Critical Examination of Methods to Determine Tibiofemoral Kinematics and Tibial Contact Kinematics Based On Analysis of Fluoroscopic Images.

Goals of knee replacement surgery are to restore function and maximize implant longevity. To determine how well these goals are satisfied, tibial femoral kinematics and tibial contact kinematics are of interest. Tibiofemoral kinematics, which characterize function, is movement between the tibia and femur whereas tibial contact kinematics, which is relevant to implant wear, is movement of the location of contact by the femur on the tibial articular surface. The purpose of this review article is to describe and critique relevant methods to guide correct implementation. For tibiofemoral kinematics, methods are categorized as those which determine 1) relative planar motions and 2) relative 3D motions. Planar motions are determined by first finding AP positions of each femoral condyle relative to the tibia and tracking these positions during flexion. Of the lowest point and flexion facet center methods, which are common, the lowest point method is preferred and the reasoning is explained. 3D motions are determined using the joint coordinate system (JCS) of Grood and Suntay. Previous applications of this JCS have resulted in motions which are largely in error due to 'kinematic crosstalk'. Requirements for minimizing kinematic crosstalk are outlined followed by an example which demonstrates the method for identifying a JCS which minimizes kinematic crosstalk. Although kinematic crosstalk can be minimized, the need for a JCS to determine 3D motions is questionable based on anatomical constraints which limit varus-valgus rotation and compression-distraction translation. Methods for analyzing tibial contact kinematics are summarized and validation of methods discussed.

Analysis of Variation in Sagittal Curvature of the Femoral Condyles.

BACKGROUND: In designing femoral components which restore native (i.e. healthy) knee kinematics, the flexion-extension (F-E) axis should match the native knee. Because the F-E axis is governed by the curvature of articular surfaces of the femoral condyles in the sagittal plane, the primary objective was to determine variations in radii of curvature. METHODS: Eleven high accuracy 3D femur models were generated from high-resolution CT scans. The sagittal profile of each condyle was created. The radii of curvature at 15 deg increments of arc length were determined based on best-fit segment circles. Results were standardized to the radius of the best-fit overall circle fit to 15 - 105 deg. RESULTS: Medial and lateral femoral condyles exhibited multi-radius of curvature sagittal profiles where the radius decreased at 30 deg flexion by 10 mm and at 15 deg flexion by 8 mm, respectively. On either side of the decrease, radii of best-fit segment circles were relatively constant. Beyond the transition angles, anterior-posterior (A-P) positions of centers of curvature varied 4.8 mm and 2.3 mm for the medial and lateral condyles, respectively. CONCLUSION: A two-radius of curvature profile approximates the radii of curvature of both native femoral condyles, but the transition angles differ with the transition angle of the medial femoral condyle occurring about 15 deg later in flexion. Owing to variation in A-P positions of centers of curvature, the F-E axis is not strictly fixed in the femur.

Risk of tibial baseplate loosening is low in patients following unrestricted kinematic alignment total knee arthroplasty using a cruciate-retaining medial conforming insert: A study using radiostereometric analysis at 2 years.

PURPOSE: Assessing the risk of tibial baseplate loosening in patients after unrestricted kinematically aligned (unKA) total knee arthroplasty (TKA) using a medially conforming insert is important because baseplates generally are aligned in varus which has been linked to an increased incidence of aseptic loosening following mechanically aligned TKA. Two limits that indicate long-term stability in patients are a change in maximum total point motion between 1 and 2 years (ΔMTPM) < 0.2 mm and anterior tilt at 2 years < 0.8°. The purposes were to determine: (1) the number of patients with ΔMTPM > 0.2 mm, (2) the number of patients with anterior tilt > 0.8° and (3) whether increased varus baseplate and limb alignment were associated with increased migration. METHODS: Thirty-five patients underwent cemented, caliper-verified, unKA TKA using a medially conforming tibial insert with posterior cruciate ligament (PCL) retention. Biplanar radiographs acquired on the day of surgery and at 1.5, 3, 6, 12 and 24 months were processed with model-based radiostereometric analysis (RSA) software to determine migration and the number of patients with migration above the two stability limits. Medial proximal tibial angle (MPTA), hip-knee-ankle angle (HKAA) and posterior slope angle (PSA) were analyzed for an association with migration in six degrees of freedom and in MTPM. RESULTS: Thirty-two of 35 patients were available for analysis at 2 years. One patient exhibited ΔMTPM > 0.2 mm. The same patient exhibited anterior tilt > 0.8°. Varus rotation (p = 0.048, r ≤ 0.34) and medial translation (p = 0.0273, r ≤ 0.29) increased with increased varus baseplate alignment. CONCLUSION: The results indicate low risk of long-term baseplate loosening in patients. Although varus rotation and medial translation increased with increased varus baseplate alignment, the magnitudes of the migrations were minimal and did not increase ΔMTPM and anterior tilt. LEVEL OF EVIDENCE: Level II, therapeutic prospective cohort study.

