Tapered Versus Inverted Body-Shift Implants Placed Into Anterior Post-extraction Sockets: A Retrospective Comparative Study

Immediate implant placement into post-extraction sockets with immediate provisional restoration or immediate tooth replacement therapy for single maxillary anterior teeth has been a viable treatment option in clinical practices for the past two decades.^1-3 Implant survival rates with this protocol are equivalent to delayed therapy, and therefore the goal in care currently is to achieve consistent esthetic outcomes.^4-7 Retrospective clinical studies on peri-implant tissue discoloration, however, have reported a range of 20% to 100% surrounding anterior implants, although in some studies it is unclear whether treated sites were delayed or immediate implants (Figure 1).^8-10 This result may be a consequence of labial plate and ridge contour loss with associated gingival recession, which can undermine a pleasing appearance (Figure 2 and Figure 3). In addition, loss of interdental papilla in the smile as a result of poor implant placement or excessive implant diameter and close proximity to an adjacent natural tooth, even with platform switching, can also detract from appearance (Figure 4 and Figure 5).^11,12

Several studies have alluded to the importance of establishing and maintaining a volume of vital bone of about at least 2 mm circumferentially around an implant whether in a healed ridge or extraction socket to avoid labial plate and papilla loss over time.^13,14 Typically, 2 mm of bone quantity is required for an endosteal blood supply, and the horizontal component of biologic width is about 0.7 mm to 1.5 mm with the latter amount associated with non-platform-switched implants.^15-17 This quantity of bone (2 mm) may be more challenging to achieve clinically in post-extraction sockets since the existing dimension may not allow adequate vital bone to be reformed relative to implant stability as a function of diameter. In addition, bone-level tapered implants have been developed to increase stability in extraction sockets yet are defined not only by a wider apical portion but also by the wider width extending to the coronal section as well. For instance, a 5 mm diameter implant is defined by the widest portion of the implant body, usually at or within a few millimeters of the implant platform, whether the implant is platform switched or not. As a consequence, the gap distance between the implant platform and adjacent teeth as well as the labial bone plate and implant body becomes diminished, which can lead to limited space for reformation and maintenance of the interproximal bone height and labial plate thickness. Collectively, these esthetic dilemmas can lead to an unacceptable result (pink esthetic score).¹⁸

A recently developed unique macro hybrid implant combines the design of a tapered implant with a narrow diameter and implant platform at the coronal aspect roughly 40% the length of the implant body.^19,20 This macro design incorporates an inverted "body-shift" or change in diameter, shape, and thread pattern in a singular form. Instead of retaining the existing wider coronal form of the implant body, the top portion is reduced to a standard (4 mm) or narrow-diameter (3 mm to 3.5 mm) implant body. The implant design is a combination of a wider 4.5 mm or 5 mm diameter implant at the apex and a narrow 3.5 mm or standard diameter 4 mm implant surrounding the implant platform (ie, 5.0/4.0 or 4.5/3.5 combinations). This implant is designed to specifically address the needs of maxillary anterior post-extraction sockets where the apical portion of the implant body is predominantly used to engage the residual socket walls and/or a few millimeters beyond for primary stability, and the reduced coronal portion allows for blood clot formation and placement of graft material. The implant design helps maintain the buccal plate thickness and interproximal bone, thereby reducing future esthetic problems such as midfacial gingival recession and papilla loss. Few studies have solely focused on the outcomes of maxillary single central incisor post-extraction socket implants.

Currently, no clinical and/or radiographic study exists that compares the outcomes of tapered versus inverted body-shift implant designs. Two hypotheses exist with the inverted body-shift design: (1) primary stability is increased due to an enhanced apical surface area of implant contacting bone, and (2) greater bone volume formation is facilitated at the coronal aspect of the implant body, which helps provide better esthetic results compared to standard tapered implants.

Therefore, the purpose of this retrospective study was to compare the design of tapered (T) versus inverted body-shift (INV) implants in maxillary single central incisor post-extraction sockets and evaluate: (1) implant primary stability and insertion torque value (ITV), (2) labial bone plate dimension (LPD), (3) tooth-to-implant distance (TID), (4) interdental marginal bone levels (MBL), and (5) pink esthetic score (PES).

