2015 ISAKOS Biennial Congress ePoster #1408

Incidence of Undesired Collateral Ligament Imbalance and Instability Between Extension and Flexion in Neutrally Aligned Total Knee Arthroplasty for Japanese Patients

Yasuo Niki, MD, PhD, Tokyo JAPAN
Kengo Harato, MD, PhD, Tokyo JAPAN
Tomoki Sassa, MD, Fujisawa, Kanagawa JAPAN
Takeo Nagura, MD, PhD, Tokyo JAPAN
Shu Kobayashi, MD, PhD, Tokyo JAPAN
Yoshiaki Toyama, MD, PhD, Shinjyuku JAPAN
Yasunori Suda, Prof, Yaita-Shi, Tochigi Prefecture JAPAN

Keio University, School of Medicine, Tokyo, JAPAN

FDA Status Cleared

Summary: Medial collateral ligament imbalance at knee extension easily occurs during neutrally aligned TKA in Japanese population, potentially due to large medial slope of the tibial joint line.

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Abstract:

Purpose

Recently, it has been reported that neutrally aligned total knee arthroplasty (TKA) resulted in a wide range of the medial or lateral collateral ligament imbalance in extension and a wide range of instability between 0º extension and 90º of flexion [1]. This study aimed to assess the incidence of undesired collateral ligament imbalance and instability between extension and 90º of flexion in Japanese osteoarthritis patients.

Materials And Methods

The present study enrolled 44 knees from 32 patients, who underwent primary TKA due to =grade III OA. CT scans were performed preoperatively and reconstructed into 3D models using ATHENA® software (SoftCube inc. Japan). When the bone cut was simulated in 3D, reference plane of the femur was defined as the plane that included femoral mechanical axis and was parallel to transepicondylar axis (TEA). Reference plane of the tibia included its mechanical axis and was perpendicular to tibial AP axis which was the line connecting medial border of tibial tubercle to the center of the PCL attachment. Bone cuts of the distal femur and the proximal tibia were simulated perpendicular to each mechanical axis on each reference plane. Bone cut line of the posterior region of the femoral condyle was parallel to mechanical axis in the sagittal plane, and also parallel to TEA in the axial plane. A femoral component with an 8mm thickness of the distal and posterior regions of the femoral condyles, and a tibial component with 9mm-thick medial and lateral condyles were used for the simulation. The minimum thickness of the bone cut was set as 8mm for femur and 9mm for tibia to reflect component design used. The thickness of the bone cut of the other condyle (=8mm) was measured. The difference in total bone cut thickness between medial and lateral compartments was calculated in both 0º extension and 90º flexion, independently, which indicates collateral ligament imbalance occurring in neutrally aligned TKA. In addition, the difference in femoral bone cut thickness between 0º extension and 90º flexion was assessed for each condyle.

Results

Magnitude of collateral ligament release required averaged 1.6±3.3mm at 0º extension and -0.2±2.6mm at 90º flexion with ranged from -7 to 9mm. When neutrally aligned TKA by using measured resection technique was simulated, 67% (medial, 56%; lateral, 11%) and 44% (medial, 22%; lateral, 22%) of the patients required =2mm of collateral ligament release at 0º extension and 90º flexion, respectively. Regarding laterality of ligament imbalance, 69% for 0º extension and 45% for 90º flexion involved medial collateral ligament tightness.
In terms of instability resulting from discrepancy of bone cut thickness between 0º extension and 90º flexion, 47% of the patients had =2mm of instability in the medial compartment. In most cases, bone cut thickness was larger in posterior region than in distal region. On the other hand, instability in the lateral compartment was found only in 8%.

Discussion

Neutrally aligned TKA with measured resection technique can create =2mm of collateral ligament imbalance at 0º of knee extension in two-thirds of our cases. Of these, frequency of medial imbalance reached =80%. Medial slope of the tibial joint line in our cases averaged 5.9±2.9º, which indicates much larger tibial slope in Japanese than in Caucasian and accounts for high incidence of medial imbalance in Japanese subjects. In addition, surgeons should be aware that the TEA-based measured resection technique frequently causes instability between 0º extension and 90º flexion in the medial compartment that is uncorrectable by collateral ligament release.