Reconstruction of the severely atrophic edentulous mandible with endosseous implants

J Oral Maxillofac Surg 53:305-320, 1995

Reconstruction of the Severely Atrophic Edentulous Mandible With Endosseous Implants: A lO-Year Longitudinal Study E.E. KELLER, DDS, MSD* Endosseous implant reconstruction of the edentulous mandible with adequate bone quantity and quality is highly successful and well documented in 7- to 25year published longitudinal studies. 1-3 This success is highly dependent on a strict surgical/prosthodontic protocol. The surgical technique includes meticulous atraumatic osseous and soft tissue manipulation, initial rigid implant stability (threaded cylinder), and an undisturbed (nonloaded) healing time. The prosthetic biomechanical design must allow long-term physiologic masticatory loading. A properly designed, machined, and biomechanically clean, pure titanium, threaded, noncoated cylinder is presumably required to predictably attain osseointegration, provided the surgical and prosthodontic procedures are properly performed (osseointegration is defined histologically but must be collaborated by long-term longitudinal clinical and radiographic examination). Research by others currently ongoing using other implant metals, designs, microsurfaces and macrosurfaces, and cleaning methods may duplicate the success of the Branemark endosseous implant protocol; however, the basic nonviolent osseous surgical manipulation mandated by this protocol will, in my opinion, continue to remain the foundation for successful outcome. To properly and consistently evaluate implant success, the criteria proposed by Albrektsson et al 4 in 1986 should be used to judge clinical outcome. Branemark 5 and o t h e r s 6'7 demonstrated early on that the same endosseous implant reconstruction used for noncompromised mandibular anatomy was also suited for compromised mandibular anatomy. Seven-millimeter

length and 3.75-mm diameter implants were used successfully in mandibles with vertical height of 10 mm or less (provided bone width was greater than 6 mm at the implant site). We know from our 11 year experience in performing approximately 400 totally edentulous reconstructions that endosseous implant reconstruction in edentulous mandibles with 4- to 5-ram height (and at least 6-ram width) are safely and predictably treated with four or five endosseous implants and a fixed or fixed-removable osseoprosthesis bridge] This article presents the indications, contraindications, complications, surgical protocol, and a 10-year experience with 61 patients with advanced mandibular bone resorption treated with a specific surgical implant protocol with or without bone grafting. This report also demonstrates the positive physiologic osseous response to surgical implant placement and endosseous loading (function) over time. The risk-benefit ratio of using this implant method in patients with advanced bone resorption will be apparent from this presentation.

Surgical Considerations PATIENTS NOT REQUIRING SIMULTANEOUS BONE GRAFTING Patients with bone height greater than 4 to 5 mm and bone width greater than 6 mm in the implant sites are generally suitable candidates for placement of 4 to 5 interformainal threaded cylindrical endosseous implants (Figs 1 through 5). Because of the advanced mandibular bone resorption in this group of patients, a number of significant modifications are recommended as compared with the surgical technique in the noncompromised mandible:

* Professor and Consultant, section of Oral and Maxillofacial Surgery, Mayo Medical Center, Rochester, MN. Address correspondence and reprint requests to Dr Keller: Department of Oral and Maxillofacial Surgery, Mayo Medical Center, 200 First St, SW, Rochester, MN 55905.

1) Make an incision on the keratinized ridge crest rather than in nonkeratinized sulcus mucosa to avoid incising through the main body of the

© 1995 American Association of Oral and Maxillofacial Surgeons 0278-2391/95/5303-001253.00/0

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FIGURE 1. Advanced mandibular resorption in 76-year-old woman treated with endosseou s implants. Increased mandibular body bone height is noted on the 5-year postprosthetic loading panoramic radiograph secondary to increased masticatory function. Panoramic radiograph before (A) and 61 months after prosthetic (B) loading. Note increased height of 5 to 9 mm in the mental foraInen region. Bone grafting was not performed on this patient.

mentalis muscle, which is attached to the ridge crest in this group of patients. 2) Use conservative periosteal elevation to preserve an already compromised blood supply to the residual bone (the mandible receives a majority of its blood supply via the periosteum in this group of patients).

FIGURE 2. Surgical technique for reconstruction of advanced mandibular resorption without a bone graft. Drawing (A) illustrates placement of endosseous implants through the inferior border to completely use the compromised mandibular height. Surgical photograph (B) shows wide spacing of implants after detachment of incisive branch of the herniated inferior alveolar nerve to allow buccal and posterior positioning of the nerve. This permits improved biomechanical osseoprosthesis loading.

3) Buccal and posterior repositioning of the mental nerve, which is herniated on the crest of the residual ridge, allows more posterior implant placement in mandibles that tend to be in a Class III relation to the maxilla, thus improving the biomechanical prosthesis loading (Fig 2). 4) Low-speed drilling of the hypovascular dense bone is critical to avoid excessive heat production, which aborts the predictability of osseointegration. 5) Use of complete bone threading of all osseous implant sites should extend through the mandibular inferior border (Fig 2). 6) Avoid countersinking the implant site, which reduces bone height for osseointegration (the patient is already bone deficient in volume). 7) Extend the implant apex through the inferior border 1 to 2 mm and place the osseous coagulum from the bone tapping instrument through the implant site before inserting the threaded implant (inferior border autogenous bone graft) (Fig 5). 8) Place the implant at right angles to the ridge crest and occlusal table, and avoid the tendency to angle implants posteriorly in the Class III mandible (poor angulation will lead to prosthesis and abutment screw overloading and difficulty in maintaining hygiene). 9) Avoid straight line placement of the implants (resorbed mandibles tend to be wide and square inferiorly), which also leads to mechanical overload and fatigue fracture of abutment and prosthesis screws (nerve repositioning allows more posterior placement of the distal implants, which helps avoid this problem. 10) Delaying interim prosthesis placement will allow a more precise prosthesis to be made and reduces the risk of premature implant loading. Implant access holes are frequently anterior to the

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FIGURE 3. Mandibular endosseous implant reconstruction of 69-year-old woman with advanced bone resorption (4 mm at mental foramen). Pretreatment panoramic (A) and lateral cephalometric (B) radiographs, and six year posttreatment panoramic radiograph (C) and clinical photograph (D) of fixed continuous bridge prosthesis. Bone height increased from 4 to 7 mm in the mental foramen region in this patient following 65 months of prosthesis loading, This patient has experienced continuous function without complications. Prosthetic treatment provided by Dr Ronald Desjardins, Section of Prosthodontics, Mayo Clinic.

anterior teeth on the final prosthesis, which has not created a problem (this actually makes it easier for the prosthodontist to place or remove the prosthesis). An increased posterior prosthesis cantilever length is also present in this group of patients due to the tendency toward a Class III jaw relationship (Fig 3). Improved posterior implant placement, or anteroposterior reduction in the posterior occlusal table when overload is evident, has obviated this potential problem. A high percentage of patients in this group will have nonkeratinized peri-implant tissue; this should not create an increased incidence of peri-implantitis, provided that the patient does adequate implant maintenance and the peri-implant tissue is nonmoveable during function. Implant or peri-implant tissue movement is the critical factor, not the presence or absence of keratinized tissue (Fig 3).

