We proposed the substitute anteroposterior (sAP) line of the tibia for medial unicompartmental knee arthroplasty (UKA), which connects the medial border of the patellar tendon at the articular surface level and the medial intercondylar tubercle of the tibia. However, it has not been shown that referencing this line improves the rotational alignment of the components. Therefore, in this study, we investigated whether the tibial component could be implanted perpendicular to the SEA by referencing the sAP line and whether referencing the sAP line could reduce the rotational mismatch between the femoral and the tibial components.

Postoperative computed tomography datasets from 60 lower limbs in 57 Japanese patients with medial UKA were used. Among these, 30 knees were operated using the sAP line for AP reference and other 30 knees using the medial intercondylar ridge (MIR) line. First, the angle between the AP orientation of the tibial component and the surgical epicondylar axis (SEA) was measured. Then, the rotational mismatch angle between the components was measured.

The tibial and femoral components placed referencing the sAP line were externally rotated 90.7°±3.2° and 91°±7.7° relative to the SEA, respectively, those referencing the MIR line were 94.9°±8.5° and 91.2°±7.7°, respectively. When referencing the sAP line, the orientation of the component was more perpendicular to the SEA (Student t-test, unpaired, P = .016) and rotational variability of the tibial component was significantly smaller (F test, P < 0 .0001). The rotational mismatch angle when referencing the sAP line and the MIR line was 0.3°±8.3°and −3.8°±6.7°, respectively. Referencing the sAP line significantly reduced the rotational mismatch between the components (Student t-test, unpaired, P = .045).

Referencing the sAP line in the medial UKA may be useful to determine the AP orientation of the tibial component.

Purpose

Despite total knee arthroplasty (TKA) is a successful surgical procedure with end-stage knee osteoarthritis, approximately 20% of the patients who underwent primary TKA were still dissatisfied with the outcome. Thereby, numerous literatures have confirmed the relationship between soft tissue balancing and clinical result to improve this pressing issue. Recently, there has been an increased research interest in patient-reported outcome measures (PROMs) after TKA. However, there is little agreement on the association between soft tissue balancing and PROMs. Therefore, the purpose of this study was to determine whether intraoperative soft tissue balancing affected PROMs after primary TKA. We hypothesized that soft tissue balancing would be a predictive factor for postoperative PROMs at one-year post-surgery.

Although the pre- or intraoperative flexion angle in TKA has been commonly considered as a predictor of the postoperative flexion angle, patients with well flexion intraoperatively cannot necessarily obtain deep flexion angle postoperatively. The reason why inconsistencies remains has been unsolved. The intraoperative compressive force between femoral and tibial components has the advantage of the sequential changes during knee motion. However, the relationship between the compressive force and the postoperative ROM has not yet been clarified. We aimed to evaluate the intraoperative femorotibial compressive force during passive knee motion, and determine the relationship between the compressive force and the postoperative flexion angle.

A total of 11 knees in 10 patients who underwent primary cruciate-retaining (CR) TKA (The FINE Total Knee System; Teijin Nakashima Medical Co., Ltd., Okayama, Japan) for osteoarthritis were studied retrospectively, with a mean age of 76 years via a measured resection technique. We developed a customized measurement device mimicking the tibial component with this platform of six load sensors arranged in two rows (medial and lateral) by three tandem sets (anterior, center and posterior): anteromedial (AM), anterolateral (AL); centromedial (CM), centrolateral (CL); and posteromedial (PM), posterolateral compartment (PL) (Fig. 1). At the step of the implant trial, this device was placed on the tibia with compressive force recorded three times, while the knee was subsequently taken from 0° to full flexion manually in 15 seconds with the flexion angle of the knee recorded simultaneously by using an electric goniometer (Fig. 2). Eligibility were evaluated for ROM using a long-armed goniometer preoperatively and at 6 months postoperatively. A p value of < 0.05 was considered significant.

The mean compressive force at AM, AL, CM, CL, PM and PL was 0.7, 0.5, 1.3, 1.2, 3.4 and 2.6 kgf, with the peak force of 4.2, 2.5, 4.1, 2.5, 7.3 and 4.7 kgf, respectively. The mean pre- and postoperative extension and flexion angles were −11° and −6°; and 115° and 113°, respectively. There were no significant correlations between the mean force in any region of interest (AM to PL) and the postoperative flexion angle. The peak force in PM showed little correlation with the postoperative flexion angle (r = −0.17, p = 0.54), however, that in PL was strongly negatively correlated with the postoperative flexion (r = −0.86, p < 0.01).

The current results suggest the presence of less force on the lateral side in flexion. We speculate that lower compressive force at the lateral side is essential for deep flexion as it has been reported that the lateral structure has more laxity than the medial side during flexion in healthy knees. Measurement between the femoral and tibial compressive force can contribute an achievement of more flexion angle following CR-TKA.