Image-Free Robotic-Assisted Total Knee Arthroplasty Improves Implant Alignment Accuracy: A Cadaveric Study

Open AccessPublished:December 31, 2021DOI:https://doi.org/10.1016/j.arth.2021.12.035

      Highlights

      • Pre-clinical evaluation of robotic-assisted total knee arthroplasty system accuracy.
      • Robotic-assisted TKA significantly improved femoral and tibial coronal alignment.
      • Robotic-assisted TKA resulted in fewer outliers with resection errors over 3°.

      Abstract

      Background

      Improving resection accuracy and eliminating outliers in total knee arthroplasty (TKA) is important to improving patient outcomes regardless of alignment philosophy. Robotic-assisted surgical systems improve resection accuracy and reproducibility compared to conventional instrumentation. Some systems require preoperative imaging while others rely on intraoperative anatomic landmarks. We hypothesized that the alignment accuracy of a novel image-free robotic-assisted surgical system would be equivalent or better than conventional instrumentation with fewer outliers.

      Methods

      Forty cadaveric specimens were used in this study. Five orthopedic surgeons performed 8 bilateral TKAs each, using the VELYS Robotic-Assisted System (DePuy Synthes) and conventional instrumentation on contralateral knees. Pre-resection and postresection computed tomography scans, along with optical scans of the implant positions were performed to quantify resection accuracies relative to the alignment targets recorded intraoperatively.

      Results

      The robotic-assisted cohort demonstrated smaller resection errors compared to conventional instrumentation in femoral coronal alignment (0.63° ± 0.50° vs 1.39° ± 0.95°, P < .001), femoral sagittal alignment (1.21° ± 0.90° vs 3.27° ± 2.51°, P < .001), and tibial coronal alignment (0.93° ± 0.72° vs 1.65° ± 1.29°, P = .001). All other resection angle accuracies were equivalent. Similar improvements were found in the femoral implant coronal alignment (0.89° ± 0.82° vs 1.42° ± 1.15°, P = .011), femoral implant sagittal alignment (1.51° ± 1.08° vs 2.49° ± 2.10°, P = .006), and tibial implant coronal alignment (1.31° ± 0.84° vs 2.03° ± 1.44°, P = .004). The robotic-assisted cohort had fewer outliers (errors >3°) for all angular resection alignments.

      Conclusion

      This in vitro study demonstrated that image-free robotic-assisted TKA can improve alignment accuracy compared to conventional instrumentation and reduce the incidence of outliers.

      Keywords

      As total knee arthroplasty (TKA) surgeons refine alignment philosophies to improve patient satisfaction and reduce the risk of revision, accurate execution of the planned resections is crucial regardless of alignment philosophy. Some clinical evidence suggests, but is not overwhelmingly compelling, that patient-specific alignment may improve outcomes [
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      ]. More recently, multiple reports demonstrate that robotic-assisted total knee arthroplasty (RATKA) systems improve resection accuracy, particularly in the coronal plane, when compared to conventional surgical instrumentation [
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      RATKA systems enable surgical resections by either constraining the saw to a specified anatomic plane during the resection [
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      Improved component placement accuracy with robotic-arm assisted total knee arthroplasty.
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      ], positioning a resection guide relative to the bone similar to previous CAS systems [
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      • Birmingham M.
      • Foran J.
      • Ogden S.
      Better accuracy and reproducibility of a new robotically-assisted system for total knee arthroplasty compared to conventional instrumentation: a cadaveric study.
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      Sequential versus automated cutting guides in computer-assisted total knee arthroplasty.
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      ], or positioning a burring tool during milling of the resection surface [
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      ]. All these approaches have proven efficacious for improving resection accuracy. Some systems require preoperative imaging to enable surgical planning and execution [
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      ], which increases cost, time, and radiation exposure for the patient. Other systems are image-free, relying on intraoperative anatomic landmarking and real-time planning [
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      ]. It is unclear if preoperative imaging is necessary to achieve the desired level of accuracy.
      The VELYS Robotic-Assisted Solution (DePuy Synthes, Warsaw, IN) for TKA was recently released. The system consists of an optical tracking system with bone-mounted arrays and a bed-mounted robotic arm that positions a surgical saw to perform bony resections (Fig. 1). This system differs from some previous robotic systems in that it does not require pre-operative imaging. The purpose of this study is to quantify the accuracy of the system compared to conventional instrumentation during simulated use. Both resection and implant positioning accuracy were evaluated. Our study hypothesized that all resection and implant alignment metrics for the RATKA cohort would be equivalent to or better than conventional instrumentation with fewer outliers.
      Figure thumbnail gr1
      Fig. 1The VELYS Robotic-Assisted solution is an imageless system that consists of an optical tracking system with bone-mounted tibial and femoral arrays, a stylus to identify anatomic landmarks, and a bed-mounted robotic arm that dynamically positions a surgical saw relative to the bony anatomy to perform the tibial and femoral bony resections.

