All-on-4: Bone density and prosthetic material put to the test

While the All-on-4 concept has established itself as a standard solution for full-arch rehabilitation, the variability of medium-term clinical outcomes remains a major concern in implantology. The practitioner is often faced with a crucial question: to what extent do initial bone quality and the choice of prosthetic material truly dictate implant survival and complication rates? This multicentric study provides concrete answers by analyzing data from a large cohort of 250 patients (125 maxillary and 125 mandibular).

The precise objective of this study was to evaluate the independent influence of bone quality (classified according to Lekholm and Zarb) and the type of prosthesis (metal-acrylic versus monolithic zirconia) on 1,000 implants followed for an average duration of 3.6 ± 0.4 years. The authors tested the hypothesis that low bone density (Type 4) and the choice of prosthetic material significantly impact primary stability, survival rate, and the frequency of mechanical or biological complications. The stakes are high: to refine clinical decision-making criteria to secure the long-term prosthetic prognosis.

Study methodology

This retrospective multicenter cohort study analyzed the clinical outcomes of the All-on-4 technique over a mean follow-up period of 3.6 ± 0.4 years. Data were collected from 250 completely edentulous patients (equal distribution of 125 maxillary and 125 mandibular cases), for a total of 1,000 implants placed between September 2018 and September 2021.

The protocol stratified the samples according to two major independent variables:

• Bone quality: implant sites were classified from Type 1 to Type 4 according to the Lekholm and Zarb classification.
• Prosthetic material: complete prosthetic rehabilitations were either metal-acrylic or monolithic zirconia.

The clinical parameters recorded included insertion torque and the implant stability quotient (ISQ). The authors evaluated the implant survival rate, the frequency of peri-implant complications (peri-implantitis, screw loosening), and prosthetic mechanical complications. The patient-reported outcome measure was assessed via a satisfaction score on a visual analogue scale (VAS). Statistical analysis compared these results based on bone density and the type of prosthesis used.

Analysis of clinical performance and implant survival

This multicentre retrospective study, involving 1,000 implants in 250 patients followed for 3.6 ± 0.4 years, confirms the viability of the All-on-4 concept while highlighting major disparities according to the clinical environment. The overall implant survival rate stands at 94.8%, but initial bone density proves to be a critical predictor of failure.

The results show that patients with Type 4 bone experience significantly more peri-implant complications, including episodes of peri-implantitis and prosthetic screw loosening. This correlation highlights the impact of primary stability and the quality of mineral support on the long-term success of the rehabilitation.

Impact of the prosthetic material on maintenance

The choice of the final prosthesis material directly influences the rate of mechanical complications. The study compares metal-acrylic structures to monolithic zirconia restorations, revealing a clear superiority of the latter in terms of structural reliability.

Despite these technical and biological differences, overall patient satisfaction remains high and consistent. The mean score on the visual analogue scale (VAS) reaches 8.7 ± 1.2, regardless of bone quality or the type of prosthesis used.

Clinical analysis and study limitations

The results of this multicentric study highlight a major clinical reality: while the All-on-4 concept is globally predictable, it is not exempt from specific risks related to the biological environment. The significant drop in the survival rate to 89.1% in Type 4 bone (compared to 98.3% in Type 1) demonstrates that low bone density is an independent risk factor for implant failure and peri-implant complications. For the practitioner, this suggests that primary stability and insertion torque, although sufficient for the initial protocol, are severely challenged by a less favourable bone biology in the medium term.

From a prosthetic perspective, the superiority of monolithic zirconia is striking. With three times fewer mechanical complications than metal-acrylic (8.9% versus 26.9%), this material has established itself as the gold standard for minimizing post-prosthetic maintenance interventions, although patient satisfaction remains high (8.7/10) regardless of the material used.

The study, however, has inherent limitations due to its retrospective design and its 3.6-year follow-up. Although the cohort of 1,000 implants is robust, these data reflect medium-term results. A longer follow-up period would be necessary to evaluate whether the trend towards complications in type 4 bone stabilizes or increases over time.

Implications for practice

These data allow for the refinement of informed consent and the therapeutic strategy in the practice. In the presence of low-density bone, increased caution is required during immediate loading. Furthermore, prioritizing monolithic zirconia appears to be the most effective strategy to ensure the mechanical durability of the rehabilitation and reduce the workload associated with prosthetic repairs.

Summary of results

After 3.6 years of follow-up, the All-on-4 concept shows an overall survival rate of 94.8%, but bone density remains the critical factor: success drops to 89.1% in type 4 bone, compared to 98.3% in type 1. The study also demonstrates the superiority of monolithic zirconia, which reduces the prosthetic complication rate to 8.9% compared to 26.9% for metal-acrylic.

In concrete terms, for the practitioner:

• Increased vigilance in type 4 bone: Low density is directly correlated with lower survival rates and more peri-implantitis; adapt your drilling protocol to optimize primary stability and strengthen post-operative follow-up.
• Prioritize monolithic zirconia: For the longevity of the restoration, prefer zirconia over metal-acrylic to reduce the risk of mechanical complications and prosthetic maintenance by three.
• Maintain confidence: Despite the increased biological risks in porous bone, patient satisfaction remains stable and high (8.7/10), confirming the clinical viability of the All-on-4 approach for complete edentulism.

Technical lexicon

Lekholm and Zarb classification: Bone quality assessment system based on the proportion of cortical and trabecular bone, ranging from Type 1 (dense cortical bone) to Type 4 (thin and low-density trabecular bone).

Implant Stability Quotient (ISQ): Measurement of implant stability by resonance frequency analysis, reflecting the stiffness of the bone-implant interface.

All-on-4: Implant rehabilitation concept allowing the support of a fixed full-arch prosthesis on four implants, of which the two posterior ones are tilted to optimize bone anchorage.

Monolithic zirconia: Prosthetic material machined from a single block of zirconium dioxide, offering superior mechanical resistance to fractures compared to layered structures.

Insertion torque: Rotational force measured during the final insertion of the implant, a major clinical indicator of primary stability.

Peri-implantitis: Inflammatory process of infectious origin affecting the tissues surrounding a functional implant, resulting in progressive marginal bone loss.

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