Minimally invasive surgery (MIS) has evolved as an alternative to conventional sternotomy (CS) when performing mitral valve (MV) surgery. In Germany, slightly more than half (55%) of patients undergoing MV surgery are treated using an MIS approach [1]. MIS techniques were introduced to reduce surgical trauma and postoperative recovery time [2]. Potential benefits of MIS techniques include less postoperative bleeding, fewer wound infections, faster postoperative recovery, shorter intensive care unit (ICU) and/or hospital stay, and the cosmetic benefits of a smaller scar [3‐9]. Meta-analyses of the available evidence also suggest that perioperative mortality, repair and reoperation rates after MIS are at least similar to those seen after CS or better [3‐9]. Few randomised controlled trials have been performed and meta-analysed [3] and most of the evidence comes from retrospective observational studies [6]. In addition, most of these studies evaluated only short- to mid-term outcomes [3‐9].

The risk profiles of patients selected for MIS often differ from those treated via CS, with the latter approach favoured for higher risk patients, making it difficult to compare outcomes for the two approaches. Propensity score matching is usually performed to control for such differences. Analyses comparing MIS and CS that use this method generally support the findings of other studies and meta-analyses [10‐20]. Only a few propensity-matched studies have evaluated long-term outcomes, with most finding similar long-term survival after MIS and CS [12, 18, 20], although one reported better 5- and 10-year survival rates with MIS among patients with degenerative MV regurgitation [13].

The purpose of our investigation, therefore, was to compare and corroborate the short- and long-term results of MV interventions after CS and MIS at our center.

This study, performed between January 2005 and December 2015, was a single-center, retrospective, propensity score matched analysis of MV surgeries. Approval for the study was obtained from the site’s ethical committee and it complied with the Declaration of Helsinki and its amendments. Written informed consent from patients was not required because the study used anonymised data that had already been collected as part of routine diagnosis and treatment.

The disposition of patients is summarised in Fig. 1. Among 1357 patients who received an MV intervention, 861 were treated using MIS and 496 underwent CS. Propensity score matching resulted in 422 MIS patients and 422 CS patients for analysis.

The results show that at our center, a higher rate of ‘short-term’ successful MV repair, a lower rate of cardiovascular death within the first 30 days, and better long-term survival, was observed with MIS compared with CS. We also observed fewer procedure-related complications, better functional outcomes and a shorter length of hospital stay, as well as lower rates of pacemaker implantation and acute renal failure within 30 days, with MIS. MIS and CS did not differ with respect to the incidence of stroke, myocardial infarction and repeat MV surgery.

MIS has been shown to be associated with low mortality and good postoperative, mid-term and long-term results [8, 9, 12, 13, 18, 20‐22]. The use of MIS instead of CS for MV surgery has increased in recent years. At our center, MIS was introduced in 2009, and the majority of patients undergoing MV procedures are now treated using this approach. CS is more likely than MIS to be used in high-risk patients and to compare outcomes after MV procedures performed using these two approaches we excluded patients that were not eligible in principle for MIS and incorporated propensity-score matching to control for differences in preoperative patient and risk profiles.

MV repair was the most common procedure, performed in more than 80% of patients irrespective of the access route. The rate of ‘short-term’ successful repair, defined as MV repair without conversion to MV replacement and hospital survival, was 96.0% in the MIS group and 76% in the CS group (p < 0.001). This is in contrast to meta-analyses which found that repair rates were similar with MIS and CS [4, 5]. In our study, however, as there was no difference in the rate of immediate procedural death, the difference observed is because of the higher rate of conversion from repair to replacement in the MIS group. Interestingly, MV replacement was planned upfront in the CS group more often than in the MIS group and the difference (95 patients) accounts for this. It appears, therefore, as if repair was usually intended in MIS patients, but repair rates were quite similar between the two groups. The median gradient was higher in the CS at baseline compared to the MIS group and this difference largely persists after treatment, albeit at a lower level. The conversion rate to sternotomy from MIS was low (2.8%), which is consistent with previous studies (1–2.6%) [23, 24].

