For more than three decades, the external hexagon has been the most commonly used implant-abutment connection in the implant industry. The 0.7 mm external hexagon on root-formed dental implants was derived from earlier non-root–formed implant designs that utilized a square or hexagonal transmucosal post to ratchet the implant in place to support a cemented prosthesis. 1–2 This external hexagon feature was not intended by the Brånemark 3 research team to be a prosthesis engaging feature. In the initial design, the prosthetic cylinders bypassed the external hexagons and were used to restore the implants with splinted, screw-retained dentures. 3 Splinting helped to maintain abutment stability and provided additional support for the implants. The non-abutment–engaging external hexagon facilitated removal of the restoration for annual monitoring, 3 but the abutment and prosthesis screws were frequently subjected to loosening under occlusal loading.
Abutment screw loosening can compromise the integrity of the implant components and disturb the health of the adjacent soft tissue. Inflammation, hyperplasia, and fistula formation have been associated with loose abutment screws. 4 Although loose screw joints are clearly involved in these complications, there is also concern that micromotion at apparently stable screw joints may contribute to an increased incidence of soft tissue problems. 5 In addition, a reliable implant-abutment connection is an economic concern. If the abutment loosens under a cemented single crown, the prosthesis must be removed to tighten the abutment screw. This usually necessitates the fabrication of a new crown.
Because the primary objective of the Brånemark 3 research was the achievement and maintenance of osseointegration, the early implant literature did not fully quantify the magnitude of the screw-loosening problem. 6 The external hexagon connection was ultimately replicated for commercial purposes by many different implant manufacturers, and associated reports of problematic screw loosening became well documented. 4,7–20 The incidence of screw loosening ranged from 2% reported in some studies to more than 45% of the abutments placed into function on implants with external hexagon connections. 4,17–20 Over the years, implant manufacturers have improved the fit between matched implant components, eg, the implant external hexagon and its respective prosthetic abutment. Their recommendations for minimum torque levels in screw tightening 21–23 have also helped to diminish, but not eliminate, the problem of screw loosening.
Given its inaugural entrance and subsequent large volume in the dental market, the original 0.7-mm–high straight external hexagon design is considered the industry’s standard implant-abutment connection. 24 Newly developed implant-abutment connections that differ from the standard external hexagon design have been developed in response to reported loosening and fracturing of implant components. 25,26 Conceptually, the newer implant-abutment connections can be categorized as 1 internal hexagon, 2 Spline, 3 Morse taper, and nonhexagon internal connection designs. 4 Internal hexagon designs are manufactured with 24,25 (Sulzer Dental Inc., Carlsbad, CA) and without 27 (Dentsply Friadent North America Division, Irvine, CA) a bevel on the coronal aspect of the implant that leads into the hexagonal recess above an internally threaded shaft. A variation of this design is the internal octagon, located between an 8-degree lead-in taper and a threaded shaft 28–30 (Sulzer Dental Inc.; Institut Straumann AG, Waldenberg, Switzerland). The Spline interface (Sulzer Dental Inc.) features six projections, or tines, that extend 1 mm coronal from the implant body, surrounding an internally threaded shaft 26 (Fig. 1). Morse taper designs have a conical opening with either an 8-degree 31,32 (Institut Straumann AG) or 11-degree 33 (Astra Tech AB, Mölndal, Sweden) taper leading to an internally threaded shaft. Nonhexagon internal connections, 34 such as the Camlog connection, have also been recently developed (Altatec Medizintechnische Elemente GmbH & Co. KG, Wurmberg, Germany).
Two studies comparing the external hexagon, internal octagon, and internal hexagon connections reported that anti-rotational stability 35 and the amount of force required to release the abutments from the implants 36 differ significantly among the three designs. Both studies documented that the best joint stability was exhibited by the internal hexagon, whereas the worst results were achieved by the internal octagon. Although the former has since been redesigned to provide frictional abutment engagement 24,25 and the latter has incorporated a new lead-in taper with an expanded abutment interface, the earlier studies demonstrated that the implant-abutment connection design played an important role in joint stability.
