Flexion Gap Balancing Techniques in Primary Total Knee Arthroplasty
Nicholas A. Bedard
Michael J. Taunton
Key Concepts
The goal of this technique is a stable knee replacement in both flexion and extension with neutral mechanical alignment of the knee.
The gap balancing technique relies on the re-creation of neutral mechanical alignment and ligament balancing in extension creating a balanced extension gap before femoral posterior condylar resection.
It is important to resect the proximal tibia perpendicular to the mechanical axis of the tibia as this cut will determine the femoral component rotation.
With equal tension on the medial and lateral ligamentous structures of the knee in flexion the femur is sized and rotated to create a balanced flexion gap. The flexion gap must equal the size of the previously established extension gap.
Sterile Instruments and Implants
Sandbag to position leg under the drapes.
Post to attach to table proximally to prevent hip abduction.
Two bent Hohmann retractors.
One posterior knee retractor.
Lamina spreader or implant-specific tensioner.
Total knee arthroplasty (TKA) implant.
Preoperative Planning
Clinical examination of the patient evaluating for fixed or correctable deformities of the knee, range of motion, and prior incisions.
Review of the anteroposterior, lateral, and patellar view radiographs to evaluate the arthritis pattern, bone quality, and bone deformities.
Evaluate weight-bearing long-leg radiographs to plan the distal femoral and proximal tibia cut. The proximal tibia cut is drawn on the long-leg radiographs at 90° to the mechanical axis of the tibia. The angle between the anatomic and mechanical axes of the femur is also measured to determine the magnitude of valgus for the distal femoral cut to ensure the cut is perpendicular to the mechanical axis of the femur (Figure 51.1).
The planned bony resections and amounts of bone planned to be removed should be noted to help ensure re-creation of a neutral mechanical alignment and a perpendicular tibial resection.
 Figure 51.1 ▪ Standing long-leg radiograph demonstrating the anatomic and mechanical axes of the femur. The magnitude of difference between these axes, in this case 5°, is used for the angle of the distal femoral resection to ensure the cut is perpendicular to the mechanical axis of the femur. |
Bone, Implant, and Soft Tissue Techniques
Introduction
One of the primary goals of TKA are symmetric and balanced extension and flexion gaps. The magnitude of bone resection, ligament balancing, tibial slope, and femoral component rotation all contribute to the size and balance of the flexion gap. The 2 primary techniques for determining femoral component rotation and balanced flexion and extension gaps are the gap balancing technique and the measured resection technique. This chapter will focus on a gap balancing technique, of which there are many. Our preferred gap balancing technique relies on first establishing neutral mechanical alignment in extension, then establishing a balanced ligamentous extension gap. The “thickness” of the flexion gap is then altered to match the thickness of the extension gap. This helps prevent altering the joint line in extension and may improve overall knee kinematics. The posterior condylar resection and thus the femoral component rotation is set based on ligament tension, not the posterior condylar axis, as is standard in many measured resection techniques.
Gap Balancing Technique
Positioning
Patient is positioned supine on the operative table.
Examination of the knee under anesthesia is performed.
A sandbag and lateral hip rest are positioned on the table to assist with keeping the knee in flexion and the leg from abducting during procedure.
Exposure
Surgical exposure of the knee is obtained utilizing the medial parapetallar approach as described in previous chapters.
Distal femoral cut
The distal femoral cut is made utilizing an intramedullary guide.
The amount of valgus in the distal femoral cut is modified depending on the preoperative planning and distal femoral alignment.
The amount of distal femoral resection should equal the thickness of the femoral implant and is typically around 10 mm off the more distal side of the knee (usually medial).
Proximal tibia cut
The proximal tibia is then exposed with knee flexion, external rotation to take the tension of the patellar tendon and subluxation of the tibia.
Proximal tibia resection is made using an extramedullary cutting guide to create a cut at a 90° angle to the mechanical axis of the tibia in the coronal plan. The amount of sagittal slope incorporated into the resection is determined by the implant utilized. The depth of proximal tibia resection is generally 8 to 10 mm off the unaffected side and is approximately 2 mm off the affected side (Figure 51.2). Our preference is to aim for a neutral slope relative to the lateral plateau of the tibia.Related posts:
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