## 1 Introduction  
Outcomes of total knee replacement (TKR) remain suboptimal, with 10-20% of patients continuing to experience pain or functional limitations after primary TKR [1], despite good long-term survivorship [2]. Approximately 45% of patients dissatisfied with TKR report anterior knee pain (AKP) affecting their daily activities [3] which is likely related to patellofemoral complications. Patients suffering from AKP have difficulties with activities that increase patellofemoral joint (PF) forces such as walking up and down stairs, rising from a seated position, exiting a car or cycling [4]. Patellofemoral complications are historically one of the most common reasons for poor TKR outcome [5]. AKP appears to occur with equal incidence as patellofemoral complications, independent to whether the patella is resurfaced or left native [6]. The patella itself is relatively poorly innervated, whereas the surrounding extensor hood of soft tissue is highly sensitive to increased strain.  
Many factors affect the balance of the PJF during TKR including the amount of trochlea resection, the rotation and size of the femoral component. The distal femoral resection and tibial resection influence the joint line, which can change soft tissue tension of the extensor mechanism. When the patella is resurfaced, the amount of patella resection, size,shape and placement of the patella component influence patella tracking and soft tissue tension. Patella under-resection and thereby over-stuffing of the PFJ can cause chronic postoperative AKP and decreased range of motion due to higher than physiologic extensor mechanism forces [7]. Patella over-resection increases the risk for fracture and may lead to under-tensioning the PFJ, resulting in lower than physiological extensor mechanism forces and muscle pain [8]. Patella maltracking is multifactorial and is defined as abnormal tracking of the patella in the femoral groove during knee flexion and extension and can lead to increased wear of the patellar component, subluxation and dislocation [9].  
Despite these findings, the PFJ and extensor mechanism's role as primary contributors to overall knee function is often overlooked, and limited attention has been paid to improving balancing and cutting techniques for the PFJ during TKR. Currently, with traditional surgical techniques, there is no method of quantifying or addressing patella compression forces. While prior studies have described changes in Q-angle and fluoroscopic patellar angle measurements to assess the PFJ postoperatively [10; 11], a dynamic, intraoperative assessment of PFJ forces during TKR has not been described. An intraoperative means of assessing PFJ forces during TKR could allow adjustments of the patella cut depth and angle to more closely replicate native PFJ kinematics, which may improve overall patient outcomes and decrease PFJ-related complications.  
The current study utilized a novel PFJ load sensor device that resembles a patella implant component. When applied to the patella, the sensor is capable of dynamically assessing PFJ forces before and after placement of tibial and femoral TKR components, which can allow for patella cut adjustments and reproduction of physiologic PFJ and extensor mechanism forces. The aims of this study were (1) to characterize changes in PFJ forces in an experimental in vitro setup and in a human cadaveric model before and after TKR component placement and (2) to assess whether the use of novel PFJ cutting guides can reproduce physiologic PFJ forces in a cadaveric model. The study hypothesized that native and postoperative resection PFJ forces can be measured and differ based on patella implant thickness and angulation of the patella cut. The goal of this translational research is to develop a system that can improve TKR outcomes by balancing the PFJ and extensor mechanism as an integral but often overlooked part of TKR surgery. Significant patient benefits might result, based on data that patella over/under-loading is closely associated with poor functional outcome [12; 13; 14].