      Materials and Methods

       Data Collection

      Forty cadaveric specimens were used in this study (age: 70.4 ± 8.2 years, height: 67.1 ± 4.1 in., body mass index: 20.6 ± 4.9). Five board-certified orthopedic surgeons participated in the experiment. All surgeons were proficient with the ATTUNE INTUITION conventional instrumentation, experienced with either CAS or robotic-assisted surgical systems, and trained in the robotic system.
      Preoperative computed tomography (CT) scans were performed with a 0.6-mm interslice distance at the knee to enable the accuracy assessment but were not used in surgical planning. Each surgeon performed bilateral TKAs on 8 specimens using the robotic system on the first knee and conventional instrumentation (CTKA) on the contralateral knee. For the RATKA surgeries, optical tracking arrays were mounted to the bones and anatomic landmarks were identified to construct anatomic coordinate systems. Surgeons planned their resection angles in real time using the system’s software. The target resection alignments were recorded, including the femoral sagittal angle (FSA), femoral coronal angle (FCA), femoral internal-external rotation angle (FRA), tibial sagittal angle (TSA), and tibial coronal angle (TCA), along with the thicknesses for the distal and posterior resections of the medial femoral condyle [femoral distal resection (FDR) and femoral posterior resection (FPR)] and tibial resection (TR). For the CTKA surgeries, alignment targets were recorded from manual instrument settings where possible (FCA, FRA, FDR, FPR, TSA, TCA, and TR). The femoral sagittal resection angle was influenced by the fit of the intermedullary rod, thus the anatomic sagittal angle of the femoral shaft was measured from the CT scans and used as the target.
      After primary resections, bone remnant thicknesses were measured using a digital caliper. The thicknesses of femur resections were measured from the resection plane to the most prominent point on the articular surface. The tibia thickness was measured to an electrocautery mark on the articular surface marking the point used in planning. After surgery, CT scans were repeated using the same imaging protocol. The proximal 300 mm of the tibia and the distal 300 mm of the femur were extracted from the specimen and denuded of soft tissue. Optical scans of the extracted femur and tibia were performed using a scanner with a reported accuracy of 0.05 mm (Space Spider; Artec 3D, Luxembourg, Luxembourg). The extracted femurs were implanted with an ATTUNE cruciate-retaining cementless femur and the extracted tibiae were implanted with an ATTUNE cemented tibial base. A second optical scan was performed to capture the orientation of the implant geometry relative to the extracted bone.

       Resection Alignment Measurements

      The intact bony anatomy was segmented from the preoperative CT scans and anatomic coordinate systems were constructed. The superior-inferior (S-I) axis of the femoral was aligned to the femoral mechanical axis and rotationally aligned to the posterior condylar axis. Likewise, the S-I axis of the tibial coordinate system was aligned to the tibial mechanical axis and rotationally aligned to the medial third of the tibial tubercle. Geometries of the resected bones were segmented from the postoperative CT scans and solid models of the resected and implanted bones were fused from the optical scans. These solid models were registered to the intact models using an iterative closest point algorithm in their respective anatomic coordinate systems [
      • Besl P.J.
      • McKay N.D.
      A method for registration of 3-D shapes.
      ].
      Planes were fit to the distal femoral, posterior femoral, and proximal tibial resections from both the CT and optical scans. Angles between the distal femoral resection and the S-I axis in the sagittal and coronal planes were used to calculate the FSA (distal reference) and FCA, respectively. Another measurement of the FSA was performed using the posterior femoral resection and the S-I axis (posterior reference). The angle between the posterior femoral resection and the anterior-posterior (A-P) anatomic axis in the transverse plane was used to quantify the FRA alignment. Like the femur, the angles between the proximal TR and the S-I axis in the sagittal and coronal plane were used to calculate the TSA and TCA, respectively. To calculate the implant alignment, equivalent calculations were performed using the implant’s bone interface surfaces to calculate the femoral implant sagittal, coronal, and internal-external rotation angles (FISA, FICA, and FIRA) and the tibial implant sagittal and coronal angles (TICA, TISA).

