distraction force in the assessment of soft tissue balance, especially when using the gapbalancing technique.
Acknowledgments The authors acknowledge Mrs. Janina Tubby for her assistance in preparation of this manuscript.
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