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Incidence and management of condylar resorption after orthognathic surgery: An overview

Objective Condylar resorption (CR) is one of the major post-surgical complications of orthognathic surgery. This systematic review (SR) aimed to evaluate epidemiological data, risk factors, and therapeutical management of CR. Methods Six databases were screened by two investigators until September 2020 to obtain all SRs. After reading the titles and abstracts, eligible SRs were determined and data extraction was performed. Using the latest version of A Measurement Tool to Assess Systematic Reviews, the methodological quality of the included SRs was determined. Results Ten SRs with low or critically-low methodological quality were included in this review. Mandibular hypoplasia on the sagittal plane and hyperdivergent growth pattern on the vertical plane were the most common skeletal alterations in which CR could occur after orthognathic surgery. Post-operative condylar changes were analyzed both on two-dimensional and three-dimensional (3D) radiographic examinations. The incidence of CR was not related to the fixation method. Based on the severity of the pathological conditions, management of CR can include conservative or surgical therapy. Conclusions Despite the limited evidence in literature, CR is considered a consequence of orthognathic surgery. However, an accurate diagnosis of CR and a better orthognathic surgical planning must include 3D radiographic examinations to improve pre- and post-surgical comparison. Well-designed studies with long-term follow-up and 3D data are needed to clarify the findings of this analysis..


INTRODUCTION
For patients undergoing orthognathic surgery, several surgical risks with different severity rates must be considered. 1 Condylar resorption (CR) is an uncommon but well-known clinical condition that can occur after surgical repositioning of the jaws. 2,3 It represents the advanced stage of condylar remodeling, irreversibly affecting the temporomandibular joint (TMJ). [3][4][5] As a result of mechanical load, the adaptive mechanisms that normally allow to balance bone resorption and bone formation on the condylar surface are completely lost, resulting in morphological alterations of the condylar structure. 5,6 Microstructural changes include a significantly reduced condylar volume and a pathological deformed profile on the condylar surface. [7][8][9] External changes have a significant influence on aesthetics and may include reduced posterior facial height, mandibular retrusion, and anterior open bite, especially if bilateral CRs occur. 7,10 Based on the etiology, CR can be distinguished into primary and secondary CR. 8 Primary CR is diagnosed when CR exists without a well-known cause. 8,11 Conversely, both local (trauma, inflammation, infection, etc.) and systemic (rheumatic immune pathologies, steroid therapy, etc.) agents can be etiological factors of secondary CR. 6 Referring to orthognathic surgery, all osteotomies can lead to CR, which can occur in both monomaxillary and bi-maxillary surgery. 2,12 Bilateral sagittal split osteotomy (BSSO) and/or Le Fort I osteotomy can modify the mechanical forces on the condylar surface, resulting in CR. 12 Several studies have focused on condylar alterations after orthognathic surgery, and some systematic reviews (SRs) aimed to determine the most crucial risk factors for CR. However, the different methodological approaches do not allow to draw firm deductions. The rationale of this overview was to summarize the most recent knowledge on CR after orthognathic surgery (CROS), emphasizing the limitations of previous studies in order to critically evaluate their conclusions and identify the gaps in knowledge that should be filled by future researches with adequate protocols.
This study aimed to systematically review all SRs and meta-analyses (MAs) to analyze CROS in order to provide evidence-based information about epidemiological data, risk factors, and management of CR, evaluating each outcome in correlation with the qualitative methodological analysis of the included reviews.

MATERIALS AND METHODS
All SRs were systematically reviewed according to the Population, Intervention, Comparison, Outcomes, and Study criteria to assess CR in patients who underwent orthognathic surgery.

Search strategy
Six databases were investigated: PubMed, Cochrane Library, Google Scholar, Scopus, LILACS, and Web of Science. The electronic search was conducted until September 2020, using keywords and MeSH terms connected by the Boolean operator "AND." The PubMed search combined the following term sequence: "orthognathic surgical procedures"[Mesh] AND "condylar resorption" AND "systematic review" and "condylar resorption" AND "orthognathic surgery" AND "systematic review." The other search databases combined the following term sequence: "orthognathic surgery" AND "condylar resorption" AND "systematic review." Additionally, a manual search was conducted in the reference lists of the selected SRs. No restriction of language or publication date was imposed.

