Abstract
Introduction
Total knee arthroplasty (TKA) implant systems offer a range of sizes for orthopaedic surgeons to best mimic the patient's anatomy and restore joint function. From a biomechanical perspective, the challenge on the TKA implants is affected by two factors: design geometry and in vivo load. Larger geometry typically means more robust mechanical structure, while higher in vivo load means greater burden on the artificial joint. For an implant system, prosthesis geometry is largely correlated with implant size, while in vivo load is affected by the patient's demographics such as weight and height. Understanding the relationships between implant size and patients' demographics can provide useful information for new prosthesis design, implant test planning, and clinical data interpretation. Utilizing a manufacturer supported clinical database, this study examined the relationships between TKA patient's body weight, height, and body mass index (BMI) and the received implant size of a well-established implant system.
Methods
A multi-site clinical database operated by Exactech, Inc. (Gainesville, FL, USA) was utilized for this study. The database contains patient information of Optetrak TKA implant recipients from over 30 physicians in US, UK, and Colombia since 1995. Nine implant sizes (0, 1, 2, 2.5, 3, 3.5, 4, 5 and 6) are seen in the database, while size 0 was excluded due to very low usage. Taking primary TKA only, a total of 2,713 cases were examined for patient's body weight, height, BMI, and their relationships with the implant size.
Results
Both patient's weight and height strongly correlate with implant size (R2»0.95 for both parameters with a linear regression). On average, the increase of one implant size corresponds to an increase of 7.4 kg in patient's weight and 7.0 cm in patient's height (Figure 1). However, there is almost no dependency between patient's BMI and implant size (R2<0.05), and the regression line is almost flat (k=-0.08) (Figure 1).
Discussion
Based on the Exactech database, this study revealed that TKA patients' weight and height increase close-to-linearly with implant size, but BMI stays fairly constant. These relationships are not all intuitive mathematically, and are likely simplified representations of higher order functions within the particular variable ranges. The most interesting finding was the independence of BMI on implant size, which provides a favorable validation of the geometry design and size selection of the Optetrak implant system. BMI (kg/m2) has the same unit dimension as stress (N/m2) excluding the constant g (9.8 N/kg). Since implant geometry is generally proportional to patient height, and joint force is generally proportional to patient weight, the mechanical stress imposed on the implant would be generally proportional to patient's BMI. The fact that BMI stays constant across sizes indicates that the implant system would experience a similar level of stress across all sizes, which has been previously observed in femorotibial contact stress analyses on the Optetrak system. This study showed that a heavier TKA patient statistically tends to receive a larger implant which, depending on implant design, will provide larger contact area and compensate for the higher load.
