Accuracy and reliability of determining the isometric point of the knee for multiligament knee reconstruction

Purpose To compare the accuracy and reliability of the anatomic and radiographic techniques for identifying the isometric point of the knee.

Methods Only four specimens were used; however, eight experienced multiligament knee injury surgeons were recruited to address this limitation. Surgeons estimated the isometric point (EIP) on the medial and lateral sides using an anatomic and radiographic method. The x and y coordinates of the EIP were compared to the true isometric point (TIP). T-tests and interclass correlation coefficients (ICC) were performed to determine the accuracy and reliability between the methods.

Results There was no difference in placement of the EIP on the medial side of the knee in the anterior/posterior (x; p = n.s.) and superior/inferior direction (y; p = n.s). The EIP was anterior (p = 0.001) to the TIP with the radiographic method on the lateral side and approached significance (p = 0.05) in the superior/inferior direction. The ICC (95 % CI) for identifying the EIP on the medial side in the anterior/posterior direction using the anatomic method was 0.64 (0.28–0.96) and 0.11 (-0.06 to 0.77) in the superior/inferior direction. Using the radiographic method, the ICC in the anterior/posterior and superior/inferior direction was 0.49 (0.14–0.94) and 0.15 (-0.47 to 0.81), respectively. On the lateral side, the ICC for the anatomic method was 0.84 (0.56–0.99) in the anterior/ posterior direction and 0.36 (0.05–0.90) in the superior/inferior direction. Using the radiographic method, the ICC in the anterior/posterior and superior/inferior direction was 0.61 (0.26–0.96) and 0.89 (0.67–0.99), respectively.

Conclusions There was no difference in accuracy on the medial side of the knee. On the lateral side, the anatomic method was more accurate in the anterior/posterior direc- tion. Reliability was greater in the anterior/posterior direction on both sides of the knee. Surgeons were most likely to place the isometric point anterior and superior to the TIP on both the medial and lateral sides of the knee with either method which has the potential to cause graft lengthening. This should be taken into consideration during reconstruction/repair of the MCL/PMC and LCL/PLC.

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