The Adductor Ratio is More Reliable Than Other Techniques in Determining Joint Line in Severe Type M Varus Gonarthrosis

Osteoarthritis (OA) of the knee, known as gonarthrosis, causes significant movement restrictions and pain in daily life activities. Total Knee Arthroplasty (TKA) is the preferred treatment method for advanced-stage OA of the knee. While various alignment techniques such as kinematic, constrained kinematic, and anatomical alignment are used, the most commonly used and preferred method by researchers is TKA performed with a technique that conforms to mechanical alignment.

Two main factors that affect patient outcomes after mechanically aligned TKA are achieving a parallel joint line and appropriate positioning of the distal femoral rotation that corresponds to the patellofemoral joint kinematics, ensuring optimal soft tissue tension. If these two aspects are not adequately addressed, patients may experience chronic pain, functional impairment, early wear at the implant interface, and ultimately, loosening. Studies have reported that 8% to 19% of patients are dissatisfied with TKA due to various reasons, including pain and unmet expectations. Problems that may arise from malrotation and/or incorrect soft tissue tension include patellofemoral instability, anterior knee pain, arthrofibrosis, and flexion gap instability.

In general, the natural joint line is not orthogonal to the tibial mechanical axis; it is varus, ranging from 87 ± 3°. When the mechanical alignment technique is applied in TKA, the proximal tibia and femur are typically cut perpendicular (90°) to the tibial and femoral mechanical axes. However, in the case of symmetric implants, the classical resection technique, especially in varus knees, results in more resection than the component thickness in the medial femoral compartment and the lateral tibial compartment. This creates an average 3° valgus joint line with respect to the tibial mechanical axis. As a result, the joint line is preserved medially, but the lateral compartment becomes more distalized.

Another issue regarding the joint line is its restoration, which involves achieving its anatomical height. Changes in the joint line can lead to instability, increased incidence of anterior knee pain, and decreased range of motion. The most commonly used bone markers for the restoration of the joint line are the epicondyles, fibular head (FH), and tibial tubercle (TT). Due to significant individual variations, some authors have suggested using the ratio of the distance between the epicondyles and the tangent to the joint line to the trans-epicondylar width (TEW) of the femur as a means of determining the appropriate value. This ratio based on femoral width allows for the calculation of an appropriate value for each individual regardless of size. However, it is not always easy to radiographically identify the epicondyles, especially in varus knees with severe metaphyseal damage.

On the femoral side, the width of the distal femoral resection should be equal to the thickness of the metal implant to restore the normal femoral joint line level, regardless of surgical techniques such as "measured resection technique" or modified "gap balancing technique." During surgery, the distal surface of the medial femoral condyle usually serves as the anatomical reference point for the distal femoral cut because in most cases, thicker bone is cut and removed from the medial femoral condyle compared to the lateral condyle. However, in patients with severely degenerated knees, significant bone and cartilage defects occur in the distal femoral condyle, and the deformed medial condyle is no longer a suitable reference point for distal femoral resection.