Previous studies have highlighted differences in the risk of periprosthetic fracture between tapered slip (TS) and composite beam (CB) stems. This biomechanical study explored periprosthetic fractures around these stems and the effect of adding a 16-gauge calcar or diaphyseal wire to TS stems on their resistance to torque.

A power analysis determined a sample size of 7 specimens per group, assuming a standard deviation of 14.8 Nm in peak torque, to provide 90% power to detect a difference of at least 30 Nm between groups. Twenty-one TS stems (eight alone, six with calcar wiring, seven with diaphyseal wiring placed 2 cm distal to the lesser trochanter) and seven CB stems were cemented into standard Sawbones. A servo-hydraulic test machine applied a 1000 N load with a 1-degree rotation per second until failure. The peak torque at failure was measured, and the fracture location recorded. Comparisons were performed using two-sample t-tests.

CB stems exhibited a significantly higher peak torque at failure (205.3 Nm) than TS stems (159.5 Nm, p=0.020). Calcar-wire-TS (148.2 Nm, p=0.036) and diaphyseal-wire-TS (164.9 Nm, p=0.036) were both weaker than CB stems. Wired-TS stems showed no significant difference from non-wired-TS stems. Additionally, the study could not conclude that calcar wiring is stronger than diaphyseal wiring. All TS fractures occurred at the mid-stem, simulating a B-type fracture, while the addition of the diaphyseal wire shifted the fracture location more distally in four of seven stems (p=0.0699).

This biomechanical study supports the clinical evidence that CB stems have stronger resistance to torque than TS stems and may explain lower risk of periprosthetic fracture. The addition of calcar or diaphyseal wires to TS stems resulted in no significant changes in peak torque to fracture. In patients at high risk of periprosthetic fracture, CB cemented stems should be considered.

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.

Background

Several studies have reported that tibial component in varus alignment can worsen the survivorship of medial unicompartmental knee arthroplasty (UKA). On the other hand, Varus/valgus inclination of the tibial component can affect the location of the contact point between femoral and tibial component especially in round on flat bearing surface design. Along with the tibial component inclination, changes in the contact point may also alter the tibial condylar bone stress, which would affect the longevity or complications after UKA.