Lino Esteve Colomina, MD, DDS
Private Practice, Alicante, Spain.

Until now it has been thought that a stress-free period was essential for the rigid anchoring of implants to bone. Adell et al.1 and Brånemark et al.,2 after an empirical review of a large number of clinical series, proposed a stress-free waiting period of 3 to 6 months. Similarly, experimental studies showed that immediately loaded implants could not obtain direct boneimplant contact, developing instead a fibrous interface.3 However, the immediate loading protocol used with overdentures on four interforaminal implants, as described by Ledermann, 4 has been successfully practiced for more than 20 years, using several implant designs.5–8 With regard to fixed prostheses, the preloading healing period has recently been questioned by several authors who have achieved similar success rates with immediately and traditionally loaded implants,9–14 reaching the conclusion that a waiting period is not an absolute necessity, but rather a “therapeutic reserve,” as defined by Henry and Rosenberg in 1994.10 Even immediately loaded single implants have given totally satisfactory results, both in experimental15 and clinical16 studies. The latest experimental work reinforces the thesis that failure to obtain direct boneimplant contact is not caused by immediate loading but by micromotion at the interface that exceeds a certain level.17–20 This level is yet to be determined, but according to Brunski21 is about 100 mm.
Although there is growing scientific evidence that implants can be immediately loaded in certain cases, the indications for immediate loading have yet to be sufficiently specified because the main factors involved are not completely understood. Given the importance to patients of safely receiving their prostheses immediately after surgery, it is vital to undertake clinical research to redefine the timing of loading and to establish reliable immediate loading protocols that can be put into general use. This paper supports the case for immediate functional loading; we present our preliminary results from a systematic application of immediate loading to fixed mandibular prostheses.

MATERIAL AND METHODS.
Between February 1999 and September 2000, 13 fixed mandibular prostheses were immediately loaded. These were the hybrid type (11 total and 2 partial) with 61 implants from four different manufacturers: 13 Klöckner (Klöckner SA, Barcelona, Spain), 17 Astra (Astra Tech, Mölndal, Sweden), 5 ITIStraumann (Straumann Institute, Waldenburg, Switzerland), and 26 Eckermann (Eckermann Laboratorium, Orihuela, Spain). The protocol was to load transitional fixed prostheses within 2 weeks of inserting the implants.

Patients.
The patients were consecutively enrolled in the prospective longitudinal trial. The selection was made using the basic criteria of an absence of risk factors such as heavy smoking, bruxism, or general ill health. Patients with lesions in the area to be implanted or with defects that needed to be treated with regenerative techniques were also rejected. The average age was 57.5 years (range, 41–74 years). Five of the 13 patients had natural teeth, fixed prostheses, or removable partial prostheses with the presence of the anterior tooth as an antagonist. The eight remaining patients were full upperdenture wearers.
All of the implants in this series were placed into bone of a density of from I to III, according to the classification systems currently in use. They required an insertion torque .25 N (the approximate maximum force of the operator’s fingers when the insertions were performed manually). With regard to the number of implants, two fixed prostheses were placed on four implants, seven prostheses were placed on five implants, one partial prosthesis was placed on four implants, and the other partial prothesis was placed on two implants. The minimum length of the implants was 10 mm, and the maximum length was 16 mm.
In six patients, the removal of residual dentition and the installation of implants were done simultaneously. In two patients, these two operations were separated by 2 months. The other five patients already had the surgical zone edentulous .12 months.

Surgical Procedure.
After infiltration of local anaesthetics, a full thickness midcrestal incision was made in the edentulous anterior mandible or, in the cases where residual teeth had to be removed at the same time, a sulcular incision was used. The mucoperiosteal flaps were raised, and the mental neurovascular bundles were identified. At that point, the extractions of the remaining teeth were carefully made, and the alveoli were thoroughly debrided until any residual soft tissues were eliminated. To optimize the ridge width for receiving the implants, the exposed bone crest was flattened by performing a horizontal osteotomy with a round bur at a low speed and under copious sterile saline irrigation. Implant recipient sites were prepared, and the implants were inserted according to each manufacturer’s instructions. The abutments were immediately connected in the case of Astra and ITI implants, and the flaps were then repositioned and sutured.
Immediately after having sutured the surgical wound and while under local anaesthesia, an interocclusal registration was taken using a customized baseplate. Transfer copings were then connected, and an impression was taken with a polyether rubber material (Impregum, Espe Dental AG, Munich, Germany) using either a custom-made tray in the case of simultaneous extractions or the preexisting or prefabricated denture in edentulous cases.
The healing caps were screwed in, and the patient was then dismissed after having received detailed instructions on postoperative wound hygiene and the use of a 0.2% chlorhexidine gel (Perio-Kin, Laboratories Kin, Barcelona, Spain). Oral antibiotics were prescribed for 2 days preoperatively and, with NSAIDs, were maintained for 1 week after surgery.

