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

Low-constraint insert with a medial pivot design as a potential predictor of favourable outcomes in cruciate-retaining total knee arthroplasty: A propensity score-matched analysis

PURPOSE

The influence of polyethylene insert conformity on the outcomes of cruciate-retaining (CR) total knee arthroplasty (TKA) with a medial pivot design remains uncertain. Therefore, this study aimed to evaluate the effects of polyethylene insert conformity in CR-TKA on patient-reported outcomes.

METHODS

The data of 255 knees (FINE Total Knee System) from 255 patients were retrospectively analysed to compare outcomes for high- or low-constraint medial pivot inserts, as determined through historical controls, over an average follow-up period of 2.2 years (range, 2.0-5.5 years). Multivariate logistic regression analysis was used to identify predictors of achieving the patient-acceptable symptom state (PASS) for the Forgotten Joint Score-12 (FJS-12). Propensity score-matched cohorts for age, sex, body mass index, Kellgren-Lawrence grade, Charlson Comorbidity Index, knee flexion contracture, FJS-12 and follow-up duration were created for between-group comparison (n = 50 in each group).

RESULTS

Low-constraint insert (p = 0.031) and age (p = 0.043) were independent predictors of achieving the PASS for the FJS-12 (>33, 153/255). After successful matching, compared to the high-constraint insert, the low-constraint insert improved patient satisfaction (p = 0.029 for pain on going up or downstairs, and p = 0.047 for the function of going upstairs) and increased the likelihood of achieving the minimal clinically important difference (p = 0.019) and PASS (p = 0.025) for the FJS-12. A significant correlation was observed between the posterior tibial slope and the FJS-12 in the low-constraint insert group (p < 0.001), indicating that a greater posterior tibial slope was associated with better functional outcomes in this group.

CONCLUSIONS

Compared with high-constraint inserts, low-constraint medial pivot inserts yielded higher functional outcomes and patient satisfaction. Therefore, insert conformity may play a crucial role in CR-TKA outcomes.

LEVEL OF EVIDENCE

Level III.

A new tibial insert design with ball-in-socket medial conformity and posterior cruciate ligament retention has low tibial baseplate migration after unrestricted kinematically aligned total knee arthroplasty: a cohort study using radiostereometric analysis

BACKGROUND AND PURPOSE

 In total knee arthroplasty (TKA), an insert with ball-in-socket (BS) medial conformity (MC) and posterior cruciate ligament (PCL) retention restores kinematics closer to native than an insert with intermediate (I) MC. However, high medial conformity might compromise baseplate stability as indicated by maximum total point motion (MTPM). Using the BS MC insert with PCL retention, we aimed to determine whether (i) the baseplate is stable as indicated by mean MTPM < 0.5 mm, (ii) baseplate stability is not strongly correlated to varus baseplate alignment, and (iii) baseplate stability, clinical outcome scores, and flexion are comparable with that of an I MC insert cohort which has demonstrated high stability, clinical outcome scores, and flexion.

METHODS

 Unrestricted kinematic alignment (unKA) TKA was performed on a cohort of 35 patients using a cemented baseplate. Biplanar radiographs acquired at timepoints up to 12 months were processed with model-based radiostereometric analysis (RSA) software to determine MTPM.

RESULTS

 At 1 year, mean MTPM of 0.35 mm was significantly below 0.5 mm (P < 0.001). MTPM was not strongly correlated to varus baseplate alignment up to 9° (r = 0.12, 95% confidence interval -0.22 to 0.44). Equivalence analyses revealed that MTPM, Forgotten Joint Score, Oxford Knee Score, and maximum flexion for the sBS MC insert were comparable with the I MC insert.

CONCLUSION

 Using the new BS MC insert with PCL retention, the tibial baseplate was stable at the group level at 1 year. Baseplate stability was not strongly related to varus baseplate and limb alignment. Comparable patient-reported outcome scores and maximum flexion/extension at 1 year were shown between the 2 insert designs.

