INTRODUCTION

During the past decade, the concept of absolute or skeletal anchorage has attracted increasing interest. Many types of skeletal anchorage, including prosthetic implants, onplants, surgical mini-plates, and mini-screws, have been suggested. Among them, mini-screws are the most popular because of their many advantages. Mini-screws are relatively cheap, require a simple implantation procedure, can be loaded immediately after implantation, and can be placed almost everywhere in the bone with attached gingiva.1–4

Despite these advantages, mini-screws suffer from an increased chance of potential failure compared with the other types. Park et al5 reported a mini-screw failure rate of 8.4%, and Kuroda et al1 reported a failure rate <20%. Several causative factors have been suggested for mini-screw failure. These include the generation of excessive heat during mini-screw implantation, wobbling of the mini-screw driver during implantation, infection around the mini-screw, lack of attached gingiva, interpositioning of soft tissue between the mini-screw and the bone, and the amount of force application.4–8 In addition, some studies have suggested that an immediate load might influence the failure rate.8–11

Mini-screw root invasion is due to proximity of the adjacent roots, anatomic variations in root form, and errors in mini-screw implantation direction.2412 A higher failure probability was reported when the mini-screw invaded the root or periodontal space.2 Therefore, many guidelines and methods have been suggested to avoid root contact, particularly surgical stents and various visual devices.3413

However, few reports have described the failure rate of mini-screws and the consequent impact when the mini-screw invades the root. This lack of knowledge raises the following three questions: (1) Does contact between a mini-screw and the tooth root predispose to a higher failure rate? (2) Does immediate loading affect the failure rate because the mini-screw touches the root? and (3) What is the histologic consequence when the mini-screw touches or damages the root?

In an effort to answer these questions, investigators in this study examined the stability of mini-screws when they invade dental roots by measuring the retention period. Histologic evaluation of the effects of contact between the tooth root and the mini-screw also was carried out.

MATERIALS AND METHODS

The protocols of this study were reviewed and approved by the Animal Ethics Committee of Kyunghee University, School of Dentistry. Sixteen mini-screws (C-Implant, Seoul, Korea) were inserted into each of three adult male beagle dogs (1.5 years old; 10 to 11 kg; 48 mini-screws). The screws had a diameter of 1.8 mm and a length of 8.5 mm, along with an intrabony length of 6.5 mm (Figure 1).14 Twenty-four mini-screws were placed to invade the roots, and the other 24 were placed in the middle of the alveolar bone as a control. Root invasion of mini-screws was defined as tangential contact of the mini-screw with the dental root during a placement procedure that mimicked the most probable situation in clinical orthodontics.

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For the mini-screw implantation procedure, the dogs were administered xylazine (Rompun, Bayer Korea, Seoul, Korea) and ketamine (Ketamin, Yuhan Corp, Seoul, Korea) intramuscularly, and the surgical site was injected with 2% lidocaine (Yuhan Corp). After the implantation procedures had been performed, gentamicin (Gentamycin, Daesung, Seoul, Korea; 0.08 to 0.1 mL/kg) was administered intramuscularly for 3 days to prevent infection. Guiding grooves filled with a radio-opaque material were prepared on the teeth, and standard radiographs were taken with an individual film holder for accurate placement of the mini-screws (Figure 2). The individual film holder was made of acrylic with tooth indentations for secure positioning of the radiographic film and reproducible film position. After the implantation site was confirmed, the mucosa was incised and the bone surface was exposed. Cortical bone perforation was performed with a 1.5 mm diameter surgical bur and a low-speed engine at 2500 rpm (rounds per minute) without the use of a reducing handpiece.6 Subsequently, the mini-screw was inserted with a hand manual driver.14 During perforation and implantation, the area was irrigated with saline to minimize heat production.

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During insertion, the sensation of sudden resistance indicated that the mini-screws were invading the root. All mini-screws were placed by the same operator. Two standard periapical radiographs were taken at different angles of each screw to confirm the positional relation between the roots and the mini-screws. Nickel-titanium (Ni-Ti) coil springs were used to exert a force of 150 g for immediately loaded screws.

Mini-screws were implanted bilaterally to invade the distal roots of the mandibular second premolars, third premolars, and first molars, as well as the mesial roots of the first molars. Noninvading screws were placed in the middle of the alveolar bone adjacent to the furcation area of the mandibular third and fourth premolars and the first molar, in addition to the distal area of the first molar (Figure 2). Half of them were loaded with Ni-Ti coil springs immediately after implantation. The screws were divided into four groups according to their position and force application (Table 1).

