This systematic review revealed seven clinical studies that examined the OHRQoL of patients after SOT. Only one study each found the OHRQoL of SOT recipients to be different compared to patients before SOT and healthy controls. Accordingly, it is difficult to evaluate whether the OHRQoL of these patients is reduced or impaired by their oral and/or systemic conditions. The majority of studies applied the OHIP 14, a valid and well-established questionnaire-based measurement that evaluates 14 different functional and psychosocial impairments that patients perceived with respect to their oral cavity (Additional file 1: Supplementary Table 2) [7, 28, 29]. Due to cross-cultural differences between different OHIP translations [29] and differences in patient groups and health systems between different countries, the comparability of the included studies might be limited. While no international reference values are available, the German reference can be used to estimate whether there is a reduction in the OHRQoL of patients after SOT. For OHIP 14, a reference value between 0 and 4 out of 56 points, whereby higher values indicate worse OHRQoL, can be stated based on the dentition of patients [30]. Three of the included studies, which were performed in Germany, are within this reference [23,24,25]. Two other studies using OHIP 14 presented slightly higher scores [26, 27]. Another study applied the OHIP 49, which is the long version of OHIP 14, of which a score between 5 and 15 out of 196 points can be seen as a reference [31]. Accordingly, the reported OHIP 49 lies slightly higher than the reference [22]. Although a general statement regarding the OHRQoL of SOT recipients is limited due to different patient cohorts (different organs, countries, mean age, oral status), the OHRQoL of these patients appears not or at most slightly reduced.
Several issues need to be discussed in this context. In general, the physical oral health findings in relation to the patients’ perception of their oral conditions seem contradictory. In a Turkish study, which applied OHIP 14, the worst OHRQoL was found [27]. This study had the lowest mean age and lowest number of missing teeth out of all included studies (Table 1). However, age and tooth loss regularly affect OHRQoL [9, 32, 33]. Accordingly, the reduced OHRQoL of this cohort of Turkish patients is surprising and might indicate a perception of oral health situation, which is not in line with the clinical situation. Because this study did not examine associations between oral health and OHRQoL of the patients, this factor remains speculative. In contrast, four German studies found the OHRQoL to be independent of insufficient dental and periodontal status [23,24,25,26]. These four studies found a high periodontitis prevalence or treatment need (Table 1). In regular cases, this should lead to an impairment of OHRQoL [10, 34]. Accordingly, the findings also argue for a patient´s perception of the oral health situation, which is not in line with the clinical situation. However, the included studies did not report in detail the extent or severity of periodontal diseases (e.g., tooth loosening, pronounced recession, active inflammation). Only one study, performed in Finland, revealed associations between physical oral findings and OHRQoL [21]. Therefore, OHRQoL was assessed by a self-composed questionnaire (OHQS), including questions on last dental check-up, toothbrushing, smoking or dry mouth [21]. This questionnaire is not comparable to regular OHRQoL assessment with the OHIP, which assesses different functional and psychosocial impacts of the patient and not oral behaviour (Additional file 1: Table 2). The study by Segura-Saint-Gerons et al. found associations between OHIP 49 and oral behaviour (tooth brushing and dental visits) but not with physical oral health [22]. Accordingly, although this conclusion must be interpreted with caution, the OHRQoL of patients after SOT, which is not or only slightly reduced, appears to be mainly independent of physical oral health conditions.
Two studies found a relationship between general HRQoL and OHRQoL [21, 26]. In general, HRQoL and OHRQoL are closely related in generally healthy individuals, where OHRQoL can be seen as a subaspect of the whole HRQoL [35]. However, specific diseases might affect this relationship to an unclear extent [36]. Furthermore, HRQoL can also be directly associated with oral health conditions [6]. This effect has not yet been considered in examinations of SOT recipients. It is well described that the general HRQoL of SOT recipients is impaired and that different important emotional and psychosocial issues are of relevance [2, 4, 5]. The general burden related to SOT, medication, psychological issues (acceptance of transplant, perceived relationship to donor) and comorbidities might affect the perception of other areas of life, e.g., oral health and dental behaviour. Based on the missing association between OHRQoL and the physical oral situation on the one hand and the association between OHRQoL and HRQoL on the other hand, a shift in patients’ oral health perception can be hypothesized.
