The success of a total knee replacement largely depends on accurate balancing of  flexion and extension gaps in the sagittal plane. The general technique to achieve this is to resect the same amount of bone that will be replaced with metal and polyethylene. The average distal femoral resection is usually between 8 mm and 10 mm. The horizontal tibial resection generally takes 2 mm off the medial tibia (low side) and 9 mm off the lateral tibia (high side) in a knee in varus alignment. These cuts should create symmetrical rectangular gaps between the tibia and femur in both flexion and extension. The collateral ligaments should be equally tensioned with symmetrical gaps.1

Although preoperative motion is the greatest predictor of postoperative motion, balancing of flexion and extension gaps is crucial for postoperative knee stability. Patients with either a loose extension gap or a tight flexion gap may have the most noticeable functional loss. A loose extension gap creates knee instability with varus and valgus stress with the knee in extension; patients may complain of the knee buckling or giving way. A tight flexion gap can result in knee flexion less than 90°, which can make climbing stairs or standing from a seated position difficult.1

Once the bone resections have been made and the trial implants are inserted, the knee is taken through a range of motion to determine if flexion and extension gaps exist. If the patient is tight in flexion and extension, more tibia is resected. If the patient is tight in extension and not flexion, more distal femur is resected. If the patient is balanced in extension and tight in flexion, the posterior capsule and posterior cruciate ligament can be released and/or more posterior slope can be resected from the tibia. If the knee is found to be loose in flexion and extension, a thicker polyethylene liner should be used.2

Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor-in-chief of the Journal of Orthopedics for Physician Assistants (JOPA).


  1. Dennis D, Daines B. Gap balancing vs. measured resection technique in total knee arthroplasty. Clin Orthop Surg. 2014;6(1):1-8.
  2. Dennis D. TKA sagittal plane balancing. OrthoBullets website. Updated May 14, 2018. Accessed December 19, 2018.
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