Reoperation, Implant Survival, and Clinical Outcome After Kinematically Aligned Total Knee Arthroplasty: A Concise Clinical Follow-Up at 16 Years.

BACKGROUND: The preceding study reported a 10-year follow-up of 222 kinematically aligned total knee arthroplasties (TKA) performed in 217 patients in 2007. As 35% of tibial components and 8% of limbs were in >3° varus, the present study assessed whether this adversely affected reoperation, implant survival, and function at 16 years. METHODS: We retrospectively reviewed a single surgeon's private practice database to determine the patients who underwent reoperation as well as Forgotten Joint Score and Oxford Knee Score. RESULTS: There were 7 patients who had a major reoperation (revision of a loose tibial component [n = 2], and revision of well-fixed component due to stiffness [n = 1], patella instability [n = 1], pain [n = 1], and infection [n = 2]). There were 5 who had a minor reoperation that retained the components, and 91 patients (94 TKAs) died. Implant survivorship was 93% using reoperation for any reason as the endpoint. The median (interquartile range) Forgotten Joint and Oxford Knee scores were 88 (57 to 100) and 45 (39 to 48) points, respectively. CONCLUSION: The kinematically aligned TKA had a 7% reoperation rate at 16 years follow-up, comparable to or lower than reports of mechanically aligned TKA, which supports the concept of the unrestricted version of kinematic alignment in which the patient's prearthritic alignment is fully restored regardless of deformity.

Guideline for RSA and CT-RSA implant migration measurements: an update of standardizations and recommendations.

Opening remarks: These guidelines are the result of discussions within a diverse group of RSA researchers. They were approved in December 2023 by the board and selected members of the International Radiostereometry Society to update the guidelines by Valstar et al. [1]. By adhering to these guidelines, RSA studies will become more transparent and consistent in execution, presentation, reporting, and interpretation. Both authors and reviewers of scientific papers using RSA may use these guidelines, summarized in the Checklist, as a reference. Deviations from these guidelines should have the underlying rationale stated.

State of the Art in Radiostereometric Analysis for Tibial Baseplate Migration and Future Research Directions.

Radiostereometric analysis (RSA) measures movement (migration) of a baseplate relative to the underlying tibia after total knee arthroplasty (TKA) and has been used extensively to evaluate safety of new implant designs and/or surgical techniques regarding baseplate loosening. Because RSA is a complex methodology which involves various choices that researchers make, including whether to use marker-based or model-based methods, which migration metric to report, how to relate short-term migrations to long-term risk, and how these choices impact error, the objectives of this review were to: (1) lay out a comprehensive structure illustrating the multiple components/considerations for RSA and their interrelations, (2) review components of the structure using the latest RSA literature, and (3) use the preceding review as a context for identifying future areas of study. The components to be reviewed were structured using the following topics: type of RSA, migration metrics, sources of error, studies/reports of error, stability limits, and studies of error in stability limits. Based on the current RSA literature and knowledge gaps which exist, the following future research directions were identified: (1) revising the ISO standard to require reporting of clinical measurement error (bias) and recommending use of a local baseplate coordinate system, (2) identifying the migration metric and associated threshold most predictive of baseplate loosening for individual patients, (3) creating a method for data sharing to improve individual patient diagnostics, and (4) determining an appropriate stability limit for model-based RSA for group stability and individual patient diagnostics.