While use of the internal hexagon, 24,25,36–41 Spline, 26,42–44 and Morse taper 32,45,46 designs have been documented through in vitro comparisons and in vivo clinical studies, many of the other newer implant-abutment connections like the Camlog (Altatec Medizintechnische Elemente GmbH & Co. KG) and Replace Select (Steri-Oss, Nobel Biocare USE, Inc.) designs have little or no documentation to support their use. In a multicenter, retrospective chart review of 76 crowns cemented over ITI conical abutments, Levine et al 32 cited loosening of 4 abutments. The overall incidence of screw loosening was 5.3% over the 2-year period. In a 5-year prospective study of Astra implants, Palmer et al 46 reported no cases of abutment screw loosening.
In 1992, the Spline connection was developed as an alternative to the traditional external hexagon implant-abutment connection. The interface of six interdigitating projections and slots (Fig. 1) was designed to improve the implant-abutment rotational micromovement and abutment tilting, two features that are the leading causes of screw loosening. 24,25,47–49 Rotational micromovement between the Spline implant and abutment has been documented to be 0.12 degrees. 26 In comparison, conventional external hexagon connections have been found to produce as much as 6.7 degrees of rotational micromovement. 35 An in vitro study of the Spline connection documented its ability to withstand three- to five-times the compression impact loading of the external hexagon connection. 50 The integrity of the Spline interface has also been reported to reduce the implant’s reliance on the abutment screw to maintain joint stability. This improves resistance to screw loosening in comparison to the external hexagon connection. 26
To date, however, limited published clinical data exists on the use of the Spline connection in dental implant restorations. One article expounded on the rationale for the Spline design and presented case reports but no clinical data. 51 The only published clinical data was collected by evaluating the failure of retaining screws of Spline implants with the external hexagonal implants as a control. 52 A total of 96 implants were evaluated for a period of 36 months starting at prosthesis delivery. The Spline implants did not present any screw loosening, whereas the external hexagonal implants presented problems in 20% of single-implant crowns and 5.7% of implant-retained fixed partial dentures.
This study calculated the rate of implant prosthetic connection complications for implant restorations restored on Spline dental implants by means of a retrospective cohort study. A retrospective cohort study is a follow-up study using secondary data (ie, patients’ treatment charts) in which the outcomes occurred in the past.
Materials and Methods
This study was conducted at The Ohio State University College of Dentistry with approval from the University’s Biomedical Sciences Institutional Review Board. The target population was patients who had implants placed at the University and restored by residents or dental students in the clinical research facility. Treatment with Spline dental implants between June 1996 and December 1998 was the sole inclusion criteria for selecting study patients from this group. A list of general implant patients treated within the specified date range was generated from the University’s computer records. From this group, postentry chart reviews were conducted to identify all patients who were treated with Spline dental implants. Indicators in the dental charts that were used to identify the Spline patients included 1 radiographs showing screw-type or cylinder dental implants in the jaw and 2 labels identifying the dimensions and serial numbers of the Spline implants. No Spline implant patients were excluded from the study, and no other inclusion/exclusion criteria were used.
A total of 68 patients (31 men, 37 women) ranging in age from 15 to 88 years (mean = 47.5 years) were identified through the postentry chart reviews. Within this group, 188 Spline implants were placed in maxillary and mandibular jaw locations and restored with 94 implant-supported prostheses. The implant-supported prostheses were divided into four groups: screw-retained dentures (N = 13), fixed partial dentures (N = 30), screw-retained crowns (N = 42) (Fig. 2), and cement-retained crowns (N = 9).
A second, more detailed postentry chart review was conducted between February and May of 2002 to gather data on all complications recorded for the Spline implant-supported prostheses (Table 1). Data collection forms were completed for each chart, then the information was entered into a spreadsheet program (Microsoft Excel 97; Microsoft Corp., Redmond, WA) on a personal computer. The entered data were validated for accuracy by one of the investigators of the study.