PATIENTS REQUIRING SIMULTANEOUS BONE GRAFHNG

Simultaneous composite onlay bone grafting is indicated for those patients whose vertical alveolar bone height is less than 4 to 5 mm and/or whose bone width is less than 6 mm in the prepared endosseous implant site (Figs 7 through 10). This group of patients can still avoid this more extensive and costly treatment if they agree to an overdenture-type prosthesis stabilized with two or three implants in the midline region, where there is invariably enough bone for two or three endosseous implants (midline lower lip and suprahyoid muscle attachment region). These patients must be willing to have compromised function and esthetics because of medical, biologic, or financial restraints. On the other hand, patients with adequate but marginal bone volume (5- to 7-ram height), who have high cosmetic

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RECONSTRUCTION OF THE SEVERELY ATROPHIC EDENTULOUS MANDIBLE FIGURE 4. Mandibularendosseous implant reconstructionof 59year-old woman with advancedbone resorptionin whom prosthetic reconstruction was accomplishedwith a fixed/removableoverdenture prosthesis on a continuous cast bar. Preprosthetic (A) and 24 months postprosthetic loading (B) panororamic radiograph. Bone height increase in this patient was from 5 to 9 mm. Clinical photograph (C) illustrating the implant connecting bar and previously placed skin graft. This patient experienced one episode of periimplant soft tissue hyperplasiarequiring surgical excisionand abutment replacement. Prosthetic treatment provided by Dr G.M. Voelker, Wisconsin Rapids, WI.

and functional expectations, may choose the composite bone graft procedure; in these patients, the prosthodontist is able to enhance esthetics with reduced prosthesis bulk and ideal tooth positioning. Also, the onlay bone graft provides increased lower lip support (cosmetics) and function. This group frequently includes patients with a younger biologic age who have worn a lower denture since early in their life and have experienced premature perioral aging secondary to extensive mandibular bone loss (Fig 8, 9, and 10). The surgical technique associated with the onestage, full arch, composite bone graft is covered in detail in a recent publication 8 (Fig 7). Corticocancellous block bone is obtained from the anterior-medial iliac crest and rigidly secured to the surgically exposed edentulous mandible with five or six endosseous threaded cylindrical implants. Important general surgical considerations for the composite onlay bone graft procedure include the following: 1) Atraumatic exposure of the superior aspect of the residual ridge preserves soft and hard tissue cellular viability, and conservative periosteal reflection preserves the already compromised blood supply. 2) Atraumatic bone graft harvesting preserves the cortical and cancellous bone cell viability; also, transfer of the bone graft immediately to the recipient site avoids graft desiccation or the toxic effects of storage fluids.

3) Provide atraumatic bone drilling and tapping before placement of endosseous implants through the bone graft into the dense residual mandible (control of heat production). 4) A watertight, everted, tension-free wound closure ensures early fluid nutrition and revascularization of the bone graft. 5) Delayed placement of the interim prosthesis avoids early composite bone graft function. 6) Placement of a continuous fixed osseoprosthesis achieves direct internal bone loading rather than the nonphysiologic bone loading via the mucoperiosteum, which has caused all previous onlay bone grafting procedures in time to partially or completely fail secondary to resorption of the remineralized and revascularized bone grafts.

Methods and Materials Medical, surgical, and prosthodontic records were reviewed for all patients with edentulous mandibles who had received full arch reconstruction with endosseous implants and/or autogenous onlay bone grafts from September 1983 through December 1993. To be included in this study group, bone height as measured on pretreatment lateral cephalograms and panoramic radiographs had to be less than 10 mm in any, one, or all the eventual endosseous implant sites. Lateral cephalograms (pretreatment) and panoramic radiographs (pretreatment and posttreatment), were avail-

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FIGURE 5. Surgical technique for reconstruction of advanced resorption using inferior border autogenous bone (osseous coagulum) from the bone tapping instrument. A, The surgical technique. B, A clinical photograph showing the bone graft material on the bone tapping instrument prior to insertion into the implant preparation site. Mandibular endosseous implant reconstruction in 55-year-old woman with advanced bone resorption using the inferior border bone grafting technique. Preloading panoramic radiograph (C) and 12 months postloading panoramic radiograph (D). Bone height increased from 5 to 7 mm in the body region secondary to increased masticatory function. Note the bone apposition at the inferior border around implants that had initially projected 2 mm through the inferior border. This patient lost two distal implants 16 months postloading and has continued to function with a fixed bridge supported by three implants and a shortened (AP) posterior prosthesis cantilever. Prosthetic treatment provided by Dr Steven Eckert, Section of Prosthodontics, Mayo Clinic.

able on all patients. The patients had to be functioning with a fixed or fixed-removable prosthesis for a minim u m of 6 months and the prosthesis had to be endosseous implant supported. All patients operated during this time frame were examined so the material can be considered a consecutively operated longitudinal series. No patients initially accepted into this study were lost to follow-up except one who died during the study. All 61 patients had surgery performed by four M a y o Clinic oral and maxillofacial surgeons (54 patients by two of the four) trained in the surgical protocol taught by the Goteberg team. 1'2 All patients (except 6) were treated by four M a y o Clinic prosthodontists, who were also trained by the Goteberg team. All implants used in this study were pure titanium, noncoated, threaded, cylindrical and manufactured by a single c o m p a n y (Nobelpharma, Goteberg, Sweden). The bone grafts used in nine patients were fresh, autogenous, corticocancellous blocks obtained from the anteriormedial iliac crest. General data were recorded and included the patients' age, sex, medical diagnoses, and dates relative

to the surgical and prosthetic procedures. Intraoperarive, perioperative, and long-term complications were recorded. The medical records were reviewed on all patients, but detailed findings are not presented in this study. In general, patients were individuals who experienced a typical incidence o f medical problems (diabetes, obesity, hypertension, coronary artery disease, malignancy, etc). None of the patients had medical problems that prevented surgical or anesthetic management, or interfered with tissue healing or follow-up maintenance. The only patient who died during the study had complications related to alcoholism (this patient was observed for 55 months). Linear measurements were made on the 1) a preoperative cephalometric radiograph (midline value); 2) a preoperative panoramic radiograph (the shortest height at or anterior to the mental foramen reduced by 20% to correct for magnification); and 3) the most recent panoramic radiograph (the same site and magnification correction as measured on the pretreatment panoramic radiograph). The latter radiograph was not always

