Is There a Force Target That Predicts Early Patient-reported Outcomes After Kinematically Aligned TKA?

BACKGROUND: Four mechanical alignment force targets are used to predict early patient-reported outcomes and/or to indicate a balanced TKA. For surgeons who use kinematic alignment, there are no reported force targets. To date the usefulness of these mechanical alignment force targets with kinematic alignment has not been reported nor has a specific force target for kinematic alignment been identified.

QUESTIONS/PURPOSES: (1) Does hitting one of four mechanical alignment force targets proposed by Gustke, Jacobs, Meere, and Menghini determine whether a patient with a kinematically aligned TKA had better patient-reported Oxford Knee and WOMAC scores at 6 months? (2) Can a new force target be identified for kinematic alignment that determines whether the patient had a good/excellent Oxford Knee Score of ≥ 34 points (48 best, 0 worst)?

METHODS: Between July 2017 and November 2017, we performed 148 consecutive primary TKAs of which all were treated with kinematic alignment using 10 caliper measurements and verification checks. A total of 68 of the 148 (46%) TKAs performed during the study period had intraoperative measurements of medial and lateral tibial compartment forces during passive motion with an instrumented tibial insert and were evaluated in this retrospective study. Because the surgeon and surgical team were blinded from the display showing the compartment forces, there was no attempt to hit a mechanical alignment force target when balancing the knee. The Oxford Knee Score and WOMAC score measured patient-reported outcomes at 6 months postoperatively. For each mechanical alignment force target, a Wilcoxon rank-sum test determined whether patients who hit the target had better outcome scores than those who missed. An area under the curve (AUC) analysis tried to identify a new force target for kinematic alignment at full extension and 10°, 30°, 45°, 60°, 75°, and 90° of flexion that predicted whether patients had a good/excellent Oxford Knee Score, defined as a score of ≥ 34 points.

RESULTS: Patients who hit or missed each of the four mechanical alignment force targets did not have higher or lower Oxford Knee Scores and WOMAC scores at 6 months. Using the Gustke force target as a representative example, the Oxford Knee Score of 41 ± 6 and WOMAC score of 13 ± 11 for the 31 patients who hit the target were not different from the Oxford Knee Score of 39 ± 8 (p = 0.436) and WOMAC score of 17 ± 17 (p = 0.463) for the 37 patients who missed the target. The low observed AUCs (from 0.56 to 0.58) at each of these flexion angles failed to identify a new kinematic alignment force target associated with a good/excellent (≥ 34) Oxford Knee Score.

CONCLUSIONS: Tibial compartment forces comparable to those reported for the native knee and insufficient sensitivity of the Oxford Knee and WOMAC scores might explain why mechanical alignment force targets were not useful and a force target was not identified for kinematic alignment. Intraoperative sensors may allow surgeons to measure forces very precisely in the operating room, but that level of precision is not called for to achieve a good/excellent result after calipered kinematically aligned TKA, and so its use may simply add expense and time but does not improve the results from the patient's viewpoint.

LEVEL OF EVIDENCE: Level III, therapeutic study.