Statistical analysis software (SAS Software; SAS Institute Inc., Cary, NC) was used to analyze the data and complete calculations. In this study, incidence density was used as the measure of complication rate for each of the prosthesis groups, as well as for all the prostheses. The incidence density is the instantaneous potential for change in complication status per unit of time at time “t,” relative to the size of the complication-free population at time “t.”53,54 Incidence density for this study is the incidence of complications related to implant-abutment joint connection per 1,000 person-years (without conferring of immunity, ie, taking into account the potential for complications to reoccur). Incidence density is calculated by dividing the complication incidences by person-years. Person-years observation was used in this study because the observation period for each subject varied. The term “person-years” is used in this study to be congruent with epidemiological terminology, although “prosthesis-years” would be a more accurate description, given that the rate of complications were calculated per prosthesis. The complication rate was calculated using the period of time “t” ending with the last clinical recall of each prosthesis. A second complication rate was calculated using the period of time “t” ending with the last chart review for this study.
Incidence density was calculated for each of the four prosthesis groups: screw-retained dentures, fixed partial dentures, screw-retained crowns, and cement-retained crowns. The “complication free” period of time for each prosthesis was determined by the date of the last clinical recall and the last chart review. The sum of all the “complication free” periods was then calculated for each of the prosthesis categories, representing the person-years for that category. The incidence of implant prosthetic complications was then calculated and divided by the person-years for that category. The person-years and incidence density were also calculated for all the prostheses as a cumulative group.
As shown in Tables 2 and 3, the number of complications in this study was extremely low, with 95.25% of the prostheses free of any complications. Of the 5 complications reported for the 94 prostheses, 1 abutment screw loosened in an edentulous patient restored with a screw-retained denture, and 4 abutment screws loosened in 3 patients with screw-retained (N = 2) and fixed partial (N = 1) dentures. In the edentulous patient, a terminal abutment screw loosened. After adjustments, no recurrence of the problems occurred. Among the partially edentulous patients with single crown restorations, one restoration was found to be in hyperocclusion, whereas the other restoration had an open interproximal mesial contact, both of which resulted in abutment screw loosening. Three of the four prostheses that had complications occurred within a few months of delivery. Only the fixed partial denture’s complication occurred after 26 months of delivery. In the patient with a fixed partial denture restoration, one abutment screw loosened twice. This prosthesis was found to be in hyperocclusion. No further abutment screw loosening occurred after appropriate occlusal adjustments.
In regards to other complications besides screw loosening, 3 out of 13 in the screw-retained denture category had an incidence of acrylic material fracturing. One prosthesis had the acrylic fracture once, another had it occur twice, and the third had acrylic fracture three times. Two subjects with three unit fixed partial dentures had porcelain fractures that occurred one time. There were no material fractures for the screw-retained and cement-retained crown categories. In fact, there were no complications at all for the cement-retained crown category. No fractures of implants, abutments, or retention screws were reported for any of the categories.
As described earlier, two sets of incidence density calculations of prosthetic complications related to implant-abutment joint connection by prosthetic type were carried out. These data are presented in Table 4. The rate of complications for screw-retained dentures was 58 incidences per 1000 person-years when calculated with the period of time ending with the last clinical recall. That rate was reduced to 21 incidences per 1000 person-years when calculated with the period of time ending with last chart review. This reduction in complication rate was similar for the fixed partial denture and screw-retained crown categories. When the cumulative incidence density was calculated with the period of time ending with the last clinical recall, the complication rate was 40 incidences per 1000 person-years. Again this rate reduced to 12 incidences per 1000 person-years when calculated with the other period of time.
The design, precision, and strength of the implant-abutment interface are among the primary determinates of component joint integrity and anti-rotational stability in implant prosthodontics, 55 thereby affecting the long-term functioning of the restoration. Although some researchers have suggested that non-axial loading may not be detrimental to long-term osseointegration, 55–58 such loading has been found to destabilize the implant-abutment connection. 49,55 In general, partially edentulous restorations, especially single-tooth replacements, are subjected to greater lateral bending loads, tipping, and elongation forces at the implant-abutment interface and abutment screw than bilaterally splinted implants in edentulous cases. 55,59–60 These forces can result in a greater tendency for component joint opening and screw loosening. 24,25,55,61,62 When subjected to non-axial forces, the narrow prosthetic platform and limited interfacial contacts between the mating hexagons 59,63 of the conventional external hexagon connection make it particularly vulnerable to screw loosening.
