|Year : 2014 | Volume
| Issue : 3 | Page : 123-129
Principles of screw - retained and cement - retained fixed implant prosthesis: A critical review
Sanath Shetty1, Aditi Garg2, K Kamalakanth Shenoy1
1 Department of Prosthodontics, Yenepoya Dental College, Mangalore, Karnataka, India
2 Department of Prosthodontics, Inderprastha Dental College, Sahibabad, Ghaziabad, Uttar Pradesh, India
|Date of Web Publication||18-Dec-2014|
Department of Prosthodontics, Inderprastha Dental College, Sahibabad, Ghaziabad, Uttar Pradesh
Source of Support: None, Conflict of Interest: None
| Abstract|| |
Dental implants in the past were mainly restricted to completely edentulous patients and to a certain extent in the distal extension situations. In the present scenario with a dramatic increase in the survival rates of the dental implants and their widespread acceptance by the patients, they are often being used in partially edentulous conditions and as well as single tooth replacements. However, longevity of the restoration, both functionally as well as esthetically depends on various factors prevailing in the clinical situation. One such factor of immense importance is the selection of connection between the implant abutment and the final prosthesis. This connection can be screw-retained or cement-retained depending on the clinical situation of the particular case. Although, screw-retained restorations have been successfully used since many years, there has been a shift in paradigm toward cement-retained restorations. This article reviews the various complications associated with screw-retained restorations and subsequently analyzes the various factors associated with the selection of the attachment mechanism in implant dentistry.
Clinical Relevance To Interdisciplinary Dentistry
- The article involves various disciplines of dentistry, particularly Prosthodontics, Periodontics, Oral Surgery and Dental Materials. As specified, in this literature, selection of connection system between implant and the prosthesis plays a significant role in reducing the prevalence of various periodontal (Peri-implantitis, bone loss), prosthetic (screw loosening or fractured prosthesis) and surgical complications associated with implant supported prosthesis. Thus, a thorough knowledge of this aspect, significantly increases the success rate of implant supported prosthesis in partially and completely edentulous patients.
Keywords: Cement, dental implants, retention, screw
|How to cite this article:|
Shetty S, Garg A, Shenoy K K. Principles of screw - retained and cement - retained fixed implant prosthesis: A critical review. J Interdiscip Dentistry 2014;4:123-9
|How to cite this URL:|
Shetty S, Garg A, Shenoy K K. Principles of screw - retained and cement - retained fixed implant prosthesis: A critical review. J Interdiscip Dentistry [serial online] 2014 [cited 2019 Sep 20];4:123-9. Available from: http://www.jidonline.com/text.asp?2014/4/3/123/147329
| Introduction|| |
Long-term success is the prime goal for any restoration in prosthetic treatment and so is it with implant dentistry. Several factors concerning the materials used as well as techniques followed in the clinical practice influence the relative outcome of the final prosthesis. One of such concerns is the connection between the prosthesis and the implant.  The attachment of the restoration to an implant can be accomplished through screw-retention, cementation, or a combination of both. 
Screw-retained prostheses have a well-documented history of successful application in completely edentulous patients because of its ease of retrievability, reduced biological complications such as bone loss, and peri-implant diseases, ease of hygiene maintenance, repairs and provision for future surgical interventions, if required. ,, However, technical complications such as screw loosening and breakage, the porcelain veneer fracture due to offset loads transmitted at implant-prosthesis connection, require very precise surgical techniques to avoid these issues [Figure 1]a. There has been a rapid switch in trends of retention system from screw-retained to cement-retained implant restorations, which provide optimal occlusal design, superior esthetics, passively fitting restorations, and axial stress distribution to the prosthetic components and bone-implant interface, with high degree of retrievability using soft access cements based on clinical judgment [Figure 1]b. ,,,
|Figure 1: (a) Screw-retained restorations (b) Cement-retained restorations|
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The purpose of this article is to discuss the various factors that regulate the decision in the selection of the attachment mechanism in implant dentistry. These include retention, retrievability, passivity, occlusion and axial loading, occlusal material fracture, esthetics and hygiene, abutment-crown crevice, ease and cost of fabrication.
