Immediate Loading: Are Implant Surface and Thread Design More Important Than Osteotomy Preparation?

For decades, dental implants have undergone perpetual innovation and refinement. Generally, high implant success/survival rates are attainable, yet clinicians continue to face limitations and struggle with unexplainable complications and failures. Albrektsson et al defined six key parameters for implant osseointegration: implant material, implant design, implant surface, bone status, surgical technique, and implant loading conditions.¹ Implant enhancement has occurred through the creation of surfaces with materials that allow for secure and long-lasting cellular contact. Modern implants feature larger surface areas for increased interactions between the implant and surrounding tissue(s). Today's implant body and thread designs are aimed at achieving higher primary stability at the time of placement, thereby promoting and supporting concepts of immediate loading.

Healthy bone tissue is essential for proper osseointegration. Many factors influence bone regeneration capacity, including patients' age and medical status, the medications they are taking, whether or not they smoke, and the sources of infections in implant areas, just to name some.² Additionally, implant bed preparation creates bone trauma. While healthy bone supports modern implant designs and protocols adequately, thermal trauma has a particularly significant impact on bone cell function and survival.

Surgical trauma during an osteotomy preparation results in a necrotic border zone immediately adjacent to the implant regardless of the precautions taken. This zone significantly enlarges if critical threshold temperatures and times are extended.³ Acceptable critical thresholds for temperature-related implant complications are 47°C for 1 minute or 50°C for 30 seconds.³ When these temperatures and time periods are exceeded, thermal bone cell damage is unavoidable. Heating the bone to 50°C for 1 minute has a significant impact on bone cells. Signs of vascular injury are clearly seen at these temperatures. About 1 week after the thermal trauma, all affected vessels disappear. Bone tissue does not remain as functional bone but is resorbed and replaced with fat cells. After an observation period of 30 to 40 days, about 30% of the bone is resorbed.⁴ The initial resorption of bone in the weeks after implant preparation can be drastically minimized by controlling temperatures generated during implant bed preparation and thereby controlling thermal damage. Thus, during the healing period a decrease in implant stability can be avoided and a high level of functional stability can be maintained.

Original concepts of osseointegration without immediate loading functioned under the premise that the layer of necrotic bone surrounding a newly inserted implant would remodel before implant loading occurred. It was assumed that 3 to 4 months were necessary for the initial implant osseointegration.⁵ Research has shown that the formation of connective tissue seen around some implants is related to early or immediate loading procedures.^6,7 Implant micro-movement is one reason for inhibited osteogenesis.⁸ Some articles have suggested that loading should not be performed until the implant screw threads are filled with bony callus.⁹

Today the time between implant placement and loading has been shortened to the point that implants can be loaded immediately after placement. Immediate loading is not a new concept, having been pioneered several decades ago.^10-12 While Tarnow et al stated that immediate loading must include at least five dental implants in edentulous mandibles,¹⁰ other authors have shown that fewer implants can be used for immediate loading. Abboud et al described successful immediately loaded single-tooth implants in the posterior mandible and maxilla.¹³

While immediate loading is not new, the association between the quality of implant bed preparation and immediate loading is a more recent revelation. To achieve high primary stability, proper osteotomy preparation is considered a prerequisite for successful immediate or early loading.¹⁴ Primary stability is a function of the mechanical retention of the implant in the bone. It is greatly influenced by implant design and the density and volume of the osteotomy bed bone.¹⁵ The presence of thicker cortical bone increases primary stability but can decrease secondary stability due to excessive bone compression if the site is not adequately prepared.¹⁵

Additionally, primary stability can be improved by under-preparing the osteotomy and using narrower drills and wider and tapered implants. The enhanced primary stability is the result of lateral compression of the bone trabeculae and increased interfacial bone stiffness. Unfortunately, this initial primary stability gain might quickly evolve into a significant stability loss due to high bone compression and resultant bone resorption. Implant surface texturing directly contributes to initial implant stability by possibly reducing the risk of stability loss and facilitating wound healing.¹⁶

With the use of an implant with greater length and wider diameter, the amount of contact surface area at the bone-implant interface is increased. This has been found to result in improved (ie, more negative) Periotest values (ie, assessment of osseointegration of dental implants).^17,18 Implant surface texture can affect the rate and extent of bone-implant fixation, expressed by the amount of bone-to-implant contact (BIC). For example, in sites with poor bone quality, implants with an acid-etched surface can achieve a significantly higher BIC than implants with a machined surface.^19,20 Surface-roughened implants have a failure rate (3.2%) five times lower than machined-surface implants (15.2%).²¹ Implant stability quotient and Periotest measurements can be effective in monitoring implant stability. Insertion torques ranging from 25 Ncm to 70 Ncm appear to be an effective criteria for good primary stability and possibly immediate loading depending on the implant design and other factors.^13,22,23

Secondary stability refers to implant stability after the osteotomy site has healed. It is related to bone formation and remodeling at the bone-implant interface and surrounding bone.²⁴ Secondary stability is a result of the host's response to the implant and is determined by biologic reactions more so than mechanical retention. It is a key aspect of an implant's success after the healing period and long-term.