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RECONSTRUCTION OF THE SEVERELY ATROPHIC EDENTULOUS MANDIBLE

during the follow-up to manage soft or hard tissue complications; 6) insertion of additional implants during the follow-up; 7) removal of nonintegrated or fractured implants (number, size, location), and 8) removal of implants from functional loading. Prosthodontic records were reviewed, and the following was recorded: 1) prosthesis type and need to remake or modify the prosthesis; 2) mechanical complications, including fracture of abutment or prosthesis attachment screws, metal framework or teeth; 3) soft tissue complications requiring treatment (hyperplasia, fistula, or nonfixed moveable tissue causing symptoms; and 4) neurolgic symptoms associated with the mental or inferior alveolar nerve. Marginal implant bone loss was recorded when noted on the most recent panoramic radiograph. Results FIGURE 6. Stress fracture that occurred through a 7-mm endosseous implant site 3 weeks after placement. Occlusal radiograph at the time of fracture (A) and after fracture remineralization (B). Previous multiple bone grafting procedures (brittle, hypovascular, residual bone) combined with inadequate osseous width predisposed to the stress fracture. Active treatment was not required and a fixed prosthesis on the remaining four implants was placed 8 months after the original procedure.

taken at the most recent clinical examination. Median height values were calculated for each group (pretreatment and posttreatment). Surgical records were reviewed, and the following were recorded: 1) intraoperative surgical complications; 2) performance of simultaneous surgical procedures such as nerve repositioning, inferior border bone grafting through implant site; 3) use of onlay composite bone graft; 4) number, size, type and location of endosseous implants; 5) visits or procedures required

GENERAL INFORMATION

Sixty-one patients (52 nonbone graft and 9 bone graft) of approximately 400 patients who had undergone full arch mandibular endosseous implant reconstruction satisfied the criteria for inclusion in this study (Table 1). The median age of the study group was 57 years (range, 26 to 81). The median follow-up was 59.5 months (range, 8 to 118 month). The study extended over a 10-year period from March 1984, when the first patient was operated, until July 1994, when the last observation period was recorded. All study patients were female except one in the non-bone graft group. All patients were totally edentulous in the mandible except one patient in the bone graft group who had advanced segmental bone loss (posterior mandible) secondary to oncologic marginal resection. The opposing maxillary arch was totally edentulous in 56 pa-

FIGURE 7. Surgical technique for advanced mandibular resorption treated with a one-stage onlay composite bone graft (autogenous block iliac crest bone graft and threaded cylindrical endosseous implants). Drawing illustrating the basic surgical technique (A) and an intraoperative photograph (B) showing an onlay iliac block bone graft secured to the residual mandible with six endosseous implants.

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FIGURE 8. Mandibular endosseous implant reconstruction of advanced resorption in a 57year-old woman with a onestage full arch composite onlay bone graft. Five 15-ram length implants were used. Pretreatment lateral cephalometric (A) and panoramic radiograph (B) and 36 months postimplant loading panoramic radiograph (C) and clinical photograph (D) after placement of a continuous fixed prosthesis bridge. This patient has not experienced any complications or marginal bone loss. Bone height increased from 5 to 15 mm in the mental foramen area secondary to full arch onlay bone grafting and increased functional loading. (Prosthetic treatment provided by Dr Timothy A. Peterson, Minneapolis, MN.)

tients, partially edentulous in two, and full arch natural teeth were present in three patients. The maxillary arch prosthetic reconstruction included: 47 patients who wore a removable mucosal-supported full denture, two patients who wore a removable partial denture, and nine patients who wore an endosseous implant-supported prosthesis (one fixed, three fixed/removable with magnet retention, and five fixed/removable with continuous bar retention). Three patients required no maxillary prosthesis. [MPLANT DATA

Three hundred three endosseous implants were placed, 22 nonintegrated implants were removed, and three osseointegrated implants were indefinitely removed from function (Tables 2 and 3). Three implants

had been taken out of function for brief periods (3, 6, and 9 months, respectively). Five implants were initially placed in 53 of the 61 patients; of the eight remaining patients, one received seven implants, two received six, three received four, and two received two implants. Three implants were placed secondarily in two patients to replace nonintegrated implants. Both of these patients had continuous prosthesis function on the remaining implants during the second surgical and prosthetic treatment periods. Three additional implants placed in the bone grafted patient who initially experienced a fracture were not entered into the study; these implants were placed into a mandible greater than 10 mm in height. An additional implant recently placed into a bone grafted mandible (now 18 mm high) was not uncovered and was also not counted in the study data.

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RECONSTRUCTION OF THE SEVERELY ATROPHIC EDENTULOUS MANDIBLE

FIGURE 9. Pretreatment (A) and 25 months post treatment (B) lateral cephalogram after placement of mandibular (and maxillary) full arch composite onlay bone grafts in a 54-year-old woman. Note placement of the endosseous implants at a right angle to the occlusal plane. This patient had a mandibular bone height increase from 4 to 15 mm secondary to full arch onlay bone grafting and increased functional loading.

A majority of the implants in the n o n - b o n e graft group were 7 or 10 m m in length (246 of 260). The 14 13-mm implants in this group were placed in the three midline positions and generally extended through the inferior border 2 to 3 mm. A majority of implants in the bone graft group were 15 m m in length (30 of 43). A majority of the implants removed (17 of 22) were in the two most distal implant positions (11 on the right side and 6 on the left). Only 5 of 183 implants placed in the remaining five anterior positions were lost in both groups. The implant survival rate for the 10-year study period was 93.1% in the n o n - b o n e graft group and 91.7% in the bone graft group, for a combined survival rate of 92.7%. If the two distal implant sites are excluded, the survival rate was 97.2% (5 lost of 183 placed) and the implant survival rate for the fight and left distal implant sites was 85.9% (17 lost of 120 placed). Implant loss relative to implant size was evaluated and did not vary significantly (7-, 10-, 13- and 15-ram implants registered an 89.4%, 94.2%, 94.5% and 90% survival rate, respectively). Implant loss was not encountered with the 18- and 2 0 - m m implants, but the numbers placed (six and two, respectively) were small.

