Alterations in patellar height and posterior tibial slope (PTS) are
frequently measured radiographic parameters associated with
postoperative impairments after total knee arthroplasty (TKA). Few
studies correlate the clinical outcome of TKA with the radiological
indices. Both the modified Insall-Salvati ratio (mISR) and the
Blackburne-Peel ratio (BPR) were hypothesised to correlate with the
clinical outcome after TKA.
A total of 282 computer navigated primary LCS-TKAs, implanted in our
institution from 2008 to 2012, were included. Data (ROM, FJS-12,
WOMAC, and revision surgery) were collected independently and
prospectively. Patellar height (mISR, BPR), joint-line position, and
PTS were measured on pre- and postoperative radiographs. Bivariate
and multiple regression analyses were performed.
Mean mISR (1.5-1.4) and BPR (0.8-0.6) decreased from preoperatively
to 1 year follow-up. Mean joint-line shift in a cranial
direction was 2 mm after TKA implantation. Analysis of
dichotomous variables (presence of PB and PPB or not) only showed
significantly lower flexion in patients with PPB
(p < 0.001). However, multiple regression revealed
that BPR was a significant positive independent predictor for FJS-12
(p = 0.016) and flexion (p < 0.001)
at 1 year follow-up. Postoperative PTS
(p < 0.01) and initial patella height
(p < 0.001) were both predictive for BPR at
follow-up, while the joint-line height was not.
LEVEL OF EVIDENCE
The BPR is a useful and reliable radiographic parameter to predict
patient outcomes 1 year after primary navigated TKA. Lowering
the BPR should be avoided, as this may lead to significant
restrictions in terms of ROM and PROMs.