The effect of knee flexion during tourniquet inflation, fat pad excision and lateral tibial release on patellar subluxation during MIS TKA.

We compared the distance of patellar subluxation (lateral patellar displacement) during MIS TKA arthrotomy among sequential variations of tourniquet application and soft tissue release in a consecutive series of 40 knees. The distance of patellar subluxation from the Whiteside's line was measured for every knee under four consecutive conditions; A) the tourniquet inflated with knee in full extension, B) no tourniquet pressure applied, C) the tourniquet inflated with knee in deep flexion, and D) the tourniquet inflated with knee in deep flexion and lateral tibial release (a limited subperiosteal soft tissue dissection including limited patellar fat pad excision and limited capsular release from the upper lateral tibial plateau). There were 28 women and 12 men with the average age of 70 years and the average BMI of 25.5. All knees had preoperative flexion more than 110 degrees . The tourniquet pressure ranged from 280 to 300 mm Hg. The average skin incision length was 9 cm. The average measured distance of condition A, B, C and D were 10, 14.5, 15.8 and 22 mm, respectively with significant difference (p<0.001). The distance of patellar subluxation correlated between conditions A) and C) (r(2), 0.67) and between conditions C) and D) (r(2), 0.72) in the studied group. However, there was no statistical difference of measured distance between group with condition B and C (p=0.40). In conclusion, when MIS TKA is performed using the tourniquet, inflating the tourniquet with knee in deep flexion provided better arthrotomy exposure than the knee in full extension. Combined inflating tourniquet in deep knee flexion and lateral tibial release provided the greatest arthrotomy visualization.