Conditions for Use and Implementation of Globally-Aligned Versus Local Baseplate Coordinate Systems When Computing Migration Using Radiostereometric Analysis.

Radiostereometric analysis can be used for computing movement of a tibial baseplate relative to the tibia (termed migration) to determine stability of fixation. Quantifying migration in six degrees of freedom requires establishing a coordinate system in which to express the movement. Establishing consistent migration directions among patients and baseplate designs remains challenging. Deviations in imaging alignment (tibia/baseplate alignment during image acquisition) and surgical alignment (baseplate alignment on tibia) will affect computed migrations when using the conventional globally-aligned baseplate coordinate system (BCS) (defined by calibration box). Computing migration using a local BCS (defined by baseplate) may be preferrable. This paper (1) summarizes the migration equations when using a globally-aligned versus local BCS, (2) proposes a method for defining a local BCS, and (3) demonstrates differences in the two BCSs for an example patient whose baseplate has rotational deviations due to imaging or surgical alignments. Differences in migration for the two BCSs ranged from about ±0.5 mm in translations and -0.4 deg to 0.7 deg in rotations. Differences were largest for deviations in internal-external rotation and smallest for deviations in varus-valgus rotation. An example demonstrated that the globally-aligned BCS resulted in migration being quantified as subsidence instead of liftoff, thereby changing fundamental interpretations. Because migrations computed using a local BCS are independent of imaging and surgical alignments and instead characterize migration using baseplate features, a local BCS enhances consistency in migration directions among patients and baseplate designs relative to the interface in which fixation may be compromised.

Caliper-Verified Kinematically Aligned Total Knee Arthroplasty: Rationale, Targets, Accuracy, Balancing, Implant Survival, and Outcomes.

Peer-reviewed studies published up to May 2022 are used to provide a comprehensive understanding of unrestricted kinematically aligned total knee arthroplasty. The intent is to cultivate the curiosity of those interested in this method of personalized alignment. The rationale of unrestricted kinematic alignment is to set the femoral and tibial components coincident with the patient's prearthritic joint lines, restore the femoral and tibial phenotypes, and coalign the three kinematic axes of the components with those of the knee. The surgical technique, learning curve, and accuracy of performing kinematically aligned total knee arthroplasty with a caliper and company manufactured manual instrumentation should be explored and compared with robotic instrumentation. Kinematic alignment restores the native knee's medial and lateral tibial compartment forces, which mechanically aligned total knee arthroplasty cannot do even after ligament release. In addition, insert conformity plays a role in restoring native tibiofemoral kinematics. A literature review of clinical outcomes, long-term durability, and the risk of varus tibial component failure and patellofemoral instability shows unrestricted kinematic alignment has comparable if not superior results when compared with mechanical alignment.

More passive internal tibial rotation with posterior cruciate ligament retention than with excision in a medial pivot TKA implanted with unrestricted caliper verified kinematic alignment.

PURPOSE: Excision of the posterior cruciate ligament (PCL) is recommended when implanting a medial pivot (MP) total knee arthroplasty (TKA) to reduce the risk of limiting flexion by over-tensioning the flexion space. The present study determined whether PCL retention (1) limits internal tibial rotation and (2) causes anterior lift-off of the insert in 90° flexion after implantation of an MP design with unrestricted caliper verified kinematic alignment (KA). METHODS: Four surgeons implanted an MP TKA design with medial ball-in-socket and lateral flat tibial insert in ten fresh-frozen cadaveric knees. Before and after PCL excision, trial inserts with medial goniometric markings measured the angular I-E tibial orientation relative to the trial femoral component's medial condyle in extension and at 90° flexion, and the surgeon recorded the occurrence of anterior lift-off of the insert at 90° flexion. RESULTS: PCL retention resulted in greater internal tibial rotation than PCL excision, with mean values of 15° vs 7° degrees from maximum extension to 90° flexion, respectively (p < 0.0007). At 90° flexion, no TKAs with PCL retention and one TKA with PCL excision had anterior lift-off of the insert (N.S.). CONCLUSIONS: This preliminary study of ten cadaveric knees showed that PCL retention restored more passive internal tibial rotation than PCL excision with a negligible risk of anterior lift-off. However, in vivo analysis from multiple authors with a larger sample size is required to recommend PCL retention with an MP TKA design implanted with unrestricted caliper verified KA.