Table 1.  Summary of Experimental Composition

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Mobility of the mini-screws was checked daily, and the surrounding tissue was irrigated with saline. The mini-screw retention period was documented until the time of failure, which was determined to occur over 1 mm of horizontal mobility. If mini-screws demonstrated visible mobility, a ruler was applied with guiding grooves used as a reference to measure mini-screw mobility. If one of the two mini-screws failed with immediate loading, an extra mini-screw was implanted in an adjacent nonexperimental area to continue the application of force on the remaining mini-screw.

Eight weeks (56 days) after the start of the study, the dogs were sacrificed with an intramuscular overdose of xylazine and ketamine. The mandible was removed and was fixed in 10% formalin, and all experimental sites were processed for histologic observation. Specimens that showed failed mini-screws were decalcified in 5% nitric acid, dehydrated, and embedded in paraffin. The buccolingual sections of each specimen were prepared with the use of a microtome set to 4 μ and stained with hematoxylin-eosin. Specimens that showed mini-screw retention were dehydrated and embedded in resin to prepare 70 μ thick sections that were stained with hematoxylin-eosin. These sections were examined by optical histology, and microphotographs were taken with the use of a Kappa Image Base version 4.5.2 program (Kappa Opto-Electronics Gmbh, Gleichen, Germany).

RESULTS

Mini-Screw Failure Rate

The root-invading mini-screws began to fail 7 days after implantation. The last one failed after 29 days. The mean retention period of the failed root-invading mini-screws was 16 days (Table 2, Figure 3). The failure rate of the root-invading mini-screws was 79.2%, and that of the noninvading mini-screws was 8.3% (Tables 3 and 4).

Table 2.  Retention Period of Mini-Screws

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Table 3.  Comparison Between Retained and Failed Mini-Screws According to Root Invasion or Not, With the Use of Chi-Square Sta tistics

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Table 4.  Comparison Between Retained and Failed Mini-Screws According to Force Application or Not, With the Use of Chi-Square Statistics^a

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The chi-square test revealed that each variable (position and force application) was independent and had no interactions with the other variables, which means that the position and force factor do not have a synergistic effect on the increase or decrease in failure rate (P > .05) (Table 3). In addition, the failure rate of the root-invading mini-screws was significantly higher than that of the noninvading mini-screws (P < .01) (Table 5).

Table 5.  Interaction of the Position and the Force Application Fac tor to the Failure Rate of Mini-Screws Evaluated by Chi-Square Sta tistics^a

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The failure rate was 45.8% for unloaded mini-screws and 41.7% for loaded screws (Table 4). Force and no-force application showed no statically significant difference (P > .05) (Table 4). This suggests that immediate loading had no effect on the success of the mini-screws.

Microscopic Findings

Mini-screw failed samples—Root-invading mini-screws (Groups 1 and 3)

Predominant features of the samples taken after mini-screw failure included an absence of inflammatory cells, regeneration of the periodontal ligament, and healing of the damaged roots by cementoid tissue (Figures 4A, B).

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One sample, taken 38 days after the mini-screw had failed, showed direct contact between the alveolar bone and the root. This sample showed minimal inflammatory cell invasion, which indicated that the inflammation had almost subsided but local resorption of the root was still in progress (Figure 5A, B). The groups with and without force application showed a similar histologic appearance.

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Noninvading mini-screws (Groups 2 and 4)

Two samples were obtained in which a failed mini-screw did not invade the root. Histologic observation of the sites where the mini-screws had been placed revealed a normal appearance that was not distinguishable from that of adjacent tissues.

Mini-screw retained samples—Root-invading group (Groups 1 and 3)

The section with root invasion but without a loose mini-screw showed good deposition of dense bone surrounding the mini-screw (Figure 6A, B). However, the invaded root showed an irregular damaged surface with some repair by cementoid calcified tissue (Figure 6C, D).

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Noninvading group (Groups 2 and 4)

Histologic examination of the sample revealed partial osseointegration of the mini-screws without inflammatory cells (Figure 7A, B). This osseointegration appeared on both force-applied and nonapplied groups, and the histology of these groups had a similar appearance.

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DISCUSSION

In this study, the mini-screw failure rate was 79.2% when the roots had been invaded with an average retention period of 16 days, which is insufficient for orthodontic treatment. The mini-screws used in this study had a sandblasted large-grit acid-etched (SLA) surface for the intraosseous portion and might show a different failure rate compared with other nontreated mini-screws. However, this is not believed to be a significant factor that would make these results comparable with those of other nontreated mini-screws. Although the mini-screws were placed on the free gingiva, the soft tissue overgrowth problem was minimal, possibly because of the tall extrabony structure and the smooth surface of the soft tissue–penetrating structure of the mini-screws, which could minimize tissue irritation.