Medial-pivot design does not provide superior clinical results compared to posterior-stabilized total knee arthroplasty despite kinematic differences during step-up and lunge activities: A prospective randomized controlled trial under medial tight soft tissue balance

PURPOSE

This study aimed to compare in vivo kinematics during weight-bearing daily activities and determine the relationship with clinical outcomes in patients undergoing total knee arthroplasty (TKA) with a medial-pivot (MP, Evolution™) versus a posterior-stabilized (PS, Persona®) design under constant conditions of intraoperative soft tissue balance.

METHODS

Forty patients undergoing MP or PS-TKA under similar conditions of soft tissue balance were enrolled in this prospective randomized controlled trial. Outcome measures included clinical knee society scores (KSS) and knee injury and osteoarthritis outcome scores (KOOS). A kinematic assessment was conducted while the participants performed lunge and step-up activities under fluoroscopic guidance.

RESULTS

Eighteen patients in each arm completed 1-year follow-up and were included in the analysis. All patients experienced pain relief and satisfactory knee function postoperatively. In kinematics, in the MP arm, the medial femoral condyle remained consistent, whereas the lateral femoral condyle gradually shifted posteriorly with increasing knee flexion. Conversely, in the PS arm, paradoxical anterior movement of the medial femoral condyle accompanied the lateral pivot motion. During lunge and step-up activities, a medial-pivot motion was observed in 83% and 72% of knees in the MP arm, respectively, compared with 22% and 11% in the PS arm. Despite these differences in kinematics, there were no statistically significant differences in the KSS and KOOS between the two groups.

CONCLUSION

Under weight-bearing conditions during flexion, knees that underwent Evolution™ MP-TKA did not show superior clinical results compared to Persona® PS-TKA, despite exhibiting in vivo kinematics closely resembling the normal in vivo pattern.

LEVEL OF EVIDENCE

Level I, therapeutic studies.

Analysis of Variation in Sagittal Curvature of the Femoral Condyles

In designing femoral components, which restore native (i.e., healthy) knee kinematics, the flexion-extension (F-E) axis of the tibiofemoral joint should match that of the native knee. Because the F-E axis is governed by the curvature of the femoral condyles in the sagittal plane, the primary objective was to determine the variation in radii of curvature. Eleven high accuracy three-dimensional (3D) femur models were generated from ultrahigh 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 segment circles best-fit to ±15 deg of arc at each increment. Results were standardized to the radius of the best-fit overall circle to 15 deg-105 deg for the femoral condyle having a radius closest to the mean radius. Medial and lateral femoral condyles exhibited multiradius 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 segment circles were relatively constant. Beyond the transition angles where the radii decreased, the anterior-posterior (A-P) positions of the centers of curvature varied 4.8 mm and 2.3 mm for the medial and lateral condyles, respectively. 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.

An Insert Goniometer Can Help Select the Optimal Insert Thickness When Performing Kinematically Aligned Total Knee Arthroplasty with a Medial 1:1 Ball-in-Socket and Lateral Flat Surface Insert and Posterior Cruciate Ligament Retention

Current surgical practices in total knee arthroplasty (TKA) have advanced and include significant changes and improvements in alignment philosophies, femorotibial implant conformities, and ligament management to replicate in vivo knee kinematics. While corrective measures have emphasized sagittal plane alignment to restore normal flexion-extension (F-E) motion and coronal plane ligament balance, internal-external (I-E) rotation kinematics in the axial plane have been largely neglected. Recent in vivo evidence indicates that the combination of factors necessary to closely restore native tibial rotation as the knee flexes and extends is kinematic alignment (KA), which resurfaces the patient's pre-arthritic knee without releasing ligaments, an insert with medial 1:1 ball-in-socket conformity and a lateral flat surface, and posterior cruciate ligament (PCL) retention. However, the inherent anterior-posterior (A-P) stability provided by the medial 1:1 ball-in-socket limits the surgeon's ability to select the correct insert thickness using manual laxity testing. Accordingly, this review presents the design and validation of an instrument called an insert goniometer that measures I-E tibial rotation for inserts that differ in thickness by 1 mm and uses rotation limits at extension and 90° flexion to select the optimal insert thickness. The optimal thickness is the one that provides the greatest external tibial orientation in extension and internal tibial orientation at 90° flexion without lift-off of the insert.

