The Bone Lid Technique in Oral and Maxillofacial Surgery: A Scoping Review

This scoping review aimed at reporting the outcomes of the bone lid technique in oral surgery in terms of bone healing, ridge preservation, and incidence of complications. Bone-cutting instruments and stabilization methods were also considered. PubMed, Scopus, and the Cochrane Register of Controlled Trials were searched using a combination of terms, including bone lid, bony window, piezosurgery, microsaw, cysts, endodontic surgery, impacted teeth, and maxillary sinus. A hand search was also performed. The last search was conducted on 30 November 2021. No date limitation was set. Searches were restricted to human clinical studies published in English. All types of study design were considered except reviews and case reports. After a two-step evaluation, 20 (2 randomized studies, 2 case-control studies, 3 cohort studies, 13 case series) out of 647 screened studies were included, reporting on 752 bone lid procedures. The bone lid technique was associated with favorable bone healing when compared to other methods, and with a very low incidence of major complications. Clinical indications, surgical procedures, study design, follow-up duration, and outcomes varied among the studies. Overall, favorable outcomes were reported using the bone lid approach, though evidence-based studies were scarce.


Introduction
The bone lid technique consists of the preparation and removal of a bone lid or window that is replaced in its original position at the end of the surgery. The aims of the technique are to achieve a valid exposure of the surgical target, to save bone otherwise lost with other more aggressive methods (ostectomy), and to improve bone healing. This technique was firstly described for the closure following opening of the maxillary antrum and for endodontic surgical treatment of lower molars, with good results [1].
The indications for this technique were then extended to the enucleation of cysts [2][3][4] and other benign lesions [5,6]. Other applications include the extraction of deeply fractured roots or impacted teeth [7,8], the removal of fracture or failed implants [9,10], the retrieval of Implants; International Journal of Oral and Maxillofacial Surgery; Journal of Oral and Maxillofacial Surgery; Journal of Oral Implantology; and Oral Surgery, Oral Medicine, Oral Pathology, Oral Radiology and Endodontology. The reference list of the retrieved reviews and of the included studies was also searched for possible additional eligible studies not identified by other search methods.  ("bone lid" OR "bony lid" OR "bony window") AND (technique OR piezosurgery OR microsaw OR micro-saw OR "bone defect" OR "bone defects" OR "oral surgery" OR maxilla OR mandible OR "maxillary sinus" OR apicectomy OR "apical surgery" OR "endodontic surgery" OR "root-end surgery" OR implant OR implants OR "impacted teeth" OR "impacted tooth" OR "impacted molar" OR "impacted molars" OR "third molar" OR "third molars" OR "inferior alveolar nerve" OR "inferior alveolar nerves" OR cyst OR cysts OR "cystic lesion" OR "cystic lesions" OR "computer-guided" OR "osteoplastic procedure" OR "osteoplastic procedures" OR "sinus surgeries") AND (LIMIT-TO (LANGUAGE, "English")) Cochrane Central Register of Controlled Trials (CENTRAL) ("bone lid" OR "bony lid" OR "bony window") AND (technique OR piezosurgery OR microsaw OR micro-saw OR "bone defect" OR "bone defects" OR "oral surgery" OR maxilla OR mandible OR "maxillary sinus" OR apicectomy OR "apical surgery" OR "endodontic surgery" OR "root-end surgery" OR implant OR implants OR "impacted teeth" OR "impacted tooth" OR "impacted molar" OR "impacted molars" OR "third molar" OR "third molars" OR "inferior alveolar nerve" OR "inferior alveolar nerves" OR cyst OR cysts OR "cystic lesion" OR "cystic lesions" OR "computer-guided" OR "osteoplastic procedure" OR "osteoplastic procedures" OR "sinus surgeries") in Title Abstract Keyword-in Trials

Inclusion Criteria
To be included, studies had to report clinical results of oral surgery procedures in which the bone lid technique was used to cover and protect the healing site in order to improve the clinical and radiographic outcome.
The search was limited to clinical studies reporting on at least 10 cases of the bone lid technique published in the English language involving human subjects. Both prospective and retrospective studies were included. The studies had to provide details on the type of clinical application, the patients' selection criteria, the procedure for applying the bone lid, the duration of the follow-up, and the number and type of complications. They also had to provide clear definitions of the clinical and/or radiographic outcomes used to assess the success or failure of the procedure.
Publications that did not meet the above inclusion criteria and those that did not deal with original clinical cases (e.g., reviews and technical reports) were excluded. Papers in which the bone lid technique was applied in combination with maxillary sinus augmentation were excluded. Multiple publications of the same pool of patients were also excluded. When papers from the same group of authors with very similar databases of patients, materials, methods, and outcomes were identified, the authors were contacted to clarify whether the pool of patients was indeed the same. In case of multiple publications relative to consecutive phases of the same study or to enlargements of the original sample size, only the most recent data (those with the longer follow-up and the larger sample size) were considered.

