The Hip - book
Home » Joints » Knee » TKR: Flexion Gap

TKR: Flexion Gap

- Definition:
    - see instability following TKR
    - is space between the posterior coronal cut on the distal femur & transverse cut on proximal tibia, while knee is in flexion;
    - effect of resection of PCL (see posterior stabilized prosthesis);
    - in report by Kadoya, et al, effect of PCL resection on tibiofemoral joint gap was analyzed in 30 patients w/ varus DJD of knee who underwent TKR;
           - medial soft tissue was released and the bone cut was made without preserving the bone segment of the tibia to which PCL was attached;
           - medial and lateral joint gaps in full extension and 90° flexion were measured before and after PCL was resected using a tensioning device;
           - after the resection, the flexion gap significantly increased in the medial and the lateral sides (4.8 ± 0.4 and 4.5 ± 0.4 mm, respectively, mean ± standard error) compared with those seen in the extension gap (0.9 ± 0.2 and 0.8 ± 0.2 mm);
           - there was no significant difference between the changes in the medial and lateral gaps;
           - mean value of the flexion gap was 2 mm smaller than the extension gap before the resection and 1.7 mm larger after the sacrifice;
           - posterior cruciate ligament resection resulted in an increase in the flexion gap and made space for approximately 3-mm thicker polyethylene;
           - ref: Effects of Posterior Cruciate Ligament Resection on the Tibiofemoral Joint Gap

- Small Flexion Gap:
    - causes of incomplete knee flexion are:
           - tibial cut that is too proximal (w/ or w/o a posterior slope);
           - an anterior femoral coronal cut which is inadequate, ie too anterior
           - patellar cut which is inadequate (too superficial) with a resultant increase in total patellar thickness (bone and implant);
           - failure to completely débride large posterior osteophytes can create a tight lateral flexion gap
    - if the knee cannot flex normally and is unstable in extension there is situation in which the gap problems are diametrically opposed;
           - this is due to surgeon error;
    - if the knee cannot fully flex but extends normally;
    - verify ROM with the patella reduced;
    - either the posterior slope is inadequate, the patella is too thick, or the anterior cut is too anteior;
    - if knee cannot fully flex nor extend, then recutting the tibia more proximally will correct the problem;
    - by increasing the posterior inclination of the proximal tibial resection, femoral rollback is facilitated, effectively increasing the flexion gap - extension gap is unaffected;

- Large Flexion Gap (unstable in flexion)
    - may cause flexion instability and even dislocation;
    - technical causes:
            - may commonly occur after release of large fixed varus or valgus contracture;
            - tibial cut which is too distal (or due to over augmentation of distal femoral augmentation implants w/ revision cases);
            - excessive posterior tibial slope
            - posterior femoral coronal cut which was too anterior; (i.e., removing too much posteior femoral condyle);
                   - this occurs with components that allow for anteriorization of the femoral component (used inorder to avoid notching the femur)
            - undersizing of the femoral component
            - popliteus tendon injury:
                   -  injury to the popliteus while cutting the posterolateral femoral condyle will increase flexion gap;

    - if extension gap is also too small, then solution is to recut the femur more proximally;
    - if flexion instability is mild, then accept the deformity;
    - management of gross flexion instability:
            - need to insert thicker tibial component, & resultant loss of extension is then corrected by recutting distal femor more proximally;
            - in such a situation the thickness of implant that is required for flexion stability is too thick in extension and the knee does not fully extend;
            - too correct this it is necessary to resect an additional amount of distal femur to equalize the spaces

The flexion gap in normal knees. An MRI study.

Flexion instability in primary total knee replacement

Total knee arthroplasty ligament balancing and gap kinematics with posterior cruciate ligament retention and sacrifice.

Flexion instability without dislocation after posterior stabilized total knees

The intra-operative joint gap in cruciate-retaining compared with posterior-stabilised total knee replacement

Flexion instability after primary posterior cruciate retaining total knee arthroplasty

Increased flexion position of the femoral component reduces the flexion gap in total knee arthroplasty.

The intraoperative gap difference (flexion gap minus extension gap) is altered by insertion of the trial femoral component.

The Clinical Consequences of Flexion Gap Asymmetry in Total Knee Arthroplasty

Instability After Total Knee Arthroplasty

Flexion Instability in Total Knee Replacement. A Comparison of Posterior Stabilised and Deep Dished Components