|Year : 2022 | Volume
| Issue : 1 | Page : 1-3
Inclined controversy: Angled implants and abutments
ProSmile Dental Clinic and Implant Centre, Dr. L H Hiranandani Hospital, Mumbai, Maharashtra, India
|Date of Submission||09-Jun-2022|
|Date of Acceptance||09-Jun-2022|
|Date of Web Publication||16-Jun-2022|
Dr. Sharat Shetty
ProSmile Dental Clinic and Implant Centre, Dr. L H Hiranandani Hospital, Mumbai, Maharashtra
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Shetty S. Inclined controversy: Angled implants and abutments. J Dent Implant 2022;12:1-3
The world is progressively witnessing an increased numbers in the aging population and in turn, rises in the edentulous (partial and complete) mouths. Prosthetic rehabilitation with dental implants has been successfully achieved for replacement of missing teeth for over seven decades with well-established protocols. They depend on many factors such as current medical health, psychological status, availability, and financial affordability of the patients. The local features are the determining and contributing factors for the choice of implant treatment including the esthetic and biomechanical risks. The available bone volume and quality in all the directions determine the surgical interventions and size and location of implants. The presence of anatomical structures influences the need for surgical modifications and/or bone augmentations for successful outcomes. Prosthesis design for stand-alone or splinted implants is responsible for good esthetic and functional outcomes with the occlusal loads that are well directed to ensure long-term stability of the rehabilitation with minimal failures and complications.
Natural teeth are suspended from alveolar bone by periodontal ligament, a unique feature which protects the teeth from vagaries of occlusal overload. They are present in extremely large numbers divided into multiple supporting fiber groups from the root surface to the surrounding corticated alveolar bone capable of stretching and compression. They along with the gingival fibers buffer the loads on the teeth. These ligamental fibers also have the distinctive trait of proprioception which provides the defense against overload through positional awareness and appropriate feedback response. They also provide cellular and hemo-defense allowing a range of reactions from pain to selective resorption and apposition of bone depending on the direction, magnitude, and time of loading on teeth. The resultant orthodontic movement of the tooth to a more stable position protects the tooth from becoming more mobile and be lost. The tooth being a single structure allows the fulcrum of movement within the apical third of the root, which is well trenched in surrounding bone, preventing crestal bone loss. The number of roots and cross-sectional designs of every tooth contribute to their ability to withstand the occlusal load in their respective position in the jaw.
The absence of periodontal ligament and direct apposition of the labile calcified bone around the implants surface and their round cross-section make them susceptible and vulnerable to microbial attack and intolerable forces and result in peri-implant inflammation and resorption of bone. The osseointegrated surfaces and the implant-abutment-restoration assembly are under the influence of the complex occlusal load and since these are in multiple units, the loas are more near the crestal bone. The resultant compressive, shear, and tensile forces should be maintained in relative balance to achieve and maintain the osseointegration.
Based on the biomechanical advantages of load distribution in natural teeth, it has been postulated and well-accepted that the masticatory forces which are directed through the long axis of the implant and within or slightly larger than the diameter of the implant ensure favorable longevity of the implant-restoration assembly with minimal bone resorption.
However, local bone anatomy and orientation and many times bone deficiencies will not allow ideal directional placement. Besides the anatomical structures like pneumatized sinuses, closer neural bundles to the crest and resorbed ridges require additional surgical managements like sinus lift with augmentation, ridge splitting, and additive bone regenerative procedures but may result in more morbidities like maxillary sinusitis, membrane perforation, oro-antral fistula, bone graft and implant displacement into the maxillary sinus, longer healing time with potential wound opening and loss of graft, and elevated patient discomfort. The other options for implant placements are change the location, shorter implants, or change the angulation.
Implants placement straying away from this vertical axis has been referred to as “tilted” or “off-axis” implants. Tilted implants are placed intentionally following accepted norms of a treatment philosophy but accidentally on many occasions due to poor planning and implementation. All-on X, pterygoid and zygomatic implants, ramus implants, and blade implants are some of them.
The proposed advantages of tilted implants in extensive edentulous situations are as follows:
- Greater diameter and length of implant engaging native bone
- Easier execution compared to complex surgical procedures
- Greater number of patients will accept this simpler procedure
- Reduced treatment time
- Possibility of immediate to early loading due to splinting and good primary stability
- Less expensive.
However, when implants are incorrectly angled or improperly positioned (buccolingually or mesiodistally) and soft-tissue defects exist, angled prosthetic components (prefabricated or customized) must be used to achieve proper restorative contours. But angled abutments result in increased stress on supporting implants, adjacent bone, and the prosthesis. Most studies have concluded that on single unsplinted implants, the straight abutments (with a zero-degree angulation) demonstrated stress at the apex, while the angled abutments showed the stress opposite to the side on which the load was applied. It has been noted that the microstrain increased three-fold with angulation of 15° and over four-fold with 25°. The response of bone depends on its density and biology. Macrostrains above 3500 με are destructive in nature and may lead to bone resorption and eventual de-osseointegration.
Hence, individual implants with greater tilts are not recommended unless loading protocols are modified and natural teeth are recruited to accept majority of loads.