In this context, the “response shift theory”, as formulated by Sprangers and Schwartz in 1999, can be quoted [37]. This phenomenon describes a cognitive change in patients with severe chronic diseases, which leads to a postponement of the internal standard due to the accommodation of the status “chronic disease” [37]. Of course, the strict transferability of this model in the context of the OHRQoL of SOT recipients is limited; assessment and interpretation of the “response shift” phenomenon is difficult [38, 39]. Previously, “response shift” assessment was applied in the context of dentistry, especially with regard to prosthodontic rehabilitation, assessment of perceived treatment effects or dentine hypersensitivity [40,41,42]. However, these deliberations were only focused on oral disease or dental therapy measures. Based on the findings of this systematic review, it might be conceivable that the accommodation of the chronically diseased status of SOT recipients might affect their oral health perception. This might lead to a shift in the perception threshold for impairment in OHRQoL and possibly HRQoL caused by oral diseases and conditions. Therefore, patients might not perceive oral diseases, such as chronic periodontitis or several missing teeth, as impairments in their OHRQoL and might be affected only if acute dental issues, such as pain or extended tooth loss, appear (Fig. 2).
Of course, this is just a hypothesis based on the nearly unaffected OHRQoL of SOT recipients independently of their oral status. However, this might be of high practical relevance for dental care. If a patient does not feel impairment of his/her oral condition, this patient might not see the necessity to visit the dentist or to increase oral hygiene behaviour. This would explain the poor clinical oral health conditions and reduced oral health behaviour, i.e., the low use of interdental cleaning devices and a switch from control- to complaint-oriented dental behaviour of SOT recipients [12,13,14,15,16]. However, early dental rehabilitation and sufficient maintenance of SOT recipients are necessary to reduce the risk of systemic infections related to their lifelong immunosuppressive medication [1, 17, 18]. This early and prevention-oriented dental care seems to not work yet [43]. Therefore, multidisciplinary oral care appears necessary. Based on the current findings and formed hypothesis, the interdisciplinary team should include dental staff and transplant centres as well as psychological teams to build awareness of the importance of healthy oral conditions for these patients (Fig. 3).
This is the first systematic review on the OHRQoL of SOT recipients. It was executed according to the PRISMA statement [19] by two independently operating individuals. While based on the search findings, the clinically relevant hypothesis of a phenomenon, which is similar to a “response shift”, could be formed, several general methodological issues of the included studies should be recognized. The included studies had certain heterogeneity regarding country, transplanted organs, age, oral examinations and OHRQoL measurements. This is important because a direct comparison between the different organ groups is not possible. However, this is the first systematic insight into the perception of OHRQoL by recipients of different SOT, which revealed common findings of clinical relevance. Because few data are available, the focus on one single organ group currently does not make sense and justifies including the heterogeneous group of different SOT recipients. It is known that oral diseases regularly affect OHRQoL [9,10,11]. To assess the real influence of oral conditions on the OHRQoL of SOT recipients, profound oral examinations, including the extent and severity of physical oral health impairment, might be necessary. This might include the severity and activity of periodontitis considering the new classification [44] or the number of remaining functional occlusal pairs instead of only assessing the number of missing teeth [9]. Furthermore, standardized and validated instruments, e.g., OHIP 14, should be applied, and future studies should aim to reveal reference values for SOT recipients. Only one study reported on the validity of the OHIP for SOT recipients [27], which should be extended in future research in the field. The reporting and analysis of different subscales, such as oral function, psychosocial impact, pain or orofacial appearance, might increase the understanding of individual patient cases [8]. Moreover, HRQoL and disease-related parameters as well as psychological issues, such as anxiety and/or depression, need to be considered to allow a complex understanding of the OHRQoL of these patients. Furthermore, longitudinal studies are needed to prove the hypothesis of a “response shift”, for which valid methods should be used [38, 39]. In general, assessment of the OHRQoL of patients after SOT can help understand the complexity of this patient group and to develop dental special care, which could allow successful, patient-oriented and multidisciplinary therapy and disease prevention.
