The purpose of this article is not to discuss the success of short dental implants versus standard/long dental implants, but to compare short dental implants with standard/long dental implants in areas that necessitated adjunctive bone grafting or augmentation procedures and as a way to avoid the need for advanced surgical procedures and their associated risks. It can be concluded that short dental implants are a viable alternative in sites that would have required additional complex and costly augmentation procedures. Short dental implants resulted in comparable survival and success rates with faster, less expensive treatment with fewer surgical complications and morbidity.
Short implants (<8 mm) have been promoted as a treatment option in many clinical scenarios with limited bone volume where long/standard length implants were otherwise contraindicated if not for complex, sophisticated and costly bone augmentation procedures that would have been required.
This article reviews the efficacy of using the currently available short implants with enhanced macrosurface and microsurface technology and abutment interfaces allowing their placement in cases previously thought ill-advised.
This allows implant treatment in a potentially faster, less expensive, less complicated manner, with decreased morbidity and comparable success rates with long/standard length implants with concomitant bone augmentation procedures.
Mankind’s desire for teeth to be able to chew and smile precedes all recorded dissertations on dentistry. Human mandibles with seashells carved into tooth shapes and placed into extraction sockets date back as far 600 ad . But, at what cost, physically, financially, and emotionally to our patients? Decreasing bone quantity and quality has traditionally necessitated increasing surgical complexity, cost, and potential complications. Is there anything we can do to mitigate these potential road blocks to successful implant treatment? Maybe short implants can be part of the solution.
What are short implants?
There is still no generally accepted length to classify implants as standard or long, short, and ultrashort. Various authors have defined implant lengths less than 11 mm, 10 mm, 8 mm, and 7 mm as short implants. When this author first wrote about this topic in 2015, 10 mm was used as the cut off for short dental implants versus long dental implants. Over the ensuing years, multiple studies have been published using an intrabony implant length of 8 mm or less as the definition of a short dental implant and has been gaining acceptance worldwide. Therefore, 8 mm or less will be used as the definition of a short dental implant for the purposes of this discussion.
Why did we favor long over short implants?
Initially, it was thought that the longer the implant the more stable and long lasting it would be because the occlusal forces would be dissipated over a greater surface area. This would prevent overloading the bone to implant interface. Implants ranged in length from as short as 7 mm to an enormous 20 mm. Evidence to support this theory that short implants (<10 mm) had inferior success and survival rates than long implants (≥10 mm) came from multiple early (1991–2005) studies done by Herrmann and colleagues, Friberg and colleagues, Wyatt and Zarb, Bahat, Attard and Zarb, and Weng and colleagues. Cumulatively, these studies showed failure rates of short dental implants ranging from a low of 15% to a high of 26%.
This notion that longer is better is also probably rooted in our dental experience of longer roots being stronger than short-rooted teeth. Perhaps this concept came from Ante’s law of fixed prosthodontics, which stated that the total periodontal membrane area of the abutment teeth must equal or exceed that of the teeth to be replaced. Additionally, it was believed that a reduced crown-to-implant ratio was an important factor in decreasing marginal bone loss and improving long-term survivability. , Implant diameter was not considered a critical factor at this time. To allow for the placement of these longer implants, there had to be adequate vertical bone height (quantity) and preferably sufficient density (quality).
Restorative biomechanical forces also have to be taken into consideration when planning an implant borne restoration. The posterior occlusion, closer to the temporomandibular joint acts as a class II lever exerting considerably more force (200–250 PSI) than is encountered anteriorly (25–50 PSI) where the jaw functions as a class III lever. This serves to highlight the need for increased bone to implant contact in the posterior mandible and especially the posterior maxilla where both quality and quantity are decreased. , , ,
What is preventing us from placing our preferred long implants wherever we want? The answer lies primarily with anatomic limitations. There are a multitude of clinical scenarios where the placement of a long implant is not possible owing to anatomic deficiencies. In the maxilla, posteriorly the pneumatized maxillary sinus and/or resorbed posterior alveolar ridge and anteriorly the nasal floor, nasopalatine canal, and/or resorbed ridge can thwart long implant placement. In the posterior mandible, the position of the inferior alveolar nerve and canal and mental nerve foramen in relation to the mandibular crest are the determining factor. These anatomic difficulties relate to the quantity or volume of the bone. The quality or density of the bone in the posterior mandible and in particular the posterior maxilla is inferior as compared with the anterior region. ,
If the posterior aspect of the mandible or maxilla is atrophied from tooth loss or trauma and there is not enough bone height to place the long implants that we would traditionally choose, what are the choices?
