Introduction

Immediate implant placement (IIP) in conjunction with immediate provisionalization, which in some scientific literature is referred to as “immediate tooth replacement therapy”, in anterior maxilla has received much attention in recent years [1, 2]. Clinical studies have confirmed that this approach offers significant benefit for maintaining alveolar ridge and peri‐implant soft tissue in this esthetically challenging zone [3–8]. When appropriately chosen and properly performed, immediate provisionalization results in shortened treatment times, fewer surgical interventions, and greater patient satisfaction. The esthetic outcome of immediate provisionalization with individual implant‐supported crowns will depend upon the original tooth shape, an appropriate three‐dimensional (3D) implant position, running room available to create a favorable emergence profile and symmetry with the neighboring teeth [9]. Specific treatment protocols, such as flapless surgery and hard tissue grafting of peri‐implant gaps, will be important. Other factors such as root length, diameter, and sagittal root position, proclination, and concavities in labial bone, available bone width and height, and the clinician’s level of expertise also will contribute [10].

Up to 1.2 mm of buccal contour collapse due to socket remodeling can be anticipated following immediate implant placement and provisionalization in the esthetic zone [10, 11]. It is thus essential that a horizontal gap (“jumping distance”) of at least 2mm be left between the thin buccal plate and buccal aspect of the implant and grafting it with a suitable bone substitute material to compensate for this inherent bone remodeling [10]. Palatal implant positioning is recommended to develop this gap, provide space for the prosthetic abutment and facilitate an adequate emergence profile and development of peri‐implant soft tissues of adequate thickness [12]. Proper implant positioning also is key to allow locating a screw access hole in the cingulum area of the prosthesis, and this can add significant complexity to the already challenging and technique‐sensitive procedures [13]. Another issue with immediate provisionalization is the role played by provisional restorations in supporting proximal and facial soft tissues. Properly contoured temporary crowns or custom healing abutments need to respect critical and subcritical contours needed to support interproximal papillae without exerting excessive pressure, which could lead to soft tissue recession [14–17].

Anatomy of Teeth, Alveolar Bone, and Sagittal Root Position

The anatomy of the anterior maxilla presents unique challenges for IIP, specifically the proclination of the alveolar ridge, which is seldom perpendicular to the occlusal plane, and therefore can complicate prosthetically driven implant positioning [18]. The long axes of conventional straight or uniaxial implant designs commonly do not coincide with the long axis of future clinical crown restorations [12]. The apical curvatures of buccal alveolar housings of maxillary anterior teeth have been assessed in cone beam computed tomography (CBCT) studies [18, 19]. These curvatures constrict caudally resulting in buccal undercuts of approximately 1 mm complicating negotiation of the osteotomy for an IIP, with possible encroachment of the thin avascular shell of buccal bone [18, 20]. Wang et al. have shown that, in anterior maxilla, the average angle of divergence between the long axis of central incisors and the long axis of their associated alveolar bone ranges between 10–20 degrees, while that of canine teeth and lateral incisors can display up to 30 degrees of divergence [18, 21, 22]. Maxillary anterior teeth also have disparities between the crown and root angulations, with the two having a biaxial relationship ranging up to 25 degrees (Figure 12.1) [23, 24]. Howes [25] reported that the offset between root and crown of maxillary incisors in establishing cingulum access ranges from 8 to 12 degrees. This often results in implants being placed with buccal–incisal inclinations, requiring costly custom abutments or cement‐retained restorations. In addition, the sagittal orientations of tooth roots frequently are positioned directly toward the buccal cortex of the alveolus [26], while immediate implants are usually placed to engage apical and palatal bone [18, 27].

To ensure sufficient initial stability of IIPs, apical bone should be available to receive 20–35% of the intended implant length [28, 29] usually requiring 4–5 mm of native apical–palatal bone. Kan et al. [26] proposed a classification of sagittal root positions of maxillary anterior teeth and determined that 81% belonged to their class I sagittal root position (i.e. with the root apex approaching the thin buccal plate of bone; Figure 12.1) This would indicate a probability of only 14% (10–24%) for successful immediate provisionalization using straight channel, screw‐retained single crowns [10]. When implants with appropriate diameters were used (3.5 mm for maxillary lateral incisors; 4.3 mm for maxillary central incisors and canines), the frequency of successful immediate provisionalization using straight screw‐channel, screw‐retained restorations was only 10–11%, although this probability was higher for maxillary central incisors at 10–24% when smaller‐diameter implants (3.5 mm) were used [10]. However, using smaller diameter implants can lead to lower insertion torque values and primary stability.

