Purpose Varus alignment of the tibial baseplate and limb > 3° might adversely afect baseplate ixation 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 signiicantly 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-ive patients underwent unrestricted KA TKA using a ixed-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 signiicantly 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.
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). Objectives were to derive measurement error and repeated measurement population parameters in six degrees of freedom (6 DOF) 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 6 DOF, 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 0.01 mm, respectively, but similar standard deviations; and 3) mean measurement error in MTPM (termed artifactual MTPM) is important for drawing conclusions about early implant stability. Because measurement error statistics are the same as repeated measurement statistics in 6 DOF but provide additional information in the form of artifactual MTPM, researchers should report measurement error instead of repeated measurement statistics. Furthermore, the ISO standard should be revised to include measurement error statistics.
Radiostereometric analysis can be used for computing movement of a tibial baseplate relative to the tibia (termed migration) to detect baseplate instability. 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 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° to 0.7° 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.
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