Centric relation: A matter of form and substance

Abstract The recent review article by Zonnenberg, Türp and Greene ‘Centric relation critically revisited – What are the clinical implications’? opens an important debate by addressing topics of central relevance in Dentistry, namely the relationship between occlusion and the condyle‐to‐glenoid‐fossa position, and the need for diagnostic assessment and therapeutic alteration of the condylar position in orthodontic patients. Zonnenberg, Türp and Greene concluded that the mandibular condyle is correctly situated in most orthodontic patients. Thus, in their view, orthodontists can disregard this aspect during treatment, and rely on the plastic properties of the masticatory supporting structures, while aiming at finishing the cases in a good occlusal relationship. We think that this approach fails to consider that biological variation of the stomatognathic structures can also be pathological and that, as dental occlusion determines condylar relative position within the glenoid fossa, changes in the occlusion are likely to alter the original condylar‐to‐glenoid‐fossa relation. Hence, we claim that whenever the occlusal relationship must be changed, the clinician should carefully monitor the condyle position and the mandibular function to prevent possible iatrogenic effects. To advance the discourse on the topic, we invite Zonnenberg, Türp and Greene to clarify their definition of ‘average patient’ and their interpretation of ‘full‐mouth orthodontic and orthognathic treatment’, their understanding of ‘biologically acceptable condylar relationship’, their justification of maximum intercuspation as reference position, the extent to which they think it is safe to rely on the TMJ resilience, and finally their alternative to centric relation in the treatment of patients needing condylar repositioning.

that 'If, however, the dentist accepts an existing MIP [maximum intercuspation]-determined jaw relationship as being biologically acceptable for the vast majority of healthy dentate patients, there would be no need to conduct such assessments [of condylar position] as a part of routine examinations of the stomatognathic system' (p. 1053). A patent confusion between what is biological (namely, occurring in nature, or phenotypically expressed in human beings) and what is clinically normal is apparent in this statement.
Advancements in food technology in modern societies have decreased the selective pressure on the human masticatory system resulting in a high degree of morphological variation of the orofacial structures, or malocclusion. 2 This fact must be acknowledged, but whether a condition is clinically acceptable should be determined by the practitioner based on an accurate and thorough assessment of the patient's signs and symptoms. Suggesting that the presenting condylar position in dentate patients is unquestionably correct because it is biologically determined is dismissive of the fact that biological variation (of the orofacial structures) can be pathological.
On the other hand, it should not be assumed that a conscientious dentist would administer unnecessary diagnostic procedures to 'healthy dentate patients'.
Zonnenberg, Türp and Greene 1 continued along the same line (p. 1053): '… we should acknowledge that the average person will have a stable, repeatable and functional MIP [maximum intercuspation] that determines where the condyles and discs are located on their articular eminences. Therefore, no special assessment of the mandibular position needs to be carried out in these subjects'. The vagueness of the expression 'average person' might lead clinicians to the wrong understanding that it is safe to operate under the assumption that condylar position is functionally correct in most people, thereby neglecting a proper diagnosis in these patients. In fact, they acknowledged that 'these so-called discrepancies [namely the sliding between CR and MIP] are found within the vast majority of the normal population, which strongly suggests that they are a normal feature of intermaxillary relationships'. This statement emphasises the high variation of the occlusal relationship found in modern humans. Despite occlusal deviations having become 'normal' (i.e. frequent) in urbanised societies, it cannot be stated that, consequently, they are also clinically or functionally normal. Since Zonnenberg, Türp and Greene 1 did not support their statement with relevant references, we refer to Pullinger, Seligman and Gornbein 3 who showed that indeed a cut-off value for sagittal occlusal slide length can be found, corresponding to 2 mm, based on their sample. In fact, none of their asymptomatic subjects had a slide longer than 2 mm, while only 6% of them had slide length between 1 and 2 mm.  On p. 1053, it is reported that The term "CR" is conceptually flawed because it is based upon the assumption that there is a place where condyles "should be". While the position of the condyles within the fossae varies greatly in modern humans and changes through the lifespan of an individual, it has been demonstrated that TMJ skeletal morphology is associated with disc derangement dysfunctions. 4 Thus, the relative position of the condyles within the glenoid fossa has functional implications, and it also seems that small changes in this relationship deriving from occlusal treatment result in an improvement of the symptoms in patients with temporomandibular disorders. 5 Accordingly, it is reasonable to assume that changes of the condylar position might also negatively affect the health of the masticatory organ.
In summary, to bring clarity into the debate, Zonnenberg, Türp and Greene 1 are invited to clarify: 1. What they mean with average patient and which morphological, functional, diagnostic, demographic, criteria they use to define the average orthodontic patient.

ACK N OWLED G EM ENTS
We dedicate this manuscript to Rudolf Slavicek (1928Slavicek ( -2022 who masterfully contributed to the understanding of the stomatognathic system. We thank Christian Slavicek for insightful discussion during the preparation of this manuscript. Two anonymous reviewers helped us improve this contribution.

CO N FLI C T O F I NTE R E S T
The authors declare no conflicts of interest.

AUTH O R S ' CO NTR I B UTI O N S
CF, IT and KP made substantial contributions to the conception of the manuscript. All authors made substantial contributions to the design of the manuscript. CF wrote the paper and all authors critically revised it. All authors approved of the version to be published and agreed to be accountable for all aspects of the work.

PEER R E V I E W
The peer review history for this article is available at https://publo ns.com/publo n/10.1111/joor.13329.

DATA AVA I L A B I L I T Y S TAT E M E N T
Data sharing is not applicable to this article, as no new data were created or analysed in this study.