Orthopedics

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Performance Comparison of Single-Radius Versus Multiple-Curve Femoral Component in Total Knee Arthroplasty: A Prospective, Randomized Study Using the Lower Quarter Y-Balance Test

Samuel S. Wellman, MD; Mitchell R. Klement, MD; Robin M. Queen, PhD

Abstract

Midflexion stability after total knee arthroplasty (TKA) is dependent, in part, on implant design. Midflexion performance of a single-radius (SR) design and a multi-radius, or J-curve (JC), design were compared using the Lower Quarter Y-Balance Test (YBT-LQ). Patient-reported outcomes and measures of physical performance were also compared. The authors hypothesized that the SR design would provide superior midflexion stability and, therefore, a greater reach distance in the YBT-LQ when compared with the JC design. Patients undergoing primary, unilateral TKA were prospectively enrolled and block randomized to receive either the SR (n=30) or the JC (n=30) implant. Patients reported outcome measures (Knee Injury and Osteoarthritis Outcome Score, Knee Society Score, and University of California at Los Angeles Activity Score), performed the YBT-LQ, and completed physical performance measures (walking speed, timed up-and-go test, and sit-to-stand test) before surgery and 1 year postoperatively. One year postoperatively, 40 patients (20 SR, 20 JC) were available for analysis. There were no significant differences between the groups regarding age, sex, body mass index, American Society of Anesthesiologists score, YBT-LQ results, or any other variable of interest. In both groups, significant improvements were observed for all variables of interest when compared from preoperatively to 1 year postoperatively. Implant type did not affect patient-reported or physical performance outcomes following TKA; however, patients showed significant improvement in most outcome measures by 1 year postoperatively. The significant limitations in strength and balance in this cohort of patients likely outweigh any subtle differences in implant design. [Orthopedics. 2017; 40(6):e1074–e1080.]

Abstract

Midflexion stability after total knee arthroplasty (TKA) is dependent, in part, on implant design. Midflexion performance of a single-radius (SR) design and a multi-radius, or J-curve (JC), design were compared using the Lower Quarter Y-Balance Test (YBT-LQ). Patient-reported outcomes and measures of physical performance were also compared. The authors hypothesized that the SR design would provide superior midflexion stability and, therefore, a greater reach distance in the YBT-LQ when compared with the JC design. Patients undergoing primary, unilateral TKA were prospectively enrolled and block randomized to receive either the SR (n=30) or the JC (n=30) implant. Patients reported outcome measures (Knee Injury and Osteoarthritis Outcome Score, Knee Society Score, and University of California at Los Angeles Activity Score), performed the YBT-LQ, and completed physical performance measures (walking speed, timed up-and-go test, and sit-to-stand test) before surgery and 1 year postoperatively. One year postoperatively, 40 patients (20 SR, 20 JC) were available for analysis. There were no significant differences between the groups regarding age, sex, body mass index, American Society of Anesthesiologists score, YBT-LQ results, or any other variable of interest. In both groups, significant improvements were observed for all variables of interest when compared from preoperatively to 1 year postoperatively. Implant type did not affect patient-reported or physical performance outcomes following TKA; however, patients showed significant improvement in most outcome measures by 1 year postoperatively. The significant limitations in strength and balance in this cohort of patients likely outweigh any subtle differences in implant design. [Orthopedics. 2017; 40(6):e1074–e1080.]

The rates of total knee arthroplasty (TKA) and total knee revisions continue to rise drastically in the United States.1 Total knee revisions alone are expected to increase by 601% from 2005 to 2030.1 Midflexion instability, defined as mediolateral instability at 30° to 60° of flexion, is an evolving concern in TKA2 that may be an underappreciated cause of postoperative pain, dissatisfaction, and instability.3,4 Elevation of the joint line,5 medial collateral ligament laxity, and femoral component design have been implicated as the most common etiologies.2 The traditional femoral component design has featured multiple radii (MR) of curvature (J curve), mimicking the geometry of the native femur6 in an attempt to restore similar kinematics.7 However, there is concern that this design allows for relaxation of the collateral ligaments in midflexion, leading to instability and poor outcomes.8,9 Newer designs have featured a single radius (SR) of curvature, which, according to the implant manufacturer, allows for constant tension of the collateral ligaments throughout the arc of motion, potentially imparting greater stability. However, clinical studies10–13 and a recent meta-analysis14 comparing the 2 implant designs have reported no difference regarding clinical or patient-reported outcome scores. Conversely, intraoperative comparison of the 2 implants using navigation has revealed differences in stability at 30° of flexion.11

