To obtain correct soft tissue balance during TKA is an important operative technique for successful clinical outcome. The soft tissue balancing has been assessed by the tibiofemoral joint gap in full extension, and at 90°. Since recent advancements in the design of femoral components, tibial articular surface and operative techniques have enabled a prosthetic deep knee flexion, the joint gap measurement in such a condition became necessary. Also it should be noted that the joint gap directly reflects on the clinical outcomes such as range of motion, laxity and instability.

In recent years, many in-vivo kinematic measurement methods were developed, which measure the 3D position and orientation from the 2D X-ray image. Among them a pattern-matching method is representative, which is the method by comparing the contour shape from the X-ray image with a predicted contour to seek the 3D position and orientation.

The objective of this study is to measure the range of motion of knee prostheses from their X-ray fluoroscopic images by using the pattern-matching method.

We analyzed 7 knee prostheses of 7 female patients, age of 59 to 77 years, height of 149.5 to 159 cm, weight of 43 kg to 72 kg. Their knee prostheses were all NRG-PS type (Striker Co., USA) with various sizes. During the fluoroscopy measurement, the patient was lying supine on a bed with her both legs free. First the patients were asked to make flexion-extension with their prosthetic knees by themselves and their fluoroscopic images were recorded for analysis. Next the following motions were done passively. Starting with 0°, the knee angle was gradually increased and fixed at 30°, 60°, 90° and up to 120° respectively. At each flexion angle, the knee was internally rotated as possible as the maximum limit of the patient capacity and then externally rotated in the same way. Similarly, the knee was made varusly and then valgusly at each flexion angle respectively.

The results of kinematic analyses were arranged by the tibial orientations relative to the femur. The range of flexion-extension angles were from 113.9° (SD=8.3°) to 5.2° (SD=8.2°). At maximum flexion for each patient, the orientation in terms of internal-external rotation and varus-valgus was measured and averaged; they were internally rotated by 6.0° (SD=0.6°) and varusly inclined by 1.2° (SD=1.0°). At full extension (minimum flexion), they were externally rotated by 4.3° (SD=1.9°) and varusly inclined by 0.1° (SD=0.7°) respectively. The maximum value of internal-external rotation range was recorded at 89.4° (SD=2.4°) of knee flexion and they were from 5.4° (SD=1.3°) of internal rotation to 12.9° (SD=6.0) of external rotation. The varus-valgus motion was small, from 1.7° (SD=1.6°) of varus to 0.1° (SD=2.2°) of valgus through the whole range of knee flexion.

Important findings were that the range of varus-valgus was smallest for the prosthesis with the thickest insert, and the knee whose collateral ligaments were loose tended to incline varusly.