       Femoral Relative Resection Accuracy Measurements

      The relative alignment of the anterior, distal, and posterior femoral resections was also characterized which affects fit of the femoral component. A femoral implant coordinate system was constructed relative to the femoral resections with the S-I axis perpendicular to the distal femoral resection and rotationally aligned to the anterior resection plane. Angles between the posterior resection and the S-I axis and between the anterior resection plane and the S-I axis in the sagittal plane were calculated, and between the anterior and posterior resections in the transverse plane (Fig. 2). Likewise, the A-P distance between the anterior and posterior resection surfaces was calculated 3 mm distal from the most proximal point on the posterior resection. Equivalent target resection angles and A-P distances were extracted from the appropriately sized femoral implant geometry.
      Figure thumbnail gr2
      Fig. 2The relative orientation of the femoral anterior, distal, and posterior resections was calculated in both the (A) sagittal and (B) transverse planes using models generated from optical scans. A-P, anterior-posterior.

       Statistical Analysis

      Alignment errors were calculated as the difference between the measured alignment metric and the corresponding target. Summary statistics, including the mean and standard deviation of the errors, the median errors, and the mean and standard deviation of the absolute errors were calculated for each alignment metric across the RATKA and CTKA cohorts. Conditional 2-sample t-tests were performed. The first t-test evaluated the null hypothesis that the RATKA cohort had superior accuracy to the CTKA cohort based on absolute errors. If RATKA was not statistically superior, a secondary t-test was performed to determine if the RATKA cohort was noninferior to the CTKA cohort within a margin of 0.5° or 0.5 mm (P ≤ .05) where appropriate. The proportion of specimen with accuracy errors less than 1° or 1 mm, 2° or 2 mm, and 3° or 3 mm were calculated. Since accuracy of the primary resections was calculated using both CT and optical scans, mean absolute differences and correlation coefficients were calculated between the redundant measures. Finally, mean absolute differences and correlation coefficients were calculated between the resection alignments and final implant alignments.

