Implant Stability: Do Dental Implant Width and Length Matter?

When placing endosseous dental implants, the goal of surgeons is to achieve apposition of bone against titanium oxide without an interpositional connective tissue layer. This is known as osseointegration. To achieve osseointegration, attaining primary stability of the endosseous implant within the osteotomy is critical. Osseointegration is accomplished during the healing period by limiting micromotion within the gap between the implant surface and the osteotomy wall to 100 µm.^1,2 Lack of initial stability due to excessive micromotion is assumed to be a cause of implant failure.^3-5 Chrcanovic et al emphasized that early and late implant failures were related to lack of initial stability and low insertion torque values.⁶ Therefore, achieving primary stability at implant placement is critical.

Primary stability, also known as mechanical stability, should be differentiated from secondary stability, or biological stability, which is needed after the 3- to 6-month healing period for the long-term survival of an implant.^7-9 Primary stability is defined as the lack of mobility of an implant, placed within an osteotomy, at the time of implant surgery.⁹ Although a secure fit of the implant within the osteotomy is desired, clinicians must be cautious to avoid applying excessive compressive stress to the bone, which can lead to necrosis and a lack of implant osseointegration.¹⁰

Many factors must be considered when attempting to achieve primary stability, including bone quality, osteotomy preparation, implant design, implant surface enhancement, and implant length and diameter.¹¹ The purpose of this article is to discuss the effects of bone density and implant length and diameter on achieving primary stability at implant placement and after osseointegration.

Use of a standardized protocol for the documentation of implant stability is advantageous to the implant team. Objective evaluation of the stability of implants can be accomplished by measuring and documenting insertion torque (IT) and implant stability quotient (ISQ) values in both the mesiodistal and buccolingual directions.

ITvalue is generally considered the parameter to evaluate initial stability of an implant at placement. IT measures implant rotational stability. Implant stability is directly related to the rotational stability at placement.^12,13 IT values of 25 Ncm to 42 Ncm have been commonly recommended to prevent implant micromotion during the healing phase,^14-16 especially if immediate loading with a provisional restoration is being considered.ISQ value is a method used to measure implant stiffness in bone.^17,18 Barewal et al stated that the ISQ provides information on axial implant stability during placement and after healing, and that most clinically valuable information is obtained when IT and ISQ measurements are used jointly.¹⁹

In 1985, Lekholm and Zarb classified bone quality. Their descriptive report examined the ratio between cortical and cancellous bone in different areas of the maxilla and mandible.²⁰ Other reports have discussed the influence of denser bone on primary stability.^21,22 It should be noted that the contemporary literature has not clearly established a consensus on the influence of bone quality on implant stability.^23,24 Other authors have shown a positive correlation between higher bone density and higher implant stability documented by ISQ values.^25-28

According to Gomez-Polo et al, bone quality has an influence on IT. In their study, the IT at implant placement was 34 Ncm, 27 Ncm, and 20 Ncm in type 1-2, type 3, and type 4 bone, respectively.²¹ Marquezan et al also supported the influence of bone quality on initial stability.²⁹ Other authors have reported a decrease in IT and initial stability of implants in softer type 4 bone,^14,30,31 confirming the influence of bone quality on initial stability. Notably, once implants are osseointegrated, bone density does not seem to have an effect on implant stability.

Gomez-Polo et al reported that primary ISQ values were shown to be unaffected by implant length.²¹ However, Hong et al stated that a statistically significant difference in ISQ value occurred by increasing the bone-to-implant contact (BIC).²² They noted that with longer implants, higher ISQ values were measured, and they concluded that ISQ value and BIC were directly proportional to each other.

Short monocortical implants may move and rotate slightly under function, causing a small deformation of the bone. When fully osseointegrated, the capacity of short implants to move slightly when submitted to load could become advantageous.

Park et al reported that ISQ value at implant placement was affected by implant diameter.³² These findings were consistent with Gomez-Polo's report that primary IT and ISQ values differed in patients with different implant diameters.²¹ At implant placement, narrower implants (3.75 mm) had lower IT compared to wider implants (4.24 mm). The IT and ISQ values reported were IT of 26 Ncm/ISQ of 74.0 and IT of 33 Ncm/ISQ of 77.0, respectively. Wider diameter has a positive influence on higher initial stability at implant placement. Additionally, the authors reported that once implants osseointegrated, secondary stability was also affected by implant diameter.²¹

Several authors have reported on the positive influence of increased implant diameter on initial stability.^25,33-35 Others have differed on the influence of increased implant diameter at implant placement (primary stability) versus the influence of increased diameter after implant osseointegration (secondary stability), noting that once implants have osseointegrated, wider implant diameter does not have a major influence on implant stability.^36-38 Pierrisnard et al reported that increasing implant diameter reduced the intensity of stresses along the implant length, and to increase the load-bearing capacity of an implant prosthesis, use of wider implants instead of longer implants was suggested.³⁹ Iplikcioglu and Akca observed lower stress values in bone with wider implants, and in osseointegrated implants, diameter appeared to play a role in force distribution.⁴⁰