What are the surgical solutions to insufficient bone height and mass?
What are we to do if satisfactory bone does not exist? The answer was bone regeneration, augmentation, or nerve transposition. To deal with these bone deficits successfully to allow for long implants in adequate numbers many different advanced bone regeneration, augmentation, grafting, and transposition procedures have been developed and upgraded over the years.
Maxillary Sinus Grafts
In the posterior maxilla, inadequate bone volume is often encountered owing to progressive sinus pneumatization coupled with typical postextraction alveolar bone atrophy. To magnify the problem, the posterior maxilla has poor bone quality, typically type IV bone.
When Boyne and James first reported on grafting of the maxillary sinus floor with autogenous bone and marrow the problem of vertical height deficiency preventing the placement of standard/long implants was overcome. Since then, the technique has been refined and studied using various autogenous, allografts, xenografts, and alloplasts alone or in combination with or without membranes, both resorbable and nonresorbable. This advanced surgical procedure, maxillary sinus floor grafting (both laterally and crestally), has been well-documented and found to be equal to conventionally placed implants without grafting.
Guided Bone Regeneration
Guided bone regeneration techniques have been used for alveolar defects, socket grafts, and lateral augmentation as well as vertical ridge augmentation. There are various protocols using a variety of grafting materials, including autogenous bone, xenografts, allografts, alloplasts, or a combination thereof. The procedure includes a cell occlusive membrane with or without tacks or miniscrews or sutures to aid in membrane stabilization to promote bone healing by space maintenance while excluding gingival cell invagination. The membranes can be resorbable or nonresorbable membranes. Nonresorbable membranes that include internal titanium struts to help maintain the underlying space and their shape and/or long screws used as tent poles to support the membrane are often used in vertical augmentations. These techniques do require advanced training and skills. Guided bone regeneration techniques also seem to yield comparable and favorable results as compared with implant placement in healed nonaugmented bone.
Block grafts for onlay or inlay/interpositional grafting involves either harvesting blocks of bone from an intraoral sites such as the chin, external oblique ridge, ramus, , and palate or from extraoral sites such as the iliac crest or calvaria. The intraoral donor sites can be accomplished as in office procedures, whereas the extraoral sites require general anesthesia in an operating room setting. The advantages of block grafts include vertical and/or horizontal bone augmentation and decreased frequency of inferior alveolar nerve injuries with the onlay technique. The inlay/interpositional graft technique improves graft revascularization with more of the graft bone block in contact with vascularized host bone at time of graft placement and less resorption compared with onlay block grafts. Implant survival and marginal bone levels were comparable with block grafted bone and ungrafted bone.
Distraction osteogenesis involves a technique of osteotomies and a distractor device that in a controlled fashion distracts or stretches the bone by repeatedly pulling the osteotomy line apart, allowing new bone to continually form in the newly created gap. During the active phase of treatment, the device is actuated twice a day to slowly elongate the osteotomized segment until the desired dimension is achieved. Then the device is left for an additional 90 to 120 days until the newly formed bone has healed sufficiently.
Distraction osteogenesis showed the greatest gain in vertical height, an average of 8.04 mm compared with guided bone regeneration and onlay blocks. Additional advantages included no donor site morbidity, less postoperative bone resorption, and gains in soft tissue along with the increased vertical bone height.
Inferior Alveolar Nerve Transposition
Inferior alveolar nerve repositioning can be carried out to increase useable vertical alveolar height and bone mass in an effort to place long dental implants. , , The stated advantages of this procedure included shorter surgical treatment time because the implants are always placed at the same operative procedure as the nerve transposition and autogenous bone grafts are superfluous; therefore, no donor site morbidity and minimal allogenic or xenograft materials are needed.
Why reinvestigate the concept of short implants?
