CLINICAL ARTICLES

J Oral Maxillofac Surg60:851-857, 2002

The Transmandibular Implant: A 5- and15-Year Single-Center Study

Greg Paton, MBBS, BDS,*

Janet Fuss, BDS, BscDent (Hons), MSc,† and

Alastair N. Goss, DDSc, FRACDS (OMS), FICD,‡

Purpose: Fifty-eight patients (average age, 60 years) with gross mandibular atrophy had transmandibu-lar implants inserted between 1984 and 1988. This study reports on their progress at 5 and 15 years.

Patients and Methods: In both 1991 and 2000, 50 cases were reviewed, with the other 8 lost tofollow-up. Detailed surveys, using exactly the same format, were conducted to determine the outcomeof transmandibular implant treatment. The records were reviewed and surviving patients with transman-dibular implants were examined.

Results: An overall success rate of 80% in 1991 had fallen to 56% by 2000, with a total of 22 implantsbeing removed.

Conclusion: Factors involved in failure were age and medical infirmity of the patients, gross mandibularatrophy, and the use of several surgeons, in particular, trainees. This survey shows that the TMI has aprogressive long-term failure rate and that extreme care needs to be taken in both patient selection andimplant placement.© 2002 American Association of Oral and Maxillofacial SurgeonsJ Oral Maxillofac Surg 60:851-857, 2002

The edentulous patient with extreme atrophy of themandible presents a considerable challenge to thetreating clinician. Over the years many prosthodonticand surgical solutions have been proposed, initiallypraised, and then fallen into disfavor.1,2 However, thissituation has been revolutionized by the advent ofseveral different types of successful dental implants3,4

that emerged throughout the 1970s and early 1980s.One such implant was the transmandibular implant(TMI) developed by Bosker of the Netherlands. Theimplant is an endo-oseous, staple type of implant,made of corrosion-resistant 18-carat 5% alloy (70%gold, 5% platinum, 12.8% silver, and 12.2% copper). Itis inserted in the anterior part of the mandible be-

tween the mental nerves and incorporates a lowerborder baseplate that is secured to the mandible by 5cortical screws. From the baseplate 4 transostealstruts pass through the alveolar crest and into themouth. Within the mouth a Dolder bar connects thesestruts and provides a point of attachment for the 3retentive sleeves embedded within the mandibulardenture. This arrangement provides a stable and re-tentive but removable mandibular prosthesis. Implantinsertion is a straightforward surgical procedure andhas been described in detail.5,6

The early results from the developer’s initial se-ries were excellent,5 and accordingly several mul-ticenter trials were established. Generally, the re-sults of these multicenter trials6-9 have reproducedthe results of the initial series.5 In the largest andlongest of these studies,9 there is an in-depth anal-ysis of the few failures. In particular, it was notedthat each individual surgeon and prosthodontistrequired training, and there was an associatedlearning curve. Interestingly, retrospective analysisof the TMI system has demonstrated the cessationof progressive mandibular bone loss and, in somecases, the induction of further bone growth.10,11

This study reports the results of a single-centerseries of TMIs. In particular, there is a detailed analysisof those cases in which the TMI failed.

*Registrar, Oral and Mxillofacial Surgery Unit, The University of

Adelaide, Adelaide, Australia.

†Senior Lecturer, Dental School, The University of Adelaide,

Adelaide, Australia.

‡Professor and Director, Oral and Mxillofacial Surgery Unit, The

University of Adelaide, Adelaide, Australia.

Address correspondence and reprint requests to Prof Goss: Oral

and Mxillofacial Surgery Unit, The University of Adelaide, Adelaide,

SA 5005, Australia; e-mail: oral.surgery@adelaide.edu.au.