Posterior rim loading of a low-conforming tibial insert in unrestricted kinematic alignment is caused by rotational alignment of an asymmetric baseplate designed for mechanical alignment.

PURPOSE: Because different targets are used for internal-external rotation, an asymmetric baseplate designed for mechanical alignment may lead to under-coverage and concomitant posterior rim loading in the lateral compartment following unrestricted kinematic alignment (KA) TKA. Recognizing that such loading can lead to premature wear and/or subsidence, our aim was to determine the cause(s) so that occurrence could be remedied. Our hypothesis was that baseplate design features such as asymmetric shape when aligned in KA would consistently contribute to posterior rim loading in the lateral compartment. METHODS: Based on analysis of fluoroscopic images of 50 patients performing dynamic, weight bearing deep knee bend and step up and of postoperative CT images, five possible causes were investigated. Causes included internal rotation of the baseplate when positioned in KA; posterior position of the lateral femoral condyle at extension; internal tibial rotation with flexion; internal rotational deviation of the baseplate from the KA rotation target; and posterior slope. RESULTS: The incidence of posterior rim loading was 18% (9 of 50 patients). When positioned in KA, the asymmetric baseplate left 15% versus 10% of the AP depth of the lateral compartment uncovered posteriorly for posterior rim loading and non-posterior rim loading groups, respectively (p = 0.009). The lateral femoral condyle at extension was more posterior by 4 mm for the posterior rim loading group (p = 0.003). CONCLUSIONS: Posterior rim loading in the lateral compartment was caused in part by the asymmetric design of the tibial baseplate designed for mechanical alignment which was internally rotated when positioned in KA thus under-covering a substantial percentage of the posterior lateral tibia. This highlights the need for new, asymmetric baseplates designed to maximize coverage when used in KA. LEVEL OF EVIDENCE: III.

The posterolateral upslope of a low-conforming insert blocks the medial pivot during a deep knee bend in TKA: a comparative analysis of two implants with different insert conformities.

PURPOSE: Tibial insert conformity in total knee arthroplasty (TKA) is of interest due to the potential effect on tibiofemoral kinematics. This study determined differences in anterior-posterior movements of the femoral condyles, pivot locations, and internal tibial rotation in different arcs of flexion for two implants with different insert conformities in kinematically aligned TKA. METHODS: Twenty-five patients treated with a medial and lateral low-conforming, posterior cruciate ligament (PCL) retaining (LC CR) implant followed by a medial ball-in-socket and flat, lateral PCL sacrificing (B-in-S CS) implant in the contralateral knee underwent single-plane fluoroscopy during a deep knee bend. Analysis following 3D-to-2D image registration determined tibiofemoral kinematics and patients completed validated outcome scores for both knees. RESULTS: The mean follow-up of 1.6 ± 0.4 years for the knee with the B-in-S CS implant was shorter than the 2.7 ± 1.2 years for the LC CR implant. From 0º to 30º of flexion, a medial pivot occurred with the tibia rotating internally approximately 5º with both implants. From 30º to 90º, the pivot remained medial and internal rotation increased to 10º with the B-in-S CS implant. In contrast, neither femoral condyle moved more than 1 mm with the LC CR implant from 30º to 60º, but from 60º to 90º degrees, a lateral pivot occurred and internal rotation increased. Internal rotation of the tibia on the femur from 0° to maximum flexion occurred about a medial pivot similar to the native knee for the B-in-S CS implant and was 4.5° greater than that of the LC CR implant (10.4° vs 5.9°). There was no difference in the median patient-reported outcome scores between implant designs. CONCLUSIONS: Tibial insert conformity is a primary determinant of a medial or lateral pivot during a deep knee bend. One explanation for the transition from a medial to lateral pivot between 30º and 60º with the LC CR implant is the chock-block effect of the insert's posterolateral upslope which impedes posterior movement of the lateral femoral condyle. Because there is no posterolateral upslope in the insert of the B-in-S CS implant, the tibia pivots medially throughout flexion similar to the native knee. LEVEL OF EVIDENCE: Level III.