With the ability to augment and regenerate bone as noted above all of the negative results in so many early studies why did clinicians and researchers continue to pursue the idea of short dental implants? First, the disadvantages and potential complications associated with these complex and invasive grafting procedures became apparent over the years. Some of these methods are extremely technique sensitive, time consuming, costly, require advanced skills and training, and increased surgical morbidity and complications. , , Second, it must be remembered that all of the previously cited early studies done by Herrmann and colleagues, Friberg and colleagues, Wyatt and Zarb, Bahat, Attard and Zarb, and Weng and colleagues were based on the original externally hexed, turned/machine surface Brånemark implants that were placed using the generally accepted drilling protocols of that era. Last, our understanding of dental implant configuration, prosthesis design, biomechanics, and surgical protocols have evolved.
Complications of advanced bone grafting techniques
With every surgical technique there are inherent risks and complications that must be considered when planning a course of treatment. Bone augmentation and related procedures are no different.
Maxillary Sinus Grafts
The complication most commonly seen with maxillary sinus elevation from both a lateral or crestal/transalveolar approach is Schneiderian membrane perforation (18%). Additional complications include postoperative bleeding (14.5%), infections (1%), pain (0.6%), abscess formation (0.2%), sinusitis (0.2%), and partial or total graft loss (0.1%).
Guided Bone Regeneration
The most common complication associated with guided bone regeneration is premature membrane exposure or membrane loss, which can cause the bone graft to become infected, leading to partial or total graft loss. This complication can occur with either resorbable membranes or nonresorbable membranes, which have the inconvenience of requiring removal as a secondary procedure. Nonresorbable membranes, polytetrafluoroethylene, and titanium mesh have higher incidences of exposure than resorbable membranes. , , Nonrigid resorbable membranes with insufficient support beneath them can collapse leading to inadequate volume of augmentation requiring supplemental grafting. A disadvantage of all of these membranes is that they can be expensive.
Block grafts are technique-sensitive procedures that demand a complete knowledge of the anatomy of the donor area. Donor site morbidity, protracted treatment time in part owing to the need for 2 operative sites and soft tissue complications, such as recipient site wound dehiscence and graft resorption and failure, make this an advanced procedure. , , Inlay or interpositional grafts in the posterior edentulous mandible have a high rate of inferior alveolar nerve temporary hypoesthesia. Additionally, there is a risk of mandibular fracture and an inability to achieve the desired vertical augmentation owing to soft tissue tension. Intraorally, the symphysis donor region has a high incidence of temporary neurosensory deficits as opposed to the ramus, which is much lower. Iliac crest grafts have their own set of potential complications, including perforation of the peritoneum or bowel, infection, ilial fracture, and gait disturbances. Calvarial bone graft minor complications include incision line alopecia and hematomas. Major complications could include cerebrospinal fluid leak, extradural hematoma, and direct intracerebral trauma if the inner table of the diploë is violated during the graft harvest.
Potential complications, both minor and major, associated with distraction osteogenesis are numerous. It is paramount that the surgeon knows how to manage these problems as much as they know how to perform the actual procedure. Mazzonetto and colleagues divided complications into minor and major categories. Minor complications were defined as those complications that had “no effect on final result, but immediate intervention required” and major complications as “lead[ing] to technique failure.” Minor complications included tilting of the transport segment (secondary to in-elastic palatal mucosa or muscle pull), soft tissue dehiscence or perforation, infection, and lack of patient cooperation. Major complications included distractor failure, transport fragment resorption, mandibular fracture, bony nonunion, neurosensory disturbances, and insufficient bone distraction.
Inferior Alveolar Nerve Transposition
The most common complication of inferior alveolar nerve transposition is neurosensory disturbance, including hypoesthesia (decreased sensitivity to all stimuli except for special senses), paresthesia (abnormal sensation even spontaneously or for no reason), and hyperesthesia (hypersensitivity to all stimuli except for special senses). These injuries are caused by mechanical trauma to the nerve and impairment of the microvascular network by direct injury and stretching of the microvascular circulation in the mesoneurium. , Putting aside the fact that surgical technique itself is technique sensitive, sophisticated, and requires general anesthesia in an operating room setting, even the postoperative care is protracted and complex. Follow-up for monitoring the return of normal neurosensory function, excluding cases that required microneurosurgical repair, can take weeks to 6 months, and has been reported to take up to 1 year. There is a percentage of cases with permanent neurosensory loss. , Additionally, low-level laser therapy, using a GaA1As laser, starting immediately postoperatively 4 times a week for 40 treatments has been advocated to hasten paresthesia recovery. , Because of the ubiquitous nature of nerve injuries, some researchers believe that sensory deviations should not be considered a complication, but a normal outcome associated with this procedure.