The present investigation is a retrospective comparative study using data extracted from two studies performed prospectively (Western Institutional Review Board #1252367 and #1142064, respectively). The studies were performed in a single private practice in New York, NY, within the guidelines of the Helsinki Declaration of 1975.

Patients' records were analyzed to collect demographic data (sex, age), social habits (smoking), relevant medical conditions, implant data (type and brand), and intraoral location. Patients who had received replacement of a single maxillary central incisor tooth with immediate implant placement and provisional restoration from 2009 to 2019 were evaluated based on the following selection criteria: advanced subgingival caries, horizontal root fracture, endodontic failure, or root resorption (internal or external). Inclusion criteria for treatment were good systemic health of the patient, intact labial bone plate, absence of periodontal attachment loss or gingival recession, and presence of adjacent natural teeth. Exclusion criteria were general medical or psychiatric contraindications, pregnancy, diabetes, smoking more than half a pack of cigarettes per day, extraction sockets with dentoalveolar and/or soft-tissue dehiscence defects, and poor patient compliance. All patients provided written informed consent for treatment.

Cone-beam computed tomography (CBCT) (i-Dixel, J. Morita USA Inc, morita.com) and digital periapical radiographs were taken pre-extraction and 1 year post-surgery in both the tapered (T) and inverted body-shift (INV) implant groups. A single operator performed all measurements.

Immediate tooth replacement therapy employing the dual-zone therapeutic concept and clinical treatment protocol, as has been previously described and reported in the literature, was applied in this study.²¹ The surgical treatment protocol entailed flapless tooth extraction, thereby maintaining the periosteal blood supply to the interproximal and labial bone plate.²² Sharp dissection of the supracrestal gingival fibers was performed with a 15c scalpel blade prior to tooth removal. The residual socket was thoroughly debrided prior to osteotomy and implant placement. All osteotomies were performed according to the respective manufacturer's recommendation for site preparation. All implants were placed in the extraction socket with a palatal bias with the implant shoulder 3 mm in depth from the midfacial free gingival margin. Tapered (T) implants of similar design varied in diameter from 3.8 mm to 6 mm (mean 4.9 mm) and were 13 mm in length with a platform switch of 0.50 mm to 0.55 mm. These implants are shown in Figure 6: (left to right) Co-Axis^®, Southern Implants, southernimplants.com; Certain^®, Zimmer Biomet, zimmerbiometdental.com; Tapered Plus, BioHorizons, biohorizons.com; SuperLine, Dentium, dentiumusa.com. Inverted body-shift (INV) implants (Inverta^®, Southern Implants) ranged in apical diameter from 4.5 mm to 5 mm (mean 4.7 mm) and coronal diameter from 3.5 mm to 4 mm (mean 3.2 mm) and were 13 mm in length with a platform switch of 0.52 mm to 0.72 mm. These implants are shown in Figure 7: (left to right) Southern Implants IV-DC40-5013, IV-DC4012d-5013, IV-DC3512d-4513, occlusal view of INV implant.

Provisional restorations were fabricated using a polyether-ether-ketone (PEEK) temporary cylinder for screw-retained restorations. Preformed submergence profile root-form shells (iShell^™, Vulcan Custom Dental, vulcandental.com) were luted with autopolymerizing acrylic resin (Super-T, American Consolidated) to the PEEK cylinder. These provisional restorations possessed the subgingival contours that conformed to the pre-extraction state of the tooth root cervix to support the soft-tissue submergence profile and help protect the blood clot and contain the bone graft particles.²³ The labial and interproximal residual gap was filled to the level of the soft tissues with a small-particle (250 µm to 500 µm) mineralized cancellous bone allograft (Puros^®, Zimmer Biomet; MinerOss^®, BioHorizons) at the time of implant placement. Once this was accomplished, the provisional restorations were steam-cleaned prior to reinsertion with hand torque, and maintenance of the periodontal architecture was ensured immediately postoperatively.^24,25