Selection of the Studies
Two reviewers (G.B. and L.S.) independently screened the titles and the abstracts of the articles initially retrieved through the electronic search. The concordance between reviewers was assessed by means of the Cohen's Kappa coefficient. In case of disagreement, a joint decision was made through discussion, or by consulting a third reviewer (M.D.F.). The full text of all studies of possible relevance were independently assessed by the same two reviewers to check if they met all inclusion criteria. For articles excluded at this stage, the reason for exclusion was recorded. The included studies were divided according to the type of clinical application: endodontic surgery, access to mandibular lesions and impacted teeth, implant explantation, access to the maxillary sinus, and other indications.

Data Charting
Data were extracted by two reviewers independently (G.B. and L.S.). Cases of disagreement were subject to joint evaluation until an agreement was reached. In case of doubts, a third reviewer was consulted (M.D.F.).
The main variables extracted from each included study were the following: study design, sample size, number of surgeons involved; patients' genders and ages, proportion of smokers, jaw (maxilla or mandible), bone-cutting devices, fixation method, any outcome variable used to evaluate treatment success, follow-up duration, number and type of complications and time they occurred, and the quality of life of patients as well as their satisfaction, as assessed by means of questionnaires or interviews.
The following methodological parameters were also recorded: selection of participants, sample size (the risk of bias was assumed to be low, medium, or high if >50, 10-50, or <10 patients were treated, respectively), length of follow-up period (it was assumed to be low, medium, or high if the mean follow-up duration was >5 years, 1-5 years, or <1 year, respectively), dropouts (it was assumed to be low, medium, or high if dropouts were <5%, 5-15%, or >15%, respectively), measurement of the outcome, and selection of reported results.
The methodological quality of the selected studies was evaluated independently and in duplicate by two reviewers (M.D.F. and S.P.). The tool reported in the Cochrane Handbook for Systematic Reviews of Interventions, version 5.1.0 was used for RCTs [30], and a modified ROBINS-I ("Risk Of Bias In Non-randomised Studies-of Interventions") tool was used for non-randomized studies [31]. All the criteria were assessed as low, moderate (uncertain), or high. The authors of the included studies were contacted to provide clarifications or missing information as needed. Studies were considered to have a low risk of bias (RoB) (green) if more than 2/3 of the parameters were judged as "low" and none as "high"; they were considered to have a moderate RoB (yellow) with 1 to 4 parameters judged as "low" and the rest as "moderate", with none at high risk. All papers with at least one score at high risk were classified as having high RoB (red).

Synthesis of Results
Descriptive statistics of the included studies were recorded by summarizing the total number of patients and cases treated with the bone lid technique, as well as the postsurgical adverse events for each surgery procedure considered.

Results
A flowchart summarizing the screening process is presented in Figure 1. The electronic search yielded a total of 610 articles. Thirty-seven additional articles were found by handsearching. After a first screening of the titles and abstracts, a total of 41 articles reporting results of clinical studies on patients that underwent oral surgery procedures in combination with the use of the bone lid technique were selected. After evaluation of the full-text of these articles, 21 of them were excluded [7,8,13,16,[23][24][25][32][33][34][35][36][37][38][39][40][41][42][43][44][45]. The reasons for exclusion are listed in Table 2. A total of 20 studies published in the years 1984-2021 were included. The kappa values for inter-reviewer agreement were 0.95 for both the title/abstract selection and full-text articles, thus indicating an almost perfect agreement between the two independent reviewers. Since a sufficient number of homogeneous studies to be aggregated could not be found, a quantitative analysis was not undertaken.
(uncertain), or high. The authors of the included studies were contacted to fications or missing information as needed. Studies were considered to hav bias (RoB) (green) if more than 2/3 of the parameters were judged as "low "high"; they were considered to have a moderate RoB (yellow) with 1 to judged as "low" and the rest as "moderate", with none at high risk. All p least one score at high risk were classified as having high RoB (red).