patients) was 6.5 nun (range, 4 to 10 mm), and in the bone graft group (nine patients) was 4.5 m m (range, 4 to 10 ram) (Table 1). The bone height in this region increased from a median of 6.5 m m to 8.5 m m (31% increase) in the n o n - b o n e graft group and from 4.5 m m to 15 m m (222% increase) in the bone graft group. PROSTHETIC TREATMENT Prosthetic reconstruction was accomplished with a continuous fixed bridge in 53 of the 61 patients. The

FRACTURES Bone fracture did not occur during the surgical procedure in any of the patients; however, two n o n - b o n e graft patients experienced bone stress fractures of the mandible in an implant site during the healing period (Fig 6), and one bone graft patient experienced a midline fracture in the implant site from facial trauma during the bone graft healing period (see Complication section). Implant fracture was not encountered during the study (Table 1). BONE HEIGHT MEASUREMENTS The median pretreatment mandible bone height (foramen position) in the n o n - b o n e grafted group (52

FIGURE 10. Marginal bone loss was noted in this 54-year-old woman at the time of implant exposure 6 months after composite bone graft onlay full arch reconstruction. After prosthetic loading the bone height has remained stable to date. Pretreatment (A) and 5 years post-prosthesis loading (B) panoramic radiographs. Initial bone resorption on the two distal implants was 2 mm and it has not increased in the 60 month postloading observation period. Bone height increase was from 4 to 12 mm in the body region. This patient has experienced four soft tissue events requiring treatment. Poor oral hygiene has complicated their management.

E.E. KELLER

Table 1.

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Demographic Data Bone Height (Foramen Area) No. of Implants Median Removed Initial Median Months No. of Median Patients Age (yr) Follow-up Implants (Nonfunctioning (mm) (Gender) (Range) (Range) Placed Implants) (Range)

Group Non-bone graft group

52 (51 F) ( 1M)

Bone graft group

9 (9F) (0M) 61 (60F)

Total

(IM)

Follow-up Median (ram) (Range)

Complications No. of Soft No. of Tissue Mechanical Events Events

%Height Increase

No. of Prosthetic Events

No. of No. of Marginal Jaw Bone Loss Fractures Implants

57 (26-81)

59.5 (8-118)

260

18 (2)

6.5 (4-10)

8.5 (5-15)

31

58

46

17

2

2

57 (54-79)

42 (8-72)

43

4 (1)

4.5 15 (4-10) (10-20)

222

7

7

1

1

10

65

53

18

3

12

57 (26-81)

303

22

5.5

(3)

(4-10)

This table provides general demographic data, bone height measurements before and after treatment, complication events, (soft tissue, mechanical, prosthetic, fracture, and number of implants exhibiting marginal bone loss). Bone height increase represents bone volume increase secondary to increased functional loading in the non-bone grafted patients, and the combined affect of bone grafting and increased functional loading in the bone graft group.

eight r e m a i n i n g patients r e c e i v e d a r e m o v a b l e overd e n t u r e - t y p e prosthesis; f o u r o f eight were totally i m plant-supported ( c o n t i n u o u s b a r with f i x e d / r e m o v a b l e prosthesis) a n d the r e m a i n i n g f o u r r e c e i v e d an overd e n t u r e partially s u p p o r t e d b y the e n d o s s e o u s i m p l a n t s ( o n e p a t i e n t i n i t i a l l y r e c e i v e d two i m p l a n t s a n d a n o v e r d e n t u r e , o n e patient r e c e i v e d an o v e r d e n t u r e p l a c e d o n the three o f five r e m a i n i n g i m p l a n t s , a n d two patients had to s w i t c h f r o m a fixed prosthesis to a r e m o v a b l e o v e r d e n t u r e prosthesis 8 a n d 12 m o n t h s , respectively, f o l l o w i n g prosthesis l o a d i n g b e c a u s e o f i m p l a n t loss). O n e o f the latter patients lost three o f

Table 2.

five o r i g i n a l l y p l a c e d i m p l a n t s a n d o n e p a t i e n t lost o n e of f o u r o r i g i n a l l y p l a c e d implants. O n e p a t i e n t w h o lost two o f five o r i g i n a l l y p l a c e d i m p l a n t s had a n e w prosthesis m a d e after two additional i m p l a n t s were inserted a n d e v e n t u a l l y u n c o v e r e d . A n a d d i t i o n a l p a t i e n t n o w f u n c t i o n s with a s h o r t e n e d o c c l u s a l table after l o s i n g two i m p l a n t s a n d h a v i n g the prosthesis cantilever s h o r t e n e d bilaterally. T h r e e patients r e q u i r e d r e m a k i n g o f the original fixed prosthesis; in two patients ( c h a n g e d to an overd e n t u r e ) this was related to i m p l a n t loss, as d e s c r i b e d in the p r e v i o u s paragraph, a n d in the third patient

Total Number and Size of Implants Implant Position* Right

Implant

Left

Group

Size

Mental Foramen

Canine

Lateral Incisor

Midline

Lateral Incisor

Canine

Mental Foramen

Total

Non-bone graft patients

7m 10 mm 13 mm Total

19 (5) 33 (6) 0 52 (11)

5 (0) 19 (0) 1 (0) 25 (0)

8 21 2 31

7 (0) 32 (1) 8 (0) 47 (1)

6 21 2 29

6 (0) 19 (0) 1 (0) 26 (0)

24 (3) 26 (2) 0 50 (5)

75 (8) 171 (10) 14 (0) 260 (18)

Bone graft patients

13 mm 15 mm

0 7 (0)

0 6 (1)

0 0

2 (l) 5 (0)

0 0

2 5 (1)

1 7 (1)

5 (1) 30 (3)

All patients

(0) (1) (0) (1)

(0) (0) (0) (0)

18 m m

1 (0)

1 (0)

1

1

1

1

0

20 mm Total

2 (0) 10 (0)

0 7 (1)

0 1 (0)

0 8 (l)

0 1 (0)

0 8 (1)

0 8 (1)

6 (0)

2 (0) 43 (4)

Total

62 (11)

32 (1)

32 (1)

55 (2)

30 (0)

34 (1)

58 (6)

303 (22)

This table illustrates the total number and size of implants placed and/or removed in the seven (interforaminal) implant sites. * The numbers in ( ) represent the implants removed. This table also separates those who did not receive onlay bone grafting (52 patients) from those who received onlay bone grafting (nine patients).

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Table 3.

