Introduction

Immediate implant placement (IIP) is becoming an integral part of day‐to‐day implant dentistry, due to the gradual development of fine‐tuned techniques and more knowledge of the relevant biology. Maxillary premolars are frequently lost and can often be replaced by immediate implants if there is sufficient palatal and apical bone for implant anchorage and if socket morphology is favorable. Indeed, IIP replacement of maxillary premolars, especially single‐rooted second premolars, is a good place to start for inexperienced implant surgeons. Even a thin gingival phenotype seems not to be a contraindication for IIP as it can be addressed via phenotype modification using connective tissue grafts [1]. Further, if such IIPs are suitable for immediate provisionalization (IIPP), optimal esthetic outcomes can be anticipated.

As with other teeth in the aesthetic zone, facial bone thickness at premolar sites needs to be respected. The greatest post‐extraction ridge resorption occurs when facial bone thickness is less than 1 mm [2]. If proper protocol for IIPs is not followed, subsequent loss in alveolar horizontal ridge dimension after extraction at sites with facial bone thickness less than 1 mm can be catastrophic (mean 7.5 mm) [3]. In contrast, where facial bone thickness is initially greater than 1 mm, overall mean bone loss after gap grafting can be as low as 1.1 mm [4]. Maxillary premolars do have far fewer sites with facial bone thickness less than 1 mm compared with incisors and cuspids. Cone beam computed tomography (CBCT) measurements of buccal bone of premolars at 1–3 mm and 4–9 mm apical to the crest show mean facial bone thicknesses of 1.28–1.84 mm [5]. Rojo‐Sanchis et al. [5] studied facial bone thickness of first and second premolars in a Spanish population, finding them to be greater at second premolars than first premolars at all measured points (Table 3.2.1). The majority of second premolars had a minimum of 2 mm facial bone thickness at 2 mm apical to the crest. In a similar study in a Turkish population [6], facial bone thickness at first premolars was 1.11 mm at 1 mm, 0.7 mm at 3 mm and 0.48 mm at 5 mm apical to the alveolar crest compared with similar measurements at second premolars (1.39 mm at 1 mm, 1.42 mm at 3 mm and 1.22 mm at 5 mm from the crest).

Overall, facial bone thickness at maxillary premolars is greater than at maxillary incisors and canines, with second premolars generally exhibiting the thickest buccal bone (Figure 3.2.1).

Sagittal Root Position and Bone Angulation

As with maxillary incisors and canines, root position also plays a fundamental role in the resorption pattern of alveolar bone following maxillary premolar extraction [7]. It also determines the amount of available bone for anchorage of IIPs. Volumetric shrinkage of alveolar bone following tooth extraction affects both its horizontal and vertical dimensions [8]. Comprehensive assessment of root morphology and position using CBCT is essential in predicting whether sufficient primary stability can be achieved. The sagittal position of the root should show 3–5 mm of apical bone for IIP stabilization, while facial bone thickness should be estimated to predict the likelihood of it being lost following implant placement. Maxillary premolar sagittal root positions can be classified into four categories according to their relationships with their osseous housings (Figure 3.2.2) [9, 10]:

• Type B roots are positioned close to the buccal cortical plate including some direct contact.
• Type M are those premolars with roots located in the middle of their alveolar housing without engaging the buccal or palatal cortical plates.
• Type N premolars have at least two‐thirds of their root lengths engaging both buccal and palatal cortices, and
• Type L premolars are closer to the palatal cortical plate with some contact

It has been estimated that 60% of maxillary premolars are angulated so far buccally as to be in contact with the buccal plate [11], as opposed to 79% of maxillary incisors [12]. This situation coupled with the fact that buccal bone can be very thin means that even if the implant can be stabilized apically, significant gap grafting and/or contour augmentation grafting will likely be needed to avoid loss of buccal plate [13].