Synthesis of Results
Descriptive statistics of the included studies were recorded by summar number of patients and cases treated with the bone lid technique, as well a gical adverse events for each surgery procedure considered.

Results
A flowchart summarizing the screening process is presented in Figu tronic search yielded a total of 610 articles. Thirty-seven additional articles w hand-searching. After a first screening of the titles and abstracts, a total of porting results of clinical studies on patients that underwent oral surgery combination with the use of the bone lid technique were selected. After eva full-text of these articles, 21 of them were excluded [7,8,13,16,[23][24][25][32][33][34][35][36][37][38][39][40][41][42][43][44][45] for exclusion are listed in Table 2. A total of 20 studies published in the ye were included. The kappa values for inter-reviewer agreement were 0.95 f tle/abstract selection and full-text articles, thus indicating an almost perfect tween the two independent reviewers. Since a sufficient number of homoge to be aggregated could not be found, a quantitative analysis was not under   The main features of the included studies are shown in Table 3 (i.e., study design, clinical indication, sample size, patients' genders and ages, jaw, number of bone lid cases, follow-up duration, osteotomy technique, and fixation method).   (14); wire (7); resorbable sutures (6); mix (2) A total of 2 RCTs (reporting on a total of 60 bone lid patients), 2 longitudinal prospective cohort studies (218 bone lid patients), 1 retrospective cohort study (30 bone lid patients), 2 case-control studies (56 bone lid patients), and 13 case series (371 bone lid patients) were found. The included studies reported on a total of 838 patients, of which 735 were treated with the bone lid approach (752 bone lids performed).
The number of clinicians who performed the surgeries was reported in nine studies. In detail, in six studies the surgeries were carried out by a single clinician [2,9,26,47,49,50], whereas only one study involved two operators [14]. One paper reported that five clinicians performed the surgeries [3], while in another study multiple surgeons provided the treatments [56].
Only 2 out of 20 studies provided detailed information on smokers. In a randomized clinical trial comparing piezoelectric surgery to the conventional surgery (rotatory instruments) in mandibular cyst enucleation, only non-smokers were included [2]. In a retrospective cohort study, approximately one third of the included patients were smokers, and smoking habit was found to be a risk factor for bone lid necrosis (p = 0.005). Indeed, necrosis was observed only in smokers [46].
In half of the included papers, the surgical procedures were performed under local anesthesia, while in the other three articles, they were also completed in combination with conscious sedation [4,46] or general anesthesia [17,46]. In three studies, the surgeries were carried out either under general or local anesthesia [48,52,54]. The remaining four articles reported no information regarding the type of anesthesia [3,51,53,56].
As shown in Table 3, in all the included papers except one [3], details on the bonecutting tools were provided. Piezosurgery and microsaws were the most frequently reported cutting methods. Rotary instruments, sometimes in combination with chisels, were also used to create the bony windows. The majority of neuronal complications were reported when disks and burs were utilized [1,25,47,48,54,55], whereas only one case of permanent paresthesia occurred with piezosurgery [4].
The stabilization of the bone lid was achieved without the need of rigid fixation in three studies. Bone lids were stabilized with mini-/microplates and screws in four articles, while the use of resorbable sutures was reported for all cases in four studies in the maxilla. Other fixation methods included resorbable miniplates and screws (one study). In the remaining eight articles, multiple fixation methods were reported. Bone lid resorption, necrosis, and removal were rather infrequent, as only 20 cases were found out of 752, of which 5 were in the maxilla [26,51,53] and 12 were in the mandible [1,3,4,46,48], while it was not specified in 3 cases [9]. In 5 cases out of 17, bone lid complications were clearly associated with suture stabilization or no fixation [1,48,53], 2 with rigid fixation methods [4,26], and 3 with resorbable plates [46]; whereas in 10 cases, the fixation method could not be identified [3,9,51]. In Sukegawa et al., different resorbable plates were utilized, and the osteosynthesis material was not found to be related to the bone lid necrosis [46].