Apical Socket Perforation Rate

Prosthodontic and biologic maintenance issues are unavoidable in implant treatment, and it is crucial that implant crowns be retrievable and therefore screw retained. It is often difficult to achieve appropriate buccopalatal implant positioning to have a cingulum screw access hole appropriately positioned [3]. Some studies have shown that the ability to deliver a straight channel, screw‐retained restoration without apical buccal bone perforation by the implant can be as low as 10–24% using conventional root form uniaxial implants [10]. Although there is inconclusive evidence pertaining to the impact of buccal fenestration/dehiscence defects and the long‐term survival of implants [30], it is best to avoid them where possible. In a 2022 study of 321 CBCTs [31] used to investigate the prevalence of adequate bone anatomy for placement of IIPs with proper angulation for screw‐retained restorations at maxillary central incisor sites, it was found that if implants were placed without considering site anatomy, 29% would end up too close to the buccal plate apically or even perforate it. This is in line with other CBCT studies, which have shown 22.6–26.07% apical perforation rates [32, 33]. Buccal bone perforation, if detected, can be immediately addressed by raising a flap and grafting with a bone substitute material and barrier over the exposed implant threads, but this adds cost, morbidity, treatment duration, and higher risk of complications.

Song et al. [18], in an observational, cross‐sectional study using virtual planning, investigated the apical socket perforation rate (ASPR) when a uniaxial implant was simulated with delivery of an immediate screw‐retained restoration in anterior maxilla (maxillary second premolar to second premolar). Their data were obtained from the INVERTA^® (Irene, South Africa) data registry (a secure repository for outcomes using an inverted body‐shift design and dual‐axis restorative interface) of immediate postoperative CBCTs (Figure 12.2). Simulated virtual surgical planning using uniaxial implants at sites previously assessed using dual‐axis implants placed with screw‐retained prostheses revealed a high ASPR (48.51%). The authors stressed the importance of careful consideration of angular disparities between the extraction socket–alveolus complex and future restorative emergence to ensure a harmonious biologic–esthetic treatment outcome.

Cement‐Retention Versus Screw‐Retention and the Issue of Buccally Inclined Implants

Screw‐retained crowns have two main advantages over cement‐retained crowns [31]. First, they are easier and less costly to retrieve [34], and second, they do not have the potential to cause biologic issues related to undetected, retained cement flash [35]. To avoid buccal bone perforation, maxillary anterior uniaxial IIPs are often placed with a slight buccal inclination, but this can necessitate restoration with cement‐retained restorations [18]. Unfortunately, this positioning has the risk of favoring soft tissue recession over time compromising esthetics [36–38]. Sanz‐Martin et al [39] published results of a systematic review and meta‐analysis in 2022, to investigate factors associated with buccal soft tissue dehiscence defects at dental implants. They concluded that buccally directed implants (odds ratio, OR, 14.37) and thin soft tissue biotype (OR 2.85) constitute significant risks [39]. The more subgingival the abutment margin of a cement‐retained implant crown, the more difficult it is to ensure complete cement removal [40], and therefore a greater risk of developing peri‐implantitis [41–44]. In a 5‐year follow‐up study, Wilson examined 42 implants that had been placed in 39 patients and found that residual cement was associated with 81% incidence of peri‐implantitis [10, 45]. It should be noted, however, that significant differences exist in the findings of systematic reviews comparing cement‐ and screw‐retained implant‐supported prostheses.

Angle‐Correcting Abutments and Angled Screw Channels

Buccally oriented implant angulations may be corrected prosthetically by using individual custom‐made abutments. However, these abutments can sometimes lead to soft tissue recessions, owing to the large space needed to accommodate them, and of course invoke additional costs [18]. With their angle‐corrections existing coronal to the buccal crest of bone, especially in the presence of thin tissues, pressure may cause recession and esthetic failure [46]. In addition, angle‐correcting abutments often limit the possibility of screw retention. Furthermore, finite element analyses, photoelastic stress analyses, and strain gauge studies have found that angle‐correcting abutments adversely affect how stresses are distributed to the surrounding bone and prosthetic components [47, 48]. Omori et al. found a statistically and clinically relevant higher implant loss (11.7 and 1.6%, respectively) at 1‐year follow‐up for implants supporting angulated compared to straight abutments. Likewise, after 1 year, implants supporting angulated abutments yielded significantly more marginal bone loss than those supporting straight abutments [49].

Implant manufacturers are aware of these angulation issues and have developed alternative solutions. For example, angulated screw channel abutments allow screws to be accessed at an angle of up to 25 degrees relative to the implant axis, and have been shown to be a viable option to overcome problems when implants cannot be placed in the ideal 3D position in anterior or posterior dentitions [50]. However, limitations include increased product costs, the need for additional space, and increased risk of soft tissue complications [50]. With angulated screw‐channel abutments, the restorative angle correction again emerges coronal to the level of crestal bone and can put unwanted pressure on supracrestal soft tissues with recession [51]. In addition, a wide channel is required to allow for proper engagement of the screwdriver, possibly causing a thinning of the buccal ceramic, again with potential aesthetic complications [52]. Also, it was recently demonstrated that achieving and maintaining appropriate screw‐tightening torque with these abutments may not be possible [53].

