Research Article| Volume 41, P150-160, March 2023

The flexion–extension gap is predictive of patient-reported outcome measures after cruciate-retaining total knee arthroplasty

Published:January 24, 2023DOI:


      • Total knee arthroplasty (TKA) can lead to an ‘artificial’ joint perception.
      • An artificial perception is associated with lower patient satisfaction.
      • A gap difference ≥1.0 mm (distraction force, 1.5 BMI) improved outcomes.
      • Soft tissue balance during TKA is predictive of patient-reported outcomes.



      We aimed to evaluate the association between the flexion–extension gap difference and patient-reported outcome measures after total knee arthroplasty (TKA).


      This was a retrospective case–control study of 60 cruciate-retaining single-radius TKAs. Soft tissue balancing was measured using an offset seesaw tensioner and centre-type digital knee balancer under joint distraction forces of 20–50 lbf and 1.5 times body mass index (1.5 BMI). At the last follow up of 2.0–6.5 (mean, 4.5) years postoperatively, patients were asked if they perceived their knee joint as ‘natural’ (26 knees) or ‘artificial’ (34 knees). Age, sex, and the flexion–extension gap were compared between the two groups. A receiver operating characteristic curve was used to determine cut-off values of variables predictive of a natural joint perception.


      Natural joint perception was associated with a greater flexion–extension gap difference under a distraction force of 1.5 BMI (P = 0.016), higher knee function (Knee Injury and Osteoarthritis Outcome Score for Joint Replacement, P = 0.019), and quality of life (EuroQol 5-Dimension, P = 0.029). A gap difference of 0.95 mm under 1.5 BMI distraction predicted a natural joint perception post-TKA (sensitivity, 97.1%; specificity, 88.5%). Using a gap threshold of 1.0 mm under a 1.5 BMI distraction force yielded significant between-group differences in postoperative flexion (P = 0.040), satisfaction (P = 0.043), knee joint function (P < 0.001), quality of life (P = 0.032), and posterior femoral condylar offset (P = 0.037)


      A flexion–extension gap difference ≥1.0 mm under a distraction force of 1.5 BMI predicted superior outcomes, including patient satisfaction, after cruciate-retaining TKA. It was suggested that posterior femoral condylar offset could influence this finding.


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