Introduction. Malrotation of the femoral component is a cause of patellofemoral maltracking after total knee arthroplasty. We have developed a balanced gap technique in posterior stabilized total knee arthroplasty using an original instrument. Patellar instability is associated with an increased the tibial tubercle and the center of the groove (TT-TG) distance > 20 mm, and TT-TG is regarded as one index of a factor influencing congruity. To assess the influence on a patellofemoral joint by a modified gap technique, the purpose of this study is to compare the TT-TG distance before surgery and after total knee arthroplasty. Material and Methods. We explored the 30 knees, 25 patients (4 male and 21 female), who underwent total knee arthroplasties (NexGen LPS-Flex, fixed surface, Zimmer; Warsaw, USA) for osteoarthritis or rheumatoid arthritis. All procedures were performed through a medial parapatellar approach and a balanced gap technique used a developed versatile tensor device. We compared the preoperative and postoperative CT at a knee flexion angle of 30 degrees. To assess the force vector of the extensor mechanism, TT-TG distance and the proximal-distal distance between the entrance of the tracheal groove and the tibial tubercle (ET-TT distance) were measured in CT. The relation of both distances (TT-TG index) that divided TT-TG in ET-TT was calculated, because the TT-TG distance was affected by the individual knee size. Three dimensional Q-angle (θ) was also calculated using TT-TG distance and ET-TT distance, sinθ = TT-TG distance/ET-TT distance. (Figure 1). Results. The mean TT-TG distance of the pre-operation and post-operation was 14.6 ± 4.9 mm and 12.4 ± 3.9 mm, respectively. The mean ET-TT distance of the pre-operation and post-operation was 68.5 ± 7.3 mm and 71.8 ± 7.5 mm, respectively. The mean TT-TG index of the pre-operation and post-operation was 0.22 ± 0.07 and 0.17 ± 0.05, respectively. The mean θ was 12.1 ± 3.96 degrees and 9.84 ± 2.98 degrees, respectively. There were statistically differences of between pre-operation and post-operation. Discussion. TT-TG distance, TT-TG index and the three dimensional Q angle decreased after total knee arthroplasty using the modified gap technique. These results indicated that patellofemoral joint was realigned and stabilized, and the force vector of the extensor mechanism was decreased by the modified gap technique

Background. The goal of patellofemoral arthroplasty (PFA) is to replace damaged cartilage, and to correct underlying deformities, to reduce pain and prevent maltracking. We aimed to determine how PFA modifies patellar height, tilt, and tibial tuberosity to trochlear groove (TT-TG) distance. The hypothesis was that PFA would correct trochlear dysplasia or extensor mechanism malalignment. Methods. The authors prospectively studied a series of 16 patients (13 women and 3 men) aged 64.9 ± 16.3 years (range, 41 to 86) that received PFA. All knees were assessed pre-operatively and six months post-operatively using frontal, lateral, and ‘skyline’ x-rays, and CT scans to calculate patellar tilt, patellar height and tibial tuberosity–trochlear groove (TT-TG) distance. Results. The inter-observer agreement was excellent for all parameters. (ICC > 0.95). Pre-operatively, the median patellar tilt without quadriceps contraction (QC) was 17.5° (range, 5.3°–33.4°) and with QC was 19.8° (range, 0°–52.0°). The median Caton- Deschamps Index (CDI) was 0.91 (range, 0.80–1.22) and TT-TG distance was 14.5mm (range, 4.0–22.0). Post-operatively, the median patellar tilt without QC was 0.3° (range, −15.3°–9.5°) and with QC was 6.1° (range, −11.5°–13.3°). The median CDI was 1.11 (range, 0.81–1.20) and TT-TG distance was 10.1mm (range, 1.8–13.8mm). Conclusion. The present study demonstrates that, beyond replacing arthritic cartilage, trochlear-cutting PFA improves patellofemoral congruence by correcting trochlear dysplasia and standardizing radiological measurements as patellar tilt and TT-TG. The association of lateral patellar facetectomy diminishes local effects of OA and improves patellar tracking by reducing the patellar tilt