López‐Jarana et al. [14] reported angulations between root axes and basal/apical bone axes in 68% of maxillary first premolars to be 10–30 degrees compared with angulations of less than 10 degrees in 52% of second maxillary premolars. Nishihara et al. [15] also investigated angulations of maxillary premolar axes with implications for tooth extraction, implant positioning and prosthetic restorations. They reported mean internal angles of 25.5 degrees for first premolars and 18.1 degrees for second premolars. These data on the morphologic relationship between premolar angulation and alveolar bone housing are relevant when planning IIPs as they will impact peri‐implant soft tissue stability, occlusion, and force distributions with implant‐supported prostheses. Implant angulation also affects emergence profiles of final restorations and may compromise aesthetics. In the presence of a steep implant angulation, buccal bone fenestration is likely.

Maxillary Sinus Proximity

Proximity of the maxillary sinus can pose challenges with immediate premolar implant placement (Figure 3.2.3). Although issues with sinus proximity mostly relate to maxillary molars, mesial extensions of the sinus have been documented [16]. This appears clinically relevant and will necessitate thorough assessment (Figure 3.2.3) [17]. If there is insufficient bone below the sinus to stabilize an IIP, an alternative approach such as using shorter/wider implants or socket preservation grafting followed by delayed implant placement will be necessary.

The frequency of maxillary premolar roots approximating maxillary sinus has been reported to change with age due to antral physiological changes [18]. First premolar roots contact the sinusfloor less often than second premolars (3% versus 13%) [18]. While mean distances of first premolar roots from sinus floor are consistently greater, their palatal roots are closer than their buccal roots [17].

Jung et al. [19] proposed a classification of relationships between premolar roots and maxillary sinus (Figure 3.2.4) using cross‐sectional CBCT images. Four types were established:

• Type 0: the root makes no contact with the sinus floor.
• Type 1: the root is in close proximity to the sinus floor.
• Type 2: the sinus floor is located below the level of root apex without apical protrusion into the sinus.
• Type 3: the root protrudes into the sinus cavity.

The relationships between maxillary premolar apices and sinus floor will affect the decision as to whether IIP is possible. Most likely, types 2, 3 and 4 sites should not receive consideration. Sinus septa also may be seen at premolar sites [20] and their presence may limit available space and alter the trajectory of implant placement [21]. Fortunately, sinus proximity with premolars is less common than with molars [22]. One study [23] evaluated the complications that occurred during apicectomy of maxillary posterior teeth and found that maxillary first premolars had the least incidence of sinus perforations (2.0%) compared with second premolars (10.4%) and molars (23.0%).

Figure 3.2.5 shows a case where sinus proximity did not allow straightforward IIP placement. The clinician chose to do flapless extraction and socket preservation grafting with delayed implant placement.

Case Selection

Suggested Surgical Protocols

Attaining Bone Anchorage

Primary stability of premolar IIPs is dependent on having adequate apical and palatal bone, and may be limited apically by the floor of the nasal cavity or maxillary sinus [35]. As pointed out above, if sufficient basal bone is not available, alveolar ridge preservation and delayed implant placement are recommended [36].

Implant Selection

Appropriate implant selection, including length, width, and macro design, will affect outcomes with premolar IIPs (Figure 3.2.8). Longer implants are generally required for maximal bone engagement apically [36]. Tapered implants with deep threads also will assist with primary stability [37]. Appropriate implant diameter depends upon the buccopalatal dimension of the socket [36], and must respect the fact that a buccal gap of at least 2 mm (“jumping distance”) must be left for hard tissue gap grafting to ensure maintenance of or an increase in facial bone thickness (Figure 3.2.9) [32,38–41]. This may require more palatal positioning of the osteotomy [42]. For first premolars, the implant often can be placed in the palatal socket, but with engagement of the interseptal bone, while at second premolar sites, the osteotomy should be initiated slightly palatal to the central fossa of the former tooth. Taking this approach will leave adequate space buccally to allow significant gap grafting, which results in 54% less horizontal alveolar ridge width shrinkage when compared with immediate implants without buccal gap grafting [43].

