In Vitro Assessment of Immediate Implant Placement in Fresh Extraction Sockets versus Healed Sites Using a Polyurethane Mandibular Bone Model

Dental implant therapy has transformed the rehabilitation of partial and complete edentulism by providing highly predictable long-term treatment outcomes. One of the key clinical decisions in implant dentistry is the timing of implant insertion after tooth extraction, with protocols ranging from immediate placement in a fresh socket to delayed placement after complete healing of the alveolar ridge.

Immediate implant placement is attractive because it can reduce treatment time, reduce the number of surgical interventions and help preserve hard- and soft-tissue contours. However, survival rates and aesthetic success should not be interpreted as equivalent to primary mechanical stability. Immediate placement may survive clinically while still presenting a more demanding initial biomechanical environment than delayed placement.

The central mechanical challenge in immediate placement is the mismatch between implant geometry and extraction socket anatomy. Fresh sockets commonly create a residual circumferential gap or “jump distance” between the implant surface and socket wall, reducing direct initial contact. In contrast, healed ridges allow a more uniform osteotomy and broader mechanical engagement of the implant threads with cortical and cancellous bone.

Primary stability reflects the mechanical stiffness of the implant-bone interface at the time of placement and is influenced by bone quality, implant macrogeometry, site preparation and recipient site anatomy. Resonance frequency analysis (RFA) estimates interface stiffness through implant stability quotient values, whereas insertion and removal torque measurements reflect rotational resistance at the interface. These measurements are complementary rather than interchangeable and none of them alone represents biological osseointegration.

Controlled in vitro testing is useful when the research question is limited to immediate mechanical behaviour. Standardized artificial bone models remove patient-related variability and allow direct comparison of ISQ, torque values and micro-CT interface measurements in the same experimental setting. At the same time, such models do not reproduce healing, remodelling, saliva or loading and their conclusions must therefore be interpreted as mechanical rather than biological.

The present study addressed a focused gap in the literature by combining RFA, insertion torque, removal torque and micro-CT assessment of bone-implant contact and gap volume in a standardized mandibular polyurethane model. The null hypothesis was that implants placed in a simulated fresh extraction socket would not differ from those placed in a healed osteotomy model with respect to primary stability or implant-bone contact parameters.

This controlled in vitro experimental study was performed between January and June 2024 using standardized polyurethane Sawbones blocks intended to simulate type II/III mandibular bone. Because no human participants, animal specimens or identifiable clinical records were used, the experiment represented bench-top laboratory research; institutional administrative approval for laboratory conduct was obtained and patient consent was not applicable.

Sample size was calculated with G*Power version 3.1.9.7 using a projected effect size of 0.85, alpha of 0.05 and power of 80%, yielding a minimum requirement of 18 specimens per group. To protect against specimen loss or measurement error, 20 specimens were allocated to each group.

Forty polyurethane foam blocks (40 × 40 × 30 mm) with a 2-mm cortical shell of 40 PCF over a cancellous core of 20 PCF were used. This model approximates mandibular type II/III bone quality and therefore should not be extrapolated directly to softer maxillary bone or other jaw conditions.

Specimens were randomly assigned to two groups (n=20 each). Group A represented a fresh extraction socket model. A custom socket former created a premolar-like socket 8 mm deep, with a coronal diameter of 6 mm tapering to 4 mm apically. Group B represented a healed site model, in which standard osteotomies were prepared directly in intact blocks according to the manufacturer’s drilling protocol.

All implants were identical SLA-surfaced titanium implants measuring 4.0 mm in diameter and 10 mm in length, obtained from the same manufacturing lot to reduce inter-implant variability. One trained operator placed all implants to minimize technique variability; this improved standardization but also means that operator-specific technique effects cannot be excluded.

For Group A, the simulated socket was prepared first, after which the implant osteotomy was extended 3-4 mm beyond the socket apex to obtain apical engagement. For Group B, sequential drilling was performed with 2.0-, 2.8- and 3.5-mm drills at 800 rpm under copious irrigation. This design compared a fresh socket condition managed with clinically relevant apical engagement against a healed osteotomy condition rather than comparing two anatomically identical sites.

Implant stability was assessed by three complementary methods. Maximum insertion torque value was recorded at final seating with a calibrated surgical motor. RFA measurements were then obtained using an Osstell ISQ device from mesial, distal, buccal and lingual directions and the mean of the four values was used. Removal torque value was recorded as the maximum reverse torque required to initiate implant rotation.