The cross‐pin retained implant‐supported restoration also has been proposed [50]. Here, a transverse screw is used to secure the crown to a milled implant abutment, allowing for prosthetic retrievability irrespective of dental implant alignment [54]. However, this solution results in a gap at the crown–abutment interface that may lead to bacterial leakage, possibly resulting in malodor and fistula formation [55, 56].

A distinction must be made between the tilting of a straight implant with supra‐crestal angled correction by way of angulated abutments and implants with sub‐crestal angular correction within the neck of the fixture.

Subcrestal Angle Correction: Development of a Root Form Dual Axis Implant

The first angulated implant was the zygomatic implant developed by Branemark and placed in 1991 [25]. It was designed with angular correction within the head of the implant. This allowed for screw retention of prostheses supported by severely tilted implants used to avoid the maxillary sinus and engage zygomatic bone. This implant has been used extensively with resorbed and compromised maxillae in oncology and trauma [25].

Howes [25] described development of an implant with a 12‐degree subcrestal angle correction (SAC) suitable for screw retention and originally designed to overcome the anatomic constraints of anterior maxilla (Figure 12.3), leading to a novel implant design introduced as the Co‐Axis® implant (Southern Implants, Irene, South Africa; Figure 12.4). Angle correction occurs within the implant neck and does not interfere with the design of the suprastructure nor occupy vertical space for the correction. Hence, a normal screw‐retained prosthetic reconstruction using normal cylinder components at fixture level is possible [50]. A sample IIP case using this implant to replace a tooth is displayed in Figure 12.5. In contrast, Figure 12.6 shows a case where the implant was used to replace a failed uniaxial implant.

Dual axis implants have been in clinical use since the early 2000s. They allow the use of longer implants at IIP sites and immediate provisionalization to optimize engagement of native bone apically ensuring adequate primary stability. They also allow screw‐retained implant restorations in class I IIP sites, as described by Kan et al. [26] (Figure 12.1), which is not commonly possible with uniaxial implants (Figure 12.7). This could help to explain the findings by others that survival of uni‐axis IIPs is often less than delayed implant placement (DIP) after previous socket preservation grafting (SPG) at the time of tooth extraction. For example, Cosyn et al. [57] published a systematic review comparing these treatment options with a meta‐analysis showing significantly lower implant survival for IIPs (94.9%) compared with SPG/DIP (98.9%; p = 0.020), all failures being early. This difference may reflect the technical difficulty in attaining primary stability in extraction sockets with uni‐axis designs.

Co‐Axis implants (Figure 12.8) have either a 12 degree (five different connections), 24‐degree (only in external hexagon connection), or 36‐degree (only in external hexagon connection) angulation correction option in the implant head, often allowing implant placement in proclined maxillary alveolar bone with the prosthetic platform aligned to enable screw‐retained restorations.

For surgical placement, the dual axis root form fixture is connected to an angle correcting mount with reciprocal inclination for the angulated implant platform, enabling the fixture to be inserted in a straight axis and orientation (Figure 12.9). The SAC design, when used in anterior maxilla, reduces the need for grafting procedures and facilitates immediate loading [25].

Screw Loosening

Abutment screw loosening is a common complication with uniaxial implants ranging from 2% to 45%, but more commonly in the range 5–10% [58–61]. The clinical consequences range from gross changes, such as mobility and separation, to microscopic issues such as micromotion at the implant–abutment joint and bacterial microleakage. These problems can be challenging and may sometimes necessitate treatment revisions [58]. In an in vitro study, Hotinsky et al. [59] assessed abutment screw loosening in dual axis implants compared with straight implants subjected to simulated non‐axial occlusal loading. Seven external hexagon connection 12‐degree angulation‐correcting implants and seven straight implants were embedded in acrylic resin and titanium abutments secured to them with titanium screws tightened to 32 Ncm. Each specimen was placed in a tooth wear machine and subjected to 1 000 000 cycles of 50‐N non‐axial load to simulate 1 year of clinical service. The resulting mean abutment screw removal torque values were calculated, as were the associations between number of cycles and abutment screw removal torque. Mean abutment screw torque loss was 59.8% for the dual axis group compared with 68.7% for the straight implant group [58].

SAC (subcrestal angle correction) implants resulted in the abutment screws being loaded at 18 degrees compared with 30 degrees with straight implants. Significantly greater mean abutment screw removal torque was recorded for the dual axis implant group compared with the straight implant group after 1 000 000 cycles (p = 0.019), and the authors suggested this difference might be explained by the reduced angle of cyclic loading to abutment screws. Based on the findings, it was concluded that: (i) a significant loss of abutment screw torque occurred in both groups with increased cycles of occlusal loading; and (ii) the dual axis implants resisted screw loosening significantly more than the straight implants after 1 year of simulated occlusal function [58].

