CONTROVERSIA I

 

Alternatives to maxillary sinus elevation: short implants

Alternativas a la elevación de seno maxilar: implantes cortos

 

 

J. González Lagunas

Servicio de Cirugía Oral y Maxilofacial e Implantología. Hospital Quirón, Barcelona. España

Dirección para correspondencia

 

 

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ABSTRACT

Poor bone quality, extreme bony resorption and a pneumatized mqxillary sinus are a severe Challenger for reconstruction of the atrophic edentuous posterior maxilla. Sinus lift is probably the gold standard for Management of that area. Short implants (<10mm) have traditionally been associated with lower survival rates. The introduction of new surfaces and designs indicate that this type of implants can adequately support dental restorations. We present the technique of installation of sinterised porous dental implants together with an update on the behaviour of short implants.

Key words: Dental implants, Tooth loss, maxillary surgery.

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RESUMEN

La mala calidad del hueso, reabsorciones extremas y la presencia de la cavidad del seno maxilar constituían obstáculos insalvables para la rehabilitación implanto-soportada en el sector posterior del maxilar atrófico. La elevación de seno es probablemente la primera opción de tratamiento en el paciente tributario de rehabilitación con implantes en ese sector. El uso de implantes cortos (menos de 10 mm) se ha asociado tradicionalmente con unas tasas de supervivencia menores que la de los implantes largos. No obstante la aparición de nuevas superficies y diseños indican que este tipo de implantes puede soportar de forma fiable restauraciones dentales. Presentamos la técnica de instalación de implantes de superficie porosa sinterizada, asi como una revisión actualizada sobre el comportamiento de los implantes cortos.

Palabras clave: Implantes dentales, Maxilar edéntulo, cirugía maxilar.

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Introduction

The surgical management of the posterior maxilla has challenged professionals since modern implantology began.¹ Poor bone quality, extreme reabsorption, and the presence of the maxillary sinus cavity were insurmountable obstacles for implant-supported rehabilitation of the sector. The maxillary sinus elevation technique introduced by Philip Boyne² in the early 1980s was quickly accepted and integrated into practice. At present, sinus elevation is probably the first therapeutic option in candidates for implant rehabilitation of an atrophic posterior maxilla.

Maxillary sinus elevation has been shown to be a reliable technique in the management of these patients. Unfortunately it can be more expensive and accompanied by more morbidity and the risk of sinusitis. In addition, certain circumstances of the patient or surgeon may make the procedure inadvisable.

On the other hand, short implants (less than 10 mm) have been associated traditionally with lower survival rates than long implants. Nonetheless, the appearance of new surfaces and designs indicate that this type of implant can support dental restorations adequately.

The use of short dental implants with a sintered porous surface is a simple and reliable alternative when any contraindication against sinus elevation exists. We report the technique for placing these implants and update our review of their use.

 

Surgical technique

The model used is a system of sintered porous-surfaced implants (Endopore, Dental Implant System, Innova Corporation, Toronto, Canada). This type of surface allows bone ingrowth into the connecting pores of the implant surface.

Figures 1-11 show the surgical sequence of the placement of this type of implants.

 

Discussion

The posterior maxilla has anatomic and morphologic characteristics that complicate the placement of dental implants. The combination of reduced bone volume with deficient bone quality and a frequently hyperpneumatic antral cavity means that the surgeon often must resort to advanced techniques.^1,3

Sinus elevation to facilitate implant placement in the atrophic posterior maxilla was introduced by Philip Boyne in 1980.² Numerous studies have been published in the literature using different approaches, graft materials, and implants with a variety of surface characteristics. ^4-6

Although sinus elevation usually is the first option in the management of atrophic posterior maxilla,⁴ a series of conditions exist (Tables 1 and 2) that make it necessary to resort to other procedures.

 

 

Short implants

In order to achieve reliable primary retention, the use of dental implants at least 10 mm long is generally recommended. When implants of less length are used, they are referred to as short implants. Threaded short implants have been used with acceptable results, but the introduction of sintered implants represented a considerable improvement in osseointegration ratios.^11-13 This type of implants, which have a truncated cone shape and are placed using a pressin technique, can be used in areas of limited bone dimensions because an interface is established with the surrounding bone that allows mechanical anchorage by bone growth into the sintered porous surface. These characteristics mean that even short implants have high survival rates.

Short sintered surface implants

The design of the implant consists of a porous surface (300-micra thick) composed of spherical particles of a titanium alloy affixed to the machined implant nucleus by high-temperature sintering. This process results in the formation of sintered necks, or regions of union between the individual spherical particles and between the particles and implant core. This neck ensures that the porous surface is an integral part of the implants and not just a coating. This creates a surface that not only increases the contact surface, but encourages bone growth to penetrate the interconnected porous network.^7,8

This type of implants has the form of a truncated cone that narrows apically and it is placed by means of a press-in technique. The implant has a smooth neck 1-2 mm high and a sintered porous surface on the rest of the implant.

