Short Implants: amazing improvement in Implant Dentistry

 Short Implants: New Horizon in Implant Dentistry

The choice of implant length in relation to the available bone quality and quantity and biting force is a critical factor in the success of implants and longevity of the prosthesis. Long implants have always been considered more desirable in this respect but in patients with advanced alveolar bone resorption their placement is problematic due to the anatomic boundaries. Anatomical limitation in resorbed maxilla includes the maxillary sinus posteriorly and nasal floor and nasopalatine canal anteriorly whereas in resorbed mandible it is inferior alveolar canal. Advanced surgical procedures such as guided bone regeneration, block grafting, maxillary sinus floor grafting, distraction osteogenesis and nerve repositioning can be carried out to gain alveolar height in these areas and permit placement of long length implants but these techniques are sensitive, challenging, costly, time consuming and increase surgical morbidity. Short implants offer a less invasive treatment alternative in resorbed ridge cases . There is no general consensus on the definition of short implant. Most of the authors have considered implants less than 10mm as short implants . 

Advantages of Short Implants

1)Bone grafting to compensate for less height is unnecessary.

2)Less money, pain and time associated with various surgical procedures before placement of implant.

3)Complex surgical techniques are often associated with complications during surgery such as bleeding, perforation of the Schneiderian membrane or nerve injury and post-operatively such as transient or permanent alteration of mandibular sensation, graft and/or membrane exposure, infections and increased peri-implant bone loss. This can be avoided.

4)Osteotomy preparation is simplified since shorter bone preparation is required at the implant site which provides direct access for water irrigation and reduces the possibility of bone overheating.

5)Implant insertion is easier.

6)Angulation to load is improved with short osteotomy site since the basal bone beyond the original alveolar ridge is not always located in the long axis of the missing tooth.

Indications for Short Implants: 

Short implants can be used in almost all types of replacements whether fixed or removable including:

Single and multiple fixed prosthesis in posterior jaw.

In the treatment of a severely resorbed edentulous mandible, with four short-length implants used to support an overdenture or six short implants used to support a fixed prosthesis.

In edentulous maxilla, two short-length implants are additionally placed in the distal area, together with longer implants in the premaxilla to support a maxillary overdenture or a fixed prosthesis.

Biomechanical Considerations

They have been categorized as follows

1. Diagnostic :

a) Implant diameter: It is more efficient than implant length for dissipation of stresses, because the area receiving maximum effort is the bone crest and very little stress is transferred to the apical portion. An increased length would therefore only improve primary stability but wider implant would not only increase the primary stability but also the functional surface area at the crestal bone level leading to better distribution of occlusal forces. Finite element analysis has also supported this concept and demonstrated that implant length might not be the principal factor influencing transfer of occlusal loads to the bone-implant interface .

b) Crown/implant ratio: Increased crown/implant ratio can act as a vertical cantilever leading to crestal bone loss and implant failure. However, improvements of surfaces and implant systems along with proper force orientation and load distribution have allowed high crown/implant ratios to be applied with success.

c) Bone quality: It is the primary factor for short implant success . Areas with type III and type IV bone show more failures regardless of the implant surface treatment. The combination of short implant length and poor bone quality reduces the implant stability during implant placement and the healing period.

d) Lack of cantilevers: A cantilever magnifies the forces directly proportional to the height of the crown. It creates six different potential rotation points on the implant body. Eliminating cantilevers favors biomechanics and increased treatment predictability.

e)Use of multiple implants will increase the functional surface area to resist occlusal forces.

f) Implant design: The implant surface area can be increased by :

Thread number: More the number of threads per unit length in the same axial plane more is the implant surface area in contact with the bone.

Thread depth: Deeper threads provide more implant surface area.

Thread shape: The square thread design has a higher bone-implant contact percent as compared to v-shape and reverse buttress thread designs 

Implant surface: As compared to turned smooth surface, rough microtopography of implant surface increases the bone-implant contact surface area and accelerates osseo-integration. It also compensates for inadequate crown/implant ratio.

2. Surgical :

a) Two step surgical protocol: A two stage surgery is advocated for short implants as it provides good primary stability during healing phase. The time elapsed between the surgical and load stage should be 4-6months for maxilla and 2-4months for mandible .

b) Adapted surgical protocol: Enhanced initial implant stability can be achieved by eliminating a step in standard surgical protocol such as eliminating the countersink drill or eliminating the final drill in the standard drilling sequence . Soft bone drilling protocol should be followed in poor quality bone whereas, the final bone drilling is done with narrow drills rather than standard size drills.

3. Prosthetic :

a) Implant to abutment connection: Morse taper connection induces less marginal bone loss as compared to external hex abutment connection and also promotes bone growth over the implant shoulder . Internal hex implant abutment connection shows a wider force distribution as compared to external hex connection . Platform switching maintains the crestal bone for the entire length of the implant up to the collar level.

Occlusal table: Small occlusal table reduces the offset loads on the implant.

c) Incisal guidance: Implants should follow a biomechanical approach similar to natural teeth to accommodate the higher bite forces in the posterior regions of the mouth. Incisal guidance of the anterior teeth eliminates lateral forces to the posterior teeth in all mandibular excursions.

d) Splinting: Splinting implants increases the functional surface area of support and transmits less force to the prosthesis, the cement, abutment screws and the implant bone interface especially when placed in soft bone.

Clinical Guidelines for Placing Short Implants

Nisand and Renourd in 2014 suggested guidelines for placement of short implants and other therapeutic options based on available bone height, bone quality and certain risk factors such as smoking, history of periodontal disease and advanced age . 

- In resorbed maxilla short implants are used if ridge hight 5-6mm in 4 bone type without hisory of periodontitis or smoking .

also in more than 6 mm for all bone types .

- In resorbed mandible greater than 8 mm but if less advanced surgical procedures is needed .