The included studies were divided according to the type of surgical procedure: rootend surgery, access to mandibular lesions and impacted teeth, implant explantation, access to the maxillary sinus, and surgery for other indications. The main results of the included studies are reported in Table 4.       Group A: bone lid as a free bone graft (10 cases); Group B: osteoperiosteal pedicled bone lid (10 cases). Bone lid consolidation: observed in all Group B cases, partial loss of bone lid in 2 cases of Group A. Bone density: significant differences in bone density between the 2 groups (p < 0.05), mean bone reduction 55% in group A, while no reduction in group B; no significant difference in bone density between pts. with and without chronic maxillary sinusitis. A total of 96% of the cases of chronic or sub-acute sinusitis were cured by the first surgical treatment; no loss of dental sensitivity; 3 out of 6 pts with residual problems were cured by appropriate medications. Complications: 3 pts. with residual problems were surgically treated again (1 case of new formation of hemangiomatous fibroma + 2 cases of polypoid sinusitis). Two studies [1,55] comprising 99 bone lids reported on the use of this technique to perform root-end surgery (apicoectomy), with successful healing except in two cases (Table 4).
Six papers [2][3][4][46][47][48], including 139 bone lids, described the use of this approach to gain access to lesions, such as cysts and impacted teeth in the mandible (Table 4). An RCT comparing the bone lid with piezosurgery versus bone removal with conventional rotary instruments showed a better postoperative recovery in the bone lid group [2]. In Sivolella et al., no cyst recurrence occurred, and in the 11 cases in which it was possible to compare preoperative and 1-year follow-up CT scans, a mean volume reduction of 93.8% in the radiolucent areas was seen [4]. In a retrospective case-control study, complications were registered in 35.3% of bone lid cases, whereas no undesired effects were observed in the not repositioned group [3]. In one of these six articles, an extensive use of customized surgical guides was reported [47].
In two articles, the bone lid technique was described for the explantation of implants owing to peri-implantitis or implant fracture (see Table 4) [9,10]. Compromised implant removal was achieved in all cases, and good healing of the reimplanted bone was obtained in 153 cases out of 156 [9].
In 11 articles comprising 290 bone lids, the clinical indication was maxillary sinus access, either for the treatment of pathologies such as cysts, or for the retrieval of displaced roots or foreign bodies [14,17,26,[49][50][51][52][53][54]56]. As evidenced in Table 4, a limited number of permanent complications occurred, and in a few cases, revision surgery, such as for recurrence, was needed.
In a randomized prospective study, the bony window healing was radiographically assessed 3 months after surgery in patients treated either with a bone lid as a free bone graft or with a pedicled bone lid [53]. Better results in terms of bone lid consolidation and bone density, as determined by means of CT scans, were observed in patients who received the pedicled one.
In the remaining cases (Table 4), the bone lid was applied for fractured teeth/roots and foreign body removal (29 cases), impacted tooth extraction (14 cases), inferior alveolar nerve lateralization (4 cases) [9], or for the removal of maxillary lesions [46].
Finally, in none of the included studies were patients' quality of life and satisfaction assessed by means of questionnaires or interviews.
The RoB summary of the two included RCTs is described in Table 5. Regarding the remaining observational studies (Table 6), the majority of the studies (8 out of 18) were judged at moderate risk of bias, while 6 studies were judged to be at high risk of bias and 4 at low risk of bias. Low = all criteria were met, and no more than one criterion was judged unclear. Moderate = two or more criteria were judged unclear, and the other criteria were met. Domains: D1, random sequence generation (selection bias); D2, allocation concealment (selection bias); D3, blinding of participants and personnel (performance bias); D4, blinding of outcome assessment (detection bias); D5, incomplete outcome data (attrition bias); D6, selective reporting (reporting bias); D7, other bias (e.g., sample size calculation).