Retention of the posterior cruciate ligament stabilizes the medial femoral condyle during kneeling using a tibial insert with ball-in-socket medial conformity

PURPOSE

Resecting the posterior cruciate ligament (PCL) increases posterior laxity and increases the flexion gap more than the extension gap in the native (i.e. healthy) knee. These two effects could lead to significant anterior displacement of the medial femoral condyle in kneeling following total knee arthroplasty even when using a tibial insert with a high degree of medial conformity. Using an insert with ball-in-socket medial conformity and a flat lateral articular surface, the primary purpose was to determine whether the medial femoral condyle remained stable with and without PCL retention during kneeling.

METHODS

Two groups of patients were studied, one with PCL retention (22 patients) and the other with PCL resection (25 patients), while kneeling at 90º flexion. Following 3D model-to-2D image registration, A-P displacements of both femoral condyles were determined relative to the dwell point of the medial socket.

RESULTS

With PCL resection versus PCL retention, the medial femoral condyle was 5.1 ± 3.7 mm versus 0.8 ± 2.1 mm anterior of the dwell point (p < 0.0001). Patient-reported function scores were comparable (p ≥ 0.1610) despite a significantly shorter follow-up of 7.8 ± 0.9 months with PCL retention than 19.6 ± 4.9 months with PCL resection (p < 0.0001). Range of motion was 126 ± 8° versus 122 ± 6° with and without PCL retention, respectively (p = 0.057).

CONCLUSION

Surgeons that use a highly conforming tibial insert design can stabilize the medial femoral condyle during kneeling by retaining the PCL. In patients with PCL resection, the 9 mm high anterior lip of the insert with ball-in-socket medial conformity was insufficient to prevent significant anterior displacement of the medial femoral condyle when weight-bearing on the anterior tibia.

Significantly better precision with new machine learning versus manual image registration software in processing images from single-plane fluoroscopy to determine tibiofemoral kinematics following total knee replacement

One common method to determine tibiofemoral kinematics following total knee replacement (TKR) is to capture single-plane fluoroscopic images of a patient activity and determine anterior-posterior (AP) positions of the femoral condyles and internal-external (IE) tibial rotation. Although JointTrack is widely used to analyze such images, precision (i.e. repeatability) in determining AP positions and IE tibial rotations using the two publicly available programs has never been quantified. The objectives were to determine the precision and reproducibility of results using both programs. Fluoroscopic images of 16 patients who performed a weight-bearing deep knee bend following TKR were analyzed. JointTrack Manual (JTM) and JointTrack Machine Learning (JTML) were used to perform 3D model-to-2D image registration after which AP positions of the femoral condyles and IE tibial rotations were determined. Precision in AP positions and IE rotations was quantified. Intraclass correlation coefficients (ICCs) for both repeatability (i.e. intraobserver) and reproducibility (i.e. interobserver) also were determined. Precision using JTM was worse than JTML for AP positions of the medial and lateral femoral condyles (1.0 mm and 0.9 mm vs 0.3 mm and 0.4 mm, respectively; p < 0.001 for both). For IE tibial rotation, precision also was worse using JTM versus JTML (1.1º vs 0.9°, p = 0.010). ICC values for JTML indicated good to excellent agreement (range: 0.82-0.98) whereas ICC values for JTM indicated only moderate to good agreement (range: 0.58-0.88). JTML has better precision and reproducibility than JTM and also is more efficient to use. Therefore, use of JTML over JTM is strongly recommended.

Posterior cruciate ligament retention with medial ball-in-socket conformity promotes internal tibial rotation and knee flexion while providing high clinical outcome scores

BACKGROUND

Although retaining the posterior cruciate ligament (PCL) is advantageous in unrestricted kinematically aligned TKA, it is often excised with a medial stabilized implant. The primary objectives were to determine whether PCL retention using an insert with ball-in-socket (B-in-S) medial conformity to maximize A-P stability promotes internal tibial rotation and flexion while providing high patient-reported outcome scores.

METHODS

Two cohorts of 25 patients each were treated with unrestricted kinematically aligned (KA) TKA using a tibial insert with B-in-S medial conformity and a flat lateral articular surface. One cohort retained the PCL; the other had it excised. Patients performed deep knee bend and step-up exercises during fluoroscopic imaging. Following 3D model-to-2D image registration, anterior-posterior (A-P) positions of the femoral condyles and tibial rotation were determined.