In a 2024 in vitro study conducted by Pellew and Dudley, a similar conclusion was drawn for 24‐degree SAC implants (i.e. angulation‐correcting implants resisted screw loosening significantly more than straight implants) [62].

Variable Platform Switching

Previous studies have shown that implants using platform‐switching show significantly greater midfacial soft tissue thickness than those with platform‐matching abutments in immediate tooth replacement therapy [63], while Canullo et al. demonstrated that the greater the platform switch, the more favorable was the maintenance of marginal bone levels [64]. Levin et al. [65] investigated any synergistic impact of dermal allograft on buccal soft tissue thickness with SAC macro design implants compared with conventional uni‐axial implants in immediate tooth replacement therapy [65]. The SAC macro design features a larger horizontal displacement of the implant–abutment junction (“platform shift”) on its buccal aspect compared with uniaxial implants, and as a result showed significantly thicker buccal soft tissue thickness (3.74 ± 1.05 mm vs. 2.85 ± 0.47 mm, respectively; p = 0.05).

Macro Hybrid Subcrestal Angle Correction Implant (Body Shift Concept)

Adequate buccal bone plate thickness and interproximal tooth‐to‐implant distance equal to 1.5–2.0 mm have been suggested as requirements for maintenance of buccal and interproximal bone [1466–68]. While smaller‐diameter implants will help to maintain buccal plate and interproximal distance, achieving adequate primary stability with them can be a challenge for immediate provisionalization. On the other hand, wider‐diameter implants are effective in increasing primary stability, but can result in inadequate buccal gap distance, together with the elevated risk of apical fenestration of the buccal socket wall [14, 69].

To address these issues, a platform‐switched, macro hybrid implant design was created with SAC (Inverta Co‐Axis, Southern Implants, Irene, South Africa) by Dr. Stephen Chu. It offers the benefits of both a tapered and cylindrical design in a single implant body. This hybrid implant is unique as it has a built‐in “body shift” in both diameter and shape [70]. It provides the primary stability of a larger‐diameter implant with apical aggressive threads (thread depth of 0.5 mm, angle of 35 degrees, and pitch of 0.6 mm) to increase cutting capability, but also the bigger gap distance crestally of a smaller‐diameter implant both labially and interproximally [14]. Because the diameter of the coronal part of the implant is reduced uniformly, gap width is increased circumferentially, providing a “bone chamber” for additional volume of hard tissue biomaterial in immediate extraction sockets compared to conventional tapered or other implant designs (Figure 12.11) [14]. A sample case using this hybrid implant is shown in Figure 12.12.

In a clinical multicentre 1 year prospective study [14, 33], single Inverta implants combined with SAC were placed immediately after extraction and temporized with screw‐retained provisionals. Mean insertion torque was recorded as 65 Ncm (range 45–100 Ncm). All definitive restorations were screw‐retained with the subgingival SAC interface. Short‐term survival rate (1 year) was 100% with a mean pink esthetic score (PES) of 12.5 of a possible 14.0 (range 9.0–14.0). Some 88% of the implant sites had a mean PES approximating 13.0, with the remainder having an average score of 10.3 and only one patient with a score of 9.0 [14]. Ostman et al. [71] continued to monitor 19 of the 33 implants to 18–24 months and reported that interproximal bone crest width, distance, and height remained stable both mesially and distally. A mean buccal plate thickness was reported as 1.82 mm with a mean PES of 13. Others published results of a prospective single‐arm cohort study [65] with this implant placed immediately into flapless extraction sockets in combination with dual zone socket grafting and immediate provisionalization reporting 12.79 as their average PES [65]. Finally, a retrospective comparative study [72] assessed PES with conventional tapered uni‐axial implants versus body‐shift dual axis implants, both groups having been placed immediately via flapless extraction protocol and dual zone socket grafting with immediate provisionalization. The average PES recorded for the conventional group was 10.33 compared with 13.29 for the body‐shift dual axis group. Additionally, significant differences (p ≤ 0.05) were found between the two groups in their primary stability and circumferential bone volume at recall, the dual axis implants being better. A final case is shown in Figure 12.13.

Conclusions

Implants with built‐in subgingival angle correction for abutment connection are one of the recent innovations developed to help in cases of IIP and immediate provisionalization at maxillary anterior sites. They allow deviations, often substantial, in tooth root angulation compared with alveolar ridge angulation to be easily overcome in implant site development. Their inherent platform switch feature helps to develop and maintain sufficiently thick buccal soft tissues to minimize the risk of their recession post‐restoration.