Immediate Transitional Prosthesis.
In the laboratory, the working cast incorporating implant analogues was transferred to the previously mounted articulator. At this point, two different methods of fabricating an immediate fixed prosthesis were used: either the direct transformation of the full denture worn by the patient or the conventional procedure. The former, based on the “conversion prosthesis” reported since 1985 by Balshi and Wolfinger26 was performed as follows: the preexisting prosthesis was modified by trimming the lingual anterior flange, thus allowing it to be used first as a surgical template and then as an impression tray. After lingual reduction was made, modified copings could be accommodated without interfering with the placement of the denture in its correct position. Then, two techniques were tried to fix the copings to the altered denture to provide the laboratory with an exact register of their relative positions: either a small amount of autopolymerizing resin (Duralay, Reliance Dental, Worth, IL) or the rubber impression material. In 24 hours, the laboratory was able to return the denture converted into a hybrid-type fixed prosthesis, highly polished and including metallic copings on the implants as well as wire reinforcement.
The conventional procedure uses a transparent acrylic surgical template and a custom impression tray. After subsequent sessions for intermaxillary registrations and try-in, the final installation of the transitional prosthesis take place in about 10 days.
In both cases, the prostheses were carefully checked clinically and radiographically to verify their correct fit on the implant shoulders. In the case of a misfit, the prosthesis was separated into two or more parts that were again rigidly connected with resin. If an increased friction or force requirement was noticed when tightening the screws, the coping inner faces were polished to better accommodate the screws.
All of the prostheses had two distal extensions from 5 to 15 mm according to clinical necessities. These transitional prostheses were substituted by the definitive ones in about 6 months (range, 4–14 months).

Follow-Up Visits and Parameters Recorded.
On the day of installation of the transitional prosthesis, a Panoramic x-ray as well as standardized periapical x-rays were taken using XCP holders (Dentsply International Inc., York, PA). These formed the baseline. After having the prostheses installed, the patients were examined after a week to take the sutures out and evaluate the postoperative condition. Subsequent visits were scheduled at 2 weeks and 2 months to check occlusal function and to reinforce oral hygiene procedures (Fig. 6). The next visit was at 4 months and was the moment when the transitional prosthesis was unscrewed for the first time since its installation, allowing individual clinical examination of each implant. After this visit, the fabrication of the final prosthesis could begin. Parallel x-rays were repeated at the moment of the placement of the final prosthesis. Afterwards, a standard maintenance protocol was instituted.
The following parameters were recorded:

• Implant stability, assessed by percussion and clinical mobility of the implant, using the handles of two dental mirrors
• Marginal bone loss, measured on parallel x-rays using a 7-X magnifying glass graded in tenths of millimeters. Measurements were taken to the nearest 0.1 mm on the mesial and distal sides using the implant shoulder level as a reference point.

RESULTS.
Implant Survival.
No patient withdrew during the follow-up period. Two of 61 implants were lost, both from the same patient. This patient had a maxillary arch fixed bridge as antagonist, and the implants were placed at the same session as the extraction of the residual mandibular dentition. The failed implants did not show signs of infection, but they had clinical mobility, and the patient had functional discomfort. The 59 remaining implants were clinically immobile, asymptomatic, and free of any radiolucency for the entire follow-up period, with a maximum of 18 months and an average of 8 months. Thus, the implant survival rate was 96.7% for the follow-up period.
The prostheses success rate was 92.3%; a previously planned fixed/ detachable prosthesis had to be converted into an overdenture because the patient refused to have the two lost implants replaced.