      Results

      During 2 of the 40 RATKA surgeries, the femoral array was dislodged intraoperatively. The damaged array was replaced, and no further issues were observed. The femoral alignment metrics for these 2 specimens were excluded from the analysis. Two TR depth measurements were inadvertently not recorded during surgery, one each from the RATKA and CTKA cohorts, and were likewise excluded from the analysis. Finally, optical scans for 1 femur implant and 1 tibial implant were corrupted, both from the CTKA cohort, and excluded. All other alignment measurements were successfully collected.
      For the RATKA cohort, absolute mean errors in resection alignment ranged from 0.63° ± 0.50° (FCA) to 1.71° ± 1.31° (FSA, distal reference). For the CTKA cohort, absolute mean errors ranged from 1.00° ± 0.70° (FRA) to 3.27° ± 2.51° (FSA, posterior reference). The RATKA cohort demonstrated statistical superiority to the CTKA cohort in accuracy of the FCA (P < .001); FSA, posterior reference (P < .001); and TCA (P = .001), and noninferiority for all other resection metrics (Table 1). RATKA had fewer outliers (errors >3°) for all angular resection alignment measures (Fig. 3, Table 2). In the RATKA cohort, 100% of the specimen had FCA, FRA, and TCA resections within 3° of the target alignments, compared to 92.5%, 97.5%, and 80%, respectively, for the CTKA cohort. Strong agreements between the CT and optical-based measurements were observed for all metrics except the FSA (distal reference), with correlation coefficients ranging from 0.91 to 0.97, and absolute mean differences ranging from 0.31° ± 0.27° to 0.91° ± 0.95° (Table 3). A weaker correlation (0.80) and increased absolute mean difference (0.94° ± 0.73°) was observed for the FSA (distal reference) between the 2 measurement modalities.
      Table 1Summary Accuracy Measurements for Differences Between Target and Measured Resection and Implant Alignments of the RATKA and CTKA Cohorts.
      Accuracy Metric CT ScansRATKACTKARATKA Accuracy Compared to CTKAP-Value
      Mean ± SDMedianAbsolute Mean ± SDMean ± SDMedianAbsolute Mean ± SD
      Femur resection
       Coronal angle (°)−0.01 ± 0.82-0.010.63 ± 0.50−0.20 ± 1.69−0.251.39 ± 0.95Superior<.001
       Sagittal angle, distal reference (°)1.23 ± 1.781.201.71 ± 1.310.47 ± 2.410.561.93 ± 1.50Noninferior.014
       Sagittal angle, posterior reference (°)0.65 ± 1.370.711.21 ± 0.900.15 ± 4.150.193.27 ± 2.51Superior<.001
       I-E rotation angle (°)−0.38 ± 1.270.021.04 ± 0.81−0.04 ± 1.230.161.00 ± 0.70Noninferior.004
      Tibial resection
       Coronal angle (°)−0.04 ± 1.180.110.93 ± 0.720.92 ± 1.900.971.65 ± 1.29Superior.001
       Sagittal angle (°)0.69 ± 1.861.071.62 ± 1.130.59 ± 2.070.651.63 ± 1.39Noninferior.036
      Accuracy Metric Optical ScansRATKACTKARATKA Accuracy Compared to CTKAP-Value
      Mean ± SDMedianAbsolute Mean ± SDMean ± SDMedianAbsolute Mean ± SD
      Femur resection
       Coronal angle (°)−0.12 ± 0.75−0.140.59 ± 0.47−0.35 ± 1.70−0.481.39 ± 1.02Superior<.001
       Sagittal angle, distal reference (°)1.42 ± 0.991.581.50 ± 0.860.25 ± 2.220.361.70 ± 1.42Noninferior.005
       Sagittal angle, posterior reference (°)0.89 ± 1.230.851.14 ± 1.000.02 ± 4.00-0.182.93 ± 2.69Superior<.001