The Lower Quarter Y-Balance Test (YBT-LQ) was originally developed to test lower extremity strength, proprioception, and balance to identify athletes at risk for injury.15 The test is multifaceted, requiring strength and dynamic balance of the stance limb while the non-stance limb reaches in 1 of 3 directions.16 The test has shown high interrater and intrarater test–retest reliability.15,17 More recently, the YBT-LQ has been used as a functional assessment tool for recovery after anterior cruciate ligament reconstruction,18,19 assessing competition level in baseball players20 and predicting injury in military personnel.21 To complete the reach testing, the patient must be able to maintain balance and strength with the knee in midflexion position. The dynamic stability observed intraoperatively may not be captured by patient-reported outcome scores alone, and no clinical test currently exists to objectively assess midflexion instability. Therefore, the YBT-LQ could provide a good assessment of dynamic stability in patients after TKA.

In this study, patients were prospectively randomized to either an MR or an SR total knee system and the YBT-LQ was used as a surrogate for clinical testing of midflexion instability in vivo. The authors hypothesized that the SR design would provide more stability in midflexion and thus lead to a greater YBT-LQ score and improvements on patient-reported outcomes and functional scores when compared with the MR design. In addition, the authors hypothesized that, independent of implant type, the patients would show significant improvements on patient-reported outcomes and functional testing from the preoperative to the postoperative assessment.

Materials and Methods

The study was conducted after institutional review board approval was obtained. Consent was obtained from all study participants.

Patients requiring primary, unilateral total knee replacement who were between 18 and 90 years old were eligible for the study. Included patients were willing to comply with required follow-up and evaluations. Patients having inflammatory arthritis, having diabetic neuropathy, and using an ambulatory assistive device prior to surgery were excluded. Specifically, patients had to be able to perform an unassisted single-leg balance for greater than 5 seconds on both limbs to be enrolled to ensure patients' safety when they were performing the YBT-LQ. Patients were block randomized to either the SR Triathlon Cruciate Retaining (CR) Total Knee Replacement System (Stryker, Mahwah, New Jersey) group or the MR NexGen CR Total Knee Replacement System (Zimmer, Warsaw, Indiana) group. The CR design was chosen to eliminate the effect of the cam and post mechanism present in posterior-stabilized knees. The MR knee design used in this study was chosen because its radii of curvature were the most different when compared with the SR design. The block randomization was completed at the time of surgical posting. All TKAs were performed by a single fellowship-trained arthroplasty surgeon (S.S.W.) using a medial parapatellar approach and gap balancing technique. All TKAs were performed using standard instrumentation without computer navigation.

The following clinical outcome scores were completed: University of California at Los Angeles Activity Score, Knee Injury and Osteoarthritis Outcome Score, Forgotten Joint Score, and Knee Society Score. In addition, each patient was asked to complete 3 walking trials along a 10-m walk-way, with walking speed assessed during the middle 5 m. Other functional outcomes completed included the timed up-and-go test,22 in which a patient rises from a chair, walks 3 m around a piece of tape, and then returns to the chair as quickly as possible; and a timed sit-to-stand test to determine how many times a patient could stand up from a standard arm chair and sit down in 10 seconds.23–25 Descriptions of these tests26 and their clinical importance27 have been previously published. The final functional test completed was the YBT-LQ—a test of dynamic balance in unilateral stance that has been deemed reliable and reproducible.15,17 This is the first study of its kind to use this test in an older TKA population. Patients were first asked to stand on 1 foot for as long as possible; the amount of time that they were able to stand on each leg was recorded as the single-limb stance time. Each patient who was able to maintain single-limb stance for a minimum of 5 seconds was then asked to complete the YBT-LQ, which is a derivative of the Star Excursion Balance Test. This test requires patients to stand on 1 leg on a central stance platform and reach as far as they can with their non-stance leg in the anterior, posteromedial, and posterolateral directions (Figure). This is performed for both the left and the right limbs.15 Reach distances were normalized to limb length for each patient. Limb length was obtained with the patient supine with legs extended. The limb was then measured in centimeters from the anterior superior iliac spine to the most distal portion of the medial malleolus as described by Plisky et al.15 Any trial that was not successfully completed was repeated with a maximum of 4 additional trials performed in each reach direction to account for the practice effect.15 These outcome measures were collected preoperatively and 1 year postoperatively. Finally, patient demographics and postoperative complications were recorded.

