Abstract
Introduction
The aim of this study was to evaluate the effects of posterior tibial slope (PTS) and posterior condylar offset (PCO) on patient-reported pain and function one year after TKA.
Methods
A total of 500 patients from 11 clinics in 6 countries were enrolled into a prospective, multicenter study. All patients were indicated for primary TKA for OA and received components from a single manufacturer. All liners were made from vitamin-E stabilized, highly crosslinked (95 kGy) polyethylene; 54.7% were posterior stabilized (PS) and the remaining were cruciate-retaining.
The Knee Injury and Osteoarthritis Outcome Score (KOOS) was administered at the one-year follow-up visit. The KOOS pain and activities in daily life (ADL) sub-scores were dichotomized and served as the primary outcomes. Dichotomization was done with the patient acceptable symptom state (PASS), defined by previous studies as the value of the PROM above which patients deem their state as acceptable (84.5 points for KOOS pain and 83.0 points for KOOS ADL).
Plain lateral radiographs were taken and assessed for PTS (Figure 1) and PCO (Figure 2). PTS was categorized as above (excessive flexion), within (ideal), or below (extension) the safe zone of 0° − 7° of flexion. PCO increases or decreases of greater than 3mm were compared against no change (≤ 3mm). Each of the two sagittal positioning metrics was tested against the KOOS pain and ADL PASS at one year.
Results
396 patients (80.3% of eligible) had completed the one-year visit. A total of 297 (75%) achieved the PASS in KOOS pain and 277 (70%) achieved the PASS in KOOS ADL (Figure 3).
PTS was closely associated with the likelihood of achieving the PASS in KOOS pain (p < 0.001) and ADL (p = 0.005) in univariate tests (Kruskal-Wallis). It was also independently predictive of achieving the PASS in multivariable models controlling for sex, body mass index, preoperative health state, and age. In a binary logistic regression for achieving the PASS in KOOS pain, a PTS < 0° (extension) was 6.3 times less likely to achieve the PASS compared to the ideal PTS (0°–7° of flexion) (p=0.004; OR=0.16). Overly flexed tibial components (>7°) were equally likely to achieve the PASS in KOOS pain as components with an ideal PTS (p=0.091). A separate model assessing independent predictors of achieving the PASS in KOOS ADL, patients with extension were 4.8 times less likely to achieve the PASS compared to those with an ideal PTS (p=0.012; OR=0.21), while patients with excessive flexion were equally likely to achieve the PASS in KOOS ADL as patients with an ideal PTS (p=0.077).
When considering the patients with a PTS > 7° (excessive flexion), PCO decrease was associated with a lower chance of achieving the PASS in KOOS ADL (p = 0.022). When considering the patients with a PTS < 0° (extension), PCO increase was associated with a lower chance of achieving the PASS in KOOS pain (p = 0.031).
Conclusions
The most influential sagittal positioning parameter affecting patient outcomes at one year after TKA was PTS. PTS had a significant, independent effect on all PROMs one year after TKA. Surgeons should be more cautious to avoid tibial component extension rather than excessive flexion. We recommend replicating the native PCO and targeting a PTS of 0°–7° of flexion.
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