Coronal alignment predicts the use of semi-constrained implants in contemporary total knee arthroplasty.

Background: Semi-constrained, or varus-valgus constrained, implants are occasionally necessary to achieve stability in primary total knee arthroplasty (TKA). However, outcomes with these implants are largely unknown. Therefore, the primary goals of this study were to determine 1) can we identify preoperatively which patients might require a semi-constrained implant and 2) are there any clinical and or radiographic differences for those that require a semi-constrained implant?

Methods: A multicenter retrospective study was performed to retrospectively review patients that had a Stryker Triathlon (Kalamazoo, MI) TKA with a Total Stabilized (TS) tibial insert (n=75). This TS cohort was subsequently matched 1:1 based on age, gender, and BMI to a cohort of patients with the same primary TKA design with a PS insert (n=75). Preoperative and postoperative radiographic and clinical data were compared between the two groups.

Results: Preoperatively, the TS cohort had significantly greater varus (9.72 vs. 3.48; p=0.0001) and valgus (14.1 vs. 7.57; p=0.0001) deformity. Post-operatively, there were no statistically significant differences in revisions (p=1), reoperations (p=1), or complications (p=1). Mean clinical and radiographic follow-ups were equivalent between groups (25.5 vs. 25.8months, p=0.8851).

Conclusions: As suspected, use of a semi-constrained insert to achieve intraoperative coronal stability was most predicted by preoperative coronal deformity (either varus or valgus). Longer follow-up and larger patient cohorts are necessary to determine.