Patient performing the Lower Quarter Y-Balance Test. Anterior reach distance (A), posteromedial reach distance (B), and posterolateral reach distance (C) are shown.

Figure:

Patient performing the Lower Quarter Y-Balance Test. Anterior reach distance (A), posteromedial reach distance (B), and posterolateral reach distance (C) are shown.

Statistical analysis was performed using SPSS version 20.0 software (IBM, Armonk, New York). The 2-sided independent t test was used to compare normally distributed continuous demographic variables, while the chi-square test was used to compare normally distributed categorical variables. A series of 2×2 repeated measures analyses of variance (implant group×time) were completed to determine significant differences between implant groups and across time. Statistical significance was defined as P<.05.

Results

Overall, 60 patients met the study inclusion criteria. Forty-seven patients were enrolled: 22 in the SR group and 25 in the MR group. Of the 60 patients meeting inclusion criteria, 13 patients were withdrawn at the time of surgery (5 required a posterior-stabilized insert because of a deficient posterior cruciate ligament, 5 received unicompartmental knee arthroplasty because of minimal anteromedial arthritis, 1 required a constrained implant because of deficient collaterals, 1 was already enrolled in another study, and 1 was unable to perform the required testing). Of the 47 patients enrolled, 7 did not present for 1-year follow-up. At final follow-up, there were 20 patients in the SR group and 20 patients in the MR group available for analysis. There were no significant differences between the groups regarding baseline age, body mass index, percent body fat, American Society of Anesthesiologist score, estimated blood loss, or operative time (Table 1). There was a significantly lower mean tourniquet time in the SR group compared with the MR group (66.2 vs 76.6 minutes; P=.002).

Baseline Patient Demographics and Surgical Details

Table 1:

Baseline Patient Demographics and Surgical Details

No significant differences existed between the 2 groups for any of the patient-reported outcome scores, measures of physical performance, or YBT-LQ results (Table 2).

Functional Testing and Outcome Measures at Final Follow-up for Surgical Side

Table 2:

Functional Testing and Outcome Measures at Final Follow-up for Surgical Side

Compared with preoperatively, there were significant improvements at 1 year postoperatively for the University of California at Los Angeles Activity Score and the Knee Injury and Osteoarthritis Outcome Score total, quality of life, sport, function in daily living, symptoms, and pain scores (all P<.001). There were also improvements from preoperatively to 1 year postoperatively for single-leg stance time (P=.020), YBT-LQ composite result (P<.001), timed up-and-go test result (P<.001), sit-to-stand test result (P<.001), and walking speed (P<.001; Table 2).

Discussion

It has been well documented that nearly 20%28–30 of patients are dissatisfied after total knee replacement and that up to 30% may have persistent anterior knee pain at mid-term follow-up.31 In addition, instability is the second most common reason for revision after total knee replacement, ahead of infection, polyethylene wear, and arthrofibrosis.32,33 On examination of revision TKAs from 2010 to 2011, 35.3% of revisions occurred within 2 years of the index arthroplasty and 60.2% in the first 5 years,32,33 most for aseptic loosening and instability. The authors of this previous work concluded that the reason for these early failures was not implant related but rather surgeon and technique dependent.

Midflexion instability has been reported by Martin and Whiteside5 using a cadaveric model and computer simulations. They found that when shifting the joint line 5 mm anteriorly and 5 mm proximally, there was laxity in midflexion with balanced flexion and extension gaps.5 Other authors have found similar effects on instability based on joint line position, but this work has been limited to cadaveric studies.34 The use of an MR femoral component may cause laxity of the collaterals in midflexion, resulting in instability and possibly the need for revision.