© 2002 American Association of Oral and Maxillofacial Surgeons

0278-2391/02/6008-0002$35.00/0

doi:10.1053/joms.2002.33850

851

Materials and Methods

Between 1984 and 1988, 58 TMIs were inserted.For all cases, the developer’s recommended methodof insertion, subsequent prosthodontic management,and follow-up were adhered to.5 The surgical andprosthodontics staff had been trained during 2 visitsby the developer of the implant, and the senior sur-geon and prosthodontist had also visited the Nether-lands. A training video outlining the method of inser-tion was made.12 Concurrently, 2 animal studies onvarious aspects of the TMI were conducted.13,14 Ini-tially, this series was part of the international multi-center trial of the TMI, but this participation waswithdrawn on the resignation of the senior surgeon.

In both 1991 and 2000, detailed follow-up surveys,using exactly the same format, were conducted todetermine the outcome of TMI treatment. Therecords were reviewed and surviving patients withTMIs were examined. Radiographic measurementswere standardized using a TMI thread distance of 1.2mm to adjust for magnification differences betweenradiographs. Three staff members who had not beeninvolved in the patient management performed theexaminations at the 5-year survey, and 2 of these staffmembers subsequently performed the same examina-tions for the 15-year review. Each examination fol-lowed a proforma, and the patient’s opinion wasassessed by questionnaire. The 1991 study resultswere not published earlier due to nonscientific issues.

Results

TMIs had been placed in 58 patients, 3 men and 55women, with an average age of 60 (range, 26 to 77years). Forty-five percent of the patients had preexist-ing medical complaints, which required concurrentmedical treatment. All patients had gross mandibu-lar atrophy, with a mean midline bone height of 10.5mm (range, 5 to 16 mm), as measured on the initialpostoperative orthopantogram. The mean premolarheight for all patients was 7.1 mm (range, 4 to 14mm).

Eight surgeons, 3 consultants, and 5 residents hadperformed the surgery. No intraoperative difficultieswere reported. Five prosthodontists were involved,all of whom confined their practice to prosthodon-tics. The average time of operation to denture inser-tion was 3.6 months (range, 2 to �6 months).

FIVE-YEAR RESULTS

A total of 45 patients were reviewed; despite adiligent search, 8 could not be contacted, and 5 haddied. All 5 patients who had died had a satisfactorilyfunctioning implant at the time of death, as deter-mined by a recent examination before death or as

reported by the next of kin. Ten implants had beenremoved, for a success rate of 80%, or if one assumesthat those lost to follow-up are still satisfactory, of83%.

The 45 available patients were reviewed at an av-erage of 5 years (range, 3 years 1 month to 8 years 7months) after implant insertion. Among the 45 pa-tients who were reviewed, there was a high fre-quency of minor untoward events, excluding totalremoval, that required further surgical treatment.Only 16 patients (35%) made uneventful progress.The numbers of further interventions per patientranged from 1 to 6 and were more common in thefirst year. However, problems occurred up to 28months after insertion. The type and frequency ofuntoward events are shown in Table 1.

The 35 patients with existing TMIs were examined.Lower denture stability was good in 32 patients (91%)and satisfactory in 3 (9%). The denture was firmlyengaged to the implant in 25 cases (71.5%), weaklyengaged in 6 (17%), and not engaged in 4 (11.5%).Lower denture retention was good in 30 cases (86%),satisfactory in 3 (8.5%), and unsatisfactory in 2 (5.5%).The results of the periodontal examination regardingthe struts are presented in Table 2. Generally therewas little relation between bleeding on probing andplaque. The deepest periodontal pocket measuredbetween 0 and 3 mm on 88% of the struts and 4 mmor greater on 12% of the struts. The amount of kera-tinized gingivae to the buccal of the struts was onaverage 2 mm (range, 0 to 7 mm).

Five patients (14%) had preoperative sensory diffi-culties that were directly related to previous prepro-sthetic surgical procedures. When examined at the5-year survey, 8 patients (23%) had ongoing sensorydifficulties related to the mental nerves. All 35 pa-tients had a satisfactory chin profile and 34 patientshad an inconspicuous scar. Seventy-seven percent ofrespondents indicated that they would have the im-plant procedure again, and 79% indicated that theywould recommend the procedure to a friend. Otherpatient opinions are shown in Table 3.