Low tibial baseplate migration 1 year after unrestricted kinematically aligned total knee arthroplasty using a medial conforming implant design.

PURPOSE: Varus alignment of the tibial baseplate and limb > 3° might adversely affect baseplate fixation after total knee arthroplasty (TKA), especially for unrestricted kinematically aligned (KA) TKA which aligns a majority of baseplates in varus. The purposes of this study were to determine whether baseplate migration at 1 year (1) was significantly less than a stability limit of 0.5 mm, (2) increased over time, and (3) was related to varus alignment of the baseplate and limb after unrestricted KA TKA. METHODS: Thirty-five patients underwent unrestricted KA TKA using a fixed-bearing, cemented, medial conforming tibial insert with posterior cruciate ligament retention. Using model-based radiostereometric analysis, maximum total point motion (MTPM) (i.e., largest displacement on the baseplate) was computed at 6 weeks, 3 months, 6 months, and 1 year postoperatively relative to the day of surgery. Baseplate and limb alignment were measured postoperatively on long-leg CT scanograms. RESULTS: At 1 year, mean MTPM of 0.35 mm was significantly less than the 0.5 mm stability limit (p = 0.0002). Mean MTPM did not increase from 6 weeks to 1 year (p = 0.3047). Notably, 89% (31/35) of tibial baseplates and 46% (16/35) of limbs were > 3° varus. Baseplate and limb alignment had no relationship to MTPM at 1 year (|r|≤ 0.173, p ≥ 0.3276). CONCLUSION: Low and non-progressive tibial baseplate migration 1 year after unrestricted KA TKA with a medial conforming design should allay any concern that unrestricted KA TKA increases risk of baseplate loosening due to varus alignment of the baseplate and limb. LEVEL OF EVIDENCE: Level II, therapeutic prospective cohort study.

Better forgotten joint scores when the angle of the prosthetic trochlea is lateral to the quadriceps vector in kinematically aligned total knee arthroplasty.

PURPOSE: The primary aim was to analyze unrestricted kinematic alignment (unKA) total knee arthroplasty (TKA) and determine the frequency of medial deviation of the prosthetic trochlear angle (PTA) of the femoral component relative to the quadriceps vector (QV) that terminates at the anterior inferior iliac spine (AIIS), and whether patients with medial deviation had a worse Forgotten Joint Score (FJS) and Oxford Knee Score (OKS) relative to those with lateral deviation. The secondary aim was to determine the frequency of medial deviation for mechanical alignment (MA) TKA simulations. METHODS: From a database of a single surgeon, the study extracted de-identified data on 147 patients with a CT scanogram showing the pelvis and AIIS, a limb with an unKA TKA, and a native (i.e., healthy) opposite limb. On the scanogram, an examiner, blinded to the PROMs, measured the PTA-QV angle on the unKA TKA and on the opposite limb simulated MA TKA by drawing the PTA at 6° valgus relative to the femoral mechanical axis and measuring the PTA-QV angle. RESULTS: Medial deviation of the PTA occurred in 86% of patients with unKA TKA, and the 126 with medial deviation had a 17/1 point worse median FJS/OKS than the 21 with lateral deviation at a mean follow-up of 47 ± 8 months, respectively (p < 0.0001, p = 0.0053). In addition, 21%, 17%, and 8% of MA TKA had medial deviation after radiographic simulation using reported surgical errors for manual, patient-specific, and robotic instrumentation, respectively. CONCLUSION: In most patients with unKA TKA and a smaller proportion with MA TKA, a PTA of 6° valgus was medial to the QV, which, by excluding the patient's Q-angle, might alter patellofemoral kinematics like an incorrectly oriented trochleoplasty. The 17-point worse FJS in the patients with an unKA TKA and medial deviation of the PTA suggests the surgical target should be to orient the PTA lateral to the QV. LEVEL OF EVIDENCE: IV.