Micro-CT scanning was performed with a SkyScan 1275 system at 100 kV, 100 μA and 15-μm voxel size. Reconstruction and quantitative analysis were completed in CTAn software. The region of interest encompassed the implant surface from the crestal level to the apical end within the prepared site. Bone-implant contact was defined as the proportion of implant surface directly contacting the surrounding bone substitute after standardized thresholding. Contact area was recorded in square millimetres and gap volume was defined as the non-contact void volume surrounding the implant inside the same region of interest. Thresholding parameters and region-of-interest selection were kept constant for all specimens.

Descriptive statistics included mean, standard deviation and 95% confidence intervals where appropriate. Shapiro-Wilk testing confirmed approximate normal distribution. Between-group comparisons were made using independent-samples t tests. Pearson correlation was used to evaluate associations among ISQ, ITV, RTV and BIC. Statistical significance was set at p<0.05.

All 40 implants were placed successfully without specimen fracture or implant damage and complete datasets were available for all specimens.

Primary Stability Measurements

Healed site models as showed in Table1, significantly higher primary stability than fresh extraction socket models. The mean ISQ was 72.4±4.8 in Group B and 64.2±5.6 in Group A (mean difference 8.2; p<0.001). Mean insertion torque was 38.6±6.2 Ncm in Group B and 28.4±5.8 Ncm in Group A (mean difference 10.2 Ncm; p<0.001). Removal torque values were 32.4±5.4 Ncm and 24.8±4.9 Ncm, respectively (mean difference 7.6 Ncm; p=0.002).

Table 1: Comparison of primary stability parameters between groups

Take-home message: the healed site model consistently outperformed the fresh socket model across all three mechanical stability measures.

Table 2: Bone-implant contact analysis results

Take-home message: the fresh socket model produced a markedly larger peri-implant gap, which likely contributed to lower BIC and lower primary stability.

Table 3: Pearson correlation coefficients between parameters

Take-home message: stability measurements and interface contact parameters were closely related, but these correlations should be interpreted as associations rather than proof of mechanism.

Bone-Implant Contact Analysis

Micro-CT analysis also favoured healed sites. As in Table 2, mean BIC was 68.3±7.4% in Group B and 52.6±8.2% in Group A (p<0.001). Contact area was higher in healed sites, whereas gap volume was substantially lower, confirming closer initial adaptation around implants placed in intact osteotomies.

Correlation Analysis

ISQ showed strong positive correlation with ITV (r=0.847, p<0.001), RTV (r=0.812, p<0.001) and BIC (r=0.792, p<0.001). ITV was also strongly correlated with RTV (r=0.884, p<0.001) and BIC (r=0.768, p<0.001), indicating that the mechanical measurements moved in the same direction across specimens (Table 3).

The present experiment demonstrated a consistent mechanical advantage for implants placed in healed site models. Healed blocks produced higher ISQ, ITV, RTV, BIC and contact area, together with markedly lower gap volume, indicating more intimate implant-bone adaptation at placement.

The explanation is biomechanical rather than biological. In the healed site model, the implant engaged an intentionally prepared osteotomy with more circumferential support from the surrounding material. In the fresh socket model, part of the implant surface remained adjacent to a simulated socket gap rather than directly contacting the bone analogue. This reduced the effective interface stiffness measured by RFA and reduced rotational resistance during insertion and removal.

The 8.2-point difference in ISQ and the 10.2 Ncm difference in insertion torque were substantial under the present model, but these values should not be over-interpreted as direct clinical thresholds. RFA reflects interface stiffness, not osseointegration and insertion torque may vary with drilling strategy, implant macrogeometry and local bone compression. The current findings therefore support relative comparison between the two models rather than absolute clinical cut-off values.

Micro-CT findings strengthened the interpretation of the mechanical data. Healed sites showed higher BIC and contact area, whereas fresh sockets showed significantly larger gap volume. This gap finding is particularly relevant to immediate placement because it highlights the central mechanical problem of fresh sockets: reduced direct contact around part of the implant surface at the moment of placement. Although grafting and membrane-based gap management were not tested in this experiment, the data support why such strategies are commonly considered clinically.

The strong positive correlations among ISQ, ITV, RTV and BIC indicate that these methods provide related but complementary information regarding initial fixation. Correlation, however, should not be interpreted as proof of mechanism. The study did not measure surface chemistry changes, biological healing or dynamic loading behaviour and these factors remain important when translating mechanical findings into clinical recommendations.

