Are There Differences in Micromotion on Radiostereometric Analysis Between Bicruciate and Cruciate-retaining Designs in TKA? A Randomized Controlled Trial.

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.