Table 1. FIVE-YEAR SURVEY OF UNTOWARDEVENTS (N � 45)

No. (%)

Mandibular fracture 4 (9)Infection

Treat with antibiotics 21 (47)Antibiotics and drainage 18 (40)Transmandibular implant removal

and reinserted 8 (18)Structural failure 6 (13)Hyperplasia near struts 14 (31)

852 TRANSMANDIBULAR IMPLANT

The results of the comparison of the immediatepostoperative radiograph and the most recent radio-graph are shown in Table 4. Importantly, the averagebone heights had not increased with time and 17% ofstruts had angular defects.

FIFTEEN-YEAR RESULTS

Of the original 58 patients, 30 were alive, 20 haddied, and 8 could not be contacted at 15 years. Of the8 patients not contacted, 5 had been lost to recall atthe 5-year survey. However, 3 of the 8 patients lost tofollow-up at 5 years were subsequently located at the15-year survey. During the observation period, 14 ofthe 30 surviving patients had implant failures neces-sitating removal. From the deceased group, 8 patientshad implants removed before death, and no deathswere caused by the implant. In total, 22 of 50 im-plants had been removed, for a success rate of 56% or,if one assumes that those lost to follow up are stillsatisfactory, of 62%. Of the 16 surviving patients withimplants, 14 were available for examination; all 14had been examined for the 5-year survey. Of theremaining 2 patients, 1 reported no previous or cur-rent problems with her implant, while the other wasunhappy with the implant and reported numerousproblems but declined to be examined.

The 14 available patients were reviewed at an av-erage of 15 years and 5 months (range, 12 years 3months to 16 years 7 months) after implant insertion.Lower denture stability was good in 9, satisfactory in1, and unsatisfactory in 4 patients. Lower dentureretention was good in 7 cases, satisfactory in 3, andunsatisfactory in 4. The denture was firmly engaged tothe implant in 7 cases, weakly engaged in 4, and notengaged in 3. Weak engagement arose from overexpanded retentive sleeves and in one case from adenture containing only two retentive sleeves. Loss ofengagement occurred in one patient due to a frac-tured denture, the other cases arose from damage tothe retentive sleeves contained within the denture,which prevented them from engaging the Dolder bar.

Results from the periodontal examination about thestruts are presented in Table 2. Once again, there waslittle relation between plaque and bleeding on prob-ing. The deepest periodontal pocket measured be-tween 0 and 3 mm on 79% of the struts, and 4 mm orgreater on 21% of struts. The amount of keratinizedgingivae to the buccal of the struts was narrow, aver-age 1.5 mm (range, 0 to 5 mm).

When examined at the 15-year survey, 3 patientshad ongoing sensory difficulties related to the mentalnerve. Preoperatively, none of the 14 patients had

Table 2. FIVE- AND 15-YEAR SURVEY PERIODONTAL EXAMINATIONS

5-yr Results (n � 35*) 15-yr Results (n � 14)

Bleeding Plaque Bleeding Plaque

n % n % n % n %

None 100 72 50 36 37 66 20 36Minor 33 24 56 40 14 25 10 18Marked 6 4 33 24 5 9 26 46

NOTE. n indicates number of struts.*One transosteal strut removed.

Table 3. FIVE-YEAR PATIENT OPINIONS (N � 34*) AND 15-YEAR PATIENT OPINIONS (N � 14)

Opinion

Better (%) Worse (%) Same (%) Unsure (%)

5 yr 15 yr 5 yr 15 yr 5 yr 15 yr 5 yr 15 yr

Speech 53 36 6 7 38 57 3 0Eating 85 71 3 14.5 12 14.5 0 0Retention

Lower 100 86 0 14 0 0 0 0Upper† 67.5 36 12 0 17.5 64 0 0

Pain‡ 47 14 6 7 6 0 0 0Appearance 55 50 9 7 24 43 12 0

*One patient did not complete a questionnaire.†One patient had natural maxillary dentition at the 5-year survey.‡Pain had never been a problem for 41% at 5 years and 79% at 15 years.

