Passive Laxity in Patients With Subjective Instability Following a Cruciate-Retaining Total Knee Arthroplasty: A Pilot Study.

Background: Postoperative knee instability is one of the major underlying causes of unsatisfactory outcomes after total knee arthroplasty (TKA). However, the thresholds between stable and unstable knees remain difficult to quantify, mainly using manual clinical assessment. This pilot study aimed to understand whether differences in passive knee laxity exist between self-reported stable and unstable knees after Cruciate retaining TKA. Methods: Eight self-reported unstable and 10 stable TKA knees were assessed using fluoroscopic imaging to accurately assess the passive tibiofemoral rotation under the application of internal/external torques using a rotometer. In addition, anteroposterior and varus/valgus laxity were measured using stress radiography with a modified TELOS device. Results: Our results showed that at 30° of knee flexion, the unstable TKA knees exhibited significantly higher joint rotational laxity between 0.9 and 2.7 Nm of internal rotation torque level (p < 0.001, Conclusions: Our findings demonstrate that unstable TKA knees have increased posterior anteroposterior laxity and higher rotational laxity at low internal rotation torques at 30° of knee flexion compared with stable joints. However, our findings should be interpreted as exploratory; larger, longitudinal cohorts are still needed to mitigate bias and validate the presented findings in subjects exhibiting postarthroplasty instability. Level of Evidence: Level IV diagnostic.