Are There Differences in Micromotion on Radiostereometric Analysis Between Bicruciate and Cruciate-retaining Designs in TKA? A Randomized Controlled Trial
Authors: Troelsen Anders , Ingelsrud Lina Holm , Thomsen Morten Grove , Muharemovic Omar , Otte Kristian Stahl , Husted Henrik 
Published: December 2019

Abstract

Background
Bicruciate-retaining TKA has been proposed to improve clinical outcomes by maintaining intrinsic ACL function. However, because the unique design of the bicruciate-retaining tibial component precludes a tibial stem, fixation may be compromised. A radiostereometric analysis permits an evaluation of early migration of tibial components in this setting, but to our knowledge, no such analysis has been performed.

Questions/purposes
We performed a randomized controlled trial using a radiostereometric analysis and asked, at 2 years: (1) Is there a difference in tibial implant migration between the bicruciate-retaining and cruciate-retaining TKA designs? In a secondary analysis, we asked: (2) Is there a difference in patient-reported outcomes (Oxford Knee Score [OKS] and Forgotten Joint Score [FJS] between the bicruciate-retaining and cruciate-retaining TKA designs? (3) What is the frequency of reoperations and revisions for the bicruciate-retaining and cruciate-retaining TKA designs?

Methods
This parallel-group trial (ClinicalTrials.gov: NCT01966848) randomized 50 patients with an intact ACL who were eligible to undergo TKA to receive either a bicruciate-retaining or cruciate-retaining TKA. Patients were blinded to treatment allocation. The primary outcome was the maximum total point motion (MTPM) of the tibial component measured with model-based radiostereometric analysis (RSA) at 2 years postoperatively. The MTPM is a translation vector defined as the point in the RSA model that has the greatest combined translation in x-, y- and z-directions. A 1-year postoperative mean MTPM value of 1.6 mm has been suggested as a threshold for unacceptable increased risk of aseptic loosening after both 5 and 10 years. The repeatability of the MTPM was found to be 0.26 mm in our study. Patient-reported outcome measures were assessed preoperatively and at 2 years postoperatively with the OKS (scale of 0-48, worst-best) and FJS (scale of 0-100, worst-best). Baseline characteristics did not differ between groups. At 2 years postoperatively, RSA images were available for 22 patients who underwent bicruciate-retaining and 23 patients who underwent cruciate-retaining TKA, while patient-reported outcome measures were available for 24 patients in each group. The study was powered to detect a 0.2-mm difference in MTPM between groups (SD = 0.2, significance level = 5%, power = 80%).

Results
With the numbers available, we found no difference in MTPM between the bicruciate-retaining and cruciate-retaining groups. The median (interquartile range [IQR]) MTPM was 0.52 mm (0.35 to 1.02) and 0.42 mm (0.34 to 0.70) in the bicruciate-retaining and cruciate-retaining groups, respectively (p = 0.63). There was no difference in the magnitude of improvement in the OKS from preoperatively to 2 years postoperative between the groups (median delta [IQR] for bicruciate-retaining 18 [14 to 23] versus cruciate-retaining 18 [15 to 21], difference of medians 0; p = 0.96). Likewise, there was no difference in the magnitude of improvement in the FJS score from preoperatively to 2 years postoperative between the groups (mean ± SD for bicruciate-retaining 46 ± 32 versus cruciate-retaining 48 ± 16, mean difference, 2; p = 0.80). Three patients in the bicruciate-retaining group underwent arthroscopically assisted manipulation at 3 to 4 months postoperatively, and one patient in the bicruciate-retaining group sustained a tibial island fracture during primary surgery and underwent a revision procedure after 6 months. There were no reoperations or revisions in the cruciate-retaining group.

Conclusions
With the numbers available, we found no differences between the bicruciate-retaining and the cruciate-retaining implants in terms of stable fixation on RSA or patient-reported outcome measure scores at 2 years, and must therefore recommend against the routine clinical use of the bicruciate-retaining device. The complications we observed with the bicruciate-retaining device suggest it has an associated learning curve and the associated risks of novelty with no demonstrable benefit to the patient; it is also likely to be more expensive in most centers. Continued research on this implant should only be performed in the context of controlled trials.

Level of Evidence
Level II, therapeutic study.