Follow=Up Data

Follow-up Months Group

Implants

Non-bone graft group

No. placed (No. removed) No. loss (time frame) No. placed (No. removed) No. loss (time frame)

Bone graft group

Totals

Stage-lI Surgery

1-12

13-24

7

10 (0) 7

24 (3) 4

5 (1)

11 (0)

15 (1)

35 (3)

25-36

37-48

49-60

61-72

73-48

85-96

97-108

109-120

30 (4)

34 (2)

33 (3)

34 (2)

45 (1)

10 (0)

20 (0)

20 (3)

10 (0)

5 (0)

7 (0) 1

5 (3)

44 (2)

38 (3)

41 (2)

50 (4)

3 30 (4)

10 (0)

20 (0)

20 (3)

Total

%Survival

260 (18)

93.1

43 (4)

91.7

303 (22)

92.7

This table illustrates the number of implants placed and the number of implants removed of those placed during the various follow-uptimes. The implant loss relative to the time (months) following loading is also illustrated. Ten implants were lost before prosthetic loading and ten (of the remaining 11) were lost during the first 17 months after prosthesis loading. Implant survival percentage is also listed.

(changed to a fixed/removable type) it was required for cosmetic reasons (tooth positioning and lip support were compromised with the continuous fixed prosthesis in the eyes of the patient). Ten of 22 lost implants were removed at stage II surgery before prosthetic loading. Eleven of the remaining 12 lost implants were removed during the first 17 months of function (the twelfth implant was lost after 63 months of function); all 12 implants removed during function were in continuous fixed prosthetic appliance patients. Fourteen of 22 lost implants opposed a removable, full-maxillary, prosthesis. Of the remaining eight lost implants two opposed a natural dentition and six opposed an implant-supported prosthesis. Of the twelve implants lost after prosthetic loading, nine opposed a removable full maxillary denture, two opposed an implant-supported prosthesis, and one opposed natural teeth.

ture was not encountered. Nerve repositioning was common (all patients during the past 5 years) and consisted of detaching the incisive branch, which allowed posterior-buccal repositioning of the herniated inferior alveolar nerve. A majority of the patients were managed under general anesthesia, but this was a relative indication in most patients. Hospitalization was generally not required except for the medically compromised or elderly patient; however, early in our learning experience, patients were routinely hospitalized for 1 day. Soft or hard tissue infection was not recorded for any of the patients.

Implant Loss

Complications were reviewed in the following categories: 1) perioperative morbidity; 2) implant loss; 3) nonfunctioning implants; 4) mandibular fracture; 5) soft tissue; 6) mechanical; 7) prosthetic; 8) implant marginal bone loss; and 9) neurologic.

Eighteen implants were lost in 13 of 52 patients (Figs 2 and 3). Seven implants were removed at stageII surgery so they could be classified as surgical failures. An additional seven implants were lost during the first 12 months of function, and two implants each were lost at 15 and 17 months, respectively, following prosthetic loading. Implant loss did not occur after 17 months of prosthetic loading in any patient. Two patients lost one implant each before loading, which was associated with a mandibular stress fracture (see Fracture section).

Perioperative Morbidity

Nonfunctioning Implants

Because the intraoral incision was placed directly over the residual ridge and periosteal reflection was minimal, the postsurgical edema was minimal and need for pain medication was almost nonexistent. Intraoperative blood loss was minimal. Intraoperative complications were minor and consisted of difficulty in drilling or tapping dense bone. Intraoperative mandible frac-

Two patients each had one osseointegrated implant indefinitely removed from prosthetic function; one midline implant had an unfavorable angle and was not needed in the final biomechanical prosthesis design, and the other midline implant was associated with chronic pain of unknown etiology and was removed from function after 28 months. One patient had an

COMPLICATIONS (NON-BONE GRAFT PATIENTS)

E.E. KELLER

implant temporarily removed from function (9 months); the implant was associated with moveable peri-implant tissue and required a gingival graft. Mandibular Fracture This complication was encountered in two patientsg: the first patient (75-year-old woman) experienced a stress fracture through an implant site in the fight parasymphyseal region 4 weeks after stage I surgery. The implant was not integrated and was removed. This patient had previous multiple bone (onlay rib grafts) and soft tissue (skin graft vestibuloplasty) surgical procedures so the overlying soft tissue was dense and effectively stabilized the nondisplaced stress (hairline) fracture. Healing occurred uneventfully without surgical intervention and, at 7 months after implant insertion, a fixed prosthesis was placed on the remaining four integrated implants and has functioned without complication for 74 months (the lost implant has not been replaced) (Fig 6). The second patient (51-year-old woman) experienced a fight body stress fracture 1 month after stage II surgery (4 months after stage-I surgery) through a nonintegrated implant. This patient previously had an infected, full-arch, mandibular subperiosteal implant that was removed at the same time the seven endosseous implants were placed. A temporary overdenture prosthesis supported by the remaining six implants was kept in position and provided adequate fracture stabilization; uneventful fracture healing occurred without surgical intervention. A permanent fixed/removable overdenture was constructed on the remaining six implants and has been functioning without complication for 57 months (the implant lost in the fracture site was eventually replaced and has functioned for 35 months). Both of these patients experienced a delay (3 and 4 months, respectively) in their final prosthesis insertion. Soft Tissue This type of complication was relatively common (58 episodes in 21 of 52 patients). The most common soft tissue problem was peri-implant tissue hyperplasia secondary to loose or fractured abutment screws, moveable peri-implant soft tissue, inadequate hygiene, leaky abutments secondary to inadequate seating or defective O-ring seal, or metal stress secondary to distorted prosthesis castings. Six of 21 patients experienced four or more episodes and four of these six had a skin graft surrounding the abutments (keratinized skin has an exaggerated response to the etiologic factors listed). One midline implant eventually required a gingival graft to provide nonmoveable peri-implant tissue. This implant was temporarily removed from function during the grafting treatment. Once the correct cause of the tissue hyperplasia was

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identified and properly treated, the clinical problem promptly resolved. The learning curve for identifying the correct etiology was lengthy (first 3 years) and, once proper treatment was realized, the incidence of soft tissue problems reduced dramatically (problems in 12 of 21 patients occurred during the first 3 years of experience). Initially, ineffective soft tissue surgical procedures were frequently performed on these patients. At this time the need for soft tissue surgical procedures for peri-implant tissue hyperplasia is relatively rare. Mechanical Fracture or loosening of abutment or prosthesis attachment screws was relatively common, affecting 25 of 52 patients (46 events were recorded). Six patients had multiple episodes (three or more) requiring treatment. Five of these patients were treated during our first 2 years of experience. Twenty-two of 25 patients experiencing fractured components had a continuous fixed prosthesis and the other three had a removable overdenture; however, the three patients with overdentures had only one event each. Eight of 25 patients with mechanical problems had an opposing maxillary arch with natural teeth (two) or a fixed or fixed/removable implant prosthesis (six). The learning curve for identifying etiologic factors for fractured abutment or prosthetic screws was quite lengthy. Once the causes of prosthesis overloading were identified and correct treatment instituted, the incidence of multiple mechanical complication events in the same patient was reduced dramatically. Prosthetic The prosthetic complications were relatively minor in nature and generally did not bring the patient to the restorative dentist for extra visits. Fractured or dislodged prosthesis teeth represented a majority of the problems (17 events in 10 patients). Two patients required distal extension reduction of the prosthesis because of bone height increase in the mandibular body region secondary to increased functional loading. This was not really a complication, but a positive effect of successful treatment and increased masticatory function. Marginal Bone Loss This was recorded in only one patient (two implants). This patient was a chronic clencher and bruxer who was diagnosed and treated (botulism toxoid injections) for oromandibular dystonia. This patient also exhibited a 100% increase in mandibular body height (5 to 10 mm) during prosthesis function (1989-1994).

