Cancellous Bone Strains Indicate Efficacy of Stem Augmentation in Constrained Condylar Knees

Abstract

Modular augmented stems of a constrained condylar knee implant are intended to improve tibial fixation under increased varus/valgus loads, but conflicting studies have not yet indicated the factors determining stem usage and performance. To address this, we combined a paired-tibiae, cadaveric experiment of unstemmed and stemmed tibial components with specimen-specific computational models. We hypothesized that the stem would improve implant stability by decreasing implant motion and compressive strains in the proximal cancellous bone due to load transfer by the stem. The models also would indicate the important factors governing stem performance. Large variations of the displacements arose because of loading and biologic variability indicating the inconclusive effects of a stem. Despite these variations, the models showed that a stem augment consistently decreased the strains (30%-50%) in the bone beneath the tray. In tibiae of sufficient stiffness, the supporting cancellous bone did not approach yield, suggesting that a stem augment may not always be necessary. On the other hand, tibial specimens with reduced bone quality and lower stiffness benefited from a stem augment that transferred load to the distal cortical bone. Therefore, patient selection and proper sizing of the implant were identified as important factors in the analyses.

Stemmed Implants Improve Stability in Augmented Constrained Condylar Knees

“…For the experiment, pairs of cadaver specimens that had been implanted with constrained condylar knee implants in our previous study [16] were revised and an oblique defect introduced on the medial side; the specimens without a stem or wedge served as a control group. The tibial pairs were then randomized to receive either a stem with a metallic augment to fill the defect or an augment alone.…”

“…All eight pairs of tibiae (mean age, 66 years; range, 52-76 years) were tested using the previous experimental protocol [16]. A 3000-N medial axial load and a 10-Nm varus moment were applied to the upper constraint spine of the constrained condylar tibial component.…”

“…A 3000-N medial axial load and a 10-Nm varus moment were applied to the upper constraint spine of the constrained condylar tibial component. The axial load represented a severe eccentric load of four times body weight; the moment was chosen as the functional limit of the polyethylene constraint spine [16]. We mounted a sixdegree-of-freedom load cell distally beneath the potted specimens to optimize tibiofemoral conformity and prevent abnormal shear loads from occurring ( Fig.…”

“…Specimen-specific finite element (FE) models were built from the CT data. As with the previous study, two sets of boundary conditions were assumed at the fixation point for the distal tibia: one in which the tibia was modeled as completely constrained and a second in which the measured forces from the six-degree-of-freedom load cell were incorporated as constraining forces [16].…”

“…In an earlier study, we integrated experimental tests with computational models to demonstrate the ability of modular tibial stem augmentation to reduce implant micromotion and stresses and strains in the underlying cancellous bone of the proximal tibia in constrained condylar knees [16]. The addition of a stem consistently reduced strains in the proximal cancellous bone from 20% to 60% depending on the stiffness of the bone.…”