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

Simulated Partial and Complete Resection of the Posterior Cruciate Ligament in Medially Conforming Total Knee Arthroplasty Causes a Graded Reduction in Femoral Rollback and Increase in Posterior Tibial Sag: A Computational Study

BACKGROUND

Medially conforming (MC) total knee arthroplasty (TKA) has seen increased clinical utilization. This design allows for either retention or resection of the posterior cruciate ligament (PCL); however, the impact of the PCL on femoral rollback and posterior tibial sag is unknown. Therefore, we developed a computational model to quantify how the PCL affects femoral rollback and posterior sag in MC-TKA.

METHODS

Computational models of 10 cadaver knees were virtually implanted with MC-TKAs. Clinical tests of passive flexion and posterior sag were simulated, and femoral rollback and posterior tibial translation (PTT) were quantified. These tests were simulated in MC-TKA with the PCL retained, partially resected, and completely resected. We then assessed how increasing the tibial insert thickness in PCL-resected MC-TKA and switching to posterior-stabilized (PS)-TKA impacted posterior sag.

RESULTS

Femoral rollback decreased medially by a median of 2.4 mm (P ≤ 0.001) and laterally by a median of 3.3 mm (P ≤ 0.001) with simulated PCL resection. For the simulated sag test, PTT increased by a median of 4.2 (P ≤ 0.05) and 7.4 mm (P ≤ 0.001) with partial and complete PCL resection, respectively. Moreover, PTT was reduced by a median of 7.1 mm (P ≤ 0.01) when converting a PCL-resected MC-TKA to a PS-TKA.

CONCLUSIONS

In a computational model, MC-TKA does not fully compensate for the function of the PCL, which facilitates femoral rollback in passive flexion and resists PTT during a posterior sag test. Resecting the antero-lateral bundle, with preservation of the postero-medial bundle of the PCL, yields more femoral rollback and less PTT than complete PCL resection in MC-TKA. Increasing tibial insert thickness in MC-TKA also does not account for the role of the PCL in reducing PTT during a sag test, while a PS-TKA does.

Does the posterior tibial slope in caliper-verified unrestricted kinematically aligned TKA using manual instruments match the slope in the contralateral healthy knee and improve function?

PURPOSE

Unrestricted kinematically aligned total knee arthroplasty (unKA TKA) strives to restore the pre-arthritic posterior tibial slope (PTS), however consistency of achieving this alignment target is unknown. The present study determined the proportion of subjects with differences in PTS less than 2° from the target and the improvement in patient-reported function after unKA TKA.

METHODS

A review of 562 postoperative scanograms identified 99 patients (51 female) with a unKA TKA in one limb, a contralateral healthy limb, and a postoperative axial CT scan. All patients were treated with a primary unKA TKA performed with mechanical instruments where the alignment target was setting the PTS to match that of the medial compartment of the contralateral healthy knee. The PTS of the TKA and the healthy medial tibial plateau were measured and the difference determined. The patient-reported Oxford Knee Score (OKS) measured pre- and post-operative function.

RESULTS

The proportion of subjects within a tolerance interval of ±2° of the contralateral healthy knee at 95% confidence was 85%. The median OKS improved from 20 points preoperatively to 47 points (range 18-48) at 15 months postoperatively. Greater differences of the PTS from healthy were unrelated to poorer Oxford Knee Scores.

CONCLUSION

Unrestricted KA TKA using manual instruments with caliper verification of resection thickness restored a high percentage of patients within a clinically acceptable tolerance of the posterior tibial slope of the contralateral healthy knee. The median postoperative OKS indicated clinically important improvement in patient-reported function.

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.

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.

The peak force to push a trial tibial insert into position cannot be used to select the correct thickness in total knee arthroplasty

An objective of a total knee arthroplasty (TKA) is to restore native (i.e. healthy) function, and a crucial step is determining the correct insert thickness for each patient. If the insert is too thick, then stiffness results, and if too thin, then instability results. Two methods to determine the insert thickness are by manually assessing the joint laxity and by using a trial insert with goniometric markings that measures the internal-external rotation of the trial with respect to a mark on the femoral component. The former is qualitative and depends on the surgeon's experience and 'feel' and while the latter is quantitative, it can be used only with an insert with medial ball-in-socket conformity. An unexplored method is to measure the force required to push a trial insert into position. To determine whether this method has merit, the push force was measured in 30 patients undergoing unrestricted kinematically aligned TKA using an insert with ball-in-socket medial conformity, a flat lateral surface, and retention of the posterior cruciate ligament. During surgery, the surgeon determined three appropriate thicknesses to test from a selection ranging from 10 mm to 14 mm in 1 mm increments. The peak push forces going from an insert 1 mm thinner than the correct thickness as determined by an insert goniometer and from the correct thickness to 1 mm thicker were measured. Mean peak forces for the different insert thicknesses were 127 ± 104 N, 127 ± 95 N, and 144 ± 96 N for 1 mm thinner, correct, and 1 mm thicker, respectively, and did not differ (p = 0.3210). As a result, measurement of peak force during trial positioning of a tibial insert cannot be used to identify the correct thickness for all insert designs.