Most heat generated by drilling is a result of the deformation and cutting of the bone. This is one reason why bone density has such a substantial effect on heat transfer. When bone is particularly dense, more of it must be deformed and cut to create an osteotomy. Drilling trabecular bone, as compared to cortical bone, has a less detrimental effect on the zone of necrosis. Heat generation during bone preparation originates from a primary deformation zone, in which energy involved in shearing the bone material is converted into heat. In a secondary zone, the heat generated dissipates into the resultant bone chips and the drill/bur. In a tertiary zone, heat results from friction between the drill bit and the surrounding bone. Heat generated at the drill bit is partly conducted into the surrounding bone and potentially results in thermal necrosis or apoptosis.²⁵

There is no measurement device or indicator available to determine the immediate level of bone damage during implant bed preparation. Also, it is impossible to visualize immediate bone damage clinically or radiologically. The clinician is essentially operating blindly, fully relying on the drill bits and hoping that high levels of thermal damage do not occur. It is, thus, critically important to always use sharp, preferably new drill bits. Drilling in dense bone results in higher wear on the drill compared to soft cancellous bone. The initial drill bit used for osteotomy preparation is likely to dull the fastest and should be exchanged more frequently. Drill usage, cleaning, and sterilization impacts cutting performance and potential thermal damage.

In addition to initial implant stability, functional stability during the transient period from primary to secondary stability, typically the first 6 weeks of healing, is also a critical parameter for immediately loaded implants.^13,24 Implant bed preparation has a significant impact on the transient stability of the implant, which is a crucial factor for clinical success. Contemporary implant surfaces and designs can only function properly when the implant is placed in healthy bone. To complement the use of modern implant systems, critical temperatures should be avoided when drilling.³

Conventional pilot drills may be inadequate for maintaining healthy bone surrounding implants. Pilot drills have poor depth-to-diameter ratios. Based on the small diameter of the pilot drill (normally around 2 mm), drilling deep holes can be problematic. Wiggins and Malkin observed this in 1976; they stated that the flutes of twist drills may tend to clog when the depth of the drill hole is increased.²⁶ This, in turn, leads to an increase in torque and specific cutting energy and may be one reason, apart from the increased drilling time, for an elevation in temperature. Additionally, volumes of broken bone chips and debris are generated by the drilling procedure. The small flute volume of the pilot drill bit does not allow fast, sufficient clearance of the debris from the bore hole, resulting in increased friction and heat generation. Use of cooling irrigation can reduce the heat and friction at the surface. However, when deeper holes are being drilled, the drill tip becomes buried deep in the bone and is fully separated from the cooling liquid.

Three studies by Strbac et al proved that thermal increase is inversely proportional to the diameter of the burs.^27-29 During drilling, with a supply of coolant, 2 mm diameter twist drills reached higher temperatures than 3.5 mm diameter drills. Larger-diameter starting drills with efficient flute-clearing capabilities well-suited for preparing osteotomies may be considered for use in standard implant bed preparation workflows.

Modern implant bodies and surfaces are designed for high primary stability, which is the ideal starting point for immediate loading procedures. Thermal damage during bone drilling and compression of the bone in the osteotomy site can cause osteocyte death followed by severe bone resorption and remodeling processes, which can ultimately compromise implant stability in the weeks after placement. For immediate loading procedures, the transient stability of the implant is mostly affected by the implant bed preparation itself. Theoretically, a minimally traumatic osteotomy will leave a maximum amount of healthy bone cells behind, which promotes osseointegration and bone healing, especially when immediate loading is being considered.

Dr. Abboud is a partial shareholder of Loocid LLC and periodically receives speaking honoraria from Medentika, Straumann, and Henry Schein. Dr. Orentlicher periodically receives speaking honoraria from Nobel Biocare.

Marcus Abboud, DMD, PhD
Adjunct Professor, College of Engineering and Applied Sciences, Stony Brook University, Stony Brook, New York

Sihana Rugova, DDS
Stony Brook University Graduate School, Stony Brook University, Stony Brook, New York

Gary Orentlicher, DMD
Section Chief, Oral and Maxillofacial Surgery, White Plains Hospital, White Plains, New York; Private Practice specializing in Oral, Maxillofacial, and Implant Surgery, Scarsdale, New York