       I-E rotation angle (°)−0.02 ± 1.130.120.97 ± 0.570.29 ± 1.330.371.10 ± 0.79Noninferior<.001
       Posterior resection sagittal angle (°)0.56 ± 1.240.460.95 ± 0.960.23 ± 2.93−0.152.08 ± 2.05Superior.001
       Anterior resection sagittal angle (°)−1.91 ± 1.05−1.901.91 ± 1.05−1.75 ± 1.51−1.811.92 ± 1.27Noninferior.028
       A-P resections transverse angle (°)−0.03 ± 1.110.100.85 ± 0.70−0.33 ± 0.86−0.190.71 ± 0.58Noninferior.007
       A-P resections A-P distance (mm)−0.16 ± 0.83−0.160.56 ± 0.63−0.17 ± 1.27−0.280.97 ± 0.82Superior.006
      Tibial resection
       Coronal angle (°)0.47 ± 1.470.891.28 ± 0.851.48 ± 1.911.481.97 ± 1.38Superior.004
       Sagittal angle (°)−0.14 ± 1.610.091.21 ± 1.05−0.04 ± 2.24−0.121.65 ± 1.49Noninferior.001
      Femur implant
       Coronal angle (°)−0.27 ± 1.19−0.300.89 ± 0.82−0.62 ± 1.73−0.421.42 ± 1.15Superior.011
       Sagittal angle (°)−0.53 ± 1.79−0.361.51 ± 1.08−2.22 ± 2.39−1.752.49 ± 2.10Superior.006
       I-E rotation angle (°)−0.24 ± 1.36−0.351.11 ± 0.800.13 ± 1.330.010.98 ± 0.90Noninferior.031
      Tibial implant
       Coronal angle (°)0.47 ± 1.500.511.31 ± 0.841.54 ± 1.971.682.03 ± 1.44Superior.004
       Sagittal angle (°)0.10 ± 1.770.371.37 ± 1.110.13 ± 2.250.361.65 ± 1.51Noninferior.005
      Accuracy Metric CalipersRATKACTKARATKA Accuracy Compared to CTKAP-Value
      Mean ± SDMedianAbsolute Mean ± SDMean ± SDMedianAbsolute Mean ± SD
      Femur resection
       Distal resection depth (mm)0.01 ± 0.87−0.020.62 ± 0.60−0.56 ± 1.18−0.300.88 ± 0.96Noninferior<.001
       Posterior resection depth (mm)−0.08 ± 0.69−0.280.54 ± 0.43−0.69 ± 0.89−0.460.76 ± 0.83Noninferior<.001
      Tibial resection
       Tibial resection depth (mm)−0.44 ± 0.86−0.300.67 ± 0.69−1.21 ± 1.79−1.301.66 ± 1.38Superior<.001
      Results of the statistical analysis are reported for each alignment metric with the associated P-values for the reported outcome.
      CTKA, conventional total knee arthroplasty; RATKA, robotic-assisted total knee arthroplasty; A-P, anterior-posterior; I-E, internal-external; CT, computed tomography; SD, standard deviation.
      Figure thumbnail gr3
      Fig. 3Histograms of femoral and tibial angular resection errors for RATKA and CTKA cohorts. CTKA, conventional total knee arthroplasty; RATKA, robotic-assisted total knee arthroplasty.
      Table 2Summary of Alignment Outliers for the RATKA and CTKA Cohorts.
      Accuracy MetricRATKACTKA
      Error From Target (% Specimen)Error From Target (% Specimen)
      ≤1° or 1 mm≤2° or 2 mm≤3° or 3 mm≤1° or 1 mm≤2° or 2 mm≤3° or 3 mm
      Femur resection
       Coronal angle (°)86.8%97.4%100.0%42.5%85.0%92.5%
       Sagittal angle, distal reference (°)34.2%63.2%89.5%35.0%60.0%85.0%
       Sagittal angle, posterior reference (°)44.7%92.1%97.4%10.0%35.0%57.5%
       I-E rotation angle (°)52.6%78.9%100.0%60.0%90.0%97.5%
       Distal resection depth (mm)78.9%94.7%100.0%77.5%90.0%95.0%
       Posterior resection depth (mm)84.2%97.4%100.0%75.0%90.0%95.0%
       Posterior resection sagittal angle (°)73.7%92.1%92.1%35.0%67.5%82.5%
       Anterior resection sagittal angle (°)15.8%55.3%97.4%27.5%57.5%80.0%
       A-P resections transverse angle (°)71.1%97.4%97.4%75.0%97.5%100.0%
       A-P resections A-P distance (mm)86.8%100.0%100.0%70.0%87.5%95.0%
      Tibial resection