All patients received antibiotic premedication that was continued through post-surgery and an analgesic as needed, and were seen 7 to 14 days postoperatively for follow-up. At least 4 months of healing was allowed before the first removal (disconnection) of the provisional restoration prior to analog impression-making. Implant-level impressions were made with a monophasic impression material (Flexitime^®, Kulzer, kulzer.com). The dental laboratory fabricated a soft-tissue cast (G-Mask, GC America, gcamerica.com) that allowed either screw-retained metal-ceramic or all-ceramic crowns to be fabricated and delivered approximately 8 weeks after final impression-making. All definitive crowns were screw-retained, seated, and torqued to the manufacturer's recommendation. After final restoration delivery, patients were placed on 6-month interval recall visits.

The following data were evaluated for the study:

Labial plate dimension (LPD): Measurements in millimeters (mm) were taken at two levels, L1 and L2, as previously described by Chu et al (Figure 8).¹⁹ L1 corresponded to the implant-abutment interface (IAI) equivalent to the midfacial labial plate bone crest; L2 matched the implant body roughly 5 mm from the IAI and bone crest coinciding with the upper portion of the INV implant transition zone where the diameter and shape shifts from a tapered to cylindrical form, roughly 40% the implant length. At each level, two reference points were identified: (1) the outermost aspect of labial bone plate, and (2) the first radiographic bone-to-implant contact point connected by a straight line perpendicular to the implant body. The distance between the two points at each level was measured (mm) using bundled CBCT digital imaging software (i-Dixel), and the bone plate thickness was recorded 1 year post-surgery (Figure 8).

Tooth-to-implant distance (TID): Since maxillary single central incisors were treated in this study, central-central (CI-CI) incisor distance as well as central-lateral (CI-LI) incisor distance was measured in mm from the implant platform to the adjacent tooth surface at the height of the implant-abutment interface proximally and perpendicular to the interdental bone using digital imaging software (ImageJ, nih.gov) (Figure 9).

Marginal bone levels (MBL): Proximal MBLs were measured from the implant platform to the interdental bone crest immediate post-implant placement and at recall on digital periapical radiographs using digital imaging software (ImageJ).

Implant primary stability: The insertion torque values were recorded in newton centimeters (Ncm) at the time of implant placement using an electric handpiece motor and manual torque wrench.

Pink esthetic score (PES): High-resolution photographic images were captured using a digital single lens reflex camera with a 105 mm macro lens and wireless twin (spot) flash system (D3200, R1C1, Nikon, nikon.com) at a 1:1 ratio and were rated by a single observer.^18,26

A single examiner (SJC) made all measurements twice and at least 24 hours apart. The intraexaminer error was calculated by comparing both first and second measurements with a paired t-testat a significance level of 5%. No statistically significance values were calculated between the values. Mean values and standard error (SE) were calculated for each category and compared between groups using paired student's t-test (P ≤ .05 = statistically significant difference) (GraphPad, graphpad.com).

Forty-two maxillary single central incisor implants were evaluated in this retrospective comparative study, including 21 implants in the tapered (T) group and an equal number in the inverted body-shift (INV) group. All implants in both groups employed a platform-switch design. The distribution of tapered implants by brand in decreasing order was as follows: Co-Axis, 43% (9/21); Certain, 29% (6/21); Tapered Plus, 19% (4/21); and SuperLine, 10% (2/21). The mean age of patients was 50 years old (range 26 to 79) with 38% males. All definitive restorations were screw-retained with 90% metal-ceramic and 10% all-ceramic in the T group, and 100% metal-ceramic in the INV group.

Implant primary stability: Mean insertion torque values were 36.19 ± 1.69 and 83.33 ± 4.99 Ncm for T and INV groups, respectively. The majority of implants (86%) in the T group were <50 Ncm whereas 71% of implants in the INV group attained ≥80 Ncm ITV with 57% reaching 100 Ncm.

LPD: Mean labial plate thickness was 1.18 ± 0.16 and 2.84 ± 0.16 mm at L1; 0.79 ± 0.14 and 2.56 ± 0.20 mm at L2 for T and INV groups, respectively.

