Abstract
Introduction
Patellofemoral joint is an important aspect of the tri-compartmental knee joint complex. Total knee arthroplasty (TKA) replaces the articulating surfaces of distal femur and proximal tibia, and often times the patella as well. Understanding the size relationship between the femur and patella bones can provide valuable information for new prosthesis design and biomechanical analysis. However, taking anthropometric measurements on a large population of patients or even cadaveric specimens could be a challenge. As a result, there are currently little quantitative data existing in the literature regarding the size relationship between TKA patient's femur and patella. This study attempted to attack this question using a novel statistical approach and a large TKA patient database.
Methods
A multi-site clinical database operated by Exactech was used in this study. The database contains patient information of Optetrak TKA implant recipients from over 30 physicians in US, UK, and Colombia since 1995. Nine femoral implant sizes (0, 1, 2, 2.5, 3, 3.5, 4, 5 and 6) and six patellar implant sizes (26, 29, 32, 35, 38, 41 mm) were seen in these patients. Due to the low usage, femoral sizes 0 and 6 were excluded from this analysis. Taking primary TKA only, a total of 2,698 cases were included in this study. The size relationship between femoral implant and patellar implant was analyzed in this patient population. Gender effect was also examined.
Results
The usage histograms showed that the most frequently used femoral implant in the database was size 3, and the most frequently used patellar implant sizes were 32 and 35 mm. In general, patients who received a larger femoral implant also received a larger patellar implant. There was a strong correlation between the anterior/posterior (AP) dimension of the femoral implant and the diameter of the spherical patellar implant, with a linear regression showing R2 > 0.9. On average, for 1 mm increase of the femoral AP dimension, the patellar implant increased by 0.36 mm in diameter. The strong correlation between the femoral and patellar dimensions exists for both male and female populations (R2 > 0.9 in both genders). The slope of the regression line was slightly greater for the males than for the females (0.38 vs. 0.33).
Discussion
By using a novel statistical approach, this study was able to provide a quantitative assessment of the size relationship between femoral and patellar implants of TKA patients. There was a strong correlation between the femoral implant's AP dimension and the patellar implant's diameter. The increase ratio of the two dimensions was about 1:0.36. There was a minor difference between genders in terms of the increase ratio, but the overall trends were similar. Statistically we can assume that the femoral implant resembled the AP dimension of the distal femur, and the patellar implant diameter resembled the short axis of the patellar oval. Thus, the results in this study also provided a meaningful anthropometric measurement of the native femur and the patella bones.