Buser et al. [44] proposed that there should be a distance of at least 1.5 mm between an implant and its adjacent tooth. Vertically, the implant shoulder should be positioned at least 1 mm below the cementoenamel junction of the adjacent teeth, which is equal to 3–4 mm below the existing soft tissue margins. Implants with internal conical connections should be positioned at least 1 mm below the buccal crest to minimize future crestal loss [45].

Soft Tissue Modification

A review conducted by Seyssens et al. [46] summarized the effects of connective tissue grafting performed together with IIP in maintaining mid‐facial soft tissue levels. Eight studies were included, of which five were randomized controlled trials and three non‐randomized controlled studies (Table 3.2.2). Only one of these reports showed a low risk of bias. Nevertheless, meta‐analysis indicated that there was a significant difference in vertical mid‐facial soft tissue changes when connective tissue grafts were used. 25.6% of patients who underwent IIP without connective tissue grafts had asymmetries in their vertical soft tissue level of 1 mm or more. In contrast, only 1% of patients who underwent IIP with connective tissue grafting had these asymmetries, indicating that it can reduce the risk of asymmetry by up to 12 times. Some clinicians always perform connective tissue grafting with IIPs.

Most studies have clinicians performing IIP and soft tissue grafting in one surgery (Figure 3.2.10) [36]. However, there are advantages in postponing soft tissue grafting. First, with time after IIP, soft tissue remodelling, including thickening, may occur. In addition, the fragile blood supply of the buccal bone wall could be negatively affected when a buccal pouch is made for the connective tissue graft at the time of tooth extraction and IIP, possibly causing unwanted bone resorption [36].

Ideally, when maxillary premolars are to be replaced using IIPs, immediate provisionalization or at least a customized healing abutment will be part of the procedure, to maintain the original gingival contours [50, 51]. The prerequisite for this non‐occlusal immediate loading of IIPs is the attainment of adequate primary stability (≥ 35 Ncm) at the time of implant placement [52], with some authors even stressing a minimum insertion torque as high as 42 Ncm [53]. Implant survival/success rates with different loading protocols were investigated in a systematic review [1]. A survival rate of 98.4% and a success rate ranging from 88% to 100% was estimated for IIPs placed with immediate restoration/non‐occlusal loading (Figure 3.2.11). IIPs followed with early loading exhibited a survival rate of 98.2% and a success rate of 93.8–100%, while those placed with delayed loading showed a survival rate of 96% and a success rate of 91.8–100%. However, only IIPs with delayed loading have been properly validated in the literature [54], so clinicians should be mindful that ideal conditions for placement including adequate torque must be achieved if immediate or early non‐occlusal loading is being contemplated. At most, only centric contacts with no lateral excursive contacts should be established [55], but many investigators will avoid occlusal contacts entirely.

Discussion

Key issues in achieving success with maxillary premolar IIPs, as with incisors and canines, is successful management of their thin buccal bone and sagittal root positions. IIPs for second maxillary premolars can be more straightforward because of thicker initial buccal bone. However, proximity of the maxillary sinus is more likely to be an issue with second premolars and may require indirect sinus elevation, even if the apical bone is sufficient for implant stabilization. For inexperienced clinicians, second premolar sites may be the best place to start doing IIPs. However, no IIP is a simple undertaking, which may explain the findings of a systematic review and meta‐analysis from Cosyn et al. comparing immediate implant placement (IIP) to delayed implant placement [56]. Lower implant survival was found for IIPs (94.9% versus 98.9%). However, all failures were early, suggesting the reason to be improper case selection or operator inexperience [1]. In another systematic review, Mareque et al. [57] showed lower survival with IIPs compared with socket preservation plus delayed implant placement (92.7% versus 98.1%), so clinicians with less surgical expertise including the use of flapless surgery and soft connective grafting would be wise to take the latter approach.