Review selection
The electronic search was independently conducted by two investigators (SB and GC), screening titles and abstracts in parallel to evaluate the reviews for eligibility. In case of missing information, full-text reading was necessary for a final decision. A third author (AG) discussed and resolved any discrepancies between the two authors. SRs and MAs were included, which allowed for the extraction of data on CROS. Narrative reviews, overview of reviews, duplicate articles, studies with no evaluation of condylar morphology, and studies on orthognathic surgery of syndromic patients or cleft lip and palate patients were excluded. The Cohen's kappa coefficient (κ) was calculated to determine the inter-rater agreement between the two investigators (SB and GC).

Data extraction
The data extraction from the eligible reviews was independently performed by the same two investigators (SB and GC), recording the following information: author, publication date, study design (SR or MA), number of included studies, number of included patients, dentoskeletal malocclusion, type of intervention, methodological data, quality assessment of primary studies, outcomes, results of reviews, and author's conclusion.

Assessment of methodological quality
The methodological quality of each SR was independently assessed by the two investigators (SB and GC) using the latest version of A Measurement Tool to Assess Systematic Reviews (AMSTAR-2). 13

Condylar resorption
Nine of the included SRs aimed to describe the main characteristics and risk factors of CROS (Table 3). 7,[15][16][17][18][19][20][21][22] Jędrzejewski et al. 19 reported that CR manifestations usually ranged from 6 months to 2 years after surgical treatment, but Catherine et al. 16 extended this time to 6 years post-operatively. Although with different diagnostic methods, CR could be identified by both radiographic and clinical signs. 8,16,20 Further, CR was recognized by 2D radiological examinations if there was a reduction of ramus or condylar height, with a minimum vertical decrease of 2 mm. 20 A 3D analysis of the condylar profile assessed that a reduction of at least 17% of the condylar volume was considered as the cut-off value for the diagnosis of CR. 8 Clinical signs of bilateral CR included an anterior open bite or a Class II malocclusion with retrognathia. 16 On the contrary, an active process of unilateral CR could identify mandibular asymmetry with an ipsilateral Class II malocclusion and a contralateral open bite. 16 There is no agreement among the included SRs about the role of sex or age in CROS. 7,[15][16][17][18][19][20][21][22] Five SRs reported that females showed a higher risk for CR than males, with an approximate female-to-male ratio of 5:1. 16,17,[19][20][21]  -Le Fort I osteotomy: from 6.7% to 9.5% of cases Fixation method: -Rigid fixation (from 6.3% to 13% of cases); -Wire fixation (9.9% of cases) What Radiological signs: -A vertical decrease of 2 mm or more of the ramus or the condylar height; -A reduction of at least 17% of the condylar volume It was estimated that 60% of cases with CROS developed a resorption that ranged between 10 and 19% of the condylar surface, 30% of cases showed CROS ranging between 20 and 29%, and 10% of cases had CROS greater than 30%. Two SRs stated that young patients may be more prone to CR, while de Moraes et al. 17 concluded that age could not be correlated with post-operative CR. 16,17,21 In terms of pre-surgical evaluation, it was mandatory to exclude an active process of condylar atrophy, the presence of severe temporomandibular disorders, and an intra-articular inflammatory damage. 8,[16][17][18] Risk factors for CR were evaluated both in the sagittal and vertical planes. 7,[15][16][17][18][19][20][21][22] Angle Class II with mandibular hypoplasia was the most common malocclusion in which CR occurred after a significant mandibular advancement (incidence rate: 1.4-31%). 7,15-22 An orthognathic plan with advancements greater than 10 mm may be a risk factor for CR. 16 Furthermore, in Angle Class II malocclusions, micrognathia often involved condylar morphology characterized by a small volume, a reduced adaptive capacity to external load, and an increased risk of CR. 8 Only one SR reported a more prone CR in Angle Class III malocclusion after a surgical mandibular setback greater than 6 mm. 8 On the vertical plane, a hyperdivergent skeletal pattern with an anterior open bite, a low posterior-to-anterior facial height, and an increased mandibular plane angle (MPA) could cause CR. 7,[15][16][17][18][19][20][21][22] However, the entity to define the critical clockwise rotation of the mandible was not unanimously accepted, but it could be described when the MPA was greater than 40°. 16,18 In patients with a dolichofacial profile, a presurgical posterior condylar inclination was exposed to CR because the antero-superior surface and the laterosuperior area were subjected to an excessive load on the sagittal and coronal planes, respectively. 7,15-22

