Sagittal gap balancing (relation between flexion and extension gaps) with placement of trial femoral components and reduction of the patella in total knee arthroplasty (TKA) is important, but it is not easy. The purpose of this study was to investigate whether (1) the flexion and extension gaps were equal when a previously suggested three-dimensional planning for a single-radius femoral component (its sagittal centre is matched with flexion-extension axis of knee movement) is executed with patient-specific instrumentation (PSI) and whether (2) PSI was done with good accuracy, which did not affect the first purpose.
Posterior cruciate ligament sacrificed (PS) TKA was performed on 12 joints. Using the patients' pre-operative computed tomography (CT) images, PSI was manufactured to fit on the bony surface of the knee joint and to simultaneously transfer pre-operative planning to the operating room. After osteotomy with PSI, gap measurements were calculated with the knee in flexion and extension. Angle deviations of both components were investigated with postoperative CT images.
The flexion gap (mean, 19.1 mm) was larger than the extension gap (mean, 12.3 mm) in all cases. Angle differences between pre- and postoperative alignments were within 3° in all cases.
Although PSI executed the pre-operative planning with good accuracy, the flexion gap is always larger than the extension gap. This finding suggests that surgeons may not aim for equal gaps of flexion and extension in PS-TKA.