       Coronal angle (°)60.0%90.0%100.0%37.5%72.5%80.0%
       Sagittal angle (°)35.0%70.0%85.0%42.5%70.0%80.0%
       Tibial resection depth (mm)79.5%94.9%97.4%41.0%71.8%92.3%
      Femur implant
       Coronal angle (°)68.4%89.5%94.7%46.2%71.8%87.2%
       Sagittal angle (°)42.1%68.4%92.1%20.5%56.4%69.2%
       I-E rotation angle (°)52.6%92.1%94.7%66.7%84.6%94.9%
      Tibial implant
       Coronal angle (°)35.0%85.0%95.0%28.2%56.4%82.1%
       Sagittal angle (°)42.5%77.5%90.0%46.2%61.5%84.6%
      CTKA, conventional total knee arthroplasty; RATKA, robotic-assisted total knee arthroplasty; A-P, anterior-posterior; I-E, internal-external.
      Table 3Correlations Between the CT and Optically Measured Resection Accuracies and Between the Resection Angles and Corresponding Implant Alignments.
      Accuracy MetricCT Measured vs Optically Measured Resection AnglesOptically Measured Resection Angles vs Implant Angles
      CorrelationMean Difference ± SDAbsolute Mean Difference ± SDCorrelationMean Difference ± SDAbsolute Mean Difference ± SD
      Femur angles
       Coronal angle (°)0.97−0.13 ± 0.400.31 ± 0.280.880.22 ± 0.900.64 ± 0.66
       Sagittal angle, distal reference (°)0.800.01 ± 1.190.94 ± 0.730.742.21 ± 1.532.24 ± 1.48
       Sagittal angle, posterior reference (°)0.91−0.03 ± 1.320.91 ± 0.950.531.82 ± 2.682.58 ± 1.95
       I-E rotation angle (°)0.950.34 ± 0.650.58 ± 0.450.920.18 ± 0.830.64 ± 0.56
      Tibial angles
       Coronal angle (°)0.960.53 ± 0.560.61 ± 0.470.970.03 ± 0.450.37 ± 0.26
       Sagittal angle (°)0.91−0.73 ± 0.810.88 ± 0.650.93−0.26 ± 0.750.61 ± 0.50
      CT, computed tomography; SD, standard deviation; I-E, internal-external.
      Femoral resection depth accuracy for RATKA was noninferior to CTKA with mean absolute errors less than 0.62 mm for RAKTA and 0.88 mm CTKA (Table 1). RATKA TR mean absolute error (0.67 ± 0.69 mm) was superior to CTKA (1.62 ± 1.39 mm, P < .001). Outliers were reduced for all resection depth measures in the RATKA cohort (Table 2).
      Accuracy of the posterior femoral resection relative to the distal resection in the sagittal plane was superior in RATKA (P = .001), along with the A-P distance between the anterior and posterior resections (P = .006; Table 1). The sagittal orientation of the anterior resection and the transverse angle between the anterior and posterior femoral resections in RATKA were noninferior to CTKA. RATKA resulted in fewer outliers in the relative orientation of the femoral resections (errors >3° or 3 mm), except for in the transvers angle between the anterior and posterior resections (Table 2).
      The accuracy of the femur implant coronal and sagittal alignment and the tibial implant coronal alignment in RATKA was superior to CTKA (P = .011, P = .006, and P = .004, respectively). In both cohorts, the femoral component was flexed relative to the target alignment and the measured femoral sagittal resection angle. In RATKA, the femoral implant was flexed an average of 0.53° ± 1.79° relative to the target, while in CTKA the femoral implant was flexed 2.22° ± 2.39° (Table 1). Strong correlations, ranging from 0.88 to 0.97, were observed between the resection alignment and resulting implant alignment for angles other than the femoral implant sagittal angle (Table 3).