The difference between longer implants and wider implants after osseointegration must be addressed. Pierrisnard et al emphasized that regardless of implant length, occlusal forces were localized at the first 3 mm of the implant platform; beyond this point, stress values were minimal toward the apex of the implant.⁴¹ Other authors corroborated these findings.^42-45 Under occlusal loads, the highest stresses were found to be at the cortical bone around the implant platform.^31,43-47 With lateral loading, the highest concentration of stresses have been found at the first 3 mm to 5 mm of the implant platform.^48,49 Pierrisnard et al reported that increased implant length did not affect or reduce the stresses transferred to the implant platform.³⁹ Iplikcioglu and Akca also reported that increased length of implants did not decrease stress levels within the implants.⁴⁰

Clinicians sometimes use longer implants to achieve bicortical stabilization in an attempt to increase initial implant stability. Although achieving bicortical stabilization may be considered beneficial at implant placement, its presence may not be favorable after implant osseointegration. Adell et al and Ivanoff et al reported increased bicortical stabilization at implant placement enhanced implant stability.^50,51 In another report, Ivanoff et al stated that bicortically anchored implants increased the long-term force distribution in implants. They used implants of the same length in monocortical- and bicortical-anchored models. Due to implant fatigue, a fourfold increase in failure of bicortically stabilized implants was observed.⁵²

Morgan et al discussed the potential for mechanical fatigue in osseointegrated implants, stating that fatigue was responsible for mechanical failure of implants and components. The stiffer anchorage of longer implants was responsible for larger loads and a higher rate of screw loosening and component fracture when compared to shorter implants.⁵³ Clinicians should differentiate between the potential positive role of longer implants and the biomechanical response of the same implants once the implant has osseointegrated.

Other reports have shown that in osseointegrated implants, shorter implants under load, due to their flexure in bone, may have fewer prosthetic complications caused by metal fatigue compared to longer implants.⁵²

Increased bone density, longer and wider implants, and bicortical stabilization all potentially increase the stability of endosseous implants. Clinicians must differentiate between primary and secondary stability when considering the importance of an increase in implant stability. The methods commonly used to measure primary stability during surgery are IT and ISQ. ISQ values may also be used to measure secondary stability after implant osseointegration.

Trisi et al reported that higher IT and higher stability of implants are linked to the density of the surrounding bone.¹⁴ Gomez-Polo et al reported that primary IT and ISQ values differed in patients with different bone quality and implant diameters. Although wider-diameter implants had a higher stability at placement, the difference was minimal compared to shorter implants. However, once implants osseointegrated, secondary stability was not substantially affected by implant length or bone quality but was affected by implant diameter.²¹ Comparing the relationship between IT and ISQ values they found that IT was closely related to primary ISQ (at implant placement), but IT was unrelated to secondary ISQ (in osseointegrated implants). In cases of very high primary ISQ values at implant placement, the primary ISQ values tended to decrease with implant osseointegration. Implants with intermediate and low ISQ values at implant placement tended to increase between the time of implant placement and implant osseointegration.

Understanding the relationship between IT and ISQ values at implant placement (mechanical stability) and at implant osseointegration (biological stability) may help clarify whether implant length and implant diameter play major roles in predicting osseointegration and maintaining osseointegration under functional loads.

A distinction must be made between the effects of bone density, implant length, implant diameter, and monocortical or bicortical stability on an implant initially placed and one that is osseointegrated. Upon review of the literature, increased bone density and use of longer and wider implants appear to be beneficial to achieving a higher initial stability at implant placement.

For osseointegrated implants, stability is not influenced by implant length or bone density. Bicortical anchorage of longer implants may lead to component fatigue and higher prosthetic component fracture when overloaded. Stresses on short monocortical implants may create a slight micromovement and rotational force under function, with a small bone deformation. This capacity for slight bone deformation may be advantageous when short implants are fully osseointegrated. As noted by Pierrisnard et al, in case of overload, long implants will demonstrate mechanical complications, while short implants may fail due to biological problems. When osseointegrated, wider implants may allow for an improved distribution of forces along the length of the implant.⁴¹

The author cautions against the absolute interpretation of conclusions reached in the literature referenced in this article and suggests that implant treatment be tailored to each individual patient. To achieve satisfactory initial stability, it is recommended that clinicians formulate an algorithm that can be modified intraoperatively based on surgical technique. Occupying the buccal and lingual alveolar volume with the widest and/or longest implant may limit the blood flow needed for the vitality of the alveolar bone. Therefore, treatment planning for the widest and longest implant may not be a prudent protocol. The author suggests planning with an implant of reasonable length and width, while allowing for increasing the length and/or width as needed to achieve initial stability.

Edmond Bedrossian, DDS
Professor, Department of Oral & Maxillofacial Surgery, Arthur A. Dugoni School of Dentistry, University of the Pacific, San Francisco, California; Diplomate, American Board of Oral and Maxillofacial Surgery; Honorary Member, American College of Prosthodontists; Fellow, American College of Dentists; Fellow, American College of Oral and Maxillofacial Surgeons; Fellow, Academy of Osseointegration