A Torn or Reconstructed Anterior Cruciate Ligament Does Not Adversely Affect Clinical Outcome Scores and the Incidence of Reoperation After Unrestricted Kinematically Aligned Total Knee Arthroplasty.

BACKGROUND: There are no reports as to whether the condition of the anterior cruciate ligament (ACL) adversely affects the 2 to 3 year function and reoperation risk of a kinematically aligned (KA) total knee arthroplasty (TKA) performed with posterior cruciate ligament (PCL) retention and an intermediate medial conforming (MC) insert. METHODS: A single surgeon's prospective database query identified 418 consecutive primary TKAs performed between January 2019 and December 2019. The surgeon recorded the ACL condition in the operative note. Patients filled out the Forgotten Joint Score (FJS), Oxford knee score (OKS), and Knee Injury and Osteoarthritis Outcome Scores for Joint Replacement at the final follow-up. There were 299 patients with an intact ACL, 99 with a torn ACL, and 20 with a reconstructed ACL. The mean follow-up was 31 months (range, 20 to 45). RESULTS: The median FJS, OKS, and Knee Injury and Osteoarthritis Outcome Score (KOOS) of the reconstructed/torn/intact KA TKAs were 90/79/67, 47/44/43, and 92/88/80 points, respectively. The median OKS and KOOS of the reconstructed ACL cohort were 4 and 11 points higher than in the intact ACL cohort (P = .003, .04). One patient who had a reconstructed ACL underwent manipulation under anesthesia (MUA) for stiffness. The 5 reoperations in the intact ACL cohort were for instability (n = 2), revision after failed MUA for stiffness (n = 2), and infection (n = 1). CONCLUSION: These results suggest that patients who have a torn and reconstructed ACL can expect high function and a low risk of reoperation comparable to patients who have an intact ACL when treated with unrestricted, caliper-verified KA, PCL retention, and an intermediate MC insert.

Ball-in-socket medial conformity with posterior cruciate ligament retention neither limits internal tibial rotation and knee flexion nor lowers clinical outcome scores after unrestricted kinematically aligned total knee arthroplasty.

PURPOSE: For a new tibial insert design with ball-in-socket (B-in-S) medial conformity (MC), posterior cruciate ligament (PCL) retention, and flat lateral articular surface (B-in-S MC + PCL), this study determined whether internal tibial rotation and knee flexion were limited and clinical outcome scores were lower during weight-bearing activities relative to an insert with intermediate (I) (i.e., less than ball-in-socket) medial conformity (I MC + PCL). METHODS: Twenty-five patients were treated with bilateral unrestricted, caliper-verified kinematic alignment (KA) total knee arthroplasty (TKA) with an I MC + PCL insert and B-in-S MC + PCL insert in opposite knees. Each patient performed weight-bearing deep knee bend, step up, and chair rise under single-plane fluoroscopy. Analysis following 3D model-to-2D image registration determined internal tibial rotation. For each TKA, knee flexion was measured and patients completed clinical outcome scoring questionnaires. RESULTS: Internal tibial rotation did not differ between conformities during chair rise and step up (p = 0.3419 and 0.1030, respectively). During deep knee bend, internal tibial rotation between 90° and maximum flexion was 3° greater in the B-in-S MC + PCL group (18° vs 15°) (p = 0.0290). Mean knee flexion (p = 0.3115) and median Forgotten Joint Score (FJS), Oxford Knee Score (OKS), and Western Ontario and McMasters Universities Arthritis Index (WOMAC) scores (p = 0.2100, 0.2154, and 0.4542, respectively) did not differ between conformities. CONCLUSION: An insert with ball-in-socket medial conformity, which maximizes anteroposterior (AP) stability, did not limit internal tibial rotation and knee flexion and did not lower patient-reported outcomes when implanted with unrestricted caliper-verified KA and PCL retention. The high AP stability provided by the medial ball-in-socket might interest those surgeons exploring the treatment of the active patient with a desire to return to high-level and athletic activities.