| Retention|| |
For screw-retained prostheses, the screw that connects the implant with the abutment and the abutment with the prosthesis is the main factor aiding in retention, as validated by the studies of the Branemark system (Nobel Biocare, Zόrich, Switzerland). ,,
Literature exhibits that screw loosening and screw-breakage is a major technical complication with these restorations. Since screw is an inclined plane, any discrepancy such as in passivity or accuracy of super structures, occlusal scheme, or existing parafunctional habit, results in emergence of fulcrums points at the junction of abutment and implant.  The consequential offset loads at the interface are of sufficient magnitude to overcome the clamping forces of the screw, as a result, screw gets stretched, broken or loosened.
To overcome this, various biomechanical considerations such as preload to be 75% of the yield strength, torque ranging from 20 to 35 N/cm, , optimum screw head design incorporating flat head, long stem length, with six threads (30° angle and V-shaped design), results in a continuum of pivot points is created around the circumference, which results in axial loading of the implant and avoids screw loosening.
Another factor of concern is the material of the screw, which is usually gold or titanium. In the case of the titanium screw, the phenomenon of galling causes slight damage to both the implant and fastening screw threads. , Conversely, gold screws have a low co-efficient of friction, allowing them to be tightened more effectively than titanium without risking galling between the threads. However, gold screws should be used only for actual seating of the prostheses and not for any laboratory procedures because of the soft structure of the material.
The factors influencing the retention of the cement-retained restoration include taper of the abutment, surface area and height of the abutment, surface texture of the abutment and type of the cement.
Taper of the abutment
Ideal taper to provide optimal retention and reduced uncementation, as proved by Jorgensen,  is 6° for a prepared natural tooth, which would by all means be an ideal taper for an implant abutment as well.
Height of the abutment
Kaufman et al.  proved that an additional 4-7 mm of abutment height, increases retention by 67%. This is due to increase in available surface area as well as increased resistance of abutment to lateral forces.  Under the lateral forces, prosthesis tends to rotate upwards on one side of the implant along the arc of rotation [Figure 2]. The height of the abutment should be greater than the arc of rotation. A wider implant requires a greater height than a smaller diameter implant to resist lateral forces. Placement of the vertical directional grooves (mesial and distal) in low profile abutments increases the retention, by decreasing the arc of rotation.
Surface area of the abutment
The wider implant has better retention than standard narrow sized implant, due to increased surface area it provides.
Width of the abutment
When compared, two abutments of the same height, the one with reduced diameter has increased resistance to lateral forces [Figure 3]a and b], due to decreased arc of rotation. At the same time, an increase in diameter in turn increases the surface area for bonding of intervening cement and thereby increases retention against vertical/axial loads. However, whether the effect of increased surface area surpasses the effect of increased arc of rotation is still debatable.
|Figure 3: (a) Larger diameter implant abutment has greater retention. (b) Smaller diameter implant abutment has lesser retention|
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Surface texture of the abutment
Enhanced retention for cemented prosthesis is acquired, by creating microretentive irregularities on the abutment surface, into which the luting agents bond. , Also, the internal aspect of the casting as well as the abutment surface is air abraded with 50 μm of alumina to enhance its retention for cementation.
Selection of cement
Provisional cementation along with ideal taper of the implant abutment and longer walls allows controlling the overall retention of the restoration as well as its retrievability. , Also, the principle of progressive cementation is used in cases where provisional cement fails to provide adequate retention.
Cement selection for prosthesis depends on the assessment of the clinical situation by the prosthodontist based on analysis of factors such as number and location of the abutments, height and width of the abutment, degree of the taper, retention and resistance form, and design of the abutment. ,, Also, the provisional restoration may be used as a guide to determine appropriate cement that is retrievable and yet will not become loose during function. 
