The act of extension in the knee—known as the “extensor mechanism”—is facilitated by the quadriceps and quadriceps tendon, the medial and the lateral retinaculum, and the patellar tendon.1 During extension, the patellar contact on the distal femur acts to change the directionality and force applied on the knee joint from the quadriceps.2 Therefore, not only does the patella act as a spacer between the distal femur and the patellar tendon, but its contact with the femur also provides a fulcrum point with which to transfer loading from the moment arm of the quadriceps.2 In consideration of the important role that the patella and patellar tendon play in the transduction of force to the knee, it may be important to reduce the patella for proper soft-tissue assessment during total knee arthroplasty (TKA).
Intraoperative sensors have recently been used to show the difference in intra-articular loading with and without the reduction of the patella, during TKA.3 In a consecutive, 53-patient case series, medial and lateral loads were evaluated during TKA with the patella in various locations:
1. With the patella located in the femoral trochlea and the medial retinaculum open
2. With the patella dislocated but not everted
3. With the patella dislocated and everted, and
4. With the patella located in the trochlea and the medial retinaculum closed with two provisional stitches in the superior and interior border of the patella.
Loading values in the medial and lateral compartment were captured at five (5) different positions, through the range of motion.
The results of this study indicate that there are several fundamental differences observed in the loading patterns of knees with a reduced patella versus non-reduced patella. For instance, higher overall loads were observed in the medial compartment with the patella reduced when compared with the patella non-reduced. Additionally, mediolateral loading differentials increased and there was a higher degree of mediolateral loading asymmetry, during mid-flexion, with the patella non-reduced. Also, from mid-flexion to flexion it was shown that absolute loading remained constant with a reduced patella, whereas loading decreased with a non-reduced patella.
Devices that allow for the quantification of intra-articular loading may allow for the surgeon to better replicate native biomechanics and ensure soft-tissue balance. Using intraoperative sensing devices, it has been shown that the reduction of the patella changes the kinematic signature of the knee, displayed through the range of motion.
Dr. Alejandro Gonzalez Della Valle is a paid consultant to OrthoSensor, Inc.
1 Wheeless CR. Extensor mechanism injuries of the knee. Wheeless’ Textbook of Orthopaedics – Duke Orthopaedics. 2012. [Web]. Accessed on 5/18/17, from: http://www.wheelessonline.com/ortho/extensor_mechanism_injuries_of_the_knee.
2 Yamaguchi GT, Zajac FE. A planar model of the knee joint to characterize the knee extensor mechanism. J Biomechanics. 1989; 22(1): 1-10.
3 Schnaser E, Lee YY, Boettner F, et al. The position of the patella and extensor mechanism affects the intraoperative compartmental loads during total knee arthroplasty: a pilot study using intraoperative sensing to guide soft tissue balance. J Arthroplasty. 2015; 30: 1348-1353.