Operating time, cardiopulmonary bypass time and cross-clamp time were longer with MIS than with CS, as has been reported previously [6, 8, 10‐12, 14, 18, 20] Despite the longer procedural time, there was no difference in the length of ICU stay, and the overall length of hospital stay was shorter in the MIS group. Previous studies have also generally reported shorter ICU or hospital length of stay after MIS compared with CS [6, 8, 10‐12, 14‐17, 20]. The shorter hospital stay suggests postoperative recovery was quicker in MIS patients and is also consistent with the lower incidence of procedure-related complications seen after MIS compared with CS in our study. Overall, the length of hospital stay reported in this study appears to be quite high by today’s standards, but this study captured data from 2005 until 2015, and a lower length of stay would currently be expected. In addition, some healthcare systems (e.g., German healthcare) dictate a minimum length of hospital stay post-treatment for reimbursement, while other healthcare systems discharge patients as early as it is safe to do so to reduce treatment costs.

In the current study, there was no difference between MIS and CS with respect to mortality within the first 72 h post-procedure, or the overall rate of mortality within 30 days. This is consistent with most other studies, all of which evaluated all-cause mortality [3, 4, 6‐8, 12].

We also evaluated long-term mortality, and after a mean of 7 years found a better survival rate among patients treated via MIS compared with CS (88.5% versus 74.8%, p < 0.001). Previous studies have generally found no significant difference in mid-term (1–3 year) [11, 12, 20, 22] or long-term (4–9 years) [12, 15, 18, 20, 22] survival between patients treated via MIS or CS. However, one propensity-matched study involving patients with degenerative MV regurgitation reported better survival after MIS, with 5- and 10-year survival rates of 90% and 84% compared with 85% and 70% after CS (p = 0.004) [13]. Our results are consistent with these values.

We did, however, find a lower rate of pacemaker implantations and less risk of acute renal failure in the MIS group. Pacemaker implantation is found to be more common in patients receiving concomitant tricuspid valve replacement [25, 26]. In our dataset, 16.0% of the patients with concomitant tricuspid valve repair versus (TVR) 8.0% of the patients with isolated MV surgery needed implantation of a pacemaker (p = 0.037). However, after adjustment for TVR amongst other variables in the present analysis the OR remained significant. Another propensity-matched study has reported a lower rate of dialysis for renal failure after MIS compared with CS [13], but most studies and meta-analyses have found no difference in these two outcomes between MIS and CS [4, 6, 14‐16, 18‐20]. Nonetheless MIS may be associated with a need for conversion to CS and we find patients with previous right lateral thoracotomy, lung adhesions and peripheral artery disease are at increased risk. Furthermore, we observed a case where the inferior vena cava was interrupted and conversion to surgery needed [27]. Finally, we gathered experience in performing MIS in patients with prior surgery of the breast, such as mammoplasty [28].

The study has several limitations. The principal ones were the retrospective nature of the analysis, the time shift with more CS being performed in the initial years and more MIS later on, the potential for a skewed group assignment based on concomitant disease having an impact on access route selection, and the definition of ‘short-term’ successful repair on study outcome. As to the first point, we considered restricting the analysis to the time of the biggest overlap and/or the second part of the time window (2010–to 2015). This would, however, increase the potential bias arising from the experience of the surgeons with the intervention (plenty for CS, less so for MIS). In addition, this would shorten the length of the follow-up considerably. As to the second point, we excluded patients from the analysis who were not eligible for either approach in principle. Our clinical standard would exclude patients with concomitant interventions CABG and/or aortic valve intervention, intervention of the ascending aorta, severe pulmonary adhesion or severe calcification of the mitral annulus and we omitted these subjects from this analysis. Furthermore, propensity score matching helped mitigate this but may not eliminate it completely and our results should be considered with this caveat. One of the 15 key baseline variable for propensity score matching was MV pathology. While the MV pathologies between the CS and MIS groups were well-matched overall, the MIS group had more patients with annulus dilatation and PML prolapse than the CS group. In an ideal scenario, patients would have been randomized for treatment and functional and mitral stenosis patients, which comprise approximately 10% of the patient populations, would have been excluded from the analysis but doing this would have limited the statistical power of the dataset. Our study defined ‘short-term’ successful repair as valve repair (without valve replacement) and hospital survival, which may introduce a bias in favor of MIS. Patients being treated with MIS are less likely to undergo repair because it would prolong the operation time and, as a result, surgeons may be more prepared to accept a small level of valve leakage. In addition to these, the analysis was based on a single center, which may limit the generalisability of the results but is usually associated with increased internal consistency.

This study demonstrates that good results for MV surgery can be obtained using an MIS approach, with a high MV repair rate, low peri-procedural morbidity and mortality, and improved long-term survival. The results support the use of a minimally invasive approach as the standard therapeutic option for MV surgery.