Several practical implications emerge from this standardized model. First, clinicians should anticipate lower initial stability in fresh sockets than in healed ridges when a gap is present. Second, immediate placement may require technique modifications aimed at improving primary fixation, such as apical engagement, careful osteotomy control or the use of implant designs intended to enhance mechanical locking. Third, stability measurements can assist with loading decisions, especially when immediate or early loading is being considered.

This study also has important limitations. Artificial polyurethane blocks do not reproduce living alveolar bone, blood supply, remodelling or the secondary stability phase. The socket model was idealized and did not simulate irregular extraction sockets, periodontal ligament remnants, thin buccal plates, infection or damaged socket walls. Only one implant size, one surface type, one socket configuration and one bone density were tested and no cyclic loading or fatigue testing was performed. Accordingly, the results should be interpreted as a controlled comparison of immediate mechanical behaviour rather than as a direct prediction of in vivo osseointegration or clinical success.

In this standardized polyurethane mandibular model, implants placed in healed sites demonstrated higher primary stability and closer interface adaptation than implants placed in simulated fresh extraction sockets. Healed site placement yielded higher ISQ, ITV, RTV, BIC and contact area, whereas fresh socket models exhibited larger peri-implant gap volumes.

These findings suggest that immediate implant placement in fresh sockets may require additional stabilization strategies and cautious loading decisions when initial fixation is limited. However, because the model did not simulate healing, remodelling, saliva or functional loading, the present results should be interpreted as mechanical baseline data rather than direct evidence of long-term clinical superiority.

