Fig. 7
The concept of manual rKA-TKA with the modified STRT: (a) Pre-arthritic knee showing 6° tibial varus angle. (b) 6° tibial varus resection angle following urKA-TKA with caliper technique. (c) Using modified STRT, the medial tibial resection level is verified in both extension and flexion (employing the medial pivot point: red circle). In the rKA-TKA, the tibial resection is made at 3° varus while maintaining a fixed medial pivot point (blue solid arrow). This creates a larger lateral space (black bidirectional dotted arrow) compared to the medial space (black bidirectional solid arrow). (d) The final angular adjustment is achieved by modifying the lateral bearing thickness. Initially, the tibial trial component is aligned with a 3° varus. The appropriate medial bearing thickness is selected after assessing the medial gap between femoral and tibial trial components. (e) Subsequently, lateral bearing thickness (black solid arrow) is selected to be less than the available lateral space (black bidirectional dotted arrow). For example, the lateral bearing is elevated only 1 mm relative to the medial bearing (e.g., medial bearing 9 mm, lateral bearing 10 mm: resulting in 1.2° joint inclination), rather than 2 mm (e.g., medial bearing 9 mm, lateral bearing 11 mm: resulting in 2.4° joint inclination). (f) This reduces the joint inclination from 6 to 4.2°, potentially decreasing the postoperative HKAA varus angle by 1.8° on whole-leg weight-bearing radiographs. In summary, when applying the rKA-TKA, the lateral tibia is cut slightly more than the component thickness and adjusted with the lateral bearing to reduce the postoperative HKAA. Therefore, a slight lateral slack in extension under non-weight bearing is acceptable. HKAA hip–knee–ankle angle, rKA-TKA restricted kinematically aligned total knee arthroplasty, urKA-TKA unrestricted kinematically aligned total knee arthroplasty, STRT soft tissue-respecting technique.