      Discussion

      The current study represents one of the largest and most comprehensive preclinical assessments of RATKA system accuracy to date. The VELYS Robotic-Assisted Solution evaluated in this study demonstrated statistically superior accuracy for many resection and implant alignments, and noninferiority for the remaining measures when compared to CTKA. In particular, the RATKA system yielded superior accuracy for the coronal alignment of the femur and tibia resections and implant alignments.
      With the improved accuracy and precision of modern RATKA surgical systems, detecting meaningful differences in resection accuracy is challenging. Some previous studies have relied on the robot’s optical tracking system to quantify resection accuracy [
      • Koulalis D.
      • O’Loughlin P.F.
      • Plaskos C.
      • Kendoff D.
      • Cross M.B.
      • Pearle A.D.
      Sequential versus automated cutting guides in computer-assisted total knee arthroplasty.
      ,
      • Sires J.D.
      • Craik J.D.
      • Wilson C.J.
      Accuracy of bone resection in MAKO total knee robotic-assisted surgery.
      ,
      • Parratte S.
      • Price A.J.
      • Jeys L.M.
      • Jackson W.F.
      • Clarke H.D.
      Accuracy of a new robotically assisted technique for total knee arthroplasty: a cadaveric study.
      ,
      • Figueroa F.
      • Wakelin E.
      • Twiggs J.
      • Fritsch B.
      Comparison between navigated reported position and postoperative computed tomography to evaluate accuracy in a robotic navigation system in total knee arthroplasty.
      ,
      • Casper M.
      • Mitra R.
      • Khare R.
      • Jaramaz B.
      • Hamlin B.
      • McGinley B.
      • et al.
      Accuracy assessment of a novel image-free handheld robot for Total Knee Arthroplasty in a cadaveric study.
      ], which neglects potentially significant errors in the registration process [
      • Davis E.T.
      • Pagkalos J.
      • Gallie P.A.M.
      • Macgroarty K.
      • Waddell J.P.
      • Schemitsch E.H.
      Defining the errors in the registration process during imageless computer navigation in total knee arthroplasty: a cadaveric study.
      ]. Other studies use preoperative and postoperative CT scans to measure resection or implant alignment angles [
      • Scholl L.Y.
      • Hampp E.L.
      • De Souza K.M.
      • Chang T.C.
      • Deren M.
      • Yenna Z.C.
      • et al.
      How does robotic-arm assisted technology influence total knee arthroplasty implant placement for surgeons in fellowship training?.
      ,
      • Sires J.D.
      • Wilson C.J.
      CT validation of intraoperative implant position and knee alignment as determined by the MAKO total knee arthroplasty system.
      ], which allow independent quantification of the system accuracy and reduce uncertainty in establishing anatomic coordinate systems. Limitations in the CT slice thickness induce errors in segmentation of the resection surfaces which are commonly parallel to the axial slices. This study used an interslice distance of 0.6 mm, which is finer than most clinical imaging protocols. Likewise, implant-induced metal artifacts cause uncertainty in alignment measures [
      • Jonkergouw F.
      • Allé F.
      • Chellaoui K.
      • Vander Sloten J.
      • Vangeneugden D.
      Three-dimensional measurement technique to assess implant position and orientation after total knee arthroplasty.
      ,
      • Campanelli V.
      • Lozano R.
      • Akhlaghpour H.
      • Brar A.S.
      • Maislin D.
      • Nedopil A.J.
      • et al.
      Implant placement accuracy in total knee arthroplasty: validation of a CT-based measurement technique.
      ]. A study by Sires and Wilson [
      • Sires J.D.
      • Wilson C.J.
      CT validation of intraoperative implant position and knee alignment as determined by the MAKO total knee arthroplasty system.
      ] found absolute differences between resection angles measured intraoperatively and the corresponding implant alignments measured using a CT scan ranged from 1.09° ± 0.75° to 1.97° ± 1.41° for tibial coronal and sagittal alignment, respectively, with 9% of the accuracy measurements deviating by >3°. The small differences between resection angles and implant angles observed in our study suggest that much of this error may be attributed to the anatomic registration step in the RATKA workflow.
      Comparisons to other robotic systems are difficult due to differences in measurement methods and reported accuracy metrics. A multicenter, prospective, nonrandomized study compared the postoperative implant alignment between image-based RATKA and CTKA cohorts using CT scans [
      • Mahoney O.
      • Kinsey T.
      • Sodhi N.
      • Mont M.A.
      • Chen A.F.
      • Orozco F.
      • et al.
      Improved component placement accuracy with robotic-arm assisted total knee arthroplasty.
      ]. The study found mean absolute errors of 1.0° (FICA), 1.7° (FIRA), 1.3° (TICA), and 1.4° (TISA) for the RATKA cohort. These errors were similar to those reported in this study using the image-free RATKA system (0.89° ± 0.82°, 1.11° ± 0.80°, 1.31° ± 0.84°, and 1.37° ± 1.11° for FICA, FIRA, TICA, and TISA, respectively). Parratte et al. [
      • Parratte S.
      • Price A.J.
      • Jeys L.M.
      • Jackson W.F.
      • Clarke H.D.
