Is Patellofemoral osteoarthritis a contraindication to lateral unicompartmental knee replacement?
ESSKA Academy. Kennedy J. May 12, 2018; 218104
James Kennedy
James Kennedy
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Objectives: Patellofemoral joint (PFJ) osteoarthritis (OA) has historically been considered a contraindication to unicompartmental knee replacement (UKR), however, with the exception of bone loss with grooving to the lateral facet, it has been shown not to affect the outcome of medial meniscal-bearing UKR. The situation in lateral UKR is unclear.

Methods: Using prospective data on the intraoperative status of the PFJ in 302 consecutive knees with mobile bearing domed lateral UKR (275 patients) with a minimum followup of 2 years, we investigated whether the presence, grade or location of PFJ OA affected outcomes. A 5-point grading system graded PFJ OA from none to extensive (>2cm2 of loss) with sub-classification based on the presence or absence of full-thickness cartilage loss (FTCL). Outcomes were evaluated independently using the Oxford Knee Score (OKS) with separate analysis of OKS Q12 "Could you walk down one flight of stairs?". Primary outcome was a comparison of OKS between groups with and without FTCL using a non-inferiority two one-sided test design performed on a change in OKS score with a clinically important difference of 4 points set as the inferiority limit. Secondary outcomes compared OKS, AKSS and OKS Q12 between subgroups using unpaired two sample t-tests.

Results: Mean follow up was 4.5 years (range 2 to 11). FTCL in the PFJ was observed at any location in 39 knees (13%): on the trochlear surface in 29 knees (10%), on the lateral patellar facet in 18 (6%), and on the medial patellar facet in 15 (5%). There were no significant differences in baseline characteristics between groups with and without PFJ FTCL.
No difference in five-year implant survival based on the presence or location of FTCL was observed. Mean improvement in OKS was 20.4 (SD 8) in those without FTCL compared to 20.3 (SD 8) for those with FTCL. For the primary outcome, the presence of PFJ FTCL was not inferior when compared to its absence (difference in means -0.1, 95% CI -3 to 3, p<0.01). No difference in functional outcomes were observed based on location of FTCL on the patella, but knees with FTCL on the trochlea reported inferior outcomes on OKS Q12. The mean postoperative OKS achieved by those with trochlear FTCL was 37.6 (SD 10), compared to 40.0 (SD 9; p=0.16) in those without trochlear FTCL. Analysis by grade of OA found extensive FTCL of the medial facet to be rare (2 knees), but associated with significantly worse functional outcomes.

Conclusions: Trochlear FTCL leads to an inferior score on OKS Q12, however, with the exception of extensive FTCL of the medial facet, the mean score achieved postoperatively by all subgroups is greater than that in large cohorts of total knee replacements. This suggests that in the surgical management of bone-on-bone lateral compartment OA with PFJ FTCL, lateral UKR is a safe and viable treatment option.
We conclude that, provided there is not extensive FTCL (>2cm2) of the medial facet, PFJ OA, however severe, is not a contraindication to the lateral domed UKR.

Unicompartmental knee replacement; patellofemoral joint; patient selection
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