The Importance of Restoring Neutral Mechanical Alignment and Cementation Technique to Primary Total Knee Arthroplasty: A Survivorship Analysis of 13 Years Follow Up

Abstract

INTRODUCTION: There are controversies in the literature regarding the effect of postoperative restoration of mechanical axis and the tibial component cementation technique on the long-term survivorship. More recent studies showed a neutral postoperative alignment within ±3° may not affect the long-term survivorship of total knee arthroplasty (TKA). Low contact stress (LCS) mobile bearing TKA has the theoretical advantage of reducing stress on the implants and lower the risk of component loosening. Based on a prospective registry of Chinese patients undergoing TKA, this study was composed of a group of patients with LCS mobile bearing TKA to primarily determine if lower limb mechanical axis within the traditional ±3° improves survivorship in comparison to alignments outside this range. Secondary, the study also addressed if full cementation technique improves long-term survivorship in comparison to surface cementation. METHODS: From 1999 to 2003, 350 knees in 240 Chinese patients (38 males; 202 females) underwent primary TKA with a LCS design. The mean age was 65.5±9.6 years (range 25-81). The primary diagnoses included osteoarthritis (202), rheumatoid arthritis (34), and others (4). Intraoperatively, bone cement was placed either on the undersurface of the tibial component without the stem (surface cementation group) or both the undersurface and the stem (full cementation group). The rest of the procedure was standardized in all patients. Perioperative standing radiographs of the lower limbs were used to determine and mechanical axis alignment. The mechanical axis of the lower limb in the coronal plane was measured by an independent observer according to the criteria defined by Crooke et al. The reliability of the measurement was assessed by comparing the intraobserver measurements at two different time points using the method of Bland and Altman on 20 randomly selected subjects. All patients’ records and x-rays were retrospectively reviewed. Clinical scores, radiological changes, and complications were documented. Descriptive analyses of patient demographics and perioperative data were performed utilizing SPSS software. Implant survivorship was estimated using the Kaplan-Meier’s method. Multivariate Cox regression model was applied to identify prognostic variables in relation to cementation technique and postoperative alignment that are significantly related to survivorship. RESULTS: The mean duration of follow up is 13.2 ± 2.8 years (range 7-18.5). Knee Society Knee Score improved from 39.6 to 89.5, Knee Society Functional Score improved from 38.1 to 55.3 (p<0.0001). Tibial stems were cemented in 154 knees and uncemented in 196 knees. Normal postoperative mechanical alignment was set at within ±3°. 4 groups of patients were identified: a) cemented stem/normal alignment (n=114), b) cemented stem/abnormal alignment (n=40), c) non-cemented stem/normal alignment (n=143), d) non-cemented stem/abnormal alignment (n=53) (p=0.8). Thirty-three TKAs were revised for aseptic loosening (17 normal alignment, 16 abnormal alignment). The 15-year survivorship was 91.5% in the well-aligned group and 88.6% in the mal-aligned group if the stem was cemented (p=0.7). The 15-year survivorship was 94.2% in the well-aligned group and 74.3% in the mal-aligned group if the stem was un-cemented (p<0.0001). All 16 loose and mal-alignmed knees were in varus, ranging from 4°-17°. Using the multivariate cox regression model, lower limb mechanical axis alone was not found to be a significant risk factor for aseptic loosening. However the combined effect of tibial stem cementation and mechanical axis were found to be significant factors in determining the survivorship (OR 5.1, p <0.0001). DISCUSSION AND CONCLUSION: We found in our study that If the postoperative mechanical alignment was normal, surface cementation technique alone would not affect the long-term survivorship of LCS TKA. But mal-alignment could cause uneven stress distribution over the tibial implant, which further led to micro-motion and loosening if tibial stem was not cemented. If combined with abnormal alignment, the risk of aseptic loosening would increase by 5 times. The traditional belief of an Ideal alignment 0°± 3° may not be necessary if good cementation technique could be achieved over the tibial component. Excessive postoperative varus malalignment should be avoided. Full cementation of the tibial stem should be advocated in all TKA prostheses to prevent the combined adverse effect of uncemented stem and abnormal alignment.