Discussion
The present scoping review set few limitations on the sources of evidence (clinical studies with at least 10 participants and English language) in order to give an overview as inclusive as possible of bone lid applications. Despite the existence of heterogeneity and the lack of comparative studies in this field, the results of the present scoping review, which included 752 cases, showed that the bone lid is a feasible and successful technique in various types of oral surgery procedures, and is accompanied by a low incidence of major complications. Indeed, it seems to be a conservative technique that could reduce the amount of bone removal in particular circumstances, such as deeply impacted wisdom teeth, in which otherwise abundant osteotomy would be required. In addition, the success of this technique might be related to the skills and the experience of the operator, and unfortunately none of the included studies analyzed this crucial aspect.
In the largest prospective cohort study included [9], 200 consecutive patients were followed for at least 4 years. The bone lid approach was applied in conservative preimplant and implant surgery. As a main result, 98.5% of bone lids healed without any complication, ensuring an adequate volume for the planned implant therapy, and limiting the need for supplementary regenerative procedure and bone donor sites. As also reported in Sivolella et al. [4], one of the most important factors outlined by the authors was the bone lid thickness, which is fundamental for its stabilization and revascularization [9,57,58].
When bone lid approach was reported for root-end surgery of lower molars [1,55], better access and intraoperative visibility to the endodontic lesion, associated with a reduced bone removal, were advocated. The remaining bony defect was reduced, allowing a better environment for complete healing. Overall, short-term results seemed to be as good as those reported historically for root-end surgery without a bone lid [59,60]. However, no comparative study was available.
As regards access to mandibular lesions and impacted teeth, one included RCT aimed to compare the use of the bone lid versus bone removal to gain access to alveolar bone lesions [2]. Short-term data on post-operative swelling, immediate neurological complications, and patients' subjective response to pain were reported. The results supported the superiority of the bone lid technique. The same technique comparison was presented in a case-control study with a longer follow-up [3]. A tendency to adopt the bone lid technique for large-sized mandibular lesions, which might be considered a selection bias, was described. The critical status of those lesions probably justified the higher rate of complications in the bone lid group. It must be noted that both studies mainly focused on the comparison of piezosurgery vs. rotary instruments, rather than on the bone lid technique [2,3].
Improvements to the technique may derive by the use of computer-designed customized cutting guides, which allow for a pre-planned accurate outline of the osteotomies and, subsequently, better lid realignment and stability [47]. This approach may further improve the outcomes of the bone lid technique [12,27,38,47,61], making it more easily applicable as well. This topic should be further investigated with future RCTs comparing free-hand vs computer-guided bone lid surgery.
Partially osseointegrated failed implants (e.g., fractured or affected by peri-implantitis) are usually surgically removed with a trephine bur, thin Lindemann bur, dedicated piezosurgery inserts, or using a reverse high-torque wrench [62]. These methods may be associated with excessive bone loss or operative difficulties. When thin bone plates or deeply located implant body are present, the bone lid technique allows the surgeon to maintain the bone plate and immediately insert a new implant [9,10,63,64]. In pre-implant cases, at the time of delayed implant placement, complete re-osseointegration of the bony window and filling of the bone defect underlying the bone lid also were frequently observed, with no need for bone grafting [4,9].
Thin and beveled osteotomy, which can be obtained with both a microsaw and piezosurgery, can provide an ideal self-retentive morphology of the lid, thus reducing the need for rigid fixation [4,9,10,47,53]. Piezosurgery might present potential advantages, including ease of handling and less danger in case of accidental contact with soft tissue [69]. Fixation devices such as microplates, screws, or metallic ligatures represent a stable fixation method, but may have some drawbacks, such as undesired tension on the lid, screw or plate superficialization, and patient complaints [3,4,26,50]. As a consequence, a second surgery for their removal may be necessary.
The recurrence rate was rarely described as a clear outcome [3,4,17,46,48,50]. No recurrence was reported after cyst removal bone lid surgeries. One recurrence was recorded in a study on bone lids associated with functional endoscopic sinus surgery for the treatment of a fungus ball of the maxillary sinus [17].
The results of this scoping review confirmed that the bone lid technique is associated with good outcomes, resulting in a bone-saving approach. However, the results of this review must be interpreted with caution. Among the limitations of the present review, it is worth mentioning the design of the included studies, the variety of instruments utilized, the advancements of the technologies available for both treatment planning and postop assessment, the different surgical applications, and the short follow-up period of the majority of the studies. Despite that this technique is deemed to be influenced by the experience of the surgeon, its role has not been evaluated. In conclusion, considering the limited number of controlled trials on this topic, the low-quality evidence, and the heterogeneity of the examined clinical studies, randomized clinical trials are needed to assess the effectiveness of the bone lid technique over other approaches. Moreover, which is the most appropriate cutting tool for bone lid fashioning has not been determined so far. Similarly, the usefulness of further fixation methods in cases of well-fitted and stable bone lids is not clear. Virtual planning and the application of customized computer-designed guides might help to improve the outcomes of the technique and its reproducibility.