Optimized Joint Coordinate System Achieves Clinically Meaningful Kinematics of the Tibiofemoral Joint as Compared to the International Society of Biomechanics (ISB) Recommendation.

Quantification of clinically meaningful tibiofemoral motions requires a coordinate system where motions are free from kinematic crosstalk errors. The objectives were to 1) develop an algorithm for assigning an optimized joint coordinate system (OPT JCS) that minimizes kinematic crosstalk errors based on a kinematic model of the tibiofemoral joint, 2) determine tibiofemoral kinematics of the native (i.e. healthy) knee during deep knee bend using OPT JCS and a coordinate system recommended by the International Society of Biomechanics (ISB JCS), and 3) determine whether OPT JCS significantly reduces kinematic crosstalk errors compared to ISB JCS. A novel algorithm was developed for assigning a patient-specific, OPT JCS based on a two rotational degree-of-freedom kinematic model, which included flexion-extension and internal-external tibial rotation axes. Single-plane fluoroscopic images of native knees of 13 subjects performing deep knee bend were analyzed to show that tibiofemoral kinematics using OPT-JCS fell within the physiological range of motion in all six degrees of freedom. Internal rotation of the tibia with respect to the femur averaged 11 deg and off-axis motions were minimal as expected based on biomechanical constraints. In contrast, off-axis motions for ISB JCS were non-physiologic; maximum valgus rotation was 12 deg, maximum posterior translation was 12 mm, and maximum distraction translation was 28 mm. Hence, an optimized JCS achieved clinically meaningful kinematics by significantly reducing kinematic crosstalk errors compared to the ISB recommendation and is the more suitable coordinate system for evaluating tibiofemoral joint function.

Measurement Error Versus Repeated Measurements: A Guide Describing Two Methods for Computing Bias and Precision of Migration Measurements From Double Examinations Using Radiostereometric Analysis.

Radiostereometric analysis is a method to measure implant migration where an ISO standard recommends double examinations (i.e., acquisition of two independent sets of biplanar images on the same day) to compute bias (i.e., mean) and precision (i.e., standard deviation) of differences in repeated migration measurements (termed repeated measurement statistics). However, repeated measurement statistics do not provide information regarding trueness of the measurements. Double examinations also can be used to compute measurement error statistics (i.e., mean and standard deviation of migration measurements relative to trueness). Our objectives were to derive measurement error and repeated measurement population parameters in six degrees of freedom (6DOF) and in maximum total point motion (MTPM), demonstrate quantitative differences by computing measurement error and repeated measurement statistics from a clinical study for an example implant, and demonstrate the importance of determining mean measurement error in MTPM. Three key findings were: (1) in 6DOF, measurement error and repeated measurement statistics were nearly identical; (2) for MTPM, measurement error and repeated measurement statistics had different means of 0.21 mm and 0.00 mm, respectively, but similar standard deviations; and (3) mean measurement error in MTPM is important for drawing conclusions about early implant stability. Because measurement error statistics are the same as repeated measurement statistics in 6DOF but provide additional information in the form of mean measurement error in MTPM, researchers should report measurement error instead of repeated measurement statistics. Furthermore, the ISO standard should be revised to include measurement error statistics.

Previously Unrecognized Source of Error in the Change in Maximum Total Point Motion to Determine Continuous Migration of Unstable Tibial Baseplates.