Marginal Bone Loss.
Marginal bone loss was generally very limited (always ,1 mm). Only two implants showed a marginal bone loss .1 mm. Excluding them, the preliminary cumulative success rate was 93.4%.

Complications.
Tissue healing occurred without any significant complications in all of the patients. During the first 4 months of using the transitional prosthesis, the most frequent prosthetic complications were three cases of fractured cantilevered portions (15%) (Fig. 7), two of them in the same patient, probably in relation to incorrect occlusal adjustment. Another prosthetic complication was the loosening of screws in 23% of the prostheses. None of the complications prevented the prostheses from functioning and, therefore, cannot be considered as failures.

DISCUSSION.
This study, carried out in a private clinic, confirms earlier reports on the immediate loading of implants in the interforaminal zone as a safe method. Despite the limited number of cases and the short follow-up period, the preliminary results of this study are significant because, apart from those presenting the aforementioned exclusion criteria, all patients in need of a mandibular prosthesis were included. These results are comparable to those obtained from similar studies on fixed/detachable mandibular prostheses, hybrid type.
In the case of the patient who lost two implants, there was occlusal pathology with considerable anterolateral discrepancy between the centric relation and the intercuspal position and some oral muscular tension. Furthermore, at the time of her treatment, she was suffering from intense emotional stress. Thus, her altered oral function cannot be ruled out as the cause of the two implant failures. Because she refused to have the two implants replaced, her prosthodontic plan was changed to a bar on the three remaining implants and an overdenture.
It should be stressed that 32 implants (53%) were placed in the same session as the extraction of the residual dentition. In another 13 implants (21%), the time between extractions and implantation was only 2 months. Only 16 implants (26%) were inserted on bone that had been edentulous for more than 12 months. Despite these different circumstances, all of the implants were immediately loaded, and no negative influence was noticed with implant success rate. Until now, this option of immediately placing and loading implants has rarely been reported in clinical studies8,11 and never discussed by their authors. The present results suggest that implants placed into extraction sockets and immediately loaded show a positive clinical response if a strong primary stability is achieved and there are no periapical infections in the surgical zone.
Some authors reported a slight improvement in marginal periimplant bone loss for immediate loading protocols. 14 Recent experimental studies have demonstrated greater density in the bone surrounding immediately loaded implants,18,19 or a better boneimplant contact in rotation-mobile implants compared with the corresponding control group of totally stable implants.27 These authors speculate that these results could be due to the immediate transmission of functional forces acting as an osteogenic stimulus. This was postulated by Frost,28 who called it regional acceleratory phenomenon (RAP). Our study confirms that marginal bone loss rates of immediately loaded implants were well within the clinically acceptable parameter: ,1.5 mm, as documented for Brånemarklike implants after 12 months of functional loading.1 This measurement is always taken at the time of prosthesis installation, thus disregarding the initial postsurgical osseous remodelling. Interestingly, in the present study, this initial bone loss is included in the measurement because our baseline is the immediate postsurgical x-ray. One of the two implants with bone loss .1.5 mm was located in the center of the mandibular arch, and the surgical wound suffered a dehiscence in a central relief incision, probably in relation to masticatory traumatism (although this hypothesis cannot be confirmed).
The fact that four different implant designs were used in this study without significant differences in success rates indicates that the results are not dependent on a particular implant design or surface, but rather that different root-form implants clinically behave in the same manner when in the same clinical conditions.
The prosthetic complications were easily managed and seem to be partly related to an imprecise working model or inaccurate intermaxillary registration. Fractured prostheses were attributed to the fatigue of wire reinforcement. (Our initial concern was the rapid placement of the fixed prostheses so that a cast framework was not made.) Technical development during the time of the study resulted in better prosthetic procedures and a reduction in the number of complications.
The rapid restoration of oral function through the immediate loading of the prostheses has been a great advantage for the patients compared with conventional delayed loading protocols. In particular, this protocol has completely eliminated decubital lesions or soreness due to pressure on the mucosa as well as uncontrolled forces transmitted to unsplinted implants through the overlying denture. These problems are frequently seen because implants are commonly placed in a nonsubmerged or semisubmerged position. A substantial improvement in function and psychological well-being was experienced by 100% of the patients included in this study, resulting in a high level of satisfaction with the treatment.

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