Studies conducted on animals of implants with a sintered surface indicate that if they meet minimal surgical requirements (avoidance of micro-movements in the initial phases and nonexposure of the sintered surface)^9-11 they are as successful as threaded implants, despite the enormous ease of placement. ¹² In addition, these implants can have a smaller length while developing a fixation equivalent to threaded implants of equivalent length,⁷ as shown by the results of Deporter.^13-15 In 2001, 151 implants were placed in 50 patients, of which 76.8% were placed in the posterior maxilla. The mean implant length was 8.7 mm. At 3 years the survival rate was 97.3% (failure of 4 implants) and all the implants were loaded with prostheses.

Crown-root ratio

The crown-root ratio is used habitually in the design of tooth implant-supported prostheses, due to the binding mechanism of the periodontal ligament. This elastic structure responds to occlusal forces and may be the source of dental mobility. A crown/root ratio of 2/1 is considered to be the most suitable for natural teeth. The same principle has been applied to threaded implants.¹⁶ However, in a recent study of 262 machined short implants with 53 months of follow-up, Tawill et al.¹⁷ observed that an unfavorable crown/implant ratio was not a risk factor for implant failure, as long as the direction of the forces, load distribution, and parafunctions are controlled.

Bone healing-reabsorption

With the proper surgical protocol, the sintered surface promotes rapid initial bone repair. Simmons et al.¹⁸ suggested that these implants could be loaded in 5-6 weeks, although the standard times are usually 4 months for the maxilla and 3 months for the mandible. The pattern of bone crest remodeling is confined to the smooth machined region of the implant, with stabilization of the bone near the union between the neck and the porous-surfaced region of the implant.¹⁹ After a year of function, mean crestal bone loss was less than 0.2 mm. Between years 2 and 10, the mean annual bone loss was 0.03 mm.^19,20

Standard threaded easy-cut implants

In a comparison of the survival of short machined implants versus acid-etched implants (Osseotite™), the difference in survival between standard and short implants was 7.1% for the machined implants and 0.7% for the acid-etched implants.²⁰ This meant that the length of the implants was not clinically significant for implant survival, as long as the implant had a treated surface.

A study of 168 implants with a length of 8 mm and diameter of 6 mm coated with hydroxyapatite yielded a survival rate of 100% with a mean follow-up of 34.9 months.²¹ Malo²² used 408 implants of 7 mm and 8.5 mm in 237 patients, with a 5-year survival of 96.2% and 97.1%, respectively.

In a study of immediate loading on 133 short implants, the same survival results were obtained (97.7%) as with a two-phase technique.²³

In a literature review,²⁴ it was observed that the higher rate of failure of short implants was associated with the learning curve of the surgeon, a non-individualized preparation of the implantation bed, the use of machined implants, and implantation in low density bone. Expert surgeons with customized preparation of the bed and placement of treated surface implants can obtain results comparable to those obtained with implants of conventional length. It has been suggested that splinting of implants, restoration with canine-protected occlusion, and the selection of implants that increase the bone-implant contact surface are important factors in increasing implant survival.²⁵

Alternatives to short implants

Among the simple alternatives are pterygoid implants, tilted implants, or the all-onfour technique (Table 3).

 

Pterygoid implants were introduced by Tulasne.²⁶ Their use is based on anchoring implants to the skull base, specifically the sphenoid pterygoid process, a bone structure that does not present the phenomenon of reabsorption associated with the edentulism. Their implantation requires the use of long drills and specific latero-medial and caudo-cranial angling, which makes the procedure very sensitive to the technique. The minimum length for reliable anchorage is 18 mm. However, tuberosity and palatine implants have been described with acceptable results.²⁷ They have a high rate of reliability with regard to their osseointegration.^28-30

Tilted implants were described by Aparicio.³¹ They are based on anchoring the dental implants where bone exists. Conceptually, they are distant from prosthesis-guided implan implantología, tology and they are a throwback to the early stage of implantology, when the surgeon placed the implants without considering the prosthetic needs of the patient. However, in selected and sufficiently informed patients, tilted implants can be an alternative when the patient does not wish to undergo any type of graft.

The "all-on-four" technique was described by Malo³² and allows the rehabilitation of a completely edentulous upper maxilla with only 4 implants. Consequently, it is not the best option for the isolated rehabilitation of the posterior maxilla.

Finally, other, more complex, techniques exist for the reconstruction of the atrophic posterior maxilla without resorting to sinus elevation, including zygomatic implants³³ and sandwich grafts of the upper jaw.³⁴ However, the description of these techniques goes beyond the objectives of this article.

 

Conclusions

Sintered, rough surfaced implants that are placed by impaction offer versatility to dental implantology. The routine use of short implants (5 mm or 7 mm) in low density trabecular bone like the maxillary tuberosity offers results comparable to those of threaded cylindrical implants. Their advantages include a surgical technique with low morbidity, rapid bone healing, the absence of any need for splinting prosthetic units can be used in situations with an unfavorable crown-root ratio. Crestal bone loss during function is insignificant.

 

 

Dirección para correspondencia:
Dr. Javier González Lagunas
Servicio de Cirugía Oral Maxilofacial e Implantología,
Hospital Quirón, Barcelona
Plaza Alfonso Comín 5-7
Planta 4ª, despachos 423-424
08023 Barcelona, España
e-mail: jgonzalez.bcn@quiron.es

Recibido: 29.10.08
Aceptado: 20.11.2008

 

 

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