To date, there have been 9 randomized trials comparing SR TKA designs with MR TKA designs. Eight included only posterior-stabilized TKAs11,12,35–40 and 1 included only CR TKAs.41 Liu et al14 performed a meta-analysis of the 8 posterior-stabilized TKA studies and found no differences regarding Knee Society Scores, including function scores, complications, isometric peak torque of the knee, and implant survivorship. The only difference found was greater range of motion with the MR design.14 Limitations of previous work have included lack of long-term follow-up and wide range of implant types (20 to 212) in the randomized studies. Similar to the results of the meta-analysis, the current study failed to identify differences in outcome scores and physical performance testing results between the 2 implant groups.

In the randomized trial using CR knees, Collados-Maestre et al41 included 118 SR and 119 MR TKAs with 5.7-year follow-up. They found better Knee Society Scores (P=.001), range of motion (P=.001), extension lag (P=.020), quadriceps strength (P=.004), chair test results (P=.032), and Western Ontario and Mc-Master Universities Osteoarthritis Index pain scores (P=.002) in the SR group. The revision rates and survivorship were similar in the 2 groups.41 The differences seen in the Collados-Maestre et al41 study compared with the current study may be attributed to longer follow-up and larger cohorts; however, nonrandomized studies with similar numbers of patients and follow-up have conflicting results.42–44 The current study also used a different set of knee outcome scores and functional testing. However, no previous randomized trial has examined midflexion instability as the primary outcome using objective and reproducible physical performance assessments.

Despite this, no differences were found between the 2 implant types on the YBTLQ, even though differences in kinematics between the 2 implants have been well documented. When comparing SR with MR in vivo using 3-dimensional and electromyography readings during a sit-to-stand test, Wang et al45 found abduction/adduction measurement differences as well as increased electromyography hamstring co-activation in the MR group. This same group also used force-plates and electromyography to analyze bilateral TKA patients with different radii of curvature. They found that SR knees had greater peak anteroposterior ground reaction force, higher anteroposterior ground reaction force, and less vastus lateralis and semitendinosus electromyography activation during the forward-thrust phase of the sit-to-stand test movement.46 Kessler et al47 used in vivo fluoroscopy during stair ascent and showed less uniform flexion–extension motion and increased varus–valgus movement in midflexion using finite helical analysis for MR knees compared with SR knees. The YBT-LQ has not been previously tested in TKA patients, and no normative data exist for this patient population. The lack of a difference may be attributed to the YBT-LQ being too challenging of a test for this patient population. The lack of difference may also be influenced by adjacent joint motion from the hip and ankle, as these have been proven to affect reach distances.48 This study did show the well-known success of total knee replacement in the treatment of knee arthritis49 with both designs having a significant improvement in all knee scores and physical performance measures from the preoperative baseline assessment to 1-year follow-up.

Strengths of this study included its prospective, randomized design. A single surgeon performed all surgeries using the same technique. The study also included several validated knee outcome scores and validated physical performance assessments. This was the first randomized controlled trial comparing SR designs with MR designs to use objective and reproducible testing that could be implemented at any institution without the need for highly specialized kinematic analysis. Finally, the study was specifically designed to address the issue of midflexion instability related only to changes in the femoral component design.

Limitations of this study included small cohorts with short-term follow-up. In addition, the study did not include a radiographic analysis of the joint line to assess changes from baseline to 1 year postoperatively. Finally, peak knee flexion of the surgical limb during the YBTLQ was not recorded; therefore, it is difficult to determine whether the patients were able to achieve a midflexion position during testing. However, it is probable that many patients were not able to achieve midflexion in the necessary 30° to 60° range to measure instability, if present, after TKA because of the previously mentioned limitations in strength and balance stability.