Fixation method
Six of the included SRs evaluated the type of fixation in relation to CR. 8,[16][17][18][19]21 Wire fixation, rigid fixation (bicortical miniscrew and miniplates), and intermaxillary fixation (IMF) are reported as the possible options. 16,17 The incidence of CROS or surgical relapse was not affected by the fixation method in 6.3-13% of the cases. 16,17,21 However, a majority of the authors recorded a high risk of CROS after a prolonged IMF rather than a rigid fixation, while no difference was found between wire and rigid fixation. 8,18,19 Although no studies have compared the two types of rigid fixation, Jędrzejewski et al. 19 reported that bicortical miniscrews could determine inferior alveolar nerve damage.

Management of condylar resorption
Two included SRs reported the main therapeutic approaches for CR. 8,16 Despite the fact that there are no precise guidelines on the management of CR to date, several suggestions were made, differentiating the preand post-surgical phases of evaluation. 8 In the preoperative phase, the aim was to annul or limit the risk factors. 8 In the post-surgical phase, the objectives included the treatment of inflammation and pain, as well as the correction of skeletal deformities and occlusal instability. 8,16 Conservative or surgical treatment were proposed. 8,16 Conservative therapy managed to avoid CR progression, stabilizing the pathologic conditions without improving dentofacial deformities. 8,16 It included anti-inflammatory drugs, occlusal splints, physiotherapy, and in some cases, orthodontic or restorative treatments. 8,16 Surgical treatment should be performed at least 6 months after orthognathic surgery, considering the severity of CR, the surgeon's experience, and the patient's intentions. 8 Disc repositioning, condylectomy, chondro-costal graft, or complete prosthetic TMJ reconstruction were more invasive therapeutic options that were reported as possible treatments for CR. 8,16

