Extremes of implant dentistry, from minis to maxes and beyond: evaluating new implant designs and techniques

Discussion

Dental implants come in a variety of shapes and sizes (Table 1 and Figure 1). More recent designs are tapered or bullet shaped with either an internal or external connection. However, parallel-sided, cylindrical implants have continued to be used, albeit by a smaller cadre of traditionalist practitioners.

Implant shapes and designs have become more standardized between manufacturers, yet the opportunities to develop implants of differing shapes, tapers and lengths have resulted in a plethora of different implant designs to fit various clinical applications in recent years.

Implant shape and length and connection type have been adapted for use in particular situations. For example, a ‘max’ implant, which may be described as short, wide and tapered, has been designed for immediate placement into a molar socket where the remaining bone is between the roots of the lost tooth. This enables the emergence profile between the prosthetic crown and the implant to be less acute, with a closer similarity to the angle between the crown and root of the natural molar tooth.

At the other extreme is the ‘mini’ implant. This has historically received poor recognition mainly because it was manufactured from a titanium alloy that may have been associated with higher failure rates with reduced levels of bony integration. Clinical misuse also contributed to premature failures.

The implant has regained popularity owing to its pure titanium construction, as used in the manufacture of conventional dental implants. Its redesign as a one-piece construction allows for one-stage placement (implant placement and simultaneous exposure of the restorative component) in atrophic edentulous ridges, a site-specific use in elderly patients.

In summary, the advantages of the mini-implant approach to the restoration of the atrophic ridge with removable overdentures can be considered as follows:

The site and shape of dental implants have evolved to fit clinical situations, and improvements in their strength and surfaces means they can be used in a more diverse range of clinical procedures. A series of case presentations is used here to outline the application of mini and max implants when placed in the maxillary/mandibular envelope in the restoration of the fully or partly edentulous arch.

Case study 1

An 84-year-old female patient had been rendered edentulous more than 30 years previously and could no longer tolerate a lower denture. She attended with a flat Class V lower ridge with a comfortable upper denture, but had used several sets of lower dentures made both on the NHS and privately without success.³

The patient initially declined treatment as she considered herself ‘too old’ and was apprehensive about undergoing any form of surgery. The different implant options were explained, and whether to proceed with either two conventional implants or two mini implants was discussed.

A randomized controlled study suggested that patients aged >70 years preferred mini-implants to conventional dental implants if they had overdentures because they were quicker, simpler to place and required less post-operative analgesia than their conventional counterparts.⁴

The surgery was carried out uneventfully under local anaesthesia (Figure 2) before mini-Rheins (Southern Implants, South Africa) implant clips were retro-fitted to her previously made lower denture some 10 weeks later (Figure 3). The new titanium ILZ implants (ILZ, Southern Implants), although they have and can be loaded immediately in this cohort of elderly patients, are often left unloaded for between 8 and 10 weeks.

The patient was delighted with the outcome and has attended annual reviews, where the mini-Rhein attachments have been replaced on occasion (Figure 4).

Case study 2

Surgery in this case was carried out in April 2018. A 78-year-old male patient had experienced a gunshot injury resulting in losing his upper left maxilla. He had also had previous chemoradiotherapy for a T2N0M0 squamous cell cancer of the right vocal cord.

The radiotherapy field for his vocal cord tumour affected the mandible, but the maxilla was spared. The remaining maxillary teeth were extracted where the patient accepted he would not be able to have any maxillary prosthesis owing to a lack of alveolar bone in the left maxillary region and the resulting scarring.

A CBCT scan of his right maxilla revealed that the alveolar ridge had a width of approximately 4.8 mm, which would have allowed for conventional implant placement with some minor bone grafting, or a ridge-splitting procedure. However, a more simple surgical procedure was advocated given his previous oncological management.

Three coaxial mini implants (ILZ12D, Southern Implants) were placed (Figure 5) in the upper right central), upper right lateral and upper right canine region (Figure 6). These were left unloaded for 10 weeks before a new half upper denture was constructed.