In this study, nearly 96% of the restorations were free of complications. Of the complications reported, all were attributable to iatrogenic factors (ie, hyperocclusion and open interproximal contacts) that represent situations in the real clinical world. The fact that the implant procedures were performed by dental students and residents does improve the external validity of this study to the actual private practice scenario, where new practitioners to implant dentistry use Spline implant systems. The lack of any complications for the cement-retained crown category may be due to two possible contributing factors. Only 9 cement-retained crowns were delivered compared with 42 screw-retained crowns. It was also possible that the clinicians took extra caution in terms of patient selection and meticulous prosthodontic procedures, knowing the consequences of a loosened screw for cement-retained crowns.
Because the cases in this study lacked a uniform restorative protocol, variations in treatments and techniques may have likely influenced the incidence of reported complications. For example, screw loosening has been attributed to such causes as inadequate screw tightening, inadequate prosthesis fit, poorly machined components, excessive loading, inadequate screw design, and the elasticity of bone. 6,24,64 Inadequate machining may be due to a lack of machining accuracy and/or variations in machining tolerances. 24 Screw loosening can also be attributed to a settling of the components in some form of plastic flow or permanent surface deformation of the mating surfaces, which results in a decrease of torque after settling. 24 Simple failure to use an appropriate torque wrench could result in abutment screw loosening. Attempts to prevent screw loosening have included the establishment of torque limits for tightening prosthesis/abutment retention screws. 22,65–67 The use of the implant systems’ prosthetic torque wrench is recommended to provide a consistent preload to the prosthesis/abutment screw. Without a universal protocol in this particular study, however, the etiology of screw loosening in some cases can only be surmised.
The subjects treated at this clinical research facility were usually referred by their dentist or were self-referred, specifically for implant-retained restorations. Once implant treatment is complete, it is not uncommon for the subjects to return to their primary dental provider (who may not practice at the College of Dentistry). For this study, it was presumed that patients’ complications with their implant-retained restorations were reported to the University for repair. If this assumption is valid, the complication rates calculated with the time period of the last chart review are accurate (Table 4). Ideally, patients in this study would have been recalled or interviewed via telephone survey to evaluate if any complications occurred or were treated elsewhere. Given limitations in funding, these additional procedures did not occur. Thus, the results of this study are weaker, and the two complication rates were calculated.
Lack of a uniform prosthodontic protocol and controls, the small sample sizes and short-term follow-up limit the validity of the data in this study, but suggest promising results. Long-term prospective clinical trials on the use of the Spline implant system will be necessary before any definitive conclusions can be reached. Questions regarding the long-term stability of this particular connection or even the possibility of one of the small Spline projections fracturing after long-term clinical function remain.
For implant therapy to become an integral part of mainstream dentistry, implant restorations must maintain a high degree of clinical success and predictability for the average clinician over a reasonable period of time. Within the scope of this short-term, retrospective study, Spline dental implants provided a stable implant prosthetic connection from 29 months to 59 months of clinical follow-up.
The authors claim to have no financial interest in any company or any of the products mentioned in this article.
This project was supported, in part, by The Ohio State University College of Dentistry Student Summer Research Program (NIH DE 07155–13 grant) and The Ohio State Clinical Research Curriculum (NIH/NHLBI grant #K30 HL 04162). This manuscript was also presented, in part, at The 2000 International Association of Dental Research Annual Session.
The authors would like to thank previous dental students from The Ohio State University College of Dentistry, Dr. Ryan Eskridge and Dr. Sun-Woo Hwang, for their assistance with this study. Appreciation is also extended to Robert Riley, CDT, and Michael D. Warner, MA, for their initial assistance with the preparation of this manuscript.