Implant Configuration and Biomechanics
Advances in the design of dental implants both externally, thread design, macrosurface, and microsurface topography and internally, the implant to abutment connection and have been dramatically transformed. All of these changes have led to enhanced implant survivability.
The implants studied in the earlier articles by Herrmann and colleagues, Friberg and colleagues, Wyatt and Zarb, Bahat, Attard and Zarb, and Weng and colleagues did show poor survival rates compared with their long counter parts. However, these were the results for turned/machine surface (smooth), externally hexed implants. A meta-analysis of observational studies by Pommer and colleagues in 2011 separated rough surface implants from smooth surface implants and “suggested that rough surfaced implants with a minimum length of 7 mm represent no risk factor for implant failure.” Annibali and colleagues in their systematic review found that a rough surface on short implants yielded superior cumulative survival rates of 99.2% as compared with machine-surfaced implants at 94.6%. They concluded there were high survival rates with low rates of biological and biomechanical complications and “rough-surfaced implants preferred.” Nisand and Renouard also concluded that rough surface implants improve bone-to-implant contact.
Changes in the abutment to implant connection have been studied since the original flat to flat, external hex interface was first used. Multiple connection configurations, including internal hexes, splines, octagons, triangular shapes, and Morse tapers or cones to list but a few, have been scrutinized. Morse taper connections have been shown to induce less marginal bone loss as compared with the external abutment connection and also can promote bone growth over the top of implant shoulder contacting the abutment surface. The lack of meaningful micromotion between the abutment and the implant, a well-documented benefit of the Morse taper connection is probably responsible. Furthermore, platform switching maintains the crestal marginal bone levels as opposed to abutment–implant platform matched components.
Finite element analyses have shown that increasing the implant diameter is more efficient than increasing implant length for reduction of stresses at the bone to implant interface, , because the greatest concentration of stress is found at the bony crest and much lower forces are found at the implant apex. Therefore, although longer implants might improve primary mechanical stability, wider implants could improve both primary stability and long-term bone-to-implant stress reduction, and result in less crestal marginal bone loss. ,
The greater crown-to-implant ratio has been a topic of concern for clinicians and researchers particularly when discussing short implants. This concern is most likely in part fueled by our prior belief in Ante’s law. However, implants are not teeth and do not have a periodontal membrane. Additionally, Ante’s law was subsequently disproven by Lulic and associates. Some early studies correlated an increase in biomechanical complications associated the greater crown-to-implant ratios. These elongated clinical crown lengths were considered vertical cantilevers, which caused greater marginal bone stresses, leading to marginal bone loss or implant loss or technical complications, including prosthetic component failure. Nevertheless, more recent studies concluded that the crown-to-implant ratio is not a reliable predictor of marginal bone loss or implant survival. The Study by Blane, which included machined surface, solid implants, and rough surface hollow cylinder and solid implants, concluded that crown to implant ratio did not affect the degree of peri-implant marginal bone loss. Nedir and colleagues conducted a 7-year life table analysis of rough surface long and short implants with varying crown-to-implant ratios and found a cumulative success rate of 99.4% and that short implants did not fail more than longer ones despite the greater crown to implant ratios with the short implants. Schulte and colleagues retrospectively determined the crown-to-root ratio of 889 implants that ranged from 0.5:1 to 3:1, and found that the guidelines of crown to root ratios of natural tooth single crown restorations have no relationship with crown to implant ratios of single implant restorations. Meijer and colleagues found that, with crown-to-implant ratios ranging from 0.9 to 2.2, there was not a high incidence of biological or technical complications.