Conclusion

Overall, both SR and MR femoral designs resulted in significant improvement from baseline in patient-reported outcome scores and during the assessment of physical performance. However, in this study, implant design did not affect the measured outcomes or the specific test used to assess knee stability. The significant limitations in strength and balance stability in this cohort of patients may likely outweigh the subtle difference seen in implant design when assessing midflexion instability. To better assess midflexion stability, additional test methods need to be developed that will allow this patient cohort to be able to complete such assessment in light of the strength and balance limitations that are present. Future studies need to assess not only patient-reported outcomes and measures of physical performance but also strength, stability, and balance during dynamic activities to determine potential differences between these 2 types of implants.

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Baseline Patient Demographics and Surgical Details

CharacteristicSingle-Radius GroupMulti-Radius GroupP
Patients, No.2020
Male42.3%53.8%
Female57.7%46.2%
White73.1%73.1%
African American26.9%23.1%
Age, mean±SD, y61.96±7.8763.15±7.24.572
Mass, mean±SD, kg91.20±15.6188.41±18.02.553
Height, mean±SD, m1.72±0.091.71±0.11.593
Body mass index, mean±SD, kg/m230.84±4.9430.32±5.30.715
Body fat, mean±SD37.76%±12.15%33.75%±11.72%.237
American Society of Anesthesiologists score, mean±SD2.58±0.502.50±0.51.587
Tourniquet time, mean±SD, min66.23±13.2576.69±8.84.002a
Estimated blood loss, mean±SD, cc150.0±20.0153.9±37.2.644
Operative time, mean±SD, min123.3±23.69120.1±15.73.679

Functional Testing and Outcome Measures at Final Follow-up for Surgical Side

TestMean±SDPInteraction

PreoperativePostoperative



Single-Radius GroupMulti-Radius GroupSingle-Radius GroupMulti-Radius GroupImplantTimea
Single-leg stance, s16.77±12.3320.42±16.7824.45±17.4723.42±17.89.782.0200.293
Anterior reach, cm49.33±9.2348.86±10.3256.35±7.8957.02±6.49.971<.0010.729
Posteromedial reach, cm77.33±23.1769.48±18.6481.13±21.4678.35±15.00.440.0030.209
Posterolateral reach, cm91.67±13.0889.10±9.7185.00±11.1080.50±16.67.408<.0010.621
YBT-LQ composite79.45±10.4679.24±7.8970.44±9.3169.42±13.39.856<.0010.822
KOOS pain51.46±14.7448.50±16.8893.49±7.6791.34±10.82.420<.0010.879
KOOS symptoms41.33±10.0739.88±9.6765.48±13.3364.85±6.22.704<.0010.819
KOOS ADL57.36±14.4151.86±17.1194.16±6.0191.41±11.51.186<.0010.609
KOOS sport16.94±17.0819.21±17.8276.99±23.2775.66±22.26.932<.0010.646
KOOS QOL27.08±13.0121.71±17.3582.24±18.5482.79±17.22.564<.0010.367
KOOS total45.60±12.3246.08±13.5067.32±35.6264.24±38.79.830<.0010.728
Walking speed, m/s1.22±0.211.16±0.281.41±0.231.28±0.23.158<.0010.365
Timed up-and-go test, s8.58±2.379.30±2.456.99±1.807.17±1.39.458<.0010.330
Sit-to-stand test, No.4.05±0.884.09±1.075.40±1.455.47±1.81.877<.0010.956
UCLA Activity Score4.19±1.333.83±1.385.90±1.555.61±1.91.416<.0010.917
Authors

The authors are from the Department of Orthopedic Surgery (SSW, MRK), Duke University Medical Center, Durham, North Carolina; and the Department of Biomedical Engineering and Mechanics (RMQ), Kevin Granata Biomechanics Laboratory, Virginia Tech, Blacksburg, Virginia.

Dr Klement has no relevant financial relationships to disclose. Dr Wellman is a paid consultant for Total Joint Orthopedics and has received grants from Stryker, Zimmer Biomet, and DePuy. Dr Queen is a paid consultant for Quest Diagnostics and has received research support from Stryker and DonJoy.

Correspondence should be addressed to: Mitchell R. Klement, MD, Department of Orthopedic Surgery, Duke University Medical Center, Box 3000, Durham, NC 27710 ( Mitchell.Klement@dm.duke.edu).

Received: June 02, 2017
Accepted: September 08, 2017

10.3928/01477447-20171020-02

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