PATON, FUSS, AND GOSS 853

sensory difficulties. The chin profile was normal forall 14 patients, however, 4 were considered to havean abnormal scar as it had become conspicuous withthe sagging of skin beneath the chin associated withageing. When questioned, 13 patients indicated thatthey would have the implant again and recommendthe implant to a friend; other patient opinions of theimplant procedures are shown in Table 3.

The results from the comparison of the immediatepostoperative radiograph and the most recent radio-graph are shown in Table 4. A very high number ofangular defects were noted and in contrast to the5-year results, the average bone heights have in-creased over time. The high number of radiographicangular defects (48%) is at odds with the lower num-ber of periodontal pockets greater than 4 mm (21%).This may have resulted from the periodontal probecoming into contact with the thread of the transostealposts during probing, rather than passing down apocket.

IMPLANT FAILURE

A comparison was made between those patientswho had a successful implant versus those whoseimplant had failed. At the 5-year survey it was foundthat implant failure was more likely if the implant wasplaced by a trainee (P � 0.5), if the mandible wasthinner than 4mm in the premolar region (P � 0.5),and if the initial implant required treatment for infec-tion and had repeated untoward events (NS).

At the 15-year survey the original number of unto-ward events recorded in the first 5 years averaged 2.7for the 22 patients who had implant failures versus1.7 for the 28 patients who retained their implant.The average time to implant removal was seven and ahalf years (range, 2 months to 15 years and 6 months).

Implant survival versus time is represented in Figure1. From this figure it can be seen that the TMI has aprogressive long-term failure rate.

Causes of implant failure at the 15-year survey arepresented in Table 5. Thirteen implants were re-moved due to recurrent or significant infections thathad not responded to conservative measures. Fourimplants were removed due to a slow but progressiveloss of bone around the struts; Figure 2 illustrates onesuch case. Two implants were removed due to man-dibular fractures, with one fracture resulting from apostoperative fall. Two implants were removed dueto complaints of pain and dysesthesia; one arose frompoor strut positioning and did not resolve despitemultiple interventions including strut removal, theother was thought to be associated with concurrentpsychiatric and drug abuse problems. One implantwas removed due to mechanical failure, which in-volved fractures of 2 transosteal posts and the Dolderbar (Fig 3).

Discussion

The results of this study show a higher failure ratethan that reported in other transmandibular implant

Table 4. COMPARISON OF IMMEDIATE AND MOST RECENT RADIOGRAPHS AT 5-YEAR SURVEY (N � 30*)AND 15-YEAR SURVEY (N � 11†)

Initial Bone Contact Most Recent Bone Contact

No. of Struts No. of struts

5 yr 15 yr 5 yr 15 yr

Good 117 (97.5%) 41 (93%) 98 (81.7%) 23 (52%)Angular defect 3 (2.5%) 3 (7%) 21 (17.5%) 21 (48%)Struts removed 0 0 1 (0.8%) 0

Initial Average Bone Height(mm)

Most Recent AverageBone Height (mm)

5 yr 15 yr 5 yr 15 yr

Midline 10.8 12 10.4 13Minimum (premolar region) 7.6 7.9 6.9 8.1

*Both radiographs not available for 5 patients.†Both radiographs not available for 3 patients.

Table 5. REASONS FOR IMPLANT REMOVAL AT15-YEAR SURVEY

Reason No. (%)

Infection 13 (59)Integration failure 4 (18)Mandibular fracture 2 (9)Pain/dysesthesia 2 (9)Mechanical failure 1 (5)

854 TRANSMANDIBULAR IMPLANT

studies.6-9 Indeed the success rates of 80% at 5 yearsbarely meets the standard of a 75% success rate set in1980,15 and a 56% success rate at 15 years is unac-ceptable. Accordingly, it is important to look at fac-tors that may account for this poor result.