The retention was excellent, and the patient reported being able to chew again to full effect (Figure 7). In the 6 years since completion of treatment, the resilient mini-Rhein clips have been replaced once.

The mini-implant approach has been around for many years, was not fully accepted as mainstream. This was partly due to their poor integration record (construction from a titanium alloy) and also possible strength issues because the original mini-implants had a diameter of <2.5 mm.^5,6,7 Poor case selection may also have contributed to the issues.

The newly designed mini-implants have a success rate of around (95%) at 5 years, and are constructed from heat-treated pure titanium rather than an alloy.⁸

However, readers should be reminded that the implants are not for every situation. They are recommended only for edentulous patients aged over 70 years with atrophic ridges, with two implants placed in the mandible and four in the maxilla, but the palate retained to reduce lateral forces on the implants.

Case study 3

This case involved a body-shifted, site-specific implant with internal angle correction used within the aesthetic zone of the maxilla.

A medically fit and well 72-year-old male patient presented with an unrestorable upper left central incisor with a root fracture (Figure 8), with associated chronic inflammatory granuloma and loss of a section of the cortical bony wall of the incisive foramen.

Treatment options were discussed with the patient. An implant-based treatment modality was chosen, with proposed immediate tooth replacement therapy using a site-specific implant with a novel, body-shifted macro geometry and an internal angled screw connection (Southern Inverta DC CoAxis, Southern Implants).^9,10,11 Temporization was carried out with a chairside customized laboratory-manufactured shell crown before definitive long-term restoration using a screw-retained zirconia crown with a lab-cemented custom abutment.

CBCT analysis confirmed that the UL1 was not predictably restorable, with an associated radiolucency involving the lateral wall of the incisive foramen and a labially displaced post-retained crown (Figure 9).

After pre-operative preparation, the UL1 was extracted using a minimally traumatic technique, with the residual chronic inflammatory granuloma curetted and the socket degranulated (Figure 10).

The UL1 socket showed a bony fenestration to the incisive foramen with a grossly intact incisive bundle visible. An osteotomy was carried out down the centre of the ridge angled similarly to the incisal edge of the UR1. This considered the 12-degree screw angle correction within the implant to allow optimal screw channel positioning at the palatal cingulum of the crown (Figure 11).^12,13

A healing abutment was placed into the implant to occlude its coronal opening while 0.5 cm³ of alloplastic bone graft material (EthOss, Ethoss Regeneration, Keighley, UK), was placed into the circumferential jumping gap and the fenestration on the wall of the incisive foramen, with direct contact onto the incisive bundle.¹⁴ Once the graft material had set, a PEEK temporary cylinder was fitted to the implant and a PMMA shell provisional crown picked up with flowable composite and polished at the chairside (Figure 12).

The provisional crown was screw retained to the implant at 15 Ncm. An immediate post-operative small-field CBCT scan was taken showing the implant position and bone graft within the residual socket (Figure 13).

At 12-week review, the clinical situation was assessed and no change in the bony ridge dimensions was found. An open custom tray impression was taken using addition-cured silicon, and a definitive zirconia crown, with a lab-cemented, custom-anodized titanium abutment and a palatal cingulum screw access channel, was manufactured and fitted to 30 Ncm (Figure 14).

At 2-year review, an optimal clinical outcome was seen, with no apparent change to the soft tissue dimensions. A peri-apical radiograph and small-field CBCT showed no reduction in the bony ridge volume, with no bone loss vertically on the implant (Figure 15).

Case study 4

This case involved a 74-year-old male patient with intermittent atrial fibrillation, but an otherwise unremarkable medical history.

The root-treated and crowned upper left first molar was not predictably restorable owing to deep distal root caries. Plain radiography and CBCT assessment confirmed the tooth's poor prognosis and a favourable bony ridge form for immediate implant placement. Treatment options were discussed with the patient and an immediate implant placement with delayed implant loading decided upon (Figure 16).