TID: Mean CI-CI distance was 2.52 ± 0.18 and 3.06 ± 0.12 mm for T and INV groups, respectively. Mean CI-LI distance was 1.44 ± 0.11 and 2.40 ± 0.19 mm for T and INV groups, respectively.

MBL: Mean MBL between CI-CI was -0.02 ± 0.02 and -0.06 ± 0.02 mm for T and INV groups, respectively. Mean MBL between CI-LI was -1.06 ± 0.40 and -0.04 ± 0.02 mm for T and INV groups, respectively.

PES: The mean PES was 10.33 ± 0.79 for the T group and 13.29 ± 0.26 for the INV group.

Statistically significant differences were found between groups within each category except for CI-CI distance and MBL CI-CI (P > .05) (Table 1).

Figure 10 and Figure 11 exemplify pretreatment and post-treatment intraoral clinical images and radiographs of a patient who had received immediate tooth replacement with an INV implant.

Primary stability of implants has been associated with implant survival, especially in immediate post-extraction sockets where engagement of residual peripheral socket walls and apical bone is challenging. Although high primary stability is not an absolute requisite for achieving implant survival, it is important in situations of immediate provisional or final restoration.^27-29 Furthermore, implant diameter is more impactful than length, particularly in soft maxillary bone; therefore, undersizing (diameter) the osteotomy is performed frequently.^30-35 Wider-diameter implants, however, may pose the challenge of reduced gap distance between the implant body and the surrounding bony walls.

INV implants in this study achieved significantly higher primary stability and ITVs than tapered implants by an average of 47 Ncm, with a range of 30 Ncm to 100 Ncm; 57% reached a maximum ITV of 100 Ncm. These range values are consistent with a prior animal study in foxhounds showing high ITVs of body-shift implants without the presence of apical pressure necrosis.³⁶ Sixty-two percent of the INV implants used were the 4.5/3.5 implant diameter combination. Tapered implants had a range of 30 Ncm to 50 Ncm, with 86% below 50 Ncm with an average implant diameter of 4.9 mm.

The apical half or 6.5 mm of the 13 mm length macro hybrid implant body has aggressive threads (0.5 mm depth, 35-degree angle) yet a small pitch or distance between threads of 0.6 mm that creates an assertive pattern for increasing cutting and self-tapping capability during placement. An essential element in design is the thread pitch (distance between threads) or the amount of threads per unit length with increased surface area that is in contact with bone, not just the depth of the threads alone.^37,38

Since 2005 it has been known that implants placed into post-extraction sockets do not alter the wound healing and remodeling process of the socket; therefore, implant position and diameter is critical for osseointegration.^39-42 Hence the treatment strategy of reducing the coronal implant diameter is prudent in that this allows further graft material to be placed into the gap circumferentially since it is not being utilized in the socket anyway. Socket grafting the gap between the implant body and internal aspect of the labial bone plate is recommended to maintain ridge dimension and soft-tissue volume for esthetics.²⁵

The results of this retrospective comparative study demonstrate that there are statistically significant differences between the labial bone plate thickness of T versus INV implants measured at L1 and L2. The mean difference in millimeters between the groups was 1.66 (L1) and 1.77 (L2), which was more than double that of the T group. In addition, 19% of implants in the T group had no visible labial plate on CBCT at follow-up, while all INV implants had an observable labial plate. The clinical relevance is that INV implants had a thicker labial plate, more than 2 mm, which may enable more sustainability over time to maintain biologic and esthetic results. It is significant that the amount of the platform switch on the INV implants was greater than that on the T implants. Studies have shown that platform switching plays a significant role in marginal bone preservation; therefore, only platform-switched implants were included in the T group.^43-45 However, INV possessed a different platform-switched design called "variable" platform switching (VPS) (Figure 7). By physically shifting the abutment-implant junction further from the platform, more horizontal space was available for biologic width formation. Combining dual-zone bone grafting over the platform of the implant with this VPS design conceptually encourages more bone and maintenance for soft tissue support.