DISCUSSION
The purpose of this review was to examine the current evidence of CROS, relating the methodological analysis of each SR included in this study. CR has been recognized as one of the major post-surgical complications occurring in orthognathic patients. 1 It can be considered an irreversible correspondence of the physiological condylar remodeling, characterized by a severe impairment of condylar morphology with or without clinical symptoms. 5 In this overview, a population of patients with skeletal deformities who had undergone orthognathic surgery was analyzed, excluding the SRs of syndromic or cleft lip and palate patients in whom different heterogeneous comorbidities can be associated with skeletal malformations. 23 This SR aimed to answer the following questions: "Can CR be considered a severe complication of orthognathic surgery?;" "What are the main risk factors for CROS?;" and "How can CROS be managed better?" Ten SRs were included in this overview. 7,8,[15][16][17][18][19][20][21][22] Quantitative analysis could be reported because no MAs were performed. 7,8,[15][16][17][18][19][20][21][22] For each SR, the qualitative analysis recorded the final score of AMSTAR-2 obtained by the evaluation of its 16 items. 13 Although the absence of MA precluded the evaluation of some critical domains, the methodological assessment of the included SRs ranged between low and critically-low quality. 7,8,[15][16][17][18][19][20][21][22] A comprehensive search strategy (item 4) and adequate details for the excluded studies (item 7) were the most common missing data in the AMSTAR-2 checklist.
The qualitative evaluation of the included SRs allowed to analyze the main characteristics of CROS, summarizing the most significant risk factors and better management reported in literature. CR related to orthognathic surgery can occur 6 months to 6 years after the surgical treatment, and both clinical and radiological signs https://doi.org/10.4041/kjod.2022.52.1.29 should be examined. It was not unanimously accepted if sex or age could influence this pathological condition. However, young female patients seem to be more prone to CROS. 7,[15][16][17][18][19][20][21][22] Despite the limited evidence in literature, altered estrogen levels could interfere with the post-operative morphological changes occurring on the condylar surface. 24 An increased inflammatory process negatively involves the synovial tissue and fibrocartilage synthesis, predisposing to TMJ instability and bone resorption. 25 CR may expose the patient to skeletal relapse, resulting in an anterior open bite and a Class II dental relationship for mandibular clockwise rotation if it occurs bilaterally. 16 Similarly, unilateral CR could be recognized by a contralateral open bite and an ipsilateral Class II malocclusion, emphasizing an asymmetric profile of the mandible. 16 These clinical manifestations are consequent to CR because suitable joint relationships are required to allow mandibular functions, even with altered condylar morphology. 21 Despite the low quality of evidence, all SRs identified Angle Class II malocclusion as the most frequent skeletal deformity in which CROS occurs. 7,15-22 Surgical treatment of mandibular hypoplasia aimed to obtain a stable advancement of the jaw without post-operative relapse. An orthognathic plan with a mandibular advancement greater than 10 mm should be considered with caution because it represents a risk factor for CR. 16 After surgery, backward suprahyoid muscle forces and reduced condylar volume are the most common problems in Class II patients, which decrease the adaptive capacity to external load, favor irreversible condylar alterations, and predispose to relapse. 21,25,26 Despite the low qualitative assessment of the included SRs, hyperdivergent facial type was recorded as a significant risk factor for CROS on the vertical plane. 7,[15][16][17][18][19][20][21][22] Concerning the clockwise rotation of the mandible, the specific cut-off for CR was not unanimously accepted, but an MPA greater than 40° was reported as a critical value. 16,18 Major condylar changes were emphasized in patients with a lower posterior-to-anterior facial height ratio due to a significant increase in perimandibular tissue stretching after surgery. 27 Furthermore, hyperdivergent facial type is often associated with a pre-operative posterior inclination of the condyle. 7,15-22 A large surgical advancement of the mandible with an important counter-clockwise rotation causes excessive forces on the antero-superior and latero-superior areas of the condylar head, predisposing to CROS because unbalanced load occurs on less dense surfaces. 8,20 Referring to the management of CR, two SRs described the main treatment options for CROS. 8,16 A conservative therapeutic approach aimed to stop the pathologic progression, stabilizing the clinical conditions with anti-inflammatory drugs, occlusal splint, physiotherapy, and orthodontic or restorative treatments. 8,16,27 Surgical therapy is reserved for severe cases of CROS, and it should be performed no sooner than 6 months after orthognathic surgery. 8,28 The goal of the surgical options is to improve the clinical situation, correct the post-operative relapse, and restore an adequate condylar morphology. 8,16,25 However, a reoperation should be considered with caution, evaluating the pathological progression, and should be meticulously planned to avoid worsening of the disease. 8 In this overview, the main limitation of the conclusions relates to the presence of SRs with low or critically-low methodological quality. All reported results should be interpreted considering an imprecise search strategy of the studies and a limited methodological assessment of CROS. Most of the SRs included studies that evaluated CR on 2D radiographs, but 3D analysis of condylar profile is fundamental to obtain more specific information about the trend and severity of CR. 7,20,29,30 As reported in literature, a reduction of at least 17% of the condylar volume was considered a significant radiological sign of active CR. 7,8,30 To date, orthognathic surgery has been a precise surgical procedure with a pre-operative 3D planning. 14 An accurate pre-surgical 3D morphometric analysis of the condyles could improve the treatment plan and avoid post-operative relapses, allowing to maximize safe movements and minimize risky ones. The extent of the jaws' movements, direction, and rotational component could be influenced by the pre-surgical condition of the condyle by evaluating the possible degree of compromise. For this reason, the pre-operative assessment of condylar morphology could also be performed on 3D scans, excluding an active process of condylar atrophy or intra-articular inflammation. Because a post-surgical 3D examination was often required, a pre-operative and post-operative comparison of the condylar surface could be achieved, allowing to recognize important details for the diagnosis and entity of CROS. 7,30 CONCLUSION CR is a possible consequence of orthognathic surgery, with an incidence rate of 1-31%. Mandibular deficiency, female hyperdivergent patients, and pre-surgical condylar alterations were the most reported risk factors for CROS. To limit inflammatory processes and improve clinical conditions, an adequate therapeutic approach involves conservative or surgical treatment, considering the severity of CR and disease progression. Although many advancements have been made with the application of 3D technologies, it is still difficult to find a standardized method for intraoperative condylar repositioning. To date, scientific evidence is scarce and limited in order to draw certain conclusions. Future perspectives https://doi.org/10.4041/kjod.2022.52.1.29 must include improvements in both the diagnostic and prognostic phases. An accurate diagnosis must be based on 3D data after an adequate standardization of the radiological characteristics of CR has been defined. Additionally, well-designed RCTs with long-term follow-up and pre-and post-operative 3D comparisons of condylar surfaces should be implemented to confirm the findings of this review.

CONFLICTS OF INTEREST
No potential conflict of interest relevant to this article was reported.