Abstract
INTRODUCTION
In computer-aided total knee arthroplasty (TKA), surgical navigation systems (SNS) allow accurate tibio-femoral joint (TFJ) prosthesis implantation only. Unfortunately, TKA alters also normal patello-femoral joint (PFJ) functioning. Particularly, without patellar resurfacing, PFJ kinematics is influenced by TFJ implantation; with resurfacing, this is further affected by patellar implantation. Patellar resurfacing is performed only by visual inspections and a simple calliper, i.e. without computer assistance.
Patellar resurfacing and motion via patient-specific bone morphology had been assessed successfully in-vitro and in-vivo in pilot studies aimed at including these evaluations in traditional navigated TKA.
The aim of this study was to report the current experiences in-vivo in two patient cohorts during TKA with patellar resurfacing.
MATERIALS AND METHODS
Twenty patients with knee gonarthrosis were divided in two cohorts of ten subjects each and implanted with as many fixed-bearing posterior-stabilised prostheses (NRG® and Triathlon®, Stryker®-Orthopaedics, Mahwah, NJ-USA) with patellar resurfacing. Fifteen patients were implanted; five patients of the Triathlon cohort are awaiting hospital admission. TKAs were performed using two SNS (Stryker®-Leibinger, Freiburg-Germany). In addition to the traditional knee SNS (KSNS), the novel procedure implies the use of the patellar SNS (PSNS) equipped with a specially-designed patellar tracker.
Standard navigated procedures for intact TFJ survey were performed using KSNS. These were performed also with PSNS together intact PFJ survey. Standard navigated procedures for TFJ implantation were performed using KSNS. During patellar resurfacing, the patellar cutting jig was fixed at the desired position with a plane probe into the saw-blade slot; PSNS captured tracker data to calculate bone cut level/orientation. After sawing, resection accuracy was assessed using a plane probe. TFJ/PFJ kinematics were captured with all three trial components in place for possible adjustments, and after final component cementing. A calliper and pre/post-TKA X-rays were used to check for patellar thickness/alignment.
RESULTS
This protocol was performed successfully in TKAs, resulting in 30 min longer TKA. Final lower limb misalignment was within 0.5°, resurfaced patella was 0.4±1.2 mm thinner than the native, and patellar cut was 0.4°±4.1° laterally tilted. Final PFJ kinematics was taken within the reference normality in both series. PFJ flexion, tilt and medio-lateral shift range were 66.9°±8.5° (minimum÷maximum, 15.6°÷82.5°), 8.0°±3.1° (−5.3°÷2.8°), and 5.3±2.0 mm (−5.5÷0.2 mm), respectively. Significant (p<0.005) correlations were found between the internal/external rotation of the femoral component and PFJ tilt (R2=0.41), and between the mechanical axis on the sagittal plane and PFJ flexion (R2=0.44) and antero-posterior shift (R2=0.45).
Patellar implantation parameters were confirmed by X-ray inspections. Discrepancies in thickness up to 5 mm were observed between SNS- and calliper-based measurements.
CONCLUSIONS
These results support relevance/efficacy of patellar tracking in in-vivo navigated TKA and may contribute to a more comprehensive assessment of the original whole knee, i.e. including also PFJ. Patellar preparation would be supported for suitable component positioning in case of resurfacing, but, conceptually, also in not-resurfacing if SNS does not reveal PFJ abnormalities., Using this procedure in the future, TFJ/PFJ abnormalities can be corrected intra-operatively by more cautious bone cut preparation and prosthetic positioning on the femur, tibia and patella.