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Abstract Translations [German, Spanish, Portuguese, Japanese]
AUTOR(EN): Alvin G. Wee, B.D.S., M.S.*, Edwin A. McGlumphy, D.D.S., M.S.**. * Stellvertretender Professor, Abteilung für wiederherstellende Zahnmedizin, Zahnersatzkunde und Endodontie, zahnmedizinische Fakultät der staatlichen Universität Ohio, Columbus, Ohio. Und ** Professor A.O., Abteilung für wiederherstellende Zahnmedizin, Zahnersatzkunde und Endodontie, zahnmedizinische Fakultät der staatlichen Universität Ohio, Columbus, Ohio. Schriftverkehr: Dr. Alvin G. Wee, zahnmedizinische Fakultät der staatlichen Universität Ohio (OSU School of Dentistry), Abteilung für wiederherstellende Zahnmedizin, Zahnersatzkunde und Endodontie (Restorative Dentistry, Prosthodontics and Endodontics), Postle Hall, 305 West 12th Avenue, Columbus, OH 43210 – 1241. Telefon: 614-292-0785 (0), Fax: 614-292-9422 (0); eMail: [email protected]
ZUSSAMENFASSUNG: Der Zugang zu den entsprechenden klinischen Daten bezüglich des Auftretens von Komplikationen bei der Gebisswiederherstellung mittels Zahnimplantaten kann behandelnden Zahnärzten wichtige Informationen zur Bewertung von Zahnimplantationssystemen liefern. So wurde bislang nur geringfügig oder gar nichts über die Schwierigkeiten berichtet, die bei Lockerung der Verschraubungen an der Prothese auftreten. Ebenso gibt es nur wenig veröffentlichte klinische Daten über den Einsatz von Spline Verbindungen bei Zahnimplantierungsbehandlungen. Daher nimmt vorliegender Bericht die Ergebnisse eines Laborversuches auf, der auf die Feststellung der Komplikationen abhebt, die bei Verwendung des Spline Zahnimplantationssystems zur Gebisswiederherstellung aufgetreten sind. Die dem Versuch zu Grunde liegenden Implantationsbehandlungen wurden von Studenten und Assistenzärzten der zahnmedizinischen Fakultät der staatlichen Universität Ohio durchgeführt. Aus der Datenbank der Universität wurde eine umfangreiche Liste von Patienten erstellt, die sich einer Zahnimplantationsbehandlung unterzogen hatten. Hieraus wurde eine ausführliche tabellarische Auflistung generiert, und alle Patienten, denen Spline Zahnimplantate eingesetzt worden waren, wurden mit in die Studie aufgenommen. Bei Erstellung einer zweiten Übersicht wurden dann alle bei diesen mit Spline Implantaten durchgeführten Implantationsbehandlungen aufgetretenen Komplikationen auf speziellen Berichtsblättern erfasst und zur Auswertung in einer statistischen Analyse in eine Datenbank eingepflegt. Die Analyse ergab, dass es in 95,75 % der Fälle zu keinerlei Komplikationen kam. Es konnten die Fälle herausgefiltert werden, in denen es aufgrund des ärztlichen Eingriffes zur Lockerung der Schrauben kam. Nimmt man die Resultate dieser, einen relativ kurzen Zeitraum (die Nachuntersuchungen fanden 29 bis 59 Monate nach Wiederherstellung statt) erfassenden Studie, so schneiden die Spline Zahnimplantate als beständige Implantatverbindung im Bereich der Zahnprothetik gut ab.