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Neurologic Temporary paresthesia of the mental nerve was commonly encountered in this group of patients, but was absent in all patients by stage II surgery (3 months after implant placement). A number of patients had paresthesia before implant surgery from other preprosthetic procedures, so changes were difficult to quantitate. One patient developed a chronic, atypical pain syndrome during the follow-up period that resulted in placing one midline symptomatic implant out of function. This implant, in retrospect, was an unlikely cause of the facial pain. COMPLICATIONS (BONE GRAFTED PATIENTS)

plant removal in this patient occurred 63 months after prosthesis loading (three additional implants have been placed in this patient). Despite these problems, this patient was never without the use of her continuous fixed prosthesis (72 months function). The fourth implant loss occurred in a recent reconstruction where it was necessary to perform a twostage, full-arch, onlay bone graft procedure; a thin .5-mm wide rim of bone extending 10 mm above a 5mm height basal bone was encountered and a twopiece graft that was in the midline was placed as a lingual onlay graft (two stage). The lost implant occurred in the midline where the two blocks of bone interfaced and significant bone resorption had occurred during the bone graft healing period.

Perioperative Morbidity

Nonfunctioning Implants

Hospitalization averaged 5 days (range, 4 to 7 days). The surgical edema and donor and recipient site pain were significantly greater in this group in comparison to the nongrafted group. All patients tolerated and accepted the surgical procedure well. A nondisplaced hairline fracture of the anterior-superior iliac spine occurred in one patient, which required no treatment other than walking cane support for 6 weeks. All patients denied long-term donor or recipient site complications. Intraoperative jaw fracture was not encountered in this group (average bone height of 4.5 mm); however, one midline fracture occurred from trauma 3 weeks after composite bone grafting (described in Fracture Section). An implant cover screw became unscrewed 4 months after composite bone grafting in one patient necessitating a minor surgical procedure for its replacement. This same patient had a small sequestrum removed between the two right distal implants 4 months after stage-II surgery. Marginal bone loss was eventually identified in this site (two implants). The onlay bone was exposed in the midline of one patient 3 weeks after composite bone grafting. This area was kept clean and a small sequestrum was eventually removed at 3 months after stage-! and 1 year after stage-II surgery, respectively (marginal bone loss around the two implants was eventually recorded in this area).

One implant was not used in the initial prosthesis biomechanical design because of unfavorable labial inclination. An angled abutment could have been used, but the remaining four implant positions gave adequate prosthesis support. A second implant was taken out of function for 3 months as tortional force caused discomfort during prosthetic treatment; at secondary uncovering and reattachment, the discomfort was absent. A second implant was taken out of function for 16 months because of mild, generalized discomfort to loading; at reattachment the implant was comfortable and stable.

Mandibular Fracture Mandibular fracture was encountered in one patient who fell 3 weeks after bone grafting. The displaced midline fracture required a closed reduction with splints and 3 months of maxillomandibular fixation (see previous implant loss section).

Soft Tissue Seven soft tissue events requiring treatment were recorded for three patients. Four events occurred in one patient necessitating two operative procedures (excision of hyperplastic tissue). Two events occurred in a second patient in whom hyperplastic tissue associated with marginal bone loss was encountered 3 months after stage-II surgery. The marginal bone loss represented partial bone graft resorption during healing.

Implant Loss Forty-three implants were placed in nine patients and four (in two patients) were removed after loss of osseointegration. Three of four lost implants were in the same patient, who fell 3 weeks after full arch onlay grafting and fractured the mandible and bone graft in the midline. This required 3 months of maxillomandibular fixation and two nonintegrated implants were removed just before prosthetic loading. The third ira-

Mechanical Seven mechanical events occurred in two patients. One patient experienced six episodes of fractured hardware, including a fractured mandibular framework. This patient had documented nocturnal clenching and brnxism, and was the one who experienced a mandibular fracture 3 weeks after bone grafting and eventually lost three of five implants initially placed.

E.E. KELLER

31 7

Prosthetic One prosthetic event was recorded and consisted of the need to replace one prosthesis tooth.

Marginal Bone Loss This occurred on 10 implants in four patients. The marginal bone loss was present at stage-II surgery in all of these patients and represented bone height reduction during bone graft revascularization and remineralization. After prosthetic loading, additional bone loss was not observed. The events associated with marginal bone loss involved early functional loading in one patient (three implants) during fracture treatment, which most likely caused the bone and implant loss. Early bone graft exposure secondary to soft tissue dehiscence occurred in two patients that predisposed the implants in this area (5) to marginal bone loss. The remaining three implants (one patient) exhibited marginal bone loss of unknown etiology at stage-I1 surgery; additional bone loss after functional loading did not occur during 70 months of function.

Neurologic The same extent of temporary mental nerve paresthesia was noted in this group as in the nonbone graft group, and was secondary to handling of the herniated nerve required in patients with advanced bone loss. Normal sensation returned by stage-II surgery in all patients unless it was present pre-implant surgery from other causes. One patient experienced a "tender" implant with loading (left distal), and this implant was kept nonfunctional for 16 months and has been asymptomatic since reattachment to the prosthesis. Discussion In 19866 the Mayo Medical Center implant team reported their initial experience with endosseous implant reconstruction of the totally edentulous mandible in three patients who had experienced advanced bone resorption. It was stated at that time that "7.0 mm of bone height is a benchmark minimum to consider for use of Branemark fixtures"; however, the three patients presented in this initial report all had bone height in the mental foramen area in the 4- to 5-mm range! After eight additional years of clinical experience and follow-up of 61 patients, it can now be stated that the benchmark minimum height is much less (4 mm) provided there is adequate width (as viewed on intraoral occlusal radiographs) at the implant sites and the bone is not exceedingly dense and brittle from previous multiple surgical procedures and/or sepsis, particularly from previous failed implants. When the previously mentioned risk factors are present, there is