In radiostereometric analysis (RSA), continuous migration denoted as ΔMTPM is the difference between maximum total point motion (MTPM) at 2 years relative to time zero and MTPM at 1 year relative to time zero. Continuous migration has been used to diagnose tibial baseplates as stable versus unstable when compared to a specified stability limit (i.e., value of ΔMTPM). If the same point experiences MTPM at 2 years and at 1 year (usually the case for marker-based RSA), then an implicit assumption is that the migration path between 2 years and 1 year is the same as the path between 1 year and time zero. This article uses vector analysis to demonstrate a source of error in ΔMTPM not previously recognized and estimates the magnitude of error based on the interplay of independent variables which affect the error. The two independent variables which affect the error are the angle between the two migration vectors (i.e., MTPM between time zero and 2 years and MTPM between time zero and 1 year) and the difference in magnitude of the two vectors. The relative error increased in an absolute sense as the angle between the vectors increased and decreased for larger differences in the magnitudes of the two vectors. For magnitude ratios ranging from 1.25 to 2, relative errors ranged from -21% to -3% at 10 deg and from -78% to -42% at 60 deg, respectively. Knowledge of these errors highlights a limitation in the use of ΔMTPM not previously recognized.

Negligible effect of surgeon experience on the accuracy and time to perform unrestricted caliper verified kinematically aligned TKA with manual instruments.

PURPOSE: Surgeons performing total knee arthroplasty (TKA) are interested in the accuracy and time it takes to make the four femoral resections that determine the setting of the femoral component. A method for quantifying the error of each resection is the thickness, measured by a caliper, minus the femoral target. The present study tested the hypothesis that the mean deviation of the resection from the femoral target, the percentage of resections with a deviation of ± 0.5, 1.0, 1.5, and 2.0 mm, and the time to complete the femoral cuts were not different between experienced (E) and less-experienced (LE) surgeons performing unrestricted caliper verified kinematically aligned (KA) TKA with manual instruments. METHODS: This study analyzed intraoperative verification worksheets from 203 patients treated by ten E surgeons and 58 patients treated by four LE surgeons. The worksheet recorded (1) the thickness of the femoral target for the distal medial (DM), distal lateral (DL), posterior medial (PM), and posterior lateral (PL) resections and the caliper thickness of the resections with a resolution of 0.5 mm, and (2) the time to complete them. The most accurate resection has a mean difference ± standard deviation of 0 ± 0.0 mm. RESULTS: The accuracy of the 1044 initial resections (261 patients) was significantly closer to the femoral target for E vs. the LE surgeons: 0.0 ± 0.4 vs. - 0.3 ± 0.5 for the DM, 0.0 ± 0.5 vs. - 0.4 ± 0.6 for the DL, - 0.1 ± 0.5 vs. - 0.2 ± 0.5 PM, and - 0.1 ± 0.5 vs. - 0.4 ± 0.6 for the PL resections (p ≤ 0.0248). E surgeons completed the femoral resections in 12 min; 5 min faster than LE surgeons (p < 0.0001). CONCLUSIONS: Because the mean difference in femoral resections with manual instruments for E vs. LE surgeons was < 0.5 mm which is within the caliper's resolution, differences in accuracy were not clinically relevant. Surgeons exploring other alignment options and robotic, navigation, and patient-specific instrumentation might find these values helpful when deciding to change. LEVEL OF EVIDENCE: III; case-control study.

Maximum Total Point Motion of Five Points Versus All Points in Assessing Tibial Baseplate Stability.

Maximum total point motion (MTPM), the point on a baseplate that migrates the most, has been used to assess the risk of tibial baseplate loosening using radiostereometric analysis (RSA). Two methods for determining MTPM for model-based RSA are to use either five points distributed around the perimeter of the baseplate or to use all points on the three-dimensional model. The objectives were to quantify the mean difference in MTPM using five points versus all points, compute the percent error relative to the 6-month stability limit for groups of patients, and determine the dependency of differences in MTPM on baseplate size and shape. A dataset of 10,000 migration values was generated using the mean and standard deviation of migration in six-degrees-of-freedom at 6 months from an RSA study. The dataset was used to simulate the migration of three-dimensional models (two baseplate shapes and two baseplate sizes) and calculate the difference in MTPM using five virtual points versus all points and the percent error (i.e., the difference in MTPM/stability limit) relative to the 6-month stability limit. The difference in the MTPM was about 0.02 mm, or 4% relative to the 6-month stability limit, which is not clinically important. Furthermore, results were not affected by baseplate shape or size. Researchers can decide whether to use five points or all points when computing MTPM for model-based RSA. The authors recommend using five points to maintain consistency with marker-based RSA.