SCHLÜSSELWÖRTER: retrospektive Kohortenstudie, Vorkommensdichte, Bruch, Verschraubungslockerung
AUTOR(ES): Alvin G. Wee, B.D.S., M.S.*, Edwin A. McGlumphy, D.D.S., M.S.**. *Profesor Asistente, Sección de Odontología de Restauración, Prostodónticas y Endodóntica, Facultad de Odontología de Ohio State University, Columbus, OH. **Profesor Asociado, Sección de Odontología de Restauración, Prostodónticas y Endodóntica, Facultad de Odontología de Ohio State University, Columbus, OH. Correspondencia a: Dr. Alvin G. Wee, OSU School of Dentistry, Restorative Dentistry, Prosthodontics and Endodontics, Postle Hall, 305 West 12th Avenue, Columbus, OH 43210-1241. Teléfono: 614-292-0785 (oficina), Fax: 614-292-9422 (oficina). Correo electrónico: [email protected]
ABSTRACTO: El acceso a datos clínicos precisos sobre la frecuencia de las complicaciones prostodónticas encontradas en las restauraciones de los implantes dentales pueden ofrecer a los dentistas importante información para la evaluación de los sistemas de implantes dentales. La complicación del aflojamiento del tornillo prostético, por ejemplo, ha menudo ha sido ignorado o poco reconocido. Solamente datos clínicos limitados publicados existen sobre el uso de las conexiones Spline en las restauraciones de los implantes dentales. Este trabajo presenta los resultados de una investigación in vitro de las complicaciones prostodónticas encontradas con el sistema de implantes dentales Spline restaurados por estudiantes y residentes en la Facultad de Odontología de la Ohio State University. De una lista general de los pacientes con implantes generadas de los archivos electrónicos de la universidad, se realizó una evaluación detallada de los registros y todos los pacientes que fueron tratados con implantes dentales Spline participaron en el estudio. Durante una segunda revisión de los registros, los datos de todas las complicaciones prostodónticas con las restauraciones de implantes Spline se recogieron en formularios de informes, luego se ingresaron a una base de datos computarizada y se sometieron a análisis estadístico. Los resultados demostraron que un 95,75% de las prótesis estaban libre de complicaciones. Los factores iatrogénicos que resultaron en el aflojamiento de los tornillos fueron identificados. Según las conclusiones de este estudio relativamente de corto plazo que varío entre los 29 a los 59 meses de seguimiento clínico posterior a la restauración, los implantes dentales Spline proporcionan una conexión prostética estable para el implante.
PALABRAS CLAVES: estudio retrospectivo de cohortes, densidad de incidencia, fractura, aflojamiento del tornillo
AUTOR(ES): Alvin G. Wee, B.D.S., M.S.*, Edwin A. McGlumphy, D.D.S., M.S.**. *Professor Adjunto, Seção de Odontologia, Prostodontia e Endodontia Restauradoras, Faculdade de Odontologia da Universidade Estadual de Ohio, Columbus, OH, e **Professor Associado, Seção de Odontologia, Prostodontia e Endodontia Restauradoras, Faculdade de Odontologia da Universidade Estadual de Ohio, Columbus, OH. Correspondências devem ser enviadas a: Dr. Alvin G. Wee, OSU School of Dentistry, Restorative Dentistry, Prosthodontics and Endodontics, Postle Hall, 305 West 12th Avenue, Columbus, OH 43210-1241. Telefone: (614) 292-0785 – Esc., Fax: (614) 292-9422 – Esc. E-mail: [email protected]
SINOPSE: o acesso aos dados clínicos precisos a respeito da freqüência de complicações prostodônticas encontradas em restaurações de implantes odontológicos pode proporcionar informações importantes aos dentistas em relação à análise de sistemas de implantes odontológicos. A complicação do afrouxamento do parafuso protético, por exemplo, tem sido muitas vezes ignorada ou a freqüência de sua ocorrência não tem sido reportada precisamente. Existe somente um número limitado de publicação de dados clínicos a respeito da utilização das conexões Spline para restaurações de implantes odontológicos. Este documento relata uma investigação in-vitro sobre as complicações prostodônticas encontradas com o sistema de implante odontológico Spline restaurado por alunos e residentes da Faculdade de Odontologia da Universidade Estadual de Ohio. A partir de uma relação abrangente e laborada com base nos registros de computador da Universidade, conduziu-se uma revisão de quadro extensiva e todos os pacientes tratados com os implantes odontológicos Spline® foram incluídos no estudo. Ao longo de uma segunda revisão de quadro, os dados de todas as complicações prostodônticas com as restaurações de implantes Spline foram agrupados em formulários de relatórios, digitados em uma base de dados de computador e então sujeitos à análise estatística. Os resultados demonstraram que 95,75 % das próteses não apresentavam complicações. Houve a identificação de fatores iatrogênicos que resultaram em afrouxamento de parafusos. Baseando-se nas descobertas deste estudo de relativamente curto prazo, com acompanhamento clínico pós-restaurador entre uma faixa de 29 a 59 meses, os implantes odontológicos Spline proporcionaram uma conexão protética estável de implantes.
PALAVRAS-CHAVES: estudo de coorte retrospectivo, densidade de incidência, fratura, afrouxamento de parafuso