an increased chance of stress fracture or fracture from trauma during the implant healing period (this and a previous report 9 describes three such patients). Mandibular fracture was not encountered intraoperatively from the forces of drilling, tapping, and implant insertion, as one might expect, but occurred from functional overload (stress fracture) during the healing period. The latter complication can be avoided by correct patient selection (minimum bone height of 4 to 5 mm and bone width of 6 to 7 mm) and avoidance of the high-risk patient with sepsis or previous multiple surgical procedures, and by using the one-stage onlay composite bone graft procedure described in this and a previous report. 8 A two-stage onlay composite bone graft procedure may be indicated in selected patients (one of our nine patients) if the residual mandible will not allow simultaneous implant insertion (extremely narrow width or height). Another factor we were not aware of at the time of our initial report 6 was the potential for increased bone volume (appositional bone on anterior inferior border and/or increased height in the body region) after placement of anterior interforaminal implants and subsequent increased masticatory functional loading over time. (Figs 1, 3, 4 and 5) This report documents a median 31% increased height (6.5 to 8.5 ram) in the mental foramen region in 52 nonbone graft patients with a median follow-up after prosthesis loading time of 59.5 months. This physiologic response of bone to increased masticatory functional loading (Wolff's law of bone physiology) occurred in all age groups (median age of 57 years with a range of 26 to 81 years) and is theoretically proportional to the increase in occlusal force generated (previous indirect bone loading via mucoperiosteum versus direct endosseous bone loading). This increased occlusal force generation in edentulous patients after endosseous implant reconstruction was previously documented by Carr. ~° All but one of our patients were women and the majority (48 of 61) were more than 50 years of age at the time of reconstruction and had previously experienced advanced mandibular bone resorption from mucoperiosteal forces generated by removable dentures. Definite risk factors (gender, age, previous severe bone loss) were present for continued bone loss 11 in our patient groups, which makes the increased bone height documented in this patient population more significant (however, osteoporosis was not diagnosed in the medical records of any of our patients). Marginal bond loss around the implant after prosthetic loading was found in only 2 of 260 implants placed in the nonbone graft patients and was not observed after functional loading in any of the onlay bone graft patients. This is in marked contrast to previous experience with onlay bone grafting of all types where bone resorption continues indefinitely (at varying rates depending on the

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type or combination of grafting materials) after placement of removable mucoperiosteal-supported dentures. It appears that the type of prosthesis loading (direct endosseous rather than indirect bone via the mucoperiosteum) rather than the type of bone grafting is the limiting factor in long-term bone graft maintenance. Because residual bone volume is directly related to physiologic functional demands, the need for bone grafting is reduced in this group of patients and bone formation will occur naturally over time after a successful physiologic (endosseous loading) reconstruction. It follows that placement of inferior border block bone grafts should be abandoned because they are out of the myofunctional envelope and will gradually resorb, particularly if endosseous implants are not eventually placed in the grafted bone. In addition, abnormal interarch space is not reduced with inferior border block bone grafting and a prosthetic-cosmetic problem is created along with unnecessary surgical morbidity (extraoral exposure from angle to angle). When intraoral onlay bone grafting is performed as described in this report (Fig 7), the block grafts are only placed where endosseous implants are positioned and bone graft volume is sized eventually to supply enough bone mass to accommodate five endosseous implants of 10to 18-mm length. Onlay bone grafting is not needed posterior to the most distal implant because this region will eventually reach the volume needed to support the prosthetic loading forces. Stress fracture due to normal loading after composite onlay bone grafting has not been observed during the healing period or during the gradual increased function that occurs in this group of patients; however, the risk of fracture from trauma is present during this time and was experienced by one of our patients 3 weeks after composite onlay bone grafting. 9 Placement of additional bone grafts (posteriorly) would not reduce this risk until some months later when there was complete bone graft incorporation and the functional response of bone to loading had occurred. Finally, the best treatment for fully edentulous patients is prevention of bone loss by placement of endosseous implants earlier in life, before pathologic bone resorption from removable mucoperiosteal-supported dentures is allowed to advance needlessly. A number of interesting observations can be made by analyzing the type and location of implants that failed to osseointegrate or were lost after functional loading. Ten of 22 lost implants were removed at the time of stage-II surgery and seven during the first year of function; these implant failures were due to failure to achieve initial osseointegration which, in turn, was due to flawed surgical technique. In this regard, the dense, hypovascular bone present in all patients in this study is highly susceptible to heat generation during

bone drilling and tapping procedures. Traumatic bone drilling may be related to dull instrumentation, excessive drilling speed, inappropriately long bone drilling time, inadequate instrument cooling, and inexperienced operator, or difficult surgical access. In regard to the last, 50% (11 of 22) of the nonintegrated implants occurred in the right distal implant site, which represents the most difficult access region for a fight-handed operator. When one considers implant loss in both distal implant site implants, 77% (17 of 22) of the nonintegrated implants occurred in these regions. Such distal dental implant sites involve additional surgical difficulty because of the need to avoid or reposition the herniated inferior alveolar nerve. The reason implant loss during the first 12 months of loading is considered to be secondary to a failure to achieve osseointegration (surgical failure) is that the dense, hypovascular bone takes more than 12 months to be completely replaced and implant mobility (loss of osseointegration) may not be evident until after this period. The opposite is noted in bone of low density (posterior maxilla and mandible), for which failure to achieve osseointegration is invariably evident to the surgeon at stage-II surgery. Failure to achieve implant osseointegration in this patient group was not related to implant length (as is the case in the maxilla) because initial implant stability was always present irrespective of implant length (7, 10 or 13 mm in the non-bone graft patients). In the bone graft patients, the number of implants lost of each length was too small to draw conclusions. The small number of implants lost after 12 months of functional loading (five implants) indicated that an osseointegrated implant in dense mandibular bone is highly resistant to overload and/or implant fracture; however, the incidence of fractured abutment and prosthesis attachment screws (53 events in 25 of 52 patients) indicated there was a relatively high incidence of overload of the overall implant system; fortunately, the weak link was not the bone implant interface (osseointegration) or the endosseous titanium implant, but the abutment or prosthesis attachment screws. The incidence of these mechanical problems reduced dramatically once the implant team realized and treated the etiologic factors, which related to prosthesis frame misfit that introduced chronic fatigue screw fractures or to inappropriate implant loading from excessive cantilevering torque from unfavorable implant position or angulation. Fortunately, these etiologic factors can be eliminated or effectively treated as experience is gained in this very technique-sensitive surgical and prosthetic treatment protocol. Soft tissue complications were also initially quite frequent (65 events in 24 of 52 patients). Peri-implant tissue hyperplasia represented a majority of the soft tissue problems and was frequently related to loose