1. Brånemark, P.I. et al. Osseointegrated Implants in the Treatment of the Edentulous Jaw. Experience from a 10-Year Period. Scandinavian Journal of Plastic and Reconstructive Surgery Supplement, vol. 16, 1977, pp. 1-132. Accessed 8 April 2026.
2. Hämmerle, C.H. et al. “Consensus statements and recommended clinical procedures regarding the placement of implants in extraction sockets.” International Journal of Oral and Maxillofacial Implants, vol. 19, suppl., 2004, pp. 26-28. Accessed 8 April 2026.
3. Araujo, M.G. and J. Lindhe. “Dimensional ridge alterations following tooth extraction. An experimental study in the dog.” Journal of Clinical Periodontology, vol. 32, no. 2, 2005, pp. 212-218. https://doi.org/10.1111/j.1600-051X.2005.00642.x.
4. Schulte, W. et al. “[The Tübingen immediate implant in clinical studies].” Deutsche Zahnärztliche Zeitschrift, vol. 33, no. 5, 1978, pp. 348-359. Accessed 8 April 2026.
5. Chen, S.T. and D. Buser. “Esthetic outcomes following immediate and early implant placement in the anterior maxilla—a systematic review.” International Journal of Oral and Maxillofacial Implants, vol. 29, suppl., 2014, pp. 186-215. https://doi.org/10.11607/jomi.2014suppl.g3.3.
6. Schwartz-Arad, D. and G. Chaushu. “The ways and wherefores of immediate placement of implants into fresh extraction sites: a literature review.” Journal of Periodontology, vol. 68, no. 10, 1997, pp. 915-923. https://doi.org/10.1902/jop.1997.68.10.915.
7. Javed, F. et al. “Role of primary stability for successful osseointegration of dental implants: factors of influence and evaluation.” Interventional Medicine and Applied Science, vol. 5, no. 4, 2013, pp. 162-167. https://doi.org/10.1556/ IMAS.5.2013.4.3.
8. Meredith, N. “Assessment of implant stability as a prognostic determinant.” International Journal of Prosthodontics, vol. 11, no. 5, 1998, pp. 491-501. Accessed 8 April 2026.
9. Botticelli, D. et al. “Hard-tissue alterations following immediate implant placement in extraction sites.” Journal of Clinical Periodontology, vol. 31, no. 10, 2004, pp. 820-828. https://doi.org/10.1111/j.1600-051X.2004.00565.x.
10. Sennerby, L. and N. Meredith. “Implant stability measurements using resonance frequency analysis: biological and biomechanical aspects and clinical implications.” Periodontology 2000, vol. 47, 2008, pp. 51-66. https://doi.org/10.1111/j.1600-0757.2008.00267.x.
11. Trisi, P. et al. “Implant micromotion is related to peak insertion torque and bone density.” Clinical Oral Implants Research, vol. 20, no. 5, 2009, pp. 467-471. https://doi.org/10.1111/j.1600-0501.2008.01679.x.
12. Anitua, E. et al. “Novel technique for the treatment of the extraction socket: a pilot study.” European Journal of Oral Implantology, vol. 8, no. 1, 2015, pp. 75-82. Accessed 8 April 2026.
13. Bernhardt, R. et al. “Comparison of bone-implant contact and bone-implant volume between 2D histological sections and 3D SRµCT slices.” European Cells and Materials, vol. 23, 2012, pp. 70-81. https://doi.org/10.22203/eCM.v023a05.
14. Lioubavina-Hack, N. et al. “Significance of primary stability for osseointegration of dental implants.” Clinical Oral Implants Research, vol. 17, no. 3, 2006, pp. 244-250. https://doi.org/10.1111/j.1600-0501.2005.01201.x.
15. Rodrigo, D. et al. “Diagnosis of implant stability and its impact on implant survival: a prospective case series study.” Clinical Oral Implants Research, vol. 21, no. 3, 2010, pp. 255-261. https://doi.org/10.1111/j.1600-0501.2009.01820.x.
16. Esposito, M. et al. “Timing of implant placement after tooth extraction: immediate, immediate-delayed or delayed implants? A Cochrane systematic review.” European Journal of Oral Implantology, vol. 3, no. 3, 2010, pp. 189-205. Accessed 8 April 2026.
17. Chen, S.T. et al. “Immediate or early placement of implants following tooth extraction: review of biologic basis, clinical procedures and outcomes.” International Journal of Oral and Maxillofacial Implants, vol. 19, suppl., 2004, pp. 12-25. Accessed 8 April 2026.
18. Novaes, A.B., Jr. et al. “Immediate implants placed into infected sites: a histomorphometric study in dogs.” International Journal of Oral and Maxillofacial Implants, vol. 13, no. 3, 1998, pp. 422-427. Accessed 8 April 2026.
19. Chen, S.T. et al. “A prospective clinical study of non-submerged immediate implants: clinical outcomes and esthetic results.” Clinical Oral Implants Research, vol. 18, no. 5, 2007, pp. 552-562. https://doi.org/10.1111/j.1600-0501.2007.01388.x.
20. Nkenke, E. et al. “Implant stability and histomorphometry: a correlation study in human cadavers using stepped cylinder implants.” Clinical Oral Implants Research, vol. 14, no. 5, 2003, pp. 601-609. https://doi.org/10.1034/j.1600-0501.2003.00937.x.
21. Sennerby, L. “Resonance frequency analysis for implant stability measurements. A review.” Integrated Diagnostic Update, vol. 1, 2015, pp. 1-11. Accessed 8 April 2026.
22. Calvo-Guirado, J.L. et al. “Peri-implant bone loss clinical and radiographic evaluation around rough neck implants: a 5-year study.” Clinical Implant Dentistry and Related Research, vol. 20, no. 1, 2018, pp. 3-8. https://doi.org/10.1111/cid.12549.
23. Raghavendra, S. et al. “Early wound healing around endosseous implants: a review of the literature.” International Journal of Oral and Maxillofacial Implants, vol. 20, no. 3, 2005, pp. 425-431. Accessed 8 April 2026.
24. Degidi, M. et al. “Primary stability determination by means of insertion torque and RFA in a sample of 4,135 implants.” Clinical Implant Dentistry and Related Research, vol. 14, no. 4, 2012, pp. 501-507. https://doi.org/10.1111/j.1708-8208.2010.00302.x.
25. Lang, N.P. et al. “A systematic review on survival and success rates of implants placed immediately into fresh extraction sockets after at least 1 year.” Clinical Oral Implants Research, vol. 23, suppl. 5, 2012, pp. 39-66. https://doi.org/10.1111/j. 1600-0501.2011.02372.x.
26. Sanz, M. and D. Cecchinato. “Current concepts on immediate loading of implants.” Periodontology 2000, vol. 66, no. 1, 2014, pp. 255-271. https://doi.org/10.1111/prd.12050.
27. Buser, D. et al. “Implant placement post extraction in esthetic single tooth sites: when immediate, when early, when late?” Periodontology 2000, vol. 73, no. 1, 2017, pp. 84-102. https://doi.org/10.1111/prd.12170.