In view of fore mentioned factors, it can be dictated that prostheses cemented over accurately fitting machined abutments establish a more stable and passive environment than screw-retained castings with microgaps and unfavorable loading characteristics,  except in the situations of limited interocclusal distance. Cemented prosthesis require a vertical component of at least 5 mm to provide retention and resistance form. In cases when abutment height is inferior to 5 mm, screw-retained prosthesis provide a considerable advantage over the cemented prosthesis. 
| Retrievability|| |
Screw-retained fixed implant prosthesis is recommended with retrievability as its major advantage, which permits hygiene maintenance, repairs, or any surgical intervention.  This is of particular advantage in multiple-unit, full-arch, or cantilevered prostheses and when interarch space allows only a low profile abutment. However, with advancements in implant surfaces and designs, prosthetic components, clinical techniques and dental materials, implant supported fixed prosthesis have been shown to have survival rate of >90% over a period of 15 years, which is quiet high, in contrast to tooth supported fixed partial dentures. ,, This is due to the fact that implant abutments are not prone to decay and do not need any endodontic therapy, thus eliminating retrievability as the matter of prime concern.
Retrievability in cement-retained restorations is possible with the use of soft access cement as a luting agent. This is in agreement with the fact that a provisional prosthesis is always cemented over natural teeth and later retrieved for final cementation.  Also, cements do not adhere as tenaciously to implant abutments as they are smooth and well-polished without any undercuts when compared to their natural counterparts.  So in case frequent debonding with soft access cement is seen, even harder cements can be used and yet still be removed readily. ,,, However, harder cement when removed results in scratches on implant abutment. This could act as the site for plaque accumulation. Hence, implant components of alloys with improved mechanical properties are preferred.
| Passivity of the prostheses|| |
Passive casting is one of the important prerequisites for long-term success of implant prosthesis to prevent biological and prosthetic complications [Figure 4].  The former includes increased transfer of load to the bone resulting in bone loss, and development of microflora at the gap between the implant and abutment. The latter includes loosening or fracture of the fastening screw, implant fracture and unretained restorations [Figure 5]. Truly passive screw-retained implant prosthesis is virtually impossible to fabricate. ,, Passive fit was described by Branemark to be ideally in the 10 μm range. However, this is not attainable in screw-retained prosthesis due to dimensional discrepancies inherent in the clinical and laboratory procedures which includes, distortion when final impression material shrinks while setting, dental stone expansion, wax pattern distortion while setting or spruing, expansion of investment material, shrinkage of metal casting, acrylic/porcelain shrinkage, soldering inaccuracies, and manufacturer variance of a number of implant component. ,
However, the passivity of casting is not an issue of as much concern in cement-retained prosthesis. In this attachment system, die spacers create 40 μm cement space that compensates for the dimensional variations of laboratory materials and permits the fabrication of a more passive casting with cement-retained restorations. Further if the casting is not passive, in case of cemented prosthesis, the casting or the abutment may be modified slightly at the same try-in appointment.  The resolution of the same difficulty in screw-retained prosthesis requires separation and soldering of the casting or an all together new fabrication of casting is needed.
| Occlusion and axial loading|| |
Axial loading of the implant is a prime requisite for proper load distribution [Figure 6]a. To achieve this, primary occlusal contacts are placed on the implant head. However, this goal is not feasible with screw-retained prosthesis because of screws and overlying occlusal obturation material, which are not capable of taking axial loading. Hence, the primary occlusal contact is often placed on the buccal cusp in the mandible that creates an offset load. According to various biomechanical principles, increased offset loads increases the stress at the bony interface resulting in increased incidence of screw loosening and breakage [Figure 6]b. ,,, In cemented prosthesis, the primary occlusal contact is directly over the top of each implant and has the ability to vertically load the prosthetic head of the implant [Figure 7].
|Figure 6: (a) Reduced stresses with axial loading of implant (b) Increased stresses with offset loading of implants|
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|Figure 7: Areas of primary and secondary occlusal contacts in screw-retained and cement-retained fixed implant restorations|
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| Occlusal material fracture|| |
An increased incidence of porcelain fracture has been observed with the presence of an access opening for an occlusal/abutment screw. In this situation, the integrity of the framework and the veneer layers are interrupted and tension might be produced while tightening the assembly and manipulations with the screw-driver, provoking stress peaks laterally in the region of the access opening and results in unsupported porcelain which fractures more often. , Wittneben et al.  analyzed the clinical performance of two prosthetic connection systems, and concluded that chipping of the resin veneer was extremely frequent in screw-retained restorations and is the second most common complication for screw-retained reconstructions. These complications were also mainly seen in full-arch reconstructions, and this should be taken in account for designing implant supported prosthesis in edentulous patients.