E.E. KELLER implant components (fractured or loosened abutment or prosthesis screws) and, on rare occasion (one patient), where there was mobile soft tissue required keratinized mucosal grafting. Marginal or inadequate oral hygiene was frequently noted, but was not related to implant loss or marginal bone loss (only 2 of 260 implants in the non-bone graft group experienced marginal bone loss) and can be classified as a nuisance factor to the implant team. The marginal bone loss (two implants) that occurred in one non-bone graft patient was associated with long-standing bruxism (this patient surprisingly exhibited only one mechanical and one soft tissue complication event requiring treatment). Marginal bone loss was relatively common in the bone graft patients (10 of 43 implants) and represented bone reduction during the nonloading (composite bone graft) healing period, and was related to bone graft exposure in two patients (five implants) during the same time frame, or overload in the one fracture patient (three implants). Once prosthesis loading occurred, marginal bone loss was not recorded; overall, the mental foramen height increased 222% in the bone grafted patients during the study period and was related to onlay bone grafting and, theoretically (based on our observations on non-bone graft patients), was due to increased functional loading. A majority of the patients were restored with continuous fixed mandibular prostheses (58 of 62). All prostheses were totally implant-supported except in four patients who had partially soft tissue (retromolar pad)supported prostheses. The relation between the type of mandibular prosthesis and soft tissue or mechanical complications was difficult to access because of the small number (four) of patients with soft tissue-supported overdentures. However, the nature of the opposing maxillary dentition was a risk factor; a high incidence of mechanical problems were encountered in patients with maxillary natural teeth or maxillary implant-supported prostheses. This was expected, and has been reported previously. 3 Continuous function, except for short interruptions to remake, modify, or repair the prosthesis, was maintained in all patients following initial placement of the prosthesis. Three patients required remaking of the prosthesis because of implant loss (two required switching from a continuous fixed bridge to a removable implant-supported overdenture and one required remaking of a continuous fixed bridge following placement of two additional implants). One patient required a prosthesis remake from a continuous fixed bridge to a fixed/removable prosthesis for cosmetic reasons. These four patients did not experience loss of prosthesis function during the additional treatment and all have subsequently functioned successfully (33, 41, 57, and 109 months, respectively). One patient required placement of a removable overdenture rather than the

319 originally planned continuous fixed bridge because of implant loss at stage-II surgery (this patient has elected to maintain the removable prosthesis during the 57 months follow-up). The remaining lost implants (14) in this study have not affected continuous prosthesis service or required prosthesis remaking. This emphasizes one clear advantage of the multiple, individual, intraorally inserted, endosseous implants; additional implants are generally not required (only three in the 52 non-bone graft patients) to support the prosthesis provided only one implants is lost (or rarely two). Another important advantage of this system is the relative lack of morbidity with implant loss (the patient should not be worse off after implant loss than before implant placement), and the relative ease of placing additional implants, if required, to support the type of prosthesis desired by the patient (three of the patients elected to function with a removable prosthesis rather than a fixed prosthesis when implant loss occurred). In the patient's eyes, continuous, comfortable, uninterrupted prosthesis service is the major criterion for success and, in this study group, this would suggest 100% success. For patients with advanced bone loss, this continuous prosthesis function is highly significant because they have all experienced multiple prosthesis remakes or adjustments with the previously worn removable mucoperiosteally supported dentures. More importantly, this group of patients have all previously experienced marginal to poor masticatory function and pathologic bone loss (they had to have less than 1 cm of bone at the implant site to be included in this study). Conclusions

Patients with advanced mandibular bone resorption secondary to pathologic indirect bone loading from a mucoperiosteally supported removable denture can be safely and predictably reconstructed to satisfactory masticatory function with transorally placed endosseous osseointegrated titanium implants supporting a dental prosthesis. The risk/benefit ratio at this point is favorable, because the surgical complications are infrequent and perioperative morbidity is low, the individual implant loss is acceptable and does not significantly alter continuous prosthesis function, the prosthetic complications are relatively minor and effectively managed without significant interruption in continuous prosthesis function, and finally, the longevity, predictability, and function over time is much improved over the alternative treatment with a removable mucoperiosteally supported prosthesis. This 10-year study documents a positive physiologic response of bone to transoral placement of mandibular interforaminal endosseous implants and eventual loading with a dental prosthesis. The bone maintenance and/or bone volume increase documented in this study

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indicates this treatment approach for advanced bone resorption states in this patent group is a preventive or functionally effective medical treatment for the masticatory disability that is invariably present. It follows that health insurance providers should provide benefits for such treatment in these patients.

Acknowledgment We acknowledge with grateful appreciation our Mayo Clinic colleagues who contributed substantially to the data reported herein: Drs Dan Tolman, William R. Laney, Ronald P. Desjardins, Ned B. Van Roekel, Steven E. Eckert, Eastwood G. Turlington, and Bruce A. Lund.

References

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1. Adell R, Lekholm U, Rockler B, et al: A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int J Oral Surg 10:387, 1981 2. Adell R, Eriksson B, Lekholm U, et al: A long term followup

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study of osseointegrated implants in the treatment of totally edentulous jaws. Int J Oral Maxillofac Impl 5:347, 1990 Tolman DE, Laney WR: Tissue-integrated dental prostheses. The first 78 months of experience at the Mayo Clinic. Mayo Clin Proc 68:323, 1993 Albrektsson T, Zarb G, Worthington P, et al: The long term efficacy of currently used dental implants. A review and proposed criteria for success. Int J Oral Maxillofac Impl 1:11, 1986 Branemark PI: Personal communication, September 1984 Keller EE, Desjardins RP, Tolman DE, et al: Reconstruction of the severely resorbed mandibular ridge using the tissueintegrated prostheses. Int J Oral Maxillofac Impl 1:101, 1986 Fugler RC, Mason M, Triplett RG: Endosseous cylinder implant placement in severely atrophic mandibles. J Dent Res 69:233, 1990 [abstr #998] Keller EE, Tolman DE: Mandibular ridge augmentation with simultaneous onlay iliac bone graft and endosseous implants: A preliminary report. Int J Oral Maxillofac Impl 7:176, 1992 Tolman DE, Keller EE: Management of mandibular fractures in patients with endosseous implants. Int J Oral Maxillofac Impl 6:427, 1991 Can" A, Laney WR: Maximum occlusal force levels in patients with osseointegrated oral implant prosthesis and patients with complete dentures. Int J Oral Maxillofac Impl 2:101, 1987 Jeffroat JK: Bone loss in the oral cavity. J Bone Mineral Res 8:5467, 1993 (suppl 2)