With a mean age of 60 years, (range, 26 to 77 years)the patient group in the Australian series is older thanthe Dutch group9 (mean, 49.8 years; range, 24 to 81years). The Australian group also had a greater pro-portion of females (Australian 95%, Dutch 68%). TheAustralian group was less healthy, as 45% of patientshad pre-existing medical conditions (no figure statedfor Dutch group) and 20 of the original 58 (34.5%)Australian patients died during the 15-year study pe-riod, whereas only 10 of the 1,356 (0.7%) patientsdied in the 13-year Dutch study period. The degree ofmandibular height was similar in these 2 studies, theaverage being 10.5 mm (range, 5 to 16 mm) in themidline for the Australian group, whereas the Dutchgroup had a greater range (4 to 24 mm), but with the

majority being less than 10 mm in height. Thus, thepatient groups were different, with the Australiangroup being older and less healthy.

There was certainly a major difference in the num-ber of surgeons involved. In keeping with normalpractice in an Australian surgical training unit, train-ees were actively involved in the placement of theimplants, but this decreased the chance of any onesurgeon passing completely through the learningcurve. It is noteworthy that the consultant surgeonshad a lower failure rate than the trainees. It must alsobe noted that the overall failure rate for the implantsystems currently being used in the same unit is lowwith no difference between the failure rates for con-sultants or trainees. In the Dutch series9 the impor-tance of using one surgeon in each center wasstressed. This point was not known when the Austra-lian trial started in 1984.

One of the advantages of the transmandibular im-plant is that it can be easily adjusted without total

FIGURE 1. Implant survival ver-sus time.

FIGURE 2. Radiograph of trans-mandibular implant removed dueto a slow but progressive loss ofbone around the transostealstruts.

PATON, FUSS, AND GOSS 855

removal. This, in part, accounts for the number ofinterventions noted at the 5-year survey. When prob-lems arose with the implant they were addressed inaccordance with the recommended management pro-tocol. The most common problem during the first 5years was infection, and this may relate to the natureof the interface between the implant and mandible. Ifthe drilling and reaming at the time of insertion is toovigorous, and insufficient cooling is used, then ther-mal damage to the bone occurs.9 It seems likely thatthis was the cause of many of the early infections thatoccurred. At the time of the 5-year review 2 patientswith satisfactory implants had a large build up ofcalculus on the implants. One week after scaling andcleaning by an experienced dental hygienist both re-presented with a submental abscess. Although thescaling had not been vigorous or subgingival an infec-tion was triggered, which within 1 week trackeddown the post indicating the absence of true os-seointegration between the implant and bone.

With advancing age, many patients developed sig-nificant medical problems that impacted on their im-plant. For example, 3 patients developed diabetesmellitus, a contraindication to transmandibular im-plant placement. Soon after the diagnosis of the dia-betes, one patient developed progressive osteolyticlesions around 3 of the 4 struts, while the other 2developed recurrent infections. As a result of thesecomplications all 3 patients had their implants re-moved. At the 15-year review several patients whohad not attended follow-up appointments were foundto be dementia suffers residing in nursing homeswhere their level of oral hygiene care was low. Onesuch patient had her TMI totally encrusted with cal-culus, which prevented her from wearing her lowerdenture. Thus, medical infirmity associated with in-creasing age can elevate the risk of TMI failure inmany different ways.

In accordance with earlier studies,16 the survey ofpatient opinions revealed a high level of satisfactionwith the TMI. In particular, at the 5-year survey 100%of patients reported better lower denture retention,85% reported improved mastication and despite thehigh number of untoward events noted at the 5-yearsurvey, 77% of patients indicated that they wouldhave the implant again if given the choice.

Despite the high levels of patient satisfaction, theTMI had several drawbacks. There was a risk of neu-rological morbidity associated with insertion, a highnumber of minor untoward events in the early post-operative period, an increasing number of angulardefects with time, and a progressive long-term failurerate mainly as a result of infection. These problemscan be minimized by careful patient selection andimplant insertion, the use of one surgeon in eachcenter, and vigilant post insertion follow-up of allpatients throughout the life of the implant, includingthose patients who become institutionalized with ad-vancing age.

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FIGURE 3. Radiograph of im-plant removed due to mechanicalfailure, which involved fracturesof 2 transosteal posts and theDolder bar.

856 TRANSMANDIBULAR IMPLANT

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sheep. MDS Thesis, The University of Adelaide, Adelaide, Aus-tralia, 1986

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PATON, FUSS, AND GOSS 857