Background: The Cementless UKA Challenge
Cementless fixation in UKA offers a path to reduced aseptic loosening, but success is device-specific. Traditional press-fit designs depend entirely on interference fit and friction to resist eccentric loading—and early implants failed by this mechanism: tray rotation under off-center femoral contact, culminating in lift-off and fibrous fixation.
Why Micromotion Matters
At the bone-implant interface, the biologic response depends on motion magnitude. Published work establishes a clear threshold: micromotion < 28 µm permits bone ingrowth; micromotion > 150 µm inhibits it. Whether an implant achieves osseointegration or fibrous encapsulation is decided in the first three months—when relative bone-implant motion determines which tissue forms.
The Clinical Context
Modern cementless UKA survivorship is strong. The published literature (2011–2026) shows 10-year cementless UKA survival of 92–97%, dominated by Oxford mobile-bearing data. Fixed-bearing results remain device-specific and depend critically on how the design resists the moment from eccentric femoral contact under flexion loading.
Sources: Mohammad et al. (Acta Orthop 2019, KSSTA 2019); J Arthroplasty 2026 systematic review; JBJS OA 2026; Bone Jt Open 2026.
Study Design
This finite element analysis examined a locked-screw, cementless tibial construct under high-flexion loading. A CT-derived tibia was loaded with 1,950 N applied at 60° flexion—within the published convention for UKA FEA studies—to evaluate:
- Interface micromotion: Paired displacement between baseplate and adjacent bone, measured anteriorly and posteriorly, decomposed into shear (ΔX, ΔY) and normal (ΔZ) components
- Implant stress: Peak von Mises stress in the baseplate and screw assembly, compared to Ti-6Al-4V yield strength (795 MPa)
- Periprosthetic bone strain: Local strain distribution in cortical and cancellous bone surrounding the implant
Key Results
Interface Micromotion
2.42 µm Posterior (shear-dominated)
No lift-off; compressive contact maintained throughout loading cycle
Peak Implant Stress
341 MPa Bulk baseplate
Both well below 795 MPa yield strength; no material yield risk
How This Compares to Published Cementless UKA Data
Interface Micromotion Under High Flexion
(Peak, Stair Descent)
Mechanism: How the Design Solves Press-Fit Instability
The published failure mode in press-fit cementless UKA is tray rotation under eccentric loading, with lift-off at the unloaded edge. Traditional designs carry the femoral loading moment through interference fit and friction alone—resources that vary with bone quality and resection precision.
The Problem: Press-Fit Geometry Alone
A press-fit construct resists the applied moment only through friction and bony geometry. Eccentric contact loads the unloaded edge in tension. If micromotion at that edge exceeds 150 µm, fibrous tissue forms instead of bone, and the implant progressively loosens.
The Solution: Screw Locking Carries the Moment to Bone
A screw-locked construct carries the loading moment directly into the tibial metaphysis, independent of friction and resection geometry. The interface remains in compressive contact through the entire loading cycle. Residual motion is shear-dominated and stays orders of magnitude below the threshold for inhibited bone formation.
This design strategy reduces dependence on interference fit and targets the alignment and resection sensitivities that the UKA finite element literature has repeatedly identified as failure drivers.
Conclusions
- Under high-flexion loading (60°, 1,950 N), the screw-locked cementless tibial baseplate produced interface micromotion two orders of magnitude below the threshold associated with inhibited bone formation, while maintaining compressive contact.
- Motion was shear-dominated and non-directional, with no interface lift-off—the biomechanical precursor to secure osseointegration rather than the fibrous fixation that characterizes loosening.
- By reducing dependence on interference fit geometry, the design mitigates the alignment and resection sensitivities that prior finite element studies have shown drive cementless UKA failure.
Study Disclosures
B. Palumbo: Royalties and consulting fees from Enovis and Restore3D; equity holder and inventor in Actuos, LLC. S. Gutiérrez: Equity holder in Actuos, LLC. The Actuos constructs evaluated in this study are investigational and not yet FDA cleared. Enovis provided implants for this research.