Anatomic esthetic studies have shown that there is no difference in papilla height between the mesial and distal papillae; therefore, TID may play a role in its preservation in post-extraction socket implant therapy.⁴⁶ In regard to TID, anatomically the space between central-lateral incisors is less than the space between the two central incisors.⁴⁷ During placement the implant tends to drift to the side of the socket where there is the least resistance and an open space, ie, the facial and distal walls. In addition, because the top portion of a tapered implant is wider, even with platform switching the implant will "bounce" off the palatal wall toward the labial-distal side. Therefore, the biologic and resultant esthetic risk is that the implant is placed too close (≤1.5 mm) to the lateral incisor tooth and the reformation of the horizontal component of biologic width (0.7 mm for platform-switched implants) can negatively affect the height of the attachment level on the adjacent natural tooth.¹⁷ One-third (7/21) of implants in the T group had a CI-LI distance ≤1 mm with MBL of about 1 mm. Comparatively, no implants in the INV group had a CI-LI distance ≤1 mm, and therefore they had stable interdental marginal bone levels. The INV implant is designed to eliminate this potential risk with the knowledge that narrow implants (3 mm to 3.5 mm) perform as well as standard-diameter implants (4 mm) in regard to survival and marginal bone stability.^48,49

Lastly, these dimensional differences on radiographs had a direct correlation to esthetic results and were evident when assessing PESs between groups, specifically midfacial recession, tissue discoloration, and papilla loss. Even though the average PESs for both groups were within the satisfactory range, the T group had a broader range (2 to 14) than the INV group (9 to 14).⁵⁰ Also, 43% of implants in the T group had a PES ≥12 compared to 95% for the INV group. The T group exhibited a higher rate of midfacial recession (7/21) and tissue discoloration (6/21) related to reduced labial plate dimension at L1 and L2 aside from papilla loss (9/21) between CI-LI with a distance of ≤1 mm.

Limitations within this study are that, first, INV implants had a shorter follow-up period than T implants, because these evaluated implants were recently introduced into the dental implant marketplace. Second, the study was performed as a retrospective comparative design without randomization of the patients. Future randomized controlled study with longer follow-up period will be needed.

This retrospective comparative study showed that an inverted body-shift designed implant may provide enhanced labial plate thickness and tooth-to-implant distance and less marginal bone loss by having a narrower coronal implant diameter, which may translate into improved clinical results.Inverted body-shift implants may offer advantages over tapered implants in maxillary anterior post-extraction sockets with regard to achieving higher primary stability and superior esthetics.

Stephen J. Chu, DMD, MSD, CDT
Adjunct Clinical Professor, Ashman Department of Periodontology and Implant Dentistry, Department of Prosthodontics, New York University College of Dentistry, New York, New York

Jocelyn H.P. Tan-Chu, DDS
Private Practice, New York, New York

Hanae Saito, DDS, MS
Clinical Associate Professor, Division of Periodontics, University of Maryland School of Dentistry, Baltimore, Maryland

Pär-Olov Östman, DDS, PhD
Adjunct Professor, Dental School University Hospital, James Cook University, Townsville, Australia; Private Practice, Falun, Sweden

Barry P. Levin, DMD
Clinical Associate Professor, Department of Graduate Periodontology and Periodontal Prosthesis, University of Pennsylvania School of Dental Medicine, Philadelphia, Pennsylvania; Private Practice, Jenkintown, Pennsylvania

Dennis P. Tarnow, DDS
Clinical Professor, Director of Implant Dentistry, Columbia University College of Dental Medicine, New York, New York

Drs. Chu, Östman, and Tarnow are paid consultants for Southern Implants. Dr. Chu is also a consultant for BioHorizons, and Dr. Tarnow receives honoraria from Southern Implants, BioHorizons, and ZimmerBiomet. The authors report that no corporate financial support was received for this study.

ACKNOWLEDGMENTS

The authors thank Guido O. Sarnachiaro, DDS, and Valentina Lyssova, DDS, for the surgical placement of implants and Adam J. Mieleszko, CDT, for the fabrication of definitive restorations in this study. The cases in Figure 1 through Figure 5 were referred to the authors.