| Esthetics and hygiene|| |
Screw-retained restorations have screw access openings, which compromise esthetics, if the implant is placed facially, as ideally required. Therefore, an implant placement is planned in a more palatal direction which allows access hole to the prosthetic screw to be placed in the cingulum area. Hence, a facial porcelain ridge lap is required to achieve proper emergence profile. This makes the cervical sulcus of the implant inaccessible for hygiene. However, this is not an issue with cement-retained restorations. Also, the emergence profile of n anterior cemented a crown does not require a facial porcelain ridge lap because the implant may be placed under the incisal edge, rather than the cingulum [Figure 8]. 
|Figure 8: Compromised esthetics in screw-retained prosthesis due to facial porcelain ridge lap|
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| ABUTMENT - CROWN CREVICE|| |
In vitro and animal studies have been conducted to evaluate technical and biologic complications in screw-and cement-retained prostheses. , Wittneben et al.  assessed that rate of biological complications, such as the presence of fistula/suppuration appeared statistically significantly more often with cemented restorations. The major problem is excess cement, which has been associated with the development of the peri-implant diseases such as peri-implant mucositis and peri-implantitis and progressive bone loss. ,, Wilson  established positive relationship between excess cement and peri-implant disease, and observed improvement after removal of excess cement.
| Ease and cost of fabrication|| |
Traditional prosthetic techniques and simple laboratory skills involved make the fabrication of the cement-retained prosthesis easier and convenient when compared to the screw-retained prosthesis. This is owing to the fact that screw-retained restorations require additional laboratory components such as impression transfer, analogs, copings, and screws. Consequently, the laboratory cost is 1.5-2 times than for the cemented restorations.
| Discussion|| |
Implant supported restorations are well-established treatment option, to replace the missing teeth. With the advancement of implant surfaces and designs, prosthetic components, clinical techniques, and dental materials, successful, functional and stable treatment can be achieved. Selection of type of connection between the final prosthesis and implant is a criterion of significant importance. Screw-retained implant restorations have advantage of predictable retrievability enabling ease of hygiene maintenance, repairs or any required surgical interventions, and require a minimal interocclusal space. These restorations require precise, prosthetically driven placement of the implant due to the position of the screw access hole. The manufacturing is more technique sensitive and demanding when compared to cement-retained prosthesis. Cement-retained restorations are more cost efficient, capable of compensating for implant position discrepancies, passivity of fit, improved esthetics and easier control of occlusion. However, a major problem of cement retention is excess cement, which creates an anaerobic niche for undisturbed growth of a biofilm and plays a significant role in the development of infections and progressive bone loss.
Various reviews on advantages and disadvantages of the two connection systems reveal conflicting information. ,,,, However, review done by Weber and Sukotjo  on effect of choice of connection between prosthesis and implant, on implant and prosthesis survival, found no statistically significant differences between screw and cement-retained prosthesis. Various animal and in vitro studies , have focused on technical and biological complications in screw- and cement-retained prostheses. These include a porcelain veneer fracture, screw loosening, loss of retention, peri-implantitis and bone loss being the most prevalent complications. A systemic review by Sailer et al.  on survival and complication rates of screw- and cement-retained restorations reported no statistically significant difference for the restoration survival. However, cement-retained restorations exhibited higher rates of biological complications with an increased incidence of bone loss and peri-implantitis. Based on their improved retrievability, the screw-retained prostheses were given preference, although they did exhibit more technical complications.
| Conclusion|| |
Retention system that is more functional and stable in the successful management of future failures and complications should be selected based on individual patient situation since both screw- and cement-retained have their advantages and disadvantages. Increased implant predictability, patient demand for high esthetic outcomes and lower cost recommend use of cement-retained restorations for implant-supported single crowns. Due to increased technical and prosthetic complications associated with screw-retained prosthesis, cement-retained restorations are preferred in patients with parafunctional habits.
For multiple-units implant supported restorations, and in patients with limited interarch space screw-retained restorations are more acceptable.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8]