Abstract
Introduction
We introduce the concept of total knee arthroplasty (TKA) fingerprinting as a tool to characterize and graphically convey the sensitivity of a TKA design to surgical variability in implant component position and patient-related anatomic factors. Identifying sensitive directions preoperatively which would cause undesirable effects may decrease revision surgery by informing surgical decisions and planning. To provide several examples of TKA fingerprinting, we estimated and compared the contact forces in a single TKA type for several configurations, simulating surgical variability and patient-related anatomical factors during a loaded deep squat. The purpose of this study is not to analyze the behavior of this specific TKA design but rather to illustrate a tool that could be used to show, in general, how surgical errors or anatomical factors can alter patello-femoral (PF) and tibio-femoral (TF) contact forces compared to its own reference configuration.
Materials and methods
Computed tomography images of one full cadaveric leg were used to generate 3D models of the bones and to obtain a physiological knee model assuming standard positions of the main soft tissue insertions.
A fixed bearing posterior stabilized knee TKA design was considered in this study. The prosthesis was a medium size, replaced both cruciate ligaments and resurfaced the patella. Following standard surgical procedure, the TKA was virtually implanted, thus defining its reference configuration. Each derivative replaced knee model was then obtained by changing the values of one parameter, or a combination of two, in a range based on literature and surgical experience (Table 1).
A 10 s loaded squat to 120° was performed for each configuration, with a constant vertical hip load of 200 N. These settings match the experimental tests performed in a previous in-vitro analysis on cadaver legs. Each replaced model was developed and analyzed using a validated musculoskeletal modeling software.
The model of the knee included TF contacts and PF contacts of the TKA components, passive soft tissues and active muscle elements. The external forces (ground reaction and weights), the muscle forces (quadriceps and hamstrings) and the frictional forces are applied to the knee joint through the machine. The mechanical properties of the tissues were obtained from literature. With these settings, for each model, both the maximum PF and TF contact forces have been evaluated.
Results
Examples of fingerprint graphs are shown presenting the main results (Figures 1 and 2). Figure 1 displays a fixed rotation femoral component and a variable rotation tibial component. FIgure 2 displays a variable rotation patella component.
Discussion and Conclusion
In general a TKA should be implanted without surgical errors to obtain the biomechanical behavior for which a TKA was designed because surgical errors can alter the functionality of a TKA. A fingerprinting tool for TKAs was developed and used to show the sensitivity of PF and TF contact forces in surgical variability.
The graphs show that PF contact forces are altered mostly by errors in positioning of the patellar component, while TF contact forces are mostly affected internal and external femoral component rotation and ligament release.