      Accuracy of a new robotically assisted technique for total knee arthroplasty: a cadaveric study.
      ] reported resection errors ranging from 0.50° to 1.29° in a cohort of 30 cadaveric knees using a RATKA system which positions a resection guide. Errors were measured using the CAS component of the robotic system which excluded errors in the registration process. Even so, the reported errors were on par with the resection accuracy found in the current study, ranging from 0.59° to 1.50°. Other studies have documented similar statistically significant improvements in coronal implant alignment when using RATKA [
      • Bollars P.
      • Boeckxstaens A.
      • Mievis J.
      • Kalaai S.
      • Schotanus M.G.M.
      • Janssen D.
      Preliminary experience with an image-free handheld robot for total knee arthroplasty: 77 cases compared with a matched control group.
      ,
      • Scholl L.Y.
      • Hampp E.L.
      • De Souza K.M.
      • Chang T.C.
      • Deren M.
      • Yenna Z.C.
      • et al.
      How does robotic-arm assisted technology influence total knee arthroplasty implant placement for surgeons in fellowship training?.
      ,
      • Mahoney O.
      • Kinsey T.
      • Sodhi N.
      • Mont M.A.
      • Chen A.F.
      • Orozco F.
      • et al.
      Improved component placement accuracy with robotic-arm assisted total knee arthroplasty.
      ,
      • Seidenstein A.
      • Birmingham M.
      • Foran J.
      • Ogden S.
      Better accuracy and reproducibility of a new robotically-assisted system for total knee arthroplasty compared to conventional instrumentation: a cadaveric study.
      ,
      • Koulalis D.
      • O’Loughlin P.F.
      • Plaskos C.
      • Kendoff D.
      • Cross M.B.
      • Pearle A.D.
      Sequential versus automated cutting guides in computer-assisted total knee arthroplasty.
      ].
      Our study leveraged redundant imaging modalities to improve confidence in our findings. Strong correlations were observed between the resection angles measured from CT scans and the optical scans, with the notable exception of FSA (distal resection). The distal femoral resection surface has a small A-P width and aligns closely with the axial slice of the CT scan, which induced errors in the segmentation and subsequent CT-based measurement. The optical scan generated higher fidelity resection surfaces, but accuracy metrics using this surface did not indicate significant differences in FSA between RATKA and CTKA. RATKA superiority was observed when comparing the FSA measured from the posterior resections from both the CT and optical scans. Future CT-based accuracy studies should consider using the anterior or posterior femoral resections to quantify the FSA due to the improved imaging accuracy.
      Small absolute mean differences (<0.64°) and strong correlations (>0.88) were observed between the resection alignment and implant alignment, except for the FSA. Cementless femoral components were implanted in our study which have an interference fit with the resected bone. The interference fit induced an average of 2.2° ± 1.5° of femoral implant flexion during impaction relative to the distal resection surface across both surgical cohorts. The observed component flexion was smaller in the RATKA cohort than the CTKA, likely due to the improved accuracy of the anterior and posterior femoral resections in the RATKA group. It should be noted that implantation of the final components took place after the bones had been extracted from the specimen. This increased the stiffness of the impaction process and improved visibility of the implant bone interface to ensure full seating of the implants. Impactions performed in a clinical setting may not result in such strong correlations between the resection and implant alignments. The small errors between resection alignments and implant alignments observed in this study (excluding femoral sagittal alignment) lend credence to retrospective clinical studies which quantify system accuracy through postoperative CT scans, where differences between intraoperative target alignments and the resulting implant alignment greater than 1° could likely be attributed to the robotic system performance.
      This study had several notable limitations. Inherent variation between cadaveric and in vivo tissue properties limits clinical applicability of these results. Cadaveric specimens were age-matched to the TKA patient population but only 33% of specimen had cartilage degeneration typical of TKA patients. Due to the bilateral comparison of RATKA and CTKA, this likely did not cause significant bias between the cohorts. Likewise due to the cadaveric tissue, we were not able to measure weight-bearing postoperative alignment. Finally, while resection accuracy is a fundamental component of implant longevity, appropriate soft tissue balance is also critical. The current study did not assess knee stability or soft tissue balance, which should be the focus of future studies.

      Conclusions

      In conclusion, the results of this study demonstrate that use of image-free RATKA can improve implant alignment, especially in the coronal plane, compared to conventional instrumentation and reduce the occurrence of outliers.

      Appendix A. Supplementary Data

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