Intra-operative Analysis of Different Patellar Implant Designs in Total Knee Replacement: A Prospective Randomised Comparative Trial

OBJECTIVES: Present scientific literature supports patellar resurfacing with regards to reduced re-operation risk and improved long term patient satisfaction in total knee arthroplasty (Schindler 2012). Two basic techniques are present for resurfacing the patella (Lachiewicz 2003); onlay and round inset (IN). Onlay designs are either symmetrical (round) (OR) or oval shaped with an asymmetric ridge (OO). When using a round patellar implant, a lateral facetectomy is often performed to avoid lateral compression syndrome and enhance patellar tracking (Zhang, Zhang et al. 2012). However, none of these implants have been found to be superior to the other. This study will compare three different types of patellar implant designs commonly used for resurfacing in TKA and report on intra-operative outcomes. We hypothesise that OO design offers improved bone coverage with less need to perform facetectomy when compared to round designs. METHODS: A prospective, randomised trial was performed on patients undergoing unilateral TKA by 2 surgeons. Patients who agreed to participate were allocated to one of the three patellar design groups (IN, OR, OO). Patients were blinded to the implant assigned to them. The prostheses were implanted with the aim to restore pre-resection thickness, uniform facet thickness, with maximal bone coverage and medialisation of the median ridge to enhance tracking. Groups were compared in terms of implant size, percentage bone coverage, lateral facet underhang and requirement for facetectomy. Paired t-tests and ANOVA were used to compare continuous outcome measures with chi-squared test for categorical variables. RESULTS: 86 patients were initially assessed for inclusion. Eight patients refused to participate and a further 18 did not meet the study inclusion criteria. A total of 60 patients were included in the final analysis. There were no differences between the 3 groups with regards to mean age, sex, pre-resection patellar thickness, patellar dimensions or pre-operative Kujala score. The asymmetrical onlay design resulted in a statistically significant larger button size (mean size 25.3 mm IN, 32.7 mm OR, 34.6 mm OO; p<0.001), reduced lateral underhang (mean 11.1mmm IN, 7.7 mm OR, 1.2 mm OO; p<0.001), and reduced need for facetectomy (85% IN, 85% OR, 0% OO). Percentage of surface bone coverage was 49%, 63.8%, 89.6% for IN, OR, OO implants respectively (p< 0.001). CONCLUSION: Oval onlay design demonstrates better surface bone coverage than round onlay or inlay implants. There were no cases in the oval group that required lateral facetectomy. Future analysis of this study group will aim to determine whether radiographic, patellar vascular and functional outcomes vary amongst the designs.