Abstract:
Objective: Compare the 6 month failure rates for stainless steel (SS) and titanium alloy (Ti) miniscrews placed in the infra-zygomatic crest (IZC). The hypothesis was that SS miniscrews will have a higher failure rate.
Materials and Methods: A total sample of 386 consecutive patients was composed of 76 males and 310 females with a mean age 24.3 yr, range 10 to 59 yr. Each subject was recruited from a patient pool, previously treatment-planned for supplemental anchorage with bilateral IZC miniscrews (OrthoBoneScrew® Newton’s A Ltd, Hsinchu City, Taiwan). Each patient who agreed to participate received a 2x12-mm SS miniscrew on one side and a Ti miniscrew in the other. The 386 pairs of screws were coded for an equal distribution of material type and side of placement. For each patient a coded pair of screws was randomly drawn, and then placed in the right or left side as specified (double blind, split mouth design). Miniscrews were placed by the same orthodontist buccal to the upper first and second molar roots, which is defined as an extra-alveolar (E-A) anchorage site. The screw heads were at least 5-mm superior to the soft tissue, which was either attached gingiva (AG) or moveable mucosa (MM). The surgeon chose the same anatomic location for the miniscrew without regard to the the type of soft tissue. Retrospectively, the soft tissue type was scored relative to the mucogingival junction (MGJ). All miniscrews were immediately loaded with pre-stretched elastomeric modules. The force ranged from 8-oz to 14-oz (227-397g, 223-389 cN) depending on the surgeon’s perception of bone density at the miniscrew site. Maxillary buccal segments were retracted for at least 6 months. Failure was defined as any miniscrew that failed to provide continuous anchorage.
Results: 49 out of 772 miniscrews failed (6.3%); failures were divided into 27 SS (7.0%) and 22 Ti alloy (5.7%). The 1.3% difference between SS and Ti failures was not statistically significant (p=.07). Failures were unilateral in 21 patients (5.4%) and bilateral in 14 patients (3.6%). The collective failure rates were similar on the right and left sides, 6.5% and 6.2%, respectively. The age of patients with a failed IZC miniscrews, 12-43 yr, with a mean of 24.2 yr, was similar to that for the entire sample. Compared to Ti, SS miniscrews had a significantly higher failure rate when placed in AG (7.4%) and on the right side (7.9%).
Conclusion: Overall, IZC miniscrews were about 94% successful for at least 6 months. SS miniscrews had a slightly higher, but statistically significant failure rate when placed in AG and on the right side. Failure was not related to patient age, but a predisposition to failure was noted in 3.6% who had bilateral failures. From a clinical perspective, all IZC miniscrews were at least 92% successful, so the slight difference between SS and Ti at some sites was not clinically significant. Since SS is less prone to screw fracture and tends to hold a sharper edge at the tip, it continues to be the preferred material for self-drilling miniscrews placed in dense cortical bone.
Chris Chang DDS, PhD.