FTFA change under valgus stress force radiography is useful for evaluating the correctability of intra-articular varus deformity in UKA

Objective

The aim of this observational study was to investigate the postoperative alignment change with Oxford unicompartmental knee arthroplasties (UKA), and clarify whether femoro-tibial facet angle (FTFA) is useful for evaluating alignment correctability with UKA.

Methods

This study evaluated 79 knees consecutive minimally invasive Oxford phase 3 UKAs performed between 2013 and 2014. Full-length weight-bearing radiographs of the lower limbs were obtained pre- and postoperatively to assess varus angle. Preoperative valgus stress radiography in the supine position was also performed. FTFA was measured on weight-bearing anteroposterior radiography and valgus stress radiography.

Results

The preoperative varus angle of 4.6° ± 3.1° reduced to 1.7° ± 2.6° postoperatively. Preoperative varus angle and postoperative varus angle change strongly correlated with the FTFA value and its change on the valgus stress radiographs, respectively (p < 0.01). Based on preoperative FTFA under valgus stress radiography, intra-articular varus corrected group (37 knees) with preoperative varus angle 2.9° ± 2.4° was corrected to − 0.3° ± 2.0° after UKA. However, intra-articular varus uncorrected group (42 knees) with preoperative varus angle 6.0° ± 3.0° was only corrected to 3.5° ± 1.7°. Thirteen knees (16.5%) were overcorrected to valgus after UKA, with a mean FTFA of − 1.2° ± 0.4° under valgus stress force, which related with a postoperative valgus angle 0.8° ± 1.2°.

Conclusion

FTFA change under valgus stress force was useful for evaluating the correctability of UKA. It could reflect intra-articular varus deformity. Intra-articular varus deformity not corrected under valgus stress would result in varus after UKA. However, intra-articular deformity which could be overcorrected under valgus stress would have a tendency to valgus after Oxford UKA.