Abstract
INTRODUCTION
Despite a large percentage of total knee arthroplasty failures occurs for disorders at the patello-femoral joint (PFJ), current navigation systems report tibio-femoral (TFJ) kinematics only, and do not track the patella. Despite this tracking is made difficult by the small bone and by its full eversion during surgery, a new such technique has been developed, which includes a new tracker, new corresponding surgical instrumentation also for patellar resurfacing, and all relevant software. The aim of this study is to report an early experience in patients of these measurements, i.e. TFJ and PFJ kinematics.
METHODS
These measurements were taken in the first ten patients, affected by primary gonarthrosis and implanted with a resurfacing posterior-stabilised prosthesis in the period July 2010 – May 2011. A standard knee navigation system was enhanced by a specially-designed patellar tracker, mounted with a cluster of three light emitting diodes. Standard procedures for femoral and tibial bone preparation were performed according to the navigation system, and the patellar was resurfaced. Relevant resection planes were taken by an instrumented verification probe. Final position of the three components and lower limb alignment were also acquired. Joint kinematics was deduced from the anatomical survey, which included anatomical landmarks on the patellar posterior aspect, and according to established recommendations and original proposals.
RESULTS
In addition to the standard assessment of TFJ kinematics, patellar tracking was performed successfully in all cases without complications, resulting in a maximum of 30 min longer operations. PFJ kinematics (see figure) after replacement and resurfacing showed a mean (± standard deviation, over the patients) range of flexion, tilt and medio-lateral shift respectively of 66.9° ± 8.5° (mean of minimum flexion ÷ of maximum flexion, 15.6° ÷ 82.5°), 8.0° ± 3.1° (−5.3° ÷ 2.8°), and 5.3 ± 2.0 mm (−5.5 ÷ 0.2 mm). Statistically significant correlations were found between the internal/external rotation of the femoral component and the range of PFJ tilt (p=0.05; R=0.64); in three patients, medio-lateral PFJ shift seemed to be affected by the medio-lateral position of the femoral component.
DISCUSSION AND CONCLUSIONS
Data obtained from our preliminary experience support the relevance, feasibility and efficacy of patellar tracking in navigated knee arthroplasty by means of a standard knee navigation system, suitably extended to track also the patellar motion.
Patellar-based measurement provides for a more comprehensive assessment of the whole knee function, not only for the resurfacing but also for a best possible positioning of the femoral and tibial components.