Modified Surgical Protocols
Many of the early failures of short implants were seen primarily in areas of poor quality, low-density bone, predominantly the posterior maxilla. These studies, although primarily using turned/machined surface implants, were also done using the standard drilling protocols of the era. There was no generally accepted differentiation in site preparation for soft versus normal bone. ,
The development and adoption of modified or adapted surgical protocols, sometimes referred to as soft bone drilling protocols, allow for improved primary mechanical stability and have been advocated by several researchers including, Tawil and Younan, Fugazzotto and colleagues, Renouard and Nisand, and Nisand and Renouard. The concept is to enhance primary mechanical stability specifically in poor quality bone by either finishing the osteotomy with a drill that is narrower than the standard final diameter drill, or eliminating a countersink or bone tap or any combination of these techniques. Many implant companies themselves have adopted and promote this idea by actually manufacturing specific narrow final drills and publishing modified or soft bone drilling protocol guidelines.
Studies Comparing Short Implants versus Long Implants with Grafting
Numerous papers, including systematic reviews, meta-analyses, randomized controlled trials, and retrospective and observational studies, have been published comparing short implants with long or standard implants in grafted, augmented, or modified sites.
Thoma and colleagues in a randomized in a randomized controlled multicenter study comparing 6-mm implants with 11- to 15-mm implants with sinus graft procedures found equal success at 1-year after loading. However, the 6-mm implant group had a substantially shorter surgical time and cost less than 50% as the long implant plus graft group.
Hadzik and colleagues published a similar study in 2018 using the same 6-mm (short) and 11- to 13-mm (long) implants. They came to the same conclusion that short implants can be used successfully in the posterior maxilla and that they decrease the need for complicated adjunctive procedures, thereby decreasing patient postoperative pain and morbidity.
The European Academy of Osseointegration Consensus Conference found predictably high implant survival for both short implants and long implants with their corresponding sinus grafting procedures. However, given the added number of biological complications, and the increased cost, surgical time, and morbidity associated with longer implants in grafted sinuses, “shorter implants may represent the preferred alternative.”
Fan and coworkers in their systematic review concluded that there was no difference in the survival rate between short (5–8 mm) implants and long (>8 mm) implants with simultaneous lateral window sinus grafts. Complication rates, cost and operating time were all less with short implants. They also suggested additional studies to confirm their findings.
Nielsen and colleagues in their systematic review of 3 randomized controlled trials comparing short (≤8 mm) with long (>8 mm) implants with lateral window maxillary sinus grafts with follow-up periods for at least 3 years found no significant difference between the 2 techniques in regards to implant survival, marginal bone loss and patient satisfaction. “Short implants seem to be a suitable alternative to standard length implants in conjunction with maxillary sinus floor augmentation.” However, they also recommended additional studies with more patients and observation times of more than 3 years to determine the superiority of one technique over the other.
Nisand and colleagues undertook a systematic review that included 4 articles comparing long implants placed in vertically augmented bone with short implants in native nonaugmented sites. They concluded that the implant and prosthetic survival rates were comparable for both techniques. Nonetheless vertical augmentation using interpositional inlay block grafts is costlier, lengthens treatment time and increases complications. They also suggested additional studies using different methods of vertical grafting, larger sample size and longer follow-ups.
Dias and coworkers performed a meta-analysis using 4 studies evaluating long (>8 mm) implants with various vertical augmentation techniques with short (≤8 mm) dental implants in ungrafted bone with at least 1 year of follow-up after final prosthesis insertion. They found a 97% versus 92.6% implant survival rate for short and long implants with grafting, respectively. Moreover, there was an increased trend of complications with bone augmentation procedures and long implants. They determined, owing to the decreased incidence of surgical complications, that short implants should be preferred when adequate bone is present.
Pieri and colleagues conducted a 5-year retrospective study comparing clinical and radiographic findings comparing short (6 mm) and standard (≥9 mm) length implants that were placed after vertical ridge augmentation with autogenous bone blocks. Although there were no statistically significant differences in the number of implant or prostheses failures, prosthetic or biological complications, the number of surgical complications was greater in the standard length implants with augmentation. Plus, the short implants had statistically significant less marginal bone loss than the long implants.
Felice and colleagues completed a randomized controlled trial comparing 5-mm length (short) implants with 10-mm length (standard) implants placed in sites augmented with anorganic bovine xenograft bone blocks using the interpositional vertical grafting technique. They found no statistically significant differences in implant or prosthetic failures or biological or prosthetic complications. There was a statistically significant difference in marginal bone loss over the 5-year study, with short implants loosing less bone than the standard length implants. They concluded that both techniques provided acceptable results up to 5 years, but that treatment with short implants less costly and quicker.