Primary failure rate for 1680 extra-alveolar mandibular buccal shelf mini-screws placed in movable mucosa or attached gingiva

Skeletal anchorage is a broad-based experimental and clinical concept.1–7 Kanomi8 introduced surgical miniscrews for orthodontic anchorage, and more specific devices soon followed.9,10 Currently there is a large range of miniscrews varying in diameter from 1.0 mm–2.3 mm and in length from 4 mm–21 mm.11–19 Interradicular (I-R) miniscrews are the most common application, but they are often problematic in the posterior mandible.11,12,15,17–26 Miniscrews in the mandibular buccal shelf (MBS) are proposed as a reliable source of extra-alveolar (E-A) anchorage for retracting the entire mandibular arch to correct severe crowding, protrusion, and skeletal malocclusion, without extractions or orthognathic surgery.1,2 

Success rates for I-R miniscrews range from 57%–95%, with a mean of approximately 84%.26–28 Failure is common in the posterior mandible, typically occurring in the first few weeks, so primary stability is the critical factor for clinical success.29–31 Attempts to improve primary stability include smaller diameter pilot holes,23 sites with increased cortical bone thickness and density,29–32 and a self-drilling protocol.33,34 Bone quality is particularly important for orthodontic miniscrews because they are retained by mechanical locking rather than osseointegration.35,36 Screw design studies show a >70% success rate for I-R miniscrews with a diameter of ≥1.2 mm, and multiple studies show success is directly related to screw length.17–21 However, increased screw size increases the probability of root damage,21 and a recent review suggests that cortical bone thickness may be the most important stability factor overall.20 Placement technique focuses on minimal root damage during screw placement. Park et al.15,17 suggested placing the screws at an obtuse angle to the bone surface to increase bone contact and lower the risk of root damage. Placing the devices in an E-A site like the MBS permits the use of larger-diameter screws that can be inserted parallel to the axial inclination of molars and not interfere with tooth roots.1,2 

A pull-out study on both arches of dogs showed greater strength for miniscrews placed in the mandible30 but in human studies, maxillary sites were more successful than those in the mandible in all26,37–40 but one study.23 More recent research confirmed the maxilla as a superior site for miniscrews,12,17,24 but inadequate AG continues to be a concern.1,2 These results suggest there may be a problem if E-A miniscrews are inserted in MM, but soft tissue considerations have not been specifically addressed.41–44 

The hypothesis tested is that MBS miniscrews are less successful short-term (<4 months) if they are placed in MM.

MBS miniscrews were installed in a consecutive series of 840 patients (405 males, 435 females; age 16 ± 5 years), inserted in private practice by the same orthodontist (senior author) from 2009 to 2012. A total of 1680 SS miniscrews (2 × 12-mm, Newton's A, Hsinchu City, Taiwan) (Figure 1) were placed without flap elevation under local anesthesia (Figures 2 and 3); 1286 were in MM and 394 penetrated the AG (Figures 4 and 5). All miniscrews were placed as nearly parallel as possible to the mandibular first and second molar roots (extra-alveolar approach). The surgical procedure began with a sharp dental explorer sounding through the soft tissue to bone at the desired skeletal site (Figures 2 and 3). The most anatomically favorable site for the miniscrew is usually at or near the mucogingival junction (Figure 4). A self-drilling bone screw was inserted and screwed into the bone perpendicular to the occlusal plane (Figures 5 and 6).43–46 After installation, the screw head was at least 5 mm above the level of the soft tissue (Figure 5) and the endosseous portion had approximately 5 mm of bone engagement (contact) (Figure 6).46 All miniscrews were immediately loaded using prestretched elastomeric modules (power chains) to deliver a relatively uniform force.45–50 The mandibular retraction force varied from 8 oz–14 oz (227 g–397 g, 231–405 cN), being proportional to the perceived density of the bone when screwing in the miniscrew. The patients were instructed in oral hygiene procedures to control inflammation. The prestretched power chains47–49 were replaced every 4 weeks. The stability of the buccal shelf screws was tested at every appointment for 4 months. Percent failure data was tested by chi-square. The Indiana University Institutional Review Board approved the protocol, assigning the number 1408974880.

