Introduction 28 10 6 4 9 4 16 21 30 8 12 13 18 By performing a prospective, randomized, blinded trial of two ligament balancing techniques for rotating platform TKA, we sought to determine if ligament balancing technique affected postoperative knee kinematics. We hypothesized that ligament balancing with a calibrated spreader/balancer would provide better controlled knee kinematics, specifically reduced anterior femoral translations with flexion, than ligament balancing with fixed thickness spacer blocks. Materials and methods 1 2 Fig. 1 All patients received a rotating platform total knee arthroplasty (TC-PLUS SB Solution, Plus Orthopedics, Rotkreuz, Switzerland)  Fig. 2 left right  22 29 2 23 14 1 Table 1 Patient demographics and clinical assessments (mean ± 1SD)  Control Spreader P Age at operation (years) 71.0 ± 8.4 72.2 ± 6.6 0.96 Height (cm) 167 ± 7.6 165 ± 7.8 0.65 Weight (kg) 75.0 ± 22.6 70.3 ± 12.9 0.72 Sex (M/F) 3/5 4/4 a Varus/valgus distribution 8/0 7/1 a Pre-op knee score 42.1 ± 10.3 50.0 ± 11.7 0.13 Pre-op function score 50.0 ± 18.9 55.0 ± 20.4 0.50 Post-op knee score 90.5 ± 5.9 93.5 ± 1.8 0.51 Post-op function score 81.3 ± 23.4 88.1 ± 15.1 0.72 Follow-up (months) 10.3 ± 3.1 11.3 ± 2.3 0.44 a Follow-up consisted of clinical and fluoroscopic assessment performed at Mayday University hospital, Croydon, United Kingdom. Fluoroscopic imaging (Siemens Polystar TOP, Siemens AG, Munich, Germany) consisted of (1) weightbearing maximum flexion lunge activity, (2) kneeling on a padded bench to maximum comfortable flexion and (3) a 4 cycles of a step-up/down on a 25 cm step. For the stair activity, the subjects faced the same direction throughout the cycle, therefore, the step-down was a backward motion that reversed the step ascent motion. Patients were instructed on the study activities prior to recording, and were given an opportunity to practice until comfortable. Lateral fluoroscopic views of the knee were recorded in the maximally flexed positions for the lunge and kneeling activities, as were four repeat trials of step-up/down on the stair. The fluoroscopic images were recorded at 15 frames per second onto an S-VHS VCR. Views of calibration targets also were acquired for distortion correction and optical calibration. 3 5 3 3 5 3 5 Fig. 3 red images  26 Researchers were unblinded to subject group membership only after all kinematic data had been produced. Statistical comparisons of the fluoroscopic images were performed (SPSS ver 13, SPSS Inc., Chicago, US) using two-way repeated measures ANOVA with post hoc pair-wise comparisons (Tukey/Kramer) at a 0.05 level of significance. All other parameters were evaluated using non-parametric tests. Results 1 2 P P P P P Table 2 Knee pose during maximum flexion kneeling (mean ± 1SD) Group Flexion (°) Valgus (°) Tibial Ext. Rot. (°) Medial AP (mm) Lateral AP (mm) Control 102.0 ± 12.8 0.1 ± 2.1 −4.7 ± 7.4 −2.7 ± 12.2 −10.5 ± 11.4 Spreader 107.9 ± 10.1 −0.5 ± 1.8 −5.3 ± 6.3 −1.8 ± 8.2 −11.1 ± 11.8 P 0.34  0.56  0.87  0.87  0.93  3 P P P Table 3 Knee pose during maximum flexion lunge (mean ± 1SD) Group Flexion (°) Valgus (°) Tibial Ext. Rot. (°) Medial AP (mm) Lateral AP (mm) Control 95.3 ± 15.1 −0.1 ± 1.4 −9.6 ± 5.9 −0.3 ± 8.2 −16.5 ± 8.7 Spreader 101.6 ± 10.6 −0.6 ± 2.1 −5.7 ± 7.3 −6.7 ± 7.7 −16.8 ± 9.9 P 0.36 0.62 0.29 0.15 0.96 P P 4 Fig. 4 Knee motions during the stair activity differed between the control and spreader groups. Condylar positions were significantly more posterior in the spreader group. There were no significant differences in tibial rotation, nor were there significant pair-wise differences for rotations or translations  5 6 P P 4 4 Fig. 5 left right P  Table 4 COR Group Extension phase (%) Flexion phase (%) Control −3 ± 19 −4 ± 2 Spreader 28 ± 41 −3 ± 3 P <0.05 >0.05 None of the subjects demonstrated valgus or varus angles larger than 2 degrees during motion, consequently there was no obvious evidence of condylar lift-off. Discussion 13 25 27 30 4 6 15 1 20 Ligament balancing technique did affect knee kinematics during the dynamic stair activity. Condylar contact locations were observed to remain more posterior on the tibia and to have a more medial center of rotation during step-up in the spreader group. These findings suggest the spreader balancing technique provided more normal balance or stability to the medial compartment of the knee, resulting in less medial contact translation during the stair activity. Medial contact in the control group was observed to move anterior with flexion on the stair activity, indicating greater functional laxity in that compartment. 2 17 4 7 11 19 24 This double-blinded prospective randomized study used fluoroscopic kinematic measurements to determine if two ligament balancing techniques would affect knee motions in several activities. Kinematics in flexion were similar, with both groups showing a more posterior femoral position than previously has been reported for similar implant designs. Knees operated with a spreader/balancer device showed a more medial center of rotation in ascending a stair activity, and both groups showed average centers of rotation that were more medial than previously had been reported for similar implant designs. Kinematics closer to the normal knee may yield improved knee performance and implant longevity. However, these kinematic differences are clinically insignificant upon short-term follow up, and their long-term significance remains to be studied.