RESULTS

For deep knee bend, mean internal tibial rotation with PCL retention was significantly greater at maximum flexion (17.7° ± 5.7° versus 10.4° ± 6.5°, p < 0.001) and significantly greater at 30°, 60°, and 90° flexion as well (p ≤ 0.0283). For step-up, mean internal tibial rotation with PCL retention was significantly greater at at 15°, 30°, and 45° flexion (p ≤ 0.0049) but was marginally not significantly greater at 60° (i.e. maximum) flexion (12.3° ± 4.4° versus 10.1° ± 5.4°, p = 0.0794). Mean flexion during active knee flexion with PCL retention was significantly greater (127° ± 8° versus 122° ± 6°, p = 0.0400). Both cohorts had high median Oxford Knee, WOMAC, and Forgotten Joint Scores that were not significantly different (p = 0.0918, 0.1448, and 0.0855, respectively) CONCLUSION: Surgeons that perform unrestricted KA TKA should retain the PCL with an insert that has B-in-S medial conformity, as this maintains extension and flexion gaps while also promoting internal tibial rotation and knee flexion as well as providing high clinical outcome scores.

Modern Total Knee Arthroplasty Bearing Designs and the Role of the Posterior Cruciate Ligament

The role of the posterior cruciate ligament (PCL) in total knee arthroplasty (TKA) surgery continues to be a source of debate among the adult reconstruction community. In native knee flexion, the PCL is comprised of an anterolateral and posteromedial bundle that work together to limit posterior tibial translation and allow adequate femoral rollback for deep flexion. In the arthritic knee, the PCL can often become dysfunctional and attenuated, which led to the development of posterior stabilized (PS) TKA bearing options. PS TKAs implement a cam-post construct to functionally replace a resected PCL. While PS designs may facilitate balancing knees with significant deformity, they are associated with complications such as postfracture, increased wear, and patellar clunk/crepitus. In recent years, newer designs have been popularized with greater degrees of congruency and incorporation of medial and lateral pivoting to better recreate native knee kinematics. The American Joint Registry has confirmed the recent predilection for ultra-congruent and cruciate-retaining TKA inserts over PS TKAs during the last decade. Studies have failed to identify an overall clinical superiority between the cruciate substituting and sacrificing designs. The literature has also failed to identify clinical consequences from PCL resection with modern, more conforming TKA designs. In this article, we review modern PCL sacrificing designs and discuss the impact of each on the kinematics after TKA. We also will delineate the role of the PCL in modern TKA in the hopes to better understand the recent surge in sacrificing but not substituting knee implants.

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.

Measurement of Tibial Orientation Helps Select the Optimal Insert Thickness to Personalize PCL Tension in a Medial Ball-in-Socket TKA

As the conformity of a medial ball-in-socket total knee arthroplasty (TKA) provides intrinsic anterior-posterior (A-P) stability, surgeons cannot rely on the manual examination of sagittal laxity to identify the optimal insert thickness. Instead, the present study determined whether measuring tibial axial orientation in extension and 90° flexion with an insert goniometer could identify the optimal thickness that, when implanted, provides high postoperative function. In twenty-two patients that underwent unrestricted caliper-verified kinematic alignment (KA) with a PCL retaining implant, two surgeons measured tibial orientation in extension and 90° flexion with 10, 11, 12, and 13 mm thick insert goniometers. Each TKA had one insert thickness that restored either the maximum external tibial orientation in extension, the maximum internal tibial orientation at 90° flexion, or both relative to 1 mm thinner and thicker inserts. In addition, the 6-month median [interquartile range] Forgotten Joint Score of 73 (54–87) and Oxford Knee Score of 42 (38–45) indicated high satisfaction and function. In conclusion, surgeons using a medial ball-in-socket TKA design can measure external tibial orientation in extension and internal tibial orientation at 90° flexion with an insert goniometer. Furthermore, implanting an insert with the thickness that provided the maximum orientation values resulted in high postoperative function, thereby personalizing PCL tension.

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.