The most important finding of the present study is that CIM TKA with mechanical alignment can achieve high levels of patient satisfaction even at mid-term with more than five years of follow-up. In fact, 94% of patients were either satisfied or very satisfied, and 90% would elect do undergo the same operation again.

Satisfaction levels in our study are located at the upper quartile compared with those reported for OTS TKA. The mean OKS for OTS implants ranges between 16.4 and 43.6, depending on the study [24,25,26]. Likewise, our results align with those reported for the FJS-knee [27]. In a recent study, Gousopoulos et al. reported a 94% satisfaction rate two years after CIM TKA using the Symbios CR implant [17]. With an OKS of 39.6 points and a FJS-knee of 69.0 points, their results align with those in the present study (OKS = 41, FJS-knee = 67). Interestingly, the mean FJS-knee of 67 points, achieved by our patients, is almost equal to that of a healthy and younger population of 2017 US citizens, reaching 66.8 points on average [28].

White et al. reported a higher rate of postoperative stiffness, which eventually required manipulation in a group of patients receiving CIM TKA using ConforMIS compared to OTS implants at two years follow-up [29]. Even though there was no personal consultation to assess the Range of Motion after five years, there were no complaints of postoperative stiffness recorded in our cohort. None of the 164 patients received manipulation under anesthesia at our institution postoperatively. This aligns with a later study from Wheatley et al. that confirmed no higher rate of manipulation after CIM TKA [30].

After proving the efficacy and safety of CIM TKA at mid-terms, one pending question remains unanswered: Does CIM TKA provide a better outcome than OTS TKA? While several RCTs have compared patient-specific to standard instrumentation [31, 32], so far no RCTs comparing CIM to OTS implants have been published. Hence, there are only cohort studies, mostly retrospective, available. In such a study, Wendelspiess et al. found no difference in outcomes after 12 months comparing the ConforMIS iTotal with the Attune® CR mobile-bearing implant (DePuy Synthes, Raynham, MA, US) [33]. In contrast, Reimann et al. found higher levels of patient satisfaction after two to three years by comparing the ConforMIS iTotal with the Triathlon® Total Knee system (Stryker, Kalamazoo, Michigan, USA). Interestingly, in patients with bilateral TKA, where one was CIM, and one was OTS, patients tended to prefer the CIM side [34].

It has to be noted that the available CIM knee implants differ greatly in terms of configuration, alignment strategy, and overall philosophy. The Origin® implant (Symbios, Yverdon les Bains, Switzerland) aims for constitutional alignment within predetermined limits, uses a single-block polyethylene Inlay, and provides individual patellofemoral configuration. In contrast, the ConforMIS® iTotalCR implant (ConforMIS Inc., Boston, Massachusetts, USA) aims to restore neutral mechanical alignment and uses two polyethylene Inlays [35]. Both systems offer a posterior stabilized version. Importantly, the notion of “fully restoring” knee kinematics with CIM implants might not be entirely accurate. First, it has to be noted, that there are different definitions (hip-knee-talus vs. hip-knee-calcaneus) of mechanical alignment [36]. Besides, when the anterior cruciate ligament (ACL) or both the ACL and posterior cruciate ligament (PCL) are resected and a cruciate-retaining (CR) or posterior-stabilized (PS) insert is used, paradoxical anterior gliding can still occur despite “restoring” bony anatomy. CIM implants, such as those by Symbios or ConforMIS, do not typically feature a medial-pivot (MP) design that could potentially address this issue.

This study has several limitations: Scores and patient satisfaction were only evaluated postoperatively and therefore change scores could not be calculated. Besides, there was no personal consultation of the patients, hence no assessment of ROM by a medical professional, and no follow-up imaging was performed. Furthermore, there was no specification of pain location in case of persistence. Knowing the exact location of pain, for instance anterior knee pain, might bring further clarity into the need for personalized patellofemoral joints since this is not provided by the implant studied. A strength of the study is the extended follow-up of at least five years, with an acceptable percentage of patients lost to follow-up (29.3%).