Retrospective analysis of the 1680 miniscrews revealed that 1286 (76.5%) were placed in MM and 394 (23.5%) were in AG. Overall, 121 of 1680 (7.2%) miniscrews failed within 4 months, and the average time of failure was 3.3 months. In the MM group, 94 out of 1286 (7.31%) failed, and 27 out of 394 (6.85%) failed in AG (Figure 7). A chi-square test failed to show a statistical significance (P > .05) between the groups, so the hypothesis was rejected.

On the other hand, there were interesting failure relationships among other variables: age, side (right vs left), and predisposition. The average age of the 121 failure patients was 14 ± 3 years, which is considerably lower than the average age of all patients (16 ± 5 years). These data suggest that the failures were more common among the younger patients who tended to have less dense cortical bone in the MBS. Regarding side of patient, 78/121 (64.5%) of the failures were on the left side and 43/121 (35.5%) were on the right (Figure 8). Overall, the failure rate on the right (9.29%), compared with the left (5.12%) side, was statistically significant (P < .001). The 121 failed screws involved only 105 patients: 89 patients had single-screw failure and the other 16 lost screws on both sides. The bilateral failures suggest a predisposition to failure in a small portion of the patients (16/840  =  1.9%).

Within the restraints of this study, placing MBS miniscrews in MM is an acceptable clinical procedure. The 4-month assessment interval was selected because all patients in the study required at least 4 months of mandibular buccal segment retraction. Additional study of the sample is indicated to determine the long-term failure rate relative to the anchorage needs of each patient.

Most studies of I-R miniscrews have shown a higher failure rate in the mandible (19.3%) than in the maxilla (12.0%).24,51,53 Furthermore, the physical stability of miniscrews tends to decrease for the first 3 weeks.31 With a primary failure rate of approximately 7%, MBS miniscrews are an attractive option for retraction of the mandibular buccal segments or the entire lower arch. Furthermore, the risk of root damage is remote when buccal shelf miniscrews are applied as described Figures 2–6.

Many patients have a minimal width of attached gingiva buccal to the molars, so >75% of optimally positioned buccal shelf screws penetrated MM.43 Attached gingiva can be moved to the buccal shelf site with an apically repositioned flap, but the present data suggest that the expense and discomfort associated with that additional surgical procedure is unnecessary. However, the elevated position of the screw head is probably an important factor in successfully maintaining the screws in MM, because oral hygiene is facilitated to control peri-screw inflammation.

The significant difference in primary failures on the left side (9.29%) vs the right (5.12%) reflects the technical sensitivity of the procedure and possibly other uncontrolled biological factors such as chewing and brushing habits. It is more difficult for a right-handed clinician to ideally position buccal shelf miniscrews on the opposite side. Additional refinement of the clinical technique is indicated to help control this variable.

Predisposition to MBS miniscrew failures is an important area for future research. Miniscrew studies of other sites have found no significant relationship between failure rate and age.11,16,17,21 However, in the current study, younger patients tended to have a higher failure rate, suggesting that a more mature skeleton may be advantageous for buccal shelf miniscrews.52 In addition, there were 16 patients who had bilateral failures, suggesting a predisposition to failure in some patients. This result is not surprising because “clustered failures” are well known both for dental implants53 and orthodontic miniscrews.54 A follow-up study of all failures in the sample is indicated to investigate predisposition based on patient age, genetic factors, and bone characteristics. Some patients may have an enhanced regional acceleratory phenomenon55 when bone is wounded with a miniscrew. This exaggerated hyperinflammatory response may be genetic, such as homozygosity for allele 2 of interleukins 1-beta, predisposing a patient to periodontitis.56 

• To provide optimal anchorage for retracting the mandibular arch, MBS miniscrews must be positioned precisely relative to tooth roots, soft tissue, and available bone.

• Assuming adequate soft tissue clearance (approximately 5 mm), screws can be positioned in attached or movable mucosa.

• Overall, the method is highly successful for most (93%) patients, but a small fraction of patients (1.9%) appears to be predisposed to failure.

This article is a research study presented to the Midwest Component of the Edward H. Angle Society of Orthodontists in partial fulfillment of the requirements for active membership.