But when implants were splinted with a rigid fixed prosthesis, lower mechanical stresses were noticed on the peri-implant bone. Branemark's landmark work on the splinted implants which were axially placed on edentulous jaws had the following deductions:
- Surgical placements after healing of extraction sites
- Nonloading healing times of 3–6 months with no external communications
- Hybrid prosthesis with acrylic occlusal surfaces to avoid overload.
Recent studies have shown good clinical outcomes with the use of distal tilted implants in combination of axially placed anterior/mesial implants but splinted passively and rigidly. However, there is no consensus-based recommendations on the maximum accepted tilt angle, number, and length of implants and opposing arch factors. Moreover, there are many reports using very narrow implants, aggressive thread designs, acute angulations of placements with unplanned biomechanics and immediate loading in all cases.
Success and long-term stability depend on the understanding of sound biomechanical principles, correct surgical planning for bone and soft tissue health, and adoption and execution of good and precise prosthetic procedures. The fact that all bones are not similar in structure and soft tissues are subject to unfavorable changes depending on the type of exits of restorative components through them, distances between implants and effects of occlusal overloads, there is a strong need to customize the implant placement and design of the prosthesis to optimize the outcome while rehabilitating extensive or fully edentulous cases with implant prosthesis.
The success of the implant prosthesis can be measured as:
- Implant survival
- Prosthesis survival
- Biological and mechanical complications
- Marginal bone loss.
In completely edentulous situations, cantilever decisions are common. The length of the cantilever concentrates the forces on the nearest implant of the cantilever and may present biological and mechanical breakdowns of various intensities ranging from progressive moderate-to-severe crestal bone loss, chipping of the ceramic, screw loosening, loss of retention, framework fracture, and implant fracture.
The survivability of the cantilevered implant prosthesis depends on managing each component of the assembly as below:
- Implant dependent: The increased diameter of the implant without reducing the width of optimum supporting bone, longer length with increased surface texture, increased number of implants with reasonable and adequate spacing will increase area of osseointegration and better load dissipation
- Prosthesis dependent: Acceptable prosthesis-to-implant ratio, greater connector areas, passive fits, preloaded torquing of abutments ensure rigidity of the prosthesis, and reduce mechanical complications and favorable transfer of loads to implant and bone
- Load dependent: Shorter cantilever length, infra-occluded cantilever arm by 30–40 μ, shallow cusps, and posterior disclusion occlusal schemes, over-engineered design and occlusal splints for control of parafunctional loads will safeguard against force-induced damages.
Although tilted but splinted implants gave been successfully used to circumvent the disadvantages of complex surgical interventions, especially in cases with anatomical limitations, the following considerations will reduce the complications and failures with their use:
- Weigh the benefits of tilting implants with the limitations of more surgical interventions and choose the most appropriate one. Avoid extremes as their immediate feasibility may result in long-term failures
- Tilt should be limited to 25° to minimize destructive microstrains
- Implants >10 mm in length must be used and more in number wherever possible with adequate distance of 8–10 mm between them
- Larger diameter implants with adequate circumferential bone along the whole length of implants must be utilized
- Extensively textured implant surfaces and adequate thread designs should be adopted
- Presurgical planning for biomechanical advantages should be practiced. The clinician should consider angle of the abutments when planning.
- Placement with the help of surgical guides is highly recommended to ensure precise three-dimensional orientation and tilt
- Tilted implants (buccolingual or mesiodistal) should be placed more deeply to allow additional vertical running room to facilitate gradual emergence profiles
- The abutment to implant connection must be robust with good anti-rotational features and long internal connections are recommended as compared to external connections to disperse loads favorably through the entire length of implants and reduce load on the abutment and prosthetic screws
- Custom abutments are preferred for its optimized emergence profile and height (high biomechanical resistance) and location of margins, better soft tissue support, and acceptable esthetic outcomes
- Preloading of the abutment screws by means of applying adequate tightening torque 10 min after initial tightening is advocated as per the manufacturer's suggestions to avoid screw loosening
- Rigid and passive superstructures are recommended. The use of multiunit abutments for all tilted implants is suggested to overcome stress arising out of nonparallel implants and their path of placement. Besides they protect the surrounding peri-implant soft tissues from repeated removal and insertion of the abutments. All prosthetic and fabrication steps should ensure passive fit of the prosthesis
- Retrievable design of the prosthesis is recommended to address any potential future complications
- Cantilever should be as short as needed (maximum: 1.5 times the A-P spread) and infra-occluded by 30 μ
- Understanding opposing occlusal conditions and presence of parafunction should be evaluated and judicious contacts should be planned. Posterior disclusion is mandatory. The use of appropriate occlusal splints should be considered
- Soft tissue exits of the abutment and superstructure should be good, maintainable for hygiene and nondamaging
- Periodic appraisals every 6 months should be undertaken to monitor and intervene when early breakdowns are seen, and corrective measures initiated.
Truth does not depend on whom we believe in but lies in understanding all the tenets to its fullest and doing using its best for our patients, understanding their current factors. Tilted implants too need these understandings and judicious applications.
“The ultimate measure of a man is not where he stands in moments of comfort and convenience, but where he stands at times of challenge and controversy.”
-Martin Luther King, Jr.