After pre-operative preparation, the UL6 crown was removed, and distal caries removed down to the crestal bone level (Figure 17).

Initial bony osteotomy was performed through the tooth as a guide for accurate inter-radicular septum preparation. After the roots were separated, further preparation of the osteotomy using sequential drills was performed before removal of the three roots; the mesio-buccal root had some buccal bone adherence (Figure 18).

The UL6 socket was degranulated and the initial osteotomy location noted on the inter-radicular septum. An elective, full-thickness muco-periosteal flap was raised with extension around the buccal of the UL5 and UL7 with no releasing incisions to assess the extent of the buccal bony dehiscence associated with the mesio-buccal root socket (Figure 19).

Versah Densah drills (Versah UK, Shrewsbury), were used counterclockwise in line with the published protocol to expand the inter-radicular bony septum and allow accurate three-dimensional positioning of the implant.¹⁵ The multi-fluted Densah bur created and expanded a pilot hole without excavating significant amounts of bone tissue. The max implant (Southern Implants) was placed to 70 Ncm insertion torque into the ideal position (Figure 20).¹⁶

After healing abutment placement to protect the open screw channel of the implant, 1 cm³ of alloplastic bone graft (β tricalcium phosphate and calcium sulphate alloplastic bone graft, EthOss) was placed into the three sockets, buccal bony dehiscence and circumferential jumping gap around the implant. This was allowed to set before a PEEK anatomical healing abutment was fitted and the flap closed transmucosally, tension free, using 4.0 PTFE sutures (Figure 21).

An immediate post-operative radiograph showed optimal implant position and the alloplastic bone graft (Figure 22).

At 12 weeks after surgery, an open tray impression was taken of the implant using an impression coping, custom tray and additional cured silicon impression material. A definitive, screw-retained zirconia crown, lab-cemented to a custom anodized titanium abutment, was manufactured and fitted to 40 Ncm insertion torque. A proprietary screw channel obturation material (Silverplug, Silveraid, Italy), was placed and covered with flowable composite resin. A peri-apical radiograph was taken showing the baseline bone levels and fit of the implant prosthesis (Figure 23).

Conclusion

The age-old question of whether size and shape matters raises its head in the implant field where one must consider whether this increased range of dental implants is necessary. When considering a reduction in the number of surgical interventions for a given case, site-specific implants can open the opportunity for fewer surgeries, by allowing both predictable immediate implant placement and immediate tooth replacement therapy.

When considering long-term outcomes, implants that facilitate optimized crown emergence profiles, both to facilitate oral hygiene around the implant crown, but also the bulk of titanium of the implant connection, support the biomechanical notion of reducing the incidence of implant fatigue failures due to a mismatch between cyclic loading and the fatigue resistance of the implant.

The long-term evidence for mini implants, coaxial implants and max implants is now showing clinical outcome data comparable with more conventionally shaped implants and therefore widens the armamentarium of implant shapes and designs to meet each clinical challenge.^17,18,19

This progress in dental implant therapy raises issues in terms of education for both the dental profession and the public at large. This evolution means that primary care practitioners, particularly those who do not work in the dental implant field, need to understand the nuances and identify the type of dental implant that may be optimal for a patient who walks through their surgery door.

It appears that site specificity of dental implants is here to stay, but there are no moves by the myriad implant companies to standardize connections and component features. This means that it is vital that communication between treating clinicians and the patient is both contemporaneous and detailed to allow future interactions to be optimized for the patient.

With the advances in dental implant design and use, the dental profession must not neglect the elderly population who may not have the disposable income to afford some of this complex work. The use of mini implants, which cost at least 50–60% less than conventional implants and require much less clinical time, affords an opportunity for the profession not to neglect the needs of this cohort.

The progress in dental implants is to be welcomed but, as a healing profession, dental professionals must balance this with the need to bring both our colleagues and patients with us on this journey.