The reason for the higher failure rate with the root-invading mini-screw is unknown but might be insufficient bone–implant contact for mechanical retention of the mini-screws. Another reason might be that the mini-screw cannot perforate the root easily, and the mini-screw slips over the root surface when it touches the root and causes damage to the surrounding tissues. In addition, physiologic movement of the root during mastication minutely mobilizes the mini-screw, which maintains contact with the root and causes inflammation.

Root-invading mini-screws showed a higher failure rate, but not a rate of 100%. It is presumed that some root-invaded mini-screws can survive for the first 8 weeks as noninvading mini-screws. However, the failure rates indicated that it is important for mini-screws to be placed clear of the dental roots to secure stability. The mini-screw samples retained despite root invasion showed a good connection between the bone and the mini-screw surface (Figure 6). These samples commonly showed less root invasion and revealed that this invasion appeared to be limited to the periodontal ligament. A minimal inflammatory reaction may have occurred with rapid onset of the healing process.

Many studies have examined the timing of force application to a mini-screw after insertion. Some reports insist that to sufficiently resist an orthodontic force, the mini-screw requires not only mechanical fitting to the bone but also osseointegration and accordingly sufficient time for osseointegration before a force is applied.89 On the other hand, some studies reported that early loading does not interfere with the stability of a mini-screw.17101115 The failure rates reported in this study suggest that early loading does not interfere with mini-screw stability, and this supports the latter opinion. Also, both loaded and unloaded samples with retained mini-screws commonly showed new bone that covered a portion of the intrabony part of the mini-screw surface.

Some reports have described the healing pattern of periodontal tissue after damage to the periodontal ligament and root cementum was caused by surgery. Many studies have reported three consequences after root damage: (1) healing without root resorption, (2) surface resorption that finally healed with cementoid tissue, and (3) replacement resorption followed by ankylosis.16–18 The pattern of periodontal tissue repair differs with the magnitude and the nature of damage, and according to the types of cells recruited for healing. The tooth-supporting apparatus can regenerate when the damage is mild and healing cells originate from the periodontal ligament. However, root resorption can occur if the damage is severe and cells are recruited from the marrow; bone formation and ankylosis may result.16 Because root damage by a mini-screw can be thought of as essentially the same as surgically caused root damage, the same consequences are expected.

The main histologic findings of this study included healing of the damaged root with cementoid calcified tissue and regeneration of periodontal ligament tissue. Asscherickx et al19 described root repair after injury from a mini-screw and reported root repair with cementum, as was described in the present study. Investigators used a bone labeling technique and concluded that complete repair of the periodontal structure occurred within 12 weeks of screw removal, compared with 6 weeks after mini-screw removal in this study (Figure 4). This time difference may have been due to variations in the area of root damage.

Only one of 19 samples of root-invaded failed mini-screws showed direct bone-to-root contact, which reveals histologic evidence of ankylosis. Andreasen and Kristerson20 observed the healing pattern after tooth reimplantation with deprivation of the periodontal ligament to a variable extent. They reported that ankylosis occurred after 2 weeks and periodontal ligament deprivation of 1 to 4 mm² caused temporary ankylosis, which disappeared after 8 weeks. The disappearance or resolution of ankylosis was revealed as the disappearance of a direct bone-to-root via bone remodeling process. Investigators concluded that successful ankylosis resolution can occur when ankylosis is caused by 1 to 4 mm² root damage, and that the probability of resolving ankylosis increases with decreasing root damage.

Given that almost all commercially available mini-screws have a diameter of 1 to 2 mm, large root defects are unlikely to occur. Therefore, resolution can be expected over time when mini-screw–induced ankylosis occurs. Also, if more time was given, resolution of ankylosis could be expected in a sample of this study population. Overall, a mini-screw–damaged root can be healed by cementoid with recovery of the original shape and form. Ankylosis will present only rarely and is expected to resolve.

CONCLUSIONS

• The success rate of the mini-screw was 91.7%, provided it did not invade the root; immediate force loading appeared to have no influence.

• It is important for mini-screws to be placed clear of the dental roots to secure stability.

• Although the dental root can be injured by mini-screws, minimal clinical side effects are expected if the injury is not too severe, because of the healing potential of surrounding tissues.