Since the introduction of bone-borne temporary anchorage devices (TADs), orthodontists are no longer solely dependent on non-predictable mechanics that necessitate the patient’s often insufficient cooperation [15]. Of all introduced TADs, orthodontic palatal implants like the Orthosystem® (Institut Straumann AG; Basel, Switzerland) are more capable of providing reliable absolute orthodontic anchorage owing to their osseointegration/low failure rate and are therefore considered to be superior to other orthodontic tooth-borne anchorage devices [17]. Yet a major downside of palatal implants was that their removal was only possible surgically using a hollow cylinder trephine. This standard method removes the implant together with a larger bone volume and is therefore considered invasive and is not free of known complications [5]. More recently, an explantation tool has been developed which allows a sufficient torque application to break the bone-implant-interface, thereby enabling to simply unscrew the palatal implant [8].
This paradigm shift in removal of osseointegrated palatal implants (i.e. unscrewing rather than trepanning) necessitates to establish an evidence base concerning the required unscrewing torque. The pertinent literature on this subject is practically non-existent, as orthodontics is presumably the only discipline where intact implant removal is seen as a treatment success. Few investigations exist for prosthodontic implant removal [1, 6, 16, 19] or miniscrew removal (e.g. [2, 3, 11, 18, 20]). All these scarce studies remain of little value for comparison, because their differences to the present investigation are too important to be ignored: different loading (prosthodontic vs. orthodontic), region of insertion, TAD dimensions in length, width and overall design, and surface characteristics differ significantly one from each other and disqualify any direct comparison.
However, Favero et al. [6] explanted 8 small screw titanium implants (Exacta MS series conical profile, with a diameter of 3.3 mm and a length of 7.0 mm) used as indirect orthodontic anchorage inserted in the palate. A large discrepancy between the torque values obtained in vitro (which are similar to ours) and in-vivo was reported. The small numbers and this uncertainty warrant caution for any attempt to generalize their observation. Our study’s strength is not only the larger sample size which makes generalizing more acceptable, but also its novel insights gained by discriminating and analysing statistically co-variates, such as type of suprastructure, duration of loading, use of anaesthesia, age and sex. Lastly, in contrast to the present findings, Favero and colleagues reported breakage of the implant at a significantly lower torque level of ca. 210Ncm.
A study analysing the MRT of sandblasted sand-blasted, large-grit, and acid-etched surface-treated mini-implant (C-implant, CImplant Co, Seoul, South Korea) [11] (1.8 mm in diameter and 8.5 mm in length) showed, however, no fracture or distortion during removal, and the mean MRT was 16.4 Ncm (range, 3.94–35.41 N per centimeter).
Implant removal with the new non-invasive method was completely successful in all but one (96.8 %) patient. Since in this single case the triangular head fractured at a very high-MRT level of 300.1 Ncm, the use of a torque control gauge might be advantageous to avoid implant head fracture. The mean MRT of successful palatal implant removal was 148.6±63.2Ncm, but it must be stressed that a large spectrum was observed (minimum 31.5Ncm, maximum 272.8Ncm). This obvious heterogeneity underlines the importance to investigate possible influencing factors.
To the best of our knowledge, only one prospective study on prosthetic implant removal has been published. Explanation reasons were periimplantitis, fracture, or malpositioning [1]. Explantation was performed by application of counter-clockwise torque using a dedicated extraction kit (BTI Biotechnology Institute, Vitoria, Spain) to break the implant–bone union. Only in cases where the torque wrench opened (the maximum of wrench torque was set at 200 Ncm), the extraction assembly was removed and specialized trephine burs were employed to cut into the first 3 to 4 mm of implant-bone contact. The mean removal torque for explantations of implants without using the trephine method was 146 ± 5 Ncm. Well aware of the due caution that must be exercised when comparing this study to our results, it is nevertheless of interest to note that the mean unscrewing MRT were very similar. Anitua and colleagues failed to present data on the dimensions of the explanted implants (length, diameter), gender, or loading time, all this further restricts the validity of the comparison. Probably of modest scientific value is the report of three implants removed in a human volunteer by reverse torque to failure at torque levels between 45 and 58 Ncm [19], which is in the lower range of the present data.
Our results clearly demonstrate that unscrewing MRT were significantly higher in male (182.0±63.0 Ncm) than in female patients (112.8±40.8Ncm). It is probably not erroneous to assume that this disparity is associated to the gender variation known in palatal bone thickness and density [24].
Perhaps somewhat surprisingly, the type of suprastructure loading (active mesializer or distalizer versus passive loading) did not have any significant effect on the measured MRT. Yet a certain trend in the data can be discerned: Implants actively loaded tended to necessitate higher MRT. It is fair to assume that the underlying reason might be associated to a higher degree of osseointegration which might have evolved during active loading. This observed tendency seems to be in accordance with previous investigations indicating that bone tissue turnover as well as the density of the alveolar bone is higher adjacent to loaded implants compared to unloaded implants [14] and might thus confirm the hypothesis posted by Frost which stipulates that mechanical agents adopt an important role in bone metabolism [7]. The importance of bone metabolism is probably mirrored in the scatterplot visualizing the explantation torque in relation to age. Although no correlation between MRT and age could be disclosed, a certain trend was evident. Here too, an association between bone density (i.e. greater brittleness with age) and higher unscrewing torque might be speculated. Of no discernible impact on explantation torque were the use of local anaesthetics and the removal direction (clockwise versus anti-clockwise). The latter demands interpretation. Assuming successful osseointegration, unscrewing the implant causes a breakage, either at the implant-bone interface or fully within the adjacent bone. Histological investigation on the exact anatomical region of breakage would certainly help to understand why rotational direction does seemingly not affect the required MRT.